JOURNAL OF APPLIED PHYSICS 108, 094506 共2010兲
Solution processable single layer organic light-emitting devices with a single small molecular ionic iridium compound Byoungchoo Park,1,a兲 Yoon Ho Huh,1 Hong Goo Jeon,1 Chan Huk Park,1 Tae Kyung Kang,2 Byeong Hyo Kim,2 and Jongwoon Park3 1
Department of Electrophysics, Kwangwoon University, Wolgye-Dong, Seoul 139-701, Republic of Korea Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea 3 National Center for Nanoprocess and Equipment, Nanoelectronics Team, Korea Institute of Industrial Technology, Gwangju 500-480, Republic of Korea 2
共Received 9 August 2010; accepted 15 September 2010; published online 3 November 2010兲 We herein report on the occurrence of bright and efficient electrophosphorescence from a simple organic light-emitting diode 共OLED兲 with a single organic layer comprised of a small molecular ionic iridium compound, formed using a solution process. The studied small molecular ionic iridium compound is 关Ir共dfppy兲2共bpy兲兴+PF−6 , which exhibits excellent film-forming properties, bright green photoluminescence, and efficient bipolar carrier transport with balanced electron and hole mobilities of about 10−5 cm2 / 共V s兲. A high performance of the device was achieved by using a phosphorescent OLED 共PHOLED兲 that was fabricated using the 关Ir共dfppy兲2共bpy兲兴+PF−6 compound, with a peak brightness of about 18 000 cd/ m2 and a peak current efficiency of 12 cd/A. A peak power efficiency of 2.5 lm/W was measured at 2800 cd/ m2. These results suggest that the small molecular ionic iridium compound is a promising material for bright and efficient PHOLEDs manufactured using a simple solution process. © 2010 American Institute of Physics. 关doi:10.1063/1.3503455兴 I. INTRODUCTION
Ever since the early pioneering studies of bright and efficient organic light-emitting diodes 共OLEDs兲, they have been the subject of considerable research interest due to their promising performance in full-color flat-panel displays and lighting.1–9 It has recently been reported that OLEDs that use phosphorescent materials, such as iridium 共Ir兲 or platinum 共Pt兲 compounds, as their emitting materials show much higher efficiencies than OLEDs that use conventional fluorescent emitting materials.3–6 In phosphorescent OLEDs 共PHOLEDs兲, strong spin-orbit coupling leads to rapid intersystem crossing and a radiative transition from the triplet states to a ground state, which enables an enhanced and efficient electroluminescent 共EL兲 emission.3–6 Given these properties, many attempts have been made in recent years to develop and improve PHOLEDs. Highly efficient PHOLEDs have commonly been fabricated using a conventional vacuum sublimation technique. Indeed, PHOLEDs have been fabricated with the use of as many as five or six different organic multilayers for hole injection, hole transport, electron injection, electron transport, hole blocking, and light emission.3–6 In the lightemitting layer, the phosphorescent materials are usually doped as an emitting guest into a host material, that has a higher triplet excited state 共T1兲 energy than that of the doped guest because of the need for T1 energy confinement on the phosphorescent guest. Some researchers have fabricated multilayer PHOLEDs that used phosphorescent dopants of fac tris共2-phenylpyridine兲 iridium 兵关Ir共ppy兲3兴其 to yield a current conversion efficiency C of 28 cd/A.3 Such a high efficiency implies that the internal quantum efficiency of the a兲
