Development of eco-friendly solar cells with undoped and copper doped zinc sulfide buffer layer

Abstract

Zinc sulfide (ZnS) is one of the eco-friendly candidate materials for the buffer layer required in CIGS and CZTS solar cells. Conventional CdS buffer layer is considered to be hazardous material and may create potential environmental damages. It also exhibits material instability when it is exposed to temperature of 300 °C or higher. ZnS have various properties for different applications. ZnS doped with copper was possible to confirm the bandgap engineering and electrical properties change for the hetero-junction of the solar cell. ZnS can give more design freedom as well as eco-friendliness in solar cell fabrication processes because it exhibits more robust characteristics against high temperature exposure. In this study, ZnS thin films were grown using an RF magnetron sputtering under various conditions (RF power, substrate temperature) to obtain a ZnS buffer layer that can replace CdS buffer layer [1]. The optical, electrical, and structural properties of ZnS films were characterized by UV-VIS-NIR spectrophotometer, x-ray diffraction (XRD), scanning electron microscope (SEM), and Hall measurement. The ZnS thin films deposited at room temperature, having cubic crystalline structure exhibit high transmittance at short wavelength range. The optical bandgap energies and the doping types were dependent on the growth temperature as well as RF power applied during the deposition. The biggest problem with ZnS as a buffer layer for CIGS solar cells is bandgap engineering due to the difference in bandgap between CIGS layer and ZnS buffer layer. In this thesis, copper-doped ZnS was used to solve the difficulty of hetero-junction formation due to the ZnS buffer layer having a wide bandgap. By doping copper into ZnS, the band gap size and electrical properties of this film could be controlled. By inserting this thin film between CIGS layer and ZnS buffer layer, it was also confirmed that the electrical properties of hetero-junction formation of the CIGS solar cell were improved. As a result, it was confirmed that Jsc improved from 11.7 mA/cm2 to 27.2 mA/cm2 and efficiency from 1.9 % to 4.7 % through this novel structure for CIGS solar cell.