Fabrication of a laser patterned flexible organic light-emitting diode on an optimized multilayered barrier Sanjeev Naithani,1,* Rajesh Mandamparambil,2,3 Henri Fledderus,2 David Schaubroeck,1 and Geert Van Steenberge1 1
Centre for Microsystems Technology (CMST), imec, and Ghent University, Technologiepark 914A, B-9052 Ghent, Belgium 2
TNO/Holst Centre, High Tech Campus 31, 5656AE Eindhoven, The Netherlands
3
Department of Mechanical Engineering, Eindhoven University of Technology, 5612 AZ, The Netherlands *Corresponding author: [email protected] Received 24 October 2013; revised 20 March 2014; accepted 21 March 2014; posted 21 March 2014 (Doc. ID 199808); published 17 April 2014
The fast-growing market of organic electronics stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning organic thin films, but only a few studies report on interaction with thin-film barrier layers. In this paper, we present an approach in which the laser patterning process is optimized together with the barrier film, leading to a highly selective patterning technology without introducing barrier damage. This optimization is crucial, as the barrier damage would lead to moisture and oxygen ingress, with accelerated device degradation as a result. Following process optimization, a laser processed flexible organic LED has been fabricated and thin-film encapsulated and its operation is shown for the first time in atmospheric conditions. © 2014 Optical Society of America OCIS codes: (220.4610) Optical fabrication; (240.0310) Thin films; (140.3390) Laser materials processing. http://dx.doi.org/10.1364/AO.53.002638
1. Introduction
Organic electronics is an emerging field where rapid advances are made in finding novel applications due to its inherent advantages, such as flexibility, stretchability, and adaptability toward roll-to-roll production. The invention of the first organic LED (OLED) in 1987 [1] generated much research attention for developing efficient materials and fabrication techniques for OLED devices [2–4]. In lighting applications, flexible OLEDs have significant advantages over conventional incandescent or fluorescent lighting devices due to their light weight, improved 1559-128X/14/122638-08$15.00/0 © 2014 Optical Society of America 2638
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durability, higher impact resistance, inherent flexibility, and energy efficiency (up to 150 lm∕W). Moreover, these devices are environmentally friendly (no heavy metals involved) and cost effective (roll-toroll fabrication). Nowadays, the research in this area is mainly focused on the development of generic technologies for cost-effective roll-to-roll production of flexible OLEDs [5]. One of the major challenges in fabricating reliable flexible OLEDs is to maintain higher device lifetimes by protecting the device from moisture and oxygen permeation [6,7]. In addition, the fabrication of an OLED on a flexible substrate is rather complicated, as it must be encapsulated from top and bottom sides, contrary to glass-based OLEDs. In such a case, the role of a flexible moisture barrier that has similar
optical properties as that of glass is vital for flexible OLEDs. In order to fabricate the flexible OLEDs in a roll-to-roll manner, different postpatterning techniques of active device layers are considered, of which laser ablation is favored. Laser ablation has several inherent advantages, such as noncontact, dry processing, high speed, less environmental pollution (no hazardous wet chemicals), and high accuracy, that make it suitable for roll-to-roll production on flexible substrates. The use of lasers for the fabrication of these devices implicates that a selective ablation process of optically thin organic device layers has to be developed. The influence of the optical absorption properties of the barrier layer, in the selective removal of the thin organic films like PEDOT:PSS or light-emitting polymer (LEP), is crucial in the process of patterning OLED layers. A polymer-based OLED consists of an organic LEP as an emissive layer, which is sandwiched between an anode and a cathode layer. Generally, an indiumtin oxide (ITO) layer is used as anode. A metal layer like Ba–Al (barium with aluminum cap) can be used as cathode. Furthermore, a layer of conducting PEDOT:PSS is commonly used as a hole injection layer in between the anode and the emissive layer. PEDOT:PSS with an integrated bus bar structure for the fabrication of flexible OLED using laser ablation has been reported [8]. Moreover, PEDOT:PSS can also be used as an anode and as a hole injection layer completely replacing ITO. PEDOT:PSS and LEP have classically been deposited using spin-coating technologies or inkjet printing. Spin coating results in significant quantities of material waste. Inkjet printing has been largely investigated for small area substrates. However, in case of large area substrates, large amounts of material have to be deposited. Hence, the layer flatness and deposition speed are questionable [9,10]. Homogeneity and high overlay accuracy of the PEDOT:PSS and LEP are essential for the functionality of an OLED, which can be attained by using large area deposition techniques for high-volume production. Slot-die coating can be considered as a fast, simple, and cost-effective deposition technique for both PEDOT:PSS and LEP layers in a roll-to-roll fashion. However, the slot-die coating requires a postpatterning technique among which laser ablation is a very suitable candidate. Large area deposition of stacked layers followed by laser processing can preserve better layer homogeneities compared to patterned deposition techniques mentioned before. Proper power density adjustment and wavelength/pulse width selection of the laser beam has to be effectively utilized for selective removal of different thin layers, which is crucial during OLED processing [11–17]. In this paper, we demonstrate a fully functional laser patterned OLED on a flexible polyethylene naphthalate (PEN) substrate with a multilayered barrier stack. Various multilayered barrier foils with different optical absorption properties are investigated, and laser processing parameters are
optimized to obtain a selective ablation process. The challenge is to selectively remove a thin-film organic layer, deposited on top of an inorganic layer, without any damage or (organic) leftovers present. The ablation process windows for different barrier foils have been compared and the barrier foil with the widest working process window has been used for the fabrication of the OLED stack in the second stage of experiments. An industrially qualified 248 nm krypton fluoride (KrF) excimer laser is used to structure and pattern flexible OLED stack layers. Subsequently, functional devices are fabricated and encapsulated using thin-film technology. A flexible substrate that is a 125 μm thick PEN is laminated on a 6 in: × 6 in: glass carrier (1.1 mm thick) with a thermally activated release layer. On top of the laminated PEN substrate, a multilayer barrier stack is deposited with various deposition techniques. The device layers are then spin coated on the barrier stack on which selective laser ablation has been carried out. The samples were fabricated in the same way for five different types of SiN layer of barrier stack. In order to find the most suited inorganic barrier layer for laser processing of thin-film organic device layers such as PEDOT:PSS and LEP, ablation experiments were carried out. As a consequence, a fully functional flexible OLED was fabricated and its operation has been demonstrated under environmental conditions. This is a verification of the selective laser process investigation for the roll-to-roll production of flexible OLEDs, and further lifetime testing will be the focus of future work. 2. Experimental Work and Methodology A. Sample Preparation
Three different types of thin-film stacks were prepared as shown in Figs. 1(a)–1(c). The thin-film stack in Fig. 1(a) consists of 125 μm PEN as a flexible substrate on top of which a multilayered barrier consisting of 150 nm inorganic silicon nitride (SiN) was deposited by chemical vapor deposition.
