Exciton Binding Energies in Organic Photovoltaic Materials: A Theoretical Perspective
Lingyun Zhu, Zhixiang Wei, Yuanping Yi
Abstract
Exciton binding energy (Eb) plays an essential role in organic electronics. For organic solar cells, the existence of Eb necessitates interfacial energy level offsets to drive exciton dissociation into free charge carriers at the donor/acceptor interfaces, which results in an extra energy loss with respect to inorganic and perovskite solar cells. Thus, it is highly desirable to reduce the Eb for improving the performance of organic photovoltaics. In this perspective, we summarize our recent advances on theoretical studies of the Eb in A–D–A small-molecule acceptors for organic photovoltaics. We highlight the importance of the polarization effect on decreasing the Eb by stabilizing charge carriers. Moreover, the polarization energies of charge carriers are found to be greatly influenced by different molecular packing structures. Remarkably, an extremely weak Eb is achieved in the state-of-the-art A–D–A acceptor of Y6 with a three-dimensional compact molecular packing, thus enabling direct photogeneration of free charge carriers even without the assistance of the donor/acceptor interfaces. We hope this perspective would be helpful to decrease the Eb and energy loss for high-efficiency organic solar cells.