Design of a high-power white light source with colloidal ... · Design of a high-power white light...

Design of a high-power white light source with colloidal

quantum dots and non-rare-earth phosphors

by

Kristopher T. Bicanic

A thesis submitted in conformity with the requirements for the degree of

Master of Applied Science

Edward S. Rogers Sr. Department of Electrical and Computer Engineering,

University of Toronto

Toronto, Ontario

Canada

Copyright © 2017 by Kristopher T. Bicanic

Kristopher Bicanic University of Toronto Page ii

Abstract

Design of a high-power white light source with colloidal quantum dots and non-rare-earth phosphors

Kristopher T. Bicanic

Master of Applied Science

Graduate Department of Electrical and Computer Engineering

University of Toronto

2017

This thesis describes the design process of a high-power white light source, using novel

phosphor and colloidal quantum dot materials. To incorporate multiple light emitters, we

generalized and extended a down-converting layer model. We employed a phosphor

mixture comprising of YAG:Ce and K2TiF6:Mn4+ powders to illustrate the effectiveness of

the model. By incorporating experimental photophysical results from the phosphors and

colloidal quantum dots, we modeled our system and chose the design suitable for high-

power applications. We report a reduction in the correlated color temperature by ~600K

for phosphor and quantum dot systems, enabling the creation of a warm white light

emission at power densities up to 5 kW/cm2. Furthermore, at this high-power, their

emission achieves the digital cinema initiative (DCI) requirements with a luminescence

efficacy improvement up to 32% over the stand-alone ceramic YAG:Ce phosphor.

Kristopher Bicanic University of Toronto Page iii

Content

Abstract .......................................................................................................................................................................................... ii

Acronyms ....................................................................................................................................................................................... v

List of Tables .............................................................................................................................................................................. vi

List of Figures ........................................................................................................................................................................... vii

Chapter 1: INTRODUCTION

1.1. Solid state light sources .......................................................................................... 1

1.2. Phosphors and colloidal quantum dots for display technology ....................... 4

1.3. Device requirements .............................................................................................. 6

1.4. Organization of thesis ............................................................................................ 8

Chapter 2: BACKGROUND

2.1. Introduction ......................................................................................................... 10

2.2. Light source fundamentals ................................................................................. 10

2.2.1. Figure of merit ........................................................................................ 14

2.3. Red phosphor materials and properties ........................................................... 15

2.4. Multilayer phosphors .......................................................................................... 17

2.5. Conclusions .......................................................................................................... 18

Chapter 3: METHODOLOGY

3.1. Introduction ......................................................................................................... 20

3.2. Photoluminescence quantum efficiency ........................................................... 20

3.3. Power conversion efficiency and photoluminescence .................................... 21

3.4. Photoluminescence lifetime ............................................................................... 23

Chapter 4: MULTI-LAYERED PHOSPHOR DESIGN AND MODELLING

CONSIDERATIONS

4.1. Introduction ......................................................................................................... 25

4.2. Design considerations ......................................................................................... 25

4.3. Optical characterization of phosphor material ................................................ 27

4.3.1. Photoluminescence lifetime ................................................................... 27

4.3.2. Power conversion efficiency ................................................................... 29

4.4. Multilayered phosphor modeling ...................................................................... 30

4.5. Conclusions .......................................................................................................... 33

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Chapter 5: FABRICATION OF MULTILAYER STRUCTURE AND PHOSPHORS

5.1. Introduction ......................................................................................................... 34

5.2. Manganese based phosphors ............................................................................. 35

5.2.1. Synthesis ................................................................................................. 35

5.2.2. Physical and optical characterization ................................................... 36

5.3. Colloidal quantum dots in matrix ..................................................................... 39

5.3.1. Synthesis ................................................................................................. 40

5.3.2. Optical characterization ........................................................................ 41

5.4. Layer fabrication .................................................................................................. 42

5.4.1. Solution-processed phosphor ................................................................. 42

5.4.2. Solution-processed quantum dots ......................................................... 43

5.4.3. Solid state film processing ...................................................................... 45

5.5. Conclusions .......................................................................................................... 45

Chapter 6: CHARACTERIZATION AND PERFORMANCE OF PHOSPHOR

DEVICES

6.1. Introduction ......................................................................................................... 47

6.2. Device performance ............................................................................................ 48

6.2.1. Correlated color temperature and luminous efficacy ........................... 50

6.3. Device application in display technology ......................................................... 53

6.3.1. Overview ................................................................................................ 53

6.3.2. Design consideration and color space .................................................... 54

6.4. Conclusions .......................................................................................................... 55

Chapter 7: CONCLUSIONS AND FUTURE WORK

7.1. Summary .............................................................................................................. 57

7.2. Original contributions ........................................................................................ 58

7.3. Future work .......................................................................................................... 58

APPENDIX A

BIBLIOGRAPHY

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Acronyms

CCT Correlated color temperature

CdSe Cadmium selenide

CdS Cadmium sulfide

CQD Colloidal quantum dot

Cs Caesium

DCI Digital cinema initiative

DMM Digital micro-mirror

Eu Europium

FWHM Full-width at half maximum

Mn4+ Magnesium ion

PCE Power conversion efficiency

PL Photoluminescence

PLQE Photoluminescence quantum efficiency

YAG:Ce Y3Al5O12:Ce3+ phosphor

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List of Tables

Table 1 | Model results for K2TiF6:Mn4+ and YAG:Ce phosphor structures. Excitation power

converted to green and red emission shown. Desired color corrected power ratio of 55%

green to 45% red emission. ........................................................................................................ 33

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List of Figures

Figure 1.1 | Design structures for solid state LED-based light sources[4] ............................. 2

Figure 1.2 | State of the art YAG:Ce phosphor emission profiles and methods of color

correction. a) CIE color coordinates of YAG:Ce compared to standard black body radiation

sources (black line). b) and c) Spectrally balanced emissions by filtering and spectral re-

engineering with a secondary down-converter, respectively. ................................................. 3

Figure 2.1 | (a) CIE 1931 XYZ standard observer color matching functions 𝑥(𝜆), 𝑦(𝜆), and

𝑧(𝜆)[19]. (b) CIE 1931 color space chromaticity plot and Planckian locus (shown in black)

[19]. ............................................................................................................................................... 12

Figure 2.2 | Display on chromaticity plot of the digital cinema initiative gamut range

requirements and locations of RGB sources and DCI white point[20]. .............................. 14

Figure 3.1 | High-power laser diode system with up to 5 kW/cm2 excitation. .................... 23

Figure 4.1 | a) Measured photoluminescence lifetime characterization of K2TiF6:Mn4+

phosphors with varying power densities. b) photoluminescence lifetime with varying

temperature environment. ......................................................................................................... 28

Figure 4.2 | a) Measured power conversion efficiency of K2TiF6:Mn4+ phosphor at varying

temperature. b) Measured emission spectra of K2TiF6:Mn4+ at varying temperatures with a

power density of 5kW/cm2. ....................................................................................................... 29

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Figure 4.3 | Phosphor 1D model setup for test structures used to calculate red and yellow

emission. ...................................................................................................................................... 32

Figure 5.1 | Red phosphor physical characterization a) XRD data for K2TiF6 and

K2TiF6:Mn4+ phosphor. b) SEM image of K2TiF6:Mn4+ phosphor powder. c) Elemental line

scanning from a typical Mn4+ doped K2TiF6 microparticle and SEM of analyzed cross

section. .......................................................................................................................................... 37

Figure 5.2 | SEM images of cross-section of K2TiF6:Mn4+ microparticles for K, Ti, F, and

Mn elemental mappings in the same selected areas. .............................................................. 38

Figure 5.3 | EDS spectrum of the microparticle displays the presence of K, Ti, F, and Mn

elements. ...................................................................................................................................... 38

Figure 5.4 | Absorption and PL emission spectra of K2TiF6:Mn4+ phosphors. Inset shows

the photographs of the K2TiF6:Mn4+ sample under UV lamp illumination ........................ 39

Figure 5.5 | Absorption and PL emission of CQDs in a silica matrix. ................................. 42

Figure 5.6 | a) Power conversion efficiency measurement showing quantum dot

degradation. b) Operation lifetime of quantum dots in optically transparent adhesive with

YAG:Ce capping layer. ............................................................................................................... 44

Figure 6.1 | Laser setup for excitation of phosphor materials and incorporation of

adjustable blue diode source for white light source. ............................................................... 48

Figure 6.2 | CQD 4-hour stability test at 5 kW/cm2. .............................................................. 50

Figure 6.3 | Spectra of the YAG:Ce compared to the QD and K2TiF6:Mn4+ structures. ..... 52

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Figure 6.4 | CIE color coordinates of the samples at high flux excitation (left) and

corresponding CCT of samples at all power densities, with Duv<0.02 (right). .................. 52

Figure 6.5 | Corresponding luminous efficacy of best Mn4+ and CQD samples at all power

densities. ....................................................................................................................................... 53

Figure 6.6 | Wavelength separation of K2TiF6:Mn4+ phosphor and YAG:Ce separated into

RGB sources................................................................................................................................. 55

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Introduction

1.1. Solid state light sources

Since the late 2000s, traditional incandescent and fluorescent light sources have been

increasingly supplanted by solid-state white light technology, which offers superior power

efficiencies, low energy consumption, and long operation lifetime[1, 2]. These benefits have

led to applications in a wide range of areas, from low-power spot illuminators to high-

power area illuminators [3]. For example, high-power illuminators can be used as light

sources for projection. For decades Xenon light bulbs have been used in this industry.

