A. OLED display

OLED displays have become a main stream technology in mobile consumer applications in the last few years. Active matrix top emitting OLED, also known as TE-AMOLED, is the technology deployed in mobile (mainly smart phones) applications. Rigid AMOLED displays fabricated on the glass substrate and encapsulated with glass were first launched in mass production by Samsung in 2010 (Samsung Electronics, Suwon, South Korea). The first commercial AMOLED display on plastic foil and TFE with a curved appearance was sold in limited numbers in 2013. Since then, thin AMOLED displays with curved edges have become popular as high-end consumer products and are commercialized by major brands.¹⁸

The large-scale mass production deployment of AMOLED displays was made possible by improvements in device lifetime, reliability, performance improvements of the organic materials used in the OLEDs, and further improved by evaporation and masking protocols when depositing the active OLED materials. In addition, the development of effective technologies for flexible substrate fabrication and TFE has played a key role. 10–20 µm thick plastic substrates are fabricated by coating PI on a carrier glass followed by depositing an inorganic layer, usually silicon nitride plasma-enhanced chemical vapor deposition (PECVD), as a barrier to moisture and oxygen permeation. This inorganic layer can be deposited at relatively high temperature because PI is stable up to temperatures in the range of 400 °C. Low-temperature polysilicon (LTPS) TFT arrays are fabricated, followed by an isolation layer, the patterning of the pixel definition layers, and other lithographic structures (e.g., banks), before the evaporation of the OLED stack. Encapsulation of devices is done by ML TFE consisting of a stack of inorganic/organic layers. In the most advanced mobile phone display technology (i.e., Galaxy S8, Samsung, Suwon, South Korea), the touch screen panel is directly fabricated on the display before detaching it from the carrier glass by laser irradiation.

The critical technical requirement in high-end mobile display applications are as follows:

1. (i) OLEDs are extremely sensitive to moisture and oxygen. OLED failure modes have been extensively discussed in scientific and technical literature. A comprehensive review is provided by Kondarov in Ref. Reference Kondakov, Gaspar and Polikarpov19. Barriers with water vapor transmission rates (WVTRs) lower than 10⁻⁶ g/(m² day) are required to avoid short-term degradation of devices by black spot formation; oxygen transmission rates (OTRs) lower than 10⁻⁴ cm³/(m² day) prevent middle/long-term degradation of light emission efficiency by oxygen.

2. (ii) In display applications, point defects cannot be tolerated.

3. (iii) A high number of particles are introduced by the evaporation process of the OLED active layers using shadow masks. To achieve the required resolution for mobile displays (540–580 PPI), fine metal masks are fabricated with special and proprietary technologies and used in strict proximity (∼10 µm) to the display glass. Advanced tools and strict procedures mitigate—but do not avoid—particle contamination on the display glass.

4. (iv) The inorganic layers have to be processed at low temperature to be compatible with the OLED low temperature constraints (80–100 °C). Low temperature deposition, however, results in poorly conformal, low density, and defect-prone inorganic layers. In addition, during plasma-aided inorganic deposition processes, the plasma energy has to be relatively low to avoid degradation by UV and particles irradiation.

5. (v) Flexibility requirements limit the thickness of ceramic inorganic layers (max ∼1 µm).

6. (vi) Encapsulation edges must be as small as possible to minimize the display bezel.

Encapsulation by an inorganic/organic ML stack (Fig. 2) is an effective way to address these challenges. The ML-TFE technology was proposed in the early 2000 by Vitex (San Jose, California)^{Reference Moro, Krajewski, Rutherford, Philips, Visser, Gross, Bennett, Graff, Kafafi and Lane20,Reference Burrows, Graff, Gross, Martin, Hall, Mast, Bonham, Bennett, Michalski, Weaver, Brown, Fogarty, Sapochak and Kafafi21} and implemented in mass production by Samsung in 2010. Since then, it has become the technology standard for the industry. Redundancy offered by the structure with multiple inorganic layers mitigates the problem of having films with sub-optimal quality and allows the use of thinner films that are more flexible. The organic layers are deposited in liquid form either by polymer vacuum evaporation or by ink-jet printing (IJP).^{Reference Moro, Krajewski, Rutherford, Philips, Visser, Gross, Bennett, Graff, Kafafi and Lane20,Reference Madigan, Van Slyke and Vronsky22} Blends of monomers cured by UV radiation are currently used. Polymeric films interlayered between inorganic films decouple defects in the inorganic layer preventing the propagation of microstructural faults (i.e., pinholes, grain boundaries, etc.) from one inorganic layer to the next (Fig. 3). Particles introduced by precedent process steps are covered, and a smooth and flat surface is made available for the nucleation of the next inorganic layer. Since fabrication defects are inevitable even on the top inorganic layer, moisture and oxygen may penetrate into the structure. Their diffusion through the ML will occur via a tortuous path created by defects (see red-line in Fig. 2), delaying its arrival to the OLED reactive layers. The mechanism has been described using theoretical modeling,^{Reference Graff, Williford and Burrows23} and experimentally tested in model structures by X-ray scattering and neutron reflection experiments.^{Reference Vogt, Lee, Prabhu, DeLongchamp, Lin, Wu and Satija24} Early in the technological development of the ML-TFE technology by Vitex, it had been demonstrated that the proper number of organic/inorganic layers is exclusively defined by the number and size of defects.^{Reference Chu, Moro and Visser25,Reference Moro, Chu and Hirayama26}

