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Control of Dopant Type and Density in Colloidal QuantumDot Films

by

Melissa Ann Sachiko Furukawa

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of EngineeringUniversity of Toronto

Copyright © 2012 by Melissa Ann Sachiko Furukawa

Abstract

Control of Dopant Type and Density in Colloidal Quantum Dot Films

Melissa Ann Sachiko Furukawa

Master of Applied Science

Graduate Department of Engineering

University of Toronto

2012

Colloidal quantum dots (CQDs) are an inexpensive and solution processable photovoltaic

material reaching modest efficiencies of 6% [20]. However, doping quantum dots still

remains a challenge. This thesis explores the level of doping in PbS CQDs by surface

ligands and bulk doping within the quantum dot lattice using metals.

In light of the knowledge that oxygen creates traps on the surface of PbS CQDs,

we have turned to the use of oxygen-free fabrication. We find that under a nitrogen

environment, PbS CQD films are n-type and tunable in doping by use of halide ions. We

show for the first time control over a wide range of doping densities (1015 to 1018cm−3)

in n-type CQD films. We also show the ability to fabricate p-type PbS films with high

doping density (1019cm−3) that are compatible with n-type films. This compatibility

enabled us to make the world’s first CQD homojunction photovoltaic device.

ii

Acknowledgements

First and foremost I would like to thank my supervisor Prof. Edward H. Sargent for

his constant guidance and encouragement. His insight and wisdom was invaluable to my

research and I am very grateful to be able to take part in such innovative work.

I would like to thank Dr. Jiang Tang for providing excellent scientific reasoning and

guidance throughout this project; I thank Dr. Huan Liu for expertise in homojunction

PV device fabrication; I thank Dr. Ratan Debnath for teaching me everything I needed

to know about TFTs when I first joined the group; I thank Graham Carey for invaluable

discussions of TFTs and hysteresis and your excellent labview programming skills; I

thank Dr. Larissa Levina for her chemistry expertise in synthesizing quantum dots; I

thank David Zhitomirsky for valuable advice and discussions; I thank Dr. Armin Fisher

for expertise in Fourier transform infrared spectroscopy (FTIR) measurements; I thank

Dr. Sjoerd Hoogland for guidance during the writing of this thesis; I thank Elenita

Palmiano, Remi Wolowiec and Damir Kopilovic for keeping the lab running smoothly; I

thank Lukasz Brzozoski for keeping the lab in line; I thank Dr. Zhijun Ning, Dr. Susanna

Thon, Dr. Xihua Wang, Alex Ip, Lisa Rollny, Daniel Paz-Soldan, Kyle Kemp, Ghada

Koleilat, Illan Kramer, André Labelle, Dr. Jennifer Flexman and Jeannie Ing for all of

their assistance and support during my MASc.

I would like to thank Mark Greiner for carrying out XPS measurements; I thank Dr.

Neil Coombs and Dr. IIya Gourevich for SEM training and help; I thank Jessica Zhang,

from CMC icrosystems, for guidance through the fabrication process of pre-patterned

silicon substrates.

I would like to thank my family and friends for their support during this journey.

Special thanks to Stephanie Furukawa for always being there for me and for all of your

encouragement.

Finally, I would like to thank David Tran for always keeping me grounded. Your

support and strength inspires me to always reach higher.

iii

Contents

1 Introduction 1

1.1 Colloidal Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Quantum Dots for Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Thesis Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Background and Literature Review 7

2.1 P-N Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Fundamentals of Photovoltaic Solar Cells . . . . . . . . . . . . . . . . . . 9

2.3 Doping Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Field Effect Transistors 13

3.1 Metal-Oxide-Semiconductor Field Effect Transistor . . . . . . . . . . . . 13

3.1.1 Accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1.2 Depletion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.1.3 Inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2 Thin Film Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3 Mobility and Doping Calculations . . . . . . . . . . . . . . . . . . . . . . 16

3.4 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4.1 Common Causes of Hysteresis in TFTs . . . . . . . . . . . . . . . 18

3.4.2 Time Dependence of Drain Current . . . . . . . . . . . . . . . . . 20

3.4.3 Gate Pulse Method . . . . . . . . . . . . . . . . . . . . . . . . . . 21

iv

3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4 Quantum Dot Thin Film Transistors 23

4.1 Device Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.1.1 Source/Drain Contacts . . . . . . . . . . . . . . . . . . . . . . . . 23

4.1.2 Quantum Dot Film . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.1.3 Gate Contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1.4 HMDS Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2 Film Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.3 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5 N-type Doping of Colloidal Quantum Dot Films 31

5.1 Doping with Halide Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2 Controlled Doping by Ion Concentration . . . . . . . . . . . . . . . . . . 34

5.3 Pb:S Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.4 Air Exposure of N-type Films . . . . . . . . . . . . . . . . . . . . . . . . 38

5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6 Control of Dopant Type Using Surface Ligands 41

6.1 Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.2 Tuning of Majority Carrier Type . . . . . . . . . . . . . . . . . . . . . . 42

6.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7 P-type Doping of Colloidal Quantum Dot Films 46

7.1 Doping with Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

7.2 Intrinsic Doping with Metals . . . . . . . . . . . . . . . . . . . . . . . . . 47

7.3 P-N Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

7.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

v

8 Conclusion 52

8.1 Thesis Findings and Conclusions . . . . . . . . . . . . . . . . . . . . . . 52

8.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Appendices 55

.1 TFT Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

.2 P-N Homojunction CQD PV Device Fabrication . . . . . . . . . . . . . . 55

.3 Fourier Transform Infrared Spectroscopy (FTIR) . . . . . . . . . . . . . . 56

.4 Absorption Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

.5 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . . . 56

.6 X-ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . . . . . . 57

Bibliography 58

vi

Statement of Personal Contributions and of Collaborations

I was the first in the research group to successfully measure high mobility PbS films using

thin film transistors (TFTs). I carried out all TFT fabrication, including the cleaning of

substrates, film fabrication and contact deposition. I also carried out all measurements

and analysis. The gate-pulse labview program was written by Graham Carey.

