Chalcopyrite Thin Films Research Articles

Identifying Key Parameters for Reducing Recombination Losses at the p - n Junction of Chalcopyrite Thin-Film Solar Cells

With a record efficiency of 23.64% achieved in the past year, Cu(In,Ga)Se2 chalcopyrites are the absorbers of choice for thin-film solar cells. In these devices, the p-type chalcopyrite absorber and the n-type window layer form the p-n junction at the interface, which is mediated by a thin buffer layer. Due to its close proximity to the p-n junction, the defect electronic properties of the absorber surface and its interface with the buffer layer are of significant importance to limit recombination losses of photogenerated carriers. Furthermore, the recent surge in the efficiency gains have been achieved with the aid of heavy alkali-metal elements, which are implemented via a postdeposition treatment of the absorber surface. Due to the complex defect physics of chalcopyrite materials, the effects and involved processes of these alkali-metal treatments are so far not well understood. As a fundamental requirement, understanding the intrinsic defect physics of chalcopyrite absorbers and how it is influenced by various surface treatments is of significant importance. In fact, it can provide a basis to develop knowledge-based strategies to reduce recombination losses due to the presence of electronic defect levels in the vicinity of the p-n junction. To experimentally access such defects at the surface, it is essential that the chosen characterization techniques possess high surface sensitivity. Furthermore, due to the pronounced crystallographic lateral inhomogeneities of the polycrystalline chalcopyrite thin films, a high spatial resolution is desirable. A combined analytical approach using scanning probe microscopy and spectroscopy or Kelvin force microscopy and photoelectron spectroscopy methods can provide valuable insights. Despite the extensive literature available on related topics, a consolidated view of the various findings made in this regard and the direct consequences for the device efficiency is still lacking. Our review addresses the significant progress made in understanding the defect electronic properties of the chalcopyrite surface in the past decade. The findings are consolidated into five sections, which illuminate several external sources of electronic defects and surface-passivation procedures. This allows to develop specific strategies to reduce interface recombination losses for device optimization and improved solar-cell efficiencies. Published by the American Physical Society 2024

An Effect of Al on the Properties of ZnIn2Se4 Thin Film

Abstract Zinc-indium-selenide ZnIn2Se4 (ZIS) ternary chalcopyrite thin film on glass with a 500 nm thickness was fabricated by using the thermal evaporation system with a pressure of approximately 2.5×10−5 mbar and a deposition rate of 12 Å/s. The effect of aluminum (Al) doping with 0.02 and 0.04 ratios on the structural and optical properties of film was examined. The utilization of X-ray diffraction (XRD) was employed to showcase the influence of aluminum doping on structural properties. XRD shows that thin ZIS-pure, Al-doped films at RT are polycrystalline with tetragonal structure and preferred (112) orientation. Where the degree of crystallinity is raised. The effect of Al content on surface roughness and average diameter was studied by atomic force microscopy (AFM), which showed that both surface roughness and average diameter increased with an increasing Al ratio until they reached their maximum values of 12.93 nm and 61.56 nm, respectively. Optical characteristics of ZIS thin film and the impact of aluminum on optical parameters have been investigated, and it was found that the optical energy ( E g opt ), absorption coefficient (α), extinction coefficient (k), refractive index (n), both the real (εr) and imaginary (εi) components of the dielectric constant have values of (1.8 eV), (3.11×104 cm−1), (0.111), (2.3), (5.36), and (0.517), respectively, for ZIS thin films at an Al ratio equal to 0.04.

Aluminum‐Based Chalcopyrite Thin Films and Solar Cells

AbstractAluminum‐based chalcopyrite materials have attracted attention because of the wide controllability range of their material properties and potential for use in energy‐conversion devices. Herein, CuAlSe2‐based thin‐film growth and solar cell device properties are discussed. Ternary CuAlSe2 thin films are relatively unstable and decompose weeks after film growth, even when preserved in a dry box. However, alloying with elemental In led to significant improvements in stability. Cu(In,Al)Se2 (CIAS) solar cells yield better photovoltaic performance than CuInSe2 cells, although the effective range of Al concentration that can improve device performance is narrower than that of elemental Ga in Cu(In,Ga)Se2 (CIGS). A decrease in the alkali metal concentration in CIAS films with increasing Al concentration is observed, indicating that the formation energy of alkali‐metal substitutional defects on the Cu site is high, and/or Al‐related complex defects formed are kinetically stable and difficult to replace with alkali metals once they form. Although the direct observation of alkali metals in the bulk (grain interior) of chalcopyrite CuInSe2 and CIGS films has been difficult to date, this result can serve as indirect evidence of the presence of alkali metals in the bulk of CuInSe2 films.

