Flexible micro-LED display and its application in Gbps multi-channel visible light communication

A flexible full-color micro-LED display with high mechanical robustness was fabricated by printing quantum dots (QDs) on a blue micro-LED array using standard photolithography. The red and green colors yielded from QDs exhibit a better color gamut than conventional color filters. The light conversion efficiency was enhanced by adding TiO2 nanoparticles to the QD-photoresist composite. This full-color micro-LED display was successfully mounted on various unusual substrates such as curved glass, fabrics, and human skin, enabling diverse optoelectronic applications. In addition, wireless multi-channel visible light communication (VLC) based on the wavelength-division-multiplexing orthogonal-frequency-division-multiplexing (WDM-OFDM) technique was demonstrated using a QD-based color micro-LED panel. A high data transmission rate of 1.9 Gbps was successfully obtained owing to the high electrical–optical modulation bandwidth of the QD-based micro-LED panel.


INTRODUCTION
Recently, micro-light-emitting diodes (micro-LEDs) have emerged as next-generation display technology because of their superior ability to withstand moisture and provide ultrahigh luminance, and long lifespan compared to organic light-emitting diode (OLED) displays and liquid crystal displays (LCDs) [1][2][3][4][5][6] . Owing to these properties, researchers have applied micro-LEDs in smart wearable displays, automobile displays, stretchable displays, and outdoor signage 4,[7][8][9] . In addition, micro-LEDs can be used as transmitters in VLC due to their high switching rate, high energy efficiency, and high modulation bandwidth 10,11 . They can wirelessly transmit data at high speeds. The radio frequency (RF) spectrum, the most used band in modern wireless communication systems, is already congested owing to the high demand for wireless data traffic 12 . VLC systems, which modulate the visible light spectrum to facilitate data transmission, have been investigated to replace the congested radio spectrum for wireless communication. In addition, the high bandwidth of the visible light spectrum can accommodate many users at high transmission rates for 5G communication applications 13 .
Micro-LEDs have the potential to integrate displays and wireless communication systems. However, the fabrication complexity increases with the decrease in the size of LED chips, which hinders the commercialization of the products 14,15 . One of the issues in micro-LED displays is the realization of a full-color RGB display that meets the standard specifications required by modern displays [16][17][18] . The most widely-used method for fabricating an RGB micro-LED display is to transfer the red (R), green (G), and blue (B) micro-LED arrays from three different epitaxial wafers on the target substrate 19 . However, this approach has several challenges, such as aligning LEDs during the transfer process and electrical bonding between the LEDs and metal electrodes of display backplanes, which could raise an issue to the production yield. Although prototypes of RGB micro-LED displays have been successfully demonstrated through the transfer method, the problem of high production costs due to low transfer yield remains a challenge 20 . Unlike traditional methods, where three different LEDs are transferred to yield an RGB display, quantum dots (QDs) have been used as a color conversion layer on a blue gallium nitride (GaN) micro-LED array 21 . These RGB LED pixels can simultaneously transmit data using different wavelengths of light, making it possible to transmit a lot of data at once. However, it is still challenging to produce an RGB micro-LED array using a QD color conversion layer on flexible substrates for VLC.
In this work, we developed a patternable QD-photoresist (PR) for photolithography. This strategy facilitates the formation of fullcolor micro-LED displays without complicated transfer of subpixels and avoids alignment issues during their assembly. We transferred a GaN-based flip-chip micro-LED array onto a flexible substrate with the micro-LED electrodes facing up. A QD color conversion layer for the red and green sub-pixels was precisely defined by photolithography to yield a flexible full-color micro-LED display. Further, the micro-LED array with QD-PR acts as the optical source for wavelength division multiplexing (WDM)-VLC and facilitates a high-speed VLC link for free-space applications using orthogonal frequency division multiplexing (OFDM). R, G, and B pixels allow for separate data transmission at different wavelengths at the same time, enabling massive data transmission that is essential for light fidelity (LiFi) communication.

