charge transport in quantum dot â€“ light emitting devices

CHARGE TRANSPORT IN QUANTUM

DOT – LIGHT EMITTING DEVICES

A DISSERTATION

SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL

OF THE UNIVERSITY OF MINNESOTA

BY

BRIJESH KUMAR

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

STEPHEN A. CAMPBELL, ADVISOR

P. PAUL RUDEN, CO-ADVISOR

AUGUST 2013

i

Acknowledgements

First of all, I would like to thank my advisors, Prof. Campbell and Prof.

Ruden for their constant support and encouragement over the last six years.

They both provided me with ideas if I was stuck or to help secure the facilities

required to conduct my research. It was a great pleasure working with both

my advisors. I cannot thank both of them enough for being my support

through both rough and smooth phases of my Ph.D.

I would like to thank all my colleagues, beginning with Sang Ho Song. I

had so many insightful discussions with him on my research and various other

topics of life. I would also like to thank Gagan Agarwal, Maryam Jalali,

Min-Woo Jang, Rick Liptak, Mohammad Yunus and Isaiah Steinke for their

help over the years. I would like to thank all the NFC staff for their prompt

help whenever requested. Teaching was an important part of my Ph.D. ex-

perience. So, I would like to thank Prof. William Robbins for assigning me

classes of my liking. I would like to thank all my teachers of different courses

for answering my endless questions.

My friends and roommates, Srijan Aggarwal, Ricky Jain and Ankur

Khare, were my support throughout my Ph.D. I am grateful to all of them

ii

(especially, Srijan) for tolerating me through all these years and making me

feel home even if I was 8,000 miles away from home. I am especially indebted

to my teacher for teaching me what is important in life.

I would like to thank my family for their support over the years. It was

hardest for my mother to allow me to go so far away from home. I cannot

thank her enough for her sacrifices for me over the years. Waking up at 4.30

am everyday so that we could get fresh lunch for school every day for 18

continuous years, even when she was sick, is just a small example of her sac-

rifices. I would like to thank my father for his love and support. He sacrificed

his own desires for getting us better education. My sisters, Kavita and

Rashmi, have been supportive of all the decisions I have taken in my life. I

thank them for being there for our parents, when I was away from home. I

would like to thank my late grandfather for teaching me so much about living

a simple life and God.

Finally, I would like to thank God for everything I have, including my

intelligence, however little it may be…

iii

Dedication

To

Krishna,

my Teachers

& my Parents

iv

Abstract

Inorganic quantum dots have excellent optoelectronic properties. But, due

in part to a lack of a suitable medium for dispersion, they have not been

extensively used in optoelectronic devices. With the advent of organic

semiconductors, the integration of quantum dots into optoelectronic devices

has become possible. Such devices are termed as hybrid organic/inorganic

quantum dot light emitting devices. In hybrid organic/inorganic quantum dot

light emitting devices, the mechanisms of charge and/or energy transfer into

the quantum dots include Forster energy transfer and direct charge injection.

Forster energy transfer involves formation of excitons on organic

semiconductors, followed by an energy transfer onto the inorganic quantum

dots, where the exciton recombines resulting in emission of a photon. Direct

charge injection is the mechanism in which the electrons and holes are directly

injected into the quantum dots and they recombine on the quantum dots to

result in a photon. Which mechanism is operating in a device has been a

subject of contention. In this work, by using various device configurations, we

show that both these mechanisms can operate independently to maximize the

quantum dot light emission in such devices.

We also propose a model for inorganic QD-LEDs, which explores the most

important parameters that control their electrical characteristics. The

v

device is divided into a hole transport layer, several quantum dot layers, and

an electron transport layer. Conduction and recombination in the central

quantum dot region is described by a system of coupled rate equations, and

the drift-diffusion approximation is used for the hole and electron transport

layers. For NiO/Si-QDs/ZnO devices with suitable design parameter the

current and light output are primarily controlled by the quantum dot layers,

specifically, their radiative and non-radiative recombination coefficients.

Radiative recombination limits the device current only at sufficiently large

bias. This model can be extended to apply to hybrid organic/inorganic

QD-LEDs.

vi

Table of Contents

Acknowledgements ................................................................... i

Dedication .............................................................................. iii

Abstract ................................................................................. iv

Table of Contents ................................................................... vi

List of Tables ......................................................................... ix

List of Figures ......................................................................... x

Chapter 1 Introduction ..................................................... 1

1.1. Introduction to Light Emitting Devices ...................................... 1

1.2. Background of Quantum Dots ..................................................... 3

1.3. Synthesis of Quantum Dots ......................................................... 5

1.3.1 Liquid phase synthesis ................................................. 5

1.3.2 Vapor phase synthesis .................................................. 7

1.4. Luminescence from Quantum Dots ........................................... 12

1.5. Structure of Dissertation ........................................................... 16

Chapter 2 Review of QD-LEDs ....................................... 18

vii

2.1. Device Structure ........................................................................ 18

2.2. Organic Transport Layers .......................................................... 20

2.3. Inorganic Transport Layers ....................................................... 25

2.4. Latest Developments ................................................................. 28

Chapter 3 Charge Transport in Hybrid QD-LEDs ......... 40

3.1. Introduction ............................................................................... 40

3.2. Experimental Method ................................................................ 42

3.3. Results and Discussion .............................................................. 47

Chapter 4 Modeling of QD-LEDs ................................... 56

4.1. Introduction ............................................................................... 56

4.2. Model Description ..................................................................... 58

4.2.1 A. Carrier injection from transport layers into

quantum dot layers ............................................................................ 59

4.2.2 Transport among the quantum dot layers ................. 60

4.2.3 Recombination in the quantum dots ......................... 62

4.2.4 Coupled rate equations .............................................. 63

4.2.5 Transport in ETL and HTL ...................................... 64

4.2.6 Carrier injection from the contacts ............................ 65

4.3. Results and Discussion .............................................................. 66

4.3.1 Device Parameters ..................................................... 66

viii

4.3.2 Simulation Results ..................................................... 68

Chapter 5 Conclusion and Scope for Future work .......... 79

5.1. Conclusion ................................................................................. 79

5.2. Scope for Future Work .............................................................. 80

Bibliography .......................................................................... 82

ix

List of Tables

Table 4.1 The parameters of NiO and ZnO used for our model device. 67

Table 4.2 The parameters used to describe transport in QD layers. ...... 68

x

List of Figures

Figure 1.1 Density of states of bulk semiconductor, quantum well,

quantum wire and quantum dot. .................................................................... 3

Figure 1.2 Nucleation and growth phases for quantum dots in hot

injection method. Adapted from [6]. ............................................................... 7

Figure 1.3 Emission of light from bulk semiconductors upon relaxation of

electrons from the conduction band to valence bands. Due to the continuity of

bands, the wavelength of emitted light is different depending on the energy

lost by electrons. ........................................................................................... 12

Figure 1.4 Discrete density of states for quantum dots. Electrons can be

excited to lowest level in the conduction “band” and thus the only

recombination takes place between highest level in the valence “band.” The

interaction between other states is highly unlikely. ...................................... 13

Figure 1.5 The spectrum of quantum dots with different size

distributions. As the variance in size increases, the spectrum becomes broader.

...................................................................................................................... 14

Figure 1.6 Silicon quantum dots of different size emitting various colors of

the spectrum. Courtesy: Dr. Rick Liptak [26]. .............................................. 15

Figure 1.7 Dependence of bandgap on size of Si quantum dots. Adapted

from [27]. ....................................................................................................... 17

xi

Figure 2.1. Schematic of a generic QD-LED. ......................................... 29

Figure 2.2 Band diagram of a generic QD-LED. .................................... 30

Figure 2.3 QD-LED with only ETL (structure A) or only HTL (structure

B). In both cases PPV acts as the transport layer. From [28]. ..................... 31

Figure 2.4 PL (solid lines) and EL (dashed lines) from two different sized

CdSe/CdS core-shell QDs (a) 2.5 nm core/ 0.5 nm shell (b) 3.4 nm core/ 0.7

nm shell [32]. ................................................................................................. 32

Figure 2.5 Energy band diagram of device used by Coe et al. containing

both electron and hole transport layers [33]. ................................................ 33

Figure 2.6 EL spectrum of (a) device with ETL and HTL (b) device with

additional hole blocking layer [33]. ................................................................ 34

Figure 2.7 Schematic of contact printing of QDs [34]. ........................... 35

Figure 2.8 Patterned QD-LED created by Kim et al. (c) Device schematic

(d) Photograph of light emission [34]. ........................................................... 36

Figure 2.9 (a) Photographs of operating QD-LEDs of different colors. (b)

EL (solid lines) and PL (dashed lines) corresponding to these devices. (c)

Photograph of luminescence of different colored QDs. (d) EQE (e) Power

efficiency and (f) J-V characteristics of these devices [35]. ........................... 37

Figure 2.10 (a) Band diagram of device with NiO inorganic HTL by

Caruge et al. (b) J-V characteristics of the device with high resistivity

(squares) and low resistivity (circles) NiO [38]. ............................................ 38

xii

Figure 2.11 (a) Device structure with inorganic HTL and ETL by Caruge

et al. (b) Band diagram of the device [42]. ................................................... 39

Figure 3.1 (a) UV/Visible Absorbance and (b) photoluminescence (PL)

spectra of CdSe quantum dots. ..................................................................... 43

Figure 3.2 Device structures for (a) Device 1, (b) Device 2 and (c) Device

3. ................................................................................................................... 44

Figure 3.3 (a) Height and (b) phase image of spin coated mixture of QDs

and TPD. ...................................................................................................... 46

Figure 3.4 Current density - Voltage (J-V) characteristics of device 1. It is

also representative of the J-V characteristics for devices 2 and 3. ................ 47

Figure 3.5 EL intensity curves at different current densities for device 1.

The peak at 460 nm corresponds to EL from TPD and the peak at 610 nm

corresponds to EL from CdSe/ZnS quantum dots. Inset shows the QD to TPD

emission ratio, which is almost constant at 0.2............................................. 48


Inset shows the QD to TPD emission ratio, which is almost constant at 1.0.

...................................................................................................................... 49


...................................................................................................................... 50

Figure 3.8 Band diagram for device 2. The band diagrams for other two

devices can be derived from this, either by removing PEDT:PSS (device 1) or

by removing TPD (device 3). ........................................................................ 51

xiii

Figure 3.9 Comparison of EL intensities from devices 1, 2 & 3 with the

sum of EL intensities of device 1 & 3 at 100 mA/cm2 showing that, EL from

QDs for device 2 and sum of device 1 and 3 overlap. Inset shows that the same

is true at 10 and 50 mA/cm2 as well. ............................................................ 53

Figure 4.1 Schematic device structure for a Quantum Dot Light Emitting

Diode. ............................................................................................................ 72

Figure 4.2 Flat band diagram for a sample device which consists of Si

quantum dots, ZnO as the electron transport layer, NiO as the hole transport

layer and ohmic contacts to both anode and cathode. The horizontal lines at

ends signify the metal Fermi energies for anode and cathode. Energies are in

eV with respect to the vacuum level. ............................................................ 73

Figure 4.3 Discretization scheme for QD-LED focusing on the QD layers.