Development of Sub‐Band in Spray‐Deposited Cr‐Substituted Chalcopyrite Thin Films for Advancing Solar Cell Efficiency

Designing effective strategies for mitigating energy crisis highlights a research gap in current solar cell technologies. This study explores the inherent physicochemical properties of thin films composed of copper gallium sulfide telluride (CuGa1−xCrx(S,Te)2) with chromium substitution, prepared through the chemical spray pyrolysis technique. The investigation showcases how the concept of intermediate band structuring contributes to enhancing solar cell efficiency. This enhancement can be attributed to the remarkable mobility (from 1.80 to 5.48 cm2 V−1 s−1) and conductivity (from 1.00 to 6.06 S cm−1) exhibited by pure and 0.1 chromium thin films, respectively. Notably, the Cr‐0.1‐substituted CGST film displays maximum photoresponsivity of 0.624 AW−1, an external quantum efficiency of 145%, and a detectivity of 2.37E10. The fabricated solar cell is subjected to theoretical simulation using solar cell capacitance simulator‐1D software, revealing an upward trend in efficiency from 7.13% to 11.23% with increasing Cr substitution from 0.025 to 0.1. This efficiency improvement can be ascribed to the reduction in bandgap (from 2.40 to 1.72 eV) and the creation of a sub‐band in CGST due to Cr incorporation, as substantiated by UV–vis and NIR spectroscopy. These outcomes suggest that Cr‐0.1‐substituted CGST holds promise as a potential thin film absorber material in solar photovoltaics.

Luminous Transmittance and Color Rendering Characteristics of Evaporated Chalcopyrite Thin Films for Semitransparent Photovoltaics

The luminous transmittance and the color rendering index of daylight through semitransparent photovoltaic glazing are essential parameters for visual comfort indoors, and they must be considered for different absorber materials that were traditionally developed for opaque solar cells, such as those of the chalcopyrite type. With this aim, various chalcopyrite compounds (CuInSe2, CuInS2 and CuGaS2) were prepared by means of evaporation and then measured to obtain their optical absorption spectra. These experimental data are used here to calculate the solar absorptance (αS), luminous transmittance (τL) and color rendering index (Ra) as a function of the chalcopyrite film thickness. The comparative analysis of the different factors indicates that 70 nm thick CuInSe2 is optimal to guarantee excellent visual comfort (τL = 50% and Ra = 93%) while absorbing as much solar irradiance (αS = 37%) as 130 nm thick CuInS2 or 900 nm thick CuGaS2. The second option (130 nm thick CuInS2) is also considered good (τL = 40% and Ra = 80%), but for CuGaS2, the thickness should be kept below 250 nm in order to obtain a suitable color rendering Ra ≥ 60%.

SILAR deposited Cu2MnSnS4 thin films for sustainable energy applications

A heavy metal-free chalcopyrite (Cu2MnSnS4) thin film has been synthesized via the SILAR method by varying the adsorption and reaction time. The fabricated thin films are characterized by structural, spectral, and optical properties that are analyzed. The X-ray diffraction studies indicated the formation of Cu2MnSnS4 with grain size ranging from (16–18 nm). The Field Emission Scanning Electron Microscopic (FESEM) analysis revealed uniform film formation. Fourier Transformed Infrared Spectral (FTIR) analysis shows that sharp peaks are observed at 552 cm−1 and 1248 cm−1, indicating the presence of metal-sulfur bonds. The peak at 810 cm−1 is due to the resonance interaction of the sulfide ion. The band gap of Cu2MnSnS4 is in the range of 1.96–2.01 eV from UV–visible absorption studies.

