CdTe has proven to be one of the most promising absorbers for producing high-efficiency and low-cost thin-film solar cells. The record efficiency of laboratory-size CdTe thin-film solar cells has dramatically improved in the last three years, from 17.3% in 2011 to 21% in 2014. However, the record efficiency is still far below the theoretical prediction of 32%. In the past few years, we have explored various approaches to improve the performance of CdTe-based thin-film solar cells. In this talk, we will review our recent progresses, including (1) the realization of efficient CdTe-based thin film solar cells using nominally Cu-free back contacts; (2) achieving high open circuit voltage via dedicated control of Cu incorporation and heat treatments; (3) enhancing short circuit currents using alternative window layers. Our results indicate that improvements of the open circuit voltage, short circuit current, and fill factor of CdTe-based thin-film solar cells through dedicated synthesis and interface engineering. The approaches applied in our research may open up opportunities for further improvement of the efficiency of CdTe solar cells.
This work was supported by the DOE-FPACE program under contract No: 0492-1662 and the Ohio Research Scholar Program.

Copper-doped ZnTe (ZnTe:Cu) is commonly used in CdTe devices as a back buffer layer to overcome the formation of a Schottky barrier in CdTe when contacted directly with a metal. Since the evaporation can be controlled individually for ZnTe and Cu during co-evaporation, co-evaporated compared to sputtered ZnTe:Cu provides more control to investigate the effect of the ZnTe to Cu ratio, since the ZnTe to Cu ratio is fixed for a given sputter target. Co-evaporated ZnTe:Cu can also provide better interfaces compared to sputtered ZnTe:Cu which can suffer from sputter damage. We have used rapid thermal processing (RTP) to activate the back contact. The precise control of time-temperature-trajectory in RTP provides improved control over Cu activation and distribution. RTP also has a minimal impact in the front end processes due to the reduced thermal budget. Both the ZnTe to Cu ratio and the RTP activation temperature provide independent control over the device performance. Maximum achievable efficiency and the optimal rapid thermal activation temperature vary with the ZnTe to Cu ratio. We have also investigated the influence of various RTP conditions to Cu activation and distribution. Current density-voltage, quantum efficiency, and capacitance-voltage measurements were used to examine the device performance in terms of ZnTe to Cu ratio and rapid thermal activation temperature. We have successfully used the optimized conditions of ZnTe:Cu and rapid thermal processing to achieve devices with certified efficiencies greater than 16%.

Controlling the introduction and redistribution of copper in the back contact region of CdTe solar cells is critical for both high efficiency and achieving good stability. In this work we describe the improved performance of ZnTe:Cu contacted solar cells through the use of rapid thermal processing (RTP). The contacts are deposited at low temperature to prevent premature Cu diffusion, and activated using short RTP steps. Under optimal conditions very good device performance (>15%) is obtained due to significant improvements in V_oc (>850 mV) and FF (>75%). Analysis of secondary ion mass spectrometry (SIMS) profiles suggests that ZnTe inhibits Cu diffusion into the CdTe absorber. Atom probe tomography (APT) is used to provide 3D elemental mapping of the back contact region, and it is shown that substantial rearrangement within the back contact ZnTe:Cu buffer layer is observed after short RTP treatments, with Cu preferentially segregating to both the CdTe and Au interfaces under optimal conditions. It is suggested that such migration passivates interface defects and enables efficient hole tunneling, which are postulated as the underlying reasons for the observed device improvements. Initial experiments employed a gold metallization layer, and SIMS revealed that a substantial fraction of the initial copper diffuses into the gold during RTP, which is unsurprising given the unlimited miscibility of these two metals. We have explored alternative metals such as chromium and titanium, and it is shown that similar performance may be obtained, however the optimal amount of Cu in the as-deposited ZnTe layer must be reduced relative to Au since these alternative metals have very limited miscibility with Cu. Comparisons of the J-V and quantum efficiency response from devices made with varying amounts of Cu and different metals are compared to improve our understanding of the roles of copper in these devices. Since Cu has been implicated as a major factor in CdTe device stability we have initiated accelerated lifetime testing of these devices as well. Initial results appear promising, and we will provide additional updates from these studies at the meeting.

Recently NREL researchers have developed a capability for operando x-ray photoelectron spectroscopy (op-XPS) measurements that can be applied to PV materials and devices. In this context, op-XPS measurements are performed under relevant conditions of electrical and/or light bias, and provide a means for connecting static (equilibrium) material properties such as band offsets with dynamic (non-equilibrium) properties such as surface photovoltage and quasi-Fermi level splitting under illumination. In light-biased op-XPS measurements under open-circuit conditions there is no photocurrent, and therefore op-XPS measurements can be performed on bare absorber materials, partially completed device stacks, or complete devices. As a result it is possible to track the development of device photovoltage as a function of process step. Our preliminary op-XPS measurements have also revealed the unexpected result that x-ray excitation in standard XPS measurements generates sufficient electron-hole pairs to create a non-trivial photovoltage in PV materials and device structures. If not properly accounted for in band-offset measurements, this stray photovoltage leads to a distorted picture of the resulting equilibrium band diagram.
We have applied the op-XPS methodology in a comparative study of CIGS and CZTSe film stacks and devices, with the goal of understanding the origin of the so-called ‘V_OC deficit&’ in CZTSe devices. For both CIGS and CZTSe, we find that the quasi-Fermi-level splitting that is present following CdS chemical-bath deposition is equal to V_OC in the completed devices, indicating that the electronic junction is complete at this stage of processing, and that the CZTSe V_OC deficit is also present at this stage. We construct equilibrium band diagrams for standard CIGS and CZTSe device structures and highlight key differences, including band alignments at the CIGS/CdS and CZTSe/CdS interfaces, and band bending in the CdS layers. Finally, we present evidence that the observed differences are symptomatic of device-performance-limiting defects at the CZTSe/CdS interface.

The relationship of grain boundaries to the performance of semiconductor devices has long remained a topic of many research efforts. Whether all randomly oriented grain boundaries within a poly-crystalline material contribute similarly to the performance or a few particularly unfavorable ones tend dominate the effects will largely depend on the atomic structure of these interfaces and their interaction with impurities and dopants. In this study we investigate atomic structures of grain boundaries using CdTe bi-crystals which are intended to mimic real grain boundaries found in thin film CdTe solar cells. Low minority carrier lifetimes, that seem to hold back further improvement of the power conversion efficiency, are largely attributed to excessive non-radiative recombination in bulk, space-charge region and grain boundaries. Thus fundamental understanding of the role that grain boundaries play in limiting the device performance is needed.
CdTe bi-crystals, with pre-defined interface planes and misorientation angles, are obtained using ultra-high vacuum (UHV) bonding of CdTe wafers. Atomic-scale imaging of the interfaces is carried out in an aberration-corrected cold-field emission scanning transmission electron microscope (STEM) using high-angle annular dark field (HAADF) and annular bright field (ABF) detectors. Chemical composition of the defect cores is examined using atomic-column resolved X-ray energy dispersive spectrum imaging (XEDS). Strain fields associated with the interface defects and dislocations are calculated using the geometrical phase analysis (GPA).
Latest results of our systematic study of grain boundaries will be presented, including analysis of low-angle tilt bi-crystal with (110) interface plane. The interface is shown to be composed of a series of undissociated Lomer pure-edge dislocations accompanied by secondary mixed-character dislocations along the boundary. Atomic investigation of As and N p-type dopant distributions in CdTe will be demonstrated which are candidates to replace Cu in conventional poly-CdTe devices that has adverse effects on grain boundaries and the p-n junction.

Thin-film PV provides a cost effective alternative to traditional wafer based PV manufacturing as the direct deposition of absorbers reduces cost associated with cutting, polishing, and processing that materials grown by other methods must undergo. The reduction in cost is an attractive factor, but producing thin-film modules with efficiencies competitive with wafer based modules remains a challenge. Even though the topic has been studied for many years, the impact of extended defects on device and subsequently module performance is not well understood for thin-film PV materials in general. There are several reasons for this that include the dimensions of absorber materials, limitations of individual characterization techniques, and sample preparation. In this work we show how multiple types of scanning probe microscopy, electron microscopy, and atomic-scale analyses can be utilized to gain a more complete understanding of the factors that limit performance in thin-film PV materials. This approach is enhanced by novel sample preparation strategies that reduce artifacts in measurements and enable the application of additional techniques to samples that would otherwise only be suitable for specific measurements. It is important to note that each material system has specific requirements in the context of sample preparation that must be addressed to insure the accuracy of characterization results. The multi-technique and multi-scale nature of the characterization methodology demonstrated in this presentation is applicable to any material system and generally results in a more thorough understanding of factors that impose limitations on device performance.

Polycrystalline thin film absorbers used in high-efficiency CuIn_1-xGa_xSe₂ photovoltaic solar cells have graded composition. Band gap in such absorbers has a well-defined depth profile. Time resolved photoluminescence (TRPL) spectroscopy for graded absorbers allows analysis of recombination at the surface/interface and in the bulk, determination of minority carrier mobility, as well as independent confirmation of depth-dependent band gap variation (Kuciauskas et al, J. Appl. Phys., 114, 154505 (2013)). In this presentation, we summarize results of such analysis applied to: (1) polycrystalline CIGS thin films that were used to produce high efficiency (>20%) PV devices, (2) high-Ga (x>0.4) polycrystalline CIGS films, and (3) CIGS PV devices. By using laser excitation at 400 nm, 600 nm, and 800 nm, we analyze grading effects and recombination in different regions of the absorber - from <60 nm depth to the bulk. Results suggest that interface recombination depends not only on defect concentration, but also on carrier dynamics (on the supply of carriers in the vicinity of the interface, which can be controlled/altered by grading). With larger band gap grading close to the surface/interface carrier drift reduces recombination, which could be one of the reasons for increased device efficiency. In the bulk, we find that disorder has large effect on recombination rate, and recombination characteristics (not only lifetime) appear to differ for absorbers with different Ga compositions. In summary, this study provides more detailed understanding of recombination in polycrystalline CIGS thin films and PV devices, and advances recombination analysis beyond a single-lifetime approximation.

Thin-film solar cells have been studied intensively in the recent decades because of their flexibility, lightweight, and ease of fabrication. CuInGaSe₂ (CIGS) semiconductors have been anticipated as a promising material for high efficient thin-film solar cells [1]. To date, CIGS-based solar cells hold the world record of 21.7% power conversion efficiency among thin-film solar cells. One of the most interesting characteristics of CIGS is the superiority of polycrystalline type over the single crystalline type. A better understanding of the optical properties in polycrystalline CIGS is important to design more efficient CIGS-based solar cells. However, owing to their complicated band-edge structures, dynamical behavior of photogenerated carriers in polycrystalline CIGS still remains unclear. In this work, we reveal the energy relaxation and recombination processes of free carriers in polycrystalline CIGS films using various optical techniques.
Recently, our group has demonstrated that the photocarrier dynamics in various thin-film solar cells can be revealed by combining photoluminescence (PL), photocurrent (PC), and transient absorption (TA) techniques [2-4]. Here, picosecond time-resolved PL and femtosecond TA measurements were used to clarify the photocarrier recombination and localization dynamics in the polycrystalline CIGS samples. We found two characteristic dynamical behaviors of free electrons, which play a dominant role in the optoelectronic conversion process, in polycrystalline CIGS: (i) Existence of long-lived electrons in the conduction band (nanosecond lifetimes). (ii) Slow energy relaxation until reaching the bottom of the conduction band (a subpicosecond time scale). The existence of long-lived free electrons is attributed to thermal excitation of trapped carriers at donor states. Slow energy relaxation of free electrons is mainly caused by the spatial electrostatic potential fluctuation, resulting in the suppression of fast energy relaxation through carrier-carrier scattering processes that occur within approximately 100 fs. We conclude that these two features are ones of the essential factors to realize the high short-circuit currents of CIGS solar cells [5].
This work was supported by JST-CREST and the Sumitomo Electric Industries Group CSR Foundation.
[1] S. Niki et al., Prog. Photo. Res. Appl. 18, 453 (2010).
[2] L. Q. Phuong et al., Appl. Phys. Lett. 103, 191902 (2013).
[3] D. M. Tex et al., Phys. Rev. B 89, 125301 (2014).
[4] Y. Yamada et al., J. Am. Chem. Soc. 136, 11610 (2014).
[5] M. Okano et al., Phys. Rev. B 89, 195203 (2014).

Scanning probe microscopy has been successfully implemented to qualitatively probe the electrical characteristics of thin-film compound semiconductor solar cells. However, a direct correlation between the measured signals and the figures of merit that define the device performance is still missing. Here we show a variant of illuminated Kelvin probe force microscopy (KPFM) to image and spatially resolve the V_oc of solar cells with truly nanoscale resolution (<100 nm), which is more than 5 orders of magnitude better than previous methods [1]. In our new method, the surface photovoltage signal is equal to the difference between the contact potential difference under illumination and in the dark, which is equal to the photo-generated voltage of the device (and is proportional to the Fermi level splitting). Additionally, we apply scanning photocurrent microscopy using NSOM probes as a local source of excitation to spatially and spectrally resolve the external quantum efficiency (EQE) within the devices, also with nanoscale resolution, while mimicking the power density operation conditions of real devices [2]. Combined, these new tools provide a full picture of the local optoelectric response of PV devices, including an indirect measurement of carrier recombination and the local collection properties of the material, respectively. We apply our novel metrology to thin-film compound PV and find that the V_oc in CIGS devices varies locally by more than 200 mV, suggesting that spatial variation of non-radiative recombination strongly affects the overall device performance. We determine the local collection characteristics of CIGS and CdTe solar cells by locally mapping EQE with nanoscale resolution. For CdTe, we find that at short wavelengths (<600 nm), when light is absorbed near the exposed surface, the EQE is relatively small because of surface recombination. However, close to the material bandgap (860 nm), the EQE enhancement at the grain boundaries is ×1.5. These novel metrologies enable new insights into the loss mechanisms that hinder solar cells and provide a new platform to image device performance with nanoscale resolution.
[1] E. M. Tennyson et al., Nature Nanotech, in review.
[2] M. S. Leite et al., ACS Nano, in press. DOI: 10.1021/nn5052585

In order to study the local collection and to extract properties as the diffusion length in individual layers and the recombination velocities at contacts and at surfaces in thin-film solar cells, electron-beam-induced current (EBIC) measurements are usually employed. In the standard EBIC setup, the solar cell is analyzed in short-circuit (see, e.g., Ref 1). Enhanced information on the device performance is available when acquiring EBIC signals at applied bias. However, the current induced by this bias (mA level) is typically several orders of magnitude larger than the EBIC current (nA scale), i.e., the EBIC signal exhibits a small signal-to-noise ratio.
A solution to this problem is to apply lock-in amplification of the EBIC signal. For the present work, the measurements were performed in a scanning electron microscope equipped with a beam-blanker. Fractured, cross-sectional specimens of Cu(In,Ga)Se₂ solar cells were analyzed by means of EBIC at bias voltages ranging from -1 to 1 V. Profiles were extracted perpendicular to the p-n junction and simulated by an approach introduced by Donolato [2]. The simulation gives values for the minority-carrier diffusion lengths in the studied Cu(In,Ga)Se₂ absorber layers as well as for the recombination velocities at the Mo back contact and at the cross-section surface. Moreover, from the detected, differing widths of the space-charge regions w at varying applied bias V_a, values for the net doping Nshy;shy;_A (acceptor density) as well as for the build-in potential were calculated using the equation w = (2ε_rε₀(V_b-V_a)/e Nshy;shy;_A)^0.5, where ε, ε₀, V_b, and e are the dielectric susceptibilities of the investigated material and the vacuum, the build-in potential, and the elemental charge.
As next step, it is planned to combine the EBIC measurements at applied bias with electron backscatter diffraction on the same identical specimen positions, in order to analyze EBIC signals at or close to the working point of the solar cell and to study influences by local orientations, strain distributions, and grain boundaries.
[1] H.J. Leamy, J. Appl. Phys. 53 (1982) R51.
[2] C. Donolato, J. Appl. Phys. 66 (1989) 4524.

Understanding the energetic distribution and density of sub-bandgap states is crucially important to understanding the transport and optical properties of photovoltaic devices. To this end, the transient photocapacitance and photocurrent spectroscopy techniques are utilized to probe the sub-bandgap density of states on completed photovoltaic devices. In addition to being performed in situ, these techniques are sensitive enough to reveal sub-bandgap defect states with densities that are as many as nine orders of magnitude smaller than conduction band states. We utilize these techniques to observe the effect of Cu on the sub-bandgap density of states of CdTe thin-film solar cells. This enables a more detailed fundamental understanding of the well-known detrimental effects of Cu on the performance of CdTe devices. We have identified two defect bands residing at optical energies of ~0.9 eV and ~1.2 eV relative to the valence band. The 0.9 eV defect band has been associated with the presence of Cu by varying the Cu content in the absorber layer and oberserving the resultant change in the density of states. Increased Cu content leads to decreased minority carrier collection efficiency and photoluminescence lifetime due to increased recombination in the absorber layer. The chemical origin of the 1.2 eV defect band was not identified, although we have shown that its presence was not associated with Cu content.

The future replacement of CuIn_1-xGa_xSe₂ by Cu₂ZnSnS(e)₄ (CZTS(e)) absorbers is still pending due to their lower photovoltaic performance. The truth is despite being similar in many aspects, relevant differences still seem to be clearly reducing the efficiency of CZTS(e) based solar cells. The degradation of the commonly used Mo back contact in the CZTS(e) case has been already extensively reported as one of the main causes. TiN barriers and ZnO intermediate layers have been already used to minimize the degradation of the back contact interface. Nevertheless, the Mo/CZTSe interface still deserves much research effort to solve key problems such as uncontrolled MoSe₂ formation and poor morphology, evidencing a clear device performance dependency on the chosen Mo deposition conditions.
In this work we propose the introduction of a thin MoO₂ layer, thermally evaporated from the pure oxide powder, on top of the Mo back contact to minimize its inherent drawbacks. Three different types of Mo were deposited: Mo_A (800 nm, low pressure, 0.2 Omega;/sq), Mo_B (bi-layer, 150 nm medium pressure + 500 nm high pressure, 1.1 Omega;/sq) and Mo_C (bi-layer, 150 nm low pressure + 500 nm high pressure, 0.8 Omega;/sq). Additionally, three different back contact designs were created using these three Mo types, combined with the absence or either 10 or 20 nm of MoO₂. Finally, a 30 nm sacrificial layer of Mo_A was deposited on top of the MoO₂ layers. The CZTSe absorbers were synthesized via a two stage process, based on metallic DC sputtered precursors deposition followed by reactive annealing under Se + Sn atmosphere. After annealing, the absorbers were etched in KMnO₄ and Na₂S aqueous solutions. Morphological, structural and compositional characterizations were conducted using SEM, XRD, Raman spectroscopy and XRF techniques. Solar cells were completed using all the back contact configurations described before and characterized with J-V AM1.5G illuminated curves and spectral response.
We observe that despite using different back contact configurations in terms of deposition conditions (monolayer versus bi-layer, highly selenized versus non selenized, highly conductive versus low conductive) as reference solar cells, with efficiencies below 8% (7.3% for Mo_A, 4.7% for Mo_B and 5.7% for Mo_C) after introducing the MoO₂ layer the efficiency exceeds 8% when using the 20 nm MoO₂ layer (8.2% for Mo_A, 8.1% for Mo_B and Mo_C).
The main change in the optoelectronic properties is related to a remarkable increase in the shunt resistance, for example from 84 Omega;middot;cm² to 610 Omega;middot;cm² for the Mo_C case. This is coupled with an improvement on the MoSe₂/CZTSe interface morphology.
In conclusion, regardless the type of Mo used, we have demonstrated that the introduction of a thin MoO₂ layer on the CZTSe back contact structure can minimize the adverse effects of the selenization process while drastically improving its electrical properties.

Thin film photovoltaic (PV) technologies have shown great promise for solar energy harvesting using inexpensive PV materials, currently reaching a power conversion efficiency of 21 % for cadmium telluride (CdTe) research cells. To achieve the maximum efficiency of 30 % possible for this technology, key PV parameters have been studied by refining fabrication processes. However, the progress is hindered by the unclear role of microstructures in their operation and performance. In this study, we investigate inhomogeneous electronic properties of CdTe solar cells using electron beam induced current (EBIC) techniques. The generation rate of excess carriers and the size of excitation bulb were systematically varied by the injected electron beam energies (3 keV to 20 keV). For comparison, we prepared CdTe devices having a power conversion efficiency of 13 % and 3 % that were extracted from a solar panel. The EBIC efficiencies at the p-n junction were compared to the standard optical quantum efficiencies obtained under 1 sun illumination. EBIC measurements were performed on a cleaved and a FIB milled CdTe device to address possible surface effects originated from the ion milling process. We performed least-squares fitting of EBIC line scans to extract material parameters (e.g., effective minority carrier diffusion length, depletion width, surface recombination rate) and estimated the product of mobility-lifetime for the high and the low performance devices. Furthermore, we will discuss distortion of EBIC signal in a highly resistive PV cell under high injection condition.

Cu₂O is an earth abundant semiconductor identified as a promising photovoltaic material due to its high absorption and good minority carrier diffusion length, and its wide bandgap of 2.1 eV makes it a suitable wide bandgap candidate for a tandem solar cell with a crystalline Si bottom cell. While the bulk properties of Cu₂O are attractive for photovoltaic applications, surface instability is one of the most pressing issues that need to be resolved to achieve a high efficiency device, and epitaxial synthesis is thus a desirable approach. In this work, we demonstrate synthesis of an epitaxial Cu₂O photovoltaic thin film heterostructure by plasma-assisted molecular beam epitaxy (MBE).
Cuprous oxide thin solar cells are inherently heterostructures, as Cu₂O is intrinsically p-type and thus requires a heterojunction window layer, but Cu₂O is susceptible to deleterious interface reactions which impede high cell performance, and thus epitaxial synthesis is an attractive route to overcome these challenges. In this work, we use MgO as a heteroepitaxial template for the growth of Cu₂O heterostructures. We discuss epitaxy of Cu₂O on conductive film templates suitable for Ohmic contact formation, including Pt and Au grown on MgO, and emitter/window layer materials such as Ga₂O₃, which can also be grown on MgO. We focus on MgO as a substrate because of its close lattice match to Cu₂O and also because biaxially textured MgO can be grown on amorphous dielectric substrates, providing a path to tandem junction solar cells on silicon with Cu₂O top cells and crystalline silicon bottom cells. Epitaxial relationship and crystallinity are confirmed by reflection high energy electron diffraction (RHEED), and x-ray diffraction (XRD). Film roughness and morphology are analyzed by atomic force microscopy (AFM). Phase purity of the interface is confirmed by ex situ x-ray photoelectron spectroscopy (XPS). Interface structure and defects are further analyzed by cross-sectional transmission electron microscopy (TEM), and device results will also be reported.

In this work, we present the deposition of Cu₂ZnSn(S,Se)₄ CZTSSe absorbers by inkjet printing and their application in thin film solar cells. Inkjet printing is a promising approach, which can be easily adapted for industrial production. One of the most attractive issue when using inkjet printing is its low raw materials wastage during processing due to its drop-on-demand feature. However, the ink properties, i.e. solvent, viscosity and surface tension are critical parameters for the applicability of a material in inkjet printing process. Another challenge of inkjet printing processing of thin films is to obtain a good ink&’s wetting behavior of the substrate and hence a homogeneous coverage of the ink on the substrate. The drying kinetics and the resulting film homogeneity is a very complex issue and need profound understanding. The viscosity of the precursor inks was tuned, which can be monitored by the evolution of contact angle of the inks on Mo substrate. We successfully obtained CZTSSe thin film absorbers with large grains up to 1.3 µm by annealing the inkjet printed Cu-Zn-Sn-S precursor under chalcogen-containing atmosphere. Structural properties of CZTSSe thin film absorbers were characterized by X-ray diffraction and Raman spectroscopy while the morphology was analyzed by scanning electron microscopy. Chemical composition was estimated by X-ray fluorescence and energy dispersive X-ray spectroscopy. Modulated surface photovoltage was used to monitor the charge separation behavior of the CZTSSe absorbers. Solar cells with a glass/Mo/CZTSSe/CdS/i-ZnO/ZnO:Al/Ni:Al grid structure were fabricated based on the printed CZTSSe absorbers. So far Efficiencies up to 6.4 % with a cell area 0.5 cm² under standard AM 1.5 Sun simulator were achieved.

Over the past few years, Cu(In,Ga)(S,Se)₂ (CIGS) has drawn a great deal of interest in the thin-film photovoltaic industry due to its desirable properties, in particular a direct bandgap, high optical absorption coefficient (α ~ 10^-5 cm^-1) and low surface recombination rate. Recently, CIGS technology has reached 21.7% power conversion efficiency on a laboratory scale using a three-stage co-evaporation process [1]. However, despite the high efficiency obtained by vacuum deposition techniques, the levelized cost of electricity remains high due to the expensive equipment and long annealing steps at high temperature usually required by these processes [2]. Moreover, high temperature processing is not suitable for low-cost flexible plastic substrates.
In this work, we explore an alternative annealing approach based on a photonic curing process that uses pulsed light from a xenon flash lamp. This technique has the advantage of delivering a light pulse in the microsecond or millisecond range that can rapidly anneal thin films without heating the substrate. Moreover, shorter annealing time leads to higher throughput and can be applied to roll-to-roll fabrication of solar cells. Photonic curing has been widely applied to sintering silver or copper nanoparticles for printed electronics, as well as annealing semiconductors such as amorphous silicon [3]. However, sintering ink-deposited CIGS nanocrystals by photonic curing into a polycrystalline film with improved structural and optical quality has not yet been reported.
Cu(InGa)S₂ nanocrystals were synthesized by hot injection technique and deposited on Mo-coated soda-lime glass substrates by spin-coating [2]. Scanning electron microscopy images show that dense nanocrystal films processed by photonic curing with fluence ranging from 4.5 to 6.6 J.cm^-2 results in the formation of a homogeneous polycrystalline CIGS film without peeling off or voids formation at the Mo/CIGS interface. In addition to ligands and residual solvents removal, photonic curing induces a restructuration and improvement of the crystalline quality of the film. In particular, defect phases (CuAu ordering), ternary (CuIn₅S₈) and binary (In₂S₃, CuS) compounds observed by Raman spectroscopy in the as-deposited films are removed or strongly reduced for the optimized processing conditions. In addition, femtosecond broadband transient absorption measurements show a clear reduction in the trap states after photonic curing, characteristic of a rather good crystalline quality.
[1] http://www.zsw-bw.de/uploads/media/pr12-2014-ZSW-WorldrecordCIGS.pdf
[2] Q. Guo, G. M. Ford, R. Agrawal, and H. W. Hillhouse Prog. Photovoltaics Res. Appl.21 64-71 (2013).
[3] H.-S. Kim, S. R. Dhage, D.-E. Shim, and H. T. Hahn Appl. Phys. A, 97 791-798 (2009).

Recently, a ternary compound semiconductor, Cu₂SnS₃ (CTS), has been considered a promising candidate for photovoltaic materials constituting Earth-abundant, low-cost, and non-toxic elements. Polycrystalline CTS has been reported to have an absorption coefficient greater than 10⁴ cm^-1 and a bandgap energy of 0.87-1.77 eV. Recent studies on CTS-based solar cells reported conversion efficiencies near 4%. However, this value is much less than the conversion efficiencies of CdTe- and Cu(In,Ga)Se₂ (CIGS)-based solar cells. This low efficiency is thought to originate from the fact that the growth process is not fully understood, electrical properties of CTS, and mismatch in the band alignment of pn heterojunctions formed with CTS. A simulation of CIGS-based solar cells indicates that a small positive conduction band offset in the range of 0.0 eV to +0.4 eV results in a suitable band alignment. There have been few reports on the band alignment of n-type semiconductor/CTS heterojunctions. The carrier concentration of photovoltaic materials plays a very important role in the performance of solar cells because it affects, for instance, the depletion layer and band bending. Some studies have reported carrier concentrations of 10¹⁷-10¹⁹ cm^-3, which is much higher than that of ideal photovoltaic materials, which is typically in the range of 10¹⁵-10¹⁶ cm^-3.
In this study, we will examine band offsets between CTS and various n-type semiconductors and the dependence of the electrical properties of CTS thin films on the Cu/Sn composition for achieving CTS-based solar cells with high conversion efficiencies.
Cu-Sn mixed precursors were prepared by co-sputtering on soda lime glass substrates using Cu and Sn targets. The precursors were sulfurized in a tubular furnace in a N₂ and S-vapor atmosphere at constant atmospheric pressure. The valence-band offsets of the n-type semiconductor/CTS heterojunction were measured through Photoelectron Yield Spectroscopy (PYS) measurements. Cu/Sn composition was measured through Energy Dispersive X-ray spectroscopy (EDX) measurements.
The PYS measurements reveal that ZnO/CTS and amorphous ZnO:In₂O₃ (a-IZO)/CTS have small positive conduction-band offsets of +0.4 eV and +0.3 eV, respectively. On the other hand, CdS/CTS has a small negative conduction band offset of -0.2 eV. In addition, the control of carrier concentration broadens the depletion layer broaden and causes band bending. This result indicates that ZnO and α-IZO may be appropriate n-type semiconductors for forming heterojunctions with p-CTS absorber to develop efficient CTS-based solar cells.

Thin films based on the quaternary AgIn(S_xSe_1-x)₂ compound with chalcopyrite type structure are suitable for absorber layers of single junction solar cells because is possible to grow them with an energy band gap (E_g) value of 1.45 eV using an adequate S/Se ratio. Semiconductor materials having E_g values close to 1.45 eV are considered optimal for single junction solar cells applications. However, the main advantage of AgIn(S,Se)₂ based compounds appears when using them as absorber layers in two junction tandem solar cells, because theoretical calculations indicate efficiencies greater than 30% can be achieved with two junction tandem solar cells fabricated using AgInS₂ (E_g=1.93 eV) as absorber layer in the top cell and AgInSe₂ (E_g=1.37 eV) as absorber layer in the bottom cell. In this work, AgInS₂ and AgInSe₂ thin films with high absorption coefficients (α> 10⁴ cm^-1) were investigated as potential candidates for use as absorber layers in single junction and tandem solar cells. The films were grown by sequential evaporation of the metallic precursors in presence of elemental sulphur or selenium, in a two stage process. Special emphasis was placed on studying the effects of the growth temperature and evaporated mass of Ag to evaporated mass of In (m_Ag/m_In) ratio on the chemical composition of AgInS₂ and AgInSe₂ thin films. The influence of these parameters on the phases formed in the films was studied through X-ray photoelectron spectroscopy (XPS) measurements. The studies revealed that the deposition conditions affect the homogeneity of the AgInS₂ and AgInSe₂ films chemical composition, as well as the phase in which the films grow. Moreover, conditions were found to prepare thin films containing only the AgInS₂ or AgInSe₂ phase, grown with tetragonal chalcopyrite type structure and good homogeneity of chemical composition in the whole volume.

Separation and coarsening of In-rich and Ga-rich metallic phases in Cu-In-Ga precursor films is a major hurdle for the fabrication of smooth and homogenous Cu(In,Ga)Se₂ films by sequential processing. If not prevented, this effect leads to solar cells with low shunt resistance due to pinhole formation, and to reduced open circuit voltages due to locally varying Ga content and compositional depth-profiles. The scale of this phase separation depends on the initial precursor microstructure, the heating rates, the temperature and the chalcogenization procedure and is assisted by a liquid phase, as expected from the calculated phase diagram of the Cu-In-Ga system [1]. Due to its complexity, this aspect has been neglected in the past and most studies are limited to the mere observation of morphologies before and after the chalcogenization process. In this study we attempt to establish a general model for the kinetics of the phase separation and coarsening of sputtered Cu-Ga-In metallic films as used for sequential high efficiency solar cell fabrication. For this we measure quantitatively the roughness and qualitatively the Ga spatial distribution by energy dispersive X-ray spectroscopy mappings of the films&’ cross sections and X-ray diffraction analysis. We study four different metallic precursor stacks of In and Cu-Ga layers heated to 170°C, 350°C and 580°C, at rates between 0.01 K/sec and 1 K/sec. This covers the experimental window relevant for selenizations in H₂Se atmosphere and for rapid selenizations in elemental Se-vapor. Based on this comprehensive data we propose optimal precursor stacks and heating profiles for pre-heating treatments before the chalcogenization takes place. The main objective is to homogenize and alloy the Cu-poor Cu-In-Ga metallic precursor without significant phase separation or increase in roughness. Finally, we present a statistical analysis of the effect of our optimized and multilayered precursor layers on the fill factor of the solar cells prepared in our atmospheric-pressure in-line and fast selenization baseline with process times of 7 min, that has led to power conversion efficiencies up to 14.8% (active area).
[1] W. K. Kim, J. Shen, M. Chu, J. H. Jung and T. J. Anderson. Prediction of the Cu-Ga-In Ternary Phase Diagram. J. Nanoelectron. Optoe. 7, 425-429 (2012), doi: 10.1166/jno.2012.1366

In recent years, there has been increasing research into intermediate band solar cells (IBSC), which utilize new semiconductor absorbers capable of generating current from sub-band gap photons. Their efficiencies could theoretically be over 20% larger than traditional single-junction devices. One particular semiconductor, V_0.25In_1.75S₃, has a nearly ideal band gap and intermediate band (IB) location for an IBSC, and has shown strong absorption for photons of sub-band gap energy. However, only powders of this semiconductor have been created thus far. Here we present for the first time a new synthesis technique for V_0.25In_1.75S₃ thin film semiconductors grown by atomic layer deposition (ALD), which allows us excellent control over uniformity, stoichiometry, and crystallinity in our films.
Our initial studies focused on ALD growth of In₂S₃ via alternating pulses of a novel indium(III) amidinate precursor and hydrogen sulfide. This technique allowed us to create oxygen-free, crystalline films of In₂S₃ with a band gap near 2.0 eV and strong electronic properties [n-type, 10¹⁷-10_shy;¹⁸ cm^-3 electron concentration, 0.1-1 cm²/(V s) mobility]. Once this recipe was established, V_xIn_yS_z thin films were synthesized by adding pulses of a vanadium(III) amidinate precursor to our In₂S₃ growth recipe. Film growth was studied in-situ using quartz crystal microbalance, proving that vanadium and indium precursors both react well with H₂S, and that no cation exchange occurs. X-ray diffraction proves that as-deposited V_xIn_yS_z films were amorphous, unless grown on crystalline In₂S₃ films at 200°C and 225°C. A similar crystal structure to Inshy;₂S₃ is observed, as expected. As we increase the ratio of V(amd)₃/In(amd)₃ precursor pulses, we increase the vanadium concentration in our films as confirmed by x-ray fluorescence. More vanadium leads to greater sub-band gap photon absorption as seen in UV-Vis-NIR measurements. Sharp, substantial peaks are observed in the absorption coefficient for crystalline V_xIn_yS_z films, suggesting the presence of an intermediate band. Films grown at 200°C show absorption peaks near 1.1 eV and 2.5 eV, and some peaks grown at 225°C show peaks at 0.9 eV, 1.9 eV, and 2.5 eV. Stoichiometry and deposition temperature affects the electronic properties we measured by Hall effect. More vanadium leads to larger carrier concentrations. Films grown at 2250C showing three absorption peaks were too resistive to measure, suggesting a shift in the Fermi level towards the valance band. Films have also been studied by photoluminescence and photoconductivity, and initial device testing has begun.

Cu₂ZnSnS₄ (CZTS) is a promising photovoltaic (PV) absorber containing abundant elements. However, efficient devices in literature are achieved with a large variation in CZTS absorber thickness. Thereby it is essential to understand the influence of the absorber thickness on CZTS solar cells. In this study, CZTS absorber layers with six different thicknesses ranging from 500 nm to 2000 nm were fabricated and devices of 0.5cm² area were prepared with the standard structure of Al:ZnO/i-ZnO/CdS/CZTS/Mo/SLG. To complement device fabrication, an identical set of samples was prepared for material analysis before and after the annealing process. X-Ray fluorescence indicates all the sputtered precursors have Cu poor, Sn rich and Zn rich composition (Cu/Sn = 1.8-1.9, and Zn/Sn = 1.0 -1.2). Upon annealing, the composition change is very limited, as we introduce sulfur excess to reduce the escape of SnS and S from the film. SEM indicates grain growth in the annealed samples as well as phase separation at the surface and backside of the CZTS films. X-ray diffraction and Raman spectroscopy reveal that despite improvement in the recrystallization and ordering of the annealed films, SnS phase also forms in the absorbers, revealing that the saturation SnS pressure is reached during the annealing process. Finally, solar cells with efficiencies ranging from 3.4% to 6.6%, depending on the thickness, are obtained. A similar trend of efficiency, open circuit voltage (V_oc) and short circuit current (J_sc) can be found, i.e. all three parameters show an initial improvement with increasing thickness, which saturates when the thickness is beyond 750 nm. Devices are analyzed by capacitance voltage, drive level capacitance profiling, current voltage and quantum efficiency (QE) measurements. QE is increased for all wavelengths with increased absorber thickness up to 1000 nm, whereas a stronger increase in the long wavelength regime is observed as the absorber becomes thicker than 1000 nm. The increase in long wavelength QE under reverse bias suggests a short depletion region and poor minority carrier collection from the bulk CZTS. In conclusion, a 750 nm thick CZTS absorber is sufficient to obtain efficient solar cells. However, V_oc and fill factor still need to be reinforced to reach high PV performance. This requires a careful design and optimization of the process to better control the material quality.

Cu-chalcogenide materials such as Cu(In,Ga)Se₂ are the basis for many high efficiency thin film photovoltaics (PV), with current research cell efficiencies over 20%. However, the use of In-containing absorbers may be problematic in the future for TW scale deployment, because In is already in high demand for other applications. CuSbS₂ is a promising alternative absorber material, with optoelectronic properties comparable to CuInS₂ because of the similar size and valence of In and Sb. There has been a significant improvement in the bulk properties of CuSbS₂ in recent years, but little work has been done to integrate CuSbS₂ thin films into PV devices, with only one report of a 3% efficient CuSbS₂/CdS device. For CuSbS₂ to succeed as a PV absorber, it will be critical to work in parallel to improve both the bulk and the interfacial properties.
Integrating an absorber material into a PV device presents many challenges, one of which is achieving the optimal conduction band offset between the absorber and the heterojunction partner. Large “spike” or “cliff” offsets can negatively impact device performance; a small spike offset (0.1-0.4 eV) is most desirable for optimal charge separation and transport. Many Cu-chalcogenide PVs use CdS as an n-type heterojunction partner. However, metal oxysulfides are interesting alternatives to CdS, because higher band gaps allow for collection of more photons by the absorber. Furthermore, the ability to tune the position of the oxysulfides conduction band edge, through changes in the O:S ratio, can be used to optimize the conduction band offset.
In the case of CuSbS₂, the high conduction band edge of this material should cause a ~0.8 eV cliff with a CdS heterojunction. In contrast, theory indicates that a II-VI based metal oxysulfide could provide the desired small spike offset with CuSbS₂. In this work, we will explore band alignments between CuSbS₂ and a range of oxysulfide alloys using x-ray photoelectron spectroscopy (XPS). The CuSbS₂ films are synthesized by RF magnetron sputtering from Cu₂S and Sb₂S₃ targets. The oxysulfide layers are then deposited without exposing the sample to the ambient environment. The binary oxide and sulfide targets are positioned such that a spatial gradient in the O:S ratio is created across the substrate. The positions of the valence band edges are measured using XPS, and the experimentally determined offsets is compared to theoretical estimates.
The properties of the oxysulfide buffer layers are also assessed by comparing the performance of PV devices made using a range of oxysulfide buffer layers on CuSbS₂, as well as devices with a CdS buffer layer. Device integration attempts are in progress, and these results will be reported.
The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

We report bias-dependent Transient Photocapacitance (TPC) and Transient Photocurrent (TPI) spectra on a variety of thin-film photovoltaic devices. Consideration of the spatial sensitivities of these techniques is used to distinguish bulk and interface defects in CIGS and CZTS. In CIGS, a 1.1 eV defect was observed in TPI but not TPC, indicating an interface defect. A 0.8 eV defect was observed in TPC and TPI. But, it appeared in TPI only under larger reverse biases. The dependence of the TPI signal at 0.8eV on applied bias was measured and found to be consistent with the interpretation of this defect as being isolated to a region away from the p-n junction near CIGS:CdS interface. Together, these observations provide a more detailed picture of the defects in CIGS and point the way towards profiling of optically observed defects in thin-film photovoltaic devices.
While, TPC and TPI spectroscopy had already proven to be valuable tools for understanding and improving photovoltaic devices based on amorphous silicon, CIGS, CZTS and CdTe they were primarily applied to uniform absorbers as this allowed a more facile interpretation of the spectra. With this work we show that understanding and utilizing bias dependent TPC and TPI spectra allows for spatial localization of optically observed defects in non-uniform thin-film PV absorbers. With this information we are better able to understand the effect of the observed defects on performance and better inform the search for their chemical origins.

