Interfacial Engineering in Solution Processing of Silicon-Based Hybrid Multilayer for High Performance Thin Film Encapsulation

Polysilazane-Coated Films Achieving Record-High Moisture Barrier Performance with Sub-10 Seconds Densification Using High-Power VUV Irradiation

An ultra-high moisture barrier compact SiNX film can be achieved from solution-processed perhydropolysilazane (PHPS) through vacuum ultraviolet (VUV) light exposure. This study investigates the photochemical reactions and photo-densification of PHPS-based barrier films under varying VUV light intensities, focusing on their effects on barrier performance. Photo-dehydrogenation of PHPS, involving N─H and Si─H bond cleavage, is efficient and unaffected by light intensity. However, photo-densification shows a strong dependence on light intensity, particularly above 290 mW cm-2. Higher intensities enhance Si─N bond cleavage, alter film dynamics, and reduce free volume through bond rearrangement, facilitating rapid network reconstruction essential for ultra-high barrier properties. High-power VUV light at 309 mW cm-2 establishes a new benchmark for ultra-high barrier films via solution processing, achieving a record-low average water vapor transmission rate (WVTR) of 1.6 × 10-5 g m-2 day-1. Films are produced in under 10 s per layer, maintaining a barrier property of 3.8 × 10-5 g m-2 day-1. The optimal refractive index for the top 30 nm layer is 1.74-1.77, controlling WVTR within 10-5 g m-2 day-1, ensuring superior barrier performance for flexible electronic devices, such as perovskite solar cells and organic photovoltaics.

Hexacarbazolylbenzene: An Excellent Host Molecule Causing Strong Guest Molecular Orientation and the High-Performance OLEDs

Hexacarbazolylbenzene (6CzPh), which is benzene substituted by six carbazole rings, is a simple and attractive compound. Despite the success of a wide variety of carbazole derivatives in organic light-emitting diodes (OLEDs), 6CzPh has not received attention so far. Here, excellent performances of 6CzPh are revealed as a host material in OLEDs regarding conventional host materials. Various strategies are implemented to improve the performance of OLEDs, e.g., triplet utilization by thermally activated delayed fluorescence (TADF) and phosphorescence emitters for maximizing internal quantum efficiency, and molecular orientation control for increasing outcoupling efficiency. The present host material is suited for both criteria. Robustness of the structure and sufficiently high triplet energy enables a high external quantum efficiency with a long device lifetime. Besides, the host material boosts the horizontal molecular orientations of several guest emitters. It is noteworthy that disk-shaped 4CzIPN marks the complete horizontal molecular orientations (Θh = 100%, S = -0.50). These results provide an effective way of improving efficiencies without sacrificing device durability for future OLEDs.

Atomic-scale stress modulation of nanolaminate for micro-LED encapsulation

Micro/nano-LEDs for augmented reality (AR) and virtual reality (VR) applications face the challenge that the edge effect in micro-LEDs becomes significant as the size of devices shrinks. This issue can be effectively addressed through thin-film encapsulation, where zero stress of the thin film is a crucial factor, apart from the barrier property. Herein, a stress-modulation strategy was developed through a binary-cycle atomic-layer deposition (ALD) process combining PEALD SiO2 (compressive stress) and thermal ALD Al2O3 (tensile stress) in the same process window. The hybrid ALD process allows avoiding extra thermal stress generation and enables precise modulation of the atomic-scale thickness, thereby allowing the fabrication of nanolaminates with modulated stress. The optical nanolaminate developed herein achieved a stress level of near-zero, representing one of the best among reported studies. The structural design, characterized by a high-low refractive index, tortuous permeation path, and ultra-thin thickness, remarkably improved the optical transmittance and barrier properties (8.68 × 10-6 g m-2 day-1) of the nanolaminate. Moreover, the micro-LED encapsulated with SA2/1 exhibited excellent stability under thermal cycling, damp heat, and applied stress conditions. The mechanical stability of the nanolaminate was due to the strong interaction between Si-O and Al-O and the abundance of Si-O-Al bonding in the interface. Overall, the ALD-coating process provides a new avenue for accurately controlling the stress on nanolaminates, and has potential application to bolster the reliability of optoelectronic devices.

Room temperature processed protective layer for printed silver electrodes

Low-temperature processed printed silver electrodes pave the way for electrical connections in flexible substrates with reduced energy consumption. Despite their excellent performance and simple process, printed silver electrodes' poor stability limits their applications. This study demonstrates a transparent protective layer without thermal annealing for printed silver electrodes, which maintains its electrical properties for a long period of time. A fluoropolymer, specifically a cyclic transparent optical polymer (CYTOP), was used as a protective layer for silver. The CYTOP is room temperature processable and chemically stable against carboxyl acid. The introduction of the CYTOP film on the printed silver electrodes mitigates the chemical reaction between silver and carboxyl acid, thereby elongating its lifetime. Under heated acetic acid, the printed silver electrodes with a CYTOP protective layer maintained their initial resistance for up to 300 hours, while the electrodes without a protective layer were damaged within a few hours. A microscopic image shows that the protective layer enables printed electrodes to maintain their shape without damage. Hence, the protective layer guarantees the accurate and reliable performance of electronic devices with printed electrodes under actual operating conditions. This research will contribute to designing chemically reliable flexible devices in the near future.

