Analysis of thermal degradation of organic lightemitting diodes with infrared imaging and impedance spectroscopy Kiyeol Kwak,1 Kyoungah Cho,1,2,3 and Sangsig Kim1,2,* 1
Department of Electrical Engineering, Korea University, Seoul 136-713, South Korea 2 These authors contributed equally to this paper 3 [email protected] * [email protected]
Abstract: We propose a route to examine the thermal degradation of organic light-emitting diodes (OLEDs) with infrared (IR) imaging and impedance spectroscopy. Four different OLEDs with tris (8hydroxyquinolinato) aluminum are prepared in this study for the analysis of thermal degradation. Our comparison of the thermal and electrical characteristics of these OLEDs reveals that the real-time temperatures of these OLEDs obtained from the IR images clearly correlate with the electrical properties and lifetimes. The OLED with poor electrical properties shows a fairly high temperature during the operation and a considerably short lifetime. Based on the correlation of the real-time temperature and the performance of the OLEDs, the impedance results suggest different thermal degradation mechanisms for each of the OLEDs. The analysis method suggested in this study will be helpful in developing OLEDs with higher efficiency and longer lifetime. ©2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (120.4630) Optical inspection; (160.4890) Organic materials; (310.6805) Theory and design; (310.6188) Spectral properties.
References and links 1.
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#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29558
11. A. B. Chwang, R. C. Kwong, and J. J. Brown, “Graded mixed-layer organic light-emitting devices,” Appl. Phys. Lett. 80(5), 725–727 (2002). 12. G. Nenna, G. Flaminio, T. Fasolino, C. Minarini, R. Miscioscia, D. Palumbo, and M. Pellegrino, “A study on thermal degradation of organic LEDs using IR imaging,” Macromol. Symp. 247(1), 326–332 (2007). 13. I. Kaya and A. Aydin, “Synthesis and characterization of the polyaminophenol derivatives containing thiophene in side chain: Thermal degradation, electrical conductivity, optical-electrochemical, and fluorescent properties,” J. Appl. Polym. Sci. 121(5), 3028–3040 (2011). 14. J. Birnstock, G. He, S. Murano, A. Werner, and O. Zeika, “White stacked OLED with 35 lm/W and 100,000 hours lifetime at 1000 cd/m2 for display and lighting applications,” SID Symp. Dig. Tech. Pap. 39, 822–825 (2008). 15. J. Kwak, Y.-Y. Lyu, S. Noh, H. Lee, M. Park, B. Choi, K. Char, and C. Lee, “Hole transport materials with high glass transition temperatures for highly stable organic light-emitting diodes,” Thin Solid Films 520(24), 7157– 7163 (2012). 16. S. Nowy, W. Ren, A. Elschner, W. Lövenich, and W. Brütting, “Impedance spectroscopy as a probe for the degradation of organic light-emitting diodes,” J. Appl. Phys. 107(5), 054501 (2010). 17. S. Nowy, W. Ren, J. Wagner, J. A. Weber, and W. Brütting, “Impedance spectroscopy of organic hetero-layer OLEDs as a probe for charge carrier injection and device degradation,” Proc. SPIE 7415, 74150G (2009). 18. C.-C. Chen, B.-C. Huang, M.-S. Lin, Y.-J. Lu, T.-Y. Cho, C.-H. Chang, K.-C. Tien, S.-H. Liu, T.-H. Ke, and C.C. Wu, “Impedance spectroscopy and equivalent circuits of conductively doped organic hole-transport materials,” Org. Electron. 11(12), 1901–1908 (2010). 19. G. Nenna, A. De Girolamo Del Mauro, R. Miscioscia, T. Fasolino, G. Pandolfi, and C. Minarini, “Electro-optical limits of organic LED investigated through temperature and applied field dependencies,” Polym. Compos. 34(9), 1477–1482 (2013). 20. H. Heil, J. Steiger, S. Karg, M. Gastel, H. Ortner, H. von Seggern, and M. Stößel, “Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode,” J. Appl. Phys. 89(1), 420–424 (2001). 21. T. Ye, S. Shao, J. Chen, L. Wang, and D. Ma, “Efficient phosphorescent polymer yellow-light-emitting diodes based on solution-processed small molecular electron transporting layer,” ACS Appl. Mater. Interfaces 3(2), 410–416 (2011). 22. S. Naka, H. Okada, H. Onnagawa, and T. Tsutsui, “High electron mobility in bathophenanthroline,” Appl. Phys. Lett. 76(2), 197–199 (2000). 23. Q. T. Le, F. M. Avendano, E. W. Forsythe, L. Yan, Y. Gao, and C. W. Tang, “X-ray photoelectron spectroscopy and atomic force microscopy investigation of stability mechanism of tris-(8-hydroxyquinoline) aluminum-based light-emitting devices,” J. Vac. Sci. Technol. A 17(4), 2314–2317 (1999). 24. D. Y. Kondakov, J. R. Sandifer, C. W. Tang, and R. H. Young, “Nonradiative recombination centers and electrical aging of organic light-emitting diodes: Direct connection between accumulation of trapped charge and luminance loss,” J. Appl. Phys. 93(2), 1108–1119 (2003). 25. Z. D. Popovic, H. Aziz, N.-X. Hu, A.-M. Hor, and G. Xu, “Long-term degradation mechanism of tris(8hydroxyquinoline) aluminum-based organic light emitting devices,” Synth. Met. 111–112, 229–232 (2000). 26. G. Vamvounis, H. Aziz, N.-X. Hu, and Z. D. Popovic, “Temperature dependence of operational stability of organic light-emitting diodes based on mixed emitter layers,” Synth. Met. 143(1), 69–73 (2004). 27. M. Matsumura and Y. Hirose, “Impedance spectroscopic analysis of forward biased metal oxide semiconductor tunnel diodes (MOSTD),” Appl. Surf. Sci. 175–176, 740–745 (2001). 28. Y. J. Lee, S.-S. Park, J. Kim, and H. Kim, “Interface morphologies and interlayer diffusions in organic light emitting device by x-ray scattering,” Appl. Phys. Lett. 94(22), 223305 (2009).
