Study of high performance organic light emitting device

• Main Authors: Peng-Yu Chen, 陳鵬聿
• Other Authors: Herng Yih Ueng
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/14237462118548002092

博士 === 國立中山大學 === 電機工程學系研究所 === 99 === The high performance organic light-emitting diodes (OLEDs) have been studied. First, we have fabricated a WOLED with AlF3 and m-MTDATA as a hybrid buffer layer. Results indicate that the turn-on voltage can be reduced to 3.1V, and the luminous efficiency can be improved to 14.7 cd/A when a hybrid buffer layer was used. Since the turn-on voltage decreases and the efficiency increases, the power consumption as well as lifespan are then improved. Moreover, the luminous efficiency of the hybrid buffer layer devices also immunes to drive voltage variations. Second, we studied the properties of transportation in OLEDs. The study presented the device of a WOLED with a combination of a graded hole transport layer (GH) structure and a gradually doped emissive layer (GE) structure as a double graded (DG) structure. The DG structure: ITO/MTDATA(15 nm/NPB(15 nm)/NPB:25% BAlq (15 nm)/NPB : 50% BAlq (15 nm)/BAlq:0.5% Rubrene (10 nm)/ BAlq : 1% Rubrene (10nm) /BAlq:1.5%Rubrene (10 nm) / Alq3 (20 nm)/ LiF (0.5 nm)/Al (200 nm) is beneficial for improving both electrical and optical performances. The luminous efficiency of the DG device is 11.8cd/A, which is larger than that of 7.9cd/A with the HJ device. This improvement is attributed to the discrete interface between hole transport layer and emissive layer can be eliminated, surplus holes can be suppressed, electron-hole pairs can obtain optimal transportation and recombination in the emissive layer, and quenching effects can be significantly suppressed. Moreover, the spectra were almost not changed with an increasing drive current. As the efficiency was improved, it is expected that the power consumption can be reduced as well. Third, high efficiency and brightness p-i-n OLEDs with a CsI-doped Alq3 layer as a n-ETL has been studied. The p-i-n WOLED with a 15 % CsI-doped Alq3 layer exhibits a luminous efficiency of 5.75 cd/A at a driving current of 20mA/cm2 as well as a maximum power efficiency of 4.67lm/W. This improved performance is attributed to the increased electron carriers of the n-ETL and the balance of electrons and holes in the recombination zone. The X-ray photoelectron spectroscopy (XPS) have shown that doping of CsI caused chemical reaction, attributing to the increase of carriers. Finally, we focus on the improvement of contrast ration (CR) of OLEDs. We successfully fabricated a conductive organic-metal light-absorbing layer with a high CR and low reflectance for use as a black cathode in an OLED. The black cathode that was fabricated using vacuum deposition has the advantages of low cost and simple fabrication. Moreover, the J-V characteristic of the black cathode device is almost identical to that of a conventional device. Additionally, the reflectance can be reduced from 66.2% to 11.3% and a small reflectance variation around 3.3% over the visible spectrum is appealed. At an ambient illumination of 250 lx, the CR can be increased from 4.2 to 10.8 at a brightness of 250 cd/m2.