Energy Transfer Directly to Bilayer Interfaces to Improve Exciton Collection in Organic Photovoltaics

Ternary blends and energy cascades are gaining popularity as ways to engineer absorption as well as exciton and charge collection in organic solar cells. Here, we use kinetic Monte Carlo simulations to investigate energy cascade designs for improving exciton collection in bilayer solar cells via a Forster energy transfer mechanism. We determine that an interfacial monolayer (C) between the donor and acceptor with a D -> A -> C energy cascade will lead to good exciton collection, allowing for >90% collection, even for energy donor layers up to 75 nm thick. We further examine how roughening the interface, increasing the exciton diffusion length, and using other energy cascade designs affect the enhancement from the energy transfer. We also propose using the inherent charge transfer states at the interfaces as energy acceptors and estimate that the Forster radius could be as large as 3.4 nm, leading to nearly 70% improvement in exciton collection, without the need for a third material.