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Cadmium telluride (CdTe) solar cells represent a commercially successful photovoltaic technology, with an annual production capacity approaching 20 GW. However, improving the open‐circuit voltage (VOC) remains challenging. This study aims to deepen the understanding of charge carrier recombination in CdTe solar cells and to explore alternative dynamical characterization methods that address the limitations found in conventionally used time‐resolved photoluminescence for CdTe solar cells. Transient photovoltage and transient photocurrent techniques are utilized to investigate charge carrier dynamics under conditions resembling real‐world solar cell operation. The results reveal that an effective nonradiative recombination lifetime of 580 ns dominates the charge dynamics at VOC values below 850 mV. Above this threshold, radiative recombination becomes significant, with a radiative recombination coefficient of 1.1 × 10−9 cm3 s−1. Additionally, the stationary charge carrier density at 1 sun is determined to be around 1 × 1014 cm−3. By accurately determining both radiative and nonradiative recombination, this work provides a comprehensive understanding of carrier dynamics in high‐performing CdTe devices and paves the way for improving the VOC and performance of CdTe solar cells. [ABSTRACT FROM AUTHOR]

High-efficiency, lightweight, and flexible solar cells are sought for a variety of applications particularly when high power density and flexible form factors are desired. Development of solar cells on flexible substrates may also offer production advantages in roll-to-roll or sheet-to-sheet processes. Here, we report device efficiencies of 17.2% and 14.6%, under AM1.5G and AM0 irradiances, respectively, for a flexible, lightweight, CdTe-based solar cell. To advance the efficiency relative to the highest previously reported AM1.5G value of 16.4%, we used an indium gallium oxide (IGO) emitter layer on a cadmium stannate (CTO) transparent conductor, which was deposited on 100-μm thick Corning® Willow® Glass. A sputtered CdSe layer was employed to incorporate Se into a CdTe absorber that was deposited by close-space sublimation, and CuSCN was used as a hole transport layer between the CdTe and the back metal electrode. The IGO and CTO layers remained intact during the high temperature film processing as seen in cross-sectional imaging and elemental mapping. This device configuration offers great promise for building-integrated photovoltaics, space applications, and higher rate manufacturing. [ABSTRACT FROM AUTHOR]

There is a great deal of interest in increasing the energy yield from thin film solar cells by implementing bifacial operation. However, deleterious band bending and high interface recombination at the back transparent electrode can cause problems. Herein, it is investigated how bifacial thin film devices perform when illuminated through the front, back, and simultaneously through both interfaces using numerical modeling. It is shown that the downward band bending near the back interface is reduced during illumination when the carrier concentration is low. This effect is not found when the doping is relatively high, but the minority carrier distribution is still modified. Under either condition, the power generated under bifacial illumination exceeds the sum of the power generated when the illumination is solely from the back or the front. It is also shown that the back‐illuminated device performance is independent of the angle at which the light enters the back of the device, which provides accommodation for scattered light. Finally, the power enhancement is calculated for bifacial devices relative to front‐illuminated devices, and is shown that any significant loss in frontside power generation is difficult to overcome with back illumination under real‐world albedo conditions. [ABSTRACT FROM AUTHOR]

Herein, antimony sulfoselenide (Sb2(S, Se)3) thin‐film solar cells are fabricated by a hydrothermal method followed by a post‐deposition annealing process at different temperatures and the impact of the annealing temperature on the morphological, structural, optoelectronic, and defect properties of the hydrothermally grown Sb2(S, Se)3 films is investigated. It is found that a proper annealing temperature leads to high‐quality Sb2(S, Se)3 films with large crystal grains, high crystallinity, preferred crystal orientation, smooth and uniform morphology, and reduced defect density. These results show that suppressing deep‐level defects is crucial to enhance solar cell performance. After optimizing the annealing process, Sb2(S, Se)3 solar cells with an improved power conversion efficiency 2.04 to 8.48% are obtained. [ABSTRACT FROM AUTHOR]

