We investigated electroluminescent (EL) characteristics of polymer and low molecular organic light emitting diodes. We synthesized new polymers and low molecular organic light emitting material containing oxadiazole unit or bis-styryl units. The synthesized polymers and low molecular organic light emitting material were soluble in common organic solvents and showed good thermal stability.
A series of electroluminescent π-conjugated polymers containing an oxadiazole group in the backbone, poly[(2,5-bis(5-hexyloxyphenyl)-1,3,4-oxadiazole)-2,2-diylvinylene-alt-1,4-phenylenevinylene] (POOXPV), poly[(2,5-bis(2-hexyloxyphenyl)-1,3,4-oxadiazole)-5,5-diylvinylene-alt-1,4-phenylenevin-ylene] (PMOXPV) and poly[(2,5-diphenyl-1,3,4-oxadiazole)-2,4-diylvinylene-alt-1,4-(2,5-dihexyloxy)-phenylenevinylene] (PPOXPV), were prepared through Heck’s coupling or Wittig’s condensation reaction. The maximum photoluminescence (PL) wavelengths of POOXPV, PMOXPV and PPOXPV appeared at 495 nm, 470 nm, and 510 nm, respectively. The electroluminescence (EL) spectra of POOXPV and PPOXPV showed maximum peaks at 500 nm and 510 nm, respectively, corresponding to greenish-blue light, with the single-layer light-emitting diodes of Al/polymer/ITO glass fabricated. To obtain high intense emissive light devices and to investigate electron-transporting ability, the POOXPV was blended with poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamin)styryl]-4H-pyran (DCM). The maximum brightness of the Al/(MEH-PPV/POOXPV=1:5)/ITO single-layer device was 2,400 cd/㎡. And compared with 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) (well known as electron-transporting material), synthesized polymers are also believed to serve as an excellent polymer electron-transporting materials in our devices.
We also synthesized two new fully conjugated alternating copolymers containing both oxadiazole and carbazole (or fluorene) units through the Wittig condensation polymerization. The maximum PL and EL wavelengths of poly[(2,5-di-phenyl-1,3,4-oxadiazole)-4-diylvinylene-alt-3,6-(9-(2-ethylhexyl)-carbazolenevinylene)] (PPOX-CAR) and the poly[(2,5-di-phenyl-1,3,4-oxadiazole)-3-diylvinylene-alt-3,6-(9-(2-ethylhexyl)-carbazolenevinylene)] (PMOX-CAR) were varied from 495 nm in the greenish-blue emission region to 450 nm in the blue emission region depending on the kink structure. The turn on voltages of PPOX-CAR and PMOX-CAR were 7.5 V and 10.5 V, respectively, when the single layer light-emitting diodes of Al/PPOX-CAR or PMOX-CAR/ITO glass were fabricated. The maximum brightness of Al/PPOX-CAR/ITO single layer device was 500 $cd/m^2$ at 20 V. Additionally, poly[(2,5-di-phenyl-1,3,4-oxadiazole)-4-diylvinylene-alt-2,7-(9,9-di-n-hexylfluornenevinyl-ene)] (PPOX-FLU), and the poly[(2,5-di-phenyl-1,3,4-oxadiazole)-3-diylvinylene-alt-2,7-(9,9-di-n-hexylfluorenevinylene)] (PMOX-FLU) were synthesized and showed almost the same optical properties as the PPOX-CAR and PMOX-CAR, however, the electrical properties of polymers containing carbazole unit showed lower turn-on voltage and higher efficiencies.
A new fully conjugated light-emitting polymer based on phenylene-alt-thiophene backbone has been synthesized by utilizing the Stille coupling reaction. And also, we synthesized poly(p-phenylene) derivative using the nickel catalyst reaction. The PL and EL peak wavelengths of the poly[2,5-bis{4-(2-ethylhexyloxy)-1-phenylvinyl}-phenylene-1,4-thiophene] (PPVPT) were 525 nm and 520 nm, respectively. The EL spectrum of PPVPT showed a broad wavelength emission. The turn-on voltage of polymer appeared at 10 V in ITO/polymer/Al device fabrication. And PL of the poly[2,5-bis{4-(2-ethylhexyloxy)-1-phenylvinyl}-phenylene] (PPVPP) was 485 nm, in greenish-blue color region.
Finally, We synthesized low molecular organic light emitting material containing both oxadiazole and triphenylamine units. The 2,5-bis{4-(triphenylamine)vinylenephenyl}-1,3,4-oxadiazole (2TPA-OX) was available to fabricate LED with both spin casting and vacuum evaporating method. The turn-on voltage of single layer device (spin casting) for light out at 8.5 V and maximum brightness is 120 cd/㎡ at 20 V. The turn-on voltage of triple layer device (vacuum evaporating) for light out at 4.4 V and maximum brightness is 520 cd/㎡ at 20 V.