...................................................................................................................... 74

Figure 4.4 Carrier densities and electrostatic potential in a device with 5

QD layers (represented by dots on the plot) at equilibrium and with an

applied voltage of 0.5 V. ............................................................................... 75

Figure 4.5 Total device current density (log and linear) plots with

increasing forward bias. The recombination current density is also plotted.

...................................................................................................................... 76

Figure 4.6 Current density vs applied voltage for different assumed values

of the carrier capture coefficients (τ). ........................................................... 77

xiv

Figure 4.7 (a) Current density vs applied voltage and (b) ratio of

recombination current to total current (Jrec/J) for various radiative

recombination coefficients (γ). ...................................................................... 78

1.1 Introduction to Light Emitting Devices INTRODUCTION

1

Chapter 1

Introduction

1.1. Introduction to Light Emitting Devices

Incandescent light bulbs still enjoy a 90% share of the household lighting

market. One of the major reasons they occupy such a large market share

despite the inefficiency of incandescent lighting, is that the light spectrum

from such sources is very close to sunlight. Hence, the color rendering in in-

candescent lighting is extremely good [1]. Lighting represents 15% of total

energy consumption in the U.S. So, if the efficiency of lighting is improved, it

will greatly reduce the total energy consumption. Compact fluorescent lamps

(CFLs) have much better efficiency but they suffer from problems of poor

color rendering and color temperature. CFLs also have a significant envi-

ronmental impact because they use mercury, which is extremely toxic to both

nature and humans. Inorganic light emitting diodes (ILEDs) usually emit

light with a reasonably sharp spectrum, so they are not as useful in lighting

applications. A broader spectrum of light emission is required for lighting

applications. Moreover, they are difficult to fabricate and tuning the bandgap

of ILEDs to get a desired color output requires great deal of engineering. Even

1.1 Introduction to Light Emitting Devices INTRODUCTION

2

upon having manufactured ILEDs of three primary colors, getting white light

out of ILED based lamps is extremely difficult because of the different effi-

ciencies of the three ILEDs, particularly as the individual sources age. This

results in poor color rendering.

In the last two and a half decades, there has been great interest in or-

ganic light emitting diodes (OLEDs). They are easier to fabricate than ILEDs

and low-cost mass-production techniques like printing can be used to deposit

organic semiconductors. Not only do they suffer from the same problems of

color rendering as ILEDs, they also emit with a wider spectrum than ILEDs,

so control of exact color is even more difficult. Incorporation of quantum dots

as the light emitters in organic diodes can result in structures called quantum

dot–light emitting devices (QD-LEDs). They have much sharper spectra and

use different sized quantum dots of a single material to create the exact colors

required. Even white light can be produced using this method by combining

quantum dots of different sizes in the correct proportions. Some of the unde-

sirable properties of organic semiconductors creep into these devices, but the

beauty of using quantum dots as light emitters is that we can use inorganic

semiconductors instead of organic semiconductors to get rid of the problems

that affect organic semiconductors. Depending on the application, one or the

other may be more suitable.

1.2 Background of Quantum Dots

The structure and functioning of QD

detail in Chapter 2. This chapter will delve deeper into the properties of

quantum dots that make them extremely desirable as light emitters.

1.2. Background of Quantum Dots

Figure 1.1 Density of states of bulk semiconductor, quantum well, quantum wire and quantum dot.

Figure 1.1 shows the density of states comparison of bulk materials and

various quantum structures. The density of states of bulk materials is co

tinuous, whereas it starts becoming quantized as we confine the directions in

which the electrons can move freely. Once all

the energy states become completely discrete as shown in

an atom. The existence of continuous bands in bulk semiconductors can be

considered a splitting of degenerate levels due to all of the atoms in a cry

talline lattice. As we reduc

the bands are no longer continuous and they have states like an atom.

Background of Quantum Dots INTRODUCTION

3

The structure and functioning of QD-LEDs will be described in more


tum dots that make them extremely desirable as light emitters.

Background of Quantum Dots

Density of states of bulk semiconductor, quantum well, quantum wire and quantum dot.

shows the density of states comparison of bulk materials and

various quantum structures. The density of states of bulk materials is co

s, whereas it starts becoming quantized as we confine the directions in

ns can move freely. Once all three dimensions are confined,

the energy states become completely discrete as shown in Figure 1.1


considered a splitting of degenerate levels due to all of the atoms in a cry

talline lattice. As we reduce the number of atoms interacting within a lattice,


INTRODUCTION

LEDs will be described in more


tum dots that make them extremely desirable as light emitters.

Density of states of bulk semiconductor, quantum well,

shows the density of states comparison of bulk materials and

various quantum structures. The density of states of bulk materials is con-

s, whereas it starts becoming quantized as we confine the directions in

three dimensions are confined,

1, just like


considered a splitting of degenerate levels due to all of the atoms in a crys-

e the number of atoms interacting within a lattice,


1.2 Background of Quantum Dots INTRODUCTION

4

The exciton Bohr radius (aB) is the size at which the transition from

bulk to quantum structure takes place. This is the typical size of a bound

electron hole pair. It is defined from Bohr’s model of a hydrogen atom. Bohr’s

radius for most semiconductors is several nanometers. Thus, the particles of

semiconductors with diameters equal to or less than the Bohr radius act as

quantum dots.

Along with the quantization of energy levels upon reducing the particle

size to the nanometer range, the value of the bandgap also changes. This can

be attributed to an increase in energy level splitting, causing the conduction

and valence band to move away from each other. The bandgap changes can be

estimated using the quantum mechanical “particle in a box” theory [2]. For

the weak confinement regime (a>aB), the energy of exciton in a spherical

quantum dot is given by

Enml

=Eg−

Ry*

n2 + ħ2χ

2

2MR2 (1.1)

where R is the radius of the quantum dot, χml

are the roots of spherical

Bessel function (m – number of the root, l – order of the function), Ry* is the

exciton Rydberg energy (Ry*= e2

2εaB

), and M=m*e+m

*h. The exciton energy

in the strong confinement regime (a≤aB) is given by

1.3 Synthesis of Quantum Dots INTRODUCTION

5

Enl

=Eg+

ħ2

2µR2χ2nl (1.2)

As can be seen from Equation 1.2, the bandgap of quantum dots is always

greater than that of bulk semiconductor and it increases as we reduce the

radius of the quantum dots.

1.3. Synthesis of Quantum Dots

There are numerous ways by which quantum dots can be synthesized.

They can be divided into two major categories. The following two subsections

describe each of those categories.

1.3.1 Liquid phase synthesis

Liquid phase synthesis is the most common technique for making quan-

tum dots. It can be further subdivided into two primary categories:

• Aerosol based synthesis: In this method, liquid precursors are

aerosolized in presence of gaseous precursors to produce the desired

quantum dots. The size of quantum dots is controlled by the con-

centration of the precursors and the size of aerosol droplets. This

method was used by both Ahonen et al. and by Kim et al. to create

TiO2 [3] and copper quantum dots [4], respectively.

• Solution based synthesis: In this method, solution based pre-

cursors are added together to allow for a chemical reaction. The


6

concentration and timing of adding the reagents is such that the

particles stop growing after a certain size. This is by far the most

commonly used technique.

Of these two the latter provides much better control of the reaction and so

the size of the quantum dots. Thus, we will only discuss solution based

methods in further detail.

There are various solution based methods to synthesize quantum dots,

but the hot injection method gives the best quality and highly monodispered

quantum dots. For most applications, monodispersity of quantum dots is de-

sired as many of the properties of quantum dots are size dependent, especially

the bandgap, which determines the luminescence wavelength. If the quantum

dots are not monodispersed, the luminescence spectra will be broad.

Murray, et al. created highly monodispersed CdE (E= sulfur, selenium,

tellurium) particles using the hot injection method [5]. They accomplish it by

rapid injection of a precursor solution into a hot solution of surfactants.

Figure 1.2 shows the different phases of a hot injection reaction. When pre-

cursors are injected into the reaction chamber, the concentration goes over the

nucleation threshold and quantum dots begin to nucleate. As time progresses,

the concentration of precursor reduces because it is being consumed in the

nucleation reaction. Once the concentration of the precursors becomes smaller

than the nucleation concentration, the nuclei begin to grow to form larger

quantum dots. If the time to reach the saturation concentration is much

1.3 Synthesis of Quantum Dots

higher than the time for nucleation, all the particles will have approximately

the same size, usually with a 10% or less standard deviation.

Figure 1.2 Nucleation and growth phases for quantum dots in hot injection method. Adapted from

1.3.2 Vapor phase synthesis

In vapor-phase synthesis of quantum dots, conditions are created such

that the vapor phase mixture is thermodynamically unstable compared to the

precursors. A supersaturated vapor is an example of such a condition. Then it

is condensed in a controlled mann

Synthesis of Quantum Dots INTRODUCTION

7


the same size, usually with a 10% or less standard deviation.

Nucleation and growth phases for quantum dots in hot injection method. Adapted from [6].

Vapor phase synthesis

phase synthesis of quantum dots, conditions are created such



is condensed in a controlled manner to create quantum dots [7].

INTRODUCTION


Nucleation and growth phases for quantum dots in hot

phase synthesis of quantum dots, conditions are created such




8

1.3.2.1 Using solid precursors

The basic philosophy for this method of synthesis of quantum dots is to

induce nucleation by supersaturation of hot gases by vaporization of solids

and then rapidly cooling the gas.

Condensation in inert atmosphere:

In this method, a solid is heated and the vapor mixed with a background

gas and then, a cooled gas is added to this vapor-gas mixture to reduce the

temperature, which results in creation of quantum dots. This method is

primarily used for synthesis of metal quantum dots because most of the metals

are relatively easy to vaporize with a reasonable vapor pressure. Wegner et al.

prepared bismuth quantum dots using this method [8]. Ohno fabricated

composite quantum dots (Al/Pb, Si/In, Ge/In and Al/Pb) by creating

quantum dots of one material using inert gas condensation in one chamber,

allowed them to flow into second chamber, where the second material was

deposited onto the quantum dots to create two material composite quantum

dots [9].

Pulsed laser ablation:

Instead of using heat to vaporize a material, pulsed laser can be used to

vaporize materials that are not easily vaporized, or, are in a confined space.

This method is primarily used to create Si quantum dots as demonstrated by


9

Nakata et al. [10] and Marine et al. [11]. Hydrogenated Si quantum dots were

also prepared using pulsed laser ablation by Makimura et al. [12].