For material characterization I carried out all SEM, absorption and FTIR measure-

ments. For XPS, all measurements were carried out by Mark Greiner in Prof. Zheng-Hong

Lu’s group and sample preparation done by David Zhitomirsky.

Dr. Larissa Levina synthesized the PbS quantum dots used in the studies reported

herein and Dr. Huan Liu carried out all solvent exchanges. Elenita Palmiano cleaned all

ITO and FTO substrates used herein. All photovoltaic devices were fabricated by Dr.

Huan Liu. The idea of ionic passivation and recipe for ligand exchange was developed

by Dr. Jiang Tang.

vii

Acronyms

CQD Colloidal Quantum Dots

PbS Lead Sulfide

PV Photovoltaic

FTIR Fourier Transform Infrared Spectroscopy

SEM Scanning Electron Microscopy

XPS X-ray Photoelectron Spectroscopy

TFT Thin Film Transistor

MOSFET Metal-Oxide-Semiconductor Field Effect Transistor

FTO Fluorine-doped Tin Oxide

ITO Indium-doped Tin Oxide

HMDS Hexamethyldisilazane

TBAI Tetrabutylammonium Iodide

TBAB Tetrabutylammonium Bromide

TBAC Tetrabutylammonium Chloride

CTAB Cetyltrimethylammonium Bromide

HTAC Hexadecyltrimethylammonium Chloride

TMAOH Tetramethylammonium Hydroxide

TMS bis-(trimethylsilyl) sulfide

viii

CdTe Cadmium Telluride

CIGS Copper Indium Gallium Selenide

PCE Power Conversion Efficiency

ix

List of Tables

1.1 Abundance of elements, used for solar cells, in the Earth’s crust [1] . . . 1

5.1 XPS measurements of atomic percentages of Pb, S and halides in quantum

dot films treated with TBAC, TBAB and TBAI . . . . . . . . . . . . . . 34

5.2 XPS measurements of atomic percentages of lead (Pb), and iodine (I) in

quantum dot films treated with different concentrations of TBAI . . . . . 36

7.1 Mobility and doping density of p-type PbS (950 nm) CQD films for differ-

ent solid state exchanges performed in air . . . . . . . . . . . . . . . . . . 47

x

List of Figures

1.1 Minimum ¢/W for 23 inorganic photovoltaic materials. Reprinted with

permission from [25]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 A)AM1.5G solar spectrum with bulk band gap energies for common semi-

conductor materials. B) Spectral tuning of the band gap of quantum dots

by quantum size effects. Reprinted with permission from [21]. . . . . . . 3

1.3 Depleted Heterojunction architecture with n-type TiO2 and p-type CQD

layers (left). Spatial band diagram showing the extraction of photo-generated

carriers by TiO2, for electrons, and gold, for holes (right). Reprinted with

permission from [16]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Spatial band diagram of a p-n junction. Reprinted with permission from

[21]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 A typical J-V plot of a solar cell in the dark and under AM1.5 illumination.

The area of the orange rectangle represents the maximum power output.

Reprinted with permission from [21]. . . . . . . . . . . . . . . . . . . . . 10

3.1 Band diagrams for MOSFETs made of a p-type semiconductor. A) Flat

band mode where the applied gate voltage is equal to the flat band voltage

and the Fermi level of the semiconductor is aligned with the gate contact,

B) accumulation mode C) depletion mode and D) inversion mode. . . . . 14

xi

3.2 TFT configuration with bottom source and drain electrodes and bottom

gate electrode. L and W, the channel length and width, are indicated. . . 16

3.3 A) Ohmic contact between an n-type CQD film and low work function

source and drain metal contacts. B) Ohmic contact between a p-type

CQD film and high work function source and drain metal contacts. . . . 16

3.4 Transient response of the drain/source current with applied gate volt-

age. A) Constant applied drain voltage with step function gate voltage.

B)Transient response of Id. Reprinted with permission from [23]. . . . . . 20

3.5 Simulations performed with ATLAS. A) Simulated transient response of

the drain/source current with applied gate voltage for pentacene TFTs.

