Quantification of electron accumulation at grain boundaries in perovskite polycrystalline films by correlative infrared-spectroscopic nanoimaging and Kelvin probe force microscopy

Ting-Xiao Qin,Xiao-Feng Huang,Zhong-Qun Tian,Mao-Xin Zhang,Bing-Wei Mao,Song-Yuan Ding,Peng Zheng,En-Ming You

doi:10.1038/s41377-021-00524-7

Abstract

Organic–inorganic halide perovskites are emerging materials for photovoltaic applications with certified power conversion efficiencies (PCEs) over 25%. Generally, the microstructures of the perovskite materials are critical to the performances of PCEs. However, the role of the nanometer-sized grain boundaries (GBs) that universally existing in polycrystalline perovskite films could be benign or detrimental to solar cell performance, still remains controversial. Thus, nanometer-resolved quantification of charge carrier distribution to elucidate the role of GBs is highly desirable. Here, we employ correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy with 20 nm spatial resolution and Kelvin probe force microscopy to quantify the density of electrons accumulated at the GBs in perovskite polycrystalline thin films. It is found that the electron accumulations are enhanced at the GBs and the electron density is increased from 6 × 1019 cm−3 in the dark to 8 × 1019 cm−3 under 10 min illumination with 532 nm light. Our results reveal that the electron accumulations are enhanced at the GBs especially under light illumination, featuring downward band bending toward the GBs, which would assist in electron-hole separation and thus be benign to the solar cell performance.Correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy and Kelvin probe force microscopy quantitatively reveal the accumulated electrons at GBs in perovskite polycrystalline thin films.

Highlights

IntroductionFirst-principle calculations suggest that while the intrinsic grain boundaries (GBs) in inorganic solar cells thin films, such as GaAs and Cu(In, Ga)Se2, generate deep levels states in band gaps and are harmful for the device performance[16], the GBs in the CH3NH3PbI3 films with shallow point defects are electrically benign and are
Organic–inorganic halide perovskites (e.g., CH3NH3PbX3,X = Cl, Br, I), featuring large absorption coefficient, high carrier mobility, and long diffusion length[1,2,3,4,5], become emerging materials for solar cells with rapidly boosted power conversion efficiency (PCE) from 3.8% in 20096 to a recently certified over 25%7
Our results reveal that the electron accumulations are enhanced at the grain boundaries (GBs) especially under light illumination, featuring downward band bending toward the GBs, which would assist in electronhole separation and be benign to the solar cell performance

Summary

Introduction

First-principle calculations suggest that while the intrinsic GBs in inorganic solar cells thin films, such as GaAs and Cu(In, Ga)Se2, generate deep levels states in band gaps and are harmful for the device performance[16], the GBs in the CH3NH3PbI3 films with shallow point defects are electrically benign and are. Qin et al Light: Science & Applications (2021)10:84 beneficial to the PCE of the perovskites solar cells[17,18]. This result is contrary to the conclusions obtained by nonadiabatic molecular dynamics studies combined with time-domain density functional theory calculations that GBs have negative influences owing to the accelerated electron-hole recombinations in CH3NH3PbI319,20. The spatially resolved imaging on photocarrier generations by scanning tunneling microscopy indicates that efficient charge separation occurs at the heterointerface of grains[27]

Methods

Results

Conclusion