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Sb2Se3 has become a promising absorber layer material for photovoltaic applications because of its attractive optical properties and rapid improvement in Sb2Se3 solar cell efficiency in recent years. The indirect bandgap and Urbach energy of Sb2Se3 have been determined to be 1.12 eV and 21.1 meV, respectively, using photothermal deflection spectroscopy. Above bandgap critical points at 1.48, 2.00, 2.35, 2.87, and 3.86 eV are identified from the complex dielectric function (e = e1 + ie2) spectra of thin film Sb2Se3 obtained from spectroscopic ellipsometry. Spectra in e serve as input for external quantum efficiency (EQE) simulations of substrate type Sb2Se3 solar cells. Comparison of experimental and simulated EQE quantifies carrier collection losses and identifies a carrier collection length in the absorber layer of 439 ± 4 nm and a 96.4 ± 0.2 % carrier collection probability near the heterojunction.

Thin film solar cells have, in general, three recombination locations that can limit the device performance – the bulk, front interface, and back interface. Unfortunately, it is difficult to determine which of these mechanisms limit any given device. Time resolved photoluminescence measurements provide an estimate for the carrier lifetime and, therefore, bulk recombination. Determining the role of the front and back interface in device performance, though, is more complicated. With this in mind, we investigated the use of current densityvoltage measurements with front and back illumination at varying intensities. Using numerical modeling, we show that recombination information about each interface can be gleaned, and the limiting interface can be determined. The response curve signatures learned from modeling are then applied to measurements of real devices.

Copper (Cu) is being widely used in CdTe solar cells to improve the p-type conductivity and reduce the back-barrier height. However, due to the defect compensate effect and the high spatial migration rate of Cu, the physical mechanism of Cu in CdTe is still a challenge. Here, three different Cu dopant source, metallic copper (Cu), and cuprous chloride (CuCl) and copper chloride (CuCl 2 ), are applied in ZMO/CdTe solar cells. Our results indicate that solution processed Cu treatment supplies lower Cu diffusion temperature and possibilities to reduce the Cu dosage and distribution profile in CdTe.

The intermittent nature of solar energy has made it necessary for photovoltaic (PV) systems to rely on external energy storage when deployed off-the-grid. In recent years, solar flow batteries (SFBs) have emerged as a potential alternative, which integrates energy production and storage in an integrated device. Here we performed an environmental assessment by highlighting potential hotspots that might hinder their acceptance, offering less pollutive alternatives. Specifically, we analyzed the environmental impacts of perovskite/silicon tandem based SFB. Our results show that the PV cell is responsible for three fourths of the impact of the device at lab scale, while the stack frame is responsible for the remaining impact. At this scale, the impact of the electrolytes is negligible. We expect the impact of the frame to reduce as the scale increases, while we anticipate an increase for the importance of the electrolytes as more energy is stored. For the future, the impact of the PV cell could be reduced by using a back electrode different than gold, and by substituting the perovskite/silicon tandem configuration with a perovskite/perovskite one.

CdSe thickness and Cu doping play significant roles in achieving a highly efficiency in CdTe solar cells. Using an evaporated CdSe/CdTe device stack to avoid vacuum breaks between layers, we investigate the role of the CdSe thickness on the device performance and show that when the CdSe thickness is above a critical value the device performance, specifically the short circuit current (J SC ), suffers. From these measurements, we determine the critical thickness of CdSe to be 120 nm. However, in some cases the CdSe thicknesses can be increased above the critical value and increasing the Cu doping process temperature can lead to an increase in J SC and an overall improvement in device efficiency. Specifically, for a device with ~270 nm of CdSe, the J SC increases from 7.9 mAcm-2 with CuCl 2 processing temperature of 200 °C to ~29 mAcm-2 when the processing temperature is increased to 250 °C.

Monolithic perovskite/perovskite tandem solar cells have attracted substantial attention owing to their promise for high power conversion efficiency (PCE) and low fabrication cost. Conventional all-perovskite tandem solar cells employ a mixed tin (Sn)-lead (Pb) low-bandgap subcell, which suffers from instability issues in the ambient air. Here, we fabricate two-terminal perovskite/perovskite tandem solar cells consisting of two solution-processed perovskite subcells based on the more airstable pure Pb-based perovskites. The champion perovskite tandem cell exhibits a PCE of 19.2% with a high open-circuit voltage of 2.15 V. We further demonstrate photovoltaicelectrolysis (PV-EC) water-splitting applications using these high-photovoltage all-perovskite tandem solar cells.

