Surface sulfurization studies of Cu(InGa)Se2 thin film

Sulfurization of copper indium gallium diselenide (CIGS) thin films solar cell absorber has been used to enhance the open-circuit voltage of the device by increasing the band gap of the absorber near the interface. Sulfurization of a homogeneous co-evaporated Cu(InGa)Se₂ thin film was studied in hydrogen sulfide and in a mixture of hydrogen sulfide and hydrogen selenide gases with the inclusion of oxygen. The structural and compositional properties of the absorber layer were investigated by XRD, EDS and AES. Sulfurization in hydrogen sulfide gas forms a fully converted sulfide layer at the top of the absorber layer, which in turn forms a barrier for the current collection. Sulfurization in a mixture of hydrogen sulfide and hydrogen selenide gases forms a wide band gap Cu(InGa)(SeS)₂ layer at the surface, but at the same time there is Ga diffusion away from the surface with the inclusion of sulfur at the surface.     

Progress in electrodeposited absorber layer for CuIn(1−x)GaxSe2 (CIGS) solar cells

Thin film solar cells with chalcopyrite CuInSe₂/Cu(InGa)Se₂ (CIS/CIGS) absorber layers have attracted significant research interest as an important light-to-electricity converter with widespread commercialization prospects. When compared to the ternary CIS, the quaternary CIGS has more desirable optical band gap and has been found to be the most efficient among all the CIS-based derivatives. Amid various fabrication methods available for the absorber layer, electrodeposition may be the most effective alternative to the expensive vacuum based techniques. This paper reviewed the developments in the area of electrodeposition for the fabrication of the CIGS absorber layer. The difficulties in incorporating the optimum amount of Ga in the film and the likely mechanism behind the deposition were highlighted. The role of deposition parameters was discussed along with the phase and microstructure variation of an as-electrodeposited CIGS layer from a typical acid bath. Related novel strategies such as individual In, Ga and their binary alloy deposition for applications in CIGS solar cells were briefed.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Performance improvement of CIGS-based modules by depositing high-quality Ga-doped ZnO windows with magnetron sputtering

Improvement of the performance of Cu(InGa)Se₂ (CIGS)-based thin film submodules by depositing high-quality ZnO:Ga (GZO) window layers with sputtering method is performed to aim for establishing deposition technology of GZO windows of CIGS submodules. In order to reduce damage onto CIGS absorber/Zn(O,S,OH)_x buffer interface due to the bombardment of high-energy particles during DC sputtering process of GZO window layers, growth of multilayered GZO window layers is developed. By using RF/DC/DC sputtered GZO window layers instead of the conventional DC sputtered GZO window layers, fill factor (FF) and conversion efficiency are increased over 10%. Increasing short-circuit current density (J_sc) of CIGS submodules is also investigated by improving the transparency of GZO window layers. Furthermore, damp heat test of the sputtered GZO films is carried out, and it is found that the GZO films have good stability of electrical properties.     

Experimental study of graded bandgap Cu(InGa)(SeS)2 thin films grown on glass/molybdenum substrates by selenization and sulphidation

High-performance Cu(InGa)(SeS)₂ (CIGSS) thin film absorbers with an intentionally graded bandgap structure grown by a two-stage method have been studied. Materials obtained from Showa Shell Sekiyu K.K., Japan have been grown using selenization and sulphidation of the Mo/Cu–Ga/In stacked precursors. Full characterizations have been carried out using X-ray diffraction, Raman, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence, inductively coupled plasma mass spectroscopy, glow discharge optical emission spectroscopy (GDOES) and photoelectrochemical (PEC) techniques to study various properties. The material layers were found to be polycrystalline with the (1 1 2) preferred orientation, and the largest grains were about 2 μm. Raman measurements show the presence of at least five different phases within the material. XPS confirmed the copper depletion and the richness of sulphur at the top surface region. Although the PEC studies indicate the overall electrical conductivity of the layer as p-type, GDOES profiling reveals the segregation of different phases at different depths suggesting the possibility of having buried junctions within the material itself. The results are presented together with suggestions for further improvements of CIGSS solar cell material.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

XPS, TEM and NRA investigations of Zn(Se,OH)/Zn(OH)2 films on Cu(In,Ga)(S,Se)2 substrates for highly efficient solar cells

Structural and compositional properties of Zn(Se,OH)/Zn(OH)₂ buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)₂ (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)₂ as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)₂ grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)₂ structure on CIGSS. Part of the Zn(OH)₂ in both layers (i.e. the Zn(Se,OH) and the Zn(OH)₂ layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)₂) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)₂/CIGSS devices exceed 14% (total area).     

