High-resolution intaglio transfer printing

For high-definition full-colour RGB QLED arrays, a novel QD integration process, known as the intaglio transfer printing, has been developed through which nanocrystal (NC) layers can be transfer-printed on various substrates regardless of the size, shape and arrangement of pixels. The procedure is illustrated in Fig. 1a. The QD layer coated on the donor substrate was quickly picked-up with a flat elastomeric polydimethylsiloxane (PDMS) stamp (Fig. 1a, process (1)). The picked-up QD layer was lightly contacted on the intaglio trench (Fig. 1a, process (2)) with a pressure of <50 g cm−2 and slowly detached<1 mm s−1 (Fig. 1a, process (3)). Only the non-contacted part of QD layer remained on the stamp and was transfer-printed on the target substrate (Fig. 1a, process (4)). This transfer printing is facilitated by the differences in surface energy between PDMS stamp and the target substrates (19.8 mJ m−2 for the PDMS and >200 mJ m−2 for the glass, organic layers and oxide layers) on which the QD layer can be tightly bound. On the basis of the same principle, multiple transfer printings are also possible (Fig. 1a, processes (5) and (6)); the second QD layer is exquisitely integrated on the first layer without any morphological changes. The resulting photoluminescence (PL) image is shown in Fig. 1b. The optical microscope images (Fig. 1c) and fluorescence microscope images (insets) show magnified views of each colour pattern in Fig. 1b, which consists of tens of micron-sized pixels (triangle, hexagon and star patterns). High-resolution aligned RGB pixels, ranging from 441 p.p.i. (30 μm pixel size) to 2,460 p.p.i. (6 μm pixel size; magnified view in inset), can be created by the multiple printing processes described above (Fig. 1d), demonstrating that the novel method is applicable to ultra-high resolution full-colour QD displays.

Figure 1: Intaglio transfer printing for high-resolution RGB QLEDs. (a) Schematic illustration of the intaglio transfer printing process. Inset images on the left of each frame show the side view. (b) The PL image of the RGB QD patterns via multiple aligned transfer printings. (c) Magnified views of selected regions of b. Each colour pattern consists of thousands of tens-of-microns-sized pixels (red: triangle (top), green: hexagon (middle) and blue: star (bottom)). Insets show further magnified PL images of pixels. (d) The PL images showing aligned RGB pixels whose resolution is between 441 p.p.i. (left) and 2,460 p.p.i. (right). Full size image

As the pixel size decreases, the intaglio transfer printing technique becomes more important. We compare the results obtained from the intaglio transfer printing (current) and structured stamping (conventional) methods (Fig. 2a–e). See Fig. 1a (intaglio printing) and Supplementary Fig. 1 (structured stamping) for comparison of the processes. The red boxes and white areas represent the designed patterns and transferred QDs, respectively (Fig. 2a). The fraction of the non-transferred area in the structured stamping method increases at higher resolution (Fig. 2a,b; representative images and statistical data). On the contrary, the intaglio transfer printing process accomplishes the transfer yield of ∼100% (see more transfer printing results of array configurations with various resolutions in Supplementary Fig. 2). The same tendency is observed in different shapes (circular dots and spaced lines; Supplementary Figs 3 and 4), demonstrating ∼100% transfer yield regardless of the size or shape of the patterns. The discrepancies from the designed patterns are particularly dominant near the edges of dot (square and circle) patterns, rather than line-and-space patterns (Fig. 2b,c, Supplementary Figs 2–4). The importances of fine dot patterns are particularly highlighted in patterning complex RGB pixels in full-colour displays.

Figure 2: Experimental and theoretical analysis of the intaglio transfer printing. (a) Pattern size scaling in the structured stamping (left) and intaglio transfer printing (right). QD transfer yields of the structured stamping dramatically decrease especially in high resolutions, while those of the intaglio printing approach ∼100% in all design rules. (b) Distribution of transfer printing yields at different pattern sizes (150, 75 and 45 μm). The transfer printing yield for the structured stamping dramatically decreases with the pattern size, while that of intaglio printing maintains ∼100%. Detailed results are shown in Supplementary Fig. 2. (c) Percentile proportion of the transferred QD line pattern area to the original pattern area. As the line width decreases from 100 to 10 μm, the structured stamping yield decreases, while intaglio printing maintains ∼100%. Detailed results are shown in Supplementary Fig. 4. (d,e) FEM simulations of the transferred area of the rectangular pattern (size: 150 × 150 μm) for the structured stamping (d) and intaglio printing (e). (f) PL image of a large-area QD dot array (7 × 7 cm) patterned by repeated aligned intaglio transfer printing on a flexible polyethylene terephthalate substrate. Full size image

