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Nickel doped indium tin oxide anode and effect on dark spot development of organic light-emitting devices
Applied Surface Science 255 (2009) 3759–3763
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nickel doped indium tin oxide anode and effect on dark spot development of organic light-emitting devices C.M. Hsu *, C.S. Kuo, W.C. Hsu, W.T. Wu Southern Taiwan University, Department of Electro-Optical Engineering, 1 Nan-Tai St, Yung-Kang City, Tainan County 710, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 July 2008 Accepted 8 October 2008 Available online 1 November 2008
This article demonstrated that introducing nickel (Ni) atoms into an indium tin oxide (ITO) anode could considerably decrease ITO surface roughness and eliminate the formation of dark spots of an organic light-emitting device (OLED). A dramatic drop in surface roughness from 6.52 nm of an conventional ITO to 0.46 nm of an 50 nm Ni(50 W)-doped ITO anode was observed, and this led to an improved lifetime performance of an Alq3 based OLED device attributed to reduced dark spots. Reducing thickness of Ni-doped ITO anode was found to worsen surface roughness. Meanwhile, the existence of Ni atoms showed little effect on deteriorating the light-emitting mechanism of OLED devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 85.60.Jb 68.37.p 68.55.J Keywords: Nickel ITO OLED Dark spot
1. Introduction Ever since the high efficiency organic light-emitting diode (OLED) was revealed by C. Tang [1], intensive researches have been motivated to further improve the characteristics of OLEDs. These efforts have indeed greatly promoted OLED’s performance in many aspects and led its application not only for displays but also for lighting devices [2], sensors [3], laser sources [4] and solar cells [5]. However, the fact that the luminance of these organic devices decays with time renders a short device lifetime and limits their practical applications. Enhancing device lifetime is then one of the core issues for these OLED related optoelectronics to become commercial and competitive devices. For this reason investigation of the degradation processes of an OLED device has been an attractive topic and widely discussed [6–11]. The causes of the reduction in device luminance with time can generally be categorized into three types: (1) development of dark spots, (2) catastrophic failure and (3) intrinsic degradation. The first two cases are related to the morphological imperfections or structural defects, which generate large joule heat as electric currents flow through and thus deteriorate the light-emitting
* Corresponding author. E-mail address: [email protected] (C.M. Hsu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.042
process. The third case is generally attributed to the gradual formation of morphology or crystalline in organics that prohibits the exciting process of electric carriers. Meanwhile, it has been demonstrated that these degradation processes would be accelerated with the incorporation of oxygen and water vapor during fabrication and operation of devices [12–14]. Encapsulating the device in extremely low oxygen and moisture ambient is then crucial and has turned out to be a standard process for manufacturing OLED devices. Indeed, efforts to explore the degradation mechanism of OLED devices and to protect devices from serious lifetime decay have successfully led OLEDs to become one of the major products in current display market. Yet, researches to achieve higher device power efficiency and reliability in a low manufacturing cost base still continue. One of the studies of interest is to elevate work function of an indium tin oxide (ITO) film that commonly serves as an anode of an OLED device. This is based on the idea that potential barrier between ITO anode and organic hole-transport layer can be reduced by the elevation of ITO surface work function. The reduced potential barrier allows more hole-carriers to inject into OLED active layers at the same voltage, leading to a lower operating voltage and enhanced power efficiency. Hence, an as-deposited ITO film is generally post-treated to raise its work function in a practical device processing. And, it has been adopted that modifying surface chemical states of an ITO film is the most effective approach for this
