Impact of Structural Dynamics on the Optical Properties of Methylammonium Lead Iodide Perovskites

Abstract

structure, [2] and changing the ratio of precursor compounds prior to the formation of the perovskite layer. [3] These approaches are based on modifying the crystallization process, for example, by controlling the composition of the precursor solution and by varying processing parameters. Such changes also led to major improvements in perovskite-based light-emitting devices within the last year. Initially, the current efficiencies were improved by optimizing the grain size to values below 100 nm, and by changing the precursor compound molar proportions. [4] More recently, efficient perovskite-based LEDs with external quantum efficiencies exceeding 10% were even obtained by expanding towards lower dimensional perovskite structures. [5] Different structures and dimensionalities resulted in altered charge-carrier dynamics affecting the radiative recombination efficiency. [5,6] By drawing strong correlations between structural properties and their electronic structure, major breakthroughs toward light-emitting devices or in photovoltaics could be achieved in very recent years. To a certain extent, this resembles the evolution in the field of organic semiconductors, where improved understanding of the interplay between morphological and electronic structure proved essential to facilitate the recent progress in organic photovoltaics, and the successful commercialisation of organic LED devices. Also, for hybrid perovskites the question of how crystal structure, shape, and grain size governs the electronic structure will remain an important topic in the future to further improve perovskite-based optoelectronic devices. Since the solution processability of hybrid perovskites enables easy tuning of material compositions, a large number of different hybrid perovskites were reported. For example, exchanging the organic cation can alter the crystal structure, shift phase transitions, as does formamidinium, or change further excited-state dynamics, as is happening with Cs or Ru. Furthermore, different halide anions change the optical and excited state properties. The optical bandgap can be changed from the UV to NIR region by replacing the halide from Cl to Br to I. Mixed-halide systems thereof even provide a way to gradually tune the respective bandgap. In fact, the recently reported, most stable and efficient perovskite solar cells (PSCs) are based on a highly optimized mixture of different anions and cations. [1,7] Nevertheless, the most investigated hybrid perovskite representative is the methylammonium lead iodide perovskite CH 3 NH 3 PbI 3 (MAPbI 3 ), which, in the meantime, has become Organolead halide perovskites have attracted a lot of attention over the recent years mostly due to their bright prospective application in photovoltaic devices. For further development, characterization of their physical properties plays a seminal role in order to gain an in-depth understanding of these mid-bandgap ionic semiconductors. Their unique optical and electronic properties are a result of their characteristic electronic stru...

Real‐Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites

“…The laser power has been optimized for fast acquisition and as low as possible introduced power. We note, that the laser excitation power may play an important role in PL quantum efficiency and dynamics . Wang et al, for example, found a photo‐driven transformation in CsPbX 3 nanostructures.…”

“…Wang et al, for example, found a photo‐driven transformation in CsPbX 3 nanostructures. Xing et al observed a decreased activation energy for ion migration under light illumination and the presence of migrating ions has direct implications on slow dynamic optical processes in perovskite‐based devices as has been recently reviewed by Panzer et al…”

Investigating the Tetragonal‐to‐Orthorhombic Phase Transition of Methylammonium Lead Iodide Single Crystals by Detailed Photoluminescence Analysis

“…[ 47 ] Based on temperature‐dependent PL measurements and comparison with associated lattice constants, it is possible to estimate the strain that would be necessary to shift the PL by 20 meV to higher energies via the Birch‐Murnaghan‐equation. [ 48 ] With the assumption that lattice dilation is the dominant factor for the temperature‐dependent shift of the PL maximum, [ 6,12 ] we find tensile strain values of 85–165 MPa (see Section S7 in the Supporting Information for details). However, recent studies demonstrated that the unit cell volume of the orthorhombic phase is smaller than the one of the tetragonal phase, [ 4,45,49,50 ] and thus there should be compressive strain instead of tensile strain to affect the tetragonal inclusions.…”

“…For thin films, typically a large temperature range of 20–40 K can be observed in which both phases coexist. [ 4,10,12,41,42 ] This is argued to be caused by the polycrystalline nature of those films, where every grain can have a slightly different transition temperature, depending, e.g., on the grain size, [ 43 ] or the strain induced by the substrate. [ 10,11,44 ] In powders, the temperature range of the coexistence was reported to be smaller with 5–12 K, [ 10,45 ] which is likely due to the absence of substrate‐induced stress on the lose particles.…”

“…[ 10 ] Accordingly, the existence of tetragonal phase even far below 160 K was reported, mostly based on temperature‐dependent optical investigations. [ 4,11,12 ] In contrast, studies on single crystals, often based on scattering investigations, suggest that the phase transition is complete at higher temperatures, i.e., it proceeds in a small temperature range of a few Kelvin. [ 13,14 ] In contrast to scattering methods, the investigation of the phase transition by temperature‐dependent optical spectroscopy is relatively easily accessible.…”