Symposium ES20 : Thin-Film Chalcogenide Semiconductor Photovoltaics

Thin-film Ag-CIGS (or ACIGS) solar cells and modules produced by MiaSolé Hi-Tech yield high efficiencies rivaling polycrystalline silicon-based modules but are based on a high-throughput, PVD-based deposition process on flexible stainless steel substrates. The process is unique among thin-film solar manufacturers in that a complete device stack is produced without a vacuum break in less than one hour. Using this manufacturing system, a high level of process control and stability is possible via real-time control of the deposition process.

To continue performance improvements that close the gap between manufacturing material and champion small devices, a better understanding of the fundamental device properties is required. This requires development of a device model that accounts for electrical performance characteristics observed under light and voltage stress over time for comparison to real-world solar cell performance.

Our recent effort to generate a fundamental device model began with employing a wide variety of characterization results from our own measurements, those of our research partners, and features of many CIGS materials that have previously been excluded in prior device models. Additionally, device parameters must be based on actual device measurements and constituents, as heterojunction ACIGS solar cell devices are complex and errors compound quickly. The overall objective was to keep the model as simple as possible and look for generally good agreement among a variety of simulated measurements.

Most of the effort has been devoted to understanding the defect structure of the ACIGS absorber layer. Conventional capacitance-voltage profiling is supplanted in our measurements by high-speed C-V profiling (HSCV) that avoids many complications of transient charge effects in highly defective materials such as ACIGS. An important consequence of these measurements is the discovery that MiaSolé absorber layers (as well as other CIGS devices) can be defined by an acceptor-rich layer (ARL) adjacent to the CdS buffer layer. This ARL has been shown to have profound consequences for device electrical performance, as it can degrade fill factor and reduce efficiency.

The physical basis for the ARL is a matter of some debate. A variation in static shallow acceptor concentration is one possible source, but it predicts neither device metastability nor measured high concentrations of DLTS-visible defects. Charge accumulation at high concentrations of deep acceptors would be possible, but accounts for neither metastability nor carrier capture kinetics observed in DLTS. Instead we hypothesize that V(Se)-V(Cu) divacancy defects are responsible for charge accumulation in the ARL, and their simulated response to light- and voltage-stressing is consistent with our measurements. This hypothesis is qualitatively consistent with positron annihilation spectroscopy (PAS) measurements of our absorber. Predicted shallow acceptor and donor levels of the divacancy also provide an explanation for the long-observed high dopant compensation in CIGS, and at the concentrations predicted by our model, it is possible that the divacancies alone can be used to explain most of the charge and doping profile of the absorber.

Emphasis of the divacancy defect in our device model can guide MiaSolé Hi-Tech to new directions for device improvement that leverage our ability to directly manipulate absorber stoichiometry in our unique thin-film solar manufacturing system.

In this work, we present a suite of characterization tools developed to accurately and systematically analyze losses in non-Si photovoltaic solar cell technologies, in particular thin-film chalcogenide devices. This characterization platform will enable easy access to the implied open-circuit voltage (iV_oc) and fill factor (iFF) of devices, to their pseudo fill factor (pFF) and thus series resistance (R_s) at maximum power point, and to their complete-stack contact resistance (R_c). Together with calculated detailed-balance performance limits and traditional J-V measurements, these metrics will enable a precise accounting of recombination and resistive losses, from fundamental efficiency limits to experimental device performance. The characterization platform includes systematic determination of the absorber optical parameters (n & k), optical modeling of the device, calculation of the iV_oc from measurement of the external radiative efficiency (ERE) of the absorber, J_sc-V_oc measurements, and measurement on the contact stack resistance using through-the-absorber transfer line method (TLM).
Cd(Se)Te solar cells, our test system, provide a prime example of the potential impact of the techniques we propose to develop and implement: although they exhibit strong photoluminescence response, record poly-Cd(Se)Te cells have bandgap-voltage deficits (W_oc) of approximately 550 mV, as compared with below 400 mV for all other mature PV technologies. Similarly, these record Cd(Se)Te devices have FFs below 80%, when other mature cells are near or above 85%. Frustratingly, a systematic identification of the origin of these sub-par performances – for example recombination or resistive losses – has been lacking, thus slowing down the development of these technologies. Similarly, it is often asserted that Cd(Se)Te cells need a better back (hole) contact. Although we believe this is true, it is currently unknown how high the V_oc and FF could be for a given cell if it had a perfect back contact.
Using the proposed set of characterization techniques, users will be able to identify the sources of inefficiencies in thin-film chalcogenide solar cells, from bulk material quality and surface passivation to contact selectivity and resistivity. Rapid absorber and contact improvement will thus be possible.

Several characterization techniques are usually performed on a localized region of any material because of convenience in a routine basis. However, lateral non-uniformities in chemical composition, structural defects and even the presence of voids can be found in a polycrystalline solar cell, i.e. CI(G)S, influencing the local carrier properties of the material. This implies that some of the material properties extracted from local characterization methods may not be representative of the full device performance complicating the correlation between measurements. For example, time-resolved photoluminescence (TRPL) is typically measured locally, and the effective lifetime values determined (including the influence of front/back recombination, etc.) are closely related to the open circuit voltage which is a macroscopic parameter that might be affected by any carrier lifetime non-uniformity. Therefore, information about the influence of spatial inhomogeneities on the carrier transport is important to assess whether it is enough to use local parameters to correlate different characterization results —and to predict the performance of the solar cell—, or to what extent it is required to consider non-uniform parameters that could be more representative of the whole device.

