A relationship between driving voltage and the highest occupied molecular orbital level of hole-transporting metallophthalocyanine layer for organic electroluminescence devices

Thin Solid Films 396 (2001) 213–218

A relationship between driving voltage and the highest occupied molecular orbital level of hole-transporting metallophthalocyanine layer for organic electroluminescence devices Lihua Zhua, Heqing Tangb, Yutaka Harimaa, Yoshihito Kunugia, Kazuo Yamashitaa,*, Joji Ohshitab, Atsutaka Kunaib a

Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739-8521, Japan b Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Received 2 February 2001; received in revised form 1 May 2001; accepted 22 June 2001

Abstract Effects of metallophthalocyanines (MPcs) are systematically investigated on electroluminescence (EL) characteristics of ITOy MPcyTPDyAlq3 yMg–Ag devices. A linear relationship is found between driving voltages of the devices and energies of the highest occupied molecular orbital (HOMO) level of MPcs, showing that the driving voltage of the device is substantially determined by the energy barrier for hole injection at the ITOyMPc interface. When an MPc having a higher HOMO level, such as CuPc or ZnPc, is selected as a hole transport material, the driving voltage of the device at a luminance of 100 cd my2 significantly reaches as low as 5.5 V compared with 7.8 V for the device without an MPc layer. The insertion of MPc layer leads to increasing of the EL output, but does not affect the EL efficiency of the devices. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Metallophthalocyanines; Hole injection; Luminescence; Optoelectronic devices

1. Introduction Organic light-emitting diodes (OLEDs) have received considerable attention in recent years because of their potential application as a new display technology, and at present OLED displays are being brought to the marketplace w1x. The operation of OLEDs is based on the injection of the negative charge carriers (electrons) and the positive charge carriers (holes) from negative and positive electrodes, respectively w2x. The charge injection from the electrodes requires that the charge carriers surmount or tunnel through the barriers at the interfaces of cathode and anode contacts. In a single * Corresponding author. Tel.: q81-824-24-6535; fax: q81-82424-0757. E-mail address: [email protected] (K. Yamashita).

layer device, it is necessary to minimize the energy barriers for the charge injection by matching well the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels in the emissive layer (EML), respectively, with the work functions of the cathode and anode materials. A multilayer structure having a hole transport layer (HTL) and an electron transport layer (ETL) can balance charge injections from the electrodes significantly w3–6x, achieving a high-performance device. Phthalocyanines (Pcs), particularly as thin films, have been extensively studied for their electrochemical w7,8x, catalytic w9x, and semiconducting w10,11x properties. Because of their p-type conductance, thin films of some metallophthalocyanines (MPcs) such as CuPc w4,12–15x and VOPc w16x have been recently used as HTLs in OLEDs. For example, insertion of a CuPc layer has been reported to lower the driving bias voltage and to

0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 2 3 2 - 9

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improve operation stability of the EL devices w4x. Utilization of MPcs as HTLs in OLEDs has been recently reviewed by Hohnholz et al. w17x. The chemical structure of MPc allows tuning of its oxidation potential (or ionization potential) by altering the central atom (M) in Pc macrocycles. A systematic change in the ionization potentials or the energies of the HOMO levels of MPcs (including metal-free H2Pc) enables us to study the influences of the ITO y MPc interface barrier on the characteristics of EL devices. In the present work, we report the influences of MPcs on the EL characteristics of the ITO y MPc y TPD y Alq3 y Mg–Ag devices, where TPD means N,N9-diphenyl-N,N9bis(3-methylphenyl)-1,19-biphenyl-4,49-diamine, one of the most frequently used HTL materials and Alq3 represents tris(8-quinolinolato)aluminum(III), a common ETL material and EML material. 2. Experimental details Prior to the organic deposition, indium–tin-oxide (ITO) coated SiO2 y glass plates (sheet resistance 8 V sqy1, from Japan Sheet Glass) were thoroughly cleaned by scrubbing, sonication degreasing, and drying. By using a Tokuda CFS-8EP vacuum deposition system, a thin MPc film was vapor deposited onto the ITO plate at approximately 5=10y6 torr, followed by a deposition of a TPD layer and then an Alq3 layer of 60 nm. Deposition rate was typically 0.3–0.5 nm sy1. The thickness of the TPD layer was 40 nm, unless otherwise specified. After the deposition of the organic layers, a top layer of Mg–Ag (10:1) alloy with thickness of 200 nm was deposited at approximately 5=10y6 torr on top of the organic layers through a mask, using an ANELVA electron beam deposition system. For simplicity, the ITO y MPc y TPD y Alq3 y Mg–Ag device is referred to as a MPc device. Its light-emitting area, defined by the overlap of the ITO and Mg–Ag electrodes, was 10=1 mm2. The EL characteristics were measured using a Takasago TP0120-06D regulated d.c. power supply, a Hokuto Denko HB-III function generator, and a Hamamatsu H957-08 photomultiplier tube, which was calibrated with a Minolta LS-110 spot luminance meter. The luminance and the driving current were recorded simultaneously with Riken Denshi F-5C x–y recorders. All measurements were performed at room temperature in a dark box. The absorption spectra of the MPc thin films deposited on glass slides were taken on a Shimadzu UV-3101PC scanning spectrophotometer. Emission spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer. 3. Results and discussion No obvious differences in AFM images were observed among the thin films of different MPcs. It is in accor-

