Is Thermal Annealing Always Capable of Enhancing the Ordering and Mobility of Donor–Acceptor Copolymer Thin Films?

The effect of thermal annealing (TA) on the backbone planarity (BP) and edge-on (EO) ordering of donor–acceptor (D–A) copolymer thin films, especially near the film–dielectric interface, was studied using complementary techniques to check the possibility of improving the in-plane charge transport, in general, and thin-film transistor properties, in particular. TA is found to deteriorate the BP of such spin-coated films, irrespective of their initial structure, which can be correlated to the thermal energy-induced twisting of the aggregates. TA is also found to deteriorate the EO ordering of the film (formed from a low polymer concentration solution) having better EO ordering initially, but improve the EO ordering of the film (formed from a high polymer concentration solution) having lesser EO ordering initially. That is, thermal energy acts as a perturbation on the well-ordered and relaxed aggregates to hamper their EO ordering, while it helps in the reorientation and organization of the poorly ordered aggregates, through relaxation, to promote their EO ordering. For the in-plane charge transport, EO ordering dominates over BP by controlling the in-plane connectivity. Accordingly, the as-grown thin film having reasonable BP but better EO ordering shows superior hole mobility, while that having better BP but lesser EO ordering shows inferior hole mobility. After TA, both thin films having lesser BP but reasonable EO ordering give rise to an intermediate hole mobility, showing improvement for one and deterioration for the other, clearly indicating that TA is not always capable to enhance the EO ordering and charge carrier mobility of D–A copolymer thin films, understanding of which is very useful in optimizing the film growth for obtaining the best device performances.