Stable Cadmium-Free Quantum Dot Optical Down-Converters for Solid-State Lighting - Nanosys, Inc. Ilan Jen-La Plante, Staff Scientist ...

Stable Cadmium-Free Quantum Dot Optical
 Down-Converters for Solid-State Lighting

 Nanosys, Inc.
 Ilan Jen-La Plante, Staff Scientist
 ijenlaplante@nanosysinc.com

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 1

Project Summary
 Timeline: Key Partners:
 Start date: 04/01/2020 Nanosys, Inc.
 Planned end date: 03/31/2023
 Key Milestones:
 1. A red InP-based QD with ≥88% QY when UC Merced
 measured at 80°C and 10 mW/mm2;
 03/31/2021
 2. A red InP-based QD with ≥ 88% QY when
 measured at 100°C and 100 mW/mm2;
 03/31/2022
 Budget: Project Outcome:
 Total Project $ to Date The objective of the project is to
 (through 06/30/2021): research, develop, and test heavy-
 metal-free Quantum Dots (QDs) that
 • DOE: $824,866.31 can be used on a Light Emitting Diode
 • Cost Share: $320,463.19 (LED) chip capable of ≥88% Quantum
 Yield (QY) at ≥ 1W/mm2 and 150°C; 30
 nm full width at half maximum (FWHM);
 Total Project $: color shift at Δu′v′ 6000hrs.
 • DOE: $2,000,000
 • Cost Share: $876,198

 First BTO peer review for project since 04/01/2020 start date
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 2

Team

 Personnel and Background Project Role
 Dr. Ilan Jen-La Plante Project Management
 Leads heavy-metal free QD development for photoemissive Quantum Dot Synthesis
 applications, project PI and technical point of contact
 Dr. Ruiqing Ma Optical Physics
 Nanosys

 Expert in emissive materials, including OLED emitters and QDs; Failure Mechanism Modeling
 Past PI on OLED DOE-funded research projects
 Ernest Lee Optical Engineering and Testing
 Leads optical test platform development for product-level
 reliability and failure mechanism testing
 Synthesis Team including Jason Tillman and Maria Bautista
 Characterization Team including Kethry Soares, Camille Peredo,
 and Nick Brockman

 Prof. David Kelley Photophysical Characterization
 Founding professor with expertise in ultrafast optical
 UC Merced

 spectroscopy
 Prof. Anne Myers Kelley Atomic-level Structural Characterization
 Founding professor with expertise in Raman spectroscopy
 Research Team including Dr. Anh Nguyen, Dr. Haochen Sun,
 and Paul Cavanaugh

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 3

Challenge
 Problem Definition:
 • Solid state lighting has greatly reduced energy consumption compared to older
 lighting technologies, but still lags the theoretical maximum for luminous efficacy.
 • Color-mixed direct LED emission offers the highest potential efficacy, but due to
 material challenges, this technology has not yet surpassed phosphor-converted LEDs.
 • The future of solid-state lighting technology requires precisely tunable emission
 spectra to improve light quality, while simultaneously increasing luminous efficiency.
 Impacts of Lighting Quality
 Color Perception Health and Productivity Safety

 Philips CRI comparison Zumtobel Ernst & Young whitepaper Cree outdoor lighting case study
 Proposed solution:
 • For phosphor-converted LEDs, developing a narrow, efficient, and tunable
 downconverter could produce high quality light at improved luminous efficacy with
 incredible flexibility across multiple lighting applications.
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 4

Approach
 Quantum dots (QDs)
 offer narrow, efficient,
 and tunable emission
 properties, but
 improvements in
 operational stability are
 required for use in LEDs.
 Nanosys QD examples DOE, EERE, 2019 Lighting R&D Opportunities

 • Red QDs (40nm) used in combination with a conventional green phosphor material can
 improve LED conversion efficiency by up to 15% (2700 K) over commercial pc-LEDs.
 K. T. Shimizu et al., Photonics Research, vol. 5, no. 2, pp. A1-A6, 2017.
 • Using conservative estimates, a mid-range improvement of 10% results in a payback of less
 than one year for incorporation of red QDs downconverters in an LED.

 • Existing commercial products harness the narrow emission linewidths of QDs to improve LED
 efficacy at high CRI (>90) but rely on cadmium-containing QDs and thus further efficacy gains
 are limited by restrictions on heavy metal content.

 Developing stable, cadmium-free quantum dot downconverters would enable
 greater gains in LED efficacy without compromising regulatory standards.
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 5

Approach: Project plan and technical approach
 Synthetic Control Confirmation of Correlation of QD
 of QD Structure QD Structure Photophysics and Performance
Synthesis Structural Characterization Best QDs Photophysical Characterization
Minimize Auger recombination Composition analysis by ICP, FTIR, Measure Auger dynamics using transient absorption
Control Auger excitation branching ratio NMR, XRD, XPS, TGA, TEM, STEM- spectroscopy and time-resolved photoluminescence
Improve excited state confinement. EDS, and Raman spectroscopy. spectroscopy of negatively charged QDs.
 Measure PL dynamics using time-resolved emission Identify
 and photoluminescence spectroscopy. Properties
 Control of QD Structure by Synthetic Modifications that Improve
 Performance Testing Performance
 Measure static QY at 0.0001 to 1 W/mm2 and
 temperatures of RT to 150°C.
 Measure QD power retention lifetime across same
 temperature and flux range as above.

 • Nanosys has demonstrated excellence in emissive materials development and has
 commercialized industry-leading InP-based QD optical downconverters for display
 applications exhibiting exceptional color tunability (PWL tunable to ±2 nm, FHWM 90% QY).
 • To address QD operational stability under high-temperature and high-flux operation,
 the project workplan is based on development cycles between synthesis, structural
 characterization, photophysical characterization, and performance testing to
 – 1) gauge current QD performance status,
 – 2) understand fundamental QD degradation mechanisms,
 – 3) control QD structure to improve performance.
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 6

Approach: Project risks
 • Risk 1: Fundamentally more difficult to improve Cd-free materials
 – Differences in the material and physical properties of heavy-metal free QDs
 including shallower band energies and core/shell materials with incompatible lattice
 constants may limit the long-term stability these materials.
 – Mitigation: Our initial project work on baseline measurements of thermal/flux rolloff
 and QD operational lifetime will benchmark the magnitude of challenge in
 increasing the stability of heavy-metal free QD downconverters.
 – Update as of 06/30/2021:
 • From our characterization of baseline performance and improvements made to QD
 structure and formulation in the first year, extrapolated T70 lifetime performance at 50°C
 and 50 mW/mm2 of our best QD examples is ~4,000 hours, an improvement of >50X
 from our starting baseline lifetime.
 • Risk 2: Adjusting to lighting application from displays
 – Quantum dot compatibility with on-chip manufacturing process
 – Mitigation: There is an existence proof of commercially viable Cd-based QD
 incorporation using existing LED downconverter deposition methods. While the
 same process may not be directly applicable to heavy-metal free QDs, our previous
 formulation experience for displays suggests that there can be an alternative
 solution for these materials.
 – Update as of 06/30/2021:
 • Not currently within project scope

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 7

Impact: Comparison to alternative technologies
 Technology comparison
 Direct Inorganic phosphor Cadmium-based
 electroluminescence downconverters Quantum Dots
 Pros:
 Low-cost material with good stability in
 high-flux excitation environments.
 Some recent examples of narrow, red Osram OSCONIQ® S 3030
 Pros: emission have been demonstrated.
 Theoretical maximum light production Cons: Pros:
 efficiency due to elimination of Opportunities remain to improve Demonstrated product readiness
 downconversion energy losses. operational stability under high-T/high- meeting required stability targets,
 Cons: flux conditions. tunable PWL, and narrow FWHM
 Color-mixed (CM)-LEDs require efficient There are no current solutions for Cons:
 emission across all emission wavelength tuning in narrow phosphor Regulations on total Cd concentration
 wavelengths; green and amber emitters emitters preventing use in green or in consumer products limits use and
 have poor demonstrated efficiency. amber downconversion applications. adoption

 Compared to competing technologies, heavy-metal free
 Quantum Dots offer significant benefit as emitter materials.
 • These benefits include 1) narrow emission wavelengths with conversion efficiencies of 90-
 100%, 2) highly tunable emission wavelengths can be mixed to maximize spectral
 efficiency, 3) demonstrated stability (>30,000 hours) under display conditions or
 environmental stress tests, and 4) elemental compositions not under regulatory restriction.

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 8

Impact: Beyond red downconversion

 Future lighting technologies require narrow, tunable
 emission with high efficiency across the visible spectrum

 DOE, EERE, 2019 Lighting R&D Opportunities

 • Successful completion of this project would enable a broad class of narrow, tunable,
 downconverters which could be deployed without regulatory restrictions.

 • A clear efficiency benefit has already been demonstrated with red-emitting Cd-based QD
 downconverters as evidenced by existing commercial LEDs in high CRI portfolios.

 • In addition to providing narrow, red emission, and improving LED efficacy at high CRI,
 improving the high-flux stability of heavy-metal free QDs can build into longer term DOE SSL
 goals including smart and human-centric lighting due to the inherent wavelength tunability
 of these materials.
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 9

Progress Highlights
 • Project is mid-stage with completion of 5 out of an anticipated 12 project quarters
 – Research planned under this project would lead to a technical feasibility demonstration of heavy-metal free
 QD downconverters for solid-state lighting
 • Project budget is on track with 50X improvement over starting materials

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 10

Progress: Benchmarking InP-based QDs under high-
 T/high-flux operating conditions
 • Current production-scale, InP-based red QD properties evaluated to determine
 baseline operating performance as a function of temperature and flux:

 - Photoemission scales linearly with flux, indicating that QY is preserved up to
 100 mW/mm2

 - Photoemission shows small drop up to 150°C, indicating that QY is mostly
 preserved up to 150°C, drop recovers when sample returns to 25°C

 Photoemission versus Incident Flux Photoemission versus Temperature
 0.05 105%
 Integrated Emission (a.u.)

 Relative Integrated
 0.04 100%

 Emission
 0.03 95% Initial QY = 90%,
 99% QY retention to
 0.02 90% 80°C and 98% QY
 0.01 85% retention to 150°C
 excited at 10 mW/mm2.
 0.00 80%
 0 50 100 150 0 100 200
 Flux (mW/mm2) Temperature (°C)

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 11

Progress: QD stability dependence on excitation flux

 • The non-linear dependence of degradation rate (T90) on flux implicates biexcitons in
 degradation mechanism

 • Biexcitons represent up to ~1-7% of all excited states at 100-1000 mW/mm2
 excitation flux at 450 nm in red-emitting InP/ZnSe/ZnS QDs

 ∝ 2

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 12

Progress: Improve InP/ZnSe/ZnS QD high-flux stability
 via graded interfaces
 • Core/shell structure modifications in red-emitting InP/ZnSe/ZnS QDs resulted in
 >10X improvements in flux stability at 50°C and 50mW/mm2 operating conditions
 compared to our baseline test materials

 - A family of synthetic modifications represented by Structure A yielded a 3X
 improvement in flux stability made by decreasing the biexciton formation rate.

 - A series of core/shell structures represented by Structure B yielded a 10X
 improvement in flux stability by modifying interfaces via etching and alloying.

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 13

Progress: Developed model of temporary hole trapping
 on non-stoichiometric In sites in InP/ZnSe/ZnS QDs
 e-
 4.0 e-
 ZnSe
 4.5
 Energy versus vacuum (eV)

 InP

 5.0
 hν -hν
 5.5

 6.0 In In h+ In

 h+ slow
 6.5 x

 h+ fast
 7.0

 Extremely Slow Trap-Mediated Hole Relaxation in Room-Temperature InP/ZnSe/ZnS Quantum Dots,
 Anh T. Nguyen, Ilan Jen-La Plante, Christian Ippen, Ruiqing Ma, and David F. Kelley,
 The Journal of Physical Chemistry C, 2021, 125, 7, 4110-4118.

 • InP/ZnSe/ZnS QDs exhibit slow photoluminescence (PL) risetimes on the order of
 50-500 ps; this is a phenomenon not previously observed in CdSe/CdS/ZnS QDs.
 • We have developed a model to explain this behavior based on temporary hole
 trapping on non-stoichiometric In sites; the concentration and location of these
 dopant sites has been correlated with elemental analysis and Raman spectroscopy
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 14

Progress: Auger dynamics as a function of non-
 stoichiometric In content in InP/ZnSe/ZnS QDs
 • Hole trapping on In sites results in a biexciton state that behaves similarly to a negative
 trion resulting in an increased Auger electron excitation fraction

 • Assuming observed biexciton times are a weighted average of the XX lifetime (80 ps)
 and negative trion (XT) lifetime (280-425 ps), we can extract electron excitation fraction
 from observed biexciton times using:
 + 2 
 = =
 + + − −

 Auger Dynamics in InP/ZnSe/ZnS Quantum Dots Having Pure and Doped Shells
 Anh T. Nguyen, Paul Cavanaugh, Ilan Jen-La Plante, Christian Ippen, Ruiqing Ma, and David F. Kelley
 J. Phys. Chem. C 2021, 125, 28, 15405–15414
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 15

