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Display process compatible accurate graphene patterning for OLED applications To cite this article: Jin-Wook Shin et al 2018 2D Mater. 5 014003
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2D Mater. 5 (2018) 014003
https://doi.org/10.1088/2053-1583/aa9cae
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RECEIVED
19 September 2017
Display process compatible accurate graphene patterning for OLED applications
RE VISED
29 October 2017 ACCEP TED FOR PUBLICATION
23 November 2017 PUBLISHED
13 December 2017
Jin-Wook Shin1,2,5, Jun-Han Han1,5, Hyunsu Cho1, Jaehyun Moon1 , Byoung-Hwa Kwon1, Seungmin Cho3, Taeshik Yoon4, Taek-Soo Kim4, Maki Suemitsu2, Jeong-Ik Lee1 and Nam Sung Cho1 1
Flexible Device Research Group, Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea 2 Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577, Japan 3 Hanwha Techwin R&D Center, Sungnam, Gyeonggi-do 13488, Republic of Korea 4 Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 5 J-W Shin and J-H Han contributed equally to this work. E-mail:
[email protected] (N S Cho) Keywords: graphene, organic light emitting diode, patterning, liquid bridging, integrated module Supplementary material for this article is available online
Abstract Graphene film can be used as transparent electrodes in display and optoelectronic applications. However, achieving residue free graphene film pixel arrays with geometrical precision on large area has been a difficult challenge. By utilizing the liquid bridging concept, we realized photolithographic patterning of graphene film with dimensional correctness and absence of surface contaminant. On a glass substrate of 100 × 100 mm2 size, we demonstrate our patterning method to fabricate an addressable two-color OLED module of which graphene film pixel size is 170 × 300 µm2. Our results strongly suggest graphene film as a serviceable component in commercial display products. The flexible and foldable display applications are expected to be main beneficiaries of our method.
1. Introduction Graphene belongs to a class of low dimensional carbon materials. In graphene, sp2 carbon atoms are bonded to form a 2D carbon hexagonal array. Since it was first isolated in 2004, the past twelve years have witnessed a huge burst in fundamental and application research activities on graphene [1–7]. Graphene, as a single entity, possesses various outstanding properties, including high electron mobility, optical transparency, and mechanical compliance. Owing to these properties graphene has been widely investigated in electronics, sensors and energy devices [8–13]. From the viewpoint of organic light emitting diodes (OLEDs), graphene is appealing as a transparent electrode [14–17]. In addition to meeting the common requirements of transparent electrodes, unlike indium tin oxide (ITO), graphene does not induce discernible internal optical effects in OLEDs. While graphene anode OLEDs have been reported elsewhere, so far commercial level OLEDs with graphene electrodes have not been reported yet. Currently, graphene may be positioned as a component, that has to be integrated into modules and systems to achieve commercial level products. The © 2017 IOP Publishing Ltd
hurdles that impede the realization of such products are not due to the characteristics of graphene rather to processing obstacles [18, 19]. In this paper, we address processing issues relevant to the integration of pixelated graphene transparent electrodes and OLED panels. In graphene active matrix (AM)-OLED displays, the most important element is the establishment of pervasive processing, which enables modular integration of graphene into existing display technologies. Processing issues of high importance in realizing graphene AM-OLEDs can be succinctly summarized as follows. Graphene films must be patterned dimensionally correct and defect free over a large area. For this task, we emphasized the use of a display technology compatible process. The patterned or pixilated graphene films were electrically connected to addressing lines and integrated with OLED pixels. In this work, we utilized the concept of liquid bridging to provide strong adhesions between the graphene film and the substrate. Strong adhesion enables the use of display technology compatible photolithography processes. Finally, we demonstrate a fully functional graphene pixilated OLED panel. We believe that our result is an important precursor toward graphene AM-
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OLED display. Our results are expected to find direct applications not only in conventional displays but also in flexible and/or stretchable display in which a thin thickness and high flexibility are both required.