B) Density of trapped holes in the channel at various time points A)30s,

B) 32s and C) 38s. Reprinted with permission from [23]. . . . . . . . . . 22

4.1 Cross-sectional SEM image of a 30 nm quantum dot film on a silicon

substrate covered with 200 nm of thermal oxide SiO2. . . . . . . . . . . 26

4.2 Top view SEM of A) n-type tetrabutylammonium iodide (TBAI) PbS film

and B) p-type Ag doped tetramethylammonium hydroxide (TMAOH) PbS

film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3 Hysteresis in transfer curves for A) as-is 10 mg/mL TBAI treated PbS

with different delay times and B) after 15 minutes of methanol soaking. . 27

4.4 (A) ID as a function of time when successive VG steps are applied, (B)

VG steps as a function of time and (C) resulting ID vs VG graph from

combining results of (A) and (B) where the current is taken at the last

point of the voltage step. . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.5 Source/drain current transient for HMDS treated SiO2 substrates. A) ID

as a function of time when successive VG steps are applied, B) VG steps

as a function of time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

xii

4.6 ID decay with constant "on" gate voltage and recovery of ID by zeroing

the gate voltage between measurements. . . . . . . . . . . . . . . . . . . 30

4.7 TFT transfer curve of TBAI treated PbS film, in the linear regime (Vd=5V),

using the gate pulse method with delay time of 100 ms and zeroing time

of 1000 ms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.1 Structure and name of salts used for halide doping. . . . . . . . . . . . . 32

5.2 Mobility and doping density comparison for PbS films treated with equimo-

lar concentrations of TBAI, TBAB, CTAB, TBAC and HTAC. . . . . . . 33

5.3 FTIR spectra of PbS films treated with equimolar concentrations of TBAI,

TBAB and TBAC. After halide treatment the removal of vibrations at

2922 cm−1 (asymmetric C-H), 2852 cm−1 (symmetric C-H), 1545 cm−1

(asymmetric COO−1 ) and 1403 cm−1 (symmetric COO−1) indicates the

removal of original oleate ligands. . . . . . . . . . . . . . . . . . . . . . . 34

5.4 Transmittance spectra of PbS films treated with equimolar TBAI, TBAB

and TBAC measured inside an integrating sphere. . . . . . . . . . . . . . 35

5.5 Controlled doping of PbS quantum dot films by iodide concentration. . . 36

5.6 Absorption spectra for PbS quantum dot solutions synthesized with dif-

ferent TMS amounts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.7 Effect of increase TMS amount used in PbS synthesis on majority carrier

mobility and doping density values. . . . . . . . . . . . . . . . . . . . . . 38

5.8 TFT output and transfer curves for n-type PbS-TBAI films fabricated

under N2 in a glove box using gold bottom contacts. A) Tested under N2

in a glove box after fabrication, B) Tested in air and C) Tested back in

the glove box under N2. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

xiii

6.1 FTIR spectra of PbS films treated with 1% n-butylamine. After treatment

the removal of vibrations at 1545 cm−1 (asymmetric COO−1 ) and 1403

cm−1 (symmetric COO−1) indicates the removal of original oleate ligands. 42

6.2 Transmittance spectra of PbS films treated with 1% n-butylamine mea-

sured inside an integrating sphere. . . . . . . . . . . . . . . . . . . . . . . 42

6.3 Output curves for A) original n-type TBAI treated PbS films, B) after

n-butylamine treatment which turns the film to p-type and C) after TBAI

treatment which turns the film back to n-type. Transfer curves for D)

original n-type TBAI treated PbS film, E) after n-butylamine treatment

which turns the film p-type and F) after TBAI treatment which turns the

film back to n-type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.4 Output curves for A) original p-type n-butylamine treated PbS film and

B) after TBAI treatment which turns the film to n-type. Transfer curves

for C) original p-type n-butylamine treated PbS film and D) after TBAI

treatment which turns the film n-type. . . . . . . . . . . . . . . . . . . . 44

7.1 Lewis chemical structure diagrams illustrating doping of PbS quantum

dots using Ag. The black lines and paired dots (color indicates the atom

that it belongs to) represent bonding electrons. . . . . . . . . . . . . . . 48

7.2 A) Linear electron mobility and B) doping density for PbS Ag-doped (815

nm) films fabricated in a nitrogen vs air environment and the effect of

annealing overnight in air. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

7.3 A) Linear electron mobility and B) doping density for PbS Ag-doped films

treated with TMAOH pre- and post-halide salt treatment. . . . . . . . . 49

7.4 Homojunction PV device architecture. . . . . . . . . . . . . . . . . . . . 50

7.5 Static measurements of Voc, Jsc and MPP and J-V curve of homojunction

PV device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

xiv

Chapter 1

Introduction

Thin film solar cells offer the advantage of lower cost due to less material use and lower

production costs. Whereas silicon is an indirect band gap material, thin film materials

such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) have a

direct band gap and higher absorption coefficient such that less material is needed to

absorb the same amount of photons. However, CdTe and CIGS contain Tellurium and

Indium respectively which are both low in abundance, see table 1.1.

Table 1.1: Abundance of elements, used for solar cells, in the Earth’s crust [1]

Element Symbol Abundance in the Earth’s crustSulfur S 260Lead Pb 14

Indium In 0.05Selenium Se 0.05Tellurium Te 0.05

In order for solar energy to compete with current fossil-fuel based electricity gener-

ation, semiconductor materials which are abundant need to be investigated. Thin film

PbS quantum dot solar cells offer a solution which is both abundant and inexpensive

due to solution processable fabrication and low material costs. Figure 1.1 shows a cost

1

Chapter 1. Introduction 2

comparison of various photovoltaic materials. PbS at 2.2×10−3 ¢/W is an order of mag-

nitude less than CIGS and CdTe at 2.3× 10−2 and 9.7× 10−2 ¢/W respectively. While

the cost for material extraction is only one component of the total cost of an installed

PV system it can be used as an indicator for future recommendations [25].

Figure 1.1: Minimum ¢/W for 23 inorganic photovoltaic materials. Reprinted withpermission from [25].

