Effects of excitons on solar cells

We have studied the effects of excitons on the two key parameters of a Si solar cell: the dark-saturation current and short-circuit current. We have found that the effect of excitons on the dark-saturation current is very sensitive to the boundary condition for excess excitons at the edge of the depletion region. With the assumption of near equilibrium between the electrons and excitons, we find that the exciton effect is rather small, which is contrary to the conclusion of significant reduction in the dark-saturation current made in previous work with the assumption of no excess excitons at the edge [J. Appl. Phys. 79, 195 (1996)]. The results for the short-circuit current are very similar to the previous work. However, the analytical results for the carrier concentrations and the corresponding currents are now presented in a simple way in which the physical meaning of each individual term is elucidated or revealed. Furthermore, we have found, for practical purposes, very accurate approximate solutions for the carrier concentrations and corresponding currents. Our conclusion is that the major effect of excitons on the Si solar cell performance relies on whether the effective diffusion length (L-1) of the coupled electron-exciton system is significantly greater than that of the electron itself (L-e). (C) 1998 American Institute of Physics. [S0021-8979(98)05519-4].