Transparent Micromesh Patterned OLED Fabricated Using the In-Situ Deposition Process of an Ultrathin Mg:Ag Interconnection Layer.

TOLEDs (transparent organic light-emitting diodes) have emerged as one of the most promising ways to implement next-generation display form factors. Transparent OLEDs can provide new added value to HMDs (head mounted displays), automobiles, smart windows, mobile devices, TVs, etc. through their transparency, which allows objects to be seen from the other side. However, previous approaches using metal thin films have faced limitations in attempting to achieve high transmittance. In this study, TOLEDs were designed using a new cathode structure consisting of an interlayer and an emission pattern layer, and these layers connect the light-emitting part and the nonemitting part by themselves without requiring the use of another interconnection layer. This structure, which was intended to improve transmittance, was implemented by applying an in situ evaporation process that adds only one shadow mask without the need to use any difficult methods. Through this process, the optimal condition was found when the light-emitting part was deposited in a mesh pattern with a length of 120 µm and a width of 80 µm, in which case the transmittance of the TOLED improved by up to 83% while maintaining electro-optical performance. It was also confirmed that this new structure can be applied to flexible devices.

355 nm Nanosecond Ultraviolet Pulsed Laser Annealing Effects on Amorphous In-Ga-ZnO Thin Film Transistors.

Bottom-gate thin-film transistors (TFTs) with n-type amorphous indium-gallium-zinc oxide (a-IGZO) active channels and indium-tin oxide (ITO) source/drain electrodes were fabricated. Then, an ultraviolet (UV) nanosecond pulsed laser with a wavelength of 355 nm was scanned to locally anneal the active channel at various laser powers. After laser annealing, negative shifts in the threshold voltages and enhanced on-currents were observed at laser powers ranging from 54 to 120 mW. The energy band gap and work function of a-IGZO extracted from the transmittance and ultraviolet photoelectron spectroscopy (UPS) measurement data confirm that different energy band structures for the ITO electrode/a-IGZO channel were established depending on the laser annealing conditions. Based on these observations, the electron injection mechanism from ITO electrodes to a-IGZO channels was analyzed. The results show that the selective laser annealing process can improve the electrical performance of the a-IGZO TFTs without any thermal damage to the substrate.

Highly Transparent Red Organic Light-Emitting Diodes with AZO/Ag/AZO Multilayer Electrode.

Free-form factor optoelectronics is becoming more important for various applications. Specifically, flexible and transparent optoelectronics offers the potential to be adopted in wearable devices in displays, solar cells, or biomedical applications. However, current transparent electrodes are limited in conductivity and flexibility. This study aims to address these challenges and explore potential solutions. For the next-generation transparent conductive electrode, Al-doped zinc oxide (AZO) and silver (AZO/Ag/AZO) deposited by in-line magnetron sputtering without thermal treatment was investigated, and this transparent electrode was used as a transparent organic light-emitting diode (OLED) anode to maximize the transparency characteristics. The experiment and simulation involved adjusting the thickness of Ag and AZO and OLED structure to enhance the transmittance and device performance. The AZO/Ag/AZO with Ag of 12 nm and AZO of 32 nm thickness achieved the results of the highest figure of merit (FOM) (Φ550 = 4.65 mΩ-1) and lowest roughness. The full structure of transparent OLED (TrOLED) with AZO/Ag/AZO anode and Mg:Ag cathode reached 64.84% transmittance at 550 nm, and 300 cd/m2 at about 4 V. The results demonstrate the feasibility of adopting flexible substrates, such as PET, without the need for thermal treatment. This research provides valuable insights into the development of transparent and flexible electronic devices.

Analyses of All Small Molecule-Based Pentacene/C60 Organic Photodiodes Using Vacuum Evaporation Method.

The vacuum process using small molecule-based organic materials to make organic photodiodes (OPDIs) will provide many promising features, such as well-defined molecular structure, large scalability, process repeatability, and good compatibility for CMOS integration, compared to the widely used Solution process. We present the performance of planar heterojunction OPDIs based on pentacene as the electron donor and C60 as the electron acceptor. In these devices, MoO3 and BCP interfacial layers were interlaced between the electrodes and the active layer as the electron- and hole-blocking layer, respectively. Typically, BCP played a good role in suppressing the dark current by two orders higher than that without that layer. These devices showed a significant dependence of the performance on the thickness of the pentacene. In particular, with the pentacene thickness of 25 nm, an external quantum efficiency at the 360 nm wavelength according to the peak absorption of C60 was enhanced by 1.5 times due to a cavity effect, compared to that of the non-cavity device. This work shows the importance of a vacuum processing approach based on small molecules for OPDIs, and the possibility of improving the performance via the optimization of the device architecture.