본 연구는 유기 발광 및 고분자 물질에 관한 합성 및 특성 연구이다. 합성된 새로운 단분자 및 고분자는 옥사다이아졸 작용기를 갖거나, 비스-스타이릴 단위를 가지고 있다. 합성 된 단분자 및 고분자는 일발적인 용매에 잘 용해되었으며, 좋은 열적 안정성을 보였다. 옥사다이아졸기를 갖는 고분자인 폴리[(2,5-비스(5-헥실옥시페닐)-1,3,4-옥사다이아졸)-2,2-다이닐비닐렌-알트-1,4-페닐렌비닐렌] (POOXPV), 폴리[(2,5-비스(2-헥실옥시페닐)-1,3,4-옥사다이아졸)-5,5-다이닐비닐렌-알트-1,4-페닐렌비닐렌] (PMOXPV) 와 폴리[(2,5-다이페닐-1,3,4-옥사다이아졸)-2,4-다이닐비닐렌-알트-1,4-(2,5-다이헥실옥시)-페닐렌비닐렌] (PPOXPV) 는 Heck 반응 혹은 Wittig 반응에 의해서 합성하였다. POOXPV, PMOXPV 와 PPOXPV의 최대 빛 발광 파장은 각각 495, 470, 510 nm 이였다. 그리고 POOXPV 와 PPOXPV 의 최대 전기 발광 파장은 각각 초록색을 띤 청색인 500 과 510 nm 에서 나타났다. 이때의 발광 다이오드의 구조는 음극으로 알루미늄 전극을 양극으로 ITO 전극을 사용하였다. 옥사다이아졸을 포함한 새롭게 합성된 고분자를 이용해서 매우 밝은 빛을 나타내는 다이오드를 얻기 위해, 폴리[2-메톡시-5-(2-에틸헥실옥시)-1,4-페닐렌비닐렌] (MEH-PPV) 와 블랜딩을 하였다. 플랜딩 비율이 1 : 5 = MEH-PPV : POOXPV 일때 단일층 구조에서 최대 휘도가 2,400 cd/㎡ 라는 밝은 빛을 얻었다. 또한 옥사다이아졸을 포함하는 고분자는 전자수송능력이 전자수송 물질로 기존에 가장 널리 알려진 2-(4-바이페닐닐)-5-(4-t-부틸페닐)-1,3,4-옥사다이아졸 (PBD) 과 비교했을 때 거의 같은 능력을 보였다.
옥사다이아졸기와 동시에 카바졸기를 혹은 플루오렌기를 갖는 고분자를 Wittig 반응을 통해서 합성하였다. 폴리[(2,5-다이-페닐-1,3,4-옥사다이아졸)-4-다이닐비닐렌-알트-3,6-(9-(2-에틸헥실)-카바졸렌비닐렌)] (PPOX-CAR)와 폴리[(2,5-다이-페닐-1,3,4-옥사다이아졸)-3-다이닐비닐렌-알트-3,6-(9-(2-에틸헥실)-카바졸렌비닐렌)] (PMOX-CAR) 의 최대 빛 발광 및 전기 발광 파장은 각각495 nm 에서 450 nm 이였다. 이때의 구동전압은 7.5 와 10.5 V이였다. 특히 PPOX-CAR 고분자는 20 V에서 500 cd/㎡ 이라는 매우 높은 휘도를 보였다. 또한 폴리[(2,5-다이-페닐-1,3,4-옥사다이아졸)-4-다이닐비닐렌-알트-2,7-(9,9-다이-n-헥실플루오렌비닐렌)] (PPOX-FLU) 와 폴리[(2,5-다이-페닐-1,3,4-옥사다이아졸)-3-다이닐비닐렌-알트-2,7-(9,9-다이-n-헥실플루오렌비닐렌)] (PMOX-FLU) 는 각각 PPOX-CAR 와 PMOX-CAR 의 광학특성과 거의 같은 특성을 보였으나, 전기적 특성 및 빛 발광 효율면에서는 약간 떨어지는 성능을 보였다.
금속촉매를 통해서 합성된 고분자 폴리[2,5-비스{4-(2-에틸헥실옥시)-1-페닐비닐}-페닐렌-1,4-싸이오펜] (PPVPT) 는 10 V에서 구동을 하였으며, 녹색 빛을 발광하였다. 또한 폴리[p-페닐렌] 유도체인 폴리[2,5-비스{4-(2-에틸헥실옥시)-1-페닐렌}-페닐렌] (PPVPP) 는 녹색이 약간 가미된 청색 빛을 발광하였다.
마지막으로, 진공 증착에 의한 박막형성 뿐만 아니라, 스핀코딩에 의해서도 박막을 형성 할 수 있는 2,5-비스{4-트라이페닐아민}-비닐렌페닐렌}-1,3,4-옥사다이아졸 (2TPA-OX)를 합성하였다. 이 물질은 녹색 발광을 보였으며, 스핀코딩에 의한 단일층 다이오드 제작 시는 120 cd/㎡, 진공 증착에 의한 삼층 다이오드 제작에 의해서는 520 cd/㎡ 라는 최대 휘도를 보였다.
