Thermodynamics aspects of charge transfer processes in organic photovoltaics materials: Insights from atomic scale modelling
Cleber Marchiori a, Leandro Franco a, Ellen Moons a, C. Moyses Araujo a b
a Department of Engineering and Physics, Karlstad University, SE-65188 Karlstad, Sweden
b Department of Physics and Astronomy, Uppsala University, Sweden, Uppsala, Sweden
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Cleber Marchiori, presentation 124
Publication date: 20th April 2022

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 donor to the 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. 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. perylene, indacenodithiophene derivatives, and some of the Y-series, 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.[2] In a following step, we assess the thermodynamic aspects of the exciton dissociation by computing the variation of Gibbs free energies when the excited molecules are oxidized or reduced, giving valuable insight on the charge transfer process. [3] Additionally, we address some aspects one should have in mind when determining energy levels and energy gaps and aim to reconcile theory and experiments.[4] The methodology shown here is a promising tool to investigate the charge separation process, as well as to provide guidelines for the development of novel molecular NFA.

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