Phosphors for LED-based Solid-State Lighting
Phosphors for LED-based Solid-State Lighting
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DOI：
10.1149/2.f04094if
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发表时间：
2009
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影响因子：
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通讯作者：
A. Setlur
中科院分区：
文献类型：
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作者：
A. Setlur
基于 InGaN LED 的固态照明 (SSL) 的功效在过去十年中提高了超过 10 倍：冷白光 LED 的功效超过了线性荧光灯 (LFL) 的功效 (>100 lm/W)，暖白光 1W LED 超过了紧凑型荧光灯 (CFL) 的功效 (>60-70 lm/W)。美国能源部已将 2015 年暖白光封装的光效目标设定为 138 lm/W，这是一项重大技术成就，将导致 SSL 在许多照明产品领域的市场渗透。在基于 LED 的 SSL 中，紫色、蓝色和绿色 LED 基于 InGaN 半导体，而红色和琥珀色 LED 基于 AlInGaP 半导体。1-4 这两种半导体系统在绿色、黄色和琥珀色光谱区域的效率往往要低得多。有关 LED 芯片效率进展的许多方面，例如绿色和琥珀色 LED 效率较低的根本原因，以及基于 LED 的 SSL 对照明能耗的影响，已在其他地方讨论过2-5，此处不再讨论。 InGaN 和 AlInGaP 效率的限制使得必须使用磷光体下转换（尽管存在固有的斯托克斯损耗）来产生绿光和黄光，以用于高效 LED 封装、灯和固定装置。此外，由于蓝光和紫光 InGaN LED 的最大效率似乎高于红光 (lmax = 600-630 nm) AlInGaP LED 的最大效率，2 使用 InGaN LED 荧光粉下转换的系统（本文中称为 pcLED）对整个白光光谱的潜在功效可能大于使用 AlInGaP LED 产生红光的系统，这进一步推动了整个可见光谱 LED 荧光粉的发展。人们对 LFL/CFL、阴极射线管 (CRT) 和 X 射线胶片中的荧光粉进行了广泛的研究和开发，6 但大多数传统荧光粉并不适合 pcLED。这通常是因为这些荧光粉不能强烈吸收紫色或蓝色 LED 辐射，从而导致 LED 封装因散射而损失。此外，许多传统荧光粉使用 Eu3+、Tb3+ 或 Mn2+ 激活剂，其跃迁被禁止，且衰减时间较长 (>1 ms)，由于荧光粉上的高 LED 辐射通量饱和，导致荧光粉猝灭。7,8 使用具有 5d1 → 4f1 Ce3+ 或 4f65d1 → 4f7 Eu2+ 发射跃迁的 Ce3+ 和 Eu2+ 荧光粉，其衰减时间为 150oC，并且许多LFL/CFL 和 CRT 荧光粉在这些温度下具有强烈的猝灭作用。这些额外的要求使得有必要开发专门用于 pcLED 的新型荧光粉。 LED 荧光粉的挑战和额外要求通过 LED 荧光粉相对于 LFL/CFL 荧光粉更大的潜在成分空间来平衡。例如，许多硅酸盐磷光体由于汞吸附而在汞等离子体中变暗，9通常阻止它们的使用，除非它们涂有保护层以防止这些反应。 LED 中不存在这些反应，因此开辟了许多潜在的荧光粉组合物。此外，荧光灯荧光粉悬浮液是水基的，防止使用在水中分解的荧光粉。同样，这些限制在 LED 中不再是问题，因为许多封装协议避免了水处理条件。然而，虽然加工问题可能会得到缓解，但由于 pcLED 不一定是密封的，因此在高温和高湿度条件下（例如 85oC 和 85% 相对湿度），荧光粉稳定性存在潜在问题。过去 10 年来，新型荧光粉的发现和开发满足了对新型 LED 荧光粉的许多需求。在此期间，LED 荧光粉领域已从单一的荧光粉成分（Ce3+ 掺杂铝酸盐石榴石）发展到各种硅酸盐、铝酸盐、氮化物、氧氮化物、硫化物和氟化物成分，从而实现了覆盖全系列白光 CCT 的商用 LED（图 1）。本文将简要讨论这些不同的 LED 荧光粉及其一些优点和缺点。
The efficacy of solid-state lighting (SSL) based upon InGaN LEDs has improved by >10x over the past decade: the efficacy of cool white LEDs surpasses linear fluorescent lamp (LFLs) efficacies (>100 lm/W) and warm white 1W LEDs surpasses compact fluorescent lamps (CFLs) efficacies (>60-70 lm/W). The U.S. DOE has set a 2015 efficacy target of 138 lm/W for warm white packages, a significant technical achievement that would lead to SSL market penetration in many lighting product segments. In LEDbased SSL, violet, blue, and green LEDs are based upon InGaN semiconductors, while the red and amber LEDs are based upon AlInGaP semiconductors.1-4 Both of these semiconductor systems tend to have much lower efficiencies in the green, yellow, and amber spectral regions. Many aspects regarding the progress in LED chip efficiency, such as fundamental causes for the lower green and amber LED efficiency, and the impact of LED-based SSL on lighting energy consumption have been discussed elsewhere2-5 and will not be addressed here. The limitations in InGaN and AlInGaP efficiency make