Heterogeneous Charge Carrier Dynamics in Organic–Inorganic Hybrid Materials: Nanoscale Lateral and Depth-Dependent Variation of Recombination Rates in Methylammonium Lead Halide Perovskite Thin Films

Abstract

We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using highresolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depthdependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic−inorganic hybrid materials.

Microstructural Characterisations of Perovskite Solar Cells – From Grains to Interfaces: Techniques, Features, and Challenges

“…Similarly, Bischak et al used high resolution cathodoluminescence (CL) microscopy to reveal substantial luminescence yield heterogeneity between individual CH 3 NH 3 PbI 3 grains. [19] The spatial resolution of CL is significantly higher than that of PL, [19] and revealed significant spatial heterogeneity in the nature of charge carrier recombination on the nanoscale (Figure 3g,h). Furthermore, varying the electron beam acceleration voltage changes the interaction volume, which allows for properties to be studied at a range of depths in the material.…”

“…[19] This means that alternative techniques such as electron microscopy and atomic force microscopy have to be used. Due to the much shorter wavelength of electrons compared to visible light, modern electron microscopes have resolutions down to 1 nm for scanning electron microscopes (SEM) (which typically operate in an energy range up to 30 kV) and below 0.1 nm for transmission electron microscopes (TEM) (which can operate between 30 kV and 1500 kV).…”

“…[19,24] CL is analogous to photoluminescence, but the excitation source is an electron rather than a photon. If a semiconductor is bombarded by electrons with sufficient energy, electrons in the semiconductor are excited from the lower-energy valence band to the higherenergy conduction band.…”

“…Reproduced with permission. [19] Copyright 2015, American Chemical Society. the photocurrent hysteresis by diffusing into the grain boundaries during long time thermal annealing, thus passivating the deep charge traps by interacting with defects such as I-Pb antisites.…”

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

“…In addition, we studied the emission characteristics of various perovskite films using cathodoluminescence spectroscopy which holds a great potential of bringing out spectral features with a nanometer resolution. [22] …”