Cd-free Buffer Layers Research Articles

Investigating the potential of WO3 and WS2 as Cd-free buffer layers in Sb2Se3-Based thin-film solar cells: A numerical study with SCAPS-1D software

In this numerical study, the performance of thin-film solar cells (TFSCs) based on Sb2Se3 with WO3 and WS2 materials as Cd-free buffer layers was investigated using SCAPS-1D software. WO3 and WS2 were chosen for their suitable band gaps, good electrical conductivity, and optical transparency. The proposed solar cell structures, Au/Sb2Se3/WO3/ITO and Au/Sb2Se3/WS2/ITO were theoretically examined, considering various factors such as conduction band offset, absorber and buffer layer thickness, doping concentrations, series and shunt resistances, temperature, activation energy, and C–V characteristic. The conduction band offset at the absorber/buffer interface showed spike-like configurations, with values of 0.38 eV and 0.23 eV for the WO3 and WS2 buffer layers, respectively. The optimized thickness for the absorber layer in both structures was 1 μm, while for WO3 and WS2 buffer layers, it was 0.1 μm and 0.05 μm, respectively. The optimized acceptor and donor doping concentrations were 1 × 1016 cm−3 and 1 × 1018 cm−3, respectively. The efficiency of the proposed photovoltaic devices was found to be 9.538 % (with Voc = 0.561 V, Jsc = 30.856 mA/cm2, FF = 55.84 %) and 10.151 % (with Voc = 0.558 V, Jsc = 29.189 mA/cm2, FF = 62.34 %) for structures with WO3 and WS2 buffer layers, respectively. These results suggest that using non-toxic materials like WO3 and WS2 as buffer layers can be an efficient and environmentally friendly alternative to toxic CdS for fabricating cost-effective and highly efficient Sb2Se3-based TFSCs.

Enhancing the properties of Cd-free MgZnS buffer for solar cells by co-sputtering ZnS and Mg targets

The buffer is an essential component that connects the window and absorber layers in thin film solar cells, which has significant impacts on the performance of cell device. CdS is widely used buffer material, but it has drawbacks of toxicity and narrow band gap of 2.4 eV. This work explores the fabrication of a novel Cd-free MgZnS buffer layer through co-sputtering ZnS and Mg targets. The grain size, morphology, phase composition, transmittance and band gap of MgZnS films are manipulated by adjusting the substrate temperature, sputtering power of ZnS target and sputtering current of Mg target. The MgZnS film exhibits remarkable characteristics, including high crystallinity with grain size of about 35 nm, high transmittance of over 80%, and high band gap of 3.44 eV. The process parameters with favorable outcomes include substrate temperature of 300 °C, ZnS sputtering power of 100 W, and Mg sputtering current of 0.05 A. Furthermore, compared with the CdS buffer, the MgZnS buffer has better compactness and transmittance. This method provides a promising new and effective avenue to fabricate Cd-free buffers for solar cells.

Simulation and performance analysis of CdTe thin film solar cell using different Cd-free zinc chalcogenide-based buffer layers

In this study, the performance of a Cadmium Telluride (CdTe) thin-film solar cell was evaluated with various buffer layers (CdS, ZnSe, ZnO, ZnS) using the Silvaco-Atlas semiconductor device simulator. The impact of these buffer layers on the functionality of the CdTe solar cell was scrutinized at a temperature of 300 K under standard AM1.5G illumination. Initially, the modeling and simulation results of CdS/CdTe solar cell were examined, in comparison with the experimental and simulation results previously reported, to validate our reference structure and to explore its performance. The baseline CdS/CdTe solar cell, serving as a benchmark, yielded a conversion efficiency of 22.14 %. Then we attempted to identify an environmentally friendly alternative buffer layer for the CdTe thin film solar cell by substituting CdS with various Cd-free zinc chalcogenide-based materials (Zn(O,S,Se)) to boost efficiency. By trimming the layer thickness to approximately 0.01 μm, conversion efficiencies of 23.04 %, 23.13 % and 24.48 % were attained for the ZnO, ZnSe, and ZnS buffer layers, respectively. In order to investigate the performance of each proposed structure, thickness and doping density of the absorber and buffer layer were varied. Best efficiency around 25.23 % and 25.04 % are achieved for the optimized solar cells for ZnS and ZnSe buffer layer, respectively. As a result, ZnS and ZnSe are promising materials can be used as an alternative to the commonly used CdS buffer layer in CdTe solar cell. The influence of the operating temperature on solar cell performance was also investigated.

