In this study, multilayer organic light-emitting diodes (OLEDs) consisting of three solution-processed layers are fabricated using slot die coating, gravure printing, and inkjet printing, techniques that are commonly used in the industry. Different technique combinations are investigated to successively deposit a hole injection layer (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)), a cross-linkable hole transport layer (N,N′-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)-hexyloxy)phenyl)-N,N′-bis(4-methoxyphenyl)biphenyl-4,4′-diamin (QUPD)), and a green emissive layer (TSG-M) on top of each other. In order to compare the application techniques, the ink formulations have to be adapted to the respective process requirements. First, the influence of the application technique on the layer homogeneity of the different materials is investigated. Large area thickness measurements of the layers based on imaging color reflectometry (ICR) are used to compare the application techniques regarding the layer homogeneity and reproducible film thickness. The total stack thickness of all solution-processed layers of 32 OLEDs could be reproduced homogeneously in a process window of 30 nm for the technique combination of slot die coating and inkjet printing. The best efficiency of 13.3 cd A−1 is reached for a process combination of slot die coating and gravure printing. In order to enable a statistically significant evaluation, in total, 96 OLEDs were analyzed and the corresponding 288 layers were measured successively to determine the influence of layer homogeneity on device performance.
11 different dialkoxy- and dialkyl-poly(para-phenyleneethynylene)s (PPE)s featuring novel branched and literature described sidechains were prepared using a Sonogashira coupling. The materials were tested as emitters in thickness dependent OLED devices.
Printing process development for fabrication of organic electronic devices is described, with focus on semiconductor layers for organic light‐emitting diodes and photovoltaic cells. This development is considerably more complex than for a graphical printing process. Key aspects are an adequate dosing and transfer of highly volatile inks, the reliable coalescence of the droplets deposited on the substrate to a closed liquid film, and the successful relaxation and leveling of the liquid–air interface in the solvent evaporation phase in the presence of Marangoni stresses and pattern formation instabilities. The conditions for successful implementation of a gravure or inkjet process, using steadily developing, new generations of polymer as well as small molecule semiconductors are, to a large extent, but not exclusively, originating from the molecular features of organic semiconductors and their printable solutions. In addition, recent developments in surface technology, and in the physics of thin‐film dynamics and spontaneous pattern formation contribute to a proper understanding of liquid layer dynamics in printed electronics. The role of ink formulation, Marangoni stresses related to concentration and temperature gradients, the effect of the disjoining pressure, and solvent evaporation are discussed.
The liquid deposition
of thin films requires a thorough understanding
of the underlying drying process, as it is an essential subprocess,
where many defects may arise. To complement experimental studies,
the present study uses a laser Michelson interferometer to visualize
the vapor cloud of evaporating liquid films. The recorded interferometric
patterns are evaluated using windowed Fourier filtering and a novel
phase-unwrapping algorithm to allow for a robust analysis. Thin solvent
stripes of different lengths are combined to yield a quantitative
two-dimensional distribution of the solvent vapor concentration along
a thin liquid stripe. The results show a considerable influence of
natural convection during evaporation.
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