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Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer
Organic Electronics 15 (2014) 2492–2498
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer Jun Liu a, Xindong Shi a, Xinkai Wu a, Jing Wang a, Zhiyuan Min a, Yang Wang a, Meijun Yang a, Chin H. Chen b, Gufeng He a,⇑ a National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b Guangdong Aglaia Optoelectronic Materials Co., Ltd., Shunde, Foshan, Guangdong, People’s Republic of China
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Article history: Received 24 April 2014 Received in revised form 4 July 2014 Accepted 15 July 2014 Available online 27 July 2014 Keywords: High efficiency Inverted OLED Phosphorescence Ultrathin EML
a b s t r a c t High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5 nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5 nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5 nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency rolloff at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5 nm doped EML has about EQE of 20 % at 10,000 cd/m2. It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, active-matrix organic light-emitting diodes (AMOLEDs) have been successfully applied in small and medium size commercial products, such as mobile phones, tablets, and high-definition television [1,2]. Due to high mobility and stability, low temperature poly silicon (LTPS) thin film transistor (TFT) [3,4] is currently used in AMOLED. However, LTPS-TFT requiring the excimer-laser annealing (ELA) process suffers from non-uniformity and high investment. For that reason, indium Gallium Zinc ⇑ Corresponding author. Tel.: +86 21 34207045; fax: +86 21 34204371. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.orgel.2014.07.027 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
Oxide (IGZO)-TFT backplane for AMOLEDs is an attractive alternative, since it provides unique advantages of uniformity, large area and low cost [5,6]. Owing to the n-type nature of IGZO-TFT, inverted OLEDs with a bottom cathode are preferred to integrate with the IGZO-TFT in AMOLED [7]. Indium tin oxide (ITO) is commonly used as the cathode of inverted OLED due to its high optical transparency and electrical conductivity, but its high work function limits electron injection into the neighboring electron-transporting layer (ETL). Most of research on inverted OLED focus on reducing barrier height between ITO and ETL. An insertion of metallic layer with low work function or n-type doped ETL by Li and Cs compounds is often used for improved electron injection from ITO [8–10]. On the
J. Liu et al. / Organic Electronics 15 (2014) 2492–2498
other hand, aluminum with low work function is usually used as anode because of its low cost and high reflectivity. An electron accepting interlayer with high work function such as MoO3 [11], WO3 [12] or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) [13] facilitates enhanced hole injection from anode Al to the hole-transporting layer (HTL). For example, the injection efficiency for inverted device with HAT-CN hole-injecting layer (HIL) on top of N,N0 -di(naphthalene-1-yl)-N,N0 -diphenylbenzidine (NPB) HTL is near unity, meaning that hole injection contact is ohmic [14]. Compared to the conventional fluorescence OLED, the inverted device using the same EIL and HIL shows significantly higher current efficiency (CE) or external quantum efficiency (EQE). Chen et al. achieved the maximum CE of 23.7 cd/A for the inverted green device with 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7,-tetra(methyl)-1H,5H,11H-[1]-benzopyrano [6,7,8-ij]quinolizin-11-one (C545T) fluorophor, almost twice as high as that of the conventional device [15]. Peng et al. also reported high CE of the inverted tris(8-hydroxyquinolinato)aluminium (Alq3)-based device while only half of that for the conventional device [16]. An EQE over 20% has often been observed for the conventional phosphorescence OLEDs (PhOLEDs), [17] however, it is rarely reported in inverted OLED. Recently, to overcome the limitation of complicated process of doping system, conventional PhOLEDs with ultrathin undoped emitting layer (EML) (<1 nm) have been introduced, which exhibited the maximum EQE about 20%, due to optimized confinement of exactions within the ultrathin EML [18]. Nevertheless, such device showed serious EQE roll-off at high brightness, owing to annihilation of high density triplets within ultrathin emission zone, and the ultrathin undoped EML has not been applied in inverted PhOLED structure. Moreover, for better understanding, the charge recombination and the effects of excitons distribution on efficiency roll-off within the ultrathin EML should be investigated. In this paper, ITO/Cs2CO3 cathode and Al/MoO3 anode were used in the inverted PhOLED, while ITO/MoO3 anode and Al/Cs2CO3 cathode were used in the conventional device. Based on current density–voltage characteristic of single carrier devices, difference of charge injection ability of ITO/Cs2CO3 cathode and Al/MoO3 anode in the inverted device was significantly larger than that of ITO/MoO3 anode and Al/Cs2CO3 cathode in the conventional device. However, the inverted PhOLED with ultrathin (0.5 nm) undoped EML showed the maximum EQE of 24.7%, and EQE of 20.9% at 1,000 cd/m2, while those of the conventional device were 13.3% and 13.0%. Moreover, the surface morphology of EML was characterized by Atomic Force Microscopy (AFM). Accordingly, the ultrathin undoped EML was simulated by a relatively broadened thin (1.5 nm) doped EML with co-host, and the amount of phosphors in both the doped and undoped EML are identical. The doped EML device demonstrated the maximum EQE of 31.1%, EQE of 24.2% at 1,000 cd/m2, and EQE of 17.0% at 10,000 cd/m2. Better charge balance and preferred horizontal orientation of emitter may contribute to the efficiency enhancement. By broadening the doped EML from 1.5 nm to 10.5 nm, the efficiency roll-off ranging from 1000 to 10,000 cd/m2 was improved. Such device with
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10.5 nm doped EML showed EQE of 23.3% at 1000 cd/m2 and 19.2% at 10,000 cd/m2, while the maximum EQE slightly dropped to 27.8%, indicating that excitons with high concentration within the ultrathin EML were dispersed within the expanded EML.
