Evidence of p- to n-type inversion at CIGS grain boundaries: A depth-dependent surface electron microscopy study*

Chalcopyrite Cu(In$_{1-x}$Ga$_x$)Se$_2$ (CIGS) is an interesting photovoltaic material because it holds the laboratory record for thin-film solar power conversion efficiency ($\eta > 20$\%) despite its disordered microcrystalline structure. However, commercialization of this technology has been limited by structural and chemical variations in CIGS films. Many microscopic and spectroscopic studies have shown built-in electric potentials ($\Phi_{\textrm{bi}}$) at CIGS grain boundaries. This may assist with electron-hole separation, but the reported magnitude and statistical distribution of $\Phi_{\textrm{bi}}$ remains inconsistent between studies. In this work, photoemission and low-energy electron microscopies (PEEM and LEEM) were used to reconcile these reported differences. Highly surface sensitive PEEM measurements showed $\Phi_{\textrm{bi}} \sim 0.5$ V, which was consistent with most other reports. However, more bulk sensitive LEEM measurements showed $\Phi_{\textrm{bi}} \sim 1.5$ V, which strongly suggests p- to n-type inversion at CIGS grain boundaries. This formation of pn junctions at CIGS grain boundaries is likely responsible for the high performance of CIGS photovoltaics.