1. Introduction In recent years, organic light-emitting diodes (OLEDs) have been actively researched since they are one of the basic components employed in commercial display devices such as smartphones, tablets, and large-size full-color OLED televisions [1–3]. Especially, improving the light-emitting efficiency and extending the lifetime of the OLEDs are major issues in the field, since the efficiency and the lifetime are seriously affected by the heat generated in the OLEDs during the operation [4–7]. The heat generation due to the thermionic emission and the electrical stress causes chemical decomposition of the materials stacked in the OLEDs and creates an undesirable reaction at the interfaces between the materials, so that the thermal degradation directly reduces the light-emitting efficiency and the lifetime of the OLEDs [8– 10]. To date, studies on the thermal degradation of OLEDs have focused on the development of device configurations and/or new organic materials with relatively higher glass transition temperatures (Tg) [11–13]. The newly proposed structures and the newly synthesized organic materials are useful for the development of the OLEDs that are less susceptible to thermal degradation, and ultimately they contribute to the improvement of the light-emitting efficiency and to the extension of the lifetime of the OLEDs. Notable examples are the adoption of a PIN structure and the application of hole transporting materials with high Tg for #192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29559
fabrication of the OLEDs [14,15]. The PIN structure, constructed with a p-doped hole transport layer (HTL), or a hole injection layer (HIL), and an n-doped electron transport layer (ETL), or an electron injection layer (EIL), has the advantage of enabling a longer lifetime of the OLEDs due to the lowering of the injection barriers at the HTL (or HIL)/ indium-tinoxide (ITO) and the ETL (or EIL)/cathode interfaces. Thus this structure prevents the heat generation originating from the electrical stress by high injection barriers; consequently, it significantly extends the lifetime of the OLEDs. Also, the OLEDs using newly synthesized hole transporting material (N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′diamine) with Tg as high as 206 °C exhibit improved lifetime and enhanced operational stability compared to the OLEDs in which conventional hole transporting materials are used with Tg as high as 96 °C. Despite the recent development of the device configurations and new organic materials, in order to gain a deep understanding of the thermal degradation, it is necessary to investigate the real-time temperature of the OLEDs under the operation and the electrical properties of the degraded OLEDs. Herein, we propose an effective method for the investigation of the thermal degradation of OLEDs by applying a real-time infrared (IR) imaging and an impedance spectroscopy. The real-time IR images of the OLEDs show the real-time temperature during the operation of the OLEDs. In addition, the impedance spectroscopy is used to estimate the changes in electrical parameters before and after the thermal degradation [16–18]. Considering that thermal degradation is mainly associated with the heat generation, the IR imaging and the impedance spectroscopy are easy non-destructive examinations for the OLEDs. In this study, in order to understand the thermal degradation of the same light-emitting material-based OLEDs with different device structures, we investigate the heat generation during the continuous operation of the OLEDs and the changes in the electrical parameters before and after the thermal degradation through the IR imaging and the impedance spectroscopy. 2. Experimental procedure We prepared the OLEDs with the following four types of device configurations: Reference Device [ITO/N,N’-bis(naphthalene-1-yl)-N,N’-bis(phenyl)-2,20-dimethylbenzidine (α-NPD) (40 nm)/ tris (8-hydroxyquinolinato) aluminum (Alq3) (40 nm)/Al (100 nm)], Device A [ITO/α-NPD (40 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al (100 nm)], Device B [ITO/α-NPD (40 nm)/Alq3 (40 nm)/ 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) (30 nm)/LiF (0.5 nm)/Al (100 nm)], and Device C [ITO/α-NPD (40 nm)/Alq3 (40 nm)/ 4,7diphenyl-l,10-phenanthroline (Bphen) (30 nm)/LiF (0.5 nm)/Al (100 nm)]. For the preparation of the OLEDs, ITO films with a thickness of 150 nm and a sheet resistance of 2530 Ω/ were first patterned on glass substrates by a photolithography process for an emission area of 2 × 2 mm2. The ITO patterned glass substrates were then cleaned by sonication in isopropyl alcohol and then rinsed in de-ionized water. Subsequently, the α-NPD (40 nm) and the Alq3 (40 nm) used as a HTL and an organic emissive layer (EML), respectively, were thermally deposited at a base working pressure of 10−7 Torr. For the Reference Device, only an Al cathode with a thickness of 100 nm was formed on the Alq3 layer by thermal evaporation, while lithium fluoride (LiF) was inserted between the Alq3 and the Al cathode as an EIL for Device A. Moreover, for Devices B and C, the TPBi and the Bphen, respectively, were deposited as an ETL on the Alq3 layer prior to the formation of the LiF/Al, without breaking the vacuum; note that the thickness of the ETL and the EIL were fixed to 30 and 0.5 nm, respectively, in order to exclude the effects of the thicknesses of the device structures on the heat generation and dissipation [19]. In this study, the encapsulation for all of the OLEDs was carried out in order to avoid the cathode degradation by moisture. Schematic and energy band diagrams with the energy levels of the OLEDs are illustrated in Figs. 1(a) and 1(b), respectively.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29560
Fig. 1. Schematic (a) and energy band diagram (b) of our OLEDs.