Argon and Oxygen Gas Flow Rate Dependency of Sputtering-Based Indium-Gallium-Zinc Oxide Thin-Film Transistors.

Oxygen vacancies are a major factor that controls the electrical characteristics of the amorphous indium-gallium-zinc oxide transistor (a-IGZO TFT). Oxygen vacancies are affected by the composition ratio of the a-IGZO target and the injected oxygen flow rate. In this study, we fabricated three types of a-IGZO TFTs with different oxygen flow rates and then investigated changes in electrical characteristics. Atomic force microscopy (AFM) was performed to analyze the surface morphology of the a-IGZO films according to the oxygen gas rate. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis was performed to confirm changes in oxygen vacancies of a-IGZO films. The optimized a-IGZO TFT has enhanced electrical characteristics such as carrier mobility (µ) of 12.3 cm2/V·s, on/off ratio of 1.25 × 1010 A/A, subthreshold swing (S.S.) of 3.7 V/dec, and turn-on voltage (Vto) of -3 V. As a result, the optimized a-IGZO TFT has improved electrical characteristics with oxygen vacancies having the highest conductivity.

Rapid photonic curing effects of xenon flash lamp on ITO-Ag-ITO multilayer electrodes for high throughput transparent electronics.

High-throughput transparent and flexible electronics are essential technologies for next-generation displays, semiconductors, and wearable bio-medical applications. However, to manufacture a high-quality transparent and flexible electrode, conventional annealing processes generally require 5 min or more at a high temperature condition of 300 °C or higher. This high thermal budget condition is not only difficult to apply to general polymer-based flexible substrates, but also results in low-throughput. Here, we report a high-quality transparent electrode produced with an extremely low thermal budget using Xe-flash lamp rapid photonic curing. Photonic curing is an extremely short time (~ µs) process, making it possible to induce an annealing effect of over 800 °C. The photonic curing effect was optimized by selecting the appropriate power density, the irradiation energy of the Xe-flash lamp, and Ag layer thickness. Rapid photonic curing produced an ITO-Ag-ITO electrode with a low sheet resistance of 6.5 ohm/sq, with a high luminous transmittance of 92.34%. The low thermal budget characteristics of the rapid photonic curing technology make it suitable for high-quality transparent electronics and high-throughput processes such as roll-to-roll.

Effects of Insertion of Ag Mid-Layers on Laser Direct Ablation of Transparent Conductive ITO/Ag/ITO Multilayers: Role of Effective Absorption and Focusing of Photothermal Energy.

From the viewpoint of the device performance, the fabrication and patterning of oxide-metal-oxide (OMO) multilayers (MLs) as transparent conductive oxide electrodes with a high figure of merit have been extensively investigated for diverse optoelectronic and energy device applications, although the issues of their general concerns about possible shortcomings, such as a more complicated fabrication process with increasing cost, still remain. However, the underlying mechanism by which a thin metal mid-layer affects the overall performance of prepatterned OMO ML electrodes has not been fully elucidated. In this study, indium tin oxide (ITO)/silver (Ag)/ITO MLs are fabricated using an in-line sputtering method for different Ag thicknesses on glass substrates. Subsequently, a Q-switched diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) laser is employed for the direct ablation of the ITO/Ag/ITO ML films to pattern ITO/Ag/ITO ML electrodes. Analysis of the laser-patterned results indicate that the ITO/Ag/ITO ML films exhibit wider ablation widths and lower ablation thresholds than ITO single layer (SL) films. However, the dependence of Ag thickness on the laser patterning results of the ITO/Ag/ITO MLs is not observed, despite the difference in their absorption coefficients. The results show that the laser direct patterning of ITO/Ag/ITO MLs is primarily affected by rapid thermal heating, melting, and vaporization of the inserted Ag mid-layer, which has considerably higher thermal conductivity and absorption coefficients than the ITO layers. Simulation reveals the importance of the Ag mid-layer in the effective absorption and focusing of photothermal energy, thereby supporting the experimental observations. The laser-patterned ITO/Ag/ITO ML electrodes indicate a comparable optical transmittance, a higher electrical current density, and a lower resistance compared with the ITO SL electrode.

A Study on Formation of Aluminum-Doped Zinc Oxide Films by Pulsed-Direct Current Sputtering for CIGS Solar Cells.

