Electrically doped, organic transport layers are important for today's high efficiency organic (opto-)electronic devices. Doped organic layers have a strongly increased free charge carrier density compared to their undoped counterparts and also improve the charge carrier injection from adjacent electrodes into the organics. For practical applications, especially in optoelectronics, these layers have to have low absorption in the wavelength range of interest. The two nearly colorless p- and n-doping materials, rhenium heptoxide and cesium carbonate, are investigated focusing on their conductivity enhancement, injection improvement, and voltage drop over doped transport layers in organic light emitting diodes. They show very good doping properties already at moderate doping concentrations and prove that they can be used in variable thicknesses without a significant voltage increase. This makes them cheap, low absorbing alternatives to today's, well-established doping systems.
Apart from usage of organic light emitting diodes for flat panel display applications OLEDs are a potential candidate for
the next solid state lighting technology. One key parameter is the development of high efficient, stable white devices. To
realize this goal there are different concepts. Especially by using highly efficient phosphorescent guest molecules doped
into a suitable host material high efficiency values can be obtained. We started our investigations with a single dopant
and extended this to a two phosphorescent emitter approach leading to a device with a high power efficiency of more
than 25 lm/W @ 1000 cd/m2. The disadvantage of full phosphorescent device setups is that esp. blue phosphorescent
emitters show an insufficient long-term stability. A possibility to overcome this problem is the usage of more stable
fluorescent blue dopants, whereas, due to the fact that only singlet excitons can decay radiatively, the efficiency is lower.
With a concept, proposed by Sun et al.1 in 2006, it is possible to manage the recombination zone and thus the
contribution from the different dopants. With this approach stable white color coordinates with sufficient current
efficiency values have been achieved.
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