2. Experimental The single carrier devices and PhOLEDs were fabricated on ITO-coated glass substrates. Prior to film deposition, the ITO glass substrates were cleaned successively using deionized water, acetone and isopropanol under an ultrasonic bath, and then pretreated with oxygen plasma after drying. The structure of hole-only device is ITO/MoO3 (6 nm)/HTL (343 nm)/MoO3 (6 nm)/Al (100 nm), where 1,3-bis(carbazol-9-yl)benzene (mCP) is used as HTL. Under forward bias, ITO/MoO3 acts as the anode, and electrons from Al can be hardly injected into mCP due to electron blocking property of MoO3. Under reversed bias, Al/MoO3 acts as the anode, and electrons from ITO are prohibited as well because of electron blocking property of MoO3. The structure of electron-only device is ITO/Cs2CO3 (1 nm)/ETL (310 nm)/Cs2CO3 (1 nm)/Al (100 nm), where ETL is 4,7-diphenyl-1,10-phenanthroline (BPhen). Under forward bias, Al/Cs2CO3 is cathode, and holes injection from ITO into BPhen are forbidden due to hole blocking property of Cs2CO3 and BPhen. Under reversed bias, ITO/ Cs2CO3 is cathode, and holes from Al are blocked because of hole blocking property of Cs2CO3 and BPhen. The device configuration of inverted PhOLED is ITO/Cs2CO3 (1 nm)/ BPhen (35 nm)/1,3,5-tris (N-phenylbenzimidazole-2yl)benzene (TPBI) (12.5 nm)/emitter (0.5 nm)/mCP (49 nm)/MoO3 (6 nm)/Al (100 nm), where the undoped phosphorescent emitter is a bis-cyclometalated iridium complex of Ir(tfmppy)2(tpip) (tfmppy = 4-trifluoromethylphenylpridine, tpip = tetraphenylimido-diphosphinate) [19–21], as depicted in Fig. 1. The EIL, ETL, EML, HTL, HIL and anode Al were successively thermally deposited within a high vacuum deposition system at a base pressure of 106 torr. The deposition rate for organic materials was monitored by quartz-crystal monitors and kept constant within the range from 0.01 to 0.1 nm s1. For comparison, the conventional PhOLED with device structure of ITO/ MoO3 (6 nm)/mCP (49 nm)/Ir(tfmppy)2(tpip) (0.5 nm)/TPBI (12.5 nm)/BPhen (35 nm)/Cs2CO3 (1 nm)/Al (100 nm) was fabricated. The surface morphologies of TPBI, Ir(tfmppy)2 (tpip) and mCP were measured by AFM. In order to maintain the same thickness of device, the multilayer TPBI (12.5 nm)/Ir(tfmppy)2(tpip) (0.5 nm)/mCP (49 nm) in the undoped EML device was adjusted as TPBI (12 nm)/TPBI: Ir(tfmppy)2(tpip):mCP (1:1:1, 1.5 nm)/mCP (48.5 nm) or TPBI (7.5 nm)/TPBI:Ir(tfmppy)2(tpip):mCP (1:1:1 or 1:1:0.5 or 1:1:0.2, 10.5 nm)/mCP (44 nm) in the doped EML device. Current density–voltage (J–V) and luminescence–voltage (L–V) characteristics were measured with a computer controlled Keithley 2400 source meter and BM-7A luminance colorimeter under ambient conditions. The EQE is calculated based on the assumption that the bottom emission inverted PhOLED is a lambertian radiator.
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Fig. 1. Schematic diagram of inverted PhOLED, chemical structure of the used phosphorescent dye, and energy level diagram of TPBI, Ir(tfmppy)2(tpip) and mCP. Edges of rectangle correspond to HOMO (bottom) and LUMO (top) energies.
Hole injection barrier is associated with the energy difference between the Fermi level of anode and the highest occupied molecular orbital (HOMO) level of HTL. Higher work function of ITO (5.1 eV) than that of Al (4.2 eV) is supposed to facilitate hole injection from ITO. However, as shown in Fig. 2, the hole-only device with Al/MoO3 anode shows significantly increased current density than that with ITO/MoO3 anode, indicating independence of hole injection on work function of anode in the presence of MoO3 injection layer. These results are consistent with the reported research that MoO3 top-contact HTL has larger hole injection efficiency than MoO3 bottom-contact HTL [14]. We present the reason of hole injection enhancement for MoO3 top-contact HTL as follows. Thermal evaporated MoO3 on top of mCP leads to diffusion of MoO3 within mCP, resulting in larger degree of charge transfer between MoO3 and mCP, compared to MoO3/mCP interface with mCP on top of MoO3. The better charge transfer between MoO3 and mCP means better p-type doping, which leads to enhanced hole injection and transporting. Likewise, Cs2CO3 on top of BPhen ETL promotes electron injection from Al/Cs2CO3 cathode, compared to ITO/Cs2CO3
cathode followed by deposition of BPhen. Just as shown in Fig. 2, the current density of electron-only device with Al/ Cs2CO3 cathode is over four orders of magnitude larger than that with ITO/Cs2CO3 cathode. The electron injection from ITO to BPhen is still weak even in the presence of Cs2CO3, due to high work function of ITO and weak n-type doping at Cs2CO3/BPhen interface. Both ITO/MoO3 anode and Al/Cs2CO3 cathode are commonly used in the conventional device, while the combination of ITO/Cs2CO3 cathode and Al/MoO3 anode are applied in the inverted device. According to the J–V characteristics of single carrier devices, difference between hole injection ability of Al/MoO3 anode and electron injection ability of ITO/Cs2CO3 cathode for the inverted PhOLED is significantly larger than that between ITO/MoO3 anode and Al/ Cs2CO3 cathode for the conventional device. Naturally, this leads to charge unbalance for the inverted device, resulting in inferior efficiency for the inverted device. However, the inverted PhOLED with an undoped EML shows sharply enhanced EQE compared to the conventional device with the same EML. As shown in Fig. 3, the maximum EQE, EQE at 1000 cd/m2 and at 10,000 cd/m2 of the inverted device are 24.7%, 20.9% and 14.0%, respectively, while those of the conventional device are 13.3%, 13.0%, and
Fig. 2. Current density–voltage characteristics of hole-only device with ITO/MoO3 and Al/MoO3 anodes and electron-only device with ITO/Cs2CO3 and Al/Cs2CO3 cathodes.
