Theory of saturation photocurrent and photovoltage in p-n junction solar cells

A theory of saturation photocurrent and photovoltage has been developed for p-n junction solar cells. The theory is based on ambipolar transport equations for electrons and holes near the junction, and on empirical models for band-gap narrowing and Fermi–Dirac integrals. It is applicable to solar cells made of nondegenerate or lowly degenerate semiconductors with position dependent band structures. Interestingly, it includes provision of both short-circuit and open-circuit configurations and involves the use of boundary conditions valid at the junction for all levels of injection. The boundary conditions automatically reduce to those of Dhariwal et al. for nondegenerate semiconductors with uniform doping. The empirical models for band-gap narrowing and Fermi-Dirac integrals are found to be significantly accurate when compared with available experiments or with exact results. Numerical calculations have been carried out for a number of silicon solar cells possessing varied doping levels, and the results have been found to be in good agreement with available experiments. The analysis shows that the saturation photovoltage developed by a p+n solar cell is higher than that developed by an equivalent n+p solar cell, and that the photovoltage is lower than the diffusion potential of the corresponding solar cell.