An efficient top-emitting electroluminescent device on

Mater. Res. Soc. Symp. Proc. Vol. 846 © 2005 Materials Research Society

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An efficient top-emitting electroluminescent device on metal-laminated plastic substrate L.W.Tan, X.T.Hao, K.S.Ong, Y.Q Li, and F.R. Zhu* Institute of Materials Research and Engineering, No.3 Research Link Singapore, 117602 ABSTRACT An efficient flexible top-emitting organic light-emitting device (OLED) was fabricated on an aluminum-laminated polyethylene terephthalate substrate. A spin-coated light-emitting polymer layer was sandwiched between a silver anode and a multi-layered semitransparent cathode. The performance of polymer OLEDs was analyzed and compared with that of the devices having a conventional structure. An optical microcavity formed in the device enables to tune the emission color by varying the thickness of the active polymer layer. The OLEDs having a 110-nm-thick active polymer layer exhibited superior electroluminescence performance, with a turn-on voltage of 2.5V and a luminance efficiency of 4.56 cd/A at an operating voltage of 10V. INTRODUCTION Organic light emitting devices (OLEDs) have recently attracted attention as display devices that can replace liquid crystal displays because OLEDs can produce high visibility by selfluminescence and they can be fabricated into lightweight, thin and flexible displays1-4. The conventional structure of OLEDs consists of a metal or metal alloy cathode and a transparent anode on a transparent substrate, whereby light can be emitted from the transparent substrate. The OLEDs may also have a top-emitting structure that has a relatively transparent top electrode so that light can emit from the side of the top electrode, which can be formed on either an opaque or a transparent substrate5-7. The top-emitting OLED structures increase the flexibility of device integration and engineering and are desirable for high-resolution active matrix displays. OLEDs have usually been built on rigid glass substrates due to their low permeability to oxygen and moisture. Over the past few years, ultrathin glass sheets8-10 and transparent plastic substrates11, 12 have been considered as the possible substrate choices for flexible OLEDs. Ultrathin glass sheets, however, are very brittle and OLEDs formed on ultrathin glass sheets have limited potential as flexible OLED displays. To make OLEDs that are lighter, thinner, more rugged and highly flexible, plastic substrates, e.g. polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), have been used for flexible OLEDs. It is apparent that PET, PEN and other commonly used plastic foils do not have sufficiently high impermeability for OLEDs. Accordingly, efforts have been made to develop highly effective barrier against oxygen and moisture permeation and hence to minimize degradation of the devices on plastic substrates13. Multilayer barrier approaches have been used to improve the barrier property of plastic substrates, such as using alternative multi-layers of organic-inorganic structures, incorporation of getter materials, thick capping metals, pinholes reduction, etc. Further optimization is required to avoid the possible exfoliation between the organic and inorganic SiON layers 14. Alternatively, metal laminated plastic substrates are promising for flexible display application due to their high mechanical flexibility and the barrier property. Metal laminated plastic foils have the potential to meet permeability standards in excess of the most demanding display and organic electronics requirements.

