Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10-10 cm3s-1 is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.