Toward Novel Flexible Display Top-Emitting OLEDs on Al-Laminated PET Substrates

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1 Toward Novel Flexible Display Top-Emitting OLEDs on Al-Laminated PET Substrates FURONG ZHU, XIAO-TAO HAO, ONG KIAN SOO, YANQING LI, AND LI-WEI TAN Contributed Paper We developed a flexible organic LED (OLED) using a top-emitting OLED (TOLED) architecture. The concept of our flexible OLED design is based on integration of a TOLED structure on an aluminum-laminated polyethylene terephthalate (Al-PET) substrate. Microcavity TOLEDs were formed for color tuning and efficiency enhancement. For a flexible TOLED with a 110-nm-thick poly (p-phenylenevinylene) light-emitting layer, it exhibited a luminous efficiency of 4.6 cd/a at 10 V. The performance of a TOLED made on a plastic substrate is analyzed and compared with that of the identical device on a rigid glass substrate. It is demonstrated that the flexible TOLED on Al-PET foil can be bent to a substantial degree without breaking. Keywords Displays, flexible substrate, organic LED (OLED). I. INTRODUCTION The demand for more user-friendly displays is propelling efforts to produce head-worn and hand-held devices that are flexible, lighter, more cost-effective, and more environmentally benign than those currently available. Flexible thin-film displays enable the production of a wide range of entertainment-related, wireless, wearable-computing, and networkenabled devices. The display of the future requires that it should be thin in physical dimension, small and large formats, flexible, and full color at a low cost. These demands are sorely lacking in today s display products and technologies such as the plasma display and liquid crystal display (LCD) technologies. Organic LEDs (OLEDs) [1] [4] have the potential to replace LCDs as the dominant flat panel displays. This is because OLEDs have high visibility by self-luminescence, do not require backlighting, and can be fabricated into lightweight, thin, and flexible displays. A typical OLED is constructed by placing a stack of organic electroluminescent and/or phosphorescent materials between a cathode layer that can inject electrons and an anode layer that can inject holes. When a voltage of proper polarity is applied between the Manuscript received October 27, 2004; revised May 1, The authors are with the Institute of Materials Research and Engineering, Singapore ( fr-zhu@imre.a-star.edu.sg). Digital Object Identifier /JPROC cathode and anode, holes injected from the anode and electrons injected from the cathode combine to release energy as light, thereby producing electroluminescence (EL). The OLED stands out as a promising technology that can deliver the above challenging flexible display requirements. Next-generation flexible displays are commercially competitive due to their low power consumption, high contrast, light weight, and flexibility. The use of thin flexible substrates for the OLEDs will significantly reduce the weight of flat panel displays and provide the ability to bend or roll a display into any desired shape. To date, much effort has been focused on fabricating OLEDs on various flexible substrates [5] [9]. The plastic substrates usually do not have negligible oxygen and moisture permeability. The barrier properties of these substrates are not sufficient to protect the electroluminescent polymeric or organic layers in OLEDs due to the penetration of the chemically reactive oxygen and water molecules into active layers of the devices. It is known that OLED structures and active electroluminescent materials degrade in the presence of oxygen and moisture. It is estimated that for OLEDs to have reliable performance lifetime, exceeding h, oxygen and moisture transmission rates must be below 10 g/m day [10]. Therefore, flexible plastic foils with an effective barrier against oxygen and moisture have to be identified before this simple vision for a flexible display can become reality [11]. A polymer-reinforced ultrathin glass sheet is one of the alternative substrates for flexible OLEDs. Although the flexibility and handling ability of an ultrathin glass sheet can be improved significantly when it is reinforced [12], [13], it still has limited application for displays that can be flexed in use. A top-emitting OLED (TOLED) has a relatively transparent upper electrode so that light can emit from the side of the top electrode. Unlike the conventional bottom-emitting OLED structure, TOLEDs can be made on both transparent and opaque substrates. A see-through or dual-sided OLED display can also be fabricated when an OLED has a transparent anode and a transparent cathode on a transparent sub /$ IEEE 1440 PROCEEDINGS OF THE IEEE, VOL. 93, NO. 8, AUGUST 2005

