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The chemical and electronic structure of the interface between a sputter-deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se)2 (CIGSSe) absorber for thin-film solar cells is investigated with X-ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X-ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk-sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface- and bulk-sensitive measurements of the buffer layer suggest the presence of S-Zn and S-O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large-area solar cells.

The Zn(O,S) thin film is considered a most promising candidate for a cadmium-free buffer layer of the Cu(In,Ga)Se2 (CIGS) thin-film solar cell due to its advantages of optical responses in the short-wavelength region and adjustable bandgap. In this paper, the thin-film growth mechanism and process optimization of Zn(O,S) films fabricated using the chemical bath deposition method are systematically investigated. The thickness and quality of Zn(O,S) films were found to be strongly affected by the concentration variation of the precursor chemicals. It was also revealed that different surface morphologies of Zn(O,S) films would appear if the reaction time were changed and, subsequently, the optimum reaction time was defined. The film-growth curve suggested that the growth rate varied linearly with the deposition temperature and some defects appeared when the temperature was too high. In addition, to further improve the film quality, an effective post-treatment approach was proposed and the experimental results showed that the microstructure of the Zn(O,S) thin film was improved by an ammonia etching process followed by an annealing process. For comparison purposes, both Zn(O,S)-based and CdS-based devices were fabricated and characterized. The device with a Zn(O,S)-CIGS solar cell after post-treatment showed near conversion efficiency comparable to that of the device with the CdS-CIGS cell. [ABSTRACT FROM AUTHOR]

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se2 interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se2 interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH)2, and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar+ ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se2 devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

We performed an investigation on the behaviour of the electrical parameters of ultrathin Cu(In,Ga)Se 2 (CIGS) solar cells buffered with Zn(O,S). Using one-dimensional simulations and by defining a new structure, we achieved a significant reduction of the absorber and the buffer layers to evaluate the changes in the cell’s performance. The simulation results revealed that a good optimisation of the thickness and sulphur content of the Zn(O,S) buffer layer could be an ingenious way to reduce the thickness of the absorber without compromising the performance of the solar cells. A high efficiency of 16.9% is obtained for 0.5 μ m of the absorber layer when the thickness and sulphur content of the Zn(O,S) layer are 10 nm and 0.9 respectively. At this configuration, we introduced p + -CIGS and SnS layers at the CIGS/Mo interface as back surface field (BSF) to reduce interface recombinations, which are very predominant in ultrathin absorber. An improvement of 2% and 5.96% on the efficiency is obtained with the p + -CIGS and SnS layers respectively. The shape of the band diagram shows good alignment and low band bending at the CIGS/OVC/Zn(O,S) interfaces and the corresponding conduction band offset is + 0.2 eV between the CIGS and the Zn(O,S) layers. Ultrathin CIGS and Zn(O,S) layers can be helpful in improving the stability of the cell with regard to the results obtained from the electrical parameters. [ABSTRACT FROM AUTHOR]

In this study, Zn (O,OH,S) films were synthesized onto ZnO seed layers by chemical bath deposition, which were annealed at 500 °C. The differences of structural, morphological and detailed optical properties of the films were investigated depending on the annealing time (between 30 min. and 90 min.). While samples of 30.min and 90 min. showed decomposed structures, sample of 60 min. showed different dimensions of nano-flower structures. Although all films had ZnO-hexzagonal crystal structure, the most obvious ZnS-related peaks were observed in the sample of 90 min. Optical absorption edge was shifted at 362 nm from Uv-Vis spectroscopy. Although ZnO, Zn(OH)2 vibration related peaks were so sharp, ZnS vibration peaks were so weak for all samples from FTIR. The PL intensities were differential depending on the annealing time but defect state-corresponding peaks were similar for each films. [ABSTRACT FROM AUTHOR]

CdS and Zn(O,S) grown by chemical bath deposition are well-established buffer materials for Cu(In,Ga)Se2 (CIGS) thin-film solar cells and modules. Typically these buffers were combined with a ZnO:Al (AZO) or ZnO:B window layer and i-ZnO or (Zn,Mg)O as high-resistive interlayer. Nowadays, alternative transparent conductive oxide (TCO) materials with higher mobility as compared to AZO like indium zinc oxide (IZO) and hydrogen-doped indium oxide (IOH) are under investigation. In the present contribution, we report on the performance of CIGS cells with solution-grown Zn(O,S) or CdS buffer layers in combination with different stacking sequences including i-ZnO, (Zn,Mg)O, IZO, and IOH. Devices with the commonly used AZO window layer are used as references. Efficiencies above 16% without anti-reflective coating (ARC) are achieved for cells with Zn(O,S)/(Zn,Mg)O combined with IZO, IOH, and AZO whereas poor results are obvious with all Zn(O,S)/i-ZnO combinations. Nevertheless, CdS-buffered reference cells with the different TCO materials exhibit efficiencies in the range of 17-18% without ARC mainly due to higher open-circuit voltages. [ABSTRACT FROM AUTHOR]

