Revealing the Band Structure of FAPI Quantum Dot Film and Its Interfaces with Electron and Hole Transport Layer Using Time Resolved Photoemission

Lead halide perovskite nanocrystals have attracted attention in the field of nanocrystal-based light-emitting diode and solar cells, because their devices showed high performances in only a few years. Among them, CsPbI₃ is a promising candidate for solar cell design in spite of a too wide band gap and severe structural stability issue. Its hybrid organic–inorganic counterpart (NH₂)₂CHPbI₃ (FAPI), where the Cs is replaced with formamidinium (FA), presents a smaller band gap and also an improved structural stability. Here, we have investigated the energy landscape of pristine FAPI, and the interface of FAPI with electron and hole selective layers using transport, photoemission, and noncontact surface photovoltage by means of time-resolved photoemission. We have found from transport and photoemission that its Fermi level is deeply positioned in the band gap, enabling the material to be almost intrinsic. Time-resolved photoemission has revealed that the interface of pristine FAPI is bended toward downward side, which is consistent with a p-type nature for the interface (i.e., hole as majority carrier). Using TiOₓ and MoOₓ contacts, as a model for the electron and hole transport layer, respectively, allows the electron transfer from the TiOₓ to the FAPI and from the FAPI to the MoOₓ. The latter is revealed by time-resolved photoemission showing inverted band bending for the two interfaces. From these results, we clearly present the energy landscape of FAPI and its interfaces with TiOₓ and MoOₓ in the dark and under illumination. These insights are of utmost interest for the future design of FAPI-based solar cell.