Effective hole-injection characteristics of organic light-emitting diodes due to fluorinated self-assembled monolayer embedded as a buffer layer

The electrical and optical properties of organic light-emitting diodes (OLEDs) are demonstrated by varying the length of the alkyl chain of a fluorinated self-assembled monolayer (F-nSAM). OLEDs containing F-nSAMs that have a long alkyl chain length were found to exhibit excellent properties in terms of current density, luminance, turn-on voltage, etc. The obtained current density at 6 V, which was the highest measurement voltage, was up to about 36 times higher for an OLED including an F-12SAM thin film with the longest chain length than for an OLED including only an indium tin oxide (ITO) substrate. With regard to luminance characteristics depending on voltage, the luminance was about 13 times higher for the OLED including the F-12SAM thin film than for the OLED including only the ITO substrate. Also, the turn-on voltage of the OLED including the F-12SAM thin film was decreased by approximately 1 V compared to that of the OLED including only the ITO substrate. Although F-nSAMs with alkyl chains have insulating film properties, F-nSAMs with long alkyl chains exhibited good electrical and optical properties because of an improvement in the hole-injection barrier due to a large positive shift of the vacuum level and smooth carrier injection resulting from a high contact angle due to strong hydrophobic properties caused by the good alignment properties of F-nSAMs resulting from strong van der Waals forces between the molecules due to the long alkyl chains. (c) 2019 Society of Chemical Industry