  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.
