title: Determining Band-Edge Energies and Morphology-Dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy.
abstract: We show for the first time that the frontier orbital energetics (conduction band minimum (CBM) and valence band maximum (VBM)) of device-relevant, methylammonium bromide (MABr)-doped, formamidinium lead trihalide perovskite (FA-PVSK) thin films can be characterized using UV-vis spectroelectrochemistry, which provides an additional and straightforward experimental technique for determining energy band values relative to more traditional methods based on photoelectron spectroscopy. FA-PVSK films are processed via a two-step deposition process, known to provide high efficiency solar cells, on semitransparent indium tin oxide (ITO) and titanium dioxide (TiO2) electrodes. Spectroelectrochemical characterization is carried out in a nonsolvent electrolyte, and the onset potential for bleaching of the FA-PVSK absorbance is used to estimate the CBM, which provides values of ca. -4.0 eV versus vacuum on both ITO and TiO2electrodes. Since electron injection occurs from the electrode to the perovskite, the CBM is uniquely probed at the buried metal oxide/FA-PVSK interface, which is otherwise difficult to characterize for thick films. UPS characterization of the same FA-PVSK thin films provide complementary near-surface measurements of the VBM and electrode-dependent energetics. In addition to energetics, controlled electrochemical charge injection experiments in the nonsolvent electrolyte reveal decomposition pathways that are related to morphology-dependent heterogeneity in the electrochemical and chemical stability of these films. X-ray photoelectron spectroscopy of these electrochemically treated FA-PVSK films shows changes in the average near-surface stoichiometry, which suggests that lead-rich crystal termination planes are the most likely sites for electron trapping and thus nanometer-scale perovskite decomposition.
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: R Clayton
authors.identifier: 0000-0003-3167-4801
authors.initials: RC
authors.lastname: Shallcross
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Yilong
authors.initials: Y
authors.lastname: Zheng
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: S Scott
authors.identifier: 0000-0002-9946-2664
authors.initials: SS
authors.lastname: Saavedra
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Neal R
authors.initials: NR
authors.lastname: Armstrong