Charge transport in amorphous low bandgap conjugated polymer/fullerene films

Abstract

The structural and charge transport properties of a low bandgap copolymer poly(3-hexylthiophene-alt-6,7-dimethyl-4,9-bis-(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P(3HT-MeTDQ)) and its blend with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) are investigated. Thermal analysis, X-ray scattering diffraction (XRD), atomic force microscopy and transmission electron microscopy (TEM) of P(3HT-MeTDQ) reveal that the polymer is amorphous in solid state. As the hole mobility of P(3HT-MeTDQ) was measured by the time-of-flight photoconductivity method, the mobility was 3.35 x 10(-4) cm(2)/Vs, which is very comparable to that of semicrystalline poly(3-hexylthiophene). When the mobility of amorphous P(3HT-MeTDQ) was analyzed according to the Gaussian disorder model, the polymer has the energetic and positional disorders with the values of sigma = 62 meV and Sigma = 1.7, respectively, indicating that the polymer has a relatively narrow Gaussian distribution of transport states. Interestingly, when P(3HT-MeTDQ) is blended with PCBM, the amorphous P(3HT-MeTDQ) becomes partially ordered, as evidenced by observation of two discernible XRD peaks at 20 = 5 degrees (d = 17.7 angstrom) and 25.5 degrees (d = 3.5 angstrom) corresponding to the interchain distance and pi-stacking distance, respectively. The bicontinuous network morphology was identified at the blend with 60 wt.% PCBM by TEM, at which the charge carrier transport changes from hole-only to ambipolar. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686633]