In Situ Solution-Processed Submicron Thick SiOx Cy /a-SiNx (O):H Composite Barrier Film for Polymer:Non-Fullerene Photovoltaics

Aiming to improve the environmental stability of organic photovoltaics, a multilayered SiO_x C_y /a-SiN_x (O):H composite barrier film coated with a hydrophobic perfluoro copolymer stop layer for polymer:non-fullerene solar cells is developed. The composite film is prepared by spin-coating of polysilicone and perhydropolysilazane (PHPS) following a densification process by vacuum ultraviolet irradiation in an inert atmosphere. The transformation of polysilicone and PHPS to SiO_x C_y and a-SiN_x (O):H is confirmed by Fourier transform infrared and energy-dispersive X-ray spectroscopy measurement. However, the as-prepared PHPS-derived silicon nitride (PDSN) can react with moisture in the ambient atmosphere, yielding microscale defects and a consequent poor barrier performance. Treating the incomplete PDSN with methanol vapor significantly densifies the film yielding low water vapor transmission rates (WVTRs)of 5.0 × 10^-1 and 2.0 × 10^-1 g m^-2  d^-1 for the one- and three-couple of SiO_x C_y /a-SiN_x (O):H (CON) composite films, respectively. By incorporating a thin hydrophobic perfluoro copolymer layer, the three-coupled methanol-treated CON film with a total thickness of 600 nm shows an extremely low WVTR of 8.7 × 10^-4 g m^-2  d^-1 . No performance decay is measured for the PM6:Y6 and PM6:L8-BO cells after such an encapsulation process. These encapsulated polymer cells show good stability storaged at 25 °C/50% relative humidity, or under simulated extreme rainstorm tests.

Design of a Flexible Thin-Film Encapsulant with Sandwich Structures of Perhydropolysilazane Layers

Perovskite solar cells (PSCs) have attracted considerable attention due to their excellent photovoltaic properties, but stability issues have prevented their widespread application. PSCs must be protected by encapsulation to extend their lifetime. Here, we show that perhydropolysilazane (PHPS)-based multilayered encapsulation improves the lifetime of PSCs. The PSCs were encapsulated by converting PHPS into silica under vacuum ultraviolet (UV) irradiation. The PHPS-based multilayer encapsulation method achieved a sandwich structure of PHPS/poly(ethylene terephthalate) (PET)/PHPS with a water vapor transmission rate (WVTR) of 0.92 × 10^-3 gm^-2 d^-1 (at 37.8 °C and 100% relative humidity). We then performed a reservoir test of the encapsulated PSCs to confirm the moisture stability of the encapsulation based on PHPS/PET/PHPS barrier films. The cell lifetime remained stable even after 1000 h of ambient-temperature operation. Finally, we analyzed the mechanical flexibility of the PHPS/PET/PHPS multibarrier through bending tests. The multibarrier exhibited high mechanical stability with no large increase in WVTR after bending.

Highly-stable PEN as a gas-barrier substrate for flexible displays via atomic layer infiltration

Polymer substrates with superior barrier properties are of great importance for the development of highly-stable flexible displays. The atomic layer infiltration (ALI) method has been utilized to integrate nanoscale inorganic materials in the subsurface of commercial PEN substrates, and the in-suit quartz crystal microbalance (QCM) is employed to study the growth behaviour as the process parameters vary, in which the nucleation and infiltration stages have been demonstrated. The O₂ plasma pre-treatment prior to Al₂O₃ infiltration was used to determine its effect on the water vapor transmission rate (WVTR), and significantly improved barrier properties were observed compared to those of the ones without the O₂ plasma pre-treatment via the electrical Ca tests, which was attributed to the surface clean and improved film adhesion. The lowest WVTR value measured was 1.28 × 10^-5 g m^-2 day^-1 for the O₂ plasma pre-treated PEN substrate coated with 100 ALI cycles, which improved 3-4 orders of magnitude compared to that of the pristine ones. Besides, the infiltrated PEN substrate with O₂ plasma pre-treatment exhibited good mechanical stability, with only a slight increase of the WVTR value which was observed after the bending fatigue test with a radius of 5 mm. Furthermore, when applied to the encapsulation of organic light-emitting diodes (OLEDs), the normalized luminance remained above 94% after storage for 800 hours.