Copper indium gallium sulfide (selenide) (Cu(In,Ga)(S,Se)₂ (CIGSSe)) is a very promising material for solar cell absorber that has 1.0~1.7 eV direct band gap and large absorption coefficient over ~10⁵ cm^-1. Recently solution processed CIGSSe devices have gained wide interest because of easy scalability and low-cost. Although hydrazine-based CIGSSe thin-film solar cell reached the record high efficiency, it is difficult to adapt this process for large-scale solar cell fabrication due to the toxicity of solvent. Therefore, non-toxic alcohol based sol-gel processes are employed to fabricate CIGSSe absorber. However, the alcohol derived methods have recently demonstrated power conversion efficiency (PCE) of 8.3% which is still not comparable to hydrazine based route. Here, we report solution-based processing of CIGSSe absorber layer using non-toxic, earth-abundant water solvent. Aqueous Cu-In-Ga-S ink was prepared by dissolving metal precursors and S powder in water with an aid of diethylenetriamine (DETA) that allows for dissolution of elemental S. The Cu(In,Ga)S₂ (CIGS) layer was fabricated by spin coating the aqueous ink, followed by drying on hot plate to remove the organic residues. The CIGS films were then annealed in a tube furnace above 500 ^oC under atmosphere of H₂S gas. Furthermore we developed new doping method of Se component into the absorber layer without the involvement of harmful Se vapor or H₂Se gas. Elemental Se can be also dissolved in DETA added water to form ionic selenium (Se^2-) ink. This aqueous selenium ink was spin-coated on top of the sulfurized CIGS to internally provide Se source. Ionic selenium doping method induced band-gap graded CIGSSe layer, which leads to develop potential differences across the absorber layer. These potential differences enhance carrier collection efficiency and decrease recombination rate in CIGSSe layer. Aqueous ink derived band-gap graded CIGSSe based solar cell exhibited PCE more than 8 % based on the effective area of 0.15 cm². In addition, we compared the optical properties and microstructure of band-gap graded CIGSSe and non-graded CIGS absorber layer. Grazing incidence XRD and composition mapping analysis confirmed the grading of Se content from the bottom (low Se concentration) to the surface (high concentration) in the CIGSSe layer.

In current literature, the instability of kesterite absorbers viz. CZTS (Cu₂ZnSnS₄) and CZTSe (Cu₂ZnSnSe₄) at elevated temperature has been widely discussed. In particular, desorption of SnS/SnSe from the film surface makes it difficult to make an absorber layer that is free of impurities. In this study, we demonstrate the preparation of CZTS absorber layer by solution approach and determine the role of tin and sulfur assisted annealing, annealing time at elevated temperatures on the quality of kesterite absorber layer.
In our work, we have successfully prepared a non-toxic CZTS sol-gel by the dissolution of the respective metal-ion precursors and thiourea in a derivative of heterocyclic based solvent in nitrogen-filled glove box. The sol-gel is spin-coated onto SLG/Mo substrate up to 6 layers, followed by a hot plate treatment at 500#730; C for 2 minutes that yields a 2 µm thick CZTS layer. Annealing conditions such as temperature (610 #730;C), time (8 min) and concentration of H₂S (6% diluted with N₂) are optimized to form large grain, pin-hole free poly crystalline absorber layer. The cross-sectional depth profile of CZTS film, however, shows considerable Sn loss from the surface which is suggestive of SnS desorption. This desorption is further, suppressed by providing metallic tin pellets and sulfur flakes externally during the annealing. The cross-sectional EDX line scan of the modified annealed sample shows homogeneous elemental distribution suggesting no SnS desorption under such annealing conditions. The device efficiency of the as fabricated solar cell with tin and sulfur assisted annealed CZTS absorber, shows a significant raise from 2.89% to 5.67%.

Along with progressively improved and quite well established Cu(In,Ga)Se₂ (CIGSe) compound semiconductors, the research on sustainable solar energy conversion technologies has further branched to another promising alternative, based on Cu₂ZnSn(S,Se)₄ (CZTSSe). The fact that the elements contained in the CZTSSe system are neither seriously toxic nor are they affected to become scarce, mediates increasing efforts in order to attain competitive, i.e. efficient and durable, solar cells. The currently highest efficiency of a CZTSSe device was reported to be 12.6 %. However, in the course of the incipient treatment as a semiconducting material, it turned out that the theoretical maximum, that is, the Shockley-Queisser limit, of attainable energy conversion efficiency of a single p-n junction device amounts eta;_max = 32.2 %. Even though this potential maximum efficiency is comparable with that of CIGSe semiconductors, the actual efficiency is still well below of those achieved for CIGSe absorber. Hence, many efforts are dedicated to further improve the kesterite-type absorber in order to become more than a promising alternative. For this purpose the efficiency gap has to be reduced as much as possible and the microstructure is might be one key aspect. This work focuses on Cu₂ZnSn(S,Se)₄ thin films deposited onto both flexible stainless steel and soda lime glass substrates respectively. While four samples have undergone a DC magnetron 2-stage process, two samples were fabricated by spray pyrolysis. The precursors of the 2-stage process consisted of a Cu/Sn/Cu/Zn metallic stack which has been annealed under Se-atmosphere at different temperatures. The samples manufactured by spray pyrolysis have been modified with respect to the [S]/([S]+[Se]) ratio. The thin films were characterized in terms of lattice constants and microstructure for various thin film depths by means of grazing incidence X-ray diffraction (GIXRD). Since every incidence angle can be attributed to a corresponding X-ray attenuation length for a given composition and for a monochromatic X-ray beam (E = 8.048 keV), the microstructure can be determined for any sample depth. In order to retrieve additional information, the samples were also analyzed by Bragg-Brentano X-ray diffraction (BBXRD) as well as by glow discharge optical emission spectroscopy (GDOES). The presentation aims to the depth-resolved characterization of the CZTSSe absorber with emphasis on microstructure, lattice constants and preferred orientation in dependence on different process conditions.

Quaternary chalcogenides have seen rapid development in recent years leading to a world record efficiency for thin film solar cells based on Cu₂ZnSn(S,Se)₄ (CZTSSe) of 12.6% [1].
CZTS belongs to the A^I₂B^IIC^IVX^IV₄ compound family and crystallizes in the tetragonal kesterite type structure, the certain ordering of the Cu, Zn and Sn cations results in the space group [2]. Typical secondary phases coexisting with a quaternary kesterite type phase are ZnS, CuS, Cu₂S, SnS, Sn₂S₃, SnS₂ and Cu₂SnS₃ [3]. Nevertheless there is still a lack of knowledge about existing phases and phase relations in the vicinity of the intersection point of CZTS. In literature [4] four off-stoichiometric CZTS compounds, named A-, B-, C- and D-type, have been proposed.
The number of publications on intrinsic point defects in kesterites is very limited. Further studies on deviation from stoichiometry, distribution of the cations and formation of intrinsic point defects are of great importance to understand solar cell performance. Therefore, our experiments focus on the synthesis of off-stoichiometric CZTS reference powder samples with cation ratios Zn/Sn and Cu/(Zn+Sn) ne; 1. All powder samples were synthesized by solid state reaction from pure elements in sealed evacuated silica tubes in a one zone furnace. The obtained samples have been well characterized regarding chemical composition, phase content and trends in lattice parameters using wavelength-dispersive X-ray spectroscopy (WDX) on an electron microprobe system and X-ray diffraction (XRD). Furthermore anomalous X-ray diffraction at the KMC-2 beam line (BESSY) was used to obtain the site occupation of the cation sites in the kesterite structure. The phase content analysis presented here considers the samples before (intermediate samples) as well as after the annealing step.
The results of the XRD and WDX analysis show that CZTS is the main phase in all samples synthesized. The CZTS grains in the intermediate samples exhibit a slight variation of the chemical composition. The presence of secondary phases like CuS, ZnS and Cu₂SnS₃ was noticed for all samples. Moreover a limited solubility of Cu and Sn in ZnS, Zn and Sn in ZnS as well as Zn in Cu₂SnS₃ was determined. After the annealing step secondary phases have been greatly reduced and the chemical composition of the off-stoichiometric kesterite phase indicates a C-D-type mixture (Cu-rich, Zn-poor and Sn-rich). These results have been assistant to understand the growth process of CZTS and to improve the solid state synthesis procedure. With increasing off-stoichiometry the lattice constants and the a/2c ratios are decreasing. This leads to smaller tetragonal distortion with increasing off-stoichiometry.
[1] Wang, et al., Adv. Energy Mater., 2014. 4(7).
[2] Schorr, Sol. Energ. Mat. Sol. Cells, 2011. 95(6): 1482-1488.
[3] Olekseyuk, et al., J. All. Com., 2004. 368(12): 135-143.
[4] Lafond, et al., ZAAC, 2012. 638(15): 2571-2577.

Cuprous oxide (Cu₂O) has high potential for application in photovoltaics due to its suitable optical and electrical properties alongside abundance and non-toxicity. Considerable progress has been reported recently in the field of Cu₂O-based solar cells, where the most efficient devices were fabricated with thermally oxidized cuprous oxide¹. In this work we employ p-type thermally oxidized Cu₂O in atmospherically produced Zn_1-xMg_xO/Cu₂O heterojunction solar cells. The p-n junction is formed via deposition of n-type Zn_1-xMg_xO on the Cu₂O substrates by atmospheric spatial atomic layer deposition (SALD), a novel oxide printing technique allowing for deposition of high quality conformal thin films of metal oxides at low temperatures outside a vacuum. Such devices produced in our laboratory have demonstrated so far the highest efficiency for atmospherically fabricated ZnO/Cu₂O heterojunctions.
¹Ievskaya et al., Solar Energy Materials and Solar Cells (2014)

Tin monosulfuide (SnS) is a promising candidate for developing solar cells that use Earth-abundant materials because it has a direct bandgap of 1.3 eV and high light absorption coefficient (>10⁴ cm^-1). Recent studies demonstrated SnS-based solar cells with conversion efficiencies near 5%. However, this value is much less than the theoretical efficiency because of the poor crystal quality and unknown physical properties of SnS, among other factors.
The sulfurization of Sn precursors under atmospheric pressure is one of the most desirable processes for the commercial production of SnS photoabsorbers. Although SnS production through the sulfurization of Sn precursors has been investigated, the control of S atoms diffusing into the Sn layer is very crucial. In fact, SnS is a metastable alloy, in contrast to the stable SnS₂ and Sn₂S₃. Therefore, in the case of S-oversupplied sulfurization, extra phases such as SnS₂ or Sn₂S₃ are obtained with SnS. On the other hand, in the case of S-undersupplied sulfurization, Sn metal remains in the SnS layers. Moreover, high-temperature sulfurization causes Sn-precursor melting because of its low melting point (232 °C). For example, in the case of selenization growth of Cu(In,Ga)Se₂ (CIGS), Cu-In-Ga alloy precursors or Se-premixed metal precursors have been used to avoid melting the precursors and to obtain single-phase CIGS without phase separation or extra phases. However, for SnS growth, there are few reports on sulfurization growth using a premixed precursor. In this presentation, the advantages of using a S-premixed precursor for the sulfurization growth of SnS will be shown.
First, Sn-S-mixed precursors were deposited on Mo-coated sodalime glass by RF sputtering. Then, the precursors were sulfurized under a sulfur vapor atmosphere at 200-400 °C for 60-150 min.
The surface morphology of the SnS film grown using the S-premixed precursor was very smooth in comparison with that grown using a Sn precursor because the S-premixed precursor did not form droplets owing to its high melting point. Moreover, S atoms tend to diffuse into the precursor homogeneously on using the S-premixed precursor. The sulfurization of Sn-S-mixed precursor layers is one of the most desirable processes for the commercial production of SnS photoabsorbers.

Modified stacking sequence of precursors and pre-annealing process on Se vapor at low temperature were applied to Cu(In,Ga)Se₂ (CIGS) solar. The remarkable improvement of efficiency (5.53 % to 10.10 % and further 11.04 %) and open circuit voltage (0.41 V to 0.53 V and further 0.56 V) comes from a compact, smooth microstructure, and modified depth profile of Ga with suitable thickness of CuGa multi-stacking layers in the top of precursors as well as surface bandgap enhancement via pre-annealing process without Se vapor followed by a specific non-toxic hydrogen-assisted solid Se vapor selenization process. The effects of microstructural, compositional, and electrical characteristics of Ga distribution including accumulation and interdiffusion were examined in detail. Finally, a large-area (40x30 cm²) CIGS solar cell efficiency with improved open circuit voltage (V_OC) and fill factor (FF) of 36 % and 14 % has been demonstrated, yielding a promising efficiency of ~11.04 %.

In order to meet future energy demands, thin film technologies continue to receive a lot of interest. In this work, we characterize a series of new synthesis routines and processes to generate photovoltaic materials whose properties are significantly better than the current state of the art. Arguably, more pressing is the need to characterize photovoltaics and their interfaces and defects. Success thereby requires a transition to a mechanistic-based approach that incorporates and extends fundamental insights into material functionalities as a function of interfaces and defects.
There are longstanding interests in the relation between microstructure, defects, electronic structure, and electro-optical activity for two reasons. The first reason is spectral luminescence studies suggest specific grain boundary orientations are more active than others. The atomistic origins however remain nascent due to the complexities associated with studying defects in connection with changes in cell efficiencies. Furthermore, beyond the generation of charged carriers, observing transient mobile behavior associated with these carriers and localized electronic energy states is of increasing interest to design next generation solar cells. The second reason is reporting on the microstructure and possibly the presence of preferential growth defects is highly relevant to the future resource utilization of developing solar technology.
To respond, we have exploited controlled material growth, sub-nanometer to micron correlative imaging, and spectroscopic capabilities associated with state-of-the-art photoluminescence and aberration corrected scanning transmission electron microscopy (S/TEM) to examine copper indium gallium selenide (CIGS) interfaces giving rise to differences in measured excited state processes.
This talk will present the correlation between direct observation and measurement of the optical properties, microstructure, and defects to assess physical mechanisms for improving enhanced generation and conduction of charge carriers in polycrystalline CIGS.
This work was partially supported by ORNL&’s Center for Nanophase Material Science (CNMS) User Facility, sponsored by the U.S. Department of Energy, Of#64257;ce of Basic Energy Sciences and the LeRoy Solid State Science Center at Arizona State University.

The design of optoelectronic devices is based on the basic understanding of non-equilibrium carrier dynamics. Cu(In,Ga)Se₂ (CIGS) film has attracted considerable attention as a material for highly efficient chalcogenide-based solar cell, however, which dynamics of photocarriers have not yet been animatedly debated. In particular, the relaxation mechanism of photocarriers at p-n junction in solar cell is also crucial issue, but not investigated at all in terms of device-relevant parameters until now. To grasp the relation between carrier dynamics and energy conversion process, probing relaxation mechanism of photocarriers at p-n junction is essentially investigated by using ultrafast optical spectroscopy. In this paper, we report on the energy-relaxation mechanism of non-equilibrium photocarriers at p-n junction by using optical pump-THz probe (OPTP) spectroscopy. For the formation of p-n junction, several types of buffer layers were applied such as chemical bath deposited (CBD)-CdS, CBD-ZnS, sputtered-Zn(O,S), and cracker-ZnS. The OPTP measurement is extremely sensitive to observe the scattering mechanism and dynamical energy transition in the ultrafast region of timescales on ~fs and ps. Two intense fs laser pump pulses of 400 nm and 800 nm were injected into the samples to excite charge carriers of buffer layer and CIGS film above excess bandgap, respectively. The carrier lifetimes induced by scattering or recombination processes were calculated by fitting the decay curves. We notified that the carrier relaxation mechanism and their lifetimes at p-n junction are significantly different with types of buffer layers deposited on CIGS film, which is closely related with solar cell efficiency. These results provide useful information for selecting the buffer layer and optimizing the design of CIGS-based solar cell.

The chalcogenide semiconductor Cu₂ZnSnS₄ (CZTS) is attracting a lot of attention as absorber layer in thin-film solar cells because of the high abundance and the non-toxicity of its primary constituents. It is a direct band gap semiconductor with high absorption coefficient (> 10⁵cm^-1) and band gap at about 1.5 eV. Recently, the related selenium compound Cu₂ZnSnS_x Se_1-x (0 le; x le; 1) has reached a promising efficiency of 12.6 %.
We present here an investigation of the optical and electrical properties of thin films of the sulfide CZTS deposited onto Mo-coated Soda-Lime glasses. The films are annealed in a sulfurized atmosphere (S + N₂) with a stack of precursors prepared by Pulsed Laser Deposition (PLD), a technique which provides a very high degree of control for depositing metal alloys and binary sulfides, which we have already produced and characterized.
So far, the record efficiency with pure sulfide CZTS solar cell is 8.4%, somewhat below that of Cu₂ZnSnS_x Se_1-x. There are at least two reasons for this lower efficiency: a narrower region of existence in phase space for CZTS compared to CZTSSe and a higher vapor pressure of SnS compared to SnSe during annealing treatment. These issues can be handled by a careful preparation.
To address the first issue, by applying PLD and different target compositions, we are able to control the stoichiometry of the precursors very well down to the limits imposed by the constraints of phase space. At the same time, the high kinetic energy (up to tens of eV) of atoms in the plasma plume, enables us to enhance the mobility also at modest substrate temperature, which improves the crystallinity. This procedure thus requires a lower temperature and/or shorter duration of the annealing process, whereby the effect of SnS evaporation is expected to be considerably reduced. This evaporation leads to a decomposition reaction of CZTS and formation of unwanted secondary phases which are detrimental for a good absorber.
Therefore, in our experiments precursors will be prepared by use of metallic targets (single elements and alloys) and binary sulfides targets, such as CuS and ZnS. The structure of the precursors will consist both of a stack of different layers or a uniform mixture of the constituents. The electrical properties will be characterized by four-point probe and Hall measurements and the optical properties by spectroscopic ellipsometry. Other characterization techniques to be employed will be: x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion x-ray analysis (EDX), and atomic force microscopy (AFM).

The changes of the photovoltaics efficiency in chalcogenide ternary films may be achieved due to effective illumination by a laser with different wavelength. We studied the large group of the CuInSe₂ films . It was established that substantial changes of the effective energy gap which play a decisive role in the efficiency of the photovoltaics can be achieved through varying the laser wavelength, its power density and the time duration. The maximum energy gap shift up to 0.24 eV was achieved during illumination by the ER:glass lasers generating at 1.54 mm. The effect was a partial reversible with relaxation time depending on the crystal non-stoichiometry. The partial substitution of the Se by S ( up to 0.2 %) initiated more stable energy gap changes. This may be a consequence of the intrinsic defect states playing a crucial role in the effect. Another interesting fact is the dependence of the energy gap on the crystalline film thickness and the substrates. The best substrates were the ZnO singe crystals. The possible tuning of the energy gap in the one synthesized films open new possibilities for creating tuneable gap photovoltaic devices under operated laser beams. The laser beam profile demonstrated high sensitivity to the effect observed. The better spectral shift parameters were demonstrated for the Voight laser beam profile. The such crucial conditions are defined by competition between the photo excited polarization of the trapping levels and by photo thermal relaxation.

A variety of sulfide and oxide-sulfide compound semiconductors are currently used or under consideration for use as absorbers, window layers, buffer layers and contact layers in thin film photovoltaics. For these materials, controlling the metal to sulfur ratio, and in the case of oxysulfides also the oxygen to sulfur ratio, is central to successful thin film synthesis. Here, we report on a new deposition approach in which an atomic sulfur source typically used for molecular beam epitaxy (MBE) growth is added to a multisource co-sputtering system to provide essentially full control over relative cation and relative anion compositions as well as the overall cation to anion ratio. This is demonstrated in proof of principle experiments through the growth of the binary metal sulfide Cu₂S, the ternary metal oxysulfide Bi-O-S and the quaternary mixed metal oxysulfide BiCuOS. In particular, an Oxford Applied Research RFK 30 radio frequency (RF) solids cracker was added to a multi-source RF co-sputtering system from AJA, Inc. configured with three 2” diameter sputter guns. The three guns are laterally offset from the substrate center normal but angled in to allow for intentional composition gradients if the substrate is not rotated or for uninform composition with rotation. The RF solids cracker is aligned normally incident to the substrate center. Typically, such RF solids crackers are used in MBE systems where the usual operating pressure is 10^-5 to 10^-6 Torr. In sputtering, the deposition pressure is much higher, typically 3 - 5 mTorr. In the RF solids cracker, atomic sulfur is created by using an RF field to crack the sulfur polymers (S₂, S₃, hellip;) evaporated from a solid sulfur loaded Knudsen cell. For all depositions, Ar is used as both the primary sputtering gas and the carrier gas for the RF solids cracker. All films were deposited on Corning Eagle 2000 glass substrates at substrate temperatures ranging 375 - 485 °C. Film compositions were measured by Rutherford backscattering and x-ray fluorescence. X-ray diffraction was used to determine the phases formed. Cu₂S films were grown from both metallic Cu and ceramic Cu₂O targets. Bi_xO_yS_z films with a tunable O:(O+S) ratio of 0.33 to 0.88 were grown from a Bi₂O₃ target. Further, the successful control of both cation and anion composition in a quaternary system was demonstrated by the growth of BiCuOS by co-sputtering from Cu₂S and Bi₂O₃ targets in conjunction with the atomic sulfur source. Attempts to grow BiCuOS without the atomic sulfur yielded highly mixed phase films. Collectively, the successful growth of these three test case materials demonstrates the viability of this hybrid deposition approach. Further, it is expected that this approach could be extended to phosphides, oxide-phosphides and phosphide-sulfides systems as well as the many sulfide materials currently used in thin film PV.

GaAs is a promising material for photovoltaics, continuing to hold the record efficiency for single-junction solar cells, but is limited by its high cost to niche applications. Thus there is a push to develop a process technology which allows for the inexpensive and rapid manufacture of thin GaAs devices while maintaining superior electronic properties.
The prevailing strategy involves depositing an epitaxial thin GaAs film via MOCVD onto a wafer, then lifting this film off, reusing the wafer for subsequent depositions. More specifically the GaAs film is grown on a sacrifical AlGaAs layer which is preferentially etched by HF. Submersion in an HF bath laterally etches the sacrificial layer and allows separation of the film from the wafer substrate. However, despite the low etch rate of GaAs by HF, the long etch times required can damage both the film and wafer.
A new process is described for liftoff of GaAs thin films which requires less time and does not cause damage to the films. An epitaxial, lattice-matched semiconductor stack of GaAs/InGaAsN is grown via MOCVD onto a GaAs wafer, then exposed to laser pulses that melt the sacrificial InGaAsN layer. The 1064 nm (1.165 eV) pulsed Nd:YAG laser is fired through the GaAs wafer toward the deposited film; because the incident photon energy is below the band gap of GaAs but above that of the InGaAsN, the photons are only absorbed by the sacrificial layer, while both the GaAs film above it and wafer substrate are unaffected. The rapid heating and subsequent obliteration of the sacrificial layer allows for the liftoff of free-standing, high-quality GaAs thin films.
Measurements show the structural and crystallographic properties of these films are preserved after the liftoff process, and their electronic properties are sufficient for use as photovoltaic devices.
As a further refinement of this process, secondary epitaxial sacrificial layers are incorporated to effect the complete removal of the InGaAsN melt debris which can be left on the GaAs surfaces after laser liftoff. These GaAs/InGaP/InGaAsN(melt debris) stacks are submerged in an HCl bath, which preferentially etches the InGaP and consequently removes the melt debris. The choice of etchant and the planar, rather than lateral, etch geometry serve to yield damage- and debris-free GaAs films and wafers, suitable for use as photovoltaic devices.

Epitaxially grown III-V compound semiconductors such as gallium arsenide (GaAs) have provided unmatched performance over silicon or other classes of materials in photovoltaics due to superior materials properties including direct bandgap, high electron mobility, appropriate bandgap energy against solar spectrum, and ability to form multiple junctions. Nevertheless, their widespread adoption in terrestrial, one-sun applications has been severely restricted by the prohibitively high cost of growing device-quality epitaxial materials. In this regard, decreasing the thickness of constituent active layers in III-V solar cells without compromising their photovoltaic performance in conjunction with a fabrication route to permit the reuse of a growth substrate provides a conceptually viable means to realize the substantial reduction of overall materials cost and hence the levelized cost of energy. Here we present a type of high performance GaAs PV system with drastically reduced active layer thickness (~0.2 mu;m) grown by molecular beam epitaxy, where hexagonally periodic dielectric nanostructures of sputter-deposited titanium dioxide (TiO₂) are heterogeneously integrated on the window layer of ultrathin GaAs solar cells by softimprint lithography for nanophotonic light management. Detailed studies of optical properties and photovoltaic performance of the integrated PV system in experiments as well as numerical modeling based on finite-difference time-domain (FDTD) method establish fundamentals of underlying materials science and optics, as well as strategies for optimized design.

It is well known that CZTS(e) presents significant phase and compositional challenges and their effect on device performance has been difficult to establish. Compositional techniques such as EDS have been able to resolve composition on the mu;m scale, while atom probe tomography has been able to resolve films at the atomic scale. TXM is able to fill the gap between these two regimes with a maximum resolution of 30 nm. For this study, thin film solar cells based on Cu₂ZnSnSe₄ (CZTSe) absorber layers with Zn/Sn ratios of 1.0 and 1.4 were characterized using element-specific X-ray transmission microscopy (TXM) and 3D tomographic reconstruction. After electrically characterizing working solar cells, areas of the full device stack of size approximately 25 x 25 mu;m² were examined using synchrotron-based TXM. The resulting data are 3D concentration fields for Cu, Zn, Sn, and Se. From these data we analyze compositional fluctuations at a heretofore inaccessible combination of sampling volume and resolution.
From the compositional data we derive phase maps, individual and averaged depth profiles, and examine the compositional fluctuations. We attempt to correlate these nanochemical data with the device current and capacitance data as functions of temperature. Admittance Spectroscopy (AS) and deep level transient spectroscopy (DLTS) were performed in order to examine defect states. This study links compositional analysis with sub-grain resolution over ensembles of a sufficient number of grains to be representative of film properties and when combined with the electrical characterization provide the critical links between the nanoscale and device performance.

We report progress made in thin film solar cells using chemically deposited antimony sulfide - selenide in the structure, FTO/CdS/Sb₂S_xSe_3-x with or without a complementary absorber of chemically deposited PbSe thin film. In such cells, a conversion efficiency eta; of 2.5% has been reported in 2013 [1]. The major attraction for this absorber is that the band gap E_g can be varied in the interval 1.1 eV (Sb₂Se₃) to 1.8 eV (Sb₂S₃). The appreciable stability observed in them and simple fabrication is another feature. The chemical deposition of the films is done at 80^oC from a solution mixture containing potassium antimony tartrate, thioacetamide, thiosulfate and selenosulfate in about 3 h. Solar cells are made on commercial SnO₂:F (FTO)-coated glass (NSG Pilkington TEC 15) constituting a structure: FTO/CdS-CdSe/Sb₂SxSe_3-x /C. The cell structures are heated at 270^oC in nitrogen for 30-90 min, depending on the value of x = 1.2-2. Such heating in the production of the cells assures cell stability under solar radiation. A higher value of x means a higher open circuit voltage, V_oc of up to 530 mV, but a lower short circuit current density, J_sc of below 10 mA/cm² and conversion efficiency eta; less than 2%. At a lower value for x, V_oc is near 450 mV, but J_sc approaches 15 mA/cm², and eta; is above 2.5%. The use of a supplementary PbS-Se absorber film constituting a cell structure, FTO/CdS-CdSe/Sb₂SxSe_3-x /PbS-Se/C, generally increases the cell performance. The solid solutions of PbS-Se are chemically deposited from a solution mixture containing lead nitrate, thiourea and selenosulfate at 60^oC in 15-60 min. These films possess crystalline grain diameters, 5-8 nm an E_g of 1.5-1.8 eV and electrical conductivity, 0.01#8486;^-1cm^-1.
[1] M. Calixto-Rodriguez, et al, ECS Journal of Solid State Science and Technology, 2 (2013) Q69

GaAs-based solar cells have been highlighted due to their high conversion efficiency compared with conventional solar cells. One of the important parameters for enhancing the efficiency of solar cells is the minimization of the optical reflectance. Therefore, a suitable antireflective surface is essential for GaAs-based solar cells to reduce its high reflectance. Antireflection coating (ARC) on solar cells has been conventionally used to reduce surface reflection. However, ARC is only effective in limited spectral ranges and has several problems such as poor adhesion and thermal mismatch. Recently, subwavelength structure (SWS), which refers to surface structures with a size smaller than the wavelength of the incident light, has been widely investigated to reduce surface reflectance. Various shaped- SWSs including pyramids, rods, cones and tips, have been demonstrated by using diverse methods like electron beam lithography and nanoimprint lithography.
In this work, a facile method is demonstrated for achieving broadband low reflectance through a two-step surface texturing technique that combines nanosphere lithography with dry-etching. Various stepped-cone nanostructures were fabricated on the surface of GaAs to suppress its reflectance. The shape and height of these nanostructures were precisely controlled by varying the diameter of the SiO₂ nanospheres and the etching time.
The effects of stepped-cone nanostructure were analyzed by measuring its reflectance spectra and compared with finite-difference time-domain calculations data. The average reflectance is reduced from 38.1 to 2.6 % at the wavelengths of 300 to 2500 nm due to enhanced light scattering and a gradual change in refractive index. We observed reflectance spectra changes upon unique shapes of step cone-like SWSs, especially in infrared ranges. The details of the fabrication procedure and results will be presented.

Ge-subcells were fabricated using the original overall InGaP-InGaAs-Ge structure. The key feature in fabrication of the present Ge-subcell is to reserve semiconductor layers of forming the InGaP-subcell and the InGaAs-subcell as dummy subcells. Thus the Ge-subcell receives the same light spectrum as that of the Ge-subcell within the triple-junction solar cell. Temperature dependences of photovoltaic parameters of the Ge-subcell within the triple-junction are directly extracted from the present Ge-subcell. In particular, two methods were employed to extract the photocurrent which is generally different from the short-circuit current in this case. Besides, determination of the junction temperature associated with the fabricated Ge-subcell was also reported in this work. Experimental results indicate that (i) Ge-subcell is no longer as a real solar cell at temperatures over 143°C; (ii) the photocurrent extracted is ~ 25.8 mA/cm² that is almost independent of operating temperature range of 30°C to 80°C. However, the measured short-circuit currents were quite different from the extracted photocurrents; (iii) maximum conversion efficiency @30°C obtained from the intrinsic Ge-subcell is 3.22% with corresponding temperature coefficient of -0.0373%/°C; and (iv) when the Ge-subcell is under irradiation, the junction temperature increases rapidly with an initial rate as high as ~1°C/s, then increases more slowly, and finally saturates at ~48°C, resulting in more than 0.8% reduction in conversion efficiency.On the other hand, the times required for the increase of 5, 10, and 15°C in the cell temperature are found to be 8, 22, and 52 s, respectively. Based on the experimental results, the maximum conversion efficiency will decrease from 3.41% to ~3.22% and ~3.03% once the Ge-subcell is exposed to the sunlight for 8 and 22 s, respectively. After 1-min irradiation, the Ge-subcell has more than 0.6% reduction in maximum conversion efficiency. In contrast, the junction temperature quickly decreases and then slowly decreases when the sunlight is removed. The recovery time is 5 minutes.

Solar cells based on Cu(In,Ga)Se₂ absorber layers have gained steady improvements in conversion efficiency up to 21.7%, exceeding the performance of market leading polycrystalline Si solar cells and other thin film solar cell technologies. For a wide distribution of the technology and competition with conventional non-renewable energy sources, however, cost competitive manufacturing processes have to be ensured. The thin nature of the solar cell layers allows for the absorber deposition by high throughput printing processes with high material utilization, high homogeneity, and low capital investment compared to state-of-the art vacuum-based manufacturing. Molecular ink-based methods are especially interesting since they facilitate molecular scale mixing of the reagents. They also have the potential to avoid toxic precursors and reagents like hydrazine and H₂S/H₂Se and avoid the need to synthesize nanocrystals.
In this paper we present a novel precursor route to deposit CuInSe₂ (CIS) absorber layers that can omit the use of toxic reagents while minimizing detrimental carbon impurities in the final absorber. The new solution chemistry enables non-vacuum deposition methods and conversion to CIS by a subsequent selenization at elevated temperatures. The ink and absorber layers are systematically characterized by TGA, ICP-MS and SEM, EDX, XRD, PL, TRPL, respectively. Solar cells are fabricated by the application of CdS and TCO layers and device efficiencies are determined by J-V and EQE measurements. We have developed this process to already yield AM1.5 power conversion efficiencies greater than 10% without an anti-reflective coating. To our knowledge, this is the highest reported efficiency for a CIS solar cell from a solution method without hydrazine solvent or nanocrystals.

Whilst the use of a post-growth CdCl₂ activation step in CdTe solar cell processing had previously been shown to be essential for realization of high conversion efficiencies (10 - 21%), recent work¹ has demonstrated that the toxic CdCl₂ can be directly replaced with a non-toxic, abundant alternative (i.e. MgCl₂) with no detriment to the device performance. This result has opened up a new parameter space in the fabrication of CdTe solar cells, as evidenced by immediate trials of, for instance, ZnCl₂.² To facilitate further progress, work should now be focused towards understanding the effectiveness of certain metal-chlorides for inducing the necessary material changes in the device stack.
Here, X-ray diffraction (XRD) is used, in both theta;-2theta; and rocking curve configurations, to probe the effect of CdCl₂, MgCl₂, NaCl and MnCl post-growth annealing treatments on the microstructure of CdTe films. It is shown that the use of CdCl₂ and MgCl₂ yield more extensive recrystallization than NaCl or MnCl₂. All treatments were successful in relaxing a sizable compressive stress that was observed for as-grown films (-164 MPa), although CdCl₂ gave the most marked relaxation (13 MPa). Two-axis XRD measurements were carried out to study the angular distribution of (111) grains, with the CdCl₂ treatment leading to a wider distribution than all other treatments, further demonstrating that it remains the most effective process for inducing recrystallization. It is posited that lower dissociation energies of the metal-Cl bond allows for more efficient Cl-treatment.
X-ray photoelectron spectroscopy was used to determine the composition of the CdTe back surface, and also to generate elemental depth profiles of CdTe/CdS/ZnO/SnO₂ stacks. The highest surface Te:Cd ratios (a Te-rich surface being desirable for forming an Ohmic contact) were found for the samples treated by CdCl₂ (Te:Cd = 1.3) and MgCl₂ (1.4) whereas more extensive oxide formation was evident for NaCl and MnCl₂ treated samples. For all samples, depth profiles demonstrated Cl segregation to the CdTe/CdS interface and predominantly within the CdS itself, with Cl content typically being ~ 1%. Notably, the CdCl₂ treatment yielded O segregation at the CdTe/CdS interface, whereas this was not observed for other samples.
The influence of the various Cl-activation treatments on the final device performances, previously reported by Major et al., are discussed, in view of the results reported here. Temperature dependent dark current-voltage measurements support the conclusions.
1. J.D. Major et al., Nature 511 (2014) p.344
2. C. Drost et al., Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.09.001

The record efficiency of a CdTe solar cell is still significantly lower than the fundamental limitations predict; this has been primarily attributed to a deficit in voltage. Incorporation of a high band gap material at the back of the cell could induce a conduction-band offset. A sufficient conduction-band offset has shown improvements in silicon HIT cells and has been modeled in CdTe to show an increase in device voltage. Due to the low lattice mismatch and the low required amount of Mg to achieve a desired conduction-band offset, close space sublimated (CSS) Cd_1-xMg_xTe is selected. Cd_1-xMg_xTe is deposited by CSS where CdTe and Mg are being sublimated simultaneously to form a Cd_1-xMg_xTe alloy. This allows for the formation of the desired cell structure of: CdS/CdTe (passivated)/Cd_1-xMg_xTe. Passivation of CdTe is a critical step in our cell manufacturing, converting the cell from ~2% efficient to ~13%. By the addition of this Cd_1-xMg_xTe layer the required passivation process has changed. We present two different ways to fabricate and passivate this structure. The first utilizes a single passivation method where Cd_1-xMg_xTe is deposited onto CdTe and then the entire CdS/CdTe/Cd_1-xMg_xTe stack is passivated. The second employs a dual passivation method where the Cd_1-xMg_xTe is deposited onto passivated CdTe and a second passivation is performed for the Cd_1-xMg_xTe layer. Both of these methods have been performed at Colorado State University and both methods have unique challenges. We have observed using transmission electron microscope, scanning electron microscope, energy-dispersive x-ray spectroscopy, and secondary ion mass spectrometry that there is magnesium diffusion from the Cd_1-xMg_xTe film, degradation of the underlining passivated CdS/CdTe cell, and back surface contamination from passivation and handling. This work will show details of these current material limitations seen in these advanced cells and investigate potential solutions.

The CBD-Zn(S,O) remains one the most studied Cd-free buffer layer for replacing the CBD-CdS buffer layer in Cu(In,Ga)Se₂-based solar cells and has already demonstrated its potential to lead to high-efficiency solar cells up to 20.9%. However, a key issue to implement a CBD-Zn(S,O) process in a CIGSe production line is the deposition time and cells stability, which depend not only on the bath chemistry of CBD-Zn(S,O) but also on a good band alignment between CIGSe/CBD-Zn(S,O)/windows layers. These past years, several bath compositions have been developed, all of them leading to very high efficiencies compared to their CdS references. However, in general it is found that devices with CBD-Zn(S,O) buffer layers exhibit more pronounced metastable changes in the IV-characteristic, which has opened the question of whether the buffer layer, the absorber layer or the windows layer are the prime origin of metastabilities in these solar cells. The present contribution will be specially focused on the effects of the bath chemistry and growth conditions of CBD-Zn(S,O) buffer layers on the properties of the resulting films and their impact on solar cell performances. In fact, even if among the many different methods available to deposit semiconductor films, chemical bath deposition (CBD) conceptually ranks as the simplest, it remains one of the least understood. The aim of this talk is first to have a better understanding of the different parameters that influence the optimization of CBD-Zn(S,O) buffer layer through a literature review. Then some suggestions will be given for the improvement of the bath chemistry, with a particular focus on introducing new additives and reactants, and analyzing their impact on the resulting thin films properties and solar cell performances.

