Many engineers make preparations for an era of Internet of everything (IOE) beyond Internet of things (IOT). It is based on needs and expectations of humans for more convenient life. To improve human-friendly techniques for IOE, transitions from hard electronics to soft electronics are required, because flexible electronics are more suited for the human body. Therefore, integrated electronic devices on clothes have started to appear. When people use their clothes as a smart device, an information display system is the one of the most significant parts to communicate with machines on e-textile. For that reason, novel wearable display designs have been proposed recently, but the outcomes of preceding works still showed insufficiencies of device performance. To be widely used, highly efficient wearable display devices with reliability are needed.
Here, we report an extremely flexible, efficient, and reliable textile-based display device using organic light-emitting diodes (OLEDs). A polymer film was thermally transferred from a guide substrate to a textile substrate in order to flatten a surface of the textile. After this process, OLEDs on the textile substrate emitted light stably on the condition that the bending radius is about 3 mm. Use of a semi-transparent metal electrode in the OLEDs contributed to strong micro-cavity effects which improved the current efficiency of OLEDs. Additive depositions of protecting layers helped prevent the transmission of water vapors and oxygens during the encapsulation process, and made out-coupling efficiency high. The encapsulation process consisted of alternative atomic layer depositions (ALD) of $Al_2O_3$ and spin coatings of a silane-based barrier solution. The calcium corrosion test results for 3.5 dyads of multi-barrier encapsulation showed an extremely low water vapor transmission rate (WVTR) value, 4.96 x $10^{-6}$ g/ $m^2$/ day. The engineering suggested in this paper may lead to advances of next-generation wearable electronics.
우리 연구진은 차세대 기술로 촉망 받는 wearable displays의 실현에 한 걸음 다가가고자 실제 직물 기반의 display device를 제작하여 장기적인 신뢰성과 함께 유연성에 대해 검증하였다.
Textile 기판은 얇은 fiber들의 weaving으로 제작하였다. 이 woven structure 때문에 textile 기판은 micrometer-scale의 roughness를 가지고 있으며, 우리는 thermal lamination 공정을 통해 Rq = 2.073 nm 수준의 planarized textile을 제작하여 surface rough-ness를 nanometer-scale로 줄였다. Planarized textile 기판 위에는 Al2O3와 silane-based film의 1.5 dyad로 이루어진 multi-barrier encapsulation을 형성하였다. 이 multi-barrier는 moisture barrier의 특성과 동시에 평탄화 기능을 함께 했다. 이 위에는 top-emitting, phos-phorescent OLED 소자가 증착되었으며, encapsulation 공정 중 소자가 degradation 되는 현상을 막기 위해 추가적으로 protecting layer($MoO_3$/ NPB/ $MoO_3$)를 증착했다. 최종적으로 OLED 소자 위에 2.5 dyads의 multi-barrier encapsulation 공정이 진행하여, real textile-based OLED 소자를 구현하였다.
본 연구에서 제작된 textile-based OLED 소자를 30 °C, 상대습도 90%의 조건을 가지는 항온항습기에 보관하여 가속수명을 측정하였으며, cyclic bending test와 ANSYS simu-lation을 통해 bending 특성을 분석하였다. 이를 통해 textile-based OLEDs는 동일 두께의 plastic 기판을 사용한 flexible OLEDs보다 유연한 성능을 가지며, 이는 textile 기판의 구조적인 특성에 기인한다는 것을 확인하였다.