Copper (Cu) doping is a crucial process to improve the hole concentration in CdTe absorber layer and reduce the back-barrier height for efficient CdTe solar cells. However, excess Cu creates detrimental defects and deteriorates device stability due to its high diffusivity. Here, we introduce a wet chemical method to incorporate Cu in CdTe using a CuCl solution. This approach allows a better control of Cu doping than the conventional method using evaporated Cu. Our investigation shows that the CuCl treatment can significantly improve the overall device performances due to an increased carrier concentration in the CuCl treated CdTe absorber layer. Furthermore, the CuCl treated devices show a longer minority carrier lifetime than the conventional Cu treated devices.

Sputtering is an efficient way to fabricate CdTe solar cells. However, the open-circuit voltage (Voc) of a sputtered device is low compared to close space sublimation or vapor deposited CdTe devices. Here, we report an outstanding Voc > 840 mV and fill factor (FF) > 71.0% for all-sputtered CdS/CdTe solar cells. These results were achieved by solution-based copper (Cu) doping of CdTe by copper (II) chloride (CuCl 2 ) in deionized water. Compared with evaporated Cu, solution-based doping increased the Voc and FF, and improved the optoelectronic properties of the sputtered CdTe thin films. Capacitance-voltage measurements indicated that the free hole concentration increased by two-fold for CuCl 2 treated devices.

In recent years, significant efforts have been paid to develop Pb-free halide perovskite and perovskite derivative absorber materials for solar cell applications. However, so far, solar cells based on these materials showed significantly inferior performances as compared to their Pb halide perovskite counterparts. Here, we provide a theoretical perspective on lead-free metal halide perovskites and derivatives for solar cell applications. We first show a theoretical understanding of the origins of the superior photovoltaic properties of Pb halide perovskites. We then show that Pb-replacing leads to reduced structural and electronic dimensionality, degrading the superior photovoltaic properties in Pb-free metal halide perovskites and perovskite derivatives.

Cu doping of CdTe has enabled reasonable device performance, but has some significant drawbacks, including limited achievable carrier concentration and high mobility in the CdTe films. Group V elements, on the other hand, are expected to facilitate higher carrier concentration and expected to have low mobility. Consequently, incorporate of As in CdTe is of great interest to the CdTe community. Here, we deposited CdTe from the CdTe:As feedstock to complete devices. We demonstrate that the As diffuses through the CdTe absorber layer and accumulates at the front and back interfaces with Cl activation. Current density-voltage and capacitance-voltage measurements indicate that the devices have low carrier concentration, even though the concentration of As is high. These results indicate that the process employed here was insufficient to activate the As doping.

We report a space tandem solar cell enabling an enhanced power conversion efficiency with radiation tolerance in low radiation environments for space photovoltaic (PV) applications. The presented space perovskite/Si 4-terminal tandem cell features an efficient semitransparent perovskite top cell with the bandgap of 1.61 eV on a thin Si space solar cell specifically designed for high radiation tolerance in space. Under a simulated air mass zero (AM0) spectrum (136.6 mW/cm2), the space tandem solar cell by combining the perovskite solar cell with a thin Si space solar cell results in a power conversion efficiency of 21.1% surpasses a limited efficiency of ~17% in a single junction Si space solar cell. Considering the potential for developing more efficient, low-cost perovskite PV technology, our approach of using the thin Si space solar for the space tandem cells makes it possible to realize cost-effective PV power generation in space.

As the efficiency of single junction devices improve, bifacial thin film (BFT) devices are being increasingly investigated as a way to increase energy yield per unit area. At the same time, increasing doping density of thin film devices are also being developed to increase the energy yield per unit area. Here, we investigate how critical parameters to improving backside illuminated devices and increasing the absorber doping density affect backside illuminated device performance. We show that reducing recombination at the back of the device through increasing initial Fermi level offset (IFLO) or decreasing back surface recombination velocity (BSRV) is far more important than increasing absorber doping density. We also show that there is a wide range of the IFLO-BSRV parameter space that can lead to high backside illumination device efficiency for highly doped thin film devices.

The high electron affinity (4.4eV) of CdTe presents a major technological challenge for forming a good back contact to CdTe, limiting the power conversion efficiency of CdTe solar cells. Commonly used metals do not possess an adequately deep work function to create an ohmic contact with CdTe. Instead, approaches such as doping the back surface of CdTe or incorporating a buffer layer between CdTe and the back-metal contact are used to lower the back-barrier height. Here, we investigate the impact of sputtered ZnTe back buffer layer on the performance of CdS:O/CdTe solar cells. We demonstrated CdTe solar cells with an efficiency of around 15% with a 35 nm ZnTe buffer layer.