Numerical modeling and performance analysis of a novel Cd-free all-Kesterite tandem solar cell using SCAPS-1D

Tandem solar cells have the potential to surpass conventional single-junction photovoltaics by harnessing a wider range of the solar spectrum and reducing losses caused by thermalization and transmission. This study examines the performance of a kesterite-kesterite tandem solar cell with two different absorber layers in each subcell using SCAPS-1D. The upper subcell comprises CZTS and ACZTS with wider bandgaps, while the lower subcell comprises ACZTSe and CZTSe with narrower bandgaps. The CZTS- and CZTSe-based solar cells were first validated using experimental data to determine the feasibility of the tandem solar cell design. The CZTS-based solar cell was modified by replacing the back contact, adding a Cd-free buffer layer of ZnMnO, and using a SnMnO2 layer as a window layer. The band gap and electron affinity of ZnMnO and SnMnO2 layers were adjusted by changing their manganese contents for optimal conduction band offset. A thin ACZTS layer was also added as a second absorber layer in the CZTS-based solar cell to enhance open circuit voltage. The CZTSe-based solar cell was thereafter modified by including an ACZTSe layer as a second absorber layer and by selecting ZnSe as a buffer layer and optimizing the thicknesses of all layers. Both upper and lower subcells were analyzed for tandem configuration using filtered spectra and current-matching techniques. The thickness of the CZTS layer in the upper subcell was adjusted to achieve current matching; consequently, the proposed tandem cell had an efficiency of ∼24%. These findings provide valuable insights for future advancements in kesterite-kesterite-based tandem solar cells.

All-Vacuum-Deposited Bifacial Cu2ZnSnSe4 Photovoltaic Cells with Sputtered Cd-Free Buffer Layer

By depositing metal precursors on fluorine-doped tin oxide substrates using evaporation and postselenisation and modifying the number of stacked metallic precursor layers, this study systematically analysed the effect of cation changes on the absorber layer and the solar cell properties of Cu2ZnSnSe4 (CZTSe). Furthermore, in this study, an all-vacuum method was adopted to prepare a cadmium-free bifacial CZTSe solar cell and conducted damp heat tests on the device. The findings indicate that the increase in the number of stacked metallic precursor layers suppresses secondary phase generation, thereby enhancing the film performance and bifacial solar cell characteristics. The cadmium-free bifacial CZTSe solar cell prepared in this study exhibited the efficiency of 4.13% under bifacial incident light. Moreover, the degradation of the device was effectively mitigated in the damp heat tests compared with previously reported devices with metal substrates. These findings can provide new directions for research and lead to novel applications for CZTSe solar cells in the future.

Device modeling and investigation of Sb-based low-cost heterojunction solar cells using SCAPS-1D

Antimony sulfide (Sb2S3) and antimony selenide (Sb2Se3) solar cells are considered as emerging photovoltaic devices due to their earth abundance, low cost, non-toxic property and high optical absorption. Also, the buffer layer for the solar cells should be non-toxic. Hence, the need to have a Cd-free buffer layer is a growing interest among the researchers. In this study, we modeled Mo/Sb2S3/TiO2/FTO and Mo/Sb2Se3/TiO2/FTO solar cells and theoretically calculated the effect of different device parameters on the properties of solar cells by SCAPS-1D (Solar Cell Capacitance Simulator) software. By optimizing different properties of Sb2S3 and Sb2Se3likethickness, hole mobility, recombination defect density and their interface, a solar cell efficiency beyond 8 % could be achieved. The optimized thicknesses for Sb2Se3 and Sb2S3absorberswere found to be 900 nm and 300 nm, respectively. A maximum efficiency of 8.67 % and 9.4 % were obtained for Sb2S3 and Sb2Se3 based solar cells, respectively after optimizing all the parameters. These results obtained by simulation study gives us useful insights about the designing and fabrication of Sb2S3 and Sb2Se3 based solar cells.

