Electronic Structures and Photoconversion Mechanism in Perovskite/Fullerene Heterojunctions

It has been generally believed and assumed that organometal halide perovskites would form type II P-N junctions with fullerene derivatives (C-60 or PCBM), and the P-N junctions would provide driving force for exciton dissociation in perovskite-based solar cell. To the best of our knowledge, there is so far no experiment proof on this assumption. On the other hand, whether photogenerated excitons can intrinsically dissociate into free carrier in the perovskite without any assistance from a P-N junction is still controversial. To address these, the interfacial electronic structures of a vacuum-deposited perovskite/C-60 and a solution-processed perovskite/PCBM junctions is directly measured by ultraviolet photoelectron spectroscopy. Contrary to the common believes, both junctions are found to be type I N-N junctions with band gap of the perovskites embedded by that of the fullerenes. Meanwhile, device with such a charge inert junction can still effectively functions as a solar cell. These results give direct experimental evidence that excitons are dissociated to free carriers in the perovskite film even without any assistance from a P-N junction.