Enargite Cu 3 AsS 4 is a promising absorber for thin-film solar cells but little is known about its optoelectronic properties. Here, we present photoluminescence (PL) spectroscopy, time-resolved terahertz spectroscopy (TRTS), timeresolved photoluminescence (TRPL) spectroscopy, and cyclic voltammetry (CV) measurements of enargite thin films. The results indicate promising defect characteristics with minority carrier lifetimes on the order of ns, and band alignments suggest that buffer layers besides CdS must be developed to achieve high efficiency Cu 3 AsS 4 solar cells.

Magnesium Zinc oxide (MZO), as a promising alternative buffer layer to replace CdS, is predicted to improve both the open circuit voltage and short circuit current, and, therefore, efficiency of CdTe solar cells due to its larger band gap and higher conduction position. However, few published works have achieved these benefits due to the S-kink in J-V curve. Here, the S-kink in MZO/CdTe films is eliminated through performing CdCl2 treatment in oxygen-free atmosphere and as a result, a significant improvement on efficiency (from 9.2% to 16.1%) is achieved.

Monolithic two-terminal (2-T) all-perovskite tandem solar cells are a promising candidate for the next generation solar cells with ultrahigh efficiency and low fabrication cost. Here, we report on the fabrication of 2-T all-perovskite tandem solar cells consisting of solution-processed wide-bandgap and low-bandgap perovskite subcells connected by a vacuum processed interconnection layer. We employed a bulk-passivation strategy to enlarge the grain size, increase carrier mobilities, reduce electronic disorder, and suppress trap-assisted recombination in the low bandgap mixed Sn-Pb perovskite films, which leads to improved performance of low bandgap perovskite solar cells with an absorber thickness over 750 nm. This advance in high-quality low-bandgap perovskite layers enables the fabrication of efficient 2-T all-perovskite solar cells with power conversion efficiencies of more than 21% and decent stability. Our approach provides a suitable pathway to realize ultra-high efficiency 2-T all-perovskite tandem solar cells.

Tandem photovoltaic (PV) cells with higher efficiency limits than current market dominated crystalline silicon PV devices are poised to be the next generation of solar cells. In this study we focus on analysis of perovskite/Cu(In x Ga 1-x )Se 2 tandem solar cells in the context of real-world conditions. Using material properties and the most recently updated atmospheric data we simulate the device energy yield for locations with different climate conditions. We use the resultant data in calculating module levelized cost and analyze the conditions under which using different forms of tracking become the cost-effective approach at each location.

The performance of Cadmium telluride (CdTe) solar cell devices is limited by the properties of front and back interfaces as well as the absorber layer, which can be influenced by the fabrication atmosphere. In this work, we report a detailed study of electrical properties of ZnMgO (ZMO)/CdTe and cadmium sulfide (CdS)/CdTe solar cells with the cadmium chloride (CdCl2) treatment performed in different atmospheres using temperature-dependent impedance spectroscopy and capacitance – voltage measurements. An equivalent circuit model consisting of two serial combinations of the front and back junctions is employed to fit the complex impedance spectra of devices measured in dark. Fitted data from equivalent circuit provides the value of each element, from which the bulk conductivity, back contact barrier height, as well as spatial inhomogeneities within the cell are extracted. Impedance spectroscopy analysis shows that there are negative and positive effects of back surface treatment in oxygen free ambient on device performance. For oxygen-free treatment, an obvious increase in the bulk conductivity is observed, suggesting an increased copper doping in the device. Additionally, ZMO devices show less junction inhomogeneity. All these improvements lead to better device performance of ZMO/CdTe solar cells.

Defect analysis of CdSe x Te 1-x alloy films prepared via close space sublimation (CSS) and sputtering have been carried out utilizing low temperature photoluminescence (PL) spectroscopy. We observe defect emission at 1.31 eV in both CSS and sputtered films which red shifts with increasing temperature. Therefore, the 1.31 eV peak is assigned to recombination of an electron in the conduction band with a hole localized in an unidentified vacancy defect state. Unlike CdSe x Te 1-x , ~1.31 eV peak has been assigned to free-to-bound (FB) peak in polycrystalline CdTe by previous researchers. Our analysis shows that a small amount of Se incorporation into CdTe has been found to change the nature of dominant native defect. Furthermore, we observe the bound exciton peak at ~1.49 eV in both films while free exciton peak (1.52 eV) is absent in latter. The presence of a free excitonic peak in CSS film confirms its better crystallinity compared to the sputtered one. Identification of such defect states and their nature are crucial to improving the efficiency of CdTe based solar cells.

