Article pubs.acs.org/JPCC
Hole Injection Enhancements of a CoPc and CoPc:NPB Mixed Layer in Organic Light-Emitting Devices Hyunbok Lee,† Jeihyun Lee,† Kwangho Jeong,† Yeonjin Yi,*,† Jung Han Lee,‡ Jeong Won Kim,§ and Sang Wan Cho∥ †
Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemoon-Gu, Seoul 120-749, Korea Department of Nano and Bio Surface Science, University of Science and Technology, 52 Eoeun-Dong, Yuseong-Gu, Daejeon 305-533, Korea § Korea Research Institute of Standards and Science, 267 Gajeong-ro, Daejeon 305-340, Korea ∥ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025, United States ‡
S Supporting Information *
ABSTRACT: The hole injection enhancement in organic light-emitting devices with the insertion of a cobalt phthalocyanine (CoPc) hole injection layer (HIL) between the indium tin oxide (ITO) anode and the N,N′-bis(1naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) hole transport layer (HTL) was demonstrated through current density−voltage−luminance measurements, in situ photoelectron spectroscopy experiments, and theoretical calculations. The CoPc HIL significantly reduces the hole injection barrier (HIB) and thus serves as an efficient HIL like the conventional copper phthalocyanine HIL. This commonality originates from their similar configurations of the highest occupied molecular orbital (HOMO), which consists of conducting macrocycle isoindole ligands, not related to the central metal. However, as the CoPc:NPB mixed HIL is inserted, the hole injection enhancements are inferior to that of a single CoPc HIL. This is due to the electron transfer from NPB to CoPc, which pulls the HOMO level of the mixed HIL down to the deeper position. The reduced hole injection with the mixed layer implies directly that the HIB between ITO and HIL dominates device performance as the so-called ladder effect of HILs.
1. INTRODUCTION During past decades, organic light-emitting devices (OLEDs) have been extensively developed by many researchers due to their promising advantages, such as a simple fabrication process, low cost, a wide viewing angle, and mechanical flexibility. As a result, consumer products with OLEDs have been released in the commercial market by mass production. However, their detailed operational mechanisms are still not clearly known, and thus in-depth investigations of their electronic structures are being emphasized to optimize the device architecture.1 Generally, OLEDs are composed of thin multilayers between two electrodes. To obtain high device performance, an efficient charge injection from both electrodes to the organic semiconducting layer must be achieved. In the case of the anode side, the insertion of an appropriate hole injection layer (HIL) is a frequently used strategy, which reduces the hole injection barrier (HIB) in both the normal and inverted OLEDs. Various organic and inorganic HILs have been reported, which can be categorized according to their different working mechanism as follows: (1) ladder effect of HILs. Their highest occupied molecular orbital (HOMO) level is situated between the Fermi level of the anode and the HOMO level of a hole transport © 2012 American Chemical Society
layer (HTL), and thus, holes effectively transfer from the anode to HTL through the HOMO level of HILs in a step. Copper phthalocyanine (CuPc) corresponds to this type.2 (2) Gap state and interfacial dipole assist of HILs. A new state emerges in the band gap of HIL deposited on the indium tin oxide (ITO) anode and a large dipole is formed at the interface between the HIL and the anode. They assist charge transfer with the new states and reduce the HIB from ITO to HTL with a large dipole. 1,4,5,8-Naphthalene tetracarboxylic dianhydride (NTCDA) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) fall into this category.3,4 (3) Charge generation of HILs. These HILs have a high work function, and their lowest unoccupied molecular orbital (LUMO) levels are located very close to the Fermi level of the anode; thus, direct electron transfer occurs from the HTL HOMO to the HIL LUMO. Transition metal oxides (MoO3, V2O5, and WO3) and 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (HAT-CN) correspond to this type.5−8 Each origin of the hole injection Received: March 28, 2012 Revised: May 29, 2012 Published: June 1, 2012 13210
dx.doi.org/10.1021/jp3029598 | J. Phys. Chem. C 2012, 116, 13210−13216
The Journal of Physical Chemistry C
Article
Figure 1. (a,b) Measured current density−voltage−luminance (J−V−L) and (c) electroluminescence (EL) efficiency-voltage characteristics of OLEDs consist of Al (100 nm)/Alq3 (50 nm)/NPB (50 nm)/CoPc (0, 5, 10, 20, 30 nm)/ITO. (d) Measured J−V characteristics of hole-only devices are composited with Al (100 nm)/NPB (100 nm)/CoPc (0, 5, 10, 20, 30 nm)/ITO, which is prepared to clearly show the hole injection enhancements by the insertion of a CoPc HIL. Schematic configuration of the devices is also depicted in each inset.