However, these sources fail to provide the brightness and color depth needed. The

unacceptably high energy cost also results in significant heat buildup in the system, which

requires further energy to cool sufficiently.

The requirements for white light emitting diodes (LEDs) are red, green, and blue

components. Ideally these will allow full and true rendering of an object's color and a full

gamut range for displays. In a first approach to combining color, one combines efficient

red, green, and blue LEDs into a single package; however, this approach results in distorted


color and incurs high-cost requirements to fabricate three LEDs[2, 4]. Alternatively, the

more widely practiced method incorporates down-converting phosphor materials, which

converts the output of a single source to lower-energy light. This is ultimately more cost-

efficient than fabricating a multi-LED system. This can be accomplished through a source

that photoexcites three down-converters; alternatively a blue light source can be used,

where a portion of its light is down-converted to red and green[4].

Figure 1.1 | Design structures for solid state LED-based light sources[4]

For applications involving high-power densities (e.g. projection), the choice of

phosphor is critical, as unfavorable photophysical properties can lead to precipitous drops

in efficiency with increased power. For high-power systems of interest, many factors can


cause a degradation in the phosphor material s emission. Once such high performing

phosphor, yellow-based Y3Al5O12:Ce3+ (YAG:Ce), can operate efficiently at high radiances

due to its high thermal conductivity and short PL lifetime. However, the

spectrum prevents it from replicating a lower correlated color temperature (CCT) [1, 5-

11]. The phosphor spectrum can be corrected using two approaches: the first method filters

out a portion of the green emission as waste (Figure 1.2b), whereas the preferred method

introduces a secondary down-converter to increase the red emission, minimizing waste

energy (Figure 1.2c) [12-15] .

Figure 1.2 | State of the art YAG:Ce phosphor emission profiles and

methods of color correction. a) CIE color coordinates of YAG:Ce compared

to standard black body radiation sources (black line). b) and c) Spectrally

balanced emissions by filtering and spectral re-engineering with a

secondary down-converter, respectively.


1.2. Phosphors and colloidal quantum dots for display

technology

Today, down-converting light-emitting materials are extensively used for television

and portable electronic backlights [16]. The main down-converting materials used are

atomic emitting phosphors, organic molecules, and colloidal quantum dots. For the

creation of a low-cost light source, solution-processed methods are desired due to the

simplicity of production and large area fabrication. The current industry standard for

down-converting lighting is made from transition and rare earth metal activated

phosphors. This is in part due to their robustness and well-established emission properties.

In this thesis, I focus instead on the use of non-rare earth metal phosphors and colloidal

quantum dot materials. I investigate them in light of their potential low-cost synthesis and

the advantageous properties including tunable emission, high color purity for a large

gamut, and high photoluminescence quantum efficiency (PLQE).

Phosphors are materials which exhibit luminescence when excited with an optical

source. Within this work, atomic emitting phosphors were used for down-conversion

sources. These phosphor materials typically consist of two parts: a host material and a

dopant species to act as an activation source. The host material forms the majority of the

phosphor and provides a matrix into which the dopant is implanted. The host material

possesses a level of influence which impacts the emission of the activation atom based on


electronegativity. The atomic centers of a phosphor are the dopant atoms embedded in the

host lattice. The dopant atoms dictate the emission properties of the phosphor material.

These atomic centers are the locations where electronic transitions occur, and these allow

for the down-conversion of incident radiation. Rare earth metals, as well as transition

metals, are typical materials which act as atomic emitters. The optical properties of these

materials are determined by the identity of the emitting center, and the tunability is limited

to variations in the host matrix composition and the concentration of the dopant used. For

red emitting centers, only a limited number of phosphors are known, each with their own

limitations, such as broad emission peaks and phosphorescent emission leading to power

saturation.

Colloidal quantum dots, in contrast with rare earth metal phosphors, are significantly

larger. They are an ensemble of many atoms, which form particles several nanometers in

diameter. These particles are of particular interest due to the unique and highly engineered

optical properties they possess. Such properties of interest are the fully tunable visible

spectrum emission, high color purity, and high photoluminescence quantum efficiency

(PLQE)[17]. These properties of the quantum dots are associated with their unique physics

resulting from their size and shape. The small size of quantum dots provides a level of

confinement for the electron and the hole present in the material. The electron and hole in

a nanocrystal are confined to a small region of space below the Bohr radius. This provides

tunable control over the excitation energy in the material that, by extension, creates a


tunable bandgap that can vary by more than 1 eV [18]. Currently, the applications in which

colloidal quantum dots have been deployed have been limited to low intensity systems, due

to instabilities and non-radiative effects. In part this is due to higher radiances that create

multi-excitons in the quantum dots, increasing the rate of Auger recombination a non-

radiative process that reduces efficiency. This process further plagues high-intensity

systems, due to the resulting generated heat. The interaction in this nano-system is more

pronounced due to the confinement on the excitons[18]. Therefore, by engineering these

materials for optimal performance at high-power densities, one can design a device

structure to take advantage of the desirable properties and provide superior performance

at these higher power levels for applications such as projection.

1.3. Device requirements

In this thesis, I investigate the design and fabrication of high-power light sources. My

goal is to provide superior color quality and efficiency. The design of a warmer white light

source necessitates a thermally conductive red emitting material that meets the high

performance requirements. The red emitter needs to satisfy the following design

requirement first: it must be stable under high illuminant fluxes, which can reach up to 5

kW/cm2. This requirement ensures that the power regime in which the emitter operates

can provide the luminous flux required for projection while maintaining high

performance. The second requirement is device stability at elevated temperatures. The


thermal requirement ties in with the high flux requirement; if the solution-processed

material is not thermally conductive, it will result in early failure. This in turn ties into the

thickness, as being thin ensures that sufficient cooling can be achieved throughout

the entire device. A limit of 250 µm was implemented in the present work in light of the

limitations of the cooling system available. Additionally, the creation of a single layer of

film is ideal to ensure that a minimal number of thermally resistive interfaces are

maintained.

With the goal to provide a white light source for projection, the red emitter needs to

provide a significant benefit over the current state-of-the-art methods. This is achieved by

providing higher color quality that meets current standards, such as the digital cinema

initiative (DCI). Incorporating red emitting sources can accomplish this, by providing

sufficient red to create an equivalent power relative to the green emission power. This

results in the full utilization of power. Taking into account the gamut of the light source, a

separation of the emission is required to provide a separate and distinguishable red and

green light source. This separation of red and green emission is needed to allow for

sufficient isolation of high quality red and green sources.

In summary, for the design of this high-power light source, the following requirements

need to be met for a superior alternative:

i. High-power density of sustained 5 kW/cm2.


ii. Conduct heat effectively away from the emitter.

iii. Maintain a thin device profile below 250 µm.

iv. Correct color balance between red and green emission for high-quality emission

sources which satisfy DCI requirements.

v. Separate the emissions of green and red light.

1.4. Organization of thesis

This thesis is separated into seven chapters to describe the process of designing,

modelling, fabricating, and characterizing red emitting materials for the creation of a high-

power white light source. Within Chapter 2, a literature review is presented with a

description of current state-of-the-art red emitting phosphors with their benefits and

drawbacks. Additionally, a background into characterizing the device performance will

also be discussed and the display standards will be explained. A clear understanding of the

standards used will elucidate the reasoning behind certain requirements described in the

previous section. Chapter 3 details the experimental methods for the photophysical

measurements of the phosphor materials investigated. These characterization methods are

essential in characterizing the raw material as well as characterizing the final device for the

implementation as a white light source. The experimental methods described pertain to the

conversion efficacy of pump lights, raw power extracted from a simulated projector setup,

and emission collection. Within Chapter 4, the photo-physical measurements described in


Chapter 3 are used to determine the fundamental properties of the chosen red manganese

phosphor and quantum dot films. Using these properties various phosphor structures were

modelled in a 1-dimensional calculation to obtain the highest performing structure.