Bandgap gradient is a proven approach for improving the open-circuit voltages (VOCs) in Cu(In,Ga)Se2 and Cu(Zn,Sn)Se2 thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved VOCs in Cd(Se,Te) solar cells using commercial SnO2 buffers. A champion device achieves an efficiency of 20.03% with a VOC of 0.863 V. Bandgap gradient is a promising approach to improve the open-circuit voltage in thin film solar cells. Here, authors incorporate a Cd(O,S,Se,Te) region to realize the bandgap gradient at front interface and demonstrate Cd(Se,Te) solar cells with reduced recombination and a champion efficiency of 20.03%. [ABSTRACT FROM AUTHOR]

Bifacial solar cells have the potential to maintain energy output higher than monofacial devices under unfavorable weather conditions. A transparent back‐buffer layer which can passivate the interface and improve the minority carrier lifetime is critical in CdTe‐based bifacial devices. Herein, solution‐processed CuxCryOz as a promising back‐buffer for CdTe/CdS solar cells is demonstrated. The carrier lifetimes measured at the front and back of the device are 31.2 and 3.1 ns, respectively, which correspond to an increase of ≈38% and 138%, respectively, compared to the reference device. This dramatic improvement in lifetime results in a 100% increase in short‐circuit current measured with backside illumination. The best bifacial device has efficiencies 7.6% and 12.5%, respectively, from back and front illumination, yielding a bifaciality factor of 0.60. [ABSTRACT FROM AUTHOR]

Employing single‐walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap‐free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond‐microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high‐performance organic solar cells. [ABSTRACT FROM AUTHOR]

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data. • Approach to determine surface recombination velocity and band bending at back of device using backside quantum efficiency. • Use values for back interface to determine recombination current densities of front illuminated device at open circuit. • Can identify location of limiting recombination in devices. • Demonstrated approach using high bifacial CdTe solar cell. [ABSTRACT FROM AUTHOR]

We fabricate and characterize methylammonium lead halide perovskite film as a novel back contact to CdTe thin‐film solar cells. We apply ~0.75 μm perovskite film at the interface of CdCl2‐activated and Cu‐doped CdTe surface and complete the device with Au back contact. We use Cu/Au back contact as a reference to compare results with novel back contact. Our investigation shows that incorporation of thin layer of perovskite film before the back contact metal reduces back contact barrier effect and improves fill factor (FF) and open‐circuit voltage (VOC) of the solar cells. Our low temperature JV results prove that thin‐film perovskite is a very necessary component in CdTe solar cells to reduce back contact barrier, to minimize interface or surface recombination, to improve collection efficiencies, and to increase the efficiency of solar cells. Our best device shows 7% increase in VOC to 0.875 V and ~7% increase in FF with the highest FF of 81%, and solar cell's efficiency finally increases by 10% with the use of MAPb(I1‐xBrx)3 as an interface layer. [ABSTRACT FROM AUTHOR]

Evolving material science and device architectures continue to drive improvements in photovoltaic solar cell performance. Herein, the synthesis and application of p-type transparent copper--zinc--sulfide (Cu--Zn--S) nanocomposite thin films for application as a semi-transparent back buffer layer for cadmium telluride (CdTe) photovoltaics is reported. Earth-abundant and low-toxicity Cu--Zn--S films are prepared at room temperature using successive ionic layer adsorption and reaction (SILAR). Transparency in the range of 500--800 nm, low resistivity, and composition-controlled bandgap energy offer a compelling material system for high performance as an electron reflector enabling bifacial cell design. Implementing the Cu--Zn--S hole transport material (HTM) at the CdTe back contact, without intentional introduction of Cu doping, converts simulated AM1.5 sunlight to electricity at an efficiency up to 13.2%, with an average device performance of 13.0%. Intentional Cu doping yields a best efficiency of 14.3% with open-circuit voltage ( VOC) of 848 mV and fill factor (FF) of 77.3% (average 14.1%). Our study shows the clear promise of this material for highly efficient and semi-transparent back contact to CdTe solar cells. [ABSTRACT FROM AUTHOR]

