Electron-hole collisions in concentrator solar cells

Point-contact solar cells are fabricated on high-resistivity silicon and consequently operate in high-level injection where both electrons and holes exist in large numbers. The authors analyze the effects of electron-hole collisions on device operation and conclude that the conventional treatments are theoretically inadequate. The reason for this is that the efficacy of electron-hole collisions in altering the current flow depends on the relative drift velocities of the two carrier, and not merely on their numbers, as is conventionally assumed. From thermodynamic considerations, the authors introduce the more general equations of current flow and consider their effect in practical solar cells. The differences between the proposed general theory and the conventional treatment are described, and both are compared to experimental data where available. A practical modeling method and the application of the general equations to the point-contact solar cell are discussed. It is shown that reductions in mobility due to electron-hole collisions should not have a significant impact on the ultimate energy conversions efficiencies obtainable.