title: Efficient small molecular organic light emitting diode with graphene cathode covered by a Sm layer with nano-hollows and n-doped by Bphen:Cs2CO3in the hollows.
abstract: Graphene is a favorable candidate for electrodes of organic light emitting diodes (OLEDs). Graphene has quite a high work function of ∼4.5 eV, and has been extensively studied when used as anodes of OLEDs. In order to use graphene as a cathode, the electron injection barrier between the graphene cathode and the electron transport layer has to be low enough. Using 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs2CO3to n-dope graphene is a very good method, but the electron injection barrier between the n-doped graphene and Bphen:Cs2CO3is still too high to be ∼1.0 eV. In this work, in order to further reduce the electron injection barrier, a novel method is suggested. On the graphene cathode, a Sm layer with a lot of nano-hollows, and subsequently a layer of Bphen:Cs2CO3, are deposited. The Bphen:Cs2CO3can n-dope graphene in the nano-hollows, and the Fermi level of the graphene rises. The nano Sm layer is very easily oxidized. Oxygen adsorbed on the surface of graphene may react with Sm to form an O--Sm+dipole layer. On the areas of the Sm oxide dipole layer without nano-hollows, the electron injection barrier can be further lowered by the dipole layer. Electrons tend to mainly inject through the lower electron barrier where the dipole layer exists. Based on this idea, an effective inverted small molecular OLED with the structure of graphene/1 nm Sm layer with a lot of nano-hollows/Bphen:Cs2CO3/Alq3:C545T/NPB/MoO3/Al is presented. The maximum current efficiency and maximum power efficiency of the OLED with a 1 nm Sm layer are about two and three times of those of the reference OLED without any Sm layer, respectively.
authors.affiliation: State Key Laboratory for Microscopic Physics, School of Physics, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Li
authors.initials: L
authors.lastname: Yao
authors.forename: Lei
authors.initials: L
authors.lastname: Li
authors.forename: Laixiang
authors.initials: L
authors.lastname: Qin
authors.forename: Yaoguang
authors.initials: Y
authors.lastname: Ma
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Hu
authors.initials: H
authors.lastname: Meng
authors.forename: Weifeng
authors.initials: W
authors.lastname: Jin
authors.forename: Yilun
authors.initials: Y
authors.lastname: Wang
authors.forename: Wanjin
authors.initials: W
authors.lastname: Xu
authors.forename: Guangzhao
authors.initials: G
authors.lastname: Ran
authors.forename: Liping
authors.initials: L
authors.lastname: You
authors.forename: Guogang
authors.initials: G
authors.lastname: Qin