1.1 Colloidal Quantum Dots

Colloidal quantum dots (CQDs) are solution processable and offer a tunable band gap.

This allows for an inexpensive yet highly efficient absorbing photovoltaic material.

Synthesized PbS CQDs are capped with oleic acid making them soluble in non-polar

solvents, such as octane, via steric forces. This colloidally stable solution of quantum

dots can be spin-cast, dip-coated or sprayed onto the desired substrate offering inex-

pensive film fabrication; however, these long bulky organic ligands limit the electronic

transport between quantum dots. In order to decrease the barrier for electron injection

a subsequent ligand exchange is needed to decrease the distance between quantum dots


while maintaining good passivation.

Figure 1.2: A)AM1.5G solar spectrum with bulk band gap energies for common semi-conductor materials. B) Spectral tuning of the band gap of quantum dots by quantumsize effects. Reprinted with permission from [21].

Quantum size effects allow tuning of the band gap by changing the size of the quantum

dot. This occurs when the electron and hole pairs (excitons) are confined to a distance

smaller than the exciton Bohr radius. In bulk materials the energy states are continuous.

When the excitons become confined, the energy states become discrete and the band

gap becomes size dependent, shifting to higher energy as the size of the quantum dot

becomes smaller. In this way we can tune the band gap to optimally absorb the sun’s solar

spectrum. Figure 1.2A shows the absorption onset of 15 common bulk semiconductors

against the AM1.5G spectrum. According to the Shockley-Queisser limit, the maximum

theoretical efficiency is 33.7% for a single band gap of 1.1eV. Instead of being limited

in material choice, in order to obtain an optimal band gap, we can use the theory of

quantum confinement described above to effectively tune the band gap of PbS CQDs.

Figure 1.2B illustrates the ability to tune the band gap of PbS quantum dots by changing

the size of the quantum dot.


1.2 Quantum Dots for Photovoltaics

The major advantages of CQD solar cells are the tunable band gap and relatively inex-

pensive material and production costs. Low band gap semiconductors such as PbS and

PbSe are ideal for photovoltaic applications because of the wide range of tunability. The

large Bohr radius of PbS, 18 nm, allows for tuning of the absorption onset from 3000 nm

to ∼600 nm [13].

Currently the highest reported power conversion efficiencies (PCEs) for a CQD PV

device are over 5% [16, 2] and most recently hit 6% [20]. These devices work under a p-n

heterojunction between n-type TiO2 and p-type PbS films, see figure 1.3. Although the

performance of CQD PVs is reaching high efficiencies little is reported in terms of long

term stability. In 2010, Nozik et al. reported a 3% AM1.5 PCE stable for 1000 hours of

continuous illumination for a ZnO/PbS heterojunction photovoltaic device [11]. Although

this device shows promise in long term stability, its performance is quite low compared

to recent advances. The need for optimized band alignment results in a limitation in

available materials to create heterojunctions in CQD PV. The poor stability of current

high PCE CQD heterojunction PV devices is attributed to interfacial reactions between

the n-type TiO2 and p-type PbS films.

Figure 1.3: Depleted Heterojunction architecture with n-type TiO2 and p-type CQDlayers (left). Spatial band diagram showing the extraction of photo-generated carriers byTiO2, for electrons, and gold, for holes (right). Reprinted with permission from [16].

A homojunction or quantum junction where the critical p-n interface is made up


of solely quantum dots with the same composition (i.e. PbS) would reduce interfacial

reactions because of the similar chemistry. By tuning the doping levels and band gaps of

the p and n-type PbS quantum dots we can easily optimize band alignment and device

performance. Furthermore, since colloidal quantum dot films are room temperature and

solution processable, homojunction CQD PV devices can also be made on any substrate

including flexible ones.

1.3 Thesis Objectives

The main objectives of this thesis are outlined as follows:

1. Measure the dopant type and density of PbS CQD films. In order to build a high

efficiency p-n homojunction photovoltaic device quantification of the doping density

in CQD films is needed. To accomplish this TFTs are used to measure the majority

carrier mobility and doping density in PbS CQD films.

2. Investigate the role of oxygen in carrier mobility and type in PbS CQD films.

Oxygen is known to create traps in the form of PbSO3 and PbSO4, 0.1 and 0.3eV

below the conduction band respectively [19]. Knowing the effect of oxygen exposure

would allow us to get a better understanding of the doping mechanisms for CQD

films.

3. Obtain control of the doping levels in p and n-type PbS CQD films in order to get

a range of doping levels while maintaining mobility. Controlled doping would allow

us to manipulate the depletion region and built-in voltage in PV devices in order

to optimize the efficiency.

4. Create compatible n and p-type PbS quantum dot films in order to form a stable

homojunction. A stable p-n junction is essential to creating high efficiency PV

devices.


All of the listed thesis objectives are addressed through experimental analysis. The thesis

is organized such that the next two chapters are devoted to background information

and literature review and chapters 4 through 7 are devoted to answering the outlined

objectives.

Chapter 2

Background and Literature Review

2.1 P-N Junction

Efficient electron and hole extraction is crucial to maximize the short circuit current (Jsc)

and power conversion efficiency (PCE) of a photovoltaic device. Transport within the

quantum dot layer is divided into two regions: the depletion region and the quasi-neutral

region, depicted in figure 2.1.

