Open circuit voltage from quasi-Fermi level splitting in polycrystalline Cu(In,Ga)Se2 films by photoluminescence with lateral sub-micron resolution

Cu(In,Ga)Se₂ thin films from a pilot/production line with nominal 12% module efficiency have been analyzed by confocal photoluminescence with lateral submicron resolution. We translate photoluminescence data into energetic splitting of the quasi-Fermi levels which indicates the maximum achievable open circuit voltage of "hypothetic" finally processed heterojunctions. We see lateral variations of the pl-yield in these polycrystalline chalcopyrites up to a factor 6 that at 70 K corresponds to an alteration of the minority quasi-Fermi level of about 11 meV. Even in heterojunctions (TCO / CdS / CIGS / Mo) despite the equipotential surface of the TCO we find a comparable lateral variation of the separation of the quasi-Fermi levels. The comparison of laterally resolved PL-data with simultaneously recorded optical reflection exhibits the granularity of the CIGS surface and shows substantially higher spatial frequencies compared to luminescence patterns.