In this contribution, we use transient PL measurements spatially resolved in the micrometer-scale range to access the carrier properties of different absorbers. The TRPL setup is coupled into a microscope enabling to operate in confocal or widefield illumination mode. Widefield mode is used to map the samples in a broader range while confocal mode attempts to resolve the influence of the grains inside the structure with the highest optical resolution of the system (< 1 µm). Besides all the mechanisms involved in the determination of the effective carrier lifetime such as surface recombination, charge separation, etc., the use of confocal mode requires the consideration of lateral diffusion of carriers as well as the proper evaluation of the sample performance due to possible high injection effects. Accordingly, the interpretation of the measurements in both modes is aided by 2D simulations, which are also compared to the macroscopic I-V parameters of the devices. Specifically, 2D device simulations allow us to quantify the lateral diffusion of carriers as well as to include other key parameters involved in TRPL measurements, i.e. surface recombination or carrier mobility. All simulations use as input measured absorption coefficient as a function of the bandgap grading for improved modelling. Finally, results from other spatially resolved techniques will be shown to correlate device performance with TRPL mapping and support the main findings.

Numerical simulation of solar cell device physics for predicting the performance, design and optimization of solar cells is a well explored active area of research. There are many tools and software packages like AFORS-HET, SCAPS-1D, AMPS-1D, PC1D, Silvaco TCAD, Sentaurus TCAD, etc., which are available freely or commercially, to perform numerical simulation of solar cells. The fundamental physics equations solved are continuity equations for charge carriers, namely electrons and holes, and the Poisson equation for electrostatic potential. In the continuity equation for electrons and holes the generation and recombination term is modeled through radiative processes, the SRH process, the Auger process etc. For the SRH generation recombination process an effective lifetime is assumed in the modeling. Combining this model with the boundary conditions, light and temperature simulation conditions, one can calculate I-V curves, efficiencies, fill factors etc., for the solar cell.
As the solar cell ages performance is affected. This can be microscopically explained through the transport of different defects present in the solar cell. The transport of defects can also cause metastability in solar cells. In our previous work we explained the metastable behavior of CdTe solar cells by studying the Cu related defect transport along with the carrier transport.
In this work, we present a novel Unified Solver for studying carrier and defect transport on an equal footing. The generation recombination term in the continuity equation for defects corresponds to the formation and transformation of defects. This formation and transformation of defects along with generation and recombination process for charge carriers is represented as a defect chemical reaction. Hence, we call our model as reaction-drift-diffusion modeling of solar cell. The drift-diffusion equations for defects require the diffusion constants and activation energies of the defect to be known and the defect chemical reaction require reaction rate constants to be known. These parameters are calculated using Density Functional Theory (DFT).
Since the main goal of our research work is to study short time metastability and long-time reliability concerns of cadmium telluride (CdTe) photovoltaics, special attention has been placed in the design of the solver to be able to produce results ranging from ns to hours/days/years. The solver gives us possibilities to explicitly account for all transient effects with free carriers (simulation of time resolved photoluminescence) and defects (simulation of performance instabilities, IV hysteresis etc). Various generation recombination processes can be represented as additional defect chemical reactions. Moreover, the Unified Solver supports accurate treatment of interfaces and grain boundaries that are crucial for the explanation of the operation of CdTe and other chalcogenide PV technologies.
The Unified Solver is benchmarked against Silvaco simulations of a homojunction and heterojunction solar cell. Excellent agreement is observed between the Unified Solver and Silvaco results for the key solar-cell parameters (short-circuit current and open-circuit voltage). Next, the Unified Solver is employed in constant temperature 2D simulation of chlorine diffusion annealing in a cadmium telluride (CdTe) system under insulating boundary conditions (isolated system). Chlorine is introduced in the system as a neutral interstitial at a half corner of the (1um×1.2um) structure. The concentration of chlorine interstitial is 1e16 cm^-3 and the system is kept at a temperature of 750K. The sample is annealed for 240s using the test case of chlorine defect reactions. The time evolution of chlorine substitutional defect (Cl_A⁺) is presented. Emulation of process temperature profiles is also presented in the talk.

For solar technology to compete with traditional energy sources, a continued decrease in photovoltaic (PV) energy generation cost is needed. Next-generation PV technology can achieve this by utilizing earth-abundant materials and low-cost processing techniques. Chalcogenide compounds are of particular interest in this area due to their prolific use in energy applications and amenability to high-throughput, low-cost processing techniques. Chalcogenides semiconductors have shown particular success in PV applications with materials such as Cu(In,Ga)(S,Se)₂ (CIGS), Cu₂ZnSn(S,Se)₄ (CZTS), and CdTe. However, developing new materials and processing techniques require extensive research efforts to achieve the required high-performance goals. Complex device processing and non-ideal optoelectronic properties associated with early stage materials often prolong material development.

