Solar cell technology is changing. New efficiency records are being set. Alta Devices has reached 28.8% efficiency in a thin film single-junction cell at 1-sun, and 30.8% efficiency in a thin-film dual junction cell at 1-sun.
Counter-intuitively, efficient external fluorescence is a necessity for approaching the ultimate limits. A great Solar Cell also needs to be a great Light Emitting Diode. Why would a solar cell, intended to absorb light, benefit from emitting light? Although it is tempting to equate light emission with loss, paradoxically, light emission actually improves the open-circuit voltage, and the efficiency.
The single-crystal thin film technology that achieved these high efficiencies, is created by epitaxial liftoff, and can be produced at cost well below the other less efficient thin film solar technologies. The path is now open to a 30% efficient photovoltaic technology, that can be produced at low cost.

Epitaxial lift off (ELO) of III-V semiconductor materials has often been used to make lightweight, ultrahigh efficiency solar cells. Unfortunately, a major promise - the ability to reuse the expensive substrate material following the removal of the epitaxial active region of the solar cell - has been elusive since the process results in irreversible damage to the parent wafer. Recently, we reported a method to avoid wafer damage caused by ELO using a combination of epitaxial protection layers and non-destructive cleaning procedures of both InP and GaAs substrates [1, 2]. This has resulted, for the first time, in the demonstration of multiple reuse of the parent wafer without incurring changes in either the chemical or morphological status of the original material. This suggests that the wafer itself may be used an unlimited number of times, employing a sequence of epitaxial growth, epitaxial removal using adhesive-free cold weld bonding to a metalized plastic foil, non-destructively cleaning the wafer surface, and then repeat. Indeed, multiple cycles of ELO followed my wafer reuse can be an effective means for achieving very low cost, flexible and extremely lightweight solar cells. In this work, we discuss the progress made in ELO growth of high efficiency p-n junction GaAs solar cells followed by adhesive-free cold-weld bonding to flexible substrates, and then once more reusing the parent wafer. We also discuss other high performance devices (e.g. LEDs, FETs, etc.) that have been grown on original and reused wafers. We will discuss the cost and complexity tradeoffs in this process multiple growths, ELO, and reuse of from a single parent wafer in achieving low cost, high power conversion efficiency III-V solar cells.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012.

Monolithically integrated III-V compound semiconductor layers on silicon substrates will enable semiconductor industry to fabricate new interesting device concepts in the near future. The device concepts that have received the most attention include CMOS compatible lasers that could be used in on-chip and chip-to-chip communication, n-channel high mobility transistors and silicon based high-efficiency solar cells. In order to realize these devices, many challenges existing in the polar-on-nonpolar epitaxy (e.g., anti-phase domains, stacking faults, lattice mismatch) needs to be overtaken. Despite the challenges however, it has recently been shown that an atomically smooth low defect density GaP buffer layer can be fabricated on silicon [1]. Therefore, an increasing amount of attention has been addressed on the properties of gallium phosphide based dilute nitride materials (E.g., GaPN, GaAsPN). The benefit of these materials is that the nitrogen incorporation enables strain compensation and the energy band structure engineering.
We have studied the growth of GaP and Ga(As)PN layers on silicon and GaP substrates by metalorganic vapor phase epitaxy. Properties of the grown layers have been examined by Raman scattering, photoreflectance, photoluminescence, IV, atomic force microscopy, Rutherford backscattering and XRD studies[2-5]. We discuss the properties of the fabricated Ga(As)PN layers and focus on the characteristics of the layers that could be utilized to realize a high-efficiency solar cells on silicon substrates. The solar cell device concepts that have drawn our attention include n-GaP heterojunction emitters, GaAsP tandem cells and GaAsPN based intermediate band solar cells (IBSC).
In this work, we present the main experimental results of the fabricated layers. The results show that defects in Ga(As)PN layers on silicon most likely arising from the III-V/Si interface degrade the quality of the layers and that high-quality layers are realized on GaP substrates. Anti-phase domain and stacking fault/threading dislocation type defects have been observed by transverse scan analysis performed by high-resolution XRD setup. Photoreflectance results of Ga(As)PN layers show the conduction band splitting which can then possibly in the future be used to realize IBSC device on silicon substrate. At the moment, we are fabricating our first IBSC devices. In addition, IV measurements performed under 0.54 SUN show that semiconductor devices such as n-GaP/p-Si heterojunction solar cell are realized.
[1]: K. Volz, et al., Journal of Crystal Growth, 315, 37, (2011).
[2]: H. Jussila, et al., J. Appl. Phys. 111, 043518 (2012).
[3]: H. Jussila, et al., Phys. Status Solidi C, 9, 1607 (2012).
[4]: S. Nagarajan, et al., J. Phys. D: Appl. Phys. 46, 165103 (2013).
[5]: H. Jussila, et al., Thin Solid Films, 534, 680-684, (2013).

Multiple batches of thin-film p-n junction GaAs thin-film photovoltaic cells were grown sequentially on a single parent wafer without loss in performance from growth to growth. Through the combination of cold-welding and a modified epitaxial lift-off (ELO) process that eliminates parent wafer damage, the device active region is transferred onto flexible plastic substrates [1]. A combination of alternating arsenide-based and phosphide-based lattice matched epitaxial layers with high etch selectivity grown on both sides of the AlAs sacrificial layer, along with thermal and plasma surface cleaning fully eliminates surface contaminants and damage, creating a pristine surface suitable for subsequent epitaxial layer growth without the use of costly and damaging wafer repolishing methods[2]. Using these methods, three iterations of lightweight and flexible GaAs thin film photovoltaic cells bonded to flexible plastic substrates were fabricated from a single GaAs substrate without any degradation in device performance after regrowth with power conversion efficiencies of ~18%. Wafer recycling without repolishing allows for an indefinite number of growth iterations on a single substrate without damage or loss in the original wafer thickness, providing a path towards cost-effective, high performance, flexible and lightweight thin-film compound semiconductor devices.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012

Mono-crystalline germanium (Ge) is the most widely used substrate in concentrated photovoltaics (CPV) high efficiency multijunction solar cells (MJSC). Increasing demand on MJSC would undoubtedly lead to a lack of this rare material, resulting in dramatic increase in the Ge wafer price, not to mention that today, the cost of the Ge substrate already represents a substantial share of the total cell cost. A typical MJSC process uses > 140 µm thick Ge wafers as substrates, whereas a few microns would be sufficient for the bottom cell to match the photogenerated currents at the top and middle subcells. Moreover, using a thin Ge film rather than a thick substrate would reduce electrical and thermal resistances, weight of the cell and increase efficiency of the Ge subcell. In order to separate the active region of the MJSC from its original substrate, a layer transfer process based on simple electrochemical porosification of Ge could be used. Similar processes have already been successfully applied to crystalline silicon thin film solar cells, yielding performances very similar to crystalline cells that are manufactured on standard thick Si wafer.
We have developed a process, using porous germanium, to separate a semiconductor structure from the Ge substrate (APL 102 (1), 011915 (2013)). First, a double porosity layer is formed on top of a p-type Ge substrate in HF based electrolyte; the topmost layer has a low porosity whereas the buried layer is highly porous. After ultra high vaccum (UHV) annealing at high temperature, the porous top layer transforms into quasi-monocristalline germanium (QMG) film and will serve later as seed for epitaxial deposition of III-V top and middle subcells. On the other hand, the buried layer forms a film with large lateral voids that weaken the interface between the QMG film and the substrate, creating a so called “separation layer”. After epitaxial deposition of the III-V MJSC device is done, the cell is bonded to a low cost host substrate, and separated from the original Ge substrate, which is then ready to be re-used in another manufacturing cycle and could potentially yield a large number of MJSC solar cells, as the active region of the Ge subcell is only a few microns thick.
The synthesis and transformation during annealing of mesoporous germanium double layer are key steps in this process. In this presentation, we show the results of electrochemical etching of Ge in HF electrolyte to form the double porosity layers, and its morphology transformation during UHV annealing at high temperature which is due to surface diffusion at constant volume. The monocrystallinity of the layers and their suitability as seed layers for epitaxy of GaAs is confirmed by X-ray diffraction rocking curves.

Upconversion of sub-bandgap photons is a promising approach to exceed the Shockley-Queisser limit in solar technologies. Calculations have indicated that ideal upconverter-enhanced cell efficiencies can exceed 44% for non-concentrated sunlight, but such improvements have yet to be observed experimentally. In this presentation, we develop both theoretical and experimental methods to understand and improve solar upconversion, considering electronic, photonic, and thermodynamic design constraints. First, we develop a thermodynamic model of an upconverter-cell considering a highly realistic narrow-band, non-unity-quantum-yield upconverter. As expected, solar cell efficiencies increase with increasing upconverter bandwidth and quantum yield, with maximum efficiency enhancements found for near-infrared upconverter absorption bands. Our model indicates that existing bimolecular and lanthanide-based upconverters will not improve cell efficiencies more than 1%, consistent with recent experiments. However, our calculations show that these upconverters can significantly increase cell efficiencies from 28% to over 34% with improved quantum yield, despite their narrow bandwidths. Then, we develop the experimental techniques to enhance upconversion efficiencies, tailoring both the optical density of states via plasmonics and the electronic density of states via pressure. Our results highlight the interplay of absorption and quantum-yield in upconversion, and provide a platform for optimizing future solar upconverter designs.

Current photovoltaic technologies harvest only a fraction of incoming solar energy since they are unable to utilize photons with energies below the cell band gap. Placed behind a solar cell, the upconverter converts transmitted low-energy photons to photons with energies higher than the cell band gap. The higher energy photons are absorbed by the solar cell and contribute to the photocurrent. We develop optical models of several state-of-the-art commercial and research thin-film solar cells incorporating the upconversion layer. We present both analytical models based on published EQE data as well as detailed finite difference time domain (FDTD) models that incorporate absorption in all cell layers. We model the improvement in absorption and overall cell performance of amorphous Si, CIGS, GaAs, CdTe, and Cu2O cells with upconverting layers. We incorporate and discuss the effect of interface texture and different cell layers on the absorption of upconverted photons and make suggestions for improving the overall cell design to get the maximum benefit from upconversion. We estimate that the cell efficiency enhancement can range from 0.5% to up to 7% absolute depending on the cell type and upconversion efficiency. This work connects to the fundamental efficiency limit analysis of narrow-bandwidth solar upconversion by our collaborators [1], but presents concrete optical models of current solar cells and discusses the promise of upconversion for particular applications.
[1] J.A. Briggs, A.C. Atre, J.A.Dionne.Narrow-bandwidth solar upconversion: Case studies of existing systems and generalized fundamental limits. Journal of Applied Physics 113, 124509 (2013).

Technological implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must be accompanied with a sufficient photocurrent generation via IB states. The demonstration of QD-IBSCs is presently undergoing two stages. The first is to develop epitaxial growth or lithography technology to fabricate high density QD arrays or superlattice with high quality heterointerface, and the second stage is to realize partially-filled or ideally half-filled IB states in order to maximize the photocurrent generation by two-step absorption of infrared photons in solar spectrum.
For the former requirement, we have developed a strain-compensation or strain-balanced technique in order to grow multi-stacks of self-organized InAs QDs in GaNAs matrix on GaAs substrate by molecular beam epitaxy (MBE). For the latter, either doping of QDs or concentrated illumination of sunlight at around 1000 suns should in principle result in a sufficient population of carriers in IB and hence production of an additive photocurrent in QD-IBSC.
We show that sunlight concentration increases the optical generation rate and hence sufficient carriers are populated in IB, which in turn increases photocurrent production from IB to conduction band. Our InAs/GaNAs QD solar cell shows a conversion efficiency of 20.3% at 100 suns increasing to 21.2% at 1000 suns. The QD-IBSCs which commonly suffer from low absorption by QDs are thus expected to improve fast and perform better under concentrated sunlight illumination.

The advantages of converting solar energy directly into clean electricity have important consequences for realizing a sustainable future. However, the conversion efficiency needs to be improved, and new solar cell concepts are believed to play an important role in order to achieve this target.
For solar cells, infrared absorption and ultra-violet relaxation are considered to be the main factors for conversion efficiency limitation [1]. To reduce these losses, several ideas have been proposed. The intermediate band (IB) solar cell reduces the infrared absorption losses [2]. This concept proposes that by upconversion, the energetic addition of two low energy quanta resulting in one high energy quanta, the infrared light can be efficiently used and the energy conversion efficiency limit can be improved significantly compared to single junction solar cells.
Quantum structures are attractive candidates for design of IB states, and a number of IB solar cells based on InAs/GaAs have been reported. Important InAs quantum structures are quantum dots (QDs) and disklike quantum structures (nanodisks) formed in the early stages of InAs deposition [3]. While the InAs quantum structures have been the focus of IB research for over one decade, the detailed mechanisms of upconversion are still unclear due to the complexity. Upconversion in InAs QDs is generally accepted to occur via the two-step two-photon-absorption mechanism. The operation principle has been verified, but the realized efficiencies are too small for practical applications [4].
In this work we focus on a novel IB solar cell concept using both QDs and nanodisks. The major upconversion mechanisms in the InAs quantum structures were determined by macroscopic measurements of upconverted photoluminescence (PL) and photocurrent (PC) [5,6]. The results showed that the nanodisks with height of a few monolayers and lateral extension up to hundreds of nm show exceptionally high upconversion efficiencies due to the activation of the upconversion via Auger process. Further exploration of the InAs quantum structures with spectroscopic imaging of PL and PC revealed that, depending on the distribution of nanodisks and QDs in different sites, the local PC generation efficiency can change drastically. We discuss the energy transfer between the quantum structures and how this relates to the upconversion efficiency. Based on our finding we propose a novel design of an efficiency enhanced IB solar cell using both QDs and nanodisks.
This work was supported by JST-CREST and by the Strategic Research Infrastructure Project, MEXT, Japan.
[1] W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961).
[2] A. Luque and A. Marti Phys. Rev. Lett. 78, 5014 (1997).
[3] R. Heitz et al., Phys. Rev. Lett. 78, 4071 (1997).
[4] A. Marti et al., Phys. Rev. Lett. 97, 247701 (2006).
[5] D. M. Tex and I. Kamiya, Phys. Rev. B 83, 081309(R) (2011).
[6] D. M. Tex, I. Kamiya, and Y. Kanemitsu, Phys. Rev. B 87, 245305 (2013).

Quantum structures are promising candidates for next-generation solar cell materials because of their unique optical properties. Among them, InAs quantum structures attract much attention because InAs/GaAs system forms intermediate bands, which absorb infrared light due to the small band gap energy, and upconversion processes are expected to improve the light-energy conversion efficiency. As previously reported, two-dimensional disklike InAs quantum structures with heights of two and three monolayers (quantum well island: QWI) display efficient upconverted photoluminescence (PL) and photocurrent (PC), which occur through a multiparticle Auger process [1]. This novel application of the QWI may be more suitable than the quantum dots for enhancing the solar cell conversion efficiency. For the further understanding of the upconversion process in QWIs, it is significant to clarify the photocarrier decay and upconversion dynamics. At low temperatures, the photocarrier dynamics can be evaluated by upconverted PL decay dynamics. However, the room-temperature photocarrier dynamics remain unclear because of the very weak PL intensity. Recently, we developed a novel measurement method for femtosecond time-resolved PC dynamics based on femtosecond excitation correlation (FEC) spectroscopy. In this study, we investigated the photocarrier dynamics of QWIs using PC-FEC measurement.
The sample structures were prepared by molecular beam epitaxy. A nominally undoped structure was grown on top of a semi-insulating GaAs (001) substrate and a GaAs buffer. A single InAs layer and a GaAs/AlGaAs quantum well were grown in an AlGaAs matrix. In PC-FEC measurement, two femtosecond laser pulses with a time delay are focused on the sample surface, as is similar to the FEC measurement of PL [2]. We measured the correlation signal of the PC induced by the two pulses as a function of delay time, which approximately corresponds to the photocarrier population in the QWIs. The light source was the optical parametric amplifier based on the KGW:Yb regenerative amplifier (repetition rate: 200 kHz).
We observed the strong FEC signal under excitation at the resonance energy of InAs QWIs with heights of two monolayers. The decay time is around 100 ps, which is weakly dependent on the excitation photon energy. We attribute the FEC decay time to the intrinsic photocarrier lifetime in the InAs QWIs. We will present the principle of the PC-FEC measurement and discuss the temperature dependence of the photocarrier intrinsic lifetime in the InAs QWIs.
Part of this work was supported by The Sumitomo Industries Group CSR foundation, JST-CREST, JSPS KAKENHI (No. 25234567), and the Strategic Research Infrastructure Project of MEXT.
References:
[1] D. Tex, I. Kamiya,and Y. Kanemitsu, Phys. Rev. B 87, 245305 (2013).
[2] D. von der Linde, J. Kuhl, and E. Rosengart, J. Lumin. 24/25, 675 (1981).

Thin-film structures are considered to be essential to lower the cost of solar cells. Thinning the active regions, however, often reduces the optical absorption efficiency. Therefore, the enhancement of optical absorption in thin-films is inevitable to maintain and improve solar cell efficiency. Among the variety of absorption mechanisms in semiconductors, excitonic absorption is one of the most promising candidates to enhance the optical absorption, because it can be added to the normal band-to-band absorption. The excitonic absorption at room temperature occurs more efficiently in the materials with higher exciton binding energies. Unfortunately, semiconductors sensitive to the main part of the solar spectrum such as Si and GaAs exhibit rather lower exciton binding energies than thermal energy at room temperature. However, the excitonic absorption can be considerably enhanced even in these materials by constructing superlattice (SL) and multiple quantum well structures. In this study, we show the absorption characteristics of the solar cells consisting of an AlGaAs / GaAs SL p-i-n structure. Al0.5Ga0.5As (2nm) / GaAs (5nm) SL solar cells are grown on n-type GaAs (001) substrates by a molecular beam epitaxy method. The total active layer thickness is fixed at 2 micrometer or 1 micrometer. In order to compare the effect the excitonic absorption, we fabricate a bulk Al0.14GaAs pin junction. The Al composition of Al0.14GaAs bulk layer corresponds to the averaged one of the SL. For the SL solar cells, the open-circuit voltage is 1.08 V, and the short-circuit current density is 23.6 mA/cm2 under the solar radiation of AM 1.5 spectrum at 100 mW/cm2. The fill factor and the conversion efficiency are 0.83 and 21.0 %, respectively. On the external quantum efficiency of the SL, two distinct maxima appear at approximately 680 and 790 nm with a dip structure in between. The calculated absorption rate with the effect of excitons shows the strong and sharp peaks at the absorption edge around 800 nm. In contrast, the step-like feature is obtained for the electron-hole pair absorption without considering the Coulomb interaction due to the SL density of states. The experimental results clearly indicate that the excitonic absorption is occurred even at room temperature. Thinning the active layer thickness reduce the optical absorption efficiency. Especially, the efficiency is easily decreased in the range near the absorption edge because of the lower absorption coefficient. For the Al0.14GaAs bulk solar cell, the decrease rate of the EQE in the range of 100 nm-wide wavelength from the absorption edge is estimated to be 27 %. On the other hand, the decrease rate for the SL solar cell is estimated to be 15 %. This result suggests the SL solar cells have high absorption coefficient compared with the bulk solar cells because of the excitonic absorptions, and the high absorption efficiency should be maintained when the active layers thickness are reduced.

Quantum dots (QDs) are an exciting group of materials because their bandgap is dependent on their nanocrystal size. The use of QDs allows for precise control over the wavelengths of light which are emitted or absorbed in applications such as light emitting diodes or photovoltaic devices. To produce efficient devices made from QD films, it is necessary to control the rates of charge transport and exciton diffusion among the QDs which make up the film. One way to affect these rates is by changing the ligand present on the surfaces of the QDs, which alters the distance and electronic coupling between neighboring dots. Large, bulky ligands serve to isolate dots from their neighbors while small, compact ligands allow for close packing and enhance electronic coupling. Regardless of whether the goal is to increase or decrease charge transport / exciton diffusion, it is important to be able to quantify properties such as the interparticle spacing and center-to-center distances in QD thin films. In this study, we characterize the morphology of lead sulfide (PbS) thin films made from highly monodisperse PbS QDs (diameter standard deviation < 5%, absorbance peak FWHM ~ 60meV). The native oleic acid ligands are exchanged for alkane monothiols, alkane dithiols, and several other commonly used ligands as well as treated with hydrazine to strip the QDs of all surface ligands. The absorbance spectra of the thin films are measured in order to characterize the shift in the first absorbance peak - which gives an indication of how much the local environment around each QD has changed. To determine the interparticle spacing, the dots are assembled into a close packed monolayer on a transmission electron microscopy (TEM) grid and subsequently have their native ligands exchanged. The TEM images are automatically analyzed to identify the boundaries of each QD as well as its center point. This information is used to calculate the average diameter and average center-to-center distance of all QDs in the image. From this analysis we are able to quantify the change in interparticle spacing as a function of the surface ligand species. These results are compared with those from small angle X-ray scattering (SAXS) on thin films, which also determines the average diameter and average center-to-center distance. We discuss the advantages and disadvantages of the TEM and SAXS measurements as well as the ease of data collection and analysis. Altogether, this study aims to provide a better understanding of how ligand chemistry can be used to control the morphology and electronic properties of a QD film.

Split spectrum photovoltaics, where incident light is divided onto multiple solar cells on the basis of wavelength, are an exciting recent development in the solar energy field. This technology has the potential to exceed record conversion efficiencies by allowing systems to incorporate a large number of active p-n junctions while mitigating the constraints that plague monolithic cells: namely lattice matching and current matching. Each cell in a split spectrum system can be designed to have a different lattice constant (allowing for a larger range or more combinations of materials) and have different operating currents (allowing for more combinations of band spacing).
In this work, we examine a split spectrum system utilizing a single spectrum splitting device, here a dichroic filter, to divide the solar spectrum onto two cells. As opposed to many other split spectrum designs where numerous filters direct light onto numerous single junction cells, in this system each cell is then comprised of multiple active junctions. Each cell is then tailored to absorb its own portion of the solar spectrum at its own operating point. The combination of the two cells allows for the constructing of a photovoltaic system which effectively has four or five active junctions while maintaining lattice and current matching conditions in each cell.
A number of different cutoff frequencies for the dichroic filter are examined. Each cutoff frequency corresponds to its own combination of ideal band placements for both the shorter and longer wavelength cells. Designs which have the potential to have both cells meet the lattice matching and current matching conditions are then simulated using TCAD Sentaurus. Designs which show promise of delivering high system conversion efficiency are then grown and tested. Testing is to be done under a variety of conditions, including that of a solar simulator and environmental conditions. Environmental testing is conducted in part using a custom-built concentration array designed for use with split spectrum solar cells.

Quantum dot (QD) based 3rd generation photovoltaic technology is important to helping the world meet its increasing energy demands. Doping CdSe QDs with impurities such as indium or gallium opens the door to increased conductivity as these elements introduce n-type donors into bulk CdSe. The growth kinetics in the heterogeneous growth-regime and temperature dependent fluorescence provide insight into the doping process. Doped CdSe QD-P3HT hybrid cells show increased photocurrent over hybrid cells fabricated with undoped CdSe QDs at room-temperature and at elevated temperatures indicating possible thermal ionization of n-type donors. Gallium doped CdSe QD based hybrid solar cells show an over-all power conversion efficiency of 2.0% under AM 1.5 solar conditions.

