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 (TiO) 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 TiO electrodes. 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.

Recent Advances in Energetics and Stability of Metal Halide Perovskites for Optoelectronic Applications

“…b) A‐site cation substitution for FA‐based perovskite films on ITO and TiO 2 contacts. Reproduced with permission . Copyright 2017, American Chemical Society.…”

Characterization of optically active perovskite electrodes

“…[152,153] A very thorough study investigated the electron injection process into formamidinium lead halide perovskite using spectroelectrochemistry. [154] It was demonstrated in this study that it is very difficult, yet possible, to probe band edge positions via charge carrier injection. During these experiments, however, an irreversible electrochemical reduction occurred, thus making the picture even…”

“…An overview of the possible solution chemistry events was given, although without taking into account the likely contribution of the electrolyte ions. [154] A similar approach was followed for mixed CsPbX3 (X = Cl − , Br − , and I − ) perovskites, where it was assumed that band edge positions can be simply determined from voltammetric data. [155] Apart from determining various physicochemical properties, coupled (photo)electrochemical techniques can offer a fast and convenient way to perform combinatorial analysis of the performance of various HOIP compositions.…”

“…Further complications arise from the dynamic exchange between the ions in the perovskite layer and ions present in the solution, which can alter the composition of the perovskite layer. [199] It has been demonstrated recently that the electrochemical properties of MAPbI3 [152,156] and related materials [153][154][155]157] can be studied in dichloromethane based electrolytes. However, special care must be exercised, because the use of an external electrochemical bias can also induce unintended side reactions (e.g., corrosion of the perovskite layer).…”

“…If there is certainly no process other than charge injection, then voltammetric curves can be employed to determine band edge positions. Similarly to organic semiconductors [210] , however, the onsets of the redox peaks [154] and not the position of the peak maxima [155] have to be correlated with the respective band edge positions. Finally, during prolonged experiments migration of mobile ions within the perovskite structure might also happen thus complicating the picture even further.…”