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Origin of Enhanced Hole Injection in Organic Light Emitting Devices with Electron Acceptor Doping Layer: P-type Doping or Interfacial Diffusion? Lei Zhang, Feng-Shuo Zu, Ya-Li Deng, Femi Igbari, Zhaokui Wang, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b01989 • Publication Date (Web): 13 May 2015 Downloaded from http://pubs.acs.org on May 19, 2015
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ACS Applied Materials & Interfaces
Origin of Enhanced Hole Injection in Organic Light Emitting Devices with Electron Acceptor Doping Layer: P-type Doping or Interfacial Diffusion? Lei Zhang, Feng-Shuo Zu, Ya-Li Deng, Femi Igbari, Zhao-Kui Wang,* and Liang-Sheng Liao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, China
Address all correspondence to the authors. Email:
[email protected];
[email protected] Abstract The
electrical
doping
nature
of
a
strong
electron
acceptor,
1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN), is investigated by doping it in a typical hole transport material N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine (NPB). Better device performance of organic light-emitting diodes (OLEDs) was achieved by doping NPB with HATCN. The improved performance could, in principle, arise from a p-type doping effect in the co-deposited thin films. However, the physical characteristics evaluations including the UV-Vis absorption, Fourier transform-Infrared (FTIR) absorption as well as X-ray photoelectron spectroscopy (XPS) demonstrated that there were no obvious evidences of charge transfer in the NPB:HATCN composite. The performance improvement in NPB：HATCN based OLEDs is mainly attributed to an interfacial modification effect owing to the diffusion of HATCN small molecules. The interfacial diffusion effect of the HATCN molecules was verified by the in-situ ultraviolet photoelectron spectroscopy (UPS) evaluations. Keywords: Organic light-emitting diodes (OLEDs); Doping; Electron acceptor; Charge transfer; 1 ACS Paragon Plus Environment
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Carrier injection; Interfacial diffusion.
1. INTRODUCTION Improving the device performance of organic light-emitting diodes (OLEDs) has drawn much attention recently owing to their remarkable application in flat panel display and solid state lighting.1,2 Lowering the driving voltage is one of most important issues from the viewpoint of low consumption. Due to the low intrinsic carrier concentration of organic semiconductors, optimization of carrier injection from the electrodes to the emitting organic semiconductors is desired with the goal of fabricating low-voltage and high-efficient organic electronic devices.3-6 Especially for the hole injection, there exists large energy barriers between the organic materials and the electrodes due to the large offset of energy levels between the work function of commonly used indium tin oxide (ITO) and the highest occupied molecular orbital (HOMO) of hole-transporting layers (HTLs).7,8 Many efforts, such as inserting pristine interlayers and/or electron acceptors doped (p-type doping) interlayers, have been carried out to tune the hole injection at the ITO/organic interfaces.9-13 Particularly, some electron accepting materials, i.e., Molybdenum trioxide (MoO3)14-17 and tetrafluoro-tetracyano-quinodimethane (F4-TCNQ)18-21, have been reported as excellent p-type dopants by modifing the energy levels of the anode and the wide band-gap HTLs. In an acceptable view, the modified energy interface and enhanced carrier injection are attributed to the doping effect in doped organic host materials through electrons transferring from organic molecules to the dopants. Similarly as MoO3 and F4-TCNQ, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) was assumed to be a p-type dopant for hole transport materials since it has six carbonitrile units with strong electron withdrawing character.22-29
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In general, p-type doping is achieved by doping strong electron acceptors to an organic host material with the electron affinity of the dopant molecule in the ionization energy range of the host. That is, charge transfer from the highest occupied molecular orbital (HOMO) of the host to the lowest unoccupied molecular orbital (LUMO) of the p-type dopant. Once a p-type doping occurs, improved hole injection and lowered driving voltage can be achieved in practical OLEDs. In recent experiments, we found that the enhanced hole injection is not simply originated from the effect of p-type doping effect for all host: dopant systems. An interface doping effect, in which some strong electron acceptors can diffuse out of the doped system by by thermal motivation during depositing or field motivation during device operation with a modification effect on the acceptor/metal interfaces, is also occurs in some mixed host:dopant systems. In this work, we investigate the injection properties as well as the “doping” behavior of HATCN in a typical hole injection and transport material N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine (NPB).30-33 The experimental results demonstrate that HATCN-doped NPB interfacial layer showed limited improvement of hole injection ability in OLEDs. Prior to the occurrence of charge transfer, the enhanced hole injection is mainly attributed to the anode modification by HATCN since the interdiffusion of HAT-CN small molecules through NPB interlayer toward the anode. The measurements of UV-Vis absorption spectra, Fourier transform infrared (FTIR) absorption as well as X-ray photoelectron spectroscopy (XPS) reveal that there is not efficient charge transfer from the HOMO of NPB to the LUMO of HATCNto form p-type doping effect. From the in-situ ultraviolet photoelectron spectroscopy (UPS) evaluations, it was verified that the HATCN molecules may diffuse through NPB layer towards ITO surface (interfacial diffusion effect) and precisely adjust the energy levels at ITO(interface modification effect), which reduces the hole injection barrier (HIB) for NPB.
