Impact of non-stoichiometry on ion migration and photovoltaic performance of formamidinium-based perovskite solar cells

Abstract

Ion migration mechanisms are often behind degradation pathways in perovskite solar cells under operating conditions. In this work we look at the effect of non-stoichiometric compositions in mixed caesium/formamidinium inverted perovskite solar cells (PSC) on their performance and short-term degradation. From impedance measurements and drift-diffusion modelling we infer that the excess of the formamidinium precursor (FAI) in the perovskite active layer leads to an acceleration of the ion dynamics due to the creation of mobile crystalline defects. In contrast to what could be expected, this feature is not correlated with faster degradation. Instead, more stable devices are demonstrated when a non-stoichiometric composition is used. Addition of excess FAI also modifies the recombination kinetics: a non-monotonous variation of the open-circuit photovoltage and the recombination rate is observed, with optimal performance found for a composition of 1.0-1.5% excess of FAI.