The electroluminescence (EL) properties of our OLEDs were examined using a Keithley 2635A voltage/current source meter and a Minolta Cs-100A luminance-meter. In addition, the lifetimes were measured from a Polaronix M6000 OLED Lifetime Test System. To obtain the real-time temperature profiles of the OLEDs, the IR images were taken using an FLIR ThermaCAMTM S45 infrared camera having an uncertainty of 2% and sensitivity of 0.08 °C at 30 °C. In addition, the impedance spectra of pristine and degraded devices were taken with an electrochemical impedance spectroscopy analyzer IviumStat. The performances of all of the OLEDs in this study were examined under ambient conditions at room temperature. 3. Results and discussion The EL characteristics of our OLEDs are demonstrated in Fig. 2 and summarized in Table 1. Compared with the Reference Device, ITO/α-NPD/Alq3/Al, the current density of Device A, ITO/α-NPD/Alq3/LiF/Al, is significantly increased in magnitude owing to the insertion of the LiF layer between the Alq3 layer and the Al cathode, which is responsible for a relatively lower turn-on voltage and operating voltage. In this study, the turn-on voltage and the operating voltage of the OLEDs correspond to the voltages giving birth to the luminance of 1 and 1000 cd/m2, respectively. For Device A with the LiF used as EIL, its superior electrical properties are associated with the facilitating of the electron injection from the Al cathode to the Alq3 layer, as understood by the energy band diagram of Fig. 1(b). In comparison with the Reference Device (without the LiF used as EIL), Device A has a lower-junction potential barrier between the work function of the Al and the lowest unoccupied molecular orbital (LUMO) level of the Alq3 by inserting the LiF. Moreover, Li ions in the LiF are practically possible to diffuse into the Alq3 layer and consequently, they act as dopants for the Alq3 layer [20]. On the other hand, the devices with the ETLs inserted between the LiF and the Alq3 layer (Device B and Device C) have inferior electrical properties to Device A. Especially, compared to Device C, Device B has higher turn-on and operating voltages, which are ascribed to the lower electron mobility of the TPBi (3.3-8 × 10−5 cm2/V·s at an e-field of 4.77 × 105 V/cm) than that of the Bphen (~3 × 10−4 cm2/V·s at an e-field of 4.9 × 105 V/cm) [21,22].
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29561
Fig. 2. J-V-L (a) and CE-L-PE (b) of our OLEDs. Table 1. EL properties of our OLEDs. at 1 cd/m2
at 1000 cd/m2
Turn-on voltage (V)
Operating voltage (V)
Current efficiency (cd/A)
Power efficiency (lm/W)
Reference device (ITO/α-NPD/Alq3/Al)
4.7
10.3
1.43
0.46
Device A (ITO/α-NPD/Alq3/LiF/Al)
2.8
6.9
2.18 ( + 52%)
0.62 ( + 35%)
Device B (ITO/α-NPD/Alq3/TPBi/LiF/Al)
4.0
10.6
4.23 ( + 196%)
1.94 ( + 322%)
Device C (ITO/α-NPD/Alq3/Bphen/LiF/Al)
2.9
7.7
1.96 ( + 37%)
0.81 ( + 76%)
The current efficiency (CE) and the power efficiency (PE) of the OLEDs are plotted as a function of the luminance, as shown in Fig. 2(b). The light-emitting efficiencies of Devices A, B, and C are higher than those of the Reference Device at the luminance of 1000 cd/m2. Especially, Device B has the highest of the CE (4.23 cd/A) and the PE (1.94 lm/W) values, owing to the zero junction potential barrier at the interface of the Alq3/TPBi for the injection of the electrons. Considering that the light-emitting efficiency depends on the charge carrier balance in the EML, the remarkably enhanced CE and PE in Device B are intimately related to the formation of the well-defined electron-hole recombination region in the EML. Although the Bphen used in Device C has higher electron mobility than the TPBi used in Device B, the relatively lower CE and PE of Device C are attributed to the LUMO offset (0.1 eV) at the Alq3/Bphen interface. The improvement of the light-emitting efficiency is closely related to the high stability in the OLED operation. In other words, the efficient movement of the charge carriers can prevent the heat generation induced by the thermionic emission as well as the electrical stress. Figure 3(a) shows the real-time temperatures obtained from the IR imaging of the devices as a function of the operation time. The temperature of the Reference Device gradually increases and rapidly rises to ~110 °C within 4 minutes, which could be associated with the excessive heat generated at the interface of the Alq3/Al resulting from the low efficiency of the electron injection. The temperature of the Reference Device is higher than the Tg value of the α-NPD (~96 °C). Hence, the heat generated in the Reference Device causes the chemical decomposition of the HTL due to the diffusion of the Al ions of the Alq3 into the α-NPD [23]. For Device C, the temperature increases to ~74 °C, which is higher than the Tg of the Bphen (~55 °C), and is attributed to the LUMO offset at the interface of the Alq3/Bphen responsible for the heat generated by the thermionic emission [15]. Thus, the thermal degradation of Device C causes the chemical decomposition of the Bphen rather than the chemical #192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29562
decomposition of the HTL. On the other hand, for Devices A and B, the temperatures of the devices are as low as ~29 and ~22 °C, respectively, indicating that Devices A and B have higher operating stabilities than the Reference Device. Note that Devices A and B have higher CEs and PEs than the Reference Device, as previously shown in Fig. 2(b). The OLEDs with poor light-emitting efficiency tend to generate excessive heat resulting from the electrical stress and/or the thermionic emission, and the heat generation shortens the lifetime of the OLEDs. The real-time temperature monitoring of the OLEDs allows the prediction of their lifetimes as well as the stabilities in the consecutive operation. Figure 3(b) shows the temperature profiles at 6 minutes after the operation of the four devices, showing the distribution of the generated heat in the OLEDs. For the Reference Device and Device C, the high temperature parts appear on the regions of the devices, and this is ascribed to the inferior electron injection at the interface of the Alq3/Al and the Alq3/Bphen, respectively.
