Dipole-Assisted Charge Separation in Organic-Inorganic Hybrid Photovoltaic Heterojunctions: Insight from First-Principles Simulations

It has been observed that the external quantum efficiency (EQE) of a hybrid organic/inorganic (P3HT/ZnO) solar cell can be tripled by inserting a monolayer of fullerene (PCBA) at the donor (P3HT) and acceptor (ZnO) interface. First-principles simulations are performed to understand the origin of the increased EQE, offering a complementary perspective to the experiment. The interfacial atomic and electronic structure as well as energy alignment are examined for both the bare and PCBA-modified interface. The presence of PCBA induces an interfacial dipole and shifts up the lowest-unoccupied-molecular-orbital (LUMO) level of P3HT relative to the conduction band edge of ZnO. Interfacial electron dynamics including forward and backward charge transfer time scales are estimated. We find that the two interfaces have similar forward electron transfer rates but their transfer mechanisms differ. The PCBA-modified interface exhibits a weaker adiabatic but a stronger nonadiabatic charge transfer. In contrast, the charge recombination time scale of the modified interface is 20 times slower than that of the bare interface, thanks to reduced donor/acceptor wave function coupling as well as increased donor/acceptor energy offset. By slowing down the charge recombination, the PCBA-modified heterojunction offers the superior performance observed in the experiment.