RESULTS
Fabrication of flexible micro-LED display Different lift-off methods have been developed to transfer the micro-LED array depending on the growth substrate (Fig. 1a). Chemical lift-off and laser lift-off (LLO) were used to release GaNbased micro-LED arrays from silicon and sapphire substrates, respectively. First, the micro-LED devices were formed on silicon and sapphire substrates using standard semiconductor fabrication, including photolithography, etching, chemical vapor deposition (CVD), and metal deposition. During the chemical liftoff, the underlying silicon was etched in a warm potassium hydroxide (KOH) solution to create a suspended micro-LED array, and the alignment of the array was supported using a set of anchors, which were defined by mesa 22 . The second approach to release the micro-LED devices is LLO, which offers high efficiency and high throughput over a large area with almost negligible damage to the devices 23,24 . A temporary substrate, pressuresensitive adhesive (PSA) film, was used to hold the micro-LED array during the LLO. After the lift-off, a roll-based transfer system was used to precisely pick up the array and print it on a foreign substrate with an adhesive layer (NOA 63) 25 . A completely automated system can offer various advantages, such as high accuracy of the alignment of the micro-LED array, which facilitates the integration of the metal interconnections and a high transfer yield owing to the well-controlled applied pressure at the rubberbased roller. A QD-based color conversion layer was defined on the blue GaN micro-LEDs using the standard photolithography for a practical micro-LED application with full-color emission (Fig. 1b). This strategy has several advantages: first, the complex transfer of red and green sub-pixels can be skipped. Second, the active color conversion layer offers higher efficiency and a better color gamut of the display than conventional color filters. Third, photolithography is compatible with standard fabrication and can be applied to large-area displays. We developed a patternable QD-PR by mixing colloidal QDs in transparent negative PR (SU-8) at a specific concentration to achieve a high color conversion efficiency. After forming the interlayers and metal interconnections on the micro-LED array, the QD-PR was patterned on the desired position to yield RGB pixels (Fig. 1c). The integrated full-color micro-LED display on a 6-μm polyethylene terephthalate (PET) substrate exhibited high mechanical flexibility (Fig. 1d). The red and green QD-PRs were well defined on the micro-LED display (inset of Fig. 1d). Finally, the data transmission using the RGB micro-LEDs was demonstrated (Fig. 1e). . Unlike passive color filters that are commonly used in current display technologies to block specific wavelengths for achieving the desired color, a QD-based active color conversion layer emits color at a specific wavelength when excited by photons 26 . Considering the absorption spectra of QD-PRs, the red and green QDs exhibit high absorption at a wavelength of 450 nm, which is the peak wavelength of the blue micro-LED device. When the QDs are excited by a photon that originates from the blue micro-LED, the QDs convert the light to red and green spectra at 630 and 530 nm, respectively (Figs. 2a, b right; Supplementary Table 1). However, blue light leakage during color conversion remains an issue. Two strategies have been investigated to resolve this issue: (1) controlling the QD concentration in the PR and (2) adding the scattering nanoparticles in the QD-PR. The color properties were investigated for different concentrations of QD-PR and nanoparticle density to achieve an optimal QD-PR. The concentration of the red QDs changed from 10 to 50 wt%. According to the electroluminescence (EL) spectra, the red peak (630 nm) increased as the concentration of the QDs in SU-8 increased, while the blue peaks almost diminished to zero. The corresponding color coordination of each concentration is plotted in a CIE diagram, as shown in Supplementary Fig. 3. To achieve a high conversion efficiency, the QDs must be well-dispersed in the PR solution without aggregation. The aggregation of QDs leads to fluorescence quenching, and the photoluminescence (PL) of the QDs significantly decreases 27 . Therefore, before mixing with the PR solution, the QDs are dispersed in cyclopentanone through ultrasonication to prevent aggregation. The light conversion efficiency (LCE) is calculated using the formula: LCE ¼ IQD I Blue ÀI Blue;Leak , where, I QD is the light intensity after the QD conversion, I Blue denotes the blue light from the GaN micro-LED chip, and I blue,Leak is the blue peak after the QD conversion. The LCE of the red QD-PR was 36% at a concentration of 50 wt%. However, a high concentration of QD in PR solution can hinder the development of the PR film, and polymer residues can remain on the substrate after the process. This issue can be resolved by blending the scattering nanoparticles into the QD-PR solution 28 . TiO 2 nanoparticles exhibit a high refractive index of 2.8 and can be utilized as scattering enhancers in the QD color conversion layers. With the addition of TiO 2 nanoparticles, the blue light absorption of the QDs can be enhanced as more light is transferred to the QDs. The emitted light of QDs increases at the peak wavelength through photoluminescence because the total internal reflection is avoided due to the scattering effect of the nanoparticles. We fixed the concentration of the green QDs at 30 wt% and mixed the TiO 2 nanoparticles from 0 to 10 wt%. The resulting EL spectra and their corresponding color coordination are shown in Fig. 2d and Supplementary Fig. 4, respectively. The LCE was 43% for 10 wt% TiO 2 in the QD-PR.

Photolithographically patternable QD-PR
For demonstrating the versatility of the QD-PRs, the red and green QD-PRs were defined on the LEDs to form RGB pixels (Fig. 2e). Three distinct colors (red, green, and blue) were clearly visible when the LEDs were turned on. The corresponding normalized EL spectra are shown in Fig. 2F. In addition, the color gamut of the RGB pixels was measured and plotted on the CIE 1976 chromaticity diagram (Fig. 2f). Notably, the QD-PRs exhibited a larger coverage area on the CIE diagram than the NTSC and BT709, which are the standard for modern ultra-high definition (UHD) displays (Supplementary Table 2).

