Electroluminescence analysis of high efficiency Cu(In,Ga)Se2 solar cells

Abstract

We compare the electroluminescence (EL) of polycrystalline $\mathrm{Zn}\mathrm{O}∕\mathrm{Cd}\mathrm{S}∕\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ heterojunction solar cells with similar band gaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent EL measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperature to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. We find further that the part of the room temperature spectrum that is due to band to band transitions in the respective cells is relatively broader than expected from a direct semiconductor with a homogeneous band gap. We analyze this spectral broadening by a model that accounts for band gap fluctuations of the absorber material. The experimental results show that the dominant part of this spectral broadening results from the intentional band gap grading and not from stochastic band gap fluctuations. We show further that the spectral EL emission is linked to the photovoltaic external quantum efficiency by electro-optical reciprocity. In a similar way, the external EL quantum efficiency is related to the open circuit voltage of the device. We verify experimentally that the difference between radiative and measured open circuit voltage determines the EL external quantum efficiency of the solar cell. The best Cu(In,Ga)${\mathrm{Se}}_2$ solar cell reaches an external light emitting diode quantum efficiency of around 0.1%.