In this work, our approach to developing next-generation semiconductors for PV is shown. We utilize low-cost, scalable solution-based processing techniques which are amenable to high-throughput optimization and a variety of chalcogenide precursors. Furthermore, we utilize advanced optoelectronic and structural characterization to guide the material development process; this provides rapid feedback for accelerated material development through accurate screening of early-stage materials for relevant optoelectronic properties and optimal synthesis parameters. In particular, we focus on the extraction of optoelectronic properties relevant for device performance – without the need for device fabrication – through optical techniques. Previous work using these techniques for developing CZTS, CIGS, and perovskites will be shown, with application to new materials currently in development. Ultimately, the successful development of new semiconductor materials requires a cross-disciplinary approach linking fundamental material properties and processing to the device-relevant optoelectronic properties.

Sb₂(S,Se)₃, is becoming a relevant thin film chalcogenide semiconductor with different technological applications such as: superconductivity, electronic components, electrode for sodium-ion batteries, photodetectors and as emerging photovoltaic absorber. In particular, and for this last application, the material has shown remarkable improvements in the last few years, demonstrating solar cells in superstrate configuration with power conversion efficiencies reaching 7.6%. In fact, and similarly to CdTe, most of the devices reported in the literature so far have been prepared using this configuration. This has opened interesting perspectives for their use in solar energy conversion applications, also taking into account the 1D crystalline organization of the material, with in principle benign grain boundaries and anisotropic conduction properties. Additionally Sb₂(S,Se)₃ has shown a high flexibility degree in terms of substrate type, due to the relatively low synthesis temperatures required for optimal high quality polycrystalline growth (300-400 C), allowing deposition onto polymeric, steel, ceramic and TCO/glass substrates. This versatility makes this compound very promising for ubiquitous applications such as building integrated photovoltaics (BIPV) (flexible, bifacial, and semi-transparent), wearables, or autonomous IOT applications among others.
In this work we present a systematic optimization study of the synthesis of Sb₂Se₃ thin films using substrate configuration solar cells, by a sequential process based on reactive annealing under Se atmosphere of thermally evaporated Sb layer precursors. The study is centered in the analysis of Sb precursor thickness and reactive thermal annealing conditions (annealing temperature, time, and pressure) on the compositional, structural and morphological properties of the layers. We observe the formation of continuous layers with large and homogeneous crystals, reporting for first time a weak photoluminescence signal close to 1.3eV in agreement with the band gap value obtained by IQE, and a systematic vibrational characterization under resonant and non- resonant Raman conditions that allows report 15 peaks of the 30 expected.
After a first optimization on Mo coated soda lime glass substrates, we report a promising power conversion efficiency of 5.3% in substrate configuration with a V_OC of 403 mV (the highest value reported for this configuration to the best of our knowledge), close to the 7.6% certified world record in superstrate one.
Additionally the study is complemented with a wide characterization of the fundamental properties of Sb₂Se₃ layers and devices using morphological and physic-chemical characterization (Photoluminescence, SEM, XRF, XRD), and with a complete analysis of the impact of the absorber stoichiometry under different regimes (Se-rich, Se-poor, Sb-rich and Sb-poor conditions). All this will be correlated with the optoelectronic characterization (JV, IQE, CV) of the solar cells. Finally, the main challenges to develop Sb₂Se₃ type solar cells in substrate configuration will be reviewed in the frame of the obtained results.

The non-cubic antimony chalcogenides, i.e., Sb₂Se₃, formed by quasi-one-dimensional ribbons can enhance light absorption and carrier transport by tuning the ribbon direction using the close-space sublimation (CSS) deposition. The power conversion efficiency (PCE) ~7% is found to be associated with the ribbon direction, which was investigated with theoretical calculation and experimental measurement in the Sb₂Se₃ films and devices. The substrate temperature and film thickness are critical for the fine-tuning of ribbon orientations during the CSS deposition. Our results show that [211]-preferred orientation leads to the minimum series resistance and highest light absorbance in the device. The device reliability measurement and in-depth elemental profiling analysis suggest that the interdiffusion between window layer and absorber layer dominate the degradation mechanism. This observation demonstrates that Sb₂Se₃-like quasi-one-dimensional materials with van der Waals boundaries can achieve scalable production at low cost and hold great potential for next-generation solar cell.

