Fundamentals of charge transfer processes in non-fullerene-based photovoltaics: Insights from atomic scale modelling
Cleber Marchiori a, Marlus Koehler b, C. Moyses Araujo a c
a Department of Engineering and Physics, Karlstad University, Sweden
b Departament of Physics, Universidade Federal do Paraná, Brazil
c Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Sweden
Online Conference
Proceedings of NFA-Based Organic Solar Cells: Materials, Morphology and Fundamentals (NFASC)
Online, Spain, 2021 February 3rd - 4th
Organizers: Natalie Banerji and Feng Gao
Oral, Cleber Marchiori, presentation 007
Publication date: 25th January 2021

In the last few years an impressive power conversion efficiency improvement has been observed for organic photovoltaic devices thanks to the advent of the so-called Non-Fullerene Acceptors (NFA).[1] Following this development, an impressive number of novel acceptors have been presented in the literature. Together with the synthesis of novel and more efficient acceptors some fundamental questions concerning the charge photogeneration start also to gain a great deal of attention. For instance, the fact that those new materials are also able of absorbing light in the visible range and, therefore, they strongly contribute to the photocurrent. In this context, not only the traditional electron transfer process from the hole transporting material (usually called donor) to the electron transporting material (for historical reasons called acceptor) should be taken into account but also the charge transfer from the acceptor to the donor, often called hole-transfer process. Both channels for charge transfer have been the focus of intensive discussion as they can be activated even with very small energy-level offsets between the donors and acceptors.[2,3] In this context, we have employed an atomistic scale modelling within the framework of density functional theory to investigate some of the most representative NFA small-molecules, viz. perilene and indacenodithiophene derivatives, aiming to rationalize the underlying charge transfer mechanism, in particular the hole transfer processes. We show that the electronic coupling between neighbor molecules along with the orbital delocalization have a significant impact on the exciton binding energy allowing its dissociation even in low-driving force system.[4] The fundamental electrochemical gap (difference between the redox potentials) are calculated from the full Gibbs free energies, which is then subtracted from the optical band gap to provide an assessment of the exciton binding energies. The crystal structure of the organic compounds was resolved by using an evolutionary algorithm, which in turn allowed us to further understand the molecular packing and electronic structure in solid-state. Such structures have been used also to construct cluster models to evaluate the effects of electronic delocalization of the energy levels. This methodology is shown to be a promising tool to a first assessment of the charge separation process as well as to provide some guidelines to the development of novel molecular NFA.

CFNM and CMA thank the Swedish National Infrastructure for Computing (SNIC) at the PDC Center for High-Performance Computing and National Supercomputer Centreat Linköping University (NSC), The Swedish Research Council (VR) (grant no. 2014-05984), Swedish Energy Agency (grant no. 45420-1).

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