Micro-mechanism study of the effect of Cd-free buffer layers ZnXO (X = Mg/Sn) on the performance of flexible Cu2ZnSn(S, Se)4 solar cell

The traditional CdS buffer layers in flexible CZTSSe solar cells lead to light absorption losses and environmental pollution problems. Therefore, the study of Cd-free buffer layer is very important for the realization of environmentally friendly and efficient CZTSSe solar cells. The Zn1–x Mg x O (ZnMgO) and Zn1–x Sn x O (ZnSnO) alternate buffer layers are studied in this study using the simulation package solar cell capacitance simulator (SCAPS-1D) numerical simulation model, and the theoretical analysis is further verified by the results of the experiments. We simulate the performance of CZTSSe/ZnXO (X = Mg/Sn) heterojunction devices with different Mg/(Zn+Mg) and Sn/(Zn+Sn) ratios and analyze the intrinsic mechanism of the effect of conduction band offsets (CBO) on the device performance. The simulation results show that the CZTSSe/ZnXO (X = Mg/Sn) devices achieve optimal performance with a small “spike” band or “flat” band at Mg and Sn doping concentrations of 0.1 and 0.2, respectively. To investigate the potential of Zn0.9Mg0.1O and Zn0.8Sn0.2O as alternative buffer layers, carrier concentrations and thicknesses are analyzed. The simulation demonstrates that the Zn0.9Mg0.1O device with low carrier concentration has a high resistivity, serious carrier recombination, and a greater impact on performance from thickness variation. Numerical simulations and experimental results show the potential of the ZnSnO buffer layer as an alternative to toxic CdS, and the ZnMgO layer has the limitation as a substitute buffer layer. This paper provides the theoretical basis and experimental proof for further searching for a suitable flexible CZTSSe Cd-free buffer layer.

N-type SnS2 thin films spray-coated from transparent molecular ink as a non-toxic buffer layer for solar photovoltaics

This paper reports the effect of solvent evaporation temperature on spray-coated tin disulfide (SnS2) thin films from molecular ink. Thiourea and tin chloride were the key chemical reagents used for the synthesis of SnS2 transparent ink under atmospheric conditions. The structural and compositional properties of SnS2 thin films revealed formation of pristine hexagonal SnS2. The films are smooth, homogeneous resulting in band gaps ranging from 2 to 2.22 eV suited for a Cd-free alternative buffer layer for Cu-based multicomponent solar cells. Thermoelectric power measurement showed that tin disulfide films exhibit n-type conductivity. Activation energy estimated from temperature variation of electrical conductivity measurement varied from 40 to 90 mV. Our results suggest that ink-processed SnS2 can be used as a potential alternative for opto-electronic devices such as thin film solar cell and photodetector devices.

Efficiency enhancement of Cd-free buffer layers on CIGS solar cell performance using WxAMPS

In this paper, the impact of three buffers structures layers on the CIGS solar cell performance was numerically investigated using the one-dimensional simulation program wxAMPS. For Cd-free buffer layers, ZnS,µc_3C_SiC and Mg0.19Zn0.81O have been proposed as alternative buffers layers materials to replace the currently toxic CdS material used for the CIGS solar cells. A scrupulous optimization strategy based on the performance analysis of the thickness and doping concentration density effects was applied to each layer of the proposed stack solar cell structure ZnO/ Buffer layer/ CIGS. Moreover, the impact of the series resistance (Rs) was also examined and taken into account in the optimization process. As known, the combination of the parameter optimization (doping concentration density, thickness) with the lowest series resistance provides a minimum loss resistivity and high optical throughput. Consequently, the maximum achieved conversion efficiencies values for the three investigated solar cell structures (ZnS, µc_3C_SiC and Mg0.19Zn0.81O) were 31.16%,30.42% and 27.52%, respectively. The optimum value of 2µm was selected for the CIGS thickness while for the CIGS doping concentration density, the optimum values chosen were: 5×1018cm−3,3×1019cm−3 and 1×1018cm−3, respectively. These efficiencies parameters were obtained with a thickness absorber of 2μm, a buffer thickness layer of 40nm, and a ZnO window layer of 20nm.