When p-type and n-type materials are brought together the diffusion of holes from

the p-type layer into the n-type layer and electrons from the n-type layer into the p-type

layer creates a region depleted of charge. The diffusion of mobile carriers leaves behind

charged ions that create a built in electric field given by,

Vbi =kT

qlnNAND

n2i

. (2.1)

The length of this depletion region is given by,

W =

[2κsε0q

(NA +ND

NAND

)(Vbi − V )

]1/2, (2.2)

where εs is the semiconductor dielectric constant, Vbi is the built-in voltage, V is

7

Chapter 2. Background and Literature Review 8

Figure 2.1: Spatial band diagram of a p-n junction. Reprinted with permission from [21].

the applied bias, and NA and ND are the acceptor and donor doping concentrations

respectively. Thus we can tune the length of the depletion region by tuning the doping

concentrations. Moreover, the depletion region is not equally distributed on both sides,

it is wider on the lower doped side in order to balance the charges on both sides. The

length of the depletion region in each side is given by,

xpo =W

1 +NA/ND

, xno =W

1 +ND/NA

(2.3)

where xpo and xno are the depletion region widths on the p-type side and n-type side

respectively. In this region the electron and hole pairs are governed by drift. The drift

length is given by,

ldrift = µEτ, (2.4)

where µ is the mobility, E is the electric field and τ is the carrier lifetime.

In the quasi-neutral region, where there is no built-in electric field, the transport of


electrons and holes is governed by diffusion. The carrier diffusion length is given by,

ldiffusion =√Dτ, (2.5)

where D is the diffusion coefficient, which, according to Einstein’s equation is related

to the mobility, µ, by,

D

µ=kT

q. (2.6)

The electric field insures efficient extraction of charge carriers in the depletion region

where as in the quasi-neutral region high mobility is crucial for efficient charge extraction.

2.2 Fundamentals of Photovoltaic Solar Cells

Solar cells convert solar radiation into electricity via the photovoltaic effect in semicon-

ductor materials. A typical current density as a function of applied voltage (J-V) plot

is shown in figure 2.2. The dark current shows the rectification of the solar cell in the

absence of light. Upon illumination, photo-generated carriers are created and the J-V

curve shifts down into the fourth quadrant. The power conversion efficiency, η, is given

by,

η =VmaxJmax

Pinc

=VocJscFF

Pinc

(2.7)

where Vmax and Jmax are the voltage and current densities at the maximum power

point (MPP), Pinc is the incident light intensity, Voc is the open-circuit voltage, Jsc is

the short-circuit current density and FF is the fill factor.

The short-circuit current density is the current density generated by the solar cell

at zero bias, it is also the maximum generated current density. The Jsc depends on the

incident light (power and spectrum), the absorption and reflection of the solar cell and


Figure 2.2: A typical J-V plot of a solar cell in the dark and under AM1.5 illumination.The area of the orange rectangle represents the maximum power output. Reprinted withpermission from [21].

the efficiency of electron/hole extraction.

The open-circuit voltage is the maximum voltage obtained from the solar cell and

occurs at zero current density. It is related to the difference between the quasi-Fermi

level in the n-type and p-type materials.

Two other important measures are the series (Rs) and shunt (Rsh) resistance. Rs

is the sum of the film resistance, electrode resistance and contact resistance (between

the film and electrode) and is minimized for optimal performance. Rsh is due to carrier

recombination and is maximized for optimal performance. In an ideal solar cell Rs is

zero and Rsh is infinite.

The fill factor is the ratio of the maximum power output to the product of the open-

circuit voltage and short-circuit current. A photovoltaic cell with a high fill factor will

have a low series resistance and high shunt resistance.


2.3 Doping Quantum Dots

As described in the previous sections, the doping of semiconductor materials is necessary

in order to make a p-n junction. Furthermore, the level of doping in solar cells controls

the built in potential and depletion region and in turn the Voc and Jsc.

There are several strategies for doping quantum dots. Hydrazine has been shown

to produce n-type PbSe nanocrystals upon ligand exchange and the ability to switch to

ambipolar and finally p-type following the removal of hydrazine by vacuum treatment or

mild heating [18]. Although hydrazine is shown to make both p and n-type PbSe films,

a p-n junction could never be realized because of the instability of n-type doping, due to

desorption of hydrazine, and reversal of p-type doping back to n-type after subsequent

hydrazine treatment. Since these two doping mechanism are conflicting, one requires the

presence of hydrazine while the other requires the absence or desorption of hydrazine, a

stable p-n junction would be difficult to fabricate.

C.B. Murray takes this one step further by creating a binary superlattices of Ag2Te

and PbTe nanocrystals to create highly conductive p-type films. Again hydrazine is used

to bring the nanocrystals closer together and increase conductivity; however, in this case

it is the Ag+ from Ag2Te that is dominantly responsible for doping the nanocrystal film.

This is evident because of the p-type behaviour of PbTe/Ag2Te films immediately after

hydrazine treatment, whereas PbTe films require annealing to switch from initial n-type

behaviour after hydrazine treatment to p-type [24].

A different approach is used by Bawendi et al. to create p and n-type InAs films.

First, they show that InAs quantum dot films, fabricated in a nitrogen environment using

ethanedithiol as the ligand, are n-type. Next they introduce Cadmium (Cd) to the InAs

quantum dots and obtain p-type behavior after ethanedithiol ligand exchange [6]. In all

cases TFTs are used to obtain mobility values and dopant type; however; no control of

doping density is shown.

