Empowering Solar Cells With Non‐Toxic Cu 2 O and Zn(O,S): A Sustainable Approach for CIGS Solar Cells

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

Experimental investigation of Cu(In 1− x,Ga x)Se 2/Zn(O 1− z,S z) solar cell performance

Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers have been intensively studied due to their high power conversion efficiencies. CIGS solar cells with Zn(O,S) buffer layers achieved record efficiencies due to their reduced parasitic absorption compared with the more commonly used CdS buffer. Accordingly, we have studied solution-grown Zn(O,S) buffer layers on polycrystalline CIGS absorber layers by complementary techniques. A bandgap energy Eg of 2.9 eV is detected by means of angle-resolved electroreflectance spectroscopy corresponding to Zn(O,S), whereas an additional Eg of 2.3 eV clearly appeared for a post-annealed CIGS solar cell (250 °C in air) compared with the as-grown state. To identify the chemical phase that contributes to the Eg of 2.3 eV, the microstructure and microchemistry of the Zn(O,S) buffer layers in the as-grown state and after annealing were analyzed by different transmission electron microscopic techniques on the submicrometer scale and energy-dispersive x-ray spectroscopy. We demonstrate that the combination of these methods facilitates a comprehensive analysis of the complex phase constitution of nanoscaled buffer layers. The results show that after annealing, the Cu concentration in Zn(O,S) is increased. This observation indicates the existence of an additional Cu-containing phase with Eg close to 2.3 eV, such as Cu2Se (2.23 eV) or CuS (2.36 eV), which could be one possible origin of the low power conversion efficiency and low fill factor of the solar cell under investigation.

Adaptation of the surface-near Ga content in co-evaporated Cu(In,Ga)Se2 for CdS versus Zn(S,O)-based buffer layers

CuGaSe2 solar cells using atomic layer deposited Zn(O,S) and (Zn,Mg)O buffer layers

Improving the performance of Cu2Zn(SnyGe1-y)(SxSe1-x)4 solar cells by CdS:Zn buffer layers

Employing Cd-free buffer layer for Cu(In,Ga)Se2 (CIGS) solar cells is very important for improving the device power conversion efficiency and for solving the environmental issues of Cd for mass production. Zn(S,O) buffer layer as one of the most promising alternatives to CdS have shown higher short-circuit current density JSC but much lower open-circuit voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> due to the improper energy alignment with CIGS. In this study, we developed a novel chemical bath deposition process that effectively removes the stationary bubbles and enables the growth of high-quality Zn(S,O) thin film at high temperatures. Devices with Zn(S,O) buffer layer grown at 95 °C have shown V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> comparable to that of devices with CdS buffer layer. Systematic characterization results suggest that our method leads to Zn(S,O) buffer layers with larger oxygen concentration and thus better band alignment with CIGS layer, which well explains the improved V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and power conversion efficiency.

Wide-gap Cu(In0.4,Ga0.6)Se2 solar cells with Zn(O,S) buffer layers deposited by atomic layer deposition (ALD) technique have been investigated. The band-gap energy (Eg) of the Zn(O,S) layer estimated by optical transmission and reflection measurements was varied from 3.2 to 3.6 eV. The solar cells with sulfur (S)-poor Zn(O,S) buffer layers showed a low open-circuit voltage (VOC) owing to the cliff nature of the conduction band offset (CBO). In contrast, the solar cells with S-rich Zn(O,S) buffer layers showed a low short-circuit current density (JSC) owing to the spike nature of CBO. Even if the CBO values were adequate, the best solar cell efficiencies were considerably low. These results suggest that the main cause for the low efficiencies is not interface recombination at the Zn(O,S)/Cu(In,Ga)Se2 interface, but mainly bulk recombination in the Cu(In,Ga)Se2 (CIGS) absorber layer.

