Investigation of sub-granular dynamics in halide perovskites via atomic force microscopy

Abstract

Halide perovskites are considered as next generation solar cell absorbers due to their several advantages over conventional solar cell absorber materials such as adjustable bandgap, high photoluminescence (PL) quantum yield, low charge recombination rate, long charge diffusion length, and defect tolerance. However, perovskite photovoltaic devices suffer from power conversion losses at interfaces. Therefore, understanding the local features and charge carrier dynamics at interfaces is crucial, making macroscopic measurements inefficient for this purpose. Among the microscopic techniques, optical microscopy and its derivatives, such as PL microscopy, are the most common ones. However, they suffer from the diffraction limit, resulting in low-resolution imaging.

In this work, I focused on studying local features in halide perovskite films and devices at internal interfaces, such as grain boundaries and ferroelastic twin domains, or external interfaces in devices where two components of a perovskite-based device meet. Here, atomic force microscopy (AFM) comes into play. To understand the nanoscale properties of perovskite interfaces, I used electrical AFM modes such as piezoresponse force microscopy (PFM), conductive AFM (C-AFM), and Kelvin probe force microscopy (KPFM).

First, I investigated the strain properties in halide perovskites by monitoring ferroelastic twin domains via PFM and x-ray diffraction (XRD). I introduced strain to halide perovskite films by changing the precursor solution. PFM measurements showed altered twin domain patterns that are correlated with strain changes within films. I used XRD measurements to support my claim for a change in overall strain and twinning behavior in the films. My investigation revealed that any chemical gradient in halide perovskites leads to a strain gradient as well.

Furthermore, I investigated the local charge carrier dynamics and conductivity at halide perovskite grains and grain boundaries via time-resolved KPFM and C-AFM. Photoconductivity and photovoltage maps I obtained suggest that grain boundaries are high-defect areas that promote faster electron-hole recombination and ion migration. Furthermore, the behavior of charge carriers at grain boundaries changes when grain size changes.

Ultimately, this work shows how sub-granular features and device interfaces affect charge carrier dynamics in halide perovskite devices. Therefore, this work may contribute to the optimization of halide perovskite devices for commercialized use.