Potential of submicrometer thickness Cu(In,Ga)Se2 solar cells

Thin-film solar cells based on Cu(In,Ga)Se2 absorbers with thicknesses similar to or below their optical absorption length are investigated with numerical simulations. The key issue for cells with thicknesses below 1.0μm is to limit back-contact recombination, which can be accomplished by the choice of back-contact material, surface modifications, or inclusion of Ga∕(Ga+In) grading. Unlike in thicker cells, the benefit of grading is maximized if it is limited to a narrow region at the back contact, where it acts as an electron reflector. The potential for optical improvement is evaluated considering variations in back-contact reflectivity and light trapping. Back-contact passivation combined with improved back-contact reflectivity should allow for thinning of the absorber material from 3 to 0.3μm with efficiencies above 17%. A sensitivity analysis with respect to material parameters illustrates that very thin cells suffer substantially stronger from nonuniformities.