Measurement and Theoretical Interpretation of Exciton Diffusion As a Function of Intermolecular Separation for Squaraines Targeted for Bulk Heterojunction Solar Cells

The efficiency of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices depends significantly upon absorption of photons and the migration of the photogenerated excited state to the heterojunction interface between the electron donor and electron acceptor. Within anilino-squaraine, molecules known for their successful use in the active layer of OPV devices, electronic aggregation strongly influences the absorption spectrum, energy transfer (EnT), and exciton migration to this heterojunction interface. Therefore, the long-range transition dipole coupling and the relative populations of the associated excited states dictate the general effectiveness of these materials in optoelectronic devices. This work presents subpicosecond transient absorption (TA) data that probe the excited-state photophysics of samples with a continuum of intermolecular separation, from monomers in solution to high-concentration solid solution thin films analogous to OPV active layers. EnT times are calculated for each squaraine concentration, and pump-power dependence provides evidence for significant EnT despite a high preponderance of H-aggregation. Theoretical modeling of essential states supports the interpretation from TA spectra that excited states relax into more tightly packed H-aggregates. This work prompts further questions regarding a far-reaching mechanistic EnT bottleneck for molecular and polymeric BHJ devices.