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2. EXPERIMENTAL SECTION 2.1 Device Fabrication and Characterization Materials
of
MoO3,
HATCN,
NPB,
tris-(8-hydroxyquinoline)
aluminum
4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran 4,4′,4′′-tri(N-carbazolyl)triphenylamine
(TCTA),
4,4′-bis(carbazol-9-yl)biphenyl
2′,2′′-(1,3,5-benzinetriyl)-Tris(1-phenyl-1-H-benzimidazole) bis(2-phenylpyridine)(acetylacetonate)iridium(III)
(Alq3),
(DCJTB), (CBP), (TPBi),
(Ir(ppy)2(acac)),
and
8-hydroxyquinolinolato-lithium (Liq) were purchased from Lumtec Company and used as received. OLED devices were prepared on ITO (110 nm, 15 Ω/square) glass substrates. The substrates were first cleaned with detergent solution and solvents, and then exposed to UV-ozone for 15 min before depositing the materials. The active area of each device is 0.09 cm2. All the layers were thermally deposited in a high vacuum deposition system with a base pressure of ~10−6 Torr. Deposition rates and thicknesses of all materials were monitored with oscillating quartz crystals. Co-deposition from individual sources with different monitors was used for the doping materials system. The deposition rate of NPB was controlled at 0.2 nm/s, and the deposition rate of the guest was adjusted according to the volume ratio doped in the host materials. All devices were encapsulated by cover glasses before the performance test. The electroluminescence (EL) and current density-voltage (J-V) characteristics were measured by a constant current source (Keithley 2400 SourceMeter) combined with a photometer (Photo Research SpectraScan PR 655). 2.2 Physical Characterization of Organic films The absorption spectra of NPB, NPB: HATCN (5 vol.%) and NPB: MoO3 (20 vol.%) films were measured with a UV-Vis spectrophotometer (PerkinElmer Lambda 750). The transmission IR spectrum was recorded from a FTIR microscopy (Bruker VERTX 70). UPS and XPS measurements 4 ACS Paragon Plus Environment
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were carried out in a Krato’s AXIS Ultra-DLD ultrahigh vacuum (UHV) surface analysis system. XPS measurements using a monochromatic Al Kα source (1486.6 eV) were conducted to study the charge transfer conditions with a resolution of 0.4 eV. UPS analysis was performed to characterize the valence states and vacuum level with an unfiltered He I (21.2 eV) lamp and a total instrumental energy resolution of 100 meV. For UPS measurements, organic thin films were in-situ thermally deposited onto UV-ozone treated ITO substrates in the interconnected deposition chamber. Samples were transferred to the analysis chamber without breaking the vacuum. The Au 4f7/2 peak position and the Fermi-level (EF) edge of an Au film were used to calibrate the binding energy (BE) scale and all the UPS and XPS spectra are referred to the EF as zero BE. 2.3 In-situ Interface Evaluation by UPS Interface studies were carried out in-situ in a KRATOS AXIS ULTRA-DLD ultrahigh vacuum (UHV) surface analysis system, consisting of a fast load lock (base pressure