Considering the relationship between thin film thickness of transparent conductive oxide (TCO) materials and the reversed pulse time in pulsed-direct current (DC) sputtering, aluminum-doped zinc oxide (AZO) films were deposited on glass substrates at different reversed pulse times by changing oxygen/argon (O2/Ar) gas ratios for window layers of large area CuIn1-xGaxSe2 (CIGS) solar cells. As a result of the reduced sputtering time, the thickness of AZO film was decreased when the reversed pulsed time was increased. The higher resistance and resistivity of the AZO film was obtained at a higher reversed pulse time. From the structural investigations of AZO such as transmittance and X-ray diffraction (XRD), it was possible to observe the relationship between the crystallinity of AZO and transmittance. Even at the short reversed pulse time of 0.5 µs, it can be concluded that the accumulated charges on the AZO target are completely cleared and the AZO layers show the highest figure of merit (FOM) with low sheet resistance and high transmittance.

Understanding of the Mechanism for Laser Ablation-Assisted Patterning of Graphene/ITO Double Layers: Role of Effective Thermal Energy Transfer.

Demand for the fabrication of high-performance, transparent electronic devices with improved electronic and mechanical properties is significantly increasing for various applications. In this context, it is essential to develop highly transparent and conductive electrodes for the realization of such devices. To this end, in this work, a chemical vapor deposition (CVD)-grown graphene was transferred to both glass and polyethylene terephthalate (PET) substrates that had been pre-coated with an indium tin oxide (ITO) layer and then subsequently patterned by using a laser-ablation method for a low-cost, simple, and high-throughput process. A comparison of the results of the laser ablation of such a graphene/ITO double layer with those of the ITO single-layered films reveals that a larger amount of effective thermal energy of the laser used is transferred in the lateral direction along the graphene upper layer in the graphene/ITO double-layered structure, attributable to the high thermal conductivity of graphene. The transferred thermal energy is expected to melt and evaporate the lower ITO layer at a relatively lower threshold energy of laser ablation. The transient analysis of the temperature profiles indicates that the graphene layers can act as both an effective thermal diffuser and converter for the planar heat transfer. Raman spectroscopy was used to investigate the graphite peak on the ITO layer where the graphene upper layer was selectively removed because of the incomplete heating and removal process for the ITO layer by the laterally transferred effective thermal energy of the laser beam. Our approach could have broad implications for designing highly transparent and conductive electrodes as well as a new way of nanoscale patterning for other optoelectronic-device applications using laser-ablation methods.

Effects of Radio-Frequency Sputtering Power on Low Temperature Formation of MoS2 Thin Films on Soda-Lime Glass Substrates.

For the realization of the economical and reliable fabrication process of molybdenum disulfide (MoS²) layers, MoS² thin films were directly formed a on soda-lime glass substrate by RF sputtering and subsequent rapid thermal annealing (RTA) at a temperature range of 400-550 °C. Using scanning electron microscopy and atomic force microscopy, it was possible to investigate more stable surface morphologies of MoS² layers at lower RF sputtering powers irrespective of the RTA temperature. Even at an RTA temperature of less than 550 °C, the Raman exhibited more distinct E12g and A1g peaks for the MoS² layers sputtered at lower RF powers. The X-ray photoelectron spectroscopy results revealed that more distinct peaks were observed at a higher RTA temperature, and the peak positions were moved to higher energies at a lower RF sputtering power. Based on the Hall measurements, higher carrier densities were obtained for the MoS² layers sputtered at lower RF powers.

Thickness Effects of Radio-Frequency-Sputtered Molybdenum Disulfide Films on Soda-Lime Glass Substrates.

Molybdenum disulfide (MoS2) films were directly formed on soda-lime glass substrates by radio-frequency (RF) sputtering and rapid thermal processing (RTP) to avoid physical exfoliation and transfer process of a MoS2 layer. The sputtering time was adjusted in the fabrication process and the thickness effects of the MoS2 films were investigated in terms of structural and electrical characteristics. The surface morphologies were not dependent on the film thickness but on the RF sputtering power after the film was annealed using RTP. The electrical mobility of the MoS2 film was more dependent on the film thickness at lower RF sputtering powers and low annealing temperatures. An investigation of the E12g and A11g peaks in the Raman spectra revealed that the two-dimensional properties of the MoS2 layers were more distinct in the case of a lower thickness.

Low-Temperature Process for Direct Formation of MoS2 Thin Films on Soda-Lime Glass Substrates.

