Theory of solar cell light trapping through a nonequilibrium Green's function formulation of Maxwell's equations

Abstract

We develop a theory of solar cell light trapping based on directly solving Maxwell's equations through a nonequilibrium Green's function formalism. This theory rigorously connects the maximum power absorbed by the solar cell to the density of states of the cell. With this theory we are able to reproduce all standard results in solar cell light trapping previously derived using approximate formalisms. Therefore our development places solar cell light trapping theory on a much firmer theoretical foundation. Moreover, here the maximum power formula is derived without the assumption of reciprocity, unlike previous theories on solar cell light trapping. Therefore, we prove that the upper bound of light trapping enhancement cannot be overcome with the use of nonreciprocal structures. As a numerical test, we simulate an absorber structure that consists of a nonreciprocal material, and show that the absorption enhancement factor is largely independent of nonreciprocity, in consistency with the theory.