Optimum morphology and performance gains of organic solar cells

Morphology of light absorbing layer is known to dictate the power conversion efficiency of organic photovoltaic (OPV) cell. The innovation of bulk heterojunction (BH) led to significant improvement for exciton harvesting, but carrier recombination at the distributed interfaces and variability of performance have been persistent concerns. The purpose of this paper is to theoretically explore the efficiency-space of OPV as a function of the degree of randomness of its morphology and to optimize the BHJ morphology for the maximum efficiency. We use systematic numerical simulation of exciton/electron/hole transport to predict the efficiency and variability associated with various OPV morphologies. We find that while (1) the efficiency of random BHJ morphology can be close to optimal, (2) the significant variability associated with cells suggests reduced efficiency in a series-connected configuration. (3) We also find that a simple fully ordered structure may also not be optimum; instead a fin like geometry (Fig. 4) may improve efficiency. (4) We also analyze the effect of each transport parameters on the cell efficiency and show that under certain configuration of transport parameters, the long-abandoned PHJ structure may re-emerge as optimal geometry.