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purpose. A variety of surface modifying methods have been reported, including the surface gaseous plasma treatment [15,16], the immersion of ITO into a surface dipole liquid [17], the formation of metal-doped indium or tin oxide layers on ITO surface [18,19] and the adding of high work function metals into ITO film [20]. Of these methods, the exposure of ITO surface to an energetic gaseous ambient has been commonly used in OLED manufacturing processes for its simplicity. However, the potential of inserting a metal oxide interlayer and adding high work function metals into ITO matrix has been highly noticed in recent years. This is due to that the existence of metal oxides can generally give rise to a much higher ITO work function than gaseous bombarding process can do. For instance, nickel oxide (NiOx) has a work function greater than 5.0 eV, much higher than that of ITO (4.5–4.7 eV). Instead of inserting a single layer of NiOx, in our previous studies we formed NiOx phases in ITO matrix by a Ni-ITO cosputter approach. With this approach, ITO work function up to 5.8 eV has been achieved and incorporating a Ni-doped ITO anode in an OLED structure was found to be able to reduce device threshold voltage by 2.3 V [21,22]. However, the effect of Ni atoms to the light-emitting mechanism of OLED devices is not yet investigated. In this article we report the influence of Ni atoms to the lifetime of an OLED device with a SiOx/Al/tris(8-hydroxyquinoline)aluminum(Alq3)/N,N0 -bis-(1-naphthyl)–N, N0 -diphenyl-1,10 biphenyl-4,40 -diamine(NPB)/ITO structure. Effects of doping level and thickness of the Ni-doped ITO anode on OLED performances are also discussed. 2. Experimental OLED bottom emission devices with a SiOx(200 nm)/ Al(100 nm)/Alq3(35 nm)/NPB(20 nm)/Ni-doped ITO(10–50 nm)/ ITO(200 nm) structure were fabricated on Corning 1735 glass and packaged with a glass back-plate. Fig. 1 schematically shows the structure of the packaged OLED panel. On the 50 mm 50 mm glass plate several devices with an active area of 1 cm2 were fabricated. Fabrication of the devices started with the deposition of a conventional ITO film using d.c. sputtering, followed by the coating of a Ni-doped ITO layer using co-sputtering of Ni (r.f. power) and ITO (d.c. power). The glass substrates were heated to a temperature of 200 8C for both processes. The NPB and Alq3 layers were then sequentially deposited in a resistive thermal evaporator, and the Al cathode layer was deposited in an E-beam evaporator afterward. These processes were conducted without the substrates being heated. To avoid electrical shorting between ITO anode and Al cathode, metallic shadow masks were used during film depositions for both the organic and Al layers. A SiOx passivation layer was finally deposited using E-beam evaporation. After being unloaded from the E-beam evaporator, all devices were immediately sealed with a glass back-plate using an UV curing method. Between the device substrate and the back-plate BaO desiccants were enclosed. The encapsulated OLED devices were characterized using a Photosearch PR650 C.I.E. (International
Fig. 1. The schematic structure of OLED device with a Ni-doped ITO anode.
Commission on Illumination) spectrometry and a HP4155B I–V meter for luminance–current–voltage (L–I–V) characteristics. The development of dark spots of the devices, treated with an electrical stress of 18 V in an environment of 80 8C and relative humidity of 80% for 1 h, was observed with a Nikon SMZ800 optical microscope. The surface topography of the Ni-doped ITO films was examined by an atomic force microscopy (AFM) to see its correlation to the dark spots. 3. Results and discussions In this study a series of Ni sputter power of 0, 10, 30 and 50 W was used to yield various Ni concentrations in ITO matrix. Meanwhile, film thickness of the Ni(50 W)-doped ITO layer was varied from 0 to 50 nm to examine the thickness effect on device performances. Fig. 2 shows the current–voltage (I–V) characteristics of OLED devices at various Ni sputter power and Ni-doped film thickness. It clearly shows that the I–V curves shift to the lower voltage end as sputter power and thickness of the Ni-doped ITO layer increase. The threshold voltages, defined as the electric current density is at 10 A/m2, are 10.8, 7.6, 6.3 and 5.5 V for the device with a 50 nm Ni-doped ITO anode deposited at Ni = 0, 10, 30 and 50 W, respectively. These threshold voltages have some discrepancies from those reported earlier [21] due to that the organic materials were supplied by different suppliers. However, we still can see the similar effect caused by the doped Ni atoms. That is, introducing Ni atoms into ITO matrix reduces device threshold voltage. The causes have been proved to be the formation of NiOx phases in ITO matrix that elevates ITO work function. This effect became prominent with the increasing amount of Ni atoms, suggesting ITO work function was still influenced by the Ni concentration for sputter conditions used in this study. For the device with a Ni(50 W)-doped ITO anode, the threshold voltage decreases from 6.8 to 5.9 V and 5.5 V when Ni-ITO film thickness increases from 10 to 30 nm and 50 nm, respectively. Two factors are considered to be the main causes for such a tendency. One is the increase in serial resistance when Ni-doped ITO film thickness decreases, which would result in a drop in voltage across the anode. Another is the effect from surface roughness of the Ni-doped ITO layer. One can see from Table 1 that the surface roughness (Ra), measured by AFM, considerably drops from 3.37 to 0.46 nm as the Ni-ITO film thickness increases from 10 to 50 nm.
Fig. 2. Current–Voltage (I–V) characteristics of OLED devices at various Ni sputter power and film thickness of Ni-doped ITO layer.
