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Two-dimensional simulations are performed to investigate the impact of grain boundaries ͑GBs͒ on Cu͑In, Ga͒Se 2 ͑CIGS͒ solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1 m, the effective grain-boundary recombination velocity must be less than 10 4 cm/ s to allow for record-efficiency devices. The majority-hole repulsion ͑additional donors at the GB͒ and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion ͑additional acceptors at the GB͒ will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block holes from the GB region and allow efficiencies comparable to GB-free material.

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