A way to reach highly efficient and stable red bottom emission organic light emitting diodes (OLEDs) is the use
of doped transport layers, charge and exciton blockers, and phosphorescent emitter materials to combine low
operating voltage and high quantum yield. We will show how efficiency and lifetime of such devices can be
further increased.
In our contribution, we report on highly efficient red p-i-n type organic light emitting diodes using an iridium-based
electrophosphorescent dye, Ir(MDQ)2(acac), doped in α-NPD as host material. By proper adjustment of
the hole blocking layer, the device performance may be enhanced to 20 % external quantum efficiency at an
operation voltage of 2.4 V and a brightness of 100 cd/m2. At the same time, a power efficiency of 37.5 lm/W is
reached. The quantum efficiency is well above previously reported values for this emitter. We attribute this high
efficiency to a combination of a well-adjusted charge carrier balance in the emission layer and a low current
density needed to reach a certain luminance due to the use of doped transport layers. High chemical stability of
the blocker material assures a long device lifetime of 32.000 hours at 1.000 cd/m2 initial luminance.
The currently starting technical exploitation of organic electronic devices requires a deep understanding of ageing and
degradation mechanisms. In addition to extrinsically caused ageing processes, such as the penetration of oxygen and
water in organic layers and subsequent (electro)chemical reactions, further degradation channels exist in such devices,
which are based on intrinsic chemical reactions of the materials used in the devices. At this time, we know the
degradation mechanisms of only few organic materials applied in organic light emitting devices (OLEDs). To detect
specific reaction products, we introduced laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS), a
method which allows to distinguish between desired and undesired compounds in thin film organic devices. We use LDITOF-
MS to detect the degradation products of different Iridium based emitter materials like Ir(MDQ)2acac (red emitter)
and FIrpic (light blue) in dc driven OLEDs and adapted test sample structures. Due to the dissociation behaviour of some
Ir complexes and the ability of their fragments to form complexes with several hole blocking materials, the degradation
mechanisms of the devices can be understood in terms of such chemical complex formation between the emitter
molecules and neighbouring materials. On the other hand, the knowledge about these mechanisms can be used to select
the right combination of materials for the benefit of long-living devices as we will show at the end of this work.
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