Properties of electronic potential barriers at grain boundaries in Cu(In,Ga)Se2 thin films

The lack of an efficiency increase with increasing Ga content in Cu(In,Ga)Se2 solar cells has attracted much scientific interest. It has been claimed that the physical properties of grain boundaries are responsible for this curious effect. Here, we present an in-depth analysis of electronic potential barriers at grain boundaries (GBs) in a series of Cu(In,Ga)Se2 (CIGSe) thin films using Kelvin probe force microscopy (KPFM) measurements, extending our previous study [Baier et al., Sol. Energy Mater. Sol. Cells 103 (2012) 86-92]. Here, (i) we show, by comparison with data of the crystal lattice orientations, that localization of GBs purely from KPFM topography data allows reliable localization of GBs. (ii) We consider the averaging effect of KPFM due to long-range electrostatic forces for the analysis of the electronic GB properties to determine the real potential barrier height for each individual GB; we determine potential variations ranging from −400 to +400 mV. (iii) We consider the different physical origin of positive and negative potential barriers and present a quantitative analysis of the results to determine charge carrier concentration and defect densities at GBs. From our data and analysis we do not observe any systematic variation of these quantities with the Ga content.