Impact of Trap Filling on Carrier Diffusion in MAPbBr$_3$ Single Crystals

TL;DR

This study demonstrates that in MAPbBr3 single crystals, carrier diffusion length and lifetime decrease with increasing photo-carrier density, influenced by trap and bimolecular recombination processes, affecting carrier transport properties.

Contribution

It provides experimental insights into how trap filling and recombination mechanisms affect carrier diffusion and lifetime in MAPbBr3 crystals, highlighting the density-dependent behavior.

Findings

Carrier diffusion length is ~13.3 μm for electrons and ~13.8 μm for holes.

Increasing light bias reduces diffusion length by factors of 2-3.

Recombination constants are approximately 9.3×10^6 s^-1 (monomolecular) and 1.4×10^-9 cm^3s^-1 (bimolecular).

Abstract

We present experimental evidence showing that the effective carrier diffusion length ($L_d$) and lifetime ($\tau$) depend on the carrier density in MAPbBr$_3$ single crystals. Independent measurements reveal that both $L_d$ and $\tau$ decrease with an increase in photo-carrier density. Scanning photocurrent microscopy is used to extract the characteristic photocurrent $I_{ph}$ decay-length parameter, $L_d$, which is a measure of effective carrier diffusion. The $L_d$ magnitudes for electrons and holes were determined to be ~ 13.3 $\mu$m and ~ 13.8 $\mu$m respectively. A marginal increase in uniform light bias ($\leq 5 \times 10^{15}$ photons/cm$^2$) increases the modulated photocurrent magnitude and reduces the $L_d$ parameter by a factor of two and three for electrons and holes respectively, indicating that the recombination is not monomolecular. The $L_d$ variations were correlated to the features in photoluminescence lifetime studies. Analysis of lifetime variation shows intensity-dependent monomolecular and bimolecular recombination trends with recombination constants determined to be ~ 9.3 $\times 10^6$ s$^{-1}$ and ~ 1.4 $\times 10^{-9}$ cm$^{3}$s$^{-1}$ respectively. Based on the trends of $L_d$ and lifetime, it is inferred that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity.