Basic Energy Sciences/Energy Efficiency and Renewable Energy
October 7, 2014
Washington, D.C.
There is a need for high efficiency temperature stable red LED, which would lead to efficient green (needed as well). RGB+ lighting is really the path to 250 lm/W warm white light since phosphors and down converters will always have Stokes loss. Down converters work if you have no other option, but if you can create photons you want directly at colors you want, you will have better efficiency. Also, RGBA (red, green, blue, amber) direct emitters give the additional functionality of color tunability. If you can get to 250 lm/W you can really compete with fluorescents and can make incandescent replacement lamps even cheaper because there can be less thermal management.
Jennifer Hollingsworth, Los Alamos National Laboratory
Next-Generation Giant Quantum Dots: Solving the Solid-State Performance Conundrum
Our work is BES funded, but it is ‘use inspired’ science with a focus on SSL. We are working on giant QDs (g-QDs) that have diameters of 15-20 nm which is much larger than traditional quantum dots. The structure of the g-QDs, a small core with a thick outer shell, gives novel functionality. Emission occurs exclusively from the core, and absorption occurs from the “antenna” shell which results in large effective Stokes shift and minimal self-reabsorption. The g-QDs do not photobleach, are non-blinking, resist saturation at high flux densities (i.e. non-radiative processes such as Auger recombination are suppressed, and efficient emission results from charged and multiexciton states), and emission is largely independent of surface ligands. The shell thickness is why the QDs are called “giant”, but we found that core size is also very important particularly with respect to blinking. Core size and shell thickness are together ‘tuning parameters’ for either high or low biexciton emission efficiencies coupled with suppressed blinking.
We found that g-QDs outperform standard QDs in direct-injection devices; with EQE greater than 10x higher, luminance more than 1000x the standard (2000 Cd/m2), and down-conversion efficiencies of up to 88%. A new automated reactor system is being employed to help meet the “scale-up” challenge for these nanomaterials. Computer controlled synthesis allows for automated precursor delivery, material sampling, and in-situ diagnostics. The system also facilitates quasi-combinatorial materials exploration which will help us more rapidly discover new g-QDs, assess and address temperature quenching, and evaluate their reproducibility and suitability for subsequent scale up.
Jeffrey Pietryga, Los Alamos National Laboratory
Nanoscale Engineering of Quantum Dots for SSL: QD Phosphors and LEDs
BES is interested in the fundamental interactions between light and QDs. Several advantages motivate the use of QDs as down-converting phosphors including bright photoluminescence, selectable energy and bandwidth over the whole visible spectrum, low cost and scalable manufacturing, and the elimination of “critical” rare earth elements. However development is still needed to retain efficiency at elevated operating temperatures, to achieve longer lifetimes, and retain full compatibility with compositing techniques. For example, silicone is a particular favorite matrix material used to apply down-converters to LEDs, but silicones formulated with a platinum catalyst can be deleterious over time due to complex interactions with QDs. We need to co-develop QDs and the matrix material.
In an alternative application (a solar photo concentrator), we have found a new way to embed QDs in plexiglass (PMMA) while maintaining 95% quantum yield for a solar window.
Another interesting approach is to use QDs as the active region in the LED. For QD-LEDs, advantages include potential efficiency gains and reducing costs at large scale, but brightness must be enhanced (through higher efficiency and less droop) and lifetimes improved. Charged QDs produce Auger losses, not because of high carrier density, but because the charge is imbalanced (causing spontaneous electron injection).
There is the potential for cadmium free QDs, and that is important. One option is a copper indium sulfide core with a zinc sulfide shell (CuInS2/ZnS). Retaining QD performance in a practical application and achieving a technologically relevant lifetime will require “hardening” of the QDs using chemical and heat treatment without sacrificing PL efficiency. Co-development of the matrix material and QDs will also be important. Bright long-lived operation at desired color point will require further engineering of reduced Auger charge-resistant QDs, use of advanced spectroscopy to analyze long term failure modes, and reduced organic content (likely to enhance stability). Additionally, reduced use of vacuum processing steps will minimize cost and scalability problems.