transport in organic semiconducting devices as MPc HILs are introduced. If the HOMO’s main contribution is from the central metal (Nd < 6), the injected holes in MPc could be trapped at the central metal due to the localized nature of the dorbital, and thus, the holes would not be transported efficiently to the HTL. However, if the HOMO of MPc is composed of isoindole macrocycle ligands (Nd ≥ 6), the injected holes in MPc can efficiently transfer to the HTL due to the delocalized nature of the π-electron states of the ligand. Accordingly, it should be critical to consider not only the energetic position but also the nature of the HOMO in order to choose an efficient MPc HIL. In consequence, it is expected that cobalt phthalocyanine (CoPc, Nd = 7) acts as a highly efficient HIL, like conventional CuPc (Nd = 10), because their main HOMO contribution comes from the delocalized macrocycle isoindole ligands.
enhancements has been well understood with in situ photoelectron spectroscopy (PES).9−17 Metal phthalocyanines (MPcs) are frequently used in many organic electronic devices as a p-type semiconducting material, and it is a common belief that all MPcs would work with the ladder effect in OLEDs. However, Grobosch et al. recently reported that the valence electronic properties of MPcs are highly affected by their central metal.18,19 If the d-orbital electron occupancy (Nd) of the central metal is smaller than 6 (0.1 nm/s. All the thicknesses and deposition rates were monitored by a quartz crystal microbalance (QCM). Hole-only devices were prepared by the same procedure. Mixed HIL (CoPc:NPB) was fabricated by coevaporation at the ratio of 1:1, and a rate of 0.01 nm/s was monitored by a QCM. J−V−L characteristics were measured under a dry nitrogen atmosphere using a Keithely 237 and 2400 source measure unit with a photodiode calibrated by PR650 spectrometer. The active areas of all devices were 0.04 cm2. In situ PES experiments were performed using a PSP RESOLVE 120 spectrometer, which is directly connected with the preparation chamber. We used ultraviolet (He I, 21.22 eV) and standard X-ray (Mg Kα, 1253.6 eV) as light excitation sources. To obtain the secondary electron cutoff (SEC), a sample bias of −10 V was applied in normal emission geometry. CoPc and NPB were deposited on a ITO coated glass substrate in a stepwise manner with a rate of 0.01 nm/s. We checked the work functions of all ITO substrates prior to the main experiments to ensure an equivalent surface condition (work function of ∼3.8 eV for all ITO used in our experiments). In monitoring the thicknesses and deposition rates, a thickness calibrated QCM in combination with the attenuation of the In 3d5/2 core level intensities upon the film deposition was used. The base pressure of the analysis and preparation chambers were 5.0 × 10−10 and 1.0 × 10−8 Torr, respectively. Theoretical calculations including single point energy, vibrational frequency, and geometry optimization were performed at the level of Becke-3 parameters exchange and Lee−Yang−Parr correlation (B3LYP) hybrid functional with a 6-311G(d,p) basis set implemented in the Gaussian09 13212
dx.doi.org/10.1021/jp3029598 | J. Phys. Chem. C 2012, 116, 13210−13216
The Journal of Physical Chemistry C
Article
of energetic shifts is equivalent to the UPS results within the experimental resolution (0 and 0.2 eV, see Figure S2 in the Supporting Information). By combining all information from the measured spectra, we drew the energy level diagrams of NPB/ITO (spectra not shown) and NPB/CoPc/ITO as shown in Figure 3. For the LUMO level position, the transport gap of NPB (4.00 eV) was taken from the reported value.43 However, the transport gap of CoPc is not yet known, and thus, we estimated the gap from the energetic separation between the C−N bonding state and its π−π* transition satellite peak in the C 1s core level (2.02 eV separation, detailed spectral deconvolution is shown in the Figure S3 of the Supporting Information).44 We remark that this energy gap from the π−π* satellite excitation is the lower limit of the CoPc transport energy gap because this excitation does not take into account the exciton binding energy of CoPc.45 The Vb was determined by the value of the HOMO peak shift, and the eD was evaluated by subtracting Vb from the total SEC shift (ΔSEC). For both measurements, the ionization energy (Eion) shows consistent values, which are in excellent agreement with the reported values within a margin of the experimental error (
