Correlating Excited States Populations, Molecular Geometry, and Efficiency for Small Molecule OPVs

The efficiency of organic photovoltaics (OPVs) depends critically on the morphology of the active layer and the nature of the excited states. A framework for tailoring binary donor:acceptor blend ratios to optimize device efficiency can be established by correlating device efficiencies, morphology, and excited state populations. To assess excited state populations the extinction coefficient for each species must be determined. Here we present a study of Squaraine:PCBM OPVs using atomic force microscopy, spectroscopy, and device efficiency data. The intrinsic extinction coefficients of Squaraine and PCBM are established by collecting spectroscopy and thickness data. Thickness measurements additionally reveal mixing geometries characterized by incorporation into the interstices, depending on blend ratio. We connect optimal blend ratios with the population of excited state species. The approach can be extended to optimize other small molecule devices.