The electrical properties such as interface energy barriers, defect energy levels, and densities dictate the performance of thin film solar cells. Here, we show that these properties can be quantified in cadmium telluride (CdTe) thin‐film solar cells using admittance spectroscopy‐based techniques. Our results reveal that the electrical properties in CdTe thin‐film solar cells depend on both buffer material and the fabrication atmosphere. We find that only a negligible front contact barrier exists at the CdS/CdTe front junction regardless of the fabrication atmospheres, while an obvious front barriers are observed at the ZnMgO (ZMO)/CdTe junctions. Both CdS/CdTe and ZMO/CdTe solar cells exhibit back contact barrier. The energy level of defects is shallower in CdS/CdTe cells than in ZMO/CdTe cells. The fabrication atmosphere influences the electrical properties, i.e., an oxygen‐free atmosphere reduces the front and back barrier heights and lowers the energy level of defects. The results provide critical insights for understanding and optimizing the performance of CdTe thin‐film solar cells. [ABSTRACT FROM AUTHOR]

Recent numerical modeling and experimental work have shown that appropriate conduction band alignment at the emitter/absorber interface is critically important for high efficiency CdTe solar cells. To date, however, the properties of the transparent conducting oxide (TCO)/emitter interface have not been thoroughly investigated. Here, we use numerical modeling to determine the role of the conduction band alignment at the TCO/emitter interface. Our findings are increasingly important to device performance as efforts are made to widen the bandgap of the absorber. We also investigate the relative importance of the barrier caused by poor front contact versus the barrier at the back contact. It is well known that a barrier at the back contact can reduce the open circuit voltage of the device and produce rollover in the current density-voltage but is not well appreciated that a barrier at the front of the device can play a completely analogous role. We also show that for optimum device performance at any absorber bandgap, the conduction band of the TCO must be no more than 0.3 eV below the conduction band of the emitter. [ABSTRACT FROM AUTHOR]

Substrate configuration CdTe/CdS solar cells were fabricated directly on 25 nm thick single wall carbon nanotube (SWCNT) back contacts using a high temperature, commercial deposition process. No copper was introduced to facilitate the formation of an ohmic back contact at the back of the device. The champion device had an efficiency of 6.5%. The thin SWCNT layer allowed the device to be illuminated through the back contact, and a comparison of the device performance with front and backside illumination revealed several novel features of the SWCNT back contact. Our findings indicate that SWCNT films may be useful as a recombination layer contact for bifacial solar cells, photovoltaic windows, or two-terminal thin-film tandem solar cells. [ABSTRACT FROM AUTHOR]

Thin film photovoltaic (PV)devices and modules prepared by commercial processes can be severely compromised by through-device low resistance electrical pathways. The defects can be due to thin or missing semiconductor material, metal diffusion along grain boundaries, or areas containing diodes with low turn-on potentials. We report the use of single wall carbon nanotube(SWCNT) layers to enable both protection against these defects and back contact formation for CdTePVdevices. Samples prepared with a SWCNT back contact exhibited good efficiency and did not require shunt protection, while devices prepared without shunt protection using a standard metal back contact performed poorly. We describe the mechanism by which the SWCNT layer functions. In addition to avoiding the need for shunt protection by other means, the SWCNTfilm also provides a route to higher short circuit currents. [ABSTRACT FROM AUTHOR]

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%. [ABSTRACT FROM AUTHOR]