Fig. 3. Real-time temperatures of our OLEDs and (b) the temperature profiles at 6 minutes after the operation.
The lifetimes of the four different OLEDs are shown in Fig. 4; note that the Reference Device has the shortest lifetime. The longer half-lifetimes of Devices A and B are significantly associated with the lower heat generation as shown in Fig. 3. The maximal temperatures of Devices A and B are as low as ~25 and ~22 °C, respectively, and the temperatures are lower than the Tg of the α-NPD. Therefore, for Devices A and B, the thermal degradation caused by the diffusion of Al ions of the Alq3 into the α-NPD layer does not occur. For Device C, although the temperature increases to ~74 °C, the half-lifetime is longer compared to Device B. This observation implies that the lifetime of the devices consisting of the α-NPD/Alq3 is determined whether or not the temperature of the device during the operation is higher than the Tg of the α-NPD. If the temperature is higher than the Tg of the αNPD, the undesirable chemical reaction occurs at the interface of the α-NPD/Alq3 and consequently, the lifetime is shortened [24–26].
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29563
Fig. 4. Lifetimes of the OLEDs.
Based on the data for the real-time temperature and the performance of the OLEDs, the thermal degradation mechanism can be explained by the investigation of changes in the impedance for the pristine and the degraded devices. Figure 5 shows the Cole-Cole plots for the pristine and the degraded states of our four different devices, in which the horizontal- and vertical-axes represent the resistance (real, Z’) and the reactance (imaginary, Z”) parts of the impedance of the OLEDs, respectively. In order to compare the impedance spectroscopy of the degraded OLEDs with those of the pristine OLEDs, we apply their different operating voltages for the different OLEDs during the continuous operation. The frequency range is from 500 mHz to 2 MHz, and the applied voltage is 8 V with a voltage amplitude of 100 mV. For the pristine devices, the resistance and the reactance of Devices A and C are lower than those of the Reference Device, while those of Device B are slightly higher than those of the Reference Device because they are assigned according to the magnitude of the operating voltages; refer to the electrical properties described in Table 1. In case of the degraded device, the impedances of Reference Device and Device C remarkably decrease, whereas those of Device A and B do not decrease. There is a possibility that the significant reduction of the impedances of the degraded Reference Device and Device C may originate from a permeation of the cathode with a consequent short with the anode. Nevertheless, this observation could be explained by different thermal degradation mechanisms, since not only the real-time temperatures of the OLEDs under continuous operation, but also the changes of the impedance spectroscopy of them after the thermal degradations are different from each other. When the Reference Device is completely degraded, the sign of the reactance is changed and the resistance and the reactance are reduced. The change in the sign of the reactance implies a change from the capacitance to the inductance (or negative capacitance) [27]. For the degraded Reference Device, we assume that the sign change of the reactance is responsible for the undesirable chemical reaction at the Alq3/α-NPD. Our assumption is supported by the higher temperature of the Reference Device than the Tg of the α-NPD, as already demonstrated in Fig. 3. The excessive heat generation above the temperature of the Tg of the α-NPD causes the diffusion of the Al ions of the Alq3 into the α-NPD, and consequently, the thickness of the Alq3 layer is reduced. Y. J. Lee et al. reported that the reaction at the interface between the Alq3 and the HTL happens at the temperature higher than the Tg of the HTL [28]. Thus, at the temperature of ~110 °C, higher than the Tg of the α-NPD, generated during the operation of Reference Device, the undesirable chemical reaction occurs at the Alq3/α-NPD and it results in the change of the thicknesses and the chemical decomposition of the Alq3 and the α-NPD, causing the sign change of the reactance for degraded Reference Device. On the other hand, for the degraded Devices A, B, and C, the sign change in the reactance is not observed in the Cole-Cole plots, and thereby their thermal degradation mechanism is not attributed to the chemical reaction at the α-NPD/Alq3 interface. For the degraded Device A, the resistance and the reactance are very similar to those of the pristine states, indicating that it is rarely degraded during the operation owing to the device configuration with the improved CE and
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29564
PE. A small change in the resistance and the reactance of the degraded Device B may be attributed to the electrical stress because it has relatively higher turn-on and operating voltages resulting from the insertion of the TPBi with lower electron mobility. In addition, the degraded Device C shows the greatly reduced resistance and reactance, which may be associated with the chemical decomposition of the Bphen. The temperature of Device C during the consecutive operation increases to ~74 °C higher than the Tg of the Bphen, as mentioned above. Because the thermal degradation of the OLEDs is mainly associated with the heat generation, the impedance measurement, based on the data for the real-time temperature and the performance of the OLEDs, is a useful way to understand the thermal degradation mechanism. Generally, the lifetime and the light-emitting efficiency of the OLEDs can be determined from the real-time temperature monitoring. If a considerable increment occurs in the real-time temperature of an OLED, the OLED has a short lifetime and poor light-emitting efficiency. Moreover, the real-time temperature monitoring of OLEDs provides some clue about the possible mechanisms for the thermal degradation, compared with the temperature during the operation and Tg of the organic materials of OLEDs. On the other hand, a possible mechanism for the thermal degradation can be drawn from the impedance spectrum of the degraded OLED. Comparing the Reference Device with Device C, in which the temperatures are elevated during operation, a significant difference is observed in the sign of the impedance spectra between the degraded devices, indicating two distinct mechanisms for the thermal degradation. The combination of the real-time temperature monitoring of OLEDs and the impedance spectra of the degraded OLEDs identifies the most suitable mechanism responsible for the thermal degradation.
