Investigating the Influence of Alkyl Chain Length in Poly(3-alkylthiophene)s Over the Thin Film Morphology by Optical and Electrical Characterization.

This paper studies the influence of alkyl-chain length in poly(3-alkylthiophene)s over the morphology of thin films and electrical parameters of the devices based on it. Regioregular poly(3-hexylthiophene) and poly(3-octylthiophene) were chosen as the semiconducting materials for the study. The morphological variations were studied by absorption spectroscopy, photoluminescence spectroscopy and X-ray diffraction study. The absorption and photoluminescence showed decreased coplanarity of main chain in poly(3-octylthiophene) over poly(3-hexylthiophene) and which was later confirmed using X-ray diffraction studies which clearly showed increased interchain spacing in case of poly(3-octylthiophene). The schottky diodes fabricated using these materials showed decreased mobility in poly(3-octylthiophene) based diodes as measured by space-charge limiting current method and photo-induced charge carrier extraction by linearly increasing voltage technique. Moreover, we observed a negative field dependence of mobility at room temperature in both the devices and attributed this to the presence of dominant positional disorder in poly(3-alkylthiophene)s. Furthermore, the photocurrent dependence on electric field too showed inferior mobility of poly(3-octylthiophene) based diodes.