CIGS-based Thin Film Solar Cells Research Articles

Numerical simulation of a new device architecture for CIGS-based thin-film solar cells using 1D-SCAPS simulator

In this study, we report on a low-cost, highly efficient, and Cd-free Cu (In and Ga) Se2 (CIGS)-based solar cell for an alternative to conventional CIGS-based solar cells. To maintain the proposed model as close as possible to the real behavior of thin-film solar cells, we produced a device with a p-Si/p-CIGS/In2S3/i-ZnO/Al-ZnO structure. The main objective of this study was to reduce the fabrication cost using an ultra-thin CIGS absorber layer with In2S3 buffer layer and obtain high performance. The solar cell performance of this proposed device was investigated using a 1D-solar cell capacitance simulator (SCAPS). The simulation was conducted in three stages. First, the cell was optimized by varying the thicknesses of the CIGS absorber and In2S3 buffer layer. The optimized structure was simulated at different operating temperatures. In each case, Voc, Jsc, FF, and the efficiency were calculated.The proposed device had a marked efficiency of ∼26 % with a 0.3 μm thick CIGS absorber layer. Varying the thickness of the In2S3 buffer layer had little effect, and as the temperature increased, the cell’s performance showed a downward trend.

Structural and microchemical characterization of Cu(In,Ga)Se2 solar cells with solution-grown CdS, Zn(O,S), and Inx(O,S)y buffers

Thin films of Cu(In,Ga)Se2 (CIGS) with a [Ga]/([Ga] + [In]) ratio of about 0.3 were deposited on Mo-coated soda-lime glass substrates by co-evaporation of the elements. In combination with three different solution-grown buffer layers, namely CdS, Zn(O,S), and Inx(O,S)y, and sputtered ZnO/ZnO:Al or Zn0.75Mg0.25O/ZnO:Al double layers as the front contact they were completed to three different types of CIGS-based solar cells. The as-fabricated cell efficiencies are 16.9%, 14.7%, and 14.6%, respectively. The microstructure of all the three devices was analyzed by transmission electron microscopy (TEM). Pores occur at the interface between CIGS absorber and Mo back electrode, and stacking faults and microtwins are observed in the CIGS absorber. Zn(O,S) and Inx(O,S)y buffers are nanocrystalline in contrast to larger grain sizes in the CdS buffer. The structure of the CIGS/CdS interface is frequently coherent, while that of the CIGS/Zn(O,S) and CIGS/Inx(O,S)y interfaces is incoherent. High-resolution TEM imaging demonstrates that CdS buffer occurs in the cubic zincblende and hexagonal wurtzite structure with a high stacking fault density in zincblende CdS. The difference in crystalline quality of the buffers and CIGS/buffer interfaces might contribute to the different efficiencies of the investigated CIGS-based thin-film solar cells. Chemical analyses by energy-dispersive X-ray spectroscopy show interdiffusion and chemical inhomogeneities in Zn(O,S) and Inx(O,S)y buffers.

Crystal Chemistry, Optical-Electronic Properties, and Electronic Structure of Cd1- xIn2+2 x/3S4 Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells.

CdIn2S4 and In2S3 compounds were both previously studied as buffer layers in CIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdIn2S4-In2S3 system, and a series of Cd1- xIn2+2 x/3S4 (0 ≤ x ≤ 1) materials were prepared and characterized. Our results show that two solid solutions exist. The aliovalent substitution of cadmium(II) by indium(III) induces a structural transition at x ≈ 0.7 from cubic spinel Fd3̅ m to tetragonal spinel I41/ amd that is related to an ordering of cadmium vacancies. Despite this transition, the variation of optical gap is continuous and decreases from 2.34 to 2.11 eV going from CdIn2S4 to In2S3 while all compounds retain an n-type behavior. In contrast with the Al xIn2-xS3 solid solution, no saturation of the gap is observed. Moreover, XPS analyses indicate a difference between surface and volume composition of the grains for Cd-poor compounds. The use of Cd1- xIn2+2 x/3S4 compounds could be a good alternative to CdIn2S4 and In2S3 to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.