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In this work, we demonstrated the feasibility of fabricating a flexible OLED on aluminumlaminated PET (Al-PET, 0.1mm) substrate using top emission device structure. In this flexible OLED, a modified Ag anode is used, OLEDs with this architecture do not require the process of ITO deposition that is needed in the conventional OLED fabrication. The ITO-free OLED may be of practical importance towards the low-cost flexible OLED displays. The Fabry-Perot planar cavity structure thus formed between a reflective Ag anode and the semitransparent cathode is also discussed. The results also show that the emission properties of OLEDs can be modified by microcavity effect 15-17. The color tuning and efficiency enhancement observed in the flexible OLEDs were examined by choosing different emitting layer thicknesses. EXPERIMENTS The surface of Al-PET film (400 Gauge Mylar 453) was cleansed sequentially with acetone, methanol, and de-ionized water. The plastic foils were then coated with a thin UVcurable acrylic layer to improve the surface smoothness and the adhesion between the anode and the substrate. A 200-nm-thick Ag electrode was deposited on the flexible substrate through a shadow mask with an array of 2 mm × 2 mm openings by thermal evaporation. The Ag contact was then modified by a 0.3-nm-thick plasma-polymerized fluorocarbon film (CFX) to improve the carrier injection property in OLEDs18. The modified Ag/CFX serves as a high conductivity anode and also a mirror to redirect the internal emitting light towards the upper semitransparent cathode to improve the light output. Phenyl-substituted poly(p-phenylenevinylene) (Ph-PPV) films with various thicknesses were then spin-coated as the emissive layer. The specimens were loaded into an evaporation chamber at a base pressure of ~10-4 Pa for semitransparent cathode deposition. The semitransparent cathode consists of LiF (0.3 nm)/Ca (5 nm)/Ag (15 nm)/tris-(8ydroxyquinoline) aluminum (Alq3, 52 nm). The EL is measured with a SPEX750M spectralphotometer, and the J-V-L characteristics are measured with a Keithley 2420 source measure unit and calibrated silicon photodiode. RESULTS AND DISCUSSIONS The layered structures of flexible top-emitting OLEDs reported in this paper are schematically depicted in Fig. 1 (a). The device structure is Al-PET /Ag (200nm) /CFx (0.3nm)/Ph-PPV(80-150nm) /semitransparent cathode. An organic microcavity, consisting of an emissive layer (EL) of Ph-PPV sandwiched between the metal anode and the semitransparent cathode, was formed color tuning and efficiency enhancement. Electron and hole injection are enhanced by interface modification at the metal/organic contacts, and color is tuned by varying the thickness of the Ph-PPV layer. The optical thickness of the active EL layer in the microcavity is in the order of few hundred nanometers and its thickness may be comparable to the emission wavelength. A typical flexible top emitting OLED with yellow emission in bending condition is shown in Fig. 1(b). The devices remained the similar emission performance after repeated bending. Such flexible electroluminescent devices enable to be bent or rolled into any shape without affecting the EL performance. This demonstrates that OLED with top emission structure on Al-PET may provide an inexpensive approach for flexible EL displays and thus make possible new product concepts.

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Semitransparent cathode Emissive layer CFx (0.3nm) Ag (200nm) Acrylic layer Al-PET substrate

Fig.1 A cross sectional view of a flexible OLED with a top emission architecture on Al-PET foil (a) and a photograph of such a flexible OLED operating in a continuous bending test (b).

Normalized EL Spectra (a.u.)

The EL spectra measured for a set of flexible microcavity OLEDs with different EL layer thicknesses and a conventional non-cavity top-emitting OLED are shown in Fig.2. The EL peak position of the OLEDs, with a Ph-PPV thickness varied from 80 to 150 nm, exhibits a clear red shift in the wavelength from 530 nm to 610 nm, showing an optical microcavity effect. The photo images taken for microcavity OLEDs and a non-cavity OLED are also illustrated on the top of the corresponding EL curves in Fig.2. The device with such a microcavity structure can be used for color tuning and efficiency enhancement. It is clear, as seen in Fig.2, that the full width at half maximum (FWHM) of EL peak for a non-cavity OLED was 137nm, The values of FWHM obtained for the microcavity OLEDs with emitting layer thickness of 80 nm, 110 nm, and 150 nm were 120nm, 77nm and 25nm, respectively. These observations are attributed to the optical microcavity effect. It is well known that the emission from the Fabry-Perot cavity is determined by the resonance modes of the cavity, and the spectral position of the cavity modes can determined by the optical thickness of the cavity: L = k (λ k / 2) (1) where k=1, 2, 3… is the mode index, L is the optical thickness of the cavity, and λk is the mode wavelength of the cavity. Ph-PPV 150nm Ph-PPV 110nm Ph-PPV 80nm Non-cavity OLED

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Fig.2 EL spectra measured for a set of structurally identical devices with different emissive layer thicknesses and a conventional non-cavity OLED, the inserted color photos are the corresponding photo images taken for the devices.

