We studied transparent organic light emitting diode (TOLED) with semitransparent cathode, Ca-Ag and
optical enhanced passivation layer by annealing. The 8-hydroxyquinolatolithium (Liq) passivation layer
improves the stability of Ca/Ag double layer, resulting in substantial increase of the efficiency by the
enhanced transmittance and change brightness as electrode.
Fabricated device was composed as followed structure, glass/ITO/2TNATA/α-NPD/Alq3:C545T
(1%)/Bebq2/Ca-Ag. The passivation layer, Liq (40nm) was deposited on the device without broken
vacuum before encapsulation with transparent glass sheets. Thermal annealing was carried out in oven
at 80oC after encapsulation.
At the device without passivation layer, annealing process made device to be deteriorated, such as to
increase driving voltage from 6.8 V to 9.0 V at 10 mA/cm2 and to drop efficiency 14.89 cd/A to 10.81 cd/A
as bottom view. This drop of properties was caused by oxidation of cathode. However, the organic
passivation layer played enough roles to protect oxidation of cathode. The efficiency as bottom side view
of device with passivation layer, it can be observed that the efficiency and driving voltage were improved
from 13.95 cd/A (@ 6.45 V) to 15.32 cd/A (@ 4.60 V) at 10 mA/cm2 by annealing.
Annealing process, in the device with passivation layer, made to increase the asymmetry of brightness
through anode and cathode. So the ratio BBot/BTop of the emitted light through each electrode was
changed from 2.26 to 4.33.
We researched a new transparent concept display, transparent organic light emitting diode (TOLED) with new
transparent cathode, Sr/Ag double layer. Sr is used as an electron injection material and Ag is adopted as a protective
material for Sr and electrical conductor. This cathode has high transmittance over 70% at 520nm and low sheet
resistance, about 12Ω/sq. We fabricated TOLED with two transparent electrodes, ITO and Sr/Ag. To improve device
properties, transparent passivation layer, CsCl is deposited on transparent cathode. CsCl, passivation layer enhances the
extraction of light emitting throughout the surface side of the TOLED by the improvement of transparency and the
microcavity effect. TOLED without passivation layer shows the large difference of brightness as electrode, 26,700cd/m2
and 10,000 cd/m2 @14V through anode and cathode, respectively. However, CsCl makes to decrease difference between
brightness through anode and cathode and come to similar brightness 20,600cd/m2 and 18,200cd/m2 @ 14V.
We simulated modification of brightness as electrode, ITO anode and semitransparent cathode in the transparent organic light emitting device (TOLED). Semitransparent cathodes were used thin metal films, Ca-Ag and LiF-Al. Ca-Ag electrode has a higher transmittance 71% and lower reflectance 13% as compared to 32% and 24% of LiF-Al at 530nm. Reflectance of these material affected brightness and spectrum. Ratio of emitted lights intensity through ITO and metal cathode was calculated theoretically to use reflectance and transmittance of each electrode. Calculated ratio of brightness is in agreement with the measured ones to each electrode. Brightness of emitted light through ITO is about 3.2 and 1.5 times higher than through LiF-Al and Ca-Ag cathodes, respectively. In case of LiF-Al, ratio of transmitted light through the electrode was very different and spectra of light through LiF-Al were accompanied with the interference fringes.