Photoluminescence, open circuit voltage, and photocurrents in Cu(In,Ga)Se 2 solar cells with lateral submicron resolution

Abstract

Thin film Cu(In,Ga)Se 2 solar cells with average efficiencies in the 14% regime have been analyzed by photoluminescence (PL) with lateral micrometer-resolution. PL has been evaluated according to Planck's generalized law with respect to splitting of quasi-Fermi levels under open circuit (OC) and short circuit (SC). Lateral patterns of luminescence yields signalize local fluctuations in quasi-Fermi energies in the range of some tens of milli-electron-volt. These patterns spatially extend to some microns and by far exceed the size of individual grains of 1 μm or less. The responses of PL, and thus the local V OC and short circuit currents turn out to be significantly anti-correlated. This behaviour is explained in terms of a two dimensional network coupling illuminated and dark hypothetic micrometer-sized diodes with different characteristic parameters such as series and parallel resistances, as well as reverse saturation currents.