An appropriate electrical back contact in CdTe solar cells is crucial to achieving high power conversion efficiency. In this work, a facile back surface treatment method for CdTe solar cells using hydroiodic acid (HI) is developed, and the effects of HI etching on the CdTe surfaces investigated. The electrical properties of the CdTe absorber and interfaces are characterized by current–voltage, capacitance–voltage, admittance spectroscopy, and complex capacitance spectroscopy measurements. The HI‐etched devices show slightly higher apparent carrier concentrations than the control devices, suggesting an increased copper doping in the CdTe absorber. The potential barrier height of the back contact is reduced from 0.430 to 0.368 eV after the HI‐treatment, accompanied by reduced contact resistance and carrier recombination. Additionally, the HI‐treatment eliminates a defect signature at 0.409 eV. The HI‐treatment effects lead to improved power conversion efficiency through enhancement of the fill factor, the short circuit current, and open circuit voltage. Electrical characterizations reveal that CdTe surface etching using diluted hydrogen iodide removes native oxides and/or chlorides produced during the CdCl2 treatment, and creates a Te‐rich surface layer, which reduces the back barrier height and improves the performance of the resulting CdTe solar cells. [ABSTRACT FROM AUTHOR]

An ex-ante life cycle inventory was developed for single walled carbon nanotube (SWCNT) PV cells, including a laboratory-made 1% efficient device and an aspirational 28% efficient four-cell tandem device. The environmental impact of unit energy generation from the mono-Si PV technology was used as a reference point. Compared to monocrystalline Si (mono-Si), the environmental impacts from 1% SWCNT was ??1/418 times higher due mainly to the short lifetime of three years. However, even with the same short lifetime, the 28% cell had lower environmental impacts than mono-Si. The effects of lifetime and efficiency on the environmental impacts were further examined. This analysis showed that if the SWCNT device efficiency had the same value as the best efficiency of the material under comparison, to match the total normalized impacts of the mono- and poly-Si, CIGS, CdTe, and a-Si devices, the SWCNT devices would need a lifetime of 2.8, 3.5, 5.3, 5.1, and 10.8 years, respectively. It was also found that if the SWCNT PV has an efficiency of 4.5% or higher, its energy payback time would be lower than other existing and emerging PV technologies. The major impacts of SWCNT PV came from the cell's materials synthesis. [ABSTRACT FROM AUTHOR]

We report the hydrothermal deposition of Sb 2 S 3 thin film on top of CdS buffer layer, and the fabrication of prototype photovoltaic devices utilizing spiro-OMeTAD as the hole transport layer. The as-deposited films were amorphous, which transformed to polycrystalline after thermal processing. The pristine films were annealed at different temperatures and showed effective recrystallization at 350 °C which resulted in larger grains, intense XRD patterns, and significantly improved device parameters. The obtained V OC of 795 mV is among the highest reported for a Sb 2 S 3 based solar cell. Deep level transient spectroscopy studies detected an electron trap with activation energy 0.61 eV in the pristine annealed absorber, which became deeper (0.66 eV) upon Na incorporation. However, the capture cross-section decreased by an order of magnitude, and the trap density halved. The reduction in the capture cross-section and trap density for the Na-incorporated device coincides with the improved EQE response in the mid- and long-wavelength regions and a 9 % increase in device efficiency. The light intensity dependence of V OC clearly demonstrated that Na incorporation reduced the trap-assisted recombination and facilitated efficient charge transport in the device. • Reporting Sb 2 S 3 based solar cell with V OC close to 800 mV, which is an achievement with respect to the state-of-the-art in this area. • With Na doping, the current collection in the mid- and long-wavelength regions improved, predominantly due to reduction in the capture cross-section and density of a deep trap, leading to 9 % relative increase in device efficiency. [ABSTRACT FROM AUTHOR]