Copper Indium Gallium di-Selenide (CIGS) solar cells are a promising approach in photovoltaic technology, having low production costs, high conversion efficiencies (> 20 %), as well as the possibility to manufacture them on flexible substrates. State-of-the-art in CIGS solar cells manufacturing is to use a stack of CdS, intrinsic ZnO (i-ZnO) and an Al-doped ZnO TCO on top of the CIGS film. Replacement of CdS by a non-toxic Cd-free layer with wider band gap (> 2.4 eV) would a) decrease the production cost by avoiding the expensive treatment of toxic wastes and b) increase the overall cell efficiency by enhancing the quantum efficiency in the blue range. Moreover, the use of a “soft” and highly conformal deposition technique is preferred to improve the electrical properties of the buffer layer/CIGS interface. In this paper we present spatial atmospheric atomic layer deposition of a Zn(O,S) buffer layer as CdS replacement for CIGS solar cells. Spatial ALD is emerging as an industrially scalable deposition technique at atmospheric pressure which combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity on large area substrates, with high growth rates (up to nm/s). Films are grown by sequentially exposing the substrate to oxygen and sulfur precursors (H2O, H2S) and the zinc metal precursor (i.e., DEZn). By controlling the kinetics of surface reactions between evaporated precursors and reactive sites at the film surface, the composition of Zn(O,S) can be precisely tuned. The incorporation of S into ZnO results in a bowing of the band gap in the range from 3.3 eV (ZnO) to 2.7 (S/O+S ~ 0.5) and 3.4 eV (ZnS), as measured by spectrophotometry. The morphology of the Zn(Ox-1,Sx) films varies from polycrystalline (for 0<x<30 and 70<x<100) to amorphous (30<x<70), as measured by X-ray diffraction. CIGS solar cells with a Zn(O,S) buffer layer show an increased spectral response around 400 nm compared to solar cells with a CdS buffer layer. The solar cells with the Zn(O,S) buffer layer had an efficiency of 15.9 %, compared to 15.5 % for the reference solar cells with a CdS buffer layer.

Actual Calculation of Solar Cell Efficiencies

Deposition of Zn(O, S) thin film as the Cd-free buffer layer is an important topic in Copper Indium Gallium Diselenide (CIGS) solar cells since it offers the potential enhancements for low cost and good for our environment. For CIGS buffer layer deposition, chemical bath deposition is the most common method in use. However, the incompatibility with in-line vacuum-based production methods is an issue of concerned. In this study, the buffer layer of Zn(O, S) thin films were fabricated by atomic layer deposition (ALD) method with alternately depositing of ZnS and ZnO thin films, using diethyl Zinc((C2H5)2Zn, DEZ), dimethyl sulfide((CH3)2S, DMS), and H2O as the precursors. As the alternative layers were deposited, atomic layer deposition technique exhibits good step-coverage, uniformity and accurate control of layer thickness. By changing the cycle ratios (m/n) of ZnS and ZnO, the oxygen (O)/sulfide(S) composition can be controlled in the Zn(O, S) buffer layer thin film as shown in Fig. 1. Various O/S composition ratios of Zn(O, S) thin films were deposited on the CIGS absorber. The optical properties, microstructure and chemical analyses of the deposited Zn(O, S) buffer layer thin films were carried out by ellipsometry, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The CIGS device performances were measured by current-voltage (I-V) measurements. Keywords: CIGS, Zn(O, S) buffer layer, atomic layer deposition (ALD) References [1] U. P. Singh, S. P. Patra. International Journal of Photoenergy, 2010, 311–330.[2] N. Naghavi, S. Spiering, M. Powalla, B. Cavana, D. Lincot. Progress in Photovoltaics: Research and Applications, 2003, 11, 437-443[3] U. Zimmermann, M. Ruth, M. Edoff. Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, 1831–1834.[4] A. Hultqvist, C. Platzer-Björkman, J. Pettersson, T.Törndahl, M. Edoff. Thin Solid Films, 2009, 517, 2305–2308.[5]J. Lindahl, J. T. Wätjen, A. Hultqvist, T. Ericson, M. Edoff and T. Törndahl. Progress in Photovoltaics: Research and Applications, 2013, 1588-1597. Figure 1

