Evolving composition of molecular aggregates during thermal annealing revealed using in situ spectroscopy

Organic photovoltaics (OPVs) are comprised of a pair of molecules or polymers that are electron donors and acceptors, and some of the most promising molecules comprise electron donating and accepting moieties in a single molecule. Thermally annealing an OPVs active area, consisting of the electron donor and acceptor molecules, can improve efficiency. Here we measure films made of squaraine (SQ) molecules, which contain an electron donor-acceptor-donor motif, embedded in an inert polymer during thermal annealing using in situ linear absorbance and single-shot transient absorption (SSTA). To better understand energy transfer in SQ films the SSTA measurements were fit using evolution associated decay spectra (EADS). The calculated amplitude spectra and experimental linear absorption spectra were fit using an intermolecular charge transfer Hamiltonian to obtain the interplanar distance between monomers. As thermal annealing proceeded the distance between monomers decreased, the probability of electron transfer increased, and energy transfer occurred from uncoupled monomers to weakly-coupled monomers until H-aggregates became the dominant species. At the end of annealing two H-aggregate species were found: one with a larger interplanar spacing and one with a smaller interplanar spacing. As annealing continued H-aggregates with a larger interplanar spacing grew closer together, while the structure of the H-aggregate species with smaller interplanar spacing stays relatively constant. This paper establishes a methodology for determining the geometry of monomers and aggregates in thin films, which can be exploited to help engineer better solution-processed materials for use in OPVs.