Excitonic Processes in Organic Semiconductors and Their Applications in Organic Photovoltaic and Light Emitting Devices

Abstract

This chapter discusses the excitonic processes in organic semiconductors and their applications in photovoltaic and light emitting devices fabricated from these materials. The mechanisms of excitonic absorption, diffusion and dissociation of excitons at the donor-acceptor interface are presented in bulk-heterojunction organic solar cells. After the formation of Frenkel excitons upon photon absorption, excitons must diffuse to the interface to dissociate into free charge carriers which are then collected at their respective electrodes. The interface must be in close proximity, of the order of the diffusion length, and for efficient dissociation the offset of the lowest unoccupied molecular orbital energy between the donor and acceptor must be at least equal to the exciton binding energy. The Forster and Dexter energy transfer mechanisms are used to calculate the exciton diffusion coefficients and exciton diffusion lengths for singlet and triplet excitons, respectively. The newly derived interaction operator between charge transfer exciton and molecular vibration energy is used to understand the mechanism and derive the rate of dissociation of excitons into free charge carriers. The exciton diffusion and dissociation in bulk-heterojunction organic solar cell are presented first followed by the radiative recombination of exciton in organic light emitting devices (OLEDs).