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PHOLED had reached almost 100% in that case.3–6 However, despite the demonstration of such efficient multilayer PHOLEDs, their application is still limited by a complex fabrication process that involves the use of high vacuum conditions. Hence, there is a real need for a simple method of fabrication of bright and efficient PHOLEDs that makes use of a solution process, for example. The simple fabrication of PHOLEDs that uses solution-processable materials could afford the possibility of a range of new applications, such as the ink-jet printing of large fine-pixel displays.7–11 To that end, several trials have been undertaken of solutionprocessed polymer PHOLEDs that have been fabricated using a small molecular phosphorescent guest dopant introduced into a polymer host. Some of these have been successful;7–11 for example, a current efficiency of 30 cd/A has been reported for solution-processed polymer PHOLEDs containing an Ir complex as dopant.7 Despite the successes, these devices still employ a solution-coated layer of a complex mixture of components, whose application may be a factor that limits the simple and homogeneous fabrication of PHOLEDs. Besides these small molecular compounds, phosphorescent polymers,10 and dendrimers11 have also been used in solution-processed PHOLEDs. However, due to difficulties in achieving their high molecular weight, satisfactory EL performance for the fabricated devices has not yet been achieved. The search therefore continues for a fabrication process that can provide a simple PHOLED structure that produces a high performance in the resulting device. To achieve this goal, a small molecular phosphorescent material system, possessing appropriate carrier-transporting properties that optimize the formation of excitons in the EL layer, may provide a new strategy for designing highly efficient and solution-processable PHOLEDs. We herein describe the development of PHOLEDs that
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F
CsF (1 nm) / Al A [Ir(dfppy) (bpy)] + PF -2 6 V Single EL layer
PEDOT:PSS ITO
N F
F Ir N
N N
PF 6 -
F
Glass substrate FIG. 1. Architecture of the device structure and chemical structure of 关Ir共dfppy兲2共bpy兲兴+PF−6 used in this study.
use a single layer of a small molecular ionic Ir compound for a new class of solution-processable carrier-transporting phosphorescent material system. In previous experiments, similar ionic Ir compounds were used for light-emitting electrochemical cells 共LECs兲 that exhibited a typical brightness of less than 400 cd/ m2, and had a long turn-on time of between several minutes and an hour.12–15 In this study, we demonstrate that the fabricated PHOLEDs emitted green phosphorescence, and their luminance reached approximately 18 000 cd/ m2 with a peak efficiency of 12 cd/A, while achieving a fast response time of ⬃10 s. II. EXPERIMENTAL DETAILS
The studied small molecular ionic Ir compound is 关Ir共dfppy兲2共bpy兲兴+PF−6 共ionic Ir compound兲, which was synthesized by modifying the previously reported procedure with a yield of 44%.16 In order to investigate the surface morphologies of the spin-coated films of the ionic Ir compound, the variation in the surface roughness of the film was monitored by atomic force microscope 共AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.兲. During the measurements, a contact mode was used with a cantilever 共CONTR-10 point probe-silicon, Nanoworld, Inc.兲. The coating solution consisted of the ionic Ir compound at an appropriate concentration 共⬃1.5 wt %兲 into a mixed solvent of 1,2-dichloroethane and chloroform 共mixing weight ratio 3:1兲 to obtain homogeneous flat films. The electrochemical properties of the ionic Ir compound were determined by cyclic voltammetry 共CV兲. In order to determine the highest occupied molecular orbital 共HOMO兲 and the lowest unoccupied molecular orbital 共LUMO兲 levels, the electrochemical reduction, and oxidation potentials of the ionic Ir compound film 共working electrode兲 on glassy carbon were measured in 0.1 M tetrabutylammonium tetrafluoroborate/acetonitrile 共ACN兲 with Ag/AgCl reference electrode and Pt counter electrode at scan rate of 50 mV/s. The optical characteristics of the spincoated films of the ionic Ir compound on quartz substrates were investigated using a Cary 1E 共Varian兲 UV-vis spectrophotometer and a calibrated S4000 Ocean Optics fiber spectrophotometer at room temperature. For the PHOLED experiments, simple sandwich-type devices were fabricated, as shown in Fig. 1. Prepatterned indium-tin-oxide 共ITO, 80 nm, 30 ⍀ / sq, active area: 3 ⫻ 3 mm2兲 coated glass substrates were used to fabricate the devices. Following routine cleaning procedures with ultraviolet-ozone treatment of the substrate, a solution of the