Investigations on the effect of buffer layer on CMTS based thin film solar cell using SCAPS 1-D

Research in the field of thin film photovoltaic has been stimulated because of the low production cost and less material consumption and it has been accelerating after the exploration of chalcogenide materials viz. CIS, CIGS, CdTe and CZTS. But, toxicity of Cadmium, high cost of Indium and Gallium and plenty of barriers in improvement of efficiency in case of CZTS gave rise to research in similar and less explored chalcogenide materials and candidate material among them. Copper Manganese Tin Sulphide (CMTS) is one of the promising chalcogenide materials for development of low-cost, thin-film solar cells. However, being a new material, there are not many reports on the optimisation of device structure for this materials system. Particularly there is no systematic studies on the material-compatibility as far as the buffer layer is concerned for the CMTS based solar cell. In this work, we studied the effect of the buffer layer (CdS, Zn(O,S) and SnS2) on the performance of the CMTS based thin film solar cell device by using SCAPS 1-D simulation. We report that the device with SnS2 as a buffer layer shows comparatively high power conversion efficiency, that is 20.26%. Subsquently, the device structure of CMTS solar cell with SnS2 buffer layer was optimized for various physical parameters, viz. Thickness of the absorber and buffer layer, acceptor density of the absorber layer, donor density of the buffer (SnS2) layer, and the defect density. These results could be helpful in designing and experimentally implementing the CMTS based low-cost thin-film solar cell with Cd-free buffer layer.

Zn(O,S) Buffer Layer for in Situ Hydrothermal Sb2S3 Planar Solar Cells.

Hydrothermal deposition is emerging as a highly potential route for antimony-based solar cells, in which the Sb2(S,Se)3 is typically in situ grown on a common toxic CdS buffer layer. The narrow band gap of CdS causes a considerable absorption in the short-wavelength region and then lowers the current density of the device. Herein, TiO2 is first evaluated as an alternative Cd-free buffer layer for hydrothermally derived Sb2S3 solar cells. But it suffers from a severely inhomogeneous Sb2S3 coverage, which is effectively eliminated by inserting a Zn(O,S) layer. The surface atom of sulfur in Zn(O,S) uniquely provides a chemical bridge to enable the quasi-epitaxial deposition of Sb2S3 thin film, confirming by both morphology and binding energy analysis using DFT. Then the results of the first-principles calculations also show that Zn(O,S)/Sb2S3 has a more stable structure than TiO2/Sb2S3. The resultant perfect Zn(O,S)/Sb2S3 junction, with a suitable band alignment and excellent interface contact, delivers a remarkably enhanced JSC and VOC for Sb2S3 solar cells. The device efficiency with the TiO2/Zn(O,S) buffer layer boosts from 0.54% to 3.70% compared with the counterpart of TiO2, which has a champion efficiency of Cd-free Sb2S3 solar cells with a structure of ITO/TiO2/Zn(O,S)/Sb2S3/Carbon/Ag by in situ hydrothermal deposition. This work provides a guideline for the hydrothermal deposition of antimony-based films upon a nontoxic buffer layer.

Optical and Structural Characterization of Cd-Free Buffer Layers Fabricated by Chemical Bath Deposition

Chemical bath deposition (CBD) is a suitable, inexpensive, and versatile synthesis technique to fabricate different semiconductors under soft conditions. In this study, we deposited Zn(O;OH)S thin films by the CBD method to analyze the effect of the number of thin film layers on structural and optical properties of buffer layers. Thin films were characterized by X-ray diffraction (XRD) and UV-Vis transmittance measurements. Furthermore, we simulated a species distribution diagram for Zn(O;OH)S film generation during the deposition process. The optical results showed that the number of layers determined the optical transmittance of buffer layers, and that the transmittance reduced from 90% (with one layer) to 50% (with four layers) at the visible range of the electromagnetic spectrum. The structural characterization indicated that the coatings were polycrystalline (α-ZnS and β-Zn(OH)2 to four layers). Our results suggest that Zn(O;OH)S thin films could be used as buffer layers to replace CdS thin films as an optical window in thin-film solar cells.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Theoretical modelling and device structure engineering of kesterite solar cells to boost the conversion efficiency over 20%

