Direct Optical Patterning of Quantum Dot Light‐Emitting Diodes via In Situ Ligand Exchange

Abstract

Precise patterning of quantum dot (QD) layers is an important prerequisite for fabricating QD light‐emitting diode (QLED) displays and other optoelectronic devices. However, conventional patterning methods cannot simultaneously meet the stringent requirements of resolution, throughput, and uniformity of the pattern profile while maintaining a high photoluminescence quantum yield (PLQY) of the patterned QD layers. Here, a specially designed nanocrystal ink is introduced, “photopatternable emissive nanocrystals” (PENs), which satisfies these requirements. Photoacid generators in the PEN inks allow photoresist‐free, high‐resolution optical patterning of QDs through photochemical reactions and in situ ligand exchange in QD films. Various fluorescence and electroluminescence patterns with a feature size down to ≈1.5 µm are demonstrated using red, green, and blue PEN inks. The patterned QD films maintain ≈75% of original PLQY and the electroluminescence characteristics of the patterned QLEDs are comparable to thopse of non‐patterned control devices. The patterning mechanism is elucidated by in‐depth investigation of the photochemical transformations of the photoacid generators and changes in the optical properties of the QDs at each patterning step. This advanced patterning method provides a new way for additive manufacturing of integrated optoelectronic devices using colloidal QDs.

Ultrahigh Resolution Pixelated Top‐Emitting Quantum‐Dot Light‐Emitting Diodes Enabled by Color‐Converting Cavities

“…By simply photopatterning the IZO phase tuning layers, our cavity can selectively convert the unpatterned QD white emission as color‐saturated and color‐stable red, green, and blue emission with a brightness of 22 170, 51 930, and 3064 cd m −2 at 5.5 V, respectively. Compared with previously demonstrated techniques including ink‐jet printing, [ 11–16 ] transfer printing, [ 17–22 ] and QD photopatterning, [ 23–27 ] our method avoids the patterning of QDs, offers ultrahigh resolution of ≈1700 ppi (that can be improved to over 8000 ppi) and is compatible with large area manufacturing. Compared with “white + CF” techniques, [ 28 ] our scheme cancels the expensive CF arrays and eliminates the energy losses caused by the CFs.…”

“…Recently, by manipulating the ligands, the QDs can be cross‐linked when exposed under ultra‐violet illumination, thereby suggesting a promising way to pattern the QDs using mature photolithography technique. [ 23–27 ] Although the resolution of the QD arrays can be higher than 1400 ppi, [ 26 ] the performance of the resultant QLEDs could be degraded due to the introduction of the photochemical additives and the damage of the QDs when patterned several times. Instead of patterning the QDs directly, the red, green, and blue emission can also be obtained by filtering the white emission with a pixelated color filter (CF) array; [ 28 ] although this method has been adopted to manufacture large area OLED televisions and high resolution OLED microdisplays, the introduction of CF not only reduces over 70% of the brightness, but also increases the material cost of the displays, thus rendering this method not ideal for energy efficient and low‐cost displays.…”

An Efficient Hole Transporting Polymer for Quantum Dot Light‐Emitting Diodes

“…Colloidal quantum dots (QDs) have received a great interest due to their distinctive features involving high photoluminescence yields (PLQYs), tunable and size‐dependent emission wavelengths, narrow full widths at half maximum, and low‐cost fabrication process, making the QD‐based light‐emitting diodes (QLEDs) promising candidates for next‐generation lighting and displays. [ 1–5 ] In the past two decades, some significant breakthroughs in improving the efficiencies of QLEDs have been achieved through engineering the materials and device architectures, leading to the external quantum efficiencies (EQEs) of QLEDs to be comparable with those of organic LEDs. [ 6–10 ] Nevertheless, the use of insulating organic ligands (such as oleic acid, OA) and zinc sulfide (ZnS) shell materials, which possess extremely deep valance bands (VBs), provides the QLEDs a long‐standing hole transporting challenge at the interface between hole transporting layers (HTLs) and QD emissive layers (EMLs).…”

Patterned‐Bank‐Free Electroluminescent Quantum Dot Emitting Array for Passive‐Matrix QLED Display

“…[21][22][23] Meanwhile, the durability of QLEDs has been also much improved. [16,18,20] Furthermore, many QD printing techniques including inkjet printing, [24,25] transfer printing, [26,27] and photolithography process [28][29][30][31] have been developed to realize the full-color pixelation of QLED for display applications.To display an image, on the other hand, the light-emitting devices or pixels must be driven individually with appropriate wiring electrodes and driver boards. For example, passive matrix (PM) [32] or active matrix (AM) [33,34] addressing system, are generally employed to control the EL of every pixel.…”

“…[21][22][23] Meanwhile, the durability of QLEDs has been also much improved. [16,18,20] Furthermore, many QD printing techniques including inkjet printing, [24,25] transfer printing, [26,27] and photolithography process [28][29][30][31] have been developed to realize the full-color pixelation of QLED for display applications.…”