Controlling aggregate formation in conjugated polymers by spin‐coating below the critical temperature of the disorder–order transition

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2018-03, Vol.56 (6), p.532-542
• Main Authors: Reichenberger, Markus, Kroh, Daniel, Matrone, Giovanni M. M., Schötz, Konstantin, Pröller, Stephan, Filonik, Oliver, Thordardottir, Margret E., Herzig, Eva M., Bässler, Heinz, Stingelin, Natalie, Köhler, Anna
• Format: Article
• Language: English
• Subjects: Aggregates; Chemical-mechanical polishing; Critical temperature; Deposition; dimer; excimer; excitonic coupling; film formation; low‐bandgap polymers; Optoelectronic devices; phase transition; Polymers; self‐assembly; solar cells; Spectrum analysis; Spin coating; Spin dynamics; Substrates; thin films; Transition temperature
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.24562

ABSTRACT Aggregates – that is short‐ranged ordered moieties in the solid‐state of π‐conjugated polymers – play an important role in the photophysics and performance of various optoelectronic devices. We have previously shown that many polymers change from a disordered to a more ordered conformation when cooling a solution below a characteristic critical temperature Tc. Using in situ time‐resolved absorption spectroscopy on the prototypical semiconducting polymers P3HT, PFO, PCPDTBT, and PCE11 (PffBT4T‐2OD), we show that spin‐coating at a temperature below Tc can enhance the formation of aggregates with strong intra‐chain coupling. An analysis of their time‐resolved spectra indicates that the formation of nuclei in the initial stages of film formation for substrates held below Tc seems responsible for this. We observe that the growth rate of the aggregates is thermally activated with an energy of 310 meV, which is much more than that of the solvent viscosity (100 meV). From this we conclude that the rate controlling step is the planarization of a chain that is associated with its attachment to a nucleation center. The success of our approach for the rather dynamic deposition method of spin‐coating holds promise for other solution‐based deposition methods. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 532–542 The formation short‐range order in thin films during spin‐coating can be controlled by choosing a substrate temperature below the critical transition temperature for the disorder–order transition in conjugated polymers. An analysis of time‐resolved in‐situ absorption spectra indicates that the formation of nuclei for substrates held below Tc is responsible for this. The growth rate of the aggregates is controlled by chain planarization associated with its attachment to a nucleation center.