A prototype active-matrix OLED using graphene anode for flexible display application

From the very first time that graphene was used as a transparent electrode for OLED applications, the emergence of active-matrix (AM)-graphene OLED displays has been envisioned. Realizing this expectation, however, turned out to be difficult. Two obstacles are the growth and transfer of a large-area graphene film and the patterning of a graphene film into pixels. To solve these problems, a process of patterning a graphene film without surface contamination was developed. The fabrication of OLED panels by the patterned graphene anode on Gen 2(370 × 470 mm)-sized and flexible substrates was successfully demonstrated. In this work, oxide TFT arrays were combined as a switching backplane, and a pixelated graphene OLED was used as an emissive layer, to realize AM-graphene OLED displays. To explore the technical feasibility of flexible AM-graphene OLED displays, the aforementioned components were formed on a flexible substrate. For commercial-level production, all the processes that were used were chosen to be compatible with the conventional display processes.


Introduction
As a single entity, single-crystalline graphene has outstanding characteristics. The typical examples are high transmittance, outstanding chemical and thermal stability, high mobility, and mechanical bendability. Thus, graphene has been proposed as an active device component in transistors, sensors, and transparent electrodes. From the perspective of organic light-emitting diodes (OLEDs), graphene has drawn attention as a material for transparent electrode [1][2][3]. Due to the trade-off relation between sheet resistance and transmittance, it is not easy to find a material that is both highly conductive and transparent. In addition, graphene has good mechanical compliance, making it a prominent candidate in the field of flexible optoelectronics [4][5][6][7]. Graphene has been reported to have excellent bending characteristics compared to indium tin oxide (ITO), which is widely used as an OLED transparent electrode [8].
Since 2012, these researchers have focused on the use of graphene in OLEDs. In particular, a multi-layered graphene film was used as an anode in bottom-emissiontype OLEDs. While graphene itself has outstanding CONTACT  properties, integrating it into a device is technically challenging. These researchers' past research achievement may be summarized as overcoming the technical issues relevant to the graphene integration into OLEDs [9,10]. In the course of the research, graphene films obtained via chemical vapor deposition (CVD) were used. Thanks to the nature of the CVD growth method, uniform and large-area graphene can be obtained. In the beginning stage of the years 2012-2014, OLEDs with graphene anode suffered serious electrical instability. The instability was due to the surface roughness of graphene and the unfitting energy alignment to its adjacent hole transport layer. Later, the transfer of large-area graphene turned out to be difficult. Also, the fine patterning of graphene films into pixels emerged as an obstacle. Graphene has poor adhesion to glass or plastic substrates. In addition, due to the nature of the physical transfer process, it is practically impossible to achieve a defect-free status at the graphene/substrate interface. The patterning issue was overcome by the use of liquid bridging [11]. Liquid bridging brought forth three beneficial effects: increased adhesion of graphene to its substrate, elimination of the air pore defects at the graphene/substrate interface, and surface planarization of the graphene film. These features allowed these researchers to implement photolithographic patterning methods with a clean graphene film surface. Figure 1 shows the two major breakthroughs these researchers have achieved. Figure 1(a) shows a fully functional Gen 2(370 × 470 mm)-sized graphene anode OLED. This result signifies the technical feasibility of graphene as a commercially serviceable component in OLED displays. Figure 1(b) shows a two-colour flexible OLED panel. In addition to the accurate patterning of graphene on a polyimide (PI) film, flexible thin-film encapsulation and a laser lift-off process were realized without damaging the organic components of the OLED pixels. The example shown in Figure 1 is a passive-matrix graphene OLED.
The aim of this work was to develop a prototype that is directly applicable to the fabrication of flexible activematrix (AM) graphene anode OLEDs. In addition to the various technical issues dealt with in the past, a thin-film transistor (TFT) array must be fabricated, and its driving scheme must be established. Briefly, an oxide TFT array was formed on a PI film/glass support. On the planarization layer of the TFT array, pixelated graphene was formed. Electrical connection between each TFT and graphene pixel was established through the standard via holes and contact pads. Figure 2 shows the detailed structure and process flow for the fabrication of a flexible AM-graphene OLED panel on a PI substrate. PI varnish was spin-coated on a cleaned glass substrate, and was thermally cured. The PI film was about 2 μm thick. A SiN x (20 nm)-SiO x (100 nm) buffer layer was deposited on the PI film via plasma-enhanced chemical vapor deposition (PECVD). Bottom-gate/topcontact (BGTC) oxide TFT arrays were prepared for driving the OLED device.

