Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity

Abstract

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V−1 s−1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

Excellent Long‐Range Charge‐Carrier Mobility in 2D Perovskites

“…In the TPC experiment, transistor‐type electrode‐gap devices, which probe the transport in a direction parallel to the plane of the substrate, are illuminated with a short (ns) above band gap laser pulse, and the transient conductivity is recorded. [ 21 ] We give full details in the Supporting Information, but briefly, during and following the photoexcitation pulse, free carriers are generated which result in the photoconductivity signal. In 3D bulk perovskites, where exciton binding energies are low, we often expect that all of the absorbed photons lead to the generation of free carriers.…”

“…An assumption of ϕ = 1 for FA 0.9 Cs 0.1 PbI 3 is valid for N < 10 16 cm –3 and corrections only need to be made for higher N , as has been described in previous work. [ 21 ]…”

“…Other reports have estimated the long‐range mobility of 3D perovskites as 0.2–6.7 cm 2 (V s) –1 (spread due to different preparation routes), using microwave conductivity, photo‐induced transmission and reflection spectroscopy, or PL quenching. [ 15,21,58,59 ]…”

“…While the assumption of ϕ = 1 is also true for the 100 s of ns, long‐range, TPC measurement of 3D perovskites, such as MAPbI 3 or FA 0.9 Cs 0.1 PbI 3 at low charge carrier densities, it certainly is not true at higher charge carrier densities due to the law of mass action, as has been investigated in a recent study. [ 21 ]…”

“…While the assumption of φ = 1 is also true for the 100 s of ns, long-range, TPC measurement of 3D perovskites, such as MAPbI 3 or FA 0.9 Cs 0.1 PbI 3 at low charge carrier densities, it certainly is not true at higher charge carrier densities due to the law of mass action, as has been investigated in a recent study. [21] 2D RP perovskites with n < 5 have exciton binding energies on the order of or greater than 100 meV and are considered to be "excitonic materials", since a significant fraction of excitons is formed following photoexcitation. [22] Excitons, as pseudo particles with a net charge of zero, do not drift in an electric field and so do not contribute to conductivity.…”