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

Structural evolution of solution-aged poly(3-hexylthiophene) [P3HT] thin films during thermal annealing (TA) was studied using complementaryin situX-ray reflectivity (XR) andex situatomic 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 pi-pi 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, Delta rho approximate to Delta rho(m)) near the substrate and negligible ordering (i.e.Delta rho -> 0) near the top surface took place following the standard decay function: Delta rho(z) = Delta rho(m) exp(-z/zeta), where the critical decay length,zeta, is the measure of the out-of-plane ordering. TA fails to improve the Delta rho(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 zeta-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 Delta rho(m)-value) along with the out-of-plane ordering (i.e.the zeta-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.