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The combined deposition of structured and homogenous layers for the device setup of OLEDs results in a structured display visible with a gray scale impression. These permanently imprinted structures are only visible during the application of an electrical field and almost invisible in the off state. Most intriguing applications of such devices are security marks, electronic watermarks but also for advertising and design applications the creation of gray scale structures might have interesting applications.
©2012 Optical Society of America
1. Introduction
Since the discovery of the electrically induced luminescence [1,2] of organic materials OLEDs are broadly investigated for display and lighting applications [3]. Most efficient materials consist of small molecules which are applied by high vacuum deposition processes [4]. Polymeric materials have attracted high interest due to their solution processibility which opens applications in the low cost area through cost effective printing processes [5]. The display of structures requires addressing in the passive or active matrix mode, fixed structures such as icons can be introduced by structuring the electrodes as achieved by photolithography of the ITO and masking of the evaporated cathode. The approach described in this paper uses non structured electrodes, for the display of information a thickness controlled deposition of the active materials is achieved by multilayer printing [6].
Security is an important issue in many fields such as brand protection as well as ID protection. Polymeric materials have been discussed for the implementation for counterfeit protection based on optical effects (gratings, holograms), surface modifications [7,8] or the implementation of electronics such as RFID based on either CMOS or organic based electronics [9,10]. In a cooperation of Samsung Technologies and the Bundesdruckerei the integration of an active matrix driven display in an ID-card has been shown [11]. The different approaches in protection require more or less advanced technologies resulting in differing costs for their implementation in the production process. Highly advanced technologies like the integration of an AM-OLED into a document can only be envisioned for highly advanced products like personal ID-cards or passports [11]. The multilayer printing technology of OLEDs presented in this paper could act as an alternative method to integrate images or signage signs as electronic security elements for brand protection or ID-cards.
2. Experimental
OLEDs have been processed on 50 x 50 mm2 ITO-glass substrates which had a four pixel structure with pixel sizes of 0.85 cm2 in the center of the substrate. The substrates have been cleaned using Deconex bath and O2-plasma and were subsequently transferred into inert atmosphere. PEDOT:PSS has been used as hole injection layer and different commercially available emitting polymers (Livilux series, Merck Chemicals) in the colors red, green blue and white were used as active emitting material. The processing of the different layers has been achieved by either spin coating or inkjet printing.
As an example the first layer of PEDOT:PSS has been homogenously coated using Clevios AI 4083 from Heraeus by either spin coating or inkjet printing. The commercial water based solution has been modified with suitable additives like isopropanol or ethyleneglycol for improved wetting and jetting properties. After a short drying period at 100 °C a second layer of PEDOT:PSS is applied via inkjet printing allowing a structured deposition. Most experiments described here have been performed with commercial singlet emitting polymers from Merck Chemicals (Livilux) of the SPG, SPR, SPB and SPW series, which were dissolved either in chlorobenzene or toluene. The active layer can either be coated by inkjet printing or spin coating. After each coating step a drying process has to be applied according to the requirements of the materials used. On glass substrates typically 180 °C for 15 min is applied for drying the hole injection layer whereas the drying temperature for the active material varies between 130 and 180 °C. The application onto flexible substrates is also possible, in this case drying temperatures and times need to be adjusted according to the temperature stability of the substrate.
3. Results and discussion
Light emitting devices are usually built on glass or plastic substrates equipped with a transparent electrode, most often indium tin oxide (ITO). For enhancement of the performance of the OLED a hole injection layer is deposited on the ITO before the active emitting material is applied. The cathode consists of a low work function element like calcium, barium, aluminum or magnesium/silver. In the approach given in this paper after the planar coating of the hole injection layer a second layer of the hole injection material is deposited by inkjet printing using a defined gray scale picture. A schematic scheme of a typical device setup is displayed in Fig. 1 .
Fig. 1 Schematic device setup (shown without backside encapsulation, not drawn to scale). The structured inkjet printed layer could either be a hole injection layer or an emitting layer (drawn in red), the hole injection and emitting can be either spin coated or applied by other uniform coating techniques such as inkjet printing or slot die coating.
