Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.363
Publication date: 7th February 2022
In this work we provide a comprehensive microscopic model for the analysis of the energetics and dynamics of the first electronic charge-transfer (CT) state in donor:acceptor (D:A) bulk heterojunction (BHJ) blends, showing wide optical bandgaps and large frontier orbital energy offset. Specifically, we focus on N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) : 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN). Although not at all appealing for their performances, these BHJ systems are nevertheless a very interesting case study for resolving the existing dilemmas about the nature and fate of interfacial CT states, as these can be probed over an unusually large energy window. By varying the blend composition and temperature, we unravel the static and dynamic contributions to the disordered density of states (DOS) of the CT states manifold, and assess their recombination to the ground state. Our accurate computational protocol entails a combination of molecular dynamics (MD) simulations, density functional theory (DFT) and time-dependent (TD) DFT calculations, and a microelectrostatic (ME) model, specifically designed to include environmental effects. We observe that the CT DOS is broadened mostly because of both conformational disorder associated with the NPB flexibility and electrostatic disorder due to the HAT-CN quadrupolar layout, and is essentially static on the timescale of charge separation/recombination. We also predict fast (~ps) non-radiative decay from the lowest CT states that should overcome charge separation. Therefore, the tail of CT DOS does not contribute to charge generation but rather to recombination. This is fully confirmed by comparison to experimental sensitive absorption versus external quantum efficiency (EQE) measurements showing that the CT DOS explored by EQE is cropped at low energy.
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