Migration of Constituent Protons in Hybrid Organic–Inorganic Perovskite Triggers Intrinsic Doping

Abstract

The crucial separation of photocarriers in solar cells can be efficiently driven by contacting semiconductor phases with differing doping levels. Here we show that intrinsic doping surges in methylammonium lead iodide (MAPbI) crystals as a response to environmental basicity. MAPbI crystals were passivated with polybases to induce the deprotonation of its methylammonium ions (MA). Stable crystals showed marked increases in photoluminescence and radiative decay, attributed to the presence of unbalanced charges acting as doped carriers. This emulates in a controlled manner the proton-withdrawing conditions of polycrystalline films, where excess basic precursors are found between grains. Raman spectroscopy showed the collective alignment of MA cations within the intrinsically doped lattices, thus revealing the buildup of electric fields. On this basis, we propose a mechanism for the formation of doping-gradients toward grain boundaries, potentially explaining the extended photocarrier lifetimes and diffusion lengths observed in perovskite solar cells.

What Remains Unexplained about the Properties of Halide Perovskites?

“…Currently, several proposals for which defect species are mobile are debated, [40,98,99] the most relevant ones being the halide [81,[100][101][102] and MA vacancies [103,104] as well as interstitial proton migration. [105][106][107][108] This is because halides and MA are part of the native HaP structure; protons can be present either as intrinsic (formed via MA deprotonation) or extrinsic (introduced during synthesis or contact with H2O) defects.…”

Ion Migration: A “Double‐Edged Sword” for Halide‐Perovskite‐Based Electronic Devices

“…A space‐charge accumulation zone was found in PVSCs under light illumination, which can also be explained by the “mobile ion” accumulation. It became clear that with the assistance of external stimulants such as light illumination and electric bias, ions such as I − and MA + , which were “loosely bonded” with the perovskite matrices, can migrate toward the interface through different channels as shown in Figure c–i, and this process was named as “ion migration.” Besides intrinsic defects such as V I− and V MA+ , the Li + dopant used in the hole transport material (HTL) and the protons (H + ) can also participate in the “ion migration” process.…”

On the Current–Voltage Hysteresis in Perovskite Solar Cells: Dependence on Perovskite Composition and Methods to Remove Hysteresis

“…Migration of ions is related to defects in perovskite, where all constituents of organic cation, inorganic lead cation, and halide anion are to be taken into consideration. Proton migration was reported by using poly‐base to deprotonate methylammonium cation in MAPbI 3 . According to the calculation, the dominant defects in MAPbI 3 are related to the Schottky defect, indicative of a stoichiometric amount of anion and cation vacancies, because of low formation energy .…”