Effect of Gallium Content on Diode Characteristics and Solar Cell Parameters of Cu(In1-xGax)(Se0.98Te0.02)2 Thin Film Solar Cells Produced by Three-stage Co-evaporation at Low Temperature

In this study, Cu(In1-xGax)(Se0.98Te0.02)2 thin film solar cells with x values of 0.17, 0.20, 0.23, and 0.26 were successfully produced by three-stage co-evaporation technique at low temperatures. The diode characteristics and solar cell parameters of thin film chalcopyrite solar cells with the structure of SLG/Mo/Cu(In1-xGax)(Se0.98Te0.02)2/CdS/ZnO/ITO/Ni-Al-Ni were investigated by current-voltage measurements. The ideality factor, series resistance, and barrier height were obtained by the Cheung-Cheung method using the current-voltage results measured in the dark at room temperature. Open-circuit voltage, short-circuit current density, fill factor, and the power conversion efficiency of the thin film solar cells were derived from the current-voltage measurements realized by a four-point measurement setup under AM1.5G standards at room temperature. It was found that the increase in the gallium content first decreased the ideality factor, however, it increased again after exceeding the x value of 0.23. While the amount of gallium was increasing, fluctuations were observed in the series resistance values. The barrier height first increased with the increasing amount of gallium and decreased after exceeding the x value of 0.23. The solar cell parameters increased by increasing the x value up to 0.23 and decreased after exceeding this point. It was found that the diode parameters have an effect on each other but the most effective diode parameter was the ideality factor. The efficiency of the Cu(In1-xGax)(Se0.98Te0.02)2 thin film solar cells was increased from 3.7% to 6.3% by increasing the x value from 0.17 to 0.23.

Evaporated Chalcopyrite Thin Films for Indoor Photovoltaic Applications

The feasibility of capturing indoor artificial light using chalcopyrite photovoltaic absorbers has been analyzed. For this purpose, various chalcopyrite compounds (CuInSe2, CuInS2 and CuGaS2) were prepared by evaporation and then measured to obtain their main structural, morphological and optical characteristics. On the other hand, several artificial light sources were selected (incandescent, halogen, fluorescent, high-pressure sodium, metal halide and LED lamps) and represented by their respective spectral radiance. The absorption characteristics of CuInSe2 and CuInS2 are optimal for collecting light from fluorescent lamps and warm or cool white LEDs, requiring only a small film thickness of about 0.6 μm to capture 90-100% of the light radiance. Otherwise, the efficiency of CuGaS2 is found to increase as the color temperature of the LED lamp increases, being always lower and more dependent on the film thickness than for the other evaporated compounds. The worst performances (collection of less than 50% of the light radiance) correspond to incandescent and halogen lamps, which have a significant emission in the infrared region outside the absorption range of the evaporated chalcopyrite films. These results provide guidelines for the design of chalcopyrite-based photovoltaic devices suitable for operation under different indoor lighting conditions.

Back Contact Plasma Treatment Enables 14.5% Efficient Solution‐Processed CuIn(S,Se)2 Solar Cells

AbstractSolution‐processed chalcopyrite thin film solar cells have made great progress but still lag behind the vacuum‐based ones due to inferior absorber quality and non‐ideal interfaces. Here, plasma treatment of the molybdenum (Mo) substrate is reported to modify the back interface and improve the efficiency of solution‐processed copper indium sulfoselenide (CuIn(S,Se)2, CISSe) solar cells. The results show plasma treatment oxides the surface Mo/MoO2 to high oxidation oxides MoO3‐x (0 ≤ x < 1). The higher surface polarity greatly improves the wettability of the substrate to the precursor solution and results in formation of more compact and uniform precursor film, which grows to void‐free larger grains absorber film with better adhesion to the substrate. In addition, the much higher chemical stability of MoO3‐x than Mo significantly reduces the thickness of high resistive MoSe2 layer. The improved absorber quality and back interface property decreases charge carrier recombination in the absorber and back contact interface, resulting in 14.53% efficiency solution‐processed CISSe solar cell, which is improved by 13% compared to device without plasma treatment.

