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

Abstract

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 CHNHPbI 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 cms 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.

Light Absorption and Recycling in Hybrid Metal Halide Perovskite Photovoltaic Devices

“…As the perovskite absorber layer becomes thicker, it will absorb more light. However, the charge carriers generated at the surface (holes in this study, as we are investigating n‐i‐p device architecture) will have to travel farther to reach the charge extraction layer (Spiro‐OMeTAD) …”

The Dominant Energy Transport Pathway in Halide Perovskites: Photon Recycling or Carrier Diffusion?

“…During the diffusion, the excited carriers may lose a small amount of energy, resulting in a redshift of emission. [18,19,[30][31][32][33][34] The photon reabsorption-emission repeats over a long propagation distance, resulting in a reduced PL photon energy and intensity. [18,19,[30][31][32][33][34] The photon reabsorption-emission repeats over a long propagation distance, resulting in a reduced PL photon energy and intensity.…”

“…[42] Tian et al measured individual single-crystal MAPbI 3 (MA = CH 3 NH 3 ) and MAPbBr 3 nanowires (NWs) and nanoplates (NPs) using PL-scanned imaging microscopy with a galvano-mirror. [19,30,[32][33][34] These reports claim that energy transport is not limited by diffusive charge transport but can occur over long distances through multiple reabsorption-diffusion emission events implying the repeated recycling between photons and electron-hole pairs. Through monitoring the time-resolved PL at different collection spots, carrier diffusion coefficient, charge mobility, recombination rate constants, and carrier diffusion length were determined based on a carrier diffusion model completely neglecting the photon recycling effect.…”

“…Besides these two contrary explanations, in many other reports, both carrier diffusion and photon recycling are ambiguously included. [30][31][32][33] Unfortunately, to date, conclusions based on the same observation are often conflicting, leading to an unresolved puzzle. [19] In addition, the same physical effects were also intensively studied by the excitation depth-dependent PL or cathode luminescence and PL of perovskite with different thickness.…”

“…[19,30,[32][33][34] These reports claim that energy transport is not limited by diffusive charge transport but can occur over long distances through multiple reabsorption-diffusion emission events implying the repeated recycling between photons and electron-hole pairs. [18,19,[30][31][32][33][34][35][36][37][38][39][40][41][42][43] This controversy commonly exists in various perovskites and device studies. [30][31][32][33] Very similar redshifts of the PL maximum were consistently observed.…”