Vladamir Bulovic, Massachusetts Institute of Technology
UV-Vis-IR Quantum Dots for SSL
QDs span the visible spectrum, and within the visible spectrum QDs are close to meeting the MYPP’s 2020 goal of less than 30 nm full width half maximum (FWHM) (Task A.1.3). QDs used in enhanced LCDs have a FWHM of 50nm, and can get down to 25nm with careful synthesis. These phosphors are only one millimeter away from the backlighting LEDs in the TV and reach temperatures of 70°C. QD synthesis can be scaled economically, but managing waste is important. With a single-step synthesis process, CdSe/CdS QDs are estimated to cost around $61/gm or $4/m2 compared to Ir(ppy)2, the emitter material used in OLEDs which costs $658/gm. A state of the art EL QD-LED with a brightness of 10,000 cd/m2 at 5 volts has an EQE of 18% and an IQE of about 90%, which is satisfactory. Beyond the visible spectrum there are infrared LED applications such as telecommunications, bio sensing and spectroscopy, bio-medical imaging, and military technologies. Other applications include sleep, food heating, UV water treatment/pasteurization, phototherapy (e.g. treating neonatal jaundice with blue light), and UV curing.
Jim Murphy, GE Research
Narrow-band Phosphors for SSL
There is a tradeoff between efficacy and CRI. It has been well-known since the 1960’s that narrower line-emission enables higher efficacy for the same CRI. This effect is stronger for red spectral regions compared to green or blue because the human visual response falls off rapidly in the deep red. Typical red phosphor material has a QE greater than 90%, but because the FWHM is > 90 nm, a large portion of the spectral emission is at wavelengths greater than 650 nm, which is beyond the eye response, leading to efficacy losses.
There are currently two possible routes for achieving narrow band phosphor red-line emission, QDs and manganese (Mn4+) fluoride based phosphors. For other colors, only QDs currently meet spectral and absorption requirements. The need for new line emitting LED phosphors requires core or basic science programs. Typical RE3+ activators have low absorption and require sensitization. Transition metal activators except Mn4+ do not meet spectral requirements. Both sets of line emitters based on these activators have slow decay times which lead to intensity saturation. Another challenge is that once you have identified a new material, reliability is not well standardized and difficult to assess. Some understanding exists regarding photo-oxidative processes and hydrolysis, but there is less understanding regarding the relationship between defects, processing, and performance. For instance a small parameter change in these areas can cause huge changes in products at 50,000 hours. Phosphor reliability is tested at 85°C/85% humidity for 150 hours. By working on reliability, the high temperature, high humidity performance can be improved. Different phosphors are currently used for high and mid power LEDs. Reliability improvements would allow for a broader phosphor portfolio in mid power devices.
http://energy.gov/sites/prod/files/2014/12/f1...bles_1.pdf
QDX tm
Looking Forward
October 7, 2014
Washington, D.C.
There is a need for high efficiency temperature stable red LED, which would lead to efficient green (needed as well). RGB+ lighting is really the path to 250 lm/W warm white light since phosphors and down converters will always have Stokes loss. Down converters work if you have no other option, but if you can create photons you want directly at colors you want, you will have better efficiency. Also, RGBA (red, green, blue, amber) direct emitters give the additional functionality of color tunability. If you can get to 250 lm/W you can really compete with fluorescents and can make incandescent replacement lamps even cheaper because there can be less thermal management.
Jennifer Hollingsworth, Los Alamos National Laboratory
Next-Generation Giant Quantum Dots: Solving the Solid-State Performance Conundrum
Our work is BES funded, but it is ‘use inspired’ science with a focus on SSL. We are working on giant QDs (g-QDs) that have diameters of 15-20 nm which is much larger than traditional quantum dots. The structure of the g-QDs, a small core with a thick outer shell, gives novel functionality. Emission occurs exclusively from the core, and absorption occurs from the “antenna” shell which results in large effective Stokes shift and minimal self-reabsorption. The g-QDs do not photobleach, are non-blinking, resist saturation at high flux densities (i.e. non-radiative processes such as Auger recombination are suppressed, and efficient emission results from charged and multiexciton states), and emission is largely independent of surface ligands. The shell thickness is why the QDs are called “giant”, but we found that core size is also very important particularly with respect to blinking. Core size and shell thickness are together ‘tuning parameters’ for either high or low biexciton emission efficiencies coupled with suppressed blinking.
We found that g-QDs outperform standard QDs in direct-injection devices; with EQE greater than 10x higher, luminance more than 1000x the standard (2000 Cd/m2), and down-conversion efficiencies of up to 88%. A new automated reactor system is being employed to help meet the “scale-up” challenge for these nanomaterials. Computer controlled synthesis allows for automated precursor delivery, material sampling, and in-situ diagnostics. The system also facilitates quasi-combinatorial materials exploration which will help us more rapidly discover new g-QDs, assess and address temperature quenching, and evaluate their reproducibility and suitability for subsequent scale up.