2. Results and discussion The basic structure of an AM-OLED can be divided into two parts: a backplane and an OLED. The backplane is composed of an array thin film transistors (TFTs), contact metal lines within via holes and a passivation layer [20–22]. The TFTs control the current flow into the OLED as on/off switches. The passivation layer is necessary to protect the TFTs from during subsequent processes and stabilize the forthcoming processes of formation of anode with pixilation and deposition of organics. In addition, the passivation layer prevents the occurrence of electrical shorts. The passivation layer was perforated to form electrical contact pads. The OLED was fabricated on the backplane by stacking up multi-functional layers such as a transparent electrode as an anode, functional organic layers, and a highly reflective electrode as a cathode. As the optical cavity effect is marginal in OLEDs, graphene film is useful for achieving color stability [23]. In order to use graphene in wellestablished AM-OLED display processing technology, adhesion of graphene to the support must be strong enough to withstand standard photolithographic process steps. In an effort to effectively investigate the technical issues associated with pixelated graphene electrodes, we adopted a simplified bottom emission type OLED structure as shown in figure 1. Addressing metal lines were installed to control the on/off states of OLED pixels. On the surface of the passivation layer, we patterned graphene films into pixelated electrodes. Various graphene patterning methods have been proposed [18, 24–26]. However, the majority of these methods yield deterioration of graphene surface quality and incorrect geometric dimensions. Thus, there is a strong technical need to establish graphene film patterning methods that, provide accurate pattern dimensions and preserve film quality. To make accurate graphene patterns without any damage, we improved the adhesion between the graphene film and the passivation layer, and used a photolithography method to pattern the graphene films with dimensional accuracy without deterioration of the surface characteristics. Figure 2 show images of actual examples we have fabricated. These images were obtained when the graphene film adhesion to the substrate was weak. We will perform quantitative analyses on the adhesion in the next section. In this work, graphene film refers to a four-layered graphene film that was obtained by sequentially stacking monolayer graphene. Monolayer graphene was grown on Cu foils using a chemical vapor deposition (CVD) method. The direct transmittance and the Raman scan results can be found in the supple2
mentary section (S1 (stacks.iop.org/2DM/5/014003/ mmedia)). After the photolithographic patterning process, the graphene films were observed to be crumbled and/or peeled off (figures 2(a) and (b)). As a result, the emission image of fabricated OLED failed to reach an acceptable standard (figure 2(c)). The emission uniformity is low and many defective regions are readily observable. Based on the observation of figure 2, we draw a conclusion that direct application of photolithographic patterning processes to graphene films is not suitable. Unlike thin-films formed by vacuum deposition methods, graphene films are physically transferred from a catalytic surface, on which graphene is grown, to a substrate of interest. Thus, due to the nature of the transfer process, graphene film will not necessarily lie globally flat on the surface. The unsuitability of direct application of photolithography seems to have originated from the weak adhesion between the graphene films and the substrate, which is Van der Waals like [27–31]. Van-der Waals forces are instantaneous dipoles interactions and far weaker than permanent chemical bonds such as covalent or ionic bonds. In photolithographic patterning processes, samples are directly exposed to various liquid solutions frequently. Thus, the chance of liquid permeation through the substrate/graphene interface is high [32]. Furthermore, the presence of undesirable conditions, such as structural defects in the graphene, surface roughness, and residues from underlying layers, can diminish the adhesion of graphene to the supporting layer [33–35]. As these imperfections may generate pores at the interface between the graphene and the substrate (figure 3(a)), the graphene adhesion based on Van-der Waals forces can be weakened by reducing the conformal contact area. In addition, these pores act as the defect seeds that, which can facilitate solution permeation during liquid solution involved processes. Therefore, to stabilize the photolithography patterning process, adhesion must be improved. In an effort to improve the adhesion and flatten the graphene film, we utilized the concept of ‘liquid bridging’. Liquid bridging provides a connection between two solid surfaces via liquid molecules and forms an attractive force between interfaces. This bridge, of which stability is dependent on the separation distance between the interfaces, is stable before the critical separation distance [36–38]. In addition, the attractive force becomes stronger as the distance decreases. By allowing water to permeate into the air pores that exist between graphene and the substrate, the formation of liquid bridge can be induced (figure 3(b)). Water was selected because it is the most widely used cleaning solvent readily available in the clean room facility and gave the best results so far. Use of other liquid medium is open for future studies. The water inside the pores is in physical contact with every part of the internal surface of the graphene film. Owing to the mechanical compliance of graphene, upon removal of the water,
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Figure 1. Graphene-pixel electrode OLED with addressing metal line.
