Benzobisthiadiazole-alt-bithiazole copolymers with deep HOMO levels for good-performance field-effect transistors with air stability and a high on/off ratio

Abstract

Benzobisthiadiazole (BBT) is a widely used building block for its high electron affinity and planar configuration. BBT-based copolymers have resulted in good performance of organic field-effect transistors (OFETs). However, devices of these polymers are usually unstable in ambient air since most of these BBT-based polymers have high-lying HOMO energy levels (−4.3 to −4.8 eV). Besides, the field-effect transistors (FET) of BBT-based polymers show relatively low on/off ratios (10²–10⁴). As a result, it is very attractive to couple BBT with an electron-deficient block to construct new polymers with air stability. 2,2′-Bithiazole (BTz) is a potential building block due to its electron deficiency and trans-planar configuration. But incorporating 2,2′-bithiazole into polymers still remains a challenge since the organic tin compound of bithiazole is difficult to synthesize. In this article, we successfully prepared 5,5′-bis(trimethylstannyl)-2,2′-bithiazole by a three-step method and then synthesized two benzobisthiadiazole-alt-bithiazole copolymers (P1 and P2). Both P1 and P2 have a low-lying HOMO energy level (−5.3 eV), leading to air stability. Moreover the resulting FETs exhibit a very high on/off ratio (10⁵–10⁷) and a good hole mobility of up to 0.11 cm² V⁻¹ s⁻¹, which represents a significant advancement for BBT-containing polymers.