Improved performance white LED

Abstract

This paper describes work leading to the development of a new packaging method for white LEDs, called scattered photon extraction (SPE). Previous work by our group showed that the traditional placement of the phosphor close to the die negatively affects the overall luminous efficacy and lumen maintenance of phosphor-converted white LEDs. The new SPE method enables higher luminous efficacy by placing the phosphor at a remote location from the die and by shaping the lens surrounding the die to extract a significant portion of the back-transferred light before it is absorbed by packaging components. Although the remote phosphor concept is not new, SPE is the first method to demonstrate efficient extraction of back-transferred light and show over 60 percent improvement in light output and efficacy compared to similar commercial white LEDs. At low currents, the prototype white LEDs based on the SPE technique showed over 80 lumens per watt. The SPE concept was tried on two types of commercial packages and both showed similar improvements. BACKGROUNDDisplacing traditional light sources with solid-state light sources is the dream of many researchers. Since the first demonstration of white light-emitting diodes (LEDs) in the mid-1990s, researchers have been striving to boost the performance of this technology so that its luminous efficacy far exceeds the commonly used incandescent light bulb and the linear fluorescent lamp. The luminous efficacies of these two traditional light source technologies are nominally around 15 lumens per watt and 85 lumens per watt, respectively. The luminous efficacy of presently available commercial white LED products is in the range of 40 lumens per watt. The target for solid-state light sources is to reach 150 lumens per watt by 2012. 1 For white LEDs to reach this target, improvements are needed in several areas, including the internal quantum efficiency, the light extraction efficiency, and the phosphor efficiency. The white LED luminous efficacy gains made during the past several years can be attributed mostly to improvements in internal quantum efficiency and light extraction efficiency from the chip. Although new phosphor blends have been used to improve color properties, they have not contributed much to the luminous efficacy gains. This may be mainly due to the fact that the commonly used YAG:Ce phosphor in white LEDs is very efficient, and most of the newer phosphor blends have added more energy in the red region of the spectrum, which is likely to lessen the efficacy. Even though the YAG:Ce phosphor is efficient, recently our group showed that the traditional phosphor placement method negatively impacts the overall efficacy of white LEDs. 2 The goal of this paper is to describe the work that led to this finding and to the development of a new phosphor placement method.

Status and prospects for phosphor-based white LED packaging

“…To make the phosphor layer with uniform plane shape in remote coating, Narendran et al [28,29] developed the scattering photon extraction (SPE) method by coating phosphor on a secondary optics. If LED packaging only concerns LE, remote coating is definitely a recommended approach.…”

Synthesis and characterization of YAG:Ce,Gd and YAG:Ce,Gd/PMMA nanocomposites for optoelectronic applications

“…It is necessary to find an embedding polymer matrix that is transparent and has no absorption in the visible spectral range. Generally, YAG:Re phosphors were embedded in silicon, epoxy resin, and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) [17][18][19][20]. When an organic matrix is chosen, for optoelectronic applications, optical (refractive index (RI), transparency, and retention of transparency over time), mechanical, chemical, and thermal properties must be taken into account.…”

Review—Electrophoretic Deposition of Phosphors for Solid-State Lighting

“…51 Nano-phosphors (<300 nm) with comparable quantum efficiency to typical micron-sized phosphors may be a means to reduce the losses and improve efficiency of the white UV-based LEDs. 52 But it is well known that the emission intensity of phosphor decreases as the crystallite size decreases. This has been attributed to surface defect states that trap the emitted photons and thus quench the emission intensity.…”