How to Optimize the Reliability of KSF Phosphor LEDs

When it comes to reliability, not all KSF phosphors are the same. Issues such as impurities or moisture in the synthesis process can affect optical performance and chemical stability of the phosphor.[1] General Electric (GE) holds the original patent for Potassium Fluorosilicate (PFS) red phosphor and has been producing it the longest, resulting in high-quality phosphors. Luminus uses KSF/PSF (K₂SiF₆: Mn4⁺) sold as TriGain® technology by Current Lighting Solutions to ensure product quality.  

The TriGain KSF phosphor red LEDs have also established an extensive track record of reliability from their initial use in the display industry since 2014. From 2014 to 2023, more than 60 billion KSF-based LEDs have been fabricated for use in displays.[2] GE reports that there are 19 worldwide licensees using these LEDs in their display products, including all of the major manufacturers in the industry.[3]

GE reports that its technology can be found in (among other products):[4]

• Smart phones by Samsung, Apple, Huawei, LG, BLU, Razer, Red
• Laptops and monitors by HP, Lenovo, Dell, Asus, Apple, Microsoft, Razer, MSI, Samsung, LG
• Tablets by Apple, Samsung, Lenovo, Huawei, Microsoft, Asus
• Televisions by Samsung, LG, Sony, Vizio, TCL, Hisense, Toshiba, Insignia, Sharp

Additionally, this same technology has been incorporated into a variety of GE Lighting products to produce more energy efficient warm white light. In 2017, GE announced that it would begin to manufacture the phosphor powder itself “to ensure a reliable source for the LED manufacturers that license the technology. This breakthrough was only possible through GE’s prevailing leadership in the advancement of LED technology,” said Jerry Duffy, GE Lighting global product general manager. “Adding to the breakthrough is GE’s proprietary improvements to phosphor synthesis, including decreased particle sizes, higher quantum efficiencies and longer life when compared to standard PFS. Many devices currently on the market offer lower-quality red components in LEDs,” he concluded.[5]

Overall, this enhanced reliability has led to the commercialization of KSF phosphor for on-chip applications. The reliability and performance enhancements outlined below are why Luminus selected TriGain phosphors for our industry-leading LUX products.

High Temperature and High Humidity (HTHH) Performance

PFS/KSF can be sensitive to temperature and humidity. KSF can darken when exposed to prolonged periods of high temperature and high humidity (HTHH) conditions. However, TriGain phosphors have improved HTHH performance compared to traditional nitride-based phosphors, exhibiting minimal temperature quenching below 150°C. To further enhance the temperature and humidity resilience of the KSF phosphors, GE outlines some processing optimization steps in a 2015 study.[6]

Improved Lumen Output & Color Stability

Through significant process improvements, TriGain provides a notable reduction (approximately nine times – 9X) in blue light photodamage to the phosphor compared to other commercial KSF samples. These process enhancements result in a combination of reduced concentration quenching (absorption of light by the manganese) and higher stability under high excitation flux and HTHH conditions.

Luminus KSF Phosphor Reliability Test Results

Luminus has performed qualification testing on our KSF product line, including a High Temperature Operating Life (HTOL) test using the IES LM-80 method to ensure performance. These tests are described in detail and the results of Luminus LUX KSF Phosphor LED tests are provided below (refer to Table 1 and Table 2).

High Temperature Operating Life (HTOL) is a qualification test where the LEDs are subjected to a set of temperature and bias current conditions to generate data that can be used for lifetime predictions. The Lumen Maintenance (LM-80) test is a variant of HTOL where there is a standardized set of test conditions and methodology (LM-80) and a standard method for lifetime prediction calculations (TM-21).

Testing LED Product Performance

There are a number of testing standards for LED product performance. For convenience, some of these are summarized below:

• LM-80, Measuring Maintenance of Light Output Characteristics of Solid-State Light Sources, defines the test procedure for lumen maintenance of LED components.[7]
• TM-21, Projecting Long-Term Luminous, Photon, and Radiant Flux Maintenance of LED Light Sources, defines the data analysis procedures for LM-80 data.[15] Lumen Maintenance (Lx) defines the percentage of lumens compared with the initial lumens. Typically, L70 data is reported.
• LM-79, Optical and Electrical Measurements of Solid-State Lighting Products, defines the test procedure for a luminaire product.[16] LM-79 is not a reliability test and is not discussed below. LM-79 testing is of interest because it is a test our customers often perform before product release.
• It is possible to calculate LxBy using commercial reliability software packages. The additional parameter, By, defines the percentage that is expected to be below the specified Lx-value at a particular time. So, for example, L70B50 indicates that half of the lamps are expected to have less than 70 percent lumen maintenance for a specific test condition. TM-21 does not include a method for these calculations.

Table 1 and Table 2 show the results of screening tests performed at Luminus as part of the process used to down select to the TriGain phosphor. These tables compare LUX with a Luminus nitride-based component and a white LED that uses a KSF phosphor other than TriGain. Table 3 shows the LM-80 qualification test data performed by a certified test lab and Figure 1 is a plot of that data. Table 4 shows the L_xB_y reportable hours calculation results using the LM-80 test data. Luminus KSF with TriGain technology has excellent reliability performance, similar to that of nitride technologies and superior when compared with other KSF technologies in the industry.

