Substrate-Independent Energy-Level Pinning of an Organic Semiconductor Providing Versatile Hole-Injection Electrodes

Tailor-made electrode work functions are indispensable to control energy-level offsets at the interfaces of (opto-)electronic devices. We show by means of photoelectron spectroscopy that several nanometer thick layers of the organic semiconductor 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) on virtually all substrates provide hole-injecting electrodes with work functions of around 5.60 eV. This substrate-independent energy-level alignment is due to a relatively large density of gap states in HAT-CN thin films, which is clearly visible in the photoemission data. Furthermore, this additional density of occupied states makes the wide-gap semiconductor thin films sufficiently conductive for electrode applications. Moreover, our study highlights a quite intriguing energy-level alignment scenario as the Fermi-level in HAT-CN thin films is located far from the midgap position, this is rather uncommon for undoped organic semiconductor thin films.