Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
Publication date: 18th December 2016
Lead (Pb)-based ‘halide perovskites’ (HaPs), have shown exceptionally high power conversion efficiencies in recent years. Structural tolerance in these systems allows incorporation of multiple cations and anions within the perovskite lattice, leading to >21% solar-cell efficiencies, which made it ‘the next big thing’ in photovoltaics. However, the toxicity of lead (Pb), which is used in the most studied cells, may affect its large-scale uses. Hence we explore completely Pb-free HaPs such as CsSnBr3, for solar-cell applications. Addition of SnF2 was observed to impart beneficial effects on several parameters of CsSnBr3-based cells, which is our major focus.Cell efficiency was observed to increase dramatically (~200 times) when SnF2 was added to the perovskite deposition solution compared to what is achieved without SnF2, possibly due to a combined effect of (1) filling of Sn2+ vacancies, (2) minimizing oxidation of Sn2+ or, (3) removal of trap states. To understand the effect of SnF2 concentration on PV parameters, various concentrations of SnF2 were added during CsSnBr3 preparation and 20 mol% was found be optimal to achieve improved device parameters such as short circuit current (JSC), open circuit potential (VOC) and fill factor (FF). In order to understand the role of SnF2 on the energetics (work function-WF and top of the valence band-EVBM) of the CsSnBr3 (with and without SnF2 addition), Ultraviolet Photoelectron Spectroscopy (UPS) was monitored on titania (TiO2) and gold (Au) substrates. SnF2 addition was found to aid in the alignment of the bands with some selective contacts in a solar cell, which explains the improvement in the critical cell parameters. Furthermore, continuous X-irradiation was found to be detrimental for pristine CsSnBr3, and resulted in metallic tin (Sn0) formation. However, addition of SnF2 protected the halide perovskite from beam damage, possibly because of a SnF2-induced change in the Sn2+ electrochemical potential that makes the reaction to form Sn0, energetically less probable.