Photogeneration and carrier recombination in graded gap Cu(In, Ga)Se2 solar cells

We investigate photogeneration and carrier recombination in Cu(In, Ga)Se₂ based thin-film solar cells with graded gap absorbers. The graded gap in the absorbers is obtained by variation of the Ga/In ratio during the coevaporation process from elemental sources. The devices exhibit conversion efficiencies up to η=16.7%. In these graded gap devices, the open circuit voltage depends on that bandgap which corresponds to the Ga content close to the absorber surface (i.e., the bandgap in the space charge region). In contrast, the short circuit current density relates to the overall minimum of the bandgap in the graded gap structure. We show that in our graded gap devices, two different bandgaps, one for recombination and one for photogeneration, are experimentally realized.