Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Despite rapid developments in both photovoltaic and light emitting device performance, the understanding of the optoelectronic properties of hybrid lead halide perovskites is still incomplete. In particular, the polarizability of the material, the presence of molecular dipoles, and their influence on the dynamics of the photo-excitations remains an open issue to be clarified as well as the control of the defect structures of the solution processed semiconductor, which currently restricts the power conversion efficiencies of solar cells from reaching their thermodynamic limit.Here, first we investigate the effect of an applied external electric field on the photo-excited species of CH3NH3PbI3 thin films, both at room temperature and at low temperature, by monitoring the photoluminescence (PL) yield and PL decays. At room temperature we find evidence for electric field – induced reduction of radiative bimolecular carrier recombination together with motion of charged defects that affects the non-radiative decay rate of the photo-excited species. At low temperature (190 K), we observe a field - induced enhancement of radiative free carrier recombination rates that lasts even after the removal of the field. We assign this to field induced alignment of the molecular dipoles, which reduces the vibrational freedom of the lattice and the associated local screening, and hence results in a stronger electron – hole interaction.Then, by using a combination of transient and steady state photocurrent and absorption spectroscopy we show that CH3NH3PbI3 films exhibit a broad distribution of electron traps. We show that the trapped electrons recombine with free holes unexpectedly slowly, on microsecond time scales, relaxing the limit on obtainable Open-Circuit Voltage (Voc) under trap-mediated recombination conditions. We find that the observed VOCs in such perovskite solar cells can only be rationalized by considering the slow trap mediated recombination mechanism identified in this work. Our results suggest that existing processing routes may be good enough to enable open circuit voltages approaching 1.3 V in ideal devices with perfect contacts.
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