• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.04a
• /
• pp.31-32
• /
• 2008

The structure of OLEDs with conventional heterostructure consists of anode, hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, and cathode. NPB used as a hole transport layer and DPVBi used as a blue light emitting layer were graded-mixed at selected ratio. Interface at heterojunction between the hole transport layer and the elecrtron transport layer restricts device's stability. Mixing of the hole transport layerand the emitting layer removes abrupt interface between the hole transport. layer and the electron transport layer. The stability of OLED with graded mixed-layer developed in this study was improved.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2016.02a
• /
• pp.229.1-229.1
• /
• 2016

본 연구에서는 Hole Transporting Layer(HTL)와 Electron Transporting Layer(ETL)의 두께에 의한 특성을 비교해보기 위해서 각각 0, 10, 20 nm로 HTL, ETL 두께를 달리한 형광 OLED소자를 제작하였다. ETL의 두께가 얇아질수록 $V_{TH}$ 값은 2.5V에서 0.9 V로 낮게 나타났고 소자의 전체 두께와 on voltage는 비례한다는 특성을 발견할 수 있었다. HTL과 ETL이 두꺼울수록 각 layer에서 carrier들의 이동에 delay가 생기고 emission layer에서 표면까지 거리가 생기기 때문이다. ETL의 두께가 두꺼울수록 높은 luminance 값을 나타내는 차이를 보여주고 있다. Hole에 비해 이동도가 작은 electron은 emission layer까지 늦게 전달되어, EML내에서 비교적 cathode쪽에 가까운 곳에서 exciton이 형성되기 때문이다. CE에도 더 두꺼운 ETL을 가진 소자가 더 높은 CE값 가짐을 확인할 수 있다. 모든 소자가 $200mA/cm^2$에서 가장 높은 CE값을 나타낸 이유는 $200mA/cm^2$에서 electron-hole 결합이 만들어내는 exciton형성이 가장 많기 때문이다. PE, QE도 ETL 두께가 두꺼울수록 특성을 향상이다. 결론적으로 ETL의 두꺼울수록 current density값이 감소함을 보이고 있는 반면 turn on voltage, luminance, efficiency 증가함을 볼 수 있다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.32 no.5
• /
• pp.393-398
• /
• 2019

A poly[bis(4-butypheny)-bis(phenyl)benzidine] (poly-TPD) and poly(9-vinylcarbazole) (PVK) bilayer was employed as a hole transport layer (HTL) in solution-processed CdSe/ZnS quantum dot light-emitting diodes (QLEDs). The thickness of the PVK layer spin-coated onto the poly-TPD layer, whose thickness was fixed to 40 nm, was varied, with PVK layer thicknesses of 0 nm, 35 nm, 45 nm, and 55 nm. Because the thickness of the PVK can determine the hole transport properties of the HTL, a PVK thickness that maximizes the performance of the HTL for the QLEDs was investigated. By employing the optimized PVK thickness of 45 nm, the current efficiency of the QLED exhibited a 1.74 times improvement when compared with that of the QLED with poly-TPD based HTL without PVK. This was mainly attributed to the decrease in the energy barrier between the HTL and the quantum dot (QD) emitting layer (EML).

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2002.11a
• /
• pp.499-501
• /
• 2002

본 연구에서는 ETL층으로 널리 알려져 있는 PBD(2-(4-biphenyl)-5-(4-tert-butylphenyl) -1.3,4oxadiazole)를 HBL(Hole-blocking layer) 물질로 이용 하고 Nile red를 사용하여 적색 발광의 EL(electroluminescence) 소자를 제작 평가하였다. 일반적인 유기 EL 소자의 구조인 Anode/HTL(Hole Transport Layer)/ETL(Electron Transport Layer)/Cathode로 이루어져 있다. 여기에 HTL과 ETL사이에 HBL를 추가하여 EL 소자의 성능을 향상 시킬 수 있으면, 이러한 구조의 최종 소자를 제작 EML(emitting layer; Nile red)의 두께 및 임계전압을 달리 하여 소자 의 특성을 평가 연구 하였다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.21 no.9
• /
• pp.853-858
• /
• 2008

The structure of organic light-emitting diodes(OLEDs) with typical heterostructure consists of anode, hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, and cathode. 4,4bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl(NPB) used as a hole transport layer and 4'4-bis(2,2'-diphenyl vinyl)-1,1'-biphenyl(DPVBi) used as a blue light emitting layer were graded-mixed at selected ratio. Interface at heterojunction between the hole transport layer and the elecrtron transport layer restricts carrier's transfer. Mixing of the hole transport layer and the emitting layer reduces abrupt interface between the hole transport layer and the electron transport layer. The operating voltage of OLED devices with graded mixed-layer structure is 2.8 V at 1 $cd/m^2$ which is significantly lower than that of OLED device with typical heterostructure. The luminance of OLED devices with graded mixed-layer structure is 21,000 $cd/m^2$ , which is much higher than that of OLED device with typical heterostructure. This indicates that the graded mixed-layer enhances the movement of carriers by reducing the discontinuity of highest occupied molecular orbital(HOMO) of the interface between hole transport layer and emitting layer.