Figure 1. Plot of the LM-80 data shown in Table 3.

Table 4 shows the calculated reportable values for L_xB_y of the LUX series using 6000 hours of LM-80 data. The TM-21 standard which specifies the calculation procedures for LM-80 data has a limit on the number of reportable hours, which in this case is 33,000 hours. The L_xB_y calculations below follow these rules but most of the calculated values are well above the reporting limit. As more test hours are acquired, the reportable hours in this table will be updated.

WHTOL Test Results

The Wet High Temperature Operating Lifetime (WHTOL) test was conducted by a certified test lab under controlled conditions of 85°C and 85% relative humidity for a duration of 1000 hours. The primary objective of LED WHTOL testing is to ensure that LEDs can maintain their functionality and reliability even in demanding operating conditions.

The WHTOL test was conducted by driving the LED samples at 80% of the maximum rated current using a one hour on/off cycle. This allows the LED package to saturate with water vapor during the off portion of the cycle.

The WHTOL results are presented in Table 5. To establish a baseline for comparison, Luminus Gen 6 LED samples were used as a control. The lumen maintenance of the tested LEDs is on par with the Gen 6 nitride control samples, demonstrating LUX with TriGain KSF reliability performance being on par with standard Nitride devices.

Design Considerations for Luminaires in Humid Environments

Although the TriGain phosphors in Luminus Lux COB arrays have demonstrated superior performance in high-temperature and high-humidity (HTHH) conditions, all narrow-band KSF (Potassium Silicon Fluoride) phosphors require careful design to ensure long-term reliability. The primary challenge is that KSF phosphors are highly susceptible to moisture, which can lead to degradation, a loss of efficiency, and ultimately failure if the phosphor is not properly protected.

To ensure luminaires with TriGain KSF-based Lux LEDs meet industry standards and achieve their rated lifetime, the following design and process recommendations are key:

Luminaire-Level Protection

• Robust Sealing: The luminaire design must incorporate robust sealing to prevent moisture ingress. This is especially critical for products intended for outdoor, industrial, or other humid environments. The integrity of the lens, gasket, and housing is paramount.
• Controlled Ventilation: For luminaires that cannot be fully sealed, proper design for ventilation and drainage is crucial. Vents with moisture-permeable membranes can equalize pressure without allowing liquid water to enter. This helps prevent condensation from building up inside the fixture, a major contributor to humidity-related failures.

Material and Process Control

• Manufacturing Cleanliness: All manufacturing processes must be meticulously clean and free of contaminants. Debris, flux residue, or other chemicals can accelerate the degradation of LED components.
• Strategic Material Selection: Designers should be cautious about the materials used in the luminaire. Certain materials, such as specific types of silicone encapsulants or adhesives, can allow moisture to penetrate and reach the phosphor. Selecting materials with a very low Moisture Vapor Transmission Rate (MVTR) is essential.
• Rigorous Testing Protocols: The degradation of KSF phosphors under HTHH conditions requires specific testing protocols to ensure the luminaire will meet its stated lifetime and performance targets. Standard LED tests may not be sufficient to account for these unique characteristics.

By following these guidelines, luminaires can benefit from the high efficacy and excellent color rendering of KSF phosphors while ensuring long life and high performance even when operated in high-temperature and high-humidity environments.

For more information about KSF Phosphors and Luminus’ LUX Series COB LEDs with KSF phosphor technology, read the white paper:  LUX Series COBs: Achieving High CRI Lighting with High Efficacy Using Luminus LUX Technology.

REFERENCES:

[1]      Reinert Verstraete, Heleen F. Sijbom, et al. “Red Mn4+-Doped Fluoride Phosphors: Why Purity Matters.” ACS Applied Materials & Interfaces 2018 10 (22), 18845-18856. DOI: 10.1021/acsami.8b01269

[2]         Murphy, J., “Market-leading wide color gamut narrow-band phosphors by GE,” presented at SID Display Week 2023, Los Angeles, CA, May 2023.

[3]         Murphy, J., “Phosphors for Next Generation Solid-State Lighting/Displays,” presented at 2021 DOE Lighting R&D Workshop Panel-Lighting & Display Cross-Cutting R&D, February 3, 2021.

[4]         “Project: Narrow-Band LED Phosphors Enabling Wide Color Gamut Displays,” GE Research website, https://www.ge.com/research/project/narrow-band-led-phosphors-enabling-wide-color-gamut-displays (Retrieved Oct. 2, 2023)

[5]         Wright, M., “GE Lighting manufactures PFS red phosphor for LED display backlight applications,” LEDs Magazine, March 13, 2015.

[6]         Murphy, J., Garcia-Santamaria, F., et al. “PFS, K₂SiF₆:Mn⁴⁺: The Red-line Emitting LED Phosphor behind GE’s TriGain Technology™ Platform. Presented at SID Display Week 2015, San Jose, CA.

[7]         Approved Method: Measuring Maintenance of Light Output Characteristics of Solid-State Lighting Sources, ANSI/IED LM-80-21. Published by the Illuminating Engineering Society

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