Tandem cells are considered as the next generation solar cell to overpass the intrinsic efficiency limitations of single junction solar cells, fixed by the Schockley Queisser limit below 33% and more probably below 30%. Silicon solar cells are already approaching the theoretical limit of about 29%, and represent 95 % of the present PV Market. Tandem solar cells on silicon would make possible to search for 43% theoretical limit, making practical module efficiencies of more than 30%. Combinations with hybrid perovskite and III-V top cells are presently focusing a lot of attention, but an alternative is to develop top cells based on chalcogenide materials, which technologies are well proved on the market (CdTe and CIGS). In the presentation we will report on researches carried out on silicon/CIGS solar cells. Based on the experience on single junction CIGS low band solar cells (1.15 eV) at about 20% efficiency, wide gap CIGS solar cells (1,6-1,8 aV) are grown on single cristal silicon substrates by coevaporation, with lattice parameters matching with silicon, thanks to compositional adjustments (Ga to In, S to Se). In order to give more flexibility in interface electrical properties engineering, the silicon substrate is functionnalized by a thin III-V buffer layer, based on GaInAlAsP lattice matched alloys¹. Epitaxial growth of CIGS has been successfully demonstrated and first devices have been elaborated to address the formation of high efficiency top cells alone, using Si/III-V as a selective back contact. Further structural, chemical and luminescence characterization will be reported.
Ref :
¹ D. Lincot et al., Proceedings of EUPVSEC 2018

A large gap still remains between the achieved conversion efficiencies and the Shockley-Queisser limit for Cu(In,Ga)Se₂ (CIGS) based solar cells. Some of the current limitations such as parasitic absorption losses are known and solutions are being developed. For some other limitations the origin is not well understood yet, but is required for knowledge based improvements. Thorough materials and device characterization of highly efficient solar cells can help to understand the origins of the remaining losses.
We used advanced materials and device characterization including high resolution transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and time resolved photoluminescence (TRPL) combined with multidimensional device modelling to confine the origins of the remaining losses. We will summarize our recent findings on optical, compositional, structural and electronic properties in multi-stage co-evaporated CIGS layers and at interfaces to adjacent layers. In particular, we will describe how surface recombination velocities, minority carrier lifetime and mobility can be extracted from TRPL measurements and what we can learn from these findings. Further, we will present the presence of undesired compositional non-uniformities & voids and discuss their formation mechanism and role on device performance. Combined with recent findings obtained on narrow band gap CIS absorbers our results indicate possible inherent limitations originating from current fabrication methods and device architecture, which leads to new strategies how the actual CIGS solar cell efficiency can be pushed closer to the Shockley-Queisser value.

Recently, 22.8% efficient Cu(In,Ga)Se₂ (CIGS) thin film solar cell has been reported, demonstrating its great potential as a competitor to silicon solar cells. Adopting solution method to prepare highly efficient CIGS light absorbing materials is crucial for reducing the cost of CIGS solar cell fabrication and achieving large-scale production. We have previously reported 10.3% copper indium selenide (CIS) solar cell with the absorber fabricated from single DMF molecular precursor solution, demonstrating the great potential of DMF as relatively environmental benign solvent for solution processed thin film solar cells. The metallic element ratio (Cu/In or Cu/(In+Ga)) is the key factor to affect the quality of absorber materials. Here, we have systematically investigated the effect of the Cu/In ratio (0.85 to 1.2, from Cu-poor to Cu-rich) in the precursor solution on the CIS device performance. We found that solar cell efficiency increases with the Cu/In ratio from 0.85 to 1.05 and then decreases from 1.05 to 1.2. The best device was achieved from a ratio of 1.05 with a peak power conversion efficiency (PCE) of 12.20%, a short circuit current density (J_sc) of 36.12 mA/cm², an open circuit voltage (V_oc) of 0.499 V, and a fill factor (FF) of 67.57%. Further, by gallium alloying, CIGS solar cell with a PCE of 13.6% has been fabricated with a V_oc of 0.600 V, J_sc of 34.78 mA/cm² and FF of 65.17% under similar Cu-rich (Cu/(In+Ga)=1.05, Ga/(In+Ga)=0.2) condition. Our results for the first time demonstrate highly efficient CIS/CIGS solar cells can be achieved from absorbers grown under Cu-rich conditions. Characterization of the absorber materials and solar cell devices are undergoing to understand the mechanism behind the phenomenon observed.

Cadmium telluride (CdTe) thin film solar cells are renowned photovoltaic materials for its high absorption coefficient, suitable band gap and low manufacturing cost. The fabrication of highly efficient devices includes reduced intrinsic defects and improved interfaces at front and back contacts. The back-contact processing includes the formation or deposition of tellurium (Te) layer to make an ohmic contact with a metal. Here, we report wet-chemical treatments of CdTe using various iodine compounds and sodium tetrafluoroborate (NaBF₄). The iodine compounds tested are elemental iodine (I₂), ammonium iodide (NH₄I), mixture of iodine and ammonium iodide (I^-/I₃^-) and formamidine iodide (FAI). The treated surfaces were studied using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The treatment with iodine compounds produced Te rich layer on the back surface based on Raman, XRD patterns and changed surface morphology. The fabrication of the devices after these treatments, except I₂-propanol, improved the open-circuit voltage (V_OC) and fill factor (FF) of the devices. The I₂-propanol etching produced tellurium iodide (TeI) in β-phase on the surface which reduced device performance. The photoconversion efficiency of CdTe devices after treatment is up to 14.0% (V_OC 841 mV and FF 78%) while untreated devices have an efficiency of about 12.7% (V_OC 814 mV and FF 73%). In case of BF₄^- treated samples, it did not produce Te rich layer, but it seems like BF₄^- ions are passivating CdTe surface with enhanced V_OC and FF compared to standard devices.

