High Performance Polymer Light-Emitting Diodes Fabricated by a Low Temperature Lamination Process**

Transcription

1 High Performance Polymer Light-Emitting Diodes Fabricated by a Low Temperature Lamination Process** By Tzung-Fang Guo, Seungmoon Pyo, Shun-Chi Chang, and Yang Yang* We report on the successful demonstration of high performance polymer light-emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue- and red-emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi-layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. Atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin-coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process. 1. Introduction The ability to process conjugated polymers from solutions simplifies the fabrication of polymer light-emitting diodes (PLEDs). [1,2] For single-layer devices, a uniform layer of the conjugated polymer, often obtained by spin coating, serves as the active layer for PLED. [3] However, the fabrication of bilayer (or multi-layer) device structures, [4,5] involves complicated materials engineering to ensure that the solvents used for the successive layers do not dissolve the preceding layer. This limits the choice of utilizable materials, and often, precludes the use of the best available materials. An alternative approach is to fabricate the PLED (or other polymer device) in two independent parts. The first part is the fabrication of the anode part, comprising the anode on the first substrate, coated with a hole-transport layer. The second part is the cathode part containing the cathode on the second substrate, coated with the luminescent polymer. These two parts can be subsequently laminated together to form a PLED. [6] Unfortunately, traditional plastic lamination is a high-temperature and high-pressure process where the materials are heated to near T m or at least to their T g. Such conditions could be potentially deleterious to the photonic and electronic properties of conjugated polymers due to interchain interactions in the conjugated polymer [7±9] and the degradation of p-electron conjugation. In addition, conjugated polymers, having aromatic rings and/or conjugated double bonds as part of their chemical structures are quite rigid, and it is difficult to soften these materials by heat compared to plastics that posses saturated ± [*] Dr. Y. Yang, T.-F. Guo, S. Pyo, Dr. S.-C. Chang Department of Material Science and Engineering University of California, Los Angeles Los Angeles, CA (USA) yangy@ucla.edu [**] The authors deeply appreciate the technical discussions with Dr. Qibing Pei and Prof. Fred Wudl. This research is partially supported by the Office of Naval Research (Grant # N ), and the Air Force Office of Scientific Research (Grant # F ). bonds. [10] It has been proposed to use an adhesive layer between the layers that need to be bonded, thereby avoiding a high temperature process. [6] However, the introduction of this additional adhesive layer increases the complexity of the device. Hence, significant processing issues have hindered the progress in fabricating laminated PLEDs. In fact, to date there are only a few articles published on the lamination process for polymer photovoltaic [11,12] and organic LEDs. [13] In this manuscript, we report the formation of high performance PLEDs using a unique low-temperature plastic lamination process. It is essential that the interface between the anode part and cathode part be formed without sacrificing the electronic and photonic properties of the device. A template activated surface process (TAS) has been invented and successfully applied to generate such an interface for the low-temperature lamination. 2. Results and Discussion Our laminated devices consisted of two components, the anode-part (or component A) and the cathode-part (or the component C). These two components were fabricated separately and then laminated together. Figure 1 shows the fabrication process and the basic structure of our laminated devices. The anode-part contains the anode and a polymer (or organic) layer, which can be either a hole-transporting layer or a luminescent polymer. The anode comprises a patterned indium tin oxide (ITO) and a thin layer of conducting polymer, poly(3,4- ethylenedioxythiophene) (PEDOT). However, the fabrication of the cathode-part was slightly complicated. To fabricate this, the active luminescent material was spin-coated on an ITO surface, followed by the sequential deposition of 5 Š LiF and 1500 Š aluminum (Al) by thermal evaporation on the polymer surface under a vacuum of <10 ±6 torr. Then, the polymer film with thermally evaporated metal electrode was carefully lifted off the ITO/glass substrate in an inert atmosphere by using an adhesive strip. Through this step, the cathode part has been Adv. Funct. Mater. 2001, 11, No. 5, October Ó WILEY-VCH Verlag GmbH, D Weinheim, X/01/ $ /0 339

