Cu(In,Ga)Se2 Absorber Research Articles

Performance enhancement of ultrathin graded Cu(InGa)Se2 solar cells through modification of the basic structure and adding antireflective layers

Traditional Cu ( InGa ) Se2 (CIGS) solar cells consist of an Mo bottom contact, p-type CIGS absorber layer, n-type CdS buffer layer, and ZnO window layer including a very thin intrinsic ZnO layer topped with an Al-doped ZnO transparent conducting oxide layer that was simulated by an Silvaco technology computer aided design simulator. The thickness of the layers and other parameters were limited to the values reported in the literature. The simulator was examined to affirm its functionality, and a step-by-step method was applied to optimize the baseline structure. The first modification was the substitution of the conventional CdS buffer layer with a nontoxic n-type ZnO buffer layer. The substituted ZnO buffer layer showed an effective band alignment and led to a remarkable improvement in cell performance. The second modification was the inclusion of different antireflective coatings, such as Al2O3, MgO, SnO2, and CdO, to enhance light trapping. Among them, CdO caused the highest efficiency. The optimization process was followed by replacing a very-thin (750 nm) CIGS layer containing three sublayers with dual-graded Ga contents. A trapezoidal gradient profile was tested and showed a significantly increased internal electric field, which assisted in raising the efficiency of the device. The last simulation concluded that the efficiency of a CIGS solar cell was 19.21% and the efficiency related to the baseline structure was enhanced by 36.24%.

Constructing a Spectral Down Converter to Enhance Cu(In,Ga)Se2 Solar Cell Performance Using Yttrium Aluminum Garnet:Ce3+ Ceramics

Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial for photovoltaic solar cells. However, more was said than done for the down‐conversion effect in the past, and quite a number of existing reports overclaimed the observed performance enhancement from down conversion either by wrongly identifying the contributions or by unfairly comparing with an inappropriate reference. Herein, Ce‐doped Y3Al5O12 ceramics with suitable absorption and photoluminescence properties are chosen as the down‐conversion material for Cu(In,Ga)Se2 (CIGS) solar cells. By properly designing optical structures to integrate the down‐conversion material with the CIGS solar cell, the fluorescent photons are efficiently guided to the CIGS absorber layer in addition to eliminating the scattering and maintaining the antireflection (AR) for incident light. The Y3Al5O12:Ce3+ ceramic down converter on CIGS solar cells can bring an enhancement of 0.53 mA cm−2 to the short‐circuit current density solely from down conversion, which is responsible for the major increase in conversion efficiency from 18.5% to 19.0%. A positive down‐conversion effect is demonstrated over AR layer‐coated CIGS solar cells without misclaiming contributions from other accompanied effects or jeopardizing any other accompanying effects.

High Efficiency CIGS Solar Cells by Bulk Defect Passivation through Ag Substituting Strategy.

Cu(In,Ga)Se2 (CIGS) is considered a promising photovoltaics material due to its excellent properties and high efficiency. However, the complicated deep defects (such as InCu or GaCu) in the CIGS layer hamper the development of polycrystalline CIGS solar cells. Numerous efforts have been employed to passivate these defects which distributed in the grain boundary and the CIGS/CdS interface. In this work, we implemented an effective Ag substituting approach to passivate bulk defects in CIGS absorber. The composition and phase characterizations revealed that Ag was successfully incorporated in the CIGS lattice. The substituting of Ag could boost the crystallization without obviously changing the band gap. The C-V and EIS results demonstrated that the device showed enlarged Wd and beneficial carrier transport dynamics after Ag incorporation. The DLTS result revealed that the deep InCu defect density was dramatically decreased after Ag substituting for Cu. A champion Ag-substituted CIGS device exhibited a remarkable efficiency of 15.82%, with improved VOC of 630 mV, JSC of 34.44 mA/cm2, and FF of 72.90%. Comparing with the efficiency of an unsubstituted CIGS device (12.18%), a Ag-substituted CIGS device exhibited 30% enhancement.

