Performance and Loss Analyses of High-Efficiency CBD-ZnS/Cu(In1-xGax)Se2 Thin-Film Solar Cells

Chemically deposited ZnS has been investigated as a buffer layer alternative to CdS in polycrystalline thin-film Cu(In1-xGax)Se2 (CIGS) solar cells. Cells with efficiency of up to 18.1 % based on CBD-ZnS/CIGS heterostructures have been fabricated. This paper presents the performance and loss analyses of these cells based on the current-voltage (J-V) and spectral response curves, as well as comparisons with high efficiency CBD-CdS/CIGS and crystalline silicon counterparts. The CBD-ZnS/CIGS devices have effectively reached the efficiency of the current record CBD-CdS/CIGS cell. The effects of the superior current of the ZnS cell and the superior junction quality of the CdS cell on overall performance nearly cancel each other. Thin-film solar cells with Cu(In1-xGax)Se2 (CIGS) absorbers have achieved high efficiencies in recent years. Cadmium sulfide (CdS) buffer layers prepared by chemical bath deposition (CBD) are commonly used in CIGS-based cells. Even though cells with the ZnO/CBD-CdS/CIGS structure have high efficiencies, manufacturing companies have sought alternatives to the CBDCdS buffer layer, which could eliminate the use of Cd and improve the collection of carriers generated by short-wavelength light. One promising alternative material is ZnS. Its band-gap energy (Eg) of 3.8 eV makes it transparent to practically all wavelengths of the solar spectrum. In contrast CdS, with its band gap of 2.4 eV, is highly absorbing for wavelengths below 520 nm. There has been a considerable progress in using CBD-ZnS in CIGS-based thin-film solar cells . ZnO/CBD-ZnS/CIGS solar cells with efficiency of up to 18.1 % have been fabricated at Aoyama Gakuin University. We present the performance and loss analyses 3) of these cells and also two CBD-CdS/CIGS and Si comparison cells made elsewhere. CBD-ZnS buffer layers were chemically grown on CIGS thin films using a ZnSO4 (0.10.3M)-ammonia (5-8 M, PH=10.5-11.0)-thiourea (0.4-0.8 M) aqueous solution at 80oC. X-ray photoelectron spectroscopy (XPS) analysis revealed that the CBD-ZnS film includes a large amount of oxygen in the form of Zn(OH)2 and ZnO. Therefore, we refer to this material as CBDZnS, since it is not pure ZnS, although its optical band-gap energy is very close to the 3.8-eV single-crystal value. The detailed fabrication process of a MgF2/ZnO:Al/CBD-ZnS/CIGS/Mo/soda-lime glass solar cell is described elsewhere. The J-V characteristics were measured at the Japan Quality Association (JQA) using a solar simulator under AM 1.5, 100 mW/cm illumination at 24oC. The