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Revealing structure formation in PCPDTBT by optical spectroscopy

Scharsich, Christina, Fischer, Florian S. U., Wilma, Kevin, Hildner, Richard, Ludwigs, Sabine, Köhler, Anna
Journal of polymer science 2015 v.53 no.20 pp. 1416-1430
absorption, cooling, optical properties, phase transition, photoluminescence, photovoltaic cells, polymers, spectroscopy, temperature
The low‐bandgap polymer poly{[4,4‐bis(2‐ethylhexyl)‐cyclopenta‐(2,1‐b;3,4‐b′)dithiophen]‐2,6‐diyl‐alt‐(2,1,3‐benzo‐thiadiazole)−4,7‐diyl} (PCPDTBT) is widely used for organic solar cell applications. Here, we present a comprehensive study of the optical properties as a function of temperature for PCPDTBT in solution and in thin films with two distinct morphologies. Using absorption and photoluminescence spectroscopy as well as Franck‐Condon analyses, we show that PCPDTBT in solution undergoes a phase transformation at 300 K from a disordered to a truly aggregated state on cooling. The saturation value of aggregates in solution is reached in PCPDTBT thin films at any temperature. In addition, we demonstrate that the photophysical properties of the aggregates in films are similar to those in solution and that a low percentage of thermally activated excimer states is present in the films at temperatures above 200 K. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1416–1430