Jeffrey Pietryga, Los Alamos National Laboratory
Nanoscale Engineering of Quantum Dots for SSL: QD Phosphors and LEDs
BES is interested in the fundamental interactions between light and QDs. Several advantages motivate the use of QDs as down-converting phosphors including bright photoluminescence, selectable energy and bandwidth over the whole visible spectrum, low cost and scalable manufacturing, and the elimination of “critical” rare earth elements. However development is still needed to retain efficiency at elevated operating temperatures, to achieve longer lifetimes, and retain full compatibility with compositing techniques. For example, silicone is a particular favorite matrix material used to apply down-converters to LEDs, but silicones formulated with a platinum catalyst can be deleterious over time due to complex interactions with QDs. We need to co-develop QDs and the matrix material.
In an alternative application (a solar photo concentrator), we have found a new way to embed QDs in plexiglass (PMMA) while maintaining 95% quantum yield for a solar window.
Another interesting approach is to use QDs as the active region in the LED. For QD-LEDs, advantages include potential efficiency gains and reducing costs at large scale, but brightness must be enhanced (through higher efficiency and less droop) and lifetimes improved. Charged QDs produce Auger losses, not because of high carrier density, but because the charge is imbalanced (causing spontaneous electron injection).
There is the potential for cadmium free QDs, and that is important. One option is a copper indium sulfide core with a zinc sulfide shell (CuInS2/ZnS). Retaining QD performance in a practical application and achieving a technologically relevant lifetime will require “hardening” of the QDs using chemical and heat treatment without sacrificing PL efficiency. Co-development of the matrix material and QDs will also be important. Bright long-lived operation at desired color point will require further engineering of reduced Auger charge-resistant QDs, use of advanced spectroscopy to analyze long term failure modes, and reduced organic content (likely to enhance stability). Additionally, reduced use of vacuum processing steps will minimize cost and scalability problems.
Vladamir Bulovic, Massachusetts Institute of Technology
UV-Vis-IR Quantum Dots for SSL
QDs span the visible spectrum, and within the visible spectrum QDs are close to meeting the MYPP’s 2020 goal of less than 30 nm full width half maximum (FWHM) (Task A.1.3). QDs used in enhanced LCDs have a FWHM of 50nm, and can get down to 25nm with careful synthesis. These phosphors are only one millimeter away from the backlighting LEDs in the TV and reach temperatures of 70°C. QD synthesis can be scaled economically, but managing waste is important. With a single-step synthesis process, CdSe/CdS QDs are estimated to cost around $61/gm or $4/m2 compared to Ir(ppy)2, the emitter material used in OLEDs which costs $658/gm. A state of the art EL QD-LED with a brightness of 10,000 cd/m2 at 5 volts has an EQE of 18% and an IQE of about 90%, which is satisfactory. Beyond the visible spectrum there are infrared LED applications such as telecommunications, bio sensing and spectroscopy, bio-medical imaging, and military technologies. Other applications include sleep, food heating, UV water treatment/pasteurization, phototherapy (e.g. treating neonatal jaundice with blue light), and UV curing.
Jim Murphy, GE Research
Narrow-band Phosphors for SSL
There is a tradeoff between efficacy and CRI. It has been well-known since the 1960’s that narrower line-emission enables higher efficacy for the same CRI. This effect is stronger for red spectral regions compared to green or blue because the human visual response falls off rapidly in the deep red. Typical red phosphor material has a QE greater than 90%, but because the FWHM is > 90 nm, a large portion of the spectral emission is at wavelengths greater than 650 nm, which is beyond the eye response, leading to efficacy losses.
There are currently two possible routes for achieving narrow band phosphor red-line emission, QDs and manganese (Mn4+) fluoride based phosphors. For other colors, only QDs currently meet spectral and absorption requirements. The need for new line emitting LED phosphors requires core or basic science programs. Typical RE3+ activators have low absorption and require sensitization. Transition metal activators except Mn4+ do not meet spectral requirements. Both sets of line emitters based on these activators have slow decay times which lead to intensity saturation. Another challenge is that once you have identified a new material, reliability is not well standardized and difficult to assess. Some understanding exists regarding photo-oxidative processes and hydrolysis, but there is less understanding regarding the relationship between defects, processing, and performance. For instance a small parameter change in these areas can cause huge changes in products at 50,000 hours. Phosphor reliability is tested at 85°C/85% humidity for 150 hours. By working on reliability, the high temperature, high humidity performance can be improved. Different phosphors are currently used for high and mid power LEDs. Reliability improvements would allow for a broader phosphor portfolio in mid power devices.
http://energy.gov/sites/prod/files/2014/12/f1...bles_1.pdf
QDX tm
Looking Forward
