Ultrathin Cu(In,Ga)Se2 solar cells with point-like back contact in experiment and simulation

We apply the concept of point contact solar cells to a model system having 190 nm thick Cu(In,Ga)Se2 (CIGSe) films by using a SiO2 back side film with periodic openings to the molybdenum layer being the electrical contact. The openings are plasma etched through a mask prepared by laser interference lithography. We find a maximum increase of the short circuit current density of 25% for a SiO2 thickness of 60 nm and structure length of 1,1 μm. This gain is due to (1) coherent optical reflection, (2) light scattering enhancing the quantum efficiency at all wavelengths and (3) an anticipated wave guide effect which boosts the quantum efficiency particularly at long wavelength. The best efficient solar cells with 190 nm thick CIGSe has a conversion efficiency of 9% (fill factor = 70%) without anti-reflection coating. These experimental results can largely be confirmed by simulation.