Chapter 5 utilizes the device models described in Chapter 4 and investigates the various

methods of film fabrication capable of generating a low cost, thin, high performance film.

In addition, selected materials are optically and physically characterized. Finally, Chapter

6 provides details into the final device characterization, post fabrication, and the potential

of the device to be a candidate for solid-state lighting, as well as digital projection. Using

the figures of merit described in Chapter 2, the designed devices will be compared to state-

of-the-art ceramic phosphor materials.


Background

2.1. Introduction

This chapter contains a review of the background information necessary to understand

the concepts described in this thesis. The first half of this chapter discusses the parameters

required for high-quality digital projection, with metrics used to quantify them and their

physical meaning. The second section will discuss the current state-of-the-art red sources

that are being considered for use in a white light source. In this section, the benefits and

drawbacks will be discussed. Finally, the last section of this chapter will detail current

literature on the modeling of multi-phosphor systems.

2.2. Light source fundamentals

The operating principle of phosphor materials, for light sources, is one that utilizes light

down-conversion. This process takes a photon of a given energy and re-emits it at a lower

energy relative to the material s bandgap and electronic states. In the design of a system,


the color produced is important and can be quantified in various ways. The most common

standard method for quantifying color relates an emission spectrum to a position in a color

space, such as the CIE 1931 XYZ color space. The CIE color space is a representation of

matching a color perceivable by the human eye to an emission spectrum. This is done using

three color matching functions �̅�(𝜆), �̅�(𝜆), 𝑧̅(𝜆) (Figure 2.1a). The color matching

functions correspond with a mathematical interpretation to represent stimuli to the human

eye [19]. This system was designed initially in 1931 to give physical meaning to the color of

plot of 1931 color space is shown below (Figure 2.1b). A few notable features are present in

this depiction. The first is that any (x) and (y) location can be reached and depicts a given

color on the plot. A pure monochromatic source would produce a color on the curved edge

of the plot, which is known as the locus of spectral colors. The colors within this curved

surface require a mixture of multiple wavelengths to accurately reproduce the closer the

color is to the center the less saturated it is. For this reason,

This idea of white can vary based on the criteria chosen.


Figure 2.1 | (a) CIE 1931 XYZ standard observer color matching functions

�̅�(𝜆), �̅�(𝜆), and 𝑧̅(𝜆)[19]. (b) CIE 1931 color space chromaticity plot and

Planckian locus (shown in black) [19].

The black curve in the CIE 1931 color space chromaticity plot (Figure 2.1b) is known

as a Planckian locus and is a representation of the colors that a blackbody radiator will emit.

These colors are identified as a white light of a given temperature, relative to the blackbody

this is the concept of a color temperature. As the Planckian locus

extends into the redder regions, the color temperature becomes colder as the blackbody

radiation is at a lower temperature, compared to a blue flame. Such temperatures can be

the following: 1000 K a yellow flame, 2740 K a 40 W gas light bulb, 6000 K is midday light

during a clear day [19].

The color space representation of color offers a simple model to visualize the gamut

provided with multiple sources. The representation of color is gaining importance,

specifically due to display technologies, where only a small number of sources are made


and they are required to reproduce a large range of colors. Based on the media, various

requirements of a gamut range need to be achieved. For the given application of our high-

power emitter, displays and projection are the main goals. For these display technologies,

the Digital Cinemas Initiative (DCI) requirements need to be achieved (Figure 2.2) [20].

This corresponds to a red, green, and blue source, as well as a balance in their power to

eliminate waste power for the DCI white point.

The current industry standard, briefly described in Chapter 1, is YAG:Ce, which has a

broad emission spectrum from 500 nm to 700 nm. The broadness of this emission spectrum

is advantageous for creating a white light source. However, a heavier weight is on the green

emission if used for display applications, this would create a plethora of powerful green

emission when compared to red. This results in a significant portion of light, upward of

40%, that is converted into waste energy to meet the DCI requirements. As a result, we need

to design a system with an improved red emitter when compared to the YAG:Ce system.


Figure 2.2 | Display on chromaticity plot of the digital cinema initiative

gamut range requirements and locations of RGB sources and DCI white

point[20].

2.2.1. Figure of merit

Correlated color temperature

The Planckian locus depicts white light emission from a blackbody radiator, which is

typically used as a reference source to compare white light. Colors emitted from a source

close to these blackbody radiators do not indicate the temperature of the source. Rather,

isothermal lines can be drawn perpendicular to the locus, to give a range of colors which

are representative of this color temperature. The sources close to the Planckian locus can

mimic the color, which is known as a correlated color temperature (CCT). A specific color


temperature can be desired, which would give a method of tuning the CCT an advantage

over materials limited to a higher CCTs.

Luminous efficacy

flux 𝜙𝑒 to a luminous flux 𝜙𝑝. This can have two meanings for the radiant emission and

the efficiency of the source. This dictates how efficient the source is at turning an incident

power (energy of the photons) into a visually perceived emission (luminous flux). This is

partly affected by the non-uniform response of the human eye to color. The luminous

efficacy of radiant emission is the theoretical maximum performance of a source based on

the emission; in other words, it is the luminous flux of the spectrum relative to the energy

of the photons emitted. Luminous efficacy of the source, converting a power to light, takes

into account the efficiency loss when down-converting and is given by:

𝐿𝐸 =𝜙𝑝

𝜙𝑒∗ 𝐸𝑄𝐸 (1)

This considers the lumens emitted by the source for a given input power.

2.3. Red phosphor materials and properties

For the advancement of both solid-state lighting and display technologies, a major

focus lies on designing high performing red emitting light sources. These red light sources

are desired to create warmer white light solutions, as well as to improve the color quality of

display technologies. Hence, an abundance of research has gone into investigating the


potential use of red light emitters. Examples of red light emitters are Eu2+ and Mn4+

activated phosphors.

Numerous host materials have been examined for Eu2+ activated phosphors; some

examples that have undergone intensive studies are sulfides, nitrides, and oxynitrides [13-

15]. These materials all have some similarities in their synthesis conditions, which are

typically quite harsh, e.g. Eu2+- -SiAlON is typically prepared under 1900 °C, 10

atm, N2 pressure [12] and for M2Si5N8:Eu2+ temperature ranges from 1400-2000°C are used

[13]. In addition to these demanding synthesis conditions, the raw materials are expensive

and lack stability when exposed to humidity [13].

Furthermore, the Eu2+ based phosphors possess a less than ideal absorption spectrum,

which overlaps

which greatly reduces efficiency [13-15, 21]

emission spectra is their wide emission range. Typically, the full width at half maximum

(FWHM) of the emission peaks ranges from 50 nm to ~175nm[13-15].

On the other hand, potential exists for using a transition manganese dopant atom

(Mn4+) to create a red emitting phosphor. Unlike the Eu2+ doped phosphors, Mn4+ doped

red phosphors have the advantage that the synthesis process does not require expensive,

high-temperature equipment, allowing for easier preparation [22, 23]. For the host

4+-


and the emission wavelength [24-28]. Previous literature states that to attain the largest

blue shift possible, fluoride compounds should be adopted, making them an ideal host for

Mn4+ to produce a highly luminous emission. With the large blue shift, the Mn4+ exhibits

the most intense excitation band located at ∼460 nm and sharp red emission line peaks at

∼630 nm [29]. Unfortunately, Mn4+-doped phosphors always suffer from a compromise

between excellent emission properties and long lifetime, which is believed to induce

saturation problems when performed at high-power densities.

Recently, Zhu et al. successfully demonstrated K2TiF6:Mn4+ as a secondary down-

converter in conjunction with YAG:Ce; however, high flux illumination, necessary for

high-power applications such as projection, was not reported [22]. While K2TiF6Mn4+ has

many advantageous properties [22-28], it also possesses a long (~5 ms) excited state lifetime

[29]. This long excited state lifetime results in saturation at high-power, which can be

deleterious for efficiency if not properly compensated.