Fabrication of High Quality Bornite and Chalcopyrite Thin Films by Aerosol-Assisted Chemical Vapor Deposition

Fabrication of High Quality Bornite and Chalcopyrite Thin Films by Aerosol-Assisted Chemical Vapor Deposition

Surface Passivation and Detrimental Heat-Induced Diffusion Effects in RbF-Treated Cu(In,Ga)Se2 Solar Cell Absorbers.

Alkali postdeposition treatments of Cu(In,Ga)Se2 absorbers with KF, RbF, and CsF have led to remarkable efficiency improvements for chalcopyrite thin film solar cells. However, the effect of such treatments on the electronic properties and defect physics of the chalcopyrite absorber surfaces are not yet fully understood. In this work, we use scanning tunneling spectroscopy and X-ray photoelectron spectroscopy to compare the surface defect electronic properties and chemical composition of RbF-treated and nontreated absorbers. We find that the RbF treatment is effective in passivating electronic defect levels at the surface by preventing surface oxidation. Our X-ray photoelectron spectroscopy (XPS) data points to the presence of chemisorbed Rb on the surface with a bonding configuration similar to that of a RbInSe2 bulk compound. Yet, a quantitative analysis indicates Rb coverage in the submonolayer regime, which is likely causing the surface passivation. Furthermore, ab initio calculations confirm that RbF-treated surfaces are less prone to oxidation (in the form of Ga, In, and Se oxides) than bare chalcopyrite surfaces. In addition, elemental diffusion of Rb along with Na, Cu, and Ga is found to occur when the samples are annealed under ultrahigh vacuum conditions. Magnetic sector secondary ion mass spectrometry measurements indicate that there is a homogeneous spatial distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after heat treatment. Depth-resolved magnetic sector secondary ion mass spectrometry measurements show that Rb diffusion within the bulk occurs predominantly along grain boundaries. Scanning tunneling and XPS measurements after subsequent annealing steps demonstrate that the Rb accumulation at the surface leads to the formation of metallic Rb phases, involving a significant increase of electronic defect levels and/or surface dipole formation. These results strongly suggest a deterioration of the absorber-window interface because of increased recombination losses after the heat-induced diffusion of Rb toward the interface.

Influence of Al dopant on structural and optical parameters of AgInSe2 thin film

Chalcopyrite thin films ternary Silver Indium Diselenide AgInSe2 (AIS) pure and Aluminum Al doped with ratio 0.03 was prepared using thermal evaporation with a vacuum of 7*10-6 torr on glass with (400) nm thickness for study the structural and optical properties. X-ray diffraction was used to show the inflance of Al ratio dopant on structural properties. X-ray diffraction show that thin films AIS pure, Al doped at RT and annealing at 573 K are polycrystalline with tetragonal structure with preferential orientation (112). raise the crystallinity degree. AFM used to study the effect of Al on surfaces roughness and Grain Size Optical properties such as the optical band gap, absorption coefficient, Extinction coefficient, refractive index, real and imaginary part of dielectric constant were calculated to inspect the influence of the Aluminum on the optical Parameters of AIS thin film. UV/Visible measure show the lowering in energy gap to 1.35 eV for AgInSe2: Al at 573 K this energy gap making these samples suitable for photovoltaic application.

Formation of Cu(In,Ga)S2 chalcopyrite thin films following a 3-stage co-evaporation process

The study aims to investigate the origin of the crystalline and compositional bi-layered structure of CuIn1-xGaxS2 (CIGS) films we previously observed for samples deposited following the 3-stage process. Therefore, the growth of films of different gallium contents (i.e. x) was interrupted at key steps of the 3-stage, namely at the end of the first stage, after half of the second stage and at the end of third stage. Compositional, morphological and structural differences at each stage were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-Ray Spectroscopy (EDX). From these investigations, we propose a model attributing composition plateaus to the formation of diverse Cu-poor crystalline phases during the second stage of the growth process.

A perspective on ordered vacancy compound and parent chalcopyrite thin film absorbers for photoelectrochemical water splitting

Chalcopyrites could fill the gap between the low-cost, poor-efficiency single junction metal oxide photoelectrochemical (PEC) water splitting cells and the high efficiency, yet costly III–V tandems. In this Perspective, we first review the key barriers that must be addressed by the community to enable economical chalcopyrite-based PEC water splitting. Then, we highlight how theoretical modeling can be used to identify promising ordered vacancy compound absorbers with improved energetics compared to their chalcopyrite parents. Finally, we describe how advanced spectroscopic analysis performed on chalcopyrite photocathodes after PEC testing uncovered a new passivation layer candidate for prolonged durability.