Figure 2. Graphene damages from the pattering process (photo-lithography method); graphene smash (a), peeled off (b), and influence of graphene damage on the actual emission image of OLED (c).
Figure 3. The concept of improvement in adhesion between graphene/substrate by liquid bridging; (a) air-pores between the graphene and the substrate, (b) the formation of water droplet inside the air-pores, and (c) the removal the water droplet.
the graphene film can be stretched to eliminate the air pores and achieve close physical contact between the substrate and the graphene film with stronger adhesion (figures 3(b) and (c)). In order to quantitatively analyze the effect of the liquid bridging method, we used a double cantilever beam (DCB) fracture mechanics to test the adhesion energy and atomic force microscopy (AFM) to examine the surface morphology (figure 4). DCB testing is a widely used method for measuring the effective adhesion energy of two dimensional materials [39, 40]. The specimen was prepared on the same surface condition with OLED panel structure. To 3
accurately evaluate our surface treatment we prepared two samples. (i) a pristine graphene film without any treatment and, (ii) a graphene film with the water and thermal treatment in the vacuum as the liquid bridge method. We have immersed the sample into deionized (DI)-water for 2 min. Immersion time longer than 2 min resulted in peeled off graphene film. In this case, water is thought to have formed a continuous layer between the substrate and acted as a detachment layer. Thus, we kept immersion time to 2 min. To remove the DI-water, thermal treatment was performed at 200 °C for 2 h in a conventional vacuum oven.
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Figure 4. (a) Schematic of DCB specimens, the delamination tendency of graphene after DCB testing; (b) the pristine graphene, (c) the liquid bridge treatment, (d) the effective adhesion energy (histograms) and average surface roughness (symbol with dotted line) of all specimen, and the Raman analysis of pristine graphene (e) and the liquid bridge treatment (f) after DCB testing.
Figure 4(a) shows cross-sections of the DCB testing specimens. The graphene film is sandwiched between a SiOx layer and an epoxy layer. The upper and lower glass substrates act as physical supports for the structure in between. We choose SiOx because it is used in OLEDs as a passivation layer material. A vertical bidirectional load was continuously applied to all specimens until the graphene was detached from the SiOx surface. When the applied force exceeds the effective adhesion energy of the weakest bond in the specimen, cracking or detachment is initiated. In this process, the crack length was measured and the effective adhesion energy was extracted [41]. After DCB testing, Raman analyses were performed on the interface where graphene cracking occurred. Figures 4(b) and (c) show the tendency of graphene to peel off under each condition after DCB testing. Figure 4(d) shows the effective adhesion energy and the surface roughness of the graphene samples. The average surface roughness (Ra) of pristine graphene was measured to be about 2.33 nm, which is about 4 times higher than the Ra of 0.52 nm for SiOx. The pristine graphene include many air pores, the surface roughness is thought to be high. The Ra values of graphene films of the graphene with the liquid bridge treatment is low as 0.65 nm. This result signifies the effectiveness of our approach to reduce the surface protrusion. The effective adhesion energy value of the graphene with pristine condition and the liquid bridge treatment was extracted as 0.9 ± 0.14 J m−2 and 1.71 ± 0.21 J m−2, respectively. Due to differences in the tendency of graphene peeling off, it is difficult to directly compare these absolute values. However, it can be interpreted that the effective adhesion energy of graphene film undergone the liquid 4