Cu(In,Ga)Se₂ (CIGSe) thin-film photovoltaic devices have recently achieved record efficiencies of 21.7% on a laboratory scale¹. These record devices consist of multiple polycrystalline layers, including a thin n-type CdS buffer layer. The energy level alignment at the CdS/CIGSe interface shows a favorably flat conduction band offset^2,3. However, to optimize transparency and avoid the use of heavy metal components, there have been significant research efforts to find alternative buffer layer materials. Most notably, Solar Frontier K. K. recently reported a 20.9% efficient device with a Zn(O,S) buffer layer³, demonstrating the potential of this alternative buffer material. For further optimization, it is very important to derive and understand the electronic and chemical structure of the Zn(O,S)/CIGSe interface and how it differs from the standard CdS-based configuration.
It is hence the goal of this study to probe the chemical and electronic structure of the Zn(O,S)/CIGSe interface. A corresponding set of Zn(O,S)/CIGSe samples with increasing Zn(O,S) layer thickness was prepared at NREL and investigated by a suite of electron spectroscopies, including x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy, as well as inverse photoemission (IPES) at UNLV. Furthermore, soft x-ray emission spectroscopy (XES) was performed at the Advanced Light Source. XPS gives insights into the chemical structure and surface composition of the samples, and monitoring the energetic positions of the core levels also allows for the determination of interface-induced band bending. Additionally, XES provides information on the chemical structure in the near-surface bulk region, making it possible to probe buried interfaces. The energy position of the valence band and the conduction band with respect to the Fermi energy is derived from UPS and IPES spectra. A complete picture of the chemical and electronic structure of the Zn(O,S)/CIGSe interface is thus experimentally derived from the combination of the employed different measurement techniques. In our contribution, we will present the results of our comprehensive study. Among others, we find a small interface-induced band bending, suggesting that the band edge positions at the absorber surface are only partially (if at all) affected by Fermi level pinning. Furthermore, we will show data that suggests a flat conduction band offset and evidence for Se diffusion at the interface.
1. www.pv-tech.org/news/zsw_sets_21.7_cigs_cell_record
2. M. Morkel et al. Appl. Phys. Lett.79, 4482 (2001).
3. S. Pookpanratana et al., Appl. Phys. Lett. 97, 074101 (2010).
4. www.solar-frontier.com/eng/news/2014/C031367.html

Raman spectroscopy is frequently applied for the analysis of phase compositions in chalcogenide thin-film solar cell materials [1]. Performed as microscopic mapping experiment, it is a powerful tool for identification and studying the spatial distribution of chemical compounds (e.g. CuInS₂vs. Cu_xS) and crystal structures (e.g. chalcopyrite vs. CuAu I-type CuInS₂) based on their characteristic vibrational modes [2]. In quaternary compounds, such as Cu(In,Ga)Se₂, determination of the exact band positions of Raman modes allows the determination of concentration gradients (e.g. [Ga]/([In]+[Ga])) that are in good agreement with elemental analysis [3]. Small band shifts also appear due to stress and strain. In chemically homogeneous CuInSe₂ changes in both, Raman shifts and relative band intensities are only due to variations in physical material properties. Former studies investigated the correlations between band positions and applied pressure [4, 5] as well as the effect of crystal orientations onto relative intensities of certain CuInSe₂ bands [6]. We employed these spectroscopic properties as contrast in Raman imaging of CuInSe₂ and correlated the results with electron backscatter diffraction (EBSD) maps.
Raman spectra of CuInSe₂ layers exhibit 3 dominant peaks at 174, 213 and 228 cm^-1. Raman microscopy was performed on surfaces of polycrystalline CuInSe₂ thin films with step sizes of 200-250 nm (purposely oversampling the optical resolution of approx. 400 nm) and Raman maps revealing the spatial distributions of Raman band intensities or peak shifts, respectively, were constructed from this data. On the same identical positions, EBSD maps were acquired, giving the local orientations of the individual CuInSe₂ grains. By this correlative approach, the three dominant Raman peaks can be assigned to specific polar and nonpolar facets of the crystals. Raman maps were acquired at various orientations of the CuInSe₂ layers with respect to the polarization direction of the incident laser beam.
It was found that at specific polarization directions and for certain crystal orientations (relatively to the polarization direction of the incident laser beam), Raman scattering was strongly reduced, which was verified by simulation of Raman spectra. The results of the present work provide the basis for enhanced information from Raman spectroscopy applied on various polycrystalline material systems.
[1] D. Papadimitriou, N. Esser, C. Xue, Phys. Stat. Sol. B 2005, 242:2633-2643.
[2] T. Schmid, C. Camus, S. Lehmann et al., Phys. Stat. Sol. A 2009, 206:1013-1016.
[3] D. Abou-Ras, R. Caballero, C.-H. Fischer et al., Microsc. Microanal. 2011, 17:728-751.
[4] H. Tanino, T. Maeda, H. Fujikake et al., Phys. Rev. B 1992, 45:13323-13330.
[5] J. González, M. Quintero, C. Ricoacute;n, Phys. Rev. B. 1992, 45:7022-7025.
[6] H. Tanino, H. Fujikake, T. Maeda, H. Nakanishi, J. Appl. Phys. 1993, 74:2114-2116.

The nature and impact of compositional variations at grain boundaries in Cu(In,Ga)Se_2shy; (CIGS) thin film solar cells is not well understood. It is widely accepted that grain boundaries can be beneficial and that polycrystalline devices outperform their monocrystalline counterparts. However, the origin of this benefit is a topic of continued research. A key challenge is the lack of statically significant data. Many STEM techniques, ideal for studying grain boundaries with high spatial resolution, require extensive sample preparation to examine only a few boundaries. Synchrotron based x-ray fluorescence (XRF) is a non-destructive technique that has the capability to scan over large areas (10-100 µm) with high spatial resolution (< 100 nm). This technique can be combined with x-ray beam induced current (XBIC) for a spot to spot correlation of carrier collection efficiency and composition in full devices. These techniques were employed to systematically study the impacts of compositional variations at grain boundaries in CIGS devices with low and high gallium contents.
We report that in both types of samples, indium segregation at grain boundaries has a direct correlation to changes in carrier collection efficiency. Indium rich (copper poor) boundaries show decreased carrier collection while indium poor (copper rich) boundaries show increased carrier collection. Compositional variations as large as 5 at. % have been observed corresponding to a 15 % change in collection efficiency. A decrease in average collection efficiency was observed for the high gallium device with an increase in the number of indium rich boundaries compared to the low gallium device. Boundaries were analyzed in a statistically meaningful way, by examining over 15 boundaries per sample and averaging over 5 line scans per boundary. These results show the dependence of carrier collection on composition at grain boundaries in CIGS, and allow for a more detailed understanding of the role grain boundaries play in the overall performance of CIGS devices.

MiaSolé produces solar modules using CIGS thin film technology. Our thin film technology is based on a "roll-to-cell" platform where all the films that comprise the CIGS solar cell are sputter deposited sequentially onto a flexible stainless steel substrate in a single pass in our all-PVD process system followed by automated cell formation and 100% inline IV testing.#12288;
This paper discusses the formation of the CIGS absorber in our system with emphasis on the repeatability and control of the Ga/III compositional grading through the thickness of the absorber layer. This is particularly important for high throughput, large area deposition manufacturing systems where CIGS deposition times are <5 minutes and deposition coverage is 1m during the continuous process.
A combination of 1D device modeling, device characterization of production deposited material, and extensive depth profiling Auger Electron Spectroscopy (AES) illustrate the importance of the ideal Ga/III composition at different depths in our absorber. We will discuss the impact of growth conditions (temperature, metal rates, selenization, Na content) on the diffusion of Ga/III during growth and their impact on subsequent device properties.

The bandgap is one of the most fundamental properties of a semiconductor and one that's critical for the application in solar cells. There are different methods to determine the bandgap: from optical transmission/reflexion measurements, from the low energy edge of the quantum efficiency spectrum, from the photoluminescence spectrum. In well-ordered crystalline semiconductors, like GaAs or CuInSe₂, the different methods lead to consistent results - not so in kesterites. This can be attributed to the strong band tailing observed in these materials. An additional complication is due to the fact, that the bandgap of kesterites depends on the history of the material, which determines whether it is ordered or disordered in the Cu-Zn planes. We will discuss the different bandgaps obtained by different methods and will clarify which one is the suitable one to be compared with the open circuit voltage extrapolation, which is used to determine the dominant recombination mechanism.

Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), interesting absorber materials for thin film solar cells, are subject to disorder by Cu-Zn exchanges in the Cu and Zn containing planes located at z=1/4 and z=3/4 of the kesterite structure. In CZTS, the observation of Cu_Zn and Zn_Cu defect clusters resulting from disorder have been reported by several technics such as neutron diffraction [1] and magnetic nuclear resonance [2]. Additionally, the order-disorder transition in CZTS has been monitored by Raman spectroscopy [3]. Besides changing the Raman spectrum of kesterite, the order-disorder transition modifies fundamental semi-conductor properties for instance the band gap which is of prime importance for solar cells [4].
We used spectrophotometry (SP), photoluminescence (PL), Raman spectroscopy and quantum efficiency (EQE) to investigate the order-disorder transition in CZTSe on bare thin film and complete solar cells.
Band gap changes induced by order-disorder transition were monitored by SP on CZTSe thin film with Cu-poor composition (Cu/(Zn+Sn)=0.86, Zn/Sn=0.96) relevant for solar cell fabrication, grown by co-evaporation on bare glass substrate. This single sample was annealed/quenched several times with varying dwell temperature to modify the degree of ordering which changes the band gap by 110 meV. The complete reversibility and the continuous band gap variation measured are expected for order-disorder transition as this is a second order transition. In addition, the evolution of the long range order parameter S (S=1 for perfect order and S=0 for complete disorder in the Cu-Zn planes) during the annealing/quenching sequence has been calculated following Vineyard&’s approach [5]. The comparison of band gap evolution during the annealing/quenching sequence and the evolution of S clearly revealed that the band gap can be used as a secondary order parameter. We find critical temperature for the CZTSe order-disorder transition at 200 ± 20°C. The change in band gap of 110 meV was confirmed for CZTSe grown on Mo coated glass by PL and for absorber in complete device by EQE (CZTSe was ordered or disordered prior to solar cell finishing). Raman spectra are in agreement with a change in symmetry between ordered CZTSe and disordered CZTSe. Measurements show that increasing the degree of order increase the correlation length of the crystal, which is explained by a decrease in Cu_Zn and Zn_Cu defect cluster density. This decrease in defect density did not lead to improvement of solar cell efficiency or open circuit voltage indicating that Cu_Zn and Zn_Cu defect clusters is not the main limiting factor of CZTSe-based solar cell.
[1] S. Schorr. Solar Energy Materials and Solar Cells, 95 (2011) 1482.
[2] L. Choubrac, et al. Phys. Chem. Chem. Phys., 15 (2013) 10722.
[3] J. J. S. Scragg, et al. Applied Physics Letters, 104 (2014) 041911.
[4] G. Rey, et al. Applied Physics Letters, 105 (2014) 112106.
[5] G. H. Vineyard. Phys. Rev., 102 (1956) 981.

Kesterite copper zinc tin sulfides/selenides/sulfoselenides (Cu₂ZnSn(S,Se)₄, CZTSSe) are attractive absorber materials due to their earth-abundant nature and high light absorbing coefficients (10⁴-10⁵ cm^-1). A molecular precursor route without the need of sophisticated vacuum system or the complex preparation of nanoparticle-inks is particularly suitable for low-cost and high-throughput deposition of CZTSSe thin films. In this study, we report a novel solution method for depositing highly uniform metal chalcogenide thin films, especially kesterite CZTSSe thin films. A versatile amine-thiol solvent mixture has been developed to dissolve elemental metals, metal oxides, elemental chalcogens, and metal salts, providing a variety of molecular precursor route to CZTSSe and other metal chalcogenide thin films. The efficacy of using this molecular solution to deposit continuous and densely-pack thin film of CuS, CuSe, SnS, and CZTSSe has been demonstrated. The dissolution mechanism of this solvent mixture was studied using Fourier transform infrared spectroscopy, thermogravimetric analysis, and mass spectrometry. For thin film fabrication, the molecular precursor is applied on the Mo sputtered (~800 nm) soda lime glass substrates using spin coating technique. An annealing process is performed after deposition of each layer. After the desired thickness is obtained, a subsequent heat treatment is performed at 500^oC for 30 min under selenium atmosphere in order to convert the as-deposited thin film into a polycrystalline large grained film with single grains in the through thickness. Kesterite CZTSSe phase is acquired after annealing and selenization, withno binary or ternary phases detected by X-ray diffraction (XRD), Raman spectroscopy, or secondary electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). The final solar cell device is fabricated by depositing CdS buffer layer (~50 nm), sputtering i-ZnO (~80 nm) and ITO (~220 nm), and e-beam depositing Ni/Al grids on the top of the absorber layer. With this molecular precursor route, a power conversion efficiency of 7.86 (8.09%) was achieved on a total area of 0.47 cm² (active area of 0.456 cm²) under AM 1.5 illumination. For further insight into device performance, the external quantum efficiency (EQE) measurement has been performed at 0 V and -1 V. The low carrier lifetime limits the longer wavelength range collection efficiency. The bandgap estimated based on the plot of [E ln(1-EQE)]² versus the photon energy is 1.08 eV, indicating a higher concentration of selenium atoms in the kesterite lattice after selenization.

Multinary materials are advantageous for many applications because they provide wide compositional parameter space in which materials properties can be tuned. Increasing the compositional complexity, however, has an important side effect—namely, the increased propensity for local decompositions that preserve the underlying crystal structure and therefore are difficult to detect and characterize experimentally. In solar absorbers such decompositions can be particularly problematic, because they increase scattering of photo-generated charge carriers or even lead to charge trapping in the band tails. The resulting decrease of the minority carrier mobility and lifetime, as well as increased recombination losses, is detrimental to the performance of photovoltaic (PV) devices. Indeed, cation disorder and formation of compositional inhomogeneities have been indicated as a possible reason for low open circuit voltage limiting the performance of CZTS PVs [1].
Using chemically intuitive model Hamiltonian and density functional calculations we study cation disorder in Cu₂ZnSnS₄ and Cu₂SnSn₃ solar absorbers [2]. We show that in both materials cation disorder may lead to formation of (sub) nanometer scale structurally coherent compositional inhomogeneities. In CZTS formation of such inhomogeneities is associated with a phase transition (Tc asymp; 450 K) in which the material transforms from a single motif phase (built from S-Cu₂SnZn tetrahedral motifs) to a multi-motif phase. Cu₂SnS₃, on the other hand, has a two-motif ground state structure, and all other motifs have prohibitively high formation energies [3,4]. However, the modeling exposes sub-nanometer scale compositional inhomogeneities that form in Cu₂SnS₃, due to the rearrangement of the 2 native motifs. We have developed experimental techniques to control these compositional inhomogeneities in Cu2SnS3.
The motif-based model of this work can serve to quantify the disorder in other multinary semiconductors. For solar cells, materials are desirable that have a single-motif ground state, and a high-energy threshold for formation of non-native motifs. These conditions should be met for the single motif ternary materials such as CuInSe₂ or Cu₃SbS₄.
The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.
[1] T. Gokmen, O. Gunawan, T. K. Todorov, and D. B. Mitzi, Appl. Phys. Lett. 103, 103506 (2013)
[2] P. Zawadzki, A. Zakutayev and S. Lany, (submitted)
[3] P. Zawadzki, L. L. Baranowski, H. Peng, E. S. Toberer, D. S. Ginley, W. Tumas, S. Lany, and A. Zakutayev, Appl. Phys. Lett. 103, 253902 (2013)
[4] L. L. Baranowski, P. Zawadzki, S. Christensen, D. Nordlund, S. Lany, A. C. Tamboli, L. Gedvilas, S. David, W. Tumas, E. S. Toberer, and A. Zakutayev, Chemistry of Materials 26,4951(2014)

Kesterite based copper zinc tin sulfoselenide (CZTSSe) semiconductors are very promising absorber materials for next generation thin film solar cells because they use earth abundant elements and have high theoretical efficiency. Forming CZTSSe thin films directly from solution is a promising method for low-cost CZTSSe solar cells due to the simplicity, high materials utilization, and easy control of the precursor ratios. The record CZTSSe solar cell with an efficiency of 12.6% was fabricated from hydrazine based ink.^[1] However, hydrazine is hepatotoxic, carcinogenic, and explosive. For safety and environmental concerns, fabricating CZTSSe absorber material from non-toxic solution would be ideal. Previously, we reported that CZTSSe films can be processed from a simple molecular-ink using an environmentally benign solvent dimethyl sulfoxide (DMSO) yielding solar cells with a power conversion efficiency of 4.1%.^[2] The solution was made by simultaneously dissolving Cu(OAc)₂#8729;H₂O, SnCl₂#8729;2H₂O, ZnCl₂ and thiourea (SC(NH₂)₂) in DMSO. Later, using the same precursor materials and ink composition, by adding precursors step by step, e.g. adding Cu(OAc)₂#8729;H₂O and SnCl₂#8729;2H₂O first to facilitate complete redox reaction followed by ZnCl₂ and thiourea, we were able to improve the CZTSSe film quality and achieve 8.3% efficient solar cells.^[3] These results reveal the importance of the chemistry in the solution. Here, we systematically study the chemistry in the DMSO molecular precursor ink and the reaction pathways from solution to solid state. We have isolated compounds formed between each metal precursor and thiourea, compounds formed in each state of the ink, and characterized compounds properties including single crystal structures. We found that Cl^- concentration is critical to stabilize Cu⁺ and enable complete redox reaction between Cu²⁺ and Sn²⁺. We have also explored different metal precursors, Cu(OAc)₂#8729;H₂O vs CuCl₂#8729;H₂O, SnCl₂#8729;2H₂O vs SnCl₄, Sn(OAc)₂ vs Sn(OAc)₄, and their combinations in the ink and studied how different cations (OAc^- vs Cl^-) and/or metal oxidation values (Sn²⁺ vs Sn⁴⁺) affect the ink stability and film composition and quality. Our results provide a whole picture of the chemistry in the ink, the reaction pathways from ink to solid state and CZTSSe absorber materials. By optimizing the ink formulation, selenization condition and using additive in the ink, we have fabricated CZTSSe solar cell with power conversion efficiency of 11.1%.
[1] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, D. B. Mitzi, Advanced Energy Materials 2013; T. K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi, Advanced Energy Materials 2013, Vol. 3, 34.
[2] W. Ki, H. W. Hillhouse, Advanced Energy Materials 2011, 1, 732.
[3] H. Xin, J. K. Katahara, I. L. Braly, H. W. Hillhouse, Advanced Energy Materials 2014, 4.

Given the terawatt-scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across many geographic regions. In addition, their supply chains should be robust to reduce concerns of price volatility. The process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials if possible. One candidate for single junction solar cells or the bottom junction of tandem solar cells is kesterite structured Cu₂ZnSn(S,Se)₄ (CZTSSe) and similar alloy materials.
Conventionally, thin film chalcopyrite and kesterite solar cells have been synthesized by evaporating or sputtering metals followed by sulfurization or selenization. Recently, a potentially low-cost high-throughput approach has been demonstrated that forms the quaternary or pentenary chalcogenide directly from a solution-phase process utilizing hydrazine as a solvent and complexing agent. Here, we report the development of a class of solution-phase routes to CZTSSe that do not use hydrazine or nanocrystals. We have developed a DMSO/thiourea solvation/complexation chemistry that yields 11.1% efficient CZTSSe solar cells. Further, we have developed a combinatorial ultrasonic spray coater and high-throughput screening method to map the optoelectronic properties of CZTSSe as a function of composition and dopants. The presentation will focus on: (1) the results of combinatorial experiments to reveal the effects of native point defects on the optoelectronic quality, (2) development of a new theory of sub-bandgap states and their effect on photoluminescence, and (3) showing that CZTSSe is capable of exceeding 70% of the theoretically possible Voc.

In this work Cu₂ZnSnSe₄ (CZTSe) thin films with lateral compositional gradients were synthesized by DC-magnetron sputtering in order to study the influence of composition on their optoelectronic and vibrational properties. Around 200 solar cells were processed with Cu/(Zn+Sn) and Zn/Sn ratios between 0.55 - 1.20 and 0.70 - 1.90, respectively. This has allowed fabrication of a set of devices with very high resolution of incremental changes in absorber composition. During cell fabrication, absorbers were chemically etched before deposition of the buffer layer to remove potential surface ZnSe secondary phases. All solar cells have been characterized by J-V and Raman measurements with the aim of correlating the vibrational and optoelectronic properties with the changes of the absorber composition. This study has allowed identification of the compositional regions where optoelectronic properties show an abrupt deterioration (Cu/(Zn+Sn) > 0.95 or Zn/Sn < 1.00), as well as the compositional regions corresponding to highest open circuit voltage (V_oc) and short circuit current (J_sc). These regions appear in the phase diagram corresponding to Cu-poor and Zn-rich compositions, where occurrence of V_Cu, Zn_Cu and Zn_Sn point defects is expected. The maximum J_sc region corresponds to a constant Sn composition close to the stoichiometric value. On the other hand, the maximum V_oc region corresponds to a slightly Sn-poor region. Also in this case, samples with highest V_oc have very similar Sn contents. This suggests that defects related to the Sn composition have a significant impact on both optoelectronic parameters. In the case of J_sc, both Sn excess defects (likely Sn-Se secondary phases) and Sn deficiency defects (mainly Zn_Sn) are likely deteriorating the J_sc. For V_oc, these results suggest the need for a small amount of Zn_Sn point defects to optimize this parameter.
Raman scattering analysis measurements performed in the surface region of the absorbers show the formation of Sn-Se secondary phase in the compositional range with Cu/(Zn+Sn) < 0.80 and Zn/Sn < 0.95, while no evidences of Cu-Se surface binary phases have been observed. Furthermore, detailed analysis of the Raman spectra has allowed investigation of variations in the position and half-width of the main A mode centered at 196 cm^-1, which involves only pure anion vibrations of Se atoms. These variations have been associated to degradation of the overall crystalline quality of the CZTSe. In contrast, systematic changes in the intensity of the E and B modes located around 170, 210 and 250 cm^-1 frequency regions, which involve mostly cation vibrations, were also observed. Changes in the relative intensity of these peaks will be systematically analyzed in relation to the occurrence of different kinds of defect clusters involving V_Cu, Zn_Cu, Zn_Sn, Cu_Zn and Sn_Zn point defects, in order to analyze the vibrational origin of the different modes.

Thin-film compound semiconductor photovoltaic devices based on chalcopyrite CuIn_1-xGa_xSe₂ (CIGSe) absorbers have reached efficiencies of up to 21.7 % [1] on the laboratory scale, and are increasingly produced on an industrial scale. However, concerns over the long-term material costs have led to a drive to produce solar cells from more earth-abundant materials. A promising alternative is the closely related kesterite material system Cu₂ZnSn(S,Se)₄ (CZTSSe), which has already reached efficiencies of up to 12.7% [2] on the laboratory scale using a hydrazine-based deposition process. A more promising method for industrial-scale production of kesterite absorber materials is to create CZTSSe layers via selenization and crystallization of metal salts, a process which has already produced solar cells of up to 10.3% efficiency [3,4]
It is reported that the dominant recombination process in selenium-free Cu₂ZnSnS₄ (CZTS) devices can be described by activation energies (E_a) well below the absorber band gap (E_g) [5], indicating that the performance is limited by recombination at the interface between kesterite absorber and CdS buffer. This conclusion is in agreement with the derived unfavorable “cliff-like” conduction band offset (CBO) at that interface [6]. Sulfur-free kesterite absorbers (CZTSe), on the other hand, have shown to form a more favorable CBO with a small “spike” at the buffer/absorber interface [7].
We will present the findings of a direct and indirect photoemission spectroscopy study of wet-chemically deposited CdS on CZTSSe absorbers prepared from metal salts. The chemical structure and electronic level alignment at this buffer/absorber interface is determined. The direct measurement of the valence and conduction band offsets (VBO and CBO, respectively) - within the uncertainty of the experiment - does not reveal any significant discontinuity of the conduction bands at the CdS/CZTSSe interface. This finding is in line with device characteristics that show that E_a (derived from V_OC(T) measurements) is very close to the absorber&’s E_g (derived from quantum efficiency measurements of reference cells), suggesting that - in contrast to CZTS-based solar cells - the performance of CZTSSe devices might at this point not be limited by recombination at the CdS/CZTSSe interface.
[1] ZSW press release 12/2014, Stuttgart, 22 September, 2014
[2] Kim et al., Adv. Mater., published online, DOI: 10.1002/adma.201402373
[3] Schnabel et al., Sol. Energ. Mat. Sol. Cells, 117, 324 (2013)
[4] ZSW press release 18/2013, Stuttgart, 25 November 2013
[5] K. Wang et al., Appl. Phys. Lett. 97, 143508 (2010)
[6] Bär et al., Appl. Phys. Lett. 99, 222105 (2011)
[7] Bär et al., MRS Spring Meeting 2013, Talk

Polycrystalline Copper-zinc-tin-sulfide/selenide (CZTS,Se) compounds have received wide research interest due to their potential as inexpensive absorber materials composed of earth-abundant elements. Photovoltaic devices fabricated on CZTS,Se have reached conversion efficiencies of 12.6 %. One of the key parameters to further boost the conversion efficiency is to control the concentration of recombination sites at the surface, secondary phase interfaces and in the grain boundaries. To determine the presence of secondary phases on the surface and composition of grain boundaries, this work has employed Auger nanoprobe electron spectroscopy (NanoAuger) with 8nm lateral resolution combined with high resolution ambient Kelvin Probe Force Microscopy (KPFM) with dual-lock-in setup. NanoAuger was performed in planar and cross-sectional modes on CZTS,Se surfaces before and after top surface oxide removal by NH₄OH clean. Elemental maps before and after NH₄OH clean show Sn-/O-rich and Cu-poor grain boundaries suggesting that grain boundaries are terminated by tin-oxide (SnOx). Secondary phases such as SnSe and ZnSe were observed in the cross-sectional maps. Kelvin probe force microscopy (KPFM) on the cleaned surfaces showed that SnOx-terminated grain boundaries have 80-200 mV larger work function than grains, resulting in upward band bending between grains and grain boundaries. The upward band bending accompanied by the large valence band offset between the SnOx and CZTS,Se lead to relatively large energy barriers for both electrons and holes to travel into the grain boundaries and recombine. Comparison with the elemental maps for CIGSe (with device efficiencies as high as 18%) revealed the absence of the grain boundary oxide passivation

Earth-abundant kesterite Cu₂ZnSn(S,Se)₄ material is a promising candidate for the mass production of low-cost thin film solar cells, as demonstrated by a 12.7% efficient device obtained by a wet chemical approach including the solvent hydrazine. Here, we synthesized Cu₂ZnSnSe₄ absorbers by a fast large area electrodeposition of metal stack precursors, followed by chalcogenization, which is a process with high potential for the industry, with reported device efficiencies up to 8% for CZTSe. Recently, we were able to set a new benchmark for the performance of electrodeposited CZTSe solar cells, obtaining a maximum device efficiency of 8.2%.
Cu/Sn/Zn metal stack precursors were sequentially electrodeposited at high speed on 15x15cm² Mo/SLG substrates from commercially available solutions. A pre alloying performed at 200°C was shown to stabilize the precursors with the formation of Cu-Sn and Cu-Zn metal alloys. 5x5cm² square parts were cut from the precursor and annealed in a tubular furnace under the presence of Sn and Se powders. At the surface of the absorbers, ZnSe and Sn_xSe_y secondary phases were detected, but could be removed by etching using KMnO₄/H₂SO₄ + Na₂S and (NH₄)₂S solutions, respectively. Next, a film of CdS was deposited by chemical bath deposition with CdSO4 or CdNO3 as cadmium sources, followed by deposition of i-ZnO/ITO window layers by sputtering. Absorbers and devices were characterized by XRF, XRD, SEM-EDX and Raman spectroscopy. To complement the characterization, I-V and EQE measurements were performed on the full 3x3mm² devices.
Varying the metal composition within and at the limits of the Cu-poor Zn-rich range, we studied the presence of secondary phases and their influence on the optoelectronic properties of the devices. The application of two etching procedures was shown to greatly reduce the negative impact of secondary phases at the surface and led to significant improvements of the device performance. Buffer layer optimization (thickness, CdS layer quality) led to a further device performance improvement until 8,2% efficiency with the cadmium nitride precursor.
Main efficiency limitations for this two-step technique will be discussed including uniformity issues related to the electrodeposition processes of tin, showing that this is one of the levels towards high-efficiency electrodeposited kesterite solar cells.

Thin-film solar cells (TFSC) hold the promise of reducing the cost of sunlight-to-electricity compared to conventional crystalline silicon. However, the development of the high-efficiency TFSC faces the challenge of finding desired earth-abundant PV absorber materials. The ideal material for TFSC application needs to satisfy a set of desired physical properties, mainly, (i) suitable band gap, (ii) strong solar absorption, (iii) low recombination loss, (iv) low effective carrier mass, (v) suitable band-offsets with contact materials, and (vi) no doping bottleneck, etc. In this talk, I will present our recent work in searching for and designing the novel Cu-based ternary chalcogenides (i.e. Cu_pM_qVI_r, where M = I, II, III, IV, V) for TFSC application according to the selection criteria based on first four physical properties. The first three (i)-(iii) properties were taken into account simultaneously in a single metric called “spectroscopic limited maximum efficiency (SLME)”, and the fourth criteria on effective mass was treated separately. A set of resulting best-of-class absorber materials will be suggested for detailed defect study and for experimental realization and validation. The potential relationships between those physical properties and local atomic structures of the compounds will be also discussed in detail. (This work is done in collaboration with Alex Zunger, Robert Kokenyesi, and Douglous Keszer. Supported by the DOE Energy Frontier Research Center "Center for Inverse Design.")

The recently reported high-efficiency Cu(In,Ga)Se₂ (CIGS) solar cells (EMPA: 20.4% [1] and ZSW: 20.8% [2]) were fabricated by post-deposition of NaF and KF on CIGS films. Alkali metals, specifically Na, which are incorporated into the CIGS absorber layer, are widely known to have significantly beneficial effects that enhance photovoltaic performance. However, alkali-metal effects of Li, Na, and K in CuInSe₂ (CIS) and Cu₂ZnSnS₄ (CZTS) related semiconductors are not yet well understood. Recently, we calculated the substitution energies and migration energies of the Li, Na, and K atoms in CIS and CZTS related semiconductors by first-principles calculations [3].
In present study, we discuss the substitution energies and migration energies of Li, Na, and K atoms in CIS and CZTS related semiconductors. We performed first-principles calculations within a density functional theory as implemented in the program package CASTEP and DMol³. The migration energies of Li, Na, and K atoms in CIS and CZTS related semiconductors are obtained by a combination of linear and quadratic synchronous transit (LST/QST) methods and a nudged elastic band (NEB) method. For CIS, the substitution energy (E_subs) of Na_Cu (0.40 eV) in CIS is smaller than that of Na_In (1.29 eV). This result indicates that Na_Cu is easily formed in CIS, but Na_In is not easily formed. For CZTS, the E_subs of Na_Cu (0.64 eV) and Na_Zn (0.51 eV) are much smaller than that of Na_Sn (2.71 eV). These results indicate that not only Na_Cu but also Na_Zn are easily formed in CZTS. If some sodium compounds (such as NaF, Na₂Se, or Na₂S) are post-deposited on a CZTS film, some amount of Na atoms can substitute for the Cu and Zn sites in CZTS, and the Na_Cu and Na_Zn atoms will be formed. The E_subs of Li_Cu in CIS and Li_Cu and Li_Zn in CZTS are smaller than those of Na_Cu in CIS and Na_Cu and Na_Zn in CZTS, while the E_subs of K_Cu in CIS and K_Cu and K_Zn in CZTS are much larger than those of Na substitution. The migration energies of (Na_Cu→V_Cu) and (Na_Zn→V_Cu) in CZTS are comparable to that of (Na_Cu→V_Cu) in CIS. On the basis of the obtained alkali-metal effects of Li, Na, and K in CIGS and CZTS related compounds, we will discuss the fabrication process of high efficiency CIGS and CZTS solar cells.
[1] A. Chiril#259; et al., Nat. Mater. 1, 1107 (2013).
[2] P. Jackson et al., Phys. Status Solidi RRL 8, 219 (2014).
[3] T. Maeda, A. Kawabata, and T. Wada, submitted to Phys. Status Solidi.

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) materials typically suffer from significant band tailing caused by alloy fluctuations and high concentrations of point defects. This may not only affect recombination and trapping of photoexcited carriers but also the mobility of charge carriers. Time resolved luminescence measurements show decay times between 1-10ns. Time resolution in these measurements is typically limited to several hundred picoseconds.
In this study we perform optical pump terahertz probe spectroscopy (OPTP) of polycrystalline CZTS and CZTSe thin films as well as a multicrystalline CZTS pellet. In OPTP the sample is excited optically and while the photoexcited carriers are monitored by Thz radiation with a resolution of 100fs, thus allowing the measurement of very fast photocarrier transients. Also, from the spectral shape of the complex conductivity the intragrain charge carrier mobility can be deduced.[1]
OPTP on Cu-poor coevaporated CZTSe and CZTS reveals biexponential transients of the pump induced conductivity with a fast 50ps and a slower 500ps decay time. By increasing the pump intensity the fast transient disappears, while the longer decay time is not affected. We conclude that the fast transient is caused by trapping of charge carriers into shallow traps possibly associated with the band tails in the material, while the longer decay times are caused by non-radiative recombination through defect states. The complex mobility spectrum measured 20 ps after excitation is indicative of trapped charge carriers and cannot be modeled by the Drude-model describing free charge carriers. DC-mobilities of about 90 cm²/Vs can be deduced from the photoconductivity spectra, which is significantly lower than the 200-1000 cm²/Vs previously observed for polycrystalline chalcopyrite thin films. On the other hand, OPTP measurements on stoichiometric Cu₂ZnSnS₄ pellets obtained by mechanical ball-milling shows photoconductivity spectra which can be well described with the Drude free carrier mobility. This indicates that the trapping into shallow states or band tails might be strongly reduced for CZTS with composition close to the point of stoichiometry.
[1] C. Strothkämper, A. Bartelt, R. Eichberger, C. Kaufmann, and T. Unold, Phys. Rev. B 89 (2014) 115204

Achieving low cost, high efficiency novel thin-film solar cells will require a thorough understanding of the efficiency limitations and reliability issues. Because the defects that cause lower efficiency or device failure are often buried within the device, it is difficult or impossible to probe these defects with traditional characterization techniques. Developing new characterization tools that can non-destructively probe surfaces, interfaces and bulk material within photovoltaics in-situ will enable researchers to zero-in on limiting factors of their devices and provide insight into potential improvements.
Both surface and bulk defects play an important roll in device performance. To probe below the surface, we have developed a two-photon microscopy technique that allows for 3D imaging of solar cells. This microscopy technique operates on the principle that many semiconductors are generally transparent to infra-red (IR) light. Since only when sub-bandgap light is focused sharply does non-linear optical absorption occur, using a pulsed IR laser enables us to locally, sub-surface carriers. In particular, we are able to measure 3D maps of local carrier lifetimes, emission spectra, and induced photocurrent with this technique. We have now successfully imaged, in 3D, carrier dynamics within grains and at grain boundaries in polycrystalline PV materials. These grains and grain boundaries are below the surface of the material, and would be otherwise inaccessible without the penetrating power of the two-photon technique. By correlating our complementary near-field “surface-only” measurements with the multiphoton depth-resolved mapping, we gain penetrating insights into the complex relationships that determine efficiency bottlenecks in PV materials

CZTS/Se is a compound semiconductor being studied for applications in photovoltaic (PV) panels since it includes only abundant and non-toxic elements. Due to recent advances in CZTS processing, the efficiency of conversion is above 12% in laboratory cells; however, greater efficiency is required for commercialization. DFT-MD simulations of CZTS_xSe_1-x materials, including their bulk and grain-boundary defects have been performed to develop an atomic level understanding of the defects which control the efficiencies in these materials.
Bulk defects in CZTS_xSe_1-x such as vacancies, interstitials, and antisites were created and simulated using DFT-MD. In general, all nearly or completely benign defects in CZTS_xSe_1-x are charged balanced. The two most important bulk defects are the one responsible for the non-stoichiometric Cu/Zn=1.8 ratio (V_Cu+Zn_Cu) and the one which is responsible for the Cu-Zn disorder (Cu_Zn+Zn_Cu). The (V_Cu + Zn_Cu) in the Cu-Zn plane produces no band gap states at low concentration. If a high concentration of V_Cu + Zn_Cu defects is simulated (i.e. Cu/Zn = 1.6) and only one of the defect pair is in the Cu-Zn plane, there is a small shift of the Fermi level towards VB. The second common defect is the exchange defect in the Cu-Zn row: (Cu_Zn + Zn_Cu). Although it is charged-balanced, it induces valence band edge states.
One of the most important influences of disorder in CZTS_xSe_1-x alloys is that the disorder (mixing entropy and associated Gibbs energy term) will stabilize the alloy against decomposition to simpler materials such as a mixture of pure sulfide and selinide, CZTS and CZTSe. In addition, disorder and symmetry braking allows more vibrational modes affecting phonon spectrum, Helmholz free energy making the alloy more stable. Total energy calculations were performed in PBE and HSE06 of CZTS_xSe_1-x and possible decomposition products (CuS, CuSe, SnS, SnSe, ZnS, ZnSe, Cu₂SnS₃, Cu₂SnSe₃). Phonon spectra were calculated for CZTS_0.25Se_0.75 alloy and secondary phases to obtain Helmholz free energy, C_v heat capacity and thermal entropy in 0-1000K temperature range. Based on these data, phase diagrams for CZTS_0.25Se_0.75 alloy have been calculated at 0K and 900K. While CZTS_0.25Se_0.75 is unstable at 0 K, the mixing entropy and vibrational energy terms stabilizes CZTS_0.25Se_0.75 alloy against simpler secondary phases making CZTS_0.25Se_0.75 stable at typical growth temperature

III-V compound semiconductors solar cells have demonstrated efficiencies of over 40% and have found use in space and concentrated photovoltaics (PV) applications. However, high manufacturing cost and very expensive and rigid wafers (Ge and GaAs) have significantly limited there use for terrestrial purpose. Other developments of c-Si, poly-crystalline and amorphous solar cells do not fulfill the criteria of low-cost high-efficiency solar devices. Therefore, a drastic reduction in the cost of III-V solar cells is required. Here we demonstrate growth of high quality GaAs thin films on inexpensive flexible metal substrate. Use of R2R process (low-manufacturing cost) and inexpensive flexible metal foils (alternative to expensive wafers) to grow high quality III-V semiconductors is promising for low-cost and high-efficiency flexible III-V solar cells.
Biaxially-textured single crystalline-like Ge thin film on polycrystalline metal substrates provided the template for epitaxial growth of GaAs thin films. Ion-beam assisted deposition (IBAD) technique was used to grow a multi-layered template consisting of biaxially textured oxides on Hastelloy (C-276) metal foil. GaAs films were grown using metal organic chemical vapor deposition and growth temperature variation was investigated to determine the different growth modes. The precursors used for the growth of GaAs were Tri-methyl gallium (TMGa) and arsine (AsH3). Biaxially textured GaAs thin films with strong (004) out-of-plane orientation and sharp in-plane texture was obtained. Strong room-temperature photoluminescence intensity and narrow peak width, comparable to GaAs wafer, suggested high optical quality of the flexible GaAs films. Cross-sectional Transmission Electron Microscopy and small-area electron diffraction pattern (SAED) confirmed the single crystal nature of the films. The influence of underlying single-crystalline-like Ge surface roughness on GaAs thin films was also studied. Further work is in progress to optimize MOCVD growth of single-crystalline-like GaAs thin films on polycrystalline flexible substrates and will be presented.

