title: Synthesis and Characterisation of Kesterite Thin Films and Single Crystals for Photovoltaics
Creators: name: Edwards, Holly
Publisher: name: University of Liverpool Repository
Year: 2022

description: This thesis presents elemental substitution investigations of Cu2ZnSnS4 to better comprehend the crystal and band structures of these materials with the goal of improving photovoltaic devices. The initial interest stemmed from the high efficiency of Cu(In,Ga)S(e)2 solar cells at 22.3% [1], however they are less desirable due to the scarcity of Indium and the expense of Gallium. Zinc and Tin were then proposed to replace these elements, given that CZTS has a suitable band gap for visible light absorption (1.5 eV) and high absorption coefficient of ~104 cm-1 [2]. The solar cell devices to date with the highest efficiencies of CZTS and CZTSe have been fabricated using sputtering [3] and a hydrazine solution approach [4] respectively. Studies in the literature of alloying CZTS with other elements including Ag/Cu [5], Cd/Zn [6], and Ge/Sn [7] have shown to improve device efficiencies by tailoring the band gap, effecting defect size, unwanted secondary phases, and carrier concentration, but have yet to surpass the efficiencies of the devices in production. Such substitutions have been investigated in this thesis. First of all, thin films were fabricated by a low-cost chemical method, exhibiting suitable grain size and phase structure, and then were compared to single crystals synthesised by a chemical vapour transport method. Traditional kesterite consisting of Cu2ZnSnS4 was synthesised first, then the more complex substitutions took place, including Se replacing S, Cd replacing Zn, Ni replacing Zn, and Ag replacing Cu. After optimisation and physical characterisation, photoemission studies including XPS, HAXPES, and IPES took place. To date, core level HAXPES has only been performed on Cu2ZnSnS4 and so this thesis provides the first account of core level HAXPES for selenised kesterite, as well as the first account for the valence states. Further to this, IPES has only been performed on Cu2ZnSnS4, never the other kesterite materials studied in this thesis. The literature is currently lacking for photoemission studies for kesterites and so this thesis provides a comprehensive study of the core level structure and valence states compared with DFT. There were also some studies in the literature whereby trends in the change in band gaps were mentioned during band tailoring in elemental substitutions, but there were never any explanations of why this mechanism was forming. For the first time, this link was explained between the bond lengths calculated from XRD measurements with the band structure from photoemission measurements, with further comparisons to DFT. Primarily, this thesis focusses on studies of the absorber layer of a thin film kesterite photovoltaic device, however the relation between the absorber and buffer layers band structure was also explored. Mismatching of the conduction band offsets of the absorber and buffer layers are common in practice and throughout the literature. It was found that with low amounts of Cd, or Ni, or high amounts of Se, CdS was the ideal buffer layer, whereas with low amounts of Ag or Se, In2S3 was most the most recommended absorber layer.