2.4. Multilayer phosphors

Phosphor down-converters have been extensively researched for their use in lighting

applications. In turn, the combination of multiple phosphors has been investigated,

including their performance based on the phosphor structure. The studies that have been

previously carried out present interesting insight into how packaging phosphors affect the

luminous efficacy. Literature has demonstrated that a separate multi-layered phosphor


structure provides the highest performance, using a calcium sulfide red phosphor and an

inorganic silicate yellow phosphor [30]. These results indicate that a heavy dependence

exists on the light extraction for their LED packaging. This highlights that grading the index

from high to low luminous flux up to 18% [30].

In addition to the light extraction enhancement, from the grading of indexes, other

various properties are shown to affect the optimal arrangement of phosphors. Such

properties include the phosphor materials used, due to their individual excitation and

emission spectra. This leads to a number of considerations in the design, including

reabsorption, device architecture, quantum efficiencies, and densities of the phosphor

materials [31].

The creation of high-quality white light sources can be accomplished with a multi-

phosphor system by designing an appropriate device architecture for high-power operation

[30]. However, a strong dependence is on the phosphor materials used, device architecture,

quantum efficiencies, and densities of the phosphor materials that need to be considered

in the design [31]. To realize a high-power source, both scattering and saturation also need

to be incorporated into the design protocol.

2.5. Conclusions

In summary, within this chapter a background on color science and light color was

provided and common red phosphors were investigated. Each red phosphor mentioned


had some complication for the inclusion in a phosphor mixture at high-power density. The

Mn4+ activated phosphor was a compelling candidate due to its favorable emission

spectrum with 10 nm FWHM, at a luminous wavelength (630 nm). This is in contrast to

Eu-activated phosphors with FWHMs ranging from 50 to 200 nm and a significant portion

of the emission >650 nm. Therefore, as a red emitter, Mn4+ would be a promising candidate

if the saturation effect can be overcome. In addition to this, previous literature showed

multi-phosphor systems have a plethora of parameters required to accurately describe the

emission characteristics. For application in high-power systems, additional parameters

need to be accounted for based on the materials selected, e.g. such as saturation and thermal

effects.


Methodology

3.1. Introduction

Within this chapter, the experimental techniques used to characterize the phosphor

devices will be discussed in detail. This section discusses the optical characterization that

was performed for analysis of the phosphor materials. Using this information, a model will

be created for designing a high-power emission source. In addition to this, performance

metrics of the device will be established.

3.2. Photoluminescence quantum efficiency

For the photoluminescence quantum efficiency (PLQE), an experimental method

adapted from literature was used to allow for higher accuracy measurements of films. The

equation for PLQE is given by:

𝜂𝑃𝐿𝑄𝑌 =𝑛𝑢𝑚𝑏𝑒𝑟 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑒𝑚𝑖𝑡𝑡𝑒𝑑

𝑛𝑢𝑚𝑏𝑒𝑟 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑(2)


For the measurement of films a difficulty in measuring the emission is due to

anisotropy, when compared to solutions [32]. To correct this, three measurements were

taken: the first was a blank of the laser itself, the second was the sample placed in an

integrating sphere and directly illuminated, and third was the sample placed in an

integrating sphere and not being directly illuminated [32]. These measurements are used

to extract the absorbed light with the absorption of scattered light within the film. This is

represented by the equations: 𝐿2 = 𝐿1(1 − 𝜇), 𝐿3 = 𝐿1(1 − 𝐴)(1 − 𝜇) [32] with L being

the integrated power of the laser collected and 𝜇 the absorption of scattered light. This gives

the equation for the absorption:

𝐴 = 1 −𝐿3

𝐿2 (3)

Using this information and the power of the emission, P, one can calculate the PLQE

of the films with the following equation[32]:

𝜂 =𝑃𝑐 − (1 − 𝐴)𝑃𝑏

𝐿𝑎𝐴 (4)

3.3. Power conversion efficiency and photoluminescence

The power conversion efficiency (PCE) measurement was performed to determine the

conversion efficiency of an excitation pump source to the emission of the sample; therefore,

the efficiency is given by the equation:


𝑃𝐶𝐸 =𝑃𝑒𝑚𝑖𝑡𝑡𝑒𝑑

𝑃𝑒𝑥𝑐𝑖𝑡𝑎𝑡𝑖𝑜𝑛=

𝐸𝑄𝐸 ∗ ℎ𝜈𝑒𝑚

ℎ𝜈𝑒𝑥

(5)

For the excitation source a high-power laser diode system was employed; the setup is

shown in Figure 3.1. A total of 16 diodes, with each capable of providing 2 W of power, in

two laser banks were combined in parallel. The emission of these diodes was collimated in

a lens system, combined, and focused to a 1 mm2 area. This pump area was confirmed

through a CCD and a 20-80 knife edge measurement. To circumvent critical thermal issues,

the pump was pulsed with a repetition rate of 60 Hz for pulse durations of 300 µs. This

setup provided a simulated environment of a phosphor projector source on a rotating ring.

This setup provided in excess of 5 kW/cm2 peak power density. Using a dichroic mirror,

the emission was separated from the excitation source and sent through a lens system; the

emission from the sample was then collected and focused onto a 30W UP55N thermal

power meter with an aperture of 55 mm. For the efficiency measurement, high temperature

environments were also investigated. To simulate these thermal effects, the sample was

mounted onto an aluminum block with a resistive heating element. This heater was

controlled with thermal couples behind the sample and a temperature controller for active

feedback.

For the purpose of obtaining spectral data from the device, the setup was manipulated

to allow for the collection of the emission spectrum. At the collection in front of the thermal

power meter, a white diffused sheet was used to reflect the emission. To ensure the


spectrometer was not saturated a neutral density filter was added. A 105 µm fiber was used

to collect the reflection from the white sheet and pass it to an Ocean Optics USB2000

spectrometer. This emission was used to characterize the high-power emission.

Figure 3.1 | High-power laser diode system with up to 5 kW/cm2 excitation.

3.4. Photoluminescence lifetime

Due to the exceedingly long lifetime of the manganese activated phosphors, the laser

diode system described above was adapted for photoluminescence lifetime. The collection

of the lens system was instead focused onto a high-speed photodetector. This

photodetector was a silicon based DET10A photodiode, with an aperture of 0.8 mm2 and a

rise time of 1 ns. This detector provided sufficient resolution when investigating long,

Dichroic Mirror

Sample Stage

High Power thermal

power meter

Fiber coupler for

Spectrometer

16 diodes, 𝜆 = 442 nm

and 2 W max power

each


millisecond regime lifetimes. The use of the 300 µs pulse at 60 Hz was sufficiently small

enough to enable the excitation to fully relax between pulses.

For better resolution, a Horiba Fluorolog TCSPC system with an iHR 320

monochromator and a PPO·900 detector was used to characterize our quantum dot

samples. However, the emission lifetime of these dots was extremely fast when compared

to the phosphors; for that reason, it was not a concern for allowing saturation effects from

occurring. In this case, other non-radiative effects that are power dependent were of

concern, e.g. Auger recombination


Multi-layered phosphor design and

modelling considerations

4.1. Introduction

Within this section, the red emitter s performance was evaluated with elevated

power and temperature conditions. These performance metrics were then investigated in

a one-dimensional model to determine the optimal device structure to obtain the highest

performance, while meeting the requirements for the device set out in Chapter 1.

4.2. Design considerations

As previously mentioned in Chapter 1, the white light source that is being created needs

to provide a stable and pure red emission. As a result, the phosphor down-converter should

be capable of high-power illumination at a peak power density of 5 kW/cm2, have a red

color purity which reaches the digital cinema initiative (DCI), and maintain a thin device

profile for integration into a projector system. With these requirements, the red phosphor


material will be integrated with the industry standard (YAG:Ce) to obtain a high-quality

white light source for display technologies. Given these requirements of a pure red source

to incorporate into the YAG:Ce, a narrow red emission spectrum is desired. Two materials

that achieve this property are manganese activated phosphors, with a narrow emission (~10

nm FWHM), and CdSe quantum dots (~20 nm FWHM). Unfortunately, each of these

materials has a drawback when illuminated under high excitation, namely a drastic increase

in non-radiative processes. To circumvent this complication, a one-dimensional phosphor

model was designed to determine a device structure that limits the non-radiative processes.

To achieve this an investigation into the phosphor material was performed under harsh

temperature and illumination conditions.

