Spatial inhomogeneities in Cu(In,Ga)Se₂ solar cells analyzed by an electron beam induced voltage technique

Spatial variations of the local open circuit voltage in Cu(In,Ga)Se₂ solar cells are analyzed by an electron beam induced voltage (EBIV) technique. The major pattern visualized by our EBIV measurements are spatial inhomogeneities on a length scale of between 5 and 20 μm. Quantitative evaluation of the EBIV signals shows that the loss of open circuit voltage due to the inhomogeneities is about 100 mV. Additional analysis of our samples by energy dispersive x-ray analysis excludes fluctuations of the Ga or Cu content as the source of the inhomogeneities. Instead, the spatial inhomogeneous supply of Na from the glass substrate turns out as a possible origin of inhomogeneities. Spatially resolved secondary ion mass spectroscopy measurements show that the Na content of our Cu(In,Ga)Se₂ samples varies between 0.03 and 0.15 at. % on a length scale of tens of micrometers.