Spark discharge generation:

This method is again used for creating quantum dots of metals. In the

presence of an inert gas, a high enough voltage so as to create a spark is ap-

plied between the two electrodes (made of the metal to be vaporized). The

spark vaporizes the metal and quantum dots are formed upon condensation of

the vapors. Weber et al. used spark discharge to create Ni quantum dots [13].

Ion sputtering:

This method involves using a target of the quantum dot material, usually

a metal, during ion sputtering using inert gas. Fabrication of quantum dots of

12 different materials using ion sputtering was demonstrated by Urban et al.

[14].

1.3.2.2 Using liquid or vapor precursors

In this case, the supersaturation of gas-vapor mixture is achieved by a

chemical reaction, rather than a physical vaporization.

Chemical vapor synthesis:

This process is very similar to chemical vapor deposition (CVD), but, in

this case a hot wall reactor is used, so that the nucleation takes place in the


10

vapor phase rather than the wall of the reactor or the substrate. The substrate

is kept at a lower temperature, so it acts as a collection location rather than a

nucleation location as used by Singh et al. while fabricating tin nitride

quantum dots [15]. This method can be used to create many different kinds of

quantum dots, with various gas and precursor combinations possible.

Laser pyrolysis:

Another method of heating the precursors to help in reaction and nucle-

ation is to use laser energy. This method consumes a lot less energy compared

to chemical vapor synthesis because only a small portion of gas is heated in-

stead of a whole chamber. Quantum dots of various elemental and compound

materials can be fabricated using this method as shown by: Borsella et al. to

create MoS2 quantum dots [16], Kamlag et al. to create SiC quantum dots [17]

and Ledoux et al. to create Si quantum dots [18,19].

Thermal plasma synthesis:

An alternate method for providing the required energy is to inject the

precursors into a thermal plasma. The thermal plasma decomposes the pre-

cursors and the radicals react to form the desired quantum dots. This method

was used by Heberlein et al. to create hard coatings using quantum dots [20].


11

Flame synthesis:

Here the precursors are combusted and the required energy is provided by

the combustion reaction. Most of the carbon quantum dots in the atmosphere

are created by the combustion of hydrocarbons in this manner. This method

was used to create ferric oxide quantum dots by Janzen et al. [21], and titania

quantum dots by Wegner et al. [22].

Low temperature reactive synthesis:

Some materials can be reacted without external application of heat. As

the energy from the reaction and injection method should be sufficient to

continue the reaction, this method is limited a few reactions. ZnSe quantum

dots were fabricated using this method by Sarigiannis et al. [23].

Non-thermal plasma synthesis:

Other methods of vapor phase synthesis suffer from agglomeration of

quantum dots, because there are no capping agents as in liquid phase syn-

thesis methods. But, non-thermal plasma synthesis can solve this problem

[24]. A non-thermal plasma is created when electrons are at a much higher

temperature than the ions, which results in quantum dots being negatively

charged. Due to this unipolar charging of quantum dots, the agglomeration of

quantum dots is highly reduced. This method was used by Gorla et al. to

create Si and Ge quantum dots [25].

1.4 Luminescence from Quantum Dots

Figure 1.3 Emission of light from bulk semiconductors upon relaxation of electrons from the conduction band to valence bands. Due to the continuity of bands, the wavelength o

different de

1.4. Luminescence from Quantum Dots

Bulk semiconductors have a conduction and a valence band due to the

interaction of atoms in a lattice. The conduction and valence bands are se

arated by a region of zero d

in the conduction and valence bands are so close to each other that they are

considered continuous. Electrons can exist in the conduction band and the

valence band, but, they cannot stay in the bandgap of t

When energy is supplied to the electrons, they can move from the valence

band to the conduction band. When the electrons are relaxed from the co

duction band to the valence band, they emit light as shown in

Luminescence from Quantum Dots INTRODUCTION

12

Emission of light from bulk semiconductors upon relaxation of electrons from the conduction band to valence bands. Due to the continuity of bands, the wavelength of emitted light is

different depending on the energy lost by electrons.

Luminescence from Quantum Dots


interaction of atoms in a lattice. The conduction and valence bands are se

arated by a region of zero density of states known as the bandgap. The states



valence band, but, they cannot stay in the bandgap of the semiconductor.


band to the conduction band. When the electrons are relaxed from the co

duction band to the valence band, they emit light as shown in Figure

INTRODUCTION

Emission of light from bulk semiconductors upon relaxation of electrons from the conduction band to valence bands.

f emitted light is


interaction of atoms in a lattice. The conduction and valence bands are sep-

ensity of states known as the bandgap. The states



he semiconductor.


band to the conduction band. When the electrons are relaxed from the con-

Figure 1.3. The


wavelength of the emitted light is given by the energy lost by the electron in

this transition. The most likely scenario for this relaxation to take place is

from the bottom of the c

there are non empty states around the conduction band edge, so some ele

trons from those states also relax to emit light at energies

bandgap as shown in Figure

for spectrum of luminescence bulk semiconductor light emitting diodes

(LEDs) is nonzero because of the “continuous” valence and conduction bands.

Figure 1.4 Discrete density of states for quantum dots. Electrons can be excited to lowest level in the conduction “band” and thus the only

recombination takes place between highest level in the valence “band.” The interaction between other states is highly unlikely.


13



from the bottom of the conduction band to the top of the valence band. But

there are non empty states around the conduction band edge, so some ele

trons from those states also relax to emit light at energies higher than

Figure 1.3. So, the full width at half maximum (FWHM)


because of the “continuous” valence and conduction bands.

Discrete density of states for quantum dots. Electrons can be excited to lowest level in the conduction “band” and thus the only

recombination takes place between highest level in the valence d.” The interaction between other states is highly unlikely.

INTRODUCTION



onduction band to the top of the valence band. But

there are non empty states around the conduction band edge, so some elec-

higher than the

. So, the full width at half maximum (FWHM)


because of the “continuous” valence and conduction bands.

Discrete density of states for quantum dots. Electrons can be excited to lowest level in the conduction “band” and thus the only

recombination takes place between highest level in the valence d.” The interaction between other states is highly unlikely.


Figure 1.5 The spectrum of quantum dots with different size distributions. As the variance in size increases, the spectrum becomes

But in quantum dots, due to the electrons being confined in a small

space (on the order of wavelength of electron wavefunction), the energy levels

in the conduction and valence bands move far apart and can no longer be

considered continuous as shown in

an electron in an energy level higher than the “conduction band” edge is

negligible. Hence, the only source of luminescence from QD

of electrons from the bottom of conduction “band” to top of the valence

“band.” If the same material is used, the bandgap is only dependent on the

size of the quantum dots. As

bandgap increases as shown in

section 1.2. So, the source of a spread in luminescence spectrum from qua

tum dots is the distribution in size of the quantum. The tighter the distrib


14

The spectrum of quantum dots with different size distributions. As the variance in size increases, the spectrum becomes

broader.

in quantum dots, due to the electrons being confined in a small



considered continuous as shown in Figure 1.4. Now the probability of finding


negligible. Hence, the only source of luminescence from QDs is the relaxation



size of the quantum dots. As the size of the quantum dots decreases, the

s as shown in Figure 1.7 and described in more details in

, the source of a spread in luminescence spectrum from qua

tum dots is the distribution in size of the quantum. The tighter the distrib

INTRODUCTION

The spectrum of quantum dots with different size distributions. As the variance in size increases, the spectrum becomes

in quantum dots, due to the electrons being confined in a small



. Now the probability of finding


s is the relaxation



creases, the

and described in more details in

, the source of a spread in luminescence spectrum from quan-

tum dots is the distribution in size of the quantum. The tighter the distribu-


tion of quantum dot sizes, the sharper is the spectrum of quantum dot l

minescence as shown in

Figure 1.6 Silicon quantum dots of different size emitting various colors of the spectrum. Courtesy: Dr. Rick Liptak

The ability to control the size of quantum dots also gives quantum dots

the ability to emit lights of different colors due to different bandgaps. An

example of different colors emitted by the quantum dots of same material

produced by the same process is shown in

quired to produce different colors is to change the growth conditions. This

property makes quantum dots extremely versatile for use in various applic

tions requiring different colors of light. Electronic displays are one such a

plication, where color accuracy and sharp spectrum are extremely important.

Quantum dot based devices can be used to creat

cause research and development cost can be highly reduced as one single

process can produce all the primary colors of the spectrum.


15

tion of quantum dot sizes, the sharper is the spectrum of quantum dot l

minescence as shown in Figure 1.5.

Silicon quantum dots of different size emitting various colors of the spectrum. Courtesy: Dr. Rick Liptak [26].



example of different colors emitted by the quantum dots of same material

produced by the same process is shown in Figure 1.6. The only change r


ntum dots extremely versatile for use in various applic

tions requiring different colors of light. Electronic displays are one such a


Quantum dot based devices can be used to create such devices cheaply b


process can produce all the primary colors of the spectrum.

INTRODUCTION

tion of quantum dot sizes, the sharper is the spectrum of quantum dot lu-

Silicon quantum dots of different size emitting various



example of different colors emitted by the quantum dots of same materials

. The only change re-


ntum dots extremely versatile for use in various applica-

tions requiring different colors of light. Electronic displays are one such ap-


e such devices cheaply be-


1.5 Structure of Dissertation INTRODUCTION

16

1.5. Structure of Dissertation

• Chapter 2 introduces quantum dot - light emitting devices

(QD-LEDs). It also details the device structure and functioning of

QD-LEDs.

• Chapter 3 details the fabrication and testing methods of QD-LEDs.

It also covers the results and analysis of charge transport in or-

ganic-inorganic hybrid QD-LEDs.

• Chapter 4 covers the model description of QD-LEDs and simulation

results for the model.

• Chapter 5 concludes the dissertation with most important results

and future work.

1.5 Structure of Dissertation

Figure 1.7 Dependence of

Structure of Dissertation INTRODUCTION

17

Dependence of bandgap on size of Si quantum dotAdapted from [27].

INTRODUCTION

quantum dots.

2.1 Device Structure REVIEW OF QD-LEDS

18

Chapter 2

Review of QD-LEDs

2.1. Device Structure

A generic device structure of quantum dot - light emitting device

(QD-LED) is shown in Figure 2.1. Any QD-LED basically consists of an

electron transport layer (ETL), which facilitates the transport of electrons

from the cathode and blocks the transport of holes. The hole transport layer

facilitates the transport of holes from the anode to the quantum dot layer and

blocks the transport of electrons. The quantum dot layer acts the recombi-

nation regime, where the transported electrons and holes recombine to create

photons. A band diagram of a QD-LED is shown in Figure 2.2. It shows

the basis on which the materials for ETL and HTL are chosen. The ETL is

chosen such that its conduction band aligns with the conduction band of the

QDs which makes it easy for the electrons to be transferred from ETL to QD

layer. The valence band of ETL is much lower than the valence band of the

QDs, so that the holes require a large amount of energy to be transferred from

QDs into the ETL. Hence, the electron transport layer also acts as a hole

2.1 Device Structure REVIEW OF QD-LEDS

19

blocking layer. Similarly, the hole transport layer is chosen such that the

electrons are transported easily from the HTL to the QD layer and electrons

are blocked from being transported from the QD layer to HTL.