In addition to obtaining the correct materials to create this light source, a design

limitation in the thickness of the active material was set to be 250 µm. This limitation was

implemented for two reasons: to ensure the cooling across the film was maintained at the

high excitation levels and to meet the limitation of the collection optics for use in

projectors. As a result of this limitation the density of the phosphor materials was required

to be high in the film to allow for sufficient absorption. However, this results in higher

levels of scattering which leads to increased absorption and poor collection in lower levels

of the film. The increased scattering would need to be incorporated into a model for

determining a device architecture of these materials.


4.3. Optical characterization of phosphor material

Within this section, an investigation into the effects of power and temperature on the

The materials underwent high temperature and high

flux conditions to understand their effects on the emission. The emission properties of

photoluminescence lifetime were investigated for the manganese-based phosphor to

observe trends, which may correspond to an increase in non-radiative processes.

Sequentially, both the quantum dot

conversion efficiency was performed at elevated powers.

4.3.1. Photoluminescence lifetime

The design process considers the extent of both scattering and saturation, allowing for

the design of a high-power illumination source. The relative scattering coefficients were

obtained by measuring the transmission of the dense phosphor layers for a given thickness.

To determine the cause and effect of saturation, the photoluminescence (PL) lifetime and

efficiency of the red phosphor were investigated as a function of power. From this

information, and incorporating the scattering effects of the dense phosphor layers, a 1D

model was built to investigate varying phosphor architectures. Once the best architecture

is obtained, a study can be carried out to determine the appropriate ratio of red and green

phosphors to achieve the desired white temperature.


The PL lifetime of K2TiF6:Mn4+ was observed to be 4.8 s; this can cause saturation as a

result of non-radiative decays. Here we investigated the power and temperature dependent

lifetime of K2TiF6:Mn4+ using 442 nm excitation source. When we adjust the peak power

density from 0.4 kW/cm2 to 5 kW/cm2, the PL lifetime decreases by 31%, from 4.8 ms to

3.3 ms (Figure 4.1 left). At 5 kW/cm2, it further decreases from 3.3 ms to 2.7 ms upon a

temperature increase from room temperature to 100 C (Figure 4.1 right). The decrease in

the PL lifetime for both high-power density and high temperature indicate an enhancement

of non-radiative processes, which are detrimental to high-power density operation at

elevated temperature.

Figure 4.1 | a) Measured photoluminescence lifetime characterization of

K2TiF6:Mn4+ phosphors with varying power densities. b)

photoluminescence lifetime with varying temperature environment.


4.3.2. Power conversion efficiency

To determine the effect of saturation on performance, the power conversion efficiency

(PCE) was measured with varying power and temperature (Figure 4.2a). Each temperature

condition shows a decrease in PCE with increasing power, which is attributed to non-

radiative decay. However, no temperature-dependent loss of efficiency occurs until both

the highest power (5 kW/cm2 , when

mixed with a yellow phosphor, the temperature induced saturation effect is expected to be

negligible, as the power will be distributed between the red and yellow phosphors.

Furthermore, the sample shows a minimal loss in its emission at 5 kW/cm2 until the highest

temperatures are reached (Figure 4.2 right).

Figure 4.2 | a) Measured power conversion efficiency of K2TiF6:Mn4+

phosphor at varying temperature. b) Measured emission spectra of

K2TiF6:Mn4+ at varying temperatures with a power density of 5kW/cm2.


4.4. Multilayered phosphor modeling

To obtain white light emission, we designed a system that incorporated a pulsed blue

excitation source used to excite down-converting phosphors YAG:Ce and K2TiF6:Mn4+

(Figure 4.3). The pulsed laser is used here to minimize saturation and thermal issues. Both

phosphors were mixed into a silicone adhesive matrix (13% wt.) to obtain a dense and

smooth film, with a thickness of less than 250 µm. The emission from the phosphors was

efficiently transmitted through a dichroic mirror. For digital projection, this emission can

be combined with an adjustable blue diode; for our analysis, we calculated the power

required from a 467 nm source to produce the desired white color temperature (Figure

4.3).

We modeled various test structures, with the goal of operating as a solid-state lighting

solution, as well as a source for high-power displays and projectors. Three different types

of designs were adopted: layering the YAG:Ce phosphor on top and the K2TiF6:Mn4+

phosphor as a base, the converse, and a homogeneous blend between the two materials

(Figure 4.3). We based our calculations on results provided by Kang [33] and extended it

to incorporate the alternate architectures, as well as scattering and saturation. The model

assumes a 1D transmission within the phosphor, using the following equations:

𝑑𝑃𝑒𝑥

𝑑𝑧= −𝛼𝑒𝑥𝑃𝑒𝑥(𝑧) (6)


𝑑𝑃𝑒𝑚

𝑑𝑧= −𝛼𝑒𝑚𝑃𝑒𝑚 +

1

2𝛼𝑒𝑥𝛽𝑃𝑒𝑥(𝑧) (7)

𝑑𝑃𝑒𝑚−

𝑑𝑧= 𝛼𝑒𝑚𝑃𝑒𝑚 −

1

2𝛼𝑒𝑥𝛽𝑃𝑒𝑥(𝑧), (8)

These three equations describe the intensity of light propagating within the phosphor

materials. Equation (5) describes the power of the excitation source, 𝑃𝑒𝑥 . In this

experiment, we used a 442 nm laser diode, focused on intensities of up to 5 kW/cm2. This

equation accounts for absorption by the red and green emitting phosphors. The absorption

of both is incorporated into the coefficient 𝛼𝑒𝑥 per unit length of the device, and thus 𝛼𝑒𝑥

depends on the phosphor composition in this layer. Equation (6) and (7) describe the

emission of the red and yellow phosphors in the forward (6) and reverse (7) directions,

respectively. Within these equations, 𝛼𝑒𝑚 is the energy loss of the phosphor emission, and

𝛽 is the conversion efficiency of the excitation source to phosphor emission

shows a power dependence based on the excitation power, which decreases as the excitation

photons are partially absorbed throughout the phosphor. on power,

𝑃𝑒𝑚± , was calculated for both red and yellow emission separately in the layered structures

with 𝑃𝑒𝑚+ phosphor emission travelling forward into the device and 𝑃𝑒𝑚

− emission travelling

backward to the surface of the device. 𝑃𝑒𝑚− at z=0 is the collected emission, as the system

architecture operates in reflection mode. As a mirror is located at the back of the device,

𝑃𝑒𝑚− (𝑡) = 𝑃𝑒𝑚

+ (𝑡) at this position. The scattering of the excitation from the rear phosphor


was calculated based on the transmission properties of the top layer; this scattering is more

pronounced in our system due to the higher loading densities, when compared to lower

power phosphor systems. The absorption saturation was incorporated into the K2TiF6:Mn4+

phosphor by using a linear fit to the PCE measured in Figure 3c, and is incorporated into

the 𝛽 term for the K2TiF6:Mn4+ phosphor only.

Figure 4.3 | Phosphor 1D model setup for test structures used to calculate

red and yellow emission.

The results of the model (Table 1) show that when a red phosphor is used as a base, the

scattering induced from the YAG:Ce phosphor coating significantly reduces the red

collection. Alternatively, when the YAG:Ce is the base, the yellow collection suffers due to

the scattering in the K2TiF6:Mn4+ layer, and saturation of K2TiF6:Mn4+ limits the red

emission. The homogeneous mixture, however, distributes the absorption and scattering

losses throughout the entire device structure. This allows for the best balance between red

and green power efficiency, which is a result of reduced red saturation.


Table 1 | Model results for K2TiF6:Mn4+ and YAG:Ce phosphor structures.

Excitation power converted to green and red emission shown. Desired color

corrected power ratio of 55% green to 45% red emission.

Mixture Red Coating/

Yellow Base

Yellow Coating/

Red Base

Green Emission 25% 18% 44%

Red Emission 22% 39% 16%

Total Converted

Power Output 47% 57% 60%

Color Corrected

Power Output 44% 31% 38%

4.5. Conclusions

In this chapter, an investigation into the emission characteristics of the phosphor

materials was performed. These emission properties were obtained under high temperature

and intensity conditions. This information presented insight into the material s

performance as a red source in a white light emitter. This information was also

implemented into a model to calculate the performance when incorporated into a device.

In this model, given the device structure, a mixture of the red phosphor materials and

YAG:Ce would provide the optimal color-corrected emission. When compared to the

layered structures, a lower emission intensity is observed. However, to obtain a balance of

red and green emission, the mixture system requires no adjustment, whereas the layered

structures will need green attenuation to emission levels below that of the mixture system.


