Limit of Exciton Diffusion in Highly Ordered π-Conjugated Systems

There are many limiting factors in the efficiency of organic photovoltaic devices. One of these factors is their diffusion length or alternatively the diffusivity of excitons, the quasi-particles responsible for the energy conversion. In this work, we investigated the limit of diffusion of excitons in pi-conjugated systems. Using a model Hamiltonian that included electronic and strong electron-phonon coupling, and including quantum corrected thermal effects through Langevin dynamics, we determined how excitons diffuse within a highly ordered polymer composed of many identical monomers. We established that the exciton follows a typical 1-dimensional random walk diffusive behavior. The diffusivity followed a Marcus behavior with a very low activation energy of 15 meV and a room temperature diffusivity constant of 6.12 x 10(-2) cm(2)/s. We obtained the maximum diffusion constant of 1.1 x 10(-1) cm(2)/s. This is the upper limit of exciton diffusivity in pi-conjugated systems as the systems modeled are highly ordered and contained no impurities and the excitons are very delocalized, justifying the low activation barrier and the high diffusion constant.