Investigation on the effect of cation substitution in Cu2ZnSn(S,Se)4 thin film solar cell

Abstract

Thin film solar cells technology based on Cu2ZnSn(S,Se)4 (CZTSSe) holds great promise due to the abundancy of its constituent elements and their environmentally benign nature.In recent years, developments in achieving higher device performance have been halted due to the inherent Voc deficit of CZTSSe. This poses a huge setback for the successful industrialization of CZTSSe thin film solar cells. Many agreed that cation disordering which emerge due to Cu and Zn similarity in size and chemical environment, is one of the main causes of this issue. Recent progress in cation substitution of CZTSSe with other metals have shown promising result such as Ag with Cu and Cd with Zn. However, incorporation of these elements in CZTSSe is not ideal as they deviate from the earthabundant and non-toxic motivation of CZTSSe.This thesis aims to explore and understand the influence of novel cation substitution in CZTSSe solar cell and focus on optimizing the promising candidates by partial substitution.Firstly, screening and comparison between suitable cations such as Mn, Mg, Ni, Fe, Co, Sr and Ba were conducted and summarized to select ideal candidate for partial substitution study. These films were prepared by chemical spray pyrolysis in stoichiometric composition to investigate its intrinsic thin film properties. Following that, nonstoichiometric composition was fabricated to optimize the solar cell device performance.The screening concluded that Manganese (Mn), Magnesium (Mg), Barium (Ba) and Strontium (Sr) emerged as the promising cations as Zn substitute.Next, Mn was partially substituted into CZTSSe in both sulfide and sulfoselenide system to optimize the photovoltaic performance. Mn has been chosen as the first novel cation based on its half-filled d-orbital in comparison with other transition metal candidates (e.g. Fe, Ni or Co) and larger ionic size mismatch with Cu. Effects of Mn substitution in Cu2(MnxZn1-x)Sn(S,Se)4