FIG. 2.

The image shows a transmission electron microscopy (TEM) cross section of a ML-TFE stack. The thin light grey layers are the inorganic barrier films and the thicker dark grey layers refer to the polymer films in between. The red arrows indicate a possible path of moisture propagating via defects in the inorganic layer towards the organic device. The diffusion through the ML will occur via a tortuous path, delaying its arrival to the OLED reactive layers.

FIG. 3.

The 2D schematic provides a simplified picture of state-of-the-art TFE on top of an organic device. The polymeric film interlayered between inorganic films decouples defects in the inorganic layer preventing the propagation of microstructural faults (i.e., pinholes, and grain boundaries) from one inorganic layer to the next.

In current commercial devices, the inorganic layer is usually SiN_x or SiO_xN_y deposited by low-temperature (LT) PECVD. The thickness of the layers for the current generation of bendable displays is about 1 µm. Among many materials and deposition techniques, PECVD SiN_x has been chosen due to availability of commercially designed tools and the ease of in situ cleaning that allows frequent short cleaning cycles of the deposition chamber, minimizing particle contamination. The polymer layer of 1–4 µm thickness was initially deposited by vacuum evaporation, using shadow masks for patterning. For high volume manufacturing, IJP has been proposed^{Reference Lin, Chu and Rosenblum27} and adopted.^{Reference Jin, Lee, Kim, An, Straykhilev, Pyo, Kim, Lee, Mo, Kim and Chung28,Reference Hebb29} IJP enables direct and mask-less patterning of the areas to be covered and hence reduces the precursor consumption. Currently, layer thicknesses of 6–10 µm are used. Figure 4 shows the coverage ability of an added particle by an IJP polymer layer.^{Reference Hebb29} Figure 5(a) shows the morphology of an uncoated display as measured with an optical profilometer. Lithographically defined wells of 1–2 µm in depth are necessary to define and isolate pixels. The integrity of the LT–PECVD SiN_x is compromised by the uneven morphology of the display (i.e., microstructural faults at the edges and vertical walls of the wells). The IJP process of the liquid polymer followed by cure creates a perfectly flat surface [see Fig. 5(b)], on which SiN_x can nucleate without defects. Today, improved fabrication techniques and procedures have enabled effective ML-TFE with a minimum stack of three layers.

FIG. 4.

This TEM cross section reveals a particle, introduced by precedent process steps, covered by the polymeric film and resulting in a smooth and flat surface.

FIG. 5.

(a) shows the morphology of an uncoated display as measured with an optical profilometer. Lithographically defined wells of 1–2 µm in depth are necessary to define and isolate pixels. (b) shows the morphology of a coated display as measured with an optical profilometer. (c) displays the two line profiles of the uncoated display (blue line) and the coated display (orange line). The IJP process of the liquid polymer followed by cure creates a perfectly flat surface.

For TE-AMOLED applications, the optical properties of the TFE stack are very stringent: transparency of the entire stack must be >90–95%, no haze, and neutral color are required. The stack structure and refractive indices of the layers are designed with the help of optical simulations to improve light out-coupling and wide angle view. These properties must be constant over the entire production glass area (>1 m²). Thickness nonuniformity and Mura must be avoided by very advanced engineering and IJP techniques.^{Reference Madigan, Van Slyke and Vronsky22,Reference Hebb29,Reference Madigan, Hauf, Barkley, Harjee, Vronsky and Van Slyke30} A differentiation point for smart phone displays is the bezel-less design, which is achieved by the implementation of advanced printing algorithms that minimize the edge transition of the TFE layers.

