Microstructural Inhomogeneity and Defect Sites in Wide Bandgap Mixed Perovskites: A Scanning Probe Microscopy Study
Jae Sung Yun a b
a Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford, Surrey GU2 7XH, UK
b University of New South Wales, Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Engineering, Sydney 2052, Sydney, Australia
Oral, Jae Sung Yun, presentation 008
DOI: https://doi.org/10.29363/nanoge.nipho.2023.008
Publication date: 3rd April 2023

Over the past few years, Organic-inorganic halide perovskites (OIHPs) have demonstrated remarkable improvements in their optoelectronic performance. The use of mixed-cations and mixed-halides in solar cell fabrication has enabled bandgap tunability for multi-junction solar cells. Nevertheless, a systematic evaluation of the microstructural behavior of wide bandgap (>1.7eV) mixed perovskites is still necessary due to the photo-instability that is dependent on composition.

This study focuses on the microstructural inhomogeneity in (FAPbI3)x(MAPbBr3)1-x, which exhibits a bandgap range from 1.55eV to 2.3eV. Advanced scanning probe microscopy techniques are employed to investigate this behavior. Contact potential difference (CPD) maps are measured by Kelvin probe force microscopy (KPFM), revealing an increase of lower CPD grains that correspond to the flat polymorph abnormal grains in the topographical map as the concentration of MAPbBr3 is increased. Chemical component analysis, performed using helium ion microscopy with secondary ion mass spectrometry, reveals that these flat grains are rich in MA, Pb, and I. Spectral photoluminescence shows clear phase segregation dependence on composition as MAPbBr3 increases, which is responsible for the formation of abnormal grains.

Bias-dependent piezo-response force microscopy (PFM) measurements confirm that ions are vigorously migrated on the flat grains, resulting in hysteretic dynamics in the piezoresponse-electric bias (P-E) loop. Finally, light-assisted KPFM measurements reveal that the flat grains contribute to phase segregation. Through various microstructural characterizations, our results indicate that the abnormal grains, as defect sites, are detrimental to phase segregation and ion migration.

In summary, this study provides new insights into the microstructural behavior of wide bandgap mixed perovskites, revealing the role of abnormal grains in phase segregation and ion migration. These findings contribute to the development of more efficient and stable perovskite solar cells, by providing a better understanding of the underlying mechanisms of the material's behavior.

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