Fig. 5. Cole-Cole plots of the pristine and the degraded OLEDs.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29565
4. Conclusion We examine the thermal degradation of the OLEDs with IR imaging and impedance spectroscopy. Our analysis of the real-time temperature measured by the IR imaging reveals that the temperature of the device with the structure of ITO/α-NPD/Alq3/TPBi/LiF/Al is much lower than that of the device without the ETL and the EIL. The insertion of the appropriate ETL and EIL lowers the junction potential barrier and consequently improves the efficiency of the electron injection. The improved electrical properties of the OLEDs prevent the heat generation produced by the electrical stress as well as the thermionic emission. The electrical parameters extracted from the impedance spectroscopy of the pristine and the degraded devices show that, for the OLED devices with negligible heat generation, the impedance does not change greatly after their degradation. The thermal degradation, in this study, is characterized by the electrical parameters extracted from the impedance spectroscopy and the real-time temperature monitoring of the OLED devices. From this point of view, whereby understanding the thermal degradation of the OLEDs is important key to fabricate the OLEDs with high efficiency and long-lifetime, the method used in this study is useful in evaluating the performance of the OLEDs. Acknowledgments This research was supported by the Future-based Technology Development Program (Nano Fields) (NRF-2007-2002746), the Basic Science Research Program (NRF2012R1A1A2042104), the Mid-career Researcher Program (NRF-2012R1A2A2A01045613) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, and a grant from Samsung Display Co., Ltd.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29566
Department of Electrical Engineering, Korea University, Seoul 136-713, South Korea 2 These authors contributed equally to this paper 3 [email protected] * [email protected]
Abstract: We propose a route to examine the thermal degradation of organic light-emitting diodes (OLEDs) with infrared (IR) imaging and impedance spectroscopy. Four different OLEDs with tris (8hydroxyquinolinato) aluminum are prepared in this study for the analysis of thermal degradation. Our comparison of the thermal and electrical characteristics of these OLEDs reveals that the real-time temperatures of these OLEDs obtained from the IR images clearly correlate with the electrical properties and lifetimes. The OLED with poor electrical properties shows a fairly high temperature during the operation and a considerably short lifetime. Based on the correlation of the real-time temperature and the performance of the OLEDs, the impedance results suggest different thermal degradation mechanisms for each of the OLEDs. The analysis method suggested in this study will be helpful in developing OLEDs with higher efficiency and longer lifetime. ©2013 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (120.4630) Optical inspection; (160.4890) Organic materials; (310.6805) Theory and design; (310.6188) Spectral properties.
References and links 1.
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#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29558
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1. Introduction In recent years, organic light-emitting diodes (OLEDs) have been actively researched since they are one of the basic components employed in commercial display devices such as smartphones, tablets, and large-size full-color OLED televisions [1–3]. Especially, improving the light-emitting efficiency and extending the lifetime of the OLEDs are major issues in the field, since the efficiency and the lifetime are seriously affected by the heat generated in the OLEDs during the operation [4–7]. The heat generation due to the thermionic emission and the electrical stress causes chemical decomposition of the materials stacked in the OLEDs and creates an undesirable reaction at the interfaces between the materials, so that the thermal degradation directly reduces the light-emitting efficiency and the lifetime of the OLEDs [8– 10]. To date, studies on the thermal degradation of OLEDs have focused on the development of device configurations and/or new organic materials with relatively higher glass transition temperatures (Tg) [11–13]. The newly proposed structures and the newly synthesized organic materials are useful for the development of the OLEDs that are less susceptible to thermal degradation, and ultimately they contribute to the improvement of the light-emitting efficiency and to the extension of the lifetime of the OLEDs. Notable examples are the adoption of a PIN structure and the application of hole transporting materials with high Tg for #192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29559
fabrication of the OLEDs [14,15]. The PIN structure, constructed with a p-doped hole transport layer (HTL), or a hole injection layer (HIL), and an n-doped electron transport layer (ETL), or an electron injection layer (EIL), has the advantage of enabling a longer lifetime of the OLEDs due to the lowering of the injection barriers at the HTL (or HIL)/ indium-tinoxide (ITO) and the ETL (or EIL)/cathode interfaces. Thus this structure prevents the heat generation originating from the electrical stress by high injection barriers; consequently, it significantly extends the lifetime of the OLEDs. Also, the OLEDs using newly synthesized hole transporting material (N,N′-di(anthracene-9-yl)-N,N′-di(naphthalene-1-yl)biphenyl-4,4′diamine) with Tg as high as 206 °C exhibit improved lifetime and enhanced operational stability compared to the OLEDs in which conventional hole transporting materials are used with Tg as