Design of Schottky Contacts for Optimum Performance of Thin-Film Silicon Solar Cells

A full-scale model combining TCAD simulations with circuit modeling of p-type window layers in thin-film silicon solar cells is validated by experiment. The results demonstrate that Schottky contacts with a barrier height greater than 0.5 eV cause a kink in the simulated J-V characteristics that represents a considerable reduction of the open-circuit voltage when thermionic emission is the dominant transport mechanism. An optimum cell can be designed by facilitating tunneling mechanism through the Schottky barrier via an adjustment of the doping concentration of the semiconductor or by introducing thin mu c-Si:H(p) in order to lower the barrier height between ZnO and a-Si:H(p). In such a case, however, the narrower bandgap of mu c-Si:H compared with that of a-Si:H, and the misalignment of the energy bands between mu c-Si:H(p) and a-Si:H(i), also compromise the short-circuit current and open-circuit voltage of the cell, respectively. Optimum interfaces for our 15-nm window layer are found when a combined mu c-Si:H(p)/a-Si:H(p) with a 4 / 11-nm thickness ratio is used. A general circuit model for solar cells that accounts for the effects of nonohmic contacts is demonstrated. The ideality factor "n(2)" of the Schottky junction of the contact indicates the transport mechanism at the interface with values less than 0.5 eV having no impact on the cell performance.