Photophysical pathways in efficient bilayer organic solar cells: The importance of interlayer energy transfer

The development of organic photovoltaic (OPV) cells has long been guided by the idea that excitons - bound electron-hole pairs created by light absorption - diffuse only 5-10 nm. True for many materials, this constraint led to an inherently complex device architecture - the bulk heterojunction - that has obscured our understanding of device physics, and handicapped rational material design. Here, we investigate the photophysics of a series of planar bilayer heterojunction devices incorporating fused-ring electron acceptors with power conversion efficiencies up to 11%. Using ultrafast optical spectroscopy, we demonstrate the importance of long-range layer-tolayer energy transfer in planar structures, isolating this effect by including an insulating layer between the donor and acceptor layers to eliminate charge transfer effects. We show that the slab geometry facilitates substantially longer-range energy transfer than between isolated molecules or small domains. Along with high molecular packing densities, high absorption coefficients, and long exciton diffusion lengths, we show that these effects amount to exciton harvesting length scales that match the light absorption lengths and thereby enable efficient bilayer devices. Our quantitative analysis of bilayer structures also accounts for large domain sizes in bulkheterojunction devices including fused-ring electron acceptors, and it quantifies the importance of strong resonant spectral overlap is for material selection and design for highly efficient OPVs