LED-based Applications Research Articles

Spectral Design of Phosphor-Converted LED Lighting Guided by Color Theory.

In the race to develop new luminescent materials for the next generation of light-emitting-diode (LED)-based solid-state lighting and display applications, it is often forgotten that color theory and human perception should be some of the principal factors guiding materials design. In this Viewpoint, we explore some of the antiquated colorimetrics established originally for incandescent and fluorescent lighting and discuss how they are still widely applied in the literature today to interpret the color quality of luminescent materials, like inorganic phosphors and quantum dots, and to analyze prototype devices, despite their shortcomings. We then shift our analysis toward contemporary ideas in color theory that more accurately describe the color quality of modern LED light bulbs and flat-panel displays. Finally, the perspective examines the opportunities and challenges of applying these new concepts to guide the design of luminescent materials used in LED-based applications.

Bismuth activated blue phosphor with high absorption efficiency for white LEDs

Trivalent bismuth (Bi3+) activated blue-emitting materials have attracted considerable attention because they effectively avoid the reabsorption demerits compared to traditional rare earth activators. Unfortunately, these materials are often plagued by mismatch to the near-ultraviolet (n-UV) chip and short blue emission peak as well as low n-UV absorption efficiency. In this work, we successfully designed a blue-emitting phosphor SrZnSO:Bi3+ with a wide n-UV excitation band ranging from 310 to 400 nm and an ideal blue emission band centered at 460 nm. Furthermore, SrZnSO:0.03Bi3+ phosphor exhibits an extraordinary absorption efficiency of 73.21% ascribed to the intense absorption of the matrix and Bi3+ in the n-UV region. The relationship between the excitation peak position of Bi3+-related phosphors and the size of the polyhedron were also summarized. A white light-emitting diode (WLED) was fabricated using a 370 nm n-UV chip, the composition-optimized sample SrZnSO:0.03Bi3+ and the commercial phosphors. Interestingly, the temperature-sensitive luminescence behavior is beneficial to the performance improvement of the WLED. The WLED demonstrates a warm white light whose Commission Internationale de L′Eclairage (CIE) coordinates are (0.3817, 0.3885), outstanding color rendering index of 93.4 and low correlated color temperature of 4051 K at current of 80 mA. The study suggests that SrZnSO:Bi3+ blue emitting phosphor has great potential in LED-based applications. Also inspires researchers to explore Bi3+-related phosphors with low energy excitation band and high absorption efficiency to develop high-quality n-UV pumped WLEDs.

A narrow-band ultra-bright green phosphor for LED-based applications.

A novel Sr2MgB2O6 (SMBO) green-emitting phosphor co-doped with Ce3+-Tb3+ was synthesized at 950 °C via solid-state reactions, and the ultra-narrow-band green emission of Tb3+ was significantly enhanced almost 20 times via energy transfer from Ce3+ to Tb3+. It was found to have a broad excitation band (250 to 400 nm), and the full width at half-maximum (FWHM) of the dominant green emission band around 544 nm was only about 10 nm. The electronic band gap of the SMBO matrix was calculated by density functional theory (DFT) to be 4.60 eV, and this was well verified by the diffuse reflection spectra results. Furthermore, the composition-optimized phosphor SMBO:0.05Ce3+,0.05Tb3+ exhibits excellent thermal quenching resistance (75.3% intensity at 423 K) and relatively high external quantum efficiency (EQE = 48.92%). Finally, two white light-emitting diode (WLED) packages were fabricated via combining a 365 nm n-UV chip, the optimal sample and commercial blue and red phosphors to assess the application potential of the phosphors. The test results indicate that the obtained WLEDs-1 fabricated with K2SiF6:Mn4+ has an outstanding color rendering index (Ra = 85.7) and Commission Internationale de L'Eclairage (CIE) coordinates (0.3242, 0.3334). Meanwhile, the color gamut can reach 87% of the National Television Standards Committee (NTSC) CIE 1931 color gamut. WLEDs-2 fabricated with red emitting CaAlSiN3:Eu2+ produced warm white light with color coordinates of (0.3792, 0.3810), a high color rendering index of 82.3, and a low correlated color temperature of 4065 K. These results reveal the broad prospects of this phosphor for LED-based applications.

3D printed plano-freeform optics for non-coherent discontinuous beam shaping

The design, fabrication, and characterization of freeform optics for LED-based complex target irradiance distribution are challenging. Here, we investigate a 3D printing technology called Printoptical® technology in order to relax or push forward both the fabrication limits and LED-based applications of thick freeform lenses with small slope features. The freeform designs are carried out with an assumption of small-sized LED source using an existing point-source-based Tailoring method, which is available in the semi-commercial software. The numerical methods of the designs are characterized by ray-tracing software. The irradiance patterns of the 3D printed freeform lenses are promising considering the average shape conformity deviation of around ± 40 µm and center area surface roughness around ± 12 nm, which is to our knowledge by far the best result reported for 3D printed freeform lenses with a thickness greater than 1 mm. Applications of freeform lenses with discontinuous target irradiance distribution patterns are expected in eco-friendly energy efficient lighting such as in zebra-cross lighting.

High-Efficiency LED Driver Without Electrolytic Capacitor for Street Lighting

High-brightness light-emitting diodes (LEDs) are considered as a remarkable lighting device due to their high reliability, chromatic variety, and increasing efficiency. As a consequence, a high number of solutions for supplying LED strings are coming out. One-stage solutions are cost effective, but their efficiency is low as they have to fulfill several purposes with only one converter: power factor correction (PFC), galvanic isolation (in some cases), and current regulation. Two-stage and three-stage solutions have higher efficiency as each stage is optimized for just one or two tasks, and they are the preferred option when supplying several strings at the same time. Nevertheless, due to their higher cost in comparison to one-stage solutions, they are used when high efficiency, high performance, and the possibility of supplying several strings are the main concerns. In addition, they are also used when high reliability is needed and electrolytic capacitors cannot be used. In this paper, a three-stage solution and its complete design guideline for LED-based applications is proposed. PFC is achieved by a boost converter, while the galvanic isolation is provided by an electronic transformer (second stage). The third stages (one for each LED string) are designed following the two-input buck schematic, but taking advantage of the load characteristics (i.e., the high value of the LED string knee voltage, approximately equal to half the string nominal voltage). Moreover, a variation of the analog driving technique is also proposed. Experimental results obtained with a 160-W prototype show an efficiency as high as 93% for the whole topology and 95% for the cascade connection of the second and third stages.