2. OLED displays for automotive applications

Another emerging OLED display application is automotive displays. This application has been made possible by the increased stability of OLED materials to higher temperature (>80 °C) and longer lifetime at relatively high luminance (>600 cd/m²). OLED displays are proposed for use on dashboards where the limited availability of flat surfaces makes OLED displays on flexible substrates and of arbitrary shapes very attractive.^{Reference Hebb29,Reference Wittmann35} OLED displays fabricated on plastic substrates might also improve the safety in the vehicle since they do not break or shutter under impact. Reduced energy consumption of up to 15% is an additional advantage compared to LCD since only lit OLED pixels consume power. Finally, the intrinsic transparency of OLED materials makes it possible to create head-up displays in the front shield where transparency higher than 70% is often required by regulatory bodies. Encapsulation of automotive displays is mostly done by hybrid or barrier-foil approach.

Full freeform involves the evolution from 2D to freeform 3D or conformal displays. While commercial displays have not been produced yet, a stretchable 9.1-inch full-color LTPS AMOLED display with the capability of arbitrary 3D shape transformation has been demonstrated by Samsung (see Fig. 6).^{Reference Hong, Shin, Kim, Joo, Park, Hwang, Kim, Park, Chu and Kim36} Stretching functionality has been obtained by using the Kirigami-based stretchable technology which makes a thin sheet stretchable by creating holes by cutting.^{Reference Yokota, Zalar, Kaltenbrunner, Jinno, Matsuhisa, Kitanosako, Tachibana, Yukita, Koizumi and Someya2} The vacuum thermoforming process was then used to give a double curved shape by laminating the stretchable AMOLED panel to a PET foil with an optically clear adhesive. Further encapsulation technology development is needed to translate this prototype display into a commercial product. The displayed prototype might include a lower grade encapsulation using thin ALD layers on the formed 3D display or engineered thin PET foil with ML barrier by thin ALD layer and stretchable polymer. For the commercialization of Kirigami-based stretchable displays, the best approach may be to deposit TFE only in selected areas excluding the sections in which holes are cut.

FIG. 6.

Photographs of the fabricated 9.1-inch AMOLED display prototype. (a) Flat AMOLED panel before applying the thermoforming process. (b) Convex and (c) Concave-shaped panel produced through the thermoforming process.^{Reference Hong, Shin, Kim, Joo, Park, Hwang, Kim, Park, Chu and Kim36}

B. OLED lighting

OLED lighting has recently attracted interest in high-end automotive applications^{Reference Mathas37} and high-end indoor lighting designs,^{Reference Nguyen38} enabling novel 3D shapes of ultra-thin and flexible light-emitting panels. BMW and Audi have demonstrated OLED rear lights provided by OSRAM OLED GmbH in selected new car models.³⁹

The TFE requirements for OLED lighting are mostly similar to those for OLED displays with respect to oxygen and moisture permeation rates: WVTR lower than 10⁻⁶ g/(m² day) and OTR lower than 10⁻⁴ cm³/(m² day). However, OLEDs for lighting are less sensitive to point defects and have more forgiving specification for edge sealing. Many of the devices are bottom emitting OLEDs; therefore, the barrier foil used as the substrate must be optically transparent, but the encapsulation can be done with opaque foils. The resulting structure has to be ultra-thin and flexible.

The barrier properties are evaluated in accelerated aging tests for >1000 h at 85 °C/85% humidity targeting shelf and operation lifetimes for OLED lighting that are expected to be 5–15 years based on the average ownership of a car and an interior architectural design. In bottom emitting devices, light extraction features can be achieved by the proper choice of the material refractive index and/or inclusion of geometrical structures on the transparent substrates. More advanced and futuristic lighting applications use top emitting devices with encapsulation that has to be transparent in the visible wavelength range and similarly includes light extraction features.

For automotive applications, the OLED light + packaging has to be durable under extreme environmental and temperature conditions. These environmental durability tests are more challenging than those for phone applications. Tests are performed by exposing the devices to H₂S and mixed gases making chemical compatibility and inertness of the encapsulation materials an additional requirement.^{Reference Wittmann35} Temperature durability under extreme usage conditions is evaluated by thermal shock tests—switching the temperature conditions from −40 °C to +85 °C every few seconds. Materials with matching coefficient of thermal expansion must be used to avoid delamination and cracking. Finally, mechanical durability and impact resistance are also tested introducing the extra specification of shock absorbance to the quality of the encapsulation layers.