Fig. 1. Thin-films stack on flexible PEN substrate and multilayered barrier, prepared for the investigation of barrier influence on patterning (a) PEDOT:PSS, (b) LEP, (c) LEP and PEDOT:PSS, at the top. (d) A detailed multilayered barrier. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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Subsequently, a thin PEDOT:PSS layer (100 nm) was spin coated on the barrier stack. Next, Fig. 1(b) shows the same layered configuration of a PEN substrate and barrier foil except the top layer is an active conjugated LEP with a thickness of about 80 nm. These two stacks were prepared to investigate the ablation behavior of the PEDOT:PSS and LEP on different barriers separately. The third thin-film stack, as shown in Fig. 1(c), was prepared in order to fabricate a flexible OLED. A LEP layer (80 nm) was spin coated on top of PEDOT:PSS (100 nm), while the flexible substrate PEN is identical to the previous two cases. A detailed schematic of the multilayered barrier consisting of organic and inorganic layers is illustrated in Fig. 1(d). B.
Characterization of Absorption Spectra
The absorption spectra of PEDOT:PSS and LEP were characterized before ablation experiments. These optical absorption spectra were measured by depositing the polymer layers, with the same thicknesses as required on the stack (80 nm LEP and 100 nm PEDOT: PSS), on a quartz substrate of 2 nm × 2 nm. The absorption spectra were obtained from a UV–Vis spectrometer (JASCO), which is able to measure below 300 nm. C.
Experimental Ablation Setup
Ablation experiments were performed with a pulsed, nanosecond KrF excimer laser at 248 nm with a pulse repetition rate of 100 Hz. The laser beam is passed through an attenuator plate and then a square mask of 2000 μm × 2000 μm as shown in the schematic experimental setup diagram (Fig. 2). In these experiments, a focusing lens of demagnification 10 is used, which is adjustable in the vertical direction, resulting in a spot size of 200 μm × 200 μm on the sample. The fluence is controlled with an attenuator plate, and the pulse energy at each fluence is measured by a pyro-electric energy meter (Coherent J25LPMUV, in combination with FieldmaxII TOP) placed at the end of the beam axis. The samples were mounted on a motorized automated translation stage, perpendicular to the beam with thin film facing the incident laser beam. The laser ablation of the hole transport layer (PEDOT:PSS) and active layer (LEP) is carried out under atmospheric conditions. The ablation setup consists of a suction head that removes the generated ablation
Fig. 2. Schematic diagram of the experimental setup for layer patterning. 2640
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by-products from the process site, leading to a controlled process. Two laser patterning steps involving organic thinfilms PEDOT:PSS (∼100 nm) and LEP (∼80 nm) have been carried out. First, in order to determine the influence of the top inorganic SiN barrier layer on the patterning behavior of thin organic films, experiments were carried out by ablating PEDOT: PSS and LEP on five different barrier stacks labeled as SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5). Afterward, selective patterning and structuring of the OLED layers were performed on an optimized barrier stack. There are two challenges to be considered for the realization of a laser patterned device. First, to selectively ablate the organics without damaging the inorganic SiN barrier layer, and second, to protect the bus bar, which is a multilayer stack of chromium and aluminum (Cr–Al–Cr) beneath, along this ablation process. The detailed schematic of the device layers with necessary involved laser process steps is given in Fig. 3. The PEDOT:PSS and LEP laser patterning steps are validated on a 15 cm × 15 cm flexible OLED device platform incorporating nine individual OLED devices. Table 1 summarizes a detailed overview of thicknesses and processing methodology of various layers. The hole injection layer (PEDOT:PSS) is deposited on top of the barrier stack by spin coating from an aqueous solution followed by a heating step for solvent removal. Laser patterning of this layer leads to electrical contact separation between anode and cathode, as well as top barrier encapsulation structures. Subsequently, LEP is deposited by spin coating followed by laser patterning, which removes the layer at specific regions for device encapsulation. The device layers are then covered by a low work function Ba cathode layer capped with Al. Finally, a barrier layer is deposited on the whole OLED stack to form the top encapsulation, which will hermetically seal the fabricated device. D.
Depth Profiling
The inspection and analysis of the samples after experiments were performed by using an optical microscope, a mechanical profiler (Tencor Alphastep 200), and a noncontact optical (Wyko NT3300) white light
Fig. 3. Flexible OLED layered structure and various selective ablation steps for patterning: ablate PEDOT:PSS on (A) a barrier and on (B) a bus bar; ablate LEP on (C) the barrier and on (D) the bus bar.
Table 1.
Layer Ba-Al LEP (Merck Livilux) Cr–Al–Cr PEDOT:PSS (Agfa Orgacon) SiN PEN
Layer Composition, Thicknesses, and Deposition Methods of the OLED Stack
Role in OLED Stack
Thickness
Deposition
Cathode Active emissive layer Bus-bar Hole injection/anode Top layer of multilayered barrier Flexible substrate
100 nm 80 nm 150 nm 100 nm 150 nm 125 μm
Evaporation Spin coating Sputtering Spin coating Plasma deposition —
interferometer. The crater depths were determined with mechanical and optical profilers. Prior to these measurements, the samples were transferred on a sticking surface (PDMS) to minimize the curvature. In the case of optical profiling, the stack of transparent layers might result in multiple reflections. Therefore, a reflective Au layer (roughly 25 nm) is applied on top of the stack using a plasma magnetron sputter coater. 3. Results and Discussion A.
