Performance of CZTX X S Se Thin film solar cells developed by simplified process and efficiency enhancement through back contact modification

Abstract

Advancement to accomplish the target efficiency in kesterite CZTX (X= S, Se) solar cells is hampered mainly due to the narrow thermodynamic stability and their experimental conditions resulting in a phase transition of sputtered Cu-Zn-Sn alloys. The conventional two-step process demands high temperature processing followed by post-annealing in high pressure Se/S atmosphere which increases the surface roughness and can reduce the performance of the device. Establishing the phase purity of the active layer with required composition through an eco-friendly process is one of the important requirements and challenging task. The main focus of this work is to achieve an optimized synthesis procedure to form a phase pure CZTX (X= S, Se) active layer in a single step process without any ex-situ high temperature selenization or sulfurization process. RF Sputter power conditions are optimized to obtain phase pure CZTX (X= S, Se) thin films with suitable composition, optical, electronic and structural properties for photovoltaic applications. Considering the industrial fabrication of kesterite solar cells, this dissertation mainly focuses on the fabrication and characterization of planar and patterned CZTX (X= S, Se) devices using a complete vacuum based approach. Effects of various deposition parameters are investigated to finely control the elemental ratio of the quaternary target which in-turn promotes the formation of highly crystalline CZTX (X= S, Se) based thin films leading to a one-step reduction in the mass production process. Prior to the actual experiments, various parameters of thin film solar cell devices are optimised using SCAPS simulation software. This simulation software package was used to find out the precise values of individual thickness values (with respect to optical and electrical parameter optimization) of absorber layer, buffer layer and window layer so as to give maximum efficiency. Insights gained from the simulation results were used as a benchmark, and the real experiments were performed to obtai