Producing white light with multiple layers of colloidal quantum dots

Recently, great efforts have been made to develop colloidal quantum dot (CQD) light-emitting technologies for visual displays. The benefits of using quantum dots (QDs) primarily depends on the narrow bandwidths they afford and their high luminescence efficiency, broad absorption, and tunable band gaps.^1–3 However, several issues associated with CQDs are hindering their wide application in commercial products. Many recent studies have focused on increasing the quantum efficiency of these nanoscale particles, or various techniques to render better white LEDs, such as co-doping QDs in phosphor or crosslinking the colloidal QD layer.^{4, 5} However, the question of how to incorporate QD layers into the device structure remains.

Spin-mist coating and inkjet printing have both been demonstrated as potential solutions to this problem.^6–8 However, both methods have shortcomings. The spin-mist-coating method suffers from loss of QDs in solution, and cross-contamination of the red, green, and blue (RGB) pixels leads to poor visual results.⁹ Meanwhile, the inkjet printing method can cause non-uniform surface covering and inaccurate diameter definition.^{10, 11} The separation of individual RGB pixels with retention of color uniformity and rendering flexibility is crucial for industrial applications. To better address these issues, we adapted several key techniques (including pulse-spray coating, multiple layered structures, and UV-distributed Bragg reflectors) into a new common platform and demonstrated its feasibility and superiority over traditional approaches.¹¹

We have made QD-based LEDs (QDLEDs), including pixelated arrays and full-wafer light sources, integrating the RGB QD layer using the pulse-spray coating method. This employs an air-atomizing control technique that allows the levels of deposited QDs to be more finely adjusted than with either spin-mist coating or inkjet printing. Polydimethylsiloxane (PDMS) was used for the interface to separate the individual CQD layers, preventing different color particles from mixing. The layered structure can also be engineered to obtain certain color rendering effects by changing the CQD dosage in each layer. In addition, a highly reflective HfO₂/SiO₂ (hafnium oxide/silica) distributed Bragg reflector (DBR) was employed to minimize loss of the UV light used to pump the RGB QDs. As our results show, our device construction provides enhancement of individual RGB colors and white-light emission.

Figure 1 shows a schematic diagram of the finished structure. An 11-pair HfO₂/SiO₂ DBR was first evaporated on a glass surface using an ion-assisted e-gun system. A DBR pair comprises two quarter-wavelength-thick layers of material with different refractive indices. A number of pairs stacked together creates a structure that is highly reflective to light of the chosen wavelength. After deposition of 11 layer pairs, CQDs were then deposited on the other side of the glass by pulse-spray coating. In the full-wafer, non-patterned sample, we can spread the CQDs without needing to use a shadow mask. For the pixelated sample, however, the shadow mask is necessary to generate pixel-like patterns. Between each CQD spray, we manually inserted a thin layer of PDMS to separate the active CQD layer.

Figure 1. (a) Schematic illustration of pixel structure from the side. (b) White-light mode. DBR: Distributed Bragg reflector.

Figure 2 shows that the peak reflectance intensity of the HfO₂/SiO₂ DBR is at 400nm. Full peak width is ∼60nm, with >90% reflectivity maintained between 365 and 430nm. We accordingly chose a pumping UV LED emitting at 380nm. Our construction reflects UV photons back into the device to maximize QD pumping, while letting visible photons through the DBR structure. This makes it ideal for displays.

Figure 2. Experimental optical reflectance spectra of 11-pair hafnium oxide/silica DBR.

In our experiment we focused on the green QD as it has the worst efficiency of the three colors. First, we had to optimize the concentration. Figure 3(a) shows the relative green light intensity of different QD concentrations, as measured under constant flow. We found that 1mg/mL has the highest intensity. Figure 3(b) shows the relative intensity of the QDLEDs with and without DBR under constant flow. This suggests that all three color QDs with DBR have higher intensity than those without. In addition, the International Commission on Illumination (CIE) color coordinates of the QDs with DBR are (0.29, 0.29), which provide a bright white color emission.

Figure 3. (a) The relative intensity of different concentrations of green quantum dots under 350mA. (b) Quantum dot LEDs with and without DBR operated under 350mA. a.u.: Arbitrary units.

Figure 4 shows the image of the pixelated structure and full RGB color under UV excitation. In Figure 4(a) each pixel is ∼2×2mm. Figure 4(b) shows a strong white color emission under UV excitation of a 2 inch full wafer. We could further engineer the amount of the QD pulse spray to adjust each layer's quantity and thus change the chromaticity coordinates and the output color.

Figure 4. (a) Image showing pixel structure. (b) Two-inch wafer-scale white-light emission under UV excitation.

In conclusion, we have demonstrated a highly efficient platform for integrating PDMS-layered CQD LEDs and DBR structures. The wide range of CIE coordinates provided by quantum dot emission can be applied in display and solid-state lighting. The extra reflection of UV photons from the DBR structure increases the utilization efficiency of pumping, and enhanced RGB color intensities were clearly observed. The layered PDMS design can be further engineered to modify the composition and augment the intensity of individual RGB colors, which holds great promise when combined with pixelated arrays and pulse-spray technology. Our future work will devote more effort to optimizing QD dispensation and DBR design to further raise the luminescence efficiency.