2 Fig. 1. Fabrication process and the basic structure of our laminated device. a) Polymer film spin-coated on a pre-cleaned ITO substrate, b) electrode deposition, c) lift-off process, d) lamination of the anode and cathode part, e) complete device structure, f) detailed configuration of laminated device. transferred to the flexible adhesive strip, forming a half-device containing the cathode and the luminescent polymer. This process, called the TAS process, creates a unique polymer surface with the desired physical property for the lamination process. Finally, the two components were laminated together by placing component C on top of component A on a pre-heated hotplate at 60 C, which is much lower than the typical temperature used for the thermal process of luminescent polymers. A minor pressure was applied for a few seconds to ensure a good contact. This lamination process can be completed within just a few minutes. Fig. 2. The I±L±V cures of the laminated PLED. The insert shows the logarithmic scale of the I±L±V curve. The device area is ca cm 2. The active material used for both component A and C is MEH-PPV Homo-Junction Laminated PLEDs Poly(2-methoxy-5-(2 -ethylhexoxy)-1,4-phenylenevinylene) (MEH-PPV) was first chosen as the luminescent material to demonstrate the lamination process for PLED fabrication. Components A and C were fabricated according to the above procedure. MEH-PPV was used as the active layer in both component A and C. The total thickness of the active layer in the laminated device was around 900 Š, which is close to the thickness of the active layer in a single-layer device. The I±L±V curves are shown in Figure 2. The charge injection occurs at 1.6 V, and the device turned on below 2 V. This is consistent with the device characteristic using traditional fabrication methods. The electroluminescence (EL) spectrum of this device, as shown in Figure 3, is similar to that of the single-layer device. The efficiency of the laminated devices using MEH- PPV as the active material can be as high as 0.34 cd/a at 6 V with a rectification ratio of The illumination of a PLED, as shown in the top inset of Figure 3, was found to be uniform across the device as examined by optical microscopy. This indicates that the charge injection and recombination occurred within the whole pixel and that the properties of the interface are uniform across the device. The bottom inset in Figure 3 Fig. 3. EL spectrum and photographs of a MEH-PPV-based laminated PLED. The upper insert shows a uniform single pixel, the bottom insert a 2 3 x±ybased passive device. shows the photograph of a 6 pixel (2 3) laminated device. This demonstration suggests that this technology can be extended to the fabrication of x±y-based passive displays. To the best of our knowledge, this is by far the best result obtained for a laminated PLED. Another experiment was conducted to compare the TAS process to the traditional plastic lamination process. MEH- PPV was spin-coated on two substrates containing ITO/PED- OT anode and a LiF/Al (or Ca/Al) cathode, respectively. These two substrates (half-devices) were then laminated using the traditional high-temperature lamination process. However, we were unable to obtain any meaningful results. The injection current was very low with no light emission. This clearly shows that the absence of proper surface treatment yields a poor interface between the two half devices. 340 Ó WILEY-VCH Verlag GmbH, D Weinheim, X/01/ $ /0 Adv. Funct. Mater. 2001, 11, No. 5, October

3 In the above comparison experiments, the major difference was the interfacial treatment prior to the lamination process. Due to the highly rough surface of the ITO substrate, as seen from the atomic force microscopy (AFM) images in Figure 4, it is anticipated that the interfacial contact area between the MEH-PPV thin film and the ITO is very high. During the liftoff of the MEH-PPV film, the ITO substrate (Fig. 4a) serves as the template for obtaining the surface roughness in the MEH- PPV film. The profile of the roughness in the MEH-PPV film is the mirror image of the ITO surface roughness. This is illustrated by comparing the AFM images of component C using MEH-PV lifted off a glass (smooth) substrate (Fig. 4b) and an ITO (rough) substrate (Fig. 4c). The surface image of a MEH- PPV film obtained by spin coating onto an ITO/PEDOT substrate is also shown for comparison (Fig. 4d). Through AFM analysis, the surface image of the MEH-PPV film (component C) in Figure 4b shows a much smoother surface than that in Figure 4c, which replicates the spikes on the ITO substrate it was deposited on. But there are no distinct features visible on the surface of the spin-coated MEH-PPV film as can be seen in Figure 4d. As we further examine the root mean square roughness calculated by AFM in Table 1, the roughness in Figure 4c shows a much higher value than those in Figure 4b and d, and close to the ITO substrate. Moreover, the unique surface properties resulting from the TSA process are also reflected in the water contact angle measurements listed in the same table. After the TSA process, the water contact angle on a spin-coated MEH-PPV film decreases by more than 15, this observation suggests that either the surface property has been modified by the TSA process, or the creation of very fine-scale Table 1. Root mean square roughness calculated by AFM and contact angle measurements of surface profiles corresponding to Figures 4a±d. [a] The contact angle depends on the pre-treatment of the ITO substrate. roughness on the