No abstract

Deformable full-colour light-emitting diodes with ultrafine pixels are essential for wearable electronics, which requires the conformal integration on curvilinear surface as well as retina-like high-definition displays. However, there are remaining challenges in terms of polychromatic configuration, electroluminescence efficiency and/or multidirectional deformability. Here we present ultra-thin, wearable colloidal quantum dot light-emitting diode arrays utilizing the intaglio transfer printing technique, which allows the alignment of red–green–blue pixels with high resolutions up to 2,460 pixels per inch. This technique is readily scalable and adaptable for low-voltage-driven pixelated white quantum dot light-emitting diodes and electronic tattoos, showing the best electroluminescence performance (14,000 cd m−2 at 7 V) among the wearable light-emitting diodes reported up to date. The device performance is stable on flat, curved and convoluted surfaces under mechanical deformations such as bending, crumpling and wrinkling. These deformable device arrays highlight new possibilities for integrating high-definition full-colour displays in wearable electronics.

In the future electronics, all device components will be connected wirelessly to displays that serve as information input and/or output ports. There is a growing demand of flexible and wearable displays, therefore, for information input/output of the nextgeneration consumer electronics. Among many kinds of light-emitting devices for these next-generation displays, quantum dot light-emitting diodes (QLEDs) exhibit unique advantages, such as wide color gamut, high color purity, high brightness with low turn-on voltage, and ultrathin form factor. Here, we review the recent progress on flexible QLEDs for the next-generation displays. First, the recent technological advances in device structure engineering, quantum-dot synthesis, and high-resolution full-color patterning are summarized. Then, the various device applications based on cutting-edge quantum dot technologies are described, including flexible white QLEDs, wearable QLEDs, and flexible transparent QLEDs. Finally, we showcase the integration of flexible QLEDs with wearable sensors, micro-controllers, and wireless communication units for the next-generation wearable electronics.

Colloidal nanocrystals have been intensively studied over the past three decades due to their unique properties that originate, in large part, from their nanometer-scale sizes. For applications in electronic and optoelectronic devices, colloidal nanoparticles are generally employed as assembled nanocrystal solids, rather than as individual particles. Consequently, tailoring 2D patterns as well as 3D architectures of assembled nanocrystals is critical for their various applications to micro- and nanoscale devices. Here, recent advances in the designed assembly, film fabrication, and printing/integration methods for colloidal nanocrystals are presented. The advantages and drawbacks of these methods are compared, and various device applications of assembled/integrated colloidal nanocrystal solids are discussed.

Large-scale colloidal synthesis and integration of uniform-sized molybdenum disulfide (MoS ) nanosheets for a flexible resistive random access memory (RRAM) array are presented. RRAM using MoS nanosheets shows a ≈10 000 times higher on/off ratio than that based on exfoliated MoS . The good uniformity of the MoS nanosheets allows wafer-scale system integration of the RRAM array with pressure sensors and quantum-dot light-emitting diodes.

An ultrathin skin-attachable display is a critical component for an information output port in next-generation wearable electronics. In this regard, quantum dot (QD) light-emitting diodes (QLEDs) offer unique and attractive characteristics for future displays, including high color purity with narrow bandwidths, high electroluminescence (EL) brightness at low operating voltages, and easy processability. Here, ultrathin QLED displays that utilize a passive matrix to address individual pixels are reported. The ultrathin thickness (≈5.5 µm) of the QLED display enables its conformal contact with the wearer's skin and prevents its failure under vigorous mechanical deformation. QDs with relatively thick shells are employed to improve EL characteristics (brightness up to 44 719 cd m at 9 V, which is the record highest among wearable LEDs reported to date) by suppressing the nonradiative recombination. Various patterns, including letters, numbers, and symbols can be successfully visualized on the skin-mounted QLED display. Furthermore, the combination of the ultrathin QLED display with flexible driving circuits and wearable sensors results in a fully integrated QLED display that can directly show sensor data.

The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.

Transparent displays lie at the heart of next generation optoelectronics [1,2] in the era of augmented reality (AR), wearable electronics, and internet of things (IoTs). [3][4][5][6][7] Being transparent for light-emitting diodes (LEDs) significantly expands their applications by displaying visual information on objects without affecting their original appearance and functionality. However, there has been a large gap in the electroluminescence (EL) performance between transparent displays and nontransparent counterparts, [8] due in large part to imbalanced injection of charge carriers into the emitter, unoptimized energy band alignment of the top electrode, and vulnerability of organic and/or polymeric light emitting materials during the deposition of transparent conducting oxide electrodes. [9][10][11][12] The previous progresses and unmet requirements for transparent displays are described in Section S2.1, Figure S1, and Table S1 of the Supporting Information. In addition, there has been much need to develop novel device architectures [13][14][15][16] that consider the carrier dynamics for high-performance transparent quantum dot light-emitting diodes (Tr-QLEDs).For high-quality transparent displays, first of all, high transparency is an absolute requirement. [17] The effect of transparency on visibility of background is examined on the university logo and a leaf (Figure 1a). For transparency below 70% (semitransparency), the color and contrast of objects behind the display are significantly deteriorated. In contrast, Tr-QLEDs of 84% transparency present clear background view in both cases. Secondly, high brightness and color purity are particularly important for vividness of "see-through" displays. The maximum brightness of conventional displays (e.g., smart phones and monitors) is around 600 cd m −2 . For see-through displays, however, the displayed information becomes blurred at this brightness (i.e., 600 cd m −2 ) because of photointerference with ambient light (Figure 1b; Figure S2a, Supporting Information). Therefore, significantly higher brightness is required to ensure clear and vivid displays (Figure 1b). In addition, chromatic aberrations can be minimized by employing engineered quantum dots (QD) emitters [18,19] that exhibit better color purity than organic and/or polymer emitters ( Figure S2b, Supporting Information). Lastly, integration of highly deformable Displaying information on transparent screens offers new opportunities in next-generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such "see-through" displays to show vivid images over clear background view. Here transparent quantum dot light-emitting diodes (Tr-QLEDs) are reported with high brightness (bottom: ≈43 000 cd m −2 , top: ≈30 000 cd m −2 , total: ≈73 000 cd m −2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplishe...