Detection and relevance of ion conduction in hybrid organic-inorganic halide perovskites for photovoltaic applications

In recent years, hybrid organic-inorganic halide perovskites have attracted much attention with respect to their potential use as sensitizers in solar cells.[1] These materials show many outstanding properties, such as high absorption coefficients, ideal bandgap for solar light absorption (1.5 eV), long electron-hole recombination lengths and high charge carrier mobilities[1-3], that leads to a photo-conversion efficiency of hybrid-perovskite-containing devices exceeding 20%.[4] However, anomalous behaviors have been reported for these materials, such as high apparent dielectric constants at low AC frequencies or photocurrent hysteresis of solar cell devices during operation.[5] In this study[6] we measure the electrical transport properties of CH3NH3PbI3, by means of DC galvanostatic polarization, AC impedance spectroscopy and open circuit voltage measurements in electrochemical cells. By using ion-blocking electrodes, we detect a clear stoichiometric polarization behavior, from which we can separate electronic and ionic contributions to the total conductivity. We show that, under certain conditions, ionic conductivity can substantially exceed electronic conductivity and we assess the nature of the migrating ions (iodine ions). It is noteworthy that such ionic conductivity can naturally explain the above mentioned anomalies (Figure 1). Moreover, from the experimental data, a strong trapping of the electronic carriers due to ionic defects is ascertained. As a natural follow-up to better understand the defect chemistry of such materials, conductivity response to different atmospheres (I2, O2) has been measured and acceptor doping has been successfully applied. Please click Additional Files below to see the full abstract