Energy-dispersive X-ray spectroscopy (EDXS) in a transmission electron microscope is frequently used for the chemical analysis of Cu(In,Ga)Se2 (CIGS) solar cells with high spatial resolution. However, the quantification of EDXS data is complicated due to quantification errors and artifacts. This work shows how quantitative EDXS analyses of CIGS-based solar cells with Zn(O,S) buffer and ZnO-based window layers can be significantly improved. For this purpose, CIGS-based solar cells and a reference sample with a stack of Zn(O,S) layers with different [O]/[S] ratios were analyzed. For Zn(O,S), the correction of sample-thickness-dependent absorption of low-energy O–Kα X-rays significantly improves the results of quantitative EDXS. Absorption of characteristic X-rays in CIGS is less relevant. However, for small transmission electron microscopy (TEM) sample thicknesses, artifacts can occur due to material changes by focused-ion-beam (FIB)-based preparation of TEM samples, electron-beam-induced damage, and oxidation of the sample surface. We also show that a Pt-protection layer, deposited on the sample surface before FIB preparation of TEM lamellae, can induce artifacts that can be avoided by first depositing a carbon layer.

Progress in fabricating Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with Zn(S,O) buffer layers prepared by chemical bath deposition (CBD) is discussed. The effect of different Zn salt precursors on solar cell device performance is investigated using production scale CIGSSe absorbers provided by AVANCIS GmbH &amp; Co. KG. The CBD process has been developed at the Hahn‐Meitner‐Institut (HMI) using zinc nitrate, zinc sulphate or zinc chloride as zinc precursor. An average efficiency of 14.2 ± 0.8% is obtained by using one‐layer CBD Zn(S,O) The dominant recombination path for well performing solar cells is discussed based on the results obtained from temperature dependent J (V) analysis. The structure and morphology of buffer layers deposited using zinc nitrate and zinc sulphate has been studied by means of transmission electron micrographs of glass/Mo/CIGSSe/Zn(S,O) structures. Results show a conformal coverage of the absorber by a Zn(S,O) layer of 15–25 nm consisting of nanocrystals with radii of ∼5 nm. XAES analysis of the buffer layer reveals a similar surface composition for buffer layers deposited with zinc nitrate and zinc sulphate. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Zinc oxide sulphide Zn (O, S) buffer layer has attracted enormous attention due to its composition of non‐toxic, low cost, and earth abundant elements, unlike cadmium sulphide (CdS) used in copper zinc tin sulphide (CZTS) solar cells. Zn (O, S) possesses a direct optical bandgap that's adjustable between 2.7 and 3.6 eV related to the mole fraction , making it a potential candidate as a buffer layer with CZTS based solar cells. In this work, the Zn (O, S)/CZTS‐based solar cell structure is numerically studied and compared with CdS/CZTS. The existence of defects and traps along the layers and interfaces is taken in consideration. To achieve optimal results, this research has focused on; effects of the element composition (x = S/(S + O)) ratio of Zn (O, S) buffer layer, the addition of a back surface field (BSF) layer, the influence of Zn (O, S)/CZTS and CdS/CZTS interfaces defect density, varying thicknesses and doping concentrations of the layers (absorber, buffer, BSF) on open‐circuit voltage (Voc), short‐circuit density (Jsc), fill factor (FF), and power conversion efficiency. Findings illustrate that the highest conversion efficiency of the Zn (O, S)/CZTS solar cell reached 14.65% with Voc = 0.9972 V, Jsc = 18.37 mA cm−2 and FF = 79.96% which is better than the CdS/CZTS structure.

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (Eg = 3·6–3·8 eV) than the conventional buffer material CdS (Eg = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS2 (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm2 in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm2 (total area: 22·5 mA/cm2). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm2 substrates have been prepared and already reach an efficiency (active area: 17·2 cm2) of above 8%. Copyright © 2006 John Wiley &amp; Sons, Ltd.