Modification of ITO anodes with self-assembled monolayers for enhancing hole injection in OLEDs

Increasing carrier injection efficiency is an important way to improve the performance of organic light-emitting diodes (OLEDs). In this work, self-assembled monolayers (SAMs) were formed on indium tin oxide (ITO) anodes with different aromatic carboxylic acids. The relationship between the molecular structure and its effect on modification was investigated. The presence of monolayers was verified by X-ray photoelectron spectroscopy. Water contact angle tests show that the surface energy of ITO has decreased after SAM modification which is beneficial to obtain a flat film of organic functional materials on ITO. In addition, the data of ultraviolet photoelectron spectroscopy reveal that the work function of SAM-ITO with different molecules modified has increased to varying degrees. Therefore, a no-hole injection layer (HIL) device whose structure is ITO/SAMs/α-naphthyphenylbiphenyldiamine (NPB) (25 nm)/tris(8-hydroxyquindino) aluminum (III) (Alq3) (60 nm)/LiF (1 nm)/Al (100 nm) was designed to explore the impact of SAMs on OLEDs. OLED performance shows SAMs of 9H-carbazole-2-carboxylic acid (CzCA) facilitating the device to obtain superior luminescence performance, with a turn-on voltage of 2.6 V and a maximum luminance of 30 418 cd·m−2. In order to study the mechanism, the highest occupied molecular orbital (HOMO) and other information of SAM molecules were calculated by Gaussian 09. According to the result, the HOMO of CzCA appears as a special “through-band,” which is beneficial to the hole transport. It is considered that when the HOMO of the SAM molecule is in a shape favorable for hole transport, hole injection will be facilitated and the performance of the OLEDs will be improved greatly.