Influence of chemical conversion parameters and resulting PbI2 content on carrier density and morphology of the p-type electrodeposited hybrid perovskite CH3NH3PbI3

Abstract

Carrier density of electrodeposited methylammonium lead iodide perovskite (CH3NH3PbI3) can be modulated inside the 1017 to 1020 cm−3 range by carefully selecting the experimental parameters used during the last two chemical conversion steps of their synthesis. This finding has been made possible by performing systematically Mott-Schottky (MS) plots in aqueous solution on various semi-conducting CH3NH3PbI3 perovskite samples. These latter were produced by a three-step synthesis method combining an electrodeposition step with two consecutive chemical conversion steps. In a first step, the galvanostatic electrodeposition of a lead dioxide (PbO2) thin film is carried out. This latter is then converted into lead (II) iodide (PbI2) by immersion in a HI/ethanol solution, this latter being then itself turned in a third and last step into the well-known CH3NH3PbI3 perovskite by a new immersion step in a methylammonium iodide (MAI)/isopropanol solution. This very simple and highly reproducible synthesis method allows the production of perovskite thin films on large surfaces, unlike many other synthesis methods of perovskite thin films. In parallel to the determination of p-type character, carrier concentration and flat band potential of the perovskite samples using Mott-Schottky plots, their chemical composition, their morphology and their crystallinity were characterized by using scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) and compared with those of the products obtained at the end of each of the two first steps, i.e. PbO2 and PbI2 respectively. Interestingly, it was shown that the final product is not always a pure p-type perovskite thin film as various amounts of lead (II) iodide (PbI2) can be detected depending on the experimental parameters used during the two consecutive chemical conversion steps. The main result of this study is that the dopant concentration of perovskite thin films was shown to be inversely proportional to the amount of the remaining PbI2 impurities detected in the bulk of perovskite thin films.