C.M. Hsu et al. / Applied Surface Science 255 (2009) 3759–3763
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Table 1 Electrical and optical characteristics of Ni-doped ITO films and OLED devices.
Ni-doped ITO thickness Half lifetime (min) Threshold voltage (V) AFM roughness (nm) Optical transmittance
Ni = 0 W
Ni = 10 W
Ni = 30 W
Ni = 50 W
Ni = 50 W
Ni = 50 W
500 A˚ 54 8.8 6.52 83.0%
500 A˚ 68 7.6 0.82 78.6%
500 A˚ 74 6.3 0.57 78.3%
500 A˚ 92 5.5 0.46 78.1%
300 A˚ – 5.9 0.71 79.3%
100 A˚ – 6.8 3.37 81.6%
Smooth surface can reduce interfacial trapping centers, allowing better hole-injection at the ITO/NPB interfaces and in turn decreasing threshold voltage. However, from Table 1 one can also see that the optical transmittance of ITO film (averaged from 400 to 800 nm) decreases as the Ni-doped film thickness increases. Therefore, further increase in Ni-doped ITO thickness may be ineffective on smoothing surface roughness. Instead, the external luminance efficiency of OLED devices could be lowered due to the loss in optical transmittance. Fig. 3 illustrates the luminance–voltage (L–V) characteristics of the devices. The L–V curves also shift to the lower operating voltage end as the Ni sputter power increases. The peak luminance measured at 12 V is 113, 271, 517 and 637 cd/m2 for the device with a Ni-doped ITO anode sputtered at 0, 10, 30 and 50 W, respectively. This indicates that at the same driving voltage more holes are injected into the emitting layer, resulting in enhanced light emission. Again the improved L–V characteristics are believed due to the elevated ITO surface work function by the adding of Ni atoms in the ITO matrix. However, it was noticed that the power efficiency of OLED devices was not enhanced by Ni-doped ITO anode in this case. The power efficiency at 250 cd/m2 is 1.8 cd/A for Ni(50 W)-doped device and 5.0 cd/A for the conventional device. This is due to the use of Al cathode by which the electron injection is limited. Unbalanced carriers injected from Ni-doped ITO anode and Al cathode resulted in a reduced electron-hole recombination probability and so forth deteriorated power efficiency. Yet, this study emphasized the effect of Ni-doped ITO on the reduction in operating voltage and the development of dark spots. Hence, the power efficiency was not particularly optimized. We have
employed LiF/Al stacked film as a cathode in our early work [21], and indeed the power efficiency of OLED devices could be better with a Ni-doped ITO anode. The inset in Fig. 3 shows the optical spectra of the OLED devices at various Ni sputter power. The spectral peak appears at 536 nm for the device with a conventional ITO anode and shifts to short wavelength region when the amount of doped Ni atoms increases. In the case with a Ni(50 W)-doped ITO anode the peak shifts to 512 nm and the intensity in the long wavelength region is lower than in the other cases. This suggests that optical absorption by ITO film in the long wavelength range of visible light increases with the amount of Ni atoms existing in the ITO matrix. This can be observed in the optical transmission spectra of ITO films as shown in Fig. 4. The optical absorption between 420 and 520 nm is lower for Ni-doped ITO films, while it becomes higher from 520 to 800 nm. Fig. 5 demonstrates results from time-dependent luminance test of OLED devices. The test was conducted with a constant
Fig. 4. Optical transmission spectra of conventional and Ni-doped ITO films.
Fig. 3. Luminance–Voltage (L–V) characteristics of OLED devices at various Ni sputter power.
Fig. 5. The half-luminance test for the OLED devices at various Ni sputter power.
