Universal Band Alignment Rule for Perovskite/Organic Heterojunction Interfaces

In contrast with the band alignment rule governing conventional semiconductor heterojunctions, the Fermi level position (i.e., work function, ϕ_psk) of the perovskite and the differences in ionization energies of the organics are shown to dictate the band alignment and band offsets at perovskite/organic heterojunction interfaces. The experimental data show that the interface band alignments follow a universal alignment rule with three different energy windows: pinning of unoccupied molecular orbital states (LUMO) to the perovskite Fermi level, band offsets mediated by perovskite work functions, and pinning of occupied molecular orbital states (HOMO) to the perovskite Fermi level. The surface-passivated perovskites and several common native perovskites are all found to follow the same band alignment rule. Data scattering in measured band offsets, however, is observed on the same type of perovskites without proper surface passivation treatment. Solar cells made on the perovskites without surface passivation are also found to exhibit a large dispersive distribution in energy conversion efficiencies.