An optical function parameterization of polycrystalline Mg x Zn 1- x O (MZO) thin films as a function of bandgap E g has been utilized in applications for the metrology of CdTe-based solar cell structures that incorporate MZO thin films as high resistivity transparent (HRT) layers. Parametric expressions are applied to facilitate mapping spectroscopic ellipsometry (M-SE) of device structures consisting of glass/SnO 2 :F/MZO. M-SE is shown to provide maps in the MZO effective thickness and bandgap within confidence limits of ± 1 nm and ± 0.003 eV, respectively. As a second application of these parametric expressions, an as-deposited glass/SnO 2 :F/MZO/CdS/CdTe device structure has been analyzed by through-the-glass spectroscopic ellipsometry (TG-SE). Such an analysis is also shown to provide the MZO effective thickness and bandgap. The outcome of the TG-SE analysis for this device structure enables simulations of the external quantum efficiency (EQE) spectrum of the resulting solar cell assuming different recombination losses within the individual layers of the structure. A comparison of these simulations with the experimental EQE spectrum reveals improved current collection from the front of the device incorporating an MZO HRT layer. • MZO thin films are prepared using RF magnetron co-sputtering. • Spectroscopic Ellipsometry (SE) used for processing-property-performance correlations. • We draw upon an optical function parameterization of the MZO genome. • Mapping-SE of a SLG/TCO/MZO structure pursued. • Through-the-glass-SE analysis of a structure of SLG/TCO/MZO/CdS/CdTe also performed. [ABSTRACT FROM AUTHOR]

Critical avenues to increase energy yield for CdTe photovoltaic (PV) technology, such as bifacial cells or tandem cells designs, rely on the need to develop transparent back contact approaches that enable high efficiency and high transparency. We report the properties of earth-abundant p-type transparent conducting nanocomposite (NC) thin film (CuS) x (ZnS) 1- x , deposited by low-cost chemical bath deposition method. Nanocomposite films of ~ 45 nm thickness transmit > 70% of the visible light and show compact grains of size ~ 10 nm. We investigate the photovoltaic performance of both opaque and semi-transparent CdTe solar cells using (CuS) x (ZnS) 1- x NC thin films as a back contact interface layer. Introduction of the NC layer between the CdTe and a standard Cu/Au metallization process yielded the best device efficiency of 13%, representing a 3.2% relative improvement over the standard Cu/Au back contact. Semi-transparent CdTe devices finished with an indium tin oxide (ITO) layer allowed for comparison of front and back illumination schemes. Experimental and modeling results confirm that the bifacial device configuration leads to significant recombination losses under back illumination due to the absence of a charge separating field, a relatively high interface recombination rate, and an insufficient electron diffusion length. Once optimized, and with an improved interface and bulk CdTe carrier lifetimes, this NC earth-abundant transparent conductor based on Cu, Zn, and S may serve as an effective, inexpensive, low-toxicity back contact layer for a bifacial CdTe solar cell. [ABSTRACT FROM AUTHOR]

We report the properties of hybrid deposited iron pyrite (FeS 2 ) thin films applied as the back contact interface layers of CdS/CdTe solar cells. The hybrid deposition process for FeS 2 optimized in this study relies on DC magnetron sputtering of iron with simultaneous thermal evaporation of sulfur. We have fabricated solar cells incorporating CdS/CdTe window/absorber layers sputter-deposited onto commercial transparent conducting oxide coated glass and have compared the performance of devices incorporating the new FeS 2 /Cu/Au back contacts with that of standard devices incorporating Cu/Au back contacts. Considering our best devices of each type, the inclusion of the FeS 2 thin film as a hole transport layer has improved the open circuit voltage V OC by 2.1%, reaching 817 mV, and the fill-factor FF by 8.3% relative, reaching 74.7%, in comparison with devices omitting the FeS 2 layer. Under standard test conditions of 100 mA/cm 2 simulated AM1.5G and 25 °C, devices utilizing the FeS 2 hole transport layer have shown a conversion efficiency η as high as 13.3% – a relative increase in η of ~10% over our current laboratory standard back contact. The attained FF exceeds previous results for high efficiency sputter-deposited CdS/CdTe solar cells. [ABSTRACT FROM AUTHOR]

We report the application of thin film nanocrystalline (NC) FeS 2 as the copper-free back contact for CdTe solar cells. The FeS 2 -NC layer is prepared from solution directly on the CdTe surface using drop-casting coupled with a hydrazine treatment at ambient temperature and pressure, and requires no thermal treatment. Copper-free solar cells based on the CdS/CdTe/FeS 2 -NC/Au architecture exhibit device efficiencies >90% that of a standard Cu/Au back contact devices. The FeS 2 -NC back contact solar cells show good thermal stability under initial tests. Devices prepared with untreated FeS 2 -NC back contacts display a strong “S-kink” behavior which we correlate with a high hole-transport barrier arising from inter-NC organic surfactant molecules. [ABSTRACT FROM AUTHOR]