Pulsed laser annealing of spray casted Cu(In,Ga)Se2 nanocrystal thin films for solar cell application

We reportambient processing of 2.5 µm thick layers of Cu(In,Ga)Se2 (CIGS) nanocrystals thin films sprayed on CdS/i:ZnO/Al:ZnO/Soda lime glass substrates with pulsed laser annealing (PLA), using 1064 nm laser at 220 ns pulse width for solar cell fabrication. The choice of the nanosecond pulse width of the near-infrared laser source is critical for the effectiveness of the laser sintering of the CIGS layer irrespective of substrate thermal stability. Solid state sintering by PLA post treatment is evidenced via morphological and structural analysis. The analysis confirms the improvement in the quality of CIGS absorber with respect to its grain size from 5 nm to 20 nm after PLA, which is expected to reduce deep level defects. The short circuit current density of photovoltaic devices prepared using ambient processed laser annealed absorber layer increased significantly from 0.75 mA cm−2 to 8 mA cm−2, resulting in reasonable power conversion efficiency exceeding 1%. The promising results realized by PLA treatment, presented in this work, have great potential to be integrated into scalable roll-to-roll manufacturing of CIGS solar cells.

Simulation analysis of functional MoSe2layer for ultra-thin Cu(In,Ga)Se2solar cells architecture

The influence of Molybdenum diselenide transition metal dichalcogenide material (p-type MoSe2TMDC) as an interfacial layer between the ultra-thin Cu (In, Ga)Se2(CIGS) absorber layer, with thickness less than 500[Formula: see text]nm, and molybdenum back contact was studied using SCAPS-1D simulation package. The possible effects of the p-MoSe2layer on the electrical properties and the photovoltaic parameters of the CIGS thin-film solar cells have been investigated. Band gap energy, carrier concentration, and the layer thickness of the p-MoSe2were varied in this study. The optimum band gap is found to be of 1.3 eV. Interfacial layers of thicknesses less than 200 nm have been found to cause deterioration for the overall cell performance. This might be attributed to the increase in the back-contact recombination current and the reduction of the built-in potential at p-MoSe2/CIGS junction. Furthermore, the MoSe2layer would form the so-called back surface field (BSF), due to the associated wider band gap with respect to that of CIGS absorber layer. Additionally, the simulation of the I–V characteristic showed a higher slope which implies that MoSe2layer at the CIGS/Mo interface acts in a beneficial way on the CIGS/Mo hetero-contact adapting it from Schottky type contact to quasi-ohmic contact. The conversion efficiency has increased significantly from 14.61% to 22.08%, without and with the MoSe2layer, respectively. These findings are very promising for future high performance and cost-effective solar cell devices.

Mechanisms of extrinsic alkali incorporation in CIGS solar cells on flexible polyimide elucidated by nanoscale and quantitative analyses

In this work, Cu(In,Ga)Se2 (CIGS) solar cells on polyimide (PI) substrates were fabricated using a low-temperature three-stage co-evaporation process. To enhance device performance, the CIGS films were extrinsically doped with alkali (Na and K) using an in-situ post deposition treatment (PDT). To account for mechanisms of extrinsic alkali incorporation in CIGS solar cells on flexible polyimide, the alkali dopant concentrations in the film bulk (intragrain and grain boundary) and the surface chemistries/band structures were quantitatively investigated with various advanced characterization methods. In addition, the effects of the PDT sequences on the resulting device performance were studied with a particular emphasis on the characteristics of CIGS surfaces. By controlling the alkali incorporation into the CIGS absorber films, flexible lightweight CIGS thin-film solar cells with an efficiency of approximately 19% were obtained.

Passivation of Deep-Level Defects by Cesium Fluoride Post-Deposition Treatment for Improved Device Performance of Cu(In,Ga)Se2 Solar Cells.