In this report, we use atom probe tomography (APT) to produce 3D compositional maps of one-step growth of perovskite films (CH 3 NH 3 PbI 3 , MAPbI 3 . Initial studies indicate that further grain size enhancement and improved crystallinity can be achieved with the addition of Cd to these films. For MAPbI 3 films, perovskite specimens for APT were prepared and analyzed, which highlighted several challenges in the development of an APT procedure that reduces the effects of sample preparation and data collection on the fundamental morphology of the perovskite being analyzed. This optimization of the APT process, specifically for perovskite films, must be achieved before meaningful comparisons can be drawn between baseline films and those with Cd additives. We present some of the observed changes in morphology of the perovskite due to the APT process, which will enable future process optimization.

A parametric optical model is developed for use in analyzing spectroscopic ellipsometry (SE) measurements of CdTe and $CdSe_{1-y}Te_{y}$ alloy films. Through fitting to measurements of a series of CdCl2 treated alloys spanning the full range of compositions, a database describing the optical response of $CdSe_{1-y}Te_{y}$ as a function of y is compiled. More specifically, spectral features are mathematically related to y, including bandgap-bowing. Additionally, the parametric model has the capability of quantitatively describing defect-induced sub-bandgap absorption through the determination of an Urbach energy (Eu). The accuracy of the SE-determined Eu is verified through complementary photothermal deflection spectroscopy measurements. For instance, Eu describing a CdTe film is measured to decrease from 42 to 20 meV after undergoing CdCl2 treatment. External quantum efficiency simulations of full solar cell structures making use of this database can be compared to actual measurements of devices to identify the source of performance gains and or losses as well as the composition of alloyed material within such devices.

Optical characterization and simulations have been applied to study formamidinium + cesium lead triiodide (FA}1}$X \mathbf{Cs} X$textbf{PbI3}\textbf{) perovskite thin films and solar cells. Near infrared to ultraviolet complex index of refraction ($N = n +_{\mathbf{\, }} ik$) spectra of varying cesium-to-formamidinium ratios in solution processed perovskite thin films have been extracted. Critical point transitions and the absorption onset are extracted, with $x =$ 0.2 showing the sharpest absorption edge and least contribution from defects to absorption near the band edge. External quantum efficiency simulations of perovskite based solar cells using optical properties as input are performed and compared to experimental device results.

Organic-inorganic perovskite solar cells have achieved power conversion efficiencies that rival the established mainstream photovoltaic technologies, however, the lack of longterm stability presents one major hurdle for their commercialization. Here, we investigate the impact of low-cost epoxy encapsulation on device stability of large-area, laserpatterned perovskite solar cells and mini-modules, yielding results relevant to large modules for commercial applications. We track the performance evolution of our devices stored under different conditions. On this basis, we employ light-beam induced current (LBIC) to spatially resolve the photocurrent degradation of these devices, showing the striking contrast between devices with and without encapsulation after three months $(\sim2000$ hours) of storage under ambient and humid conditions.

Thin film CdTe solar cells were prepared using close-space sublimation followed by a standard cadmium chloride activation annealing treatment. After the activation process, some of the samples were subjected to a brief etching process using a 10% hydrogen iodide solution in methanol while the others were only rinsed with methanol. Completed solar cells from both sample types were prepared and characterized using current and capacitance-voltage scanning methods. The results show that the etching process increases the open circuit voltage by 10-20mV and the fill factor by 1-5% compared to the devices without the etching, thus increasing the efficiency from $\sim$ 13% to $\sim$ 15%. Capacitance-voltage measurements indicate that the bulk carrier (hole) density of the CdTe films increase by approximately a factor of three, which could partially explain the increase in device performance. The cause of the change in carrier density is still under investigation.

Here we quantify and spatially resolve the carrier collection enhancement of CdSe window layers in CdTe photovoltaic devices by scanning photocurrent microscopy. We compare the photogenerated current of CdTe solar cells with both CdS and CdSe window layers. Our results show that, despite notable spatial variation in current in both cases, the device with the CdSe window layer outperforms the CdS one, in agreement with macroscopic analysis previously performed. The external quantum efficiency (EQE) enhancement occurs near the band edge of CdTe.

Thin film CdTe x Se 1-x solar cell devices are fabricated using close-space sublimation from homogenized mixtures of CdTe and CdSe powders. Various window layer combinations of CdS and/or CdSe are also explored. The performance of the finished devices, in terms of current-voltage and spectral response, are evaluated while the cross sectional structures and compositional profiles are examined for selected devices. The observations from this study suggest that independent control of the Se alloying content into thin CdTe absorber layers can be independently controlled both near the front interface and within the film bulk while stile using a simple, one-source close-space sublimation film deposition arrangement.