Fabrication of multilayer structure

and phosphors

5.1. Introduction

Chapter 5 delves into the fabrication and synthesis processes that were performed for

the high-power light sources used. For the first section, the synthesis procedure for the

manganese activated phosphor is established. The physical and optical characterization are

provided. Since the materials are created in microparticle sizes, and the device thickness is

a few hundreds of nanometers, the particle size is critical. A similar procedure was

described for the implementation of colloidal quantum dots into a silica matrix, and optical

characterization was carried out. Finally, the processing methods to incorporate the red

emitters with YAG:Ce powder was described. An initial goal of pure solution-processing is

desired; however, a method of implementing the CQDs in a highly conductive matrix was

described.


5.2. Manganese based phosphors

In Chapter 2 various rare and non-rare earth metal phosphors were investigated. Each

type of phosphor material has benefits and drawbacks with applications at high-power.

Short photoluminescence lifetimes are desired to have lower impact when operating at high

flux; however, many rare earth metals in literature with short PL lifetimes do not have the

desired emission spectrum to create a high quality red source for displays. Such examples

are Eu and Cs based phosphors with emission bandwidths of 80 nm and a large portion of

the wavelengths around 650 nm, a regime insensitive to the human eye, as shown by �̅�(𝜆)

in Figure 2.1a. Manganese based phosphors, on the other hand, depicted very narrow

emission peaks with a bandwidth of 50 nm [22]. When implementing the manganese

activated phosphor into a fluoride based matrix, the emission peak is near 630 nm, an

optimal wavelength for the sensitivity to the human eye. The downside with implementing

a manganese based phosphor was the long PL lifetime, which at the power regime required

typically enters roll off and a significantly decreased efficiency.

5.2.1. Synthesis

The synthesis of the manganese-based phosphor was conducted in two steps: the first

was the preparation of the manganese into a matrix compatible with the host, and the

second step involved the doping process of the manganese atoms into the host matrix. The


synthesis was adapted from previous literature in the doping of manganese phosphor into

a K2TiF6 matrix to obtain high quantum efficiencies [22].

For the synthesis of the K2MnF6 powders, a mixture of 9.0 g KHF2 and 0.45 g KMnO4

was dissolved in 30 mL 48% HF to form a purple solution. A 0.1 mL of 30% H2O2 solution

was added dropwise to the solution . The solution gradually produced a yellow

precipitate at the bottom. The obtained yellow powder was washed with acetone several

times and then dried at room temperature.

Then, the K2MnF6 powder was used to dope into the K2TiF6 matrix. To perform this,

36 mg of K2MnF6 was dissolved in 2 mL 48% HF solution, and then commercial K2TiF6

powder (0.5 g, 0.75g, 0.9 g, 1g, 2.5g, 5g) was added to the HF solution separately. The liquid-

solid mixtures were stirred at room temperature for about 20 minutes. The white powder

turned to light yellow after the liquid-solid exchange reaction, indicating that the Mn4+

doped successfully into the K2TiF6 matrix. The obtained light yellow powder was washed

with 5% HF twice and then several times with acetone. Finally, it was dried at room

temperature.

5.2.2. Physical and optical characterization

The red emitter K2TiF6:Mn4+ was synthesized as previously reported [22, 34]. Pure

K2TiF6 and Mn4+ doped powders were characterized by XRD measurements (Figure 5.1a):

diffraction peaks of the two samples can be indexed into tetragonal-phase K2TiF6 (JCPDS


No. 00-008-0488). The particle size of the phosphors from SEM images (Figure 5.1b) ranges

from 30-500 microns and shows a wide size distribution. Elemental line scanning for a

typical Mn4+ doped K2TiF6 microparticle highlights a similar distribution between

manganese and titanium along the cross section (Figure 5.1). This is in agreement with the

mapping measurements from scanning transmission electron microscopy (STEM, Figure

5.2). The molar ratio of manganese to titanium was estimated to be 10% based on the

calibrated signal intensities of energy dispersive spectroscopy (Figure 5.3). These results

demonstrate Mn4+ ion doping in the K2TiF6 matrix with uniform distribution throughout

the microparticles.

Figure 5.1 | Red phosphor physical characterization a) XRD data for K2TiF6

and K2TiF6:Mn4+ phosphor. b) SEM image of K2TiF6:Mn4+ phosphor

powder. c) Elemental line scanning from a typical Mn4+ doped K2TiF6

microparticle and SEM of analyzed cross section.


Figure 5.2 | SEM images of cross-section of K2TiF6:Mn4+ microparticles for

K, Ti, F, and Mn elemental mappings in the same selected areas.

Figure 5.3 | EDS spectrum of the microparticle displays the presence of K,

Ti, F, and Mn elements.

The intense absorption band of the K2TiF6:Mn4+ microparticles (Figure 5.4), positioned

at 468 nm, overlaps well with the emission of conventional blue diodes. The overlap

between the absorption of K2TiF6:Mn4+ and emission of YAG:Ce is negligible, thereby

avoiding the re-absorption problem that usually occurs between (oxy)nitride red

phosphors and yellow (or green) phosphors [21]. The red emission of K2TiF6:Mn4+ (Figure

5.4) comprises three sharp peaks with the main peak positioned at 631.5 nm (FWHM: 10


nm), with PLQEs up to 95%, similar to literature values [28, 29]. Their narrowband spectra

and the high PLQEs enable high efficiency and excellent color quality for white LEDs [14].

Figure 5.4 | Absorption and PL emission spectra of K2TiF6:Mn4+ phosphors.

Inset shows the photographs of the K2TiF6:Mn4+ sample under UV lamp

illumination

5.3. Colloidal quantum dots in matrix

The secondary phosphor material that was used as a red emitter was cadmium selenide

(CdSe) core cadmium sulfide (CdS) shell quantum dots. These colloidal quantum dots were

prepared through solution-processing means, maintaining a low cost. For high-power

applications, a thin shell was used to reduce the absorption cross section and thereby

maintain high efficiency at elevated powers. This reduced absorption cross section is

important for high-power applications to prevent excessive absorption within the QDs,

leading to an increase in non-radiative effects. For quantum dots, the process that causes


the non-radiative effects is Auger recombination. Unfortunately, the thin shell is normally

undesired since a precipitous drop in the PLQE results when in film. This is due to energy

transfer between quantum dots. To correct this issue, the dots used were engineered into a

silica matrix. This matrix provides passivation of the quantum dots from the environment,

and a means by which to separate the dots to ensure the high efficiency is maintained. Here

we will describe the synthesis procedure to incorporate colloidal quantum dots into a silica

matrix.

5.3.1. Synthesis

For the synthesis of the colloidal quantum dots, an alternate method was implemented

to improve their optical properties at high-power. This process provided a thin passivating

shell to the quantum dots, in order to reduce the absorption across section, thereby

reducing the potential of multi-excitons from generating within a single dot. The synthesis

of the colloidal quantum dots is described in Appendix A. The quantum dots are then

implemented into a passivating silica matrix to prevent quenching by energy transfer. The

matrix process is outlined as follows:

For the incorporation of CQDs into the silica matrix, a solution of 100 µL QDs (Aexciton

~ 2.5) was used. By using acetone, centrifugation, and decanting, the QD samples were

precipitated and extracted. The precipitate was re-dispersed in a mixture of 300 µL pyridine

and 50 µL 6-mercapto-1-hexanol. The solution was stirred for 48 hours until transparent.


Following this, hexane was used to precipitate the quantum dot from the pyridine/6-

mercapto-1-hexanol solution. After extracting the quantum dots, 200 µL ethanol was used

to re-disperse into solution. With the ethanol solution, 50 µL 3-MPS, 200 µL TEOS, and

100 µL propylamine were added. Following a minute of stirring, 20-30 µL of water was

incorporated and the solution was centrifuged for 10 s. This solution was then sealed in a

vial and dried under room temperature conditions for 4~5 days.

5.3.2. Optical characterization

After incorporation into a matrix, the optical characteristics of the colloidal quantum

dots were analyzed. The emission band can be adjusted based on the growth during the

synthesis of the CdSe cores. For that reason, a tolerance for the emission band must be

established. For the creation of pure red emission, wavelengths spanning 620 630 nm

were desired to provide a highly luminous red from the entire emission spectrum, as

emission at 630 nm is twice as luminous as emission > 640 nm as shown in the color

matching function �̅�. The PL emission and absorption of one such dot batch is given in

Figure 5.5. The measured emission is shown to be around 621 nm and possesses a FWHM

of 26 nm. This narrow emission in a luminous region of the spectrum shows strong

luminous flux. In addition, the absorption band overlaps well with the 442 nm excitation

source used. The dots in film also exhibit a PLQE of 67% in film, which was determined to

be sufficiently high for our application where PLQE>60% was required.


