Abstract
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•The critical understanding and the origin of VOC as high as 1.5V is presented.•The charge extraction layers exhibit minimal effect on charge carrier dynamics.•The quasi Fermi level splitting within the absorber layer determines the VOC in PSC.
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)2PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)2PbBr3 films yield VOC as high as 1.5V 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 535nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)2PbBr3 films. Therefore, we contend that the VOC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer.