This article presents an investigation of the modulation bandwidths of quantum-dot (QD) light-emitting diodes (QLEDs). The QLEDs used in our study are red-emissive CdSe/ZnS QLEDs, which have a structure of indium tin oxide (ITO)/poly(3.4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)/TFB/QD/ZnO/Al and an emitting area of 2 or 4 mm². We find that at a small injection current (below ~10 mA), the effects of the resistance-capacitance (RC) time constant and the carrier lifetime on the bandwidths of the QLEDs are comparable, while at a large injection current, the bandwidths are mainly determined by the carrier lifetime. The response time of the QDs is not a limiting factor. The bandwidths of the QLEDs increase with the injection current and are eventually limited by the damage threshold current of the devices. At the same injection current, the QLED that has a smaller emitting area provides a larger current density, and thus exhibits a larger bandwidth. At an injection current of 28 mA, the 2-mm² QLED provides a bandwidth of 11.4 MHz and a luminance value of 156,000 cd/m², and the 4-mm² QLED provides a bandwidth of 8.2 MHz and a luminance value of 97,000 cd/m². Our investigation provides a guideline for QLED-bandwidth optimization and useful information for the further development of QLEDs for lighting, display, and communication applications.

We study a trichromatic white light-emitting diode (WLED) system that consists of a blue LED covered with layers of red emissive CdSe/ZnS quantum dots (QDs) and yellow emissive YAG:Ce ³⁺ phosphors. With the knowledge of the individual emission spectra of the blue LED, the QDs, and the phosphors, we develop a method of determining the right amounts of QDs and phosphors to optimize the illumination performance of the WLED. The predicted performances of the WLEDs obtained from our method agree well with experimental results. With this method, we are able to produce white light with, for example, a high color-rendering index of 95.4 and an R9 value of 95.0 at a correlated color temperature (CCT) of 3640 K. We also analyze the modulation characteristics of this WLED system for visible light communication (VLC). By combining the frequency responses of the blue LED, the QDs, and the phosphors, we are able to accurately predict the overall frequency responses and the bandwidths of the WLEDs that have various CCTs and, thus, optimize the bandwidth of the WLED. Our approach of optimizing the performance of a WLED, as verified by our measurement data, is simple and general and can facilitate the selection of suitable light-conversion materials for combined illumination and VLC applications.

Visible light communication (VLC) is a kind of high-efficiency wireless communication technology. As one of the key issues in VLC, the modulation bandwidth of light-conversion materials based light-emitting diodes (LEDs) has gained considerable attention in recent years. However, few people focus on theoretical analyses of modulation on light-conversion materials, which show significant influence on the modulation bandwidth of white LED. In this work, we proposed low-pass filter based models of light-conversion materials, including YAG:Ce phosphor and CdSe/ZnS quantum dots, to build the relationship between fluorescence lifetime of these light-conversion materials and their modulation bandwidth. Furthermore, the modulation bandwidth of light source with polychromatic light was obtained and analyzed based on the proposed model. The experimental results of the modulation bandwidth showed a highly consistency with the results from simulation models. These analyses were believed to help realizing white-light illumination with high bandwidth. To the best of our knowledge, this is the first time the modulation mechanisms of light-conversion materials in VLC have been comprehensively discussed in literature.

We realize signal transmission with a miniature light source fabricated by a 4 m m 2 red-emissive CdSe/ZnS quantum-dot light-emitting diode (QLED) in visible light communication (VLC). The light emitted from the 60°-designed QLED transmits in free space with a data rate of 4 Mb/s at a 3 m transmission distance by using a simple modulation scheme of non-return-to-zero on-off keying. The maximum data rate of 2.5 Mb/s with a bit error rate below the forward-error-correction (FEC) limit is achieved with the optical angles of ± 20 ∘ . The influences caused by the voltage, distance, and optical angle of emitting light are taken into consideration during communication. The performance of the QLED-based light source compares favorably with other solution-processed devices in efficiency, luminance, bandwidth, transmission speed, and distance. Additionally, to the best of our knowledge, this is the first report of an investigation on the application of QLED in VLC. Our results should be instructive for further investigation on QLED communication.

To clarify the positive effect of the down-conversion process for ultraviolet (UV) and deep ultraviolet (DUV) light detection and visualization, we choose, synthesize, and characterize a spectrum of direct-bandgap CdSe-based colloidal quantum dot (QD) solvents and color-conversion layers (CCLs) across blue, green, yellow, orange, and red hues. Their optical absorption, emission, and response speeds under various UV and DUV wavelength of 280 nm, 372 nm, and 405 nm are evaluated. The blue QD CCL demonstrated the highest quantum yield up to 0.68. By integrating this blue QD CCL directly onto a silicon-based photodiode, the responded optical power to 280-nm DUV light is significantly enhanced by 27 times; this data decreases slightly to 23 times when using orange QDs, due to the comparatively lower quantum yield. For the optimal result in a communication system, the orange QDs help exhibit the highest response of 520 mV when stimulated with 372-nm UV light, compared with a substantial improvement over the original response of 120 mV. This enhancement makes the orange QDs significantly reduces the BER, especially at data rates below 70 Mb/s, due to the stronger response of the avalanche photodiode (APD) at 600 nm. Furthermore, to demonstrate the potential application of QDs for patterning and visualization, we have also produced CdSe-based QDs through inkjet printing, showcasing their printability, high stability in air, and pure color emission under DUV illumination. These results underscore the significant potential of CdSe-based QDs for full-color anti-counterfeiting solutions and their integration into flexible, printable wearables for a variety of visualization and DUV detection applications.

In this study, we have investigated the trade-off between the color rendering index (CRI, Ra) and the scotopic/photopic ratio (S/P) for color-tunable phosphor-coated white light-emitting diodes (LEDs) at two CRI limitations (Ra ≥ 70 and Ra ≥ 96). First, luminescent spectra measurements have been conducted to determine experimental results of Ra and S/P under various correlated color temperatures (CCTs). Then, a nonlinear programming method has been adopted for the optimization of Ra and S/P by varying spectral shapes through adjusting spectral parameters, such as peak wavelengths, full-width at half-maxima, and relative intensities. Therefore, polynomial curves of optimal S/P versus CCT at two Ra limitations have been discovered, enabling users to obtain optimal S/P under arbitrary CCTs within [2700 K, 6500 K]. In addition, a comparison study between the present work and our previous work has also been conducted at Ra = 70, and a fair agreement of optimal S/P has been observed.