Evaluation of Kelvin probe force microscopy for imaging grain boundaries in chalcopyrite thin films

In view of the outstanding performance of polycrystalline thin film solar cells on the basis of Cu(In,Ga)Se₂, the electrical activity at grain boundaries currently receives considerable attention. Recently, Kelvin probe force microscopy (KPFM) has been applied to characterize the properties of individual grain boundaries, observing a drop in the work function in many cases. We present finite element simulations of the electrostatic forces to assess the experimental resolution of KPFM. Depending on the tip-sample distance, the observed drop in the work function amounts to only a fraction of the real potential drop. The simulations are considered for different grain boundary models and consequences for the quantitative evaluation of experimental results are discussed.