In spite of previously reported stannite type structure for CZTSe, CZTS and CZTSe crystallize both in the kesterite type structure (space group ) [1, 2] which can be derived from the cubic sphalerite type structure by doubling the unit cell in the direction of the crystallographic c-axis and ordering of the cations. A differentiation between the isoelectronic cations Cu⁺ and Zn²⁺ and consequently kesterite and stannite type structures is not possible using X-ray diffraction due to their similar scattering factors. But neutrons diffraction can solve this problem; the coherent scattering lengths are sufficiently different for these cations (b_Cu=7.718(4) fm, b_Zn= 5.680(5) fm [3]). It was shown by this method that both Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ occur in the kesterite structure. [1, 2] Another method which can solve the problem is anomalous X-ray diffraction on the Cu-K (8979 eV) and Zn-K (9659 eV) edges. The usage of multiple wavelengths above, below and between the absorption edges of Cu and Zn ensures significant over determination, so that the Cu-, Zn-, and vacancy concentrations can be refined reliably for the independent crystallographic sites.
A detailed structural analysis of stoichiometric Cu₂ZnSn(S_1-xSe_x)₄ (Cu/Zn+Sn=0.99 and Zn/Sn=0.99) powder samples with x= 0.58, 0.70, 0.79, 0.90, 1, grown by solid state reaction in evacuated silica tubes, was performed by both neutron diffraction and anomalous X-ray diffraction at the fine resolution neutron powder diffractometer E9 at BER II (lambda; = 1.7986 Å, RT) and the Diffraction station at KMC-2 beamline at BESSY II respectively [4]. Rietveld refinement of both sets of diffraction data using the FullProf suite software [5] lead to accurate values of a and c lattice constants, which are fullfilling Vergard&’s low, and site occupancy factors. The latter have given insights into the cation distribution within the crystal structure of Cu₂ZnSn(S_1-xSe_x)₄ solid solutions with different x values. The correlated information about changes in lattice parameters and cation site occupancies, details on the existing intrinsic point defects and their amounts obtained by both methods will be discussed.
This research was supported by projectsIRSES PVICOKEST 269167 and KESTCELLS 316488, FP7-PEOPLE-2012 ITN, Multi-ITN.
[1] S.Schorr, Solar Energy Materials and Solar Cells, 95 (2011)1482.
[2] S. Schorr, H.-J. Hoebler, and M. Tovar, Eur. J. Mineral. 19, 1 (2007).
[3] V.F. Sears, Neutron News 3 (3), 26-37, (1992).
[4] A. Erko, I. Packe, C. Hellwig et al., AIP Conference Proceedings (2000), 521, 415-418
[5] Juan Rodriguez-Carvajal and Thierry Roisnel, www.ill.eu/sites/fullprof

The material CuInSe₂ has a wide existence range in regard to the Cu content during growth, even under conditions of severe Cu deficiency it still forms in the chalcopyrite phase. If grown under a surplus in copper a phase separation occurs and the CIS will form in stoichiometry with an additional Cu_xSe on top, which can subsequently be removed by a selective KCN etch. As shown previously¹ this stoichiometric material exhibits some beneficial properties over the Cu-poor CIS, which used as a standard in solar cell manufacturing. However, very high doping densities reduce not only the voltage, but also decrease the current of these devices, due to incomplete collection within the space-charge region². We have shown that the doping can be decreased by limiting the Se supply during growth³, therefore alleviating the problem of low current. Additionally a surface treatment, consisting of the deposition of In-Se on these improved absorbers can be applied in order to form a Cu-poor surface, while still keeping the stoichiometric bulk. By this we were able to produce improved Cu-rich solar cells, on-par with the Cu-poor devices⁴.
In this work we performed admittance measurements in order to study the electrical defects in devices comprised of a stoichiometric bulk with a Cu-poor surface. With this technique it is possible to probe the different defects&’ energies and attempt-to-escape frequencies. This technique was applied to solar cells made from Cu-rich absorbers with and without a Cu-poor surface. We previously reported two admittance signals for Cu-rich absorbers⁵, one with a small activation energy in the range of 50-100meV and one around 200-250meV. The one at higher energies is not detectable anymore and the lower energy signal shifts downwards in the solar cells with a Cu-poor surface. Due to the special sample geometry we conclude, that this higher energy signal might be related to an interfacial defect in the Cu-rich material, which is not present in the Cu-poor cells and therefore one of the reasons for the poor performance of Cu-rich grown CIS solar cells.
¹ S. Siebentritt, L. Gütay, D. Regesch, Y. Aida, V. Deprédurand, Solar Energy Materials and Solar Cells 119, 18, 2013
²V. Deprédurand, D. Tanaka, Y. Aida, M. Carlberg, N. Fèvre , S. Siebentritt, J. Appl. Phys. 115, 044503, 2014
³V. Deprédurand, T. Bertram, D. Regesch, B. Henx , S. Siebentritt, submitted, 2014
⁴T. Bertram, V. Deprédurand, S. Siebentritt, Proceedings of the 40th IEEE PVSC Conference, 3633, 2014
⁵T. Bertram, V. Deprédurand, S. Siebentritt, in preparation, 2014

Emerging materials for photovoltaics often require multiple iterations of device fabrication and characterization to establish process-property relationships essential to pinpoint loss mechanisms and improve device efficiencies. Traditionally, device quantum efficiency (QE) measurements have provided insight into depth-resolved carrier collection and thus bulk minority carrier transport properties. We enhance existing approaches for analyzing QE measurements without assuming a dominant charge collection mechanism (drift vs. diffusion), allowing us to determine minority carrier diffusion length (L_diff) for materials that exhibit a mix of both charge collection mechanisms.
As inputs into our analysis framework, we measure QE, optical transmission and reflection, and film thickness. By using an inverse transfer matrix method routine, we obtain quantitative estimates of the refractive index and extinction (or absorption) coefficient. Employing an analytical model, we then use this optical data to reconstruct QE plots from wavelength-dependent generation and collection profiles, resulting in estimates for L_diff for specific devices solely using data from devices and control samples. We constrain the fit by incorporating voltage bias dependent QE measurements into the analysis, making basic but generous assumptions about device properties, including the voltage independence of L_diff and the square-root dependence of depletion width on bias voltage.
To demonstrate the benefits of this approach, we apply this analysis to tin monosulfide (SnS) solar cell devices processed under conditions designed to suppress bulk and interface recombination. We analyze QE on these devices to extract values of L_diff and correlate these with device fabrication processing conditions. We further explore the use of bias light in QE measurements to assess the value of illumination level as an additional analysis input parameter. These methods expand the utility of QE measurements as an indicator of both device performance and material quality in non-traditional materials for photovoltaics.

In recent years, the efficiencies of CdTe-based solar cells have significantly increased, both on the laboratory scale as well as for modules.¹ For further improvement, efforts focus on replacing the currently used CdS buffer layer by an alternative buffer layer. A fundamental understanding of the chemical and electronic interface properties of the absorber/buffer interface is needed, with the ultimate goal of correlating the chemical and electronic properties with device performance. Among many buffer material candidates, (Zn_,Mg)O (ZMO) is chosen as a test platform due to its band gap tunability and a wealth of prior knowledge about its use as an alternative buffer layer in chalcopyrite thin-film solar cells. In this contribution, we thus describe a study of the CdTe/ZMO interface using a combination of surface-sensitive characterization methods (i.e., x-ray, UV, and inverse photoemission - XPS, UPS, and IPES) and a series of CdTe/ZMO layer stacks of varying thickness. The data set allows the determination of the chemical environment at the various surfaces, the band alignment at the CdTe/ZMO interface, and the impact of the interface formation on the band bending in the two junction partners. The analysis of the chemical and electronic structure, with particular emphasis on the band alignment and its implications on device performance, will be discussed in this presentation.
¹Green, M. A.; et al. Prog. Photovolt: Res. Appl.2014,22, 701.

The level of performance recently achieved by Cu(In,Ga)Se₂-based thin film technology allows optimistic expectations for the increase of industrial production capacity within the coming decade. The viability of Cu(In,Ga)Se₂-based solar devices industry depends on several complex economical parameters; one can nevertheless believe the level of production costs is an important issue. In order to decrease production costs, both machine/process related expenses and scrap rates should be reduced. Optimizing production yields implies the use of stable machines and robust processes. Too rarely addressed in the literature is the issue of the purity of raw materials used to fabricate the devices. Indeed, to minimize the risks of production performance fluctuations, academics as well as industrials use highly pure raw materials, although using lower purity materials would have a direct impact on both economical and ecological production costs.
The present contribution aims at investigating the impact of the use of lower purity source materials during the coevaporation of Cu(In,Ga)Se₂ thin films. The case of Cu is particularly interesting because, contrary to In and Ga, it is mined for what it is and purified according to the targeted application. The experimental approach that we followed consists in comparing the Cu(In,Ga)Se₂ layers and related solar cells fabricated from Cu source of different purities (the nature and the density of impurities have been determined previously). Moreover, the devices have been prepared on both standard Mo-coated soda-lime glass (SLG) and SLG/Si_xN-barrier/Mo substrates.
The results obtained on standard SLG/Mo substrates show that, as expected, the contaminated Cu yields lower efficiency (all parameters are hindered). The most surprising result is that the cells fabricated on the substrates with the Si_xN diffusion barrier show, after Na-PDT, similar high performance, independently of Cu source purity. In order to better understand the impact of using a diffusion barrier on the activity of impurities, all cells have been further investigated by secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM) and atom probe tomography (APT). Combining these techniques together with advanced electro-optical characterizations drove us to the following model: the impurities detected in the Cu source material do not impact directly the Cu(In,Ga)Se₂ semiconductor properties but rather the characteristics of Mo back contact layer, strongly increasing its permeability to alkali species contained the SLG. Finally, the presence of too important amounts of alkali during the absorber growth yields poor photovoltaic performance. By using a diffusion barrier, one prevents the migration of these alkali species; the system thereby tolerates lower purity of Cu source material.

Cu₂ZnSnS₄ (CZTS) has gained a lot of attention as a promising absorber for thin film solar cells. However, due to the chemical instability of the material interesting challenges arise in fabrication. In this work CZTS absorbers were prepared by annealing of Cu-Zn-Sn-S precursors deposited by reactive sputtering. One of the challenges of the annealing process is the well-known thermal decomposition reaction that causes loss of S and SnS from the absorber surface. To prevent the decomposition a sufficiently high SnS and S partial pressure must be supplied during annealing. Aiming to yield more flexibility in the annealing process, this paper investigates an alternative approach to prevent surface decomposition. The CZTS precursor was capped with a thin Cu_xS layer prior to annealing. The cap was subsequently removed with a KCN etch after annealing, before device finishing. Absorbers and devices made from capped and uncapped reference precursors are compared. By electron microscopy it was found that the cap coverage decreased during annealing, exposing a part of the absorber surface, which could indicate limited potential of CuS to completely prevent surface decomposition. Based on energy dispersive X-ray spectroscopy (EDX) measurements it was, however, found that the cap reduced the amount of Sn lost from the absorber. At the same time, the initially Cu poor absorber took up Cu from the cap, ending up with a stoichiometric Cu content after annealing. The presence of the cap on the absorber surface during annealing had a dramatic impact on the device behaviour. While the uncapped reference devices showed device efficiencies up to 7.4%, the devices made from capped precursors had poor device characteristics and a tendency of shunting. From capacitance-voltage measurements it was found that the devices from capped CZTS had a doping density on the order of 10¹⁸ cm^-3, almost one order of magnitude higher than the reference. The high doping density is accompanied by a very shallow space charge region which in combination with possible remaining Cu_xS phases could be the reason for the very poor performance and tendency of shunting for the devices form the capped materials. Photoluminescence measurements on the completed devices showed a seven-fold reduction in the luminescence intensity of the material that was capped during annealing compared the uncapped reference. This strong decrease in intensity can be interpreted as increased non-radiative recombination in the capped material. This measurement further confirms that that capped CZTS has poorer absorber properties than the reference. In conclusion, despite indications that surface decomposition is reduced, Cu_xS is not a suitable cap material for CZTS. All measurements indicate that the capped CZTS is a poorer absorber material than the uncapped reference. Other cap materials must therefore be investigated to protect the CZTS absorber surface during annealing.

Flexible and lightweight solar cells manufactured via cost-effective, reel-to-reel processes have the potential to revolutionize photovoltaic technology by reducing both the hardware and non-hardware (balance of system) costs associated with solar energy systems. Unlike traditional rigid solar cells, flexible solar cells can be seamlessly integrated with infrastructures of various shapes and sizes. In addition, flexible solar cells are lightweight and suitable for various terrestrial applications as well as for providing power in space. However, solar cells fabricated on flexible substrates have historically been associated with lower efficiencies compared to the traditional rigid cells. Consequently, much research is currently focused on improving the efficiency of flexible solar cells. In this paper, we review recent developments concerning flexible copper indium gallium diselenide (CIGS) solar cells. In 2008, the National Renewable Energy Laboratory (NREL) demonstrated 19.9% efficient CIGS solar cells. More recently, researchers at Stuttgart&’s Centre for Solar Energy and Hydrogen Research (ZSW) achieved 21.7% efficiency with a new CIGS solar cell design. Both the NREL and ZSW cells were fabricated on rigid substrates. However, scientists at EMPA, the Swiss Federal Laboratories for Materials Science and Technology, have developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%. The PVMC team at the College of Nanoscale Science and Engineering, SUNY Polytechnic Institute has demonstrated 17.3% efficient CIGS cells on flexible yttria-stabilized zirconia substrates. We have also demonstrated flexible CIGS photovoltaic cells with a specific power higher than 275 W/kg on ultra-lightweight and highly durable titanium foil. Utilizing advanced device designs employing nanostructured optical coatings and exploiting optical cavity effects, we are currently working to develop greater than 20% efficient flexible lightweight CIGS solar cells.

The new emerging p-type compound semiconductor Cu₂SnS₃ consisting of only earth abundant and non-toxic elements has a high absorption coefficient (> 5×10^4 cm^-1) above its 0.93 eV band gap, which makes it suitable for photovoltaic application [1]. It is known as a secondary phase in the Cu₂ZnSnS₄ (CZTS) system, where it is detrimental for the solar cell properties due to its lower band gap than CZTS. However, Cu₂SnS₃ by itself is suitable for PV and since it contains less elements than CZTS, less secondary phases can be formed. Here we present a novel method to synthesize monoclinic Cu2SnS3. Photoluminescence (PL) and XRD measurements show that the material has a high purity and combined with optical measurements it is concluded that monoclinic Cu₂SnS₃ has two band gaps.
Cu₂SnS₃ was synthesised by annealing a solution based Cu/Sn precursor. The solution was prepared by dissolving stoichiometric amounts of Cu(II)formate and Sn(II)methoxide in methanol and 1,1,3,3-tetramethylguanidine. The ink was coated on soda lime glass or molybdenum glass by blade coating. The organic constituents are decomposed into gasses at 170 degrees, leaving mostly elemental copper and tin as precursor. The precursors are annealed at 550 °C for 30 minutes in a sulphur and SnS environment to avoid Sn loss [2].
XRD diffractograms of the resulting absorber layers shows the existence of monoclinic Cu₂SnS₃, but this does not rule out the existence of the cubic or triclinic polymorph. Etching with KCN to remove possible copper sulphide phases shows no difference in XRD diffractrogram, meaning that during the synthesis no significant copper sulphide phases are formed. Reflection and transmission measurements of the layers on glass show that the material has two optical gaps at 0.92 and 0.98 eV obtained by fitting the absorption spectrum confirming previous photocurrent spectroscopy measurements [1].
Room temperature PL spectra shows only the lower band gap at 0.93 eV and a large tail at long wavelengths. Etching in KCN and HCl to remove possible copper and tin sulphide phases neither changed the peak position nor the tail implying that the tail is due to non-etchable secondary phases or defects within the band gap. Micro PL mapping of a 40x40 µm area with a 1 µm spot size showed no deviation in peak position or shape. Therefore cubic or triclinic Cu₂SnS₃ polymorphs are ruled out suggesting the tail is due to defects in the gap and the two band gaps are a property of the monoclinic Cu₂SnS₃ phase. The consequences of two optical gaps on the performance of photovoltaic devices will be discussed.
[1] Dominik M. Berg et. al Thin Solid Films 520 (2012) 6291-6294
[2] A. Weber, R. Mainz, H. Schock, J. Appl. Phys. 107 (2010) 013516

Minority carrier lifetime (tau;) is an important figure of merit for any photovoltaic (PV) absorber, and much ongoing research in PV can be fairly described as attempts to improve the lifetime. Therefore, reliable lifetime measurements are essential to enable rapid and rational engineering of PV technologies. For legacy PV materials such as Si and GaAs, measurements of tau; are relatively well established. The bulk lifetimes tend to be fairly long (over 1 ms), and passivation techniques to reduce the surface recombination velocity are widely known. The same cannot be said for most thin film PV materials. The bulk tau; is typically on the order of 1 ns, even for high performing materials such as polycrystalline CdTe [1]. Furthermore, surface passivation is generally not well understood and therefore it is difficult to experimentally distinguish the time scales for non-radiative recombination in the quasi-neutral, space charge, and interface regions.
Here we report minority carrier lifetime measurements on SnS thin films. SnS is an attractive material for thin film PV. It has strong manufacturing advantages and has seen recent efficiency gains, including a current record 4.36% certified, 0.24 cm² area device [2]. Numerical modeling suggests that bulk tau; is on the order of 100 ps in the record devices, and efficiencies of 10% or greater could be readily achieved with bulk tau; on the order of 1 ns [3].
We use an optical pump and a THz probe to measure time-dependent free carrier absorption (FCA), which is a direct measure of the excess-carrier concentration. We model our data with diffusive charge dynamics, and we distinguish surface from bulk recombination by simultaneously fitting the model to multiple data sets acquired with different pump colors. We use these first-ever lifetime measurements on SnS to provide essential feedback to defect engineering techniques including post-growth annealing and surface oxidation. Our results point to specific bulk and interface processes that improve the minority carrier lifetime and may enable further PV efficiency gains.
1. T. A. Gessert, et al., 37^th IEEE Photovoltaic Specialist Conference, 1271-1274 (2011).
2. P. Sinsermsuksakul, et al., Adv. Energy Mater. (2014). DOI: 10.1002/aenm.201400496
3. N. Mangan, et al., 40^th IEEE Photovoltaic Specialist Conference (2014).

Thin-film based photovoltaics, an emerging technology, is a clean and potential choice for the future energy harvesting. Among various thin-film technologies, Cu(In,Ga)Se₂ (CIGS) thin-films are the most studied semiconductor absorber layers for the application in solar cells due to their suitable direct bandgap (asymp; 1.20 eV) and large optical absorption coefficient (asymp; 10⁵ cm^-1). Owing to these advantages CIGS cells exhibited higher efficiencies (21.7%) in the laboratory scale. These high efficient CIGS devices are often fabricated using expensive vacuum based technologies; however, efforts to seek an economical and scalable method for the production of stoichiometric CIGS thin-films have been ongoing to realize the commercialization of these devices. In pursuit of this, electrodeposition has demonstrated to produce CIGS devices with high efficiencies and it is easily amenable for achieving large area films of high quality with efficient material utilization and high deposition rate. The use of multi-steps for the deposition using a three-electrode system followed by a conventional selenization step have often been employed to achieve chalcopyrite compact CIGS which make the process more complex and expensive.
In appreciation of the above, a simplified approach for the fabrication of compact stoichiometric copper indium gallium selenide (CIGS) thin-films is reported. It employs a two-electrode system with a high purity graphite plate anode and Mo sputtered glass cathode during a simplified pulsed current electrodeposition which avoids the use of a reference electrode during deposition and eliminates the conventional selenization step. Cu-In-Ga-Se films are deposited using a solution containing CuCl₂, InCl₃, GaCl₃, H₂SeO₃, LiCl and tri-sodium citrate as complexing agent. The as-deposited films are annealed in Argon atmosphere at 550 °C for 30 min. The effect of concentration of tri-sodium citrate on the morphology and stoichiometry of the films is studied. Optimized concentration of tri-sodium citrate during the deposition has resulted in the formation of a single phase chalcopyrite CIGS with a compact morphology and well-controlled composition of individual elements. The flat-band potential and carrier density of CIGS thin-films deposited using optimized condition are -0.17 V and 5 × 10¹⁶ cm^-3, respectively, as determined by Mott-Schottky studies. The bandgap of the CIGS thin-films is obtained to be asymp; 1.28 eV, from diffuse reflectance measurements. The photoelectrochemical performance of CIGS films shows a photocurrent density of 0.25 mA/cm² at -0.4 V vs. SCE, almost three fold increment compared to our previous reported value. This simplified preparation method using pulse plating gives superior quality CIGS films which are promising for application in thin-film solar cells and photoelectrochemical cells.

One potential avenue for reducing the open circuit voltage deficit in CZTSe is through improved control of cation defects. Recent literature reports have shown an abundance of cation anti-sites, as well as the ability to manipulate cation ordering for CZTSe via low temperature treatments. Theoretical arguments suggest potential beneficial results from increased cation ordering by reducing defects that may cause bandtails, bandgap fluctuations, and recombination, however few direct electrical measurements have been reported and improved device efficiencies have hitherto not been realized. This work details device results of quenched and slow-cooled single crystals which demonstrate improved Voc consistent with increased cation ordering. Electrical and optoelectronic properties were investigated to corroborate the changes in defect structure underlying device improvements.
Single crystals were grown through a solid state reaction below the melt in a sealed ampoule without a flux agent. Single crystals >3mm in diameter, as confirmed by Laue diffraction and Raman spectra, were polished, etched and then made into devices with a modified device process using chemical bath deposited CdS, followed by sputtered ZnO/ITO. Hall measurements and temperature and intensity dependent PL were performed on both quenched and slow-cooled samples to better understand defect properties resulting from of the equilibrating at high or low temperatures.
Devices fabricated on slow-cooled single crystals showed improved efficiencies with open circuit voltage over 350mV compared to less than 300mV for quenched samples. Carrier concentrations were 100x higher for quenched samples, while mobility was a factor of ten lower than slow cooled samples consistent with increased disorder. PL spectra for both quenched and slow cooled crystals show a peak below the band-gap likely related to the dominant acceptor level. Intensity and temperature-dependent spectra will be analyzed and compared in more detail to help explain defects that may be limiting Voc. Finally, structural evidence of disorder in quenched samples was examined in Raman measurements as a metric for disorder. Proposed origins for the changes in electrical properties are discussed and examination of thermodynamics shows the critical role of entropy in defect creation in CZTSe. These results suggest a new potential avenue for mediating the Voc deficit in CZTSe.

Quaternary Cu₂ZnSnSe₄ is a promising low cost alternative absorber material for solar cells. Record efficiency 12.6% was reported for a CZTSSe based thin film solar cell [1]. The polycrystalline absorber layer exhibits an off-stoichiometric composition which causes intrinsic point defects (vacancies, anti-sites, interstitials). These defects determine the electronic properties of the material significantly.
This work focuses on the synthesis and characterization of off-stoichiometric CZTSe. In literature [2] off-stoichiometric kesterite types have been suggested, amongst them are A-type Cu-poor/Zn-rich, Cu_2-2xZn_1+xSnSe₄; B-type Cu-poor/Sn-poor, Cu_2-2yZn_1+3ySn_1-ySe₄ and C-type Cu-rich/Sn-rich, Cu_2+2zZn_1-3zSn_1+zSe₄ materials. In our study we have synthesized powder samples of these types by solid state reaction from pure elements in sealed evacuated silica tubes in a one zone furnace. First reaction took place at 750°C with temperature steps (250°C, 450°C, 600°C) in between. After reaction all samples were ground, pressed in pellets and annealed again at 750°C.
In the ternary phase diagram Cu₂Se-ZnSe-SnSe₂ the single phase kesterite region is indicated to be very narrow [3], therefore the formation of secondary phases is highly expected. To determine phase content and chemical composition of the obtained samples, an electron microprobe system equipped with wavelength dispersive X-ray analysis was used. The measurements proved the presence of CZTSe as main phase within all the different off-stoichiometric type synthesized samples. The lattice parameters a and c of the CZTSe main phase were determined by Rietveld analysis from XRD data collected using a PANalytical diffractometer. Refinements were performed using FullProf software [4] with the kesterite structure model, because stoichiometric CZTSe crystallizes in the kesterite type structure [5].
However, isoelectronic cations Cu⁺ and Zn²⁺ cannot be distinguished by XRD. Therefore, anomalous X-ray diffraction near to the Cu-K and Zn-K edges have been performed at the diffraction station KMC-2 beamline of HZB-BESSY II. Anomalous scattering coefficients are highly wavelength-dependent close to the absorption edges. The usage of multiple wavelengths above, below and between the absorption edges of Cu and Zn, results on intensity variation from the reflections which contain information on cation distributions. Collected data was refined and the atomic positions within the kesterite type structure have been determined. The presentation will give a trend of the cation distribution in the crystal structure of the CZTSe phase concerning the different off-stoichiometry types.
Acknowledgments: Financial support from KESTCELLS EU-project 316488.
[1] Wang, et al., Adv.Energy materials (2013).
[2] Lafond, et al., ZAAC 638, (2012) 2571-2577.
[3] Oleksyuk, et al., J. Alloys Comp. 368 (2004) 135-143.
[4] Carvajal, et al., www.ill.eu/sites/fullprof/.
[5] Schorr, Sol. Energ. Mat. Sol. Cells 95 (2011) 1482-1488.

Molybdenum is the preferred element that is used as back contact layer in Cu₂ZnSnS₄ (CZTS) synthetize. However, lately studies revealed that high temperature sulfurization process leads to formation of MoS₂ phase, which causes current losses in the device characteristics since it acts as a blocking barrier. This shows an urgent alternative chemically inert back contact back should be developed for further studies of CZTS.
We aimed to investigate the role of the flexible titanium and molybdenum foil substrates in the growth mechanism of CZTS thin films. In this study, CZTS absorber layers were fabricated using a two-stage process. Sequentially deposited Cu-Zn-Sn thin film layers on metallic foils were annealed in an Ar + S_2(g) atmosphere above 500°C. A soda lime glass substrate was also used for optical and electrical characterizations of the CZTS layers. Moreover, the back contact behavior of these metallic flexible substrates was investigated and compared. Structural characterizations of CZTS layers were done through Raman Spectroscopy, X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and X-Ray Photoelectron Spectroscopy (XPS). Optical properties of CZTS layers were investigated by UV-Vis spectrophotometer and electrical characterizations were carried out by four-probe and Hall Effect measurements. Secondary phase formations near to CZTS/foil interface and film morphology on distinct foil substrates were investigated via Raman and SEM analysis after sulfurization. Raman spectra and XPS analysis of the sulfurized thin films revealed that nearly pure CZTS thin films were obtained, without any trace of secondary phase formations. Additionally, the intense and sharp XRD diffraction peak from the (112) plane provided evidence of good crystallinity. SEM images of CZTS film grown on titanium foil revealed dense and homogeneous structure and no trace of interface instability have detected at the interface. However, in the case of molybdenum foil substrate usage deep cracks on the CZTS film and MoS₂ formation at the CZTS/Mo foil interface was detected. It is concluded that, the crack formation in the CZTS layer on the Mo foils is an indication of the incompatible thermal expansion coefficient of Mo with the CZTS structure. We demonstrated the application of magnetron sputtering technique for the fabrication of CZTS thin films on Ti foil, which is lightweight, flexible and suitable for roll-to-roll manufacturing for high throughput fabrication.

CIGS solar cells have excellent radiation tolerance, which means their electrical properties do not degrade by 1MeV electrons. Conversely, cell performance is impaired with exposure to 10MeV proton irradiation, similar to other solar cell types. The radiation damage to the cells caused by proton irradiation gradually recovers when the irradiated cells are kept even at room temperature and the recovery rate is temperature-dependent. The radiation defect in CIGS solar cells, which decrease the cell performance, was reported as an III atom antisite defect. However, it remains unclear whether the other types of defects, namely Cu, Ga and Se Frenkel-pair in CIGS, which are simultaneously generated by radiation, impair cell performance or not. Therefore, we investigated these defects in CIGS solar cells induced by low energy electrons, enabling the type of radiation defect in the solar cells to be selected.
Electrons (up to 1×10¹⁶ cm^-2) with the energy of 100 keV or 250 keV improved the electrical performance due to the change in conductive type in the metastable defect, which is an effect equivalent to the light-soaking effect. However, the electrons (over 1×10¹⁶ cm^-2) caused the cell performance of the CIGS solar cell to deteriorate. The high electron fluence decreased the carrier density in CIGS solar cells. This result indicates that the radiation defects are donor like defects.
The electron-induced defects would be DX-like properties of III_Cu (Cu substitutional defect at a column III atom site) defects and/or III_Cu-V_Cu complex defects analyzed by the red on bias treatments. However, the origin of defects has not been clearly revealed.
The defects were characterized by photoluminescence (PL) for thin films and electroluminescence (EL) for solar cells. PL intensity of CIGS thin films as well as EL intensity of CIGS solar cells decreased after 100 or 250 keV electron irradiation. In addition, new signal with the energy level of around 0.7 eV was observed on the PL spectrum from CIGS thin films. The signal would be III_Cu related antisite defects, since the energy level is the same as that of the antisite defects reported by the spectrum of PL on CIGS thin films. Therefore, it may be suspected that the electron-induced defects in CIGS are III_Cu related antisite defects.

We have applied a well-known, scalable technique such as spray pyrolysis for the synthesis of copper-zinc-tin-sulfide (CZTS) nanostructures. CZTS is a quaternary semiconducting material that shows promise as a replacement to common semiconductors such as CdTe and CIGS for use in photovoltaic devices. In our approach, we synthesize single-phase nanostructures starting from precursors containing the elements required to make the desired material dissolved in an organic solvent. For the production of CZTS, we start with zinc-, copper-, and tin-diethyldithiocarbamate precursors in a toluene solvent. The precursor solution is aerosolized using a Collison-type nebulizer wherein the droplets are carried to a tube furnace. After nebulization, the droplets can be accelerated through an aperture and impacted onto a heated substrate to grow quasi-1D CZTS nanostructures. In another approach, the aerosol is carried through the length of a tube furnace where nucleation of nanocrystals occurs at atmospheric pressure. The powder is then collected in a series of solvent-filled bubblers. This technique continuously converts the chemical precursor into high-purity nanopowder with a production rate of ~50 mg/hour for an unoptimized lab-scale reactor. We reproducibly synthesize kesterite, Cu₂ZnSnS₄, nanocrystals, as verified by Raman spectroscopy, x-ray diffraction and TEM analysis. The nanoparticle composition can be precisely controlled and tuned by varying the relative ratio of the molecular precursors in solution. Moreover, FTIR confirms that these particles are free of organic ligands. Preliminary results on the coating and sintering of these particles into thin films will be presented, and other potential applications in solar-to-fuel conversion will be discussed.

Low temperature growth of CIGS films below 500 ^oC has been recently of much interest in the CIGS photovoltaics community. It is beneficial for polyimide (PI) substrates due to its low heat tolerance, for steel substrates due to its impurity (Fe) out-diffusion, and for highly challenging subjects such as transparent back contact, tandem solar cells, and low thermal budget processes. Since the sodium (Na) diffusion may be suppressed at the low temperature, easier ways than using soda-lime glass (SLG) substrate are required for sodium (Na) doping of CIGS films that is essential to highly efficient CIGS solar cells. In this work, two approaches for the Na doping - employing the Na-doped Mo (Mo:Na) as part of a back contact and applying NaF post-deposition treatment (PDT) - were investigated in terms of their effects on the CIGS films grown at low temperature (450 ^oC) and their cell efficiencies. Three kinds of back contact structure were prepared on SLG such as Mo/Mo:Na/SiO_x, Sb(20nm)/Mo/Mo:Na/SiO_x, and Mo/SiO_x where the composition of the Mo:Na sputtering target was 10 wt% Na. Basically, three-stage coevaporation processes were used for the CIGS growth. Ga vapor flux was intentionally controlled for optimal Ga gradient within CIGS films. Otherwise, Ga depletion was very severe during the Cu deposition stage, because Ga diffusion is highly retarded at 450 ^oC. In the case of Mo:Na back contact, Sb precursor on Mo back contact improved the cell efficiency by 1-2% primarily arising from the increment of open-circuit voltage (V_OC), which is due to the increase of grain size of CIGS films. The intentional Ga grading was more effective in achieving high efficiency, which reached up to 15.7%. In the case of NaF PDT, Na-free growth of CIGS on Mo/SiOx/SLG resulted in much higher efficiency up to 17.2%, while the CIGS growth on Mo:Na back contact deteriorated the efficiency considerably. We found that the CIGS grain structure was improved significantly by the Ga grading process, but degraded by employing Mo:Na back contact. Our results showed that the cell efficiency of the CIGS solar cells on the basis of low temperature CIGS growth was determined by the V_OC limitation and strongly correlated with the grain size of CIGS films. Rather, the Na concentration showed less correlation with the efficiency. In this regard, Mo:Na back contact approach would be unfavorable for high efficiency routes despite of its advantage in pursuit of simpler process, because high intensity of Na existing during CGS growth may degrade the crystallinity of CIGS films. In order to make the Mo:Na approach more feasible for low-temperature process, innovative approach for improving grain structure of CIGS films is required.

Solution based nanoparticle methods are an interesting strategy for the development of kesterite Cu₂ZnSn(S,Se)₄ based solar cells. These processes have lead to a current record of 8.6 % of the conversion efficiency, involving spray-deposition of a nanocrystalline Cu₂ZnSnS₄ (CZTS) layer followed by partial selenisation¹. These methods are especially interesting for their potential for lowering of production costs and their scalability for fabrication of large area modules. On the other hand, optimization of the optoelectronic properties in order to improve performance of the solar cells requires tuning of the absorber band gap with the solar spectrum. Typically band gap tuning is achieved with the Se annealing of CZTS nanoparticle precursors, which result in formation of the Cu₂ZnSn(S,Se)₄ (CZTSSe) solid solutions.
The aim of this work is to analyze the Se incorporation dynamics during the solid solution formation, using sprayed CZTS nanoparticle thin film precursors. The mechanism of Se incorporation in the precursor CZTS films was investigated by variation of annealing temperature (from 475 to 575 ^oC) and time (from 5 to 60 minutes) in the Se/Sn atmosphere. The obtained CZTSSe absorbers have been characterized by X-ray diffraction (XRD) and Raman spectroscopy under different excitation wavelengths.
The results have shown that Se incorporation into the films is mostly controlled by the annealing time, while the temperature is mostly affecting the crystalline quality of the absorber films. These results suggest that the diffusion mechanism inside the CZTS nanoparticle precursor is mainly controlled by the annealing time and not the partial pressure of Se which changes with the annealing temperature. Furthermore, in all cases complete recrystallization of the nanoparticle precursor films is achieved.
Comparison between the Raman analysis of the surface and XRD analysis of the bulk of the samples have shown that all CZTSSe films have a gradient [Se]/([S]+[Se]) composition through the thickness of the film, with the front of the film being more Se rich than the back. Additionally, analysis of secondary phases show the formation of Mo(S,Se)₂ layer in the back of the films, while there are no evidences of other secondary phase formation either in the bulk or the surface of layers, although a ZnS phase was detected on the surface of the CZTS precursor film, before selenization.
Based on this results a model for Se incorporation in solution-processed CZTS nanoparticle thin films is proposed and discussed. These results suggest that it possible to fine tune absorbers with desired crystal quality and Se concentration, by controlling the time and temperature of annealing. Control in the [Se]/([S]+[Se]) gradient profile opens a possibility for band gap engineering of these absorbers.
(1) Larramona, G. J. Phys. Chem. Lett.2014.

By now, the progress in manufacturing of Cu(Ga,In)Se₂ thin films as absorber layer in thin film solar cells has led to conversion efficiencies of more than 20% [Ref.a]. In general, compound semiconductors own the advantage of adjusting the band gap by changing the composition of the solid solution, like in CuIn_1-xGa_xSe₂, giving the advantage to exploit the solar energy spectrum optimally. During the 3-stage co-evaporation process of Cu(In,Ga)Se₂ thin film synthesis [Ref.b], the depth distribution of indium and gallium within the thin film depends on several process parameters and can be adjusted during the first and second stage. However, the third stage triggers recrystallization and ultimately redistributes the trivalent cations, i.e. Ga and In [Ref.c]. To understand how this system works, is it useful to reduce the number of variables to analyze what happens throughout the entire process.
In this study we focused on 12 similar samples of CuInSe₂ thin films, half of them with NaF precursor. Fabricated at 6 different temperatures of the substrate during the 2^nd and 3^rd stage of the 3-stage-co-evaporation process in the range from 330°C to 525°C. The thin films have been characterized with respect to both, the microstructure as well as the phase content for various thin film depths, by means of grazing incidence X-ray diffraction (GIXRD). The microstructural analysis refers to the separation of size and strain induced peak broadening after the Williamson-Hall method [Ref.d]. Through measuring a standard reference material (LaB6, NIST-SRM 660b) the finite resolution of the XRD instrument was taken into consideration to solely obtain the microstructural information of the XRD pattern.
The presentation will give an overview of the variation of strain and apparent grain size in dependence on the substrate temperature in the 2^nd and 3^rd stage of the 3-stage-process as well as on the probed thin film depth.

Cu₂ZnSnS_4xSe_4(1-x) (CZTS) is a promising absorber material for the development of solution processed thin film solar cells due to a number of significant advantages, including inexpensive, non-toxic, and earth abundant elemental constituents.
Over the past 5 years our research group has focused on the synthesis of compositionally controlled CZTS nanoparticles (NPs) using chemically benign and scalable processes [1,2]. The as-prepared NPs are then formulated into polar “NP inks” and utilized within Mo/CZTS/CdS/i-ZnO/ITO/Al devices, which exhibit efficiencies > 7% [3]. Unlike other existing NP inks, polar NP inks permit the direct solubilization of bene#64257;cial dopants (e.g. Na⁺, K⁺) in controllable amounts.
At present the bottleneck to higher device efficiencies is the poor understanding of the optoelectronic properties of CZTS, including the role defect states have on the underlying device physics. In this presentation we focus on elucidating the effect of various dopants and NP ink compositions on the underlying device physics. Temperature and illumination dependent properties were probed using a custom-built cryogenic thin film characterization suite.
Illumination dependent current-voltage (J-V) curves as well as light and voltage biased incident photon converted electron (IPCE) spectra were measured from 77-310K. The temperature/illumination dependence of critical device parameters was analysed to give insight into the recombination processes, device operation, and possible routes towards improved performance. E.g. extrapolation of the temperature dependent V_OC to absolute zero (at standard illumination conditions) indicates reasonably low recombination losses within some of our devices (E_g - V_OC (0K) < 0.4 eV).
[1] A. S. R. Chesman, N. W. Duffy, S. Peacock ,L. Waddington, N. A. S. Webster, J. J. Jasieniak, RSC Advances, 2013, 3, 1017
[2] A. S. R. Chesman, J. van Embden, N. W. Duffy, N. A. S. Webster, J. J. Jasieniak, Cryst. Growth Des. 2013, 13, 1712.
[3] J. van Embden, A. S. R. Chesman, E. D. Gaspera, N. W. Duffy, S. E. Watkins, J. J. Jasieniak, J. Am. Chem. Soc., 2014, 136, 5237

The effect of laser treatment on the composition, morphology and microstructure of kesterite Cu₂ZnSnS_xSe_4-x (CZTSeS) thin films manufactured during sulfurization-selenization of metallic layers was explored. It was established that the S/Se ration is defined on the diagram sulphur pressure/ laser power density. Better laser parameters were achieved for the 1320 nm Nd:YAG laser beam The CZTSeS thin films were manufactured at low sulphurization pressures and laser power densities varying within the range 200 MW/cm²-700 MW/cm². The beam diameter varied from 2-4 mm . The principal role here was played by deviation of the crystalline film non-stoichiometry with the appearance of some charge density non-centrosymmetry playing a crucial role in the charge separation. Structural and TEM studies indicate occurrence of the acentric nanocrystalline with sizes varying between 14 nm-116 nm. Additional control was achieved through nonlinear optical methods which are very sensitive to local disordering. The reversibility of the effects is discussed. The solar cell fabricated for such CZTS thin film under 10 Torr sulfurization pressure shows the best conversion efficiency up to 1.82%. The formation of such devices may be optimized by the additional tehrmoannealing up to 1500⁰ C. The studies of the photovoltaics was performed versus the nanoparticle sizes and their size dispersion. However, due to additional acoustical sonification at frequencies around 12 kHz the size dispersion was less and the shapes of the nanoparticles was more spherical-like. Possible ways of further improving the parameters mentioned are discussed.

Copper sulfide is a simple binary chalcogenide with great promise for low-cost thin film photovoltaics. However, stable Cu₂S-based device efficiencies approaching 10% free from cadmium have yet to be realized. For the first time, we employ transient absorption spectroscopy to investigate the dynamics of the photoexcited state of isolated Cu₂S thin films prepared by atomic layer deposition or vapor-based cation exchange of ZnS. While variables including film thickness, carrier concentration, surface oxidation, and grain boundary passivation were examined, grain structure alone was found to correlate with longer lifetimes. A map of excited state dynamics is deduced from the spectral evolution from fs to us. This study provides new insights into why such high device efficiencies may have been achieved in CdS/Cu₂S heterostructure devices fabricated through the 1970s topotaxial exchange process. We posit that highly efficient and stable Cu₂S thin film photovoltaics will not only require a deviation from CdS as the n-type mate but also a novel fabrication route that significantly augments grain size and prioritizes surface passivation.