When a power supply is connected to the device, with the positive ter-

minal connected to the anode and the negative terminal connected to the

cathode, electrons are injected into the ETL and holes are injected into the

HTL. Electrons and holes are finally transported into the QD layer, where

they recombine to emit light. The electrons and holes are blocked by HTL

and ETL respectively, from overshooting out of the QD layer (which reduces

the efficiency of light emission). As explained in Chapter 1, the wavelength

of light emission can be changed by changing the size of QDs, everything else

staying the same, as long as the band alignment remains reasonable.

QD-LEDs can be divided into two major types:

• Organic transport layers.

• Inorganic transport layers.

Each of these will be explored in detail in the following sections.

2.2 Organic Transport Layers REVIEW OF QD-LEDS

20

2.2. Organic Transport Layers

Colvin et al. were the first to demonstrate a working QD-LED based on

organic transport layers [28]. p-paraphenylene vinylene (PPV) was used as

the transport layer and the QDs were made of CdSe. Indium tin oxide (ITO)

acted as the transparent anode and Mg was used as the opaque and reflective

cathode. ITO coated glass was used as the substrate for device fabrication.

CdSe QDs were dissolved in toluene and spin coated 5 times, to create ordered

sheets of CdSe QDs. 100 nm PPV layer was spin coated on top to reduce

electric fields in the device at the voltages required for device operation.

They used two device configurations (shown in Figure 2.3):

• structure A: PPV acted as electron transport layer

• structure B: PPV acted as hole transport layer

In structure A, once the electrons are injected from Mg cathode into the

PPV layer, they get transported into the QD layer, but there is no barrier for

electrons to stop them from overshooting into the ITO anode. Similarly,

once holes are injected from ITO anode into QD layer, they can easily over-

shoot into the PPV layer. Thus, the light emission efficiency of this device is

very poor. In structure B, both electrons and holes experience a barrier at

the QD/PPV interface, thus the probability of overshoot of carriers is highly

reduced. As a result structure B has higher light emission compared to

structure A.


21

There was a lot of advancement in core/shell quantum dot growth be-

tween 1994 and 1997 [29-31]. Core/shell quantum dots show increased

photoluminescence (PL) efficiency, primarily due to resistance to oxidation of

the core. As discussed in Chapter 1, the light emission wavelength of QDs is

dependent on the size of the quantum dot. Upon oxidation the emission

wavelength decreases due to reduction in effective size of the QDs. QD-LEDs

should be able to give consistent performance at the desired wavelength for

long periods of time, so, maintaining a constant size is essential. For com-

pound semiconductors, however, there is a second problem. Oxidation is al-

most always preferential to one of the elements leaving a high concentration of

defects at the oxides/semiconductor interface. These centers often provide a

high concentration of states in the gap, acting as effective recombination

centers. Thus, resistance to oxidation one of the most important qualities of

QDs required for QD-LEDs.

In 1997, Schlamp et al. improved the efficiency and lifetime of the device

used by Colvin et al. (described above) by using core/shell CdSe quantum

dots [32]. The core was composed of CdSe and the shell was made of CdS.

The CdS shell helps avoid oxidation of CdSe core, which results in increased

luminescence efficiency of devices based on core/shell quantum dots. CdS also

separates the carriers, which are confined to the core, from defects that are

highly likely to occur at the shell/air or shell/matrix interface. They used two

different QD sizes to demonstrate that different colored LEDs could be cre-


22

ated using the same structure and materials, just by changing the QD size.

The PL and EL (electroluminescence) spectra for the two different sized QDs

are shown in Figure 2.4. An increase of a factor of 20 was observed in

quantum efficiency and device lifetime increased by a factor of 100 when the

device structure used was the same as structure B in Figure 2.3.

In 2002, Coe et al. improved upon the above designs, to include both

electron and hole transport layers, which could confine the electrons and holes

in the quantum dots, increasing the light emission efficiency [33]. The en-

ergy level diagram of the device is shown in Figure 2.5. They used a novel

technique of phase separation to create a monolayer of quantum dots. So,

instead of using polymers as the hole transport layer, small molecule organic

material, N,N’-diphenyl-N,N’-bis(3-methylphenyl)-(1,1’- biphenyl)-

4,4’-diamine (TPD) was used, which would not trap the quantum dots in the

organic semiconductor-QD solution. A TPD-QD solution was spin coated on

ITO, which results in phase separation of QDs from TPD. If the right ratio

of QDs and TPD are used, it results in a monolayer of QDs on top of TPD

HTL. Coe et al. were able to achieve 22% - 100% coverage (a complete

monolayer of QDs) using this method, by varying the ratio of QDs and TPD

in the TPD-QD solution. After growing QD and HTL layers, the ETL which

again consisted of small molecular organic semiconductor,

tris-(8-hydroxyquinoline)aluminium (Alq3), was deposited using thermal


23

evaporation. A cathode consisting of 10:1 Mg:Ag followed by Ag cap was

deposited using thermal evaporation.

Figure 2.6(a) shows the electroluminescence (EL) spectrum of the device.

As shown in Figure 2.5, the barrier for the holes to be overshot into the Alq3

ETL is very small, which results in a significant recombination of electrons

and holes in Alq3 layer. Thus, only 62% of the light output is a result of

emission from QDs. But, upon introducing an additional hole blocking

layer, 3-(4-biphenylyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole (TAZ), between

QDs and Alq3 increases the light output from quantum dots to 85% as shown

in Figure 2.6(b). By using this device structure, Coe et al. improved the

efficiency 25 fold compared to the device structure used by Schlamp et al.

[32,33]. So, it is extremely important to control the location of recombina-

tion in QD-LEDs to maximize the light output efficiency.

In the phase-separation method as described by Coe et al., the materials

that can be used are limited by the ability of QDs to form a solution with the

HTL and to phase-separate during spin-coating. This limits our flexibility in

choosing materials. We may not be able to choose a material which has

better band alignment, but not chosen because QDs cannot phase-separate

during spin-coating, or QDs and HTL are not soluble in the same solvent.

Kim et al. presented a solution to this problem by using contact printing of

QDs [34]. A schematic of this contact printing method is shown in Figure

2.7. The major steps of contact printing are:


24

1) A patterned silicon master is created by using photolithography

and plasma etching. Poly(dimethylsiloxane) (PDMS) is molded

using the silicon master mold.

2) The PDMS stamp is coated with parylene-C using chemical vapor

deposition (CVD). Parylene-C is more resilient to chemicals and

moisture than PDMS, so, it increases the longevity of PDMS

stamp.

3) Then, the PDMS stamp is spin-coated with QDs dissolved in a

solvent and allowed to dry.

4) Finally, the PDMS stamp is brought in contact with the substrate

which results in the transfer of QDs from the stamp to the sub-

strate. The QDs in the trenches in the PDMS stamp are not

transferred because they do not come in contact with the sub-

strate.

A side, but important, benefit of using the contact printing method is that

QDs can be transferred onto the substrate in a pattern, which can be used to

create different colored pixels on the same substrate if aligned properly. An

example of a patterned matrix of different colored QD-LEDs is shown in

Figure 2.8. Figure 2.8(c) shows a schematic of the patterned device with

perpendicular red and green patterns and Figure 2.8(d) shows a photograph of

a working patterned QD-LED. The light blue color is emitted by TPD,

whereas red and green color is emitted by QDs.

2.3 Inorganic Transport Layers REVIEW OF QD-LEDS

25

Anikeeva et al. extended the contact printing technique to vary the

QD-LED color over the entire visible spectrum, by changing the QD size and

composition [35]. Figure 2.9 shows various photographs of QD-LEDs of

different colors. Their PL, external quantum efficiency (EQE), power

efficiency and current density – voltage characteristics are also shown in

Figure 2.9.

2.3. Inorganic Transport Layers

QD-LEDs with organic transport layers suffer from all the problems that

ail organic light emitting diodes (OLED) such as, limited lifetime, degradation

on exposure to air and moisture, and instability of metal contacts [36,37].

Thus, Caruge et al. were the first to explore the possibility of using wide band

gap inorganic semiconductors as transport layers for QD-LEDs [38]. In their

work, instead of replacing both electron and hole transport layers, they just

replaced the hole transport layer by NiO. The electron transport layer was

kept organic (Alq3). NiO was chosen as HTL because it had previously been

used as hole transport/injecting layer in optoelectronic devices [39-41].

The role of transport layers is to avoid quenching of QD

electroluminescence due to proximity to the contacts, due to the infinite

recombination velocity at the contacts. The NiO transport layers should not

have too many charge carriers so as to cause substantial quenching of QD EL.

At the same time, the carrier density in NiO should not be so low that it


26

causes a dramatic increase in the overall resistance of the device. Sheet

resistance is inversely proportional to carrier density so it can be used a good

estimator of carrier density assuming that the mobility is nearly constant.

Caruge et al. conducted PL experiments by depositing QDs on NiO films of

varying resistivity [38]. It was found that for NiO films with high resistivity

( cm⋅Ω≈1ρ ), the PL of QDs remains high, whereas for NiO with low

resistivity ( cm⋅Ω×≈ −4105ρ ), it was quenched.

NiO resistivity can be varied by changing the oxygen to argon ratio

during sputtering. The controllability of resistivity is useful not only to

avoid quenching of EL of QDs, but, also gives us control on charge balance in

the QDs. If the hole concentration in QDs is higher than electron

concentration, the resistivity of NiO can be increased to reduce the hole

concentration in QDs to achieve better charge balance. If the hole

concentration in QDs is lower than that of electrons, the resistivity can be

decreased to achieve better charge balance and consequently, better efficiency.

The device is fabricated by sputtering NiO on ITO coated glass, followed

by spin coating of CdSe QDs, followed by thermal evaporation of Alq3 and Mg

contact. The band diagram of this device is shown in Figure 2.10(a). Two

different devices were fabricated by Caruge et al. with different NiO

resistivities; the current-voltage characteristics of which are plotted in Figure

2.10(b). At low voltages, the current density is limited by the resistivity of

NiO layer, hence the current density for low resistivity NiO device is much


27

higher than the current density for the device with high NiO resistivity. But

at high voltages, the current is limited by transport through the quantum

dots, hence, the two J-V curves overlap each other at high voltages.

In 2008, Caruge et al. made a device with both transport layers being

organic [42]. NiO was used as HTL and ZnO:SnO2 was used as ETL. The

device structure and corresponding band diagram is shown in Figure 2.11.

