Numerical calculations have beee made of the effect of grain size on the three-dimensional carrier density, the quantum efficiency, and the AMI efficiency of 30$mu$m polycrystalliue silicon p-n junction solar cells. Quantum efficiencies calculated for the polycrystalline silicon solar cells are compared to the monocrystalline cases. An efficiency of 12% can theoretically be obtained with grain size 100$mu$m, and 6% for 5$mu$m grains.