After the structured printing the active layer is again coated as a homogenous film either by spin coating or inkjet printing. This process is followed by the deposition of the cathode and the encapsulation of the device. As examples two devices are shown in Fig. 2 displaying the abbreviation of the Institutes name and a detail of a simple icon. For both displays PEDOT:PSS has first been applied by inkjet printing as a homogenous layer with a typical layer thickness of 80 to 100 nm followed by the second layer PEDOT:PSS printed using a layout file based on the given structures. In both cases the layer of the red emitting polymeric material is applied by spin coating. In principle, the structure can also be visualized by twofold printing of the emitting layer. In this case, the two processes need to be adjusted such that the dissolving of the first layer is prevented during the second step by e.g. using orthogonal solvents for the deposition of the two layers.
Fig. 2 Multi-layer printed OLED in the on mode, the structure is permanently written in by inkjet printing a second hole injection layer. In the off-mode the structure is not visible. The left hand picture shows the logo of the Institute, the right hand picture shows a detail of a sketch. Around the printed contours (blue arrows) several points (black arrows) are visible showing failure of the print head with not accurately printing nozzles leading to a contrast outside the wanted contour lines of the image layout.
Through the multilayer printing including one structured layer thickness differences in the individual layer lead to the grey scale contrast as visualized in Fig. 2. This structure is only visible upon application of an electrical field while in the off mode no structure is visible. Around the printed lines as visible in the detailed enlargement of a simple icon structure a number of point defects are visible outside the actual picture. These result from not accurately printing nozzles due to nozzle clocking or nozzle printing at wrong angles.
As an example for a security application an imprinted personal picture is shown in Fig. 3 and Fig. 4 in different colors. Such a feature e.g. in a smart card could act as invisible security sign for the user, which would be hard to copy or counterfeit.
Fig. 4 Printed image with a green emitting layer in the on- (left) and off-mode (right). Almost no contrast variations are visible in the off-state, in contrast to the pixel structure resulting from the photolithographic structuring of the ITO-electrode.
The fabrication of devices in the fashion described above has several advantages. First, no substrate treatment is required except the cleaning process. The grey scale image is written in by inkjet printing; layout, contrast and resolution are easily controlled by the printing process but also limited by it with respect to the resolution and the limitation to printable inks. The layout can be transferred to the printer using e.g. CAD files but also image files like jpg or tiff can be implemented. For changing the layout of the device no substrate patterning is required, only the new print file needs to be loaded. The OLED characteristic is not influenced by this device setup, thus requirements with respect to driving schemes and power supply are the same as for conventional OLEDs of the same composition of materials.
The printed image is only visible in the on-mode, thus such devices are well suited for security applications. Images such as those shown in Fig. 3 and Fig. 4 could be implemented in personal documents such as smart cards or ID-cards as an additional security feature for the certification of the user. Logos or signs could act as electronic watermark. The advantage of the use of OLEDs lies in the advanced processing technique which is only accessible to selected producers limiting the possibility of imitation. The manipulation of the display will lead to its destruction, so the misuse of documents containing such a display could be largely excluded. The simple processing technology as compared to the production of passive matrix or active matrix OLEDs makes it easy to integrate in a production process at considerably lower costs.
4. Conclusion
The investigations in this paper give a new procedure for the implementation of security features based on organic light emitting diodes. This exhibits a high degree of safety to the technological requirements for the implementation of the structure which complicates the manipulation or imitation. A change of the image is not possible without its destruction. Besides the application of this printing technique for security issues its use in the advertisement industry could be of interest including grey scale images of products e.g. in shelf displays to gain a higher attraction of the products.
Acknowledgments
Financial support by the Fraunhofer Society, the States of Berlin and Brandenburg as well as of the Bundesdruckerei in the Framework of the innovation network cluster “Sichere Identität” (Secure Identity) is gratefully acknowledged.
References and links
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