Fig. 3. Luminance-EQE-power efficiency characteristics of conventional and inverted undoped EML and inverted doped EML PhOLEDs. Inset: Current density–voltage characteristics.
3. Results and discussion
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11.0%. The power efficiencies of the conventional and inverted devices at 1000 cd/m2 are about 35 and 31 lm/ W, respectively. In order to find the reason for higher EQE of inverted device than that of conventional device, the emission distribution of inverted device at different view angles has been checked. As Fig. S1 shows, the absolute intensity of electroluminescent (EL) spectra of device does not show color shift with view angle. Moreover, the EL intensity from the surface of inverted device varies as the cosine of the angle h between that direction and the perpendicular to the surface: E(h) = E(0°) cos h, which is the same as emission distribution of conventional device, as shown in Fig. 4. This means that the inverted device is a lambertian radiator, and high efficiency for the inverted device is not due to microcavity effect enhancement as often observed in top-emission OLEDs. Furthermore, we used a red phosphorescent material tris(1-phenylisoquinolinoline)iridium (Ir(piq)3) as sensing layer (0.5 nm) in the mCP layers of conventional and inverted devices to estimate the location of recombination zones. The Ir(piq)3 sensing layer is 3 and 6 nm away from green phosphorescent emitter for both conventional and inverted devices. The structure of conventional device is ITO/MoO3 (6 nm)/ mCP (46 or 43 nm)/Ir(piq)3 (0.5 nm)/mCP (3 or 6 nm)/ Ir(tfmppy)2(tpip) (0.5 nm)/TPBI (12.5 nm)/BPhen (35 nm)/ Cs2CO3 (1 nm)/Al (100 nm), and that of inverted device is ITO/Cs2CO3 (1 nm)/BPhen (35 nm)/TPBI (12.5 nm)/ Ir(tfmppy)2(tpip) (0.5 nm)/mCP (3 or 6 nm)/Ir(piq)3 (0.5 nm)/mCP (46 or 43 nm)/MoO3 (6 nm)/Al (100 nm). As shown in Fig. 5, when the Ir(piq)3 sensing layer is 3 nm away from the mCP/TPBI interface, only small amount of red emission is observed in conventional device, while the red emission is the major contribution of the spectrum in inverted device. And with the sensing layer further away from the interface, the red contribution gets smaller as expected, and almost no red contribution is observed in conventional device and much more red emission in inverted device. It indicates that the recombination zone is narrower and closer to mCP/TPBI interface in conventional device. Most likely, the excitons are directly formed on Ir(tfmppy)2(tpip) molecules by carrier trapping.
Fig. 4. The relationship between the EL intensities of conventional and inverted devices and measured angle.
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Fig. 5. The EL spectra of conventional and inverted devices with sensing layer inserted at different position of mCP layer. 1 and 2 represent sensing layer 3 and 6 nm away from the green emitter in conventional devices, respectively. Also, 3 and 4 represent sensing layer 3 and 6 nm away from the green emitter in inverted devices, respectively.
Since the spin-forbidden phosphorescence is a slow decay process (ls), the triplet quenching induced by strong exciton-aggregation will seriously reduce the efficiency of conventional device. In inverted device the recombination zone extends more in depth in mCP layer. More excitons are formed on mCP molecules and then transfer energies to Ir(tfmppy)2(tpip) at mCP/TPBI interface and emit light. Therefore, effective energy transfer from host mCP to the emitter contributes to high EQE for the inverted device. In view of device structure, the phosphorescent dyes form a neat layer with 0.5 nm thickness, which may not be a continuous film. The film morphology on top of TPBI layer has been characterized by AFM as shown in Fig. 6: (a) TPBI (10 nm) on the glass substrate, (b) Ir(tfmppy)2 (tpip) (0.5 nm) on the TPBI (10 nm), and (c) mCP (0.5 nm) on the TPBI (10 nm)/Ir(tfmppy)2(tpip) (0.5 nm). The TPBI film surface exhibits a root mean square (RMS) roughness of 0.672 nm, indicating that the surface is not an absolute plane. This may be originated from the aggregation of TPBI molecules at the surface within a certain thickness. The deposition of Ir(tfmppy)2(tpip) on TPBI occupies the concave and convex sites of TPBI surface and leads to a slightly higher RMS of 0.737 nm, demonstrating that the Ir(tfmppy)2(tpip) molecules are not in the form of a neat layer but partially penetrate into TPBI. Next, the following deposition of mCP with 0.5 nm on the TPBI/Ir(tfmppy)2 (tpip) surface forms a homogeneous film with RMS roughness decreasing from 0.737 nm to 0.366 nm, referring that mCP molecules are prone to fill in the concave sites of TPBI/ Ir(tfmppy)2(tpip) surface. Therefore, Ir(tfmppy)2(tpip) molecules with 0.5 nm thickness could be considered as guest partially doped into TPBI and mCP hosts. Accordingly, another inverted PhOLED with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 1.5 nm) doped EML system was fabricated to simulate the undoped EML with 0.5 nm thickness. The total amount of Ir(tfmppy)2(tpip) in the doped 1.5-nm-thick and undoped 0.5-nm-thick EML is identical. In the doped EML, direct contact probability of mCP with electron blocking property and TPBI with hole blocking property is larger
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Fig. 6. AFM images of (a) TPBI (10 nm) on the glass substrate, (b) Ir(tfmppy)2(tpip) (0.5 nm) on the TPBI (10 nm), and (c) mCP (0.5 nm) on the TPBI (10 nm)/ Ir(tfmppy)2(tpip) (0.5 nm).
