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 earth-abundant 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, non-stoichiometric 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 (CMZT(S,Se)) thin films, with x = 0.0–1.0, on the film morphology, structure, and device performance was investigated. The highest device performance and the least defect density at the heterojunction interface was obtained at x=0.05. Following that, double layered structure was fabricated by sol-gel spin coating method with variation in the CMZT(S,Se) layer thickness to further optimize the performance. In this structure, the sulfoselenide film showed the highest efficiency in this study due to reduced interface defect density, and increased hole mobility and photoluminescence intensities.

Mg was chosen as the second candidate for partial substitution as it is most stable as Mg2+, unlike other multivalent transition metal cations. It is hypothesized that the single oxidation states help to reduce defects or secondary phases that could be formed due to the multivalent nature of other cations. In this thesis, the effect of Mg in CZTS solar cell was investigated by varying the amount of Mg from Mg/(Mg+Zn) = 0.0 – 1.0. Systematic phases transformation was observed from kesterite CZTS to monoclinic Cu2SnS3 as the Mg amount increase based on XRD and multiwavelength Raman study. The device performance improvement by Mg was apparent at Mg/(Mg+Zn) = 0.05. It is attributed to the improved bulk properties such as better charge collection, lesser amount of ZnS secondary phase and shallower acceptor defects level.

To summarize, this thesis provides systematic investigation of the effect of various cation substitutes for Zn. Our investigations on the positive effects of Mn and Mg as partial substitutes of Zn, which are among the first in literature, reveal that both cations reduced different types of defects in CZTS. Finally, based on our investigation, we conclude that the following design criteria should be considered when choosing a cation substitute for Zn: 1) it should possess d-orbital to form stable quaternary compound; 2) multiple oxidation states and partially filled d-orbital cations affect the conductivity and photovoltaic performance; 3) large cation radius difference with Zn result in structure and optoelectronic properties transformation; 4) partial (instead of full) substitution of Zn could lead to CZTS efficiency improvement.