Absorption Spectra
In order to investigate the influence of barrier absorption on the selective ablation processes, five different types of SiN inorganic barriers were evaluated. The absorption spectra of the inorganic barrier foils were characterized with a UV–Vis spectrometer as illustrated in Fig. 4. These barrier foils were referred to as SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5). Figure 4 shows that a significant difference in absorption at 248 nm (wavelength of operation) can be observed for five different barrier foils. The order of absorption can be clearly observed at 248 nm, that is, SiN1 < SiN2 < SiN5 < SiN3 < SiN4. A variation in the ablation behavior of LEP and PEDOT:PSS on these different SiN barriers is expected because of the difference in their absorption. Hence, we prepared samples for each of the 3 thinfilm stacks depicted in Figs. 1(a)–1(c) with 5 different barriers in each case, leading to 15 samples in total.
Fig. 4. Optical absorption spectra of different SiN barrier foils, indicating significant difference in the absorption at the wavelength of operation, i.e., 248 nm.
The optical absorption spectra of thin organic films LEP and PEDOT:PSS as obtained are shown in Fig. 5. From the spectral information, it is observed that LEP absorbs quite heavily from 200 to 450 nm. However, PEDOT:PSS is almost transparent for the wavelengths higher than 300 nm. This means that the selection of a laser source for the ablation of these two layers in the nanosecond pulse regime lies in their absorption band. The natural choice would be either 193 or 248 nm excimer lasers, because of the better absorption of these wavelengths by thin polymer films. Since 248 nm excimer lasers are well industrialized, this laser has been chosen for these experiments. B. Barrier Damage Thresholds
In the first stage of this research, experiments were conducted on samples covered with 100 nm PEDOT: PSS, 80 nm of LEP, and LEP on PEDOT:PSS (180 nm), as shown in Fig. 1. The laser fluence was varied from 75 mJ∕cm2 to 250 mJ∕cm2 . For each value of the fluence, the number of laser shots was varied by changing the speed of the automated translational stage. These experiments were carried out with one pulse, two pulses and five pulses per location. Experiments were performed on different types of barrier foils [SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5)], and the depths of the ablated craters were measured. Since the ablation threshold of the bus bar (Cr–Al–Cr) is quite higher than that of organic thin films (PEDOT:PSS, LEP), it is not very relevant to discuss the ablation of bus bar in more detail.
Fig. 5. Optical absorption spectra of the thin organic films indicating higher absorption by LEP compared to PEDOT:PSS at 248 nm. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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Fig. 6. Selective ablation for PEDOT:PSS on two different kinds of barrier stacks: least absorbing SiN(1) in (a) clean removal, (b) and onset of damage at overlapping; and highest absorbing SiN(4) in (c) clean removal, and (d) start of damage at edges.
It was observed that the variation in the absorption properties of the barrier has a major influence on the patterning of these particular thin films. Figure 6 illustrates optical microscopic images after patterning of PEDOT:PSS thin films on two barrier stacks, SiN(1) and SiN(4). SiN(1) is the least absorbing and SiN(4) the most absorbing barrier foil at 248 nm (Fig. 4). In case of SiN(1), a clean selective removal of PEDOT:PSS is possible with a fluence of 125 mJ∕cm2 with two pulses [Fig. 6(a)]. Furthermore, there is noticeable damage starting at a fluence of 200 mJ∕cm2 with two pulses, particularly at the overlapping regions. These observations of clean ablation as well as detectable damage at overlapping regions on barrier foils have been verified by analyzing the crater depths. In a similar fashion, in the case of SiN(4), clean removal of the PEDOT:PSS is depicted in Fig. 6(c). In this case, it is interesting to note that even at a fluence of 75 mJ∕cm2 with two pulses per location [Fig. 6(d)], there is detectable damage at the edges. These results suggest that PEDOT:PSS can be selectively removed on SiN(1) barrier foil at a fluence ranging from 125 mJ∕cm2 to 200 mJ∕cm2 . However, it is difficult to find a selective patterning process window for SiN(4) barrier foil, as the damage starts even at fluence lower than 75 mJ∕cm2 with two pulses. The ablation behavior of LEP, PEDOT, and LEP on PEDOT:PSS for five different types of multilayered
Table 2.
SiN barrier stacks has been studied, and the damage thresholds of various SiN barriers are determined. A detailed comparison of damage threshold of various SiN layers for different organics is illustrated in Table 2. From Table 2, it is clear that SiN(1) has the highest value of the damage thresholds and therefore it has the widest process window for the selective ablation of PEDOT:PSS, LEP, and, LEP on PEDOT:PSS. However, SiN(4) has the lowest values of damage thresholds compared to other SiN barrier foils, implying the most narrow process window. These values are in agreement with the absorption spectra of respective SiN barrier foils (Fig. 4). Hence, in order to fabricate the OLED device, SiN(1) barrier foil has been chosen for the next stage of experiments. In Fig. 7, the onset of layer removal starts only beyond a laser fluence of 200 mJ∕cm2. These results show that for selective patterning of organic thinfilms PEDOT:PSS and LEP on SiN(1) barrier, the ablation thresholds of thin films should be lower than 200 mJ∕cm2 . C.
Thin-Film Ablation Thresholds
Ablation experiments of PEDOT:PSS and LEP layers deposited on SiN(1) barrier stack showed almost identical thresholds for layer removal. Figure 8 shows that the threshold of layer removal in both cases is found to be 75 mJ∕cm2 from single pulse experiments. This information is important to define a process for patterning both layers in subsequent steps on the SiN(1) layer of the barrier. Since the threshold of ablation of SiN(1) barrier is found to be more than twice that of the fluence required for ablating any of these organic layers, there is a high probability of patterning these layers without significant damage of the underlying layer. Therefore, a wide working process window is available for selective ablation. Multipulse laser ablation studies at various effective fluences were carried out to determine the process windows. D.
Patterning Thin Films
In order to pattern large areas, multishot ablation is required. Laser multishot scribe tests were carried out at various pulse overlap schemes to optimize the process. Figures 9(a) and 9(b) show the multishot ablation of PEDOT:PSS and LEP layer on top of SiN(1) barrier foil, respectively. It is found that at a fluence of 125 mJ∕cm2 with two shots per location, the depth of the ablated PEDOT:PSS crater is about 100 nm,
Damage Thresholds for Different Types of Multilayered SiN Barrier Foils
Damage Threshold (mJ∕cm2 ) of SiN Layers with Two Pulses Top Layer(s) of Sample Stack (See Fig. 1) PEDOT:PSS LEP LEP on PEDOT:PSS
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SiN(1)
SiN(2)
SiN(3)
SiN(4)
SiN(5)
200 200 250
125 150 125
100 100 125
75 100 100
100 100 125
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Fig. 7. Graph of the ablation depth versus the laser fluence for multilayered SiN(1) barrier stack applying two pulses per location.