CdTe solar technology is producing electricity at costs less than conventional fuels in many regions. Yet despite its commercial maturity, there is still headroom to increase performance significantly by addressing fundamental material challenges including compensation, hole density, carrier lifetime, and interfaces. This presentation will describe ongoing work to understand and overcome these challenges.

Last advances on kesterite (Cu₂ZnSn(S,Se)₄) corroborate the importance of doping and alloying strategies, not only for tuning the properties of this family of materials, but also for achieving higher efficiencies and broadening their possible range of application. Alkaline (Li, Na, K) and Ge doping; as well as Ag, Cd and Ge alloying are demonstrating a high potential by enhancing different aspects of these materials. In particular, the partial or total substitution of Sn by Ge in kesterites is an emerging approach aiming to improve their efficiency and broaden their band gap up to 2.2 eV with direct application in advanced solar cell tandem concepts, or semi-transparent photovoltaics for building integrated photovoltaics (BIPV) among others. Many studies have proved that the use of Ge as doping or alloying element influences drastically the overall parameters of Sn-based kesterite devices improving carrier lifetime, boosting the V_OC, and increasing the conversion efficiency in solar cell devices. The highest efficiency reported until now for pure Cu₂ZnGeSe₄ (CZGSe) compound is a promising 7.6% achieved with an annealing under H₂Se environment and employing very similar process parameters to those typically used for the pure Sn alloy. This work aims to study and optimize the main parameters affecting the synthesis of CZGSe comparing their main characteristics with the pure Sn compound.
For this purpose, Cu₂ZnGeSe₄ thin films were synthesized onto Glass/Mo substrates by a sequential process, based on the sputtering of Cu, Zn and Ge metallic layers, followed by a reactive annealing under a Se+Ge atmosphere inside a graphite box. Parameters like metallic stack order; annealing temperature (450-550 C), pressure (1-1000 mbar), routine (one or two-step annealing) and time; as well as the composition and of the use of chemical etchings were investigated. Additionally, a combinatorial sample with compositional gradients was prepared covering the full Cu/Zn, Cu/Ge and Zn/Ge range of interest, in order to correlate composition with the formation of secondary phases and defects. All the samples were fully characterized using a complete set of techniques (XRF, Raman spectroscopy, XRD, SEM and EDX) and by preparing and measuring solar cell devices.
After trying several stack orders, Cu/Zn/Ge is selected as the most promising one, leading to the formation of less secondary phases. The optimization of the annealing parameters confirms that a two stage process is the best suited for high quality absorbers. This includes a stage at relatively low temperature (330 C) for the synthesis of the CZGSe compound, and a second step at relatively high temperature (480 C) for its crystallization, revealing that lower annealing temperatures than in the case of the pure Sn compound are required. The compositionally graded sample shows that best efficiencies are obtained for absorbers with Cu/Ge ratio around 0.67 and Zn/Ge around 1.10, suggesting that Cu and Zn-poorer conditions than for pure Sn kesterite are required. Using this compositionally graded sample, we will present a deep analysis of the correlation of the different optoelectronic parameters with composition, including the analysis of secondary phase and possible point defects formation. With a first optimization which includes an etching of the absorbers in KCN, a champion cell with an efficiency of 6.5% (V_OC = 556 mV, FF = 59.6%, J_SC = 19.6 mA/cm²) is reported. Additional experiments introducing different sources of Ge during the annealing process (GeSe₂, GeSe, and pure Germanium) in order to change the partial pressure inside the graphite box will also be presented.

Recently, different works related to the improvement of SnS-based thin film for solar cells applications have been reported. These solar cells (SCs) are built with complex configurations and fabricated using expensive techniques to obtain high short circuit current densities, J_sc, above of 20 mA/cm², and power conversion efficiencies of 4.36%. However, the SCs often have low values of open circuit voltage, V_oc, below of 400 mV, which limit their application to industrial scale. In order to incresase the V_oc, some researchers have developed either transition metals-doped SnS films or the synthesis of novel semiconductor ternary materials based in Sn and S such as Sn-Sb-S (TAS): SnSb₂S₄, SnSb₄S₇, and Sn₂Sb₂S₅. Conventionally, ternary compounds are synthetized by deposition of individual intercalated layers of SnS and Sb₂S₃, followed by annealing at high temperatures.
In this work, SnS and Sb₂S₃ thin films has been obtained by in-situ chemical bath deposition. The effect of the sulfur load, time deposition and pH bath solution has been studied. Antimony and tin chloride were used as metal ion sources, thioacetamide as sulfur ion source, and tartaric as the complexing agent. Chemical bath deposition was carried out at 80°C varying the concentration of sulfur and time of deposition while the pH of the solution was kept between 8 and 9. Then, the resulting samples were heated in nitrogen at temperatures from 300 to 500 °C to make them crystalline and form the ternary compounds. The results showed that the film thickness depends of the sulfur load, deposition time, pH and the temperature during the heating. The thickness of the as-deposited film without excess of sulfur was of 117 nm for a time deposition of 2 h. An increase on the sulfur load produced thicker films with thickness above of 250 nm. The SEM analysis of the as-deposited films showed a morphology of elongated grains of 100 nm, which increase their size after heating. The optical band gap of the as-deposited films was of 1.6 eV, which decreased to 1.46 and 1.24 eV after heating the films at 300 and 500°C, respectively. All the TAS films exhibited optical absorption coefficients higher than 10⁴ cm^-1. Moreover, the photo-conductivity of the as-deposited films was of 10^-9 Ω^-1 cm^-1 while those heated at 300 and 500 °C showed photo-conductivities of 10^-8 to 10^-6 Ω^-1 cm^-1. Finally, the evaluation of these ternary absorbers into the FTO/CdS/TAS/C structure gave a V_oc of 462 mV and a J_sc of 2.37 mA/cm² under white-LED light. These preliminary results demonstrated that TAS absorbers could be good candidates to obtain higher V_oc values than those obtained with the SnS-based SCs.