high as 96 °C. Despite the recent development of the device configurations and new organic materials, in order to gain a deep understanding of the thermal degradation, it is necessary to investigate the real-time temperature of the OLEDs under the operation and the electrical properties of the degraded OLEDs. Herein, we propose an effective method for the investigation of the thermal degradation of OLEDs by applying a real-time infrared (IR) imaging and an impedance spectroscopy. The real-time IR images of the OLEDs show the real-time temperature during the operation of the OLEDs. In addition, the impedance spectroscopy is used to estimate the changes in electrical parameters before and after the thermal degradation [16–18]. Considering that thermal degradation is mainly associated with the heat generation, the IR imaging and the impedance spectroscopy are easy non-destructive examinations for the OLEDs. In this study, in order to understand the thermal degradation of the same light-emitting material-based OLEDs with different device structures, we investigate the heat generation during the continuous operation of the OLEDs and the changes in the electrical parameters before and after the thermal degradation through the IR imaging and the impedance spectroscopy. 2. Experimental procedure We prepared the OLEDs with the following four types of device configurations: Reference Device [ITO/N,N’-bis(naphthalene-1-yl)-N,N’-bis(phenyl)-2,20-dimethylbenzidine (α-NPD) (40 nm)/ tris (8-hydroxyquinolinato) aluminum (Alq3) (40 nm)/Al (100 nm)], Device A [ITO/α-NPD (40 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al (100 nm)], Device B [ITO/α-NPD (40 nm)/Alq3 (40 nm)/ 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) (30 nm)/LiF (0.5 nm)/Al (100 nm)], and Device C [ITO/α-NPD (40 nm)/Alq3 (40 nm)/ 4,7diphenyl-l,10-phenanthroline (Bphen) (30 nm)/LiF (0.5 nm)/Al (100 nm)]. For the preparation of the OLEDs, ITO films with a thickness of 150 nm and a sheet resistance of 2530 Ω/ were first patterned on glass substrates by a photolithography process for an emission area of 2 × 2 mm2. The ITO patterned glass substrates were then cleaned by sonication in isopropyl alcohol and then rinsed in de-ionized water. Subsequently, the α-NPD (40 nm) and the Alq3 (40 nm) used as a HTL and an organic emissive layer (EML), respectively, were thermally deposited at a base working pressure of 10−7 Torr. For the Reference Device, only an Al cathode with a thickness of 100 nm was formed on the Alq3 layer by thermal evaporation, while lithium fluoride (LiF) was inserted between the Alq3 and the Al cathode as an EIL for Device A. Moreover, for Devices B and C, the TPBi and the Bphen, respectively, were deposited as an ETL on the Alq3 layer prior to the formation of the LiF/Al, without breaking the vacuum; note that the thickness of the ETL and the EIL were fixed to 30 and 0.5 nm, respectively, in order to exclude the effects of the thicknesses of the device structures on the heat generation and dissipation [19]. In this study, the encapsulation for all of the OLEDs was carried out in order to avoid the cathode degradation by moisture. Schematic and energy band diagrams with the energy levels of the OLEDs are illustrated in Figs. 1(a) and 1(b), respectively.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29560
Fig. 1. Schematic (a) and energy band diagram (b) of our OLEDs.
The electroluminescence (EL) properties of our OLEDs were examined using a Keithley 2635A voltage/current source meter and a Minolta Cs-100A luminance-meter. In addition, the lifetimes were measured from a Polaronix M6000 OLED Lifetime Test System. To obtain the real-time temperature profiles of the OLEDs, the IR images were taken using an FLIR ThermaCAMTM S45 infrared camera having an uncertainty of 2% and sensitivity of 0.08 °C at 30 °C. In addition, the impedance spectra of pristine and degraded devices were taken with an electrochemical impedance spectroscopy analyzer IviumStat. The performances of all of the OLEDs in this study were examined under ambient conditions at room temperature. 3. Results and discussion The EL characteristics of our OLEDs are demonstrated in Fig. 2 and summarized in Table 1. Compared with the Reference Device, ITO/α-NPD/Alq3/Al, the current density of Device A, ITO/α-NPD/Alq3/LiF/Al, is significantly increased in magnitude owing to the insertion of the LiF layer between the Alq3 layer and the Al cathode, which is responsible for a relatively lower turn-on voltage and operating voltage. In this study, the turn-on voltage and the operating voltage of the OLEDs correspond to the voltages giving birth to the luminance of 1 and 1000 cd/m2, respectively. For Device A with the LiF used as EIL, its superior electrical properties are associated with the facilitating of the electron injection from the Al cathode to the Alq3 layer, as understood by the energy band diagram of Fig. 1(b). In comparison with the Reference Device (without the LiF used as EIL), Device A has a lower-junction potential barrier between the work function of the Al and the lowest unoccupied molecular orbital (LUMO) level of the Alq3 by inserting the LiF. Moreover, Li ions in the LiF are practically possible to diffuse into the Alq3 layer and consequently, they act as dopants for the Alq3 layer [20]. On the other hand, the devices with the ETLs inserted between the LiF and the Alq3 layer (Device B and Device C) have inferior electrical properties to Device A. Especially, compared to Device C, Device B has higher turn-on and operating voltages, which are ascribed to the lower electron mobility of the TPBi (3.3-8 × 10−5 cm2/V·s at an e-field of 4.77 × 105 V/cm) than that of the Bphen (~3 × 10−4 cm2/V·s at an e-field of 4.9 × 105 V/cm) [21,22].