2 strate. One of the important applications of the top device structure is to achieve monolithic integration of a TOLED on polycrystalline or amorphous silicon thin-film transistors used in active matrix displays [14]. The TOLED structures therefore increase the flexibility of device integration and engineering. In this paper, we present the results related to the flexible OLEDs on a plastic substrate using a TOLED architecture. The flexible substrate consists of a plastic layer laminated to or coated with a metal layer. High-performance single and bilayer anodes and an upper semitransparent cathode were used for the TOLEDs. The performance of the TOLEDs made on flexible plastic foils and rigid glass substrates is analyzed and compared. II. DEVICE FABRICATION Thin films of high-performance low-temperature indium tin oxide (ITO) anode for TOLED were deposited on an aluminum-laminated polyethylene terephthalate (PET) substrate (Al-PET), from Bright Silver Polyester (thickness: 0.1 mm), by RF magnetron sputtering using an oxidized target with In O and SnO in a weight ratio of 9 : 1. The substrate was not heated during or after the film deposition. The actual substrate temperature, which might be raised due to the plasma process during the film deposition, was less than 60 5 C. Sputtering power was kept constant at 100 W. The base pressure in the sputtering system was approximately Pa. During the film deposition, an argon hydrogen gas mixture was employed. The use of a hydrogen argon gas mixture allowed a broader process window for preparation of the ITO films having high optical transparency and high conductivity [15], [16], e.g., a 130-nm-thick ITO film with a sheet resistance of 25 2 sq and an optical transparency of 80% in the visible light range can be fabricated at a substrate temperature of 60 5 C. An acrylic layer was spin-coated on Al-PET to form the smoother surface for the subsequent film deposition process. It also is found that the presence of an acrylic layer increases the adhesion between the film and the substrate. Our TOLED design includes an anode, a stack of polymeric layers of poly (ethylenedioxythiophene) (PEDOT) and poly ( -phenylenevinylene) (Ph-PPV), and an upper semitransparent cathode. A 200-nm-thick Ag electrode was deposited through a shadow mask by thermal evaporation. It was then overlaid on a 130-nm-thick ITO or modified with a 0.3-nm-thick fluorocarbon (CFx) by plasma polymerization [17]. Both ITO and a bilayer anode of Ag/ITO and Ag/CFx were used for the OLEDs. Prior to the spin coating of active polymeric materials, the ITO specimens were treated by oxygen plasma. In our work, spin-coated PEDOT and Ph-PPV were used as a hole transporting layer (HTL) and an emissive layer, respectively. The samples with polymeric layers were then transferred to an electrode chamber for semitransparent cathode depositions. The semitransparent cathode was deposited on the active polymeric stack to form a TOLED. The semitransparent cathode has a multilayer Fig. 1. (a) Cross-sectional view of a typical TOLED on an Al-PET foil. (b) Control device made on a rigid glass substrate. architecture consisting of organic and inorganic layers. The semitransparent cathode is readily prepared by thermal evaporation without incurring radiation damage to the OLED layer structure, particularly the underlying active polymeric emissive Ph-PPV layer. In a previous work [18], we have demonstrated that anode modification plays a critical role in determining the EL efficiency and stability of the OLEDs. The ITO surface modification is also applied for TOLED fabrication. The deposition of organic and cathode materials was controlled at a constant rate of s, and the thickness of the organic and metal layers was estimated and controlled by the deposition time. Fig. 1 shows a cross-sectional view of a TOLED on an Al-PET and a control device with a configuration of ITO/HTL/emissive layer/semitransparent cathode on a glass. In a control device, light can be emitted from both the upper semitransparent cathode and bottom