Enhancing exciton diffusion in organic photovoltaics cells incorporating dilute donor layers

In this work we measure the exciton diffusion length (L_D) of the electron donor material boron subphthalocyanine chloride (SubPc) as a function of concentration in a wide energy gap host material, effectively modulating the intermolecular separation. It is shown that the L_D of neat SubPc (L_D = 10.7 nm) can be increased by ~50% at a concentration of 25 wt.% (L_D = 15.4 nm). The enhancement in L_D is attributed to the optimization of the parameters that control Förster energy transfer. Furthermore, we show that enhanced L_D can be translated to dilute donor OPVs that demonstrate an enhanced power efficiency of η_P = 4.4%, a ~30% increase relative to OPV devices based on neat SubPc and rivaling the efficiency of corresponding bulk heterojunction devices based on SubPc and C₆₀. Kinetic Monte Carlo modeling of exciton dynamics in these devices suggests that optimal incorporation of dilute donor layers with enhanced L_D depends intimately on the interface. Specifically, an imbalance in energy transfer across the dilute donor interface imparts inhomogeneity in the energy transfer landscape, dramatically affecting exciton motion. Overall, this work highlights the opportunity for designing future organic semiconductors that have longer L_D as well as OPV architectures that are directly optimized for enhanced exciton diffusion.