Enhanced reliability by diamond-like carbon in single-layer organic light emitting diodes S.-S. Yap, R.-B. Yang, H.-K. Yow and T.-Y. Tou Diamond-like carbon (DLC) layers with thickness of 1.5 nm were incorporated, by pulsed laser deposition technique, in organic light emitting diodes (OLEDs), which consisted of a single light-emitting layer of polystyrene (PS) as the host material, but co-doped with N,N0diphenyl-N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine (TPD) and Tris-(8-hydroxyquinoline)-Aluminium (Alq3). For unstable ITO=doped-PS=Al OLEDs, which easily failed with poor light emission when the doped-PS layer used was too thin, the insertion of DLC in the ITO=doped-PS=DLC=Al structure restored the device operation to maximum operating voltage and hence full brightness.
Introduction: Electron injection in organic light emitting diodes (OLEDs) is usually improved by using low work function metal cathodes such as Ca and Mg [1, 2], but these are reactive materials. In recent years, diamond-like carbon (DLC), which is chemically inert with excellent ambient stability, has been investigated for OLED improvement. Lmimouni et al. [3] obtained a two-order of magnitude of current increase for the ITO=P3OT=DLC=Al device with a 100 nm DLC layer, which was more stable than the ITO=DLC=P3OT=Al, but without light emission. Han et al. [4] also obtained a two-order of magnitude improvement in the injection current by inserting a thin 1.5 nm DLC layer in the ITO=PEDOT:PSS=MEH-PPV=DLC=Al device, but very weak emission was reported. Only recently, the role and performance improvement by DLC in the ITO=DLC= PEDOT:PSS=MEH-PPV=LiF=Al device were reported by Choi et al. [5] as a diffusion barrier to prevent indium-ion contamination of the polymer layer and the device brightness was doubled, although the injection current was reduced. In another recent paper [6], ultra-thin DLC (ta-C) of 0.5 nm significantly improved the brightness and operation lifetime of an ITO=DLC=PEDOT=PPV=Ca=Ag device, although the injection current was hardly increased. On the other hand, the operation and efficiency of molecularly-doped, single-layer OLEDs were reported to be very much dependent on the thickness of the doped-polymer layer between the cathode (Al) and the anode (ITO). At doped-polymer layer thickness of 50 nm and less, the stability and brightness of the devices drastically deteriorated [7]. In this work, the effect of DLC was investigated for the similar singlelayer OLED, in which the molecularly-doped polymer layer consisted of polystrene (PS) as the host material and co-doped with N,N0diphenyl-N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine (TPD) and Tris-(8-hydroxyquinoline)-Aluminium (Alq3).
Experiment: Four OLEDs with a light-emitting area of 2.6 3.1 mm were fabricated. The layer structures of these OLEDs are summarised in Fig. 1. Three of the devices are of the structure ITO=doped-PS=Al (structure A) with different doped-PS layer thicknesses at 65 nm (OLED-1), 55 nm (OLED-2) and 45 nm (OLED-3), respectively, and the fourth device is of the structure ITO=doped-PS=DLC=Al (structure B) with a 45 nm-thick doped-PS layer (OLED-4). The PS was used as the host material but co-doped with both the hole transport material TPD and the electron transport material Alq3. The weight percentages of PS, TPD and Alq3 were 80, 12 and 8%, respectively. The OLEDs were fabricated using commercially available indium-tin-oxide (ITO)-coated glass substrates (30 O=square), which were cleansed with acetone and isopropyl alcohol (IPA) in an ultrasonic bath sequentially, followed by deonised water rinsing, and nitrogen gas blow-drying. The cleansed samples were then heated on a hotplate at 100 C for 10 min. The doped-PS solution was subsequently spin-coated onto the ITO (as anode) substrates to form a single polymer thin film, followed by solvent drying on a hotplate in a nitrogen-filled glove-box at 50 C for 1 h. Immediately after the thermal drying process, the samples were transferred into an Edwards Auto 360 thermal evaporator for 2 h of vacuum drying. The thickness of the doped-PS layer, as measured by a Zygo optical interferometer, was controlled by the speed of the spin-coater. For devices of structure A (OLED-1 to OLED-3), a 50 nm-thick aluminium layer (as cathode) was subsequently evaporated on top of the polymer film at a pressure of 4 106 torr. For the device of structure B (OLED-4), a DLC layer was deposited, via a shadow mask, onto the ITO=doped-PS surface by
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a pulsed, third-harmonic Nd:YAG laser (355 nm) before the aluminium deposition. The DLC deposition, at a rate of 0.025 nm per shot, was carried out in high vacuum of about 2 106 mbar. For these conditions, the visible Raman spectra for a 50 nm DLC layer indicated that the sp3 content could be 40–50%. The current–voltage characteristics of the test devices were measured using a Keithley 238 sourcemeter. For the measurement of the electroluminescent intensity against input voltage, a silicon photodetector (Oriel) and a power meter were used. The electroluminescent spectra was measured using an Ocean Optics S2000 (UV-Vis-IR) spectral analyser. All the test devices were emitting in the green region with a peak wavelength at 520 nm. The green emissions were visible to the naked eye in a lighted room for all devices except for OLED-3. +