We report the role of copper iodide (CuI) nanoparticles (NPs) as a hole transport layer (HTL) in cadmium sulfide/cadmium telluride (CdS/CdTe) photovoltaics. These CuI NPs were prepared using solution processing at room temperature and used to fabricate monofacial and bifacial CdTe solar cells with different back contacts. Using CuI/Au as the back contact, the device efficiency reached to 14.8% with outstanding fill factor (FF) of 79.2%. Replacing the gold (Au) electrode with sputtered transparent indium tin oxide (ITO), a CuI/ITO back contact yielded photoconversion efficiencies (PCEs) of 11.6% and 5.5% under front and back illumination respectively. Bifacial devices (CdTe/ITO) without the CuI NP HTL have an efficiency of 7.0% and 1.0% for front and back illumination, respectively. For CuI/ITO, a current collection of 12.0 mAcm−2 was observed upon back illumination which significantly improved over an ITO-only back contact (5.0 mAcm−2). The PCE obtained from back illumination was enhanced when using CuI NPs as the HTL due to the reduced back barrier height, and improved back interface as determined by temperature dependent current vs. voltage characteristics and impedance spectroscopy analysis. The improvement in device performance of the bifacial configuration is a significant step forward toward realizing thin film photovoltaic modules which harvest energy incident on the rear of the module. [Display omitted] • Investigated CuI Nanoparticles (NPs) as Hole Transport Layer (HTL) for Monofacial and Bifacial CdS/CdTe devices. • CuI NPs helps to improve photo conversion efficiency by significant improvement in open-circuit voltage (V oc) and fill factor (FF) of the devices. • Bifacial devices with absorber layer thickness (3.3 μm) showed excellent short circuit current density (J sc = 12.0 mAcm−2) with back illumination reaching the device efficiency of 5.5% (bifaciality of 0.47). [ABSTRACT FROM AUTHOR]

Thin film CdTe superstrate solar cells have been fabricated by sputtering starting from CdS/CdSe front layers deposited on transparent conductor coated glass. The performance of such devices is sensitive to the fabrication details including the temperature-time profile, which leads to CdSe/CdTe interdiffusion and formation of a CdTe 1-x Se x bandgap-graded absorber. Mapping spectroscopic ellipsometry (M-SE) has been applied to the CdS and CdSe thin films for process calibration, which involves determining the deposition rate in terms of effective thickness (volume/area) versus spatial position on the sample. The goal is to optimize the performance of the devices by correlating cell parameters with these two effective thicknesses. Intended variations in the thicknesses along with unintended spatial non-uniformities enable coarse and fine-scale optimization, respectively. Using these methods, the highest performance solar cells from the CdS/CdSe/CdTe structure are obtained with 13 nm CdS and 100 nm CdSe. An increase in the CdS thickness above 13 nm leads to a decrease in open-circuit voltage and fill-factor attributed to the formation of a CdSe 1-z S z interdiffusion region with z approaching 0.5, where the alloy electronic properties are likely to suffer. Our results demonstrate that M-SE, exploited in conjunction with deposition non-uniformities, serve as a viable approach for process optimization of complex solar cell structures. • Mapping spectroscopic ellipsometry was used to calibrate the deposition processes for the sputtered CdS and CdSe thin films. • The goal was to maintain the V oc without a significant loss in J sc for the CdS/CdSe/CdTe solar cells. • The performance was optimized by correlating device parameters with the effective thicknesses of the CdS and CdSe layers. • Different ranges for CdS were used along with a 65–175 nm thickness range for CdSe in the fabrication of the CdS/CdSe/CdTe device. • The results reveled that spectroscopic ellipsometry exploited in conjunction with deposition non-uniformities serve to optimizie the fabrication process. [ABSTRACT FROM AUTHOR]