Among the several obstacles which impede efficiency enhancement of CZTS solar cells, the sub-optimized Mo back contact and the unfavorable conduction band offset at CZTS/CdS interface are the critical ones, contributing largely to high surface recombination and low open circuit voltage. In this article, a numerical simulation model was used for device structure analysis and performance optimization. A substrate configuration of CZTS solar cell was considered. Benchmarking study of the reference cell gives a conversion efficiency of 8.3% which matches well with the reported experimental value, validating the accuracy of our simulation model. Thereafter, we have implemented several modifications in the experimental device structure including an alternative back contact, a back surface passivation layer, a Cd-free buffer layer with tunable conduction band minimum, and a wide band gap ZnO-based alloy material which enhances the spectral response to shorter wavelength contributing to higher efficiency. A systematic optimization of device structure results in the increase of JSC and VOC due to the increment in diffusion length and optimal free carrier collection which ultimately increases the power efficiency from 8.4 to 20.6%.

Theoretical evaluation of emerging Cd-free Cu3BiS3 based solar cells using experimental data of chemically deposited Cu3BiS3 thin films

The present research involves a combined experimental and theoretical study to evaluate the optoelectronic properties of Cu3BiS3 (CBS) semiconductor as an effective solar absorber in thin film photovoltaics. Our study consists of the synthesis and characterization of CBS films, followed by the execution of performance analysis as absorber material in a single heterojunction solar cell using numerical simulation by loading the individual material parameters for each layer of CBS solar cell into SCAPS-1D software. In the case of CBS layer, we considered the physical properties of CBS obtained experimentally in this work. The active layer was obtained by thermal diffusion of chemically deposited Bi2S3/CuS binary stacks at 300 °C and 350 °C in an inert atmosphere. Energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy studies confirm the appropriate composition as well as the formation of well-crystalline and single-phase orthorhombic Cu3BiS3 structure. Promising electrical properties were revealed by Hall effect measurements. The performance of the CBS layer was investigated considering the traditional CdS buffer layer and a Cd-free buffer layer such as Bi2S3. The higher conversion efficiency of 7.6% was obtained for CBS/Bi2S3 heterostructure compared with 6.8% efficiency using CdS buffer. A suitable band alignment at the CBS/Bi2S3 interface and ohmic back contact at Mo/CBS junction resulted to be mainly responsible for the enhanced efficiency.

Promising Cd-free double buffer layer in CZTSSe thin film solar cells

Zn(O,S) film is widely used as a Cd-free buffer layer for kesterite thin film solar cells due to its low-cost and eco-friendly characteristics. However, the low carrier concentration and conductivity of Zn(O,S) will deteriorate the device performance. In this work, an additional buffer layer of In2S3 is introduced to modify the properties of the Zn(O,S) layer as well as the CZTSSe layer via a post-annealing treatment. The carrier concentrations of both the Zn(O,S) and CZTSSe layers are increased, which facilitates the carrier separation and increases the open circuit voltage (VOC). It is also found that ammonia etching treatment can remove the contamination and reduce the interface defects, and there is an increase of the surface roughness of the In2S3 layer, which works as an antireflection layer. Consequently, the efficiency of the CZTSSe solar cells is improved by 24% after the annealing and etching treatments. Simulation and experimental results show that a large band offset of the In2S3 layer and defect energy levels in the Zn(O,S) layer are the main properties limiting the fill factor and efficiency of these CZTSSe devices. This study affords a new perspective for the carrier concentration enhancement of the absorber and buffer layers by In-doping, and it also indicates that In2S3/Zn(O,S) is a promising Cd-free hybrid buffer layer for high-efficiency kesterite solar cells.

Over 11 % efficient eco-friendly kesterite solar cell: Effects of S-enriched surface of Cu2ZnSn(S,Se)4 absorber and band gap controlled (Zn,Sn)O buffer