To obtain molybdenum disulfide (MoS2) patterns without any mechanical problems caused by the transfer process, direct current (DC) sputtering and rapid thermal processing (RTP) were used to form MoS2 instead of the conventional chemical vapor deposition (CVD) process. To form MoS2 on a soda-lime glass substrate at temperatures below 600 °C, MoS2 films were deposited at various DC sputtering powers and annealed at various temperatures from 400 °C to 550 °C. From the scanning electron microscope (SEM) and atomic force microscope (AFM) results, the surface morphologies of the MoS2 films can be observed, depending on the sputtering power and the film thickness. The Raman spectrum results showed that the E12g and A1g mode peaks appeared at approximately 372 cm-1 and 400 cm-1, respectively, and the MoS2 surface was crystallized in the in-plane direction. The X-ray photoelectron spectroscopy (XPS) results showed noticeable S 2p (2p3/2, 2p1/2) peaks and Mo 3d (3d5/2, 3d3/2) peaks at stable binding energies after RTP at temperatures below 600 °C. The high mobilities and carrier densities of all the MoS2 films can be investigated from the Hall measurements.

A Study on the Formation of 2-Dimensional Tungsten Disulfide Thin Films on Sapphire Substrate by Sputtering and High Temperature Rapid Thermal Processing.

As direct formation of p-type two-dimensional transition metal dichalcogenides (TMDC) films on substrates, tungsten disulfide (WS2) thin films were deposited onto sapphire glass substrate through shadow mask patterns by radio-frequency (RF) sputtering at different sputtering powers ranging from 60 W to 150 W and annealed by rapid thermal processing (RTP) at various high temperatures ranging from 500 °C to 800 °C. Based on scanning electron microscope (SEM) images and Raman spectra, better surface roughness and mode dominant E12g and A1g peaks were found for WS2 thin films prepared at higher RF sputtering powers. It was also possible to obtain high mobilities and carrier densities for all WS2 thin films based on results of Hall measurements. Process conditions for these WS2 thin films on sapphire substrate were optimized to low RF sputtering power and high temperature annealing.

Surface Functionalization of Liquid-Phase Exfoliated, Two-Dimensional MoS2 and WS2 Nanosheets with 2-Mercaptoethanol.

In this work, the UV-Vis-NIR absorption spectrum of liquid-phase exfoliated two-dimensional (2D) MoS2 nanosheets, revealed two prominent peaks at 608 nm (2.04 eV) and 668 nm (1.86 eV). These peaks were blue-shifted compared to the reported literature values and are attributed to the quantum confinement effect. Interestingly, the WS2 nanosheets exhibited the same characteristic absorption peak at ~624 nm (1.99 eV). Raman spectroscopy analysis revealed that both nanosheets displayed distinctive peaks [377.8 cm-1 and 405.6 cm-1 for MoS2, 348.3 cm-1 and 417.9 cm-1 for WS2] that originate from optical phonon modes (E12g and A1g). These peaks are shifted toward higher wavenumbers (i.e., blue-shift or phonon-stiffening) compared to bulk MoS2 and WS2, probably due to enhanced Stokes Raman scattering. Subsequently, surface functionalization of the nanosheets with 2-Mercaptoethanol was successfully performed and confirmed using optical characterization techniques, including FT-IR spectroscopy. In addition, we determined the spectral broadening after functionalization, which would be attributed to photon confinement of the nano-sized layer structure, or to inhomogeneous broadening.

Direct Ablation by Laser of Single Graphene Monolayer and Graphene/Photopolymer Double Layer.

A diode-pumped Q-switched neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) laser was applied to obtain graphene patterns on a photopolymer layer by direct ablation. In the transfer process of the graphene layer, the photopolymer was employed as a graphene supporting layer and it was not removed for the simplification of the process. The laser ablation was carried out on graphene/photopolymer double layers for various beam conditions. The results showed that the laser-ablated widths on the graphene/photopolymer double layer were much greater than those on the graphene monolayer, especially at lower scanning speeds and at higher repetition rates. The photopolymer layer was not removed by the laser ablation, and the thermal energy was considered to have been dissipated in the lateral direction of graphene instead of being conducted vertically to the glass substrate. The Raman spectrum results showed that the graphene layer was clearly removed on the laser-ablated region of interest.

Laser Direct Ablation of Indium Tin Oxide Films on Both Sides of Various Substrates.

