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Title: Thermodynamics for practical kesterite photovoltaics
Author: Jackson, Adam
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2016
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Copper zinc tin sulfide (CZTS) is a photovoltaic absorber material used in thin-film devices. Research into the kesterite absorbers Cu2ZnSn(S1-xSex )4 has been particularly active in the last decade: this is motivated by the abundance of the constituent elements, which presents a long-term advantage over existing thin-film technologies including CIGS and CdTe. However, the efficiency of CZTS-based devices has been lower than expected and among other factors this is attributed to is the presence of secondary phases. In this thesis, a novel thermochemical model is developed for the mixed sulfur vapour phase that is present during the annealing of CZTS films. Temperature and pressure-dependent phase diagrams are then constructed for the Cu-Zn-Sn-S system from first-principles calculations within density-functional theory. While research-grade CZTS is produced in both low and high-pressure processes, it is found that the phase diagram is similar in both regimes except for the oxidation state of tin sulfide. The dominant ternary phase, of those modelled, is a monoclinic phase of Cu2 SnS3 . A small temperature/pressure envelope appears to exist in which this phase is unstable relative to CZTS and the binary phases. This potentially offers a route to copper-rich CZTS without the presence of ternary phases. Rapid, high-temperature annealing for industrial deployment appears to be feasible without decomposition of the kesterite, provided that an adequately high system pressure is maintained. This work demonstrates that with high-performance computing it is possible to explore the thermodynamic space around a quaternary compound before traditional thermochemical data has been obtained. The methodology developed here is applicable to other multi-component systems.
Supervisor: Walsh, Aron ; Patterson, Darrell ; Peter, Laurence Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available