Progress: Correlation between Auger electron excitation
 fraction and InP/ZnSe/ZnS high-flux stability
 Auger electron excitation fraction Current best performing materials,
 as a function of PL risetime >50X improvement from project start
 0.90
 0.85
 100%

 Power retention (%)
 0.80
 fraction electron excitation

 0.75 80%
 0.70 λem = 631 nm
 0.65 60% FWHM = 37 nm
 0.60 r = 0.95
 40% PLQY = 90%
 0.55
 T70 = 4,000 hours*
 0.50
 0.45
 20% *extrapolated from test conditions
 at 450 nm, 50°C, 50 mW/mm2
 0.40 0%
 0.35
 0 200 400 600 800
 0 10 20 30 40 50 60
 fraction * risetime / ps Time (hours)
 • Across the range of non-stoichiometric In studied (In:P 1.0-1.6), the Auger electron excitation
 fraction more than doubles from 0.40 to 0.85.
 • We observe a correlation between higher Auger electron excitation fractions and increased
 operational lifetime under high flux indicating that Auger excited electrons are less damaging
 to long term stability than Auger excited holes.
 • These findings have led to our longest lifetime samples to date representing a >50X
 improvement in T70 since the project start
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 16

Stakeholder Engagement
 • Nanosys, Inc. has deep synthetic expertise in the development and scale-up of heavy-
 metal free QD materials and is the leading supplier of QD emissive materials for the
 display industry. The current research team represents all aspects of QD
 downconverter development including synthesis, structural characterization, and
 optical testing under conditions relevant to deployment in solid-state lighting
 applications.
 – Engagement: Weekly internal staff meetings including data presentation and
 discussion between QD materials development, optical test development and
 results, and failure mechanism model refinement.
 • The UC Merced team (including Profs. David Kelley and Anne Myers Kelley and their
 academic trainees) represents combined expertise in semiconductor photophysical
 and structural characterization.
 – Engagement: Biweekly technical meetings between Nanosys and UC Merced teams
 including data presentations and discussion; physical shipments of QD samples
 from Nanosys to UC Merced for characterization occurs multiple times per quarter
 – Collaboration has enabled publication of 3 peer-reviewed journal articles since the
 project start date (04/01/2020)

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 17

Remaining Project Work
 Project Year 2 Project Year 3
 Activity Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
 SOPO Task 3.0 Improve excited state confinement - 1
 High purity, controlled growth ZnS shell
 Wide bandgap metal oxides as outermost shell
 Wide bandgap metal fluorides as outermost shell
 Go/No-Go Decision 2 (red InP with >88% QY when measured at 100°C and 100 mW/mm2) ◊
 SOPO Task 4.0 Improve excited state confinement - 2
 Evaluation of inorganic ligands to increase confinement and stability
 Evaluation of matrix-reactive ligand systems with covalent bonding
 Final project report to DOE ◊

 • In the upcoming Project quarters, we plan to investigate the degree to which the
 excited state can be confined within the QD thereby limiting the damaging effects of
 QD and ligand oxidation following Auger charge ionization (see chart above).
 • The overall project approach of synthesis, structural + photophysical
 characterization, and performance evaluation will continue to achieve the final
 project targets of heavy-metal-free QDs capable of ≥88% QY at ≥1W/mm2 and
 150°C; 30 nm FWHM; Δu′v′ 6000hrs.
 • Once technical feasibility of a heavy-metal free QD downconverter is demonstrated,
 we plan to partner with an LED manufacturing company to optimize and fully qualify
 a QD product that can be integrated into an LED package with a time estimate to
 market availability of enhanced efficiency LEDs of approximately two years.
 •
U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 18

Thank You

 Nanosys, Inc.; University of California, Merced
 Ilan Jen-La Plante, Staff Scientist
 (408) 240-6734, ijenlaplante@nanosysinc.com

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 19

REFERENCE SLIDES

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 20

Project Budget
 Project Budget: $2,000,000 federal share; $876,198 non-federal cost share
 Variances: Delays in hiring due to statewide public health guidelines lowered
 project expenditures in FY2020, we have increased optical test capacity and
 project personnel staffing in FY 2021 and current budget variance is

Project Plan and Schedule
 Project Start: 04/01/2020 Project End: 03/31/2023

U.S. DEPARTMENT OF ENERGY OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY 22