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-C61-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 pπ-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 cm2/Vs, while that of electrons increased from 10−5 to 10−2 cm2/Vs. These results demonstrate that the flat-on orientation strongly reduces the charge carrier transport for holes because two-dimensional charge transport assisted by π–π interaction is interrupted by the presence of the flat-on structures.

Field-induced ESR is a powerful tool to investigate the interfacial molecular orientations for P3HT domains in field-effect devices of P3HT/PCBM composites. As increasing PCBM mixing fractions, the unique co-existence of both edge- and flat-on domains was directly determined by g-value anisotropy and simulating the split ESR signals. The demonstrated flat-on domains strongly reduce the field-effect mobility for holes.Figure optionsDownload as PowerPoint slideResearch highlights
► Field-induced ESR directly determines interfacial molecular orientations of P3HT.
► Introduced PCBM modulates the original edge-on orientation partly into a flat-on one.
► Co-existence of both edge- and flat-on orientations depends on PCBM mixing fractions.
► Flat-on orientations strongly reduce the field-effect mobility of holes.