Dopant optimization for triplet harvesting in polymer photovoltaics

Abstract

We investigate the dependence of the characteristics of polymer photovoltaic cells on the concentration of doping of the active layers by heavy-metal complexes. We wish to exploit the strong spin-orbit coupling of such complexes to induce a relatively fast exciton intersystem crossing from the singlet to the longer-lived triplet manifold, to increase in turn the lifetime of the exciton, its diffusion length, and thus the probability of dissociation and the overall device conversion efficiency. We combine current–voltage curves, photocurrent spectra, and atomic force micrographs to show that there is an optimum doping load above which the device performance severely deteriorates. We find, for an Ir-pyridine dopant, a maximum load of approximately 5%.