Because, the ETL is deposited by sputtering, there is a lot of flexibility in

choosing the material for ETL. Thus, a ZnO:SnO2 alloy was used as the

ETL, because it acts as a good transporter of electrons with high electron

mobility as well as the conduction band aligns well with the Fermi level of the

cathode and the conduction band of quantum dots. At the same time, the

valence band is deep enough that it blocks the overshoot of holes from the

QDs into the ETL. They observed that inorganic QD-LEDs were more ro-

bust to atmospheric conditions as compared to QD-LEDs with organic

transport layers. They retested after storing unpackaged devices in air for

four days and saw no degradation in performance, which is impossible to see

for unpackaged QD-LEDs based on organic transport layers. They were also

able to drive higher currents through the device compared to organic

QD-LEDs.

2.4 Latest Developments REVIEW OF QD-LEDS

28

2.4. Latest Developments

QD Vision (http://www.qdvision.com/) is a startup that exclusively

manufactures QD-LEDs. Sony is set to use QD technology in their latest

line of televisions, Triluminos [43]. Conventional LCD TVs use a blue LED

coated with phosphors to convert the blue light to white which acts as the

backlight. In case of QD based LCD TVs, the blue LED will be uncoated

and will be placed in a thin glass tube containing QDs. The QDs will be

such that they will absorb blue light and emit pure red and green light. The

resultant white light is more intense at these wavelengths (which are the

primary colors of LCD TVs) compared to conventional phosphor coated

LEDs. Thus, the color is brighter and the TV is more efficient compared to a

conventional LCD TV.


29

Figure 2.1. Schematic of a generic QD-LED.


30

Figure 2.2 Band diagram of a generic QD-LED.


31

Figure 2.3 QD-LED with only ETL (structure A) or only HTL (structure B). In both cases PPV acts as the transport layer. From [28].


32

Figure 2.4 PL (solid lines) and EL (dashed lines) from two different sized CdSe/CdS core-shell QDs (a) 2.5 nm core/ 0.5 nm shell (b) 3.4 nm core/ 0.7 nm shell [32].


33

Figure 2.5 Energy band diagram of device used by Coe et al. containing both electron and hole transport layers [33].


34

Figure 2.6 EL spectrum of (a) device with ETL and HTL (b) device with additional hole blocking layer [33].


35

Figure 2.7 Schematic of contact printing of QDs [34].


36

Figure 2.8 Patterned QD-LED created by Kim et al. (c) Device schematic (d) Photograph of light emission [34].


37

Figure 2.9 (a) Photographs of operating QD-LEDs of different colors. (b) EL (solid lines) and PL (dashed lines) corresponding to these devices. (c) Photograph of luminescence of different colored QDs. (d) EQE (e) Power efficiency and (f) J-V characteristics of these devices [35].


38

Figure 2.10 (a) Band diagram of device with NiO inorganic HTL by Caruge et al. (b) J-V characteristics of the device with high resistivity

(squares) and low resistivity (circles) NiO [38].


39

Figure 2.11 (a) Device structure with inorganic HTL and ETL by Caruge et al. (b) Band diagram of the device [42].

3.1 Introduction CHARGE TRANSPORT IN HYBRID QD-LEDS

40

Chapter 3

Charge Transport in Hybrid

QD-LEDs

This chapter has been reprinted with the permission from Kumar,

Brijesh, et al. "Comparing direct charge injection and Forster energy transfer

into quantum dots in hybrid organic/inorganic quantum dot light emitting

devices." Journal of Applied Physics 112.3 (2012): 034501-034501, Copyright

2012, AIP Publishing LLC. [44]

3.1. Introduction

The advent of organic electronics has allowed the integration of inorganic

materials which have good optoelectronic properties. The ability of materials

such as quantum dots (QDs) to be deposited from a solution allows one to

incorporate them into low-cost optoelectronic devices. Over the last few years

several groups have demonstrated such devices.[32,45,46]

There are various factors that influence the emission from Quantum Dot

Light Emitting Devices (QD LEDs). One of the most important of those is the

mechanism for energy and/or charge transfer to the recombination sites

3.1 Introduction CHARGE TRANSPORT IN HYBRID QD-LEDS

41

(QDs). This transfer generally takes place either by direct charge injection of

electrons and holes or by Forster energy transfer of excitons from organic

molecules onto the quantum dots.[46-48] Anikeeva et al. suggest that the

light output from QDs can be increased by confining carriers near the QDs to

promote both mechanisms, but asserts that Forster energy transfer is the

primary mechanism.[48] Chin et al. suggest that light output is maximized by

using an electron transport layer that enhances direct charge injection and

reduces Forster energy transfer.[47] In this work, we investigate the charge

transfer mechanisms into CdSe quantum dots. In agreement with Chin, we

show that carrier confinement increases QD electroluminescence (EL) effi-

ciency. We also find that, for the devices studied here, the optical outputs of

the two mechanisms are independent. That is, the total optical output is well

described by the sum of the two mechanisms, independent of current injection

density. This may be a property of devices in which the Forster mechanism

does not change the population of carriers. In such devices both mechanisms

can be combined to increase the amount of light output from hybrid QD LED.

3.2 Experimental Method CHARGE TRANSPORT IN HYBRID QD-LEDS

42

3.2. Experimental Method

Cadmium selenide quantum dots with an outer shell of zinc sulfide were

synthesized using the SILAR method modified from Reiss et al. [49] The

successive ion layer adsorption and reaction (SILAR) technique deposits a

monolayer at a time of the shell on to the core nanocrystals, and gives ex-

cellent thickness control.[50] This technique has the advantage of avoiding the

nucleation of shell particles, since only one precursor is introduced at a time

and the process can be done in the same reaction vessel as the growth.

To prepare the quantum dots, a mixture of cadmium oxide (CdO, 0.411

g), dodecylphosphonic acid (DPA, 1.6 g) and hexadecylamine (18 g) in

trioctylphosphine oxide or TOPO (8.4 g) was heated to 270°C under a ni-

trogen atmosphere. A solution of trioctylphosphine:selenium (TOP:Se, 4

mL) in TOP (10 mL) was rapidly injected and the particles were grown for 15

minutes at 250°C. The solution was then cooled to 220°C for SILAR injec-

tions of 0.1 M zinc stearate and sulfur solutions. Each monolayer was grown

for 15 minutes per precursor, for a total of two ZnS monolayers. After

completion, the particles were precipitated and washed with methanol, cen-

trifuged, and redispersed in chloroform. Figure 3.1 (a) and (b) show the

UV/Vis and photoluminescence (PL) spectra of the CdSe nanoparticles. The

UV/Vis spectrum showed a first absorption peak at 626 nm, corresponding to

5.6 nm nanocrystals. The second and third absorption peaks of CdSe are also


43

Figure 3.1 (a) UV/Visible Absorbance and (b) photoluminescence (PL) spectra of CdSe quantum dots.

visible at 550 and 510 nm respectively. The PL spectrum showed emission at

635 nm.

The substrates used for the fabrication of the devices, were glass slides

precoated with indium tin oxide (ITO), obtained from Delta Technologies

Limited. ITO is a transparent conducting oxide commonly used as an elec-

trode in light emitting devices. In our device, it acts as an anode, injecting

holes into the device. Before use the substrates were cut to squares of about

half an inch, then cleaned by sequentially ultrasonicating in water, acetone,

methanol and iso-propanol with a N2 blow dry step after each solvent. After


44

Figure 3.2 Device structures for (a) Device 1, (b) Device 2 and (c) Device 3.

the cleaning, the ITO coated glass slides were exposed to UV-ozone for 10

minutes to remove any residual carbon contamination from the solvents.

Meanwhile, a solution of N,N’-Bis(3-methylphenyl)-N,N’-diphenyl-

benzidine (TPD) (obtained from Sigma-Aldrich Co.) was prepared in chlo-

roform. The suspension of CdSe/ZnS quantum dots in chloroform was added

to this solution such that the final concentration of dots was 50 mg/ml and

that of TPD was 10 mg/ml. A 50 mg/ml solution of quantum dots in chlo-

roform with no added TPD was also prepared. PEDT:PSS

(Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) which is available

in solution form (with water as a solvent) from H.C. Starck GmbH under the

commercial name CLEVIOS™ P VP CH 8000, acts as a hole transport layer.

It was filtered just before deposition using a syringe filter (pore size: 0.2 µm)

to remove any particles formed during transport and storage.

Three sets of devices were prepared, one without the PEDT:PSS layer

(device 1, Figure 3.2(a)) and two with the PEDT:PSS layer. The two devices

with the PEDT:PSS layer had different emission layers, one with a matrix of


45

TPD and quantum dots (device 2, Figure 3.2(b)) and one with just quantum

dots (device 3, Figure 3.2(c)). The procedure for three sets of devices was the

same except for the deposition of the PEDT:PSS layer. To avoid degradation

due to exposure to air and moisture, all of the processing was done inside a

nitrogen-filled glove box. A 30 nm thick PEDT:PSS layer was deposited by

spin coating at 3000 rpm. It was then baked on a hot plate at 120°C for about

1 hour to remove any residual solvent. This bake also hardens the film so that

the next layer to be deposited does not affect the morphology of the film. An

advantage of using PEDT:PSS layer as the hole transport layer is that it is

soluble in water, but not chloroform. The deposition of the next layer from a

chloroform solution does not affect the PEDT:PSS film.

For device 2, a 40 nm thick film of TPD matrix embedded with quantum

dots (emission matrix) was deposited from the solution by spin coating on top

of the PEDT:PSS film. For device 3, a film of quantum dots was deposited by

spin coating a suspension of quantum dots. In both cases this was followed by

baking on hot plate at 120°C for about 45 minutes to remove residual solvents.

This was followed by the deposition of a 40 nm thick bathocuproine (BCP)

film by thermal evaporation of BCP powder (Sigma-Aldrich Co.) BCP acts as

the electron transport layer. A stencil mask with one mm diameter holes was

placed on top of the substrate followed by the deposition of 400 nm thick Al

cathodes by thermal evaporation of Al pellets (Sigma-Aldrich Co.) The

thickness of the spin coated films was measured using Variable Angle Spec-

3.2 Experimental Method

Figure 3.3 (a) Height and (b) phase image of spin coated mixture of QDs and TPD.

troscopic Ellipsometer (VASE, J.A. Woollam Co.). The thermally evaporated

film thickness was measured in real time using a quartz cry

power to the deposition boats was controlled such that the deposition rate was

constant to get high quality films. The devices were taken out of the glove box

just before testing to minimize exposure to air and moisture.

Testing of the devices was done using an Agilent 4156A parameter an

lyzer attached to two probes: a hard probe (tungsten) and a

gold wire). The hard probe is used to contact the anode and so pierces the

deposited films, making contact with the ITO. The soft probe is chosen to

make contact with the cathode so that it does not pierce the thin Al layer. As

a constant electric current is supplied to the device using the parameter a

alyzer, the light spectrum is measured using USB2000 spectrometer (Ocean

Experimental Method CHARGE TRANSPORT IN HYBRID QD

46

(a) Height and (b) phase image of spin coated mixture of


film thickness was measured in real time using a quartz crystal monitor. The



just before testing to minimize exposure to air and moisture.

