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 C
₆₀
. 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.