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voltage of 18 V applied to the device. The half-luminance time for the conventional, Ni(10 W), Ni(30 W) and Ni(50 W) doped devices is 54, 68, 74 and 92 min, respectively. Devices with a Ni-doped ITO anode clearly exhibited better lifetime performance. However prior to the test, the Ni-doped OLED devices were expected to have shorter half-luminance time because at the same driving voltage (18 V) much higher current flowed into Ni-doped devices and should accelerate joule-heat caused degradation process [23,24]. The result being contrary to the initial expectation implied that the determinative factor for the light-emitting degradation process should differ from the general joule-heating process. Since organic materials used for all devices were the same, we suspected that surface morphology should play an important role, in other words formation of dark spots should be responsible for the degradation process. To promptly investigate the development of dark spots, OLED devices were driven at a forward high voltage of 18 V for 1 h in an environment of 80 8C and relative humidity of 80%. Fig. 6 shows the optical microscope images of the electrically stressed OLED devices at various Ni sputter power. It is clear that the amount of dark spots decreases with the increasing Ni sputter power. The generation of dark spots generally relates to the surface topography of ITO anode [25]. This is because rough surface provides large numbers of local spikes through which a high electric current flows, yielding dark spots faster than a smooth surface due to the local joule-heat degradation. From Table 1, one can see the surface roughness of ITO films decreases with the increasing Ni sputter power. The surface roughness is 6.52 nm for the conventional ITO film and dramatically drops to 0.46 nm for the Ni(50 W) sputtered film. The fact that the development of dark spots is less for the devices with highly Ni-doped ITO anode should therefore be attributed to the improved surface roughness of Ni-doped ITO matrix. Observations from AFM images (Fig. 7) also showed that the conventional ITO surfaces appeared a large amount of spikes whereas the Ni-doped ITO surfaces were much smoother. Furthermore, these devices were conducted with the time-dependent luminance test, and were found to exhibit better half-luminance time when with a Nidoped ITO anode. As shown in Fig. 5, devices with the conventional
Fig. 6. Optical microscope images showing dark spots of packaged OLED at Ni = (a) 0 W, (b) 10 W, (c) 30 W and (d) 50 W after an electrical stressing at 18 V for 1 h. Fig. 7. AFM surface morphology of (a) the conventional ITO film and (b) the Ni(50 W)-doped ITO film.
C.M. Hsu et al. / Applied Surface Science 255 (2009) 3759–3763
ITO and the Ni(10 W)-doped ITO anode died before the luminance could be actually measured. This indicates that the existence of Ni atoms should not deteriorate the light-emitting mechanism of OLED devices. We can then conclude that introducing Ni atoms into ITO matrix could improve the surface roughness of an ITO anode and eliminate the formation of dark spots without degrading the light-emitting process of an OLED device. 4. Conclusions This work has demonstrated that introducing Ni atoms into an ITO anode can largely eliminate the development of dark spots of an OLED device and lead to an improved lifetime performance. Considerable decrease in ITO surface roughness and local spikes by the added Ni atoms was thought to be the main cause for the reduced dark spots. The surface roughness of ITO film could drop from its conventional case of 6.52 nm to the case with a Ni(50 W)doped of 0.46 nm. Reducing thickness of Ni-doped ITO layer was found to degrade surface roughness of ITO film although its optical transmittance could be elevated. Results from half-luminance test suggested that Ni atoms had little effect on deteriorating the light-emitting mechanism of OLED devices. However with a Ni-doped ITO anode the structure of OLED devices needs to be optimized to enhance power efficiency. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this work under Contract No. NSC-95-2221-E-218-047. Thanks also go to Center for Advanced Optoelectronics of National Cheng Kung University
