Theoretical Investigation of Charge-Transfer Processes at Pentacene–C₆₀ Interface: The Importance of Triplet Charge Separation and Marcus Electron Transfer Theory

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Abstract

Photovoltaic devices have great potential in harvesting solar energy. In organic solar cells, the role of the donor–acceptor heterojunction is critically important; here optical excitation leads to charge separation (CS) and subsequently photocurrent generation. Charge recombination (CR) may also happen, which reduces the overall efficiency of the device. After light absorption, the singlet excitation of a pentacene can achieve CS at the interface, or it may also undergo singlet fission and produce two triplet excitons, which may further undergo CS at the interface. In this work, we study charge-transfer processes at the pentacene–C60 interface, including CS rates in both singlet and triplet surfaces, and the CR rate back to the ground state. We first constructed two different pentacene–C60 interfacial structures by force field optimization. With pentacene–C60 molecular pairs derived from the structures, we calculate the electronic couplings of CS and CR reactions. We found that the electron-transfer couplings have a systematic preference on the configurations. Both singlet and triplet CS coupling prefer to take place in the “head-on” configurations, where the C60 is located at the top of the pentacene, whereas CR coupling prefers the “inserted” configuration, where the pentacene is located between two C60 and is slightly lifted from the layer. In estimating the electron-transfer rates, we found that the interfacial energy shift has a determining effect. When an interfacial energy derived from experimental results is included, we found that the calculated triplet CS rate can reach 1013 s–1, while the singlet CS rate is only ∼105 s–1. The CR rate was calculated to be 1011 s–1 for a singlet ion pair. Our results indicate that triplet CS is fast. Because triplet CS can proceed after singlet fission, with its immediate recombination being spin-forbidden, it is important to photovoltaic performance in the pentacene–C60 system.

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