고분자 나노 구조를 이용한 플렉서블 백색유기발광소자의 전기적 및 광학적 특성에 관한 연구

Abstract

유기발광소자는 얇은 유기 박막에 전압을 인가하여 가시광선 영역의 빛을 발광하는 소자로서, 기존의 디스플레이 기술에 비해 빠른 응답속도, 넓은 시야각, 얇은 두께의 특성으로 전색 디스플레이 및 백색 광원 기술로 많은 주목을 받고 있다. 특히 플렉서블 백색 유기발광소자의 기술은 디스플레이의 배경조명 및 플렉서블 광원으로 큰 활용 가능성을 가지고 있기 때문에 높은 연구 가치를 가진다. 이에 본 연구에서는 전색 디스플레이의 배경조명 및 일반조명으로 응용할 수 있는 고분자 나노 구조와 청색 저분자 유기물을 적층한 플레서블 백색 유기발광 소자를 제작하였으며 백색 유기발광 소자의 전기적 및 광학적 특성을 분석하였다. 고분자 poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene (MEH-PPV)와 polystyrene (PS) 혼합물을 스핀코팅 방법을 사용하여 박막을 형성한 후 열처리에 의한 상분리 현상과 두 고분자 물질의 용해도 차이를 이용해 선택적으로 PS 물질을 제거하여 MEH-PPV 적색 다공성 나노 구조를 가진 고분자 발광층을 형성하였다. 고분자 MEH-PPV와 PS의 혼합 비율과 혼합층 두께에 따른 MEH-PPV 적색 고분자 다공성 박막의 변화를 원자힘 현미경을 통하여 관찰할 수 있었다. MEH-PPV 적색 다공성 고분자 발광층의 미세구조의 형태는 MEH-PPV와 PS 혼합물의 혼합 비율의 변화에 따라 PS 혼합비가 높아지면 미세구조의 밀도가 높아지고, 혼합된 두 고분자 물질의 분자량의 차이에 의한 응집도의 차이로 인하여 MEH-PPV와 PS 혼합물 박막의 두께가 얇아지면 미세구조의 경사도가 높아진다. 이렇게 형성 된 적색 나노 구조의 고분자 박막 위에 청색 저분자 4,4’-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) 물질, 저분자 보조층 및 음극을 열증착으로 형성하여 플렉서블 백색 유기발광소자를 제작하고 발광 스펙트럼 및 발광 특성을 측정하였다. 혼합 고분자 박막의 스핀코팅 조건이 4000rpm 및 5000rpm으로 제작된 소자에서 각각 15V에서 (0.36, 0.25)와 (0.28, 0.24)의Commission international de l’Eclariage 1931 (CIE 1931) 좌표를 가지는 백색 발광 특성이 발견되었다. 최고 휘도는 스핀코팅 조건 5000rpm로 제작된 소자가 19V에서 136 cd/m2의 발광 특성을 나타냈다. 본 연구 결과는 고분자-저분자 혼합 발광층 구조를 사용하는 플렉서블 백색 유기발광소자의 색안정성과 효율 향상에 대한 기초자료로 활용할 수 있다. |Organic light-emitting devices (OLEDs) have emerged as potential candidates for applications in commercial displays and light sources. However, relatively few works have been performed on light sources based on OLEDs have been developed in comparison with displays based on OLEDs because of the delicate problems encountered in the fabrication process. Flexible OLEDs have been particularly attractive because of interest in their promising applications in flexible electronic and optoelectronic devices. Flexible white OLEDs (WOLEDs) with a porous red polymer emitting layer (EML) and a blue polymer EML on flexible substrates were fabricated by using a spincoating method. The EML for the flexible WOLED consisted of a porous poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and 4,4’-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) layer. The porous MEH-PPV layer was fabricated by using the selective etching of a blend layer consisting of MEH-PPV and polystyrene (PS). When the blend layer with MEH-PPV and PS was spincoated and annealed, the phases of MEH-PPV and PS were separated and formed the entangled structure. The selective etchant used in this work was propylene glycol mono-methyl ether acetate, which selectively melted the PS of the entangled blend layer, resulting in a formation of the porous structured MEH-PPV layer. The DPVBi layer was spin-coated on the porous MEH-PPV layer, and the hole injecting channels were formed through the pores to enhance hole injection to the DPVBi layer. The ratio between red and blue color peak intensities of the electroluminescence spectra for the flexible WOLEDs with a porous red polymer EML and a blue polymer EML was adjusted by changing the pore size of the MEH-PPV layer. These investigations provide applications possibilities for such flexible WOLEDs in large-scale flexible light sources.; Organic light-emitting devices (OLEDs) have emerged as potential candidates for applications in commercial displays and light sources. However, relatively few works have been performed on light sources based on OLEDs have been developed in comparison with displays based on OLEDs because of the delicate problems encountered in the fabrication process. Flexible OLEDs have been particularly attractive because of interest in their promising applications in flexible electronic and optoelectronic devices. Flexible white OLEDs (WOLEDs) with a porous red polymer emitting layer (EML) and a blue polymer EML on flexible substrates were fabricated by using a spincoating method. The EML for the flexible WOLED consisted of a porous poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and 4,4’-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) layer. The porous MEH-PPV layer was fabricated by using the selective etching of a blend layer consisting of MEH-PPV and polystyrene (PS). When the blend layer with MEH-PPV and PS was spincoated and annealed, the phases of MEH-PPV and PS were separated and formed the entangled structure. The selective etchant used in this work was propylene glycol mono-methyl ether acetate, which selectively melted the PS of the entangled blend layer, resulting in a formation of the porous structured MEH-PPV layer. The DPVBi layer was spin-coated on the porous MEH-PPV layer, and the hole injecting channels were formed through the pores to enhance hole injection to the DPVBi layer. The ratio between red and blue color peak intensities of the electroluminescence spectra for the flexible WOLEDs with a porous red polymer EML and a blue polymer EML was adjusted by changing the pore size of the MEH-PPV layer. These investigations provide applications possibilities for such flexible WOLEDs in large-scale flexible light sources.