Morphology-Dependent Excited-State Dynamics of Squaraine Thin Films during Thermal Annealing

Thermal annealing is a widely used technique to enhance organic photovoltaic (OPV) efficiencies in bulk heterojunction devices. Combining annealing studies and spectroscopic measurements with theoretical modeling provides a more complete understanding of how aggregation influences energy transfer, an essential factor for photovoltaic performance. Here, we use in situ absorbance and single-shot transient absorption (SSTA) spectroscopy to characterize the electronic structure and excited-state dynamics of squaraine molecules embedded in an inert polymer matrix during thermal annealing. Analysis with a Hamiltonian based on the essential-states model reveals a stepwise transformation from disordered to ordered species, with energy transfer occurring preferentially from aggregates with larger interplanar spacing to more tightly packed aggregates. This study demonstrates how annealing-dependent changes in charge transfer coupling drive energy transfer dynamics in heterogeneous films. This work establishes a broadly applicable methodology for engineering solution-processed materials for applications in OPVs, field-effect transistors, and next-generation optoelectronic devices.