Excitonic solar cells: The physics and chemistry of organic-based photovoltaics

The physical principles underlying the photovoltaic effect in organic-based solar cells are described and compared to those in conventional solar cells. The photo induced chemical potential gradient created by the interfacial dissociation of excitons is an important, and frequently overlooked, driving force for charge separation in organic photovoltaic cells. The photovoltage is proportional to the sum of the chemical potential gradient and the electrical potential gradient (band bending or built-in potential). Photovoltages up to similar to1 V have been obtained in cells without any built-in potential, despite misconceptions that this is impossible. The importance of the photo generated interfacial electric field is emphasized, and the difficulties in disentangling exciton transport length measurements from interfacial quenching rates are discussed. The advantages and disadvantages of increasing the exciton-dissociating. surface area by structuring the substrates, or by structuring the films on planar substrates, are considered.