Trade-off between collection efficiency and open-circuit voltage

There is trade-off between collection efficiency and open-circuit voltage as the absorber carrier density of a solar cell is varied. Calculated results are presented for polycrystalline thin-film CuInSe ₂ and crystalline Si cells. Variation in carrier density will not significantly affect collection efficiency for Si cells because of low absorption coefficient and high minority-carrier diffusion length. Collection efficiency of CuInSe₂ cells, however, has a significant decrease with increasing density. It also has a nonnegligible voltage dependence. When carrier density is less than 10 ¹⁷ cm^-3, open-circuit voltage will increase with increasing carrier density for both types of cell. Si efficiency will increase gradually with increasing carrier density below 10¹⁷ cm^-3, but Auger recombination at higher densities will reduce the efficiency. For CuInSe₂ cells at carrier densities below 10¹⁷ cm^-3, the efficiency increases with density as compensation is reduced but at higher densities, a relatively flat open-circuit voltage should lead to a shallow maximum in efficiency. For both crystalline Si and polycrystalline thin-film CuInSe₂ cells, optimum carrier density of approximately 10¹⁷ cm^-3 is predicted