than that in the undoped EML, which is responsible for the slightly decreased current density of the inverted doped EML device, as shown in inset of Fig. 3. As co-host further facilitates hole and electron balance in the recombination zone, the equally doped EML device shows significantly enhanced EQE. As shown in Fig. 3, the device achieves the maximum EQE of 31.1%, EQE of 24.2% at 1000 cd/m2, and EQE of 17.0% at 10,000 cd/m2. Recently, the conventional PhOLED without outcoupling was reported to have EQE over 30% due to the horizontal orientation of phosphorescent molecules within the co-host [17]. The ultrahigh EQE of our device may be relevant to the molecule orientation of Ir(tfmppy)2(tpip) in the co-host. To the best of our knowledge, the EQEs of device are among the best ever reported for an inverted PhOLED. Fig. 7 shows current density–voltage, luminance–voltage, current efficiency-current density characteristics and EL spectra of inverted PhOLEDs with various EMLs. It is observed that the device with 1.5 nm EML shows higher EQE at low luminance region but a larger EQE roll-off at high luminance region (especially higher than 1000 cd/ m2), compared to that with 10.5 nm EML. The device with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 10.5 nm) exhibits EQE of 23.3% and EQE of 19.2% at 1000 and 10,000 cd/m2, respectively. In the co-host EML system, the hole density
decreases gradually from HTL/EML to EML/ETL interface for both thin EML and thick EML. However, the hole density reduction at EML/ETL interface is smaller with thin EML, i.e. the hole density is close to the electron density for thin EML, while the difference is much larger for thick EML. The same situation applied to electrons as well, as shown in Fig. S2. The charge carriers are more balanced within thin EML, which leads to higher EQE. With increasing the current density, more holes and electrons are injected into the EML, the holes and electrons penetrate deeper into the EML. The recombination efficiency at both sides of EML is getting higher for thick EML, while higher exciton density in thin EML leads to strong triplet–triplet annihilation (TTA). Hence, the efficiency roll-off is larger with thin EML. As doping ratio within the 10.5 nm EML increases from 10.0% to 33.3%, the increased exciton density should lead to self-quenching of phosphors. However, as shown in Fig. 7, the efficiency roll-off is independent of phosphorescent dopant ratio, which is contradictory to TTA commonly existed in PhOLED. The phosphor Ir(tfmppy)2(tpip) featured with aggregation-induced phosphorescent emission (AIPE) [19], which shows weak phosphorescence in solution and enhanced phosphorescence emission in the solid state, is likely to be responsible for the phenomenon.
J. Liu et al. / Organic Electronics 15 (2014) 2492–2498
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Fig. 7. Current density–voltage (a), luminance-voltage (b), current efficiency-current density characteristics (c) and EL spectra of inverted PhOLEDs with various EMLs (d).
4. Conclusion In summary, we have achieved high efficiency inverted PhOLED with EQE of 24.7% based on ultrathin (0.5 nm) undoped EML. The enhancement of EQE of inverted device compared to conventional device is attributed to effective energy transfer from mCP excitons to the emitter. AFM image of Ir(tfmppy)2(tpip) on top of TPBI indicates that the emitter partially dopes into TPBI and mCP hosts. The inverted PhOLED with doped EML system TPBI:mCP: Ir(tfmppy)2(tpip) (1:1:1, 1.5 nm) exhibits EQE of 31.1%, which is originated from optimized charge balance and preferred horizontal orientation of Ir(tfmppy)2(tpip). Due to TTA process caused by high exciton density, the inverted device with the ultrathin EML shows large EQE roll-off, which is improved by broadening doped EML. The inverted device with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 10.5 nm) has about EQE of 20% at 10,000 cd/m2. Our research about the inverted PhOLED is expected to promote development of high efficiency AMOLED based on the n-type IGZO-TFT. Acknowledgements This research work was supported by National Natural Science Foundation of China (61377030), Science and Technology Commission of Shanghai Municipal (12JC1404900). Chin H. Chen thanks Guangdong province for funding (2012B050300012). The authors would also
like to thank Mrs. Huiqin Li from the Instrumental Analysis Center of the SJTU for helping with AFM measurement. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.07.027. References [1] M.G. Helander, Z.B. Wang, J. Qiu, M.T. Greiner, D.P. Puzzo, Z.W. Liu, Z.H. Lu, Science 332 (2011) 944. [2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234. [3] J.K. Jeong, J.H. Jeong, H.W. Yang, T.K. Ahn, M. Kim, K.S. Kim, B.S. Gu, H.J. Chung, J.S. Park, Y.G. Mo, J. SID 17 (2009) 95. [4] J.H. Lee, W. Nam, B.K. Kim, H.S. Choi, Y.M. Ha, M.K. Han, IEEE Electron Dev. Lett. 27 (2006) 830. [5] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [6] S. Yamazki, J. Koyama, Y. Yamamoto, K. Okamoto, SID Symp. Digest Tech. Paper 43 (2012) 183. [7] H. Fukagawa, K. Morii, Y. Arimoto, M. Nakata, Y. Nakajima, T. Shimizu, T. Yamamoto, SID Symp. Digest Tech. Paper 44 (2013) 1466. [8] T.-Y. Chu, J.-F. Chen, C.H. Chen, Jpn. J. Appl. Phys. 45 (2006) 4948. [9] T. Xiong, F. Wang, X. Qiao, D. Ma, Appl. Phys. Lett. 89 (2006) 053518. [10] H. Lee, C.-M. Kang, M. Park, J. Kwak, C. Lee, ACS Appl. Mater. Interfaces 5 (2013) 1977. [11] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, A. Kahn, Appl. Phys. Lett. 95 (2009) 123301.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer Jun Liu a, Xindong Shi a, Xinkai Wu a, Jing Wang a, Zhiyuan Min a, Yang Wang a, Meijun Yang a, Chin H. Chen b, Gufeng He a,⇑ a National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b Guangdong Aglaia Optoelectronic Materials Co., Ltd., Shunde, Foshan, Guangdong, People’s Republic of China