Fig. 8. Graph of the ablation depth versus the laser fluence for PEDOT:PSS and LEP thin films on multilayered SiN(1) barrier (single pulse).
without any visual detectable damage on the SiN(1) layer. Similarly, at 100 mJ∕cm2 with two shots per location, the depth of the ablated LEP crater is 100 nm without any detectable SiN(1) barrier damage. These mechanical profiler depth measurements are also supported by the depth measurements obtained from the noncontact optical profiler. Next, Fig. 10(a) shows the optical microscopic image of the patterned PEDOT:PSS layer at a laser fluence of 125 mJ∕cm2 with two pulses per location. On the right [Fig. 10(b)], the corresponding depth profile indicating the PEDOT:PSS layer removal of approximately 111 nm is shown. Similarly, Figs. 11(a) and 11(b) show the results for LEP removal. The pictures are representative images for removal of LEP using five shots per location at fluence of 100 mJ∕cm2. E.
Fig. 9. Multishot ablation of (a) PEDOT:PSS and (b) LEP on SiN (1) barrier indicating the depth measurement at various laser fluences.
Fig. 10. (a) PEDOT:PSS layer removed at a fluence of 125 mJ∕cm2 with two pulses per location on SiN(1) barrier. (b) Corresponding white light interferometer (Wyko) depth profile.
OLED Stack Patterning Process Flow
Sequential ablation steps of PEDOT:PSS and LEP are required for processing the OLED device. It is important to remove both the organic layers (LEP and PEDOT:PSS) from the SiN(1) barrier layer. After laser processing of the device layers, thin-film encapsulation is made on the top side by depositing
Fig. 11. (a) LEP layer removed at a fluence of 100 mJ∕cm2 with five pulses per location on SiN(1) barrier. (b) White light interferometer profile showing a depth of ∼89 nm and indicating the removal of LEP. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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OLED. Since LEP serves as an isolation layer between the anode and the cathode, there is no need to print an additional isolation layer. F.
Fig. 12. Process flow for laser patterned flexible OLED. Process 1: PEDOT:PSS is deposited by spin coating on flexible barrier substrate. Process 2: PEDOT:PSS is removed by laser process forming encapsulation ring and contact isolation. Process 3: LEP is deposited by spin coating. Process 4: LEP is removed from encapsulation ring and contact cleaning for top electrode deposition. Process 5: metal cathode (Ba-Al) is evaporated through a mask. Process 6: device is thin-film encapsulated.
identical SiN(1) layer over the whole device surface. This process should ensure that the top and the bottom SiN(1) barrier stack should form a hermetical seal. If the laser process leaves an incompletely removed layer, this acts as a moisture ingress path, which leads to damage of the device. Figure 12 is a detailed description of the process flow that has been followed for the fabrication of a flexible OLED device. After the uniform deposition of the PEDOT:PSS on the flexible barrier substrate (process 1), the first ablation step is the removal of PEDOT:PSS from the SiN(1) barrier layer and Cr– Al–Cr bus bar, forming a 2 mm wide track as shown in process 2. Next, LEP is deposited uniformly by spin coating over the entire sample surface (process 3). Afterward, a 1 mm wide track is ablated exactly in the middle of the 2 mm track removing LEP on SiN(1) and the Cr–Al–Cr metal bus bar simultaneously (process 4). Finally, the deposition of the cathode (process 5) and subsequent thin-film encapsulation (process 6) leads to a functional flexible
Flexible OLED Fabrication
As described in the process flow section above, both PEDOT:PSS and LEP layers are laser ablated in subsequent steps. Afterward, a top SiN(1) layer (encapsulation) is deposited so that it forms a hermetical seal with the bottom SiN(1) layer (barrier). Figure 13 demonstrates a laser patterned flexible OLED, which lights up on application of an electrical power supply. It can be seen from this figure that, after the device is turned on, no black spots are visible in the active region or dark fronts originating from the laser ablated regions. 4. Conclusion
The process investigation on selective ablation of thin organic films on a multilayered barrier foil has been performed. It is concluded that the influence of barrier absorption plays a significant role in the selective ablation process. The barrier with minimum absorption at the wavelength of operation has the widest working process window. After selection of an optimized barrier foil, feasibility of the laser process for patterning and structuring of various OLED layers has been investigated. Minimal debris field and layer delamination are observed in the ablated zone. Multipulse laser processes were optimized for the removal of both PEDOT:PSS and LEP organic layers on a flexible substrate containing a multilayer barrier stack. A functional flexible laser processed OLED device has been fabricated and its operation has been demonstrated under environmental conditions. 5. Perspectives
This work suggests future study where device performance on accelerated test conditions is evaluated for process optimization, which will be discussed in subsequent publications. The authors thank Mr. Steven Van Put for assisting in laser ablation experiments. This work is partly supported by the Research Foundation-Flanders through the project GA04711N. References
Fig. 13. Functional flexible OLED device incorporating PEDOT: PSS and LEP selective ablation using excimer laser; demonstration under environmental conditions. 2644
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1. C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51, 913–915 (1987). 2. A. N. Krasnov, “High-contrast organic light-emitting diodes on flexible substrates,” Appl. Phys. Lett. 80, 3853–3855 (2002). 3. M. Eritt, C. May, K. Leoa, M. Toerker, and C. Radehaus, “OLED manufacturing for large area lighting applications,” Thin Solid Films 518, 3042–3045 (2010). 4. D. A. Pardo, G. E. Jabbour, and N. Peyghambarian, “Application of screen printing in the fabrication of organic Lightemitting devices,” Adv. Mater. 12, 1249–1252 (2000). 5. www.holstcentre.com. 6. White paper on the characterization of thin-film barrier layers for protection of OLEDs, available for download at http://www .fast2light.org/news.html.
