| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 87 === A new method for preparing organic electroluminescent (OEL) polymer thin films has been presented in this study. The characteristic of this method is that the OEL-active molecule can be co-deposited during its vapor deposition polymerization (VDP) process, without going through traditional complicated synthesis. Moreover, emission color can be changed freely from blue to red simply by changing the luminescent materials.
The temperature-dependent deposition rates of the OEL-active molecule and two polymer thin film forming monomers (dianhydride and diamine) were separately determined to acquire the optimal deposition conditions. The structure of the resultant films was characterized to be poly(amic acid) (PAA) by using Fourier transform infrared (FTIR) spectrometry. It could be converted into the polyimide (PI) counterpart after curing.
A device structure of ITO(indium tin oxide）glass/ molecularly doped polyimide/ Al（aluminum） was employed. Carrier injection from the electrodes to the co-deposition layer through the dopants and concomitant electroluminescence from light emitting molecule was observed. The blue device, with an emission maximum at 435nm, was triggered at a driving voltage of 17 V. The green device, with an emission maximum at 515nm, was triggered at a driving voltage of 14V.
Two primary degradation behaviors were identified. First, hydrolysis of the thin films led to the reduction in EL intensity. Second, non-uniform structure formation of the top Al metal in the Al and co-deposition film interface caused reduction in the active emission area and increased the driving voltage required.
Optical and atomic force microscopy observation of the molecularly doped polymer system revealed that crystallization of the TPD was remarkably suppressed by dispersing TPD into the polymer matrix. The lifetime of the device with the TPD dispersed in the polymer film matrix was improved by more than two times when comparing with that without a polymer matrix.
The characteristics of the multi-layer OEL device were presented, and the mechanism of the carrier recombination and emission was discussed. The carrier recombination and emission were located not in the narrow interface regions between the emitting layer and the transport layer but in the entire region crossing the emitting layer. According to this result, white light was observed from the device（ITO/TPD/TPB/Alq3: DCM/ Al） having an Alq3 layer doped with DCM.
To improve the luminance efficiency and lifetime of the device, two types of device structure were proposed. The single-layer type was composed of PI, m-MTDATA, and Alq3 in a single layer and was triggered at a driving voltage of 9V. The Al metal was replaced by using Mg：Ag alloy in thin device. When the driving voltage was 13 V, the brightness and current density were 80 cd/m2 and 15 mA/cm2 respectively, the efficiency was 0.53cd/A (energy conversion efficiency was 0.04﹪), and the external quantum efficiency was 0.23﹪. The lifetime was 200h.
The multi-layer type was composed of one layer of m-MTDATA in PI, one layer of TPB, and one Alq3. This device was triggered at a driving voltage of 5 V. When the driving voltage was 9V, the brightness and current density were 100cd/m2 and 11mA/cm2 respectively, the efficiency was 0.9 cd/A (energy conversion efficiency was 0.1 ﹪), and the external quantum efficiency was 0.4﹪. The lifetime was 400 h.
|
|---|