dance with SEM observations that any kind of vacuum deposited MPc film has rod-like crystallites with width of approximately 20 nm w18x. The deposited molecules are aligned in crystal grains with the molecular plane nearly perpendicular to the surface of the ITO substrate w19x. The chemical structural characteristics of MPcs suggest that their common physical properties will not seriously change with the alternation of the central atom in Pcs. In our EL devices, only the MPc film is altered. Thus, we may assume that in these devices, most physical parameters at the related ITO y MPc and MPc y TPD contacts, such as the adhesion strengths, are not strongly dependent on the type of MPc. Different MPcs have different oxidation potentials, i.e. different ionization potentials. These permit us to focus on the influence of the energy barrier at the ITO y MPc on EL characteristics of the devices. The absorption spectra of the thin MPc films were in good agreement with those reported in the literature w8x. The Q-band absorption is located at wavelengths longer than the emission envelop in the EL spectrum of the related devices. Although the oxidation of the thin film results in an electrochromic change, the Q-band shifts somewhat to shorter wavelength, the absorption at about 525 nm is still weak. Moreover, the MPc films used in our devices are very thin. These permit the absorption of the emission by the MPc film to be neglected. Indeed, the EL spectra of our devices were found to be independent of luminance levels and the central atoms of MPcs, and the inserted thin MPc layer did not interfere with the EL of the emissive layer (Alq3). Fig. 1 shows EL characteristics of the ZnPc device with a TPD layer of different thickness. It is easily found from Fig. 1a that the current is not so strongly affected by the insertion of the TPD layer, and increases only slightly as the TPD layer increases in thickness. The weak dependence of the current on the TPD film thickness can be explained with the aid of the energy level diagram of the EL devices shown in Fig. 2. Because the HOMO level of ZnPc is close to the Fermi energy level of ITO, the energy barrier of hole injection at the ITO y ZnPc interface becomes very small (vide infra). In this way, the insertion of a ZnPc layer greatly favors the injection of holes and the flowing of the current. Hence the alternation of the TPD layer thickness does not considerably affect the current–field strength (I–E) property of the device, where E is defined as the field strength averaged over the thickness of all organic layers. The slightly improved I–E property resulting from the increased TPD layer thickness may be related to higher voltages and greater hole mobility in TPD than in ZnPc. The same field strength E means a higher voltage for the device with a thicker TPD layer. One can expect that the field is unevenly distributed through the individual layers and interfaces. A higher voltage and a greater thickness of the TPD layer with greater

L. Zhu et al. / Thin Solid Films 396 (2001) 213–218

Fig. 1. Characteristics of (a) I–E, (b) L–E, and (c) luminescence efficiency - E for ITOyZnPc (20 nm)yTPDyAlq3 (60 nm)yMg–Ag devices. The TPD layer thickness is (h) 0, (s) 10, (∑) 20, (n) 30, and (d) 40 nm.

hole mobility favor increasing of E across the ITO y ZnPc interface. Consequently, the I–E property of the device is improved somewhat as the TPD layer increases in thickness. In contrast, the luminance of the device is significantly influenced by the insertion of the TPD layer (Fig. 1b). When a very thin (10 nm) TPD layer is put between ZnPc and Alq3 layers, the luminance increases greatly compared to the device without a TPD layer. As the TPD layer thickness increases from 10 to 40 nm, the luminance–field strength (L–E) property is improved further, especially at greater luminance. Similarly, the relative luminescence efficiency (cd Ay1), which was evaluated by dividing the luminance (cd my2) by the current density (A my2), is very low for the device without using a TPD layer, whereas it increases by more than two orders of magnitude when only a very thin

