ABSTRACT
Organic-based solar cells convert light to electricity by a mechanism involving exciton formation, transport, and dissociation. The same factors that cause exciton formation upon light absorption, as opposed to the formation of free electron–hole pairs, also control the doping process and carrier transport in organic semiconductors (OSCs). A unified approach has been developed to describe the excitonic processes, doping, and transport in these “excitonic” semiconductors. A simple equation is proposed that semiquantitatively distinguishes between excitonic semiconductors (XSCs) and conventional semiconductors (CSCs). The most essential qualitative difference between them is that Coulomb forces can often be neglected in CSCs, whereas they often dominate the behavior 140of XSCs. The Coulomb force between photogenerated electrons and holes in XSCs causes exciton formation. Mobile excitons dissociate into free electrons and holes primarily at heterojunctions, thus producing a large interfacial chemical potential energy gradient that drives the photovoltaic (PV) effect even in the absence of, or in opposition to, a bulk electric field. This force is usually insignificant in CSCs. Electrostatic considerations also control the doping process in XSCs: most added charge carriers are not free but rather are electrostatically bound to their conjugate dopant counterions. A superlinear increase in conductivity with doping density is thus expected to be a universal attribute of XSCs. An analogy is drawn between purposely doped XSCs and adventitiously doped XSCs such as π-conjugated polymers: in both cases the number of free carriers is a small, and field-dependent, fraction of the total carrier density. The Poole–Frenkel (PF) mechanism accounts naturally for the interactions between carriers bound in a Coulomb well and an applied electric field. Together with a field-dependent mobility, this mechanism is expected to semiquantitatively describe the conductivity in doped XSCs.
