Abstract title:Polarons in CH3NH3.PBI3—Formation, Transport and Recombination
Authors:Jarvist Frost(1); Jonathan Skelton(1); Aron Walsh(1)
Presenting author: Jarvist Frost
Institutions:1. University of Bath, Bath, United Kingdom.
Abstract body: Abstract Body
Hybrid halide perovskites have rich solid state physics. A unique characteristic is their soft nature, with response processes over timescales on many orders of magnitude. The key question to understand is how a solution processed (and thus inevitably defective) can have such long recombination times, and thereby long minority charge carrier diffusion lengths and high photovoltaic performance.
In this work we present a multi-scale approach to understanding this problem. We combine solid state models for Frohlich polaron location, with quantitative lattice dynamic calculations. The multi time scales of response requires an extension of standard solid-state models, developed for more simple tetrahedral semiconductors.
We propose that the uniquely low energy optical modes, and soft zone boundary acoustic modes are responsible for carrier scattering and the modest mobility. We build a model for the formation of the polaron, and its migration through the material, based on our prior monte-carlo simulation method of the disordered material. We quantify the beneficial decrease in recombination rate due to segregation of electrons and holes in the 'ferroelectric highways', versus the detrimental decrease in mobility due to disorder. Our new model quantifies the contribution of short-range ferroelectric order on carrier stability and electron-hole recombination in this unique class of materials.
Reduced recombination can occur due to the spin-split indirect-gap. Local ferroelectric distortions generating a crystal field interacts with the high spin-orbit coupling of the lead and iodide atoms. We have directly calculated the reduction in recombination due to this band-structure effect.
We implement mode-following to recover a potential energy landscape from our lattice dynamic calculations, inputting this to a numeric quantum oscillator solution, which is then used within the frozen-phonon approximation to calculate the electron-phonon coupling for the phonon modes.
This work has benefited from funding by the EPSRC and close collaboration with the groups of Mark van Schilfgaarde (King's College London), Piers Barnes (Imperial College London), and Simon Billinge (Columbia, New York).
 J.M. Frost, A Walsh, Acc. Chem. Res., 2016, 49 (3), pp 528–535 (2016).
 P Azarhoosh, et al., ArXiv: 1604.04500 (2016)
 J. M. Frost, K. T. Butler and A. Walsh, APL Mater. 2, 081506 (2014).
 J.M. Frost et al, Nano letters 14 (5), 2584-2590 (2014).
 A.M.A. Leguy et al, ArXiv: 1606.01841 (2016)
 F. Brivio, J. M. Frost et al., Phys. Rev. B 92, 144308 (2015).