Testing of the devices was done using an Agilent 4156A parameter an

lyzer attached to two probes: a hard probe (tungsten) and a soft probe (thin




electric current is supplied to the device using the parameter a


HYBRID QD-LEDS

(a) Height and (b) phase image of spin coated mixture of


stal monitor. The



Testing of the devices was done using an Agilent 4156A parameter ana-

soft probe (thin




electric current is supplied to the device using the parameter an-


3.3 Results and Discussion CHARGE TRANSPORT IN HYBRID QD-LEDS

47

Figure 3.4 Current density - Voltage (J-V) characteristics of device 1. It is also representative of the J-V characteristics for devices 2 and 3.

Optics Inc.). The spectrometer is attached using a fiber optic cable, with a

core diameter of 600 µm, to the back of the device, so as to collect the light

from the transparent side of the device. Care was taken in placement so as to

cover the end of the optical fiber completely by the device that was being

tested.

3.3. Results and Discussion

Figure 3.3 shows the height and phase atomic force microcopy (AFM)

images of the spin coated mixture of TPD and QDs. Both height and phase

images show a uniform pattern. In a similar process, Coe [46] observed ex-


48

Figure 3.5 EL intensity curves at different current densities for device 1. The peak at 460 nm corresponds to EL from TPD and the peak at 610 nm corresponds to EL from CdSe/ZnS quantum dots. Inset shows the QD to TPD emission ratio, which is almost constant at 0.2

tremely smooth (rms roughness 0.57 nm by atomic force microscopy) regions

and extremely rough (no value given, but obviously very rough) regions and

ascribe the behavior to phase segregation. Our films appear to be uniform,

with no observable rough regions. The rms roughness of 300 nm by 300 nm

areas varied from 1.6 to 2.1 nm. We take this value to be consistent with in-

dividual nanoparticles rather than agglomerates. Although the reason for this

difference is uncertain, we note that our suspensions were far more concen-

trated than those of Coe, 50% by volume rather than 10%. Thus, our analysis


49

Figure 3.6 EL intensity curves at different current densities for device 2. Inset shows the QD to TPD emission ratio, which is almost constant at 1.0.

assumes a uniform mixture of QDs and TPD in the QD+TPD layers shown in

Figure 3.2.

Figure 3.4 shows the J-V characteristics of device 1 which is representa-

tive of J-V characteristics of other devices as well. The semilog plot in Figure

3.4 shows the diode like characteristics of our device. As we increase the ap-

plied voltage, the series resistance of the device plays a more significant role

and thus the slope of the curve constantly decreases with increasing voltage.


50


Figure 3.5 shows the EL spectrum of the device without the PEDT:PSS

layer (device 1) for the three different current densities (10 mA/cm2, 50

mA/cm2 and 100 mA/cm2). There are two peaks in the EL spectrum. The

peak at 460 nm corresponds to EL from TPD. [51] The peak at 610 nm cor-

responds to EL from CdSe/ZnS quantum dots. When compared to the PL

spectrum, the peak is shifted by about 25 nm. This small shift may be due to

different spectrometers being used for measuring solution based PL and solid

state EL, or it could be due to some minor oxidation of the QDs when testing


51

Figure 3.8 Band diagram for device 2. The band diagrams for other two devices can be derived from this, either by removing PEDT:PSS (device 1) or by removing TPD (device 3).

them in devices. The EL intensity from TPD is much stronger than that of

quantum dots. The inset shows that the ratio of QD emission to TPD

emission is almost constant (~0.28). Figure 3.6 shows the EL spectrum of

device with the PEDT:PSS layer (device 2) for three different current densi-

ties. Here again two peaks are observed, but this time the intensity of EL from

quantum dots is comparable to that of TPD. The inset showing the QD/TPD

emission ratio again shows that the emission ratio (~1.0) is not dependent on


52

current density. Finally, Figure 3.7 shows the EL spectra for three current

densities for the device with just the quantum dot layer and no TPD (device

3). As expected, there is only one EL emission peak observed at 610 nm,

which corresponds to emission from the quantum dots. The intensity of

emission increases with increasing current density as one would expect.

Figure 3.8 shows the band diagram for device 2. The band diagrams of the

other two devices can be deduced from Figure 3.8. The carrier transport

mechanism can be most easily understood for device 3. Electrons are injected

by the Al cathode and transported via the BCP layer to the CdSe quantum

dots. Holes are injected by the ITO anode and transported via the PEDT:PSS

layer to the quantum dots. Electrons and holes recombine on the quantum

dots resulting in the emission spectrum observed in Figure 3.7. There is no

possibility of Forster energy transfer from PEDT:PSS layer into the CdSe

quantum dots because the bandgap of the quantum dots is higher than that of

PEDT:PSS. Thus, the only mechanism of charge transfer into the quantum

dots in device 3 is direct charge injection. Figure 3.9 shows a comparison of

EL from devices 1, 2 & 3 and the sum of EL from devices 1 & 3 at 100

mA/cm2. In devices 1 and 2, there is a possibility of both direct charge in-

jection and Forster energy transfer from TPD into the quantum dots. Looking

at TPD emission first, we find that the addition of the PEDT:PSS hole

transport layer appears to decrease the emission from TPD. This is may be

due to competition between the emission mechanisms. Indeed, one can clearly


53

Figure 3.9 Comparison of EL intensities from devices 1, 2 & 3 with the sum of EL intensities of device 1 & 3 at 100 mA/cm2 showing that, EL from QDs for device 2 and sum of device 1 and 3 overlap. Inset shows that the same is true at 10 and 50 mA/cm2 as well.

see that the QD emission for device 2, where both carrier types are confined

near the QDs, is much stronger than it is for devices without the confining

layers.

An unexpected result however, is that the EL emission from the QDs for

device 2, which has both TPD and the confining layers, is exactly the same as


54

the EL of the device with only TPD plus the EL of the device with only the

confining layer. This is not a numerical fluke. It is true for all values of current

density that were measured (see inset). Thus, it can be concluded that the two

charge transfer mechanisms in device 1 and 3 act in parallel to give the light

output in device 2. Thus, the EL from quantum dots in device 1 is completely

due to Forster energy transfer from the TPD layer. The amount of light

output by the QDs in device 1 is limited by the efficiency of Forster energy

transfer. This depends on various factors including the materials used and the

device configuration as demonstrated by Jing et al. [52] Some of the excitons

in TPD are transferred onto QDs by the Forster process, some radiatively

recombine, and the remaining non-radiatively recombine. In device 2, the

amount of light output by QDs is increased as we are no longer limited by the

Forster energy transfer efficiency. Here, direct charge injection provides car-

riers that can radiatively recombine. Hence, instead of competing, these two

mechanisms, direct charge injection and Forster energy transfer, act inde-

pendently to maximize the light output from the QDs. [47,48]

The above conclusions are also supported by the band diagram drawn in

Figure 3.8. For device 1, the barrier for hole transfer from ITO to CdSe is 1.8

eV whereas the barrier for hole transfer from ITO to TPD is 0.7 eV. Thus,

most of the holes are first transferred into TPD and then, upon formation of

excitons, are transferred into the quantum dots via the Forster mechanism. In

device 2, there is an intermediate PEDT:PSS layer about 30 nm thick, so the


55

only place for the holes to go is into this layer even though the barrier is 1.7

eV. Once holes are in this layer, they can either go into the CdSe quantum

dots by overcoming a small (0.1 eV) barrier or into TPD by losing energy. The

holes that travel directly into the quantum dots recombine there and account

for the direct charge injection component of the EL intensity, whereas, the

holes traveling into the TPD layer account for the Forster energy transfer

component of the EL intensity.

4.1 Introduction MODELING OF QD-LEDS

56

Chapter 4

Modeling of QD-LEDs

This chapter has been reprinted with the permission from Kumar,

Brijesh, et al. "Modeling Charge Transport in Quantum Dot Light Emitting

Devices with NiO and ZnO transport layers and Si Quantum Dots" Journal of

Applied Physics (2013): Copyright 2013, AIP Publishing LLC.

4.1. Introduction

Colloidal quantum dots (QDs) are well suited for display technologies

because of their tunable band gap and hence wavelength tunable luminescence

properties. [32,33,44,53] Optimization and other development efforts

potentially can be minimized for different colored light emitting diodes

(LEDs) because essentially the same structure may be used to create LEDs of

various colors. If appropriately chosen, electron and hole transport layers

and their contacts need not be varied. The only structural feature that

determines the emission wavelength is the size of the QDs. The procedure to

prepare the QDs may stay the same (although the conditions need to be

varied to create different sizes), the procedure for deposition of the QDs onto

the device could stay the same, the electrode deposition could stay the same,

4.1 Introduction MODELING OF QD-LEDS

57

and the transport layer deposition could stay the same. Hence, all the design

and optimization required for each of these individual device components can

be useful for LEDs of different colors.

Quantum dot light emitting devices (QD-LEDs) were first presented in

the form of hybrid organic/inorganic light emitting devices by Colvin et al.

[28] and were later improved upon by Coe-Sullivan et al.[54] For the hybrid

devices, the transport layers were fabricated from organic semiconductors and

the QDs from inorganic semiconductors. However, organic transport layers

suffer from sensitivity to air exposure and stability problems. Hence,

inorganic semiconductor transport layers, NiO and ZnO, were developed and

used in a device by Caruge et al. [42] This work deals with modeling of

QD-LEDs with ZnO and NiO transport layers and Si QDs. It can readily

be extended to other inorganic transport layers and QDs. The model can also

be used for hybrid QD-LEDs by modifying the transport model to allow for

the field and density dependence of the carrier mobility that usually need to

be considered for organic semiconductors.

QDs have been a subject of intense study ever since their discovery by

Ekimov et al. in 1981.[55] Most of the studies concerning transport in

quantum dots are related to microscopic single electron transport at

extremely low temperatures.[56-59] Relatively few studies deal with charge

transport at room temperature, which is the range of primary interest for

applications like light emitting or memory devices.[60,61] In this work, a

4.2 Model Description MODELING OF QD-LEDS

58

complete model for modeling charge transport in QD-LEDs is proposed. It

incorporates charge transport in the bulk, followed by charge transfer into the

quantum dot layers and finally, the transfer amongst the various QD-layers

and recombination in the QDs.

4.2. Model Description

The schematic device structure for a QD-LED is show in Figure 4.1.

Figure 4.2 shows the corresponding band diagram. The device essentially

consists of a number of quantum dot layers sandwiched between two thin

transport layers, an electron transport layer (ETL) and a hole transport layer

(HTL). The HTL is contacted by an indium tin oxide (ITO), transparent

conducting layer, which acts as a hole injector, whereas, the ETL is connected

to an Aluminium cathode, which acts as an electron injector. The device

functions as follows. Electrons are injected from the cathode and holes from

the anode and they travel through their respective transport layers to the

QDs, where they recombine. If the recombination is radiative, photons are

emitted. The photon wavelength depends on the QDs and is completely

independent of the transport layers.

