High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

To understand the cause of the high open circuit photovoltage (VOC) achieved by todays' state of the art perovskite solar cells (PSCs), we examine formamidinium lead bromide CH(NH2)(2)PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)(2)PbBr3 films yield VOC as high as 1.5 V ascertaining their high quality. TAS establish that the presence of charge extraction layers has very little influences on the nature of a negative band at 535 nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)(2)PbBr3 films. Therefore, we contend that the V-OC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer. (C) 2017 Published by Elsevier B. V.