Energetics of the Charge Generation in Organic Donor-Acceptor Interfaces

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 (V OC ). In this paper we present a coarse grained mixed quantum-classical model that provides insights into the charge generation process for small energy offset interfaces. We have studied the influence of the energetic driving force and the vibronic effects on the charge generation and photovoltaic energy conversion. By analyzing the effects of the Holstein and Peierls vibrational couplings, we find, quite generally, 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. By examining the electron-hole binding energy as a function of time, we evince two distinct regimes for charge dissociation: the temperature independent excitonic spread on the sub-100 fs timescale whereas the dissociation of the charge-transfer state occurs on the timescale of tens to hundreds of picoseconds, depending on the temperature, so that when the electron-hole pair reaches the interface its binding energy is much smaller that the initial excitonic binding energy.