A solution-processed and low threshold voltage p-type small molecule based on indolocarbazole- and benzothiophene-fused rings

A novel planar heteroacene, 8,17-dihexadecyl-8,17-dihydrobenzo[4′,5′]thieno[2′,3′:5,6] indolo[3,2-b]benzo[4,5]thieno[2,3-h]carbazole (ICzDBT), was designed and synthesized. The thermal, optical, electrochemical and field-effect transistor properties were investigated systematically. The highest occupied molecular orbital energy levels (EHOMO) measured by cyclic voltammetry and photoelectron yield spectra were −5.15 and −5.29 eV, respectively, which are close to the work function (WF) of Au (∼5.2 eV). This suitable energy level alignment between the EHOMO of ICzDBT and WF of Au is favorable for hole carrier injection from the Au electrode to the ICzDBT layer. The ICzDBT showed a hole mobility as high as 0.17 cm2 V−1 s−1 and a current on/off ratio of 1.2 × 106, with a very low threshold voltage of −0.8 V. Moreover, the device displayed excellent stability with little roll-off of hole mobility in air. Hence, this kind of molecule is a promising candidate for p-type organic field-effect transistors (OFETs) with high mobility and air stability.