it necessary to use phosphor downconversion (in spite of the inherent Stokes losses) to generate green and yellow light for high efficacy LED packages, lamps, and fixtures. In addition, since the maximum efficiency of blue and violet InGaN LEDs appears to be higher than the maximum efficiency for red (lmax = 600-630 nm) AlInGaP LEDs,2 the potential efficacy for a system that uses phosphor downconversion of InGaN LEDs (termed pcLEDs in this article) for the entire white spectrum could be greater than systems using AlInGaP LEDs to generate red light, further motivating the development of LED phosphors across the visible spectrum. There has been extensive research and development for phosphors in LFLs/CFLs, cathode-ray tubes (CRTs), and X-ray films,6 but most of these traditional phosphors are not suitable for pcLEDs. This is usually because these phosphors do not strongly absorb violet or blue LED radiation, leading to LED package losses from scattering. Also, many traditional phosphors use Eu3+, Tb3+, or Mn2+ activators whose transitions are forbidden with long decay times (>1 ms), causing phosphor quenching due to saturation from the high LED radiation flux on the phosphor.7,8 Using Ce3+ and Eu2+ phosphors with 5d1 → 4f1 Ce3+ or 4f65d1 → 4f7 Eu2+ emission transitions that have decay times of 150oC, and many LFL/CFL and CRT phosphors have strong quenching at these temperatures. These additional requirements make it necessary to develop new phosphors specifically for pcLEDs. The challenges and additional requirements for LED phosphors are balanced by larger potential composition spaces for LED phosphors versus that for LFL/CFL phosphors. For example, many silicate phosphors darken in the Hgplasma due to Hg-adsorption,9 generally preventing their use unless they are coated with a protective layer to prevent these reactions. These reactions are not present in LEDs, opening up many potential phosphor compositions. In addition, fluorescent lamp phosphor suspensions are water-based, preventing the use of phosphors that decompose in water. Again, these restrictions are less of an issue in LEDs since many packaging protocols avoid aqueous processing conditions. However, while processing issues might be alleviated, there are potential issues with phosphor stability at high temperatures and high humidity conditions (e.g. 85oC and 85% relative humidity) since pcLEDs are not necessarily hermetically sealed. Many of the needs for new LED phosphors have been met by the discovery and development of new phosphors over the past 10 years. During this time, the field of LED phosphors has moved from a single family of phosphor compositions—the Ce3+-doped aluminate garnets—to a variety of silicate, aluminate, nitride, oxynitride, sulfide, and fluoride compositions, leading to commercial LEDs that cover a full range of white CCTs (Fig. 1). This article will briefly discuss these various LED phosphors with some of their advantages and drawbacks.