Opto-electronic property gradients in CdTe solar cells can lead to improved photovoltaic (PV) device performance. Near infrared to ultraviolet complex optical response (e) is measured for polycrystalline CdSe1-xSx and CdSe1-yTey alloys. Monotonic variation in e and the bandgap are observed for CdSe1-xSx, whereas bandgap bowing is observed for CdSe1-yTey. Spectroscopic e serves as input for simulations of external quantum efficiency (EQE) for CdTe PV incorporating Se. Comparison with experimental EQE indicates increases at short wavelengths are due to current generated in CdSe1-xSx and CdSe1-yTey components and increases at long wavelengths arise from red-shifting of the CdSe1-yTey bandgap relative to CdTe.

Thin film CdTe solar cells devices are fabricated using homogenous alloys of CdS-CdSe, deposited by a co-sputter process and of varying compositions, as the window layer materials. After close-space sublimation of the CdTe layer and subsequent device finishing processes, the cells are characterized in terms of electo-optical performance as well as structural and compositional properties.

We demonstrate a solar cell based on a thin layer of metal directly deposited on an n-doped semiconductor structure, forming a Schottky barrier acting as a semi-selective contact for an electron temperature driven cell. Absorption of light is shown to be by free carrier absorption in the metal layer for illumination with wavelengths of light below the semiconductor band gap, giving rise to an IV characteristic with a reverse bias slope dependent on electron temperature. Illumination with light in excess of the band gap reveals a standard Schottky cell IV characteristic.

$\mathbf{Zn}_{3}\mathbf{P}_{2}$ is a promising candidate as an absorber in thin film solar cells because of the earth-abundant constituents and the optical properties. In the present study, we first report on formation of a novel Mg-P-Zn ternary phosphide semiconductor, $\mathbf{Mg}_{x}\mathbf{Zn}_{y}\mathbf{P}_{2}$ , in $\mathbf{Mg}/\mathbf{Zn}_{3}\mathbf{P}_{2}$ junction which was utilized in the champion device of $\mathbf{Zn}_{3}\mathbf{P}_{2}$ -based solar cells. $\mathbf{Mg}_{x}\mathbf{Zn}_{y}\mathbf{P}_{2}$ has a certain range of Mg/Zn ratio and a different crystal structure from that of $\mathbf{Zn}_{3}\mathbf{P}_{2}$ . $\mathbf{Mg}_{x}\mathbf{Zn}_{y}\mathbf{P}_{2}$ must play a key role in $\mathbf{Zn}_{3}\mathbf{P}_{2}$ solar cells since the cells show much higher efficiency compared to $\mathbf{Zn}_{3}\mathbf{P}_{2}$ -based cells with other structures.

The rapid improvement in the performance of photovoltaic (PV) solar cells has resulted in an overall reduction in the environmental impacts. Simultaneously, the source of the impacts has shifted from the processing energy to the materials themselves. We analyzed the potential toxicity of new materials to provide insight into material and technology selection. We evaluated metal releases to different ecological compartments and estimated human and ecological toxicity potentials using the life cycle assessment (LCA) approach. We observed that leaching of metals to the environment during the use or end-of-life phase can significantly increase the toxicity.

Thin film CdTe solar cells devices are fabricated using bi-layer stacks of CdS and CdSe with the intent of increasing the photocurrent generated within the CdTe layer. Variations in device spectral response with varying thickness of the CdS layer within the bi-layer stack are examined. Also, the impacts of processing conditions used for CdTe deposition and CdCl 2 annealing treatments are studied with respect to the CdS/CdSe/CdTe interface characteristics.

Close-space sulfurization (CSS) of sputtered metal precursors is developed for the first time to fabricate Cu2ZnSnS4 (CZTS) thin-film solar cells. To optimize the CSS parameters, a series of CZTS thin films are prepared by sulfurizing stacked metal precursors with various sulfurization temperatures and durations. These films are studied by different characterization techniques and then fabricated into solar cell devices. According to the XRD, Raman and SEM, single-phase CZTS thin films with densely packed large grains are formed after CSS. The best cell performance with an efficiency of 5.2% is achieved for the CSS at 590°C for 30 min.

We used Kelvin probe force microscopy to study the potential distribution on cross-section of perovskite solar cells with different types of electron-transporting layers (ETLs). Our results explain the low open-circuit voltage and fill factor in ETL-free cells, and support the fact that intrinsic SnO 2 as an alternative ETL material can make high-performance devices. Furthermore, the potential-profiling results indicate a reduction in junction-interface recombination by the optimized SnO 2 layer and adding a fullerene layer, which is consistent with the improved device performance and current-voltage hysteresis.