The superposition principle for homojunction solar cells

The superposition principle for solar cells states that the current flowing in an illuminated cell subject to a forward bias is given by the algebraic sum of the short-circuit photocurrent and the current which would flow at bias in the dark. Several authors have published arguments establishing the validity of this principle for homojunction cells operated so that the minority-carrier concentrations in the quasi-neutral regions do not exceed low injection levels. All these arguments depend on the assumption that the quasi-Fermi levels are constant across the depletion region of a forward-biased, illuminated cell. The accuracy of this assumption is examined in detail in the present paper. It is found that the quasi-Fermi levels do, in fact, vary significantly across the depletion region of an illuminated cell operated at short-circuit or low forward bias. However, it is shown that if the carrier mobilities are reasonably high and the carrier lifetimes reasonably long, the superposition principle still provides an excellent description of the cell characteristics at all bias levels. The superposition principle may seriously overestimate the efficiency of cells fabricated on poor-quality substrates with very short lifetimes and low mobilities.