The two-dimensional short-circuit AM1.5 collection efficiency is studied in thin multicrystalline silicon solar cells with optical confinement. The collection efficiency is calculated by linking an optical analytical generation profile with the two-dimensional collection probability in pn junction solar cells. The calculations are carried out for variable grain boundary recombination velocity, cell thickness, grain width, diffusion length, and back surface recombination velocity. The role of optical confinement leading to a strong dependence of the collection efficiency on the cell thickness in very thin cells is confirmed. The optimum cell thickness for maximum collection efficiency increases in cells with low back reflection or poor back surface passivation. Also, the optimum thickness in very thin cells increases significantly with increasing the diffusion length. It is also found that the effect of grain boundary recombination is predominant if the cell thickness is larger than the diffusion length and if the diffusion length is larger than half the grain width, especially, in cells with unpassivated grain boundaries. On the other hand, back surface recombination dominates the response in cells with unpassivated back surface if the thickness is smaller than or comparable to the diffusion length.