7. J. S. Park, H. Chae, H. K. Chung, and S. I. Lee, “Thin film encapsulation for flexible AM-OLED: a review,” Semicond. Sci. Technol. 26, 034001 (2011). 8. S. Harkema, H. Rooms, J. S. Wilson, and T. van Mol, “Large area ITO-free flexible white OLEDs with Orgacon PEDOT: PSS and printed metal shunting lines,” Proc. SPIE 7415, 74150T (2009). 9. G. E. Jabbour, R. Radspinner, and N. Peyghambarian, “Screen printing for the fabrication of organic light emitting devices,” IEEE J. Sel. Top. Quantum Electron. 7, 769–773 (2001). 10. S. C. Chang, J. Liu, J. Bharathan, Y. Yang, J. Onohara, and J. Kido, “Multicolor organic light-emitting diodes processed by hybrid inkjet printing,” Adv. Mater. 11, 734–737 (1999). 11. D. G. Lidzey, M. Voigt, C. Giebeler, A. Buckley, J. Wright, K. Böhlen, J. Fieret, and R. Allott, “Laser-assisted patterning of conjugated polymer light emitting diodes,” Org. Electron. 6, 221–228 (2005). 12. Y. H. Tak, C.-N. Kim, M.-S. Kim, K.-B. Kim, M.-H. Lee, and S.-T. Kim, “Novel patterning method using Nd:YAG and
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Centre for Microsystems Technology (CMST), imec, and Ghent University, Technologiepark 914A, B-9052 Ghent, Belgium 2
TNO/Holst Centre, High Tech Campus 31, 5656AE Eindhoven, The Netherlands
3
Department of Mechanical Engineering, Eindhoven University of Technology, 5612 AZ, The Netherlands *Corresponding author: [email protected] Received 24 October 2013; revised 20 March 2014; accepted 21 March 2014; posted 21 March 2014 (Doc. ID 199808); published 17 April 2014
The fast-growing market of organic electronics stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning organic thin films, but only a few studies report on interaction with thin-film barrier layers. In this paper, we present an approach in which the laser patterning process is optimized together with the barrier film, leading to a highly selective patterning technology without introducing barrier damage. This optimization is crucial, as the barrier damage would lead to moisture and oxygen ingress, with accelerated device degradation as a result. Following process optimization, a laser processed flexible organic LED has been fabricated and thin-film encapsulated and its operation is shown for the first time in atmospheric conditions. © 2014 Optical Society of America OCIS codes: (220.4610) Optical fabrication; (240.0310) Thin films; (140.3390) Laser materials processing. http://dx.doi.org/10.1364/AO.53.002638
1. Introduction
Organic electronics is an emerging field where rapid advances are made in finding novel applications due to its inherent advantages, such as flexibility, stretchability, and adaptability toward roll-to-roll production. The invention of the first organic LED (OLED) in 1987 [1] generated much research attention for developing efficient materials and fabrication techniques for OLED devices [2–4]. In lighting applications, flexible OLEDs have significant advantages over conventional incandescent or fluorescent lighting devices due to their light weight, improved 1559-128X/14/122638-08$15.00/0 © 2014 Optical Society of America 2638
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durability, higher impact resistance, inherent flexibility, and energy efficiency (up to 150 lm∕W). Moreover, these devices are environmentally friendly (no heavy metals involved) and cost effective (roll-toroll fabrication). Nowadays, the research in this area is mainly focused on the development of generic technologies for cost-effective roll-to-roll production of flexible OLEDs [5]. One of the major challenges in fabricating reliable flexible OLEDs is to maintain higher device lifetimes by protecting the device from moisture and oxygen permeation [6,7]. In addition, the fabrication of an OLED on a flexible substrate is rather complicated, as it must be encapsulated from top and bottom sides, contrary to glass-based OLEDs. In such a case, the role of a flexible moisture barrier that has similar
optical properties as that of glass is vital for flexible OLEDs. In order to fabricate the flexible OLEDs in a roll-to-roll manner, different postpatterning techniques of active device layers are considered, of which laser ablation is favored. Laser ablation has several inherent advantages, such as noncontact, dry processing, high speed, less environmental pollution (no hazardous wet chemicals), and high accuracy, that make it suitable for roll-to-roll production on flexible substrates. The use of lasers for the fabrication of these devices implicates that a selective ablation process of optically thin organic device layers has to be developed. The influence of the optical absorption properties of the barrier layer, in the selective removal of the thin organic films like PEDOT:PSS or light-emitting polymer (LEP), is crucial in the process of patterning OLED layers. A polymer-based OLED consists of an organic LEP as an emissive layer, which is sandwiched between an anode and a cathode layer. Generally, an indiumtin oxide (ITO) layer is used as anode. A metal layer like Ba–Al (barium with aluminum cap) can be used as cathode. Furthermore, a layer of conducting PEDOT:PSS is commonly used as a hole injection layer in between the anode and the emissive layer. PEDOT:PSS with an integrated bus bar structure for the fabrication of flexible OLED using laser ablation has been reported [8]. Moreover, PEDOT:PSS can also be used as an anode and as a hole injection layer completely replacing ITO. PEDOT:PSS and LEP have classically been deposited using spin-coating technologies or inkjet printing. Spin coating results in significant quantities of material waste. Inkjet printing has been largely investigated for small area substrates. However, in case of large area substrates, large amounts of material have to be deposited. Hence, the layer flatness and deposition speed are questionable [9,10]. Homogeneity and high overlay accuracy of the PEDOT:PSS and LEP are essential for the functionality of an OLED, which can be attained by using large area deposition techniques for high-volume production. Slot-die coating can be considered as a fast, simple, and cost-effective deposition technique for both PEDOT:PSS and LEP layers in a roll-to-roll fashion. However, the slot-die coating requires a postpatterning technique among which laser ablation is a very suitable candidate. Large area deposition of stacked layers followed by laser processing can preserve better layer homogeneities compared to patterned deposition techniques mentioned before. Proper power density adjustment and wavelength/pulse width selection of the laser beam has to be effectively utilized for selective removal of different thin layers, which is crucial during OLED processing [11–17]. In this paper, we demonstrate a fully functional laser patterned OLED on a flexible polyethylene naphthalate (PEN) substrate with a multilayered barrier stack. Various multilayered barrier foils with different optical absorption properties are investigated, and laser processing parameters are
optimized to obtain a selective ablation process. The challenge is to selectively remove a thin-film organic layer, deposited on top of an inorganic layer, without any damage or (organic) leftovers present. The ablation process windows for different barrier foils have been compared and the barrier foil with the widest working process window has been used for the fabrication of the OLED stack in the second stage of experiments. An industrially qualified 248 nm krypton fluoride (KrF) excimer laser is used to structure and pattern flexible OLED stack layers. Subsequently, functional devices are fabricated and encapsulated using thin-film technology. A flexible substrate that is a 125 μm thick PEN is laminated on a 6 in: × 6 in: glass carrier (1.1 mm thick) with a thermally activated release layer. On top of the laminated PEN substrate, a multilayer barrier stack is deposited with various deposition techniques. The device layers are then spin coated on the barrier stack on which selective laser ablation has been carried out. The samples were fabricated in the same way for five different types of SiN layer of barrier stack. In order to find the most suited inorganic barrier layer for laser processing of thin-film organic device layers such as PEDOT:PSS and LEP, ablation experiments were carried out. As a consequence, a fully functional flexible OLED was fabricated and its operation has been demonstrated under environmental conditions. This is a verification of the selective laser process investigation for the roll-to-roll production of flexible OLEDs, and further lifetime testing will be the focus of future work. 2. Experimental Work and Methodology A. Sample Preparation
Three different types of thin-film stacks were prepared as shown in Figs. 1(a)–1(c). The thin-film stack in Fig. 1(a) consists of 125 μm PEN as a flexible substrate on top of which a multilayered barrier consisting of 150 nm inorganic silicon nitride (SiN) was deposited by chemical vapor deposition.
