Energetics of the Charge Generation in Organic Donor-Acceptor Interfaces

TL;DR

This study uses a mixed quantum-classical approach to analyze charge generation in organic solar cell interfaces, revealing how vibronic effects and energy offsets influence photovoltaic efficiency.

Contribution

It introduces a combined Ehrenfest and Redfield theoretical framework to elucidate vibronic effects on charge separation in small energy offset interfaces.

Findings

Vibrational couplings generally enhance charge generation.

Holstein relaxation reduces charge separation efficiency.

Peierls coupling consistently promotes charge generation.

Abstract

Non-fullerene acceptor (NFA) materials have posed new paradigms for the design of organic solar cells (OSC), whereby efficient carrier generation is obtained with small driving forces, in order to maximize the open-circuit voltage. In this paper we use a coarse-grained mixed quantum-classical method, that combines Ehrenfest and Redfield theories, to shed light on charge generation process in small energy offset interfaces. We have investigated the influence of the energetic driving force as well as the vibronic effects on the charge generation and photovoltaic energy conversion. By analyzing the effects of the Holstein and Peierls vibrational couplings, we find that vibrational couplings produce an overall effect of improving the charge generation. However, the two vibronic mechanisms play different roles: the Holstein relaxation mechanism decreases the charge generation whereas the Peierls mechanism always assists the charge generation. Moreover, by examining the electron-hole binding energy as a function of time, we evince two distinct regimes for the charge separation: the temperature independent excitonic spread on a sub-100 fs timescale and the complete dissociation of the charge-transfer state that occurs on the timescale of tens to hundreds of picoseconds, depending on the temperature. The quantum dynamics of the system exhibits the three regimes of the Marcus electron transfer kinetics as the energy offset of the interface is varied.