Effect of thickness on silicon solar cell efficiency

Semiconductor material cost is one of the factors which determines the performance-cost ratio and economical feasibility of silicon solar cells for terrestrial power generation. Decreasing the cell thickness would lower the silicon material cost. The energy conversion efficiency of a back-surface field solar cell will have a peak as the silicon film thickness is reduced due to two opposing factors: 1) the open-circuit voltage increases and 2) the short-circuit current decreases with decreasing cell thickness. A computer-aided-design study on the dependence of this efficiency peak on the concentrations of the recombination and dopant impurities is presented in this paper. The illuminated current-voltage characteristics of over 100 cell designs were obtained using the transmission line circuit model to numerically solve the Shockley equations. Using an AM1 efficiency of 17 percent as a target value, which is the highest encapsulated silicon cell efficiency used in the Block IV modules of the Low-Cost Solar Array Project, it is shown that the efficiency versus thickness dependence has a broad maximum which varies less than 1 percent over more than a three-to-one range of cell thickness from 30 to 100 µm. Optical reflecting back surface will give only a slight improvement of AM1 efficiency, about 0.7 percent, in this thickness range. The sensitive dependence of efficiency on patchiness across the back-surface field low-high junction in thin cells is noted.