Sputtering of Zn(O,S) from ZnO/ZnS compound targets has been proven to be a promising buffer layer process for Cd-free CIGS modules due to easy in-line integration, low cost and high efficiency on lab scale. In this publication, we report on successful upscaling of the lab process to pilot production. A record aperture efficiency of 13.2% has been reached on a 50 × 120 cm2 sized module. Neither a non-doped ZnO layer nor additional annealing steps are required. Moreover, this very reproducible process yields a standard deviation comparable with that of the CdS base line. In contrast to lab experiments, strong performance gain after light soaking has been observed. The light-soak-induced power increase depends on the preparation of the window layer. Accelerated aging tests show high stability of module power. This is confirmed by outdoor testing for 20 months. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The electronic band alignment of the Zn(O,S)/Cu(In,Ga)Se2 interface in high-efficiency thin-film solar cells was derived using X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, and inverse photoemission spectroscopy. Similar to the CdS/Cu(In,Ga)Se2 system, we find an essentially flat (small-spike) conduction band alignment (here: a conduction band offset of (0.09 ± 0.20) eV), allowing for largely unimpeded electron transfer and forming a likely basis for the success of high-efficiency Zn(O,S)-based chalcopyrite devices. Furthermore, we find evidence for multiple bonding environments of Zn and O in the Zn(O,S) film, including ZnO, ZnS, Zn(OH)2, and possibly ZnSe. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Zn(O,S) films were fabricated by oxidizing ZnS thin films deposited by electron beam evaporation method onto glass substrates at temperatures of 350-500℃ for 2 h in an atmosphere of oxygen. The XRD and EDX confirmed that the Zn(O,S) films were obtained successfully. The influence of the oxidization temperature on the optical and electrical properties of the Zn(O,S) thin films was investigated. The experimental results show that the Zn(O,S) thin film oxidized at the temperature of 400℃ exhibits better properties than others, with the transmittance of 86% in the visible region, the band gap energy of 3.36 eV and the resistivity of 3.22 × 103 Ω·cm, which makes it a potential buffer layer of solar cell. [ABSTRACT FROM AUTHOR]

High-efficiency Cu(In,Ga)Se2 (CIGSe) solar cells generally require a CdS buffer layer formed by chemical bath deposition (CBD). However, the incompatibility of this process with in-line vacuum-based production methods is a matter of concern. In this contribution, ZnO1 −xS x thin films synthesized by RF co-sputtering of ZnO and ZnS targets in a confocal configuration are studied as a buffer layer in CIGSe solar cells. It was found that the sulfur content x can be easily tuned by controlling the power applied to the ZnO and ZnS targets. The band structure of the Zn(O,S) compound as a function of the sulfur content was measured by combining X-ray photoelectron spectroscopy and optical analysis techniques and found to follow the trends described in the literature. CIGSe solar cells using sputtered Zn(O,S) buffer layers with different sulfur contents were fabricated. The variation of the electrical parameters of the devices as a function the x ratio was attributed to the evolution of the conduction-band offset (CBO) at the CIGSe/ZnO1 −xS x interface, as confirmed by means of SCAPS numerical simulations. However, even with an optimized band alignment between the two materials, relatively low Voc values were obtained, likely due to defect assisted interface recombination at the buffer/window layer interface. [ABSTRACT FROM AUTHOR]

ABSTRACT In an effort to reduce the complexity and associated production costs of Cu(In,Ga)Se2 (CIGSe)-based solar cells, the commonly used sputtered undoped ZnO layer has been modified to eliminate the requirement for a dedicated buffer layer. After replacing the ZnO target with a mixed ZnO/ZnS target, efficient solar cells could be prepared by sputtering directly onto the as-grown CIGSe surface. This approach has now been tested with high-quality lab-scale glass/Mo/CIGSe substrates. An efficiency of 18.3% has been independently confirmed without any post-deposition annealing or light soaking. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Zn(O,S) is a promising candidate to replace the commonly used CdS buffer layer for Cu(In,Ga)Se2 (CIGS) thin-film solar cells due to its non-toxicity and its potential to enhance the conversion efficiency of the CIGS solar cell. The composition of chemical bath deposited (CBD) and sputtered Zn(O,S) layers with thicknesses well below 100 nm was determined by sputtered neutral and secondary ion mass spectrometry (SNMS and SIMS). Despite numerous mass interferences of double-charged atoms and dimers with single Zn, O and S isotopes, we developed an evaluation algorithm for quantification of SNMS depth profiles of Zn(O,S) layers. In particular, the superposition of double-charged S and Zn atoms with O and S isotopes is accounted for numerically in the quantification procedure. For sputtered Zn(O,S) layers, the S/(S + O) atomic ratio and the vertical composition profile can be controlled by the O2 content in the gas flow and the substrate temperature during sputtering whereas for CBD Zn(O,S) the S/(S + O) ratio is constant around 0.7-0.8. A Cu-depleted layer of about 5 nm on the CIGS surface after buffer deposition was observed for both preparation methods. With negative SIMS, we found more hydroxides and carbon residues in CBD Zn(O,S) as compared to sputtered layers. Best cell performance with sputtered Zn(O,S) layers was achieved for S/(S + O) ratios of 0.25-0.40, yielding efficiencies up to 13%. Our solar cells with CBD Zn(O,S) buffers exhibit higher efficiencies due to an improved open-circuit voltage. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a study of synthesis of thin films of Zn(O;OH)S and In(O;OH)S deposited by chemical bath deposition (CBD) is presented. The thin films of Zn(O;OH)S and In(O;OH)S were deposited from different chemical bath systems on absorber layers of CuInS2 (CIS), indium tin oxide substrates (ITO) and soda lime glass substrates (SL). The differences on the growth rate, optical, morphological and structural properties of the thin films Zn(O;OH)S and In(O;OH)S are studied. The Growth studies showed that thin films of Zn(O;OH)S and In(O;OH)S grown faster on CIS than on SL and ITO substrates. The optical and morphological studies showed that both thin films present high transmittance in visible electromagnetic spectrum and covered uniformly the surface of the substrate, furthermore it was observed that thin films of Zn(O;OH)S and In(O;OH)S were polycrystalline. Finally, the results suggest that thin films of Zn(O;OH)S and In(O;OH)S obtained in this work could be used as buffer layer to replace the thin films of CdS, which are conventionally used as buffer layer in chalcopyrite based solar cells. [Copyright &y& Elsevier]