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) semiconductors, composed of non-toxic and earth-abundant elements, have great potential as low cost and mass production photovoltaic absorber materials. However, the best CZTSSe solar cell, obtained from hydrazine based ink by IBM in 2014, only has an efficiency of 12.6%[1], much lower than similar structured Cu(In,Ga)Se₂ (CIGS) solar cell (22.6%)[2]. The development of kesterite Cu₂ZnSn(S,Se)₄ thin-film solar cells is currently hindered by the large open-circuit voltage deficit (V_oc-def). The possible reasons for the large V_oc-def include large tail states (electrostatic potential fluctuation and bandgap fluctuation), Cu-Zn antisite order-disorder, formation of deep defects and interface recombination, which is dominate still under debating. Here, we have explored different strategies to understand the V_oc deficit in DMSO solution processed CZTSSe solar cells. First, we found that the V_oc-def as well as efficiency of DMSO solution processed CZTSSe solar cells is strongly affected by the precursor tin oxidation state (Sn²⁺/Sn⁴⁺) in the solution. The power conversion efficiency achieved from Sn⁴⁺ solution was 13.2% with a V_oc-def of only 0.570 V, much lower than the current world record CZTSSe device (0.617 V), whereas the efficiency obtained from Sn²⁺ solution was only 7% with V_oc-def higher than 0.600 V. Investigation by solution chemistry and film morphology characterization reveals that the huge difference comes from different complexation of the Sn with ligand (thiourea) and solvent (DMSO) which lead to different reaction pathways from solution to solid state film and thus dramatic absorber quality and device performance. Second, we have used Ag or Ge alloying to further investigate the their effects on the V_oc-def of Sn⁴⁺ solution processed CZTSSe solar cells. Characterization of CZTSSe absorber materials including XRD, Raman, SEM, EDX, TEM, and PL and device performance show positive effect of the alloying on V_oc-def issue. Our results shed new lights on how to suppress CZTSSe solar cell V_oc-def and improve their efficiency to higher level.

Crystalline antimony-based nitrides have long posed fabrication challenges due to the high nitrogen chemical potential required for formation and the tendency of antimony to react with oxygen and moisture to form amorphous oxynitrides. To date, Zn₂SbN₃ is the only reported crystalline antimony nitride in which Sb functions as a cation with a positive oxidation state. This material warrants further investigation not only due to its unique nature, but also due to its advantageous optoelectronic properties, which may be tunable like those of other ternary nitrides. Theoretical calculations predict this material to have a direct band gap (1.7eV), low effective mass for electrons (0.15-0.19m_e) and moderate effective masses for holes (2.4 m_e), which suggests that Zn₂SbN₃ has promise as a photoactive absorber. [1] Experimental measurements show room-temperature photoluminescence activity and band edge alignment appropriate for the hydrogen evolution reaction.

In this work we report on the detailed investigation of the optoelectronic properties of thin-film Zn₂SbN₃ and their correlation to the films’ composition. Films were grown through combinatorial co-sputtering and characterized by spatially-resolved measurements. Experimental results show that the wurtzite-derived crystal structure is stable over a large range of cation composition (Zn/(Zn+Sb) = 60-80%) as determined by X-Ray Fluorescence. 4-point-probe measurements reveal 200-400 kOhm resistance, tunable with composition. UV-visible spectroscopy shows an absorption onset around 1.7 eV. This will be further investigated through photoluminescence and Hall effect measurements. Growth parameter space will be mapped by varying factors such as growth rate, substrate temperature, and the use of a N₂-cracker to enhance the nitrogen chemical potential. A fuller understanding of the relationship between fabrication conditions and film properties will help evaluate the attainability of this new ternary nitride’s predicted utility as a photovoltaic material.

