Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy.

Sofiia Kosar,Miguel Anaya,Christopher E Petoukhoff,Samuel D Stranks,Stuart Macpherson,Andrew J Winchester,Michael K L Man,Kyle Frohna,Tiarnan A S Doherty,Nicholas S Chan,Julien Madéo,Keshav M Dani

doi:10.1039/d1ee02055b

Abstract

With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. Here, by correlating photoemission and synchrotron-based scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells.

Highlights

With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for generation, low-cost photovoltaic technologies
To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies
We used photoemission electron microscopy (PEEM) and scanning electron analytical techniques to visualize nanoscale defect clusters and obtain some understanding relating to the structure and composition of the surrounding grains.[8]

Summary

Introduction

With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for generation, low-cost photovoltaic technologies. The authors have directly imaged the nanoscale defect clusters, and uncovered the presence of multiple types of defects in state-of-the-art triple cation mixed halide perovskite thin films Depending on their nature, these defect clusters play a surprisingly varied roles in charge carrier trapping – from highly detrimental to relatively benign. An inherent downside to low-cost solution processed hybrid perovskite photovoltaic thin films is the generation of a variety of defects that impact device operation These defects can include atomic vacancies and interstitials,[1,2] unreacted precipitates from the solution processing steps,[3,4] as well as alternative crystalline phases,[5,6] which limit device performance.[7,8,9,10] Designing successful strategies to mitigate these defects strongly depends on our fundamental understanding of their identity and role in performance. The presence of multiple defect types, and their varied roles in device efficiency and stability, suggests the need for targeted approaches to improve the performance of perovskite devices

Methods

Results

Conclusion