Article pubs.acs.org/JPCC
Charge Generation Mechanism of Metal Oxide Interconnection in Tandem Organic Light Emitting Diodes Kihyon Hong† and Jong-Lam Lee*,† †
Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 789-784, Korea
ABSTRACT: The mechanism of charge generation in metal oxide-based charge generation layers (CGLs) in tandem organic light emitting diodes (OLEDs) was studied via in situ synchrotron radiation photoelectron spectroscopy (SRPES) and in situ ultraviolet photoemission spectroscopy (UPS). The energy band structure and interface dipole energy of a CGL architecture comprising Ca doped tris(8-hydroxyquinoline) aluminum (Alq3), 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPD), and various kinds of metal oxides are studied. The charge generation property is contributed to the amount of work function and interface dipole energy of metal oxide CGLs. The hole injection barrier at the metal oxide/α-NPD interface decreased as a function of the work function of the metal oxide. However, contrary to common belief, the large interface dipole resulted in a small hole injection barrier and low operation voltage of the device. Using data on interface energetics measured by in situ SRPES and UPS, it is shown that the work function of the metal oxide is a key factor in determining the charge generation process. The low work function (1.0 eV). Meanwhile, due to the high work function of AgO (5.40 eV), the hole injection barrier at the AgO/α-NPD interface could be reduced to 0.36 eV. Thus, the tandem OLEDs with AgO showed the lowest turn-on voltage (15 V) and highest current efficiency (41 cd/A) out of all the OLEDs studied in this work.
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INTRODUCTION Tandem organic light emitting diodes (OLEDs) are attractive for flat panel displays and solid-state lighting, owing to their high current efficiency and long lifetime.1−5 In tandem OLEDs, multiple electroluminescent (EL) units are electrically connected in series via a charge generation layer (CGL). Under an applied electric field, the CGL should facilitate effective charge injection and hole blocking into one of the connected EL units. It is obvious that a key factor in the performance of tandem OLEDs is an efficient CGL for charge generation and photon output.6,7 High performance tandem OLEDs require the formation of a CGL that has high optical transmission and superior charge generation and injection properties. Thus, understanding the physical properties of the CGL materials is not only important fundamentally but essential to achieve high efficiency tandem OLEDs. Recently, various efforts have been devoted to the exploration of high performance CGL materials, which typically consist of a p-n junction.8,9 Various kinds of CGL structures have been reported, including junctions between chemically n© 2012 American Chemical Society
and p-doped charge transport layers, the insertion of thin metal or transparent conductive oxide layers, and the insertion of transition metal oxides. As n-type CGL materials, alkali metal or low ionization energy organic molecules doped organic thin films such as Cs2CO3 doped BPhen, Mg doped tris(8hydroxyquinoline) aluminum (Alq3), and Ca doped Alq3 have been used.10−12 Device characteristics were found to be insensitive to the work function of the dopant, as supported by the ultraviolet photoemission spectroscopy (UPS) results, which showed that the lowest unoccupied molecular orbitals (LUMOs) of various n-type CGLs have similar energy levels. Meanwhile, the electron injection barriers from the n-type CGLs depend on their electrical conductivities, which can be improved by using an electron-transporting host with higher electron mobility.13,14 Received: December 15, 2011 Revised: February 15, 2012 Published: February 17, 2012 6427
dx.doi.org/10.1021/jp212090b | J. Phys. Chem. C 2012, 116, 6427−6433
The Journal of Physical Chemistry C
Article
Figure 1. Schematic diagram of (a) single unit and (b) tandem OLEDs considered in this work.
p-type metal oxide CGL materials. Thus, it is necessary to provide the general charge generation mechanism of a p-type CGL including not only n-type metal oxides but also p-type metal oxides having relatively low work functions (9.5 eV) and a high work function (WF > 6.8 eV), energy level alignment with a large interface dipole (>1.5 eV) can be formed. This energy level alignment can lead to a small charge injection barrier between the organic and the metal oxide, which allows for both electron and hole transfer (known as generation) from the highest occupied molecular orbital (HOMO) level of the organic to the conduction band of the metal oxide. In this case, the injected electrons were transported to the LUMO state of the n-type CGL by a field-induced tunneling process. Thus, the charge generation process occurs at the interface between the hole transport organic material and the n-type CGL. However, the charge generation process in this model is restricted to the kind of p-type CGL materials. This model is based on the assumption that the charge injection occurs between the conduction band of the metal oxide CGL and the HOMO level of hole transport organic materials. According to the model, the electrons are injected from the HOMO level of the hole transport organic material into the conduction band of the metal oxide CGL, resulting in a generation of holes in the organic material. Considering the large ionization potential energy (∼6.0 eV) of organic materials, only an n-type transition metal oxide having a deep lying conduction band (electron affinity: >6.5 eV), such as WO3 and MoO3, can support this charge generation process. Although this model successfully explains the charge generation mechanism of n-type transition metal oxides, it has been difficult to reconcile with high luminous efficiency of tandem OLEDs with various well-known
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EXPERIMENTAL SECTION 1. Fabrication of Tandem OLEDs. The tandem OLEDs were fabricated with two individual EL units, each consisting of a hole transport layer (HTL) and an emissive layer (EML) (Figure 1). The two EL units were separated by a CGL 6428
dx.doi.org/10.1021/jp212090b | J. Phys. Chem. C 2012, 116, 6427−6433
The Journal of Physical Chemistry C
Article
consisting of 10 wt % for Ca-doped tris(8-hydroxyquinoline) aluminum (Alq3, 10 nm) as a n-type CGL and a variety of metal oxide (10 nm) heterostructures as a p-type CGL. For comparison, a standard device with a single EL unit was also prepared. We grew all devices on precleaned indium tin oxide (ITO) coated glass substrates and fabricated them using high vacuum thermal evaporation. We used 4,4′-[bis[N-(1-naphtyl)N-phenylamino]biphenyl (α-NPD) as the HTL, Alq3 as the EML (it also acted as an electron transport layer), and 10-(2benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) as the green fluorescent dye. The current density− voltage−luminance (J-V-L) characteristics of the devices were measured in nitrogen ambient. 2. In-Situ SRPES and UPS Analysis. We analyzed the energy band structure and the interface dipole energy at the interfaces of p-type CGLs (metal oxides) with organic materials (α-NPD), by taking in situ SRPES and UPS measurements at the 4B1 beamline in the Pohang Acceleration Laboratory (PAL). The various metal oxide CGL materials, (10 nm) AgO, NiO, V2O5, In2O3, and MoO3, were ex-situ deposited on the ITO coated glass substrate, and α-NPD was in situ evaporated in steps onto the substrates in the preparation chambers (
