Direct observation of copper depletion and potential changes at copper indium gallium diselenide grain boundaries

We have used micro-Auger electron spectroscopy, cathodoluminescence spectroscopy, and work function measurements in copper indium gallium diselenide polycrystalline solar cell films cleaved in ultrahigh vacuum. We establish that, relative to the grain interior, the grain boundary shows (1) a Cu composition decrease, as large as a factor of two, (2) a work function decrease of up to 480 meV, and (3) no additional radiative recombination centers despite a high concentration of grain boundary (GB) defects. These results confirm theoretical predictions that (i) polar GB interfaces are stabilized by massive $(∼50%)$ removal of Cu atoms, leading to (ii) a valence band offset between GB and grain interiors that (iii) repels holes from the GB, thus likely reducing GB electron-hole recombination and improving photovoltaic (and other photonic) device operation.