Experiments
The structure of the BGTC TFT is shown in Figure 3, and the fabrication procedure is as follows. The OLED pixel driving circuit was composed of two TFTs and one capacitor (2T-1C). An Mo gate electrode was deposited on the buffer layer through a sputtering process, and was patterned. A SiO x gate insulator was deposited on top of the gate electrode to a thickness of 200 nm, using PECVD. Then an aluminum-doped indium zinc tin oxide (Al-IZTO) active layer was deposited using the sputtering process. The composition of Al-IZTO was Al:In:Zn:Sn = 1.7:24.3:34:40, respectively (total thickness: 30 nm). After the patterning of the active layer, thermal annealing was done at 200°C in a vacuum atmosphere. The gate insulator layer was patterned for source/drain (S/D) electrode-gate electrode metal contact. The S/D electrode was deposited using Mo metal. The passivation layer for the planarization and protection of the TFT array was designed with an organic/inorganic multilayer structure. On the top of the passivation layer, SiO x was deposited for facilitating the graphene transfer.
To contact the graphene anode with TFT, a via hole and a transparent contact pad were formed at the top of the passivation layer. The contact via holes were opened from the top of the passivation layer to the driving TFT. The transparent contact pad was deposited via sputtering. The graphene film was transferred to the top of the prepared passivation layer. The four-layered graphene film was used as an anode; it had 70 /sq sheet resistance and 80% transmittance. The adhesion to the substrate was improved by the combined liquid bridge-vacuum thermal treatment of the graphene film [11][12][13][14]. This treatment prevented the graphene film from peeling off  during the photolithography process. To fabricate the OLED anode, a graphene film was patterned via photolithography. The graphene surface was coated with photoresist (PR) using spin coating, and baking was performed on a hot plate. To pixelate the graphene film, a photomask was placed on top of the PR, after which it was exposed to ultraviolet (UV) light, followed by the development process. The opened area of the graphene film was removed by O 2 plasma at 50 Watts, and the PR residual was cleaned with PR remover.
Red bottom emission OLEDs were fabricated on the PI substrate to evaluate the graphene patterning process at the OLED device level. The thermal evaporation method was used for fabricating the organic and metal cathode layer of the OLED. The structure of the OLEDs consisted of a pixelated graphene anode/hole transport layer ( . The OLED was encapsulated using an Al 2 O 3 layer and was bonded with a commercial barrier film. A 30-nm-thick Al 2 O 3 layer was formed via atomic layer deposition (ALD). Finally, the AM-graphene OLED panel was peeled off from the glass substrate using the laser lift-off process.

Pixelated graphene anode
The purpose of this work was to pave the way for the use of graphene as the anode for the active-matrix OLED (AMOLED). Therefore, it is necessary to accurately pattern graphene on the AMOLED backplane. In a commercial display process, the transparent electrode is generally patterned via photolithography. Transferred graphene, however, has weak adhesion, and as such, it can be easily peeled off and damaged during the photolithography process.
The "liquid bridging process" was developed to overcome the aforementioned technical issue. This process improves the adhesion of the transferred graphene, enabling the subsequent photolithography patterning process to be carried out [11]. Figure 4 shows the optical microscope (OM) and scanning electron microscopy (SEM) images of the pixelated graphene electrodes formed on the planarization layer. The graphene pixels had a geometrically accurate size, as designed. As shown  in Figure 4, these images demonstrate the technical feasibility of forming graphene pixels on top of the AMOLED backplane in a stable state, without failure.

Device characteristics
As a preliminary test, phosphorescent red OLEDs with an array of patterned graphene films were fabricated as pixel anodes ( Figure 5). Figure 5(a) shows the current density (J)-voltage (V)-luminance (L) plot. The J-V and L-V characteristics showed the typical behavior of a normally functional OLED. The J and L levels saturate around 10 −1 A/cm 2 and 3 × 10 4 cd/cm 2 . The inset of Figure 5(a) shows the actual emission images of the OLED with pixelated graphene anodes. As can be seen, all the pixels are working without deteriorated marks. The luminance efficiency (LE, lm/W) exhibits the typical roll-off behavior ( Figure 5(b)). A 25 lm/W LE was achieved at the 7.5 × 10 −3 A/cm 2 J level. The Figure 5(b) inset shows the normalized EL spectra of the OLEDs. The EL spectra of the graphene OLED show an aberrational feature, with the central peak at 625 nm. If the graphene surface has contaminants, the OLED device characteristics are expected to be either aberrant or non-working. The study results in Figure 5 show that the proposed graphene film patterning process can be used to fabricate OLEDs.
The fabricated oxide TFT exhibited 25 cm 2 /Vs mobility (μ), as shown in Figure 6. Such μ may suffice for driving an AMOLED pixel. The leakage level does not exceed 10 −12 A up to an applied voltage of 10 V, indicating the outstanding property of the proposed gate dielectrics.  Figure 7(c,d) are the unit pixel layout and schematics of the 2T-1C circuit, respectively. In the panel layout, test pixels and align keys were placed for process defect confirmation. Figure 8 shows the operating image of the fabricated AMOLED panel prototype with the graphene anode and red bottom emission OLED. The unit pixel size of the AMOLED panel was 222×222 μm. The panel resolution was 320×240 QVGA, and the screen size was 3.5 inches. From this panel, a flexible display could be obtained by separating PI from the glass using the LLO process.

Conclusion
In the past, these researchers successfully demonstrated passive-matrix-type organic light-emitting diode (OLED) panels with pixelated graphene films as transparent electrodes. To be specific, fully functional Gen 2(370 × 470 mm)-sized and flexible graphene anode OLEDs were demonstrated. With the aim of developing fully functional flexible AM-graphene OLED panels, the relevant technical issues were presented, and associated solutions were suggested, both on the integration and component levels. The importance of graphene film fine patterning and the use of the laser lift-off technology for realizing flexible panels on a realistic-sized substrate was emphasized. For demonstration purposes, an active-matrix OLED (AMOLED) panel with a QVGA (320 × 240) resolution and a 2T-1C per-pixel scheme was prepared. On a PI/glass support, an oxide thin-film transistor (TFT) array, a planarization layer, and a pixelated graphene array were sequentially formed. The fabrication steps are compatible with those of the existing display processes. It is believed that the proposed approach suggests a meaningful advancement in realizing the ultrathin flexible display, in which the unique properties of graphene are crucial.

Funding
This work was supported by the Ministry of Trade, Industry, and Energy/Korea Evaluation Institute of Industrial Technology (MOTIE/KEIT 10044412) research program "Development of basic and applied technologies for OLEDs with graphene," and by an Electronics and Telecommunications Research Institute (ETRI) grant funded by the South Korean government (19ZB1220, Development of Core Technologies for Implantable Active Devices). research on OLED materials and devices. He has led the Reality Devices Research Division of ETRI since 2017 and has worked on the convergence of display and sensor technologies.