In this paper, we examine the properties of two series of hole transport materials. One is FTPD, having triphenylamine and fluorene structure. The FTPDs have high glass transition temperatures above 80°C because of their rigid fluorene moiety. The ionization potentials (Ips) of the FTPDs were correlated with the Hammett constants of the substituents of the FTPDs. The driving voltages of the double layer organic light emitting diodes (LEDs) fabricated with FTPDs and tris (8- quinolinolato) aluminum complex (Alq
3) were affected by the Ips of the FTPDs. The other is OTPAC, which has a triphenylamine oligomeric structure that improves thermal stability further. The glass transition temperatures of the OTPACs are higher than those of the FTPDs. The driving voltages of the double layer LEDs fabricated with OTPACs and Alq
3 were lower than those of the devices with FTPDs, and they have no relationship to the Ips of the OTPACs. It is likely that the morphological character of the hole transport layers mainly affects the performance of the LEDs in the case of the OTPACs.
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