As the demand for flexible, rollable, and foldable displays grows, various state-of-the-art component technologies, including thin-film transistors (TFTs), electrodes, thin-film encapsulations (TFEs), and touch screen panels, have been developed based on organic light-emitting diodes (OLEDs) with flexible organic layers. Developing highly reliable flexible OLEDs is essential to realize flexible displays, but the flexible encapsulation technology still has technical difficulties and issues to be addressed. This review covers the recent developments in encapsulation technologies, particularly their material and structural designs, for highly reliable, flexible OLEDs. The solution concepts for the existing technical hurdles in flexible encapsulations are addressed. Among the various advanced flexible encapsulation technologies developed so far, neutral-axis engineering with a thin metal layer and a crack arrester is introduced.

Wearable electronic devices are being developed because of their wide potential applications and user convenience. Among them, wearable organic light emitting diodes (OLEDs) play an important role in visualizing the data signal processed in wearable electronics to humans. In this study, textile-based OLEDs were fabricated and their practical utility was demonstrated. The textile-based OLEDs exhibited a stable operating lifetime under ambient conditions, enough mechanical durability to endure the deformation by the movement of humans, and washability for maintaining its optoelectronic properties even in water condition such as rain, sweat, or washing. In this study, the main technology used to realize this textile-based OLED was multi-functional near-room-temperature encapsulation. The outstanding impermeability of TiO2 film deposited at near-room-temperature was demonstrated. The internal residual stress in the encapsulation layer was controlled, and the device was capped by highly cross-linked hydrophobic polymer film, providing a highly impermeable, mechanically flexible, and waterproof encapsulation.

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Organic light-emitting diodes (OLEDs) have become mainstream as the next generation of wearable displays that will outperform lasers and light-emitting diodes (LEDs). Recently, OLED-based platforms used for light applications were introduced. [1][2][3] In particular, deep red to nearinfrared (DR/NIR) OLEDs have emerged in the past few years because they can be applied for night vision displays, optical sensors, and phototherapy. [4] However, DR/NIR emitters exhibit intrinsic defects: these emitters are prone to undesired nonradiative decay pathways owing to the narrow bandgap. According to the energy gap law, the nonradiative decay rate (k nr ) is inversely proportional to the bandgap. In addition to this, the radiative rate (k r ) has a cubic dependence on the transition energy, so that it is expected to get smaller for lower energy emitters, implying that DR/NIR emitters have poor quantum efficiencies because of the incorporation of ground-and excited-vibrational energy states. [5][6][7][8][9] Consequently, research on high efficiency DR/NIR OLEDs has lagged behind that on visible-region OLEDs. To overcome this limitation, diverse ways of boosting the DR/NIR efficiency were introduced. In general, there are three kinds of DR/NIR emitters for high efficiency DR/NIR OLEDs: donor-acceptordonor (D-A-D) type, [10,11] thermally activated delayed fluorescence (TADF), [12][13][14][15] and transition-metal complexes. [16,17] DR/ NIR fluorophores of the D-A-D type have been researched to provide DR/NIR OLEDs with cost advantage and versatility. However, most DR/NIR OLEDs based on the D-A-D type show extremely low external quantum efficiency (EQE), radiance, and unexplained operational lifetime for practical application. [10,11] TADF DR/NIR emitters utilize nonradiative triplet excitons that move to a singlet region and achieve 100% internal quantum efficiency (IQE). [18] Nonetheless, there are chronic constraints with fluorescence quenching. Therefore, TADF-based DR/NIR OLEDs have a low luminance value. Meanwhile, high-efficiency DR/NIR OLEDs using transition metal complexes such as osmium (Os), [19][20][21] platinum (Pt), [22][23][24][25][26] and iridium (Ir) [27][28][29][30][31][32][33][34][35][36][37][38]

Smart displays have been integrated into our daily life, providing new concepts of interconnections between humans and interactivity with close collaboration

Recently, as state-of-the-art displays, wearable electronic devices and free-form displays have begun to attract attention, so that much research on textile-based organic light-emitting diode (textile OLED) displays as their ultimate platform has been actively conducted. However, although many studies have reported textile OLEDs with high current e ciency and maximum wearability, most research has left for the future improvements that require the use of transparent OLEDs to improve aesthetics and applications in various elds such as fashion displays and military products. Herein, RGB full color transparent organic light emitting diodes (trOLEDs) were designed using a dielectric-metal-dielectric transparent electrode to overcome the limitations of previous works and improve the aesthetic elements of textile displays. Opto-electronic performance and mechanical characteristics of textile RGB trOLEDs were analyzed; furthermore, to quantify the normalized acquisition rate of information that can be obtained from the textile substrate when transparent OLEDs on textile substrate are not operated, this study suggests a new method and novel equation for optical analysis.

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Flexible see-through displays are considered to be the next generation smart display, providing improved information flow, safety, situational awareness, and overall user experience in smart windows, automotive displays, glass-form biomedical displays, and augmented reality systems. 2D titanium carbides (MXenes) are promising material as electrodes of the transparent and flexible displays due to their high transparency, metallic conductivity, and flexibility. However, current MXene-based devices have insufficient air stability and lack engineering schemes to develop matrix-addressable display forms with sufficient pixels to display information. Here, we develop an ultraflexible and environmentally stable MXene-based organic light-emitting diode (OLED) display by combining high performance MXene electrodes, flexible OLEDs, and ultrathin and functional encapsulation systems. The MXene material was synthesized and used to fabricate a highly reliable MXene-based OLED that can stably operate in air condition for over 2000 h, endure repetitive bending deformation of 1.5 mm radius, and maintain environmental stability for 6 h when exposed to wet surroundings. The RGB MXene-based OLEDs were fabricated, (1691 cd m–2 at 40.4 mA cm–2 for red, 1377 cd m–2 at 4.26 mA cm–2 for green, and 1475 cd m–2 at 18.6 mA cm–2 for blue) and a matrix-addressable transparent OLED display was demonstrated that could display letters and shapes.