In recent years, the electron accepting diketo pyrrolopyrrole (DPP) moiety has been receiving a remarkable attention for constructing donor−acceptor (D−A) type organic semiconductors for a variety of applications, particularly for organic thin film transistors (OTFTs) and organic photovoltaics (OPVs) as compared to commercialize polymers like poly alkylthiophenes. However, weak van der Waals interaction within the polymers is one of the major reasons for their rather low charge carrier mobility. A copolymer consists of both electron-accepting (DPP) and electron-donating (thionothiophene or TT) units would have large intermolecular overlapping through π-π stacking. Further the strong interactions between the donors and acceptors, shorten the distance between polymer chains, results in more crystalline structures which would facilitate the charge carrier transport.
  The actual performances of D−A copolymer-based devices strongly depend on the conjugation length, i.e., backbone planarity or exciton bandwidth (EB) and edge-on oriented (EO) ordering of the copolymer aggregates and, thus, their understanding and improvement are of paramount importance, which were carried out here using complementary techniques such as optical absorption spectroscopy, X-ray reflectivity, and atomic force microscopy and by tuning the pre-deposition parameters such as solvent, polymer concentration (c), and spinning speed (ω) of the spin-coated D−A copolymer thin films. The high c-value films show unusual improvement of EB with increasing ω, unlike low c-value films, for the first time. The overlapped and entangled aggregates, which are known to form in the high c-value solution of high viscosity, get disentangled in the film during deposition due to a large centrifugal force arising from their relatively large effective mass along with the increasing ω-value. On the contrary, the well separated less entangled aggregates, which formed in the low c-value solution of low viscosity, get entangled in the film due to the ω-related fast evaporation of the solution. Ultimately, improved EB is observed when both c and ω are either small or large for the films prepared using chlorobenzene (CB) and only large for the films prepared using chloroform (CF), which can be realized considering effective evaporation rate and viscosity of the solutions during deposition, while improved EO ordering is found when both c and ω are small (i.e., when diffusion and organization of aggregates are easy) for the films prepared using both the solvents. Altogether, the films prepared using CF with high c and ω values show crystallites with high EB but low EO ordering, while those with low c and ω values show better EO ordering but low EB. The films prepared using CB with low c and ω values show improvement of both EB and EO ordering and thus expected to provide better device performances compared to others [published in Macromolecules 56, 7065 (2023)].
  The effect of thermal annealing (TA) on the backbone planarity (BP) and EO ordering of 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 wellordered 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 [published in ACS Appl. Polym. Mater. 7, 16707 (2025)].