Probing Electrical Transport across Single and Multiple grains in Organic-Inorganic Perovskites
Giovanni Azzellino, Sam Stranks, Farnaz Niroui, Vladimir Bulović
As the light-harvesting layer in solar cells, organic-inorganic perovskites combine the promise of solution processing and bandgap tunability by chemical substitution, yielding devices with remarkable efficiencies. Perovskites also exhibit high radiative recombination yield, with long carrier lifetimes of hundreds of nanoseconds to microseconds. Ensuring that all recombination is radiative is critical for approaching the thermodynamic efficiency limits of solar cells. Carrier recombination lifetimes measured by photoluminescence are commonly taken as a hallmark of perovskite film quality, with longer decay lifetimes used as indicators of better-performing materials. To assess the local PL lifetime enhancement or quenching, confocal fluorescence mapping has been adopted to investigate the presence of bright and dark grains composing the thin film. That map constitutes a powerful tool by which we can evaluate the influence of changes in film processing on carrier recombination and electrical transport in films.
The aim of this work is to investigate the lateral conductivity across single and multiple grains of which the perovskite film is composed, and correlate the lateral mobility to the quality of the perovskite film. Despite their tremendous promise as PV materials, it has been recently shown that there remains a substantial level of heterogeneity in emission on the microscale in thin perovskite films, with grain boundaries exhibiting particularly prominent non-radiative recombination. It remains unclear whether the electrical transport exhibits similar heterogeneity. In order to address this question, we use gated devices as a test bench to contact single grains and investigate their electronic properties in different conditions: temperature, humidity, and chemical post-treatment of the perovskite film.