bridge treatment is improved by about 2 times relative to that of a pristine graphene film. The graphene with the liquid bridge treatment shows the highest effective adhesion energy with lowest surface roughness. Without any treatment, the graphene film was observed to detach completely from the glass surface. Raman scans yielded no graphene signal on the glass surface, whereas a clear graphene peak was observed on the counterpart surface (figure 4(e)). In contrast, Raman scans of graphene films undergone the treatment showed clear graphene peaks on both sides. This means that the multilayered graphene film was cleaved. In other words, our method improves the adhesion of the graphene to the SiOx higher than of the graphene interlayer binding. More importantly, by combining liquid bridging and water removal, it is possible to not only reduce the surface roughness of graphene film but also improve adhesion. Conversely, we can draw a conclusion that the weak adhesion of untreated graphene film is due to the defects at the graphene film/substrate interface. For this reasons, untreated pristine graphene is easily damaged and delaminated by external factors. This improvement in the effective adhesion energy can be explained by Van-der Waals force, as graphene and SiOx have chemically stable surfaces. From this theoretical framework, the energy required for separating the two surfaces as the effective adhesion energy (Γ) is expressed as the following [42, 43]. ∞ A −2 r Γ= Fdr = 12π r F is the Van-der Waals force. A and r is Hamaker constant and the distance between the surfaces [44].
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Figure 5. (a) SEM image of graphene pattern edge by the photolithographic patterning processes and (b) the Raman analyses before and after the pattering process.
The value of Γ is proportional to the inverse square of the distance between the two surfaces. In our method, the overall distance between the graphene film and the substrate was effectively reduced by eliminating pores. In summary, the improvement in adhesion can be interpreted as an increase in Van-der Waals force which due to the very close contact between the graphene film and supporting layer. In addition, Raman analysis of each condition was carried out as shown in the supplementary section (S2). No significant changes in the intensity and shift of graphene peaks were observed, which means that our approach is applicable to enhance the graphene film adhesion to the support without deteriorating the graphene film quality. The AFM images in S2 show the flattened surface morphology of graphene film which has undergone treatment. Figure 5 shows typical results obtained after graphene film patterning. By comparing the SEM images of figures 2(a) and (b), one can readily notice the surface smoothness and dimensional accuracy of the patterned graphene film. In particular, the pattern edge is clearly straight (figure 5(a)). Raman analyses show that the graphene film surface is well preserved after the patterning process (figure 5(b)). After the patterning process the small shoulder close to the G (graphite) peak (1591 cm−1) disappears. Because this shoulder is reported to be associated with organic residues, its disappearance strongly implies the absence of organic residues, such as photoresist (PR) or other containments, on the surface, which is expected to contribute to inhibiting aberrational behavior in OLEDs [45, 46]. In order to evaluate our patterning process on the OLED device level, we fabricated phosphorescent green bottom emission OLEDs with patterned or non-patterned graphene electrodes. The organic stack structure of OLED is listed in the experimental section. In the case of non-patterned graphene-OLED, the emitting area was defined by forming a non-conductive organic material banks on the graphene film. Figures 6(a)–(c) shows the current density (J)-voltage 5
(V)-luminance (L), luminous efficacy (LE), and electroluminance (EL) spectra characteristics of OLEDs with non-patterned or patterned graphene. The graphene-OLEDs have a green emission with its main peak at λ = 515 nm. The optical and electrical properties of both devices are almost identical. In the J–V–L curves (figure 6(a)), the characteristics of OLEDs with patterned and non-patterned graphene electrode almost superimpose in the on-state region (V > 4 V). Significant luminance (L > 5 × 103 cd m−2) is observed at an almost identical current density level. These results strongly indicate that the liquid bridging does not induce significant changes in the values of sheet resistance and work function of the graphene film. The luminance efficacy of these devices also have the same characteristics (figures 6(b)). The EL spectra show no distinguishable feature (figure 6(c)). The actual emission image of the OLED with patterned graphene electrode shows no black spot or overly charged white region. We attribute the results of figure 6 to our surface treatment that yields strong adhesion of graphene film to its support and enables accurate and defect free patterning of graphene films into pixelated electrodes. The successful implementation of photolithographic patterning opens the door to utilizing established display compatible patterning processes, which is of high technical importance for realizing commercial level graphene electrode OLED products. Figure 7 shows the actual implementation of our patterning and surface treatment processes on a glass substrate of 100 × 100 mm2 in size. We integrated a graphene pixel array with an active area of 80 × 60 mm2 (figure 7(a)). The basic architecture of individual unit is identical to that of figure 1. The array is composed of 33 000 graphene electrode pixels, connected by each column and weaved into odd–even pairs. This array corresponds to a resolution of 151.8 pixel/inch. Figure 7(b) captures two graphene pixels from the overall layout plot. The area of one pixel is 170 × 300 µm2. The graphene electrode has an area
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Figure 6. (a) J–V–L and (b) luminance efficacy of characteristics of graphene-OLED with the passivation wall and the pattering process. (c) EL spectra characteristics at normal direction. Actual emission image of graphene-OLED with (d) the non-patterned graphene film and (e) the patterned graphene film.
Figure 7. Graphene film patterning on a glass substrate of 100 × 100 mm2 size. (a) Actual image of a fabricated backplane with graphene film pixels. (b) Lay out drawing of pixels including the electrical pad, via hole and addressing metal line. (c) A SEM image of graphene film pixels.
of 100 × 250 µm2, representing an aperture ratio of 49%. Electrical contacts between the Mo addressing lines and the graphene pixels were established using via-holes and ITO contact pads. The ITO contact pads occupy only 6.8% of the area, resulting in marginal influence on the overall display. Figure 7(c) shows a SEM image corresponding to the figure 7(b). It can be readily observed that our patterning process and surface treatment yields accurate graphene dimensions in the integration scheme. Figure 8 shows a fabricated two-color OLED module. The OLED was formed by a thermal evaporation method. We used phosphorescent red (λ = 620 nm) and phosphorescent yellow–green (λ = 560 nm) as our emissive layer materials. The fabricated OLED 6
panels were glass encapsulated and connected to a driving board. Our OLED was fully operational without any noticeable defect. A short movie clip can be found in the supplementary section (S4). To date there have been difficulties in overcoming the hurdles imposed by various processing difficulties with graphene. Many graphene applications have been unsuccessful beyond the ‘proof of concept’ stage. The fine operation shown in figure 8 is an actual demonstration of graphene application on practically meaningful level. Because all the implemented processes are compatible with existing display fabrication methods, our approach can be readily applied at the commercial level. As graphene films have mechanical compliance, they can be very useful alternatives to brittle ITO. In
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Figure 8. Addressable two-color OLED module with pixelated graphene films as transparent electrodes.
the near future, we believe emergence of graphene in flexible OLED display on a commercial large area scale.
3. Summary With the aim of utilizing graphene films as transparent electrodes for OLED display applications, we patterned a graphene film into 170 × 300 µm2 pixels over a large area. To overcome the weak adhesion of graphene to glass substrates and enable standard photolithography process, we combined liquid bridging and thermal treatment to improve the effective adhesion to a great extent. Our patterning approach yielded dimensionally correct graphene pixels without surface contamination. We applied our graphene patterning process to fabricate a fully operational two-color OLED module. Our results signify the technical possibility of using graphene films as actual components in commercial OLED products.
