Abstract
An electron-only device was realized by using 20 nm Bphen layer to modify ITO anode in NPB/AlQ3 heterojunction organic light-emitting diode (OLED). Different from the usual application as an anode buffer layer, up to 10 nm thick MoO3 layer was inserted at Bphen/NPB interface to recover the electroluminescence (EL). Interfacial charge generation and transport are suggested as the mechanism of such luminescence recovery. Experimental results showed that mobile holes generated in NPB transported to NPB/AlQ3 interface, resulting in light-emitting, while electrons in MoO3 transported to ITO anode through Bphen layer under continuous current condition. The suggested mechanism explains how inserted MoO3 layer modifies Bphen/NPB interface.
Highlights
Molybdenum trioxide (MoO3), with deep-lying energy band structure,1 has been widely used as anode buffer layer in organic light-emitting diodes (OLEDs) to facilitate hole injection from indium tin oxide (ITO) anode to hole transport layer (HTL)
A series of Bphen thin layers with several-nanometerthicknesses were inserted at the ITO/NPB interface as the anode buffer layer to realize the electron-only device
It can be seen that the B-V characteristics of Device A are much more sensitive to the additional Bphen layer inserted at the ITO/NPB interface, as shown in Fig. 1(b) and (c)
Summary
Molybdenum trioxide (MoO3), with deep-lying energy band structure (work function: 6.9 eV, conduction band minimum: 6.7 eV, valence band maximum: 9.7 eV), has been widely used as anode buffer layer in organic light-emitting diodes (OLEDs) to facilitate hole injection from indium tin oxide (ITO) anode to hole transport layer (HTL). In 1996, Tokito et al inserted 3 nm MoO3 thin layer between ITO and hole transport N,N’-diphenylN,N-bis(3-methylphenyl1)1,1’ -biphenyl-4,4’-diamine (TPD) layer, which reduced the threshold voltage (corresponding to electroluminescence brightness of 1 cd/m2) from 4.7 V to 3.7 V.2 Afterwards, plenty of work showed that thin MoO3 anode buffer layer could improve hole injection in both small molecular and polymeric OLEDs. It is generally accepted that hole injection into the organic film results from that electrons transfer from the highest occupied molecular orbital (HOMO) of hole transport material to the conduction band of MoO3 because of its very high electron affinity, and holes are left in organic material. Besides the application as an anode buffer layer, MoO3 is used as HTL dopant to improve the conductivity of HTL. Kroger et al. Observed five orders of magnitude current increase in MoO3 doped 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP) layer.. More than an order of magnitude hole current increase was observed.14 They considered holes were generated in vacuum-deposited MoO3 via spontaneous electron transfer from defect states to the conduction band.. Under constant bias/current condition, mobile holes could be generated continuously in organic material and collected by the cathode, the electrons accumulated in MoO3 should be either injected into the neighboring organic material or recombined with holes injected from the anode. A relatively complete mechanism for hole current increase by modifying the interface between the electron transport layer (ETL, Bphen) and HTL (NPB) with the thin MoO3 layer is suggested
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