The performance of the light-harvesting active layer of several photovoltaic (PV) technologies (e.g. amorphous silicon and organic photovoltaics) is limited by the overlap of the absorption spectrum with the solar spectrum. Over the last decade the phenomenon of triplet-triplet annihilation-assisted photon upconversion (UC) has been demonstrated for a wide variety of molecular triplet sensitizer/annihilator systems. More recently these efforts have focused on efficient UC of photons from the near-infrared to visible regions of the solar spectrum. These observations suggest that photons below the bandgap of amorphous silicon (a-Si) can be converted to photons with higher energies, which can then be absorbed in the active layer of a-Si devices and converted into carriers, thereby enhancing the photocurrent.
We will outline recent efforts to synthesize novel light-harvesting chromophores that (i) possess large near-infrared extinction coefficients, (ii) manifest ultrafast intersystem crossing to generate the triplet state at unit quantum yield, and (iii) possess triplet lifetimes exceeding several microseconds. We will subsequently discuss factors affecting the optimization of the photon UC efficiency of these sensitizers in combination with various triplet annihilators, the incorporation of the best-performing systems into device architectures employing transparent a-Si devices, and the observed performance of the coupled UC-PV system.

Wide-bandgap metal oxide Nanowires (NW) sensitized with quantum dots (QD) is a promising third generation concept due to their tunable electro-optic properties leading to high efficiency and low cost quantum dot sensitized solar cells (QDSSCs). CdSe/ZnS colloidal QDs have the advantage of band gap tuning, high stability, broad-range photon absorption and multiple carrier generation. Zinc oxide (ZnO) NWs with the advantage of high electron mobility, directionality, and ease of fabrication is a good candidate to use as the conducting electrode. ZnO NWs provides a transparent, high surface area medium for the QD absorbers and a directional path for the carriers. Among challenges faced in practical realization of QDSSs such as slow hole transfer, interface carrier recombination and the poor counter electrode performance, charge transfer dynamics from the absorber QD to the NW is key to better performance. To date, the carrier transfer mechanism has not been thoroughly explored for ZnO NW/QD structures. In this work the QD-to-ZnO NW charge transfer dynamics is analyzed for QDSSs.
In this work, ZnO NWs were synthesized by two methods; bottom-up and top-down. In the first approach, ZnO NWs were grown by hydrothermal method on sputtered zinc oxide (ZnO) coated glass wafer. In the second approach, arrays of ZnO nanowire were fabricated using silica nanosphere colloidal crystal formed by spin coating on a sputtered ZnO film on glass slide and etching in CF4:H2 plasma. The synthesized structures were studied by scanning electron microscope (SEM) and transmission electron microscope (TEM). Upright ordered arrays of ZnO NWs with a diameter range of 50-350nm and length of 1µm with a high crystalline structure were obtained.

The fabricated ZnO NW arrays were sensitized with hydrophobically ligated colloidal CdSe/ZnS QDs. QDs with two different ligands, octadecylamine (ODA) and trioctylphosphine (TOPO), and two various sizes were incorporated in the ZnO NWs using the drop casting and dipcoaing methods. The structural properties of the ZnO NW/QD were studied using high resolution-SEM and TEM. The results show that the QDs are fully filled the gaps between wires and orderly assembled onto the wires.
Charge transfer mechanism in ZnO NW/QD structures was examined by photoluminescence (PL) using steady state and lifetime measurements. The results compared for the structures containing NWs synthesized by two methods. The effects of QD size and ligands on charge transfer dynamics were also studied. A profound quenching of the QD emission peak and lowered average lifetime in ZnO NW/QD structure with respect to the control sample, which is defined as a layer of QD deposited on glass slide, were attributed to the deactivation of the excited QDs via electron transfer to ZnO nanowires. These studies provide insight on charge transfer dynamics at NW/QD interface and better designs for high performance QDSSCs can be engineered accordingly.

Semiconductor bandgap establishes a fundamental upper limit for the conversion efficiency of the photovoltaic (PV) devices. Lattice thermalization and sub-band gap transmission contribute to more than 50% of the total losses in photovoltaic devices. To reduce spectral mismatch losses, two basic solutions might be adopted: designing a novel solar cell to better use the solar spectrum, or modifying the solar spectrum to better match the solar cell. The fabricated solar cells are known as “Third-Gen PV” and considered as leading concepts that are mainly obtained with quantum dots (QDs). However, the toxicology of nanocrystals, health and safety information about nanomaterials and QDs as dominant material for research on advanced PV, are yet relatively unexplored.
In this work a review study on the Third-Gen PV, key factors in QD toxicity, potential pathways by which humans can become exposed to toxic substances, summarized studies on potential exposure pathways were presented. Since the greatest potential for aerosolization of QDs likely arises during the synthesis and manipulation phases of QD manufacturing, workplace exposure to aerosolized QDs requires careful consideration. Experimental results on the detection of various type of QDs and nanoparticles ranging from 6-300nm in the workplace using a particle counter were also presented.
Toxicity of QDs can, at least to some extent, be minimized through selection of an appropriate shell coating, by modulating surface charge or surface coating, selecting a lower overall dosage of QDs, or modulating the overall size of the QD. Airborne inhalation, dermal/ocular absorption, ingestion and injection are a number of potential means by which humans can become exposed to toxic substances. QDs have shown a preference for deposition in organs and tissues and that they do not remain circulating in the bloodstream. QDs are more likely to accumulate in the liver, spleen, kidneys, lymph nodes, and bone marrow. However, the scope of the migration of QDs in vivo is effectively limited by their size. Capping of QDs with ZnS significantly enhances their stability. Exposure of CdSe QDs to air and ultraviolet light leads to the degradation of the QD and the consequent release of free cadmium ions which is been greatly addressed as the major QDs methodological issue.
Nanoparticles follow airstreams very easily and will be collected and retained in standard ventilated enclosures equipped with HEPA filters. Nanomaterials need to be synthesized in enclosed reactors or glove boxes under vacuum, nitrogen and exhaust ventilation, which prevent exposure during the actual synthesis as well as nanodust expulsion. These studies and the obtained experimental results on size selection and counted particles suggest the possibility of handling the QDs for PV applications in such a way to minimize their negative impact on the human health while utilizing them for PV manufactures.

A complete understanding of the reliability for ZnO-based transparent conducting oxides is essential for actual applications in photovoltaic devices or displays requiring long-term stability. The stability and degradation mechanisms under damp-heat environment (humid and hot atmosphere) for sputter-deposited aluminum-doped zinc oxide (ZnO:Al) thin films were systematically studied. The continuous degradations of carrier concentration and mobility with the Fermi-level shift were observed, and these behaviors were resolved by separating the changes in the carrier-transport characteristics of grain boundary and intragrain. By correlating the temperature dependence of electrical characteristics with x-ray photoelectron spectroscopy, the degradation is well explained by the increase of chemisorbed OH^- in the grain boundaries. [1] J. Steinhauser, S. Meyer, M. Schwab, S. Fayuml;, C. Ballif, U. Kroll, and D. Borrello, Thin Solid Films520, 558 (2011). [2] Y. Kim, W. Lee, D.-R. Jung, J. Kim, S. Nam, H. Kim, and B. Park, Appl. Phys. Lett.96, 171902 (2010). Corresponding Author: Byungwoo Park: [email protected]

In this research, we suggest a facile method to synthesize nanoporous NiO thin films for p-type dye-sensitized solar cells (DSSCs). Preparation of porous NiO thin films was successfully achieved by simultaneous sputter deposition of Al and Ni, followed by selective etching of Al and subsequent heat treatment. The initial sputtering power of Al determined the porosity of NiO, and the resulting photovoltaic properties were correlated with the various nanostructures of NiO. Furthermore, we observed that additional NiO coating by nickel acetate (Ni(CH₃COO)₂) had positive effects on surface passivation, showing ~50%-increase in power-conversion efficiency compared to the bare cell. [1] H. Choi, J. Kim, C. Nahm, C. Kim, S. Nam, J. Kang, B. Lee, T. Hwang, S. Kang, D. J. Choi, Y.-H. Kim, and B. Park, Nano Energy (2013). [2] A. Nattestad, A. J. Mozer, M. K. R. Fischer, Y.-B. Cheng, A. Mishra, P. Bäuerle, and U. Bach, Nat. Mater.9, 31 (2010). Corresponding Author: Byungwoo Park: [email protected]

Silicon nanocrystal assembly is an interesting material which has a potential applicability to various functional devices. Its photo-conductive property, however, has not been understood sufficiently, although its knowledge is inevitable in opto-electronic device designing. In this paper, we investigated photo-conductive properties and studied the transport mechanism in Si nanocrystal assembly. We found that Auger excited carriers play important roles in the photo-conduction at room temperature operation.
We deposited Si nanocrystals having a diameter of about 5nm on n-type Si substrate by using pulsed laser ablation method in hydrogen atmospheres [1]. The deposit shows columnar structure with a thickness of 100nm. The photo-conduction properties of the Si nanocrystal assembly between the electrodes deposited on both sides of the sample were measured in the temperature range between 30K and 300K. As for the excitation light source, we used 325nm He-Cd laser line and 488nm Ar laser line.
The current-voltage (I-V) characteristics for the photo-current from 30K to 140K indicated a clear voltage threshold, suggesting Coulomb blockade behavior. We analyzed the experimental date and found that it can be expressed by Middleton and Wingreen&’s scaling law [2]. Thus, the carrier conduction in this temperature region is found to be governed by the percolation transport mechanism. When the operation temperature is increased higher, the I-V characteristics changed to the exponential function behavior with the activation energy of several tens of meV. This indicates that the transport mechanism in this temperature range is variable range hopping. When the temperature is raised further to around room temperature, the photo-current showed steep increase. The activation energy of this temperature range was evaluated to be several hundreds of eV. This phenomenon can be well explained by assuming that the photo-current is caused by the Auger excited carrier transport mechanism.

In conclusion, we elucidated the transport properties of photo-carriers Si nanocrystal assembly in the wide temperature region. It is found that the Si nanocrystal assembly behaves as a network of tunnel junctions between quantum dots when the operation temperature is lower, while the current is governed by variable range hopping in the higher temperature range. When the temperature approaches to room temperature, the steep increase of the photo-current was observed, which can be well explained by the Auger excited carrier transport mechanism. This result suggests that we can expect very high efficiency solar cell operation exceeding Shockley-Queisser limit using Si nanocrystal system.
Reference
[1] MRS Proc.832 F7.24(2005)
[2] A. Middleton and N. Wingreen, Phys. Rev. Lett. 71, 3198 (1993).

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because incorporation of small amount of O results in formation of a narrow, O-derived intermediate band IB (E-) well below the upper conduction band CB (E+) edge [1]. Previously, we have reported fabrication of ZnTeO-based IBSC structures using an n-ZnO window layer, and demonstrated a photogenerated current by two-step photon absorption [2]. However, because of the large conduction band offset between ZnTe and ZnO, a small open circuit voltage (Voc) of approximately 0.4 V was observed in this structure, [2]. In order to achieve higher Voc, it is necessary to develop a new n-window layer blocking electrical transport IB(E-). ZnS has a large direct band gap of 3.7 eV with a smaller conduction band offset against ZnTe, and therefore, a suitable n-window material for ZnTeO IBSC. Here, we have grown n-type ZnS thin films by molecular beam epitaxy (MBE), and demonstrated heterojunction ZnS/ZnTe solar cells with an improved Voc.
n-type ZnS thin films were grown by MBE on ZnTe(100) substrates using Al as a dopant. The substrate and the Al cell temperatures were changed between 100 and 300C and between 810 and 950 C, respectively, to optimize the growth conditions of n-type ZnS. Electrical properties of the thin films were characterized by Hall effect measurements. Finally, the solar cells with n-ZnS/i-ZnTe/p-ZnTe structures were fabricated.
At the substrate temperature of 250 C, n-type conductivity was confirmed under a wide range of Al cell temperatures between 810 and 910 C. The highest electron concentration of 4.2 × 10^19 cm^-3 and the lowest resistivity of 5.5 × 10^-3 Omega;cm were obtained at the Al cell temperature of 890 C. Rapid decrease in the electron concentration was observed when the Al cell temperature exceeds 910 C, probably due to self-compensation effects. A current-voltage (J-V) curve of n-ZnS/i-ZnTe/p-ZnTe solar cell under 1 sun illumination showed that the improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm^2, and the fill factor of 0.60, yielding the power conversion efficiency of 3.1 %.
[1] K. M. Yu et al. Phys. Rev. Lett. 91 (2003) 246403.
[2] T. Tanaka et. al. Appl. Phys. Lett. 102 (2013) 052111.

Nanocrystal thin films have been widely applied to photovoltaic devices, optical devices, electronic sensors, etc. As building blocks of nanocrystal thin films, nanoparticles are normally capped by organic ligands preventing aggregating and providing solubility in solutions. However, this insulating organic layer attaching to the surface of nanoparticle influences the performance dramatically such as conductivity, inter-particle charge coupling and most importantly, carrier mobility, etc. In this work, we developed a new theoretical model to study the dependence of ligand chain length V.S. carrier mobility in nanocrystal thin films.
Other than considering surface ligands as an energy barrier for inter-particle carrier transportation, a model of electron mobility in Nanocrystal ligand complex systems is proposed based on theoretically calculated microscopic cross sections of electron and lattice phonon interactions, which are in good agreement with the experiment. Different ligand has the special pattern of connecting with nanocrystal. The ligand chain length determines the effective mass and carrier mobility. This method can be broadened to model electron transport of other nano-crystal and ligand complex materials.

A modified polysulfide redox mediator with incorporation of cyanocobalamin (vitamin B12) was employed in quantum dot-sensitized solar cells (QDSCs), which can improve the electrocatalytic activity and reduce the charge transfer resistance caused by sulfureted contamination on the platinum counter electrode. The enhancement in the electron transfer properties could be regarded as a cause of a new formed vitalizing chelation which could behave as an ohmic characteristic interface between modified electrolyte and platinum counter. As a result, QDSCs with high filling factor of 65% with energy conversion efficiency more than 6% have been achieved.

Converting solar energy to electrical power or chemical potential has long been viewed as one of the greatest challenges of our time. Water-splitting photoelectrochemical cells (PEC) represent an efficient means to harvest solar energy but necessitate the design of photocatalytic constructs with discrete light-harvesting and catalytic components. Towards this end, we have interfaced CdSe quantum dots to lead vanadium oxide (β-Pb_xV₂O₅) nanowires to study their feasibility for photoelectrochemical cells. β-Pb_xV₂O₅ crystallizes with Pb²⁺ ions residing within the channels of a tunnel-like V₂O₅ framework. This material is characterized by mid-gap states that arise between the conduction and valence bands. The mid-gap states are located only slightly positive relative to the water-oxidation potential yet still negative relative to the valence band edges of CdSe quantum dots. Consequently, we propose their use within photocathodes wherein photogenerated holes can easily transfer across the CdSe QD/ β-Pb_xV₂O₅ interface and be propagated to water-oxidizing catalysts.
We have successfully functionalized β-Pb_xV₂O₅ nanowires with CdSe quantum dots using two separate approaches: successive ionic layer adsorption and reaction (SILAR) and linker-assisted attachment via bifunctional linker molecules. Control has been established through the SILAR process as shown by the CdSe layer thickness correlating directly to the number of SILAR cycles. Selective functionalization using linker-assisted attachment allows for direct control of the amount of CdSe on β-Pb_xV₂O₅ nanowires.
The CdSe QD/ β-Pb_xV₂O₅ nanowire heterostructures have been characterized by scanning electron microscopy and transmission electron microscopy to understand surface morphology and layer thickness. X-ray diffraction and Raman spectroscopy have been performed to further characterize the functionalized wires. Experiments examining the photo catalytic degradation of methylene blue have also been performed using our functionalized nanowire photocatalysts. This work was supported by the Research Corporation for Science Advancement through a Scialog Award.

The conversion efficiency of single junction solar cells is limited due to a necessary compromise between current and voltage that can be generated with a single semiconductor material. One way to overcome this limitation is the use of multi-junction solar cells. Today&’s record cell efficiencies of 44% have been achieved with triple junction cells made of III-V compound materials and put under concentration of 942 suns [1]. However, such cells typically are very expensive because of the use of Ge as substrates, and are typically used in concentrating devices. An alternative approach that promises reasonably high efficiencies is the use of a double junction cell made of GaAs on top of Si, which is cheaper and more abundant than Ge. The advantage of this approach is that it uses mature devices and gives a potentially significant efficiency boost compared to a single junction device. However, the disadvantage of this concept is that the lattice constant of silicon and GaAs do not match and a technique has to be found to combine these two cells.
In this project we concentrate on two different approaches:
1. Wafer bonding of the two sub-cells
2. Mechanical stacking of the sub-cells
These techniques can be used to create 2-, 3- or 4-terminal devices.
In this paper we present a theoretical study of GaAs - Si tandem solar cell stacked mechanically or by wafer bonding and investigate the most important parameters that affect the efficiency of this system. These parameters include tunnel junction property, surface recombination, light trapping and photon recycling, etc. This investigation will incorporate both extensive optical and electrical simulation to determine realistic device performance. With this analysis, a comprehensive design rule can be given for designing GaAs on Si cells for different applications in different meteorological regions.
1. Green, M.A., et al., Solar cell efficiency tables (version 41). Progress in Photovoltaics: Research and Applications, 2013. 21(1): p. 1-11.

Surface textures on solar cells can confine light and compensate light absorption losses resulting from a thin absorber layer. Conventional light trapping structures are pyramidal textures on monocrystalline- or isotextures on multicrystalline silicon wafer solar cells. The features of these textures have sizes that are typically in the micrometre range. Conventional textures, however, are not applicable for thin silicon-based solar cells as the absorber layer thickness itself is no larger than a few micrometres [1]. Nanoscale surface textures are hence explored catering for solar cells with a thin absorber layer. This project investigates the optical properties of nanoscale “muffin” structures fabricated from a lithography procedure and simulated using the Rigorous Coupled Wave Analysis (RCWA) method. The “muffin” structures are modelled as a periodic nanostructure in RCWA and study of its optical properties shows a potential for light trapping application, especially in thin silicon solar cells.
The objective of this investigation is to demonstrate a structure with a thickness of a few hundred nanometres that enables light trapping comparable to that of conventional textures with a much larger thickness and is, therefore, applicable, to very thin silicon layers. Preliminary results show that the muffin structure has a high potential for light trapping by concentrating light within a volume close to the texture.
Using RCWA, we investigate theoretically the path length- and absorption enhancement that can be observed for silicon films with a “muffin” surface texture over a broadband wavelength range. The results are compared to those of a planar silicon film and other types of textures. The “muffin” surface textures are further optimized by studying the optical properties for structures with varying shape parameters. By simulation, the nanoscale periodic muffin structures with a small aspect ratio are able to achieve a substantial absorption enhancement.
1. Mavrokefalos, A., et al., Efficient Light Trapping in Inverted Nanopyramid Thin Crystalline Silicon Membranes for Solar Cell Applications. Nano Letters, 2012. 12(6): p. 2792-2796.

Future photovoltaic systems can be greatly benefited by cell and module design that enables simultaneously ultrahigh efficiency (> 50%) and low-cost (< $0.50/Wp) to propel sharp reductions in the levelized cost of electricity. The photonic environment of the cell should be designed to maximize the external radiative efficiency and the open circuit voltage, and key factors that affect open circuit voltage will be discussed. I will also discuss ‘full spectrum&’ photovoltaic modules, which take advantage of advances in low-cost III-V compound cell fabrication and emerging optical and electronic fabrication/assembly methods, features 6-15 independently connected subcells in a spectrum splitting, concentrating photovoltaic receiver. Module architectures utilizing independently connected single junction and multijunction subcells allow flexibility in subcell selection for optimal energy bandgaps and fabrication, and also reduce the constraints posed by current matching requirements. Several different spectrum-splitting optical architectures designed for systems with many (>6) subcells are possible, including designs based on holographic spectrum splitting, specular reflection in dielectric polyhedra and light trapping textured filtered dielectric slabs that perform as nonimaging concentrators.

In order to realize more than 45% efficiency, new approaches for novel materials and structures are necessary. III-V lattice-mismatched material is one of appropriate materials for high efficiency multi-junction solar cells with efficiency of more than 45% due to optimum band gap energy combinations of sub cell materials. However, lattice-mismatching-related dislocation generation in active solar cell layers is one of major problems for high efficiency. InGaAsN is also another appropriate material for 4- or 5-junction solar cell configuration because this material can be lattice-matched to GaAs and Ge substrates. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers and low carrier mobility due to alloy scattering and \non-homogeneity of N.
This paper presents our recent results in 1) understanding of lattice-mismatching-related strain relaxation and dislocation behaviour in the InGaAs/GaAs lattice- mismatched system, and 2) understanding of majority and minority carrier traps in GaAsN grown by chemical beam epitaxy (CBE) and their relationships with the electrical properties of the materials and solar cells.
In order to understand dislocation behaviour and reduce dislocation density in lattice-mismatched InGaAs/GaAs, in-situ measurements of reciprocal space mapping of X-ray diffraction during molecular beam epitaxial growth of InGaAs/GaAs have been studied. As a result, mechanism of dislocation motion in lattice-mismatched materials and effects of buffer layer upon strain relaxation and dislocation behaviour have been discussed.
Under the Project of the Europe-Japan Collaborative Research on Concentrator Photovoltaics, high efficiency inverted epitaxially grown InGaP/GaAs/InGaAs 3-junction solar cells with efficiencies of 37.9% under 1-sun and 43.5% under 240-306 -suns have been demonstrated by Sharp. Co. as a result of these studies.
By adapting CBE technique to grow (In)GaAsN thin films, higher mobility and longer minority-carrier lifetime compared to those grown by the other growth methods have been achieved. As a result, 13.7% efficiency (Jsc=20.3mA/cm2, Voc=0.721V, FF=0.815) GaAsN single-junction cells have been obtained. Further improvement in efficiency is necessary for realizing high-efficiency 4-junction solar cells.
To solve above problems, we have characterized deep levels in CBE-grown GaAsN films by Deep-Level Transient Spectroscopy (DLTS). In conclusion, the following results have been obtained in this study:A well-known electron trap E2 (Ec-0.33eV) center in n-GaAsN and p-GaAsN has been for the first time confirmed to be non-radiative recombination center by using double-carrier pulse DLTS. (N-As)As is thought to be a possible origin of the E1 center.

Optimization of the conversion efficiency in solar cells requires addressing both entropy and energy loss mechanisms simultaneously. Multi-junction solar cells are able to achieve higher performance over conventional single-junction cells, where the energy loss due to thermalization and the limited absorption spectrum are addressed. Further efficiency improvement can be achieved through appropriate light management architectures, which can reduce losses due to reflection, or enhance absorption of the transmitted light at energies just above the bandgap of the absorbing material inside the solar cell. Several light management approaches are possible due to advances in nano- and micro-fabrication techniques. These include single- and multi-layer anti-reflection coatings (ARCs), and surface texturing via periodic gratings, nanowire and nanopore arrays.
In this work we focus on the design of multi-layer ARCs to minimize reflection at the cell top surface, in GaAsP/Si tandem cells. The developed model is based on the Transfer Matrix Method (TMM) and takes into account coherent and incoherent reflections from the various layers. The calculated reflectance is mapped over a broad wavelength range, for different material combinations and thicknesses. The final ARC designs are determined by the largest broadband reflection suppression.
For each optimized ARC design, the enhancement of the quantum efficiency of the bottom Si cell is also estimated. The reference quantum efficiency curve for the Si cell is measured from a Si cell adapted with a GaAsP/GaP filter layer. This combination mimics the target GaAsP/Si cell. Dispersion and absorption are considered for all materials, based on experimental data and literature. The reflectance spectrum of the bare air/GaAsP interface is experimentally obtained and is used for assessing the performance of the proposed ARC designs.
The model indicates that multi-layer ARCs provide reflection reduction over a wider portion of the spectrum, when compared to their single-layer counterparts. Optimized double-layer (DLAR) and triple-layer (TLAR) coatings implemented with Al₂O₃/SiC, SiO₂/SiC and SiC/HfO₂/MgF₂, respectively, provide a significant reflection reduction over a broad spectrum. The layer thicknesses vary between 50-120 nm. The results show that the proposed DLAR coating designs reduce reflectance at the air/GaAsP interface to below 8% over the wavelength range of 450 nm to 1000 nm, compared to about 30-35% reflectance measured at the bare interface. The proposed TLAR coating design shows a significant further reflectance reduction in the near ultraviolet (300-390 nm) when compared to the DLAR coatings. The estimated reflectance for the TLAR coating is about 10% at the near ultraviolet wavelengths (as opposed to about 45% measured at the bare interface), and about 5% from visible to near-infrared (390-1000 nm) wavelengths.

