Flexible Cu(In,Ga)Se2 solar cells with reduced absorber thickness

Reduction of the absorber thickness combined with deposition on a flexible substrate is a technically viable strategy to allow lower cost manufacturing of Cu(In,Ga)Se-2 solar modules. Flexible plastic substrates, however, require a low-temperature deposition process and appropriate control of the band gap grading for achieving high efficiencies. In this work, we developed solar cells on polyimide films using evaporated Cu(In,Ga)Se-2 absorbers with thickness of 0.8-1.3 mu m. The double Ga-grading profile across the absorber thickness was modified by varying the maximum Cu excess at the end of the second stage or by adapting the In and Ga evaporation flux profiles during the growth process. By minimizing the Cu excess during the intermediate stage of the growth process, no loss in open circuit voltage and fill factor is observed compared with a device having a thicker absorber. Efficiency of 16.3% was achieved for cells with an absorber thickness of 1.25 mu m. Insufficient absorption of photons in the long wavelength region is mainly responsible for current loss. By changing the In and Ga evaporation profiles, the shape and position of the Ga notch were effectively modified, but it did not lead to a higher device performance. Modifications of the Ga compositional profile could not help to significantly reduce absorption losses or increase charge carrier collection in absorbers with thickness below 1 mu m. Changes of open circuit voltage and fill factor are mostly related to differences in the net acceptor density or the reverse saturation current rather than changes of the double Ga grading. Copyright (c) 2013 John Wiley & Sons, Ltd.