Tuning the edge-on oriented ordering of solution-aged poly(3-hexylthiophene) thin films

Abstract

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 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 the 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_m exp(−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 the 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.