transparent substrate, as illustrated in Fig. 1(b). After the device fabrication, the samples were transferred to a connected glove box with oxygen and moisture levels lower than 1.0 ppm for current density voltage, luminance voltage, and EL efficiency voltage characteristics. III. RESULTS AND DISCUSSION The surface morphology of the flexible substrate was examined using atomic force microscopy (AFM). The typical AFM images generated for bare PET, PET with an acrylic layer and a 130-nm-thick ITO film on an acrylic-layer-coated PET are shown in Fig. 2(a) (c), respectively. The surface of ZHU et al.: TOWARDS NOVEL FLEXIBLE DISPLAY TOP-EMITTING OLEDs ON AL-LAMINATED PET SUBSTRATES 1441

3 Fig. 2. AFM images of: (a) bare PET; (b) PET with an acrylic layer; and (c) a 130-nm-thick ITO film on an acrylic-layer-coated PET. bare PET has an rms roughness of nm. PET with an acrylic layer has a much lower rms roughness of nm. It shows clearly that the ITO-coated PET foil thus prepared also has a very similar smooth surface with a rms roughness of nm, which is suitable for OLED fabrication. In a separate experiment, it reveals that the presence of an acrylic layer improves the adhesion between the anode contact and the substrate when subjected to bending as a function of number of cycles from flat to a fixed radius of curvature 12.5 mm. The response of cycles of Al-PET/anode to bending shows that there are more than 5% of anode layer delaminated from the substrate, but no anode delamination can be observed for Al-PET/acryliclayer/anode after the same bending test. This is consistent with the adhesion analyses made for ITO/polymer substrate, which shows an enhancement of the adhesion between the oxide layer and the polymer substrate through an interfacial modification [19]. It is known that most metals possess lower gas permeability than plastics by six to eight orders of magnitude [10]. Therefore, a several-micrometers-thick metal layer can serve as a highly effective barrier to minimize the permeation of oxygen and moisture. Hence, the combination of plastic metal materials is extremely promising for flexible OLED applications. The flexible substrate consists of a plastic layer laminated to or coated with a metal layer could be one of the possible solutions for flexible OLEDs. The,, and EL efficiency voltage of TOLED fabricated on both Al-PET and glass substrates are plotted in Fig. 3. The results indicate that TOLEDs made with both Ag/ITO and Ag/CFx anodes on Al-PET exhibit similar EL behavior. However, in comparison to an OLED made with a bilayer anode of Ag/ITO, the identical device with an Ag/CFx anode exhibited higher luminance at the same current density. This demonstrates that an efficient flexible ITO-free OLED can be fabricated using Ag/CFx on Al-PET foil. A luminous efficiency of cd/a at an operating voltage of 10 V can be obtained. A polymeric emissive Ph-PPV layer sandwiched between a bilayer anode of Ag/CFx and a semitransparent cathode forms an optical microcavity. The TOLED with a microcavity structure showed a higher efficiency than that of a conventional bottom-emitting OLED on transparent glass substrate [20]. In this work, microcavity TOLEDs with a configuration of Al-PET/Ag(200 nm)/cfx (0.3 nm)/ph-ppv( nm)/semitransparent cathode were formed for 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 TOLED is in the order of few hundred nanometers and its thickness can be chosen close to that of the emission wavelength. By tuning the cavity resonance to wavelengths near the EL peak (noncavity), one can increase the amount of light collected outside the device [21]. The EL spectra measured for a set of flexible microcavity TOLEDs with different EL layer thicknesses and a conventional noncavity TOLED are shown in Fig. 4. The EL peak position of the TOLEDs, with a Ph-PPV thickness varied from 80 to 150 nm, exhibits a clear red shift in the wavelength from 530 to 610 nm, showing an optical microcavity 1442 PROCEEDINGS OF THE IEEE, VOL. 93, NO. 8, AUGUST 2005

