Since the first organic light-emitting diode (OLED) was reported in 1987 by Tang and VanSlyke[9] great efforts have been made to optimize both materials and OLED devices. This has led to the first commercial full-color flat panel displays of the next generation.[103, 104] For this issue new red, green and blue emitter materials had to be developed. Until today the preparation of efficient red[105] and green[106] electroluminescent OLED devices has been described in literature. However the production of pure blue light-emitting devices is still a big challenge. External quatum yields of about 2.4 % and lifetimes of ca. 10 000 h have been reported so far from blue fluorescent emitters.[107] In order to achieve quantum efficiencies of 10.4 %, so-called phosphorescent emitters like iridium (III)bis[(4,6-di-fluoropheny)-pyridinato-N,C2’]picolinate (FIrpic) have been introduced.[108] However the device lifetimes are shorter and it has not been possible to obtain a deep-blue light emission from devices based on phosphorescent dyes. Since current blue light emitting devices still have some drawbacks, there is still a strong demand for the development of new high performance blue emitters.
Recently we have reported on novel bisindenocarbazoles from which a strong blue fluorescence together with quantum yields up to 63 % were obtained. For this reason we have tested the new material as blue emitter in OLED devices. In typical setups for blue OLEDs the blue fluorophore is often doped into a wide band gap host material in order to avoid quenching of the EL emission and to adjust the HOMO energy levels of the different materials used in the OLED setup. For this issue we decided to test 4,4'-dicarbazolyl-1,1'-biphenyl (CBP), 1,3-bis(9-carbazolyl)benzene (mCP) and 4,4',4''-tri(N-carbazolyl)-triphenylamine (TCTA) as host materials.
Before OLEDs could be prepared, the host/guest energy transfer had to be investigated. This was done by fluorescence spectroscopy and in the case of the CBP matrix, additionally by time resolved photoluminescence (PL). These preliminary experiments proved an efficient energy transfer between the matrix materials and the blue bisindenocarbazole dye.
Furthermore it was found that doping concentrations of 1 % yielded the highest fluorescene intensity. Higher amounts of the dopant lead to a quenching of the PL intensity. The results are discussed in detail in paper 5.
Alq3 (40nm)
PEDOT (40nm)
ITO (80nm)
BCP (10nm) BCP (20nm)
BCP (30nm) BCP (40nm)
TCTA:Emitter BCP
For the preparation of the OLED devices a combinatorial evaporation setup was used in order to dope the different host systems by co-evaporation of the guest material. By using this technique it was also possible to evaporate step gradients of the hole blocking layers in a single experiment. In conclusion it was found that a pure, deep-blue emission at CIE coordinates of x = 0.19 and y = 0.17 can be obtained from the bisindenocarbazole emitter by using TCTA as host material doped with 1 % of 1,1-Dimethyl-1’, 1’-dimethyl-bisindeno[3,2-b:2’3’-h]-9-sec-butyl-carbazole (Figure 6-2, inset) as emitter. At a hole blocking layer (BCP) thickness below 40 nm, a shoulder at 500 nm is detected in the EL spectra. The intensity of the shoulder increases with decreasing thickness of the BCP layer. This can be explained by the fact that the excitons are not confined in the TCTA/emitter layer if the blocking layer is too thin and therefore are able to diffuse into the Alq3 layer which leads to a green emission.
The corresponding EL spectra are shown in Figure 6-2.
Luminance values of 200 cd/m2 at a current density of 100 mA/cm2 and a maximum luminance efficiency of 1.60 cd/A were obtained from the TCTA containing OLEDs at a BCP layer thickness of 40 nm. The turn on voltage was recorded at 5 V. The energy level diagram and the setup of the device using TCTA as matrix material are shown in Figure 6-1.
Figure 6-1. Energy level diagram of the OLED containing TCTA as host material doped with 1 % of the blue bisindenocarbazole emitter (above) and device architecture with different BCP layer thicknesses (below).
42 6. Bisindenocarbazole as new deep-blue emitter for OLED applications
250 300 350 400 450 500 550 600 650 0,0
Figure 6-2. EL spectra of the TCTA containing OLEDs with different BCP blocking layer thicknesses. For comparison the PL spectrum of the pure bisindenocarbazole emitter is added (blue dashed curve). The inset shows the chemical structure of the bisindenocarbazole emitter molecule.
The current-voltage characteristics of the TCTA containing OLEDs (Figure 6-3) show a saturation of the luminescence values at driving voltages higher than 10 V. This could be explained by the fact that the TCTA/emitter layer was solution processed and therefore no HTL could be used. Thus a huge mismatch of the HOMO energy levels of the ITO anode and the TCTA layer occurs and therefore an efficient charge carrier injection is no longer ensured.
In future experiments the emitting layer will be prepared by evaporation what might solve the problem of the low brightness values.
The application of CBP and mCP as host materials resulted in the emission of green light with a maximum around 500 nm. Therefore the emission can be exclusively assigned to Alq3. In conclusion it was found that TCTA is the most suitable matrix for the novel emitter. These experiments have shown that the bisindenocarbazole is a promising candidate to be used as blue light emitting material in OLED applications.
0 2 4 6 8 10 12 14 16 18 0,1
1 10 100 1000
10nm BCP 20nm BCP 30nm BCP 40nm BCP
Luminance [cd/m2 ]
Bias [V]
Figure 6-3. Current-voltage characteristics of the OLEDs with TCTA matrix containing 1 % of the bisindenocarbazole dopant. Device setup: ITO/PEDOT (40 nm)/TCTA:bisindeno-carbazole (40 nm)/BCP (10, 20, 30 and 40 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (150 nm).
44 7. Summary