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A new electrode design for ambipolar injection in organic semiconductors

 Tohoku University and Tokyo Institute of Technology announced on 18 October, that the research group led by Katsumi Tanigaki (Professor at WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University) has developed air-stable and highly efficient electron-injection electrode for organic semiconductors. The results were published in Nature Communications*.

 Organic semiconductors (OSC) are promising materials for coming optoelectronic device, as they are flexible and low-cost. However, OSC devices suffer from the problems in electrodes, such as the low electron injection efficiency. In the conventional OSC devices, gold (Au) electrode is used for hole injection and calcium (Ca) metal is used for electron injection as the most efficient electrode, but Ca metal is unstable in the air. In this study, a novel electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal layer is proposed. Field effect transistors (FETs) were fabricated by a thin single crystalline OSC (sc-OSC) and electrodes were successively deposited TTC layer, pc-OSC layer, and metal (Au or Ca) layer. The new Au/pc-OSC/TTC electrodes show not only higher hole injection than that of a simple Au electrode but also higher electron injection than that of a simple Ca electrode. These improvements of injection efficiency for both carriers are caused by the low contact resistance for carrier injection. In addition, in case of rubrene FET, the highest field-effect mobilities of holes (22cm2V-1s-1) and electrons (5.0cm2V-1s-1) are observed, when Ca/pc-OSC/TTC and Au/pc-OSC/TTC are employed for electron injection and hole injection, respectively.

 This study shows the possibility of the air-stable high-performance OSC devices such as organic electro- luminescence devices, organic logic circuits and organic lasers in the future.

*T. Kanagasekaran, H. Shimotani, R. Shimizu, T. Hitosugi, and K. Tanigaki, "A new electrode design for ambipolar injection in organicsemiconductors", Nature Communications, Vol.8, Article number: 999 (2017), doi: 10.1038/s41467-017-01047-9; Published online: 17 October 2017