Heavy-alkali post-deposition treatments (PDTs) utilizing Cs or Rb has become an indispensable step in producing high-performance Cu(In,Ga)Se2 (CIGS) solar cells. However, full understanding of the mechanism behind the improvements of device performance by heavy-alkali treatments, particularly in terms of potential modification of defect characteristics, has not been reached yet. Here, we present an extensive study on the effects of CsF-PDT on material properties of CIGS absorbers and the performance of the final solar devices. Incorporation of an optimized concentration of Cs into CIGS resulted in a significant improvement of the device efficiency from 15.9 to 18.4% mainly due to an increase in the open-circuit voltage by 50 mV. Strong segregation of Cs at the front and rear interfaces as well as along grain boundaries of CIGS was observed via high-resolution chemical analysis such as atomic probe tomography. The study of defect chemistry using photoluminescence and capacitance-based measurements revealed that both deep-level donor-like defects such as VSe and InCu and deep-level acceptor-like defects such as VIn or CuIn are passivated by CsF-PDT, which contribute to an increased hole concentration. Additionally, it was found that CsF-PDT induces a slight change in the energetics of VCu, the most dominant point defect that is responsible for the p-type conductivity of CIGS.

Functional Data Analysis of Electrical Measurements on Thin-Film Photovoltaic Devices

Cu(In,Ga)Se2 (CIGS)-based photovoltaic cells were fabricated under varying deposition conditions for the Mo back contact and CIGS absorber layers. Specifically, O2 concentration and sputtering pressure were varied during the Mo deposition, as well as the final Mo-layer thickness. The maximum deposition temperature was then varied during the subsequent deposition of the CIGS-layer by thermal coevaporation. The JV response curve of the finished devices were recorded and used to calculate the usual solar performance characteristics (efficiency, J sc, V oc, fill-factor, R oc, and R sc). The effects of the varied fabrication conditions on these characteristics were then investigated, and CIGS-deposition temperature alone was found to have had a significant influence on J sc. Following this, the entire JV -curve was analyzed using functional data analysis (FDA). The curve shape was revealed to be affected not only by temperature, but by O2 conc. and Mo sputter pressure as well. The effect of these fabrication parameters was greatest in regions of the curve larger than V oc. External quantum efficiency measurements on the finished devices were also studied using FDA, indicating a strong influence of temperature in reducing collection across most of the visible spectrum. Mechanisms that aim to explain these observations are proposed. This work details, according to the authors’ best knowledge, the first-ever use of FDA in the analysis of photovoltaic devices.

Improvement of the CIGS solar cell performance: structure based on a ZnS buffer layer

Cu(In,Ga)Se2 (CIGS) based thin film solar cells are the most efficient thin-film solar cells today. The non-toxic and wide band-gap zinc sulphide (ZnS) is a promising material to replace the cadmium sulfide (CdS) as the buffer layer in CIGS based solar cells. In this work we present a simulation study of a CIGS based solar cell with a buffer layer of ZnS, using the simulator Silvaco-Atlas. Our primary simulation shows a 22.6% efficiency of the CIGS solar cell with the CdS buffer layer which is comparable to reported and highest experimental results. However, the simulated efficiency of the CIGS solar cell with the ZnS buffer layer as high as 23.54% was achieved. The effects of layer parameters like the thickness, the acceptor and donor densities of the CIGS absorber and ZnS buffer layers and the CBO on the photovoltaic parameters of the ZnS/CIGS solar cell are optimized in order to improve the performance of the ZnS/CIGS solar cell. The highest efficiency of 27.33% is achieved when the ZnS buffer and the CIGS absorber layers have thicknesses of 0.025 µm and 4 µm with acceptor and donor densities of 6 × 1017 cm−3 and 1018 cm−3, respectively and a CBO in the range − 0.05 to 0.05 eV. The present results of simulation can help the development of the solar cells with higher conversion efficiency and low cost.

Giant Voc Boost of Low‐Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers

Large‐scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium‐free dry vacuum process suitable for in‐line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum‐based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module‐size glass/Mo/CIGS stack. The buffer‐window stack consisting of Zn(O0.75S0.25)/i‐ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open‐circuit voltage (Voc). Temperature‐dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.