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29561
Fig. 2. J-V-L (a) and CE-L-PE (b) of our OLEDs. Table 1. EL properties of our OLEDs. at 1 cd/m2
at 1000 cd/m2
Turn-on voltage (V)
Operating voltage (V)
Current efficiency (cd/A)
Power efficiency (lm/W)
Reference device (ITO/α-NPD/Alq3/Al)
4.7
10.3
1.43
0.46
Device A (ITO/α-NPD/Alq3/LiF/Al)
2.8
6.9
2.18 ( + 52%)
0.62 ( + 35%)
Device B (ITO/α-NPD/Alq3/TPBi/LiF/Al)
4.0
10.6
4.23 ( + 196%)
1.94 ( + 322%)
Device C (ITO/α-NPD/Alq3/Bphen/LiF/Al)
2.9
7.7
1.96 ( + 37%)
0.81 ( + 76%)
The current efficiency (CE) and the power efficiency (PE) of the OLEDs are plotted as a function of the luminance, as shown in Fig. 2(b). The light-emitting efficiencies of Devices A, B, and C are higher than those of the Reference Device at the luminance of 1000 cd/m2. Especially, Device B has the highest of the CE (4.23 cd/A) and the PE (1.94 lm/W) values, owing to the zero junction potential barrier at the interface of the Alq3/TPBi for the injection of the electrons. Considering that the light-emitting efficiency depends on the charge carrier balance in the EML, the remarkably enhanced CE and PE in Device B are intimately related to the formation of the well-defined electron-hole recombination region in the EML. Although the Bphen used in Device C has higher electron mobility than the TPBi used in Device B, the relatively lower CE and PE of Device C are attributed to the LUMO offset (0.1 eV) at the Alq3/Bphen interface. The improvement of the light-emitting efficiency is closely related to the high stability in the OLED operation. In other words, the efficient movement of the charge carriers can prevent the heat generation induced by the thermionic emission as well as the electrical stress. Figure 3(a) shows the real-time temperatures obtained from the IR imaging of the devices as a function of the operation time. The temperature of the Reference Device gradually increases and rapidly rises to ~110 °C within 4 minutes, which could be associated with the excessive heat generated at the interface of the Alq3/Al resulting from the low efficiency of the electron injection. The temperature of the Reference Device is higher than the Tg value of the α-NPD (~96 °C). Hence, the heat generated in the Reference Device causes the chemical decomposition of the HTL due to the diffusion of the Al ions of the Alq3 into the α-NPD [23]. For Device C, the temperature increases to ~74 °C, which is higher than the Tg of the Bphen (~55 °C), and is attributed to the LUMO offset at the interface of the Alq3/Bphen responsible for the heat generated by the thermionic emission [15]. Thus, the thermal degradation of Device C causes the chemical decomposition of the Bphen rather than the chemical #192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29562
decomposition of the HTL. On the other hand, for Devices A and B, the temperatures of the devices are as low as ~29 and ~22 °C, respectively, indicating that Devices A and B have higher operating stabilities than the Reference Device. Note that Devices A and B have higher CEs and PEs than the Reference Device, as previously shown in Fig. 2(b). The OLEDs with poor light-emitting efficiency tend to generate excessive heat resulting from the electrical stress and/or the thermionic emission, and the heat generation shortens the lifetime of the OLEDs. The real-time temperature monitoring of the OLEDs allows the prediction of their lifetimes as well as the stabilities in the consecutive operation. Figure 3(b) shows the temperature profiles at 6 minutes after the operation of the four devices, showing the distribution of the generated heat in the OLEDs. For the Reference Device and Device C, the high temperature parts appear on the regions of the devices, and this is ascribed to the inferior electron injection at the interface of the Alq3/Al and the Alq3/Bphen, respectively.
Fig. 3. Real-time temperatures of our OLEDs and (b) the temperature profiles at 6 minutes after the operation.
The lifetimes of the four different OLEDs are shown in Fig. 4; note that the Reference Device has the shortest lifetime. The longer half-lifetimes of Devices A and B are significantly associated with the lower heat generation as shown in Fig. 3. The maximal temperatures of Devices A and B are as low as ~25 and ~22 °C, respectively, and the temperatures are lower than the Tg of the α-NPD. Therefore, for Devices A and B, the thermal degradation caused by the diffusion of Al ions of the Alq3 into the α-NPD layer does not occur. For Device C, although the temperature increases to ~74 °C, the half-lifetime is longer compared to Device B. This observation implies that the lifetime of the devices consisting of the α-NPD/Alq3 is determined whether or not the temperature of the device during the operation is higher than the Tg of the α-NPD. If the temperature is higher than the Tg of the αNPD, the undesirable chemical reaction occurs at the interface of the α-NPD/Alq3 and consequently, the lifetime is shortened [24–26].
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29563
Fig. 4. Lifetimes of the OLEDs.