It is well known that the layering of multiple active junctions can increase solar cell efficiency by maximizing the amount of energy that is harvested from solar photons of varying energy. The monolithic, or two-terminal tandem (2TT), has been developed for a number of material systems, but the non-series, or three-terminal tandem (3TT), structure remains largely unexplored (here, the third terminal is a transparent conductor inserted between the two cells of a 2TT). In this work we use the detailed balance analysis to compare the losses and limits of both device architectures, highlighting cases in which the 3TT can outperform the 2TT. For example, we find that the 3TT will be less susceptible to current matching issues that may arise from film thickness fluctuations common in many thin film processing techniques. Most importantly, however, we show that compared against the monolithic tandem, the 3TT exceeds 40% theoretical power conversion efficiency (PCE) for five times as many band-gap combinations, significantly expanding the materials phase-space available to researchers.

Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. We evaluate inorganic thin-film top cells in a tandem stack with a high-efficiency c-Si bottom cell. We show that a tandem cell efficiency of 30% requires minimum top cell efficiencies ranging from 14% with 2eV bandgap, to 22% for a bandgap of 1.5eV. Including the typical parasitic light absorption from transparent conducting layers further increases the required top cell efficiency to 17% at 2eV and 24% at 1.5eV.
To investigate these limits in more detail, we develop a simple model to identify the requirements for the top cell in terms of optical absorption, electronic bandgap, carrier transport and luminescence efficiency. In particular, we show that even relatively low quality earth-abundant semiconductor materials with luminescence efficiencies of 10-5 and diffusion lengths below 100nm are compatible with tandem cell efficiencies above 30%. Introducing light trapping in the top cell can increase the efficiency beyond 32%, and reduce the required diffusion length below 50nm. Detailed Figure of Merit calculations of light trapping mechanisms deliver clear design recommendations for optimising light distribution in tandem devices. Wavelength-selective intermediate reflectors are observed to be detrimental to tandem performance, and single-pass absorption in the top cell is found to be preferable to Lambertian light trapping with analytically derived losses.
This analysis establishes clear research targets for high-bandgap semiconductor materials and novel thin film solar cell concepts that can be combined with existing c-Si technology. Such tandem approaches could enable the rapid development of a new generation of low cost, high efficiency solar cells.

Solar modules that involve large, interconnected collections of small, ultrathin photovoltaic cells offer unique options in engineering design. This talk describes assembly strategies and optical aspects for this type of technology, as implemented with silicon and compound semiconductors derived from wafer-scale sources of material. Examples include modules that exploit optical concentration based on (1) focusing micro-optics to achieve ultrahigh efficiencies for utility-scale power generation, and (2) thin, plastic luminescent waveguides to enable flexible, rugged mechanics for portable energy.

Advances in new photovoltaic active materials have led to both increased efficiency and decreased cell cost. For these trends to continue however, additional consideration must be given to non-solar module components, including tracking and concentration mechanisms. While traditional tracking and concentration methods are expensive and require complex structural foundations, new techniques are being developed to circumvent these limitations. One such technique utilizes tuneable origami substrates, in which individual features can be oriented as a function of feature geometry and sample strain. By incorporating active photovoltaic materials into or onto these tuneable structures, strain-controlled feature manipulation can be used as a low-power, integrated solar-tracking method.
This talk will briefly review the principles of tracking systems that minimize power loss due to changes in oblique light incidence, and further detail the design and implementation of flexible, polyimide-mounted GaAs origami solar cells. Two origami designs will be examined, comprising 1) offset, repeating linear cuts, and 2) repeating square cuts, both exhibiting different stress-strain behavior. The linear pattern is capable of 45 degree solar tracking, with a maximum feature angle of 45 degrees at 25% strain in the plane of the module, whereas the square pattern provides solar tracking up to 90 degrees at 25% strain in the plane of the module. We will discuss the impact of feature geometry on cell architecture and mechanical behavior, as well as the effect each design has on solar tracking performance. Simulations and experimental results for optimized device layout and potential methods for autonomous tracking operation will also be presented.

We exploit microscale, bifacial thin-film GaAs solar cells embedded in luminescent waveguides to double the output power under standard solar illumination. For GaAs cells, efforts to reduce costs focus on techniques to reuse the growth wafers and concentrate incident sunlight. We have demonstrated that thin-film GaAs micro cells can be epitaxially lifted off (ELO) from the grown wafers, and embedded on soft, flexible luminescent solar concentrators (LSCs). The ELO process enables the reuse of the expensive GaAs substrates for re-growth. Luminescent dyes in the LSC substrate help absorb the incoming light, with waveguided re-emission concentrated on the backside of the GaAs cells. The LSC operates independent of incident angle, in principle surpassing the fundamental ergodic limits due to the down-shift in photon energy. Experimental cells that incorporate a free-standing waveguide layer with a diffuse backside reflector deliver the highest photocurrent, due to simultaneous capture of both waveguided luminescence and diffuse scatter. Numerical studies suggest that higher performance and greater cost can be obtained by implementing optimized LSC parameters and improved cell designs. The ELO and transfer printing techniques represent methods for cell/LSC integration with potential for large scale module production.

Nanowire core-shell heterostructures have gained much attention for photovoltaic applications because they can provide enhanced light trapping effects, elastic strain relief, and a decoupling of the absorption axis from the carrier collection axes. For novel solar cell architectures that incorporate significant structural, chemical, and electronic heterogeneities, a detailed mechanistic understanding of spatial variations in external and internal quantum efficiency is essential to understand and improve device performance. Advances in spatially resolved measurements of electronic properties are therefore necessary to the development of new generations of devices. Here we apply spectrally resolved scanning photocurrent microscopy (SPCM) to characterize a nanowire solar cell consisting of an array of GaN-InGaN core-shell nanowires connected at the tops by a p-InGaN canopy layer and fabricated using scalable device fabrication techniques.[1] The spatially and spectrally resolved photovoltaic performance measured by SPCM is correlated with structure and composition inferred from (1) atomic force microscopy (AFM) topography; (2) scanning transmission electron microscopy (STEM) imaging; and (3) Raman microspectroscopy. SPCM images reveal the contributions of individual nanowires to the external quantum efficiency within an array of ~500 nm pitch, and identify structural defects that limit device performance. We find that the quantum efficiency is highest for illumination of the regions in between the nanowires, likely due to a combination of photonic effects and enhanced carrier collection in the generation region and this conclusion is supported using Finite Difference Time Domain (FDTD) simulations. The analysis is used to spatially decompose the overall device EQE and identify promising directions for optimization of performance.
References
1. Wierer, J. Jr., Wang, G.T., et al, Nanotechnology 23, 194007 (2012).
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Radial p-n junction is one of the most suitable structures realizing concurrent maximization of light absorption and photogenerated carrier separation in photovoltaic (PV) devices. As a platform of radial p-(i)-n junction, semiconductor nanowire (NW) has attracted great attention because NW synthesis is more facile way than micromachining for preparing radial or orthogonal p-(i)-n junction. However, high-performance PV devices based on radial p-n junction NWs have not been successfully demonstrated though high efficiency of single NW PV device has been reported. The main problems of NW-based PV devices are huge surface recombination of photogenerated carriers, short carrier diffusion length in radial shell, difficulty in optimal top-electrode formation, and manufacturing cost. Here we present high-performance crystalline Si radial p-i-n junction NW PV devices on Si and stainless steel substrates. Our comprehensive study of epitaxial growth of Si radial p-i-n junction on Si NWs and characterizations of electrical and optical properties of Si radial p-i-n junction reveals the design rule of high-performance PV cells based on Si radial p-i-n junction arrays and the feasibility of Si PV cells on economical and conductive substrates such as stainless steel.
Si radial p-i-n junctions consisted of core Si NWs and Si shells. Core Si NWs were prepared by both of top-down and bottom-up approaches. Top-down approach consisting of lithographic technique and deep reactive ion etching was employed to study design rule of Si radial p-i-n junction NW PV devices. Bottom-up approach of Au-catalyzed chemical vapor deposition (CVD) growth was used to prepare Si NWs on stainless steel substrate. Dimensions and electrical doping profiles of epitaxial Si shells were precisely controlled by low-pressure CVD growth. The doped and undoped Si shells were grown in the range of 500 to 800oC to avoid autodoping problem at high growth temperature. Single crystalline doped and undoped shells were obtained at the temperatures in the range of 700 to 800oC. The physical properties of Si radial p-i-n junction NW arrays were investigated by current-voltage characterization, quantum efficiency measurement, photocurrent microscopy, and photovoltaic response measurement. Through the series of characterizations we elucidate optimal window of material parameters and decoupling effects of enhanced optical absorption and carrier extraction along radial direction on PV performances of Si radial p-i-n junction NWs. Additionally we discuss the evolution of PV response of Si radial p-i-n junction NWs from single to arrays. Moreover, the PV efficiencies of crystalline Si radial p-i-n junction arrays with optimized geometry and marginal Si radial p-i-n junction NWs on stainless stell substrate will be presented.

CdTe has enjoyed the strongest commercial success of all thin film solar cell technologies despite it's moderate conversion efficiency performance. The success has come largely from a strong cost-performance ratio. CdTe's excellent manufacturability attributes, including extremely high deposition rates, ease of uniform large area processing, and forgiving process tolerances as compared to CIGS and other compound thin-film materials, have been instrumental in establishing capable manufacturing capacity. With low-cost high-throughput manufacturing firmly in place, the industry is now turning it's attention to closing the relatively wide performance gap between lab results and theoretical limits. The highest reported CdTe research cell efficiency at the time of this writing was 19.05% by First Solar, as compared to the S-Q limit of nearly 33%. Device performance improvements over the past 20 years have been dominated by Jsc and FF increases which are now at the 28mA/cm2 and 78% respectively. However Voc has been largely stagnant around 850mV and has only recently been taken to 872mV in a high efficiency device and 903mV in a polycrystalline high voltage device. Recent research indicates that Voc at these levels is strongly correlated to minority carrier lifetime which it is believed can be improved through techniques currently in development. If successful, Voc of 925mV with modest improvements in Jsc and FF should enable a 22% CdTe cell in the near-term without assuming any radical change to materials or device architecture. Based on this visible trajectory for research cell performance and CdTe's relatively narrow gap between research cell and large area efficiency, module performance can be expected to reach 17% in the coming few years. Such conversion efficiencies will put CdTe field performance significantly ahead of commodity crystalline Si module technologies at real operating temperatures which can average 50C to 60C on a power-weighted bases in typical dessert conditions. Given that the underlying cost structure of thin-film CdTe is area-based and that performance improvements are derived from process, not architecture changes, the anticipated conversion efficiency trajectory should result in a 30% reduction in module manufacturing costs with an additional savings of 15cent; to 25cent; in component and labor costs at the system level from improved areal power density.

Understanding the effect of chlorine-related defects on the CdTe electric properties is important both for obtaining high resistivity (HR) CdTe-based detectors and high efficiency CdTe-based thin-film solar cells. However the actual mechanism of the effect of Cl on electric properties of CdTe is not clear and is still under discussion. It is known from the experiments that the resistivity of CdTe:Cl increases up to 1e8-1e10 Omega; cm with Cl concentration. To simulate HR of CdTe, early defect chemistry calculations included either deep donors or deep acceptors. However recently in work by Biswas and Du it was claimed that shallow donors rather than deep donors are responsible for high resistivity in detector-grade CdTe.
In order to resolve this contradiction we derive analytically the limits of Fermi level values on the stage of sample preparation/annealing at high temperature, that lead to HR at RT in a system with shallow defect levels only (i.e. defect which ionization degree does not change upon cooling). We show that the experimentally observed HR in CdTe:Cl samples cannot be explained by shallow defect levels only and non-shallow defects that are able to change their charge state upon cooling are needed to diminish the excess charge (and resulting conductivity) that is formed in the material at high temperature. Obtaining the experimentally observed HR is possible only with a deep defect with thermodynamic charge transition level in the range ~ 0.6-0.9 eV above VBM. Among Cl-related defect and complexes, Cl_Te-Cl_Cd double acceptor complex meets this criterion with 0/-2 transition level at about 0.9 eV according to our HSE hybrid functional calculations. We present a defect chemistry model with intrinsic CdTe defects, Cl_Te, Cl_i defects and Cl_Te-Cl_Cd complex, which is capable, with the same set of parameters of defects, to reproduce qualitatively the available in the literature experimental data on high temperature Hall data of pure CdTe, high temperature conductivity of CdTe:Cl as well as the increase of resistivity with increase of Cl content and the formation of high resistivity in CdTe:Cl at RT. The presented model can provide an insight into the effect of Cl on CdTe properties which is important both for CdTe-based detectors and CdTe-based solar cells.

Chalcogenide photovoltaic materials are attractive options for thin film solar cells due to their effective optical absorption and inexpensive fabrication processes. However, the power conversion efficiency of commercial cadmium telluride (CdTe) solar modules is 13 %, well below the theoretical maximum value of 30 %. The underlying physical mechanisms for the low efficiency are presently not well understood. Here, we investigate local photovoltaic properties with a focus on the difference between the grain bulk (< a few µm in size) and the grain boundary in the CdTe absorber. We use electron beam induced current (EBIC) to measure the charge carrier collection efficiency. By using low energy electron beams, we achieve a high spatial resolution (asymp;20 nm) on both the top surface and in the cross-section of the device. We additionally use patterned contacts to confine the collected current path. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk. These measurements are supplemented with 2-dimensional drift-diffusion simulations. We find that in this material, there is downward band bending near grain boundaries (with typical magnitude of 0.2 eV) which is responsible for the enhanced charge collection of grain boundaries. More generally, we find that the EBIC signal in this low energy regime can be used as a probe of local electric fields.

The rapid development of thin-film solar cells has largely focused on alternative absorber materials, while the choices for buffer layers remain somewhat limited.
The most common buffer layer material is cadmium sulfide (CdS), which exhibits very good electrical properties, but whose band gap (2.4 eV) is too small to be transparent to the entire solar spectrum and leads to a loss of useful photocurrent. The use of a wider band gap material with good electrical properties is desired, but the precise material characteristics dictating the electrical properties are not fully understood. Here we present a first principles study to benchmark the thermodynamic and electrical characteristics of intrinsic and common extrinsic point defects in CdS and ZnS, a larger band gap alternative buffer layer. We use density functional theory to calculate the structures, formation energies, and energy levels of each defect, from which we analyze the electrical nature, carrier compensation, and carrier recombination properties of the defects, in the context of buffer layer electrical properties in a thin-film solar cell. Correlation of defect properties with growth conditions is made in terms of film stoichiometry and presence of impurities. We also calculated the band alignments in a conventional Cu(In,Ga)Se₂ solar cell, showing why CdS performs well and why Zn(O,S) is a promising alternative buffer layer for high-efficiency devices.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The development of light concentrating techniques and setups for Cu(In,Ga)Se₂ thin film solar cells can help to improve their overall device efficiency further whilst also reducing raw material usage. Combined with a cell minimization from millimeter to micrometer scale, the heat transfer and cooling in the solar cells is optimized as well as limitations caused by serial resistances in the transparent front contact and the absorber layer itself can be reduced in general [1]. By focusing on new concepts in low concentrating optics, it is possible to increase the tolerance against different solar altitudes while the demand in cost expensive two-axis tracking systems is alleviated [2].

Light concentration in photovoltaic devices will cause an adjustment of several key parameters: the electrical contacts, the heat management, material stability and the efficiency of optical elements. In this work micrometer sized Cu(In,Ga)Se₂ chalcogenide solar cells were investigated under concentrated and non-concentrated AM 1.5 light using two different cell and circuitry geometries.
Linear shaped Cu(In,Ga)Se₂ solar cells completed with the, until now mostly in industry used, P1/P2/P3-patterning technique are compared with point like cells utilizing lapped joint interconnects. By a stepwise reduction of the cell width from 1900 to 100 mu;m, characteristic shunt-profiles of each circuitry approach could be analyzed in detail, what finally allowed a quantification and evaluation of their impact on cell performance. Especially for the line shaped solar cells, a basic re-design of the connection is proposed in order to preserve a high diode quality behavior. The results are introduced into a new IV-simulation model that allows an optimized calculation of future geometries. To investigate material issues like changes in (photo)-conductivity, stoichiometry, defect concentration and charge accumulation effects, both cell geometries are studied with low and high concentrated light between 10 and 1000 suns with full spectral illumination.
[1] M. Paire, A. Shams, L. Lombez, N. Pere-Laperne, S. Colling, J.L. Pelouard, J.F. Guillemoles, D. Lincot, Energy & Environ. Sci., 2011, 4, 4972.
[2] K.A. Baker, J. H. Karp, E.J. Tremblay, J.M. Hallas, J.E. Ford, Applied Optics, 51, 8, 2012, 1086.

CIGS absorber films incorporating self-assembled nanostructures can be uniformly formed over large areas by the Reactive Transfer Synthesis (RTS) method used in HelioVolt&’s module manufacturing process. Comparative analysis of the photocurrents of devices made with and without these nanostructured surfaces, as well as with conformal and non-conformal junction contact layers, suggest that light-trapping and three-dimensional carrier collection play an important role in the device physics of their operation. These characteristics are important contributors to reduced manufacturing cost because they enable the use of thinner CIGS layers, requiring less raw material inputs and significantly improving the productivity of the equipment used for semiconductor deposition. Our synthesis process can be tuned either to form planar surfaces or nanoscale morphological asymmetries in the CIGS surface region. We believe nanostructure formation is driven by Cu-Se flux-assisted recrystallization during the second stage of the RTS process, leading to coalescence and coarsening of both the CIGS grains and the voids formed between them by reactive mass transport. Emitter structures on these absorbers have been fabricated by Chemical Bath Deposition (CBD) of CdS, followed by the deposition of a bilayer of undoped and doped ZnO:Al (AZO). The CBD CdS layer is conformal on both types of CIGS absorber surfaces, but two methods have been used to deposit the undoped ZnO: sputtering (which results in a nonconformal layer on nanostructured CIGS due to shadowing effects), and Atomic Layer Deposition (ALD, which is conformal). The thicker, conducting AZO contact layer is nonconformal on the nanostructured absorbers. We find a dramatic increase in photocurrent for the ALD compared to sputtered ZnO devices on nanostructured absorbers, but not on planar ones, providing nearly perfect internal quantum efficiency over a broad spectral range in the former case. Device efficiencies between 14-15% without anti-reflection coatings have been demonstrated using both planar and nanostructured CIGS absorbers from our production line. Materials, optical, and electrical characterization data will be presented, and the prospects for further improvement of PV devices incorporating nanostructured CIGS absorbers described.

Solar is one of the fastest growing renewable energy sources in the world today. According to Green Tech Media, the U.S. now has over 8.5 GW of installed capacity. This is enough to power 1.3 million average American homes. Distributed residential photovoltaic applications, while currently the smallest portion of the U.S. installed capacity, are in particular poised to expand. In the U.S., distributed residential photovoltaic applications can broadly be fragmented into building applied photovoltaics (BAPV), which are attached to a building and serve the purpose to generate electricity, and building integrated photovoltaics (BIPV) such as faccedil;ade and roofing material applications that are by definition multifunctional in that they generate electricity and serve to function as a building material. While BAPV and BIPV approaches are vastly different, they do share some common attributes, safety requirements, and performance metrics that must be demonstrated to enable customer adoption. Here, we share insights related to these requirements and in relation to next generation technologies present how a keen understanding of the interplay between product design, manufacturing process, raw materials, and geographic specific end-use environment can catalyze the transition of new technologies from the lab bench into commercialization.

Thin film solar cells on flexible substrates such as Cu(In,Ga)Se₂ on polyimide foil open up new markets like building integrated photovoltaics. Today, efficiencies up to 20 % can be reached on lab scale for this material system on polyimide foil.
All samples in this contribution are from an industrial roll-to-roll process developed at Solarion AG which yields efficiencies up to 15 %. For further solar cell optimization it is crucial to determine the main recombination processes which limit the path to 20% on an industrial scale.
As the polyimide substrate does not contain sodium - in contrast to soda lime glass which is commonly used as substrate - sodium fluoride is co-evaporated during CIGS deposition. In this study we look at solar cells with a broad range of sodium contents and analyze the interdependency with the CdS buffer layer properties.
We used temperature dependent current-voltage measurements in the range of 300 to 100 K to distinguish among interface-, tunneling enhanced interface- and bulk recombination. These results are supported by capacitance-voltage profiling and apparent quantum efficiency measurements.
We show that the amount of sodium and the properties of the CdS buffer layer strongly influence the recombination processes. These results can significantly facilitate the development of highly efficient industrial Cu(In,Ga)Se₂ solar cells on polyimide foils.

Although the Cadmium Chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the CdTe layer are not well understood. In this investigation we apply advanced micro-structural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis, X-ray Photoelectron Spectroscopy (XPS) depth profiles to show the effect of annealing time on the diffusion of chlorine into the CdTe. Grain orientation data as well as grain size change has been obtained using Electron Backscatter Diffraction (EBSD) on Focused Ion Beam (FIB) prepared planar sections providing grain texture and grain size measurements. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the CdTe layer.

Currently, Cu(In,Ga)Se2 (CIGS) solar cells tend to use CdS as an n-type buffer layer. However, CdS has several disadvantages, including the toxicity classification of Cd, the non-vacuum chemical bath deposition (CBD) process, and the absorption of blue light. Therefore, several groups have proposed alternative n-type layers for CIGS solar cells using simple and conventional dry processes, such as sputtering.
We have proposed the use of sputtered amorphous Zn-Sn-O (ZTO) for CIGS solar cells. In general, amorphous thin films are interesting because they reduce the number of grain boundaries both in the n-type layer and at the interface around the pn-junction. Moreover, lattice mismatch with the p-type photoabsorber is able to be ignored in amorphous thin films. As a result, the number of interface defect states may be reduced, which may thereby lower the interface recombination. Therefore, amorphous ZTO could be a suitable candidate for the n-type layer in CIGS solar cells.
One serious problem of sputtering techniques is the sputtering damage that occurs at the surface of the CIGS layer when the n-type layer is deposited. In fact, it is difficult to directly examine the sputtering damage by performing electrical measurements because CIGS solar cells have several inhomogeneous interfaces. To investigate the several interfaces of CIGS solar cells, we previously used electrochemical impedance spectroscopy (EIS). This simple technique enables the observation of defects, grain boundaries, and the diffusion of atoms around several interfaces. In this presentation, we discuss the growth of alternative amorphous n-type layers for CIGS solar cells. In addition, we discuss the investigation of sputtering damage around the pn-interface by EIS.
A conventional ITO/ZTO/CIGS/Mo/glass solar cell structure was fabricated by selenization (CIGS) and RF sputtering (ITO, ZTO, Cu-In-Ga precursor, and Mo). EIS clarifies the electrical properties and defect physics of the region around the pn-interface of CIGS solar cells.

