Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation

Charge carriers are photogenerated with very different spatial distributions in conventional inorganic photovoltaic (IPV) cells and in organic photovoltaic (OPV or excitonic) cells. This leads to a fundamental, and often overlooked, mechanistic difference between them. Carriers are generated primarily at the exciton-dissociating heterointerface in OPV cells, resulting in the production of electrons in one phase and holes in the other—the two carrier types are thus already separated across the interface upon photogeneration in OPV cells, giving rise to a powerful chemical potential energy gradient ∇μhv that promotes the photovoltaic effect. This occurs also in high-surface-area OPV cells, although their description is more complex. In contrast, both carrier types are photogenerated together throughout the bulk in IPV cells: ∇μhv then drives both electrons and holes in the same direction through the same phase; efficient carrier separation therefore requires a built-in equilibrium electrical potential energy difference ∅bi across the cell. The open-circuit photovoltage Voc is thus limited to ∅bi in IPV cells, but it is often greater than ∅bi in OPVs. The basic theory necessary to compare IPVs to OPVs is reviewed. Relevant experiments are described, and numerical simulations that compare semiconductor devices differing only in the spatial distribution of photogenerated carriers are presented to demonstrate this fundamental distinction between the photoconversion mechanisms of IPV and OPV devices.