Flexible full-color micro-LED display
We developed a prototype of full-color micro-LED display by patterning the optimized QD-PRs on a large-area micro-LED array (20 × 20 pixels) after the metal interconnection process (Supplementary Fig. 5)  substrate, which provides low mechanical stiffness. Further, the fabricated display was connected to an external driving circuit via a flexible printed circuit board (FPCB) for display operations ( Supplementary Fig. 6, Supplementary Fig. 7). Moreover, an additional graphene heat spreading layer can be inserted between the micro-LED array and the plastic substrate for better heat dissipation. Graphene distributes the central temperature of micro-LED heating evenly on a two-dimensional plane to facilitate uniform heat dissipation ( Supplementary Fig. 8) 29 . The letters 'L,' 'E,' and 'D' were encoded to the display driving unit and presented on the micro-LED display through line addressing ( Fig. 3a; Supplementary Movie 1). The full-color display operated stably without mechanical failure owing to its low bending stiffness when peeled off from the glass handling substrate and bent by a tweezer (Fig. 3b, c; Supplementary Movie 2). To further validate the mechanical robustness of the micro-LED display, cyclic bending tests were performed. No significant change in the luminance of the display was observed when it was bent and rolled on a rod with a bending radius of 3 mm (Fig. 3d; Supplementary Movie 3). In addition, the electrical properties of the micro-LEDs were investigated during the cyclic bending test (Fig. 3e). The variation in the operating voltage was less than~4.5% after 1000 bending cycles under operating current of 100 μA (Fig. 3f). The change in operating voltage could be recovered when the display was flattened. In addition, the change in the luminance of the micro-LEDs and EL peak is negligible after the cyclic bending test under currents of 100 μA, 500 μA, and 1 mA (Supplementary Fig. 9). Therefore, we can conclude that the slight variation in electrical properties and mechanical bending did not affect the performance of the micro-LED display. The high mechanical flexibility could be attributed to the low bending stiffness of the display substrate and the isolation of each micro-LED pixel.
The good mechanical feature of the micro-LED display allows it to attach to various types of unusual surfaces, such as thin glass, fabric, and human skin, and provide invariant properties even under deformation. A 3 µm-thick double-sided adhesive layer was utilized to give the display the ability to perform like a sticker that can attach on target surfaces. As a demonstration, the ultrathin display was attached to a curved glass with a bending radius of 4 cm (Fig. 4a). Recently, augmented reality (AR)-embedded headup displays have grabbed much attention as the next-generation displays for automobiles. Micro-LED displays promise to deliver multiple advantages for automobile displays over conventional OLED displays, such as high brightness, better reliability against moisture and high temperatures, and low power consumption.
The flexible sticker-like micro-LED display offers a strategy to integrate the displays on vehicles by simply attaching the display on curved windshields. A wearable display is another prominent application as it can effectively deliver the output information to the users 30 . We attached the flexible micro-LED display on a knitted fabric to produce a wearable display (Fig. 4b). A wearable micro-LED display integrated with LiFi applications is a promising approach to let users interact with the Internet of Things (IoT), in which the physical objects are connected to the digital network. In addition, Fig. 4c shows the operation of this sticker-like micro-LED display on a soft surface like human skin. A conformal contact of the electronics systems and the human skin surface is strongly dependent on the mechanical properties of the device including, Young's modulus, thickness, stiffness, adhesion, and the Poisson's ratio of each layout in the display system 31,32 . To achieve flawless contact with the skin, the micro-LED display was fabricated by integrating materials with low stiffness that can adhere to the wrinkles of the skin surfaces. The low bending stiffness of the ultrathin micro-LED display allows the system to perform with minimum degradation in both luminance and electrical properties due to deformations induced by human skin. As a result, the ultrathin micro-LED display is conformally mounted on the human skin and operates robustly without failure.