C. Photovoltaics

In 2017, perovskite solar cells reached certified record efficiencies of 22.7%, exceeding the record efficiencies of various inorganic PV technologies: CIGS (22.6%), CdTe (22.1%), and multicrystalline silicon (22.3%).⁴⁰ High efficiencies paired with the potential of low-cost device fabrication on flexible substrates have spurred interest in commercialization. To date, the biggest hurdles are device instability and rapid performance degradation.^{Reference Saliba, Buonassisi, Grätzel, Abate, Tress and Hagfeldt41,Reference Roesch, Faber, Von Hauff, Brown, Lira-Cantu and Hoppe42} Ongoing research is aimed at developing perovskite-inspired absorber materials with improved intrinsic stability.^{Reference Steinmann, Brandt and Buonassisi43,Reference Brandt, Stevanović, Ginley and Buonassisi44} Improved device lifetime has been demonstrated by inserting an ultra-thin alumina film into a conventional perovskite solar cell structure.^{Reference Koushik, Verhees, Kuang, Veenstra, Zhang, Verheijen, Creatore and Schropp45} The alumina film directly deposited on top of the perovskite active layer serves as a passivation layer against surface defects, reducing the hysteresis behavior of the device. In addition, the alumina provides some protection against moisture and oxygen entering the perovskite film. To achieve long-term stability of perovskite solar cells, encapsulation will still be needed. Key contributors to perovskite degradation are oxygen, moisture, UV light, and thermal effects. While oxygen only decomposes the perovskite under illumination, moisture causes rapid degradation even in the dark.^{Reference Wang, Wright, Elumalai and Uddin7}

Organic solar cells (based on small molecule or polymer active layers) reveal a similar sensitivity to water and oxygen. Oxidation at the aluminum contacts can result in an oxide layer between the photoactive layer and the contact, hindering the effective charge collection.^{Reference Gevorgyan, Madsen, Roth, Corazza, Hösel, Søndergaard, Jørgensen and Krebs12} In addition, oxygen and water diffusion into the photoactive layer can trigger chemical and physical degradation mechanisms and hence deteriorate the effectiveness of charge carrier generation and separation in the photoactive layer. In a more specific example, water diffusion can cause aggregation of the fullerene, affecting the metastable morphology of electron donor and acceptor domains.^{Reference Cheng and Zhan46} Lifetime degradation processes in organic PVs have been studied for decades. A comprehensive overview can be found elsewhere.^{Reference Gevorgyan, Madsen, Roth, Corazza, Hösel, Søndergaard, Jørgensen and Krebs12,Reference Grossiord, Kroon, Andriessen and Blom47–Reference Neugebauer, Brabec, Hummelen and Sariciftci50}

To encapsulate organic solar cells or perovskite-based PV panels, the lamination of barrier foils is likely to be the preferred method. To achieve operation lifetimes competitive with the inorganic counterparts, we estimate that a WVTR of <10⁻² g/(m² day) will be required.^{Reference Hauch, Schilinsky, Choulis, Rajoelson and Brabec51} The materials used as barriers and substrates must be UV stable to avoid degradation upon exposure to the UV portion of the solar spectrum. Photochemical and photophysical degradation has been observed upon long-term irradiation.^{Reference Cheng and Zhan46} Planarization layers are to be included in the encapsulation structure to avoid stress-induced cracking of the inorganic barrier layers during the lamination process. Additional characteristics to be considered are electrical isolation, breakdown field, exposure to extreme temperature, and physical impact.

Current PV products on the market offer typical warranties up to a few years for mobile applications and up to 20–25 years for grid installations.^{Reference Roesch, Faber, Von Hauff, Brown, Lira-Cantu and Hoppe42,Reference Jordan and Kurtz52} Hence, the encapsulation requirements are strongly governed by the application.

Extremely low cost^{Reference Powell, Fu, Horowitz, Basore, Woodhouse and Buonassisi53–Reference Powell, Winkler, Goodrich and Buonassisi55} is required for packaging of large-area PV modules in grid installations. To date, the packaging cost of organic PV devices is estimated to be 60% of the entire module cost.^{Reference Mulligan, Wilson, Bryant, Vaughan, Zhou, Belcher and Dastoor56} Reducing the packaging costs—including the encapsulation—will be a major driving force towards making organic and hybrid organic/inorganic perovskite solar cells competitive for grid installations.^{Reference Deibel and Dyakonov57}

For some high-end PV roof-top installations, ultra-thin and flexible panels integrated in traditional building fabrication components could become very attractive. Barrier encapsulation must be combined with architectural functions to complement environmental durability with the aesthetics of high-end PV system designs.