Next-generation solar cells will be fabricated from low-cost and earth abundant elements using streamlined processes that depart from conventional batch processing. This talk will delve into the opportunity of utilizing compositionally and structurally controlled colloidal nanocrystals as building blocks of such devices. Our recent efforts in developing scalable methods for synthesizing kesterite Cu2ZnSnS4 (CZTS) nanocrystals, one of the most promising materials to emerge in this area, enable the deposition of CZTS thin-films using a variety of solution-processed methods. Such nanocrystalline thin films possess poor electronic properties, which obviates their use in solar cell devices. To overcome this, chemical and thermal treatment steps are applied to modify the surface chemistry, include Se into the lattice and induce large scale crystallite growth. We will discuss each of processing factors in detail, highlighting the significant challenges that need to be overcome in order to fabricate working CZTSSe thin film solar cells from nanocrystals

Thin-film silicon photovoltaic is expected as an alternative to traditional crystalline silicon solar cells because it costs less and uses less silicon. Plasma-enhanced chemical vapor deposition (PECVD) from SiH4 gas is a widely-used growth technique for silicon film. SiH3 and SiH2 radicals, which are decomposed from SiH4 molecules in the plasma, are main precursors for the film growth. Many experimental results suggest that SiH3 radicals produce higher quality films than SiH2 radicals. To control the atomic-scale structure and obtain higher performance solar cells, the detailed understanding of silicon CVD process for SiH3 and SiH2 radicals is strongly required. Here, classical molecular dynamics method has been often employed to simulate the silicon crystal growth process. However, it cannot describe the electron transfer dynamics. Therefore, we newly developed a crystal growth simulator based on quantum chemical molecular dynamics (QCMD) method [1, 2], which enables us to clarify the chemical reaction dynamics during silicon CVD. In this study, our aim is to clarify the atomistic growth mechanisms of silicon thin films for SiH3 and SiH2 radicals during PECVD by using the crystal growth simulator based on QCMD method. We perform film growth simulations of H-terminated Si(001) by the continuous irradiation of each 80 radicals. We find that the growth process by SiH3 radical irradiation consists of two stages. Concretely, the first SiH3 radical abstracts a surface hydrogen atom and produces a dangling bond on the surface. Then, the second SiH3 radical is adsorbed on the dangling bond. Therefore, at least two SiH3 radicals are essential for the film deposition. On the other hand, one SiH2 radical is enough to generate a new Si-Si bond on the surface. We find that a SiH2 radical is directly adsorbed on a hydrogen terminated site and it consists of two chemical reactions. Firstly, a SiH2 radical abstracts a hydrogen atom and produces a SiH3 radical and dangling bond. Secondly, the produced SiH3 radical is adsorbed on the dangling bond. Next, we investigate the effects of the different growth mechanisms for SiH3 and SiH2 radicals on the growth rates and film qualities. The growth rate of SiH2 radical deposition is higher than that of SiH3 radical deposition. However, SiH2 radical deposition produces a defective film with silicon polymers. On the other hand, during SiH3 radical deposition, a lot of Si-Si bonds and ring structures are formed. We suggest that the abstraction-adsorption mechanism by SiH3 radicals prompts the layer-by-layer and epitaxial growth, while the direct adsorption mechanism by SiH2 radicals causes the 3-dimentional growth. Therefore, our QCMD simulations conclude that SiH3 radicals have an important role on the formation of a higher quality film than SiH2 radicals.
[1] T. Kuwahara, H. Ito, M. Kubo et al., J. Phys. Chem. C. 116, 12525-12531 (2012).
[2] H. Ito, T. Kuwahara, M. Kubo et al., Jpn. J. Appl. Phys. 52, 026502 (2013).

The development of highly efficient, low-cost solar cells based on abundant and non-toxic materials is a long-term target of present worldwide research. Although silicon is an abundant material, it exhibits a relatively low absorption in a constricted spectral range. For a more efficient use of the solar spectrum, multijunction thin film solar cells with absorber layers from highly absorbing materials with different band gaps are considered. An elegant way to combine excellent optical and electrical properties of a large variety of materials with the well-established thin film silicon technology is the embedding of nanoparticles into thin film silicon devices. In the present study, spherical α-Fe₂O₃ (hematite) nanoparticles with diameters of 40 nm to 80 nm are embedded into the intrinsic amorphous silicon (a-Si:H) layer of a state of the art p-i-n amorphous thin film silicon solar cell structure. This system allows the investigation of the absorption in the nanoparticles, as well as the charge transfer to the surrounding matrix or an adjacent layer, depending on the position of the nanoparticles in the film. Furthermore, the impact of nanoparticles on the growth of the matrix material can be studied. Therefore, the hematite nanoparticles used in the present study serve as a suitable model system for embedding experiments. Since the substrate texture influences the nanoparticle distribution on the surface and the layer growth, the solar cells were prepared on aluminum doped zinc oxide substrates with various textures. A spin-coating process was used to apply monolayers of hematite nanoparticles from a dispersion onto the textured surface of a p-type amorphous silicon layer deposited in a plasma enhanced chemical vapor deposition process. Scanning electron microscopy investigations of the resulting nanoparticle layers show a widely homogeneous distribution with a well-controlled surface coverage. To study the growth of the intrinsic a-Si:H layer on the nanoparticles, scanning transmission electron microscopy measurements were performed on a lamella cut out of the solar cell layer stack, revealing voids underneath the nanoparticles and areas of reduced a-Si:H thickness in their vicinity. Measurements of the device&’s I-V characteristics show a working solar cell and therefore indicate, that the nanoparticles do not compromise the insulating properties of the intrinsic a-Si:H layer. Furthermore, constant photocurrent measurements were performed to investigate the contribution of charge carriers generated by absorption of light in the nanoparticles, as well as the contribution of defects in the a-Si:H(i) layer induced by the implementation of the hematite nanoparticles. The presented results show that semiconducting nanoparticles can be successfully implemented as absorber material in thin film silicon solar cells.

Recently, a ternary Cu₂SnS₃ (CTS) thin film has been considered as a promising candidate for photovoltaic applications. CTS has a high absorption coefficient of over 10⁴ cm^-1, and its bandgap energy has been reported in the range of 0.9-1.8 eV. In fact, small-area solar cells with efficiencies of less than 3 % have been fabricated. The use of nontoxic and abundant elements is the main reason for the interest in this material as an alternative to Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells. Furthermore, CTS thin films also can be grown via the chalcogenization process. Therefore, it is possible for CTS thin films to be utilized for large-area applications. In addition, it should be easy to control the composition of CTS compared with CIGSSe or Cu₂ZnSn(S,Se)₄ (CZTSSe) because CTS consists of only three elements. However, only a few reports on the preparation of CTS thin films have been reported. Without understanding the growth process of CTS thin films, it is impossible to make a breakthrough similar to what was achieved with CIGSSe solar cells. In this presentation, the growth mechanism of CTS by sulfurization will be clarified. In addition, CTS thin films will be grown to control the carrier density and grain size for fabricating high efficiency CTS solar cells.
Cu-Sn precursors were prepared by RF magnetron sputtering on soda lime glass substrates using a Cu-Sn alloy target. The precursors were sulfurized in a tubular furnace in a N₂ and S vapor atmosphere at a constant atmospheric pressure.
Phase-transition and change in the surface morphology of CTS thin films were observed as functions of sulfurization time, temperature, and supplied S molar fraction. The proposal reaction mechanism of CTS thin films was described. The result is the first step toward realizing CTS related solar cells.

All-solid-state mesoscopic thin film solar cells were fabricated with the CH3NH3PbI3-sensitized ZnO nanorod as a photoanode and the spiro-MeoTAD as a hole transporting layer. ZnO nanorods were grown on a fluorine-doped tin oxide (FTO) conductive glass by two-step solution process. Length and diameter of ZnO nanorod were controlled by changing concentration of zinc nirate contained solution and growing period. Cross-sectional scanning electron micrograph confirmed that length was increased from 0.5 to 1.5 micrometer with increasing the growing period and diameter was increased from 50 to 90 nm without altering length as solution concentration was increased. CH3NH3PbI3 perovskite light absorber was deposited on the ZnO nanorods by one-step spin-coating technology, which was in contact with organic hole transporting material by infiltration of spiro-MeOTAD. Photovoltaic performance was found to be influenced by length and diameter. For the given length, photocurrent density was improved from 13.8 to 16.8 mA/cm2 as diameter was increased from 60 to 90 nm, whereas photovoltage was decreased from 906 to 747 mV. As a result, power conversion efficiency was better for larger diameter than for smaller one. Dependence of photovoltaic performance on length and diameter of ZnO nanorod was investigated in detail using transient photocurrent and photovoltage spectroscopy and electrochemical impedance spectroscopy.

Silicon-based optoelectronic devices that operate at photon energies that are less than the silicon bandgap (1.1 eV) have been fabricated using femtosecond (fs) laser doping. This process drives concentrations of chalcogen atoms in silicon to levels several orders of magnitude beyond their equilibrium solubility limit and produces hyper-doped silicon that exhibits near-unity absorption in the infrared (IR) region, at wavelengths to which crystalline silicon is transparent. Such properties are not achievable through equilibrium doping techniques, and the ability to tune the band structure of silicon with fs-laser doping makes this process of great interest for IR-photodetectors and photovoltaics. Aside from the drastic change in the absorption properties, researchers also observed visible luminescence in silicon structured by femtosecond laser in air [Wu et al, Applied Physics Letter 2002,81,1999-2002]. This broadband luminescence was attributed to the formation of SiOx and quantum confinement. To fully characterize and optimize the material in the pursuit of obtaining novel nanophotonics devices, we study the visible luminescence properties of sulfur-doped micro-structured black silicon by cathodoluminescence (CL) mapping and spectroscopy.
The spectroscopic studies have been carried out in a scanning electron microscope (SEM) to study the large area light emission properties and in a transmission electron microscope (TEM) to evaluate the light emission properties at the nanoscale by examining the cross-section of the material.
We fabricate sulfur-hyperdoped microstructured black silicon by irradiating a silicon (100) wafer with a train of Ti:Sapphire femtosecond laser pulses in an environment containing sulfur precursors (SF6). Room-temperature SEM-CL spectroscopy reveals a broad emission, peaked at about 2.4 eV. This broad emission is composed of three main components: an emission at 2.7 eV related to silicon oxide oxygen deficiency center (SiODCII) [Skuja L. Journal of Non-Crystalline Solids 1998,239,16-48] an emission at 2.4 eV attributed to the incorporation of sulfur in silicon oxide [Fitting H.J. Journal of Luminescence 2009,129,1488-1492], and an emission at 1.9 eV related to the non-bridging oxygen hole center (NBOHC)[Skuja L. Journal of Non-Crystalline Solids 1998,239,16-48]. This analysis reveals that the visible emission is related to an oxidation process occurred during the material processing. In addition the sum of these three different components results in a white emission. A comparative study of TEM-CL (spectroscopy and panchromatic mapping) and energy disperse X-ray (spectroscopy and mapping) reveals that the silicon oxide related luminescence centers exist in the form of amorphous SiOx pockets.

A newly extended chemical bath deposition (e-CBD) method for binary metal oxides such as highly magnesium-doped zinc oxide (Zn(1-x)Mg(x)O) is reported. In contrast to conventional CBD, the two parameters, the temperature and the pH value in the solution of two cations, are simultaneously controlled in e-CBD. The phase diagram of aqueous solutions of the two cations, Zn2+ and Mg2+, is first surveyed in the three dimensional space (the temperature, the concentrations of the cations pH value). And the two following regions are determined: region-1 where the both cations are both stable and region-2 where they precipitate as hydroxides. Then, the closest boundaries (or contact points) between the two regions are found. Via the closest boundaries, the representative point of the solution condition is shifted from region-1 to region-2. In Zn2+ and Mg2+ aqueous solutions, region-1 occupies a lower-temperature and higher-pH space, while region-2 does high-temperature and low-pH one. As the temperature of the solution of Zn2+ and Mg2+ was raised gradually from 278K to 333K, the pH value could be lowered the simultaneously from 11 to 9 through controlled vaporization of ammonia (NH3) from the aqueous solution. The two cations precipitated on a substrated as an uniformly mixed hydroxide gel film. Thus, single phase ZnMgO thin films with large values of x up to 0.48 could be formed after poly-condensation at 573K. The thickness as well as the Mg content can be controlled from 10 to 100 nm by the initial concentrations of the cations. The films made by this method are highly transparent over 90 % at visible rigion and wide band gap energies from 3.2 to 4.3 eV.

Although the Cadmium Chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the CdTe layer are not well understood. In this investigation we apply advanced micro-structural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis, X-ray Photoelectron Spectroscopy (XPS) depth profiles to show the effect of annealing time on the diffusion of chlorine into the CdTe. Grain orientation data as well as grain size change has been obtained using Electron Backscatter Diffraction (EBSD) on Focused Ion Beam (FIB) prepared planar sections providing grain texture and grain size measurements. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the CdTe layer.

The combination of optical concentration, spatial spectral splitting and the use of multiple cells of suitable bandgap allows achieving high PV conversion efficiency [1] without any constraint in terms of material choice. The overall idea builds on dividing the broad solar spectrum into smaller energy ranges and converting each range with a cell of appropriately matched bandgap, obtaining thus high electric conversion efficiencies[2]. We propose here to apply this framework to multiple inexpensively developed Cu(InxGa1-x)Se2 solar cells[3]. The cells absorbers are developed in a single step by Pulsed Electron Deposition method [4] and different bandgaps are achieved by suitable In-Ga balancing. CIGS solar cells are designed for concentration and for limited-range wavelength applications[5]. Theoretical framework, simulations [6] and preliminary experimental results are presented and discussed.
1. Green, M.A. and A. Ho-Baillie, Forty three per cent composite split-spectrum concentrator solar cell efficiency. Progress in Photovoltaics: Research and Applications, 2010. 18(1): p. 42-47.
2. Kim, G., et al., Increased Photovoltaic Power Output via Diffractive Spectrum Separation. Physical Review Letters, 2013. 110(12): p. 123901.
3. Zhang, D., J.M. Castro, and R.K. Kostuk. Commercial CIGS Solar Cells for Concentrator Applications. 2010. Optical Society of America.
4. Rampino, S., et al., 15% efficient Cu (In, Ga) Se 2 solar cells obtained by low-temperature pulsed electron deposition. Applied physics letters, 2012. 101(13): p. 132107-132107-4.
5. Maragliano, C., et al., Three-Dimensional Cu(InGa)Se2 Photovoltaic Cells Simulations: Optimization for Limited-Range Wavelength Applications. Photovoltaics, IEEE Journal of, 2013. PP(99): p. 1-7.
6. Maragliano, C., et al., Realistic simulation of polycrystalline CIGS absorbers and experimental verification. MRS Online Proceedings Library, 2013. 1493.

The copper indium gallium diselenide (CIGS) absorber-based photovoltaic device arises nowadays as one of the most promising routes to cost-efficient solar cells based on thin film technology. However the actual efficiency of both lab cells and of industrial solar arrays is significantly lower than the theoretical limit value. As reported in the literature, several mechanisms could be responsible for this loss of efficiency, notably carrier recombination at the various interfaces of the structure. In order to shed light on the microscopic origin of efficiency loss, a comprehensive study of the electrical response of the CIGS-based heterostructure is highly desired. In this work, the electrical characteristics of a chalcopyrite-based solar cell heterostructure have been simulated for both the dc and ac regime by numerically solving the basic semiconductor equations by means of a finite differences method based on a Scharfetter-Gummel discretization scheme. The electric potential, electric field, carrier concentrations, current densities and recombination rates are obtained as functions of the space coordinate, the bias voltage and the frequency of the signal in ac regime. Starting with the analysis of a single absorber layer structure sandwiched between a molybdenum back contact and a front electrode made of another metal, a systematic study of the impact of the barrier heights has been conducted by linking features in the obtained electrical characteristics to microscopic parameters of the material system. We subsequently investigated the role of the CIGS/ZnO pn heterojunction from the perspective of the issue of carrier recombination at the interface and we clarified the influence of the CdS buffer layer thickness on the cell electrical response. A special focus was also given to the properties of the window layer, the concentration of impurities and the quality of the interface with the buffer layer and with an Al contact on the window material side. Finally, since the cell efficiency loss can be attributed to different mechanisms at the microscopic scale, the influence of each of them on the cell response was assessed from the point of view of their detectability in the dc and ac electrical characteristics.

Current solar-cell device configurations employ diffusion to extract photogenerated minority carriers. For efficient extraction, absorbers must have high carrier mobilities and long minority carrier lifetimes. Absorber materials that exhibit strong optical absorption (α > 10^5 cm^-1) allow for a reduced absorber thickness of less than 1 mu;m without compromising the device efficiency. In a cell with such an ultra-thin film, a strong intrinsic electric field can drive minority carrier extraction, reducing mobility and lifetime constraints. Thus, our objective is to identify new materials capable of enhancing solar conversion efficiencies in an ultra-thin drift-based thin-film solar cell (TFSC).
Using chemical insights along with the recently introduced figure-of-merit, spectroscopic-limited maximum efficiency (SLME) led us to the materials system Cu-V-VI (V = As, Sb, Bi) (VI = S, Se), which includes two compounds that exhibit stronger absorption than CuInSe2. Polycrystalline thin films of CuSbS2 (EG = 1.44 eV) and Cu3SbS4 (EG = 0.88 eV) exhibit exceptionally high absorption (α > 10^5 cm^-1 at EG+1eV). Device simulations of TFSCs utilizing CuSbS2 and Cu3SbS4 as the absorber layer will be compared to CuInSe2 to illustrate the necessary properties and tolerances of these materials in a high efficiency TFSC. Emphasis will be placed on the device configuration, absorber thickness and minority carrier lifetime. While both materials exhibit efficiencies greater than 15%, efficiencies approaching 20% can be obtained when the thickness of the Cu3SbS4 absorber layer is reduced to less than a micron. Variation of the minority carrier lifetime suggests that high purity, defect free thin-films exhibiting long lifetimes may not be required for high performance. Coupled with the possibility for continuous band gap tuning across the solar spectrum, the Cu-V-VI materials family provides new opportunities for exploiting the use of ultra-thin absorbers in both single-junction and tandem TFSCs.

To substitute CdS by cadmium-free material and to enlarge spectrum window to absorber layer, utilizing zinc-based buffer is favored in CIGS solar cell structure [1-2]. The general CBD-processed CdS thin film has been used due to several reasons such as the advantages of simple process with low cost on large area, the good conformal coverage without damage on surface, and the ability to produce high efficient CIGS solar cell. However, due to the process incompatibility with in-line vacuum-based production methods and the limited current generation in the spectral range of 350-550 nm, which is originated from the relatively low CdS band gap of ~ 2.4 eV [3], there are still margins to enhance CIGS solar cell performance by replacing CdS buffer.
For CIGS solar cell application, Zn(S, O) thin films have prepared by atomic layer deposition (ALD) to reveal the optical and electrical properties of the film and also to have clues to the optimal energy band alignment at the interface according to element compositions. In particular, the electrical properties of the thin film are largely affected by S:O pulse ratio in ALD system, which are related to the non-linear incorporation in the actual films.
In this study, Zn(S, O) thin films are investigated by changing pulse ratios of ALD sources and their S:O supply ratio are compared to the actual elemental compositions of the films. The performance of CIGS thin film solar cells with Zn(S, O) buffer are analyzed and the effect of heat and light soaking of the device with ALD-grown Zn(S, O) is also discussed.
[1] U. P. Singh and S. P. Patra, Int. J. Photoenergy, 2010, 468147 (2010).
[2] N. Naghavi, D. Abou-Ras, N. Allsop, N. Barreau, S. Bücheler, A. Ennaoui, C.-H. Fischer, C. Guillen, D. Hariskos, J. Herrero, R. Klenk, K. Kushiya, D. Lincot, R. Menner, T. Nakada, C. Platzer-Björkman, S. Spiering, A.N. Tiwari, and T. Törndahl, Prog. Photovolt: Res. Appl., 18, 411 (2010).
[3] B. T. Ahn, L. Larina, K. H. Kim, and S. J. Ahn, Pure Appl. Chem., 80, 2091 (2008)

In recent years, nanostructured materials have attracted a great interest because they exhibit specific physical and chemical properties that cannot be found in their bulk counterparts [1]. So, they have been sought for different applications in optics, biotechnology, and optoelectronic devices such as gas sensors and solar cells. In the solar energy conversion area, nanostructured materials can be seen as a concept to reach the goal of reducing costs and to improve solar cell conversion efficiencies. It is desirable to have the semiconductor nanomaterials on a suitable substrate such as a template i.e. an imposed pattern on a surface that acts as a guide for the material to be deposited. Nanodots or nanowires of metals and semiconductors have been obtained using templates of porous alumina (Al2O3) or porous silicon (PSi), which represent two viable templates [2]. In this work, we report the electrodeposition of CuInSe2 nanostructured on PSi templates. PSi can be prepared by electrochemical etching of p-type (100) c-Si wafers with a resistivity value 0.007-0.013 Omega; cm and using an electrolyte composed of HF diluted with ethanol (C2H6O). The pore diameter of PSi can be designed by varying the concentration of the electrolyte, applied current density, and the c-Si substrate resistivity. The aim of this work is to increase the absorption cross section of CuInSe2 taking advantage of the large specific surface area of porous silicon. The ED of CuInSe2 onto the PSi templates was carried out using an acidic aqueous buffered bath that contains the three active ionic species. The deposition conditions to obtain nanostructured CuInSe2 on the PSi templates were established after performing a systematic study that consisted in varying the deposition parameters. In this way, we were able to obtain nanostructures of CuInSe2 on c-Si and PSi templates.
Acknowledgements: This work was partially supported by CONACYT under grant No. 167993.
References
[1] P. Zhang, P. S. Kim, and T. K. Sham, Nanostructured CdS prepared on porous silicon substrate: Structure, electronic, and optical properties, J. of Applied Physics, 91, Num 9 (2002), p. 6038.
[2] T. Soga, Nanostructured Materials for Solar Energy Conversion, Elsevier (2006), ISBN-13: 978-0-444-52844-5

The low cost, non-toxic, earth abundant elements based kesterite material Cu2ZnSnS4 (CZTS) possesses promising characteristic to be a conventional absorber for thin-film solar cell due to its optimal band gap of 1.45-1.51 eV and high optical absorption coefficient (>104 cm-1). Recently, it has attracted massive attentions of researchers and various techniques have been developed for the preparation of CZTS thin film such as sulfurization followed co-sputtering, chemical vapor deposition (CVD), electrodepostion, and hydrazine based solution process which leads to current highest performance CZTS solar cell (>11% efficiency). Nevertheless, the non-vacuum low-toxic preparation of CZTS inks forms from quaternary nanoparticles is still a potential method that could produce low cost thin-film with superior homogeneous composition.
The major obstacle of nanoparticles process is that the surfaces of nanoparticles are typically stabilized by surfactants which are generally long-chained electrically insulating organic ligands. Therefore, to get rid of the organic ligands, the “ink” method requires a high-temp (>500 oC) annealing procedure as well as all other techniques. Unfortunately, during this aggressive process, drawbacks are raised: cracking and void occur in the film formation due to high weight losses; Sn losses through desorption of SnS from CZTS due to high vapor pressure of SnS; sulfur diffusion into molybdenum back contact forms MoS2 and left secondary phases at the CZTS|Mo interface lower the performance. Therefore, low temperature process should be developed to solve the above issues meanwhile extend the possible application of CZTS to flexible substrate like plastic which cannot stand over 400oC.
Here, we&’re reporting a room temperature, large scale thin film process via ligand exchange of CZTS nanoparticles followed by spray casting. By switching different ligands, we improved the hole mobility 20 times which is competitive to high-temp process. Besides, the theoretical study revealed that the influence of ligands length on performance of CZTS thin film which has excellent agreement to experimental results.