Fig. 1. Thin-films stack on flexible PEN substrate and multilayered barrier, prepared for the investigation of barrier influence on patterning (a) PEDOT:PSS, (b) LEP, (c) LEP and PEDOT:PSS, at the top. (d) A detailed multilayered barrier. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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Subsequently, a thin PEDOT:PSS layer (100 nm) was spin coated on the barrier stack. Next, Fig. 1(b) shows the same layered configuration of a PEN substrate and barrier foil except the top layer is an active conjugated LEP with a thickness of about 80 nm. These two stacks were prepared to investigate the ablation behavior of the PEDOT:PSS and LEP on different barriers separately. The third thin-film stack, as shown in Fig. 1(c), was prepared in order to fabricate a flexible OLED. A LEP layer (80 nm) was spin coated on top of PEDOT:PSS (100 nm), while the flexible substrate PEN is identical to the previous two cases. A detailed schematic of the multilayered barrier consisting of organic and inorganic layers is illustrated in Fig. 1(d). B.
Characterization of Absorption Spectra
The absorption spectra of PEDOT:PSS and LEP were characterized before ablation experiments. These optical absorption spectra were measured by depositing the polymer layers, with the same thicknesses as required on the stack (80 nm LEP and 100 nm PEDOT: PSS), on a quartz substrate of 2 nm × 2 nm. The absorption spectra were obtained from a UV–Vis spectrometer (JASCO), which is able to measure below 300 nm. C.
Experimental Ablation Setup
Ablation experiments were performed with a pulsed, nanosecond KrF excimer laser at 248 nm with a pulse repetition rate of 100 Hz. The laser beam is passed through an attenuator plate and then a square mask of 2000 μm × 2000 μm as shown in the schematic experimental setup diagram (Fig. 2). In these experiments, a focusing lens of demagnification 10 is used, which is adjustable in the vertical direction, resulting in a spot size of 200 μm × 200 μm on the sample. The fluence is controlled with an attenuator plate, and the pulse energy at each fluence is measured by a pyro-electric energy meter (Coherent J25LPMUV, in combination with FieldmaxII TOP) placed at the end of the beam axis. The samples were mounted on a motorized automated translation stage, perpendicular to the beam with thin film facing the incident laser beam. The laser ablation of the hole transport layer (PEDOT:PSS) and active layer (LEP) is carried out under atmospheric conditions. The ablation setup consists of a suction head that removes the generated ablation
Fig. 2. Schematic diagram of the experimental setup for layer patterning. 2640
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by-products from the process site, leading to a controlled process. Two laser patterning steps involving organic thinfilms PEDOT:PSS (∼100 nm) and LEP (∼80 nm) have been carried out. First, in order to determine the influence of the top inorganic SiN barrier layer on the patterning behavior of thin organic films, experiments were carried out by ablating PEDOT: PSS and LEP on five different barrier stacks labeled as SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5). Afterward, selective patterning and structuring of the OLED layers were performed on an optimized barrier stack. There are two challenges to be considered for the realization of a laser patterned device. First, to selectively ablate the organics without damaging the inorganic SiN barrier layer, and second, to protect the bus bar, which is a multilayer stack of chromium and aluminum (Cr–Al–Cr) beneath, along this ablation process. The detailed schematic of the device layers with necessary involved laser process steps is given in Fig. 3. The PEDOT:PSS and LEP laser patterning steps are validated on a 15 cm × 15 cm flexible OLED device platform incorporating nine individual OLED devices. Table 1 summarizes a detailed overview of thicknesses and processing methodology of various layers. The hole injection layer (PEDOT:PSS) is deposited on top of the barrier stack by spin coating from an aqueous solution followed by a heating step for solvent removal. Laser patterning of this layer leads to electrical contact separation between anode and cathode, as well as top barrier encapsulation structures. Subsequently, LEP is deposited by spin coating followed by laser patterning, which removes the layer at specific regions for device encapsulation. The device layers are then covered by a low work function Ba cathode layer capped with Al. Finally, a barrier layer is deposited on the whole OLED stack to form the top encapsulation, which will hermetically seal the fabricated device. D.
Depth Profiling
The inspection and analysis of the samples after experiments were performed by using an optical microscope, a mechanical profiler (Tencor Alphastep 200), and a noncontact optical (Wyko NT3300) white light
Fig. 3. Flexible OLED layered structure and various selective ablation steps for patterning: ablate PEDOT:PSS on (A) a barrier and on (B) a bus bar; ablate LEP on (C) the barrier and on (D) the bus bar.
Table 1.