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for partially funding this project and providing analytical instruments. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] R. Bathelt, D. Buchhauser, C. Ga¨rditz, R. Paetzold, P. Wellmann, Org. Electron. 8 (4) (2007) 293. [3] R. Shinar, D. Ghosh, B. Choudhury, M. Noack, V.L. Dalal, J. Shinar, J. Non-cryst. Solids 352 (2006 1995). [4] H. Nakanotani, H. Sasabe, C. Adachi, Appl. Phys. Lett. 86 (2005) 213506. [5] C.J. Yang, T.Y. Cho, C.L. Lin, C.C. Wu, Appl. Phys. Lett. 90 (2007) 173507. [6] P.E. Buyyors, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E. Thompson, Appl. Phys. Lett. 65 (1994) 2922. [7] J. Shen, D. Wang, E. Langlois, W.A. Barrow, P.J. Green, C.W. Tang, J. Shi, Synth. Met. 111–112 (2000) 233. [8] S.F. Lim, W. Wang, S.J. Chua, Mater. Sci. Eng. B 85 (2001) 154. [9] Z.D. Popovic, H. Aziz, IEEE J. Sel. Top. Quant. Electron. 8 (2002) 362. [10] P. Melpignano, A. Baron-Toaldo, V. Biondo, S. Priante, R. Zamboni, M. Murgia, S. Caria, L. Gregoratti, A. Barinao, M. Kiskinova, Appl. Phys. Lett. 86 (2005) 041105. [11] S. Gardonio, L. Gregoratti, P. Melpignano, L. Aballe, V. Biondo, R. Zamboni, M. Murgia, S. Caria, M. Kiskinova, Organ. Electron. 8 (2007) 37. [12] T. Zyung, J.J. Kim, Appl. Phys. Lett. 67 (1995) 3420. [13] D.G. Sutherland, J.A. Carliste, P. Elliker, G. Fox, T.W. Hagler, I. Jimenez, H.W. Lee, K. Pakbaz, L.J. Terminelle, S.C. Williams, F.J. Himpsel, D.K. Shuh, W.M. Tong, J.J. Jia, T.A. Callcot, D.L. Ederer, Appl. Phys. Lett. 68 (1996) 2046. [14] B.H. Cumpston, I.D. Parker, K.F. Jensen, J. Appl. Phys. 81 (1997) 3716. [15] C.C. Wu, C.I. Wu, J.C. Sturm, A. Kahn, Appl. Phys. Lett. 70 (1997) 1348. [16] J.A. Chaney, P.E. Pehrsson, Appl. Surf. Sci. 180 (2001) 214. [17] J. Schwartz, E.L. Bruner, N. Koch, A.R. Span, S.L. Bernasek, A. Kahn, Synth. Met. 138 (2003) 223. [18] H. Kim, J.S. Horwitz, G.P. Kushto, S.B. Qadri, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 78 (2001) 1050. [19] I.M. Chan, T.Y. Hsu, F.C. Hong, Appl. Phys. Lett. 81 (2002) 1899. [20] T. Minami, T. Yamamoto, Y. Toda, T. Miyata, Thin Solid Films 373 (2000) 189. [21] C.M. Hsu, W.T. Wu, Appl. Phys. Lett. 85 (2004) 840. [22] C.M. Hsu, J.W. Lee, T.H. Meen, W.T. Wu, Thin Solid Films 474 (2005) 19. [23] M. Ishii, Y. Taga, Appl. Phys. Lett. 80 (2002) 3430. [24] K.K. Lin, S.J. Chua, W. Wang, Thin Solid Films 417 (2002) 36. [25] G. Liu, J.B. Kerr, S. Johnson, Synth. Met. 144 (2004) 1.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nickel doped indium tin oxide anode and effect on dark spot development of organic light-emitting devices C.M. Hsu *, C.S. Kuo, W.C. Hsu, W.T. Wu Southern Taiwan University, Department of Electro-Optical Engineering, 1 Nan-Tai St, Yung-Kang City, Tainan County 710, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 July 2008 Accepted 8 October 2008 Available online 1 November 2008
This article demonstrated that introducing nickel (Ni) atoms into an indium tin oxide (ITO) anode could considerably decrease ITO surface roughness and eliminate the formation of dark spots of an organic light-emitting device (OLED). A dramatic drop in surface roughness from 6.52 nm of an conventional ITO to 0.46 nm of an 50 nm Ni(50 W)-doped ITO anode was observed, and this led to an improved lifetime performance of an Alq3 based OLED device attributed to reduced dark spots. Reducing thickness of Ni-doped ITO anode was found to worsen surface roughness. Meanwhile, the existence of Ni atoms showed little effect on deteriorating the light-emitting mechanism of OLED devices. ß 2008 Elsevier B.V. All rights reserved.
PACS: 85.60.Jb 68.37.p 68.55.J Keywords: Nickel ITO OLED Dark spot
1. Introduction Ever since the high efficiency organic light-emitting diode (OLED) was revealed by C. Tang [1], intensive researches have been motivated to further improve the characteristics of OLEDs. These efforts have indeed greatly promoted OLED’s performance in many aspects and led its application not only for displays but also for lighting devices [2], sensors [3], laser sources [4] and solar cells [5]. However, the fact that the luminance of these organic devices decays with time renders a short device lifetime and limits their practical applications. Enhancing device lifetime is then one of the core issues for these OLED related optoelectronics to become commercial and competitive devices. For this reason investigation of the degradation processes of an OLED device has been an attractive topic and widely discussed [6–11]. The causes of the reduction in device luminance with time can generally be categorized into three types: (1) development of dark spots, (2) catastrophic failure and (3) intrinsic degradation. The first two cases are related to the morphological imperfections or structural defects, which generate large joule heat as electric currents flow through and thus deteriorate the light-emitting
* Corresponding author. E-mail address: [email protected] (C.M. Hsu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.042
