Model for the dynamics of carrier injection in a band with polaronic states: Application to exciton dissociation in organic solar cells

TL;DR

This paper presents a quantum model for carrier injection dynamics in bands with polaronic states, applied to exciton dissociation in organic solar cells, revealing high injection yields despite strong electron-vibration coupling.

Contribution

It introduces a combined quantum scattering and DMFT approach to model carrier injection with polaronic effects in organic solar cells.

Findings

High injection yield achievable with strong electron-vibration coupling

Spectral density analysis reveals pseudo-gaps and polaronic states

Model aligns with experimental observations of vibrationally cold charge transfer states

Abstract

We develop a quantum model for the dynamics of carrier injection in a band that presents a strong carrier-vibration coupling. This coupling modifies the spectral density of the band and can even create pseudo-gaps that sign the onset of polaronic states. The injection of a carrier that interacts with many vibration modes is a complex many-body process that is treated by combining the quantum scattering theory and the Dynamical Mean-Field Theory (DMFT). For the model analysed here, which is adapted to compact phases, the number Z of neighbors of a given site is large and in this limit the DMFT becomes exact. The model is applied to the excitonic dissociation at the donor-acceptor interface for organic solar cells. The main ingredients are the electron-hole Coulomb interaction, the recombination process and the existence of polaronic states in the acceptor band. Using parameters extracted from ab-initio calculations we analyze the spectral density on the charge transfer state (CTS), the average energy transfered to phonons on the CTS and the quantum yield of the injection process. We find in particular that, even with a strong electron-vibration coupling, one can get a vibrationally cold charge transfer state with a high injection yield as often observed experimentally.