Slot coating minimum film thickness in air and in rarefied helium

Abstract

This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower "thin film" viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.

Ink Design Enabling Slot‐Die Coated Perovskite Solar Cells with >22% Power Conversion Efficiency, Micro‐Modules, and 1 Year of Outdoor Performance Evaluation

“…The periodic height variations in the wet‐film thickness is a phenomenon referred to as “ribbing”, which occurs when the positive pressure gradients and surface tension forces acting on the meniscus during slot‐die coating are unbalanced. [ 12 ] Ribbing originates from inhomogeneous flow and may persist even the final coating result if coated wet films cannot self‐equilibrate to a homogeneous wet film prior to solidifying. [ 28 ] In our case, we observed the layer inhomogeneity caused by ribbing to persist and cause inhomogeneity effects in coated perovskite thin films after annealing, as illustrated in the photo of a slot‐die coated larger‐area sample shown in Figure 2c.…”

“…[10,11] Among them, the slot-die coating is one of the most promising processes enabling sheet-to-sheet, continuous roll-to-roll coating, and solution-processable optoelectronic device technology such as organic electronic devices and polymer solar cells. [12][13][14] In our previous work, we demonstrated efficient inverted PSC fabricated by SDC with power conversion efficiencies (PCEs) over 20%. [11] However, it remains far behind the champion device from blade coating.…”

Reverse roll coating with a deformable roll operating at negative gaps

“…Table 1, an operating guide for this coating method, summarizes the range of film thickness that can be achieved [1-40 μm] at a coating speed range up to 150m/min. This is a remarkable achievement for such a simple coating method, out-performing all other coating techniques, including slot coating (see Benkreira and Ikin, 2016;Lee et al, 1992).…”

Controlling Thin Film Morphology Formation during Gas Quenching of Slot-Die Coated Perovskite Solar Modules

“…25,26 Simultaneously, it is transferable to roll-to-roll (R2R) fabrication unlike vacuum-induced quenching. 16,24 Altogether, gas quenching is a facile, reproducible, low-cost, and easily transferable technique, thereby showing great potential in the application of highly efficient large-area PSCs. 11,13,24,27 For the induced crystallization during quenching, a specifically high mass transfer is needed at the position where the wet thin film passes the critical solute concentration.…”

“…Thus, one major challenge of the technology is upscaling the deposition of homogeneous high-quality thin films to large areas (≥100 cm 2 ) without performance loss using industry-relevant deposition techniques. ,− On this account, the present work deals with the solution processing of hybrid perovskite thin films by the scalable technique of slot-die coating (SDC). SDC promises scalable, material, and cost-efficient deposition of perovskite thin films on a wide range of substrate types. , Recent publications successfully integrate the industrially adaptable process of SDC for two-terminal wide bandgap halide perovskite/c-Si tandem solar cells. , To date, slot-die coated methylammonium-free perovskite modules reach a PCE of up to 16.2% (active area of 57.5 cm 2 ), while the current record in PCE of a solution-processed module, 17.9%, is held by Panasonic who inkjet-printed the perovskite absorber thin films. , These values still lag behind the record of spin coated laboratory-scale PSCs of 25.8%. When scaling PV, loss of active area due to module scribing and interconnection resistances cannot be avoided. , However, compared to concurrent thin film modules involving similar architectures, the upscaling losses in perovskite PV modules remain high .…”