a r t i c l e
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Article history: Received 24 April 2014 Received in revised form 4 July 2014 Accepted 15 July 2014 Available online 27 July 2014 Keywords: High efficiency Inverted OLED Phosphorescence Ultrathin EML
a b s t r a c t High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5 nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5 nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5 nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency rolloff at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5 nm doped EML has about EQE of 20 % at 10,000 cd/m2. It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, active-matrix organic light-emitting diodes (AMOLEDs) have been successfully applied in small and medium size commercial products, such as mobile phones, tablets, and high-definition television [1,2]. Due to high mobility and stability, low temperature poly silicon (LTPS) thin film transistor (TFT) [3,4] is currently used in AMOLED. However, LTPS-TFT requiring the excimer-laser annealing (ELA) process suffers from non-uniformity and high investment. For that reason, indium Gallium Zinc ⇑ Corresponding author. Tel.: +86 21 34207045; fax: +86 21 34204371. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.orgel.2014.07.027 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
Oxide (IGZO)-TFT backplane for AMOLEDs is an attractive alternative, since it provides unique advantages of uniformity, large area and low cost [5,6]. Owing to the n-type nature of IGZO-TFT, inverted OLEDs with a bottom cathode are preferred to integrate with the IGZO-TFT in AMOLED [7]. Indium tin oxide (ITO) is commonly used as the cathode of inverted OLED due to its high optical transparency and electrical conductivity, but its high work function limits electron injection into the neighboring electron-transporting layer (ETL). Most of research on inverted OLED focus on reducing barrier height between ITO and ETL. An insertion of metallic layer with low work function or n-type doped ETL by Li and Cs compounds is often used for improved electron injection from ITO [8–10]. On the
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other hand, aluminum with low work function is usually used as anode because of its low cost and high reflectivity. An electron accepting interlayer with high work function such as MoO3 [11], WO3 [12] or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) [13] facilitates enhanced hole injection from anode Al to the hole-transporting layer (HTL). For example, the injection efficiency for inverted device with HAT-CN hole-injecting layer (HIL) on top of N,N0 -di(naphthalene-1-yl)-N,N0 -diphenylbenzidine (NPB) HTL is near unity, meaning that hole injection contact is ohmic [14]. Compared to the conventional fluorescence OLED, the inverted device using the same EIL and HIL shows significantly higher current efficiency (CE) or external quantum efficiency (EQE). Chen et al. achieved the maximum CE of 23.7 cd/A for the inverted green device with 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7,-tetra(methyl)-1H,5H,11H-[1]-benzopyrano [6,7,8-ij]quinolizin-11-one (C545T) fluorophor, almost twice as high as that of the conventional device [15]. Peng et al. also reported high CE of the inverted tris(8-hydroxyquinolinato)aluminium (Alq3)-based device while only half of that for the conventional device [16]. An EQE over 20% has often been observed for the conventional phosphorescence OLEDs (PhOLEDs), [17] however, it is rarely reported in inverted OLED. Recently, to overcome the limitation of complicated process of doping system, conventional PhOLEDs with ultrathin undoped emitting layer (EML) (<1 nm) have been introduced, which exhibited the maximum EQE about 20%, due to optimized confinement of exactions within the ultrathin EML [18]. Nevertheless, such device showed serious EQE roll-off at high brightness, owing to annihilation of high density triplets within ultrathin emission zone, and the ultrathin undoped EML has not been applied in inverted PhOLED structure. Moreover, for better understanding, the charge recombination and the effects of excitons distribution on efficiency roll-off within the ultrathin EML should be investigated. In this paper, ITO/Cs2CO3 cathode and Al/MoO3 anode were used in the inverted PhOLED, while ITO/MoO3 anode and Al/Cs2CO3 cathode were used in the conventional device. Based on current density–voltage characteristic of single carrier devices, difference of charge injection ability of ITO/Cs2CO3 cathode and Al/MoO3 anode in the inverted device was significantly larger than that of ITO/MoO3 anode and Al/Cs2CO3 cathode in the conventional device. However, the inverted PhOLED with ultrathin (0.5 nm) undoped EML showed the maximum EQE of 24.7%, and EQE of 20.9% at 1,000 cd/m2, while those of the conventional device were 13.3% and 13.0%. Moreover, the surface morphology of EML was characterized by Atomic Force Microscopy (AFM). Accordingly, the ultrathin undoped EML was simulated by a relatively broadened thin (1.5 nm) doped EML with co-host, and the amount of phosphors in both the doped and undoped EML are identical. The doped EML device demonstrated the maximum EQE of 31.1%, EQE of 24.2% at 1,000 cd/m2, and EQE of 17.0% at 10,000 cd/m2. Better charge balance and preferred horizontal orientation of emitter may contribute to the efficiency enhancement. By broadening the doped EML from 1.5 nm to 10.5 nm, the efficiency roll-off ranging from 1000 to 10,000 cd/m2 was improved. Such device with
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10.5 nm doped EML showed EQE of 23.3% at 1000 cd/m2 and 19.2% at 10,000 cd/m2, while the maximum EQE slightly dropped to 27.8%, indicating that excitons with high concentration within the ultrathin EML were dispersed within the expanded EML.
