There is a demand of highly efficient, thin, light and flexible solar cells. Polycrystalline GaAs could satisfy this need. However, effect of grain size on performance has to be evaluated. Here, the effect of grain boundaries under dark and illuminated conditions is quantified. A 2-D simulation of single junction GaAs solar cell was done using SILVACO software. Columnar grain structures were assumed for simplicity and also because these kinds of grain structures give maximum efficiency. Device performance was simulated for various grain sizes, assuming Gaussian distribution and varying density of defect states at the grain boundary. Carrier lifetime of at least 10 ns was required to have proposed efficiency of greater than 20% for a grain size lesser than 100 μm.