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TPD layer is inserted between ZnPc and Alq3 layers. The greatly increased luminance and luminescence efficiency are related to the electron blocking ability of the TPD layer. If the TPD layer is not included, ZnPc directly contacts Alq3. In such a device, the electron transport energy barrier becomes missed at the ZnPc y Alq3 interface, because the LUMO of ZnPc lies below Alq3. Therefore, electrons cannot be blocked from flowing through the Alq3 layer. When the device is fabricated using a layer of TPD, which has LUMO level higher than Alq3, the flowing of electrons is blocked at the TPD y Alq3 interface. As a consequence, even if a very thin (10 nm) TPD layer is used, the luminescence efficiency increases by more than two orders of magnitude. As the TPD layer thickness increases, its electron blocking ability is improved, and thus the L–E property is improved further. Alternatively, the generated Alq3 excitons are possibly quenched by ZnPc contacting directly the Alq3 layer, leading to lower luminance and luminescence efficiency in a device without the TPD layer. Because of the electron blocking ability of the TPD layer, the luminescence efficiency of the ITO y ZnPc y TPD y Alq3 y Mg–Ag devices is much less sensitive to the thickness of the ZnPc layer, as shown in Fig. 3c. As mentioned above, the inclusion of a ZnPc layer yields a decreased energy barrier for hole injection. Therefore both the I–E and L–E curves shift to a direction of lower field strength as the ZnPc layer thickness increases up to 20 nm, as shown in Fig. 3a,b. When the ZnPc layer is too thick (30 nm), the I–E and L–E properties worsen somewhat compared to the ZnPc layer of 20-nm thickness, possibly due to the smaller hole mobility in ZnPc than in TPD. The above-mentioned results and discussion indicate that the EL efficiency of our devices is primarily

Fig. 2. Energy diagram of ITOyMPcyTPDyAlq3 yMg–Ag EL device. The shadow ranges represent the alternation ranges of the HOMO and LUMO levels of MPcs, respectively.

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energy barriers at the ITO y MPc interface. This must influence the EL characteristics of the devices. Estimation of the energy barrier at the ITO y MPc interface requires knowledge of the HOMO levels of MPc films deposited on ITO substrate, which can be directly determined using photoelectron emission w20x and ultraviolet photoelectron spectroscopy (UPS) w21x. The HOMO levels can also be derived from experimental first oxidation potentials of the MPc. Light transitionmetal complexes of Pcs, such as ZnPc, MgPc, CuPc, NiPc, and FePc, have been extensively studied w22x. Some MPcs have been found to show reversible oxidation and reduction under appropriate conditions w22,23x. For example, both ZnPc and CuPc films exhibit reversible oxidation and reduction, hence, we can derive the HOMO levels (EHOMO) of ZnPc and CuPc from their first oxidation potentials (Eox) in non-aqueous solvents using the following relation, EHOMOŽeV.s4.3ŽeV.qeEoxŽV vs. SCE.y0.3ŽeV.

Fig. 3. Characteristics of (a) I–E, (b) L–E, and (c) luminescence efficiency - E for ITOyZnPcyTPD (40 nm)yAlq3 (60 nm)yMg–Ag devices. The ZnPc film thickness is (h) 0, (s)5, (∑) 10, (d) 20, and (n) 30 nm.

determined by the energy barrier for blocking electron transport at the TPD y Alq3 interface, while the driving voltage is substantially influenced by the inserted MPc layer. The insertion of the MPc layer results in two new interfaces, the ITO y MPc and MPc y TPD interfaces. In principle, the barriers at both of these interfaces will affect the driving voltage. In our discussion, it is assumed that the energy barrier for hole injection at the ITO y MPc interface determines predominantly the driving voltage relative to the barrier at the MPcy TPD interface. The influence of the latter may become considerable as discussed later. It is well known that the oxidation potential or ionization potential of MPc can be changed considerably by displacement of different central atoms. Thus, different MPcs will yield different

where 4.3 eV represents the energy of the saturated calomel electrode (SCE) referred to the vacuum w24x, and the term of ‘y0.3 eV’ is a correction in consideration of the fact that in a number of cases the polarization energy in solution is large approximately by 0.3 eV compared to the solid state value w25x. The oxidation potentials of ZnPc and CuPc in 1-chloronaphthalene solution were reported as 0.68 and 0.98 V vs. Ag y AgCl, respectively w26x, thus their HOMO level energies (or ionization potentials) are roughly estimated as 4.7 and 5.0 eV correspondingly. These two values are tabulated in Table 1 along with other HOMO data available from the literature w20,21,27–30x. Some scattering is observed in Table 1 for the HOMO energies measured with different methods by different research groups. Average values of the available data are used in our later discussion. In this way, we can observe a trend from Table 1 that the HOMO levels of the MPc films against a vacuum are lower in the order of ZnPc)FePc fCoPcfCuPc)PbPc)NiPc)VOPc)SnPc)H2 Pc. The EL characteristics of the MPc devices are depicted in Fig. 4. As expected above, the luminescence efficiencies (not shown) change only slightly among different MPcs because the luminescence efficiency is determined primarily by the use of the TPD layer. The dependencies of the current–voltage (I–V) and luminance–voltage (L–V) characteristics on MPc are very similar, and the EL performances of the MPc devices are improved. For instance, the driving voltages decrease in the order of ZnPc-CuPc -FePc-CoPc-PbPc-NiPc-VOPc-SnPc-H2 Pc, which is in good agreement with that of their HOMO levels of MPcs. This demonstrates firmly that the MPc layer can alter the hole injection barrier height at the ITO y MPc interface, and then affect the driving voltage of the fabricated devices. This relationship between the

