The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells

TL;DR

This study evaluates how KPFM operation modes and environmental conditions affect the measurement of grain boundary band bending in perovskite and CIGSe solar cells, revealing limitations of common methods and environmental influences.

Contribution

It demonstrates that amplitude modulation KPFM is unsuitable for rough samples and shows that environmental exposure significantly impacts grain boundary measurements.

Findings

AM-KPFM is unsuitable for rough samples like CIGSe and perovskite.

Air exposure alters grain boundary contrast, leading to potential misinterpretation.

FM-KPFM indicates negligible band bending at grain boundaries in non-air-exposed samples.

Abstract

An in-depth understanding of the electronic properties of grain boundaries (GB) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very high and hence limit the solar cell device performance. Kelvin Probe Force Microscopy (KPFM) is the method of choice to investigate GB band bending on the nanometer scale and thereby helps to develop passivation strategies. Here, it is shown that amplitude modulation AM-KPFM, which is by far the most common KPFM measurement mode, is not suitable to measure workfunction variations at GBs on rough samples, such as Cu(In,Ga)Se2 and CH3NH3PbI3. This is a direct consequence of a change in the cantilever-sample distance that varies on rough samples. Furthermore, we critically discuss the impact of different environments (air versus vacuum) and show that air exposure alters the GB and facet contrast, which leads to erroneous interpretations of the GB physics. Frequency modulation FM-KPFM measurements on non-air-exposed CIGSe and perovskite absorbers show that the amount of band bending measured at the GB is negligible and that the electronic landscape of the semiconductor surface is dominated by facet-related contrast due to the polycrystalline nature of the absorbers.