Introduction of Tin and Indium impurities during quantum dot synthesis of CdSe has


been shown to produce n-type doping [17]. More recently Banin et al. have introduced

metal impurities into InAs nanocrystals for active n and p-type doping; however, assum-

ing a quantum dot contains 1000 atoms, adding just one impurity atom per quantum dot

results in a heavily doped film on the order of 1019 cm−3[14].

The novelty of our doping technique is the control of doping we show by creating a

range of doping density values and the ability to make a good PV device out of our p

and n-type PbS layers. Our p and n-type materials are not only compatible but they

also form a stable p-n junction.

Chapter 3

Field Effect Transistors

3.1 Metal-Oxide-Semiconductor Field Effect Transis-

tor

In a traditional metal-oxide-semiconductor field effect transistor (MOSFET) there are 3

modes of operation: accumulation, depletion and inversion, see figure 3.1. In order to

understand the difference between these modes it is important to define the flat band

voltage (Vfb). The flat band voltage is the applied gate voltage needed in order to align

the Fermi level of the semiconductor with the work function of the gate metal. In the

following sections these modes will be explained for a p-type semiconductor.

3.1.1 Accumulation

With the application of a gate voltage less than the flat band voltage holes will accumulate

at the interface between the oxide and the semiconductor creating a positive channel.

13

Chapter 3. Field Effect Transistors 14

A B

D C

SiO2 Gate Semiconductor

Ef’ Ef

CB

VB


Vg>Vt

Ef

CB

VB


Ef

CB

VB

Vg<V<


Vg>V<

Ef

CB

VB

Vg=V<

Ef’

Ef’

Ef’

Figure 3.1: Band diagrams for MOSFETs made of a p-type semiconductor. A) Flat bandmode where the applied gate voltage is equal to the flat band voltage and the Fermi levelof the semiconductor is aligned with the gate contact, B) accumulation mode C) depletionmode and D) inversion mode.

3.1.2 Depletion

The application of a gate voltage between the flat band and threshold voltage causes

the depletion of positive charges from the oxide/semiconductor interface. This depletion

layer width increases with increasing gate voltage. The threshold voltage (Vth) is given

by

Vth = ±qn0d

Ci

+ Vfb (3.1)

where q is the elemental electron charge, n0 is the free carrier density, d is the semi-

conductor thickness, Ci is the dielectric capacitance and Vfb is the flat band potential.

The sign indicates the charge of the channel (negative in the case of a p-type material as


it forms an electron channel).

3.1.3 Inversion

In the inversion regime a gate voltage above the threshold voltage is applied which at-

tracts electrons toward the oxide/semiconductor interface creating a negative channel.

In this regime current from the conductive channel can be extracted from n+ source and

drain electrodes. This is considered the "on" state for the MOSFET.

3.2 Thin Film Transistor

Thin film transistors (TFTs) are used to measure the conductivity, mobility and doping

density of quantum dot films. The typical configuration of a quantum dot TFT (QD

TFT), shown in figure 3.2, consists of three electrodes, the semiconductor film and an

insulator. The flow of electrons and holes is controlled by the source-drain voltage and

gate voltage. Unlike the MOSFET the TFT is turned "on" in the accumulation mode

where majority carriers make up the conductive channel. Using the TFT in this manner

allows for the doping level and mobility of the majority carrier to be investigated. Source

and drain electrodes which are ohmic to the semiconductor material are used (i.e. high

work function metals such as gold for p-type material and low work function metals

such as silver or aluminum for n-type material). Unlike traditional MOSFETs there is

typically no inversion regime because, for the case of a p-type semiconductor, the Fermi

level of gold is far off from the conduction band of the p-type material, see figure 3.3.


Figure 3.2: TFT configuration with bottom source and drain electrodes and bottom gateelectrode. L and W, the channel length and width, are indicated.

N‐type Semiconductor Low work func5on metal

A

B

e‐

P‐type Semiconductor High work func5on metal

h

vacuum

vacuum

CB

VB

CB

VB

Ef

Ef

Figure 3.3: A) Ohmic contact between an n-type CQD film and low work function sourceand drain metal contacts. B) Ohmic contact between a p-type CQD film and high workfunction source and drain metal contacts.

3.3 Mobility and Doping Calculations

By measuring output curves (ID vs VD) and transfer curves (ID vs VG) the majority

carrier mobility and the doping density can be calculated. Although MOSFETs operate


in the inversion regime and TFTs operate in the accumulation regime the equations used

to model the current are the same. This is due to the fact that the source and drain

contacts are ohmic to the channel and the charge accumulated at the oxide interface is

given by the oxide capacitance and the applied gate voltage in both cases. However, the

threshold voltage must be redefined as the onset of accumulation of majority carriers for

TFTs.

When the transistor is turned on, a channel of charge is created and allows for current

flow between the source and drain electrodes. In this linear regime the TFT operates like

a resistor and the measured current across the source and drain increases linearly with

the gate voltage following the equation,

ID =

(WCiµlin

L

)(VG − VT −

VD2

)VD, (3.2)

where L and W are the channel lengths and widths, Ci is the capacitance per unit

area of the insulating layer (SiO2), VT is the threshold voltage and µlin is the linear

mobility.