Abstract For high efficiency kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell, CdS thin film was usually used as a buffer layer. However, due to the toxicity of Cd and pollution problems involved from the solution-based chemical bath deposition, eco-friendly high efficiency CZTSSe solar cell with Cd-free buffer is necessary. As an Cd-free buffer layer, we investigated (Zn,Sn)O (ZTO) film deposited by sputtering method. In order to achieve high power conversion efficiency, we controlled energy band gaps of CZTSSe absorber as well as ZTO buffer, which was required to optimize conduction band offset (CBO) between the absorber and the buffer and to increase open circuit voltage (Voc) and fill factor (FF). The CBO was optimized by controlling the band gap of ZTO. By varying the Sn/(Zn + Sn) ratio and its deposition temperature, band gap of ZTO was successfully adjusted. Experimental and computational calculation results showed that solar cell performance was strongly affected by the CBO between absorber and buffer. Besides CBO matching, larger band gap of ZTO improved short circuit current density (Jsc) with enhanced external quantum efficiency value in blue photon spectrum range. As an additional way to improve power conversion efficiency of solar cell, band graded CZTSSe absorber was developed by using spray-based two-step process. The sprayed CZTSSe film was engineered to have S-enriched surface, which makes surface band gap widened and surface passivated, and resultantly increases Voc, Jsc and fill factor (FF). By controlling band gaps of both CZTSSe absorber and ZTO buffer, we obtained 11.22% environment-friendly CZTSSe solar cell without MgF2 anti-reflection coating.

Impact of Hydrogen flow rate on physical properties of ZnS thin films: As potential buffer layer in solar cells

The exceptional need of potential Cd-free buffer layer in thin film solar cell devices motivated us to study the role of post-deposition Hydrogen annealing for the optimization of physical properties of ZnS thin films. The deposited films of thickness 200 nm were hydrogenated within the flow rate range of 50.0–150.0 sccm at 200 °C. XRD analysis revealed transformation of amorphous into cubic phase with maximum crystallinity at 150.0 sccm for films deposited on glass substrate while into wurtzite structure for films on ITO substrate with enhanced crystallinity. A mixed phase (cubic and hexagonal) at 150.0 sccm also appeared. Electrical behaviour (I–V) exhibits ohmic nature with maximum carrier concentration at 100.0 sccm. The blue shift in absorption edge and maximum of 95% transmittance were recorded in the visible region with optical energy band gap of 3.41 eV at 150.0 sccm. The reduction in surface roughness is observed in surface topographical analysis while the photoluminescence (PL) study indicated a sharp peak at 2.95 eV with strongest emission for 150.0 sccm attributed to reduction of defects at interstitial sites and passivation of grain boundaries. These results are useful to understand the Hydrogen related impurities in ZnS films and the improvement caused by hydrogenation to physical properties suited for buffer layer in solar cells.

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

High efficiency chalcopyrite thin film solar cells generally use chemical bath deposited CdS as buffer layer. The transition to Cd-free buffer layers, ideally by dry deposition methods is required to decrease Cd waste, enable all vacuum processing and circumvent optical parasitic absorption losses. In this study, Zn1−xMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se2(CIGS) absorbers. A specific composition range was identified for a suitable conduction band alignment with the absorber surface. We elucidate the critical role of the CIGS surface preparation prior to the dry ALD process. Wet chemical surface treatments with potassium cyanide, ammonium hydroxide and thiourea prior to buffer layer deposition improved the device performances. Additional in-situ surface reducing treatments conducted immediately prior to Zn1−xMgxO deposition improved device performance and reproducibility. Devices were characterised by (temperature dependant) current-voltage and quantum efficiency measurements with and without light soaking treatment. The highest efficiency was measured to be 18%.

Unraveling interface characteristics of Zn(O,S)/Cu(In,Ga)Se2 at nanoscale: Enhanced hole transport by tuning band offsets

Environment-friendly Cu(In,Ga)Se2 (CIGS) solar cells requires the replacement of Cd-containing buffers with non-toxic materials. Zn(O,S) buffers have been developed and yielded even better efficiency than CdS-buffered CIGS thin-film solar cells [23.35%, Ref. [6]]. In this work, we studied band offsets of Zn(O,S) and CIGS interfaces. The Cd-free buffer layers were deposited with 1.0%, 1.3%, and 1.6% oxygen (O2) gas partial pressure during the deposition. Effects of the oxygen partial pressure on the structure and electronic properties of the devices were investigated by micro-Raman scattering spectroscopy and Kelvin probe force microscopy, respectively. We achieved depth-profiling of spatial work function mapping across the interface between the absorbers and the buffers. The best efficiency sample, grown using 1.3% of oxygen, showed 80 mV spike-like band offsets. We propose that the efficiency can be improved through tailoring of the band offsets at the interface as well as improving the absorber and the buffer materials.