2. Experimental The single carrier devices and PhOLEDs were fabricated on ITO-coated glass substrates. Prior to film deposition, the ITO glass substrates were cleaned successively using deionized water, acetone and isopropanol under an ultrasonic bath, and then pretreated with oxygen plasma after drying. The structure of hole-only device is ITO/MoO3 (6 nm)/HTL (343 nm)/MoO3 (6 nm)/Al (100 nm), where 1,3-bis(carbazol-9-yl)benzene (mCP) is used as HTL. Under forward bias, ITO/MoO3 acts as the anode, and electrons from Al can be hardly injected into mCP due to electron blocking property of MoO3. Under reversed bias, Al/MoO3 acts as the anode, and electrons from ITO are prohibited as well because of electron blocking property of MoO3. The structure of electron-only device is ITO/Cs2CO3 (1 nm)/ETL (310 nm)/Cs2CO3 (1 nm)/Al (100 nm), where ETL is 4,7-diphenyl-1,10-phenanthroline (BPhen). Under forward bias, Al/Cs2CO3 is cathode, and holes injection from ITO into BPhen are forbidden due to hole blocking property of Cs2CO3 and BPhen. Under reversed bias, ITO/ Cs2CO3 is cathode, and holes from Al are blocked because of hole blocking property of Cs2CO3 and BPhen. The device configuration of inverted PhOLED is ITO/Cs2CO3 (1 nm)/ BPhen (35 nm)/1,3,5-tris (N-phenylbenzimidazole-2yl)benzene (TPBI) (12.5 nm)/emitter (0.5 nm)/mCP (49 nm)/MoO3 (6 nm)/Al (100 nm), where the undoped phosphorescent emitter is a bis-cyclometalated iridium complex of Ir(tfmppy)2(tpip) (tfmppy = 4-trifluoromethylphenylpridine, tpip = tetraphenylimido-diphosphinate) [19–21], as depicted in Fig. 1. The EIL, ETL, EML, HTL, HIL and anode Al were successively thermally deposited within a high vacuum deposition system at a base pressure of 106 torr. The deposition rate for organic materials was monitored by quartz-crystal monitors and kept constant within the range from 0.01 to 0.1 nm s1. For comparison, the conventional PhOLED with device structure of ITO/ MoO3 (6 nm)/mCP (49 nm)/Ir(tfmppy)2(tpip) (0.5 nm)/TPBI (12.5 nm)/BPhen (35 nm)/Cs2CO3 (1 nm)/Al (100 nm) was fabricated. The surface morphologies of TPBI, Ir(tfmppy)2 (tpip) and mCP were measured by AFM. In order to maintain the same thickness of device, the multilayer TPBI (12.5 nm)/Ir(tfmppy)2(tpip) (0.5 nm)/mCP (49 nm) in the undoped EML device was adjusted as TPBI (12 nm)/TPBI: Ir(tfmppy)2(tpip):mCP (1:1:1, 1.5 nm)/mCP (48.5 nm) or TPBI (7.5 nm)/TPBI:Ir(tfmppy)2(tpip):mCP (1:1:1 or 1:1:0.5 or 1:1:0.2, 10.5 nm)/mCP (44 nm) in the doped EML device. Current density–voltage (J–V) and luminescence–voltage (L–V) characteristics were measured with a computer controlled Keithley 2400 source meter and BM-7A luminance colorimeter under ambient conditions. The EQE is calculated based on the assumption that the bottom emission inverted PhOLED is a lambertian radiator.
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Fig. 1. Schematic diagram of inverted PhOLED, chemical structure of the used phosphorescent dye, and energy level diagram of TPBI, Ir(tfmppy)2(tpip) and mCP. Edges of rectangle correspond to HOMO (bottom) and LUMO (top) energies.
