Abstract
To fully unlock the potential of metal halide perovskites (MHPs) for use in optoelectronic devices, a comprehensive understanding of their electronic properties is in strong demand but presently lacking. This photoelectron spectroscopy study reveals that the thin films of three important mixed-cation/mixed-halide MHPs behave like intrinsic semiconductors with a very low defect concentration. The Fermi level position in the bandgap can be varied by almost 1 eV by choosing substrates of appropriate work function for samples that were handled under inert conditions. Upon oxygen exposure, two organic/inorganic-cation MHPs become strongly p-doped due to oxygen diffusion into the bulk, a process that is fully reversible when storing the samples in ultrahigh vacuum. In contrast, all-inorganic CsPbI1.8Br1.2 exhibits no electronic property changes upon oxygen exposure. Nonetheless, oxygen is found to effectively remove (light-induced) lead-related surface states of CsPbI1.8Br1.2.
Highlights
IntroductionThe successful application of metal halide perovskites (MHPs) in optoelectronic devices, most notably in solar cells, has sparked considerable interest due to their fundamental material properties. Accessing the electronic properties at the surface and devicerelevant interfaces of MHPs allows for a better understanding of the material itself and provides the insight needed for further enhancing device performance. In this context, the electronic properties of MHPs have been intensively investigated; the pronounced variations of their apparent electronic characteristics have been reported, such as huge discrepancies in the Fermi level (EF) position in the energy gap. A probable underlying mechanism was reported recently for the prototypical methylammonium lead triiodide (MAPbI3), where the reversible diffusion of oxygen into and out of thin films was revealed, which goes in hand with the reversible p-doping of MAPbI3 by oxygen molecules. In this study, it was further shown that MAPbI3 thin films prepared under inert conditions and without oxygen exposure behave like an intrinsic, i.e., undoped, semiconductor
The electronic properties of mixed-cation/mixed-halide metal halide perovskites, i.e., Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3, FA0.83MA0.17Pb(I0.83Br0.17)3, and CsPbI1.8Br1.2, deposited onto substrates with widely varying work functions were studied by ultraviolet photoelectron spectroscopy (UPS)
The dependencies of the perovskite work function and the position of EF in the gap on Φsub show the behavior expected for an intrinsic semiconductor, with a regime following the Schottky–Mott limit and regimes where EF-pinning close to the conduction/valence band edges occurs
Summary
The successful application of metal halide perovskites (MHPs) in optoelectronic devices, most notably in solar cells, has sparked considerable interest due to their fundamental material properties. Accessing the electronic properties at the surface and devicerelevant interfaces of MHPs allows for a better understanding of the material itself and provides the insight needed for further enhancing device performance. In this context, the electronic properties of MHPs have been intensively investigated; the pronounced variations of their apparent electronic characteristics have been reported, such as huge discrepancies in the Fermi level (EF) position in the energy gap. A probable underlying mechanism was reported recently for the prototypical methylammonium lead triiodide (MAPbI3), where the reversible diffusion of oxygen into and out of thin films was revealed, which goes in hand with the reversible p-doping of MAPbI3 by oxygen molecules. In this study, it was further shown that MAPbI3 thin films prepared under inert conditions and without oxygen exposure behave like an intrinsic, i.e., undoped, semiconductor. It was further shown that MAPbI3 thin films prepared under inert conditions and without oxygen exposure behave like an intrinsic, i.e., undoped, semiconductor This was concluded from the observation in photoemission experiments that the EF position followed the substrate work function (Φsub), unless Fermi level pinning at the conduction and valence band edges stopped the movement of EF. Different cation and halide combinations can affect the carrier density and doping so that an extrapolation of results obtained for MAPbI3 to modern MHPs should be substantiated by experiments In this regard, it was reported that the EF position in the gap can vary with Φsub by as much as 1.4 eV for bromide-only perovskites, i.e., Cs0.05FA0.85MA0.1PbBr317 and CsPbBr3,29 and by as much as 0.5 eV for a mixed-halide perovskite, i.e., FA0.83MA0.17Pb(I0.83Br0.17)3.22 the Φsub range used in these studies was not sufficiently wide to investigate the full possible EF-movement range, and a more comprehensive study is still missing.
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