Based on the data for the real-time temperature and the performance of the OLEDs, the thermal degradation mechanism can be explained by the investigation of changes in the impedance for the pristine and the degraded devices. Figure 5 shows the Cole-Cole plots for the pristine and the degraded states of our four different devices, in which the horizontal- and vertical-axes represent the resistance (real, Z’) and the reactance (imaginary, Z”) parts of the impedance of the OLEDs, respectively. In order to compare the impedance spectroscopy of the degraded OLEDs with those of the pristine OLEDs, we apply their different operating voltages for the different OLEDs during the continuous operation. The frequency range is from 500 mHz to 2 MHz, and the applied voltage is 8 V with a voltage amplitude of 100 mV. For the pristine devices, the resistance and the reactance of Devices A and C are lower than those of the Reference Device, while those of Device B are slightly higher than those of the Reference Device because they are assigned according to the magnitude of the operating voltages; refer to the electrical properties described in Table 1. In case of the degraded device, the impedances of Reference Device and Device C remarkably decrease, whereas those of Device A and B do not decrease. There is a possibility that the significant reduction of the impedances of the degraded Reference Device and Device C may originate from a permeation of the cathode with a consequent short with the anode. Nevertheless, this observation could be explained by different thermal degradation mechanisms, since not only the real-time temperatures of the OLEDs under continuous operation, but also the changes of the impedance spectroscopy of them after the thermal degradations are different from each other. When the Reference Device is completely degraded, the sign of the reactance is changed and the resistance and the reactance are reduced. The change in the sign of the reactance implies a change from the capacitance to the inductance (or negative capacitance) [27]. For the degraded Reference Device, we assume that the sign change of the reactance is responsible for the undesirable chemical reaction at the Alq3/α-NPD. Our assumption is supported by the higher temperature of the Reference Device than the Tg of the α-NPD, as already demonstrated in Fig. 3. The excessive heat generation above the temperature of the Tg of the α-NPD causes the diffusion of the Al ions of the Alq3 into the α-NPD, and consequently, the thickness of the Alq3 layer is reduced. Y. J. Lee et al. reported that the reaction at the interface between the Alq3 and the HTL happens at the temperature higher than the Tg of the HTL [28]. Thus, at the temperature of ~110 °C, higher than the Tg of the α-NPD, generated during the operation of Reference Device, the undesirable chemical reaction occurs at the Alq3/α-NPD and it results in the change of the thicknesses and the chemical decomposition of the Alq3 and the α-NPD, causing the sign change of the reactance for degraded Reference Device. On the other hand, for the degraded Devices A, B, and C, the sign change in the reactance is not observed in the Cole-Cole plots, and thereby their thermal degradation mechanism is not attributed to the chemical reaction at the α-NPD/Alq3 interface. For the degraded Device A, the resistance and the reactance are very similar to those of the pristine states, indicating that it is rarely degraded during the operation owing to the device configuration with the improved CE and
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29564
PE. A small change in the resistance and the reactance of the degraded Device B may be attributed to the electrical stress because it has relatively higher turn-on and operating voltages resulting from the insertion of the TPBi with lower electron mobility. In addition, the degraded Device C shows the greatly reduced resistance and reactance, which may be associated with the chemical decomposition of the Bphen. The temperature of Device C during the consecutive operation increases to ~74 °C higher than the Tg of the Bphen, as mentioned above. Because the thermal degradation of the OLEDs is mainly associated with the heat generation, the impedance measurement, based on the data for the real-time temperature and the performance of the OLEDs, is a useful way to understand the thermal degradation mechanism. Generally, the lifetime and the light-emitting efficiency of the OLEDs can be determined from the real-time temperature monitoring. If a considerable increment occurs in the real-time temperature of an OLED, the OLED has a short lifetime and poor light-emitting efficiency. Moreover, the real-time temperature monitoring of OLEDs provides some clue about the possible mechanisms for the thermal degradation, compared with the temperature during the operation and Tg of the organic materials of OLEDs. On the other hand, a possible mechanism for the thermal degradation can be drawn from the impedance spectrum of the degraded OLED. Comparing the Reference Device with Device C, in which the temperatures are elevated during operation, a significant difference is observed in the sign of the impedance spectra between the degraded devices, indicating two distinct mechanisms for the thermal degradation. The combination of the real-time temperature monitoring of OLEDs and the impedance spectra of the degraded OLEDs identifies the most suitable mechanism responsible for the thermal degradation.
Fig. 5. Cole-Cole plots of the pristine and the degraded OLEDs.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29565
4. Conclusion We examine the thermal degradation of the OLEDs with IR imaging and impedance spectroscopy. Our analysis of the real-time temperature measured by the IR imaging reveals that the temperature of the device with the structure of ITO/α-NPD/Alq3/TPBi/LiF/Al is much lower than that of the device without the ETL and the EIL. The insertion of the appropriate ETL and EIL lowers the junction potential barrier and consequently improves the efficiency of the electron injection. The improved electrical properties of the OLEDs prevent the heat generation produced by the electrical stress as well as the thermionic emission. The electrical parameters extracted from the impedance spectroscopy of the pristine and the degraded devices show that, for the OLED devices with negligible heat generation, the impedance does not change greatly after their degradation. The thermal degradation, in this study, is characterized by the electrical parameters extracted from the impedance spectroscopy and the real-time temperature monitoring of the OLED devices. From this point of view, whereby understanding the thermal degradation of the OLEDs is important key to fabricate the OLEDs with high efficiency and long-lifetime, the method used in this study is useful in evaluating the performance of the OLEDs. Acknowledgments This research was supported by the Future-based Technology Development Program (Nano Fields) (NRF-2007-2002746), the Basic Science Research Program (NRF2012R1A1A2042104), the Mid-career Researcher Program (NRF-2012R1A2A2A01045613) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, and a grant from Samsung Display Co., Ltd.
#192873 - $15.00 USD Received 26 Jun 2013; revised 3 Sep 2013; accepted 1 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029558 | OPTICS EXPRESS 29566