Hole injection barrier is associated with the energy difference between the Fermi level of anode and the highest occupied molecular orbital (HOMO) level of HTL. Higher work function of ITO (5.1 eV) than that of Al (4.2 eV) is supposed to facilitate hole injection from ITO. However, as shown in Fig. 2, the hole-only device with Al/MoO3 anode shows significantly increased current density than that with ITO/MoO3 anode, indicating independence of hole injection on work function of anode in the presence of MoO3 injection layer. These results are consistent with the reported research that MoO3 top-contact HTL has larger hole injection efficiency than MoO3 bottom-contact HTL [14]. We present the reason of hole injection enhancement for MoO3 top-contact HTL as follows. Thermal evaporated MoO3 on top of mCP leads to diffusion of MoO3 within mCP, resulting in larger degree of charge transfer between MoO3 and mCP, compared to MoO3/mCP interface with mCP on top of MoO3. The better charge transfer between MoO3 and mCP means better p-type doping, which leads to enhanced hole injection and transporting. Likewise, Cs2CO3 on top of BPhen ETL promotes electron injection from Al/Cs2CO3 cathode, compared to ITO/Cs2CO3
cathode followed by deposition of BPhen. Just as shown in Fig. 2, the current density of electron-only device with Al/ Cs2CO3 cathode is over four orders of magnitude larger than that with ITO/Cs2CO3 cathode. The electron injection from ITO to BPhen is still weak even in the presence of Cs2CO3, due to high work function of ITO and weak n-type doping at Cs2CO3/BPhen interface. Both ITO/MoO3 anode and Al/Cs2CO3 cathode are commonly used in the conventional device, while the combination of ITO/Cs2CO3 cathode and Al/MoO3 anode are applied in the inverted device. According to the J–V characteristics of single carrier devices, difference between hole injection ability of Al/MoO3 anode and electron injection ability of ITO/Cs2CO3 cathode for the inverted PhOLED is significantly larger than that between ITO/MoO3 anode and Al/ Cs2CO3 cathode for the conventional device. Naturally, this leads to charge unbalance for the inverted device, resulting in inferior efficiency for the inverted device. However, the inverted PhOLED with an undoped EML shows sharply enhanced EQE compared to the conventional device with the same EML. As shown in Fig. 3, the maximum EQE, EQE at 1000 cd/m2 and at 10,000 cd/m2 of the inverted device are 24.7%, 20.9% and 14.0%, respectively, while those of the conventional device are 13.3%, 13.0%, and
Fig. 2. Current density–voltage characteristics of hole-only device with ITO/MoO3 and Al/MoO3 anodes and electron-only device with ITO/Cs2CO3 and Al/Cs2CO3 cathodes.
Fig. 3. Luminance-EQE-power efficiency characteristics of conventional and inverted undoped EML and inverted doped EML PhOLEDs. Inset: Current density–voltage characteristics.
3. Results and discussion
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11.0%. The power efficiencies of the conventional and inverted devices at 1000 cd/m2 are about 35 and 31 lm/ W, respectively. In order to find the reason for higher EQE of inverted device than that of conventional device, the emission distribution of inverted device at different view angles has been checked. As Fig. S1 shows, the absolute intensity of electroluminescent (EL) spectra of device does not show color shift with view angle. Moreover, the EL intensity from the surface of inverted device varies as the cosine of the angle h between that direction and the perpendicular to the surface: E(h) = E(0°) cos h, which is the same as emission distribution of conventional device, as shown in Fig. 4. This means that the inverted device is a lambertian radiator, and high efficiency for the inverted device is not due to microcavity effect enhancement as often observed in top-emission OLEDs. Furthermore, we used a red phosphorescent material tris(1-phenylisoquinolinoline)iridium (Ir(piq)3) as sensing layer (0.5 nm) in the mCP layers of conventional and inverted devices to estimate the location of recombination zones. The Ir(piq)3 sensing layer is 3 and 6 nm away from green phosphorescent emitter for both conventional and inverted devices. The structure of conventional device is ITO/MoO3 (6 nm)/ mCP (46 or 43 nm)/Ir(piq)3 (0.5 nm)/mCP (3 or 6 nm)/ Ir(tfmppy)2(tpip) (0.5 nm)/TPBI (12.5 nm)/BPhen (35 nm)/ Cs2CO3 (1 nm)/Al (100 nm), and that of inverted device is ITO/Cs2CO3 (1 nm)/BPhen (35 nm)/TPBI (12.5 nm)/ Ir(tfmppy)2(tpip) (0.5 nm)/mCP (3 or 6 nm)/Ir(piq)3 (0.5 nm)/mCP (46 or 43 nm)/MoO3 (6 nm)/Al (100 nm). As shown in Fig. 5, when the Ir(piq)3 sensing layer is 3 nm away from the mCP/TPBI interface, only small amount of red emission is observed in conventional device, while the red emission is the major contribution of the spectrum in inverted device. And with the sensing layer further away from the interface, the red contribution gets smaller as expected, and almost no red contribution is observed in conventional device and much more red emission in inverted device. It indicates that the recombination zone is narrower and closer to mCP/TPBI interface in conventional device. Most likely, the excitons are directly formed on Ir(tfmppy)2(tpip) molecules by carrier trapping.
Fig. 4. The relationship between the EL intensities of conventional and inverted devices and measured angle.
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Fig. 5. The EL spectra of conventional and inverted devices with sensing layer inserted at different position of mCP layer. 1 and 2 represent sensing layer 3 and 6 nm away from the green emitter in conventional devices, respectively. Also, 3 and 4 represent sensing layer 3 and 6 nm away from the green emitter in inverted devices, respectively.