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217

Table 1 HOMO and LUMO energies of MPcs films available from the literature w20,21,27–30x. MPc

HOMO (eV)

LUMO (eV)

Determination method

Reference

ZnPc

5.0 4.7 5.7 5.2 5.0 5.1 5.6 5.0 5.0 5.2 5.3 5.4 5.7 5.7 5.9 5.9

3.4

Photoelectron emission Cyclic voltammetry UPS Photoelectron emission Cyclic voltammetry UPS UPS Photoelectric emission Photoelectron emission Photoelectron emission Photoelectron emission Photoelectron emission UPS Photoelectron emission Photoelectron emission VEH-UPS and VEH-XPS

w20x See text w27x w20x See text w28x w27x w29x w20x w20x w20x w20x w27x w20x w30x w21x

CuPc

FePc CoPc PbPc NiPc VOPc SnPc H2Pc

3.6

3.4 3.9 3.6 3.9 4.2

driving voltage and the HOMO level becomes clearer as shown in Fig. 5, where the driving voltages of the MPc devices at a luminance of 100 cd my2 are plotted against the HOMO level energies of MPcs. It has been reported that an MgPc film has an oxidation potential as low as 0.60 V vs. SCE w23x. This hints that the device performance should be improved by inserting a layer of MgPc. However, we found that compared to other MPcs, both the I–V and L–V curves of the MgPc device shift to much higher bias voltage, its driving voltage at 100 cd my2 increased to 7.1 V, and its maximum luminance is only approximately 4000 cd my2 whereas the other MPc devices approach or exceed 10 000 cd my2. The reason is not yet clear why the MgPc device shows poor EL performances. As a tentative explanation, we considered the following. When the HOMO level of MPc is considerably higher than the Fermi level of ITO in the energy diagram, the

Fig. 4. (a) I–V and (b) L–V characteristics of ITOyMPc (20 nm)y TPD (40 nm)yAlq3 (60 nm)yMg–Ag devices.

Fig. 5. Plot of driving voltages against the HOMO level energies of MPcs at luminance of 100 cd my2 for ITOyMPc (20 nm)yTPD (40 nm)yAlq3 (60 nm)yMg–Ag devices.

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hole injection barrier at the ITO y MPc contact becomes missed, whereas the barrier at the MPc y TPD contact becomes too great. The latter is unfavorable to the successive hole injection. As a consequence, the observed apparent hole injection is depressed, and the EL characteristics of the device becomes poor. Some other parameters should also be considered, such as the adhesion strengths at both the ITO y MPc and MPc y TPD interfaces. We noted that at 100 cd my2, the driving voltages of the devices for SnPc and H2Pc were 8.3 and 9.4 V, respectively. These driving voltages are higher than that (7.8 V) required by the ITO y TPD (60 nm) y Alq3 y Mg–Ag device, where no MPc layer was inserted. Furthermore, the I–V and y or L–V curves of the devices for such as NiPc and SnPc are different from the others, and crossed with the curves of some others. These suggest that beside the hole injection barrier at the ITO y MPc contact, some other parameters also affect the EL characteristics of the devices. 4. Conclusions The HOMO level energies of MPcs are dependent on their central metal atoms. MPc films show p-type conductance and may function as HTL materials. Use of MPcs having low HOMO level energies results in a decreased energy barrier for hole injection, leading to improvement of the EL performances of MPc devices, especially the decreasing of the driving bias voltage. The driving voltages of the MPc devices are found to decrease in the order of ZnPc -CuPc -FePc-CoPc-PbPc-NiPc-VOPc-SnPc-H2 Pc, which is in agreement with the order of HOMO levels of MPcs except MgPc. The finding of such a linear relationship between the driving voltage and the HOMO energy is expected to serve as a guide for developing better HTL materials. MPcs often exhibit a HOMOLUMO band gap of 1.7 eV or less. Thus, MPc films have very poor, if any, electron-blocking ability, which accounts for the very low EL efficiency without use of a TPD layer. This is consistent with the observed low EL efficiency for ITO y VOPc y Alq3 y Al devices fabricated by Blochwitz et al. w16x. In order to increase the EL efficiency of the device, it is necessary to insert a thin electron blocking layer with a higher LUMO than MPc but a similar HOMO level. This is easily achieved by use of TPD, a traditional HTL material. Acknowledgements This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.

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