Light management design in ultra-thin chalcopyrite photovoltaic devices by employing optical modelling

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se2 (CIGS) devices. For structures with an ultra-thin (500 nm) CIGS absorber, we study the improvements related to the introduction of (i) highly reflective metal back reflectors, (ii) internal nano-textures applied to the substrate and (iii) external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al2O3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external) is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm) CIGS absorber.

Optimization of Intrinsic ZnO Thickness in Cu(In,Ga)Se2-Based Thin Film Solar Cells.

The typical structure of high efficiency Cu(InGa)Se2 (CIGS)-based thin film solar cells is substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al(AZO) where the sun light comes through the transparent conducting oxide (i.e., i-ZnO/AZO) side. In this study, the thickness of an intrinsic zinc oxide (i-ZnO) layer was optimized by considering the surface roughness of CIGS light absorbers. The i-ZnO layers with different thicknesses from 30 to 170 nm were deposited via sputtering. The optical properties, microstructures, and morphologies of the i-ZnO thin films with different thicknesses were characterized, and their effects on the CIGS solar cell device properties were explored. Two types of CIGS absorbers prepared by three-stage co-evaporation and two-step sulfurization after the selenization (SAS) processes showed a difference in the preferred crystal orientation, morphology, and surface roughness. During the subsequent post-processing for the fabrication of the glass/Mo/CIGS/CdS/i-ZnO/AZO device, the change in the i-ZnO thickness influenced the performance of the CIGS devices. For the three-stage co-evaporated CIGS cell, the increase in the thickness of the i-ZnO layer from 30 to 90 nm improved the shunt resistance (RSH), open circuit voltage, and fill factor (FF), as well as the conversion efficiency (10.1% to 11.8%). A further increas of the i-ZnO thickness to 170 nm, deteriorated the device performance parameters, which suggests that 90 nm is close to the optimum thickness of i-ZnO. Conversely, the device with a two-step SAS processed CIGS absorber showed smaller values of the overall RSH (130–371 Ω cm2) than that of the device with a three-stage co-evaporated CIGS absorber (530–1127 Ω cm2) ranging from 30 nm to 170 nm of i-ZnO thickness. Therefore, the value of the shunt resistance was monotonically increased with the i-ZnO thickness ranging from 30 to 170 nm, which improved the FF and conversion efficiency (6.96% to 8.87%).

Impact of heat‐light soaking and heat‐bias soaking on NaF‐treated CIGS thin film solar cells

AbstractThe effects of heat‐light soaking (HLS) and heat‐bias soaking (HBS) were investigated on NaF‐treated Cu (In,Ga)Se2 (CIGS) solar cells. The HLS improved the cell efficiency with increased open‐circuit voltage (VOC) for NaF‐treated CIGS solar cells, whereas the short‐circuit current density (JSC) slightly decreased. The reduced JSC was caused by the significantly high carrier concentration due to the HLS, leading to a fairly narrow space charge region. However, the reduced JSC could be successfully recovered by the subsequent moderate heat soaking (HS) in the dark, thereby improving cell efficiency. On the other hand, no beneficial effect was observed for NaF‐free solar cells. Secondary ion mass spectrometry (SIMS) measurements revealed that the Na and K ions increased especially near the CIGS surface region after the HLS process. This result suggests that these ions were shifted by the photo‐induced electric field. The thermally and electrically activated alkali ions may eliminate the donor‐like defects in CIGS absorber, resulting in an increased acceptor concentration. Furthermore, we found that an external forward bias without light irradiation (HBS) resulted in the same effect as HLS. Based on these results, a possible mechanism of HLS and HBS effects was proposed on NaF‐treated CIGS solar cells.