The photovoltaic thin films of CdS/CdTe were prepared by pulsed laser deposition (PLD) on indium tin oxide (ITO) coated glass. The local structural variations in the thin films around Cd atom upon variations in the thin film growth parameters were investigated by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) and x-ray diffraction. X-ray absorption spectroscopy measurements were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. The effect of the thicknesses of the CdS and CdTe layers, laser energy and the substrate temperature on the local crystal structure and coordination around the Cd atoms were investigated through quantitative multiple scattering analysis and modeling of the x-ray absorption spectroscopy data. The induced local structural modifications upon varying synthesis conditions are correlated with the electrical performance of these photovoltaic thin-films. The quantitative multiple scattering analyses and modeling of X-ray absorption spectroscopy data revealed the local environment around the Cd atoms are highly sensitive to thin film deposition parameters and the variations of the Cd local structure influences interface quality consequently, affect the electrical performance of these photovoltaic thin films.
This work is supported by NSF Award #:DMI-0420952 and Research Corporation Award #:CC6405

We show that metal oxide band-tails play a crucial role in the performance of thin film heterojunction solar cells and demonstrate this with the PbSe - ZnO quantum dot and Cu₂O - ZnO systems. While the efficiency of quantum dot solar cells has rapidly increased, their performance is limited by their open-circuit voltage being much lower than theoretical limits. Using absorption, photoelectron spectroscopy, photoluminescence and light-dependent measurements, we show that the band-tail in the metal oxide component is responsible for a previously unidentified open-circuit voltage loss mechanism through electron thermalization. We reduced this loss mechanism by raising the ZnO conduction band tail through Mg incorporation (Zn_1-xMg_xO), increasing both the open-circuit voltage and device efficiency to some of the highest values reported for PbSe - ZnO quantum dot solar cells.^[1] However, in Cu₂O - Zn_1-xMg_xO solar cells, metal oxide band-tails were found to be essential for overcoming another loss mechanism: the Schottky barrier formed between the Zn_1-xMg_xO and indium tin oxide (ITO) transparent top contact. Impedance spectroscopy, absorption, current - voltage, and device performance measurements show that the band-tail allows electrons to hop through the Schottky contact, as if it were ohmic. This feature allowed Cu₂O - Zn_1-xMg_xO devices with Schottky ITO top contacts to be more efficient than devices with ohmic Al-doped ZnO top contacts, in addition to being the most efficient Cu₂O solar cells with an open-air fabricated heterojunction. These studies show that the metal oxide band-tail is a crucial parameter that needs to be controlled when designing more efficient solar cells.
Reference:
[1] Robert L. Z. Hoye, et al., Improved Open-Circuit Voltage in ZnO-PbSe Quantum Dot Solar Cells by Understanding and Reducing Losses Arising from the ZnO Conducting Band Tail. Adv. Energy Mater., 4(8), 1301544.

Sample material was created for full stack copper indium gallium diselenide (CIGS) solar cells from the Miasole production line; this material included a layer of CdS grown on the surface of the CIGS. Bare CIGS material was also created without the CdS or TCO layers. Both materials were etched in HCl; the HCl removed the CdS and TCO layers from the full stack material. The work functions of the CIGS surfaces were measured by Kelvin probe in nitrogen at atmospheric pressure. A work function of 4.7 eV was measured for the CIGS sample that had been exposed to CdS, and 4.5 eV for the CIGS sample not exposed to CdS. The difference in work functions could be explained by diffusion of sulfur from the CdS into selenium vacancies on the surface of the CIGS.

Depending on the bath temperature and bath composition, SnS films of orthorhombic structure with E_g of 1.1-1.3 eV [1-3] or of cubic zinc blende structure with optical band gap E_g of 1.5-1.7 eV [2,4]have been reported previously [1-4]. Chemical deposition of SnSe thin films of orthorhombic structure are also reported from our group [5]. In this work we present the transformations that can take place in these thin films during chemical or heat treatments which brings-in many desirable optical and electrical properties for solar cell applications:
SnS thin films of all the above types may be converted either totally or partially into p-type SnSe₂ by immersion into chemical bath containing selenosulfate. GIXRD and EDX studies are presented to illustrate such conversion. The SnSe₂ thin film produced this way have E_g of 1.5 eV and electrical conductivity, σ, 0.001 #8486;^-1cm^-1.
SnSe thin films are converted into n-type SnSe₂ when heated at 300-400^oC in presence of Se powder, with E_g of 1.5 eV and σ, 0.01 #8486;^-1cm^-1.
It is possible to deposit stratified layers of SnS of orthorhombic structure on SnS of cubic structure, thus having E_g variation of 1.7 to 1.1 eV through the film thickness.
Solid solutions of p-type Sn₂SSe of orthorhombic structure are formed by depositing an SnS film on SnSe followed by heating. These films have an E_g of 1.35 eV and σ, 0.1 #8486;^-1cm^-1.
We shall present some cell structures using these thin films.
[1] M.T.S.Nair and P.K.Nair, Semicond. Sci. Technol. 6 (1991) 132; [2] Ana Rosa Garcia-Angelmo et al, Solid State Sciences 30 (2014) 26; [3] M. Safonova et al, J. Mater. Sci.: Mater. Electronics, 25 (2014)3160; [4] David Avellaneda et al, J. Electrochem. Soc.155 (2008) D517; [5] Enue Brrios-Salgado et al, ECS J. Solid State Science and Technology, 2 (2014) Q169-Q175.

The absorption of light by ultra-thin films poses interesting questions for theoretical electromagnetism and is of practical interest for applications such as photodetectors and next generation solar cells. A suite of recent studies have focused on exotic structures such as nanopatterned graphene, graphene coupled to a photonic crystal, arrays of core-shell plasmonic nanoparticles, optical metamaterials and tunable phase change materials. We here show that total light absorption (TLA) can also be achieved using simple lamellar gratings composed of common materials that are compatible with standard nanofabrication techniques.
To absorb 100% of the incident light within an ultra-thin structure (thickness h much smaller than the wavelength) the modes of the structure must destructively interfere with the incident light, and their absorptive decay rate must be equal to the incident flux.
We consider light with a wavelength of 400-800 nm and ultra-thin absorbers with h <= 50 nm placed above a silver mirror with separation equal to a quarter wavelength. In this configuration a homogeneous film can attain TLA when the real and imaginary parts of its refractive index (n", n' respectively) are approximately equal. As the thickness of the film is reduced the magnitude of n", n' required increases proportional to h^1/2. This condition rapidly precludes materials for ultra-thin layers.
Here we report that ultra-thin lamellar gratings (LGs) can realise TLA without this restriction on n. LGs are highly polarisation sensitive; for TM light (E-field across grating rulings) we observe TLA in materials with n" > n', while for TE light (E-field along rulings) TLA occurs when n" < n'.
The TLA of TM light is driven by the excitation of surface plasmon polaritons (SPPs). It is well known that SPPs exist only for TM light and require the permittivity to be negative (n" > n'). To couple to the SPPs the incident plane waves require additional transverse momentum, which they gain from the gratings reciprocal lattice vector.
The mechanism driving TLA of TE light has not been fully established, but appears to be associated with the excitation of a resonant bound mode of the rulings. Such a resonance traps light in the absorber, which intuitively explains how TLA can occur with small values of n", as the absorption culminates over many oscillation.
An advantage of LGs is that the target wavelength can be easily tuned, being slightly greater than the period of the grating. This relationship ensures the grating doesn't excite propagating higher diffraction orders. Being close to the Wood anomaly is crucial to achieve the correct scattering properties. The scale and simplicity of LG TLA contrasts metamaterial TLA structures that generally requires complicated, deeply subwavelength structures.
For 50 nm thick gratings made of perovskites or chalcogenides we have designed LGs that absorb > 99.989% of 600 nm light, of which > 99.3% is absorbed within the lamellar grating.

We have previously presented work on the creation of an equivalent AC circuit based in device physics used to model, as well as fit, data obtained on CZTSe cells using temperature admittance spectroscopy. It was the conclusion of that work that the energy state derived from the observed step in capacitance was due to a process outside of the active absorber layer of the cell. We have further investigated possible causes of this observed step and found that it is likely due to a variable back contact resistance which we represent as a Schottky barrier. Here, we elaborate on our device model in order to make it more accurately represent the physics of a device while maintaining a minimal number of fitting parameters.
The proposed base circuit consists of a series resistor and two RC loops, which represent the depletion region and the quasi-neutral region. We further elaborate on that to include a distribution of depletion widths as well as a variable back contact resistance which we represent through device physics as a non-Ohmic Schottky barrier. The physics of the barrier is such that the current over the barrier is driven by thermionic emission. A simple, normalized Gaussian type distribution of Schottky barrier heights was also introduced in order to represent likely inhomogeneities at the back contact surface. Taking the derivative of the resulting current equation with respect to voltage gives the temperature dependent conductance over the Schottky barrier, as well as the associated resistance. Although we have modeled the back-contact resistance using a Schottky barrier model, we understand the physics of a back contact barrier may be more complicated and further work is being done to more accurately characterize the behavior at the back contact interface.
We will also show the effect of fundamental device parameters on the behavior of the main capacitance step. Impedance data was simulated using basic device properties together with the described model. The effects of the resistivity of the depletion width, the main dopant activation energy, and the effect of the back contact Schottky barrier were all studied. These parameters were chosen because they were found to exhibit unique responses within the system. The results agreed with our hypothesis in that it was found that the most notable effect on the transition frequency of the main capacitance step is due to the addition of a temperature dependent series resistance. By showing that the resistance of the back contact is the only device parameter that can cause the observed shift in the capacitance step we have been able to add the proper circuit elements and device physics to our model and thus have a more accurate representation of the cell.

1. Introduction
Magnesium half silicide, Mg₂Si, having anti-CaF₂ structure, is a semiconductor with an indirect energy gap of about 0.6 eV at room temperature [1]. Recently, we have succeeded to observe the photo response from Mg₂Si pn-junction diode formed by thermal diffusion of Ag acceptor into n- Mg₂Si substrate (n ~10¹⁵ cm^-3) [2,3]. In order to evaluate the diffusion profile of Ag and study the pn-junction property, development of observation technique for Mg₂Si pn-junction depth is very important, practically. In this study, we report the convenient evaluation technique of Mg₂Si pn-junction depth using a conventional sputter etching.
2. Experimental procedure
Samples of Mg₂Si pn-junction diode were fabricated by rapid thermal diffusion of Ag at 550 °C in Ar ambient [2,3]. Diffusion source of Ag-metal layer (400 nm) was deposited on a mirror polished n-Mg₂Si substrate (n ~10¹⁵ cm^-3) with Au capping layer through the 0.8 mm diameter metal mask. The diffusion period was varied between 3 min and 30 min. To observe the diffusion depth, surface of the pn-junction diode was polished at slant angle of 3 ° and etched by Ar ⁺ RF-sputtering. After the sputter etching, the etched surface was observed using Nomarski-type optical microscope. The Ag diffusion profile was also evaluated by secondary ion mass spectroscopy (SIMS).
3. Result and discussion
After the sputter etching on the polished surface of the pn-junction diode, circular shaped bright and dark contrast was appeared on the surface owing to the different surface roughness between the Ag-diffusion region and the substrate. Typical sputter etching condition was 40 W RF-power for 1 min. We determined the Ag diffusion depth from the width of Ag diffusion region observed as the contrast. As the result, the diffusion depth increased with increasing the annealing periods and temperature.
We analyzed the result of Ag diffusion depth at the annealing temperature of 550 #8451; using the conventional thermal diffusion equation,
N_Ag = N₀ erfc {W/(2(Dt)^1/2)}
where N_Ag is Ag concentration; W the diffusion depth; t the diffusion periods; D (= 5 × 10^-9 cm²/s) and N₀ (= 2 × 10¹⁹ cm^-3) the diffusion coefficient and solubility limit of Ag at 550 #8451;, respectively[2]. From the analysis of the data, we found that the observation limit of Ag concentration by the sputter etching was approximately 1.1 × 10¹⁷ cm^-3. This observation limit is two orders of magnitude lower than the detection limit of SEM-EDX observation. The estimated hole concentration of 1.6 × 10¹⁶cm^-3 is as low as the electron concentration (7.0 × 10¹⁵cm^-3) in the substrate. Thus, the sputter etching method is very useful to evaluate the diffusion depth of Mg₂Si pn-junction diode.
References
[1] D. Tamura et al., Thin Solid Films, 515(2007)8272.
[2] H. Udono et al., J. Phys. Chem. Sol, 74(2013)311.
[3] M.Takezaki et al., Phys. Stat. Sol. C, 10(2013)1812.

CdS has proved to be the best n-type layer in p-n hetero-junction thin films solar cells, such as CuIn_1-xGa_xSe₂ (CIGS), Cu₂ZnS(S_1-ySe_y)₄ (CZTSSe) and CdTe based devices. Nevertheless, CdS has sustainability concerns mainly for the Cd toxicity and several attempts have been made for its replacement using Cd-free buffer layers such as Zn(O,OH)S and In₂S₃. Despite the efficiencies reported using these alternative buffer layers are very promising, all the certified efficiency records in CIGS and CZTSSe have been achieved so far using CdS layers between 60 nm and 120 nm in thickness. Moreover, as a major drawback, Cd-free buffers require additional activation steps (thermal annealing and light soaking) increasing the processing demands. One strategy to minimize the Cd content on the final devices (and in consequence the environmental impact of Cd) is to go towards extremely thin CdS buffer layer. Currently, 50-60 nm thick CdS layers can be considered as a good standard for the CIGS and CZTSSe technologies. These thicknesses seem to be the lowest acceptable limit to allow for a good match between performance and good surface coverage. Further thickness reduction is usually coupled to a dramatic decrease of device performance.
In this paper, we present an approach based on the use of a Cu doping agent on the chemical bath during the deposition of CdS, which allows a remarkable reduction of the buffer layer thickness, keeping its quality and improving the absorber surface coverage. We have deposited standard CdS (using CdSO₄ as Cd source) by chemical bath deposition, with thicknesses between 20-69 nm by changing the deposition time. Alternatively, we prepared similar set of samples but adding CuSO₄ to the solution as a Cu-doping source. Varying the nominal Cu concentration in the chemical bath from 1x10^-4 M to 5x10^-3 M allows to obtain thicknesses between 50 nm to 21 nm respectively. We study the morphological (SEM and AFM), optical (PL) and structural (Raman spectroscopy) properties of the undoped and CdS:Cu layers showing that, with the Cu doping, higher quality layers with reduced pin holes density can be obtained. Starting from a CIGS reference sample with 12.1% efficiency using CdS with 69 nm in thickness, we observe that the efficiency drops towards values lower than 10% for thicknesses lower than 45 nm. Conversely, using CdS:Cu as buffer layer, the efficiency is still in the 10-12% level with thicknesses as low as 27 nm. The same trends are observed for CZTSe based solar cells where the efficiency drops from 7% to values lower than 5% when the CdS thickness is reduced from 60 nm to 45 nm. For CdS:Cu buffer layer, the efficiency is still above 6% even for thicknesses as low as 27 nm. Finally, a phenomenological model based in the solubility product constant of CdS and Cu_xS will be presented to show why it is possible to use ultra thin CdS:Cu buffer layers, while keeping good device performance.

Metamorphic GaAs_xP_1-x solar cells with a direct bandgap of 1.7eV are ideal top junction candidates for tandem silicon solar cells with potential efficiencies in excess of 37% [1]. The lattice mismatch between Si and GaAs_xP_1-x (nearly 3% at room temperature) can be effectively accommodated with Si_yGe_1-y composition graded buffers while retaining low threading dislocation density (TDD) (<1E6cm^-2). Despite being lattice matched, the best GaAs_xP_1-x solar cells grown on Si_yGe_1-y graded buffers to date have 10x to 100x greater TDD (9.2E6 cm^-2 - 2E8 cm^-2) than the underlying Si_yGe_1-y graded buffer due to defect nucleation at the III/V-IV heterointerface [2], [3]. We investigated the GaAs_xP_1-x/Si_0.35Ge_0.65 interface to elucidate the cause.
We identified two sources of dislocation nucleation—the deleterious interaction of PH₃ with the Si_0.35Ge_0.65 virtual substrate surface and point defect condensation at the heterointerface into dislocation loops. We explored the effect of pre-GaAs_xP_1-x growth exposure of arsine (AsHshy;₃) and phosphine (PH₃) to the Si_0.35Ge_0.65 virtual substrate surface and the subsequent effect on GaAs_xP_1-x film quality. Additionally we studied the use of thin strained layers at or above the heterointerface to confine and suppress dislocation loops that were observed to nucleate at the GaAs_xP_1-x/Si_0.35Ge_0.65 interface.
Having identified these mechanisms we developed particular initiation conditions and strained layers that suppress the dislocation nucleation. We demonstrate GaAs_xP_1-x films grown on Si_0.35Ge_0.65 virtual substrates with a TDD of 1.2E6 cm^-2. Using these techniques and the high quality material they enable, single junction 1.7eV bandgap GaAs_xP_1-x solar cells were fabricated on silicon substrates.
[1] J. F. Geisz, J. M. Olson, C. S. Jiang, and A. G. Norman, “Lattice - mismatched GaAsP Solar Cells Grown on Silicon by OMVPE,” 2006.
[2] J. R. Lang, J. Faucher, S. Tomasulo, K. Nay Yaung, and M. Larry Lee, “Comparison of GaAsP solar cells on GaP and GaP/Si,” Appl. Phys. Lett., vol. 103, no. 9, p. 092102, 2013.
[3] T. J. Grassman, M. R. Brenner, M. Gonzalez, A. M. Carlin, R. R. Unocic, R. R. Dehoff, M. J. Mills, S. A. Ringel, S. Member, and A. Gaas, “Characterization of Metamorphic GaAsP / Si Materials and Devices for Photovoltaic Applications,” Education, vol. 57, no. 10, pp. 3361-3369, 2010.

Low temperature deposition of Cu(In,Ga)Se₂ followed by a KF post-deposition treatment (PDT) allows to process highly efficient thin film solar cells with > 20% efficiency on flexible polyimide substrates.
It is well known that the final copper content expressed as cation ratio [Cu]/([In]+[Ga]) (CGI) plays an important role on absorber properties and pn-junction formation. Usually a CGI of 0.8-0.9 is used in highest efficiency devices, despite all indications that underline the superior material quality of absorbers with composition very close to Cu stoichiometry such as lower defect concentration and higher mobility. A phenomenological explanation for this is provided by the observation that stoichiometric grown devices without KF PDT show dark current activation energies < Eg which points towards the dominance of interface recombination.
The application of a KF PDT, where KF is evaporated onto a CIGS absorber after growth, has been shown to lead to the formation of a Cu-depleted CIGS surface on absorbers grown with a CGI of 0.8. Reduced interface recombination for such a treated CIGS surface layer was found as a reason for enhanced photovoltaic parameters and improved junction quality .
In this contribution we investigate the effect of increased CGI on absorbers grown by a low temperature (< 500°C) multi-stage process and subjected to a KF PDT, as a possible route towards CIGS devices with enhanced efficiency that combine the apparent advantages of an absorber bulk with composition close to a CGI of 1 with the improved interface quality of a KF PDT.
Therefore, different strategies to achieve higher Cu content while maintaining a favourable Ga grading are pursued on absorbers with baseline efficiencies > 18 %. Material characterization techniques such as SEM, XRF, and SIMS are combined with electronic and optical characterization techniques (T-IV, CV, TRPL) to assess the interplay of Cu content, alkaline distribution and Ga grading. We especially focus on the study of the effect of different CGI on carrier concentration, transport and recombination mechanism in the absorber, and compare our results to previous studies available for CIGS absorbers without surface modification with KF PDT.

Thin-film Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising absorber material for low-cost, scalable photovoltaic applications. There has been substantial progress recently in the performance of CZTSSe devices, in spite of a complex native point defect chemistry and a limited understanding of the kinetic processes during film growth and processing. We have performed photoelectron spectroscopy measurements combined with CZTS growth and sulfur anneals to determine the band alignments at the CZTS back-contact between CZTS, MoS₂ and Mo.
First-principles calculations predict that chalcogen vacancies form deep levels in CTZSSe materials that could negatively impact critical opto-electronic properties, including minority-carrier lifetime and electron and hole mobilities.[1] As a consequence, CZTSSe materials synthesized under low or high vacuum conditions, or via liquid-phase precursors, are often subjected to a post-deposition annealing process under relatively high chalcogen partial pressures. Diffusion of chalcogen through the CZTS(e) layer results in conversion of some of the Mo back-contact to MoS(e)₂.[2-4] While there have been several reports that a thick MoS(e)₂ layer can be detrimental to device performance, the band alignments between the Mo/MoS(e)₂/CZTS(e) interfaces have yet to be reported.
The band alignment between molybdenum and MoS₂ was determined via annealing Mo-coated glass at various temperatures in 1x10^-4 Torr sulfur. All transfers between the CZTS growth chamber and the spectrometer utilized a UHV transfer system to eliminate the effects of airborne contamination on the results. The Kraut method[5, 6] of band-alignment was employed to yield the conduction band offset and a valence band offset at the Mo/MoS₂ interface. The data indicates there is not a barrier to holes present at the Mo/MoS₂ interface. The band-alignment between MoS₂ and CZTS was determined by creating a thick film of MoS₂ on the Moly-coated glass in the growth chamber by annealing in 1x10^-4 Torr of sulfur at 500 °C. A thin crystalline CZTS film was subsequently deposited on this MoS₂ at lower temperature (~400 °C). Subsequent growth of thin CZTS films continued until there was no evidence for further band-bending in the CZTS layer. This allowed determination of the band alignment between MoS₂ and CZTS, allowing for a clear picture of the band alignment at the back contact which is typically present in CZTS devices that have been subjected to a high-chalcogen annealing treatment.
1. Chen, S., et al., Phys Rev B, 2010. 81(24): p. 245204.
2. Nishiwaki, S., et al., J. JAP. 37 p. L71.
3. Scragg, J.J., et al., Journal of the American Chemical Society, 2012. 134(47): p. 19330-19333.
4. Shin, B., et al., Applied Physics Letters, 2012. 101(5): p. 053903.
5. Bosco, J.P., et al., JAP, 2013. 113(20): p. -.
6. Kraut, E.A., et al., PRL, 1980. 44(24): p. 1620-1623.

Inexpensive and reasonably efficient copper sulfide (Cu₂S) solar cells studied in 1970s benefitted greatly by the move to ternaries (Cu-III-VI₂), and their subsequent alloys (CIGS). Less explored ternaries include the Cu-V-VI₂ compounds. Despite the quite different crystal structure that results from the group V³⁺ element, compounds like CuSbS₂ appear to have electro-optical properties very similar to well known CuInS₂; such as a direct band gap of 1.5 eV, high absorption coefficients (>10⁵ cm^-1), moderate ~10¹⁶ cm^-3 p-type doping, and an electron affinity of 4.0 eV. These similarities permit diode formation with the mature n-type heterojunction partner CdS/ZnO. Here, we present accelerated photovoltaic (PV) device development of CuSbS₂ absorbers, including an analysis of bulk defects, which currently limit the solar cell performance.
We synthesize CuSbS₂ absorbers by RF magnetron co-sputtering of Cu₂S and Sb₂S₃ targets onto a variety of back contacts (Mo,W,Ni,Pt,Pd,Au,Mo/MoO_x). At substrate temperatures between 350-425°C, excess Sb₂S₃ flux creates a deposition window where-in phase pure material can be deposited with tunable carrier concentrations between 10¹⁵-10¹⁸ cm^-3 [1]. Junctions are then completed with uniform deposition of CdS (chemical bath) and front contacts (i-ZnO/ZnO:Al/metal grid). The resulting 2"x2" sample is subdivided into 44 ~0.5 cm² devices, each prepared at unique combinations of Sb₂S₃ flux, substrate temperature, and absorber thickness. Measured solar cell parameters (Jsc, Voc, FF) can then be studied as a function of growth condition, and correlated with absorber material properties such as morphology, orientation, composition etc.
The devices are found to have good diode characteristics, but relatively low photoresponse (~1% efficiency). The best photocurrent is observed for a thin (800 nm) absorber grown at 400°C in 2X Sb₂S₃ overflux on a Mo/MoO_x back contact. Comparing the measured JV and EQE to a device simulation points to bulk defect states as the primary photocurrent limiting factor. We therefore probe defects via temperature- and excitation-dependent photoluminescence (PL), finding a broad peak 0.2 eV below the band gap associated with band-to-defect recombination. These experimental results are then examined in the context of a calculated CuSbS₂ defect theory. We find that the dominant shallow acceptors and donors are V_Cu and Sb_Cu respectively, and that V_S forms deep states but has a large formation energy. Overall, this study concludes that CuSbS₂ requires a reduction in bulk defect densities before higher energy conversion efficiencies can be achieved.
The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.
[1] Welch, A. et. al. Solar Energy Materials and Solar Cells, in press (2014), doi:10.1016/j.solmat.2014.09.041

The compound semiconductor Cu₂ZnSnS₄ (CZTS) is a promising alternative for absorber layers in thin film solar cells. Replacement of some elements with chemically similar ones, in particular (S,Se), allows to optimize desired characteristics; Cu₂ZnSn(S,Se)₄ have already achieved efficiencies above 12% [1]. Moreover, off-stoichiometric composition allows introducing vacancies in the structure. Concentration and distribution of point defects depend on thermal history, in particular low-temperature thermal history, of the compound [2].
Besides chemical composition and phase purity, the efficiency of CZTSSe thin film solar cells depends strongly on the concentration of these vacancies and Cu- and Zn-antisites. However, Cu(I) and Zn(II) are isoelectric and thus cannot be distinguished by conventional X-ray diffraction. In prior work [3] we determined Cu-Zn-distribution successfully using the different neutron scattering lengths of Cu and Zn. Anomalous X-ray diffraction can also be used to this effect. Anomalous scattering coefficients are highly wavelength-dependent close to the absorption edges of the respective element. This can be utilized for contrast enhancement.
The Diffraction station at KMC-2 beamline at BESSY is well suited for these experiments. The Cu-K edge (8979 eV) and Zn-K edge (9659 eV) are in the center of the available energy range. The narrow energy width of the monochromator allows to use energies very close to the absorption edge, where f' contrast enhancement is at a maximum. Changes in the diffraction pattern at different wavelengths are highly significant at the attainable counting statistics.
However, even in effective total absence of uncertainty from counting and particle statistics an unusual high degree of variance was found in the values for cation site occupation factors resulting from straightforward Rietveld refinement of data sets taken at multiple wavelengths. Detailed analysis reveals this to be the result of extremely high correlation between various structural parameters. These in turn result from the extreme crystallographic similarity of the different sites, which in the Kesterite structure are non-equivalent only due to the very cation ordering analyzed. While this can be handled comparatively easy in single-crystal experiments, it is amplified by the loss of information in powder diffraction. We present a recipe to circumvent this problem by use of selected derived parameters. This allows quantification of Cu and Zn site occupancies, going beyond currently published papers, which either restrict themselves to qualitative changes [4,5] or use single crystal data [6].
[1] W. Wang, et al., Advanced Energy Materials, (2014) 4(7)
[2] J.J.S. Scragg, et al., Appl. Phys. Let. (2014) 104, 041911
[3] S. Schorr, Sol. Energy Mat. and Sol. Cells. (2011) 95, 1482
[4] H. Nozakia et al., J. Alloys Comp. (2012) 524, 22
[5] T. Washio et al., J. Appl. Phys. (2011) 110, 074511
[6] A.0. Lafond et al., Acta Cryst. B (2014) 70, 390

Long-known critical issues for the reaction of Cu-Ga-In metal precursor layers to form Cu(InGa)(SeS)₂ alloy thin films include control of the through-film composition, void formation and poor adhesion at the Mo/Cu(InGa)(SeS)₂ interface. Processing advances to address these issues using sputter-deposited Cu-Ga-In precursor layers and reaction in H₂Se and H₂S will be described. The reaction time-temperature-gas flow sequence can be used to control the film formation by controlling the reaction pathways. In particular, the Ga and S distributions can be manipulated by changing the concentrations of chalcopyrite and intermetallic phases through the reaction process. The effects of Ag addition on the morphology and phase formation in the sputtered precursor layers and the subsequent hydride gas reaction will be described. Benefits of the Ag-alloying include elimination of elemental In agglomeration in the precursor layers and improved adhesion in the reacted film.

Cu(In,Ga)Se₂ (CIGSe) thin-film photovoltaic devices are well known and studied. In these materials, the substitution of In for Ga widens the band gap, which improves module performance due to reduced power losses, and increases the utility of this material as a top cell in tandem solar cells. However, as the Ga content in the film is increased, the device performance decreases. By replacing some of the Cu with Ag to make (Ag,Cu)(In,Ga)Se₂ (ACIGSe), improved device performance at wider band gaps has been achieved.^1,2 While ACIGSe materials show promise as a wide bandgap absorber, their chemical and electronic structures have not been extensively evaluated.
In this work, the chemical structure and electronic level alignment at the ACIGSe/Mo interface are characterized. A set of ACIGSe films, deposited on Mo/soda-lime glass substrates, were prepared at IEC and cleaved at UNLV under inert conditions to create two samples, allowing the study of the ACIGSe/Mo interface with surface-sensitive spectroscopies. All samples were investigated by X-ray and UV photoelectron spectroscopy (XPS and UPS) and inverse photoemission spectroscopy (IPES) at UNLV, as well as X-ray emission spectroscopy (XES) at the ALS. By combining these complementary techniques, we derive the chemical “surface” composition on both sides of the absorber/back contact interface, as well as the position of the valence band maximum (VBM) and conduction band minimum (CBM) with respect to the Fermi energy. By combining the VBM and CBM data, we can directly derive the electronic surface band gap for each of the two cleaved surfaces. The full chemical and electronic analysis of the cleaved samples will be presented, providing detailed information on the chemical properties and an estimation of the electronic level alignment at this deeply buried ACIGSe/Mo interface. The results will also be compared with the similarly made and characterized CIGSe/Mo interface.
1. J.H. Boyle, B.E. McCandless, W.N. Shafarman, and R.W. Birkmire, J. Appl. Phys.115, 223504 (2014).
2. W. Shafarman, C. Thompson, J. Boyle, G. Hanket, P. Erslev, and J. D. Cohen, Proc. 35^th IEEE PVSC, 325 (2010).

In this work, p-type Cu_0.5Ag_0.5InSe₂ (CAIS) and n-type ZnInSe₂ (ZIS) polycrystalline thin films were analyzed for photovoltaic applications. CAIS polycrystalline thin film has similar characteristics with CuInSe₂ (CIS) and AgInSe₂ (AIS) ternary chalcopyrite semiconductor compounds belong to a group of I-III-VI₂ compounds. Moreover, it has direct band gap, and high absorption coefficients. This indicates that it can be suitable to use as an absorber layer in the photovoltaic applications. On the other hand, ZIS polycrystalline structure is a ternary chalcopyrite semiconductor belongs to the group of II-III-VI compounds with the interest of II-VI binary analog of ZnSe structure. To investigate their film characteristics, they have been deposited on soda lime glass substrates with the evaporation of pure elemental sources by using physical thermal evaporation technique. During the deposition process, the substrate temperature was kept at about 200°C. The thin films were characterized firstly in as-grown form, and then under the nitrogen environment the post-annealing was applied to some of the samples to deduce the effects of annealing on the structural, electrical and optical properties of the deposited thin films. In addition to this, ITO/n-ZIS/p-CAIS/In hetero-structure were fabricated as a solar cell application of the combination of these film structures. Detailed electrical characterization of this hetero-junction was performed by the help of temperature dependent current-voltage (I-V) and frequency dependent capacitance-voltage (C-V) measurements to investigate the device characteristics and to determine dominant conduction mechanism in this sandwich structure. Wavelength dependent I-V measurements were also performed to investigate the photo-transport properties. To determine photo-spectral working range of the junction the spectral photo-response measurements were carried out in the spectral range of 300-1200 nm. This measurement was also performed in order to see the effects and contributions of the film layers on this device structure. Moreover, at room temperature, the photovoltaic characteristics of the deposited hetero-junction were investigated under different illumination intensities varying in between 20 to 115mW/cm²

During the last decade, an increasing attention has been brought to the preparation of thinner absorber layers in Cu(In,Ga)Se₂ (CIGSe) based solar cells, in order to reduce the consumption of indium per Wp and thus the costs and availability for large production. However, most of these works have been done on co-evaporated CIGSe absorbers. In this work, for the first time Cu(In,Ga)Se₂ absorbers with thicknesses from 2µm down to 350nm have been prepared by a two-step process using sequential electrodeposition of Cu-In-Ga followed by annealing under pure Se atmosphere. Electrodeposition is a very promising technique enabling lower production costs, high deposition rate and effective material use. In order to obtain thinner CIGSe layers, the deposition time of metallic Cu-In-Ga was shortened.
Based on compositional characterizations, we will show that the reduction of the electrodeposited time of Cu-In-Ga layers not only leads to the decrease of the thickness of the precursors but can impact the composition of deposited films. After the optimization of annealing conditions, the properties of the absorbers and solar cells with three different thicknesses (2µm, 0.7µm and 0.35µm) will be compared and the impact of absorber layer thickness and composition on device performance will be analyzed. We will show that, in spite of the decreasing thickness and so the decrease of Jsc, the Voc of ultrathin CIGSe electrodeposited solar cells can be improved due to an increase of the Ga content and lateral compositional non-uniformity of electrodeposited absorbers. Without deliberate light trapping and anti-reflecting coating from the very thin absorber layer of 0.35 mu;m, an efficiency of 8.7%, similar to the one obtained using high-efficient co-evaporation methods, with V_OC of 685 mV, J_SC of 19 mA/cm², and FF of 67 % has been achieved. These results show that just by adjusting the composition of the electrodeposited precursors, it is possible to improve highly the Voc in ultrathin CIGSe solar cells due to the spontaneously increase of Ga toward back contact and its passivation effect. These results point out the high potential of electrodeposition technique for preparing ultrathin CIGSe absorbers.

Cu(In,Ga)Se₂ (CIGSe) solar cells have achieved the highest efficiency of 21.7 % amongst the polycrystalline solar cells. Considering the consumption of the rare element In and resulting manufacturing cost, reducing the In consumption is relevant to enhance the competitiveness in the market. One approach is to greatly reduce the thickness of CIGSe absorbers down to below 0.5 mu;m (ultra-thin). However, the ultra-thin solar cells fail to maintain the high efficiencies due to a substantial drop of short circuit current density (J_sc). This is arising from the incomplete absorption of the incident solar spectrum due to the CIGSe thickness reduction. For the incomplete absorption, light-trapping nanostructures based on dielectric materials show the potential and favourability to enhance the absorption in CIGSe solar cells. Besides, the transparent conductive oxide (TCO) back contacts allow a more effective working of light-trapping structures than the typical Mo. However, the solar cells on TCO contacts mostly don&’t perform electrically well. In this work, we firstly investigated the light-trapping structures (SiO₂ nanopatterns at CIGSe/Mo) for ultra-thin solar cells on Mo substrates, then the solar cells on the transparent In₂O_3:Sn (ITO) contacts were optimized for the further enhancement of J_sc.
SiO₂ nanopatterns at the CIGSe/Mo were prepared by substrate conformal imprint lithography (SCIL) with a diameter of 205 nm, 210 nm in height, 513 nm in pitch. Simulations show that the SiO₂ nanopartterns can enhance the absorption in the CIGSe absorber and reduce the parasitic absorption in Mo due to the Mie resonances. The experimental external quantum efficiency (EQE) agrees well with the simulation results. J_sc is enhanced from 28.6 to 31.6 mA/cm² and the efficiency from 11.2 % to 12.4% for the 460-nm-thick absorber solar cell without an anti-reflection layer.
For the solar cells on ITO, lowering the substrate temperature and using the post treatment of Na doping were proved to improve the electrical performance of cells by restraining the formation of the n-typed Ga₂O_x at the CIGSe/ITO interface. Further inserting a 10 nm thick MoO₃ is confirmed to be able to reduce the back recombination by improving the match of working function between ITO and the CIGSe layer. Finally, the ultra-thin CIGSe solar cells on ITO achieved a comparable electrical performance to the ones on Mo. Incorporating the SiO₂ nanopatterns at the CIGSe/ITO interface, the parasitic absorption in ITO was further largely reduced compared to Mo. The transmitted light is reflected back into the solar cell and a comparable J_sc to the thick cell is achieved.

The p-type doping concentration and minority carrier lifetime (MCLT) in the absorber layer are quoted the main limiting parameters for further performance improvement of CdTe thin film solar cells. In conventional CdTe devices grown in superstrate configuration several mechanisms which are responsible for the p-type doping as well as the MCLT are coupled and a correlation to processing parameters is very difficult. In particular, in superstrate configuration the junction formation (intermixing of CdS and CdTe) and the healing of crystalline defects and grain boundary passivation in Cl ambient occur simultaneously. Furthermore, the p-type doping of CdTe by diffusion of Cu and the back contact formation are overlapping. To overcome these obstacles we developed a process sequence which has advantages in studying each mechanism separately.
In this contribution we will present the effect of doping concentration (effective hole concentration) on the device performance, discuss the basic mechanism of p-type doping with group Ib elements Cu and Ag and present results on doping with the group V element As in polycrystalline CdTe layers. In the second part the distribution of S, O, and Cl impurities in grains, grain boundaries, and at interfaces and their effect on the MCLT will be presented by correlating SIMS imaging techniques and atom probe tomography with TRPL measurements. We will show that sulfur plays an important role at CdTe interfaces. In the last part we will present stability aspects derived from accelerated stress test at elevated temperature and illumination of the CdTe solar cells fabricated with the substrate configuration process.

The use of polyimide (PI) foils as a substrate for Cu(In,Ga)Se₂ (CIGS) thin-film solar cells has great potential for the production of cost-effective, flexible, low-weight solar modules. The substrate has also drawbacks which require careful control of the CIGS growth process in order to make efficient solar cells. First, the growth temperature on the polyimide substrate is limited which generally leads to a reduced efficiency and influences the defect structure. Further the use of PI substrates needs distinct incorporation of the alkaline metals (Na and K) which are necessary to process high efficiency solar cells. It is known [1] that K and Na increase the carrier concentration in CIGS, presumably by compensation of donor defects. However, only recently an additional beneficial effect of a sequential post deposition treatment (PDT) with NaF and KF was reported and enabled for the first time the production of a flexible CIGS solar cell with efficiency exceeding 20% [2]. The origin of the beneficial role of these elements is however still not completely clear.
In this study the effect of various Na and K introduction methods (during third stage of CIGS deposition and/or PDT) on the defect structure of CIGS layers deposited at low growth temperature on PI is investigated by photoluminescence (PL). In PL spectra up to five emission peaks between 0.8 and 1.3 eV can be observed. PL measurements as a function of temperature and excitation power do not exhibit large peak shifts which indicates that potential fluctuations in the absorbers are rather small. The addition of Na or K does not give rise to additional PL peaks but leads to a change in the relative intensities of certain existing peaks. The role of K and Na is distinct: Na appears to suppress the PL peak related to a rather deep donor defect, whereas introduction of K lowers the intensity of a PL emission peak related to a slightly more shallow donor defect. The highest efficiency is reached by the sample with NaF PDT followed by KF PDT. Excitation wavelength dependent PL measurements on this sample indicate an increase in the concentration of the deeper donor defects near the interface. This effect was not found in samples in which K or Na have been added during third stage, or for Na included by NaF PDT.
[1] M. A. Contreras, B. Egaas, P. Dippo, J. Webb, J. Granata, K. Ramanathan, S. Asher, A. Swartzlander, R. Noufi, On the role of Na and modifications to Cu(In,Ga)Se₂ absorber materials using thin-MF (M=Na, K, Cs) precursor layers. In Photovoltaic Specialists Conference, 1997, Conference Record of the Twenty-Sixth IEEE, pp. 359-362,1997
[2] A. Chirila, P. Reinhard, F. Pianezzi, P. Patrick Bloesch, A. R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler, and A. N. Tiwari. Potassium-induced surface modification of Cu(In,Ga)Se₂ thin films for high-efficiency solar cells. Nature Materials,12:11071111, 2013

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. In an EBIC experiment, a beam of high energy electrons excites electron-hole pairs, some fraction of which are collected by contacts and measured as charge current. Ideally, an EBIC measurement reflects the spatially resolved quantum efficiency of the device. However, experiments on polycrystalline CdTe solar cells reveal that the EBIC collection efficiency is substantially lower than the quantum efficiency of the device under optical excitation. In order to reliably extract intrinsic material properties from EBIC signals, these differences must be reconciled.
Two important differences between an EBIC experiment and normal device operation are: 1. the high generation rate density associated with the electron beam excitation (which may exceed 1 sun illumination by up to 5 orders of magnitude), and 2. the substantial effect of the exposed surface in an EBIC experiment. By developing numerical and analytical models which account for both of these effects, the difference in the material response under EBIC and normal device operation conditions can be understood. These models highlight the role of nonlinearities in the material response under some regimes of EBIC experiments. The nonlinearities arise from the nonequilibrium charge accumulation, which screens the built-in electrostatic field, and leads to increased rates of radiative and Auger recombination. Additionally the model accounts for the electrostatic fields present at the exposed surface, and recombination within the depletion region.
We show that the analytical models demonstrate good agreement with full numerical simulations of the drift-diffusion-Poisson equation. Comparison between the model and experiment show good agreement between quantities such as maximum EBIC collection efficiency versus charge generation rate. Accounting for these nonlinearities enables the intrinsic material properties to be reliably extracted from high-resolution EBIC experiments. We finally apply these models to interpret the difference in EBIC signal between grain interiors and grain boundaries in CdTe.