The Zn(S,O)/Zn1-xMgxO structure is a promising candidate to replace the conventional CdS/i-ZnO buffer layer in Cu(In,Ga)Se2 based solar cells and has the highest reported efficiency among all Cd-free alternatives. Here, we report the optimization of this cadmium free buffer layer. The Zn(S,O) layer was grown by chemical bath deposition and the Zn1-xMgxO layer was deposited by radio-frequency co-sputtering. The properties of the Zn(S,O) and Zn1-xMgxO films, including their chemical composition, optical transmittance, and electric transport have been measured and their influences on the cell performance have been systematically evaluated. Using this novel structure, we have achieved comparable efficiency with the reference solar cell based on CdS buffer layer. The band profile of Cu(In,Ga)Se2/Zn(S,O)/Zn1-xMgxO stacks has also been investigated by scanning probe microscopy and its influence on the cell performance will be discussed.

We study the structural and electronic properties of Cu3XY4 (X=P, As, Sb and Bi; Y= S, Se, and Te) compounds using first-principles density-functional theory with the HSE06 hybrid functional. Four different wurtzite-based and zinc-blende-derived crystal structures, enargite, wurtzite-PMCA, famatinite and zinc-blend-PMCA, have been considered. We find that Cu3PS4 and Cu3PSe4 prefer energetically energite structure, whereas, other compounds favor famatinite structure. Band structure calculations reveal that the bandgap depends on both the anion and the cation X. Among the compounds and structures considered, the energite(structure) Cu3PSe4, and famatinite (structure) Cu3AsS4, are suitable for photovoltaic solar cell applications due to their bandgaps, i.e. 1.29 eV, 1.33 eV, respectively.

Tin monosulfide (SnS) is an ideal and a promising semiconductor for solar cell applications because it has a high absorption coefficient of 104 cm-1 and a direct bandgap energy of 1.3 eV. In addition, the constituent elements of SnS are cheap and nontoxic. These favorable properties, however, the current record efficiency of SnS solar cells is still low as 2.4%, which can be attributed to the poor crystal quality of SnS thin films and to the anisotropy of the effective masses of holes along axes. Therefore, in order to obtain high-efficiency SnS solar cells, obtaining orientated SnS films is important for realizing high mobility of minority carrier transportation. Sulfurization is the most desirable process for commercial preparation of SnS photoabsorbers. However, the growth mechanism of SnS by sulfurization has not yet been clarified.
In this presentation, the impact of orientation and/or defects on the electrical properties of SnS thin films has been investigated. For the realization of a high-efficiency solar cell, the relationship between the sulfurization conditions and the orientation or mobility of polycrystalline SnS has also been investigated.

A Sn precursors were sulfurized under a sulfur vapor atmosphere at 150-540 °C for 10-80 min. In general, the SnS thin films exhibited the [111] orientation (or random orientation), in addition to exhibiting a low mobility in the range of 0.1-5.0 cm2/Vs. On the other hand, we obtained a high mobility in the range of 10-13 cm2/Vs in completely b-axis orientated SnS films. The orientated films, which were obtained by controlling supplied S vapor, were used in SnS solar cells. These results represent a step toward in the realization of high-efficiency SnS solar cells by the use of a simple fabrication technique.

CdI2 has been proven as a low temperature flux material for the synthesis of Cu2ZnSnS4 (CZTS) absorber materials, used in monograin membrane solar cells, where each grain works as an individual solar cell. It means that the final solar cell performance depends strongly on homogeneity of absorber material powder. In the monograin technology, the isothermal recrystallization of semiconductor polycrystalline powders in the presence of the liquid phase of a suitable solvent (flux) material in sufficient amount, aids the formation of powders with single crystalline structure of powder grains. However, to reach single phase absorber material in monograin powder form, the problems of the presence of by-products need to be solved. Also the incorporation of Cd and I from CdI2 into CZTS crystals affects the properties of solar cell absorber materials. CZTS powder samples were synthesized in CdI2 as flux material. The obtained materials were analysed by EDX, SEM, XRD and Raman methods. The optimal Cd content and possible chemical treatments with the aim to remove off the by-products to reach the single phase absorber material are reported.

Thin film silicon (TF Si) photovoltaics (PV) based on amorphous (a-Si:H) and nanocrystalline (nc-Si:H) silicon have matured through three decades of advances in the design and processing of high-quality materials, solar cells and modules. The current record TF Si PV efficiencies of the conventional a-Si:H/nc-Si:H double junctions and a-Si:H/nc-Si:H/nc-Si:H triple junctions are 12.5% (Tokyo Electron) and 13.4% (LG), respectively. Several aspects as the relative small gap between record efficiencies between cell and produced panels (12.5 % vs 11%); high production yields (>97%) on industrial scale, the low costprice among PV technologies (0.35 euro;/Wp), utilization of solely earth abundantly available materials and the demonstration of mature flexible TF Si PV products, demonstrates the maturity of the TF Si PV technology. However, to compete with silicon wafer based PV technology a reduction of the non-modular costs of a TF Si based PV system, a significant increase in the conversion efficiencies is required.
In this contribution I will discuss the vision how to arrive at TF Si PV devices having conversion efficiencies beyond 20%. As basis I will use the general design rules for PV devices, which are 1) optimizing the spectrum utilization (choice of materials and band gap), 2) optimizing the utilization of band gap energy (defect engineering) and 3) optimizing light management (reduction of reflection, transmission and parasitic absorption losses). Optimizing these three aspects is delicate interplay, demonstrated by enhancing light trapping due to texturing of interfaces, which subsequently will deteriorate the electrical properties of the PV active material again.
A better spectral utilization is achieved by the development of low band gap materials. Both the mature processing and the low cost price per Wpnot;, in combination with cheap low band gap materials allows to move to more complex solar cell concepts as triple- and quadruple junctions, having conversion efficiencies above 20%. Secondly, the amorphous alloys suffer from metastable light induced defects (LIDs). Tackling LID is an important issue for achieving further breakthroughs in the performances of devices, as the amorphous alloys are responsible for 50% up to 75% of the power output of the variety of TF Si devices. We report on progress in the understanding of the origin of LIDs and defect engineering to tackle the metastability issues. Thirdly, I will discuss the latest light trapping technologies (textured, plasmonic, and flat light scattering substrates) which combine both high-quality electronic properties of the TF Si and light trapping close to the Yablonovitch limit.
Finally, examples of thin film silicon PV technology integrated in variety of photoelectrochemical water splitter devices will be demonstrated. The crucial advantages of low cost-price, resistance against aqueous environments and the possibility to process platinum free devices will be presented.

In recent years, thin-film silicon tandem solar cells, consisting of amorphous silicon (a-Si) top cells and microcrystalline silicon (mu;c-Si) bottom cells, have been established as an industrial low-cost photovoltaic technology. However, their efficiencies are considerably lower than those of wafer-based crystalline Si cells. In addition, the price of Si wafers has been rapidly declining, such that thin film Si solar cells have to become more efficient to remain competitive. One of the major factors limiting the a-Si/mu;c-Si tandem cell power conversion efficiency is the light induced degradation of the a-Si sub-cell. This degradation can be minimized by thinning down the a-Si cell, which requires an intermediate reflector layer (IRL) to allow for current matching between the two sub-cells. At the same time, the a-Si/mu;c-Si tandem cell optimization involves a trade-off between light trapping, typically requiring rough scattering interfaces, and defect-free growth of the photo active semiconductor layers, which is usually disturbed by surface roughness. Conventional IRLs, such as ZnO and SiO_x thin films deposited by sputtering or chemical vapor deposition form a conformal layer. As a result, the morphologies of both sub-cells are typically linked, and it is not possible to achieve optimal light trapping while still allowing for the growth of high-quality Si layers.
Here, we present an advanced IRL that does not only act as a highly efficient light reflector, but also as a smoothening layer to decouple the morphologies of the two sub-cells. Specifically, an IRL based on a solution-processed self-patterned SiO₂ nanoparticle (NP) layer will be presented. By adjusting the SiO₂ NP dispersion formulation and spin-coating parameters, a valley-filling effect is obtained on rough substrates, such that small surface features are covered by a SiO₂ NP layer with a flat surface while large surface features penetrate the electrically insulating SiO₂ NP layer, allowing for efficient charge transport between the two sub-cells. This valley-filling effect is confirmed by conductive atomic force microscopy and cross-sectional scanning electron microscopy. Due to the large refractive index mismatch between the SiO₂ NPs and the Si layers, very thin a-Si top cells can be utilized, which, in the case of mu;c-Si current-limited tandem cells, leads to very high fill factors of >80%. At the same time, the smoothening effect leads to an enhanced mu;c-Si material quality, resulting in an increase in open-circuit voltage by up to 60 mV. Overall, the best SiO₂ NPs IRL solar cell has an efficiency of 13.0%, a significant enhancement compared to the reference cells without IRL with efficiencies of up to 11.6%.
In summary, we show that self-patterned, insulating nanoparticle layers can be integrated into inorganic thin-film solar cells for selective smoothening of rough surfaces, as demonstrated by the example of an SiO₂ NP IRL for thin-film Si tandem solar cells.

One of the most persistent costs in the manufacture of silicon solar cells has been that of the silicon substrate. The natural abundance of silicon suggests that the material is promising for the long-term future of photovoltaics, but it will be essential to further reduce the cost of the input materials. As an indirect bandgap material, however, silicon for photovoltaic cells has a significant handicap in that it does not absorb light well, particularly in the red and near infrared wavelengths. Consequently, a thin silicon solar cell on a planar wafer would be quite inefficient without providing a strategy to trap photons in the material.

At MRS 2013, we will present our latest progress in our effort to fabricate efficient crystalline silicon photovoltaics less than 10mu;m thick. We will discuss the challenges that have been faced using projection lithography to produce the light-trapping structures critical to the device efficiency. Although the fabrication approach taken here - using projection lithography and an ion-implanted junction on a silicon-on-insulator substrate - is not commercially oriented, it demonstrates the viability of thin silicon substrates for photovoltaics and lays the foundation for developing a competitive manufacturing process to realize them commercially. The overarching goal of this research effort is to prove that thin-film c-Si solar cells can be made to be as efficient as their conventional wafer-based counterparts.

Liquid phase crystallization of Silicon thin-film on glass via continuous wave diode laser has allowed fabrication of high quality Si thin-film on glass, with solar cell voltages surpassing the voltages previously realized on poly-silicon based cells grown on foreign substrates. To further improve the electronic properties of the film, characterization tool is required to analyse the effects of individual processes on the electronic properties of the material. In this study, photoluminescence (PL) imaging technique was applied to investigate the influence of phosphorus phosphorous gettering and remote hydrogen passivation on p-type diode laser crystallized silicon thin-film on glass. PL setup was modified to suit thin-film on glass substrate. Phosphorus gettering parameters were split: temperature, time and concentration of phosphorus spin-on source. Samples diffused with high concentration phosphorus dopant source out performed samples diffused with low concentration dopant source, with PL intensity of 598 counts/s and 368 counts/s respectively. PL images showed hydrogen passivation was more effective at improving the electronic properties of diode laser crystallized films than phosphorus gettering, improving the PL intensity on average by a factor of 1.5. Measurements of the Pl intensity were compared with Suns-Voc results for phosphorus gettered and hydrogenated p-type laser crystallized Si thin-film on glass. A correlation between the Photoluminescence intensity and open-circuit voltage was found: high PL intensity (1700 counts/s) were only measured with samples with high voltages (530 mV), in samples with low PL intensity relatively low voltages were observed. Hall mobility at the area of high PL intensity was as high as 450 cm2/Vs while low PL intensity area was 310 cm2/Vs. Photoluminescence imaging technique showed the influence processing techniques have on the electronic properties of the laser crystallized Si thin-film on glass substrate.

Thin kerfless single-crystal silicon wafers using multiple regrowth substrates provide a promising opportunity to reduce the manufacturing cost of crystalline silicon photovoltaic modules [1]. However, manufacturing cost is most sensitive to module efficiency [1,2], which underscores the risk of low electrical performance to eliminate the advantages of a kerfless process. Minority carrier lifetime is a good measure for the electrical performance of the material and ultimate device efficiency potential [3]. We investigate performance-limiting defects in two generations of epitaxially-grown silicon wafers for solar cells, and apply defect engineering tools during growth and processing to minimize their impact on minority carrier lifetime. We demonstrate high minority carrier lifetimes (> 300 µs, effective) in thin p-type kerfless material grown with a high-throughput epitaxial process, that through device simulation are predicted to enable cell efficiencies greater than 22% with a 50 µm thick planar device. With our cost and price model for c-Si module manufacturing [1,2], we predict that this performance could achieve the U.S. Department of Energy SunShot module price targets [4].
References
1. D.M. Powell, M.T. Winkler, H.J. Choi, C.B. Simmons, D. Berney Needleman and T. Buonassisi, Energy and Environmental Science, 2012, 5, 5874-5883.
2. D.M. Powell, M.T. Winkler, A. Goodrich and T. Buonassisi, IEEE Journal of Photovoltaics, 2013, 3, 662-668.
3. G. Coletti, Progress in Photovoltaics: Research and Applications, 2012, (Early view) DOI: 10.1002/pip.2195.
4. U.S. Department of Energy, SunShot Vision Study, R. Margolis, C. Coggeshall, and J. Zuboy, Ed., 2012.

Defect engineering in silicon has been used in the past decades for enhancing peculiar properties as light emission or sub-bandgap absorption. Introducing deep-level impurities in silicon in sufficiently high concentrations can produce an impurity band within the band gap. Such intermediate band silicon are of great interest for photovoltaic and IR photodetection applications. Silicon can be hyperdoped with sulfur well above the equilibrium solubility limit using ion implantation followed by pulsed laser melting (PLM), and such treatment causes drastic changes in the optical and electronic properties of the bulk material: the PLM S:Si has anomalously high absorption down to 0.5 eV, and exhibits impurity-mediated transport at low temperatures. The PLM process induces the appearance of surface structures with peculiar properties. These surface structures have an external quantum efficiency >100%.
In this work, we study the morphological, electrical, electronic and structural properties of surface topographical structures in sulfur hyperdoped silicon. This study is carried out using electron microscopy-based techniques to study properties of the structures. The electrical properties are studied by means of electron beam induced current (EBIC), showing that the structures, under analysis, have a higher conductivity in comparison with the reference material, with an EBIC contrast equal to 7.5%, surrounded by a carrier recombination region. Cathodoluminescence mapping and spectroscopy are used to evaluate the electronic properties of the material. This analysis demonstrates a higher emission from the surface structure, mainly related to a higher intensity of the silicon band-to-band recombination (1.1 eV). In addition, CL spectroscopic study reveals that the signal related to sulfur-related states is less intense inside the structures. A transmission electron microscopy (TEM) lamella has been prepared by the same structure, previously analyzed, by focused ion beam process. The TEM analysis reveals that the structure is made of single-crystalline silicon, excluding the formation of extended defects. In addition, TEM analysis shows that there is a transition region between inside and outside the structure in which the formation of ripples, suggests a gradient of stress. TEM energy dispersive X-ray (EDX) analysis shows that inside the structure the sulfur concentration drops of about the 50% in comparison with reference material. This work makes progress towards the understanding of surface structures in sulfur hyperdoped silicon opening a new scenario in the possible engineering of this material in the field of photovoltaic and IR photodetection applications.

We discuss the implication of nanophotonics for the control of current and voltage of a solar cell. In particular, we relate the properties of open circuit voltage to the electromagnetic modes of the nanophotonic structures, and show that one can control both the voltage and current of a solar cell by photonic engineering.

To achieve the highest conversion efficiencies in thin-film photovoltaics, light management is of key importance to increase absorption and quantum efficiency leading to higher conversion efficiency. Recently, multiple strategies have been proposed that require nano-patterning or nano-structuring to achieve better optical performance in thin-film solar cells [1]. Realized together, these strategies could achieve conversion efficiencies greater than 40% for a multi-terminal multi-junction solar cell with spectral splitting. However, at this time, high-volume, low-cost implementation of these strategies has not been realized -- something necessary for current photovoltaic manufacture. Successful development and adoption of these ideas by the solar industry will require a demonstrated roadmap of realistic efficiencies for cell architectures; beginning from the industrial standards of today and leading to ultra-high efficiency photovoltaic cells and modules.
In this contribution, we will examine a number of light management strategies including periodic and random resonators for light trapping, nano-patterned contacts, and structures for spectrum conversion or spectral splitting. These structures are integrated into existing models of high performing solar cell architectures (i.e. both mono- and mc-Si, ultra-thin bifacial and HIT, all back contacted, and high performance multi-junction solar cells) to explore a realizable increase in conversion efficiency compared to a baseline of cells available today.
Using this framework we propose a series of cell architectures based upon current state-of-the-art technologies with incremental advances, which build upon each other, to achieve more complex high-efficiency solar cell designs. This roadmap will act as a guide to identify promising areas for future research in fundamental concepts of light management based upon potential efficiencies, as well as, necessary developments in materials for thin-film photovoltaic devices.
[1] Polman, A. and Atwater, H.A., Nature Materials, 11, 174 (2012).

Efficient solar energy conversion requires collecting as many photons as possible in a given area via means such as solar tracking and concentration. Although flat-plate photovoltaic (PV) panels are attractive for their low costs and direct compatibility for building integration, they are often installed in fixed positions, making solar tracking or/and concentration extremely challenging. This work concerns using origami-inspired microstructures to achieve autonomous solar tracking in the flat-plate architecture. The basic principle lies in morphing the semiconductor solar materials into paper-like thin films and creating origami structures as one would do on an actual sheet of paper. Unlike conventional MEMS whose system functions rely on collective motions of each individual parts, origami structures morph themselves in a collective and continuous way that is governed by folding geometry and microscopic texture of the “paper”. The force applied in an origami structure is not localized but is distributed to the entire structure, again governed by topology and micro-textures. This work exploits these unique properties of origami arts to enable autonomous solar tracking in a flat-plate PV panel.
In our design, we consider a semiconductor thin-film absorber being transferred onto a paper-like template, for example a micron-thin GaAs PV structure transferred onto a kepton film. The hybrid film is then cut and folded in a way that the resulting film comprises of an array of microstructures which further consist of three parts: the light absorbing surface (LAS), the hinges, and the actuator. In spite of possessing distinct functions, all parts are connected in a way that the actuation of a local area will morph the entire structure with the objective to guide the LAS directly facing the Sun at all times. The PV cell will be electrically connected before the cutting and folding. The actuation is enabled by heat-induced volume expansion and contraction in a chamber located inside the actuator part. As the LAS is misaligned from the direction of direct sunlight, the actuator will generate an asymmetric force to realign the LAS. As compared to a planar GaAs solar cell of the same total surface area, the proposed origami structure can generate up to 30% more electricity according to our numerical modeling. We will present our design and modeling of optical, mechanical, thermal, and electrical properties of the origami PV. Proof-of-concept experimental results will also be presented.

Conventional optical coatings, which consist of one or more films of dielectric materials, rely on Fabry-Perot-type interference, involving multiple optical passes through transparent layers with thicknesses of the order of the wavelength. Highly absorbing dielectrics are typically not used because usually light propagation through such media minimizes interference effects.
We show that under appropriate conditions strong interference can instead persist in ultrathin, highly absorbing films, and demonstrate a new type of optical coating comprising such a film on a metallic substrate, which absorb a large fraction of the incident light. These coatings rely on nontrivial optical phase shifts at interfaces between highly-absorbing materials, and as a result have a low sensitivity to the angle of incidence and can be as thin as 5-20 nm.
As these wide-angle coatings can be designed to have a large degree of optical absorption within a film of deeply-subwavelength thickness, they can potentially be useful in a variety of applications which demand light harvesting including photodetectors and solar cells. We focus on several material systems including germanium and amorphous silicon on various metallic substrates, and also show how extra nanometer-scale transparent layers can be used to enhance optical absorption, protect against environmental erosion, or as a transparent electrode for optoelectronic devices.

Economic roll-to-roll printed thin film solar cells require flexible and cheaper materials than the most commonly used indium tin oxide (ITO) for their transparent conducting electrode layer. Here we introduce a high performance silver nanowire electrode on transparent paper with optoelectronic properties competitive with ITO, yet with greater diffuse optical haze. The impact of the optical properties of ITO and our silver nanowire paper is compared experimentally and theoretically, and it is demonstrated that the higher diffuse optical haze in the silver nanowire paper induces greater light absorption in a thin film absorber. According to this analysis we conclude that our silver nanowire paper promotes a better conversion efficiency than ITO for thin film solar cells, making it a compelling option for roll-to-roll printed solar cells.

Kesterite Cu2ZnSn(S,Se)4 (CZTS) thin films are attracting a lot of interest as an alternative system to Cu(In,Ga)Se2 (CIGS) thin films, owing to their majority carrier type (p-type), proper band gap energy (1.0-1.5 eV), and high optical absorption coefficient (> 10^4 cm-1). More promisingly, the CZTSSe is composed of earth-abundant (cf. In in CIGS), environmentally-friendly (cf. Cd in CdTe), and relatively cheap elements. Here, we fabricated metallic Cu-Zn-Sn (CZT) precursor thin films via electrochemical deposition from aqueous metal salt solution on Mo-coated soda-lime glass substrates, and the influence of the subsequent sulfurization/selenization condition on the structural, electrical, and photovoltaic properties of the CZTSSe thin films was investigated. The as-deposited films are composed of binary metallic alloys, which can be converted to the highly crystalline CZTS phase after sulfurization at temperatures above 500 oC. The composition of the CZT film barely changes during the sulfurization, and small amount of CuS-based secondary phases exists even at 550 oC. However, a quick post-annealing KCN treatment effectively and selectively removes the secondary phase, evidenced by the Raman spectroscopy. The formation of the CuS-based secondary phase can be suppressed by slowing down the hearting rate during the sulfurization process, leading to an increased conversion efficiency of ~ 4%. The selenization process has been found to accelerate the crystallization process to CZTSe and the grain growth compared to the sulfurization process, and thus, to enhance the photovoltaic properties, exhibiting a high conversion efficiency of ~ 8%.

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 several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be kesterite structures of 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. More recently, two potentially low-cost high-throughput approaches have been demonstrated that form the quaternary or pentenary chalcogenide directly from solution-phase processes. One is based on first synthesizing multinary sulfide nanocrystals and then sintering them to form a dense layer. The other approach utilizes molecular precursors dissolved in hydrazine. Both approaches reach their highest device efficiencies by incorporating Se to form Cu2ZnSn(Sx,Se1-x)4 devices, and each has yielded higher efficiency devices than the best vacuum deposited absorbers. The hydrazine route has yielded the most efficient CZTS-based devices thus far.
The presentation will focus on our development of the nanocrystal-ink based routes to materials and devices and a new molecular-ink route that utilizes benign solvents (avoiding the use of hydrazine). For both systems we will show the results of composition spread experiments coupled with spatially resolved photoluminescence and Raman scattering to reveal the effects of native point defects and doping on material quality and device performance. Finally, the current state-of-the art and performance limitations for the material will be review.

