Improving the Aesthetics and Performance of Perovskite Materials for Photovoltaics

Within the last decade, lead halide perovskite solar cells have rapidly evolved to the cusp of commercialisation. Current record device efficiencies have surpassed 25% however; a principal limitation of these materials is their instability on exposure to ambient conditions. Methylammonium lead tri-bromide (MAPbBr3) perovskite has shown superior stability over other lead halide perovskite materials, yet the efficiencies of MAPbBr3 devices are significantly lower with a record efficiency of 10.4%. This research investigates the treatment of MAPbBr3 perovskite solar cells with organic dyes of complementary absorbance in a bid to maximise the light harvesting, increase the photocurrent and improve the device efficiency. Initial investigations focused on developing an optimised build method capable of manufacturing MAPbBr3 devices which consistently achieve above 1% efficiency. The optical characterisation of six organic dyes revealed a red indoline dye, D205 and a blue squaraine, SQ2 (which both absorb strongly between 300-700 nm) would offer the best complementary absorbance to MAPbBr3 perovskite. On adding the dyes, the perovskite layer underwent an evident colour change highlighting the potential for coloured perovskite cells which could be beneficial for building-integrated applications. MAPbBr3 cells co-sensitised using a novel method (which sensitises the film after perovskite crystallisation) show improved efficiency (2.6% SQ2, 3.1% D205) over perovskite-only devices (2%) with a 10% photocurrent contribution from the dye. Whilst increases in the photocurrent are observed with co-sensitisation, increased device efficiencies are mainly derived from improvements in the fill factor. We also see lower series resistance and increased photoluminescence lifetime with co-sensitisation where control and co-sensitised MAPbBr3 thin-films produce average lifetimes of 0.44 ns and 0.80 ns, respectively. Further investigation has revealed the dye solvent, toluene, and the dye both help to improve device performance acting as both a treatment and a second sensitiser in the device by passivating defects and lowering recombination losses whilst providing additional photocurrent through increased absorbance. As a result, co-sensitised devices show slower recombination kinetics resulting in increased open-circuit voltage under lower light levels. These effects have proven beneficial for thicker co-sensitised devices (>0.7 µm) where they have often translated into large increases in device efficiency. In future, this may be beneficial for indoor or lower light level PV systems including within the rapidly expanding internet of things market.