When the drain voltage exceeds the gate voltage (VD>(VG-Vth)) the channel becomes

broader extending deeper into the semiconductor. In this saturated regime, the channel

becomes pinched off and follows the equation,

ID =

(WCiµsat

2L

)(VG − VT )2 . (3.3)

From these two equations the linear and saturated mobility can be calculated as,

µlin =∂ID∂VG

L

WCiVD, (3.4)

µsat =

(∂√ID

∂VG

)2WCi

2L. (3.5)


Furthermore the doping density is calculated using the equation,

ρ =σ

eµ, (3.6)

where ρ is the doping density, σ is the conductivity, e is the electron charge and µ is the

majority carrier mobility.

3.4 Hysteresis

Typically in TFT devices there exists some hysteresis between the forward (off to on) and

reverse (on to off) transfer curve (ID vs VG). But what is the origin of this hysteresis?

It can be quantified as the change in the threshold voltage (Vth) given by,

Vth = ±qnod

Ci

+ Vfb (3.7)

where no is the density of free carriers, Vfb is the flat band voltage and d is the

thickness of the semiconductor. From this equation it is evident that a change in Vth

can occur if free carriers are trapped changing ni, charges are injected into the dielectric

changing Ci, or there is some structural change in the semiconductor changing Vfb.

Some defining features of hysteresis include the direction and change in hysteresis

with scan speed. We define the direction of the hysteresis in the transfer curve as either

lower back sweep current, forward scan (off to on) measures higher current than reverse

scan (on to off), or higher back sweep current, forward scan(off to on) measures lower

current than the reverse scan (on to off).

3.4.1 Common Causes of Hysteresis in TFTs

There are many causes of hysteresis in TFTs. In this section we will discuss some of the

major contributions to hysteresis and how to characterize them.


Traps present in the semiconductor or on the interface between the dielectric and

semiconductor can lead to hysteresis effects in TFTs. As charge carriers get trapped the

measured current decreases, causing lower back sweep current. It is typically thought

that the hysteresis due to traps arises because of a faster scan rate than the release rate of

charges from trapped states. Thus, normally slower scan rates can reduce the hysteresis;

however, this is only true if trap states are filled quickly. If the trapping rate is quite

slow, than a faster scan can benefit by avoiding the trapping of charge carriers altogether.

Mobile ions in the semiconductor can also cause lower back sweep current. Mobile

ions of the same polarity as majority carriers slowly move into the channel as an "on"

gate voltage is applied. This decreases the number of charges that can accumulate in

the channel, since the total charge for a given VG is fixed, resulting in a lower ID for the

back sweep. Since mobile ions move slowly, the hysteresis is expected to increase with a

slower scan rate.

Mobile ions in the dielectric have the opposite effect. The following mechanism

is described for n-type semiconductor TFTs. Applying an "on" gate voltage (+’ve

for n-type semiconductors) results in the accumulation of cations towards the dielec-

tric/semiconductor interface and anions towards the gate electrode. Since ions move

slowly, the electric field is retained even after the gate voltage is sweeped to 0 V and as a

result a higher ID is measured for the back sweep. A slower scan rate results in a larger

hysteresis since more ions will accumulate for longer applied "on" voltages.

Charge injection from the semiconductor into the dielectric has a similar effect on

hysteresis as traps. The only difference is that the "traps" are located inside the dielec-

tric. Charge injection from the gate electrode to the dielectric, on the other hand, causes

higher back sweep current. For an n-type semiconductor TFT, applying an "on" gate

voltage (+’ve VG) will inject holes into the dielectric. After reducing VG back to 0 V the

electrons remain in the dielectric thus retaining a high ID.


3.4.2 Time Dependence of Drain Current

Unlike traditional silicon MOSFETs, TFTs made from organics or quantum dot films

often do not exhibit constant drain currents with constant applied gate and drain-source

voltages. Time traces of the source-drain current exhibit a spike and then exponential

decay towards its value at VG=0 V when the gate voltage is stepped from off (0 V) to on.

This transient behaviour is attributed to a slow screening of the applied gate voltage by

trapped charges [7, 3, 12, 8, 23, 22]. Figure 3.4 depicts the source/drain current transient

seen when applying a negative "on" gate voltage for a p-type semiconductor TFT. During

the forward sweep, negative gate bias, holes in the accumulation layer become trapped.

This reduces the number of free charges in the channel and thus the source/drain current

decays. When the negative gate bias is removed trapped holes are slowly released and

this leads to a rise in the source-drain current. It is this trapping/de-trapping mechanism

that gives rise to the hysteresis in transfer and output curves.

Figure 3.4: Transient response of the drain/source current with applied gate voltage. A)Constant applied drain voltage with step function gate voltage. B)Transient response ofId. Reprinted with permission from [23].


By passivating the oxide surface with hexamethyldisilazane (HMDS), charging due to

traps at the oxide surface is ruled out [12, 8, 10]. Thus it is the presence of bulk traps in

the semiconductor that are the source of the transient current.

Simulation of pentacene TFTs with bulk traps 0.1 eV below the HOMO band at a

density of 2×1019cm−3 show agreement in the decay and rise of the source/drain current

with trapping and de-trapping of holes as the gate voltage is ramped on and then off.

Figure 3.5A shows the simulated transient response of the source/drain current which

is in agreement with experimental results. Furthermore the decay of the source/drain

current with constant negative bias is in accordance with simulated results which show

an increase in trapped holes in the channel over time, figure 3.5B.