We examine photoluminescence spectra (PL) of Cu2ZnSnSe4/CdS/ZnO solar cells via temperature-dependent and illumination power-dependent measurements. Our cells are fabricated by H2Se selenization of sputtered Cu, Zn, Sn multilayers and show a total area efficiency of 9.2 % with an open circuit voltage of 416 mV and a short circuit current density of 38 mA/cm2. PL measurements offer an opportunity to study the defect states in the absorber layer.
The experimental results lead us to propose a recombination model for our cells that is able to explain both temperature dependent PL as well as power dependent PL results. At low temperatures and moderate excitation power, the quasi-donor-acceptor recombination (Q-DAP) between electrons localized at distorted donor states and holes at distorted acceptor states dominates. At higher temperatures and/or higher excitation power, the recombination involves electrons in the conduction band and distorted acceptor levels and the transition changes from Q-DAP to a quasi-free to bound transition (Q-FB). We can link the low Voc values generally observed in CZTSe solar cells to the presence of strong potential fluctuations in the absorber layer, as these potential fluctuations will give rise to tunneling enhanced recombination, thereby increasing the recombination currents as compared to the case without potential fluctuations.

Copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) solar cells with the highest power conversion efficiencies are fabricated on Mo-coated soda lime glass (SLG), a carryover from Cu(In_xGa_1-x)Se₂ (CIGS) solar cells. In CIGS solar cells, Na diffusion from the SLG into the CIGS film has been shown to enhance the power conversion efficiency. Na diffusion is also expected when CZTS is deposited on Mo-coated SLG. In fact, SLG hosts a variety of other impurities such as K, Ca, Mg, and Al that may also diffuse into CZTS. However, a systematic investigation of whether these impurities diffuse into CZTS and how they affect the film properties has not yet been conducted. To this end, we have investigated the effects of the substrate and the intentional addition of individual impurities on the microstructure and electronic properties of CZTS films. Thin CZTS films were synthesized via ex situ sulfidation of Cu-Zn-Sn films co-sputtered on a variety of substrates, including, crystalline quartz, amorphous quartz, sapphire, SLG and Pyrex. These Cu-Zn-Sn precursor films were then loaded into quartz ampoules with 1 mg of S, evacuated to 10^-6 Torr, sealed and sulfidized at 600 ^oC for 8 hours. The sulfidized films were then characterized using a suite of techniques including X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Concentration depth profiles were examined using time-of-flight secondary ion mass spectrometry (TOF-SIMS). CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. Furthermore, we found that by simply including a bare additional piece of SLG in the sulfidation vessel, the grain size of films grown on impurity-free quartz increases from 100's of nm to greater than 1 mu;m. This conclusively demonstrates that the impurity atoms found in SLG are volatilized in a sulfur atmosphere and transported via the vapor phase to neighboring films. TOF-SIMS experiments implicated Na, K and Ca as the impurities responsible for this enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of either Na, K, or Ca into the sulfidation ampoule. By including impurities at levels as low as 10^-6 moles of Na, or 10^-7 moles of K, in the 8 cm³ sulfidation ampoule, the grain size of CZTS on quartz substrates was increased dramatically from 100's of nm to greater than 1 mu;m, while Ca loading had little effect. The electronic properties of CZTS films synthesized with different amounts of impurities in the sulfidation tube were also studied using temperature dependent conductivity and Hall effect. The effects of alkali metal impurities on the microstructure and electrical properties of CZTS films will be discussed in the context of their applications to solar cells.
Work supported by National Science Foundation through CBET-0931145 and in part by the UMN Initiative for Renewable Energy and the Environment (IREE).

Tin monosulfide (SnS) is a candidate Earth-abundant solar cell absorber material. It is attractive due to its high absorption coefficient (α > 10⁴ cm^-1)¹, suitable band structure (1.1 eV indirect and 1.3 eV direct) and potentially high carrier mobility (Hall mobility reported above 100 cm²/Vs).² SnS has sufficient elemental abundance to reach terawatt levels of photovoltaic module production, which cannot be reached by CdTe and CIGS because of the limited availability of Te and In.
Industrial production of both CdTe and CIGS involve crucial efficiency-boosting annealing steps that increase grain size, improve electrical properties and reduce interface defects. The analogous process for tin monosulfide has yet to be explored and optimized.
It was previously shown that RF sputtered SnS films annealed in H₂S ambient at 400 °C show promising grain growth from near amorphous, ~10 nm grains to a 40-200 nm range. The Sn/S ratio shifted from sulfur-rich (Sn/S < 1) to a stoichiometric ratio of ~1.0 after annealing. Additional thermodynamic simulations suggest a phase evolution during annealing³, indicating a natural tendency of this material toward SnS “phase purification.” This is a significant result, because minority phase formation can be difficult to control in films grown rapidly by industrial methods. The effects of annealing in a sulfur-containing, or H₂S gas, ambient are suggested to promote grain growth and control of the Sn/S ratio.
We hypothesize that annealing in a sulfur-containing atmosphere may fill sulfur vacancies, which have been calculated to lie near mid-gap for SnS.¹ Filling these sulfur vacancies should improve solar cell performance by reducing Shockley-Reed-Hall carrier recombination. The use of a mixed H₂S + H₂ gas environment allows fine control over the S₂ gas partial pressure, offering the possibility of point defect control in SnS films.

We will report the effects of annealing thermally evaporated SnS thin films in 4% H₂S + 4% H₂ (N₂ balance). We characterize changes in grain structure, optical properties, and electronic properties using SEM, XRD, 4-point-probe resistivity measurements, Hall effect measurements, and spectrophotometry. Current methods for producing thermally evaporated SnS solar cells reach an efficiency of approximately 2%. We will present the results of H₂S + H₂ annealing on solar cell performance, using an established device stack: glass/Mo/SnS/ZnO_xS_y/ZnO/ITO/Ag.⁴

[1] J. Vidal, S. Lany, M. d&’Avezac, A. Zunger, A. Zakutayev, J. Francis, J. Tate, Appl. Phys. Lett. 100 (2012) 032104.
[2] K.T. R. Reddy, N. K. Reddy, and R.W. Miles, Sol. Energy Mat. Solar Cells 90 (2006) 3041.
[3] V. Piacente, S. Foglia, P. Scardala, J. Alloys Compd. 177 (1991) 17.
[4] P. Sinsermsuksakul, K. Hartman, S. Bok Kim, J. Heo, L. Sun, H. Park, R. Chakraborty, T. Buonassisi, R. G. Gordon, Appl. Phys. Lett. 102 (2013) 053901.

In order for photovoltaic (PV) technology to contribute significantly to society&’s energy supply, device components must be abundant, cheap and environmentally benign. One candidate that satisfies these criteria as well as exhibiting almost ideal electronic properties is the photo-absorber tin sulfide. SnS is reported to have a higher optical absorption coefficient and a more suitable band gap for light absorption at peak intensity than current commercially available materials. However, the record device efficiency for SnS PV cells is only 2.0 % to date,[1] far below those obtained for similar absorbers.

We employ a combination of first-principles calculations and single crystal growth to study the multiphasic tin sulfide system with the goal of identifying the limiting attributes for PV technology. Our methods provide insight into thermodynamic stabilities, reaction pathways and electronic configurations, which allow us to ultimately comment on the photovoltaic applicability of a given structure. We are also able to predict the characteristic signatures of intrinsic chemical and physical phenomena and suggest measurements in order to identify them.
Our key results include the prediction that a recently reported structure of SnS, zinc-blende, has been mis-assigned. This phase is unstable, with large negative phonon modes and spontaneous distortions upon introduction of moderate conditions (e.g. 300K) in simulation.[2] We have developed a clear synthetic route for obtaining phase pure materials, which have been characterised using X-ray diffraction, Raman spectroscopy and time-resolved microwave conductivity measurements. Finally, our electronic calculations reveal a band mismatch between SnS and common PV device components; i.e. the molybdenum metal contact. Beyond this, we are able to suggest optimal contacts that would allow for the full photovoltaic potential of tin sulfide to be achieved.[3]
1) P. Sinsermsuksakul, K. Hartman, S. B. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, R. G. Gordon; Appl. Phys. Lett., 102, 053901 (2013). http://dx.doi.org/10.1063/1.4789855
2) L. A. Burton and A. Walsh; J. Phys. Chem. C 116, 45, 24262 (2012). http://dx.doi.org/10.1021/jp309154s
3) L. A. Burton and A. Walsh; Appl. Phys. Lett. 102, 132111 (2013). http://dx.doi.org/10.1063/1.4801313

Bulk photovoltaic systems can provide a solution for decentralized and grid-compatible electricity generation. Tin sulfide (SnS), a layered metal chalcogenide, showing high optical absorption, has been identified as an attractive material for photovoltaic absorbers [1]. We used density functional theory methods to study trends in the electronic and optical properties of model single-layer, double-layer and bulk structures of SnS.
We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them. For instance, the calculated band gap is wider for fewer layers and increases with tensile strain along the layer stacking direction [2]. We conclude from these results that either a restriction of the number of layers or the application of uniaxial strain could be used to obtain improvements in the performance of SnS as absorber material.
[1] H. Noguchi, A. Setiyadi, H. Tanamura, T. Nagatomo, and O. Omoto, Solar Energy Materials and Solar Cells 35, 325-331 (1994)
[2] G. A. Tritsaris, B. D. Malone, E. Kaxiras, Journal of Applied Physics 113, 233507 (2013)

Materials with physical properties that are insensitive to the presence of defects in crystal structure are rare but very desirable in solar energy conversion applications. For example Cu(In,Ga)Se2 (CIGS) can have enormous deviations from nominal compositions yet still result in good performance. Unfortunately, Cu(In,Ga)Se2 contains elements that may constrain its large scale fabrication in the future, so development of the next generation of Earth-abundant absorbers with similar properties is highly desirable. Here we demonstrate defect tolerance in copper nitrides, a new class of next-generation Earth-abundant solar absorber materials, using a binary copper nitride Cu3N. This prototypical copper nitride material is known to have optical absorption and electrical transport properties that are reasonably suitable for solar absorber applications and achievable at low substrate temperature synthesis conditions: 1.5 eV absorption onset, 0.9 eV band gap and 10^15 - 10^17 p-type doping at ~100 C synthesis temperature.
Defect tolerance of Cu3N is experimentally manifested by insensitivity of its electrical conductivity to the presence of point defects as well as low-angle and high-angle grain boundaries identified by the means of diffraction, microscopy and spectroscopy measurements. In the pure semiconducting Cu3N electrical conductivity is 10^-3 -10^-2 S/cm, regardless of these of crystallographic imperfections. Defect tolerance of electrical conductivity in Cu3N is theoretically supported by first principles calculations of point defects in this material. According to the theoretical results, Cu3N has no deep bulk point defects that can act as scattering centers for majority charge carriers in the experimental conductivity measurements or as recombination traps for minority charge carriers in solar energy conversion applications.
We propose that the observed defect tolerance of electrical properties in Cu3N originates from its anti-bonding character of the valence band maximum. In semiconductors with such electronic structure, crystallographic defects states are likely to fall in the energy bands, not in the energy band gap. This in turn leads to effective-mass-like shallow defect states close to the band edges that cause minimal scattering to both majority and minority electric charge carrier transport. This explanation of defect tolerance in Cu3N invokes only the character of the involved atoms and their electronic energy states. Therefore, it is likely that other copper nitrides, in particular ternaries, will also have similar defect tolerant properties. This leads to a conclusion that copper nitrides are a new class of defect-tolerant next-generation Earth-abundant solar absorber materials.
This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy, as a part of a Next Generation PV II project “Ternary copper nitride absorbers” within the SunShot initiative.

A series of copper transition metal nitrides have been synthesized by using different methods, including ion-exchange and high pressure reactions. A combination of theoretical calculations and experimental studies were applied for the analyses of electronic structures and optical properties of these materials. Our results indicate that ternary copper nitrides may have considerable potential as absorbers in earth abundant solar cells. For example, layered CuTaN2 was synthesized by an ion exchange reaction of CuI and NaTaN2 as previously reported. Based on the results of EDX analysis, the Cu:Ta ratio of the CuTaN2 sample was1:1 within the overall errors when examining powders of ±10 % and no Na was detected. The crystal structure and thermal stability of CuTaN2 was accurately determined by Rietveld analysis of the powder X-ray Diffraction profile and by TGA analysis, respectively. CuTaN2 crystallizes in a rhombohedral structure with space group R-3mH as shown in [Figure 1]. CuTaN2 possesses a band gap of 1.53(x) eV, which is in reasonable agreement with density functional theory calculations of Cu containing nitrides. Similar materials may be even better suited for solar cell application.

During the last two decades there has been a proliferation of solution-phase batch colloidal nanocrystal synthesis methods. Solution-phase processes offer excellent control over the size and composition of nanocrystals but there is a perspective gaining momentum that the ligands attached to the nanocrystal surfaces and the presence of residual solvent degrades performance when these nanocrystals are incorporated into thin films and optoelectronic devices. Using gas-phase synthesis and gas-phase deposition of nanocrystals, it is possible to form films with nearly the ideal random close packed density without exposure to solvents or need for ligands. Our group&’s work to date has focused on nonthermal plasma synthesis of silicon and germanium nanocrystals followed by dense film formation via inertial impaction. However, very little attention has been paid to compound semiconductors. In particular, the plasma synthesis of technologically important metal-sulfide nanocrystals remain completely unexplored.
We have developed a new generalizable approach for making metal-sulfide nanocrystals in the gas phase using a nonthermal sulfur-argon plasma. A metalorganic precursor and cyclic sulfur molecules are dissociated by electron impact reactions to form metal sulfide nanoparticles with controllable composition and controllable size. For example, zincblende (cubic) ZnS nanocrystals, were formed using diethyl zinc (DEZ) as the Zn precursor. Scherrer analysis of the X-ray diffraction (XRD) peak broadening and transmission electron microscopy (TEM) agreed and indicated nanoscrystals with diameters less than 10 nm. The metal to sulfur ratio could be controlled through the DEZ feed rate. Surprisingly, the elemental sulfur feed rate, in the range we explored, had little effect on the sulfur content of the particles. Sulfur rich products (low DEZ feed rate) exhibited visible light absorption while stoichiometric ZnS (high DEZ feed rate) showed negligible absorption in the visible range of the spectrum and a clear absorption onset near the bulk bandgap of ZnS (3.7 eV). Crystalline copper sulfides were made using hexafluoroacetylacetone Cu(+1) vinyltrimethylsilane (HFAC)Cu(VTMS) as the copper precursor. Depending on the feed rate of the (HFAC)Cu(VTMS) relative to sulfur, various phases were observed by XRD, from Cu metal, to Cu2S, to CuS. This particular copper precursor exhibits a rich plasma chemistry. Interestingly, if the present results on ZnS and CuxS are compared to previous experiments on Si and Ge, both the CuxS and ZnS nanocrystals are much smaller than expected from the precursor partial pressures and the residence time. This suggests that the metal sulfide growth is dominated by nucleation, in contrast to the Si and Ge which exhibit characteristics of surface growth. Finally, the synthesis of tin sulfides and iron sulfides will be discussed.

Thin film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. Solar cells with a structure of glass/Mo/SnS/Zn(O,S)/ZnO/ITO/Ag are studied by varying treatments of the SnS and Zn(O,S) layers. Annealing SnS in pure hydrogen sulfide increased the mobility by more than one order of magnitude, and improved the open-circuit voltage up to 273 mV, fill factor up to 52%, and power conversion efficiency up to 2.2%, which are higher than our current champion cell [2]. A short-circuit current density of 16 mA/cm² was achieved. Solar cell performance will be further optimized by adjusting the oxygen to sulfur ratio of Zn(O,S) [1,2] and by in situ nitrogen doping using ammonium hydroxide to serve as both an oxygen and nitrogen source. [1] H. H. Park, R. Heasley, and R. G. Gordon, Appl. Phys. Lett.102, 132110 (2013). [2] P. Sinsermsuksakul, K. Hartman, S. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, and R. G. Gordon, Appl. Phys. Lett.102, 053901 (2013).

The solid solution between ZnO and GaN is the best single system capable of utilizing visible light to stably drive solar water splitting (to renewably produce H₂ fuel) developed to date. The lowest band gaps observed for this system are found for Zn-rich samples, suggesting that samples which are rich in the earth-abundant cation may be the most desirable for water splitting applications. We find that defects are common in samples with high Zn contents, and describe methods for identifying and quantifying defects in bulk samples. Our results provide a framework for exploring the influence of synthesis and post-processing conditions on defect formation, and for testing the impact of defects on photoelectrochemical activity.

Cu₂ZnSn(S,Se)₄ (CZTSSe) currently offers the highest performance, measured in terms of power conversion efficiency (PCE), among thin-film PV devices based on “earth-abundant” metals, with PCE now reaching above 11%. Despite rapid increase in demonstrated performance, the current levels are not yet sufficient for commercialization when compared to other more established thin-film materials, such as Cu(In,Ga)(S,Se)₂ (CIGS) and CdTe, which have efficiencies of as high as 20%. Among the three device characteristics that determine PCE—J_sc, FF and V_oc—modest improvements in efficiency can be gained by addressing J_sc and FF. However, the most important deficiency in the current generation of devices (relative to either analogous CIGS or CdTe device performance or the Shockley-Queisser limit) can be found in V_oc. In this talk we will examine progress in understanding and mitigating limitations that are holding back device performance in the current generation of CZTSSe devices. Key issues include the nature of bulk/surface defects and phase purity of the CZTSSe absorbers.

The heterojunctions formed between solution phase grown Cu2ZnSn(SxSe1- x)4(CZTS,Se) and a number of important buffer materials including CdS, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) and photovoltage spectroscopy. Time synchronized pump (1.55eV/photon) and probe (20-40 eV/photon) light pulses from an amplified 50 fs Ti:sapphire laser were employed for these experiments. Under static conditions (no pump pulse) standard UPS spectra were collected. With the pump pulse present, a dense electron-hole plasma screens the depletion field and flattens the CZTS,Se bands. With this approach we extract the magnitude and direction of the CZTS,Se band bending, locate the Fermi level within the band gaps and measure the band offsets between the absorber and buffer under flatband conditions. This approach is particularly useful when studying chemical bath deposited buffer layers. We find spike conduction band offsets (CBO) for the CdS/CZTS,Se heterostructure over all ranges of S and Se content. We find a somewhat smaller spike CBO for the In2S3/CZTSSe heterostructure and have achieved the highest reported device efficiency (7.6%) for that system. For ZnS/CZTS,Se, the large CBO (1.1 eV) results in a device with no short circuit current (Jsc) flow while for ZnO, the ~0 eV CBO produces good Jsc but low open circuit voltage (Voc). More detailed correlations between heterostructure electronic properties and device performance will be discussed. We will also discuss two-color pump/probe experiments in which the band bending in the buffer layer can be independently determined. Finally, studies of the bare CZTS,Se surface will be discussed including our observation of mid-gap Fermi level pinning and its relation to Voc limitations and bulk defects.

Having a high absorption coefficient and an optimal bandgap similar with that of CIGS solar cells, CZTS solar cell is intrinsically even attractive owing to a large reserve in the earth crust and non-toxic feature.[1] Since CZTS is a complex material system and a single phase production is not yet controllable, the thermal co-evaporation approach is now holding the champion efficiency 8.4% for CZTS solar cells,[2] in contrast with over 20%[3] for its CIGS counterpart. CZTS at this stage mainly imitates the relevant device structure of CIGS as a shortcut for development, which may not be the optimal due to the difference between CZTS and CIGS. In fact, parasitic secondary phases are un-avoidable yet and detrimental to device performance in CZTS materials, which form either carrier transport barrier like ZnS and SnS2 or shunting path such as Cu2-xS and CuxSnSy.[4] One of the major challenges for phase control is the defects inducing Mo/CZTS interface due to the lower Gibbs free energy of MoS2 compared with that of CZTS or Cu2SnS3.[5] Voids, detrimental secondary phases, thick MoS2 (in the order of hundreds of nm) are generally observed for untreated Mo/CZTS interface due to the reaction between Mo and CZTS (a decomposition reaction for CZTS).[5, 6] To conquer such similar problem both TiN[7] and ZnO[8] barrier layers have been proved to be effective in reducing the thickness of MoSe2 and the amount of defects at Mo/Cu2ZnSnSe4 interface, and therefore enhancing the cell efficiency; To suppress or even avoid the decomposition of CZTS at the back contact, four potential solutions have been proposed and investigated as follows. (1) Ultrathin ZnS overcoating has been introduced as it is relatively stable in comparison with MoS2; (2) Rapid thermal annealing (RTA) of the Mo layer was also explored as this increases the grain size of Mo and crystallinity of CZTS grown on such layer; (3) RTA treated Mo with ultrathin Ag overcoating is attempted aiming to block the reaction between Mo and CZTS. Ultrathin ZnO overcoating is adopted in this investigation for comparison purpose. The results indicate that these solutions reduce the amount of voids, secondary phases at the interfaces and the thickness of MoS2 substantially, and improve the device performance consequently. Notice that the solutions should not be limited to a specific CZTS preparation procedure and in this investigation the photoactive layer is prepared by sputtering metal stack precursor followed by sulfurisation.
References:
1 Ji, S., Ye,C., Rev. Adv. Sci. Eng.1, 42 (2012).
2 Shin, B., et al., Prog. Photovolt., Res. Appl. 21, 72 (2013).
3 Jackson, P., et al., Prog. Photovolt., Res. Appl. 19, 894 (2011).
4 Platzer-Björkman, C., et al., Sol. Energy Mater. .Sol. C. 98,110 (2012).
5 Scragg, J.J., et al., J. Am. Chem. Soc. 134,19330 (2012).
6 Wätjen, J.T., et al., Thin Solid Films 535, 31 (2013).
7 Shin, B., et al., Appl Phys Lett 101,053903-4 (2012).
8 Lopez-Marino, S., et al., J Mater Chem A, 2013.

The earth abundant kesterite Cu₂ZnSnS₄ material with a high absorption coefficient, direct band gap, and good long-term stability has become an attractive candidate for use in solar cell absorber layers, as compared to the traditional CdTe and Cu(In,Ga)(S,Se)₂ (CIGS) thin-film absorber layers. However, the narrow compositional window for obtaining a stable single phase in the phase diagram leads to difficulties in the manufacturing process of Cu₂ZnSn(S,Se)₄. Additionally, the existing substrate-type device configuration for these solar cells uses a molybdenum (Mo) back contact, which becomes a critical bottleneck for further increasing the power conversion efficiency because of serious disadvantages suffered by the (i) presence of a Schottky barrier at the Mo/Cu₂ZnSn(S,Se)₄ interface and (ii) decomposition of Cu₂ZnSn(S,Se)₄ at the Mo interface. Therefore, we here introduce a low-cost and Mo free superstrate-type device configuration of Au/Cu₂ZnSn(S,Se)₄/CdS/TiO₂/ITO/glass, which includes a bifunctional interlayer of CdS to circumvent the formation of Mo(S,Se)₂ and avoid the occurrence of potential decomposition pathways. We also demonstrate the tunable properties of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals followed by a relatively mild sulfurization process, as compared to the harsher selenization reaction. Using a facile hot-injection approach for synthesizing Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals with varied Se to (S+Se) ratio, not only the role of Se in Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals but also the evolution of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals to Cu₂ZnSn(S_xSe_1-x)₄ film during the sulfurization step are systematically investigated. It is found that minimizing the possibility for the loss of Sn during the heat treatment and producing a compact film with large grain size are beneficial for the device performance. As a proof-of-concept, our superstrate-type architecture without using any binder has exhibited the conversion efficiency of 1% with V_oc of 353 mV, J_sc of 7.75 mAcm^-2, and FF of 36.66%. It indicates that a low-cost, Mo(S,Se)₂-free superstrate-type architecture is potential for earth abundant Cu₂ZnSn(S,Se)₄ solar cell applications.

