Carola Diez

Highly stable charge generation layers using caesium phosphate as n-dopants and inserting interlayers

Highly stable and efficient charge generation layers (CGLs) comprising caesium phosphate (Cs3PO4) doped 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as n-type organic semiconductor and molybdenum trioxide (MoO3) doped N,N′-di-(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD) as p-type organic semiconductor, respectively, are presented. By inserting narrow-gap organic copper-phthalocyanine (CuPc) and wide-gap insulating aluminum oxide (Al2O3) as interlayer (IL), we show that the long-term stability of the CGL can be improved. The variation of the CuPc IL thickness yields an optimum of 8 nm as a trade-off between minimal operating voltage and maximum voltage stability of the CGL. Luminance-current density-voltage characteristics and lifetime measurements of stacked green organic light emitting diodes (OLEDs) confirm the functionality and high voltage stability of the presented CGL. The luminous efficacy of the stacked OLED compared to the non-stacked reference device is nearly unchanged. However, the...

Fluorinated copper(I) carboxylates as advanced tunable p-dopants for organic light-emitting diodes.

Volatile copper(I) benzoates with variable degrees of fluorination are used for p-doping of organic hole-transport layers in single-carrier devices, charge-generation layers, and in organic light-emitting diodes. The charge-transport abilities of the doped materials correlate with the degree and position of the fluorination on the aromatic ring of the carboxylate groups.

Investigation of energy transfer mechanisms between two adjacent phosphorescent emission layers

The investigation of energy transfer mechanisms between two adjacent phosphorescent emission layers comprising the green emitter molecule fac-tris(2-phenly-pyridin)iridium (Ir(ppy)3) and the red emitter molecule iridium(III)bis(2-methyldibenzo[f,h]quinoxaline(acetylacetonate) (Ir(MDQ)2(acac)) is presented. We show that the performance can be enhanced by a variation of the emission layer thickness and the emitter concentration. By inserting different interlayer materials between the emission units, we demonstrate that triplet excitons are formed on the Ir(ppy)3 and subsequently transferred to the Ir(MDQ)2(acac) molecules via the hole transporting host material N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine of the red emission layer. The variation of the interlayer thickness shows that the triplet diffusion length is several tens of nanometers. After optimization of the guest-host system an efficiency enhancement by 15% was achieved and the lifetime of the red-green emissive unit could be enhanced by ...

Degradation induced decrease of the radiative quantum efficiency in organic light-emitting diodes

The efficiency decrease during electrical operation of organic light-emitting diodes is a crucial issue for both applied and fundamental research. In order to investigate degradation processes, we have performed an efficiency analysis for phosphorescent state-of-the-art devices in the pristine state and after an accelerated aging process at high current density resulting in a luminance drop to less than 60% of the initial value. This loss in efficiency can be explained by a decrease of the radiative quantum efficiency of the light-emitting guest/host system from 70% to 40%, while other factors determining the efficiency are not affected.