|| The purpose of this research is to study the effect of the thermal treatment on the devices. We dissolved polymer light emitting materials in different solvents to discuss the influence on polymer thin film and device efficiency at different thermal conditions. We confirmed that the best thermal condition was changed as that of solvent changes.|
In this study, we dissolved polymer blue light material－BP105 in Toluene and o-xylene. The glass transition temperature (Tg) of BP105 is 120.7℃, and the boiling point of Toluene and o-xylene were 110 and 145℃. That is namely the boiling point of Toluene and o-xylene are lower and higher, respectively, than the Tg of BP105. This makes us to compares the thermal treatment conditions on different influence from different boiling point of solvents. The interrelations between the thermal treatment temperature, the boiling point of solvents and the glass transition temperature of polymer is an interesting topic to study, because it does affect the surface morphology of polymer thin films and the characteristic of devices. The device structure is as follows: ITO/ PEDOT:PSS/ BP105/ LiF/ Ca/ Al.
Known from the experimental results that the spectra and the morphologies of polymer thin films will change in the different thermal treatment condition, and the choice of different solvent will also affect the best thermal treatment condition for device processing. We observed the surface roughness of polymer thin film is one of the important factors to affect the device efficiency in this study. We found that if the boiling point of used solvent was higher than the thermal treatment temperature, which was higher than the Tg of polymer, the surface roughness of polymer thin film is more smooth resulting in higher current injecting and higher stability of the device.
The best thermal treatment temperature is 130℃ by using o-xylene as solvent. The surface roughness of polymer thin film is 0.393 nm, and the maximum brightness of the device is 8593 cd/m2 at 12.5 V as a configuration of ITO(1500Å)/PEDOT:PSS(800Å)/BP105(650Å)/LiF(10Å)/ Ca(100Å)/ Al(2000Å). The luminous and the power efficiencies are 3.98 cd/A, and 1.43 lm/W, respectively, at the current density 100 mA/cm2.