High-efficiency Cd-free CIGSS thin-film solar cells with solution grown zinc compound buffer layers

Zn-compounds Zn(X,OH) (X=S,Se) buffer layers have been deposited by chemical bath (CBD) process on Cu(In,Ga)(S,Se)₂ (CIGSS) with the aim of developing Cd-free CIGSS-based devices. The films are produced in alkaline aqueous solution containing ZnSO₄, ammonia NH₃ and XC(NH₂)₂. Optimum deposition conditions were established. The temperature (T_sub) of the chemical bath is found to be critical for the device quality. The thickness and good surface coverage were controlled by XPS-UPS photoemission spectroscopy. SEM study showed that the growth of ZnSe nuclei on CIGSS proceeds in lateral direction. Once the surface is covered the growth takes place in vertical direction . The ZnSe clusters grow in size and their elongated shapes cover the CIGSS surface. High efficiency of over 13% was obtained for both CIGSS/Zn(S,OH) and CIGSS/Zn(Se,OH)-based solar cells. Solar cells with CIGSS/Zn(Se,OH)_x/ZnO/MgF₂ structure show an active area efficiency up to 15.7%. Using Zn(Se,OH) buffer layer, efficiency of 11.7% was achieved with a 20 cm² aperture-area monolithic minimodule.     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Cu(In,Ga)Se2 thin film solar cells with buffer layer alternative to CdS

Progress in fabricating Cu(In,Ga)Se₂ (CIGS) solar cells with ZnS(O,OH) buffer layers prepared by chemical bath deposition (CBD) is discussed in this paper. Such buffer layers could potentially replace CdS in the CIGS solar cell. Total-area conversion efficiency of up to 18.6% has been reported previously using ZnS(O,OH) prepared by CBD. The reported 100 nm CBD ZnS(O,OH) layer was prepared by at least three consecutive depositions, which would make it a relatively expensive replacement for CdS. The recent development of a ZnS(O,OH) layer that enabled to obtain high-efficiency devices using a single-layer CBD is reported in this paper. A 14.4%-efficient device is obtained by using one-layer CBD ZnS(O,OH) on commercial-grade Shell Solar Cu(In,Ga)(S,Se)₂ (CIGSS) absorber and an up to 17.4% device is obtained by using two-layer CBD ZnS(O,OH) on an NREL CIGS absorber.     

Fabrication of pentanary Cu(InGa)(SeS)2 absorbers by selenization and sulfurization

The objective of this study is to find the key factors to improve V_oc. In this study, pentanary Cu(InGa)(SeS)₂ absorbers were prepared by selenization and sulfurization or a sulfurization after selenization (SAS) method. It is found that the “sulfurization degree” defined as a function of temperature and holding time at the sulfurization step is a key factor to enhance the Ga diffusion and improve V_oc. It is also verified that increase in the temperature difference between selenization and sulfurization enhances the incorporation of S into the selenide absorber. Applying these findings related to Ga and S, V_oc of 642 mV/cell and efficiency of 14.3% are achieved on a 30 cm×30 cm-sized soda-lime glass substrate.     

Simulation studies on photovoltaic response of ultrathin CuSb(S/Se)2 ternary compound semiconductors absorber-based single junction solar cells

Copper-based ternary CuSb(S/Se)₂ compound semiconductors are showing promise for ultrathin photovoltaic devices. The high absorption coefficient of these semiconductors makes them suitable for very thin absorber, where maximum absorption can be achieved in a photovoltaic device with only nanometers thick CuSb(S/Se)₂ based thin films. The device structure under consideration consists of AZO/i-ZnO/n-CdS/absorber layer/back contact, as the constituent material layers. The device structure is simulated using one dimensional solar cell capacitance simulator (SCAPS 1D) under one sun illumination and considering flat band approximation for the back contact and CuSb(S/Se)₂ interface. The optimized single junction device efficiencies are approximately 14% and approximately 10.18% with CuSbS₂ and CuSbSe₂ absorbers, respectively. Further, the impact of various material parameters such as thickness, acceptor concentration of bulk absorber layer, donor concentration of CdS buffer layer, and defects present at bulk absorber layer and at the buffer/absorber interface is discussed in correlation with the photovoltaic performance of the considered devices. The bandgap of CuSb(S/Se)₂ reduces linearly with Se alloying, and their impact on device performance is quantified in terms of capacitance voltage (CV), capacitance frequency (Cf), and impedance spectra of the photovoltaic device.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Ga homogenization by simultaneous H2Se/H2S reaction of Cu-Ga-In precursor