Influence of Cu and Na incorporation on the thermodynamic stability and electronic properties of β-In2S3

We studied the incorporation of a Na and Cu in β-In2S3 buffer layer and addressed the stability of the absorber/buffer interface in CIGS-based thin film solar cells.

Numerical thickness optimization study of CIGS based solar cells with wxAMPS

In this paper, a numerical investigation of the effects of thickness of absorber and buffer layers for a typical CIGS based solar cell was carried out for the first time, employing the wxAMPS software which is a new custom-designed software package for solar cell simulation. We have examined how the output parameters change with absorber and buffer thicknesses and suggested four different cell structures. By using the baseline structure, an optimum band gap value of 1.4eV was determined for the CIGS absorber layer prior to do thickness study. Optimum absorber thickness value has been determined as 2500nm from the simulation results. Spectral response of the cell decreased with increasing thickness in blue region. The influence of buffer layer thickness was investigated and it was found that most of the photo-generated carriers are collected by a thinner CdS layer. A significant decrease in current density, open circuit voltage and conversion efficiency occurred with increasing buffer layer thickness. This behavior was attributed to a decrease in the number of collected carriers due to the recombination of electron-hole pairs proceeded from the absorbed photons in the CdS bulk for large thickness. As a result, efficiencies around 27% were obtained through optimization while that of baseline structure was 25%. Our results have shown that the suggested structures both produce higher efficiencies than that of baseline structure and may lead to produce more efficient CIGS-based thin film solar cells.

Invited Paper: CIGS-based thin film solar cells and modules: Unique material properties

Although CIGS solar cells consist of a polycrystalline thin film grown on a glass substrate, more than 20% conversion efficiency has been achieved. The efficiency has reached the same level as polycrystalline silicon solar cells. This high efficiency has not yet been observed in other thin film solar cells including thin film Si and CdTe. Therefore, it is important to understand the mechanisms that allow CIGS solar cells to exhibit high conversion efficiencies. This paper discusses the origin of the high efficiency and demonstrates that it is caused by the unique material properties of CIGS films.

13·7%-Efficient Zn(Se,OH)x/Cu(In,Ga)(SSe)2 thin-film solar cell

Cu(In,Ga)Se2 (CIGS) and related semiconducting compounds have demonstrated their high potential for high-efficiency thin-film solar cells. The highest efficiency for CIGS-based thin-film solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, Zn(Se,OH)x buffer layers were deposited by CBD on polycrystalline Cu(In,Ga)(S,Se)2 (CIGSS). A total-area conversion efficiency of 13·7% was certified by the Frauenhofer Institute for Solar Energy Systems. The CIGSS absorber was fabricated by Siemens Solar Industries (California). For device optimization, the thickness and good surface coverage were controlled by XPS–UPS photoemission spectroscopy. A Zn(Se,OH)x thickness below 7 nm has been found to be optimum for achieving a homogeneous and compact buffer film on CIGSS, with open-circuit photovoltage Voc=535 mV, fill factor FF=70·76% and a high short-circuit photocurrent density Jsc=36·1 mA cm−2. Copyright © 1998 John Wiley & Sons, Ltd.

Large area ZnO films optimized for graded band-gap Cu(InGa)Se 2-based thin-film mini-modules

In this study, two deposition methods (i.e. MOCVD and sputtering methods) to prepare n-type ZnO window layers for CIGS-based thin-film solar cells are discussed. In order to make ZnO : Al transparent conductive oxide (TCO) films prepared by DC magnetron sputtering comparable to ZnO : B TCO prepared by MOCVD, a new ZnO sputtering process is proposed by introducing a multilayer structure. Using these films, CIGS thin-film solar cells with efficiencies of greater than 14% have been fabricated with an active area of 3.2 cm 2. This structure was adapted to fabricate CIGS thin-film mini-modules with efficiencies around 11% having aperture area of 50 cm 2.