process. The third case is generally attributed to the gradual formation of morphology or crystalline in organics that prohibits the exciting process of electric carriers. Meanwhile, it has been demonstrated that these degradation processes would be accelerated with the incorporation of oxygen and water vapor during fabrication and operation of devices [12–14]. Encapsulating the device in extremely low oxygen and moisture ambient is then crucial and has turned out to be a standard process for manufacturing OLED devices. Indeed, efforts to explore the degradation mechanism of OLED devices and to protect devices from serious lifetime decay have successfully led OLEDs to become one of the major products in current display market. Yet, researches to achieve higher device power efficiency and reliability in a low manufacturing cost base still continue. One of the studies of interest is to elevate work function of an indium tin oxide (ITO) film that commonly serves as an anode of an OLED device. This is based on the idea that potential barrier between ITO anode and organic hole-transport layer can be reduced by the elevation of ITO surface work function. The reduced potential barrier allows more hole-carriers to inject into OLED active layers at the same voltage, leading to a lower operating voltage and enhanced power efficiency. Hence, an as-deposited ITO film is generally post-treated to raise its work function in a practical device processing. And, it has been adopted that modifying surface chemical states of an ITO film is the most effective approach for this
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purpose. A variety of surface modifying methods have been reported, including the surface gaseous plasma treatment [15,16], the immersion of ITO into a surface dipole liquid [17], the formation of metal-doped indium or tin oxide layers on ITO surface [18,19] and the adding of high work function metals into ITO film [20]. Of these methods, the exposure of ITO surface to an energetic gaseous ambient has been commonly used in OLED manufacturing processes for its simplicity. However, the potential of inserting a metal oxide interlayer and adding high work function metals into ITO matrix has been highly noticed in recent years. This is due to that the existence of metal oxides can generally give rise to a much higher ITO work function than gaseous bombarding process can do. For instance, nickel oxide (NiOx) has a work function greater than 5.0 eV, much higher than that of ITO (4.5–4.7 eV). Instead of inserting a single layer of NiOx, in our previous studies we formed NiOx phases in ITO matrix by a Ni-ITO cosputter approach. With this approach, ITO work function up to 5.8 eV has been achieved and incorporating a Ni-doped ITO anode in an OLED structure was found to be able to reduce device threshold voltage by 2.3 V [21,22]. However, the effect of Ni atoms to the light-emitting mechanism of OLED devices is not yet investigated. In this article we report the influence of Ni atoms to the lifetime of an OLED device with a SiOx/Al/tris(8-hydroxyquinoline)aluminum(Alq3)/N,N0 -bis-(1-naphthyl)–N, N0 -diphenyl-1,10 biphenyl-4,40 -diamine(NPB)/ITO structure. Effects of doping level and thickness of the Ni-doped ITO anode on OLED performances are also discussed. 2. Experimental OLED bottom emission devices with a SiOx(200 nm)/ Al(100 nm)/Alq3(35 nm)/NPB(20 nm)/Ni-doped ITO(10–50 nm)/ ITO(200 nm) structure were fabricated on Corning 1735 glass and packaged with a glass back-plate. Fig. 1 schematically shows the structure of the packaged OLED panel. On the 50 mm 50 mm glass plate several devices with an active area of 1 cm2 were fabricated. Fabrication of the devices started with the deposition of a conventional ITO film using d.c. sputtering, followed by the coating of a Ni-doped ITO layer using co-sputtering of Ni (r.f. power) and ITO (d.c. power). The glass substrates were heated to a temperature of 200 8C for both processes. The NPB and Alq3 layers were then sequentially deposited in a resistive thermal evaporator, and the Al cathode layer was deposited in an E-beam evaporator afterward. These processes were conducted without the substrates being heated. To avoid electrical shorting between ITO anode and Al cathode, metallic shadow masks were used during film depositions for both the organic and Al layers. A SiOx passivation layer was finally deposited using E-beam evaporation. After being unloaded from the E-beam evaporator, all devices were immediately sealed with a glass back-plate using an UV curing method. Between the device substrate and the back-plate BaO desiccants were enclosed. The encapsulated OLED devices were characterized using a Photosearch PR650 C.I.E. (International
Fig. 1. The schematic structure of OLED device with a Ni-doped ITO anode.