ionic Ir compound was spin coated on top of the ITO, precoated with a poly共3,4-ethylenedioxythiophene兲:poly共4styrenesulphonate兲 共PEDOT:PSS, Clevios PVP. Al 4083, H.C. Starck Inc.兲 buffer layer 共40 nm兲. The spin-coated solution consisted of the ionic Ir compound at a concentration of about 1.5 wt % in the mixed solvent of 1,2-dichloroethane and chloroform 共mixing weight ratio 3:1兲. The fabricated EL layer was about 100 nm thick. A cathode layer of CsF 共1 nm兲:Al 共100 nm兲 was then formed on top of the EL layers via thermal deposition at a rate of 0.5 nm/s under a base pressure below 2.7⫻ 10−4 Pa. Thus, the sample PHOLED structure consisted of the sequence 共ITO with PEDOT:PSS/关Ir共dfppy兲2共bpy兲兴+PF−6 EL layer/CsF and Al cathode兲. The sample device was fabricated and characterized at room temperature under ambient conditions, without encapsulation. The performance of the PHOLEDs was measured using a Chroma Meter CS-200 共Konica Minolta Sensing, Inc.兲, and a source meter 共Keithley 2400兲 was used to measure the EL characteristics. III. RESULTS AND DISCUSSION
We first investigated the film-forming ability of the solution of the small molecular ionic Ir compound 共关Ir共dfppy兲2共bpy兲兴+PF−6 兲 using AFM, as shown in Fig. 2共a兲. The investigation clearly showed that the topography is fairly uniform; the root mean square roughness for the fabricated film was only approximately 0.6 nm, which is comparable to that 共approximately 1.0 nm兲 of the used substrates. Moreover, the surface roughness was identical for the coated films at different positions. It was therefore self-evident that, using solution-coating, a film of the ionic Ir compound of high quality could be formed. The electrochemical properties of the ionic Ir compound were also investigated using CV measurement, as shown in Fig. 2共b兲. From CV curves, the measured oxidation and reduction potentials of the ionic Ir compound were 1.46 V and ⫺0.87 V 共versus Ag/AgCl兲, respectively. From the oxidation and reduction potentials, we determined the HOMO and the LUMO energy levels. The HOMO and LUMO levels were estimated to be ⫺5.8 eV and ⫺3.5 eV, respectively. We then obtained the UV-vis absorption and photoluminescence 共PL兲 spectrum of the spincoated ionic Ir compound film at room temperature 关Fig. 2共c兲兴. The intense absorption bands in the ultraviolet region below 300 nm are assigned to 1共 – ⴱ兲 transitions of the ligands and the peaks near 350–400 nm may be ascribed as spin-allowed metal-to-ligand charge transfer 共 1MLCT兲 transitions.15–17 The film was also characterized by broad and almost featureless PL emission spectra at room temperature, which indicated that the emissive excited states of the compound has predominantly characters of spin-forbidden metalto-ligand charge transfer 共 3MLCT兲 and/or ligand-to-ligand charge transfer 共 3LLCT兲 transitions other than ligand-tocentered 共LC3兲 – ⴱ transition, which always shows the vibronic structure in emission spectra.15–17 Next, in order to study the device performance of the fabricated PHOLED, the ionic Ir compound was used to fabricate single-layered PHOLEDs 共Fig. 1兲. Figure 3共a兲 shows the schematic energy level diagram of the PHOLED, in
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6.2 nm
-9.7 nm
ITO 4.4 ~ 4.7
X (3 μm) Y (3 μm)
(b)
5.1 ~ 5.4
3.5
CsF
Mean fit 15.7 nm
(a) PEDOT:PSS
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--
[Ir(dfppy)2(bpy)] PF6
Al 4.2
5.8
(b)
(c)
(c)
FIG. 2. 共Color online兲 共a兲 Three-dimensional topographical AFM images of the spin-coated film of 关Ir共dfppy兲2共bpy兲兴+PF−6 . 共b兲 Cyclic voltammogram of 关Ir共dfppy兲2共bpy兲兴+PF−6 in 0.1 M tetrabutylammonium tetrafluoroborate/ACN with Ag/AgCl reference and Pt counter electrodes. 共c兲 UV-vis absorption and PL spectra for the film of 关Ir共dfppy兲2共bpy兲兴+PF−6 .