-
structure A
Al doped-PS ITO
glass substrate
a +
-
structure B
Al
DLC doped-PS
ITO
glass substrate
b
Fig. 1 Layer structures for molecularly-doped, single-layer OLEDs a Structure A of ITO=doped-PS=Al b Structure B of ITO=doped-PS=DLC=Al
Fig. 2 Comparison of electroluminescent intensity against applied voltage between three structure A OLEDs with different doped-PS thickness and no DLC, and one structure B OLED with 1.5 nm-thick DLC layer
Results and discussion: The electroluminescent (EL) intensities against voltage for the OLEDs are compared in Fig. 2. The characteristic features from these four OLEDs are presented in Table 1. A stable and efficient OLED-1 was found to operate at a doped-PS thickness of 65 nm, which usually starts to light up at 17 V and breaks down at about 31 V with a maximum EL intensity at 36 000 a.u. (arbitrary unit). When the doped-PS layer was reduced to 55 nm as in the OLED-2, the maximum operating voltage decreased to 27.5 V, and thus the EL intensity dropped to 15 000 a.u. However, when the thickness of the doped-PS layer was reduced to 45 nm, not only the initial threshold voltage of OLED-3 was increased to 23 V, but the device also broke down immediately after 25 V with merely a maximum EL intensity at 1000 a.u. In comparison, by depositing a 1.5 nm DLC on top of the
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doped-PS surface prior to the aluminium cathode, the OLED-4 of structure B could be repeatedly tested up to 31 V without electrical breakdown and no reduction in the electroluminescent intensity was recorded. The OLED-4 was as stable as OLED-1 although there were some differences in their intensity curve characteristics (Fig. 2). Furthermore, the maximum current densities (Table 1) of 50 mA=cm2 for OLED-1 and 56 mA=cm2 for OLED-4 were sufficiently close to suggest that the DLC layer was unlikely to enhance nor did it reduce electron injection significantly, as observed in other work [3–6]. For this work, we may attribute the role of the 1.5 nm DLC to its known strong electrical insulation property, which could largely be responsible for mitigating the weak spots in the 45 nm doped-PS layer that were prone to electrical stress-induced breakdowns. At the same time, DLC might moderate the highly energetic electrons which were injected into the doped-PS layer under high electric field from the aluminium cathode, in particular from the tip of any sharp point.
Table 1: Effects of doped-PS thickness and DLC on OLED performance
injected from sharp points in the aluminium; these explanations may befit its electrical insulating property. Acknowledgments: We are grateful to the Malaysia Ministry of Science, Technology and Innovation (IRPA 09-99-01-0001-EA001) and the Motorola Foundation in Chicago for financial support. # IEE 2006 Electronics Letters online no: 20063947 doi: 10.1049/el:20063947
S.-S. Yap, R.-B. Yang, H.-K. Yow and T.-Y. Tou (Faculty of Engineering, Multimedia University, Persiaran Multimedia, Cyberjaya, 63100 Selangor, Malaysia) E-mail:
[email protected] References 1
Device Layer structure Thickness of doped-PS (nm)
OLED-1
OLED-2
OLED-3
Structure A: ITO=doped-PS=A1 65
55
45
OLED-4 Structure B: ITO= doped-PS=DLC=A1
2
45
Maximum voltage (V)
31
27.5
25
> 31
Light intensity (a.u.)
36 000
15 000
1000
36 000
Current density at maximum voltage (mA=cm2)
50
27.5
17
56
Threshold voltage for light emission (V)
17
17.5
23
16
3 4
5
Conclusions: We have confirmed that the stable operation of singlelayer OLEDs depends very much on the thickness of the lightemitting layer, but this can be resolved by incorporating a thin DLC layer. There was no evidence of significantly enhanced electron injection by DLC, but its roles were likely to mitigate weak spots in the doped-PS layer and possibly moderate the energetic electrons
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Tang, C.W., and Vanslyke, S.A.: ‘Organic electroluminescent devices’, Appl. Phys. Lett., 1987, 51, pp. 913–915 Jabbour, G.E., Kawabe, Y., Shaheen, S.E., Wang, J.F., Morrell, M.M., Keppelen, R.B., and Peyghambarian, N.: ‘Highly efficient and bright organic electroluminescent devices with an aluminum cathode’, Appl. Phys. Lett., 1997, 71, pp. 1762–1764 Lmimouni, K., Legrand, C., Dufour, C., and Chapoton, A.: ‘Diamondlike carbon films as electron-injection layer in organic light emitting diodes’, Appl. Phys. Lett., 2001, 78, pp. 2437–2439 Han, D.W., Jeong, S.M., Lee, S.J., Yang, N.C., and Suh, D.H.: ‘Electron injection enhancement by diamond-like carbon film in organic electroluminescence devices’, Thin Solid Films, 2002, 420–421, pp. 190–194 Choi, S.H., Jeong, S.M., Koo, W.H., Jo, S.J., Baik, H.K., Lee, S.J., Song, D.W., and Han, D.W.: ‘Diamond-like carbon as buffer layer in polymeric electroluminescent device’, Thin Solid Films, 2005, 483, pp. 351–357 Chen, B.J., Sun, X.W., Tay, B.K., Ke, L., and Chua, S.J.: ‘Improvement of efficiency and stability of polymer light-emitting devices by modifying indium tin oxide anode surface with ultrathin tetrahedral amorphous carbon film’, Appl. Phys. Lett., 2005, 86, p. 063506 Leung, L.M., Kwong, C.F., Kwok, C.C., and So, S.K.: ‘Organic polymer thick film light emitting diodes (PTF-OLED)’, Displays, 2000, 21, pp. 199–201
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