Low-Voltage Organic Transistors Based on Tetraceno[2,3‐b]thiophene: Contact Resistance and Air Stability

The small-molecule organic semiconductor tetraceno[2,3-b]thiophene has been synthesized through an environmentally friendly synthetic route, utilizing NaBH4, rather than Al/HgCl2, for the reduction of the quinone. Lowvoltage organic thin-film transistors (TFTs) have been fabricated using tetraceno[2,3-b]thiophene and, for comparison, pentacene and anthradithiophene as the semiconductor. The tetraceno[2,3-b]thiophene TFTs have an effective fieldeffect mobility as large as 0.55 cm V−1 s−1 and a subthreshold swing of 0.13 V/decade. In addition, it has been found that the contact resistance of the tetraceno[2,3-b]thiophene TFTs is substantially smaller than that of the anthradithiophene TFTs and similar to that of the pentacene TFTs. The long-term air stability of TFTs based on all three semiconductors has been monitored over a period of 12 months. The initial charge-carrier mobility of the tetraceno[2,3-b]thiophene TFTs is ∼50% smaller than that of the pentacene TFTs, but as a result of the greater ionization potential and better air stability induced by the terminal thiophene ring condensed at the thiophene-b-bond, the tetraceno[2,3-b]thiophene TFTs outperform the pentacene TFTs after continuous exposure to ambient air for just 3 months. ■ INTRODUCTION Organic thin-film transistors (TFTs) are a promising technology for the realization of flexible electronic systems, such as rollable or foldable displays or stretchable sensor arrays. The conjugated organic semiconductor that forms the active TFT layer must meet a significant number of requirements, and to date, more than 700 different materials have been evaluated for this purpose. Pentacene was one of the first and remains among the most popular small-molecule semiconductors for organic TFTs, because of its large carrier mobility. Electronic systems that have been successfully demonstrated using pentacene TFTs include flexible active-matrix displays, sensor arrays, radio-frequency identification (RFID) tags, and microprocessors. However, pentacene has a pronounced sensitivity to chemical oxidation in ambient air that typically leads to a rapid, irreversible degradation of the electrical performance of pentacene TFTs exposed to ambient air. One strategy for overcoming this problem is to replace pentacene with a material that has a larger ionization potential, i.e., a lower-lying highest occupied molecular orbital (HOMO) level. This can be accomplished, for example, by reducing the size of the conjugated core or by replacing some of the benzene rings with fused heteroaromatic rings, such as thiophene. This study focuses on pentacene analogues with one or both of the terminal benzene rings replaced with thiophene rings condensed at the thiophene-b-bond: tetraceno[2,3-b]thiophene and anthradithiophene. The reorganization energies (∼100 meV), the packing structure (herringbone motif), and the lattice parameters of these semiconductors are all similar to those of pentacene, which is beneficial in view of efficient charge transport. Compared with pentacene, they provide better oxidation resistance, because of their lower-lying HOMO levels (−4.6 eV for pentacene, −4.7 eV for tetraceno[2,3-b]thiophene, and −4.8 eV for anthradithiophene). To the best of our knowledge, TFTs based on tetraceno[2,3-b]thiophene and anthradithiophene have so far been fabricated only using thermally grown SiO2 as the gate dielectric, which is unsuitable for flexible electronics because of the high process temperature. In addition, in all previous reports on tetraceno[2,3-b]thiophene and anthradithiophene TFTs, the gate dielectric was quite thick, so that the TFTs had to be operated with relatively high voltages. Also, there are no previous reports of the contact resistance of tetraceno[2,3b]thiophene and anthradithiophene TFTs. Here, we present a comparison of the performance and long-term air stability of TFTs based on tetraceno[2,3-b]thiophene, anthradithiophene, and pentacene with a gate dielectric that is obtained at a sufficiently low temperature to be suitable for flexible plastic Received: November 24, 2014 Revised: January 5, 2015 Published: January 5, 2015 Article