Wireless VLC based on QD-based micro-LED array
Micro-LEDs exhibit a high electrical-optical modulation bandwidth, making them a suitable optical source for VLC applications. We demonstrated high-speed data transmission using QD-based color micro-LEDs enabled by the QD-PRs through the WDM-OFDM scheme. The OFDM is a kind of digital multi-carrier scheme in which the bandwidth of an optical source is completely utilized to encode digital data on multiple subcarriers 33 . To realize the 3-channel RGB communication link, WDM-VLCs that enabled the data to be individually carried by the three wavelengths of the RGB micro-LEDs were implemented. Figure 5a shows the experimental setup of the communication link that consisted of the micro-LEDs with the QD-PRs as the optical sources and a commercial avalanche photodiode (APD) as the receiver. The VLC is performed wirelessly in a free-space of 30 cm. The OFDM signals were generated using an arbitrary waveform generator (AWG) after MATLAB coding, and the signals at the receiver end were received by an oscilloscope and processed in MATLAB, including fast Fourier transform and demodulation. For the high throughput of the WDM VLC link, dichroic filters were placed in front of the APD to prevent optical crosstalk between the wavelengths. The frequency response of the GaN blue micro-LED was from 5 to 250 MHz, indicating that the system is above the noise level. Therefore, it can be completely utilized by the OFDM scheme and can provide a large bandwidth for data transmission (Fig. 5b, Supplementary Fig. 10). We implemented the OFDM signals that contained 256 subcarriers to occupy the bandwidth of the system. The bit allocation on each subcarrier is shown in Fig. 5c, which correlates with the achieved SNR values in Fig. 5d. Notably, the red and green channels enabled by the QD-PR exhibit fewer allocated bits than the blue channel because the long fluorescence lifetime of the QDs limits the optical bandwidth of the channel 34 . However, it is noteworthy that the QD-based color conversion layer exhibits shorter fluorescent lifetime than that of phosphor-based color filter, which provide not only better color purity, but also improve the modulation bandwidth 35 .
The BER for each subcarrier is shown in Fig. 5e. The total BER of the link was 5.41 × 10 -4 , which is below the forward error correction (FEC) target (2 × 10 -3 ). As a result, the blue channel showed the highest data rate of 964 Mbps, while the red and green channels exhibited the data rates of 529 and 460 Mbps, respectively. Hence, a total of 1.9 Gbps data transmission was achieved when the optical filters were used at the receiver to allow the desired wavelength to be processed in the WDM VLC link (Supplementary Table 3).

DISCUSSION
We successfully fabricated a flexible full-color micro-LED display with high mechanical flexibility using QD-PR as the color conversion layer on an array of GaN micro-LEDs. The fabrication of the flexible micro-LED display through the roll-transfer method provides high accuracy in the alignment of the LEDs and high productivity while preserving the performances of the devices during the process. In addition, the QD-based color conversion layers enable display size scalability as they can be defined via a foundry-compatible photolithography process. The developed QD-based color micro-LED application in an OFDM-based WDM VLC system was analyzed, and a high data transmission rate of 1.9 Gbps was achieved. The outstanding electrical-optical modulation properties of the micro-LEDs enable Gbps multi-channel VLC, which is difficult to be achieved by conventional light sources. A combination of flexible micro-LED display and wireless data transmission may accelerate the development of next-generation technologies, such as the IoT, LiFi, and intelligent wearable electronics.

Fabrication of micro-LEDs
GaN was epitaxially grown on silicon and sapphire wafers. n-GaN was etched by inductively coupled plasma-reactive ion etching (ICP-RIE) using BCl 3 and N 2 plasma. To form an n-type ohmic contact, metal layers (Cr/Au: 3/50 nm) were deposited and annealed at 300°C under a N 2 atmosphere. Next, p-type ohmic contacts (Ni/Au: 10/50 nm) were defined on p-GaN. A 100-nm thick silicon nitride passivation layer was deposited using plasmaenhanced chemical vapor deposition (PECVD). An etch mask (PR 4620) was generated to define the geometry of the micro-LEDs, and the exposed regions were etched using ICP-RIE. Further, the micro-LED array was released from the substrate either using a chemical or laser-assisted method. An adhesive layer (NOA 63) was spin-coated onto a thin PET substrate, which was attached to a handling substrate with a layer of polydimethylsiloxane (PDMS). The micro-LED array was then printed on the target substrate using an automated roll transfer system.

Preparation of QD-PR
CdSe/ZnS core-shell QDs (Fine Lab) are used. The QD/toluene solution was stirred using a magnetic bar to evaporate the solvent at room temperature. Further, the QDs were dispersed in SU-8 Thinner (MicroChem) in an ultrasonic bath. After confirming the absence of the aggregation of the QDs, a transparent negative photoresist, SU-8 2015 (MicroChem), was added to the solution and stirred at 150 rpm overnight in an ambient environment.

Photoluminescence and electroluminescence measurement
The PL of the QDs was obtained using a Raman spectrometer (Horiba, LabRAM ARAMIS) with an external laser source of 325 nm. The EL characteristics of the micro-LEDs with QD-PRs were obtained using a spectroradiometer (Konica Minolta, CS2000) with Keithley 2635A source meter.

VLC link
The OFDM signals are generated using an AWG (Tektronix, AWG7002A). These signals were DC-biased with a bias-tee (Mini-Circuits, ZFSCJ-2-2-S) and fed to the micro-LEDs. At the receiver, an APD (Hamamatsu, C5331-11) detected the optical signal and analyzed them using a digital oscilloscope (Tektronix, MSO71604C). The signals were processed offline through MATLAB.