We have investigated the electrical properties of red organic light-emitting diodes (OLEDs) featuring a conventional structure comprising N,N¡¦-bis-(1-naphthl)-diphenyl -1,1¡¦- biphenyl-4,4¡¦-diamine (NPB) as the hole transporting layer (HTL) and 8-hydroxy -quinoline aluminum (Alq3) as the electron transporting layer (ETL). There are many methods for improving the efficiency of OLED with a configuration of ITO/NPB/Alq3/LiF/Al, such as increasing electron mobility of Alq3 or decreasing hole mobility of NPB. We can¡¦t obtain such improvement of the efficiency in the OLEDs composing Alq3:DCJTB as the emitting layer. From the measurement of hole-only and electron-only devices, the electrical characteristics such as hole concentration is greatly changed after doped DCJTB to Alq3 as the emitting layer. In our study, we also found that the efficiency increased upon the decreasing the deposition rate of NPB, presumably because the improved packing of NPB enhanced the carrier mobility, which therefore improve the carrier recombination rate. We deduced that the emitting layer of the device will become electron rich when the Alq3 is doped by DCJTB. It means that if we want to improve the efficiency of the red OLED, with Alq3:DCJTB as the emitting layer, we have to increase the hole concentration in the device. A red OLED device having the configuration of ITO/NPB/Alq3/LiF/Al exhibited a current efficiency of 7.4 cd/A at 20 mA/cm2 with Commission International de l'Eclairage coordinates of (0.62, 0.38). The current efficiency reached to 10.8 cd/A by putting optical film, which is a 50% improvement.

The purpose of the research is probing for the cause that red shift of green-host Alq3 is doped by red dopant with concentration increasing. Generally speaking, this phenomenon of red shift is resulted from the excimer that complex formed by the interaction of an excited molecular entity with a ground state partner of the same structure. And that excimer maybe formed by cyaniding group of red dopant because of the molecular polarity. We will survey the difference of spectra when Alq3 doped guest with different doping concentration in order that research the circumstance of energy transfer. We also success to synthesize the novel material of red dopant from green-dopant C545P,and name the red dopant ¡§RC545P¡¨. Then, we will measure the performance of the RC545P to compare with DCJTB that is often used to be the red dopant in OLED. Finally, we prove the assumption that excimer formed by cyaniding group is result in the red shift of spectra. Moreover, the emission efficiency of RC545T (9.28) in solution is better than DCJTB (7.96), but the efficiency of Alq3 energy transfer to RC545T (17.88%) is worse than DCJTB (36.72%). In thin film, the emission efficiency of Alq3 doped RC545T (42.3) is closer than Alq3 doped DCJTB (48.9). the optimal concentration ratio of Alq3 doped RC545T is between 1% and 2%.

In this research, we used the novel material 9-cyano-10-(2-benzothiazoly1)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro- 1H,5H,11H- benzo[l]pyrano[6,7,8-ij]quinolizin-11-one(RC545T) as a red dopant, which was synthesized from the green dopant material 10-(2-benzothiazoly1)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro- 1H,5H,11H- benzo[l] pyrano[6,7,8-ij]quinolizin-11-one(C545T). First, we used aluminum tris(8-hydroxyquinoline)(Alq3) as a hostmaterial and doped with different concentration of RC545T, with the configuration of ITO(1300Å)/NPB(650Å)/Alq3:x%RC545T(300Å)/Alq3(300Å)/LiF(8Å)/Al(2500Å). We obtained the maximum efficiency at 2% of RC545T. Second, 5,6,11,12-Tetraphenylnaphthacene (rubrene) and RC545T as a co-dopant material were doped into the Alq3 layer. The energy transfer from Alq3 to RC545T was mainly assisted by Rubrene. Therefore, we obtained the luminance yield of 0.54 cd/A under the current density of 20mA/cm2, and the peak emission wavelength at 632nm with a CIExy color coordinate of (0.642,0.347).The improved color purity render RC545T a good red-emitting dopant in OLED application.