4 Fig. 4. EL spectra measured for a set of structurally identical devices with different emissive layer thicknesses and a conventional noncavity OLED; the inserted color photos are the corresponding photo images taken for the devices. spectral position of the cavity modes can be determined by the optical thickness of the cavity where is the mode index, is the optical thickness of the cavity, and is the mode wavelength of the cavity. In this case, the optical thickness of the cavity can be calculated, taking into account a substantial penetration depth into the semitransparent mirror, by (1) (2) Fig. 3. (a) J V, (b) L V, and (c) EL efficiency voltage characteristics measured for TOLEDs on glass and Al-PET. TOLEDs made on glass and flexible substrates had an identical semitransparent top cathode (TC). effect. The photo images taken for microcavity TOLEDs and a noncavity OLED are also illustrated on the top of the corresponding EL curves in Fig. 4. The device with such a microcavity structure can be used for color tuning and efficiency enhancement. It is clear, as seen in Fig. 4, that the full-width at half maximum (FWHM) of EL peak for a noncavity OLED was 137 nm. The FWHM values obtained for the microcavity TOLEDs with emitting layer thickness of 80, 110, and 150 nm were 120, 77, and 25 nm, respectively. These observations are attributed to the optical microcavity effect. It is well known that the emission from the Fabry Pérot cavity is determined by the resonance modes of the cavity, and the The first term is the effective penetration depth in the semitransparent mirror layer, is the vacuum wavelength, is the effective refractive index of the semitransparent mirror, is the difference between the indexes of the materials of the such layer, and and are the refractive index and the thickness of organic layer. The last term in (2) is the optical thickness contributed by the phase shift at the interface of the metal layer and the Ph-PPV layer, and is the phase shift at the interface, depending on the refractive indexes of the metal and the Ph-PPV layer at the interfaces where is the refractive index of Ph-PPV in contact with the metal and, are the real and imaginary parts of the refractive index of the metal. The,, and EL efficiency-voltage characteristics of the TOLEDs with different Ph-PPV thickness are shown in Fig. 5(a) (c), respectively. The turn-on voltage for the TOLEDs with Ph-PPV thickness of 80 and 110 nm is around 2.5V; it is increased to 7.5 V when a thicker Ph-PPV layer of 150 nm was used in the device with the identical configuration. This is because the presence of the thicker polymer (3) ZHU et al.: TOWARDS NOVEL FLEXIBLE DISPLAY TOP-EMITTING OLEDs ON AL-LAMINATED PET SUBSTRATES 1443

5 Fig. 6. Photograph of a flexible TOLED, demonstrating an emissive image, with a configuration of Ag/CFx/Ph-PPV/semitransparent cathode on a Al-PET substrate. Fig. 5. Characteristics of: (a) current density versus the operating voltage; (b) luminance versus operating voltage; and (c) efficiency versus voltage of the flexible OLEDs with different Ph-PPV thickness. makes the whole device more resistive and hence a higher driving voltage is expected. The luminance of 6000 cd/m is obtained at voltage of 12 V for the TOLED with Ph-PPV thickness of 110 nm. It also can be seen from Fig. 5 that the EL efficiency of the devices is varied quite substantially with different Ph-PPV thicknesses used in the devices. The maximum EL efficiency of cd/a was obtained for a TOLED with a Ph-PPV layer thickness of 110 nm at the operating voltage of 10 V. The EL efficiency measured for the devices with Ph-PPV thickness of 80 nm and 150 nm is cd/a and cd/a, respectively. EL devices with optical microcavity structures offer a promising means to achieve the higher performance organic EL diodes that exhibit very high luminance and can be driven with low dc voltages. The TOLEDs with optical microcavity structure offer the possibility to control the spectral properties of emission. By replacing the single ITO anode with a highly reflective bilayer anode of Ag/CFx, a Fabry Pérot microcavity can be introduced into usual OLEDs. Planar microcavity structures can be used to improve the performance of OLEDs. The emitting layer in organic microcavity devices is embedded between a transparent electrode and a highly reflective distributed Bragg reflector or a quarter wave stack lead to strong modulation of the emission spectrum and angular dependence. In some applications, the microcavity effects are desired to achieve directionality and color saturation. Fig. 6 is a photograph of a flexible TOLED on Al-PET showing an emissive image. It is found that the performance of TOLEDs made on Al-PET does not deteriorate after repeated bending. Although only the Al-PET foil was tested for