Structural characterization of coevaporated Cu(In,Ga)Se2 absorbers deposited at low temperature

This paper focuses on low temperature (∼390 °C) growth of Cu(In,Ga)Se2 (CIGS) for high efficiency solar cells. The process developed in this work allows the growth of high quality CIGS thin films on flexible substrates such as polymer foils with lower impurity out-diffusion from the substrate. We carry out advanced analysis of the microstructure of CIGS absorbers prepared using multi-stage coevaporation on polyimide at low temperature leading to state-of-the-art efficiencies exceeding 18%. We also develop a method that can be used to easily interpret the x-ray diffraction data using compositional analysis, when asymmetric or multi-feature peaks are present. This method proves to be particularly useful to rule out in-plane inhomogeneities and undesired phases when in-depth gradients are present. This analysis is combined with electron microscopy and more advanced x-ray analysis (grazing incidence diffraction and pole plots) to create a global model of the film microstructure. We show that the film is composed of at least three stacked layers with different properties and that the microstructure of the layers has an impact on the cell performances. In particular, the short circuit current Jsc is strongly related to the intensity of the (112) peak.

Role of surface microstructure of Mo back contact on alkali atom diffusion and Ga grading in Cu(In,Ga)Se2 thin film solar cells

AbstractWhile the major function of molybdenum (Mo) back contact on soda‐lime glass (SLG) substrate in Cu(In,Ga)Se2 (CIGS) solar cells is to provide good adhesion and good conductivity, its role as the transportation channel for alkali elements from SLG substrate to CIGS layer has often been overlooked. In this work, we have intentionally fabricated tri‐layered structure in contrast to the conventional bi‐layered structure for Mo back contact with the microstructure of the thin topmost Mo layer manipulated by the Ar pressure and water vapor during sputtering deposition. It has been discovered that the CIGS solar cell conversion efficiency can be greatly affected by this thin topmost Mo layer of the back contact. Investigations on the elemental depth profiles in the CIGS devices and the chemical states of Mo on the back contact surfaces have revealed that the surface layer of Mo back contact has a strong effect on the diffusion of alkali elements from SLG into CIGS absorber layer, which in turn influences the formation of Ga gradient in the CIGS layer and thus the solar cell performance. It was revealed that Mo(OH)6 and MoO3 formed on the surface of Mo back contact play a key role in determining the K atom distribution and Ga gradient. A reaction mechanism was also proposed.

Influence of RbF post deposition treatment on heterojunction and grain boundaries in high efficient (21.1%) Cu(In,Ga)Se2 solar cells

Post deposition treatments (PDT) by alkali fluorides applied to chalcopyrite-based absorbers have produced record efficiencies in thin-film solar devices in the past few years and recently the efficiency of 22.6% was achieved with Cu(In,Ga)Se2 (CIGS) using rubidium fluoride (RbF) PDT. However, the effects of RbF-PDT towards changes in its interfacial and grain boundary (GB) properties are still not fully understood. In this work, cells with efficiency higher than 21% are investigated by combination of atom probe tomography (APT) and transmission electron microscopy (TEM) to show how changes in GB and interface chemistry may facilitate high efficiencies. APT studies, carried out at the interface between CIGS absorber and solution-grown CdS buffer layer, show In enrichment and Cu depletion along with traces of Rb. Our APT studies reveal higher amounts of Rb (1.5 at. %) and lower amounts of Na and K (<0.5 at. %) at GBs as compared with previous studies (on non-PDT samples) thus indicating substitution of Na and K by Rb. However, concentration of all alkali elements inside the grain bulk is below detection limit of APT. The concentration of Rb at the GBs in CIGS is measured depth-dependent using both APT and TEM, which consistently shows the increase in Rb towards the Mo back contact. In addition, a pronounced Cu depletion is observed at the GBs which might enhance hole-barrier properties of the GBs, thus improving charge carrier collection and hence the overall efficiency of the device. Thus, understanding effects of RbF-PDT at the atomic scale provides new insights concerning the further improvement of CIGS absorber and interfaces.