For mobile PV applications, e.g., military or rescue applications, portable consumer electronics, outdoor and camping, or other off-grid energy requirements, lightweight ultra-thin encapsulation is generally preferred to minimize the weight and size of the product. Many mobile applications might be desired to be flexible, stretchable, or even of arbitrary shapes. In addition, the product needs to be durable and robust. In general, the shelf lifetime of a solar cell integrated in mobile applications needs to be several years. However, much shorter operation lifetimes of months to a few years can be tolerated. Hence, the moisture and oxygen permeation requirements might be less aggressive compared to PV grid installations. An example for a mobile PV application could be an ultra-thin and stretchable solar cell integrated in a tent cover, offering shelter and a source of electricity at the same time.^{Reference Beckman58} In this application, the encapsulated solar cell has to withstand rough handling (folding and tight packing of the tent cover in storage). When in use, it has to withstand a wide range of weather conditions. Here, lamination of multiple barrier foils engineered with single layer evaporated inorganic films of poor barrier performance is often used for encapsulation to comply with the above requirements and still be cost-effective. With this approach, an effective ML structure is created.

Most high-efficiency hybrid organic/inorganic perovskite solar cells absorb strongly in the visible wavelength range (∼400–800 nm). However, perovskite absorber materials are unstable under UV light.^{Reference Wang, Wright, Elumalai and Uddin7} State-of-the-art barriers are highly transparent to visible light, but absorb the UV portion of the light. Hence, these could serve well to protect the solar cell from UV light damage.

D. Sensors

Sensors are used across a wide range of applications, e.g., in digital health,^{Reference Belluck59} biomedical devices,^{Reference Tricoli, Nasiri and De1,Reference Someya, Bao and Malliaras60,Reference Kaltenbrunner, Sekitani, Reeder, Yokota, Kuribara, Tokuhara, Drack, Schwödiauer, Graz, Bauer-Gogonea, Bauer and Someya61} environmental monitoring,^{Reference Karim and Zeadally62–Reference Srbinovska, Gavrovski, Dimcev and Krkoleva64} and agriculture and food industry.^{Reference Ruiz-garcia, Lunadei, Barreiro and Robla65,Reference Khanal, Fulton and Shearer66} To date, however, most of the commercialized sensors are rigid and/or bulky.

Recent progress in the development of flexible organic field effect transistors (OFETs) and photodetectors provides a promising platform to enable ultra-thin and stretchable sensors. While the device performances are approaching those of their inorganic counterparts, OFETs and organic photodetectors are still lacking in device stability and reliability.^{Reference Zang, Huang, Di and Zhu67,Reference Vuuren, Armin, Pandey, Burn and Meredith68} Further material and device development might yield improved device stability. To commercialize devices with shelf and operational stability and reliability suitable for the application, proper encapsulation will be required. For most sensor applications, long shelf lifetimes are required, meaning that the user has to be able to rely on the functionality of the device even after storing it for several months to years. Operation lifetimes, however, can be much shorter: in biomedical applications, the product might only be used for a few days to weeks. Shelf-life requirement and operational requirement should therefore be decoupled by using proper packaging materials, e.g., metalized ML bags.

While there are no optical transparency requirements for the encapsulation on OFETs, the application of photodetectors requires the encapsulation to be transparent to visible and IR light. One of the key challenges for designing encapsulation for sensors is that the material compatibility requirements strongly depend on the specific application: e.g., for biomedical implants, the encapsulation materials need to be benign for the human body, but themselves resistant to the human body aggressive environment, i.e., sweat, acids, etc. By being chemically inert and available with high purity, silicones and vacuum deposited fluorinated parylene coatings are often used as external encapsulation layers for this type of devices.

In field installation for environmental monitoring, the device packaging has to be very durable in extreme weather and temperature conditions and mechanically reliable. For example, for strain sensors to be used as engineering and architectural monitoring of building and structures, the encapsulation is required to be highly stretchable and to be consistent and reliable. Polymeric coating must be used. For some types of sensors, permeation and diffusion of specific compounds through the encapsulation is required for functioning. Semiporous media must be included in the encapsulation to allow selective permeation. Many of the proposed applications of organic sensors, some in the form of fiber or yarn, involve integration with textile products used in sport or medicine. In this case, breathability (as special case of selective permeation), comfort, and—in some cases—resistance to repeated washing are necessary for successful commercialization. Most of the currently deployed solutions sandwich the sensor between engineered plastic sheets and then laminate it on the textile in a specific position. Better solutions including encapsulation by directly coating on the sensor-fiber/yarn are required.