Charge carrier injection at the heterointerface in CH3NH3PbI3 perovskite solar cells studied by time-resolved photoluminescence and photocurrent imaging spectroscopy

摘要

Summary form only given. Organic-inorganic halide perovskite solar cells are attracting much attention from the photovoltaic community because of their high conversion efficiencies exceeding 20%. So far, intrinsic superior optoelectronic properties of this material class have been revealed through comprehensive studies on the thin films and single crystals [1,2]. For further improvement of the device architecture and conversion efficiency, the carrier recombination and transport dynamics in actual solar cell devices have to be clarified. The perovskite solar cell is usually implemented as a heterojunction structure consisting of a perovskite absorber layer and charge transport layers as selective contacts, and the carrier-injection properties at these heterointerfaces play a crucial role for the device performance. Time-resolved photoluminescence (PL) techniques are usually adopted to investigate carrier injection and transport properties [3]. However, PL is also additionally affected by traps and defects within the perovskite layer and also at the heterointerface. On the other hand, the photocurrent (PC) measurement can directly assess the net charge-carrier flow through the whole device. Therefore a combination of PL and PC enables us to investigate the details of the carrier injection. In addition, perovskite solar cells are prepared by a fast and cost-effective low-temperature solution-process, but this simple preparation method also causes a spatial nonuniformity in the optical and electrical properties [4]. Thus, the spatial imaging is invaluable for statistical evaluation of the solar cell characteristics.In this work, we first performed microscopic mapping of time-resolved PL and PC on CH3NH3PbI3 perovskite solar cells. The devices structure was FTO/compact TiO2/mesoporous TiO2/CH3NH3PbI3/SpiroOMeTAD/Au. For photoexcitation of the perovskite layer, a pulsed picosecond laser with wavelength of 688 nm was used. The PL intensity, PL lifetime, and PC intensity were measured at room temperature, and varied spatially on the order of several tens of micrometers. We found that the fluctuation of the photoelectrical properties such as PC and electroluminescence is directly linked to the spatial nonuniformity of the mesoporous TiO2 substrate, which was prepared by the spin-coating method [5]. Furthermore, we found a positive correlation between the PL intensity and PL lifetime, and a negative correlation between the PL and PC intensities. These correlations can be only explained with dominant carrier injection of the photogenerated carriers into the charge transport layers.