Very high absorption coefficient and cost effective production makes Copper Indium Gallium Selenide (CIGS) semiconductor one of the most promising thin film device used in the photovoltaic industry, with efficiency > 20% it is currently the most efficient thin film solar cell produced. It is well known that polycrystalline CIGS is more efficient than the monocrystalline form because of Na segregation along the Grain Boundaries (GBs) in polycrystalline CIGS. Efficiency of CIGS is very sensitive to Ga content in CIGS and is most efficient for x asymp; 0.25. Herein we make a first attempt to explain the variation in efficiency of CIGS as a function of Ga content by exploring the nanochemistry of GB using Atom Probe Tomography (APT). APT provide highly resolved 3D atomic mapping at sub nanometer resolution and can accurately characterize the composition profile across GB. Previous atom probe studies also reveal the modification of the GB chemistry at different stages of the CIGS-growth process suggesting an important role of the In/Cu-CIGSe-content on its final efficiency.
In the present study, we investigate the influence of Ga-content on the GB composition and CIGSe-efficiency. GB nano-chemistry of different Ga-concentrated samples (From CuInSe₂ to CuGaSe₂) has been investigated by atom probe and will be discussed. The influence of Ga-content on GB composition and its impact on efficiency of CIGS solar cells will be discussed.

Closed space sublimation (CSS) of CdTe absorber layers has been proven to produce high efficiency CdTe solar cells after CdCl₂ and Cu diffusion heat treatments (HTs). In order to better understand the high efficiencies achieved using the CSS growth technique with post-deposition HTs, a series of electron beam induced current (EBIC), scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS), and atom probe tomography (APT) measurements have been performed to understand the correlation between carrier separation properties of grain boundaries (GBs) and the CdTe/CdS interface with elemental segregation at these regions. The EBIC results pictorially reveal how the CdCl₂ HT and Cu diffusion increase device efficiency. The STEM EELS and APT measurements show Cl segregation within GBs and at the CdS/CdTe interface, while only the APT measurements show <0.1% Na segregation within these regions. APT is able to detect these low Na concentrations at these regions because it is a high sensitivity, high resolution technique. In addition to the segregation of Na and Cl at these interfaces, S diffusion at the CdS/CdTe interface is investigated using APT and STEM EELS. Both techniques show inter-diffusion of S and Te of ~5 nm between the two layers, in which the S concentration drops from 50% in the CdS grain to 5% in the CdTe grain for both a CdCl₂ and non-CdCl₂ treated cell. STEM EBIC was performed in this region of a CdCl₂ treated cell to reveal an EBIC current maximum located ~10 nm within the CdTe layer. This suggests that the CdS_xTe_1-x region close to the CdTe/CdS interface is doped n-type, which forms a p-n junction within the CdTe grain. The combination of S diffusion within the CdTe grain and Cl segregation within this region are important for device performance. This allows for the p-n junction to be located inside a layer matched to the CdTe grain crystal structure, which reduces non-radiative recombination and improves the device efficiency.
This research was supported by the US Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy, Foundational Program to Advance Cell Efficiency (F-PACE), and ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

CdTe record device efficiencies have increased dramatically in recent years, and there is tremendous interest in understanding the fundamental limits of the technology. CdTe is commercially appealing because of the low manufacturing costs demonstrated by First Solar. However, CdTe efficiencies are still lower than silicon solar cells. In order to compete against conventional silicon modules, CdTe cell and module efficiencies must increase significantly. Increasing CdTe efficiency requires higher open circuit voltages (V_oc). V_oc is limited by low lifetime and net acceptor density in polycrystalline CdTe. We use epitaxial CdTe as a model system to study the fundamental properties of the material in order to understand how to achieve higher V_oc and efficiency. Here, we present results on increasing lifetime and doping in epitaxial CdTe and show a significant reduction in surface recombination velocity.
The low cost and relatively high efficiencies of CdTe may also enable new applications for solar cells. We have recently demonstrated 16% flexible CdTe cells on flexible glass and have a pathway to higher efficiencies. These cells are made in the conventional superstrate architecture using high-rate, manufacturable processes. Through this work, we have also demonstrated the mechanical robustness of CdTe cells and the high-temperature TCOs that they often rely on.

Potassium post-deposition treatment (K-PDT) on co-evaporated Cu(In,Ga)Se₂ (CIGSe) absorber layer have recently yielded increased conversion efficiency of the related thin film solar cell. It has been reported in the literature that K-PDT induces a Cu and Ga depletion in the very near-surface region of the absorber and an enhanced in-diffusion of Cd into the CIGSe [1]. It has also been observed a widening of the energy band-gap in this region [2], which is consistent with the Cu depletion. The model proposed by Chirila et al. to explain the origin of such improvement (mainly increasing the open circuit voltage) is the formation of a p-n homojunction in the CIGSe due to the presence of Cd_Cu donor defects. The formation of these defects could therefore be enhanced by the Cu depletion. However, the origin of such K-induced Cu depletion is still not clear. In this study, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements have been performed on CIGSe layers treated with K (PDT) and Cd (partial electrolyte). We confirmed the depletions and the enhanced Cd in-diffusion in the CIGS near surface region. Based on transmission electron microscopy microstructural characterizations and X-ray diffraction measurements we propose a model in order to explain the origin of the Cu depletion induced by the potassium treatment in the near-surface CIGSe region.
[1] Chiril#259;, A. et al. Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells. Nat Mater 12, 1107-1111 (2013).
[2] Pistor, P. et al. Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment. Applied Physics Letters 105, 063901 (2014).

Cu₂ZnSnS₄ (CZTS) has many desirable properties for solar cell applications - low-cost abundant materials, an optimal bandgap, and promising initial solar energy conversion efficiencies. A major difficulty, however, is in understanding the structures being formed within the material. The main diffraction peaks of CZTS are at nearly the same position for several related compounds and components - and in thin films the grain size is small, leading to broader peaks. We therefore use the EXAFS technique to explore the local structure in CZTS.
We investigate the local structure about the metal atoms in CZTS nanoparticles. The EXAFS data for the Cu, Zn, and Sn K edges look nearly identical in shape. In addition, the metal-S bond lengths are nearly the same (maximum difference = 0.1 Å), as expected for CZTS, but not the individual sulfides. These similarities suggest the environment is similar for all the elements.
The kesterite and stannite CZTS structures contain no Sn-Sn second neighbor bonds, so the Sn atoms theoretically are in a purely Cu and Zn environment (with twice as much Cu as Zn). Yet our EXAFS analysis of the CZTS nanoparticles shows Sn-Sn clustering. In the Sn data, the second peak contains about one-third Sn second neighbors. The step height analysis shows a slight Sn excess and Zn deficiency in the relative metal concentrations. Our results suggest that the Sn may be substituting on to the Cu and Zn sites, excess SnS may be present, or clustering may occur for each element within the CZTS structure. We conclude by addressing how these local structures impact the device performance of CZTS.

The earth abundant photovoltaic thin film absorber Cu₂ZnSn(S_xSe_1-x)₄- (CZTS,Se) has attained an efficiency over 12.5% primarily through increases in short circuit current; at present this value is >85% of the SQ limit. Conversely, increases in open circuit voltage, Voc, have been hampered primarily by bulk point defects such as Cu and Zn antisites and Cu vacancies, to name just a few. We have studied the details of the front CdS/CZTS,Se p-n junction with a number of spectroscopic techniques including femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) and XPS and suggest a model in which Cd²⁺ ions occupy Cu vacancies and form a high quality p-n junction. For the back contact Mo has been almost exclusively employed since it provides a low resistance path for hole transport. In an effort to improve Voc and overall efficiency we have investigated the use of high work function (WF) back contact materials such as MoO₃ in conjunction with careful absorber thickness control. Using fs-UPS we demonstrate that the dramatic WF difference between MoO₃ (6.5eV) and CZTS,Se (5.2eV) drives electron transfer that negatively charges the back contact relative to the CZTS,Se, producing an internal electric field that can enhance Voc and act as an electron mirror. Hall measurements on MoO₃ were carried out as a function of anneal temperature to determine the carrier density in the oxide film. In order to exploit these back contact modifications, we have also investigated more robust front contact materials intended to survive high temperature processing of CZTS,Se. The results of these investigations and the promise for increasing Voc will be described.

Recently Cu(In,Ga)Se₂ (CIGS) solar cells have been achieved more than 20% by surface treatment process and many researchers have focused on the surface condition of CIGS such as potassium fluoride treatment, sulfurization, and exist of ordered vacancy compound layer. We have indicated that the CIGS solar cells with single graded band profile with an average band gap (E_g(avg)) of 1.4 eV have a potential to achieve more than 23%, when a wide gap layer with the valence band offset (DE_V) of 0.3 eV was inserted at a CdS/CIGS interface. This is because interface recombination was suppressed due to repelling holes from the interface by DE_V. In this study, we theoretically and experimentally investigated the influence of quality of this surface layer on CIGS solar cell performance.
In a calculation, the CIGS absorber has single grading and E_g(avg) of 1.4 eV. The interface defect at CdS/CIGS was reproduced by inserting a defective layer (DL) with a defect density of 10¹⁸ cm^-3 and a thickness of 10 nm, which was comparable to the surface-defect density of 10¹² cm^-2. In order to investigate the DE_V effect on the cell performance, a surface layer (SL) was inserted at the DL/CIGS interface. Carrier density and DE_V of SL were set at 10¹³ cm^-3 and 0.3 eV, respectively. Defect density in the SL was varied from 1.0 × 10¹⁴ to 1.0 × 10¹⁷ cm^-3 and the thickness of the SL was changed from 0 to 50 nm. Furthermore, we fabricated CIGS solar cells with single grading and E_g(avg) of 1.4 eV. The solar cells had a Cu(In,Ga)₃Se₅ layer at CdS/CIGS interface as the SL, and the thickness of the Cu(In,Ga)₃Se₅ layer was changed from 0 to 30 nm.
Calculation results revealed three followings. Firstly, conversion efficiency was significantly improved from 13 to 23.5% at a maximum due to the effect of the insert of the SL. This maximum conversion efficiency was obtained when the density and thickness of the SL were 1.0 × 10¹⁴ cm^-3 and 50 nm, respectively. Secondly, as defect density in the SL became higher, optimum SL thickness where maximum efficiency was obtained was decreased. Thirdly, the defect density in the SL did not influence the performance of CIGS solar cells significantly if the optimum thickness of the SL layer was set. The last result is most important, and it shows that we can enhance the conversion efficiency of CIGS by controlling the valence band maximum at the CdS/CIGS interface even though the SL is defective. At the defect densities of the SL of 5.0 × 10¹⁶ and 1.0 × 10¹⁷ cm^-3, maximum efficiency of 23.2% at 30 nm and 22.9% at 10 nm were obtained, respectively. The SL can be formed by the surface treatment or the insertion of a Cu(In,Ga)₃Se₅ layer.
We experimentally investigated the influence of the insert of the Cu(In,Ga)₃Se₅ layer, and we have confirmed remarkable improvement of the conversion efficiency with increasing the thickness of Cu(In,Ga)₃Se₅ from 0 to 30 nm. The maximum efficiency of 14.4% was achieved at the thickness of 30 nm with a E_g(avg) of 1.4 eV.

Antimony selenide (Sb₂Se₃) is a simple binary compound with orthorombic crystal structure. The constituent of Sb₂Se₃ is earth abundant, low toxicity and inexpensive. Moreover, Sb₂Se₃ has a direct band gap of approximately 1.1 eV with large absorption coefficient (>1E5 cm^-1 at short wavelength) and low grain growth temperature (~300 ^oC), facilitating easy fabrication of flexible thin film solar cells. Theoretical analysis indicates the efficiency limit is >30% for single junction Sb₂Se₃ solar cells. Although very attractive, solar cells using Sb₂Se₃ as the absorber material only appeared in 2014. In this talk I will focus on the latest research progress achieved within our group: i. using a hydrazine solution process, we fabricated high quality Sb₂Se₃ thin film and built a 2.26% TiO₂/Sb₂Se₃ heterojunction device; ii. due to their large vapor pressure, we employed thermal evaporation to deposite Sb₂Se₃ thin films and obtained a FTO/Sb₂Se₃/CdS/ZnO/AZO/Au substrate device and achieved 2.1% device efficiency. iii. We further applied a post-selenization step to reduce the concentration of harmful selenium vacancies (V_Se) in the thermally evaporated Sb₂Se₃ film and resulted in a FTO/CdS/Sb₂Se₃/Au superstrate device with 3.7% device. All these devices showed excellent reproducibility and stability. Understanding of film growth, device operation and efficiency bottleneck will also be presented, as well as perspective for future development of Sb₂Se₃ thin film solar cells.

Tin sulfide is a candidate p-type solar cell absorber material. It is Earth-abundant, has high majority carrier mobility (90 cm²/Vs in single crystals at room temperature¹), and strong absorption characteristics. 1 mu;m of material is enough to absorb 83% of incoming above band gap light on a single pass. SnS has a 1.1 eV indirect band gap and 1.3 eV direct gap. Recently, SnS devices have shown notable efficiency improvements, with the NREL-certified record at 4.36 % for SnS grown by atomic layer deposition (ALD)² and 3.88 % for a thermally evaporated device³. Both of these devices employ annealing in H₂S gas as a crucial grain-growth step. However, this step has yet to be optimized and there exists great promise for further efficiency gains.
SnS devices are produced with the following substrate stack design: Si / SiO₂ (0.5-1 mu;m)/ Mo (600 nm) / SnS (1 mu;m) / ZnO_xS_y:N (30 nm)/ ZnO (10 nm) / ITO (250 nm)/ Ag (500 nm). After SnS absorber deposition by thermal evaporation, the partial stack is typically annealed in 4% H₂S (96% N₂) for 1 hour at 400 °C and 28 torr pressure³. After annealing, the stack experiences a SnO_x growth step, which is believed to passivate the surface of SnS. Next, the ZnO_xS_y:N n-type layer and ZnO layer are deposited by ALD. The device is finished with a sputtered ITO layer and E-beam deposition for the Ag metal fingers.
In this study, we find that by increasing the temperature, time, and total pressure of the annealing conditions, further improvements of film properties are achieved. For anneals performed at 450 °C for 3 hours, at 80 torr, majority carrier concentration increases from an as-deposited average of 4.2×10¹⁵ cm^-3 to an annealed average of 8.1×10¹⁶ cm^-3. Average grain area also increases from 0.06 to 0.21 mu;m².
Using different H₂S gas environments and temperatures, we are able increase majority carrier concentration by 2 orders of magnitude. Curiously, the carrier concentration cannot be subsequently reduced by further thermal treatments, in contrast to previous single-crystal studies¹.
Finally, we also present improvements in device performance on thermally evaporated SnS cells with an adjusted annealing method. Efficiency improvements can be correlated with film properties such as increased grain size and higher majority carrier doping density. These improved film properties have primarily led to an increase in J_sc and fill factor. Device modeling is also used as an aid to further elucidate reasons for increased efficiency.
[1] W. Albers, C. Haas, H.J. Vink, J.D. Wasscher. J. Appl Phys., 10 (1961) 2220.
[2] P. Sinsermsuksakul, L. Sun, S.W. Lee, H.H. Park, S.B. Kim, C. Yang, R.G. Gordon. Adv. Energy Mater., (2014) DOI: 10.1002/aenm.201400496.
[3] V. Steinmann, R. Jaramillo, K. Hartman, R. Chakraborty, R.E. Brandt, J.R Poindexter, Y.S. Lee, L. Sun, A. Polizzotti, H.H. Park, R.G. Gordon, T. Buonassisi. Adv. Mater., (2014) DOI: 10.1002/adma.201402219.

The substitution of Ag for Cu in CIGS absorber layers shifts the bandgap energy higher and towards the optimal value while decreasing materials cost. With this substitution and higher bandgap energy, an alternative buffer layer material to CdS is needed to retain a favorable band alignment with the absorber. Zinc Oxysulfide (ZnO_1-xS_x) is a candidate buffer layer material since its electrical and optical properties are tunable by varying the oxygen to sulfur ratio in the film. Furthermore, the wider bandgap of ZnO_1-xS_x compared to CdS and the thinner film possible with conformal ALD compared with conventional chemical bath deposited CdS, promises higher transmission.
In this work, ZnO_1-xS_x thin films were deposited, characterized, and tested as an alternative buffer layer for wide-bandgap CIGS thin film solar cells. The CIGS substrate was manufactured by Global Solar Energy (GSE) using co-evaporation on Mo/stainless steel sheet. The buffer layer was deposited using an atomic layer deposition reactor with reactants diethylzinc, H₂S, and H₂O at 150 °C. For, The reactant sequence for depositing ZnO_1-xS_x film was set as (diethylznic-purge-H₂O-purge)_x (diethylznic-purge-H₂S-purge)_y, where x and y are number of times each Zn-O and Zn-S cycle are respectively executed. The fabricate test cells, aluminum-doped zinc oxide (AZO) was sputtered on the buffer layer, followed by deposition of an Al/Ni contact grid using e-beam evaporation. The buffer layer thickness is precisely controlled by the number of deposition cycles.
XPS studies of the deposited films indicated that the concentration of sulfur in the buffer layer film deviated negatively from the precursor pulse ratio x/(x+y). The surface morphology and film structure were examined using SEM and XRD, respectively. Results showed that the deposited film had a polycrystalline structure and the conformality of the deposited film was excellent. Cells were fabricated and tested with variable buffer layer composition and thickness. The highest efficiency, 14.2%, was achieved with 12% S/(S+O) ratio. Compared with the reference cell fabricated with a traditional CdS buffer layer, most improvement of the cell with the ZnO_0.88S_0.12 buffer layer occurred in the short circuit current density, J_SC. The efficiency decreases with decreasing and increasing S concentration, reaching 0 % at S/(S+O) ratio of 0.5. A lower fill factor is primarily responsible for the reduced performance, which may be caused by parasitic resistances induced by recombination at the absorber/buffer layer interface. The effect of deposition temperature, post annealing and the buffer layer thickness on the performance of the cell was also studied by annealing the sample under N₂ or vacuum and varying the cycle number and other operation parameters during ALD.

Kesterite Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) has been considered as a promising candidate material class for scalable photovoltaic applications, due to its elemental abundance and controllable band gap (E_g asymp; 1.0 - 1.5 eV). Power conversion efficiencies up to 12.6% were demonstrated using Se-rich CZTSSe absorbers prepared by solution-based deposition process. However, the devices have shown significant open-circuit voltage (V_OC) deficit problem, largely due to deep trap states and band tailing. Theoretical calculations predict that the pure selenide Cu₂ZnSnSe₄ (CZTSe) phase possesses shallower defect levels than pure sulfide Cu₂ZnSnS₄ (CZTS) phase. To date, CZTSe-based solar cells with efficiencies up to 9.7% were reported, while the highest efficiency reported for the CZTS-based solar cell was 8.4%.
In this contribution, we study pure selenide phase CZTSe-based thin-film solar cells fabricated by thermal co-evaporation method. A certified power conversion efficiency of 11.6% is demonstrated, which is the highest efficiency among CZTSe-based thin-film solar cells reported to date. Quantum efficiency and capacitance-based measurements of the device indicate that the enhanced efficiencies are enabled by significantly improved minority carrier diffusion length over 2 µm. The device also shows a record V_OC-deficit below 600 mV, which can be attributed to reduced band tailing or the occurrence of shallower defect related states in the CZTSe layer. Microstructural analysis of the CZTSe thin-film and a comparative study of photoluminescence properties between CZTSe and CZTS phases are discussed.

KCN etching has been used for removing secondary phases from Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film absorbers, which are, otherwise, detrimental to solar cell performance [1]. It has also been shown that KCN etching affects the surface composition of CZTSSe thin film absorbers [2]. Moreover, improvements in device characteristics, particularly in open circuit voltage (V_OC), have been reported for solar cells that are fabricated with KCN-etched CZTSSe absorbers [3]. On the other hand, NH₄OH is one of the chemicals that is used in CdS buffer layer deposition on absorber by chemical bath deposition method, and the effect of NH₄OH on CZTSSe surface chemistry has been reported [4, 5]. Consequently, all these studies demonstrate that NH₄OH and KCN treatments of CZTSSe absorbers may have various outcomes, and efforts towards better understanding of the relation between these surface treatments and the properties of CZTSSe absorbers and solar cells are necessary.
In this study, we examine the effects of NH₄OH and KCN treatments on CZTSSe thin films and solar cells. First, we report on the effect of ambient on CZTSSe absorbers by using thin films that were oxidized and contaminated by exposure to ambient for a long time. Secondly, we demonstrate the effects and extensiveness of NH₄OH and KCN treatments on the ambient-exposed CZTSSe absorbers. Simultaneously obtained topography and surface potential images by Kelvin probe force microscopy (KPFM) demonstrate that ambient-exposed absorbers have an overlayer that possibly consists of oxide and contamination layers. Although the overlayer is removed completely by KCN etching, NH₄OH treatment partially removes it. Characteristics of solar cells fabricated with KCN-etched and NH₄OH-treated CZTSSe absorbers demonstrate that KCN-etched devices outperform their NH₄OH-treated counterparts with notable increase in short circuit density. In addition, the KCN-etched devices also demonstrate higher V_OC than the NH₄OH-treated ones. We also report that positive charge at the grain boundaries of CZTSe thin film disappears, after the film is etched with KCN, and the absorber surface becomes almost equipotential according to KPFM.
[1] Tanaka et al., Existence and removal of Cu₂Se second phase in coevaporated Cu₂ZnSnSe₄ thin films, J. Appl. Phys. 111 (2012).
[2] Bär et al., Impact of KCN etching on the chemical and electronic surface structure of Cu₂ZnSnS₄ thin-film solar cell absorbers, Appl. Phys. Lett. 99 (2011).
[3] Bär et al., Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu₂ZnSnS₄ thin-film solar cell heterojunction, Appl. Phys. Lett. 99 (2011).
[4] Choi et al., Dielectric function spectra and critical-point energies of Cu₂ZnSnSe₄ from 0.5 to 9.0 eV, J. Appl. Phys. 111 (2012).
[5] Timmo et al., Chemical etching of Cu₂ZnSn(S,Se)₄ monograin powder, Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE. (2010) 001982-001985.

Nanocrystals (NCs) provide an ideal means to generate semiconductor layers from solution wherein the nanocrystals act as a template for the formation of the desired microcrystalline semiconductor. These nanocrystal templates can be synthesised cheaply and on large scales. The structure of colloidal nanocrystals is such that the ratio of (the desired) inorganic semiconductor component to organic contaminants is hundreds of times greater than in molecular precursor inks.
The exchangeable surface chemistry of nanocrystals permits the native organic ligands used during synthesis to be readily swapped with ligands of the desired functionality. This culminates in the ability to mitigate the effects of the organic contaminants while permitting their dispersion in a variety of solvents that are desirable for printing applications.
A particularly attractive candidate for thin film solar cells is Cu₂ZnSnS_4xSe_4(1-x) (CZTSSe). In this presentation we build on the scalable synthesis of Cu₂ZnSnS₄ nanocrystals recently published by our group [1] and their use in the formation of high quality thin films, and high efficiency CZTSSe solar cells (>7%). This presentation will elaborate and expand on results recently published in the Journal of the American Chemical Society entitled “Cu₂ZnSnS_4xSe_4(1-x) Solar Cells from Polar Nanocrystal Inks”[2]. Specifically, we explore the effects of adding beneficial dopants (such as Na⁺, Sb³⁺) directly to the NC ink on the CZTS device performance. Such doped NC inks have the potential to circumvent the need for post deposition dopant incorporation or complicated syntheses that aim to dope the surface of CZTS
NCs. Furthermore, the effect of selenization conditions (a process used to grow the nanocrystal films into microcrystalline semiconductor films) on device performance will be elucidated. Finally we also explore the effects of using composite Cu₂ZnSnS₄/Cu₂ZnGeS₄ ink mixtures and their advantages within thin film devices [3].
References
[1] Chesman, A.S.R., van Embden, J., Duffy, N.W., Webster, N.A.S., Jasieniak, J.J.. (2013) Crystal Growth and Design, 13 (4), pp. 1712-1720.
[2] van Embden, J., Chesman, A.S.R., Della Gaspera, E., Duffy, N.W., Watkins, S.E., Jasieniak, J.J. Journal of the American Chemical Society (2014), 136 (14), pp 5237-5240
[3] Chesman, A.S.R., van Embden, J., Della Gaspera, E., Duffy, N.W., Webster, N.A. Jasieniak, J.J.“Cu₂ZnGeS₄ Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys” (2014) Chemistry of Materials, 26 (19), pp 5482-5491

With better electronic and transport properties Cu-rich based CuInSe₂ (CIS) material represents a promising candidate for better thin film solar cells¹. Nevertheless these solar cells are limited by two major problems. Firstly, the devices perform worse due to interface and tunneling enhanced recombination: the first one is located between the absorber and the buffer layer and leads to a huge Voc loss while the second one, due to a too high doping of the absorber, has a direct impact on the Jsc²: the expected good performance of the resulting solar cells is then not reached. The second major issue for these devices is linked to their fabrication process: growth under excess of Cu leads indeed to a stochiometric crystal plus a secondary layer of Cu₂Se³ which has to be removed before finishing of the solar cell. The selective removal of this secondary phase represents a toxic step in the fabrication process as it is done by KCN etching⁴. The goal of this work is to replace this toxic etching step while suppressing the interface recombination by smoothing the surface of the absorber. As we observed by AFM, the roughness of the Cu-rich absorbers is indeed two times higher than the Cu-poor ones. With a better surface, we expect to reduce the interface recombination. We tried thus two alternative etchings known in the literature⁵ to remove both the Cu₂Se and the CIS: bromine in methanol (Br₂MeOH) and in aqueous solution (HBr/Br₂) etching. While the first etching destroyed the obtained solar cells, the HBr/Br₂ etching on the contrary leads to an improvement of the solar cells performance: the efficiency increased up to 8,9% compare to 5,4% for the Cu-rich solar cell etched by KCN. Top view and cross section electron micrographs exhibit that after HBr/Br₂ etching a smooth surface can be obtained while etch pits can be observed for the Br₂MeOH etching ones. For the absorbers etched by HBr/Br₂ solution, the increased efficiency results from an improvement of both the FF and the Voc, while the Jsc remains stable. We suggest that the change of the absorber&’s surface suppresses the interface recombination leading to this increased Voc and FF. However the diode quality factor of these cells is still above 2, indicating that the issue of tunnelling enhanced recombination remains. However it was meanwhile found that the latter can be suppressed by controlling the Se pressure during the growth⁶, thus we will test soon the new etching on these optimised absorbers. In conclusion, we here find a non-toxic alternative etching for Cu-rich based CIS solar cells, which improved the performance of the cells by suppressing the interface recombination.
¹S. Siebentritt et al., Sol. Energie and Mat., 2013.
²V.Deprédurand et al., J. Appl. Phys.115, 044503, 2014.
³T. Gödecke et al., Z. Metallkd. 91, 622-634, 2000.
⁴Y. Hashimoto, Jap. J. Appl. Phys., 35, 4760-4764, 1996.
⁵Z. Jehl et al., Thin Solid Films 519, 7212-7215, 2011.
⁶ V. Deprédurand et al., APL, in press, 2014

The implementation of an optically efficient top-cell material that is lattice matched with crystalline silicon (c-Si) for tandem photovoltaics has been an enduring problem. It would be transformative to develop a material that has similar optoelectronic properties to the III-V&’s but also has characteristics that make it suitable as a monolithic top-cell on c-Si: a direct band gap that is close to the dual-junction ideal 1.7 eV, lattice matching, and thermal expansion matching. ZnSiP₂ is member of the II-IV-V₂ chalcopyrites, and because of its structural similarity to both c-Si and the III-V&’s, it is expected to have good optoelectronic performance. This material is known to have a pseudodirect band gap of about 2.1 eV and a lattice mismatch with c-Si of 0.5%, making it potentially applicable as a monolithic top-cell on c-Si. Additionally, it consists of earth abundant elements. Although ZnSiP₂ has received scientific attention since the late 1950's, photovoltaic-relevant characterization to date has been insufficient to thoroughly determine its potential as an absorber material.
We have grown bulk single crystals of photovoltaic-quality ZnSiP₂ and found it to have many properties required of a top-cell material for use with c-Si photovoltaics. The crystals form in a fully ordered chalcopyrite structure and are stable at typical film growth temperatures (< 800 C). We have found the parasitic sub-band gap absorption to be negligible (α < 10 cm#65293;1) and that the reflectance at a ZnSiP₂/c-Si interface should be less than 1% for light with energy less than the 2.1 eV band gap. Photoluminescence (PL) strongly indicates high material quality, evidenced by the presence of an exciton peak at low temperatures. ZnSiP₂ luminesces brightly, and the PL spectrum is dominated by the radiative recombination of shallow donor/acceptor pair transitions. These shallow donor and acceptor levels indicate the potential for p- and n-type doping. We have tested these single crystals in a photoelectrochemical configuration using a liquid junction and measured a strong photoresponse and open circuit voltages of ~ 1 V. Measurement of the time resolved PL gives a minority carrier lifetime of 8 ns, resulting in a predicted diffusion length of approximately 6 micrometers. These measurements all indicate excellent optical and minority carrier transport properties, suggesting that ZnSiP₂ is a promising material for Si-based tandem photovoltaics.

Synthesis of the photovoltaic absorber material of the nominal composition Cu₂ZnSnX₄ (CZTS; X = S, Se), via non-vacuum, solution based processing techniques is receiving considerable attention due to the low cost, earth abundance and non-toxicity of the elements. In this study, CZTS and CZTSe were synthesized at near room temperature conditions using elemental metals and chalcogens using ball-milling. Both dry and wet ball-milling of elemental metals (Cu, Zn, Sn) and chalcogens was done in a planetary ball-mill, for various durations (i.e. 15h, 20h, 25h and 30h) at a fixed rotation speed. XRD, Raman and TEM studies confirmed the formation of single phase, nano-crystalline powders of CZTX (where, X = S, Se). Optical bandgap of CZTS samples was observed in the range of 1.4-1.6 eV and that of CZTSe was observed in the range of 1.08-1.18 eV, as confirmed by UV-Vis. The obtained particles were nano-sized and the distribution was narrower in case of wet ball-milling compared to dry ball-milling. TGA analysis provides the information on the appropriate annealing conditions to avoid the formation of secondary phases due to evaporation of the materials like Zn, Sn and S at annealing temperature. Further, the effect of annealing temperature on the properties of the material has also been investigated in the temperature range of 300-600 ^oC. The thermodynamic calculations for Gibbs free energy during CZTX formation support the phase selection of the quarternary phase (CZTX), in phase competition with the binaries and ternaries (ZnX, Cu₂X, SnX₂ etc.) at room temperature. Our work provides a route for developing cheap, scalable, ink-based techniques for low cost solar PV.

Highest efficiencies of Cu(In,Ga)Se₂ solar cells have been achieved with absorbers deposited by a 3-stage-based co-evaporation, including a Cu-rich intermediate growth stage but ending with an overall Cu-poor composition. The Cu-poor/Cu-rich transition during the growth process as well as the presence of small amounts of alkali metals, such as Na and K, in Cu(In,Ga)Se₂ are known to be favorable for the photovoltaic device performance. It was found recently - in particular at low substrate temperatures - that a post-deposition supply of Na is favorable compared to presence of Na during Cu(In,Ga)Se₂ deposition [1,2]. However, the reason for this behaviour remains unclear. We investigate the influence of Na on the structural evolution of CuInSe₂ by interrupting the growth process at overall Cu-poor and at overall Cu-rich film compositions. A series of CuInSe₂ layers without and with varying NaF precursor thicknesses on Mo/glass substrates with Na-diffusion barrier was prepared and analyzed by X-ray diffraction (XRD), X-ray fluorescence, energy-dispersive X-ray spectrometry (EDX) and glow discharge optical emission spectroscopy (GDOES). Low growth temperatures below 400°C facilitated the detailed observation of the influence of Na on the evolution of planar defects, such as stacking faults, and of elemental interdiffusion. For the samples without NaF precursor a XRD signal characteristic for planar defects [3] was observed only for the Cu-poor samples, while for the Cu-rich samples the signal for planar defects completely vanished. In contrast, for the Na-containing absorbers the characteristic signal was present also for the Cu-rich samples. For Cu(In,Ga)Se₂ layers without Na precursor real-time XRD shows that the annihilation of planar defects coincides with the Cu-poor/Cu-rich transition. GDOES and EDX measurements clearly reveal the reduction of Cu-diffusivity due to the presence of Na, leading to pronounced Cu-gradients. At the same time grazing incident XRD measurements suggest a higher density of planar defects in areas with lower Cu-content close to the back contact. Thus, it is shown that the presence of Na during growth impedes the annihilation of planar defects during the Cu-poor/Cu-rich transition in CuInSe₂ at low growth temperatures, which might offer an explanation for the poorer performance of Cu(In,Ga)Se₂ solar cells with Na present during growth as compared to cells with post-deposition-treatment or Na supply after the second stage.
#12288;
[1] A. Chirila et al. “Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells” Nature Materials 12, 1107-1111 (2013)
[2] D. Rudmann et al., “Sodium incorporation strategies for CIGS growth at different temperatures”, Thin Solid Films 480-481 (2005) 55-60 (2004)
[3] H. Rodriguez-Alvarez et al. “Recrystallization of Cu(In,Ga)Se2 thin films studied by X-ray diffraction”, Acta Materialia 61 4347-4353 (2013)

We demonstrate the use of continuous wave (CW) lasers as an annealing tool for post-deposition thermal treatment for CdTe. We will show that the heat treatments such as CdCl₂ treatment can be carried out in the transient regime where the substrate or superstrate does not reach steady-state conditions, thus opening the possibility of using more temperature sensitive glass or other substrate materials. The annealing time by laser annealing can be tuned to be in tens of seconds as compared to many minutes needed for conventional annealing methods, thus making higher production throughputs possible. This makes CW laser annealing a potential candidate for post-deposition treatment of CdTe that could lead to high throughput and reduced manufacturing costs.
In this study a sub-bandgap 1064nm, 1064 nm CW Nd:YAG laser is used to anneal out sub-bandgap defects and to achieve chorine activation of CdTe. Surface photovoltage spectroscopy (SPS) along with time-resolved photoluminescence (TRPL) measurements are used to demonstrate that recombination at the surface of a CdTe layer is reduced by laser treatment. SPS phase spectra shows that the surface band bending has been reduced to below our detection level, as relatively no phase change in bulk and near surface region is observed. TRPL shows a doubling of the minority carrier lifetime for laser treated samples caused by reduced surface recombination. These changes occurred with only an annealing time of 20s. X-ray diffraction studies indicate an improvement in structural quality of CdTe via a reduction in stacking fault defects, both with and without CdCl₂. Further, a reduction in non-radiatve defects of the laser treated samples is observed which is inferred from increased luminescence yield of the laser treated samples in low temperature spectrally resolved photoluminescence spectroscopy. Laser assisted chlorine treatment of CdTe samples (with 40s dwell time) show the evolution of the A&’ center peak (1.45eV) in the PL signal which is associated with chlorine activation of the sample. This study demonstrates that CW lasers can be used for post-deposition treatment of CdTe and can be achieved in tens of seconds (20-40s) as compared to hours required by conventional thermal treatment of CdTe.

The development of thin-film photovoltaics has largely focused on alternative absorber materials, while the choices for other layers in the solar cell stack have remained somewhat limited. In particular, cadmium sulfide (CdS) is widely used as the buffer layer in typical record devices utilizing absorbers like Cu(In,Ga)Se₂ (CIGSe) or Cu₂ZnSn(S,Se)₄ (CZTS) despite leading to a loss of solar photocurrent due to its band gap of 2.4 eV. While different buffers such as Zn(S,O,OH) are beginning to become competitive with CdS, the identification of additional wider-band gap alternatives with electrical properties comparable to or better than CdS is highly desirable.
Here we use hybrid functional calculations to characterize Cd_xZn_1-xO_yS_1-y candidate buffer layers in the quaternary phase space composed by Cd, Zn, O, and S. We focus on the band gaps and band offsets of the alloys to assess strategies for improving absorption losses from conventional CdS buffers while maintaining similar conduction band offsets known to facilitate good device performance. We also consider additional criteria such as lattice matching to identify regions in the composition space that may provide improved epitaxy to CIGSe and CZTS absorbers. Lastly, we incorporate our calculated alloy properties into simulations of typical CIGSe devices to identify the Cd_xZn_1-xO_yS_1-y buffer compositions that lead to the best performance.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency & Renewable Energy (EERE) through the SunShot Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program.

We performed a comparative study of Cu₂ZnSnSe₄ absorber based solar cells prepared by co-evaporation or sequential processes. The power conversion efficiency of the devices, the temperature dependent current voltage (IVT) and admittance spectroscopy (AS) is compared to the microstructure as measured by scanning electron microscopy and transmission electron microscopy.
The studied devices include absorber layers grown by co-evaporation with the following substrate temperatures during growth and with or without an annealing process (AP):
470 °C
470 °C + AP
320 °C + AP
100 °C + AP
The AP includes a heat treatment at 520 °C with the addition of Se powder and SnSe chunks. The process number 3) is the known CAPRI process ^[1] and detailed studies of IVT and admittance were already published^[2,3]. It was found that the photo and diode current are blocked at low temperatures and correlate with at double capacitance step at these temperatures^[2,3]. This current blocking can be attributed to the series resistance which could arise due to the ZnSe network^[1] or by a back contact barrier^[5]. In addition a deep broad defect distribution was identified by AS which was shown to be detrimental for the V_OC^[4].
The devices from process 1), where the absorber layers were grown by co-evaporation only neither show the current break down, nor the double capacitance step at low temperatures. So far no ZnSe network was detected in these samples, while only ZnSe patches were observed at the back. Moreover, these samples do not show a MoSe₂ layer, which could be the origin of a back barrier. In addition, these co-evaporated samples do not show the detrimental deep broad defect distributions and the best sample could reach a V_OC-deficit of 500 mV with a V_OC of 365 mV and a bandgap of 865 meV.
After annealing of these absorber layers, see process 2), a small MoSe₂ layer forms at the back. The low temperature capacitance step appears as well as the broad deep defect distribution. This effect shows the detrimental effect of the annealing process which is independent of the precursor.
Absorber layers grown by 4) only show efficiencies below 1% and thus were not studied further in detail. A possible reason for bad efficiency might be insufficient crystal structure as revealed by STEM measurements.
[1] M. Mousel, T. Schwarz, R. Djemour, T. P. Weiss, J. Sendler, J. C. Malaquias, A. Redinger, O. Cojocaru-Mirédin, P.-P. Choi and S. Siebentritt, Adv. Energy Mater. 2014, 4, 1300543
[2] T. P. Weiss, A. Redinger, J. Luckas, M. Mousel and S. Siebentritt, 39^th IEEE PVSC, p. 3066-3070, 2013, Tampa
[3] T. P. Weiss, A. Redinger, J. Luckas, M. Mousel and S. Siebentritt, Appl. Phys. Lett. 102, 202105 (2013)
[4] T. P. Weiss, A. Redinger, D. Regesch, M. Mousel and S. Siebentritt, IEEE Journal of Photovoltaics, DOI: 10.1109/JPHOTOV.2014.2358073
[5] O. Gunawan, T. K. Todorov and D. B. Mitzi, Appl. Phys. Lett. 97, 233506 (2010)

All high-performance kesterite based solar cells employ a p-type absorber/ n-type buffer/window heterostructure to form the p-n junction for the separation of light generated electron hole pairs. Up to now, the highest efficiencies are reported for CZTSSe solar cells with CdS buffer layer grown by chemical bath deposition (CBD), reaching certified power conversion of 12.6%. The role of the buffer layer is to provide an optimal conduction band alignment of absorber/buffer/window heterostructure which is crucial for high device performance and a so called small spike like band alignment is favorable. Due to the high toxicity of cadmium and the relatively low band gap of CdS of 2.4eV there is a high interest to replace this buffer layer with a more environment friendly material with higher transparency in the short wavelength region of the solar spectrum. ZnS(O,OH) is a promising alternative to CdS due to its low toxicity, higher bandgap and, in addition, its possibility of tuning the band alignment with the absorber layer depending on the sulfur to oxygen ratio. In this study we present a variation of the thiourea concentration during the CBD of ZnS(O,OH) buffer layers, and show its large influence on device performances and stability of selenide CZTSe/ZnS(O,OH)/i-ZnO/ZnO:Al heterostructure solar cells. Variation of the thiourea (TU) concentration in the chemical bath between 0.5M, 0.4M, and 0.3M results in devices with efficiency of 0.6%, 2.1%, and 5.6%, respectively compared to 6.9% for a CdS reference device. The 0.5M and 0.4M TU devices show strong kinks in the illuminated JV curves indicating the presence of a too high spike in the conduction band alignment as expected for ZnS(O,OH) with a high S/O ratio. Only in the JV curves of 0.3M TU samples no distortions are detected, however a lower Voc than in a CdS reference cell is observed (332mV compared to 401mV, respectively). This loss in Voc could be explained assuming the existence of a cliff like conduction band alignment in this case. Jsc of the 0.3M TU ZnS(O,OH) device is higher than for the CdS case, as it is expected for a higher bandgap buffer. Furthermore, the effects of light soaking (LS) on the different ZnS(O,OH) buffer are investigated. Whereas the 0.3M TU sample is only slightly affected by LS, the two other samples show large efficiency improvements after LS treatments. The LS improves temporarily the distortions of the JV curves, resulting in efficiencies of up to 5.7% for the 0.4M TU device. On the whole, we present the importance of the chalcogen source concentration for tuning the S/O ratio in the ZnS(O,OH) buffer, i.e. band alignment with CZTSe and discuss its large influence on the device performance and stability that opens up opportunities to further optimize Cd free kesterite solar cells with alternative buffers.

This work examines the effects of Trioctylphosphine Sulfide (TOP:S) passivation deposited conformally on the sputtered CuInSe₂ (CIS) thin films on Na diffusion throughout the film thickness. X-ray photoelectron spectroscopy (XPS) analysis of the sample surface and cross-sectional Energy-dispersive X-ray spectroscopy (EDS) measurements demonstrate that the presence of TOP:S pasivation layer enhances the concentration of Na at the film surface by up to 40%. These analyses revealed a distinct concentration gradient of Na along the film height in passivated films that was not present in as-fabricated ones. An increase in the surface Na content in CIS has been previously linked to a significant improvement in device performance.
To investigate the atomic-level mechanism of Na transport through the CIS lattice we employ ab initio density functional theory (DFT) calculations to calculate the formation energies of different defects within the CIS lattice. These simulations reveal that Na-Cu substitution is the most energetically favorable in bulk CIS. Comparison with formation energy at film surfaces that are of different orientation and Cu content show (110) Cu-poor surfaces are the most favorable destinations for Na. The overall lower formation energy for Na-Cu substitution at film surfaces suggests an inherent driving force for Na diffusion towards film surfaces via Cu vacancies. We hypothesize that TOP:S is promoting the equilibrium of Na diffusion in CIS films, resulting in increased Na concentration on passivated film surface.

The greatest opportunity to improve CdTe device efficiency is through increasing the open circuit voltage, which is limited by a combination of low carrier concentration and lifetime in conventional thin film structures. Here we introduce plasma enhanced vapor transport deposition (PE-VTD) as a technique to potentially address these issues. This combines the benefits of VTD, which is more amenable than close spaced sublimation for the introduction of dopants and passivation agents, with a non-equilibrium plasma that provides the energy required to activate precursors that have high formation energies. For example, tellurium, like most Group VI elements, sublimes as a dimer (Te2). Before Te2 can incorporate into the CdTe lattice, it must first break its internal chemical bond. The plasma will be used to dissociate such dimers and alter the chemical potential of the reactants impinging on the deposition surface. The efficiency of these approaches will be evaluated using measures of carrier density, lifetime, and device performance. In this presentation we describe the design and construction of the PE-VTD reactor, which introduces an inductively coupled plasma source between the CdTe source material and the substrate. Ar is used as the carrier gas due to its ability to stabilize the plasma. Initial results compared the material properties of CdTe deposited in this reactor with and without plasma activation. The use of plasma leads to textured growth, as well as significant changes in film morphology. Preliminary device results suggest that use of plasma is beneficial, as device efficiencies were significantly improved over thermal VTD (12 vs. 8%).

Nitride materials have played a key role in many important technological advances in the modern world. Group III-nitrides have revolutionized solid-state lighting and power electronics, and newer, more exotic nitride materials have recently garnered attention for their relatively unstudied, yet promising properties. Of these novel nitrides, ZnSnN₂ is a particularly promising candidate for Earth-abundant, thin film PV absorber applications due to its direct band gap, with predicted tunability from 1-2 eV depending on cation ordering. To date, development of ZnSnN₂ into a useful optoelectronic material has been hindered by high free electron concentrations on the order of 10²⁰ cm^-3. Such high carrier concentrations are likely caused by a combination of poor nitrogen incorporation due to the inert nature of molecular nitrogen, oxygen incorporation, Zn interstitials, and Sn_Zn point defects due to the higher volatility of Zn.
In this work, we present a novel method for facile synthesis of ZnSnN₂ thin films at a wide range of deposition conditions via combinatorial reactive sputtering using a nitrogen plasma source. This novel deposition method has resulted in fast deposition rates, up to 550 nm/hr, and films with dense microstructure and smooth surfaces as determined by SEM. Complementary TEM images and SAED patterns reveal columnar grains and polycrystalline growth with evidence of preferential orientation. Preliminary EBSD analysis has yielded Kikuchi patterns indexed with very high coincidence using a hexagonal lattice. Moreover, calculated interplanar spacings based on SAED suggest a wurtzite crystal structure with fully disordered cation sub-lattice, which is in agreement with previous works. With this approach, we are able to achieve ZnSnN₂ thin films with the lowest free electron concentration reported to date, 3 x 10¹⁸ cm^-3 at a 10% zinc-rich composition, and mobility on the order of 10 mu;V/K. An inverse relationship between carrier concentration and mobility is observed, indicating ionized impurity scattering as opposed to scattering from grain boundaries or lattice vibrations. Additionally, 10% zinc-rich samples display temperature-activated conductivity, which underscores our claim of non-degeneracy. These films also display strong absorption onset with no free carrier absorption below the bandgap. Furthermore, we have compelling evidence in support of a Burnstein-Moss shift contributing to the wide range of reported bandgaps for this material. Estimated bandgaps in our material range from 1.4 to 2.1 eV depending on cation composition. Along with these exciting results, a discussion of the current and future challenges facing ZnSnN₂, such as small grain size and device integration concerns, will also be included. We believe our demonstration of facile synthesis leading to the findings discussed above reaffirm the promise of ZnSnN₂ as an Earth-abundant PV absorber.

Solution processed kesterite-based Cu₂ZnSnS₄ (CZTS) absorbers have emerged as a potential candidates due to the high power conversion efficiencies of 12% up to date. One of the major challenges in CZTS solar cells is due to their significant numbers of intrinsic defects which cause strong carrier recombination and limit the photo-generated carrier transport capability. Based on this concern, defect engineering of CZTS absorbers governs an important role for improving the device performance. In this study, sodium-assisted post treatment was performed on top of CZTS thin films to manipulate defect level energies, the corresponding carrier transport dynamics and device performances were also comprehensively studied. Using admittance spectroscopy and impedance measurements, we observed that sodium quantity has significant effects on the energetic distribution of defect states or traps within CZTS thin films, which have substantial impacts on the carrier transport properties. Devices with an optimized sodium post treatment shown a prolonged electron lifetime from 0.9 us to 1.8 us benefited from the elimination of deeper defect states. Our results also shown that sodium segregated near the top of CZTS thin films improved p-n junction properties by providing a negative potential for attracting electrons and repelling holes, which hence reduced carrier recombination rate and improve photo-generated carrier extraction and transportation. By optimizing the content of sodium cooperation in post annealing process, our champion device achieved a power conversion efficiency of 5.6% which is about 37% enhancement compared to the devices without sodium-assisted post treatment. The significance of sodium assisted post treatment on CZTS based photovoltaics can be attributed to the capability of passivating the deeper defect states and improving carrier transport behaviors, which ultimately enhance the photovoltaic performance.

Because the diffusion length of photo-generated minority carriers in thin film solar cells is shorter than the width of the absorber the carrier recombination mechanisms will control the electronic transport of the solar cell under forward bias. Since these mechanisms have a strong impact on the solar cell efficiency, their identification is of utmost importance for the further development. It is well known that different recombination processes can contribute to the total recombination current [1] and different parallel parasitic current pathways can be present [2]. Consequently the description of the dark I-V characteristics for such structures involves a lot of parameters. Analysing the thermal activation of the processes can help to elucidate the mechanism that is limiting the solar cell performance.
In this work we demonstrate how to identify the dominant mechanism via regression analysis. Rival models corresponding with alternative recombination mechanisms were assessed against experimental data acquired on three different materials, i.e., Cu(In,Ga)Se₂ (EMPA Switzerland) solar cells, Cu(In,Ga)Se₂ absorbers with an aluminium Schottky barrier and on IV characteristics generated with SCAPS simulations [3].
It is shown that the shunt current is mainly determined by tunneling enhanced recombination [4] and the space charge limited current [5] while no strong influence of an ohmic shunt for both solar cells and Mo/CIGS/Al Schottky barriers could be observed.
[1] U. Rau, A. Jasenek, H. W. Schock, F. Engelhardt, and Th. Meyer. “Electronic loss mechanisms in chalcopyrite based heterojunction solar cells” Thin Solid Films, 361:298-302, 2000
[2] B. L. Williams, S. Smit, B. J. Kniknie, N. J. Bakkers, W. M. M. Kessels, R. E. I. Schropp, M. Creatore “Identifying Parasitic Current Pathways in CIGS Solar Cells by Modelling Dark JV Response”, Presented at 29th European Photovoltaic Solar Energy Conference and Exhibition in Amsterdam, 2014
[3] M. Burgelman, P. Nollet and S. Degrave, "Modelling polycrystalline semiconductor solar cells", Thin Solid Films, 361-362, 527-532, 2000
[4] V. Nadenau, U. Rau, A. Jasenek, and H. W. Schock. “Electronic properties of CuGaSe2 - based
heterojunction solar cells. Part I. Transport analysis” J. Appl. Phys., 87:584 -593, 2000
[5] A. A. Grinberg, S. Luryi, M.R. Pinto, and N.L. Schryer “IEEE transactions on electron devices, 36 (6), 1162-1170 (1989)

Cu₂Zn(Sn,Ge)S₄ (CZTGS) compounds are promising candidates for absorber layers in thin-film solar cells. These materials are derived from the better known Cu₂ZnSnS₄ (CZTS) ^[1] compounds by partial substitution of Ge for Sn. CZTS has a band gap of around 1.5 eV ^[2] and a large absorption coefficient (> 10⁴ cm^#8209;1) ^[3] which are similar to those of Cu(In,Ga)Se₂ (CIGS), one of the most successful thin-film photovoltaic materials. In contrast to CIGS, which requires relatively rare and expensive In and Ga, CZTGS is composed of earth abundant elements ^[4]. The Ge harr; Sn substitution allows tuning of the band gap. Another degree of freedom becomes available when CZGS crystallites are prepared in the nanometer region where optical and electrical properties can easily be changed by size variation. This is enabled by the synthesis of CZTGS nanocrystal dispersion ^[5] from solution based processes ^[6]. The resulting so called “solar paint” can be directly printed, sprayed or dip-coated without cost- and energy-intensive postdeposition processing.
CZTGS nanocrystal samples with different Sn:Ge ratio were prepared by the hot-injection method similar to the procedure described in ref. ^[5]. TEM images showed predominantly round shaped particles but also a fraction of hexagonal shaped particles. Number weighted means of the particle size were in a range of around 15-30 nm. The elemental distribution was found to be homogeneous in the investigated areas by TEM-EDX. XRD measurements revealed the presence of at least two structurally different CZTGS phases (tetragonal kesterite/stannite and hexagonal wurtz-kesterite/wurtz-stannite). Distinction between kesterite-type and stannite-type structure (which differ by the ordering of Cu and Zn ^[7]) is not possible at this state due to the low crystallinity of the samples. Thermal annealing of the samples is expected to increase the crystallinity and to form single phase (tetragonal) kesterite ^[8] which is suggested to lead to the most efficient light conversion. The mentioned features like particle size, elemental distribution, structure and also opto-electronic properties were furthermore investigated and discussed with respect to various synthesis parameters (Sn:Ge ratio, metal ratios, sulfur source, synthesis temperature).
[1] H. Katagiri, Thin Solid Films 2005, 480, 426.
[2] K. Ito et al., Jpn. J. Appl. Phys. 1 1988, 27, 2094.
[3] H. Katagiri et al., Jpn. J. Appl. Phys. 1 2001, 40, 500.
[4] S. Siebentritt et al., Prog. Photovoltaics 2012, 20, 512.
[5] G. M. Ford et al., Chem. Mater. 2011, 23, 2626.
[6] Q. J. Guo et al., J. Am. Chem. Soc. 2009, 131, 11672; S. C. Riha et al., J. Am. Chem. Soc. 2009, 131, 12054; C. Steinhagen et al., J. Am. Chem. Soc. 2009, 131, 12554.
[7] S. Schorr, Sol. Energ. Mat. Sol. C. 2011, 95, 1482.
[8] X. Z. Lin et al., RSC Adv. 2012, 2, 9894.

Cu₂ZnSnS₄ (CZTS) as a promising low cost and environmentally friendly p-type light absorber material has recently attracted significant attention for applications in thin film photovoltaic devices. CuZnSn (CZT) film is normally the essential precursor for formation of CZTS film in number of deposition techniques. Among them, electrochemical deposition is considered as a facile, scalable and low cost technique. Generally speaking, there are two kinds of strategies to prepare CZT thin film via electrochemical deposition method: (i) stacked Cu/Sn/Zn elemental layers (SEL) are deposited from separated electrolyte solutions and (ii) CZT film is synthesized by a co-electrodeposition where all three metal elements are deposited together. Compared to the SEL method, the co-deposition approach is more favorable for time saving and high throughput production of CZTS films from a practical application point of view.
It is well known that CZTS film with copper poor and zinc rich content is the most effective composition for the high performance solar cells. Therefore, it is crucial to control the content of copper, zinc and tin in the electrodeposited CZT precursor film in order to fabricate CZTS films with desired composition. This can be easily achieved by controlling separately for the deposition potential, current, and duration or each metal cation concentration in the SEL electrodeposition. However, it is very challenging for the co-electrodeposition method because the three metal cations are mixed in an electroplating solution and they have different reduction potentials and deposition rates.
In this work, CuZnSn (CZT) films were co-electrodeposited on the Mo coated lime glass from electroplating solutions containing copper, tin and zinc salts and a complexing agent. The study on the concentration-dependent charge density consumed in the co-electrodeposition has indicated that the co-electrodeposition process of the CZT film was controlled by diffusion related mass transfer. Moreover, copper tin and zinc contents in the CZT thin film could be controlled by adjusting the concentrations of the precursor salts in the electroplating solution. Both SEM morphology and EDS mapping results confirmed the uniformity of the CZT and CZTS films and the homogeneous distribution of all elements in the films. By optimization of CZT film composition, a copper poor and zinc rich CZTS film has been synthesized. The XRD and Raman spectra indicated that the synthesized CZTS film was kesterite phase without impurities detected. Finally, a Cu₂ZnSnS₄ (CZTS) solar cell showed an energy conversion efficiency of 2.15% under the illumination intensity of 100 mW/ cm².

Among the many types of thin film solar cells, Cu(In,Ga)Se₂ thin film solar cells are considered important, due to their high visible light absorption. The typical structure of CIGS based thin film solar cell consists of multiple layered stacks of bottom electrode, p-type CIGS absorber, n-type buffer layer, n-type intrinsic ZnO, Al-doped ZnO, and top electrode. Due to this stacked structure, it is very imperative to optimize the electronic structure of the interface and surface of each layer from which the charge recombination at these interfaces can be minimized, which acts as one of the major factor in the decreased power conversion efficiency (PCE). In particular, the electronic band structure of CIGS-buffer layer interface is considered to be determining factor in improved PCE. Therefore, a systematic spectroscopic analysis is needed to construct the band alignment of interfaces based on conduction band offset (CBO) and valence band offset (VBO) measurements, which in turn can provide important guidelines to improve PCE of CIGS based solar cells.
Changing the oft-used CdS buffer layers and the use of Cd-free buffer layers is being actively explored due to toxity of Cd, and Zn(O,S) is considered as one of the main alternative buffer materials. At the interface of CIGS and Zn(O,S), ordered vacancy compound (OVC) in CIGS surface and S contents in Zn(O,S) are the significant factors in controlling CBO. The conductivity in OVC, which is In-rich CuIn₃Se₅ subphase of CIGS surface, is controlled by whether the type of major vacancy is either Cu acting as acceptor or Se acting as donor. In order to achieve CBO at 0-0.4 eV, at which the carrier recombination is minimized and high PCE is attained, both electronic structures of CIGS and Zn(O,S) surfaces should be investigated.
In this report, in an effort to correlate PCE of CIGS based solar cell to electronic structures, we investigated (1) electronic structures of CIGS depending on In-final and Ga-final process and (2) Zn(O,S)/CIGS interfacial band alignment and CBO using various spectroscopic analysis such as XPS, UPS, UV-Vis spectrometer, and CL. Our studies revealed that, a rapid change of Cu content and binding energy at the near-surface of CIGS and Ga content affects the increase in CIGS bandgap. Based on the extracted bandgap of CIGS from CL, Zn(O,S) from UV-vis measurements and XPS valence band edge analysis on CIGS/Zn(O,S) interface, we suggest that CBO value at Zn(O,S)/CIGS interface is a function of S content and is a crucial factor in determining the solar cell efficiency as high as (~14.6%) or no PCE (0%) in a reference to 20% of Sulfur.

Using cost-effective, safe, and easily handled materials, a thin film solar cell comprised of polycrystalline Cu₂ZnSnS₄ (CZTS) and related materials shows high conversion efficiency. Accordingly, significant efforts have been devoted to fabricating CZTS solar cells. However, only a few degradation studies have been conducted on the CZTS thin film and/or solar cell, which has resulted in limited knowledge of both the mechanisms responsible for “how to degraded” or “what part is weak” the CZTS solar cells. Researchers have gradually come to realize that the mechanism governing solar cell degradation is a complex phenomenon. For example, the performance of Cu(In,Ga)Se₂ (CIGS) solar cells is known to be influenced by environmental conditions, such as damp heat or light soaking. Our group previously studied the degradation properties of CIGS solar cells and thin films using alpha-ray, proton, gamma-ray, and electron irradiation. We found that particle irradiation tended to cause degradation primarily around the interfaces comprising CIGS solar cells. These results suggest that the interfaces play an important role in defining solar cell performance.
From a scientific perspective, investigating the degradation mechanism of CZTS-based solar cells is crucial. Similarly, from an industrial perspective, this investigation is necessary for determining the long-term reliability of CZTS solar cells before these cells are applied commercially. In this study, the effects of electron irradiation on the properties of CZTS-based solar cells are examined by irradiating each thin film comprising the solar cell&’s structure.
Conventional CZTS solar cells with a Mo/CZTS/CdS/ZnO:Al structure were prepared on soda lime glass substrates. The average efficiency of solar cell was about 3%. The irradiating electron energy was fixed at 2 MeV, and the fluence was varied between 1 × 10¹³ and 2 × 10¹⁷. All irradiated CZTS solar cells exhibited the same PL peaks, which originated from donor-to-acceptor transitions. Although the PL intensity decreased with increasing irradiation fluence, neither a new peak nor a change in the relative peak intensity ratio was observed. The efficiency and Jsc decreased dramatically for 1 × 10¹⁶-cm^minus;2 irradiation. These results may indicate that several heterojunction interfaces tend to degrade in response to electron irradiation. However, Voc remained constant at approximately 2 × 10¹⁷ cm^minus;2. In general, the efficiency and Jsc of Si solar cells decrease by about 1 × 10¹⁴ cm^minus;2 when exposed to identical type of irradiation. Therefore, this result implies that, like a CIGS solar cell, a CZTS solar cell has excellent radiation tolerance and durable materials. These results are the first step toward realizing practical applications for CZTS-based solar cells in space and toward clarifying their degradation mechanism

In recent years, the interest in solar cells based on kesterite materials has increased as non-toxic, earth abundant, alternatives to those based on CdTe and CIGS. Efficiencies up to 12,6% [1] have been demonstrated for Cu₂ZnSn(S,Se)₄ (CZTSSe) based devices. To produce high efficiency kesterite devices requires a Cu-poor and Zn-rich absorber composition [2]. However, further investigations are required to understand the relationship between compositional change in the Cu-poor Zn-rich region and the properties of the absorber layers and the JV characteristics.
Absorber layers were synthesized by reactive thermal annealing of Cu-Zn-Sn sputtered metallic precursors with three different [Cu]/[Zn+Sn] ratios of 0.65, 0.73 and 0.90. The [Zn]/[Sn] ratio was kept constant around 1.1. The precursors were selenised at 500°C in a rapid thermal processing (RTP) system and resulted in absorber layers with [Cu]/[Zn+Sn] ratios ranging from ~0.7 to ~1 as measured by X-ray fluorescence (XRF). The crystal structure was investigated using X-Ray diffraction (XRD) showing that the tetragonal CZTSe phase was formed for all the compositions studied and no secondary phases were detected within the resolution of XRD. Raman spectra of the samples showed the presence of the modes characteristic of CZTSe. As the Cu content is decreased, the relative intensity of the modes in the frequency region around 170 cm^-1, in relation to the main mode at 197 cm^-1, decreases indicating an increase of defect concentration.
The solar cells produced from the absorber layers with different Cu content were characterized using external quantum efficiency (EQE) and current density-voltage (J-V) measurements. J-V results showed a dependence of the V_oc values on the Cu content, increasing from 367 to 434 mV as the Cu concentration decreased. The best solar cell was based on the absorber layer with the lowest Cu content and yielded an efficiency of 8.1% with a fill factor of 59.8 %, J_sc = 31.1 mA/cm² and V_oc = 434 mV. This V_oc value corresponds to a V_oc deficit ( E_g/q-V_oc; E_g=1.0eV extracted from EQE) of only 566 mV, which is lower than that of current record devices with efficiencies over 12%, which show a V_oc deficit of 593mV [1].
In this work we demonstrate the importance of the Cu content in CZTSe absorbers for high performance devices, and especially its strong influence on V_oc. Possible origins of the strong V_oc dependence on the Cu content, such as defect concentration variations observed in the Raman spectra, will be discussed. This could be a key parameter to overcome the presently high V_oc deficit in kesterite based solar cells.
[1] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, W. Wang, H. Sugimoto, D.B. Mitzi, High Efficiency Cu ZnSn(S,Se) Solar Cells by Applying a Double In S /CdS Emitter, Advanced materials, (2014).
[2] S. Siebentritt, Why are kesterite solar cells not 20% efficient?, Thin Solid Films, (2013).

The enhancement of photo-carrier collection, aided by a drift field, is a promising route for approaching the fundamental limits in photovoltaic (PV) performance, while maintaining a simple device design. In a drift-aided cell the absorber layer must be very thin, exhibit an absorption coefficient α > 10⁵ cm^-1, and an abrupt absorption onset at the band gap. Few inorganic materials exhibit this type of superabsorption. To enhance the conventional Shockley-Queisser model for materials selection and design, we have recently introduced a new metric, Spectroscopic Limited Maximum Efficiency (SLME), which incorporates the full absorption spectrum of the absorber. By applying SLME to Cu-V-VI systems, new materials design principles have been formulated to guide the design and selection of absorbers for use in drift-aided cells. Here we show that synthetic mineral tetrahedrite compounds, typified by Cu₁₂Sb₄S₁₃, exhibit exceptionally high absorption, as guided by these important principles. In addition, optical and electrical properties have been tuned via substitution of Zn, In, Te, and Se. Two materials derived from the tetrahedrtie family, Cu₁₀Zn₂Sb₄Se₁₃ and Cu₁₀Te₄S₁₃, exhibit maximum absorption coefficients > 10⁵ cm^-1 at E_G + 0.2 eV (E_G = 1.4 eV). From device simulation, these materials are predicted to enable high-efficiency (> 20 %) Thin-film solar cells (TFSCs) in a drift-based operation mode with absorber layers as thin as 250 nm. This family of compounds thus open new opportunities for tuning band gaps and electrical properties in pursuit of a new generation of single-junction and tandem TFSCs containing ultra-thin absorber layers.

Recently, tin monosulfide (SnS) was shown to be a promising alternative p-type absorber material to conventional Cu(In,Ga)(S,Se)₂ (CIGS) and CdTe because of its Earth abundancy, non-toxicity, suitable band gap of 1.1-1.5 eV, and high absorption coefficient of above 10⁴ cm^-1. Furthermore, SnS is a binary compound that involves simpler growth chemistry compared to Cu₂ZnSn(S,Se)₄, which is another Earth-abundant substance investigated for use as an absorber material.
Despite these promising properties, the conversion efficiency of SnS solar cells is still low. One reason for the low conversion efficiency may be the influence of defects existing around the pn interface of SnS solar cells. Surface inhomogeneity and the existence of extra phases affect electrical properties around the pn interface and depletion layer. Although interfaces are known to play an important role in carrier transfer/recombination processes and solar cell properties, there are few reports on their influence on the performance of SnS solar cells.
Electrochemical impedance spectroscopy (EIS) analytic theory is being increasingly applied as an analytical tool in chemical materials research and has been shown to be invaluable for studies of polycrystalline semiconductor materials and devices such as CIGS-, CdTe-, and Cu₂ZnSnS₄-based solar cells. To improve the performance of SnS solar cells, we investigated defects around the pn interface of SnS solar cells using EIS, which is a simple and promising method for determining pn-interface properties such as uniformity, defect characteristics, and carrier-recombination time.
Sulfur-premixed Sn precursors were prepared using the RF sputtering method on a soda-lime glass substrate. The precursors were then sulfurized using S vapor in a quartz-tube reactor. The sulfurization temperature and time were 200-400°C and 40-120 min, respectively. We present the relationship between the sulfurized precursors and defect properties of SnS films. On using sulfurized S-premixed Sn precursors, Sn droplets on the surface of SnS thin films disappeared, and SnS thin films with a flat surface were obtained. Further, we investigate the relationship between defects around pn interface measured using EIS and the conversion efficiency of SnS solar cells.

Recently, BaCuQF (Q=S, Se, Te) were found to have wide band gap energies and show high p-type conductivities [1]. We deposited BaCuSeF films for solar cells applications by Pulsed laser deposition (PLD) [2] and characterized their optical and electrical properties [3]. BaCuSeF film deposited at substrate temperature (Ts) of 300°C showed high conductivity of 24 S/cm. Then, BaCuSeF films were deposited on back side of CdS/CdTe solar cells [4] at low substrate temperatures. The CdS/CdTe solar cells with BaCuSeF back contact deposited at Ts=200°C showed a highest conversion efficiency of 3.2%. [3]. In this study, we fabricated BaCuSF films for solar cells applications by PLD and characterized their optical and electrical properties.
BaCuSF films with thickness of 500-1000 nm were prepared by PLD. The BaCuSF ceramic target was fabricated by a solid-state reaction of BaF₂ (99.9%), BaCO₃ (99.9%) and Cu₂S (99.9%) powders at 800^oC in 5% H₂S/N₂ gas atmosphere. A KrF excimer laser with a wavelength of 248 nm was focused on the ceramic target and the BaCuSF films were deposited on borosilicate glass at Ts = 150°C - 400°C. Crystallographic analyses were performed by X-ray diffraction (XRD). Optical properties were characterized by UV-VIS-NIR spectroscopy and electric properties were evaluated by the van der Pauw method.
All the BaCuSF films did not show clear diffraction peak. Therefore, BaCuSF films were considered to be amorphous or microcrystalline. The transmittance of the films increased with increasing the deposition temperature (Ts) up to 400°C. The maximum transmittance at visible region of about 60% was obtained for the film deposited at Ts=400°C. The band gap energy of the film was estimated to be 3.1 eV. All the BaCuSF films showed p-type conductivity. The maximum conductivity of 30.4 S/cm was obtained for the film deposited at Ts=150°C, while the conductivity of the film deposited at 400°C was 3 orders of magnitude smaller than the maximum value. Cu vacancies in BaCuSeF are considered to contribute to their p-type conductivity [4]. We will fabricate CdS/CdTe solar cells with BaCuSF back contact.
Acknowledgements
#12288;A part of this work was supported by the In-corporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).
References
[1] H. Yanagi et al., J. Appl. Phys. 100, 083705 (2006).
[2] M. Yoshikawa et al., Jpn. J. Appl. Phys. 51, 10NC40 (2012).
[3] K. Yamamoto et al., Jpn. J. Appl. Phys. 53, 05FX02 (2014).
[4] T. Okamoto et al., Jpn. J. Appl. Phys. 52, 102301 (2013).
[5] Sakakima et al., to be presented at WCPEC-6 (2014).

We investigated the characteristics of Ag nanowire (NW) network covered by graphene sheet for use as flexible and transparent electrodes. To correlate the number of graphene layer and performance of graphene/Ag NW hybrid electrodes, we prepared Ag NW network covered by monolayer of graphene and double layers of graphene sheet, respectively. Compared to sheet resistance (95.41 Ohm/square) of Ag NW network, the Ag NW network covered by graphene sheet showed lower sheet resistance of 85.88 Ohm/square (graphene monolayer) and 84.03 Ohm/square (graphene double layer) because the graphene on Ag NW network effectively connected the conduction path in Ag NWs. It was noteworthy that the decreased sheet resistance of Ag NW network covered by graphene sheet could be attributed to increase in carrier mobility. In general, the graphene layer has high mobility, graphene covered Ag NW network showed much higher mobility than Ag NW network. The Ag NW networks electrodes covered by monolayer and double layer of graphene exhibited a mobility of 210.75 cm²/V-sec and 112.75 cm²/V-sec, respectively. In addition, an optical transmittance (87.47 %) of Ag NW network covered by monolayer of graphene is higher than that (84.51 %) of Ag NW network covered by double layer of graphene. In addition, the graphene covered Ag NW network electrode showed a similar mechanical flexibility to Ag NW network electrode due to high strain failure of graphene sheet. Therefore, Ag NW network covered by graphene sheet is a promising hybrid transparent electrode with low sheet resistance, high optical transmittance and good mechanical flexibility apt for flexible organic light emitting diodes and organic solar cells.

Cu₂ZnSnS₄ (CZTS) monograins approach is considered to be a potential candidate for the future low-cost production of solar panels. For CZTS solar cell, CdS deposited by chemical bath deposition (CBD) is still the most efficient buffer layer with 12.6 % solar cell efficiency (reported by IBM). However, the development of a Cd-free buffer layer is highly desired in view of scaling-up production of CZTS solar cells due to the harmfullness of Cd.
A sulfur-rich Zn(O,S) buffer layer is suitable for 1.4 - 1.5 eV band gap absorber material such as CZTS. Furthermore the band gap of sulfur-rich Zn(O,S) is wider than CdS and conduction band edge is higher compared to CdS and ZnO. For finding the optimum conduction band offset for CZTS/Zn(O,S) monograin solar cells it is important to precisely control the oxygen to sulfur ratio. In this work we have employed atomic layer deposition (ALD) for growing 10 - 30 nm thin buffer layers. The effect of S/(S+O) pulse ratio in ALD Zn(O,S) and thickness of ALD ZnS buffer layer was investigated and compared to current voltage measurements (I-V) and external quantum efficiency (EQE) results of respective CZTS monograin layer solar cells.
No clear effect to solar cell jV parameters was observed in case of same thickness of the buffer layer with various S/(S+O) content, except for the S/(S+O) = 1 (ZnS), which resulted in drop of all jV parameters. By theory, ZnS/CZTS junction has too high positive conduction band offset (spike) resulting in barrier to current flow. Decreasing ZnS film thickness from 25 nm to 15, 10 or 5 nm improved the jV results to same level as rest of S/(S+O) ratios in Zn(O,S) buffer layers. EQE results showed highest quantum efficiency for ZnO and lowest for ZnS buffer layers in blue light region.
The Zn(O,S) buffer layer for CZTS monograin solar cell, with S/(S+O) = 0 (ZnO), showed an efficiency of 5.4 %, open-circuit voltage of 692 mV, and fill factor of 52.9 %. CZTS with ZnO buffer layer has improved all jV parameters compared to reference CdS buffer, which resulted in 4.4 % solar cell efficiency. Those results do not match fully with results found in literature, probably due to smaller buffer layer thicknesses are used in current studies and in case of thinner buffer layer the conduction band offset theory is not pre-dominant mechanism.

In this work, the quaternary system AgGa_xIn_1-xTe₂ (AGIT) have been worked out as a new alternative instead of Cu based chalcopyrite (CIS,CIGS) since it allows tailoring of the optical band gap and other properties. In order to investigate the thin film hetero-junction behavior of the AGIT thin films, ZnInSe₂ (ZIS) thin films is fabricated to get the In/n-ZIS/p-AGIT/ITO sandwich structure. p-AGIT polycrystalline thin films have been deposited on ITO coated glass substrate by the combination of e-beam and thermal evaporation, sequentially in the same deposition system without interrupting the vacuum conditions. To obtain the hetero-structure, n-ZIS thin film have been deposited on the AGIT thin films then In dot front contacts were evaporated to complete device structure. Dark temperature dependent Current-Voltage (I-V) measurements and I-V measurements under illumination were carried out. Additionally the room temperature Capacitance-Voltage (C-V) measurements were performed to understand the device characteristics of this sandwich structure. The series and shunt resistances were calculated by the help of parasitic resistance analysis for forward and reverse bias voltages, respectively. The dominant conduction mechanisms were investigated for different temperature and bias regions.

The absorbing kesterite material, Cu₂ZnSn(S_xSe_1-x)₄ (CZTS), is very promising for future thin film solar cells. The material is non-toxic, the elements abundant, and it has a high absorption coefficient. These properties make CZTS a potential candidate also for large-scale applications. Here, solution processing allows for comparatively fast and inexpensive fabrication, and also holds the record efficiency in the kesterite family. Unfortunately, the record cell is deposited with a highly toxic solvent, hydrazine. This toxic solvent can be avoided through the nanocrystal ink approach, but to maintain good control of the nanocrystal formation during the synthesis, it is necessary to have organic ligands on the surface of the particles. These ligands are often long alkyl chains that potentially limit the quality of the film and degrade its electronic properties.
For nanocrystal solution processing to be a feasible fabrication route in the future, the amount of carbon in the film has to be limited. Today, several methods are employed in order to surpass this barrier, for example ligand exchange. A successful ligand exchange was carried out by Carrete et al. [1], where they replace the organic ligands by an antimony sa< however the efficiency is 1.4% for a cell annealed in Se-atmosphere.
In our work, we try to limit the carbon amount in the film by synthesizing larger nanoparticles. The bigger the particles are the smaller surface-to-volume ratio they have, which might decrease the amount of ligands necessary to stabilize the particles in solution. Today, CZTS nanoparticles synthesized through the so-called hot-injection method vary between 2 nm and 60 nm in diameter. In our group, we have synthesized particles larger than 200 nm. Transmission electron microscopy (TEM) allows us to image the faceted/hexagonal nanoparticles and determine their individual composition.
Densification of the film will also improve the film-quality. The optimal packing density will be calculated, and size-selective methods can be carried out in order to try to isolate the desired particle sizes. Films will be deposited through wet-chemical means, e.g. doctor-blading, spin-coating and spray-coating. The annealing time required can be minimized when starting with larger nanoparticles, and thus the elemental losses associated with annealing at higher temperature reduced. The films are characterized by TEM and scanning electron microscopy (SEM) as well as other surface characterization techniques.
A photovoltaic device of the structure soda lime glass (SLG)/Mo/CZTS/CdS/ZnO is built, and the power conversion efficiency will be determined. Our first CZTS solar cell made from doctor blading of approx. 20 nm Cu₂ZnSnS₄ nanoparticles in octanethiol, annealed in Se-atmosphere, had an efficiency of 1.4%.
[1] Carrete, A., Shavel, A., Fontane#769;, X., Montserrat, J., Fan, J., Iba#769;n#771;ez, M., Saucedo, E., Pe#769;rez-Rodríguez, A., Cabot, A.; J. Am. Chem. Soc. 2013 (135), 15982-1598.

Kesterite based solar cells (Cu,Zn,Sn,S,Se:CZTS) are an alternative to other thin film photovoltaic semiconductors. Materials used for their synthesis are not dangerous and abundant, leading to a low cost sustainable photovoltaic technology. Encouraging results have been achieved by different groups in a short time, efficiency keep climbing up to 12.7%. Nevertheless, there is much work to be done for controlling the process of fabrication of CZTS layers to push-up the efficiency values towards those obtained for conventional photovoltaic technologies.
One of the fundamental aspects for the improvement of the efficiencies is the formation and control of the junction interface. This particular region is very difficult to analyze, since is formed by few layers of atoms where the absorber and the buffer layer are in contact. By electrical measurements it is easy to assess whether the junction is functioning or not, but from a process point of view, it is interesting to have knowledge of the mechanisms that lead to a good working junction. The formation of intermediary layers as it has been proposed for the other thin film technologies (OVC for CIGSe or CdS_xTe_1-x for CdTe) can be a mechanism for efficiency improvement also for kesterites, but it is not straightforward to characterize.
While main used optical techniques such as Photoluminescence (PL) and Raman scattering rely in the absorption of light and subsequent reemission of photons, which need a certain amount of material to interact with the light, reflectance based techniques rely in the presence of an interface that can effectively modify the reflection of light. This means that a few nanometer layer is easier to detect with a reflectance measurement than with an absorption/emission technique. Photoreflectance (PR) is a contactless technique that allows the characterization of a semiconductor and the properties of the junction it forms. The PR spectrum is defined as the relative change of the probe reflection, DR/R, induced by the laser pump. Because of its derivative-like features, the spectrum is sharp and very sensitive to fundamental and higher optical transitions of semiconductors such as band transitions.
CZTSSe samples were obtained by reactive annealing under Se+Sn atmosphere of sputtered metallic stacks. The absorbers were prepared with the composition that allows to obtain high efficiency solar cells: Cu/(Zn+Sn) ~ 0.7-0.8 and Zn/Sn ~ 1.15-1.25. Solar cells were prepared using the standard structure and processes, giving efficiencies around 3-8%. Simultaneous PR and PL measurements on the different solar cells have been performed in all the samples. For high efficiency cells, it has been observed a secondary transition at 1.4 eV that cannot be assigned to any secondary phase. This can be explained by the formation of the S-Se solid solution Cu₂ZnSn(S_xSe_1-x)₄ due to sulphur diffusion from the CdS layer. Finally, the possible formation of CdSe layers has been also explored.

Rapid annealing of coatings cast from nanocrystal dispersions to form polycrystalline thin films has the potential to decrease the production cost of thin film solar cells by both increasing throughput and by enabling manufacturing on flexible substrates. Intense pulsed light (IPL) annealing is one possible alternative to current thermal processes which typically require heating to high temperatures (> 500 °C) for 30 to 60 minutes. IPL annealing utilizes high intensity electromagnetic radiation pulses produced by a xenon flash lamp with wavelengths ranging from the ultraviolet to the infrared. Energy densities are adjustable from 3 to 15 J/cm² with flash durations less than 4 milliseconds. We used IPL to heat coatings cast from copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) nanocrystal dispersions onto molybdenum-coated soda lime glass (Mo-coated SLG), quartz, and molybdenum foil. CZTS is an emerging solar absorber comprised of earth-abundant elements for low-cost sustainable thin film solar cell production. However, little is known about the effects of IPL annealing on the coating integrity, microstructure development, and chemical stability of CZTS nanocrystal coatings. To further understanding, we studied the effects of the number of flashes, flash duration, nanoparticle size, and substrate on the structure and composition of the films. The films were characterized using a battery of methods that included x-ray diffraction, electron microscopy, and Raman spectroscopy. The coatings were sealed in evacuated quartz ampoules with or without sulfur to ensure a well-controlled environment. At low intensities, the CZTS coatings cracked with no other noticeable physical changes. Increasing the intensity to 9-13 J/cm² caused the coatings to develop raised areas reminiscent of blisters. The blisters were accompanied by cracking, localized coating delamination, and decomposition. The source of blister formation was linked to the decomposition of CZTS at the molybdenum-CZTS interface. Specifically, we identified secondary phases within the blisters through imaging with confocal Raman spectroscopy. CZTS coatings on quartz substrates developed cracks after IPL annealing but exhibited no blisters, further evidence that blistering is due to molybdenum-induced decomposition. Replacing the Mo-coated SLG with Mo foil shifted the onset of blistering and material decomposition to higher energy densities and longer annealing durations. The introduction of sulfur vapor (>50 Torr) during annealing prevented the decomposition, blistering and cracking in CZTS coatings on Mo foil. Coatings comprised of 5 nm nanocrystals could be transformed into thin films with large grains easier than coatings comprised of larger nanocrystals (e.g., 35 nm). Finite element modeling was used to complement the experiments. The simulations show that the substrate thermal properties have a significant effect on the temporal evolution of the CZTS layer temperature during IPL annealing.