The compositional distribution of Ga and S in Cu(InGa)(SeS)₂ films fabricated by a simultaneous selenization and sulfization process was systematically investigated. At low H₂Se/H₂S reaction temperature (490 °C), most Ga remains at the back of the film adjacent to the Mo back contact. However, the Ga/III ratios at the top and bottom of the Cu(InGa)(SeS)₂ layer monotonically increase and decrease with reaction temperatures, respectively. At T>550 °C, homogeneous distribution of elemental Ga and In through film is achieved. Further increase of the reaction temperature (e.g., T>550 °C) causes phase segregation on the surface of the Cu(InGa)(SeS)₂ film confirmed by XRD, GIXRD and EDS analysis.     

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se₂ (CIGS) thin-film solar cells using evaporated In_xSe_y and ZnIn_xSe_y buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnIn_xSe_y and the CdS buffer layers.     

Replacement of the CBD–CdS buffer and the sputtered i-ZnO layer by an ILGAR-ZnO WEL: optimization of the WEL deposition

As shown earlier the window extension layer (WEL) concept for thin film solar cells based on chalcopyrites results in device performances exceeding those of corresponding chemical bath deposited cadmium sulfide (CBD–CdS) buffered reference cells. The WEL concept is extended and it will be demonstrated, that now a single WEL successfully replaces both, the conventional buffer and the intrinsic part of the window bi-layer usually deposited by sputtering. Thus, one part of the window is deposited directly onto the absorber by a soft process called ion layer gas reaction (ILGAR). The optimization of ILGAR-ZnO WELs on Cu(In,Ga)(S,Se)₂ absorbers with respect to the efficiencies of the completed solar cells is presented. This effort results in ‘total area’ efficiencies of 14.5% (best cell) which are comparable to those of devices with CBD–CdS buffer (14.7%—best cell) without any antireflecting coating.     

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

We have developed the flexible Cu(In,Ga)Se₂ (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO₂/stainless steel. The insertion of the SiO₂ insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.     

Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

Prospects of wide-gap chalcopyrites for thin film photovoltaic modules

The development of a standard process for the fabrication of Cu(In,Ga)Se₂ thin film solar cells by coevaporation leads to highly efficient devices with band-gaps in the range of 1–1.2 eV. The comparison of recent module efficiencies and the corresponding performance of small test cells demonstrates the high uniformity of the cell performance over large areas. In view of their higher open circuit voltage the use of absorbers with higher band-gaps is advantageous for the production of solar modules. However, to date the efficiencies of cells employing chalcopyrite absorbers with higher band-gaps lag behind. A consistent picture of the problems encountered in the case of wide-gap absorbers is drawn on the basis of results from photoelectron spectroscopy in conjunction with electrical measurements on the devices. This picture appoints the high efficiency of the cells with lower band gaps to the successful suppression of recombination at the heterointerface, resulting from a stabilized type inversion at the surface of the absorber. The formation of such a type-inverted surface appears to be a prerequisite for the achievement of high efficiency with wide band-gap absorbers.     

Microdefects and point defects optically detected in Cu(In,Ga)Se2 thin film solar cells exposed to the damp and heating

Optically detected changes have been studied in Cu(In,Ga)Se₂ (CIGS) thin film solar cells, which were exposed to the damp and heat test by IEC 1215 international standard recommendations. High-resolution optical microscopic images at T=300 K and emission properties at T=20 K of ZnO/CdS/CIGS devices were characterized and compared to the tested non-incapsulated device. The near-gap photoluminescence peak at 1.191 eV for the baseline device drastically decreases after the test. Long wavelength emission bands at 1.13 and 1.07 eV, associated with optical transitions through defect levels in absorber, retain their intensity and spectral position. Microscopic surface morphology deteriorates after the test: appearance of micro-scale defects and reduction of optical reflectivity have been observed in blue-violet light and polarization with good contrast. A decrease of conversion efficiency of the exposed solar cell is caused by the degradation of upper wide-gap films and heteroboundary between CdS and CIGS.