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This study aimed to investigate the effects of thermal annealing on a film of squaraine (SQ) molecules in a polymethyl methacrylate (PMMA) matrix. Molecular aggregation is inferred from in situ absorption measurements, and excited state dynamics are measured using a spatially encoded transient absorption (TA) spectroscopy. TA spectra were well-replicated using a kinetic model that evolves as a function of annealing time and extent of aggregation. While linear absorbance spectra indicate that the SQ molecules are primarily uncoupled or weakly-coupled when initially deposited in a PMMA matrix, the kinetic model shows that some pi-stacked aggregates are already present. Excitons are funnelled by energy transfer to these aggregates in just a few picoseconds. The amount of pi-stacked aggregates increases during thermal annealing, further increasing the population of excitons that end up in these aggregates. 

Block copolymers with donor and acceptor conjugated polymer blocks provide an approach to dictating the donor–accepter interfacial structure and understanding its relationship to charge separation and photovoltaic performance. We report the preparation of a series of donor‐linker‐acceptor block copolymers with poly(3‐hexylthiophene) (P3HT) donor blocks, poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(thiophen‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (PFTBT) acceptor blocks, and varying lengths of oligo‐ethylene glycol (OEG) chains as the linkers. Morphological analysis shows that the linkers increase polymer crystallinity while a combination of optical and photovoltaic measurements shows that the insertion of a flexible spacer reduces fluorescence quenching and photovoltaic efficiencies of solution processed photovoltaic devices. Density functional theory (DFT) simulations indicate that the linking groups reduce both charge separation and recombination rates, and block copolymers with flexible linkers will likely rotate to assume a nonplanar orientation, resulting in a significant loss of overlap at the donor–linker–acceptor interface. This work provides a systematic study of the role of linker length on the photovoltaic performance of donor–linker–acceptor block copolymers and indicates that linkers should be designed to control both the electronic properties and relative orientations of conjugated polymers at the interface. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2018,56, 1135–1143