which energy levels of ITO, PEDOT:PSS, and Al were taken from previous reports.18 As shown in the figure, the HOMO level of the ionic Ir compound was slightly lower than the work function of ITO; hence, the holes may be efficiently injected and transported into the compound layer from the ITO/PEDOT:PSS anode. Similarly, because the LUMO level, which is slightly higher than the work function of Al, efficient electron injection, and transportation into the compound layer from the CsF:Al cathode is also possible. It may thus be expected that the ionic Ir compound layer could be used as the EL layer, in order that a single compound layer PHOLED device structure could be achieved. Using the fabricated device, we observed the operation of the EL with the naked eye. Figure 3共b兲 is a photograph of the PHOLED in operation, under an applied bias of +11 V. The figure clearly shows bright EL emission from the active area of the PHOLED. We then investigated the EL spectrum obtained from the PHOLED 共under the same +11 V bias兲. Figure 3共c兲 shows that the EL emission spectrum 共solid curve兲 of the
FIG. 3. 共Color online兲 共a兲 Schematic energy band diagram of the studied PHOLEDs. 共b兲 A photograph of the operating device using 关Ir共dfppy兲2共bpy兲兴+PF−6 under an applied bias of 11 V. 共c兲 Normalized EL spectra from the sample device at +11 V. The dotted curve shows the PL spectra for the film of 关Ir共dfppy兲2共bpy兲兴+PF−6 .
sample devices has a peak wavelength of max = 517 nm with full widths at half maximum 共FWHM兲 of approximately 80 nm. The characteristics of this spectrum are very similar to those of the PL spectrum 共dotted curve兲 for the ionic Ir compound films, which indicates that the same mechanism is responsible for the emissions in both cases. In order to determine the optimal operating conditions of the PHOLED under test, we observed the variation in the characteristics of the device with the thickness of the EL layer of the ionic Ir compound. The results are summarized in Table I, which shows that all the devices were characterized by relatively bright EL emissions, implying an effective confinement of excitons in the single layer of the ionic Ir compound. It is noteworthy that even without fulloptimization of the operation of the PHOLEDs, the performance of the single layer PHOLEDs of the ionic Ir com-
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TABLE I. EL characteristics of the devices. VON: turn-on voltage, Lmax: maximum luminance, max: emission peak wavelength, CIE: Commission Internationale de l’Eclairage coordinates, FWHM: full width at half maximum, P,max: maximum power efficiency, C,max: maximum current efficiency. Layer thickness Characteristics VON 共V兲 Lmax 共cd/ m2兲 关V at Lmax 共V兲兴 max 共nm兲 CIE 共x, y兲 FWHM 共nm兲 P,max 共lm/W兲 C,max 共cd/A兲 L 共cd/ m2兲 a P 共lm/W兲a C 共cd/A兲a
120 nm
100 nm
85 nm
70 nm
10.5 14 530 共18.0兲 517 0.293, 0.583 80 2.0 10.7 10 180 1.8 6.5
7.0 17 810 共17.0兲 517 0.293, 0.583 80 2.5 12.0 11 560 2.3 11.8
7.5 12 268 共14.5兲 517 0.282, 0.575 79 2.1 8.0 8 600 1.8 7.9
6.5 8 202 共14.5兲 517 0.280, 0.561 79 1.1 4.0 4 760 1.0 3.8
a
Measured at a current density of 100 mA/ cm2.
pound shows the potential attractiveness of these devices. In particular, the PHOLED with a 100 nm thick EL layer exhibits the highest luminous and power efficiencies among the OLEDs tested. Figure 4共a兲 shows its current-voltage 共J − V兲 and luminance-voltage 共L − V兲 characteristics. As shown in the J − V curve, the PHOLED exhibited a typical diodelike of current flow, with the current density rapidly increasing with applied positive bias, with good rectification and clear reverse breakdown 共near ⫺15 V兲. As the applied voltage was increased from zero, the EL luminescence also increased 共L − V curve兲. The sharp increase in the L − V curve, even under a low current density, suggests that the efficient injection of both holes and electrons into the EL layer is possible. Operating voltages of about 11.3 V and 12.7 V were required to
(a)
(b)
FIG. 4. 共Color online兲 共a兲 J − V and L − V characteristics and 共b兲 current efficiency-voltage and power efficiency-voltage characteristics for the sample PHOLED devices using 关Ir共dfppy兲2共bpy兲兴+PF−6 .
