Abstract:The morphological and structural features of the conjugated polymer films play an important role in the charge transport and the final performance of organic optoelectronics devices [such as organic thin-film transistor (OTFT) and organic photovoltaic cell (OPV), etc.] in terms of crystallinity, packing of polymer chains and connection between crystal domains. This review will discuss how the conjugated polymer solidify into, for instance, thin-film structures, and how to control the molecular arrangement of such functional polymer architectures by controlling the polymer chain rigidity, polymer solution aggregation, suitable processing procedures, etc. These basic elements in intrinsic properties and processing strategy described here would be helpful to understand the correlation between morphology and charge transport properties and guide the preparation of efficient functional conjugated polymer films correspondingly.

A fibrillar morphology was obtained, compared to the featherless pristine films, via solvent annealing the films of a series of derivatives of poly(3-alkylthiophene)s (P3ATs): poly(3-dodecylthiophene) (P3DDT), poly(3,3'''-didodecyl-quaterthiophene) (PQT12), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT12). Among the solvents used, including dichloromethane, chloroform, tetrahydrofuran, and carbon disulfide (CS2), CS2 was the best to induce fibril formation because its solubility parameter is closest to those of the P3AT derivatives. It was found that higher critical CS2 vapor pressures were needed to form crystal nuclei with increasing conjugation length and molecular weight of the P3AT derivatives; i.e., the critical vapor pressures for P3DDT 13.9k and PQT12 15.5k were 59.0% and 80.7%, respectively, and there were no nuclei of fibrils for pBTTT12 15.6k with the highest conjugation length, even at a CS2 vapor pressure of 98.3%. Meanwhile, at the highest vapor pressure, the fibril density decreased with increasing conjugation length and molecular weight of the P3AT derivatives. This is attributed to the rod-like conformation prevailing for polymers with larger conjugation length and higher molecular weight during solvent annealing, making the conformational transition toward coils more difficult and hindering diffusion of molecules. The results presented here are expected to be helpful for the design and processing of conjugated semiconductor polymers.