Figure 5.5 | Absorption and PL emission of CQDs in a silica matrix.

5.4. Layer fabrication

With the phosphor and quantum dot material characterized, the model set, and a device

architecture designed, the process for fabricating these devices was established. The main

goal of fabrication was two-fold. The first was to ensure that a low cost was maintained by

not relying on epitaxial growth or high-temperature procedures. The second goal was the

demonstration of a material which could be scaled up easily. These goals were easily

achieved by solution-processeing methods and as such were the first investigated.

5.4.1. Solution-processed phosphor

For the red phosphor, a solution-processed film preparation method was used. The

phosphor was combined with the commercial YAG:Ce which has a mean particle size of


~10 µm, a peak emission at 550 nm, and a width from 500 nm to 780 nm. This process used

an optically transparent silicone with a refractive index of 1.7. This silicone was shown to

have good index matching with the phosphor material, allowing for better extraction. This

silicone was mixed in low concentrations with the phosphor (13% wt.) to obtain a thick

heavily loaded phosphor paste. The paste was loaded uniformly into a stencil of a desired

shape, size, and thickness. The solution was placed on a 1-inch aluminum mirror, with a

dielectric reflective coating. Using a razor blade, the film was smoothed to give a 250 µm

or 125 µm thickness. This was used to create both mixed and layered films with the desired

thickness. The film was then heated to 70°C for 2 hours to fully cure the silicone.

5.4.2. Solution-processed quantum dots

For the solution-processing of the quantum dots in the film, the method previously

described was also implemented; however, the films produced were not fully cured and so

an alternative method was required. The complications with curing were attributed to

excess chemicals in the quan Thus, screen printing methods

were used with a UV curable adhesive.

The quantum dots, in a silica matrix, were crushed to make a fine powder and mixed

in Norland optical adhesive 81. At this stage the YAG:Ce was added in varying ratios. Using

a silkscreen for processing, and Kapton tape as a mask, the quantum dot and YAG:Ce

solution was loaded and printed onto the high reflectivity aluminum mirror. Placing the


sample under a UV lamp for 5 minutes was sufficient for curing, and films (~250 µm thick)

were created.

Unfortunately, the quantum dots in the UV curable adhesive showed signs of

degradation when tested at elevated powers. For quantum dots alone a large drop (~6%

PCE) was lost when increasing power up to 3 kW/cm2. After this power level, the quantum

dots were irreversibly damaged due to thermal buildup (Figure 5.6 left). In addition to this,

when protecting the quantum dots with a top capping layer of YAG:Ce phosphor, a drop

in red emission was seen over a time span of 40 minutes (Figure 5.6 right). This indicated

poor thermal transport resulting in very short operation lifetime.

Figure 5.6 | a) Power conversion efficiency measurement showing quantum

dot degradation. b) Operation lifetime of quantum dots in optically

transparent adhesive with YAG:Ce capping layer.


5.4.3. Solid state film processing

Due to the short comings of quantum dots in the screen printing method, an

alternative matrix was investigated to improve the thermal transport and achieve higher

power. This method utilized a pellet press and sodium chloride. This process was initiated

similarly to the processing quantum dots for screen printing. This involved grinding the

dots in matrix using a mortar and pestle with sodium chloride (anhydrous). Using a

hydraulic press, the sodium chloride, quantum dot, and phosphor mixture was pressed into

a 13 mm diameter pellet using 8 tonnes of pressure. This created a highly scattering film

that was 200 µm thick. By applying a high refractive index optically transparent silicone,

the voids within the pressed pellet were filled with the use of a vacuum. This resulted in

significantly lower scattering and higher extraction of red emission. Using this method a

top coating layer was added of our YAG:Ce phosphor to reduce the fraction of light

incident on the quantum dots and allow long term stable operation at 5 kW/cm2.

5.5. Conclusions

In this chapter an in-depth description into the device fabrication process was

presented. The K2TiF6:Mn4+ phosphor and CQDs in silica matrix were synthesized were

shown to have ideal emission characteristics for our application. Such characteristics

include an intense absorption at the excitation wavelength, narrow PL emission

wavelength, and the correct wavelength for a luminous red source and high PLQE.


Following this the processes to form the device structures was also described; for the

manganese activated phosphors a solution-processed film was created in silicone. For the

CQDs, an additional fabrication process was required to account for their intrinsic low

thermal conductivity. As such, a process of incorporating the CQDs into a thermally

conductive NaCl matrix was demonstrated, and a significant increase in device stability

was realized.


Characterization and performance of

phosphor devices

6.1. Introduction

In this chapter, the K2TiF6:Mn4+ phosphor and colloidal quantum dot device structures

will be characterized for their performance in an assembled device structure. The test setup

of this device will be in the high-power laser system described in Chapter 3, resulting in a

sample which operates in back reflection mode, with powers reaching 5 kW/cm2. Their

potential for application in solid state lighting will be investigated in terms of color quality

and efficiency at elevated powers. Following this the device structure will be analyzed for

its quality as a light source for digital projectors.


6.2. Device performance

As already mentioned, the phosphor samples were fabricated onto a highly reflective

aluminum mirror with a dielectric coating to provide a reflection of 99%; this allows for

the phosphor to efficiently operate in reflection mode (Figure 6.1). This structure allows

for the sample structure to be a densely packed phosphor film, to provide the broadband

emission spectrum of the YAG:Ce.

Figure 6.1 | Laser setup for excitation of phosphor materials and

incorporation of adjustable blue diode source for white light source.

Given the test setup and based on the calculations performed in Chapter 4, we chose to

design the high-power devices on the red and yellow phosphor mixture. For the quantum

dot samples, however, a YAG:Ce capping ceramic was used to reduce incident power and

improve the operation lifetime. This structure allows us to equally distribute the scattering

of both phosphors along the length of the device without placing the red emitter under

intense illumination, avoiding saturation problems. The highest performing phosphor

devices were composed of ~60% K2TiF6:Mn4+ by weight. For quantum dot samples placed

in the NaCl matrix encapsulated with silicone, they operated optimally at 15% QD


concentration, with a 40% YAG:Ce phosphor by weight (remaining weight was NaCl filler).

The coating for this device was loaded at the highest possible ratio of YAG:Ce to NaCl

(75%-25%). Potential for improvement are present by utilizing a pure YAG:Ce capping

layer pressed and sintered into a ceramic with no NaCl filler.

For the K2TiF6:Mn4+ samples, no degradation was observed when testing the phosphor

alone at high-power. However, when solution-processed techniques were used to test the

CQDs a large degradation was observed at moderate powers. As a result, a stability test was

performed on these devices to ensure color quality and efficacy are maintained. The results

can be shown in Figure 6.2 below. These results indicate that within the 4-hour period

tested, there was no degradation in red emission. In fact, there was a slight increase in red

emission which can be attributed to the filling of trap states.


Figure 6.2 | CQD 4-hour stability test at 5 kW/cm2.

6.2.1. Correlated color temperature and luminous efficacy

The spectra of these samples (Figure 6.3) were used to calculate the color coordinates

of the phosphor mixtures with different red K2TiF6:Mn4+ concentrations at 5kW/cm2

(Figure 6.4), in addition to the color temperatures (Figure 6.4). Increasing the

concentration of red phosphor raises the saturation point, thereby increasing the overall

red emitted power and by extension the balanced external quantum efficiency. At

K2TiF6:Mn4+ concentrations of 60%, the yellow phosphor still absorbs sufficient light to

maintain its emission intensity at high-power densities. This allows for high external

efficiency, while also allowing the red emission to be increased, thereby correcting the

spectrum (Figure 6.3).


Contrary to the red phosphor, the CQD samples prepared showed a concentration

dependence, where increasing the percentage past 15% shows diminishing returns. In fact,

a decrease in emissions results when higher concentrations are reached. This decrease in

emission intensity can be attributed to reabsorption due to the high concentration of

CQDs. At this 15% concentration a sufficient level of absorption is present to adequately

alter the emission profile. This is due to the higher absorption cross section of the CQDs

relative to the Mn4+ activated phosphor.

The CCT was determined for each of the samples using the spectra (Figure 6.3). The

CCT obtained indicated negligible fluctuation in the CCT at varying power for the

K2TiF6:Mn4+ phosphor and CQD mixtures, 4369 K and 4248 K, respectively (Figure 6.4

warmest CCT with Duv<0.02 from Planckian locus). This implies that the effect of any

saturation with both K2TiF6:Mn4+ and CQD is insignificant in this mixed system, since the

effective pump power was decreased by the absorption of the yellow phosphor. Due to the

invariance in color emission, the device structure is able to alter successfully the YAG:Ce

white emission closer to that of a warmer black body radiator.