obtain brightnesses of 100 cd/ m2 and 1000 cd/ m2, respectively, with a peak luminescence of approximately 17 800 cd/ m2 being produced at 17.0 V. The luminance of the device under test was comparable to that of a previously reported polymer PHOLED 共⬃20 000 cd/ m2兲 共Ref. 7兲 that was co-doped with a hole-transporting molecule, a phosphorescent dopant, and a large band-gap polymer. It is noted that the Commission Internationale de L’Eclairage 共CIE兲 coordinate of 共x = 0.29, y = 0.58兲 of the EL emission from the devices is nearly independent of current density. In order to confirm the high performance of the PHOLEDs, we also deduced the efficiencies of the device that had an EL layer 100 nm thick 关see Fig. 4共b兲兴. A current efficiency 共C兲 of 4.0 cd/A was obtained at 100 cd/ m2, reaching C = 8.8 cd/ A at 1000 cd/ m2 and C = 12.0 cd/ A at 7200 cd/ m2. We also determined the power efficiency P, which increased, reached a maximum of 2.5 lm/W, and then slowly decreased with increasing bias voltage. These results imply that a balance in charge injection was achieved for the single EL layer of the ionic Ir compound. In order to understand the EL characteristics of the single layer of the ionic Ir compound, the mobilities of the charge carriers were investigated using a time-of-flight 共TOF兲 transient photocurrent technique19 by irradiation of a pulse laser light 共355 nm, neodymium-doped yttrium aluminium garnet laser, pulse width; 4 ns兲 at room temperature. Figure 5 shows the representative TOF transients of the electrons for the ionic Ir compound layer 共1.59 m thick兲. No clear plateaus are observed, which indicates the dispersive transport behavior of the electrons 共i.e., with carrier trapping兲. Nevertheless, in the logarithmic representations of the TOF transients 共inset in Fig. 5兲, the mobility of the carriers is reflected in the carrier transit time 共tT兲, which can be evaluated using the intersection point of the two asymptotes. The measurements show that the film of the compound has mobilities of about 10−5 cm2 / V s for both electrons and holes 共electron mobility approximately 1.8⫻ 10−5 cm2 / V s and hole mobility approximately 8.1⫻ 10−5 cm2 / V s兲. These values are about one order of magnitude higher than the electron mobility 共approximately 10−6 cm2 / V s兲 of a typical electron transporting material, such as tris共8-
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face potential22 and could thus lead to a decrease in the potential barrier, which leads to a larger injection of current. The large hole and electron injections that occur with the bipolar carrier-transporting processes of the ionic Ir compound layer may result in the efficient formation of excitons and a charge recombination inside the single compound layer, both of which are critical for the high performance of simple PHOLEDs. IV. CONCLUSIONS
FIG. 5. 共Color online兲 TOF transients of electrons for the 关Ir共dfppy兲2共bpy兲兴+PF−6 film 共1.59 m thick兲 at an electric field E = 3.77 ⫻ 105 V / cm. The inset shows a double-logarithmic plot. tT: transit time.
