Significant decrease in the ionization energy of dinaphtho[2,3b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) solid induced by a pinacolborane group

Abstract

The low carrier density in organic semiconductors leads to high resistivity and contact resistance in electronic devices. Doping has been implemented to solve these issues. We describe herein a molecular modification approach to increase the carrier density. A representative p-type organic semiconductor, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), was modified with a pinacolborane (Bpin) group, a reactive functional group in the Suzuki-Miyaura cross-coupling reaction. The resulting Bpin-modified DNTT (Bpin-DNTT) has a low-lying HOMO energy level at the single molecular level (5.4 eV below the vacuum level) and excellent transistor characteristics with mobility of greater than 2 cm ² V ^-1 s ^-1 . However, the Bpin-DNTT solid was easily oxidized upon exposure to ambient air, generating hole carriers. To clarify this unprecedented behavior, we investigated Bpin-DNTT in detail through single-crystal field-effect transistor (SC-FET), electron spin resonance (ESR) spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and theoretical calculations. The SC-FET and ESR spectra demonstrated that the surface of the Bpin-DNTT solid in air was readily oxidized, which was due to the significantly decreased ionization energy of 4.58 eV, confirmed by UPS. These results unveil the potential of the Bpin group to increase the carrier density in p-type organic semiconductors.