Abstract
Molecular orientations and interfacial morphologies have critical effects on the electronic states of donor/acceptor interfaces and thus on the performance of organic photovoltaic devices. In this study, we explore the energy levels and charge-transfer states at the organic donor/acceptor interfaces on the basis of the fragment-based GW and Bethe–Salpeter equation approach. The face-on and edge-on orientations of pentacene/C60 bilayer heterojunctions have employed as model systems. GW+Bethe–Salpeter equation calculations were performed for the local interface structures in the face-on and edge-on bilayer heterojunctions, which contain approximately 2000 atoms. Calculated energy levels and charge-transfer state absorption spectra are in reasonable agreements with those obtained from experimental measurements. We found that the dependence of the energy levels on interfacial morphology is predominantly determined by the electrostatic contribution of polarization energy, while the effects of induction contribution in the edge-on interface are similar to those in the face-on. Moreover, the delocalized charge-transfer states contribute to the main absorption peak in the edge-on interface, while the face-on interface features relatively localized charge-transfer states in the main absorption peak. The impact of the interfacial morphologies on the polarization and charge delocalization effects is analyzed in detail.
Highlights
The charge-transfer (CT) states across the interface between electron-donating and electronaccepting materials play central roles in the charge photogeneration process and the power conversion efficiency of organic photovoltaics [1,2,3]
We being by presenting the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) quasiparticle energies, which represent the energy levels for the localized charge carriers
We investigated the energy levels and CT states of the edge-on and face-on orientations of the PEN/C60 interfaces
Summary
The charge-transfer (CT) states across the interface between electron-donating and electronaccepting materials play central roles in the charge photogeneration process and the power conversion efficiency of organic photovoltaics [1,2,3]. The energy offset between the interfacial CT state and an exciton state provides the driving energy for charge separation, but constitutes energy loss in the open-circuit voltage [5]. The CT state can decay to the ground state by radiative or nonradiative charge recombination. The nonradiative recombination is responsible for a significant fraction of the energy loss in the open-circuit voltage [6]. Because of the central importance, understanding and engineering of CT states are essential for improving the power conversion efficiency of organic solar cells
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