Two-dimensional numerical simulations of polycrystalline Cu(In,Ga)Se2 thin-film solar cells showed that grain boundary recombination could deteriorate the photovoltaic power conversion efficiency of these devices by about 9% absolute with respect to a starting value of 21.7% that would hold for a material without grain boundaries. The achieved record efficiencies of 19% were only possible if the recombination velocity S at grain boundaries was kept below S = 103cm/s. Comparing devices that had all defects homogeneously distributed in the bulk to devices where the same number of defects was concentrated at grain boundaries only revealed that the latter situation was more favorable because of kinetic restrictions. The efficiency difference between the homogeneous and the concentrated cases is, however, only 1% (absolute). Further modelled was the possible effect of an additional hole barrier at the grain boundary. This was done by assuming asymmetrical capture cross-sections for electrons and holes. It was found that the positive consequences of this feature were limited and depended upon the specific properties of the grain boundary defects. The efficiency thus improved by 2% upon introducing a hole barrier of 0.12eV at a grain boundary with mid-gap defects. The same improvement would result from a reduction, of grain boundary defects, by a factor of 2.5.

Numerical Simulation of Grain Boundary Effects in Cu(In,Ga)Se2 Thin-Film Solar Cells. K.Taretto, U.Rau, J.H.Werner: Thin Solid Films, 2005, 480-481, 8-12