The device model can be divided into the following parts:

A. Carrier injection from transport layers into the quantum dot layers

B. Transport among the quantum dot layers

C. Recombination in the quantum dots


59

D. Coupled rate equations

E. Transport in the ETL and HTL

F. Carrier injection from the contacts

Each of the different components of the model are described in the

following subsections.

4.2.1 A. Carrier injection from transport layers into quantum dot

layers

Focusing on the QD layer adjacent to the HTL, the quantum dots closest

to the HTL are considered to be traps for holes. In equilibrium, using

detailed balance, the rate of capture of holes by these QDs is the same as the

rate of emission,

(4.1a)

where, NT is the density of trap states in the quantum dot layer (equivalent to

the density of quantum dots since each QD can only accommodate one hole),

p(0) is the hole density in the quantum dot layer adjacent to the HTL, p(0-) is

the hole density in the HTL very close to the quantum dot layer, ẽ is the

emission rate coefficient for the trap states, and, c is the capture rate

coefficient for the trap states. Whenever a subscript (0) is used it stands for

equilibrium

The ratio of emission and capture coefficients is,

)0()0(

)0(~

~

00

0

0

0−−

=ppN

p

e

c

T (4.1b)

)0()0(~)0(~00000 pNpcpe T −= −


60

Under non-equilibrium conditions, the rate of change of carrier

concentration is given by the difference of capture and emission, which can be

described as:

)0(~)0()0(~ pepNpct

pT −−=

∂∂ −

(4.1c)

If the device is not too far out of equilibrium, the capture and emission

coefficients can be taken to be the same as for equilibrium. Furthermore, if

the carrier concentration is non-degenerate, eqn. 4.1c can be simplified to,

−=∂∂

−

−

)0()0()0(

)0(10

0

ppp

p

t

p

plτ (4.1d)

where τpl (= 0~/1 e ) is capture/emission time constant, a parameter which

depends on the materials used. Equations for electrons at the interface

between the ETL and the adjacent QD layer are entirely analogous.

4.2.2 Transport among the quantum dot layers

The quantum dots can be considered as semiconductor particles with an

insulating layer surrounding them.[62] The insulating layer may consist of

silicon dioxide[63] or organic ligands,[64] depending on the method used to

prepare the quantum dots. Figure 4.3 shows the model potential energy

diagram for the quantum dots. The transport from one QD to another is

viewed as a direct tunneling process, where the QDs act as potential wells and

the insulating layers as tunnel barriers.


61

Using a one-dimensional WKB approximation, the tunneling probability

from 1→2 can be written as,

( ) insdT 22exp12 κ−= , (4.2)

where kappa is the inverse characteristic length for tunneling, and, dins is the

thickness of the insulating layer around the QDs.

The electron is ‘oscillating’ in the well with ‘frequency,’ Sith dv 2/=ν ,

where, vth is the thermal velocity of electrons, dSi is the diameter of the QD,

and it has probability T12 of making a transition to the neighboring QD layer.

The total density of electrons per second tunneling from 1→ 2, assuming an

unoccupied layer 2 is,

12121 TnN ν=→ , (4.3a)

where n1 is the number of electrons in layer 1.

The ‘oscillation frequency’ is the same for both the layers because the

particle size and temperature is the same in layers 1 and 2. Thus, the total

number of electrons per second tunneling from 2→1, assuming an unoccupied

layer 1, can be written as,

21212 TnN ν=→ , (4.3b)

Consequently, the net flow of electrons from 1→2 is,

)( 2211121221 nTnTNN −=− →→ ν (4.3c)

For elastic tunneling T12 = T21, and again by detailed balance we may write

−=− →→ 2

2

11121221 n

n

nnwNN

o

o

(4.3d)


62

where, )2(2exp

21212 oxSi

th dd

vTw κν −==

and n1o and n2o are equilibrium

concentrations of electrons in layers 1 and 2, respectively. Similar equations

can be written for transport of holes in the QDs.

4.2.3 Recombination in the quantum dots

There are two types of recombination in quantum dots, radiative and

non-radiative. Radiative recombination can be either monomolecular or

bimolecular depending upon the doping of quantum dots. If the quantum

dots are doped, then the recombination rate is dependent only on the minority

carrier density, hence it is monomolecular. But if the quantum dots are

undoped, or if both non-equilirium carrier densities are comparable to the

equilibrium carrier density, then the recombination rate depends on both

kinds of carriers, i.e., it is bimolecular. In the rest of this work, we assume

that the quantum dots are undoped and hence the recombination is

bimolecular, as also shown by Kočka et al.[65]

Bimolecular recombination rate can be defined as follows:

Ur = γ(np – ni2), (4.4a)

where γ is recombination rate coefficient, n and p are electron and hole

concentrations, respectively, and ni is intrinsic carrier concentration.

Non-radiative recombination can be modeled as Shockley-Read-Hall

(SRH) recombination,[66,67] .


63

( ) ( )11

2

nnnp

npnU

pn

inr +++

−=ττ (4.4b)

Assuming, the electron and hole recombination lifetimes to be equal, τp =τn

and p,n >> n1 , the total non-radiative recombination rate can be written as,

( )np

npnU

nr

inr +

−=τ

2

(4.4c)

Both, radiative and non-radiative recombination take place in parallel in the

quantum dots, hence, the total recombination rate can be written as,

( ) ( )np

npnnpnUUU

nr

iinrr +

−+−=+=τ

γ2

2

(4.4d)

4.2.4 Coupled rate equations

The discretization scheme for the quantum dot layers is shown in Figure

4.2. From what has been described in the above subsections, the transport

equations for holes can be written in terms of coupled rate equations:

))1()1((

)1()1()1()1()2(

)2(

)1()1()1()1(

)0(

)0(1)1( 212

110

0120

01,0 np

nnpnpnp

p

ppwpp

p

p

dt

dp

nr

ii

p

p +−−−−

−−

−=τ

γτ

(4.5a)

))2()2((

)2()2()2()2()3(

)3(

)2()2()2(

)2(

)1()1(

)2( 222

220

023

0

012 np

nnpnpnp

p

ppwp

p

ppw

dt

dp

nr

ii

pp

+−−−−

−−

−=τ

γ

(4.5b)

…..


64

))()((

)()()()(

)1()1(

)()()(

)(

)1()1(

)(

22

0

01,

0

0,1

NnNp

nNnNpnNpNn

NpNp

NpNpwNp

Np

NpNpw

dt

Ndp

nr

iNiNN

pNN

pNN

+−−−−

++

−−

−−−= +−

τγ

(4.5c)

…..

))1()1((

)1()1()()(

)()1(

)1()(

1)(

)(

)1()1(

)(

212

001,0

0,1

np

nnpnLpLn

LpLp

LpLpLp

Lp

LpLpw

dt

Ldp

nr

iiLL

LpL

pLL

+−−−−

++−−

−−−=

+−

τγ

τ

(4.5d)

The transport equations for electrons can be written in analogous fashion. It

is important to note that each of the above equations contain a product of

electron and hole densities, thus, we need to employ a solution based on

non-linear system of equations.

4.2.5 Transport in ETL and HTL

Bulk drift and diffusion is used to describe the transport in the two

transport layers. It is represented by the following set of equations:

UGx

J

qt

p p −+∂

∂−=

∂∂ 1

(4.6a)

UGx

J

qt

n n −+∂∂=

∂∂ 1

(4.6b)

( )npNNq

x ad −+−−=∂∂

εψ

2

2

(4.6c)


65

where,

xqp

x

pqDJ ppp ∂

∂−∂∂−= ψµ

(4.6d)

xqn

x

nqDJ nnn ∂

∂−∂∂= ψµ

(4.6e)

Generation is assumed to be negligible and thus neglected from our

calculations (G=0). Recombination is assumed to be of Shockley-Read-Hall

form,[66,67]

( ) ( )ipin

i

nnnp

npnU

+++−=

ττ

2

(4.7)

4.2.6 Carrier injection from the contacts

The contacts at both the anode and cathode are assumed to be ohmic.

Thus, the carrier concentrations inside the transport layers, very close to the

contacts (positions 0 & w), are equal to the equilibrium carrier

concentrations.

( ) ( ) ( ) ( ) 22 )()( ;000 wnwnwpnnp ii == (4.8a)

There is no space charge at electrodes,

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 0 ;00000 =−+−=−+− wnwpwNwNnpNN adad (4.8b)

The electrostatic potential at the electrodes is thus given by:

( ) ( ) ( ) ( )

∆+∆−

=

−=

2)0(ln ;

)0(

0ln0 VC

ii

EE

n

wn

q

kTw

n

p

q

kTV ψψ

(4.8c)

4.3 Results and Discussion MODELING OF QD-LEDS

66

The last term in Eqn. 4.8c accounts for the discontinuities in the

conduction (∆EC) and valence (∆EV) band, which are estimated from

Anderson's rule[68] as described in more detail by Smith.[69] The

conduction band and valence band edges of quantum dots do not affect the

potential boundary conditions because the electric field in the device is

controlled by the difference in the Fermi levels of the transport layers, just like

a p-i-n diode where the electric field in the i-region is controlled by the

difference in the Fermi levels of p and n-regions.

4.3. Results and Discussion

4.3.1 Device Parameters

To illustrate the above described model, the example device structure

shown in Figure 4.4 is used. A 47 nm thick layer of NiO, which is naturally a

p-type, is used as the HTL, while a 50 nm thick ZnO layer, a naturally n-type

material, provides the ETL. The emission layer consists of 5 layers of silicon

QDs, each 5 nm in diameter including a 0.5 nm thick SiO2 shell acting as the

passivation layer. Thus the total device thickness is 122 nm. The

parameters for the transport layers are presented in Table 4.1. The majority

carrier mobilities are taken from the literature, but, the minority carrier

mobilities are not easily available in the literature, so, the minority carrier

mobilities are assumed to the same as majority carrier mobilities. As

demonstrated in the section below, the minority carrier mobility does not


67

matter because the current is dominated by majority carriers. Using the

method described by Smith[42,69] and the electron affinity and band gap

values given by Caruge et al.,[42] the discontinuities in the bands are

calculated to be ∆EC = 2.5 eV and ∆EV = 2.2 eV. The carrier lifetimes were

not available for NiO, so they were assumed to be the same as for ZnO as

reported by Liu et al.[70]

Table 4.1 The parameters of NiO and ZnO used for our model device.

