Doping in Polymers for Organic Light Emitting Diodes

• Main Author: 張興華
• Other Authors: 李正中
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/85849911651688008672

博士 === 國立中央大學 === 光電科學研究所 === 89 === Doping is an important technology for electronic imaging and optoelectronic devices based on molecular or polymeric materials. Emissive doping is usually used for tuning emission colors and enhancing luminescence efficiency. For the versatility in the device structures and the integration of different devices on a substrate, it usually requires the capability to laterally and vertically modulate the distribution of dopants in organic films. In molecular devices fabricated by vacuum deposition, lateral modulation of compositions can be implemented by switchable shadow masking, and vertical modulation is achieved by controlling the co-deposition sequences. However, in organic devices incorporating molecularly doped polymers (MDPs), the conventional blending process can only produce uniform dispersion of dyes throughout the polymer layer, providing no spatial selectivity of dopant distribution. In this thesis, we propose finite-source dye-diffusion thermal transfer (FS-D2T2) for performing controllable doping of polymer films. In this process, the polymer receiver film is placed in direct contact with the dye-dispersed polymer donor film to permit direct dye-diffusion thermal transfer. We perform a series of experiments of dye diffusion in the poly(N-vinylcarbazole) (PVK) or the poly (N-vinylcarbazole): 2-(4—biphenyl)-5-(4-tert- butylphenyl)-1,3,4-oxadiazole (PVK: PBD) matrix systems and show that it can be modeled by Fick’s diffusion theory. We have also shown that in an atmosphere of organic solvent vapor, effective dye-diffusion thermal transfer may be enhanced at temperatures much below Tg of host glassy polymers. Such an effect may permit polymers of higher Tg and wider ranges of dye molecules to be used in the D2T2 process for electronic imaging applications. By using finite-source dye-diffusion thermal transfer process , we demonstrate lateral and vertical modulation of dopant distribution in polymer films. Lateral modulation of dopant distribution was applied to the color integration of OLED devices. Vertical modulation of dopant distribution was used to reduce carrier trapping effect caused by the emissive dopants in the device. To permit repeated use of the D2T2 source plate, a method of rechargeable thermal transfer stamping was also introduced. Furthermore, we demonstrated the enhancement of device performance with small molecule/FS-D2T2 doped polymer hybrid heterostructures. In order to achieve low driving voltage and high efficiency in OLED devices, it is necessary to facilitate the injection of charges. One effective approach to enhance carrier injection is to conductively dope the organic layer. In this thesis, we investigated the SbCl5-doped PVK as a conductively doped polymer, and its use as a hole-injection material in OLEDs. It is found that SbCl5-doped PVK is an effective hole-injection material for both polymer or small-molecule based OLED devices.