Quantum Dots: Next Generation Downconverters for High Efficiency and High Color Quality SSL

Jonathan S. Owen, Department of Chemistry, Columbia University, New York, NY

Quantum Dot Down Converters in Solid State Lighting

Critical Need: High Efficiency, Stable,

Narrow Red and Amber Down Converters

Solid State Lighting R&D Plan, 2015, U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solid State Lighting Program.

Slide Courtesy of Seth Coe-Sullivan

Photodegradation Mechanism I:

Multiexciton Mediated Photoionization

Bawendi, Chem. Soc. Rev., 2014, 43, 1287

Auger Recombination

Reduces PLQY

Leads to Photoionization

Carriers trap at mid-gap electronic states derived from surface atoms.

Ligands “passivate” these traps by removing them from the optical gap.

Photoluminescence quantum yield very sensitive to surface structure.

Photodegradation Mechanism II:

Surface “Trap State” Mediates Photoionization

Trap Mediated Photoluminescence Blinking

Owen and Sfeir, J. Phys. Chem. C, 2015, 119(49), 27797.

Trap Mediated Photoluminescence Blinking

Long lived “OFF” states arise from rapid Auger recombination

Bawendi, Chem. Soc. Rev., 2014, 43, 1287

Nanocrystal ionization and internal charge reorganization / surface

trapping can both lead to long-lived “OFF” states.

Ionized Nanocrystals Undergo

Fast, Nonradiative Auger Decay

Klimov, ACS Nano, 2014, 8(7), 7288.

Each type of multiexciton has its own decay dynamics and PLQY.

Core-Shell Nanocrystals Isolate Excitons From Surfaces

Energy level landscape localizes excited electron and hole within the crystal core

Photoluminescence quantum yields remain sensitive to surface chemistry

Auger recombination dominates PLQY at high flux ”on chip”

Graded Alloy QDs with Suppressed Auger Recombination

- Alloy leads to increased FWHM.

- Lengthy stepwise synthesis

difficult to optimize.

- Atomic structure difficult to

determine and optimize.

Dubertret, Nano Lett., 2015, 15(6), 3953.

- Large, graded CdSe/CdS

interface reduces Auger

recombination.

- Reduced Auger rate gives

100% PLQY of Biexciton,

Non-Blinking fraction of QDs.

Opportunities in The Synthesis of Core Shell QDs

• difficult to control precursor reactivity leads to irreproducibility.

• linear sequence compounds irreproducibility

• managed with engineering controls, little knowledge of

underpinning chemical reactions.

• Nanosys: “25 tons of QD material annually” http://www.nbclearn.com/nanotechnology

Tunable QD precusors crystallization kinetics

controlled by

Owen and Alivisatos, JACS, 2007. Owen and Alivisatos, JACS, 2010. Owen, et al. Science, 2015.

N N

EH

PhPh

HN N

EBu

BuBu

HNH

N

EH

Bu

X

X = MeO, Me, H, Cl, F, CN

FASTERSLOWER

N NR R

E

Fine Control Over QD Se/S Alloys with Precursor Library

Graded Alloykselenium > ksulfur

OR

Single Step Synthesis of

Graded Alloy QDs

Homogeneous Alloykselenium = ksulfur

[CdSe]i [CdS]i+

N N

S

R4

R3

R1

N NR4

R3

R2

R2

R1

Se

+

CdX2

CdX2

+

kselenium

ksulfur

[CdSe]i [CdS]i+

O

O

OOO

OR

O

R

O

O

R

O R R

O

R

O

O OR

O

R

O

L + + Ln–Cd(O2CR)2

L

Keq

Prevailing model of Quantum

Dot surface chemistry has

changed dramatically.

Metal ion coverage crucial to

blinking statistics and PLQY.

Surfaces and trapping remains

largely a mystery.

Surface Metal Ions Prove Labile

Owen et al., JACS, 2013.

Owen et al., Science, 2015.

Owen et al., J. Phys. Chem. C, 2015.

–X

MX2

+[HB]

L-typeneutral donor

X-type chemisorbed ion pair

Z-typeneutral acceptor

X-type terminates metal enriched QD

XX

L

M = Cd, Pb., E = S, Se

X = O2CR, Cl, SR, etc.

L = PR3, NH2R, etc.

MX2 = Cd(O2CR)2, CdCl2, Pb(SCN)2, etc.

[X]-[HB]+ = [Cl]-[HPBu3]+, [S]2-2[H4N]+,

M E

ME

QDs with improved stability under high flux are being discovered.

• Increasing shell and core dimensions and graded alloy interfaces

reduce Auger recombination kinetics, improve flux stability.

• Understanding of synthesis and reagents have improved, allowing

performance to be optimized by design rather than by an Edissonian

approach.

• Cd-free QDs (InP, CuInS) lag far behind Cd-QDs, (FWHM > 40 nm,

PLQY < 90%). Greater focus on fundamental understanding of

synthesis is needed to gain fine control.

Long-Term Environmental Stability by Encapsulation.

• Surface passivation remains largely a mystery. Fundamental studies

of surface ligand interactions needed as well as R&D on new surface

passivation strategies.

• Ultra-barrier layer surface coatings need to be developed.

Opportunity Areas for QD Down-Converters