surface. It was also observed that the activated surface was noticeably tackier after the lift-off process, which tends to easily stick onto other organic component. Unfortunately, it is beyond our instrumentation capability to quantitatively characterize this adhesive property. This suggests that the lift-off process is more than a pattern transfer process. It is strongly suspected that the lift-off process also plays a role in activating the surface of the MEH-PPV film by creating nano-scale polymer chains that stick out of the surface like tiny antennas. This is consistent with our contact angle measurements. The rough and activated surface topology can provide many benefits for the fabrication of the laminated device. Firstly, due to the high surface area in the MEH-PPV interface, the injected charges have a greater likelihood of crossing over the laminated interface. [14] Secondly, it is quite likely that the spikes in the polymer surface facilitate the enmeshing of the polymer chains at the interface during the short laminating annealing, resulting in a better interface. This step results in a tremendous improvement of the interface compared to polymer films without the TAS treatment. Thirdly, since the activated surface (cathode part in Fig. 1) is very sticky, no additional adhesives are required during lamination. Our lamination process requires a much lower temperature and mechanical pressure than that used for the traditional lamination process Hetero-Junction Bi-Layer Laminated PLEDs Fig. 4. AFM tapping mode images of a) surface of pre-cleaned ITO, b) surface of MEH-PPV film with thermally evaporated metal electrode (component C) after lift-off from the glass substrate, c) surface of MEH-PPV film with thermally evaporated metal electrode (component C) after lift-off from the ITO substrate, and d) surface of MEH-PPV film spin-coated on an ITO/PEDOT substrate. In addition to its simplistic fabrication, the other advantage of plastic lamination is that it allows the independent fabrication of the anode and cathode parts. This concept creates a new approach to fabricating polymer electronic devices. It can eliminate the troublesome issue of solvent compatibility with the different organic layers in the fabrication of multi-layer devices. We illustrate this advantage by fabricating a bilayer PLED consisting of poly(9,9-bis(octyl)-fluorene-2,7-diyl) (BOc-PF) and N,N -diphenyl-n,n - bis(l-naphthyl)-1-1 -biphenyl-4,4²-di- Adv. Funct. Mater. 2001, 11, No. 5, October Ó WILEY-VCH Verlag GmbH, D Weinheim, X/01/ $ /0 341

4 amine (a-npd). BOc-PF is a very good blue-emitting material, but is usually used along with a hole-transporting layer for achieving balanced charge injection and increasing the quantum efficiency. [15] a-npd, an aromatic diamine has been widely used as a hole-transporting material in organic light emitting diodes. [4,16] However, a-npd is also soluble in the organic solvents used to process BOc-PF. Hence, the organic solvent used for BOc-PF can potentially wash away the underlying hole-transporting layer during the spin-coating process, which impedes the fabrication of bilayer PLEDs. Hence without blending, the excellent properties of many of the ideally suited hole-transport layers for BOc-PF, such as a-npd, can hardly be realized. [17] We fabricated hetero-junction bilayer PLEDs using a similar process as shown in Figure 1. A 300 Š layer of a-npd was deposited on a pre-patterned ITO/PEDOT surface, which served as the anode-part. The cathode-part consisting of BOc- PF/LiF/Al was prepared using the TAS process. The anode and the cathode part were then laminated together to form a PLED. When biased, this laminated PLED uniformly emitted blue light between 425±450 nm, with a turn-on voltage of around 5 V. The EL spectrum and the device photograph of the laminated BOc-PF/a-NPD device are shown in Figure 5. The EL spectrum of a single layer BOc-PF device fabricated by the traditional PLED process and the photoluminescence (PL) spectrum of a-npd are also shown in the same figure. From the above luminescence spectra, it is most likely that the holes injected into the a-npd layer, cross over the interface to recombine with electrons near the interface or in the BOc-PF layer. The I±L±V curves of the laminated device are shown in Figure 6. The brightness of the device is about 700 cd/m 2 at 15 V with a maximum efficiency of 0.26 cd/a. This performance is as good, if not higher than the published data for the polyfluorene-based, blue-emitting PLEDs fabricated by the traditional method. [18] T.