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In this case, the optical thickness of the cavity can be calculated, taking into account a substantial penetration depth into the semitransparent mirror, by L=

λv ⎛ neff ⎞

Φ ⎜⎜ ⎟⎟ + ∑ ni d i + m λ v 2 ⎝ ∆n ⎠ i 4π

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the first term is the effective penetration depth in the semitransparent mirror layer, where λv is the vacuum wavelength, neff is the effective refractive index of the semitransparent mirror, ∆n is the difference between the indices of the materials of the such layer, ni and di are the refractive index and the thickness of organic layer. The last term is the optical thickness contributed by the phase shift at the interface of the metal layer and the Ph-PPV layer, and Φ m is the phase shift at the interface, depending on the refractive indices of the metal and the Ph-PPV layer at the interfaces: ⎛ ⎞ 2nm k m ⎟ Φ m = arctan⎜⎜ 2 2 2 ⎟ ⎝ n s − nm − k m ⎠

(3)

where ns is the refractive index of Ph-PPV in contact with the metal, and nm, km are the real and imaginary parts of the refractive index of the metal.

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Fig.3 Characteristics of (a) current density vs the operating voltage, (b) luminance vs operating voltage and (c) efficiency vs voltage of the flexible OLEDs with different Ph-PPV thickness.

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The current density-voltage, luminance-voltage and efficiency-voltage characteristics of the devices with different Ph-PPV thickness are shown in Figs. 3 (a), (b) and (c), respectively. The turn-on voltage for the devices with Ph-PPV thickness of 80 and 110nm is around 2.5V, it is increased to ~7.5V when a thicker Ph-PPV layer of 150nm was used in the device with the identical configuration. This is because the presence of the thicker polymer makes the whole device more resistive as hence a higher driving voltage is required. The luminance of 6000cd/m2 is obtained at voltage of 12V for the OLED with Ph-PPV thickness of 110nm. It also can be seen from Fig. 3 that the EL efficiency of the devices is varied quite substantially with different PhPPV thicknesses used in the devices. The maximum EL efficiency of 4.56cd/A was obtained for OLED with a Ph-PPV layer thickness of 110 nm at the operating voltage of 10V. The EL efficiency measured for the devices with Ph-PPV thickness of 80nm and 150nm is 3.4cd/A and 1.2cd/A, respectively. Present OLED technologies are focused on rigid substrates but flexible devices are fast gaining attention due to its lightweight, low cost and physical flexibility. To date, the research efforts surrounding flexible OLEDs have been centered on fabricating OLED on transparent flexible plastic substrates. We are currently extending our work to include design and fabrication of a flexible OLED using a top emission OLED architecture. The flexible substrate consists of a plastic layer laminated to or coated with a metal layer. This substrate has the potential to meet permeability standards in excess of the most demanding display requirements. The robustness of this substrate is also very high. This technology may provide a cost-effective approach for mass production, such as roll-to-roll processing, which is a widely used industrial process. CONCULSION In summary, a flexible ITO-free OLED on Al-PET substrate is demonstrated. When a top emitting OLED is formed on a metal surface of a flexible substrate, the metal surface can serve as a part of the anode for the top emitting OLED as well as a barrier to minimize oxygen and moisture permeation. The structure of our flexible OLED consists of 1) an opaque flexible substrate, which can be a metal (Al)-laminated plastic (PET) or a metal film sandwiched between two plastic foils, 2) a planarisation/isolation layer, 3) bilayer Ag/CFX anode, 4) an active lightemitting layer, and 5) an upper semitransparent cathode. A microcavity structure was formed between a modified Ag anode and a semitransparent cathode. The performance of the flexible OLEDs can be modified by selecting an appropriate emissive Ph-PPV layer thickness. REFERENCES: 1. C.W. Tang, and S.A. Vanslyke, Appl. Phys. Lett. 51 (12), 913 (1987). 2. L.S. Hung, and C.H. Chen, Mater. Sci. Engi. R 39 143 (2002). 3. J.H. Burroughes, D.D.C. Bradley, A. R. Brown, R.N. Marks, K. Mackay, R. H. Friend, P.L. Burn, and A.B. Holmes, Nature 347, 539 (1990). 4. R. H. Friend, R.W.Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, and W.R. Salaneck, Nature 397, 121 (1999). 5. M.H. Lu, M.S. Weaver, T.X. Zhu, M. Rothman, R.C. Kwong, and J.J. Brown, Appl. Phys. Lett. 81, 3921 (2002).

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