• Journal of Institute of Control, Robotics and Systems
• /
• v.14 no.4
• /
• pp.365-368
• /
• 2008

Feasibility test for radical reactions in organic light emitting diode(OLED) has been applied on OLED consisting of hole transport layer(HTL) and electron transport layer(ETL). Organic molecules such as 4,4',-Bis[N-(1-naphthyl)-N-phenylamino] biphenyl(NPD) and 4,4',4"-tris(3-methylphenylphenylamino)triphenylamine(m-MTDATA) are chosen for hole transport layer(HTL) and Bathocuproine(BCP) for electron transport layer(ETL) in this study. Informations on energy and shape of frontier orbitals and data on radical reactions of simple aromatics from semiconductor($TiO_2$) photocatalysis have provided basis for determining feasibility for radical reactions in OLED. The outcome of our feasibility test would be useful in designing optimum molecule for organic layer with a view to extending the lifetime of OLED.

• Journal of the Korean Society of Manufacturing Technology Engineers
• /
• v.24 no.4
• /
• pp.363-367
• /
• 2015

This paper reports on thermally adjusted graphene oxide (GO) as the hole transport layer (HTL) for organic light-emitting diodes (OLEDs). GO is generally not suitable for HTL of OLEDs because of intrinsic specific resistance. In this paper, the specific resistance of GO is adjusted by the thermal annealing process. The optimum specific resistance of HTL is found to be $10^2{\Omega}{\cdot}m$, and is defined by the maximum current efficiency of OLEDs, 2 cd/A. In addition, the reasons for specific resistance change are identified by x-ray photoelectron spectroscopy (XPS). First, the XPS results show that several functional groups of GO were detached by thermal energy, and the amount of epoxide changed substantially following the temperature. Second, the full width at half maximum (FWHM) of the C-C bond decreased during the process. That means the crystallinity of the graphene improved, which is the scientific basis for the change in specific resistance.

• Journal of Adhesion and Interface
• /
• v.23 no.2
• /
• pp.17-24
• /
• 2022

Colloidal quantum dots (QDs) have gained attention for applications in quantum dot light emitting diodes (QLEDs) due to their high photoluminescence quantum yield, narrow emission spectra, and tunable bandgap. Nevertheless, non-radiative recombination induced by electron and hole imbalance deteriorates the device efficiency and stability. To overcome the problem, researchers have been trying to enhance hole transport properties of hole transporting layers (HTL) and/or slow down the electron injection in electron transport layer (ETL). Here, we summarize two approaches: i) development of interfacial materials between QD and ETL (or HTL); ii) engineering of HTL by blending or multi-layer approaches.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2014.02a
• /
• pp.227-227
• /
• 2014

OLED 소자에 사용되는 유기물들은 대부분 전자에 비해 정공의 이동속도가 매우 빨라 소자 효율의 손실이 일어난다. 본 연구에서는 이러한 전하 이동도의 불균형에 의한 OLED 소자 성능의 감소를 개선하기 위해 HBL (hole blocking layer) 물질로 BCP (HOMO : 6.5 eV, LUMO : 2.83 eV)를 도입하였다. 그러나 BCP의 LUMO 값이 약 3 eV를 가지기 때문에 전자의 이동에 영향을 미치는 것으로 예상되어 더 높은 효율을 가지는 소자를 제작하기 위해 host 물질을 상용물질(PGH02)로 교체하였다. PGH02의 HOMO 값은 약 5.86 eV로 소자에 사용된 HTL (hole transport layer)의 HOMO 값(5.54 eV)에 비해 높은 값을 가지기 때문에 HBL의 역할 역시 가능하여 소자의 성능이 상당히 개선되는 것을 확인할 수 있었다. 또한 전하 이동도의 균형을 맞추기 위해 ETL 물질로는 기존에 많이 사용되고 있는 Alq3 (${\mu}{\sim}10-5cm2/Vs$)에 비해 이동도가 10배 이상 빠른 Bebq2 (${\mu}{\sim}10-4cm2/Vs$)를 사용하였다. HTL (hole transport layer) 물질로는 상용물질(LHT 259)를 사용하였고, LHT 259의 전하 이동도는 FET (field effect transistor)를 제작하여 측정하였다. 이를 기반으로 하여 ETL과 HTL의 두께를 조절하여 전하 이동도가 균형을 이루는 OLED 소자를 제작하기 위해 실험을 진행하였다.

• 한국정보디스플레이학회:학술대회논문집
• /
• 2006.08a
• /
• pp.1024-1027
• /
• 2006

The electrical and the optical properties of organic light-emitting devices (OLEDs), with and without various kinds of multiple heterostructures were investigated. The efficiency in green OLEDs were significantly enhanced by the structure of the multiple heterostructures acting as a hole transport layer (HTL) rather than by the number of periods. These results indicate that highly efficient green OLEDs utilizing multiple heterostructures acting as a HTL can be fabricated.