4. Experimental section 4.1. Graphene preparation In this work, we used four layer-graphene films supplied from Hanwha Techwin R&D Center. Graphene film was grown on the Cu film by a chemical vapor deposition (CVD) process. Cu film was etched by using an etching solution containing H2O, H2SO4 and benzimidazole. During the Cu etching process, benzimidazole p-dopes the graphene to have low sheet resistance. Four layer-graphene films were transferred by the thermal release tape (TRT) on the passivation layer (SiOx)/glass substrate layer by layer. The sheet resistance of the graphene films was ~65 Ω/sq. 4.2. Characterization of graphene The adhesion energy of graphene was measured by DCB fracture mechanics testing with a micromechanical test system (Delaminator Adhesion Test System; DTS Company, Menlo Park, CA). Specimen were prepared by cutting a 6 mm × 28 mm 7
rectangular beam from a four layer graphene/SiOx/ glass and bonding it to a glass counterpart by using epoxy (Epo-Tek 353ND consisting of bisphenol F and imidazole; Epoxy Technology). The epoxy was cured for 2 h at 125 °C in an electric oven. Raman facility (Model: NTEGRA spectra, NT-MDT) was used to assess the quality of the graphene film surface. The surface morphology of graphene was observed using an atomic force microscope (Model: PSIA XE-100) and a scanning electron microscope (FEI Sirion), respectively. 4.3. Device structure In this work, we used an alternating hole transport layer (HTL) of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) (10 nm) and 1-Bis[4[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC) (40 nm). The number of HAT-CN/TAPC pairs correspond to the 350 nm of HTL thickness. The devices were composed of a stack structure of four-layer graphene/the alternating structure of HTL (350 nm)/emission layer (EML)/1,3bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB) (60 nm) (electron transport layer; ETL). Lithium fluoride (LiF)/aluminum (Al) was evaporated on ETL for a high reflective metal cathode. In the case of graphene-OLED for pattering test, the organic material of 2,6-bis[30-(N-carbazole)phenyl] pyridine:Tris[2-phenylpyridinato-C2,N]Iridium(III) (DCzPPy:Ir(ppy)3) (20 nm) was used for EML with the main peak at λ = 515 nm (as shown in figure 6). The EML of graphene-pixel electrode OLED (as shown in figure 8) offers red (λ = 620 nm) and yellow-green (λ = 560 nm). The electrical and optical properties of graphene OLEDs were measured by using a source meter (Keithley 238) and a goniometer-equipped spectroradiometer (Minolta CS-2000). 4.4. Graphene pattering process Photolithography process was used for the graphene film patterning. Positive type photoresist (PR) was
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coated on graphene surface by using the spin coater (Karl Suss Micro Tec 80T) and soft baking was carried out at 90 °C for 120 s on a hot plate. To define an anode area, a photomask was aligned on the PR/graphene and it was then exposed to ultraviolet (UV) light by using the contact aligner system (Karl Suss MA6 mask aligner) followed by the development process. The exposed portion of graphene was etched by O2 plasma at 40 Watt with 30 sccm of O2. After etching process, the PR residue on the graphene was removed by the PR remover. 4.5. Integration of backplane with graphene pixel electrode For the formation of addressing metal line, molybdenum was evaporated on glass substrate and patterned by a photolithography process. The SiOx passivation layer was formed on the addressing metal line. In order to contact with addressing metal line and graphene pixel, the via hole and ITO contact pad was formed into the passivation layer. The graphene film was transferred on the passivation layer with the contact pad following the graphene film pattern process. The process flow is described in the supplementary section (S3).
Acknowledgments This work was supported by Ministry of Trade, Industry and Energy/Korea Evaluation Institute of Industrial Technology (MOTIE/KEIT), research program ‘Development of basic and applied technologies for OLEDs with Graphene’.
ORCID iDs Jaehyun Moon 6562
https://orcid.org/0000-0002-8625-
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