Slot Die Coated and Flexo Printed Highly Efficient SMOLEDs

Abstract

A basic solution‐processable organic light‐emitting diode (OLED) stack comprising two organic layers is presented in this study. The hole injection layer (poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate), PEDOT:PSS) is slot die coated and flexographically printed for process comparison in terms of film homogeneity and the device performance. The emissive layer consisting of a small molecule host–guest system with a green triplet emitter is slot die coated. The single layer thicknesses are mapped by an in‐house developed imaging color reflectometry enabling the precise measurement of the layer thicknesses over the whole coating area (>50 cm2). Luminance, current density, and light and current efficiency are defined for two different pixel sizes with the active area of 6 × 4 and 14 × 14 mm2. A total of 100 OLEDs are measured showing a very low standard deviation for the different pixels. The solely slot die coated OLEDs show much higher current and power efficiency than the OLEDs with flexographically printed PEDOT:PSS. Small molecule OLEDs reach efficiencies up to 30 cd A−1 and 8 lm W−1. Additionally OLED demonstrators with an active area of ≈27 cm2 are fabricated.

Comparative Study of Printed Multilayer OLED Fabrication through Slot Die Coating, Gravure and Inkjet Printing, and Their Combination

“…On the one hand, oxygen or argon plasma as a pretreatment step to increase the surface energy of the substrate is widely spread and is commonly used in literature [9][10][11]. On the other hand, wettability can be improved by reducing the surface tension of the ink by adding surfactants, such as Triton-X for water-based formulations [12,13]. The material class of small molecules is particularly interesting due to the higher efficiency compared to polymers but also more challenging regarding solution processing [14].…”

“…If the coating is not applied continuously but in a S2S mode, it has to be considered that it takes a few seconds until the coating solution has bridged the gap between slot die and substrate and until a state of equilibrium between the delivered and applied volume is reached [12]. In order to achieve a steady-state mode as quickly as possible, the substrate movement can be started delayed to the pumping process; additionally, the inlet zone can be extended (if possible) or glass instead of plastic syringes may be used [13,31]. Otherwise, a layer thickness gradient in the coating direction can occur; for example a thickness increase from 66 nm to 88 nm was observed for a slot-die-coated green emissive material over a coating length of 131 mm [13].…”

“…In order to achieve a steady-state mode as quickly as possible, the substrate movement can be started delayed to the pumping process; additionally, the inlet zone can be extended (if possible) or glass instead of plastic syringes may be used [13,31]. Otherwise, a layer thickness gradient in the coating direction can occur; for example a thickness increase from 66 nm to 88 nm was observed for a slot-die-coated green emissive material over a coating length of 131 mm [13]. In addition to coating defects, nonuniform thickness variations may also occur due to inhomogeneous drying conditions, especially when solvent mixtures are used as we have already reported for PEDOT:PSS layers [13].…”

Toward high volume solution based roll-to-roll processing of OLEDs

“…Research aimed at results more directly applicable in an industrially relevant production environment therefore focuses on other techniques, like slotdie coating. 8,[11][12][13][14] This approach exhibits its own specific challenges not encountered in vacuum processing, but there are also distinct benefits associated with it, which, for certain application areas, show good prospects for replacing dry deposition technologies on the mid-term. Many of these beneficial characteristics are related to the possibility of processing at atmospheric pressure, with more efficient materials use, high throughput, and at low overall production cost.…”

“…15 Several recent publications have demonstrated that slot-die coating can be used for the production of highly efficient OLED devices by solution based techniques. [11][12][13][14] These studies, however, have all made use of sheet-to-sheet (S2S) processing. An especially interesting aspect of slot die coating and several other solution processing methods is production upscaling by roll-to-roll (R2R) manufacturing techniques.…”