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X-ray and electron diffraction analyses were carried out under various geometries to investigate the role of alkyl side chain length on the structural properties of regioregular poly(3-alkylthiophene) (RR-P3AT) thin films by sequentially increasing the alkyl side chain length from n-pentyl till n-octyl. P3AT thin films with different thicknesses were drop cast at room temperature (RT) and low temperature (−30 °C) for the structural analysis. A thorough X-ray diffraction, transmission electron microscopic, field effect transistor and photoluminescence (PL) investigations revealed the following: (i) texturing of RT cast P3AT thin films along the out-of-plane direction on decreasing the alkyl side chain length, (ii) poor π–π stacking of P3AT crystallites on increasing the alkyl side chain length, which results from the increased steric hindrance between the grafted alkyl side chains, (iii) such poor π–π stacking of P3ATs on increasing the alkyl side chain length eventually found to hinder the mobility of charge carriers, and (iv) evolution of well defined vibronic side bands in the PL spectra of −30 °C cast films confirm the formation of highly crystalline films as compared to the RT cast ones. Highly edge-on oriented and uniform thin films can be fabricated by optimizing the cast temperature and concentration of the solution. [ABSTRACT FROM AUTHOR]