Organic light-emitting devices based on different hole transport materials

Abstract

The characteristics of N, N '-biphenyl-N, N '-bis-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and four derivatives, 2,7-bis(p-methoxyphenyl-m'-tolylamino)-9,9-dimethylfluorene (TPF-OMe), 2,7-bis-(phenyl-m'-tolylamino)-9,9- dimethylfluorene (TPF), 2,7-bis(p-fluorophenyl-m'-tolylamino)-9,9-dimethyl fluorene (TPF-F), and 9,9'-(9,9-dimethyl- 9 H -fluorene-2,7-diyl)bis(9 H -carbazole) (TPG) in double layer organic light-emitting devices (OLEDs) are systemically studied in the paper. The devices were fabricated by conventional thermal vacuum deposition using TPD and its four derivatives as hole transport layer (HTL), and tris(8-hydroxyquinolinato) aluminum (Alq) as an electron transport and emissive layer. Monolayer Alq (50 nm) OLED was also fabricated to compare the HTL role and its effect on device performance. The results show that HTL assisting hole injection and transport improved device luminance performance except TPG material. The monolayer device has a maximum current density of 1270 mA/cm 2 at 15 V just due to the thinnest film thickness but a low luminance of 172 cd/m 2 at 15 V, and a turn-on voltage (when device luminance is 1 cd/m 2 ) of 6.2 V. The devices of TPD and TPF-OMe as HTL have the maximum luminance of 9813 cd/m 2 and 2880 cd/m2 at 15 V, the turn-on voltage of 3.4 V and 4.1 V, and maximum luminance efficiency of 0.52 lm/W at 5.5 V and 0.14 lm/W at 6 V, respectively. The devices of TPF and TPF-F as HTL show the maximum luminance of 2068 and 2515 cd/m 2 at 15 V, the turn-on voltage of 4.8 and 4.4 V, and maximum luminance efficiency of 0.12 lm/W at 7.5 V and 0.34 lm/W at 6 V, respectively. TPG material showes incompetent hole transport capability and no redound to device luminescence performance, that device has the maximum current density of 29 mA/cm 2 at 15 V , the maximum luminance of 123 cd/m 2 at 15 V, turn-on voltage of 10.5 V, and maximum luminance efficiency of 0.10 lm/W at 14 V.