Understanding deposition temperature dependent photovoltaic characteristics of Cu(In,Ga)Se2 solar cells: A study with thermally stable alkali aluminosilicate glass substrates

Abstract

During 3-stage thermal co-evaporation of Cu(In,Ga)Se2 (CIGSe), alkali aluminosilicate (AAS) glass has proven to be stable at process temperatures Tmax of up to 650 °C. This is considerably higher than the maximum endurable Tmax for soda-lime glass (SLG), which is in the range of 530–550 °C. As the Na-content of the AAS glass used here is comparable to that of SLG, growing CIGSe at elevated Tmax is expected to promote Na diffusion from the glass substrate through the Mo back contact (BC) into the absorber layer. An increased Na concentration in the CIGSe absorber would be expected to result in an improved device performance as it is mostly associated with an increased open-circuit voltage (Voc) and fill factor (FF). This paper discusses the influence of varying Tmax from 530 to 650 °C during the 2nd and the 3rd stage of the 3-stage process on the morphological, electrical, and photovoltaic performance of CIGSe solar cells. Upon raising Tmax from 530 to 650 °C, the short-circuit current density (Jsc) decreased due to bandgap (Eg) widening. Glow discharge optical emission spectrometry analysis reveals that the Na concentration in the absorber is gradually decreased when elevating Tmax. As will be seen, we attribute this in part to a densification of the Mo BC during the high temperature process. A maximum conversion efficiency (η) was realized at Tmax = 600 °C. An increased Voc at Tmax = 600 °C is due to the wider Eg and to an increased carrier concentration despite the fact that the Na concentration in the CIGSe thin film was low compared to lower Tmax. Admittance spectroscopy analysis is performed to access information on defect energy level and density in the finished devices. In the light of present findings, ways to further improve η at elevated Tmax are suggested. © 2020