-F. Guo et al./polymer LEDs by Low Temperature Lamination Fig. 6. The I±L±V curves of the BOc-PF-based laminated PLED. The insert shows the I±L±V curve on a logarithmic scale. The component C in this device is BOc-PF, while component A is a-npd. 3. Conclusions In summary, we have successfully demonstrated the template activated surface process for enhancing the interface property of the laminated polymer device. We demonstrated a laminated MEH-PPV-based PLED (homo-junction) and a laminated bilayer polyfluorene PLED (hetero-junction) using this method with excellent device performance. The TAS method appears to be a promising approach for the independent fabrication of the anode and the cathode part of the device that can be later laminated together to complete the device. Even though a promising pathway has been realized for fabricating laminated PLEDs, several issues need to be investigated. Since our lamination process is carried out in a nitrogen environment, it is anticipated that a considerable amount of nitrogen is trapped in the interface. A vacuum laminated system should significantly improve our device performance. The current lift-off process needs to be replaced by a direct surface activation method to create the high surface area needed for the lamination process. It could be done, ideally, by placing a roller on top of the fresh finished polymer film and activate the surface. Finally, the lamination process discussed in this manuscript can be used as the final step in a continuous and large area polymer electronic device fabrication process. An ideal fabrication scheme is shown in Figure 7. Fig. 5. EL spectra of a BOc-PF-based laminated device and the EL and PL spectra of a single-layer BOc-PF device fabricated by the traditional PLED process and a-npd, respectively. The insert shows a photograph of the blue-emitting laminated PLED. Fig. 7. An ideal fabrication scheme for a continuous polymer coating process combined with the lamination process. Here, a surface activated process is introduced by a roller with desired function. 342 Ó WILEY-VCH Verlag GmbH, D Weinheim, X/01/ $ /0 Adv. Funct. Mater. 2001, 11, No. 5, October

5 4. Experimental The basic device structure of a traditional PLED is a spin-coated MEH-PPV thin film sandwiched between an anode and a cathode. Usually, a bilayer anode, PEDOT/ITO, and a bilayer cathode, Ca/Al, are used as the anode and cathode respectively. Due to the high reactivity of Ca with oxygen and moisture, a LiF/Al bilayer cathode was instead used for the AFM analysis. The detailed device fabrication procedures of typical PLEDs can be found in reference [19]. For the formation of laminated PLEDs, the process consists of the fabrication of the anode part and the cathode part and the subsequent low temperature lamination process. For the anode part, a polyethylene terephthalate (PET) or glass with patterned ITO (2 12 mm) were used as the substrate. A thin layer of PED- OT (Bayer Corp. V4070) was spin coated on top of the substrate to form the bilayer anode. Finally, a thin layer of active material, such as MEH-PPV for the homo-junction device or a thin layer of a-npd layer (300 Š) for the hetero-junction device, was coated on top of the ITO/PEDOT anode. For the cathode part, a conjugated luminescent polymer solution, such as MEH-PPV or BOc-PF, was spin-coated onto a pre-cleaned ITO substrate. As described in the results section, the ITO substrate was used as the mold for the formation of the high roughness surface of the polymer. Finally, LiF/Al cathode was thermally evaporated onto the polymer layer at 10 ±6 torr vacuum to finish the cathode part. The finished anode part was then placed on a hot plate at a baking temperature of 60 C for 10 min before the lamination. In the meantime, a piece of Scotch tape (3M Magic tape 810) was placed on the top of the finished cathodepart, and a proper pressure was applied to ensure a good contact between the Scotch tape and the cathode-part. Subsequently, this tape was removed such that the cathode and the polymer luminescent film was lifted off from the ITO/glass substrate and transferred to the Scotch tape, forming a half-device containing the cathode and the luminescent polymer. Finally, these two components were laminated together by placing the cathode part on top of the warmed anode part. This last step can be completed within just a few minutes with a minor pressure applied on the device for a few seconds to obtain a better contact. The total thickness of the device was controlled to be around 900±1100 Š. The cross section of the metal electrode and the ITO stripe is the pixel, its area is 4 mm 2. The current±brightness±voltage (I±L±V) measurements were carried out using a Keithley 236 source-measure units and Keithley 195 digital multimeter along with a calibrated silicon photodiode functioning as optical sensor. The PL and EL emission were measured with an S2000 fiber optic spectrometer (Ocean Optic Inc.) and Spectra-scan PR 650 respectively. The thickness of the polymer film was determined using an Alpha-step profilometer. Contact angles were obtained at room temperature with a contact angle meter from March Instruments, Model Cam-F1. All the AFM images (512 pixels wide) of the different polymer surface were taken with a Digital Instruments NanoScope III operating in tapping mode in air. The cantilevers were 125 lm long etched Si probes. The images were automatically plane-fitted to account for the sample tilt. The optimization of the tip± sample forces is the main issue of most AFM measurements of polymers. Low force imaging, which means a small tip±sample contact area will be beneficial for high-resolution imaging of polymers. Tapping at elevated forces is extremely useful for the compositional imaging of heterogeneous polymer systems. 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