[1] Arca et al., J. Am. Chem. Soc., 2018, 140, 4293

Currently CZTS sulfide absorbers for solar cells with the highest efficiency have all been produced by vacuum techniques which thus seem to be superior to solution processing for CZTS cells. Sputtering and pulsed laser deposition are typical vacuum techniques, which are employed for solar cell absorbers of complicated stoichiometry. Both techniques are well-known for stoichiometric transfer of chemical compounds such as metal oxides and nitrides from a target to a substrate. The two techniques are non-thermal since the emitted atoms from the target are quite energetic and may have an energy distribution with a tail up to more than 10 eV. This non-thermal arrival energy is also known to be beneficial for growth of layers with high crystallinity.
In the photovoltaic literature there seems be a limited number of examples of a comparison between the absorbers and cells produced by either sputtering deposition (SD) or pulsed laser deposition (PLD) except for an inconclusive study without production of solar cells by Sun et al. [1].
We have recently produced solar cells by co-sputtering deposition and PLD with a record efficiency of 6.6 % for SD, while that by PLD reached 5.2 % [2]. While the efficiency was comparable, there were several processing differences, in particular in the thermal annealing steps. It is not clear how different the ideal thermal profiles for the two techniques are under similar deposition conditions, i.e., same type of Mo-coated substrate and precursor films of the same thickness and composition.
Furthermore, for single-target SD the composition of the films can be very difficult to tune because of preferential sputtering from the growing film induced by the arrival of fast atoms or ions from the target. In contrast, for standard PLD (with a single target) the composition can be varied by adjusting the laser fluence, since the transfer of copper atoms from target to film is increasing with fluence (and with 0.8 J/cm2 as the optimum fluence) [2,3].
We will discuss a few other PLD experiments: The CZTS-films can be doped with additional elements, e.g. by Ag using doped targets without any additional complications in the PLD process. Also the production of oxide precursor films is comparatively easy with an oxide target for PLD, while it is not yet clear how successful a detour via metal oxides to sulfides would be for co-SD.
Detailed results for the comparison between films made by SD and PLD as well as examples PLD-films of other compositions will be shown in this contribution.
[1] L. Sun et al., Journal of Crystal Growth 361 (2012) 147–151
[2] A. Cazzaniga, A. Crovetto et al., Solar Energy Materials & Solar Cells 166 (2017) 91–99
[3] J. Schou et al., Applied Physics A) 124 (2018) 78

Low temperature-processed Cu(In,Ga)Se₂ (CIGS) thin film absorber is important for demonstrating a functional CIGS solar cell on plastic substrates. However, control of the band profile mainly dominated by the Ga concentration profile in the CIGS film has been challenging due to slow atomic diffusion at low temperatures. Here, a systematic engineering of Ga profile in CIGS films is reported by employing a thin Ag precursor layer prior to CIGS co-evaporation. By increasing the Ag precursor layer thickness, Ga gradient was mitigated along with improved CIGS grain size, which enhanced the overall solar cell performance. Formation of liquid-phase Ag-Se could provide mobile channels for Ga diffusion along the grain boundaries and expedite CIGS recrystallization process at such a low temperature.

Various chalcogenides (Cu(In, Ga)(S, Se)₂, CdTe, Cu₂ZnSn(S, Se)₄) have been adopted as successful absorber materials for application in thin-film photovoltaic (PV) and photoelectrochemical (PEC) devices. However, despite being commercialized (e.g. Cu(In,Ga)(S,Se)₂, CdTe) or showing performance advances (e.g., Cu₂ZnSn(S,Se)₄), several concerns and material limitations remain regarding scarcity/toxicity (e.g., In and Ga/ Cd) and Cu-Zn anti-site disordering in earth-abundant Cu₂ZnSn(S,Se)₄(CZTSSe) chalcogenides.Cu₂BaSn(S, Se)₄(CBTSSe) has recently gained substantial attention as an alternative semiconductor material due to desirable properties for solar energy conversion applications, and reduced tendency for antisite disorder relative to Cu₂ZnSn(S, Se)₄. In this study, as an alternative to more expensive vacuum-based film-deposition processes, we report a low-toxicity solution-based process for the fabrication of high-quality CBTSSe absorber layers with µm-scale film thickness and grain size. The facile process involves spin-coating an environmentally benign solution of highly soluble, inexpensive and commercially available precursors, Ba(NO₃)₂, Cu(CO₂CH₃)₂and SnI₂ in DMSO, followed by sequential sulfurization/selenization annealing. A high-temperature pre-baking step under sulfur vapor is needed for each film layer to avoid forming the impurity phase, Ba(SO₄) when starting from the soluble Ba(NO₃) reagent. Our reproducible approach forms a dense, 1-µm-thick, single phase CBTSSe absorber layer with large grains (0.9-4.5 µm) and a tunable band gap (e.g., 1.68 eV under the typical processing conditions employed). Additionally, we demonstrate the first prototype solution-deposited CBTSSe PEC device, exhibiting a photocurrent of ~10mA/cm²at 0 V_RHE((increasing to ∼12 mA/cm²during the stability test), comparable with analogous devices based on vacuum-processed CBTSSe films, as well as stable hydrogen evolution for more than 10 hours. These results demonstrate the prospects for low-cost solution-processing of high-quality CBTSSe film and absorbers for thin film PV and PEC cells.

