Statistical copolymers of 3-hexylthiophene and thiophene: Impact of thiophene content on optoelectronic and thermal properties

Abstract

Polythiophenes have attracted much attention for their application in flexible, low-cost, tunable, and comparatively efficient optoelectronic devices due to their highly-controlled synthesis, modifiable microstructures, and favorable optoelectronic properties. Grignard Metathesis (GRIM) polymerization is widely used for synthesizing poly(3-substituted thiophenes) with highly ordered microstructures. This route has been applied to synthesizing copolymers of 3-substituted thiophene monomers yielding copolymers with desirable combinations of homopolymer properties such as enhanced charge transport and solubility. In the present work, we synthesize an extensive statistical range of copolymers of 3-hexylthiophene and thiophene and examine the resulting thermal and optoelectronic properties. We find incorporation of thiophene units leads to defects to the chain regioregularity as thiophene lacks the steric hindrance of a 3-position substituent group for directing monomer addition to the polymer chain. This decreasing regioregularity is tied to the inclusion of structural defects and thereby systematically decreasing melting temperature at low thiophene content. However, at increasing thiophene content the thermal behavior is complicated by more complex regiochemistry before ultimately increasing as the composition approaches polythiophene homopolymer. Interestingly, while incorporation of small amounts of thiophene (up to 15 percent) dramatically lowers the melting temperature, the optoelectronic properties (namely optical band gap) are found to remain constant and analogous to that of P3HT. This behavior provides an opportunity to tune the thermal behavior for improved processability and stability while maintaining desired optoelectronic properties.