1. Introduction

Micro-LEDs have attracted numerous attentions in academic and industrial fields since its invention, and it has considered as the most promising luminescent device and new display technology [1–4]. However, several challenges need to be solved in the commercial market, and full-color technology is one of the most important issues [5]. Two approaches have been proposed to achieve full-color micro-LED displays. One of them is the RGB method, which integrates red, green and blue micro-LEDs on a driver circuit substrate [6,7]. An alternative way is the color conversion method, which uses UV or blue micro-LEDs and color conversion materials to realize full-color displays. This method can reduce the complexity of the fabrication process as a single-color micro-LEDs is needed, and the excellent color conversion materials can achieve a high display performance [8,9]. Quantum dots are excellent nano-size materials with outstanding optoelectronic performance, so they are highly suitable for color conversion [10,11]. Because quantum dots are usually synthesized by chemical solution method, it is easy to apply different deposition methods [12–16]. As micro-LEDs have micro-size chips, the precision of the deposition is vital for an efficient color conversion layer. Electrohydrodynamic (EHD) inkjet printing is the application of high voltage static electricity between the nozzle and substrate, and it can control the amount of ink sprayed by adjusting the voltage; thus, it can print inks with a size of several micrometers [17–19]. Therefore, EHD printer is suitable for high-resolution and high-precious quantum dots deposition.

The color conversion structure has micro-LEDs and fluorescent materials, and crosstalk will occur between different materials. One of the crosstalk comes from the micro-LEDs and color conversion layer because of the large divergence angle of micro-LEDs. This leads to a color shift and a low contrast ratio [20,21]. The most effective way is to use a barrier between chips to reduce the micro-LED divergence angle [22]. Other crosstalk occurs at the color conversion layer, as different colors will affect each other. A bank is used to reduce the crosstalk in the color conversion layer and confine the color conversion materials to a sufficient thickness [23]. As blue micro-LEDs have high efficiency and brightness, the color conversion layer cannot absorb all the blue light, and a color filter film is always used to remove extra blue light. However, red and green light cannot fully transmit the color filter; thus, the light intensity will decrease [24].

In this paper, we propose an effective way to make an ultra-thick black matrix and print an ultra-thick QDs color conversion film. The ultra-thick black matrix was fabricated using semiconductor processing technology, and this high thickness of the black matrix can remove the crosstalk in the color conversion film and form a QDs film with sufficient thickness for color conversion. Then different thicknesses of QDs films were printed using an EHD printer, and their blue light leakage was measured. Finally, we printed a red/green color conversion layer, then it was integrated on blue micro-LEDs. The film can achieve effective color conversion, and the blue leakage is reduced greatly without a color filter. The brightness of green and red films can reach 257.64 nit and 140.51 nits at the current density of 2 A/cm², and their color gamut is 71.19% NTSC. The results show that this method can achieve excellent full-color micro-LED displays.

2. Experimental

The ultra-thick black matrix was fabricated using the following steps. First, black resist was spin-coated on glass and repeated several times to obtain a certain thickness. After each spin coating, the film needs to be baked at 230°C for 30 min. Next, a layer of 150 nm Al was deposited on the black resist by electron beam evaporation. Then, a photoresist layer was spin-coated on the Al layer and a pattern was formed by photolithography process. After that, Al in the pattern was etched using an Al etchant, then the black resist was etched using inductively coupled plasma to form a thick black matrix. All green and red QDs inks were purchased from Suzhou Mesolight company. The ink is an acrylate mixture containing 40 wt% of QDs (CdSe/ZnS) and 10 wt% of 200 nm scattering particles. The QDs ink was printed into the thick black matrix using an eNanoJet-Eco printer from Enjet company. After printing, the QDs ink were solidified under UV light. Finally, the QDs film was integrated onto the blue micro-LEDs.

3. Results and discussion

The color conversion method needs a QDs color conversion layer which was integrated onto blue micro-LEDs. The schematic of QDs color conversion layer is shown in Fig. 1(a). A bank which is usually fabricated by photolithography method is needed to confine the QDs ink and reduce crosstalk in the layer. A conventional photoresist can easily get a high-resolution bank, but it also has a certain transmittance in the visible light range. The optical density (OD) is used to describe the ability of a material to block light. The optical density value is higher, then it can block more light. In this work, the optical density of the black resist is 4/µm, while that of the conventional photoresist is less than 0.5/µm. Therefore, the black matrix, which is made of black resist, can remove the crosstalk because of its high optical density and can be a QDs ink bank for the color conversion layer. In our previous work, because of the high efficiency and brightness of blue micro-LEDs, extra blue light is inevitable which will have blue light leakage in the color conversion layer, and an alternative way is to use a thick QDs layer to reduce extra blue light [25]. An ultra-thick bank is needed to confine the QDs ink. However, the thickness of the commercial black matrix obtained by the photolithography process is approximately 1 µm because of the high optical density of the black resist [26], which is not enough to obtain a sufficient QDs ink thickness. An etching method was proposed to make a black matrix with high thickness. Multiple spin coatings can increase the thickness of the black resist, and a high thickness of black matrix is fabricated by inductively coupled plasma etching using an Al layer as a mask. The 3D morphology image of the black matrix and the printed QDs ink are shown in Fig. 1(b). The thickness of the black matrix is 12 µm, and the pixel size and pitch are 15 µm and 18.85 µm, respectively. The QDs ink can fill the ultra-thick black matrix through multiple inkjet printing.