flexible TOLEDs in this work, the concept also applies to other plastics. For example, the flexible substrate can be a plastic layer laminated to or coated with a metal layer or a metal film sandwiched between the two plastic foils. When TOLED is formed on a metal surface of a flexible substrate, the metal surface can serve as part of the anode for a TOLED and a barrier to minimize the permeation of oxygen and moisture. This substrate has the potential to meet permeability standards far in excess of the most demanding display requirements of 10 g/m day [10]. A flexible TOLED using Al-PET substrate may provide a cost-effective approach for mass production, such as roll-to-roll processing, which is a widely used industrial process. Most current OLEDs are based on rigid substrates, such as glass, which limits the mouldability of the device, restricting the design and spacing where OLEDs can be used PROCEEDINGS OF THE IEEE, VOL. 93, NO. 8, AUGUST 2005

6 The use of Al-PET in OLEDs will significantly reduce the weight of flat panel displays and endow the ability to bend a display into any desired shape. Imagine displays that can be wrapped around the circumference of a pillar of foldable and roll-able televisions. Flexible OLEDs using Al-PET, demonstrated in this work, will also make it possible to fabricate displays by continuous roll processing, thus providing the basis for very low-cost mass production. IV. CONCLUSION A high-quality bilayer anode and an upper semitransparent cathode were developed for flexible TOLEDs. A sheet resistance of 25 2 sq at a film thickness of 130 nm was obtained for ITO film prepared at a deposition temperature below 60 C. For a flexible TOLED made with a bilayer anode of Ag/CFx on Al-PET, an EL efficiency of cd/a was obtained at an operating voltage of 10 V. When a TOLED is formed on a flexible Al-PET substrate, an aluminum layer can serve as a barrier to minimize oxygen and moisture permeation. 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Chua, Flexible polymer light emitting devices using polymer-reinforced ultrathin glass, Thin Solid Films, vol. 477, pp , Furong Zhu received the B.Sc. and M.Sc. in physics from Fudan University, China, in 1983 and 1987, respectively, and the Ph.D. degree in physics from Charles Darwin University, Australia, in He joined the Institute of Materials Research and Engineering (IMRE), Singapore, in He is a Senior Research Scientist at IMRE. His research interests include materials-oriented research, organic electronics, and transparent oxide semiconductors for optoelectronic applications. Xiao-Tao Hao received the B.Sc. and Ph.D. degrees in physics from Shandong University, China, in 1997 and 2002, respectively. He is currently a Research Associate at the Institute of Materials Research and Engineering (IMRE), Singapore. His research interests are in the field of novel transparent conducting oxides and organic electronics. Ong Kian Soo received the M.Sc. degree in polymer and polymer composites from the University of Sheffield, U.K., in He is currently with the Institute of Materials Research and Engineering (IMRE), Singapore. He has more than 18 years of experience related to LCD technologies. His research interests are mainly focused on the science and technology of organic LEDs. ZHU et al.: TOWARDS NOVEL FLEXIBLE DISPLAY TOP-EMITTING OLEDs ON AL-LAMINATED PET SUBSTRATES 1445

7 Yanqing Li received the B.Sc. degree from Zhejiang University in 2001 and the M.Sc degree from City University of Hong Kong in She was a Research Officer at the Institute of Materials Research and Engineering (IMRE), Singapore, from 2003 to She is currently a full time Ph.D. student with the Department of Physics and Materials Science, City University of Hong Kong. Her research interests are mainly focused on organic electronics, photonic devices, synthesis, and characterization of nanostructured semiconducting materials. Li-Wei Tan received the M.Sc. degree in chemistry from Universiti Teknologi Malaysia in 2002 and the M.Sc. degree in material science from Singapore-MIT Alliance, National University of Singapore, in She is currently a Research Officer at the Institute of Materials Research and Engineering (IMRE), Singapore. Her research interests include organic electronics, especially in organic electroluminance, photovoltaic, and device applications PROCEEDINGS OF THE IEEE, VOL. 93, NO. 8, AUGUST 2005