Md Saifuddin,^a Mala Mukhopadhyay,^b Arindam Biswas,^a Lara Gigli,^c Jasper R. Plaisier^c and Satyajit Hazra^a
aSaha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata 700 064, India
^bB R A Bihar University, Muzaffarpur 842001, India
^cElettra - Sincrotrone Trieste S.C.p.A., 34149 Basovizza, Trieste, Italy

Structural evolution of solution-aged poly(3-hexylthiophene) [P3HT] thin films during thermal annealing (TA) was studied using complementary in-situ X-ray reflectivity (XR) and ex-situ atomic force microscopy (AFM) and optical absorption (UV-Vis) techniques to understand the possibility of obtaining enhanced edge-on oriented (EO) ordering for the better device properties. The presence of P3HT nanofibers (NFs), which were formed through π−π stacking within solution during aging, is evident in the films. Such NFs are well-organized near the film-substrate interface and less-organized near the film-air interface due to the respective slow and fast evaporation rates of the solvent during spin-coating. Accordingly, prominent EO ordering (i.e. the electron density contrast between polymer backbone and side chains is maximum, Δρ ≈ Δρ_m) near the substrate and negligible ordering (i.e. Δρ → 0) near the top surface took place following the standard decay function: Δρ(z) = Δρ_mexp(−z/ζ), where the critical decay length, ζ, is the measure of the out-of-plane ordering. TA fails to improve the Δρ_m-value, i.e. the EO ordering near the substrate and also the total crystalline aggregates or NFs, rather deteriorates both, when annealed near the melting temperature of P3HT. TA improves the ζ-value, i.e. the EO ordering of more out-of-plane region due to thermal energy induced alignment of the NFs, however, lack of improvement of the EO ordering near the substrate is of concern. A relatively low viscous polymer solution and low spin-coating speed play important roles in the formation of a smooth film-substrate interface and better EO ordering near that interface. Though solvent vapor annealing (SVA) fails to improve the structure, the combination of SVA and TA, i.e. SVTA, improves the in-plane EO ordering near the substrate (i.e. the Δρ_m-value) along with the out-of-plane ordering (i.e. the ζ-value) of the film. Such improvements, which are probably through the alignment and growth of NFs, promoted by SVTA induced proper diffusion, are of immense importance for obtaining better device properties.