Fe-doped CuGaS2 (CuGa1-xFexS2) - Detailed analysis of the intermediate band optical response of chalcopyrite thin films based on first principle calculations and experimental studies

We have theoretically and experimentally studied the tailoring of intermediate bands in the CuGa1-xFexS2 chalcopyrite system. An ab initio density functional theory calculations within GGA + U approach have been performed to model pristine and Fe-doped CuGaS2 to understand their structural and electronic properties and thereby understand the origin of intermediate states produced with Fe doping. Thin films of the pristine and Fe-doped CuGaS2 of various compositions are deposited using chemical spray pyrolysis for the experimental studies. The crystal structure, morphology, and topography of the deposited thin films are examined using Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), respectively. The binding energy and chemical composition of CuGa0.95Fe0.05S2 thin films are determined using X-ray Photoelectron Spectroscopy (XPS). The direct and intermediate band optical responses have been probed through UV–Vis–NIR Spectroscopy. Pristine CuGaS2 exhibits a direct bandgap of 2.43 eV, with formation of intermediate bands in Fe-doped CuGaS2 at 1.82 eV, 1.75 eV, 1.54 eV and 1.49 eV with varying Fe atomic percentage (X = 0.025, 0.05, 0.075, and 0.1 at. %). The optimum Fe concentration is determined to be X = 0.05 (CuGa0.95Fe0.05S2) as it exhibits the highest photocurrent (1.46 mA), electrical conductivity, and intermediate band optical response. The IB formation is further supported by the calculated density of states of the Fe doped CuGaS2 structure.

(Invited) Potentials and Challenges of Wide Bandgap Copper Chalcopyrite Thin Film Absorbers for Photoelectrochemical Water Splitting Applications

Photoelectrochemical (PEC) water splitting has the potential to become an efficient method for renewable hydrogen production. However, the efficiency, projected cost, and durability of lab-scale systems are not yet at the level required to make this technology economically feasible. Amongst all materials studied to date, the chalcopyrite class is arguably one of the best classes for PEC water splitting, as it has already demonstrated low-cost and high photoconversion capabilities as a photovoltaic material. In the context of PEC, our group has shown that co-evaporated 1.65 eV bandgap (EG) CuGaSe2 is capable of evolving hydrogen with Faradaic efficiency greater than 85% and generate photocurrent densities over 15 mA.cm-2, as measured in a 3-electrode configuration in 0.5M H2SO4 under simulated AM1.5G illumination. Unfortunately, CuGaSe2’s narrow EG limits its integration as top absorber into a dual junction stacked PEC device (also known as hybrid photoelectrode, HPE). In the present communication, we report on our latest efforts to synthesize wide-EG (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating saturated photocurrent densities greater than 10 mA/cm2. We present specifically results on EG tunable Cu(In,Ga)(S,Se)2, Cu(In,Ga)S2, CuGa(S,Se)2 and CuGa3Se5. We also discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of bandgap tunable MgZnO n-type buffer layers. We also include recent durability testing of chalcopyrite photocathodes coated with metal oxide protection layers. Finally, we present solid-state techniques and theoretical modeling used by our team to identify possible pitfalls in these material systems and discuss potential paths for improvement.

Mechanistic study of the gas-phase chemistry during the spray deposition of Zn(O,S) films by mass spectrometry