Figure 6.3 | Spectra of the YAG:Ce compared to the QD and K2TiF6:Mn4+

structures.

Figure 6.4 | CIE color coordinates of the samples at high flux excitation (left)

and corresponding CCT of samples at all power densities, with Duv<0.02

(right).

Furthermore, the devices exhibit high luminous efficacies (Figure 6.5). These efficacies

were calculated by combining the power conversion efficiency with the calculated

luminous flux, obtained from the spectra. When compared to the ceramic YAG:Ce, the

samples show a significantly lower efficacy; however, when increasing the power to 5


kW/cm2 the samples show a slower decrease in efficacy making the samples relatively

comparable.

Figure 6.5 | Corresponding luminous efficacy of best Mn4+ and CQD

samples at all power densities.

6.3. Device application in display technology

6.3.1. Overview

To operate the phosphor mixture as a source for displays, the spectrum must be split

into green and red sources that meet the Digital Cinema Initiatives (DCI) color space

As described in Chapter 3, the gamut range achieved by the separated

source dictates the range of colors that can be achieved by a given light source. With the

creation of an optical filter to split the emission of the device s source, the white light source

can generate a red and green source. Furthermore, in this section a comparison of the


generated light source to the industry standard YAG:Ce ceramic phosphor will be made to

determine the luminous efficacy of the emission.

6.3.2. Design consideration and color space

To reach the DCI color space requirements, the white light emission must be separated

into red and green components. To do this a 590 nm long and short pass filter can be

introduced to separate the spectrum into green and red components, while removing a

portion of the yellow emission.

reflection profile was implemented artificially with the spectrum. This was then used to

calculate the red and green sources for the Mn4+ phosphor and CQD samples (Figure 6.6).

The onset of the short and long pass filters was adjusted to accurately obtain the DCI red

point (x: 0.680, y: 0.320) and green point (x: 0.265, y: 0.690). In a digital projector, pixels

are controlled by a digital micro-mirror (DMM) and can control the quantity of red and

green source used. This can be used to adjust the individual source s level to balance the

power and obtain the correct DCI white point, which is a slight modification of the D65

white-point (daylight). This allowed for the full gamut range required in digital projection.

For the devices fabricated, a balanced emission between red and green allowed for the

rendering of a DCI white point with no waste energy.


Figure 6.6 | Wavelength separation of K2TiF6:Mn4+ phosphor and YAG:Ce

separated into RGB sources

Using these separated spectra an effective luminous flux can be calculated for operating

a projector at the DCI standard. Unlike the devices created, when separating the emission

for the standalone YAG:Ce system, a significant reduction (~49%) of the green emission is

necessary to obtain the correct emission profile for the DCI white point. Similarly, when

compared to the mixture system, we achieved ~21% higher luminous efficacy in the Mn4+

activated phosphor when rendering white light, and ~32% higher luminous efficacy for the

CQD samples.

6.4. Conclusions

In this Chapter, the quantum dot and Mn4+ activated phosphor were successfully

implemented into a device structure with YAG:Ce phosphor. These devices displayed a

large decrease in their CCT of ~650 K, while maintaining a high luminescence efficacy at

high-powers. These devices also showed superior effective luminescence efficacy for


applications in display technologies when meeting the DCI requirements. Improvements

of 21% and 32% in the effective luminescence efficacies were shown.


Conclusions and future work

7.1. Summary

We established a design protocol for analyzing device architectures, with the goal of

producing a highly efficient white light source. The effectiveness of this design protocol

was demonstrated through the combination of an efficient YAG:Ce phosphor with a red

emitting K2TiF6:Mn4+ phosphor. The structure was shown to have superior performance

for adjusting the temperature of white light at high fluxes. This enables the device, one

which conventionally would suffer from saturation and scattering of the red emitter, to

exhibit improved color emission as a source for higher power applications. This mixed

phosphor system, when compared to the stand alone YAG:Ce at 5 kW/cm2, provides a 21%-

32% luminescence improvement when used in cinematography; meanwhile, the

illumination is capable of reducing the correlated color temperature to produce a warmer

white light source.


7.2. Original contributions

1. A design protocol was created for the incorporation of red light emitting

materials into highly efficient broadband source. A one dimensional model was

created to account for the high-power saturation effects. Using known

properties of the material, various device architectures were investigated. This

allowed for the successful design and implementation of a high-power (5

kW/cm2) red emitting source using both a transition metal phosphor and

colloidal quantum dots [35].

2. We report the first solid-state phosphor and CQD system that creates warm

white light emission at powers up to 5 kW/cm2. Furthermore, at this high-

power

requirements with a luminescence efficacy improvement of 32% over the stand-

alone ceramic YAG:Ce phosphor.

7.3. Future work

Potential future directions for this research are as follows:

1. Within this thesis a measurement of the phosphor and colloidal quantum dot

performance was investigated. For future application, it would be important for

the device to display long term stability (shelf life) and a long operation lifetime.

It would be important to demonstrate the full lifetime of the material, longer


than what has presently been demonstrated. During the synthesis of the

material a shelf time of a month was demonstrated. This can potentially be

improved through encapsulation methods and further refinement of the CQD

in silica matrix process.

2. In addition to the long term stability of the CQDs, an improvement in the

design of the dots should be investigated, to minimize deleterious effects that

occur at high-power, i.e. Auger. Furthermore, manipulating the dots

synthetically offers the potential to stave off the onset of Auger, or the level to

which it affects the system. This would drastically improve the performance of

the CQD device, giving it a bright future in large area display technologies.


Appendix A

Synthesis of CdSe core QDs: CdSe QDs were synthesized according to a modified

method reported previously [40]. Typically, CdO (240 mg), tri-octylphosphine oxide

(TOPO, 24 g) and octadecylphosphonic acid (ODPA, 1.12 g) were mixed in a Schlenk

flask (100 mL). The mixture was heated to 150 ˚C for 0.5 h under vacuum with continuous

stirring. Then the system was refilled with N2 and heated at 320 ˚C for 2 h. Subsequently,

tri-octylphosphine (TOP, 4 mL) was injected into the Schlenk flask, and the resulting

mixture was further heated to 380˚C. Upon reaching 380˚C, 2 mL of TOP solution

containing selenium (60 mg mL-1) was injected into the system. CdSe QDs with the first

excitonic peak at 586 nm were formed after about 2 min growth. Finally, the reaction was

terminated by cooling and adding acetone. The resultant CdSe QDs were redispersed in

hexane for shell growth.

CdS and ZnS shell growth on CdSe cores: The shell growth was carried out as described

previously [22]. CdSe core QDs were quantified by measuring absorbance (A1st exciton peak)

at exciton peak (586 nm) in a cuvette with path length of 1 mm. A hexane solution of CdSe

core QDs (8.8 mL, A1st exciton peak = 1) was mixed with oleylamine (OLA, 12 mL) and

octadecene (ODE, 12 mL) in a 250 mL Schlenk flask. The mixture was heated to 100 oC

under vacuum to remove hexane. The Cd-oleate and octanethiol were dissolved seperately

in ODE to obtain desired concentations, and then were injected simultaneously and

continuously into the system at a rate of 12 mL h-1 whilst ramping the system temperature

from 100 oC to 310 oC. Different amounts of Cd-oleate (1, 3, 6, and 7.5 mL) and octanethiol

(106, 320, 640, 800 L) were used for growth of different shell thickness.


To further grow ZnS shell on CdSe/CdS QDs, the above solution was cooled down to

290 oC. 1.5 mL of as-prepared Zn-OLA diluted in 10.5 mL of ODE was mixed with 0.03

g of sulfur dissolved in 2 mL of OLA. The mixture was slowly and continuously injected

into the system for 1 h at 290 oC. After injection, the solution was annealed at 290 oC for

10 min, followed by an injection of 4 mL of oleic acid (OA) and further annealing for 10

min. The reaction was terminated by cooling and adding acetone at 80 oC. The resultant

CdSe/CdS/ZnS QDs were purified by 3 cycles of centrifugation at 6000 rpm, redispersion

in hexane, and precipitation by acetone. The final core-shell-shell CdSe/CdS/ZnS QDs

were redispersed in toluene (A1st exciton peak = 2.5).


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