Semi-transparent photovoltaics using ultra-thin Cu(In,Ga)Se2 absorber layers prepared by single-stage co-evaporation

Semi-transparent thin-film solar cells with ultra-thin Cu(In,Ga)Se2 (CIGS) absorber layers were fabricated on fluorine-doped tin oxide coated glass substrates using a single-stage co-evaporation process. The effects of the deposition conditions, such as the absorber thickness, deposition temperature, and post-deposition treatment using NaF on the material properties of the CIGS films were evaluated. In addition, the power conversion efficiency (PCE) and average visible transmission (AVT) in the wavelength range of 420–720 nm for semi-transparent CIGS solar cells using these absorber layers were systematically investigated. The AVT of the solar cells was directly dependent on the thickness of the CIGS absorber films. When the film thickness of the absorber layers increased from 300 nm to 950 nm, the AVT decreased gradually from 9.04% to 0.30%, while their PCE values did not show a clear dependence on the absorber thickness. Narrow columnar grains were observed in the microstructure of the ultra-thin CIGS absorber layers, irrespective of the film thickness and deposition temperature. Doping the ultra-thin CIGS absorber layers with Na after film deposition enhanced solar cell performance (the open circuit voltage (VOC) and fill factor (FF) were improved). The enhanced cell performance was attributed to the lack of a resistive GaOx film at the CIGS/FTO interface and thermal damage to the FTO back contact. The PCE values of semi-transparent solar cells with ultra-thin CIGS absorber layers prepared under the optimal deposition conditions varied from 6.46% to 9.78% when the CIGS absorber thickness was varied from 200 nm to 300 nm, respectively, while the corresponding AVT values changed from 18.59% to 9.04%.

Growth improved of CIGS thin films by applying mechanical perturbations to the working electrode during the electrodeposition process

Cu(In,Ga)Se2 (CIGS) is a semiconductor which has been extensively studied for photovoltaic applications. With this semiconductor, efficiencies of solar cell greater than 20% have been achieved by physical vapor deposition. There are different techniques to make the CIGS absorber layer. However, electrodeposition is the most adequate technique to make it for different technical and economic reasons. In this paper, the performance of CIGS solar cells are compared using different strategies to synthesize the CIGS films on Mo-coated glass substrates by one-step electrodeposition employing an electrolytic cell with horizontal electrodes. For the above, two strategies are presented, these are: (i) DC electrodeposition and (ii) DC electrodeposition plus mechanical perturbations to the working electrode during the CIGS film growth. In the as-electrodeposited CIGS film a grain growth in selenium atmosphere process was applied and photovoltaic devices were completed with CdS, ZnO and ZnO:Al layers. The photovoltaic devices that were made with the CIGS films obtained by DC electrodeposition plus mechanical perturbations exhibit better absorber morphology with respect to that where only DC electrodeposition was used. The short circuit current density obtained with the absorber synthetized only by DC electrodeposition was of 7.35 mA/cm2 while for the absorber synthetized by DC electrodeposition plus mechanical perturbations was of 28.38 mA/cm2, both under 1000 W/m2 (AM 1.5).

Voltage-Induced Charge Redistribution in Cu(In,Ga)Se2 Devices Studied With High-Speed Capacitance–Voltage Profiling

Devices made from Cu(In,Ga)Se2 (CIGS) solar cell material have been evaluated with high-speed capacitance–voltage profiling after stepwise voltage changes. The changes primarily affect near-interface charge at deep acceptors within the CIGS absorber layer and generate temperature-dependent capacitance changes observed in deep-level transient spectroscopy measurements. SCAPS device modeling indicates that the deep acceptor concentration is up to the two orders of magnitude higher than the shallow doping level. High deep acceptor concentrations are found in all materials studied here. The large deep defect levels are high enough to limit minority carrier lifetime and cell efficiency.

Effect of Window-Layer Materials on p-n Junction Location in Cu(In,Ga)Se2 Solar Cells

We report on measurements of junction location in Cu(In,Ga)Se2 (CIGS) solar cells with different window-layer materials by nm-resolution electrical potential/field profiling across the junction using Kelvin probe force microscopy imaging on cross-section of the devices. The results illustrate that the device with a CdS window layer (CdS/CIGS) has a buried homojunction located inside the CIGS absorber with ∼40-nm junction depth, whereas the ZnOS/CIGS devices with and without partial electrolyte treatment prior to the window-layer deposition are similar, exhibiting a heterointerface junction. This junction location may contribute in part to the highest efficiency of the CdS/CIGS device among the three devices.