Considerations for modeling heterojunction transport in solar cells

Heterojunctions are frequently employed in modern solar cell device structures as both wide-gap window layers and in place of heavily doped back surface field layers. The common approach to numerical modeling of such structures is based on the drift-diffusion formulation, where the Poisson equation is solved self-consistently with the current continuity equations. A key assumption in such an approach is that the quasi-Fermi energies are continuous across the interface. However, thermionic emission and tunneling can ultimately be responsible for determining the transport across a heterointerface, especially in the presence of a band spike. This paper describes the incorporation of a combined thermionic emission/tunneling expression for current through heterojunction interfaces into a finite-difference drift-diffusion solar cell simulation code. The modified program can be used to provide a more accurate numerical analysis of a heterojunction solar cell design