Layer Ba-Al LEP (Merck Livilux) Cr–Al–Cr PEDOT:PSS (Agfa Orgacon) SiN PEN
Layer Composition, Thicknesses, and Deposition Methods of the OLED Stack
Role in OLED Stack
Thickness
Deposition
Cathode Active emissive layer Bus-bar Hole injection/anode Top layer of multilayered barrier Flexible substrate
100 nm 80 nm 150 nm 100 nm 150 nm 125 μm
Evaporation Spin coating Sputtering Spin coating Plasma deposition —
interferometer. The crater depths were determined with mechanical and optical profilers. Prior to these measurements, the samples were transferred on a sticking surface (PDMS) to minimize the curvature. In the case of optical profiling, the stack of transparent layers might result in multiple reflections. Therefore, a reflective Au layer (roughly 25 nm) is applied on top of the stack using a plasma magnetron sputter coater. 3. Results and Discussion A.
Absorption Spectra
In order to investigate the influence of barrier absorption on the selective ablation processes, five different types of SiN inorganic barriers were evaluated. The absorption spectra of the inorganic barrier foils were characterized with a UV–Vis spectrometer as illustrated in Fig. 4. These barrier foils were referred to as SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5). Figure 4 shows that a significant difference in absorption at 248 nm (wavelength of operation) can be observed for five different barrier foils. The order of absorption can be clearly observed at 248 nm, that is, SiN1 < SiN2 < SiN5 < SiN3 < SiN4. A variation in the ablation behavior of LEP and PEDOT:PSS on these different SiN barriers is expected because of the difference in their absorption. Hence, we prepared samples for each of the 3 thinfilm stacks depicted in Figs. 1(a)–1(c) with 5 different barriers in each case, leading to 15 samples in total.
Fig. 4. Optical absorption spectra of different SiN barrier foils, indicating significant difference in the absorption at the wavelength of operation, i.e., 248 nm.
The optical absorption spectra of thin organic films LEP and PEDOT:PSS as obtained are shown in Fig. 5. From the spectral information, it is observed that LEP absorbs quite heavily from 200 to 450 nm. However, PEDOT:PSS is almost transparent for the wavelengths higher than 300 nm. This means that the selection of a laser source for the ablation of these two layers in the nanosecond pulse regime lies in their absorption band. The natural choice would be either 193 or 248 nm excimer lasers, because of the better absorption of these wavelengths by thin polymer films. Since 248 nm excimer lasers are well industrialized, this laser has been chosen for these experiments. B. Barrier Damage Thresholds
In the first stage of this research, experiments were conducted on samples covered with 100 nm PEDOT: PSS, 80 nm of LEP, and LEP on PEDOT:PSS (180 nm), as shown in Fig. 1. The laser fluence was varied from 75 mJ∕cm2 to 250 mJ∕cm2 . For each value of the fluence, the number of laser shots was varied by changing the speed of the automated translational stage. These experiments were carried out with one pulse, two pulses and five pulses per location. Experiments were performed on different types of barrier foils [SiN(1), SiN(2), SiN(3), SiN(4), and SiN(5)], and the depths of the ablated craters were measured. Since the ablation threshold of the bus bar (Cr–Al–Cr) is quite higher than that of organic thin films (PEDOT:PSS, LEP), it is not very relevant to discuss the ablation of bus bar in more detail.
Fig. 5. Optical absorption spectra of the thin organic films indicating higher absorption by LEP compared to PEDOT:PSS at 248 nm. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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Fig. 6. Selective ablation for PEDOT:PSS on two different kinds of barrier stacks: least absorbing SiN(1) in (a) clean removal, (b) and onset of damage at overlapping; and highest absorbing SiN(4) in (c) clean removal, and (d) start of damage at edges.
It was observed that the variation in the absorption properties of the barrier has a major influence on the patterning of these particular thin films. Figure 6 illustrates optical microscopic images after patterning of PEDOT:PSS thin films on two barrier stacks, SiN(1) and SiN(4). SiN(1) is the least absorbing and SiN(4) the most absorbing barrier foil at 248 nm (Fig. 4). In case of SiN(1), a clean selective removal of PEDOT:PSS is possible with a fluence of 125 mJ∕cm2 with two pulses [Fig. 6(a)]. Furthermore, there is noticeable damage starting at a fluence of 200 mJ∕cm2 with two pulses, particularly at the overlapping regions. These observations of clean ablation as well as detectable damage at overlapping regions on barrier foils have been verified by analyzing the crater depths. In a similar fashion, in the case of SiN(4), clean removal of the PEDOT:PSS is depicted in Fig. 6(c). In this case, it is interesting to note that even at a fluence of 75 mJ∕cm2 with two pulses per location [Fig. 6(d)], there is detectable damage at the edges. These results suggest that PEDOT:PSS can be selectively removed on SiN(1) barrier foil at a fluence ranging from 125 mJ∕cm2 to 200 mJ∕cm2 . However, it is difficult to find a selective patterning process window for SiN(4) barrier foil, as the damage starts even at fluence lower than 75 mJ∕cm2 with two pulses. The ablation behavior of LEP, PEDOT, and LEP on PEDOT:PSS for five different types of multilayered
Table 2.
SiN barrier stacks has been studied, and the damage thresholds of various SiN barriers are determined. A detailed comparison of damage threshold of various SiN layers for different organics is illustrated in Table 2. From Table 2, it is clear that SiN(1) has the highest value of the damage thresholds and therefore it has the widest process window for the selective ablation of PEDOT:PSS, LEP, and, LEP on PEDOT:PSS. However, SiN(4) has the lowest values of damage thresholds compared to other SiN barrier foils, implying the most narrow process window. These values are in agreement with the absorption spectra of respective SiN barrier foils (Fig. 4). Hence, in order to fabricate the OLED device, SiN(1) barrier foil has been chosen for the next stage of experiments. In Fig. 7, the onset of layer removal starts only beyond a laser fluence of 200 mJ∕cm2. These results show that for selective patterning of organic thinfilms PEDOT:PSS and LEP on SiN(1) barrier, the ablation thresholds of thin films should be lower than 200 mJ∕cm2 . C.
Thin-Film Ablation Thresholds
Ablation experiments of PEDOT:PSS and LEP layers deposited on SiN(1) barrier stack showed almost identical thresholds for layer removal. Figure 8 shows that the threshold of layer removal in both cases is found to be 75 mJ∕cm2 from single pulse experiments. This information is important to define a process for patterning both layers in subsequent steps on the SiN(1) layer of the barrier. Since the threshold of ablation of SiN(1) barrier is found to be more than twice that of the fluence required for ablating any of these organic layers, there is a high probability of patterning these layers without significant damage of the underlying layer. Therefore, a wide working process window is available for selective ablation. Multipulse laser ablation studies at various effective fluences were carried out to determine the process windows. D.