The conversion efficiency of Cu₂ZnSnS₄ (CZTS) based solar cells significantly depends on deposition conditions and specifically the composition of the material. It is known that the chemical potential of the constituent elements during deposition plays an important role in the formation of point defects, because it directly affects their formation energy. In most processes the chemical potentials during growth are a result of the composition of the deposited CZTS. Although best material qualities are observed for Cu-poor and Zn-rich conditions, it has been shown that the compositional region of single phase CZTS is rather small, inevitably leading to a segregation of secondary phases, especially ZnS for Zn-rich material. Uncontrolled segregation of secondary phases has been shown to result in electronically unfavourable structures such as current blocking ZnS at the back contact or low-bandgap Cu₂SnS₃ at the front of the absorber.
To overcome these problems, but still maintain Cu-poor and Zn-rich chemical potentials we have developed a two stage co-evaporation process including a thermal treatment. In this process CZTS is formed by a cation interdiffusion process during a solid state reaction of ZnS with ternary Cu₂SnS₃. By oversupplying ZnS the chemical potential of Zn is higher than needed for stoichiometric CZTS, while an uncontrolled segregation of ZnS within the CZTS layer is avoided. Excess ZnS rather remains as a sacrificial top layer on the CZTS absorber, which subsequently can be removed by an etching process.
The method allows the synthesis of single phase CZTS as shown by X-ray absorption spectroscopy. Elemental composition maps of cross sections of CZTS layers are obtained by energy dispersive X-ray spectroscopy (EDX) from partially reacted as well as fully reacted CZTS films to investigate the cation diffusion processes. The final absorber layers show a homogenous distribution of atoms indicating that the solid state reaction is fully completed. To investigate the diffusion kinetics as well as the recrystallization mechanism in-situ real-time X-ray diffraction measurements were performed, which show the reaction to be limited by the diffusion of the Zn cation in CZTS.

In this work is presented the fabrication and characterization of Sb₂S₃ and SnS thin film absorbers obtained by thermal evaporation. These materials are integrated into a solar cell structure, SnO₂:F/CdS/Sb₂S₃/SnS/C-Ag, which has a V_oc of 607 mV, J_sc of 4.1 mA/cm² and conversion efficiency of 0.7%. The SnO₂:F (FTO) is from NSG Pilkington (TEC 15) with a sheet resistance of 15 #8486;; the CdS thin film is of 90 nm in thickness, which is deposited on the FTO by chemical bath deposition at 80 °C during 60 min. The CdS film has an optical band gap E_g of 2.4 eV and photoconductivity σ_p of 10^-3 #8486;^-1 cm^-1. A thin film of Sb₂S₃, 450 nm in thickness, is deposited by thermal evaporation of commercial Sb₂S₃ powder (Aldrich) at a deposition rate of 1-5 nm on FTO/CdS at substrate temperature 400^oC. Such Sb₂S₃ film has a band gap of 1.5 eV and photoconductivity, σ_p of 10^-6 Omega;^-1 cm^-1 (n-type). Thin film of SnS 100-600 nm in thickness is deposited by thermal evaporation of SnS precipitates on to FTO/CdS/Sb₂S₃ films held at 200-400 °C. The SnS film has E_g of 1.1-1.3 eV and σ_p of 10^-3 Omega;^-1 cm^-1 (p-type). Colloidal graphite-acrylic paint is applied on SnS film of the above cell structure to produce six electrodes, each with area of 0.36 cm². Finally, the whole structure FTO/CdS/Sb₂S₃/SnS/C is heated in nitrogen at 300 °C during 30 min to stabilize the electrodes. These preliminary results are presented to illustrate that the interfaces in these cell structures are stable; production of the cell structure by thermal evaporation is compatible; and that reasonable cell parameters are obtained. Further work on the optimization of the film thicknesses, thermal processing, choice of electrode materials are expected to improve the cell parameters.

Iron pyrite (FeS₂) is a potential material for thin film solar-energy-conversion devices due to its suitable bandgap (optical bandgap of 0. 95 eV and electronic bandgap of 0.8 eV), its absorption depth of less than 300 nm for hnu; < 1.3 eV, and its previously reported high photon-to-electron conversion efficiency under intense illumination of monocrystalline n-type FeS₂. However, pyrite-based photovoltaics and photoelectrochemical half-cells have consistently displayed low photovoltages le; 200 mV. It is believed that FeS₂ surface chemistry has played an important role in limiting previously reported open-circuit voltages. In this poster we present our investigations into how different pre-treatments including electrochemical activation of the FeS₂ surface, surface passivation by coordinating Lewis acids and bases (e.g., Zn²⁺ and CN^-), and redox potential affected the photoresponse of pyrite in a nonaqueous electrolyte with a non-coordinating redox couple. We will also report most recent photoelectrochemical results from our current efforts to grow Zn-doped single crystal and homoepitaxial layers of FeS₂ as an alternate way of increasing the photovoltage.

Thin-film solar cell provide a competitive alternation to traditional silicon solar cell because it possesses the advantages of high efficiency, low cost, and flexibility. Recently metal chalcogenides, such as CdTe, PbS(e), CuIn_xGa_(1-x)Se_2, Cu₂ZnSnS(e)₄(CZTSSe) have attracted considerable interests as absorber for thin-film solar cell. However, the toxicity of Cd, Pb, and the scarcity of element In and Te, and the impurities in CZTS are the obstacles restrict the development of above materials. Recently, Sb₂S₃, another V-VI binary chalcogenide has attracted a lot of attention. It has direct band gap of 1.7 eV, high absorption coefficient, and simple relatively low-toxic binary compound with fixed composition and phase avoiding the defect control as encountered in CZTSSe, therefore it doesn&’t suffer from all the disadvantages of previous materials which makes it holds great potential for absorber of high-efficiency, low-cost, low-toxicity solar cells. Different fabrication approaches to fabricate Sb₂S₃ thin-film solar cell are reported like thermoevaporation, chemical bath deposition, and hydrazine-based solution deposition. However, as a vacuum-base deposition method, thermoevaporation suffers from relatively low throughput and high cost, and the chemical bath deposition will inevitably create impurity in film, while the other solution method adopting hydrazine which makes it impossible for large-scale commercial fabrication.
Here, we reported a low-cost, facile chemical rout to fabricate Sb₂S₃ thin films via a thermal decomposition sol-gel solution method. We carefully designed the sol-gel solution recipe, therefore the side products are theoretically volatile matters during a mild annealing process which leads to a very condensed uniform film with large grain size. The optoelectronic properties of this film was validated with a photoresponse device. Also, as the nature of solution method, it&’s extremely easy to dope other element into final product, so the effect of sodium doping was studied as well.

Development of gallium nitride (GaN) with p-type conductivity has been a major roadblock in the development of economical growth technologies for III-V nitride LED&’s and solar cells. Significant progress toward the development of P-type gallium nitride has been made in a growth system using low temperature, pressure, and material flow rates. Not only do these growth regimes offer superior materials costs, but particularly, the temperature and pressure conditions of p-GaN growth (550C and ~1000mtorr) used in this work are compatible with films containing indium - which normally desorbs out under typical commercial GaN growth conditions. Indium-preserving growth of high quality doped gallium nitride allows for the realization of nitride heterostructures utilizing InN and InGaN - candidates for use in high efficiency solar cell designs and LED&’s from the green to near UV.
By utilizing an amorphous, 7nm thick aluminum nitride buffer layer and the process of migration-enhanced metal modulation epitaxy, magnesium doped Ga-polar gallium nitride (0001) has been grown by RPECVD with superior electrical and morphological characteristics to prior growths under similar low temperature and pressure conditions. The system utilized uses a hollow cathode driven by 600W RF power to supply excited state molecular nitrogen plasma species.
During growth the gallium metal precursor and active nitrogen species are pulsed in alternatively to reduce mixing and hence there is a reduction in powder-forming reactions during transit to the substrate surface. This also allows for evacuation of the H and C impurities from the metal precursors before nitridation of the metal - resulting in reduced numbers of these species in the film. Their reduction has been confirmed by cross-sectional EDS measurements. A metal grid biased with an electrostatic field prevents the vertical growth of the gallium droplet species on the film surface toward the cationic plasma species above; hence promoting film growth in the lateral direction.
Growth of the buffer layer was performed atop (0001) sapphire which was treated with nitrogen plasma of 600W before deposition. Buffer layer RMS roughness was less than 0.2nm in smooth regions, but demonstrated sparse pitting of 1nm. These defects did not appear in the subsequent p-GaN films, which measured an RMS roughness of less than 0.5nm for a 150nm film. Thorough coalescence during plasma nitridation of the deposited metal droplets was observed. XRD measurements of a particular p-GaN film grown atop amorphous AlN using metal modulated epitaxy gave a FWHM of 0.24 in 2theta;-omega; scans. These films demonstrated a hole concentration of the order of 10E18/cm^3 found by measurement of P-N junction threshold with a well characterized n-GaN overlayer. Overall these figures represent a considerable improvement in the morphology of magnesium doped gallium nitride films grown with low temperature and pressure RPECVD over previous results.

Persistent photoconductivity (PPC) - a metastable increase of doping after prolonged illumination around room temperature - is a phenomenon which has significant impact on the performance of the Cu(In,Ga)Se2-based solar cells. While existing theoretical predictions indicate that its source might be point defects with negative Hubbard correlation energy such as double vacancies V_Se-V_Cu, more experimental work is necessary in order to provide direct evidences on the nature of PPC. The task becomes the more important the higher efficiency is reached in CIGS solar cells. Metastabilities in the cells parameters should be reduced to minimum at efficiency levels beyond 20%, hence finding out their source should help to design most appropriate processing route for absorber deposition.
In this work we will present the experimental data on the amount of PPC in CIGS thin films fabricated by using various processes: two-stage, three stage, with various Na content, with selenium added at various stages of the process. We evaluated the magnitude of the effect by light soaking at room temperature and cooling down under illumination. Then, after switching off the light, the increase of conductivity is persistent up to around 250 K when it starts to anneal out and its magnitude can be measured without contribution of other time-dependent emission processes. The increase corresponds then to a frozen metastable doping concentration created at room temperature by illumination.
We observed a dependence of the magnitude of PPC on sodium content, on annealing in selenium vapour, and in vacuum. We found that sodium-containing samples behaved differently upon Se annealing than those without sodium indicating that presence of sodium was a decisive factor for selenium incorporation. The amount of PPC depended also on the Fermi-level position in thermal equilibrium, which is predicted by the Lany-Zunger model. Thus the analysis of the results will include calculations based on this model in which an increase of doping due to electron capture by metastable defects is obtained as a function of these defects and shallow acceptors concentrations. The results confronted with the experimental findings will serve as a test of the validity of the model.

The intermediate band solar cells (IBSC) is a third generation photovoltaic technology with a theoretical efficiency that exceeds the Shockley-Queisser limit. IBSCs are characterized by the existence of one or more broad bands of energy located within an otherwise conventional semiconductor bandgap. According to the standard Shockley-Read-Hall recombination theory, the states corresponding to these energy levels behave as non-radiative recombination centers. However, when the density of states increases so that wavefunction of electrons become delocalized, nonradiative recombination is inhibited. These intermediate bands can act as stepping stones for low energy photons to be promoted to the conducting band. Hence an intermediate band solar cell can have a high photocurrent by absorbing a much broader spectral range of the solar spectrum leading to power conversion efficiency well above the Shockley-Queisser limit for a single-junction solar cell.
Highly mismatched alloys (HMA) are materials in which is it possible to create the needed intermediate bands. We are investigating producing IBSCs using HMAs of the II-VI semiconductor zinc telluride, which has a band gap of 2.25eV. When small quantities (1-3%) of oxygen are incorporated as ZnTe:O, localized oxygen levels form just below the conduction band interact with the extended states of the conduction band forming an intermediate band while the original conduction band is pushed upward. We describe the hot wall evaporation (HWE) procedure used to produce thin films with columnar structures. This method permits controlled formation of unconventional compositions via simultaneous doping of multiple elements. Uniform films can be grown over relatively large areas and on a variety of substrates. The physical and electronic properties of the films and devices will be described, and the potential of this approach to produce efficient solar cells will be discussed.

BiFeO₃ (BFO) is one of the most interesting materials in the area of magntoelectric-multiferroics. This is due to the fact that this single phase material BFO exhibits long range ferroelectric and ferromagnetic ordering at room temperature. More importantly, recent discovery of photovoltaic effect in BFO has triggered numerous interests in research community. There have been constant efforts made by researchers to further improve the magnetic, electrical, and optical properties by doping at the Bi or Fe site. Here, we report on the optical properties and Raman studies of Cr doped BiFeO₃ thin films prepared by pulsed laser deposition technique on MgO substrates. In this case Cr is doped at the Fe site at a doping level of Cr varying from 5 to 15%. In our synthesis process we maintained the growth condition identical in order to have samples of nearly same thickness. From the X-ray diffraction studies, the single phase formation for all Cr doped samples was confirmed and no secondary phase was noticed. The UV-visible absorption spectroscopy measurements were carried out to understand the effect of Cr doping on the band gap of the material. The electron-phonon interaction of the Cr doped BFO thin films were investigated using Raman spectroscopy. The excitation energy of the laser radiation was varied from 2.33 to 5.08 eV to understand the electron-phonon interaction at resonance and off-resonance conditions. Moreover, temperature dependent Raman studies of these films was utilized to shed light on the anharmonicity involved in these systems. Photovoltaic measurements on these samples were carried out both in sandwich and planar electrodes geometries.

In_xGa_1-xN₂ alloys are employed in light-emitting diodes [LEDs] and sensors, where alloying enables tuning of the band gap energy, allowing material applications such as colored LEDs. However, challenges arise in achieving a complete tunable band-gap alloy series due to the large lattice mismatch between InN and GaN. Indium content above ten atomic percent leads to phase segregation, limiting the ability for an In_xGa_1-xN₂ series that encompasses the whole visible-light regime and application in LED and photovoltaic devices.
The II-IV-nitrides are an emerging alloy series, analogous to the well-characterized III-nitrides, where a Group II and Group IV element replace the Group III element. We report on the structural and optoelectronic characterization of earth-abundant II-IV-nitrides: zinc tin germanium nitrides (ZnSn_xGe_1-xN₂). We demonstrate that for 0le;xle;1, ZnSn_xGe_1-xN₂ alloys are a non-phase segregated, tunable band-gap alloy series throughout visible-light wavelengths.
We have fabricated ZnSn_xGe_1-xN₂ thin films by RF reactive sputtering on both (0001) sapphire and GaN thin films on sapphire. Sputtered ZnSn_xGe_1-xN₂ films are nanocrystalline by x-ray diffraction with an orthorhrombic lattice, and have preferential orientation to the (002) direction. The (002) peak shifts linearly with decreasing Sn content and increasing Ge content, indicating no phase segregation throughout the alloy series.
Electronic measurements indicate ZnSnN₂ sample resistivities are in the 0.01 Ohm-cm range, exhibiting high n-type doping. Conversely, sputtered ZnGeN2 thin films were found to be insulating. Measurements of the ZnSn_xGe_1-xN₂ alloy series have demonstrated the resistivity of the films to increase with decreasing Sn content [and increasing Ge content]. Furthermore, investigations of the alloy series with photoelectron spectroscopy and ellipsometry show shifting work function values and band energetics relative to vacuum with decreasing Sn content. Device architectures of moderately doped samples will be discussed.

Tin (II) sulfide (SnS) is a promising earth-abundant thin-film solar absorber due to its strong optical absorption (α > 10⁴ cm^-1) [1], suitable bandgap (1.1 eV indirect, 1.3 eV direct) [1,2], and ease of film deposition. We recently demonstrated a thermally evaporated SnS-based solar cell with an NREL-certified conversion efficiency of 3.88% [3], the highest published for this growth method. However, the growth conditions for the thermal evaporation step have not been optimized, leaving room to improve bulk carrier collection.
In this work, we investigate the effects of substrate temperature on device-relevant parameters including grain size and mobility. Grain size and majority-carrier mobility are measured by image analysis of scanning electron micrographs and Hall effect measurements, respectively. We also measure the impact of a standard anneal as a function of substrate temperature and correlate the results to device performance.
Both grain size and mobility of the as-deposited films increase monotonically with substrate temperature from 150°C to 285°C. Films that are subsequently annealed at 400°C in 4% H₂S for 1 hour also show a monotonic increase in grain size and mobility with substrate temperature. The anneal causes grain growth for substrate temperatures up to 240°C, but has limited additional benefit at higher temperatures. In contrast, the anneal improves Hall mobility significantly (>100% increase) for all substrate temperatures, suggesting that the anneal may reduce the density of carrier scattering sites other than grain boundaries (e.g., bulk defects).
Devices are fabricated with the annealed films using a previously developed device stack [3]. J-V measurements show that J_sc increases monotonically with substrate temperature in the range 200-285°C. Internal quantum efficiency measurements show that the J_sc increase is mostly due to improved collection at long wavelengths (650-950 nm), suggesting an increase in the minority carrier collection length.
We quantify minority carrier collection lengths by numerically fitting the collection probability profile to quantum efficiency data. The fitted collection lengths, in combination with Hall mobility data, allow us to estimate the minority carrier lifetime as a function of substrate temperature. This analysis decouples the mobility and lifetime components of carrier collection, giving us a more complete understanding of how carrier collection improves with growth temperature.
References
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[2] J. Vidal, S. Lany, M. d&’Avezac, A. Zunger, A. Zakutayev, J. Francis, J. Tate, Appl. Phys. Lett. 100 (2012) 032104. doi:10.1063/1.3675880.
[3] V. Steinmann, R. Jaramillo, K. Hartman, R. Chakraborty, R.E. Brandt, J.R. Poindexter, Y.S. Lee, L. Sun, A. Polizzotti, H.H. Park, R.G. Gordon, T. Buonassisi, Adv. Mater. (2014). doi:10.1002/adma.201402219.

Since 2011, the efficiencies of CdTe solar cells have significantly increased from 17.3% to 21%. A survey of record efficiency CdTe cells over the past decade[i] indicates remarkable improvement on not only short-circuit current (J_sc), but also fill factor (FF) and open-circuit voltage (V_oc). Presently, J_sc is approaching the theoretical limit of CdTe. And further fill factor improvement will scale with device V_oc. There is no doubt that device V_oc improvement is of paramount importance to future higher efficiency CdTe device research effort. Through experimental results and device modeling, we show that reduction of Shockley-Read-Hall recombination and enhanced absorber doping are two critical enablers of high V_oc. Following this approach, First Solar have demonstrated 945 mV device on single crystalline CdTe using Nakazawa structure[ii]. We also reported 916 mV Voc on poly-crystalline CdTe devices.
At device level, accomplishment on individual IV parameters (Voc, Jsc and FF) has led us to believe that 22.8% cell efficiency can be achieved in foreseeable future. At material level, further progress has been made to-date to bring CdTe material properties on par with GaAs, in particular on the reduction of interface recombination velocities, minority carrier lifetime improvement, as well as the increase of p-type doping in CdTe. Given the critical CdTe material properties already accomplished in lab research, we have confidence that 25% CdTe solar cell efficiency can be realized.
[i] Markus Gloeckler, Igor Sankin, and Zhibo Zhao, CdTe solar cells at the threshold to 20% efficiency, 39^th IEEE Photovoltaic Specialists Conference, June 16-21, 2013, Tampa, Florida, USA
[ii] Gang Xiong, Opportunities to increase CdTe solar cell efficiency above 18.7%, 2013 U.S. Workshop on the Physics and Chemistry of II-VI Materials, Oct 1-3, 2013, Chicago, IL, USA

We report on the properties of high efficiency thin-film Cu(In,Ga)Se₂ (CIGS) solar cells. The introduction of a potassium post deposition treatment procedure after the CIGS thin-film absorber deposition has enabled us to reach a device efficiency of 20.8 %. This new process step increases the doping density of the solar cells and consequently also the conversion efficiency of the finished devices. Further process optimization has helped us to push the performance of these promising thin-film solar cells beyond the 21 % mark. We describe the properties of such high efficiency devices which includes the interaction of the new treatment procedure with the possible composition of the absorber material. We also show a detailed diode analysis, and further material analysis of these devices.

As one of the 2nd-generation thin-film PV technology, CuInSe₂ (CIS)-based thin-film PV technology is keeping a competitive position on the performance against the p-type mono-Si wafer-based PV technology. Such understanding is based on the production and sales results as well as the continuous progress of R&D activities, mainly performed by Solar Frontier K.K. This PV technology has passed the milestone of 20 % as small-area cell efficiency. However, it is important to increase the cell efficiency up to 23 %, but more important and urgent issue is to attain the module efficiency of 16 % in the commercialization by transferring the R&D achievement smoothly. Furthermore, as widely understood, this PV technology is recognized not only as a mass-production technology through the annual production results especially of 1000 MW production by Solar Frontier K.K., but also has some advantages that 1) an environmentally-friendly device structure is possible to prepare without using both CdS buffer and Pb solder, 2) competitive performance against the p-type mono-Si wafer-based PV technology shall be attainable, 3) lower production cost is realistic without employing any Si materials, 4) comparable production cost against the CdTe thin-film PV technology is foreseeable, and 5) meanings and purpose to recycle the end-of-life products are essentially different from the CdTe thin-film PV technology. In the presentation, current status and future prospect of this PV technology would be reported mainly through the activities in Solar Frontier K.K. from the standpoint of competitiveness.

The world annual production of PV modules exceeded 30GW in 2011, and is predicted to keep increasing again after a minor standstill in 2012. The crystalline Si solar modules still dominate the market of about 90% though the expectations for thin film technologies are still very high.
Thin film solar cells based on chalcogenide materials such as CdTe and CIGS have emerged and have been leading thin film solar cell technologies. First Solar (CdTe) and Solar Frontier (CIGS) dominate the market of each thin film technology demonstrating great potential to compete with crystalline Si.
In this presentation, the current status of CIGS solar cell technologies will be introduced and potential of CIGS solar cells and modules will be discussed. A significant improvement in solar cell performance has been reported with conversion efficiencies of as high as eff=21.7% by utilizing K-effects. By the development of K-effects, CIGS solar cells can be considered to have stepped up to the next level.
In order to further improve the competitiveness in the PV market, the cell and module efficiencies have to be improved. In the CIGS technology, the instrumental cost is expensive indicating that the development of fast, simplified and low cost processes can contribute to cut down the production cost.
In addition, the material cost occupies the biggest part in the production cost. Reduction of the usage of materials and development of alternative materials can have the strongest impact.
Furthermore, improvement in long term stability of CIGS modules (longer lifetime) is very important and the stability and reliability of CIGS modules will be also discussed.

Chalcopyrite Cu(In,Ga)Se₂ (CIGS) based thin film solar cells have recently exceeded the power conversion efficiency of 21 %. For solar modules the efficiency still is significantly lower primarily caused by free carrier absorption in the state-of-the-art transparent conducting oxides (TCOs). Novel TCOs as front electrodes with matched work function and enhanced transparency are promising to further enhance the performance of CIGS solar modules. We investigate In₂O₃:H as front contacts and compare their performance with the state-of-art ZnO:Al (AZO) electrodes in CIGS solar cells. Thin films of In₂O₃:H were deposited without intentional heating by RF sputtering in an unbalanced magnetron configuration where the substrate is located off-axis from the target normal for reducing the impact of high energetic ions. H₂O vapor was injected via a needle valve for achieving the n-type doping of In₂O₃ with a carrier density of ~3 x 10²⁰ cm^-3. As-deposited amorphous films showed electrical mobility close to 50 cm²/Vs on soda lime glass. After annealing hydrogenated In₂O₃ had a highest electrical mobility of ~137 cm²/Vs with a low optical absorptance of <1 % in the wavelength range of 400-1300 nm. For a comparative study both TCOs were implemented as front electrodes in Mo/CIGS(Na post-treated)/CdS/i-ZnO/TCO solar cells. Devices with as-deposited In₂O₃:H yielded an increased open circuit voltage (V_OC) by ~20 mV compared to those with AZO. In order to distinguish the effect of the i-ZnO/TCO layer interface and the bulk TCO properties, devices with In₂O₃:H/ZnO:Al bilayers of different thickness ratios but the same total sheet resistance were fabricated. A small improvement of the V_OC was already detected if a thin In₂O₃:H layer of 5 nm was introduced and V_OC further increased for higher thickness ratio of In₂O₃:H to ZnO:Al. This indicates the importance of the i-ZnO/TCO interface which can be influenced by a higher work function of In₂O₃ and the sputtering process itself (lower substrate temperature and lower ion bombardment in the case of In₂O₃:H). The fill factor of the CIGS solar cells could also be improved from 73.7 to 75.2% by using In₂O₃:H, whereas the short circuit current remained constant for both front electrodes in this comparative study on small area solar cells with thin TCO layers. In total the power conversion efficiency of the best solar cells were improved by absolute 0.8% to 16.9 % by introducing the In₂O₃:H front contacts. Additionally In₂O₃:H implemented in K post-treated CIGS devices led to a maximum power conversion efficiency of 19.0 %. This proof of concept study indicates that additional improvement of the Jsc can be expected for solar modules with significantly thicker TCO layers.

Copper Zinc Tin Sulfide-Selenide (Kesterite/CZTS-Se) absorbers based on earth-abundant non-toxic materials are being investigated by many research groups currently for optimizing electronic properties and enhancing thin-film solar cell performance. Short-circuit current of the order of 85% of the theoretical Shockley-Queisser value has been reached in these devices but they still lack in the open-circuit voltage with only close to 60% of the theoretical limit being achieved. The open-circuit voltage (V_OC) of the device can be improved by developing a rectifying junction with suitable band alignment between the absorber and the front n-type contact. Indium sulfide is a favorable front contact material in comparison to the toxic cadmium sulfide. Coupled with an indirect band gap of 2.1 eV which helps in transparency to the solar spectrum, tunability of the conductivity, the bandgap, and the absorber to buffer conduction band offset, might make indium sulfide (In₂S₃) an ideal n-type buffer material in combination with CZTS-Se.
We have synthesized stochiometric almost carbon-free indium sulfide thin films in polycrystalline form by atomic layer deposition (ALD) using indium acetylacetonate and hydrogen sulfide. The crystal structure, chemical composition and stoichiometry of the films have been characterized. The thickness of indium sulfide thin films has been tailored to suit the trade-off between obtaining a thin continuous film to reduce contact resistance and avoiding the diffusion of copper through the buffer for the duration of ALD at the deposition temperature. Spectrophotometry measurements have been conducted to determine optical band gap. Derivatives of In₂S₃ like indium oxysulfide (In₂O_xS_3-x) and tin-doped indium sulfide as alternative non-toxic high-temperature crystalline buffers in CZTS-Se devices shall be discussed. Indium oxysulfide as a wide bandgap material with band tunability has been previously reported in the literature. Oxygen incorporation in In₂S₃ using hydrogen peroxide and fine tuning of stoichiometry by varying ALD precursor pulse ratios have been performed. Femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) measurements on In₂O_xS_3-x samples to determine band offset will be discussed. Tin doping in indium sulfide increases conductivity and potentially helps improve fill factor and short circuit current of the devices. Tin incorporation in In₂S₃ has been studied to control conductivity and obtain graded buffer layers with lower electron concentration near junction to negate interface recombination and higher electron concentration near the transparent conductive oxide to reduce contact resistance. Preliminary results on CZTS-Se device performance with these novel buffers will be presented with discussions about future directions to further improve V_OC.

Cu₂Zn(Sn,Ge)(S,Se)₄ (CZTGSSe) solid solutions have a strong interest for photovoltaic (PV) applications. Cu2ZnSn(S,Se)₄ (CZTSSe) kesterites have already been proposed as medium term alternative to more mature
Cu(In,Ga)(S,Se)₂ (CIGSSe) based technologies, that are already entering industrial production stage, avoiding the
use of scarce elements as In. Addition of Ge gives the possibility to control the bandgap of the absorber in a wider
region from 1 to 2.3 eV, expanding the range of values available by varying the anion compositions in CZTSSe.
This is of paramount interest for the development of new multi-junction devices where photovoltaic conversion is
optimized by improving the device efficiency in different spectral regions. Recently device prototypes based in
these compounds have been reported, with efficiencies of 2.7% for Cu₂Zn(Sn,Ge)Se₄ [1] and 6.3% for
Cu₂Zn(Sn,Ge)S₄ [2]. Further development of these technologies requires a much deeper knowledge on the
fundamental properties of these complex compounds, which requires for the study of reference single crystals with
high crystalline quality and well known composition.
In this work we provide an in depth analysis of the fundamental optical and vibrational properties of the Cu₂ZnSn_1-
xGe_x(S,Se)₄ solid solutions. For this purpose Cu₂Zn(Sn,Ge)S₄ (CZTGS) and Cu₂Zn(Sn,Ge)Se₄ (CZTGSe) high
quality single crystals, as confirmed by XRD measurements, have been grown by chemical vapour transport with
different [Ge]/([Sn] + [Ge]) atomic ratios (x varying from 0 to 1) and have been fully characterized using optical,
structural, and Raman scattering techniques. The optical characterization has been performed by variable angle
spectroscopic ellipsometry in the full visible to near infrared range (0.7-4.7 eV) [3] in order to obtain the dielectric
functions (ε1 and ε2). Analysis of the dielectric response shows that by varying the composition indeed solid
solutions with a wide band gap variation can be obtained ranging from 1.5 to 2.3 eV for the CZTGS, and a more
moderate variation only from 1 to 1.3 eV for CZTGSe. Finally, vibrational properties were thoroughly studied by
Raman spectroscopy in resonant and non-resonant conditions under different excitation wavelengths (325, 532, 633
and 785 nm). This has allowed to describe the evolution of the Raman modes as a function of the [Ge]/([Sn]+[Ge])
accurately. The results reveal one-mode and bi-mode behaviour of the main modes for the CZTGSe and CZTGS
systems, respectively.
This characterization provides us a deeper knowledge of the fundamental properties of the CZTGS and CZTGSe
systems. Additionally, this work shows the way to evaluate the [Ge]/([Sn] + [Ge]) ratio of these compounds using
only non-destructive optical methods.
[1] M. Morihama et al., Jpn. J. Appl. Phys. 53(2014)04ER09 1-6.
[2] I. Kim et al., Chem. Mater. 26(2014)3957-3965.
[3] R. Caballero et al., Acta Materialia 79 (2014) 181-187.

Achieving high conversion efficiency in thin-film solar cells requires thorough understanding of the recombination mechanisms, and localizing and quantifying the various types of recombination involved. Recombination rates at the surfaces/interfaces, grain interior and at grain boundaries do not affect the open-circuit voltage equally, and decupling them can provide insight into the dominant loss mechanism and guide the growth. Determining where the dominant recombination occurs requires detailed measurement and accurate interpretation of the results. One photon excitation (1PE) time-resolved photoluminescence (TRPL) measurements are quick and nondestructive, but the surface is inevitably affecting the decay, and the bulk lifetime might be obscured. In a two-photon excitation (2PE) TRPL measurement, one can generate carriers at an arbitrary location in the material. This provides a possibility to probe various regions in the material, to distinguish bulk and surface recombination, and between grain boundaries and grain interiors. Spatial probing, both lateral and in depth can also reveal spatial nonuniformities in carrier lifetime. We present numerical simulation analysis of carrier dynamics during 2PE TRPL decays. We show how the experimental conditions such as spot size and injection rate affect the measured decays and how the diffusion of carriers outside of the collection volume alters the measurement results. The analysis is used to determine requirements of the sample properties such as thickness and grain size in relation to the minority-carrier diffusion length for unambiguous separation between surface and bulk recombination. This determines the limits and the potential of 2PE TRPL microscopy. While the analysis presented here is for CdTe, it could be generalized to other materials.

In the past 2 years the implementation of a KF post-deposition treatment (PDT) of Cu(In,Ga)Se₂ (CIGS) thin films has triggered the announcement of several new world record solar cell efficiencies. First introduced by Empa for CIGS absorbers evaporated at low-temperature on a flexible plastic substrate, the KF PDT allowed an improved junction quality (higher V_oc and FF) and reduced optical losses in the CdS buffer layer (higher J_sc), enabling the fabrication of a 20.4% flexible CIGS solar cell. The process step, involving the evaporation of KF along with Se onto an as-grown film at a substrate temperature of about 350°C, was quickly adopted by other groups, and led to several improvements, finally bringing the CIGS technology to its current record efficiency of 21.7%.
Whereas we observe modifications of both bulk and surface of the absorber layer subjected to KF PDT, it appears that a significant part of the positive effect of the treatment can be mostly related to changes in the surface region of the absorber, hence interface with the buffer layer. This is supported by the fact that the enhancements in efficiency have not only been observed in solar cells with evaporated absorbers, but also in solar cells with absorbers grown with a higher Ga content or in a 2-step selenization process. Those typically suffer from pronounced recombination at the absorber-buffer interface, which can be significantly reduced by using a KF PDT.
In this contribution, we discuss our current understanding of the effect of KF PDT, combining results of several material and electronic characterization techniques. Special attention is given on the challenge in characterizing a rough, polycrystalline surface, subjected to chemical- and diffusion-based modifications due to KF PDT, subsequent air exposure and growth of the buffer layer by chemical bath. Whereas several advanced characterization techniques are available, difficulties arise when trying to combine high spatial resolution and sufficient level of detection, while avoiding any sample alteration induced during preparation. Suggestions for further improvements of the junction quality and modifications of the absorber-buffer interface will also be discussed.

Film-based photovoltaic (PV) modules (e.g. CdTe, CuInGaSe₂, organic systems) employ monolithic interconnects to minimize series resistance losses and enhance module voltage via series connections. Generally, these interconnects are made sequentially, with a scribing step following deposition of the bottom electrode, a second scribe after absorber deposition, and a third following deposition of the top electrode. Here, an alternate method, performed after deposition of all PV layers utilizing printed dielectric and metal materials, provides the same functionality with several advantages. First, an integrated scriber/printer apparatus envisioned makes interconnects in a single pass, eliminating two scribing steps in the conventional process flow, therefore streamlining production and reducing manufacturing capital expense. Second, using a self-aligned printing and scribing tool facilitates construction of narrower interconnects (width less than 100 µm compared to 500 µm). This increases the geometric fill factor and provides a relative power output increase of about 5%. Initial analysis taking these advantages into account within an industry-scale CdTe PV manufacturing model projects a manufacturing-cost savings of about $0.02/watt. Mini-module experiments on CdTe, CuInGaSe₂, and organic polymer:fullerene systems, demonstrate the expected voltage addition and photoconversion efficiency using printed interconnects.

First reports of light and voltage bias-induced changes of the electrical characteristics of CuIn_1-xGa_xSe₂ solar cells have been published more than 25 years ago[1]. Nevertheless, despite the proposal of a number of different models to explain them, a detailed quantitative and conclusive description of the underlying physical processes is still missing. This is partially due to the fact that usually rather singular measurements are performed on a very limited number of samples, e.g. only CuIn_1-xGa_xSe2 with one Ga- content. In this study we have looked at the effect of light soaking with white, red and blue illumination on the open-circuit voltage and junction capacitance on a representative series of samples with varying Ga content, ranging from pure CuInSe₂ to CuGaSe₂. All measurements are performed at different temperatures, ranging from 320K- 380K, thus allowing the extraction of activation energies for the creation and also relaxation of metastabilites in the different samples. The combination of open-circuit voltage and capacitance measurements allows us to discriminate between different models proposed to explain the metastabilities.
We find that red light soaking causes the strongest increase in V_oc (~6% after 10⁴ seconds) for CuInSe₂, a smaller increase for CuIn_0.7Ga_0.3Se₂ , and even decreases V_oc for CuGaSe₂. While the effect of white light soaking is comparable to red light soaking for all devices, the illumination with blue light generally causes a different device response. For all samples we find that red light soaking increases the capacitance, consistent with a metastable increase in the carrier density in the space-charge region. By comparing simultaneously recorded V_oc and capacitance data, we show that the increase in V_oc for CuInSe₂ and CuIn_0.7Ga_0.3Se₂ can be quantitatively explained with dominating recombination in the space charge region, while the decrease of V_oc for CuGaSe₂ is explained by dominating interface recombination.
The activation energies found for the creation and relaxation of the metastable states in CuIn_0.7Ga_0.3Se₂ are ~500meV and 600meV, respectively, and considerably larger for CuInSe₂ with values ~1000meV. This partially agrees with values reported by Rau et al., but disagrees strongly with theoretical predictions [2] and other experimental reports [4]. Implications of these results for our current understanding of metastability in Cu(In,Ga)Se₂ will be discussed.
[1] M.N. Ruberto and A.Rothwarf. Journal of Applied Physics, 61 (1987) 4662
[2] S.Lany and A.Zunger. Journal of Applied Physics, 100 (2006) 113725
[3] U.Rau, M.Turcu, and A.Jasenek. Thin Solid Films, 515 (2007) 6243
[4] A.Urbaniak and M.Igalson. Journal of Applied Physics,106 (2009) 063720

One of the reasons for Cu(In,Ga)Se₂ based thin film solar cells sucess is the remarkable flexibility of its chalcopyrite type crystal structure [1,2]. This flexibility is a key point also for quaternary kesterites like Cu₂ZnSnS₄/Se₄ (CZTS, CZTSe). Nervertheless the absorber layer exhibits an overall off-stoichiometric composition, thus the existence of intrinsic point defects is strongly correlated with the chemical potential and therefore depend on the composition of the compound. Finally these structural defects also influence the electronic properties of the final device sensitively.
Both compounds, CZTS and CZTSe, crystallize in the kesterite type structure (s.g. I-4) [3]. The structure can be described by cation layers Cu/perpendicular to the crystallographic z-direction with sequence Sn-Cu/Zn-Sn/Cu-Cu/Zn-Cu/Sn. An off-stoichiometric composition, for instance Cu-poor, originates from the prospensity of the crystal structure to stabilize e. g. copper vacancies, the charge balancees beeing commonly insured by appropriate substitutions on the cationic sites. If the oxidation states of the cations and anions are retained going form stoichiometric to off-stoichiometric CZTS/Se, a number of specific substitutions can be envisioned to account for the charge balance in the off-stoichiometric compound. These substitutions lead to the formation of point defects in the crystal structure of the material. Lafond et al. [4] suggested four cation substitution reactions (off-stoichiometry types) which lead to off-stoichiometric CZTS/Se: a Cu-poor and Zn-rich composition (A- & B-type) and for a Cu-rich and Zn-poor composition (C- & D-type). These types include kesterites with compositional ratios Cu/(Zn+Sn)<1 and Zn/Sn>1 as well as Cu/(Zn+Sn)>1 and Zn/Sn<1. Thus the compositional ratio pairs with both ratios > 1 or <1 are missing. To fill these gaps in the Cu/(Zn+Sn)-Zn/Sn map, two other off-stoichiometry types have been derived by considering the Cu₂S/Se-SnS/Se-ZnS/Se phase diagramme and appropriate cation substitution reactions, named E- and F-type.
The presentation will give an overview of the cation substitution reactions and the connected point defects, which are considered to form off-stoichiometric kesterites by keeping the charge balance. The basis for our studies is a number of CZTS, CZTSe and CZTSSe powder samples, synthesized by solid state reaction. The results of our chemical (WDX spectroscopy) and structural analysis (neutron diffraction and anomalous synchrotron X-ray diffraction) demonstrate the ability of the kesterite type phase to tolerate deviations from stoichiometric composition without collapse of the structure. Moreover the revealed cation distribution indicates the presence of point defects expected from the according off-stoichiometry type.
[1], [2] C. Stephan et al. APL 98 (2011) 091906 and 101 (2012) 101907
[3] S. Schorr, Sol En Mat Sol Cells 95 (2011) 1482
[4] A. Lafond et al. Z Anorg Allg Chem 638 (2012) 2571

One of the main objectives of this work is to give an overview and analyze the low market share of thin film based technologies and in particular CIGS from an industrial point of view. CIGS is far from being mature at industrial level in spite of all the investments that have been done during the last 20-30 years.
Technological aspects of present CIGS-based PV module technologies are presented, with emphasis on the main scientific/technological routes leading to record efficiencies at laboratory and industrial scale. A benchmark analysis of scientific and technical advanced that have being leading to increasing in performance at laboratory level as well as at industrial one is done. Main conclusions lead to an industrial guided R&D efforts, mainly from a technology and engineering point of view in materials science; to develop robust and reproducible fabrication tools and processes. A deep analysis of efficiency gap between record cells and modules will be presented.