Commission on Illumination) spectrometry and a HP4155B I–V meter for luminance–current–voltage (L–I–V) characteristics. The development of dark spots of the devices, treated with an electrical stress of 18 V in an environment of 80 8C and relative humidity of 80% for 1 h, was observed with a Nikon SMZ800 optical microscope. The surface topography of the Ni-doped ITO films was examined by an atomic force microscopy (AFM) to see its correlation to the dark spots. 3. Results and discussions In this study a series of Ni sputter power of 0, 10, 30 and 50 W was used to yield various Ni concentrations in ITO matrix. Meanwhile, film thickness of the Ni(50 W)-doped ITO layer was varied from 0 to 50 nm to examine the thickness effect on device performances. Fig. 2 shows the current–voltage (I–V) characteristics of OLED devices at various Ni sputter power and Ni-doped film thickness. It clearly shows that the I–V curves shift to the lower voltage end as sputter power and thickness of the Ni-doped ITO layer increase. The threshold voltages, defined as the electric current density is at 10 A/m2, are 10.8, 7.6, 6.3 and 5.5 V for the device with a 50 nm Ni-doped ITO anode deposited at Ni = 0, 10, 30 and 50 W, respectively. These threshold voltages have some discrepancies from those reported earlier [21] due to that the organic materials were supplied by different suppliers. However, we still can see the similar effect caused by the doped Ni atoms. That is, introducing Ni atoms into ITO matrix reduces device threshold voltage. The causes have been proved to be the formation of NiOx phases in ITO matrix that elevates ITO work function. This effect became prominent with the increasing amount of Ni atoms, suggesting ITO work function was still influenced by the Ni concentration for sputter conditions used in this study. For the device with a Ni(50 W)-doped ITO anode, the threshold voltage decreases from 6.8 to 5.9 V and 5.5 V when Ni-ITO film thickness increases from 10 to 30 nm and 50 nm, respectively. Two factors are considered to be the main causes for such a tendency. One is the increase in serial resistance when Ni-doped ITO film thickness decreases, which would result in a drop in voltage across the anode. Another is the effect from surface roughness of the Ni-doped ITO layer. One can see from Table 1 that the surface roughness (Ra), measured by AFM, considerably drops from 3.37 to 0.46 nm as the Ni-ITO film thickness increases from 10 to 50 nm.
Fig. 2. Current–Voltage (I–V) characteristics of OLED devices at various Ni sputter power and film thickness of Ni-doped ITO layer.
C.M. Hsu et al. / Applied Surface Science 255 (2009) 3759–3763
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Table 1 Electrical and optical characteristics of Ni-doped ITO films and OLED devices.
Ni-doped ITO thickness Half lifetime (min) Threshold voltage (V) AFM roughness (nm) Optical transmittance
Ni = 0 W
Ni = 10 W
Ni = 30 W
Ni = 50 W
Ni = 50 W
Ni = 50 W
500 A˚ 54 8.8 6.52 83.0%
500 A˚ 68 7.6 0.82 78.6%
500 A˚ 74 6.3 0.57 78.3%
500 A˚ 92 5.5 0.46 78.1%
300 A˚ – 5.9 0.71 79.3%
100 A˚ – 6.8 3.37 81.6%
Smooth surface can reduce interfacial trapping centers, allowing better hole-injection at the ITO/NPB interfaces and in turn decreasing threshold voltage. However, from Table 1 one can also see that the optical transmittance of ITO film (averaged from 400 to 800 nm) decreases as the Ni-doped film thickness increases. Therefore, further increase in Ni-doped ITO thickness may be ineffective on smoothing surface roughness. Instead, the external luminance efficiency of OLED devices could be lowered due to the loss in optical transmittance. Fig. 3 illustrates the luminance–voltage (L–V) characteristics of the devices. The L–V curves also shift to the lower operating voltage end as the Ni sputter power increases. The peak luminance measured at 12 V is 113, 271, 517 and 637 cd/m2 for the device with a Ni-doped ITO anode sputtered at 0, 10, 30 and 50 W, respectively. This indicates that at the same driving voltage more holes are injected into the emitting layer, resulting in enhanced light emission. Again the improved L–V characteristics are believed due to the elevated ITO surface work function by the adding of Ni atoms in the ITO matrix. However, it was noticed that the power efficiency of OLED devices was not enhanced by Ni-doped ITO anode in this case. The power efficiency at 250 cd/m2 is 1.8 cd/A for Ni(50 W)-doped device and 5.0 cd/A for the conventional device. This is due to the use of Al cathode by which the electron injection is limited. Unbalanced carriers injected from Ni-doped ITO anode and Al cathode resulted in a reduced electron-hole recombination probability and so forth deteriorated power efficiency. Yet, this study emphasized the effect of Ni-doped ITO on the reduction in operating voltage and the development of dark spots. Hence, the power efficiency was not particularly optimized. We have
employed LiF/Al stacked film as a cathode in our early work [21], and indeed the power efficiency of OLED devices could be better with a Ni-doped ITO anode. The inset in Fig. 3 shows the optical spectra of the OLED devices at various Ni sputter power. The spectral peak appears at 536 nm for the device with a conventional ITO anode and shifts to short wavelength region when the amount of doped Ni atoms increases. In the case with a Ni(50 W)-doped ITO anode the peak shifts to 512 nm and the intensity in the long wavelength region is lower than in the other cases. This suggests that optical absorption by ITO film in the long wavelength range of visible light increases with the amount of Ni atoms existing in the ITO matrix. This can be observed in the optical transmission spectra of ITO films as shown in Fig. 4. The optical absorption between 420 and 520 nm is lower for Ni-doped ITO films, while it becomes higher from 520 to 800 nm. Fig. 5 demonstrates results from time-dependent luminance test of OLED devices. The test was conducted with a constant
Fig. 4. Optical transmission spectra of conventional and Ni-doped ITO films.