The mass spectrometer, is a powerful tool to identify species and investigate reactions in the gas phase. In this work, the mechanism of aerosol assisted chemical vapor deposition (AACVD) of Zn(O,S) films prepared from H2S and zinc acetylacetonate (Zn(acac)2) precursor solutions is elucidated by mass spectrometry. The thermochemical behavior of Zn(acac)2 is investigated by characterizing the influence of the solvent (H2O or ethanol), the pH value of the precursor solution and the effect of the reactant H2S, and by tracking gaseous intermediate products using mass spectrometry. Based on these results, a proton-promoted thermolysis mechanism for the AACVD Zn(O,S) film formation is then proposed, which is initiated by the hydrolysis with H2O as the first stage, followed either by the rearrangement with an intramolecular proton or by the reaction with an extramolecular proton to produce ZnO or Zn(O,S). A real time mass tracking of the AACVD process reveals that only an adequate amount of H2S promotes the chemical gas-phase decomposition and sulfurization process, while an excess of H2S depletes the gaseous Zn(acac)2 concentration and consequently reduces the film growth rate. The knowledge of the thermal decomposition process helps to optimize synthesis conditions and to adjust film properties to meet the requirement of the application in chalcopyrite or kesterite thin film solar cells.

High Efficiency Solution-Processed Aluminum-Alloyed Chalcopyrite Thin Film Solar Cells

CuInGaSe2 (CIGSe) chalcopyrite is one most efficient materials for light conversion to electricity with a record efficiency surpassing all other thin film photovoltaic technologies, achieving 23.4% efficiency. Record efficiencies of CIGSe are typically achieved using 1.16 eV absorbers even though Shockley-Queisser (SQL) limit predicts an optimum bandgap of around 1.4 eV for a single junction solar cell. In theory, higher efficiencies can be expected from wider gap chalcopyrite, however, all attempts to increase efficiency with wide bandgap Ga-rich CIGSe have led to defective materials. Another alternative to increase the band gap of chalcopyrites is by partial replacement of In with other group-V atoms, such as aluminum (Al). The band gap energy of CuInAlSe2 (CIASe) can be tuned from 1.0 eV (CISe) to 2.7 eV (CASe). In addition, Al is also very appealing to reduce the raw materials' cost since it is much more abundant in the earth’s crust than Ga (Al: 82,000 ppm, Ga: 19 ppm) and significantly cheaper (Al: ~$2/kg, Ga: ~$200/kg).In an attempt to fabricate high-efficiency CIASe devices, we demonstrated an alternative and low cost method by fabricating bandgap tunable Cu(In,Al)(S,Se)2 (CIASSe) utilizing molecular ink consisted of Cu, In, Al, and sulfur containing salts dissolved in methanol. In this study, Cu-In-Al-S containing ink was spin coated into a molybdenum substrate and followed by selenization at high temperature to form selenium-rich CIASSe. We are able to achieve a device with maximum efficiency of 11.6% using CIASSe having relatively low-content of Al (Al/(Al+In)~0.1), significantly higher efficiency than CISSe (no Al) fabricated with the same process (8.3%-efficient). To our understanding, this study is the first demonstration of a relatively high-efficiency Aluminum-alloyed chalcopyrite solar cells through solution-based process. Details of the fabrication process and materials characterization will be discussed.

Electrochemically growth and characterization of CuInTe2 chalcopyrite thin films

Copper indium tellurite (CIT) chalcopyrite compounds were electrochemically grown from an aqueous electrolyte including water-soluble Cu, In, and Te molecular sources onto indium thin oxide-coated glass substrates. CuSO4·5H2O, InCl3, and Na2TeO3 were used as copper, indium, and tellurium sources, respectively. Deposition mechanisms of the CIT thin films are explained by cyclic voltammetry (CV) studies. It is also noted that the effect of deposition potential on the electrical, optical, and structural facilities of the electrodeposited CIT thin films. Energy bandgap of the electrodeposited CIT films is in the range of 0.97–1.83 eV. Stoichiometry of the CIT films deposited at − 0.5, − 0.6, − 0.7, and − 0.8 V is near to CuInTe2. We report that the produced CIT films is polycrystalline nature, and CuInTe2 is a major chalcopyrite phase corresponding to (1 1 2), (2 0 4), and (1 1 6) directions at 2θ ~ 25°, 41°, and 49°, respectively. Hall-effect measurements show that the produced CIT thin films have p-type semiconducting conductivity with the acceptor concentration range of 2.8 × 1017 and 2.8 × 1018 cm−3. The variation of the mobility within 20.4–60.2 cm2/V s can be explained by the variation of Cu/In ratio within 2.19–0.59. The resistivity of the films is found to vary within 0.011–0.036 Ω cm, which is in good agreement with the literature data.

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%.