Effects of processing additive on bipolar field-effect transistors based on blends of poly(3-hexylthiophene) and fullerene bearing long alkyl tails

Abstract

Bipolar FETs (BiFETs) based on the bulk heterojunction system comprised of various ratios of P3HT and soluble fullerene derivatives are demonstrated. We studied the effect of addition of small concentrations of the processing additive, 1,8-octanedithiol (ODT), on gate-induced transport properties. The control blend system consisting of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) showed enhanced hole transport properties with the addition of the ODT additive. However, electron transport properties were diminished in the presence of ODT additive because of the relatively isolated PCBM phase between the large-scale segregation of the P3HT amorphous phase. The BHJ BiFET based on P3HT and soluble fullerene derivatives bearing long alkyl tails (FP-Ph-OC10) showed enhanced performance in both hole and electron transport when the ODT additive was applied. We attribute the enhancement of hole mobility to well-formed P3HT fibrilla structures of P3HT caused by the alkyl–alkyl interaction assisted by both the ODT additive and alkyl side chain in FP-Ph-OC10. As the P3HT forms fibrilla structures, connection to the isolated FP-Ph-OC10 phase be formed, resulting in a continuous electron pathway, thereby improving electron mobility. This suggests that not only the selective solubility, but also the alkyl-alkyl interaction between the side-chain and ODT additive may affect the phase segregation of BHJ mixtures.