Modification of an ITO anode with a hole-transporting SAM for improved OLED device characteristics

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Abstract
Modification of inorganic electrodes has attracted much attention in the study of organic semiconductor devices. An ethoxysilane functionalized hole-transporting triphenylamine (TPA–CONH–silane) was synthesized and was self-assembled to form a monolayer on an indium tin oxide (ITO) anode. The modified surface was characterized by water contact angle, X-ray photoelectron spectroscopy (XPS), ellipsometry and atomic force microscopy (AFM). The increase in surface work function is expected to facilitate hole injection from the ITO anode. To investigate the effect of a self-assembled monolayer (SAM) on the characteristics an organic light-emitting diode (OLED), a typical OLED device [SAM-modified ITO/TPD (50 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm)] was fabricated. The SAM-modified device could endure a higher current and showed a much higher luminance (6300 cd m−2) than the bare ITO device (2700 cd m−2). The external quantum efficiency was also shown to improve as a result of the presence of the SAM. In addition to these device characteristics, a study of the TPD film morphology revealed an enhanced thermal stability for the SAM-modified device. The variation of the terminal group of the SAM and possible further optimization of SAM-modified OLEDs is under investigation.