Fig. 3. Luminance–Voltage (L–V) characteristics of OLED devices at various Ni sputter power.
Fig. 5. The half-luminance test for the OLED devices at various Ni sputter power.
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voltage of 18 V applied to the device. The half-luminance time for the conventional, Ni(10 W), Ni(30 W) and Ni(50 W) doped devices is 54, 68, 74 and 92 min, respectively. Devices with a Ni-doped ITO anode clearly exhibited better lifetime performance. However prior to the test, the Ni-doped OLED devices were expected to have shorter half-luminance time because at the same driving voltage (18 V) much higher current flowed into Ni-doped devices and should accelerate joule-heat caused degradation process [23,24]. The result being contrary to the initial expectation implied that the determinative factor for the light-emitting degradation process should differ from the general joule-heating process. Since organic materials used for all devices were the same, we suspected that surface morphology should play an important role, in other words formation of dark spots should be responsible for the degradation process. To promptly investigate the development of dark spots, OLED devices were driven at a forward high voltage of 18 V for 1 h in an environment of 80 8C and relative humidity of 80%. Fig. 6 shows the optical microscope images of the electrically stressed OLED devices at various Ni sputter power. It is clear that the amount of dark spots decreases with the increasing Ni sputter power. The generation of dark spots generally relates to the surface topography of ITO anode [25]. This is because rough surface provides large numbers of local spikes through which a high electric current flows, yielding dark spots faster than a smooth surface due to the local joule-heat degradation. From Table 1, one can see the surface roughness of ITO films decreases with the increasing Ni sputter power. The surface roughness is 6.52 nm for the conventional ITO film and dramatically drops to 0.46 nm for the Ni(50 W) sputtered film. The fact that the development of dark spots is less for the devices with highly Ni-doped ITO anode should therefore be attributed to the improved surface roughness of Ni-doped ITO matrix. Observations from AFM images (Fig. 7) also showed that the conventional ITO surfaces appeared a large amount of spikes whereas the Ni-doped ITO surfaces were much smoother. Furthermore, these devices were conducted with the time-dependent luminance test, and were found to exhibit better half-luminance time when with a Nidoped ITO anode. As shown in Fig. 5, devices with the conventional
Fig. 6. Optical microscope images showing dark spots of packaged OLED at Ni = (a) 0 W, (b) 10 W, (c) 30 W and (d) 50 W after an electrical stressing at 18 V for 1 h. Fig. 7. AFM surface morphology of (a) the conventional ITO film and (b) the Ni(50 W)-doped ITO film.
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ITO and the Ni(10 W)-doped ITO anode died before the luminance could be actually measured. This indicates that the existence of Ni atoms should not deteriorate the light-emitting mechanism of OLED devices. We can then conclude that introducing Ni atoms into ITO matrix could improve the surface roughness of an ITO anode and eliminate the formation of dark spots without degrading the light-emitting process of an OLED device. 4. Conclusions This work has demonstrated that introducing Ni atoms into an ITO anode can largely eliminate the development of dark spots of an OLED device and lead to an improved lifetime performance. Considerable decrease in ITO surface roughness and local spikes by the added Ni atoms was thought to be the main cause for the reduced dark spots. The surface roughness of ITO film could drop from its conventional case of 6.52 nm to the case with a Ni(50 W)doped of 0.46 nm. Reducing thickness of Ni-doped ITO layer was found to degrade surface roughness of ITO film although its optical transmittance could be elevated. Results from half-luminance test suggested that Ni atoms had little effect on deteriorating the light-emitting mechanism of OLED devices. However with a Ni-doped ITO anode the structure of OLED devices needs to be optimized to enhance power efficiency. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this work under Contract No. NSC-95-2221-E-218-047. Thanks also go to Center for Advanced Optoelectronics of National Cheng Kung University
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