Patterning Thin Films
In order to pattern large areas, multishot ablation is required. Laser multishot scribe tests were carried out at various pulse overlap schemes to optimize the process. Figures 9(a) and 9(b) show the multishot ablation of PEDOT:PSS and LEP layer on top of SiN(1) barrier foil, respectively. It is found that at a fluence of 125 mJ∕cm2 with two shots per location, the depth of the ablated PEDOT:PSS crater is about 100 nm,
Damage Thresholds for Different Types of Multilayered SiN Barrier Foils
Damage Threshold (mJ∕cm2 ) of SiN Layers with Two Pulses Top Layer(s) of Sample Stack (See Fig. 1) PEDOT:PSS LEP LEP on PEDOT:PSS
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SiN(1)
SiN(2)
SiN(3)
SiN(4)
SiN(5)
200 200 250
125 150 125
100 100 125
75 100 100
100 100 125
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Fig. 7. Graph of the ablation depth versus the laser fluence for multilayered SiN(1) barrier stack applying two pulses per location.
Fig. 8. Graph of the ablation depth versus the laser fluence for PEDOT:PSS and LEP thin films on multilayered SiN(1) barrier (single pulse).
without any visual detectable damage on the SiN(1) layer. Similarly, at 100 mJ∕cm2 with two shots per location, the depth of the ablated LEP crater is 100 nm without any detectable SiN(1) barrier damage. These mechanical profiler depth measurements are also supported by the depth measurements obtained from the noncontact optical profiler. Next, Fig. 10(a) shows the optical microscopic image of the patterned PEDOT:PSS layer at a laser fluence of 125 mJ∕cm2 with two pulses per location. On the right [Fig. 10(b)], the corresponding depth profile indicating the PEDOT:PSS layer removal of approximately 111 nm is shown. Similarly, Figs. 11(a) and 11(b) show the results for LEP removal. The pictures are representative images for removal of LEP using five shots per location at fluence of 100 mJ∕cm2. E.
Fig. 9. Multishot ablation of (a) PEDOT:PSS and (b) LEP on SiN (1) barrier indicating the depth measurement at various laser fluences.
Fig. 10. (a) PEDOT:PSS layer removed at a fluence of 125 mJ∕cm2 with two pulses per location on SiN(1) barrier. (b) Corresponding white light interferometer (Wyko) depth profile.
OLED Stack Patterning Process Flow
Sequential ablation steps of PEDOT:PSS and LEP are required for processing the OLED device. It is important to remove both the organic layers (LEP and PEDOT:PSS) from the SiN(1) barrier layer. After laser processing of the device layers, thin-film encapsulation is made on the top side by depositing
Fig. 11. (a) LEP layer removed at a fluence of 100 mJ∕cm2 with five pulses per location on SiN(1) barrier. (b) White light interferometer profile showing a depth of ∼89 nm and indicating the removal of LEP. 20 April 2014 / Vol. 53, No. 12 / APPLIED OPTICS
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OLED. Since LEP serves as an isolation layer between the anode and the cathode, there is no need to print an additional isolation layer. F.
Fig. 12. Process flow for laser patterned flexible OLED. Process 1: PEDOT:PSS is deposited by spin coating on flexible barrier substrate. Process 2: PEDOT:PSS is removed by laser process forming encapsulation ring and contact isolation. Process 3: LEP is deposited by spin coating. Process 4: LEP is removed from encapsulation ring and contact cleaning for top electrode deposition. Process 5: metal cathode (Ba-Al) is evaporated through a mask. Process 6: device is thin-film encapsulated.
identical SiN(1) layer over the whole device surface. This process should ensure that the top and the bottom SiN(1) barrier stack should form a hermetical seal. If the laser process leaves an incompletely removed layer, this acts as a moisture ingress path, which leads to damage of the device. Figure 12 is a detailed description of the process flow that has been followed for the fabrication of a flexible OLED device. After the uniform deposition of the PEDOT:PSS on the flexible barrier substrate (process 1), the first ablation step is the removal of PEDOT:PSS from the SiN(1) barrier layer and Cr– Al–Cr bus bar, forming a 2 mm wide track as shown in process 2. Next, LEP is deposited uniformly by spin coating over the entire sample surface (process 3). Afterward, a 1 mm wide track is ablated exactly in the middle of the 2 mm track removing LEP on SiN(1) and the Cr–Al–Cr metal bus bar simultaneously (process 4). Finally, the deposition of the cathode (process 5) and subsequent thin-film encapsulation (process 6) leads to a functional flexible
Flexible OLED Fabrication
As described in the process flow section above, both PEDOT:PSS and LEP layers are laser ablated in subsequent steps. Afterward, a top SiN(1) layer (encapsulation) is deposited so that it forms a hermetical seal with the bottom SiN(1) layer (barrier). Figure 13 demonstrates a laser patterned flexible OLED, which lights up on application of an electrical power supply. It can be seen from this figure that, after the device is turned on, no black spots are visible in the active region or dark fronts originating from the laser ablated regions. 4. Conclusion
The process investigation on selective ablation of thin organic films on a multilayered barrier foil has been performed. It is concluded that the influence of barrier absorption plays a significant role in the selective ablation process. The barrier with minimum absorption at the wavelength of operation has the widest working process window. After selection of an optimized barrier foil, feasibility of the laser process for patterning and structuring of various OLED layers has been investigated. Minimal debris field and layer delamination are observed in the ablated zone. Multipulse laser processes were optimized for the removal of both PEDOT:PSS and LEP organic layers on a flexible substrate containing a multilayer barrier stack. A functional flexible laser processed OLED device has been fabricated and its operation has been demonstrated under environmental conditions. 5. Perspectives
This work suggests future study where device performance on accelerated test conditions is evaluated for process optimization, which will be discussed in subsequent publications. The authors thank Mr. Steven Van Put for assisting in laser ablation experiments. This work is partly supported by the Research Foundation-Flanders through the project GA04711N. References
Fig. 13. Functional flexible OLED device incorporating PEDOT: PSS and LEP selective ablation using excimer laser; demonstration under environmental conditions. 2644
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