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Continuous increase in the device performance of lead halide perovskite-based solar cells is strongly related to better understanding of the optoelectronic processes occurring in the perovskite layer and its interfaces. There are many of these processes that are critical to device performance, but are not yet fully understood, which include charge carrier accumulation and recombination, trapping of electrons and holes, and ionic movement. Here we report our recent results of methylammonium lead iodide (MAPI)-based photovoltaic devices identifying the origins of different open circuit voltages and their potential loss mechanisms in conventional and inverted device structures. We have investigated in detail the energetics and the illumination generated surface photovoltage (SPV) and its transient behaviour at the perovskite layer and its heterointerfaces with various charge extracting interlayers.
A MAPI layer with different thicknesses was deposited on top of the various underlayers including ITO, n-type TiO2, p-type PEDOT:PSS and many oxides and organic semiconductors. We found that the work function of MAPI is strongly influenced by the underlayer showing generally p-type semiconductor character. The results of thickness dependent SPV measurements indicate that there is an increase in the hole concentration at both PEDOT:PSS/MAPI and TiO2/MAPI interfaces, which leads to an increased interfacial charge recombination. In this talk, I will discuss how these observations are related to different open circuit voltages and their loss in conventional and inverted devices. I will also discuss the temperature dependent transient SPV results, which is used to distinguish different processes governed by charge carrier generation, ion migration, and charge trapping – three processes taking place at three different timescales.
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