Nanoscale Structural Heterogeneity and Efficient Intergrain Charge Diffusion in a Series of Mixed MA/FA Halide Perovskite Films

Abstract

Perovskite solar cells have made tremendous technological progress, but knowledge of the fundamental photophysical properties of perovskite materials is equally important for further advancement of the field. We use fluorescence microscopy to study the nanoscale properties of a series of mixed-cation perovskite MA 1−x FA x PbI 3 films, an important photovoltaic material. Measuring photoluminescence spectra on submicrometer scales reveals the compositional heterogeneity of the films. The heterogeneity is largest for the FA 50% fraction films which contain purely MA domains, purely FA domains, as well as domains composed of mixed MA/FA cations of varying ratios. The films also show photoluminescence intensity fluctuations (blinking), which reflects dynamic nonradiative quenching. The quenching is most suppressed for the FA 50% films which contain the truly mixed MA/FA domains. Further, the blinking is correlated between locations that are micrometers apart, indicating that the grain boundaries do not function as traps and are transparent toward efficient charge migration.

Air-stable mixed cation lead halide perovskite films and microscopic study of their degradation process

“…This range covers the whole spectral range of all compositions from FA 0% to FA 100%, as reported before. 15 This result is evidence of the local compositional heterogeneity of the films and means that the FA 50% (EA) samples contain purely MA domains (FA 0%-like spectra), purely FA domains (FA 100%-like spectra) and domains composed of mixed MA/FA cations of varying ratios. The wavelengths corresponding to the FA 0% and FA100% compositions are marked in the histograms by red lines.…”

“…13 Here, we report the preparation of mixed cation perovskite films in air with long-term stability and characterization of their degradation process by photoluminescence (PL) microscopy and spectroscopy over extended periods of time. PL microscopy has been a powerful tool to reveal the relationship between the nanoscale structure and properties in a variety of perovskite materials, 14 including films [15][16][17][18] and micro-/nanocrystals. 19,20 We show that MA 1−x FA x PbI 3 films with x of 0.3 and 0.5 prepared in air using EA antisolvent are compositionally stable in ambient air for more than a year.…”

Self‐Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

“…The phenomenological model of metastable supertraps in MHPs readily explains many transient effects of MHPs. [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 35 ] Although we cannot yet tell their chemical nature, we know with confidence that the supertraps are extremely efficient NR centers in comparison to any others. Indeed, for the PL intensity to drop by a factor of two by adding one supertrap, the induced NR rate should be at least equal to the total recombination rate (the radiative rate plus NR recombination rate due to all other NR centers already present before adding of the supertrap).…”

“…We have the following crucial observations: i) PL blinking in MAPbI 3 individual crystals [ 24 , 26 ] and films [ 34 , 35 , 68 ] and ii) PL quenching under strain likely occur due to stabilization of supertraps in their active state. These two observations strongly suggest that the metastable NR centers are indeed responsible for the majority of NR recombination in MAPbI 3 .…”

“…[ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ] When comparing classical semiconductor quantum dots [ 21 , 32 ] with MHPs we note that PL blinking and flickering is not only observed in MHP nanocrystals, [ 33 ] but also in sub‐micrometer crystals, grains in thin films and even in micrometer‐sized objects. [ 26 , 30 , 34 , 35 ] The current understanding is that the PL intensity fluctuations occur due to the presence of very efficient, yet metastable nonradiative (NR) recombination centers, also called supertraps (Figure S1 , Supporting Information). Due to the long‐range charge carrier diffusion in MHPs and large capturing and recombination cross sections of the supertraps, just one active supertrap can dominate the NR recombination in a whole sub‐micrometer MHP crystal.…”