As is common in other thin film photovoltaic technologies, the most common form of Cu2ZnSnS4 (CZTS) in solar cell applications is polycrystalline, i.e., with grain boundaries of various crystallographic orientations. To date, there is still insufficient evidence to determine the role of grain boundaries in CZTS. Investigating grain boundary-free CZTS (grown epitaxially on a low resistivity substrate) would unambiguously answer this question. Fortunately, CZTS and Si have a nearly perfect lattice match; the reported a-axis lattice constant, a of CZTS ranges from 0.5426 - 0.5435 nm (c-axis lattice constant, c is twice of a in CZTS) while that of Si is 0.5431 nm. We have demonstrated epitaxially-grown CZTS films on Si(001) for the first time using a thermal co-evaporation technique with a valved cracker as a sulfur source. A substrate temperature as high as 370 °C and proper substrate cleaning (HF-dip followed by thermal desorption of surface hydrogen) are found to be necessary for the epitaxial growth. Theta-2Theta X-ray diffraction measurements reveal that the overall orientation of the CZTS films follows the underlying Si(001) substrate. Direct evidence of the epitaxial relationship between the CZTS and the Si is provided by transmission electron microscopy measurements. Formation of structural defects such as twinning and the effect of the growth temperature on the defects are also discussed.

The rapid rise in power conversion efficiencies of thin-film solar cells based on the p-type semiconductors Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) attests to their potential as low-cost, earth-abundant alternatives to CdTe and CuInGaSe₂, the two leading commercial solar absorber materials. To date, many CZTS and CZTSe thin film formation methods have been developed, and solar cells with efficiencies exceeding 11 % have been demonstrated. However, increasing solar cell efficiencies relies heavily upon trial-and-error optimization of multiple film properties during the film&’s synthesis. One of these properties is the film&’s microstructure. Ideally, a monolayer of single crystal grains with sizes on the order of the film thickness is required to reduce charge recombination at grain boundaries. This talk will focus on elucidating the factors that affect the microstructure of thin CZTS films by comparing two synthesis methods: (i) annealing, in a sulfur containing atmosphere, of thin films deposited from colloidal CZTS nanocrystal dispersions, and (ii) high-temperature sulfidation of copper-zinc-tin precursor films deposited by co-sputtering. In the first method, 20 to 30 nm diameter CZTS nanocrystals are synthesized in hot (150-300 ^oC) oleylamine from copper, zinc, and tin diethyldithiocarbamates. Following this, thin coatings (2-3 mu;m) are drop cast from colloidal dispersions of these nanocrystals, which are then annealed in a well-controlled closed system to form large grain microcrystalline films. In this approach, CZTS already forms prior to annealing and the large surface energy of the nanocrystals drives the microstructure development. Abnormal and normal grain growth compete with one another, leading to bimodal grain size distributions. The factors that affect the abnormal and normal grain growth rates include, the annealing temperature, the substrate, and the sulfur and tin sulfide partial pressures. In the second method, CZTS formation and microstructure development takes place simultaneously during the sulfidation of the copper-zinc-tin precursor films. In this approach, abnormal grain growth is not observed. In both methods, however, the film microstructure is a strong function of the underlying substrate. Specifically, substrates containing alkali metal impurities enable the formation of significantly larger grains at lower processing temperatures.

Kesterite thin film solar cells made from Cu, Zn, Sn, Se and S (CZTS/Se) are promising for their earth abundant materials and solution processability. To date, these devices have achieved efficiencies of around ~11%, underperforming compared to other thin film technologies such as CIGS or CdTe. A major limiting factor is the low open circuit voltage resulting from recombination losses. Interestingly, polycrystalline devices have shown higher efficiencies than the single crystal counterparts. The nature of these recombination pathways and the role of grain boundaries are not fully understood. Additionally, the beneficial role that grain boundaries provide may be important for improving device performance. We seek to characterize the defect physics of these devices using scanning Kelvin probe microscopy (SKPM). SKPM is a non-contact technique that provides information about surface potential and topography. We correlate local electrical properties and structure on the nanoscale using this instrument. We analyze cross sections of CZTS devices to probe both the grain boundaries and interfaces in working devices processed under different conditions.

The sustainable PV material, Cu2ZnSnS4 (CZTS), can form in two different tetragonal polymorphs, known as kesterite and stannite respectively. These differ in the ordering of Cu and Zn cations within the (004) planes. The energy difference between the two phases is only ~3 meV/atom, meaning that the material is prone to Cu, Zn cation disorder, as evidenced by neutron diffraction experiments. The disordering can potentially take place at the unit cell level in the form of point defects (vacancies and/or anti-site atoms) or on a much larger scale where the material can be viewed as consisting of domains of kesterite and stannite. The latter scenario is not expected to have a significant effect on device performance, since theoretical calculations have shown the optical properties for pure kesterite and pure stannite to be similar. However, point defects are important because they can dope the material as well as act as recombination centres. Luminescence measurements on CZTS have indicated that the material contains a high density of charged point defects which result in a fluctuating electrostatic potential.
In this contribution direct evidence for Cu, Zn disordering in CZTS is obtained by measuring the chemical composition of individual atom columns using aberration-corrected scanning transmission electron microscopy. Many of the measurements indicated that Cu and Zn were inter-mixed; this may be a genuine feature of the sample or an experimental artefact due to spreading of the electron probe as it propagates through the specimen, resulting in a measurement that represents the average composition over several neighbouring atom columns. However, evidence for atom columns consisting entirely of ZnCu anti-site atoms was also found. The composition inhomogeneity results in local degenerate doping and causes strong electrostatic potential fluctuations due to uncompensated ZnCu donors. This is the first time such composition inhomogeneities have been observed directly. The role composition fluctuations have on device performance will also be discussed. For example, uncompensated donors are expected to be stronger recombination centres compared to uncompensated acceptors in p-type CZTS material.

The interest in Cu₂ZnSn(S,Se)₄ (CZTS) for photovoltaic (PV) applications is motivated by the similarities to the promising material Cu(In,Ga)Se₂ (CIGS) while being comprised of non-toxic and earth abundant elements. Competition between the kesterite (necessary for PV applications) and the stannite phase of CZTS, as well as a number of binary and ternary competing phases affects the power conversion efficiencies of CZTS devices. However, the structural similarities of a number of these phases make their identification through standard x-ray diffraction challenging. Furthermore, the strong possibility of site-swapping of the Zn and Cu between their respective sublattices may also lead to a high level of defects which reduce solar cell efficiency. The tunable energy x-rays available at modern synchrotron sources provide a site and element specific probe to investigate such disorder. We have used resonant x-ray diffraction techniques to quantitatively determine the crystallographic phases and level of site-swapping disorder present in thin films of polycrystalline CZTS in order to shed light on the relative success of different growth conditions. Our goal is to understand and characterize the structural differences and defect levels of films grown under different conditions. By comparing with device efficiencies for these films, we can identify those structural features with the greatest effect on PV performance and the growth conditions to effectively control them.

Grain boundaries are ubiquitous in inorganic thin-film PV and, unless passivated, reduce the device efficiency by lowering the open circuit voltage through Shockley-Read-Hall recombination within the space charge region. In the sustainable PV material, Cu2ZnSnS4 (CZTS), secondary phases are also commonly observed. Typical secondary phases include sulphides of copper and tin, Cu2SnS3 and ZnS which has a high exothermic heat of formation. Secondary phases can affect device efficiency through a number of different mechanisms (e.g. variation in band gap and hence absorption, effect on series and shunt parasitic resistances), but one of the factors that needs to be considered is the recombination rate at the heterointerface between CZTS and the secondary phase. This is similar to the role of grain boundaries on device performance. The recombination velocity measures the ability of an interface to act as a minority carrier sink; interfaces with a larger recombination velocity are more deleterious to device performance. An interface typically has a large recombination velocity if it contains deep electronic states within the band gap, the origin of these defect states being due to lattice mismatch and/or segregation at the interface.
Cathodoluminescence (CL) is a technique that measures the light emitted by a solid when illuminated by an electron probe, such as that in a scanning electron microscope (SEM). The CL intensity is lower in the vicinity of an interface due to recombination. By analysing the variation of CL intensity as a function of distance from the interface the recombination velocity can be extracted. This technique has been applied to heterointerfaces for a number of different secondary phases in CZTS. SnS was found to have a high recombination rate, but heterointerfaces for ZnS and Cu2SnS3 were electrically passive, i.e. the recombination velocity was lower than the bulk carrier diffusion velocity. To examine the cause for the low recombination velocity the atomic structure of the heterointerface was examined using a transmission electron microscope. The interface between CZTS and ZnS for example, showed registry of the c-planes of tetragonal CZTS with the cubic planes of ZnS. The lattice registry is facilitated by the similarity of crystal structure and lattice parameters between CZTS and ZnS, Cu2SnS3 secondary phases, which are based on tetrahedral bonding. Due to the relatively small lattice mismatch secondary phases such as ZnS and Cu2SnS3 do not lower device efficiency through recombination, although they may have an effect through other processes, such as light absorption.

Alkylammonium lead halides with 3-dimensional perovskite structure have recently attracted great attention due to their superb light harvesting property. In this talk, high efficiency mesoscopic thin film solar cells based on CH3NH3PbI3 perovskite are presented. CH3NH3PbI3 was deposited in-situ on nanocrystalline TiO2 surface using spin-coating technique. Bandgap energy of CH3NH3PbI3 was determined to be 1.5 eV. Electrochemical junction between CH3NH3PbI3-sensitized 3.6-mu;m-thick TiO2 film and iodide-based redox electrolyte exhibited a power conversion efficiency (PCE) of 6.5% at AM 1.5 G one sun illumination, which was two times higher than the conventional N719-based device. Substitution of methylammonium with ethylammonium led to 2H perovskite structure with enlarged bandgap energy from 1.5 to 2.2 eV. Perovskite sensitizers were found to be chemically unstable in the presence of electrolyte, which was overcome by replacing liquid electrolyte with spiro-MeOTAD as a solid hole transporting material (HTM). A PCE as high as 9.7% at one sun was achieved from CH3NH3PbI3-sensitized 0.6-mu;m-thick mesoporous TiO2 film. Moreover, excellent long-term stability was demonstrated even without encapsulation. Sub-micrometer-long rutile TiO2 nanorods were modified with CH3NH3PbI3 perovskite, which resulted in a PCE of 9.4%. Well aligned nanorod structure was found to be better in pore filling of HTM, resulting in better fill factor compared to mesoporous structure. Compared to one-step spin-coating method, a modified method for coating CH3NH3PbI3 into the mesoporous network improved PCE to 12.7%.

Lead halide perovskites and Semiconductor quantum dots (QDs) currently attracted widespread attention for photovoltaic devices due their large absorption coefficients, high carrier mobility and the possibility of controlling their optoelectronic properties.
In this work two possibilities of hole conductor free heterojucntoin solar cells are presented the lead iodide perovskite/TiO2 heterojunction solar cell and the PbS QDs/TiO2 heterojunction solar cell.
The lead iodide perovskite/TiO2 heterojunction solar cell was produced by deposition of perovskite nanoparticles from a solution of CH3NH3I and PbI2 in butyrolactone on a film of TiO2 (anatase) nanosheets exposing (001) facets. Importantly, the CH3NH3PbI3 nanoparticles assume here simultaneously both the role of light harvester and hole conductor, rendering superfluous the use of an additional hole transporting material. The simple mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cell shows impressive photovoltaic performance with short circuit photocurrent (Jsc) of 16.1 mA/cm2, an open circuit photovoltage (Voc) of 0.631 V and a fill factor (FF) of 0.57, corresponding to a light to electric power conversion efficiency (PCE) of 5.5% under standard AM 1.5 solar light of 1000 W/m2 intensity. At a lower light intensity of 100W/m2 a PCE of 7.3 % was measured.
The Solid state PbS QDs/TiO2 heterojunction solar cell were produced using layer-by-layer deposition of PbS QDs on a film of TiO2 (anatase) nanosheets exposing (001) facets. The TiO2 nanosheets are known for their slightly higher conduction band and their higher surface energy compared to normal anatase TiO2 nanoparticles (NPs). Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO2 heterojunction solar cell produces a short circuit photocurrent (Jsc) of 20.5 mA/cm2, an open circuit photovoltage (Voc) of 0.545 V and a fill factor (FF) of 0.38, corresponding to a light to electric power conversion efficiency (eta;) of 4.7% under AM1.5 illumination. The advent of such simple solution processed mesoscopic heterojunction solar cells paves the way to realize low cost, high-efficiency solar cells.

In recent years, colloidal quantum dot (CQD) based photovoltaics received great attention due to their promise for low-cost, high-efficiency photovoltaics deriving from solution-phase processibility and quantum-size-effect tunability for tandem and multijunction devices.1

The efficienciency of CQDs based photovoltaics has been increasing quickly recently. A critical factor for the efficiency increase is the improvement of the performance of CQDs. By using solution-phase halides passivation strategy, we succesfully achieved better charge-balance and thus lower trap state density. Improved passivation in the solution phase was found to be critical to reduce midgap traps.
The hybrid passivation strategy, which combined solution-phase halides treatment and solid-state thiol ligand exchange, led to a record certified efficiency of 7%.2 While for all-inorganic homojunction devices, we found that the halide passivation in the solution phase is more efficient than solid state treatment alone.3 By reducing midgap traps with solution phase halides treatment, a record carriers mobility of 0.24 cm2 V-1s-1 for PbS CQDs was achieved, and the performance of all-inorganic homojunction device was significantly improved.3
Based on the material progress, we further developed a novel approach to improve the CQD photovoltaic architecture by taking advantage of the doping dexterity achieved with our CQDs materials. By judiciously choosing the halides ligand and optimizing the condition, we engineer n-type, device-grade films, in which we carefully engineer the doping spatial profile to produce a doping gradient within the n-type absorber. The doping gradient greatly improves carrier collection and enhances the voltages attainable by the device, leading to a 1 power point efficiency improvement, that is a record efficiency of 7.4% for all inorganic CQDs photovoltacis.
1. Grtäzel, M.; Janssen, R. A. J.; Mitzi, D. B.; Sargent, E. H. Nature 2012, 488, 304-312.
2. Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H. Nat Nano 2012, 7, 577-582.
3. Ning, Z.; Ren, Y.; Hoogland, S.; Voznyy, O.; Levina, L.; Stadler, P.; Lan, X.; Zhitomirsky, D.; Sargent, E. H. Adv. Mater. 2012, 24, 6295-6299.
4. Ning, Z.; Zhitomirsky, D.; Aldinolfi, V.; Sutherland, B.; Xu, J.; Voznyy, O.; Maraghechi, P.; Lan, X.; Hoogland, S.; Ren, Y.; Sargent, E. H. Adv. Mater. 2013, 25, 1719-1723.

It has been shown over the last two decades that conversion efficiencies over 13% can be achieved using TiO2 based solar cells, such as in liquid electrolyte dye sensitized solar cells. There has been a push to replace the hazardous electrolytes with organic material to create environmentally friendly devices. In this work we introduce core-shell nanostructured hybrid solar cells with a single crystalline core in order to increase electron mobility and light scattering without additional recombination effects. As organic hole transport material we use poly(3-hexyl thiophene)(P3HT). The core consists of hydrothermally grown Sn:TiO2 wires conformally coated with a thin TiO2 shell. The energy differences in the conduction band edge, due to the different oxide layers, provide a cascade for electrons to migrate to the core of the nanowire, potentially decreasing the charge recombination at the organic-inorganic interface. Additional modification of the surface by organic molecules can further improve electron injection into the metal-oxide and suppress charge recombination. In this presentation we will demonstrate how the core-shell structure and the interface modification influence the device physics of our hybrid solar cells.

Colloidal semiconductor nanocrystals (NCs) are promising new material systems that allow for solution-processed devices retaining the benefits of quantum confinement effects. The benefit of a tunable bandgap via quantum confinement is hindered by the transport barriers bulky organic ligands impose. Inorganic ligands can lower the barrier to the point where band-like transport is possible and allow the effective exciton size to become larger than 2x the NC diameter meaning the exciton is coherently delocalized over multiple nearest-neighbor NCs. In transport through these NC systems, the carriers encounter the semiconductor\surface termination (ligand) interface thousands of times, whereas in bulk systems, this interaction might only occur once. Therefore the NC surface and how it is terminated plays a dominant role in these systems. By utilizing inorganic (rather than organic) ligands and introducing various chemical species to the surrounding media, the properties (e.g. majority carrier type, PLQY, mobility, catalysis, magnetism, etc.) of NCs arrays and solutions can be tuned and, perhaps more importantly, these properties are not overtly hindered as is the case with organic ligands. Then through screening basic physical properties such as charge carrier mobility and lifetimes of the NC/ligand combinations in stand-alone films (as opposed to finished devices) a direct comparison between NC/ligand systems independent of device optimization or lack thereof is possible. We aim to find synergistic behavior between the NC/ligand combination which will increase the mobility and lifetime of carriers needed for improved devices. In this talk, we will present findings on the transport properties of arrays of CdTe, CdSe, PbS, and PbSe NCs treated with inorganic metal chalcogenide complexes such as In2Se4, In2Te3, Sn2S6, and metal free ligands such as S2- and Se2- treated with various metal ions. The properties of these ligands have not yet been fully studied and understood on p-type materials (such as PbS, PbSe, and CdTe) relevant for photovoltaic devices.

Colloidal quantum dot (CQD) solids are promising semiconductor materials for electronic and optoelectronic applications offering several distinct advantages like inexpensive and facile processing and the ability to tune the bandgap via the quantum size effect. Solar cells are one of the main applications for CQD materials. A successful approach to fabricate CQD photovoltaic devices, employs a p-n hetero-junction realized combining a p-type PbS CQD solid with the n-type titanium dioxide semiconductor. This architecture presents high efficiency due to the capability of collecting photo-generated carrier offered from the depletion region. On the other hand this structure introduces a severe limitation in the choice of the CQD&’s bandgap, due to band alignment requirements. This limitation has been recently overcome by introducing the Quantum Junction (QJ) solar cell, a device consisting of both p-type and n-type CQD solids to form the p-n junction. This architecture extends the range of design opportunities for CQD photovoltaics, since the bandgap can be tuned, in a spatially-varying fashion, across the light-absorbing semiconductor layer by changing the CQD size. Here we report a novel approach for increasing the width of the depletion region and the built-in electric field through the p-n QJ. We efficiently use the quantum size effect for introducing a large bandgap layer at the far side of the n-type film (N-layer). Proper design of the structure generates increased electric field and consequently higher Voc, better Jsc and higher fill factor improving the PCE up to 12%, compared to a non optimized device. In this way we demonstrate how an all-CQD hetero-structure can be used for efficient photovoltaic applications. Further, by using a computational model, we show the profile of the electric field and the potential within the solar cell and the curves of PCE, FF, VOC and JSC as a function of the bandgap of the N-layer. This theoretical study offers fundamental indications from a design prospective and also precious insights on the physics governing the device and a path for future high-efficiency devices. The described approach is than compared with a recently realized graded structure where control of CQD films doping is effectively used to realize a p+/ n/ n+ structure. Even in this case the architecture determines a wide depletion region within the solar cell resulting in currents as high as 24.5mA/cm2 and a PCE of 7.4%. A further discussion on combining the two aforementioned approaches in a fully optimized device is also presented by using the simulated model.
We show how to wisely adjust the material&’s parameters, bandgap and doping and exploit the several advantages offer from CQD technology to create new high efficiency photovoltaic devices and overcome the intrinsic limitations of this semiconductors.

Lead sulfide (PbS) colloidal quantum dots have become the dominant material for solution-processed nanocrystalline solar cells due to their amenability to layer-by-layer processing and an optical absorption spectrum that is well-matched to the sun. However, it remains unclear whether photogenerated excitons in ligand-exchanged PbS films are stable to spontaneous dissociation. Further, the weak, infrared fluorescence from these films has made it experimentally challenging to measure the effective diffusion length of photogenerated species -- a critical parameter for device optimization. Here, we employ recently-developed superconducting single-nanowire single-photon detectors (SNSPDs) to demonstrate that photogenerated exictons in PbS films diffuse intact under flat-band conditions, and determine that the diffusion length is >100nm. This suggests an alternate operating paradigm for nanocrystalline PbS solar cells, and emphasizes that that the key limits on the performance of present-day devices are practical, rather than fundamental.

ZnO - PbSe colloidal quantum dot solar cells (CQDSCs) are promising technology because they have yielded encouraging efficiencies and the properties of the quantum dots (QDs) can be controlled using solution-processable techniques. However, the open-circuit voltage (Voc) is restricted by the low acceptor level of the ZnO, which limits the useful bandgap range of the quantum dots. We have overcome this limitation by using Atmospheric Atomic Layer Deposition (AALD) to produce Mg-doped ZnO in which the conduction band position can be continuously tuned. AALD implements the conventional ALD process in atmospheric conditions, allowing uniform, compact films to be rapidly deposited in atmospheric conditions and low-temperatures. XPS measurements have shown us that we were able to incorporate up to 80 at% Mg into the ZnO films and continuously tune the band gap from 3.3 eV to 5.5 eV. By continuously tuning the conduction band level, we have increased the Voc of Mg:ZnO - PbSe CQDSCs from 408 mV to 608 mV, the highest value reported for this combination of materials. This allowed us to also achieve one of the highest efficiencies reported as well, despite the open-atmosphere nature of our deposition process.

Photovoltaic (PV) cells that utilize quantum dots (QDs) in the active layer have gained much attention recently because of their potential as high-efficiency, low-cost, and air-stable devices for sustainable solar energy conversion. The power conversion efficiencies of QD-PVs based on lead sulfide (PbS) have been enhanced dramatically in only several years to the current record 7.0%, owing to the favorable optical properties of PbS QDs including facile tunability of band gaps with the variation in dot sizes or shapes, wide spectral responses, and multiple exciton generation.
Yet, despite these beneficial optical properties, the performance of QD-PVs is still limited by both poor electronic transport properties and unfavorable energy level alignment at the donor-acceptor interface. In order to overcome these limitations, recent work has focused on the ability to engineer ligands that are typically present on QD surfaces, eventually to control the electronic properties of the QD film.
Here, we use a combination of ab initio density functional theory calculations and ultraviolet photoemission spectroscopy measurements to demonstrate that ligands induce dipole moments that shift band-edge energy levels (conduction band minimum and valence band maximum) of QD films. Our focus is on sulfur-terminated ligands including 1,2-ethanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol. Surprisingly, we find that these seemingly similar ligands behave quite differently in terms of the induced dipole moments, significantly affecting photovoltaic device efficiency both in schottky- and heterojunction-architectures. Our work indicates that indeed the choice of ligands is extremely important, and ligand engineering offers room for large performance improvements in photovoltaic devices based on QDs.

