Direct determination of interfacial molecular orientations in field-effect devices of P3HT/PCBM composites by electron spin resonance

Field-induced electron spin resonance (FI-ESR) measurements have been performed on metal-insulator-semiconductor diode structures using regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) composites with various mixing ratios on a parylene gate insulator. For lower PCBM mixing ratios, clear FI-ESR signals of hole carriers were observed around g = 2.0031, with the external magnetic field normal to the substrate plane, indicating the edge-on orientation of P3HT lamellar structures at the device interface. On the other hand, for higher PCBM mixing ratios above 50 wt.%, a new additional peak was observed at g = 2.0022, which is characteristic of the p pi-orbital axis, indicating the occurrence of flat-on P3HT domains. Ambipolar charge transport was also observed in field-effect transistors using P3HT/PCBM on parylene. With increasing PCBM fraction, from 0 to 100 wt.%, the field-effect mobility of holes decreased from 10(-3) to 10(-6) cm(2)/Vs, while that of electrons increased from 10(-5) to 10(-2) cm(2)/Vs. These results demonstrate that the flat-on orientation strongly reduces the charge carrier transport for holes because two-dimensional charge transport assisted by pi-pi interaction is interrupted by the presence of the flat-on structures. (C) 2011 Elsevier B.V. All rights reserved.