Since the spin-forbidden phosphorescence is a slow decay process (ls), the triplet quenching induced by strong exciton-aggregation will seriously reduce the efficiency of conventional device. In inverted device the recombination zone extends more in depth in mCP layer. More excitons are formed on mCP molecules and then transfer energies to Ir(tfmppy)2(tpip) at mCP/TPBI interface and emit light. Therefore, effective energy transfer from host mCP to the emitter contributes to high EQE for the inverted device. In view of device structure, the phosphorescent dyes form a neat layer with 0.5 nm thickness, which may not be a continuous film. The film morphology on top of TPBI layer has been characterized by AFM as shown in Fig. 6: (a) TPBI (10 nm) on the glass substrate, (b) Ir(tfmppy)2 (tpip) (0.5 nm) on the TPBI (10 nm), and (c) mCP (0.5 nm) on the TPBI (10 nm)/Ir(tfmppy)2(tpip) (0.5 nm). The TPBI film surface exhibits a root mean square (RMS) roughness of 0.672 nm, indicating that the surface is not an absolute plane. This may be originated from the aggregation of TPBI molecules at the surface within a certain thickness. The deposition of Ir(tfmppy)2(tpip) on TPBI occupies the concave and convex sites of TPBI surface and leads to a slightly higher RMS of 0.737 nm, demonstrating that the Ir(tfmppy)2(tpip) molecules are not in the form of a neat layer but partially penetrate into TPBI. Next, the following deposition of mCP with 0.5 nm on the TPBI/Ir(tfmppy)2 (tpip) surface forms a homogeneous film with RMS roughness decreasing from 0.737 nm to 0.366 nm, referring that mCP molecules are prone to fill in the concave sites of TPBI/ Ir(tfmppy)2(tpip) surface. Therefore, Ir(tfmppy)2(tpip) molecules with 0.5 nm thickness could be considered as guest partially doped into TPBI and mCP hosts. Accordingly, another inverted PhOLED with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 1.5 nm) doped EML system was fabricated to simulate the undoped EML with 0.5 nm thickness. The total amount of Ir(tfmppy)2(tpip) in the doped 1.5-nm-thick and undoped 0.5-nm-thick EML is identical. In the doped EML, direct contact probability of mCP with electron blocking property and TPBI with hole blocking property is larger
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Fig. 6. AFM images of (a) TPBI (10 nm) on the glass substrate, (b) Ir(tfmppy)2(tpip) (0.5 nm) on the TPBI (10 nm), and (c) mCP (0.5 nm) on the TPBI (10 nm)/ Ir(tfmppy)2(tpip) (0.5 nm).
than that in the undoped EML, which is responsible for the slightly decreased current density of the inverted doped EML device, as shown in inset of Fig. 3. As co-host further facilitates hole and electron balance in the recombination zone, the equally doped EML device shows significantly enhanced EQE. As shown in Fig. 3, the device achieves the maximum EQE of 31.1%, EQE of 24.2% at 1000 cd/m2, and EQE of 17.0% at 10,000 cd/m2. Recently, the conventional PhOLED without outcoupling was reported to have EQE over 30% due to the horizontal orientation of phosphorescent molecules within the co-host [17]. The ultrahigh EQE of our device may be relevant to the molecule orientation of Ir(tfmppy)2(tpip) in the co-host. To the best of our knowledge, the EQEs of device are among the best ever reported for an inverted PhOLED. Fig. 7 shows current density–voltage, luminance–voltage, current efficiency-current density characteristics and EL spectra of inverted PhOLEDs with various EMLs. It is observed that the device with 1.5 nm EML shows higher EQE at low luminance region but a larger EQE roll-off at high luminance region (especially higher than 1000 cd/ m2), compared to that with 10.5 nm EML. The device with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 10.5 nm) exhibits EQE of 23.3% and EQE of 19.2% at 1000 and 10,000 cd/m2, respectively. In the co-host EML system, the hole density
decreases gradually from HTL/EML to EML/ETL interface for both thin EML and thick EML. However, the hole density reduction at EML/ETL interface is smaller with thin EML, i.e. the hole density is close to the electron density for thin EML, while the difference is much larger for thick EML. The same situation applied to electrons as well, as shown in Fig. S2. The charge carriers are more balanced within thin EML, which leads to higher EQE. With increasing the current density, more holes and electrons are injected into the EML, the holes and electrons penetrate deeper into the EML. The recombination efficiency at both sides of EML is getting higher for thick EML, while higher exciton density in thin EML leads to strong triplet–triplet annihilation (TTA). Hence, the efficiency roll-off is larger with thin EML. As doping ratio within the 10.5 nm EML increases from 10.0% to 33.3%, the increased exciton density should lead to self-quenching of phosphors. However, as shown in Fig. 7, the efficiency roll-off is independent of phosphorescent dopant ratio, which is contradictory to TTA commonly existed in PhOLED. The phosphor Ir(tfmppy)2(tpip) featured with aggregation-induced phosphorescent emission (AIPE) [19], which shows weak phosphorescence in solution and enhanced phosphorescence emission in the solid state, is likely to be responsible for the phenomenon.
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Fig. 7. Current density–voltage (a), luminance-voltage (b), current efficiency-current density characteristics (c) and EL spectra of inverted PhOLEDs with various EMLs (d).
4. Conclusion In summary, we have achieved high efficiency inverted PhOLED with EQE of 24.7% based on ultrathin (0.5 nm) undoped EML. The enhancement of EQE of inverted device compared to conventional device is attributed to effective energy transfer from mCP excitons to the emitter. AFM image of Ir(tfmppy)2(tpip) on top of TPBI indicates that the emitter partially dopes into TPBI and mCP hosts. The inverted PhOLED with doped EML system TPBI:mCP: Ir(tfmppy)2(tpip) (1:1:1, 1.5 nm) exhibits EQE of 31.1%, which is originated from optimized charge balance and preferred horizontal orientation of Ir(tfmppy)2(tpip). Due to TTA process caused by high exciton density, the inverted device with the ultrathin EML shows large EQE roll-off, which is improved by broadening doped EML. The inverted device with TPBI:mCP:Ir(tfmppy)2(tpip) (1:1:1, 10.5 nm) has about EQE of 20% at 10,000 cd/m2. Our research about the inverted PhOLED is expected to promote development of high efficiency AMOLED based on the n-type IGZO-TFT. Acknowledgements This research work was supported by National Natural Science Foundation of China (61377030), Science and Technology Commission of Shanghai Municipal (12JC1404900). Chin H. Chen thanks Guangdong province for funding (2012B050300012). The authors would also
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