Semiconductor quantum dots (QDs) of PbS play an important role in solar energy conversion, due to their unique new physics that arise from quantum confinement effects such as tunable bandgap as a function of QD size and multiple exciton generation (MEG). The power conversion efficiencies (PCEs) of solar cells using thin, densely packed layers of PbS QDs have increased from 2% just a few years ago to approaching 9% today. Furthermore, more than 45% PCE can be achieved theoretically using MEG effect. Therefore, the combination of high efficiency and low cost makes PbS QDs revolutionary materials for the next generation thin film solar cells.
In this talk, I will focus on the fundamental device physics in PbS QD solar cells including carrier generation, transport and recombination of PbS QD-layers characterized by steady-state photoluminescence spectroscopy and open circuit voltage(Voc) dependence with temperature in in situ PbS QD solar cells. We discovered that photoluminescence is due to bimolecular recombination from the band tail state, where carriers follow variable range hopping transport mechanism.
Reference:
1. J. Gao, C. L. Perkins, J. M. Luther, H. Y. Chen, M. C. Hanna, O. E. Semonin, A. J. Nozik, R. J. Ellingson, and M. C. Beard, Nano Lett. 11, 3263-3266(2011).
2. J. Zhang, J. Gao, J. M. Luther, and M. C. Beard, submitted to JACS (2013)
3. J. Gao, J. Zhang, J. C. Johnson, J. VandeLagemaat, Arthur J. Nozik, and Matthew C. Beard, under preparation(2013).

The field of colloidal quantum dots (CQDs) and their applications got much attention in the last few years. Their size-tunable bandgap, solution processability and the consequent low production cost together make them suitable materials for active layers of optoelectronic devices. Cross-linking quantum dots result in new and unique assemblies, quantum dot solids, which are claimed to show band-like transport due to their coupling, while the optical properties still indicate quantum confinement. While the early results were promising [1], state-of-art devices have not reached the expected performances so far. The best field effect transistors show mobility not larger than 10^-2 cm²/V.s and on/off ratios in the range of 10² using conventional SiO₂ gate [2]. Recently it has been pointed out that charge carrier loss due to trap states is one of the main enemies of quantum dots solids [3,4].
In general, these trap states stem from unbalanced surface stoichiometry or chemical doping of the sample. The crosslinking process itself can affect the stoichiometry donating sulfur atoms to the quantum dots. The final effect depends ultimately on the amount of ligands and their binding to the surface. Contaminants that adsorb on the CQD surface such as water have similar effect on the electronic structure acting as trap especially for electrons. Thoroughly controlling these parameters would allow removing and preventing the formation of intragap states, thus achieving better performances.
Here we report high mobility quantum dot solids obtained by optimizing the crosslinking process. Field-effect transistors prepared using cross-linked PbS quantum dots show mobility as high as 3x10^-2 cm²/V.s on conventional SiO₂ gate. Importantly, the devices demonstrated on/off ratio of 10⁶, which is the highest for PbS CQD FETs. FET parameters are extracted to elucidate the trapping behavior in the CQD solids using different crosslinking agents. The obtained results indicate the importance of the quality of the CQD surface, before and after crosslinking, in order to control the trap density in this new class of semiconductors.
[1] K. Szendrei, W. Gomulya, M. Yarema, W. Heiss, and M. A. Loi, Appl. Phys. Lett. 2010 97 203501
[2] S.Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M.A. Loi, Adv. Mater. 2013, Published online, DOI: 10.1002/adma.201205041
[3] P. Nagpal, V.I. Klimov, Nat. Comm. 2011 2 486
[4] K. Szendrei, M. Speirs, W. Gomulya, D. Jarzab, M. Manca, O.V. Mikhnenko, M. Yarema, B.J. Kooi, W. Heiss, M.A. Loi, Adv. Funct. Mater. 2012 22 1598

With a size and composition tunable band-gap (i.e., absorption onset and emission color) of 1.0 - 2.5 eV, unusually large absorption coefficient, and lack of toxic or low-abundance elements, alloyed CuInSexS2-x quantum dots (QDs) show great promise for a range of applications from photovoltaics and solar photocatalysts to light emitting diodes (LEDs) and down converters. We are utilizing these newly developed QDs as absorbers in record-efficiency QD-sensitized solar cells (QDSSCs). The QDSSC architecture is particularly attractive because it doesn&’t require QD doping and avoids QD-QD charge transport, which are major limitations on the performance of more traditional p-n heterojunction QD solar cells. In these new CuInSexS2-x colloidal QDs, we incorporate controlled amounts of Se to reduce the QD band-gap into the near-IR, which allows for improved broad-band absorption of the solar spectrum. Then, we use cation exchange to create a thin barrier layer at the QD surface that improves stability and reduces recombination, resulting in dramatically enhanced photovoltaic performance. Finally, exchanging the bulky native ligands of as-synthesized QDs with short amines enables a greater loading of QDs into the electron-accepting mesoporous TiO2 film, which results in increased photocurrent. This multi-faceted material optimization relies on detailed characterization of the photoexcited charge carrier dynamics in isolated QDs, as well as in complete devices, using transient absorption and time-resolved photoluminescence spectroscopies. By using these enhanced QDs and optimizing other elements of the solar cell (e.g., electrolyte, cathode), we demonstrate NREL-certified record QDSSC performance of >5%, and record-long stability of several months.
Our rapid development of this new material capitalizes on design principles established by previous generations of QDs (e.g., CdSe, PbS), which are typically hazardous. Non-toxic CuInSexS2-x QDs constitute a potentially disruptive technology in not only sunlight harvesting but also light emission applications (e.g., LEDs) because they can achieve high performance safely at very low-cost.

Chalcopyrite-type CuInS2 and CuInSe2 nanocrystals (NCs) synthesized via solution route have recently attracted much attention because of their utilization for printable solar cells. Colloidal NCs that are synthesized in a hot organic solvent with surfactant molecules possess high quality; however, the compositions of colloidal CuInS2 and CuInSe2 NCs usually deviated from their stoichiometric composition, because there are several compounds in the Cu2S-In2S3 and Cu2Se-In2Se3 pseudo-binary systems. In this presentation, we report synthesis of stoichiometric CuInS2 NCs.
Copper iodide and indium iodide respectively dissolved in oleylamine and sulfur dissolved in octadecene were prepared as starting solutions. 0.5 mL of the each solution was mixed, and then heated at 240 oC for 1 ~ 30 min. After the NCs were extracted from the solution, they were subjected to the powder XRD, high resolution TEM observation, ICP chemical analysis to characterize their phase purity, size, composition and crystal quality.
The XRD indicated that obtained NCs consisted of single phase with a chalcopyrite structure, and their average diameters were 8.1 ~ 9.4 nm. The atomic ratios of Cu:In:S of the NCs were 0.99:1.00:1.75 for the 1 min reaction, 1.02:1.00:1.79 for the 2 min reaction and 1.01:1.00:1.86 for the 30 min reaction. These values indicate that the NCs are stoichiometric within the experimental error. High resolution TEM images clearly showed that the NCs were well crystallized with a very small defect density. This feature is attributable to their stoichiometric composition of the present CuInS2 NCs. We further checked their quality by photoluminescence (PL) spectroscopy. Although the NCs were stoichiometric and highly crystallized, no PL emission appeared for the colloidal solution even at 10 K. Therefore, we fabricated CuInS2/ZnS core/shell NCs to suppress the surface effect of NCs. After coating with thin ZnS, we successfully observed PL emission at a peak wavelength of 784 nm. The Stokes&’ shift was calculated to be 120 meV, because their absorption appeared at 720 nm. For the CuInS2 NCs, a PL emission with a Stokes&’ shift of ~500 meV was usually observed. The Stokes&’ shift for the present NCs was smaller than that for the usual CuInS2 NCs. This means that the defects relevant to the emission formed shallower level than the usual case; this feature also attributable to their stoichiometric composition and well crystallization nature of the present CuInS2 NCs.

Sb₂S₃ is widely considered as an attractive photovoltaic material, based on abundant, non-toxic elements. However, the maximum efficiency reported for solar cells based on this semiconductor does not exceed 6.5%. In order to understand the main loss mechanisms responsible of these quite low performances, we have measured light intensity dependent J-V curves, Transient Microwave Photo-Conductivity, Steady State Photocurrent Grating, Modulated Photocurrent, and Photoconductivity on Sb₂S₃ based devices. All techniques converge toward the same observation: The main recombination route controlling the density of charge carriers in the absorber is of secondary order and appears to stem from an exponentially decaying density of tail states within the conduction band of the material. This conclusion has direct and drastic implications upon the performances of Sb₂S₃ based solar cells.
Another interesting metal sulphide that is currently largely studied consists in a quaternary compound comprising Copper, Zinc, Tin and Sulfur (CZTS). We have developed a new, simple synthesis route to produce a liquid, non-toxic CZTS source that we spray on Molybdenum substrate. The resulting devices have been characterized with many different techniques revealing the strength and weaknesses of our active layers. The loss mechanisms identified and ways forward will be discussed.

Thousands of inorganic materials have band gaps between 1 and 2 eV, a necessary precondition for consideration as a absorber materials for thin-film PV. This band-gap range covers the popular Schokley-Quisser (SQ) criterion for identifying candidate absorbers, but the SQ approach is not a sufficient initial filter. In this talk, I will present our recent work in searching for novel highly absorbing inorganic materials according to the new metric “spectroscopic-limited maximum efficiency” (SLME)[PRL, 108, 068701(2012)]. We focus on main group Cu-based ternary chalcogenides, i.e., CupMqVIr, where M = I, II, III, IV, V (main group) and p,q,r are stoichiometric ratio numbers. Thin-film solar cells hold the promise of reducing the cost of sunlight-to-electricity compared to conventional crystalline silicon. In a solar cell with an ultra-thin film absorber, a strong intrinsic electric field can drive photo-generated carrier extraction, mitigating the constraints on the mobility and lifetime of minority carriers and thus being more defect-tolerant. The realization of such cell is largely predicated on the availability of materials that exhibit both a strong absorption across the solar spectrum and an abrupt onset at the band gap. Based on first-principles quasiparticle theory, we have calculated SLME, band gap, and absorption spectra for all CupMqVIr materials that have been reported in the inorganic crystal structure database. Tens of materials (e.g., CuSbS2 and Cu3SbS4) exhibit higher absorption strength than CuInSe2. By analyzing the fundamental physical factors that control absorption strength in these compound semiconductors, new insights into “absorption design principles” have been developed. Theory- driven experimental work on some of those candidate materials will also be presented. (This work is supported by the DOE Energy Frontier Research Center "Center for Inverse Design.")

The development of earth-abundant photovoltaic materials is critical if photovoltaics are to generate a significant fraction of the world&’s energy in the future. Ternary Cu-Se absorbers have demonstrated both high efficiency and long-term stability; we believe these effects can also be achieved in the ternary Cu-S system. In this work, we determine optimized synthesis parameters for Cu₂SnS₃ through combinatorial sputtering, and explore device integration of this material.
Theory indicates that several compounds within the Cu-Sn-S phase space may be suitable for absorber applications, including Cu₂SnS₃, Cu₄SnS₄ and Cu₄Sn₇S₁₆. For example, the Cu₂SnS₃ phase has a calculated 0.8 eV band gap and an absorption coefficient of up to 10⁵ cm^-1; the other two Cu-Sn-S phases are calculated to have similar properties. However, in contrast to the other two phases, Cu₂SnS₃ has been calculated to have no defects that can lead to Fermi level pinning. Therefore, we have focused our experimental efforts on the Cu₂SnS₃ phase.
There has been limited prior work on the synthesis and characterization of Cu₂SnS₃. The measured optical gaps of Cu₂SnS₃ span a wide range from 0.83-1.6 eV, with absorption coefficients of 10⁴-10⁵ cm^-1. The current record efficiency for a cell with a Cu₂SnS₃ absorber layer is 2%; a 6% cell was recently reported using an alloyed Cu₂Sn_0.83Ge_0.17S₃ absorber.
We have synthesized thin films of Cu₂SnS₃ using combinatorial RF sputtering from Cu₂S and SnS₂ targets with spatial gradients in temperature and composition. When coupled with spatially resolved characterization techniques, this has allowed us to quickly explore the optimal growth conditions for the Cu₂SnS₃ phase. Using this understanding, we have synthesized Cu₂SnS₃ films under both Cu-rich and Sn-rich conditions. The properties of the films grown under Sn-rich conditions were more advantageous for absorber applications, especially when the high sub-gap absorption of the Cu-rich films is considered.
The films grown under Sn-rich conditions were p-type with carrier concentrations of about 1*10¹⁸ cm^-3. The films had an absorption edge at 0.8 eV, with an absorption coefficient up to 10⁵ cm^-1. The work function was measured to be about 5 eV. Conductivity measurements under dark and illuminated conditions showed the films to be photoactive. Temperature dependent Hall effect measurements indicated a 150 meV shallow acceptor activation energy. The films had low values of mobility (1-2 cm²/Vs), which we believe to be due to the small grain size (~100 nm). Current research is focused on increasing the grain size and integrating the optimized Cu₂SnS₃ absorber layers into PV device prototypes; the results of these efforts will also 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.

Today's solar cell technologies based on crystalline silicon wafers and thin film CdTe and CIGS are limited for deployment at the grid parity level (TW production scale). These technologies are restricted by expensive refinement to solar grade or raw materials supply, confining production levels to few GW per year. Hence, new solar absorber materials need to be recognized to solve the growing energy demands with an environmentally conscious, renewable and rapidly scalable technology, using earth-abundant elements found in concentrated deposits. Moreover, materials with rapid onset to high absorption allow the development of thin film PV devices of superior efficiency, with drift field aided extraction of photogenerated carriers.
Utilizing a collaborative experimental and computational approach using optical band gap and absorption as design metrics (SLME), the Cu3-V-VI4 material system was identified as photovoltaic absorbers that satisfy the above criteria. We demonstrated Cu3SbS4 (Eg=0.88 eV) polycrystalline thin films exhibiting exceptionally rapid onset to high absorption (α>10^5 1/cm at Eg+0.6 eV), as well as high hole carrier mobility mu;=15 cm^2/Vs. In this contribution we explore the formation energies of intrinsic defects in Cu3-V-VI4 materials using DFT methods and provide experimental support to the findings.

Cu2ZnSn(SxSe1-x)4 (CZTSSe) solid solution has attracted considerable excitement in photovoltaic (PV) community since it is earth abundant and possesses a reasonably high cell efficiency (> 10%). However, routes to synthesis of these PV materials are limited by very little fundamental knowledge of thermochemistry and reaction pathways. In the present work, phase stability of CZTSSe due to partial disordered Cu and Zn atoms as well as S and Se atoms has been studied in terms of first-principles calculations within the density functional theory. The temperature and pressure dependences of binodal and spinodal decompositions of the pseudo-binary CZTS-CZTSe system have been studied based on the cluster expansion and the partition function approaches with input from first-principles. It is found that the low energy structures due to partial disordered Cu and Zn are the ones with space groups I-4 (#82) and P-42c (#112).

The U.S. Department of Energy (DOE) aggressively supports the development of low-cost, high-efficiency photovoltaic (PV) technologies through the SunShot Initiative, which seeks to make solar electricity cost-competitive with fossil fuel based sources of electricity, without subsidies, by 2020. For solar energy to become competitive with other energy resources, the installed cost for utility-scale photovoltaic (PV) solar systems must reach $1 per watt or, equivalently, 5-6cent; per kilowatt hour (kWh). The SunShot Initiative currently supports research and development of PV materials and devices with earth-abundant constituents to enable terawatt scale deployment. Current projects span materials such as CZTS; pyrites (FeS2); Cu-M-N; (Zn, Mg)Cu- oxysulfides; etc. A brief overview of projects addressing challenges of earth-abundant materials and devices will be presented. Current and forthcoming programs to support materials science and research for development of PV devices and systems will also be described.

We demonstrate that one important factor limiting hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching above 11% is the existence of band-edge tail states due to electrostatic potential fluctuations. Detailed comparison of internal quantum efficiency (IQE), photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) data indicates that the electrostatic potential fluctuations for CZTSSe devices are roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. The formation of compensating charged defects, such as [Cu_Zn + Zn_Cu], and the lower dielectric constant of CZTSSe are suggested as the main factors determining the amplitude of these fluctuations.

CZTS (Cu2ZnSnS4) and related alloys like CZTSSe (Cu2ZnSn(SxSe(1-x))4) have potential as attractive options for low-cost, high-efficiency thin-film solar cells. With optimal band-gaps (1.1 eV to 1.5 eV) and high absorption coefficients (>10^4 cm^-1), these quaternary direct-gap semiconductors can be produced with earth-abundant elements. However, the record laboratory efficiency for a CZTS or CZTSSe cell is about 11 %, significantly trailing that of CIGS (Cu(In,Ga)Se2) and CdTe thin-film solar cells. Further improvement of the cell performance can benefit from a deeper understanding of the photovoltaic properties of the CZTS materials at the level of their microstructures. Here, we characterize the local carrier collection properties of a CZTSSe solar cell prepared from solution precursors. The p-type CZTSSe absorber layer was synthesized employing a colloidal ink comprised of nanoparticle metal sulfides in an organic vehicle. The ink was coated onto a Mo coated glass substrate and annealed in the presence of elemental selenium as described previously. The device construction was completed by deposition of CdS emitter, i-ZnO buffer, ITO transparent conductor and silver grid lines by standard methods. Electron beam-induced current (EBIC) measurements were performed to map the local carrier collection efficiencies with a spatial resolution as high as asymp; 20 nm on the top surface and in cross-section of the devices. The spatial maps of current are compared for low- and high-efficiency CZTSSe devices. The cross-sectional EBIC images reveal that the large-grain top layer (asymp; 700 nm) is electrically active, while the particulate-like bottom CZTSSe layer (asymp; 500 nm; carbon-rich) remains inactive. We will discuss optimal designs of the device and interface engineering at the p-n junctions.

Copper zinc tin sulfide (Cu₂ZnSnS₄, or CZTS), copper zinc tin selenide (Cu₂ZnSnSe₄, or CZTSe), and their alloys are being considered for light absorbing materials for thin film solar cells because they are comprised of abundant, nontoxic and inexpensive elements. We synthesize colloidal dispersions of CZTS nanocrystals (inks) using rapid thermal decomposition of copper, zinc, and tin diethyldithiocarbamates in presence of hot (150-300 °C) oleylamine and then use these inks to drop cast nanocrystal coatings on various substrates which are then annealed to form large grain microcrystalline films. Specifically, the substrates with the nanocrystal coatings are placed in quartz tubes with Sulfur, Selenium, or a mixture of the two, evacuated to 10^-6 Torr, sealed, and then heated for 1-8 hours at temperatures from 500 °C to 800 °C. Isothermal annealing in a closed system provides good control of the annealing temperature and known Sulfur and/or Selenium pressures over the film. Moreover, any decomposition products such as tin sulfide cannot leave the system, minimizing tin loss from the film. After annealing, the microstructure, the elemental composition and the phase composition of the microcrystalline films are examined using a battery of characterization methods including X-ray diffraction (XRD), Raman spectroscopy, various electron microscopies and energy dispersive X-ray spectroscopy (EDS). Within the detection limits of these techniques, both the starting nanocrystals and the annealed films are CZTS or CZTSSe, if annealed under Selenium vapor. We have studied the extent to which Sulfur and Selenium vapor pressures, substrate, carbon content in the nanocrystal coating, annealing time, and annealing temperature affect the mechanisms by which the microcrystalline films form and how their microstructure evolves. Depending on the annealing conditions, the CZTS nanocrystals sinter and grow to sizes ranging from a hundred nanometers to a few microns. In addition to sintering, we observe abnormal grain growth, which can lead to formation of single-crystal CZTS grains up to 10 microns in size. The surface energy difference between the nanocrystals and the large grains is the driving force for abnormal grain growth, which appears to be enhanced at high temperatures but reduced significantly at high Sulfur pressures and on soda lime glass. Annealing under selenium vapor and sulfur vapor yield different microstructures. In particular, annealing in Selenium leads to carbon segregation either as a continuous several hundred-nanometer thick film under several micrometer thick large CZTSSe crystals or as thin fibrous coatings weaving in between smaller CZTSSe grains. In contrast, annealing with S does not show distinct carbon-rich layers or fibrous coatings after annealing.

We report on the low-temperature (4K) photoluminescence (PL) spectra of Cu2ZnSnS4 (CZTS) as a function of excitation intensity. The radiative recombination is characteristic of heavily-compensated material with a high quasi donor-acceptor pair density, as determined by the changes in peak position and peak height with excitation intensity. This is further supported by measurements of carrier lifetimes at different wavelengths. The blue-shift in the defect peak position can be used to estimate the average donor-acceptor pair spacing. Probing a large number of samples with varying compositions indicates a relationship between sample composition and donor-acceptor pair density. Additionally, we will show how the PL spectra of completed devices are correlated with their photovoltaic performance.

Presently commercialized thin-film solar PV technologies based on CIGS and CdTe have attracted considerable attention due to high power conversion efficiencies. Nevertheless, the limitation in scalability due to the use of rare and expensive elements is a barrier to achieving terawatt-scale fabrication. Tin monosulfide (SnS) is a promising earth-abundant alternative because of its suitable bandgap (1.1 eV indirect, 1.3 eV direct) [1,2], strong optical absorption (α > 10⁴ cm^-1) [2], good transport properties (reported Hall mobilities of > 100 cm²/V#9679;s) [3], and ease of film deposition.
We demonstrate congruent evaporation of SnS in a CdTe-like manufacturing process. SnS can be thermally evaporated at rather low temperatures (> 1 Å/s at le; 600 °C). The precursor can be phase-purified by distillation in the growth chamber. These properties allow for potentially low-cost and readily scalable fabrication. We anneal commercially available Sn_xS_y powder to obtain phase-pure SnS feedstock (confirmed by XRD) prior to thermal evaporation. Congruent evaporation of SnS is confirmed by comparing deposition rates to the equilibrium vapor pressures of SnS. We obtain phase-pure SnS thin-films on varied substrates including glass, metals (e.g., Mo and Au), and metal oxides (e.g., ITO). This enables a variety of PV device architectures. We characterize the grain morphology, texture, optical and electronic properties of the as-deposited films by SEM, XRD, spectrophotometry, and Hall effect measurements and compare for the different underlying metal and metal oxide substrates. The thin-film properties are further evaluated as a function of different processing parameters (e.g., deposition rate, substrate temperature and film thickness).
We present a proof-of-concept thin-film solar cell based on thermally evaporated SnS, following the previously developed substrate device stack (glass/metal/SnS/ZnO_xS_y/ZnO/ITO/Ag) [4]. We also demonstrate SnS-based superstrate devices (glass/metal oxide/buffer/SnS/metal), and we compare the two stack architectures. Upon device optimization, our best performing SnS-based devices yield power conversion efficiencies near 2%, similar to the performance of the current champion device [4]. We will present J-V and quantum efficiency measurements, demonstrating considerably high open-circuit voltages up to 300 mV and short-circuit current densities approaching 20 mA/cm². In closing, we present state-of-the-art SnS devices fabricated using a manufacturing process with proven scalability, harnessing the underlying physical property of congruent evaporation intrinsic to only a handful of binary PV candidate compounds.
1. Vidal, J. et al., Appl. Phys. Lett. 100, 032104 (2012).
2. Sinsermsuksakul, P. et al., Adv. Energy Mater. 1, 1116-1125 (2011).
3. Ramakrishna Reddy, K., et al., Sol. Energy Mater. Sol. Cells 90, 3041-3046 (2006).
4. Sinsermsuksakul, P. et al., Appl. Phys. Lett. 102, 053901 (2013).