hydroxyquinoline兲 aluminum,19,20 and two orders of magnitude lower than the hole mobility 共approximately 10−3 cm2 / V s兲 of a typical hole transporting material, such as 1,4-bis共1-naphthyl phenyl amino兲 biphenyl.21 In our single compound layer PHOLED structure, both injected carriers can therefore be effectively transported into the EL layer of the ionic Ir compound with balanced hole-current and electron-current flows, thus producing the EL performance shown here. We also observed the dynamic responses of the PHOLEDs in operation, 共Fig. 6兲. The observed dynamic responses of the device are nearly the same as those of a typical multilayer OLED, in that the rising and falling times are about 14 s and 3 s, respectively, making them suitable for practical applications such as flat-panel displays. These fast dynamic responses of the studied PHOLED reflect the fast bipolar transportation of electrons and holes, and the ionic parts of the ionic Ir compound in the EL layer remain stable. The results presented herein reflect the distinctive characteristics of our PHOLEDs in terms of current flow and luminance, which in turn implies clear differences between them and typical LECs.12–15 Finally, we offer a comment on the large injections of current into the EL layer of the PHOLEDs under test. One possible reason for such a large injection of current is that the ionic parts of the compound molecules may form dipolar interfacial layers between the EL layer and the electrodes. These dipolar interfacial layers may cause a change in sur-
FIG. 6. 共Color online兲 Dynamic responses for the operating sample PHOLED device using 关Ir共dfppy兲2共bpy兲兴+PF−6 .
In conclusion, we have demonstrated that bright and efficient electro-phosphorescence can be obtained using solution-processable single compound layer PHOLEDs that use a small molecular ionic Ir compound of 关Ir共dfppy兲2共bpy兲兴+PF−6 . A high brightness of about 18 000 cd/ m2, and a high current efficiency of up to 12 cd/A with a fast response time 共approximately 10 s兲 was obtained from the single compound layer PHOLED. This performance may be attributed to a balanced transportation of charges into the single compound EL layer with large hole and electron injections at the interfaces between the EL layer and electrodes. From these results, it is demonstrated that simple, efficient, and bright PHOLEDs can be produced through a combination of a simple solution process that uses a highly luminous small molecular ionic Ir compound material. ACKNOWLEDGMENTS
This work was supported by Korea Institute of Industrial Technology 共Grant No. 09-IK-2-0002, Development of transparent conducting polymer for OLED applications兲 and by The Ministry of Knowledge Economy 共MKE兲, Korea, under the Information Technology Research Center 共ITRC兲 support program supervised by the National IT Industry Promotion Agency 共NIPA兲 共NIPA-2010-C1090-1011-0002兲. C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett. 51, 913 共1987兲. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradly, D. A. Dos Santos, J. L. Bredas, M. Logdlund, and W. R. Salaneck, Nature 共London兲 397, 121 共1999兲. 3 M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest, Appl. Phys. Lett. 75, 4 共1999兲. 4 M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Appl. Phys. Lett. 79, 156 共2001兲. 5 C. Adachi, M. E. Thompson, and S. R. Forrest, IEEE J. Sel. Top. Quantum Electron. 8, 372 共2002兲. 6 G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, and J. Salbeck, Appl. Phys. Lett. 85, 3911 共2004兲. 7 X. H. Yang and D. Neher, Appl. Phys. Lett. 84, 2476 共2004兲. 8 H.-M. Liu, J. He, P.-F. Wang, H.-Z. Xie, X.-H. Zhang, C.-S. Lee, B.-Q. Sun, and Y.-J. Xia, Appl. Phys. Lett. 87, 221103 共2005兲. 9 Y.-H. Niu, H. Ma, Q. Xu, and A. K.-Y. Jen, Appl. Phys. Lett. 86, 083504 共2005兲. 10 S. Tokito, M. Suzuki, F. Sato, M. Kamachi, and K. Shirane, Org. Electron. 4, 105 共2003兲. 11 D. Ma, J. M. Lupton, R. Beavington, P. L. Burn, and I. D. W. Samuel, Adv. Funct. Mater. 12, 507 共2002兲. 12 J. C. deMello, N. Tessler, S. C. Graham, and R. H. Friend, Phys. Rev. B 57, 12951 共1998兲. 13 Q. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, Science 269, 1086 共1995兲. 14 J. Gao, G. Yu, and A. J. Heeger, Appl. Phys. Lett. 71, 1293 共1997兲. 15 L. He, J. Qiao, L. Duan, G. Dong, D. Zhang, L. Wang, and Y. Qiu, Adv. Funct. Mater. 19, 2950 共2009兲. 1 2
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