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.
In recent years, higher power conversion efficiencies have been measured using "push-pull" or Donor-Acceptor (D-A) type compounds designed to specifically address bandgap and energy level requirements. Yet, a strong prescription is fundamentally lacking that improves materials for the set of all critical properties (including exciton diffusion rate and charge transport/ mobility) that combine to provide optimal performance. We will present our newest theoretical models that simulate the morphology-based spectroscopy for a series of squaraines, compounds representative of the total set of D-A type OPV-targets. The theory will describe how morphological and molecular structure influences i) the absorption spectrum, ii) the excited states and iii) the intermolecular charge transfer integral (ICTI). In particular, the ICTI's role in exciton diffusion and carrier mobility will be explored. Using device data that correlates with the ICTI variation, we will explain how this parameter must be considered in future design of new easily-purified, consistently processable, low-band-gap small molecules targeted for large scale OPV manufacture.
Squaraines are targeted for organic photovoltaic devices because of their high extinction coefficients over a broad wavelength range from visible to near infra-red (NIR). Moreover, their side groups can be changed with profound effects upon their ability to crystallize, leading to improvements in charge mobility and exciton diffusion.
The broadening in squaraine absorption is often qualitatively attributed to H- and J-aggregates based on the exciton model, proposed by Kasha. However, such assignment is misleading considering that spectral shifts can arise from sources other than excitonic coupling. Our group has shown that packing structure influences the rate of charge transfer; thus a complete and accurate reassessment of the excited states must be completed before the true charge transfer mechanism can be confirmed.
In this work, we will show how squaraine H-aggregates can pack in complete vertical stacks or slipped vertical stacks depending upon sidegroups and processing conditions. Hence, we uncover the contribution of an intermolecular charge transfer (IMCT) state through essential states modeling validated by spectroscopic and X-Ray diffraction data.
We further show external quantum efficiency data that describe the influence of the IMCT state on the efficiency of our devices.
This comprehensive understanding of squaraine aggregates drives the development of more efficient organic photovoltaic devices, leading towards a prescription for derivatives that can be tailored for optimized exciton diffusion, charge transfer, higher mobilities and reduced recombination in small molecule OPV devices.
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