Fabrication of transparent conducting electrode is widely demonstrated with indium tin oxide (ITO) by sputtering method. However ITO is not suitable for flexible substrate, so recently many other kinds of method are researched for making flexible transparent electrodes. As one of them, electrospray method for spraying conducting nanowire or nanoparticle and coating onto the flexible substrate is being applied for transparent conducting electrode. The electrospray is one of the liquid atomization techniques by electrostatic means. When high voltage is applied to a conducting liquid which is fed through a capillary, the meniscus formed at the tip of the capillary becomes a cone shape due to the hydrostatic balance between the electrical forces and the surface tension. Subsequently, the liquid jet comes out at the apex of conical meniscus due to the momentum transfer by the accelerated free charges in the conducting liquid by the electric field formed between the meniscus apex and the ground substrate. Finally, the jet breaks into fine droplets due to the flow instability. The generated droplets have electrical charges, so they self-disperse (through Coulomb’s repulsive forces). Using the electrospray method we can produce mono-dispersed size droplets and size of droplets can be controlled easily. A novel slit-type electrospray nozzle was developed and characterized. The nozzle consisted of two parallel slits, one of which had sawtooth-like tips. We visualized the flow rates through the nozzle and determined the current？flow rate relationship, and found that there was a stable cone-jet mode at each sawtooth-like tip. The sizes of the droplets generated and the flow rate at each spraying tip were determined using a phase Doppler particle analyzer. The nozzle gave mono-disperse droplets with diameters of 18？20 μm and a geometric standard deviation of 1.06？1.10 ？m. The stable cone-jet mode was produced at flow rates of up to 160 mL/h. The tip-by-tip variations in the droplet sizes and the flow rates at the tips were less than 5% and 6%, respectively. And using this saw-tooth type electrospray nozzle, large area deposition of silver nanowire onto flexible substrate (PET film) was achieved. By varying the flowrate of spray from 30 ~ 60 mL/h, changing the surface temperature from 140°C to 150°C, and varying the deposition time from 5 ~ 30 min., silver nanowire coated PET film was made or transparent electrode. The optical and electrical characteristics are measured by UV/VIS/NIR spectrophotometer and 4-point probe system resistance meter. The sheet resistances can be obtained to ~5.5 ohm/sq, and the optical transmittance can be obtained to ~78% respectively. The bending test was conducted for confirming the flexibility and the durability. In spite of bending of more 2,000 cycles, the sheet resistances were increased by 18% comparing with those of the TCE before bending. This TCE was also applied into touchpad simulation, which consist of 2-TCE and 4-electrode, the controller and monitor. The position of contact between two TCE can be measured by the controller, and the point or the line are displayed at the monitor. Handwriting recognition test was conducted by writing letters onto TCE, and the software can recognize the input letters.
In this study, we confirmed that a stable cone-jet mode was achieved at each spraying tip and the new nozzle generates mono-disperse droplets. The tip-by-tip variation of the droplet size and the flow rate was less than 5% and 6%, respectively. The nozzle can therefore be thought of as an array of identical spraying nozzles. The developed nozzle has several advantages over an array of conventional electrospray nozzles. It is easy to construct with a simple tubing sys-tem for delivering the liquid, and it has the ability to generate mono-dispersed droplets with high throughput. The small tip-by-tip variation is beneficial to uniform surface coating. These advantages combined with a suitable manipulation stage system, could be used for the fabrication of large areas of thin films and for producing surface coatings. Our results show good agreement with previous Q1/2 and Q1/3 laws for current and droplet size. And this study indicates that a silver nanowire electrode manufactured by electrospray deposition system using saw-tooth type slit nozzle can become flexible transparent electrode. By measuring the optical transmittance and the sheet resistance, we confirmed that TCE made by ESD method and saw-tooth type slit nozzle can be operated as transparent conducting electrode. Finally we also con-firmed that the practical touchpad panel can be assembled and well operated by consisting this TCE, and by replacing commercial product of transparent touch-panel.
투명전극의 제조는 주로 ITO 물질을 사용하여 스퍼터링 방법으로 제조되고 있다. 하지만 ITO는 유연한 기판에 적용되기 어려운 단점이 있어, 최근에는 휘어지는 투명전극을 만들기 위한 여러가지 다른 방법들이 연구되고 있다. 그 중 한 방법으로 메탈 나노입자 또는 나노와이어를 정전분무법을 사용하여 유연한 기판에 코팅하여 투명전극을 제작하는 방법이 적용되고 있다. 정전분무는 액체를 미립화하는 방법 중 하나로써 정전기력을 이용한다.
캐필러리 튜브로 주입되는 전도성 액체에 고전압이 인가되면, 캐필러리 출구부분 팁에서 콘 형상의 메니스커스가 형성되는데, 이는 유체의 표면장력과 전기력이 균형을 이루어 발생하는 것이다. 콘의 끝에서 액주가 형성되고 액주의 끝에서는 유체의 불안정성에 의해서 액적으로 분리가 된다. 이렇게 발생한 액적은 서로 같은 극의 전하를 띠고 있기 때문에 반발력을 가지게 되고, 따라서 미립화된 액적이 서로 분리된다. 정전분무법을 이용하여 단분산분포를 가지는 액적을 제조하고 이것을 이용하여 표면 코팅에 적용을 한다. 대면적의 표면 코팅을 위해서 기존의 슬릿 노즐에 스프레잉 팁을 장착하여 정전분무법을 적용하였다. 안정된 콘젯 모드의 정전분무를 가시화를 통하여 확인하였고, 또한 전류, 직경, 유량의 관계를 확인함으로써 콘젯 모드를 확인하였다. 또한 톱니형상의 슬릿 노즐의 각 팁에서 발생하는 액적의 크기가 편차 5% 이내로 일정함을 확인하였고, 유량 또한 6% 이내로 일정함을 확인하였다. 발생된 입자의 기하표준편차는 1.10 미만으로 단분산 분포를 가짐을 확인하였다. 이렇게 제안되고 개발된 톱니형상 슬릿 노즐을 이용하여 정전분무법으로 투명전극을 제조하였다. 은 나노 와이어를 이소프로필 알콜에 분산시킨 후 정전분무를 사용하여 PET 필름에 코팅하였다. 분무 유량은 30~60 mL/h를 설정하였고, 해당 유량에서 콘젯 모드를 발생시킬 수 있는 인가 전압을 설정하여 적용하였다. 이렇게 완성된 투명전극의 표면저항과 투과도를 측정하였다. 표면저항은 최소 5.5 Ω/sq, 투과도는 최대 78%를 나타내었다. 또한 유연성을 확인하기 위해서 최소곡률반경 2,4,6 mm의 조건으로 2,000회의 굽힘 시험을 수행하였다. 굽힘 시험 이후 최대 18%의 표면저항 변화가 있는 것을 확인하였다. 완성된 2매의 대면적 투명전극을 이용하요 터치 패널을 제작하였다. 터치 패널로 손글씨를 입력하였고, 입력된 글씨가 문자인식 프로그램에서 잘 인식됨을 확인하였다.