Cd chalcogenide films are direct band gap semiconductors that can harvest photon energy in a wide energy range from 2.42 eV (CdS) to 1.74 eV (CdSe) by compositional tuning or chemical gradation. Growth of such films using chemical bath deposition is advantageous due to low production costs and the ability to easily control bath composition and resulting film microstructure. We have recently demonstrated monocrystalline CdS and CdSe deposited on GaAs substrates by chemical bath deposited.^{1, 2} In this work we present the chemical, structural and photovoltaic properties of Cd(S,Se) solid solution thin films.³ We present evidence for the formation of chemically graded films which can be expected since thiourea (S precursor) decomposes faster than sodium selenosulfate (Se precursor) and as a result, S anions are first to react with the substrate surface. The physical and chemical properties of the films were characterized using x-ray powder diffraction, scanning electron microscopy, analytical transmission electron microscopy and energy dispersive spectroscopy mapping. Finally, current-voltage measurements of a GaIn-eutectic/GaAs/Cd(S,Se)/In device were conducted at room temperature under 1 sun. We demonstrate increasing photo-response with S/Se ratio in CdS_xSe_1-x based photovoltaic cells. This work presents a comprehensive and applicable study on chemical bath deposited Cd chalcogenide thin films.

1. O. Friedman, A. Upcher, T. Templeman, V. Ezersky and Y. Golan, Journal of Materials Chemistry C, 2017, 5, 1660-1667.
2. O. Friedman, D. Korn, V. Ezersky and Y. Golan, CrystEngComm, 2017, 19, 5381-5389.
3. O. Friedman, O. Moschovitz and Y. Golan, CrystEngComm, 2018, 20, 5735-5743.

Optically transparent p-type conductors enable development of transparent electronics, and opportunities to develop high-efficiency bifacial photovoltaic devices. Sulfide materials offer an interesting alternative to oxides for the photovoltaic applications due to better hole transport properties. We report on the structural, optical, and electronic properties of earth-abundant p-type transparent conducting barium copper sulfide (BCS) thin films fabricated using solution-processing. The BCS thin films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis–NIR spectroscopy, and spectroscopic ellipsometry (SE). A BCS film of thickness 100 nm can transmit > 70% of the visible light with band gap ∼ 2.4 eV. Based on SEM images, initial BCS films look compact with a grain size of ∼15 nm. Our studies show that these films are conductive with a carrier concentration of ∼10¹⁹ cm^-3, making them promising hole transport materials for photovoltaic applications. We will discuss cadmium telluride (CdTe) devices fabricated using BCS layer as an interfacial layer between CdTe and a standard Cu/Au. Additionally, we will extend our study to fabricate bifacial CdTe devices employing the transparent BCS buffer layer and a conducting oxide as the back contact.

Polycrystalline II-VI and silicon are the most mature material systems for photovoltaic applications, both produced and used at the GW-scale by the photovoltaic industry. Their combination in tandem devices – with a silicon bottom cell and a wide-bandgap (1.70-1.75 eV) II-VI top cell – is, thus, a promising pathway. However, such a development has so far been stymied by the absence of a suitable wide-bandgap polycrystalline II-VI absorber, with high enough material quality to ensure a performant top cell. In the particular case of the wide-bandgap alloy Cd_1-xZn_xTe, this is largely due Zn out-diffusion during the CdCl₂ process – required to “activate” the material by passivating internal defects and, thus, to achieve high-efficiency polycrystalline CdTe and CdSeTe devices – leading to a final film with a bandgap closer to 1.5-eV CdTe than 1.7-eV Cd_1-xZn_xTe. Reducing the intensity of the CdCl₂ treatment while still achieving activation of the material is believed to be a key step to enable high material quality 1.7-eV II-VI cells.
In the case of non-Zn-alloyed materials, CdSe_xTe_1-x films exhibit substantially stronger luminescence than their CdTe counterparts before the CdCl₂ passivation treatment step, indicating a higher as-deposited material quality and, correspondingly, a longer minority carrier lifetime after CdCl₂ treatment. Following these results, we theorize that incorporation of Se into Cd_1-xZn_xTe would lead to higher material quality of as-deposited wide bandgap films. We expect that such films would require a lower intensity CdCl₂ treatment to activate, leading to limited Zn out-diffusion. Hence, we deposited the quaternary alloy Cd_1-xZn_xSe_yTe_1-y using a modified close space sublimation (CSS) process from sources containing CdSe_yTe_1-y and Zn. The deposited films were characterized by Transmission and Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, and X-ray Diffraction in order to assess their material properties and composition as a function of deposition parameters. Optoelectronic properties were assessed with Transmittance, Photoluminescence, and Time-Resolved Photoluminescence measurements. Preliminary device results will be reported.

High bandgap kesterite Cu₂ZnSnS₄ (CZTS) has attracted world-wide attention in recent years owing to the earth-abundance and non-toxicity of its constituents and the realization of high and stable performance [1]. The large deployment of photovoltaic (PV) in the future would demand stable, abundant and non-toxic materials similar to Si, in either flexible/rigid single junction thin film solar cells or tandem cells with Si bottom cell. In this regards, CZTS is more advantageous over other single junction cells and their associated top cell for tandem cells. For its industrial-scale deployment, obtaining high efficiency CZTS is the first prerequisite. Until recently, however, state-of-the-art CZTS de