 

Fig. 1. (a) The schematic of QDs color conversion layer, (b) the 3D morphology image of black matrix and printed QDs ink.

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Figure 2 shows the spectra of the blue micro-LEDs and the QDs ink. The blue micro-LEDs have a pixel size of 12 µm and pitch of 18.85 µm, and the peak wavelength and full-width at half maximum (FWHM) are 455 nm and 15 nm, respectively. The PL spectra under UV light illumination show that the green QDs have the peak wavelength of 530 nm with a 22 nm of FWHM, and the red QDs have a peak wavelength of 630 nm with a 21 nm of FWHM. The color conversion layer was fabricated by EHD inkjet printing on a black matrix substrate. By adjusting the voltage of the printer, a suitable size of QDs ink can be formed and preciously printed into the black matrix. A certain thickness of the QDs film can be reached after repeating the printing process.

 

Fig. 2. The spectra of blue micro-LED and QDs.

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The different thickness of QDs films were printed in a black matrix, and their spectra were measured under blue LED to check their blue light leakage. The spectra of the green and red QDs films are shown in Fig. 3, and the inset shows their luminous images under blue LED. In Fig. 3(a), the unabsorbed blue light decreased as the thickness of the green QDs film increased, indicating that the blue light leakage was reduced. At higher thicknesses, the peak wavelength of blue light has a red shift. This is attributed to the absorption of the green QDs being smaller in the long wavelength region of the blue light. The green QDs film absorbs less photons at long wavelength of blue light, and a longer wavelength of blue light remains; therefore, a red shift appears in the thick green QDs film. The blue light leakage was also reduced as the red QDs film increased in Fig. 3(b). However, the peak wavelength of blue light does not change as the thickness increases because the red QDs have a high absorption in the blue light region. Therefore, we can print a high thickness QDs film to remove blue light leakage without further processing, which will simplify the process of the color conversion film.

 

Fig. 3. The spectra of green (a) and red (b) QDs films with different thickness. The inset shows their luminous images under blue LED.

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The color conversion film was fabricated using red and green QDs inks. As shown in Fig. 4, the color conversion layer was integrated onto blue micro-LEDs, and red and green light were achieved by blue light illumination. A higher thickness can remove extra blue light, but the color conversion efficiency will decrease as the film thickness increases. According our previous study, a thickness of 12 µm QDs film was adopted in this work [25]. Figure 4(a) shows the RG array of the printed QDs film and the illuminous image. The thickness of QDs film is 12.83 µm and 12.69 µm for green and red, respectively. Figure 4(b) shows the spectrum of the converted light. Although it has clear red and green light peaks, there is still a blue light peak. The blue light peak originates mainly from the blue light leakage. We also attempted to increase the thickness of the QDs film to further reduce the blue light leakage, but a thicker QDs film decreased the light intensity of the converted light, especially for the green QDs film. The main reason for this is the reabsorption of the QDs ink caused by the high concentration, which is consistent with our previous work [23]. Therefore, an effective way is to synthesize a lower reabsorption of QDs ink. The luminous performance was measured using an integrated sphere. After calculated, the brightness of the converted light is 257.64 nits and 140.51 nits at the current density of 2 A/cm² for green and red light, respectively. As shown in the spectrum, the brightness of blue light still has 8.66 nits. Figure 4(c) shows a RGGB array for full-color demo, including QDs color conversion layer and blue micro-LEDs. There is a large amount of blue light from blue micro-LEDs, and even the converted red and green light is affected by blue light. The spectrum of this structure is shown in Fig. 4(d), which shows that the blue light intensity is much higher than that of the red and green light. The brightness is 635.34, 169.19, 114.39 nits at the current density of 2 A/cm² for blue, green, red light, respectively. This is attributed to the large divergence angle and high brightness of the blue micro-LEDs, and we can adjust the blue light intensity using an IC driver to eliminate this phenomenon. According to this color conversion film, the color gamut was calculated and plotted in the CIE 1931 chromaticity diagram in Fig. 5, which shows that the region can reach 71.19% of the NTSC color space.

 

Fig. 4. The illuminous images of the RG (a) and RGGB (c) array and their spectra (b) (d). The scale bar is 30 µm.

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Fig. 5. The color gamut in the CIE 1931 chromaticity diagram.

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4. Conclusion

In summary, full-color micro-LEDs was achieved by integrating blue micro-LEDs and QDs color conversion layer. To confine the thick QDs ink and reduce the crosstalk in the layer, an ultra-thick black matrix with a thickness of 12 µm was fabricated using an etching method. The QDs color conversion layer was then formed by inkjet printing. Different thickness of QDs film was printed and their blue light leakage was measured. After printing the red and green QDs films, their luminous performance was analyzed, and the blue light leakage was reduced significantly without a color filter. Their brightness can reach 257.64 nits and 140.51 nits for green and red light, respectively. Finally, a RGGB array was demonstrated with a pixel size of 12 µm and pitch of 18.85 µm, and its color gamut is 71.19% NTSC. These results show a simple way to make a full-color micro-LED display.