Theoretical analysis of the enhanced yields of high-resolution patterning in the intaglio transfer printing over the structured stamping was performed using the finite-element method (FEM). Supplementary Fig. 5a–d compares two methods by simulating the transfer printing of a square pixel (size: 150 × 150 μm). In the structured stamping method, the shape is determined by the pick-up process (process (1) and (2) of Supplementary Fig. 1). As the contacted structure stamp is rapidly retrieved, the delamination between the stamp and the QD layer is initiated from edges of the stamp structure and propagates into the centre of the stamp structure, which induces stresses and generates cracks in the QD layer (Supplementary Fig. 5e). Cracks of the QD layer, therefore, occur at the inside of designed pixel edges and result in a reduced pixel size (Fig. 2d). On the contrary, in the intaglio transfer printing method, the pixel shape is determined by the QD release process from the flat stamp to the intaglio trenches (process (2) and (3) of Fig. 1a). Cracks of the QD layer occur at sharp edges of intaglio trenches (Supplementary Fig. 5f). Therefore, the obtained pixel pattern precisely matches the original design (Fig. 2e). The QD/intaglio trench interfacial energy, which is much higher than the QD/stamp interfacial energy, further helps the high definition and yield. See Supplementary Methods for details of FEM simulations and related mechanical analysis.

The intaglio printing process can be generalized to transfer various QD layers (Supplementary Fig. 6) regardless of QD materials (CuInSe and PbS) or sizes (2–18 nm). Furthermore, the current method is readily expanded over large areas by the repetitive aligned transfer printing, which is a critical technology for the mass production (Fig. 2f). Often, distances between pixels should be variable depending on pattern designs. The structured stamping method shows the sagging and leaning of structures in elastomeric stamps, thereby showing low yields, particularly with a large pattern spacing (Supplementary Fig. 7a,c). However, the intaglio stamping method does not exhibit these defects (Supplementary Fig. 7b,c).

White LEDs fabricated by transfer printing of RGB QDs

Our intaglio transfer printing technique can be utilized to create high-performance pixelated white QLEDs (PWQLEDs) on flexible substrates (Fig. 3a). Conventional white QLEDs have employed a mixture of several kinds of QDs and phosphors of different characteristic wavelengths45,46,47,48,49. However, these white QLEDs have been proven to be inefficient owing to the inevitable energy transfer between the different QDs/phosphors (for example, Förster energy transfer)50,51. In the mixed system, it is difficult to obtain balanced white light because the energy transfer occurs from B to G, R and from G to R. Therefore, it is desired to realize white emission by controlling the injected current of each RGB subpixel in the pixelated LED arrays, rather than by controlling RGB luminophore content in the mixed system.

Figure 3: True-white light emission based on pixelated RGB QLEDs. (a) Optical images of the flexible white QLEDs under the bias. (b) Magnified view (PL image) of the RGB QD pixels of white QLEDs. (c) Energy band diagram of white QLEDs estimated by the ultraviolet photoelectron spectrometry. (d) EL spectra of PWQLEDs and each monochromatic (R, G and B) QLED. (e) CIE 1931 x–y chromaticity diagram showing the true-white colour (0.39, 0.38) of PWQLEDs. (f) Brightness versus voltage of PWQLEDs and MWQLEDs. PWQLEDs show the higher efficiency than MWQLEDs, particularly at the high brightness. (g) External quantum efficiency of PWQLEDs and MWQLEDs. (h) Electrical properties (J–V characteristics) at different bending angles. (i–k) Time-resolved PL spectra of aligned RGB (PWQLED), mixed (MWQLED), and monochromatic (R, G and B) QD layers. Full size image

On the other hand, the current flexible PWQLEDs utilize aligned RGB fine pixels (Fig. 3b), whose colour can be tuned to be the true white with high efficiency. We unify QD materials using CdSe/ZnS alloyed QDs (Supplementary Fig. 8) to minimize variations in the RGB EL brightness and to prevent the inefficient blue EL of CdS-based QDs35,42. All the CdSe/ZnS alloyed QDs have the same type of ligand, oleic acid (Supplementary Fig. 9). Figure 3c shows the band diagram for PWQLEDs, which is estimated from the ultraviolet photoelectron spectra (Supplementary Fig. 10). Band alignments and efficient electron and hole injections are enabled by the careful selection and integration of inorganic/organic materials for each layer.

