Volume 30, No. 1
January 2005

The Photoconversion Mechanism of Excitonic Solar Cells
Brian A. Gregg

Abstract
Excitonic solar cells (XSCs) function by a mechanism that is different than that of conventional solar cells. They have different limitations on their open circuit photovoltages, and their behavior cannot be interpreted as if they were conventional
p–n heterojunctions. Exciton dissociation at the heterojunction produces electrons on one side of the interface already separated from the holes produced on the other side of the interface. This creates a powerful photoinduced interfacial chemical potential energy gradient that drives the photovoltaic effect, even in the absence of a built-in electrical potential. The maximum thermodynamic efficiency achievable in an XSC is shown to be identical to that of a conventional solar cell, with the substitution of the optical bandgap in the XSC for the electronic bandgap in the conventional cell. This article briefly reviews the photovoltaic mechanism of XSCs, the limitations on their photovoltage, and their maximum achievable efficiency.

Keywords: efficiency limitations, excitonic mechanism, organic solar cells.