We demonstrate ablation of indium tin oxide (ITO) films onto both glass and polyethylene terephthalate (PET) substrates, using a Q-switched diode-pumped neodymium-doped yttrium vanadate laser (Nd:YVO4, λ = 1064 nm) incident on both the front and back sides of the substrate. From scanning electron microscope (SEM) images and depth profile data, ITO patterns that were laser-ablated onto glass from the back side showed a larger abrupt change in the ablated line width than those ablated from the front. However, there were only slight differences in ablated line widths due to the direction of the incident laser beam. We provide a possible explanation in terms of several factors: dispersion of laser beam energy through the substrate, overlapping of each laser beam spot due to scanning speed, and the thickness of glass and PET substrates.

Ablation of graphene film by direct Nd:YVO4 laser under various beaming conditions.

Recently, graphene is gaining increasing popularity as one of the most functional materials for advanced electronic and optical devices owing to its high carrier mobility and optical transparency. Patterning the graphene calls for particular cares in line definition without carbon (C)-based residues that might be working as a leakage path. Thus, realization and processing of the graphene monolayer are very complicated and need to be stringently controlled. For this reason, in accordance, processing technology should be evolved with higher reliability and accuracy, and compatibility with the conventional unit processes including electron beam (e-beam) lithography, plasma etching, and nano-dimensional optical lithography. In this work, a reliable, simple, and cost-effective technique for patterning the graphene is proposed. Graphene film transferred on glass substrate is directly patterned by a quality factor (Q)-switched neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) pumped laser diode (LD). In order to optimize the process condition, various beaming conditions of repetition rate and scanning speed are experimented. From the optical microscope images, it has been shown that graphene film was more easily etched by direct laser patterning technique at higher repetition and faster scanning speed. It was confirmed by Raman spectrum where 2-dimensional (2-D) and graphite (G) peaks were closely investigated that graphene residue was also completely removed after the proposed processing technique.

Laser direct patterning of indium tin oxide for defining a channel of thin film transistor.

In this work, using a Q-switched diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4, lambda = 1064 nm) laser, a direct patterning of indium tin oxide (ITO) channel was realized on glass substrates and the results were compared and analyzed in terms of the effect of repetition rate, scanning speed on etching characteristics. The results showed that the laser conditions of 40 kHz repetition rate with a scanning speed of 500 mm/s were appropriate for the channeling of ITO electrodes. The length of laser-patterned channel was maintained at about 55 microm. However, residual spikes (about 50 nm in height) of ITO were found to be formed at the edges of the laser ablated area and a few ITO residues remained on the glass substrate after laser scanning. By dipping the laser-ablated ITO film in ITO diluted etchant (ITO etchant/DI water: 1/10) at 50 degrees C for 3 min, the spikes and residual ITO were effectively removed. At last, using the laser direct patterning, a bottom-source-drain indium gallium zinc oxide thin film transistor (IGZO-TFT) was fabricated. It is successfully demonstrated that the laser direct patterning can be utilized instead of photolithography to simplify the fabrication process of TFT channel, resulting in the increase of productivity and reduction of cost.

Effects of the duty ratio on the niobium oxide film deposited by pulsed-DC magnetron sputtering methods.

Niobium oxide (Nb2O5) films were deposited on p-type Si wafers and sodalime glasses at a room temperature using in-line pulsed-DC magnetron sputtering system with various duty ratios. The different duty ratio was obtained by varying the reverse voltage time of pulsed DC power from 0.5 to 2.0 micros at the fixed frequency of 200 kHz. From the structural and optical characteristics of the sputtered NbOx films, it was possible to obtain more uniform and coherent NbOx films in case of the higher reverse voltage time as a result of the cleaning effect on the Nb2O5 target surface. The electrical characteristics from the metal-insulator-semiconductor (MIS) fabricated with the NbOx films shows the leakage currents are influenced by the reverse voltage time and the Schottky barrier diode characteristics.

Nd:YVO4 laser direct ablation of indium tin oxide films deposited on glass and polyethylene terephthalate substrates.

A Q-switched diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4, lambda = 1064 nm) laser was applied to obtain the indium tin oxide (ITO) patterns on flexible polyethylene terephthalate (PET) substrate by a direct etching method. After the ITO films were deposited on a soda-lime glass and PET substrate, laser ablations were carried out on the ITO films for various conditions and the laser ablated results on the ITO films were investigated and analyzed considering the effects of substrates on the laser etching. The laser ablated widths on ITO deposited on glass were found to be much narrower than those on ITO deposited on PET substrate, especially, at a higher scanning speed of laser beam such as 1000 mm/s and 2000 mm/s. As the thermal conductivity of glass substrate is about 7.5 times higher than that of PET, more thermal energy would be spread and transferred to lateral direction in the ITO film in case of PET substrate.