The EL of PWQLEDs consists of three distinct peaks that match each monochromatic RGB EL (Fig. 3d). The EL location of PWQLEDs in Commission International de l'Éclairage coordinates is (0.39, 0.38) under 6 V bias, which indicates the emission of true-white light (Fig. 3e). The EL spectra at different applied voltages are presented on Supplementary Fig. 11. Furthermore, EL efficiencies are compared between PWQLEDs and mixed white QLEDs (MWQLEDs) in which the active layer is created by mixing RGB QDs in the solution phase (Supplementary Methods for fabrication details). The brightness of PWQLEDs is enhanced over MWQLEDs by ∼10 to ∼52% depending on the applied voltage (Fig. 3f), and the EQE of PWQLEDs is higher than that of MWQLEDs in entire operating voltage (Fig. 3g). In addition, flexible PWQLEDs present stable current density versus voltage (J–V) characteristics under various bending angles (Fig. 3h).

For the better understanding of the enhanced performance of PWQLEDs, time-resolved PL measurements were conducted for QD layers employed in MWQLEDs and PWQLEDs (Fig. 3i–k; data at the blue, green and red wavelengths and a summarized plot, respectively). The time-resolved PL of each RGB QD layer was also measured for the comparison. In MWQLEDs, the carrier lifetime of blue and green QDs significantly decreases, while that of red QDs increases, which implies the energy transfer between QDs50,51. Because QDs with different band gaps are adjacent to each other in the close-packed (mixed) layer, they transfer energy to neighbouring QDs with lower energy band gaps instead of emitting photons. The energy transfer between QDs of the same colour is neglected for analysis. In PWQLEDs, on the contrary, the carrier lifetime of pixelated QD arrays does not change from that of individual RGB QDs. These results demonstrate that the geometrical separation of pixelated configurations effectively suppresses the energy transfer process, enabling highly efficient true-white emission.

Electronic tattoos based on ultra-thin and wearable QLEDs

The current QLED technologies are applied in electronic tattoo demonstrations (Fig. 4). Ultra-thin form factors (total thickness of ∼2.6 μm, including ∼300-nm-thick active and ∼1.1-μm-thick encapsulation layers; inset of Fig. 4a) enable various deformations and conformal integrations with soft, curvilinear epidermal tissues2,7. The detailed device structures and the magnified view of active layers (electron transport layer (ETL), QDs and hole transport layer) are shown in Fig. 4a,b, respectively. The ultra-thin encapsulation consists of a Parylene-C and epoxy bilayer. Electronic tattoos show outstanding device performances, such as a high brightness of 14,000 cd m−2 at a driving voltage of 7 V and EQE of 2.35% at 4.5 V bias (J–V–L characteristics, Fig. 4c). The electronic tattoo exhibits EQE above 1% in the range of 3.6–6.9 V applied voltages (current density: 3.4–1,132 mA cm−2) as shown in Supplementary Fig. 12a. To the best of our knowledge, the brightness is higher than the previously reported values of the wearable LEDs at the same driving voltage6,10,14,15. The high device performance at the low driving voltage, which can be obtained by commercial mobile batteries, is particularly beneficial to wearable device applications. The high EL performance remains stable after 1,000 cycles of uniaxial stretching (∼20% applied strain, Fig. 4d). For stretching tests, ∼20% prestrain, which is similar with the skin stretchability7, is applied to ultra-thin QLEDs to form a wavy structure9. Moreover, as shown in Supplementary Fig. 12b, the lifetime of electronic tattoo is about 41.7 h at 3 mA applied current (initial brightness=4,554 cd m−2), which corresponds to device lifetime of 12,815 h at 100 cd m−2 (lifetime × initial brightness1.5=constant)20. Furthermore, these ultra-thin QLEDs can be laminated on various curvilinear substrates, such as the crumpled Al foil, human skin, round glass, metal can and sharp edges of a slide glass (Fig. 4e–g, Supplementary Fig. 13a–d). Various deformations, such as bending, folding or crumpling, as well as moistures (water droplets) do not cause mechanical/electrical damages or any decrease in the EL performance (Fig. 4e–g, Supplementary Fig. 13e, Supplementary Movie 1). The current electronic tattoo platform can be extended to wearable PWQLEDs that are laminated on the human skin (Fig. 4h).