Photoinduced absorption of aggregate-coupled intermolecular charge transfer states and their impact on small molecule photovoltaic device efficiency (Conference Presentation)

Squaraines continue to attract attention for their use in non-linear optics, fluorescence bioimaging and organic photovoltaics applications because of their strong, broad NIR absorbance and optoelectronic properties that depend on both excitonic and intermolecular charge transfer (ICT) couplings in the solid state. Our previous theoretical work demonstrates splitting of the H-aggregate with coupling to the ICT that goes beyond Kasha’s exciton model. This ICT splitting leads to the panchromatic absorption profile in the solid state, but the impact of the ICT on excited state diffusion and dynamics remains unclear. Here, we employ subpicosecond transient absorption spectroscopy to probe the excited state photophysics of an anilino-squaraine and its aggregates. Our samples are designed with a continuum of intermolecular separation from monomers in solution, through solid solution thin films, to the fully condensed phase, demonstrating the increasing contribution of short-range intermolecular charge transfer. We measure excited state kinetics that confirm species assignments and we show the effect of ICT states on exciton diffusion. The experimental results are in excellent agreement with our theoretical modeling. Finally, we correlate this combination of theory and excited state characterization with the measured efficiency in small molecule organic photovoltaic devices. Our remarkable results explain the importance of excitonic and ICT couplings for future applications driven by rational optoelectronic material design.