NiO ZnO

Hole mobility, µp[71] 0.2 cm2/V-s 8.6 cm2/V-s

Electron Mobility, µn[71,72] 0.2 cm2/V-s 8.6 cm2/V-s

Doping concentration p-type, NA=1017 cm-3 n-type ND=1017 cm-3

Bandgap, Eg[42] 3.7 eV 3.4 eV

Intrinsic carrier concentration, ni 6.106 × 10-13 cm-3 2.0 × 10-10 cm-3

Electron affinity, χ[42] 1.8 eV 4.3 eV

Carrier Lifetime, τp=τn [70] 1.2 µs 1.2 µs

Table 4.2 shows the important parameters relating to the transport in the

QD layers. The intrinsic carrier concentration, ni, is based on the band gap

provided by Ligman et al.[73] The QDs are assumed to be undoped. The

tunneling coefficient is calculated using WKB approximation assuming a 0.5

nm thick oxide shell on the Si QDs. The capture lifetime of an electron/hole

by a QD is not easily available in literature. So, the capture lifetimes are

varied from 1 ms to 1 ns and the effect of such a change is studied on the

device. This is discussed in further detail in the following section. The


68

non-radiative SRH recombination lifetime is assumed to be ~1 µs as observed

by Sobolev et al. [74]

Table 4.2 The parameters used to describe transport in QD layers.

Intrinsic carrier concentration, ni 1.42 × 104 cm-3

Tunneling coefficient, w 1.25 × 1010 s-1

Capture lifetime, τ 1 × 10-3 s – 1 × 10-9 s

Bimolecular Recombination Coefficient, γ[65] 4 × 10-10 cm3 s-1

Non-radiative recombination lifetime, τnr [74] 1 × 10-6 s

Band gap, Eg[73] 1.8 eV

Electron affinity, χ[73] 3.5 eV

4.3.2 Simulation Results

Figure 4.4 shows the electron and hole carrier densities and electrostatic

potential at equilibrium and for an applied potential of 0.5 V. The built-in

potential for the device is 0.98 V as can be seen from Figure 4.4. Upon

applying the 0.5 V forward bias, the electrostatic potential across the device

goes down to 0.48 V as expected. At equilibrium, the carrier densities

decrease as we move away from the contacts and deeper into the device.

Because of the high band gap of the ETL and HTL, only the majority carriers

play any significant role in the overall current of the device As can be seen

from the Figure 4.4, minority carrier densities in both ETL and HTL are more

than 20 orders of magnitude lower than the majority carrier densities. In the

QD layers, both electron and hole carrier densities are at comparable levels

because of the much smaller band gap. When a 0.5 V forward bias is

applied, the majority carrier densities in the transport layers increase, both


69

electron and hole densities in the QD layers increase, and as a result, a current

of 9.47 µA/cm2 flows through the device.

The total current density is plotted in Figure 4.5 as semilog and linear

plots. At low voltages, the current is limited by diffusion and thus, the current

density increases exponentially with increasing forward bias. At high

voltages, the current is limited by the series resistance of the device and hence

the current increases linearly with forward bias. A semilog plot of the

radiative recombination current is also shown in Figure 4.5 to compare with

the total current. At low voltages, the radiative recombination current is much

smaller than the total current, because the SRH recombination dominates

over the bimolecular radiative recombination. When a higher forward bias is

applied, the carrier densities in the QD increase, thus, the total bimolecular

radiative recombination increases (Eqn. 4.4a). The total SRH recombination

(Eqn. 4.4b) in QDs also increases, but it increases at a lower rate than

bimolecular radiative recombination because p and n terms appear both in the

numerator and denominator, whereas in bimolecular radiative recombination,

they appear only in the numerator. Because of the different rates of increase

in the two recombination mechanisms, at around 1 V forward bias, SRH

recombination in QDs becomes negligible compared to bimolecular radiative

recombination

As described in section III.A, it is difficult to estimate the capture

lifetime, τ, of carriers from the bulk to the quantum dots, so, the capture


70

lifetime is varied from 1 ms to 1 ns, to assess the effect it has on the device

characteristics. Figure 4.6 shows the current density vs applied voltage plot

for the device with varying carrier capture lifetimes, keeping all others

parameters constant. The plots at τ = 1 ns and τ = 1 µs completely overlap

each other, and the current density only reduces slightly if τ = 1 ms. Hence,

it can be concluded that over a reasonably large range of values the carrier

capture lifetime has negligible effect on the current-voltage characteristics of

the device.

We have assumed a radiative recombination coefficient of γ = 4 × 10-10

cm3 s-1. Depending on the fabrication conditions, this coefficient may vary..[75]

Thus, we next explore the effect of changing the value of γ. Keeping all the

other parameters constant, the effect of the changing γ from 4 × 10-12 cm3 s-1

to 4 × 10-8 cm3 s-1 on current density and ratio of recombination current to

total current is studied and is plotted in Figure 4.7 (a) and (b) respectively.

At high forward bias, the total current density increases upon increasing the

recombination coefficient, which again indicates that the total current is

limited by the radiative recombination current at high forward bias. At low

voltages, the total current is limited by the non-radiative recombination

current, so the total current density stays nearly constant at voltages below

0.5 V for γ between 4 × 10-12 cm3 s-1 and 4 × 10-10 cm3 s-1. Both these

observations are consistent with the our conclusions from Figure 4.5 that at

low voltages, the current is primarily due to non-radiative recombination and


71

at high voltages, the current is primarily a result of radiative recombination.

As observed in Figure 4.7(b), the ratio of recombination current to total

current increases upon increasing the applied voltage. With increasing γ, the

transition point of no recombination current to a positive value of

recombination current (the turn on point of the LED) shifts to the left. For

γ between 4 × 10-12 cm3 s-1 and 4 × 10-10 cm3 s-1, the turn-on point is beyond or

around 0.5 V forward bias. But, for γ =4 × 10-8 cm3 s-1, the turn-on point is at

almost 0 V. Thus, the current density plot corresponding to for γ =4 × 10-8

cm3 s-1 is higher than the other values of γ even at smaller voltages, as the

recombination current is not negligible. Thus, our model can be used to

explore the turn on-voltage of QD-LEDs.


72

Figure 4.1 Schematic device structure for a Quantum Dot Light Emitting Diode.


73

Figure 4.2 Flat band diagram for a sample device which consists of Si quantum dots, ZnO as the electron transport layer, NiO as the hole transport layer and ohmic contacts to both anode and cathode. The horizontal lines at ends signify the metal Fermi energies for anode and cathode. Energies are in eV with respect to the vacuum level.

1.8

NiO

3.5

Si QDs

4.3

ZnO

Anode

Cathode

4.3 Results and Discussion

Figure 4.3 Discretization scheme for QDlayers.

Results and Discussion MODELING OF QD

74

Discretization scheme for QD-LED focusing on the QD

MODELING OF QD-LEDS

LED focusing on the QD


75

0 20 40 60 80 100 120 140100

102

104

106

108

1010

1012

1014

1016

1018

Car

rier

Den

sity

(cm

-3)

x(nm)

p (0.5 V) n (0.5 V) p (0.0 V) n (0.0 V)

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

V (0.5 V) V (0.0 V)

Pot

entia

l (V

)

Figure 4.4 Carrier densities and electrostatic potential in a device with 5 QD layers (represented by dots on the plot) at equilibrium and with an applied voltage of 0.5 V.


76

0 1 2 3 4 5 6 7

10-10

10-8

10-6

10-4

10-2

100

102

C

urre

nt D

ensi

ty (

A/c

m2 )

J(log) J

rec(log)

Forward Bias (V)

0

200

400

600

800

1000

1200

J(linear)

Cur

rent

Den

sity

(A

/cm

2 )

Figure 4.5 Total device current density (log and linear) plots with increasing forward bias. The recombination current density is also plotted.


77

0.0 0.5 1.0 1.5 2.010-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Applied voltage (V)

Cur

rent

den

sity

(A

/cm

2 )

τ=10-3 s τ=10-6 s τ=10-9 s

Figure 4.6 Current density vs applied voltage for different assumed values of the carrier capture coefficients (τ).


78

0.0 0.5 1.0 1.5 2.0

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Cur

rent

Den

sity

(A

/cm

2 )

Voltage applied (V)

γ=4x10-12 cm3s-1

γ=4x10-11 cm3s-1

γ=4x10-10 cm3s-1

γ=4x10-8 cm3s-1

0.0 0.5 1.0 1.5 2.0

0.0

0.2

0.4

0.6

0.8

1.0

J rec/

J

Voltage applied (V)

γ=4x10-12 cm3s-1

γ=4x10-11 cm3s-1

γ=4x10-10 cm3s-1

γ=4x10-8 cm3s-1

Figure 4.7 (a) Current density vs applied voltage and (b) ratio of recombination current to total current (Jrec/J) for various radiative recombination coefficients (γ).

5.1 Conclusion CONCLUSION AND SCOPE FOR FUTURE WORK

79

Chapter 5

Conclusion and Scope for

Future work

5.1. Conclusion

Two charge transfer mechanisms in hybrid organic/inorganic QD-LEDs,

direct charge injection and Forster energy transfer were investigated. With the

help of AFM, it was shown that in our devices phase separation does not take

place. By using different device configurations, it was shown that direct

charge injection and Forster energy transfer act independently to produce the

light output from QDs in a hybrid QD LED where the QDs do not get phase

separated from the organic matrix. So, based on our findings and that of

Anikeeva et al. [48] we can conclude that depending on the distribution of the

QDs in the organic matrix, it is possible to have one or both of direct charge

transfer and Forster energy transfer mechanisms play a significant role in the

light output of QD LEDs.

5.2 Scope for Future Work CONCLUSION AND SCOPE FOR FUTURE WORK

80

We also proposed a model for inorganic QD-LEDs. Our model is able to

incorporate the most important parameters that affect the current density

and light output of such devices. The device current is mostly limited by the

QD layers, where the current is primarily due to recombination of two kinds,

radiative and non-radiative. At low voltages, radiative recombination

dominates the device operation, whereas as the voltage increases, radiative

recombination dominates. As the voltage is increased even further, the

current is limited by the series resistance of the device and the current voltage

plot becomes essentially linear. There is practically no effect of capture

lifetime from the transport layers to the QD layers because the device

operation is limited by the QD layers, not how the carriers got into the QDs.

Our model is also able to explore the turn-on voltage of QD-LEDs based on

the ratio of radiative recombination current to the total device current.

5.2. Scope for Future Work

There is great potential for future work on modeling of QD-LEDs.

QD-LEDs with various device configurations have been fabricated, but their

functioning is not fully understood. The model proposed by us can be

extended to model organic transport layer based QD-LEDs, by using field

dependent mobility to model the transport layers.

The model for charge transport in quantum dots is a first-order model,

where quantum dots are simplified to multiple layers of uniform sheets of

5.2 Scope for Future Work CONCLUSION AND SCOPE FOR FUTURE WORK

81

quantum dots. Thus, only one dimensional charge transport is considered by

us. A more complex model can be developed where charge transport from

individual quantum dots is considered in all three dimensions.

We only considered charge transport in QD-LEDs but the light output

was not modelled. In a future work, the model can be extended to include the

light output from QD-LED along with the other complex phenomenon such as

reflection of light from the electrodes, the loss of light from the sides of the

QD-LED, absorption of the light by the transport layers and glass substrate,

etc.

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