3.4.3 Gate Pulse Method

In order to avoid hysteresis due to the trapping/de-trapping of charges, the gate pulse

method is used for measuring TFTs. This technique was first proposed by Leroux et al

to reduce the hysteresis due to high K dielectrics [9] and is widely accepted and used

to reduce hysteresis in QD TFTs [6] and organic field effect transistors (OFETs) [3, 5].

After each ID measurement the gate voltage is zeroed for a prescribed amount of time

related to the delay time of the ID measurement. By zeroing the gate voltage we allow

for the trapped carriers to de-trap between each measurement point.

3.5 Conclusion

In conclusion, TFTs offer a valid way to measure the mobility and doping density of thin

films. Hysteresis can be an issue due to current transients; however, by implementing the

gate-pulse method the hysteresis in transfer and output curves is minimized. The current

transient is often associated with bulk traps in the semiconductor and is not associated

with interface traps on the oxide.


Figure 3.5: Simulations performed with ATLAS. A) Simulated transient response ofthe drain/source current with applied gate voltage for pentacene TFTs. B) Density oftrapped holes in the channel at various time points A)30s, B) 32s and C) 38s. Reprintedwith permission from [23].

Chapter 4

Quantum Dot Thin Film Transistors

In this section the method used to fabricate CQD TFTs is described as well as the

technique used to acquire hysteresis free output and transfer curves.

4.1 Device Fabrication

TFTs were fabricated on highly doped (boron) p-type silicon wafers with 100 or 200 nm

of thermally grown SiO2. For our bottom contact TFTs 100 nm of SiO2 was used and

for our top contact devices 200 nm of SiO2 was used. The wafers were purchased from

Silicon Valley Microelectronics Inc. and Silicon Quest International Inc.

4.1.1 Source/Drain Contacts

For TFTs we used both bottom and top contact devices. Bottom gold contact TFTs

were processed by NRC-CPFC (Canadian Photonics Fabrication Centre). Contacts were

patterned via a photolithography lift-off process. A thin layer of titanium (7.5 nm) was

used as an adhesion layer between gold (37.5 nm) and underlying silicon dioxide (100

nm). These TFTs were used for measurements of p-type materials using a channel length

of 5 µm and channel width of 1 mm. Contact resistance measurements were done in order

23

Chapter 4. Quantum Dot Thin Film Transistors 24

to ensure good electrical contact between the quantum dot film and the source and drain

contacts. The resistance of QD TFTs were measured for various channel lengths and by

extrapolating to a channel length of zero we find that the contact resistance is negligible.

Top contact TFTs were used for n-type material characterization. Silver top contacts

were evaporated through a shadow mask to create contact lengths of 25, 50, 100 and 250

µm and contact widths of 0.5, 1 and 1.5 mm. Typically contacts lengths of 25 or 50 µm

and contact widths of 0.5 or 1 mm were used for measurements. An evaporation rate of

1 Å/s was used to obtain a total thickness of 50 nm.

4.1.2 Quantum Dot Film

Prior to CQD film fabrication, silicon substrates were cleaned via sonication in acetone

and isopropyl alcohol (IPA) for 20 minutes each and then dried with a N2 gun. In order

to fabricate a thin (30 nm) continuous layer of quantum dots, a 2 layer spin-coating

recipe was used. A quantum dot concentration of 25 mg/mL in octane was used for

both layers. After dropping 1-2 drops of quantum dots onto the substrate, the substrate

was immediately spun for 10 s at 2500 rpm with 1000 rpm acceleration. Subsequently,

our ligand exchange process was carried out by dropping the ligand solution onto the

quantum dot film (i.e. 0.5 mL of 10 mg/mL tetrabutylammonium iodide in methanol

for 1 minute). After the alloted treatment time the film was spun dry (2500 rpm 10 s)

and this ligand exchange process was repeated a second time. The film was then washed

three times with methanol (drop 0.5mL of methanol then spin dry) to remove any excess

ligands (i.e. free iodide). The whole process was repeated a second time to build a crack-

free thin film of quantum dots. This procedure is similar to that used for photovoltaic

device fabrication.


4.1.3 Gate Contact

The gate contact was created by using a diamond scriber to scratch away top quantum

dot film and SiO2 layer and applying silver paste to connect to underlying the silicon.

These top gate contacts were used for simplicity of probing. The resistance of the silicon

was measured using two silver paste gate contacts and was found to be in good agreement

with the provided information from the manufacturer, indicating good electrical contact.

4.1.4 HMDS Treatment

In order to passivate any hydroxyl groups on SiO2, which can act as electron traps, we

implement a self-assembling monolayer (SAM) of hexamethyldisilazane (HMDS). After

cleaning the Si/SiO2 substrate, 3-4 drops of HMDS were used to cover the surface and

then the solution was spin-coated at 3500 rpm for 15s. The film was then dried at 150C

for 30 minutes on a hot plate in air. Contact angle measurements of a water droplet

were used to confirm the presence of HMDS on the surface which makes the surface more

hydrophobic.

4.2 Film Morphology

Scanning electron microscopy images of our QD TFTs were taken to verify the film

thickness and absence of cracks in our quantum dot films. Smooth crack free films are

essential for creating TFTs. In particular, because of the solid state exchange of long

organic ligands for shorter less insulating ligands, films are prone to cracking due to the

decrease in volume. Figure 4.1 illustrates a smooth and uniform thickness of roughly

30 nm of PbS quantum dots on a silicon wafer with 200 nm of SiO2 obtained using the

standard 2 layer recipe for 25 mg/mL of PbS CQDs in octane. Figure 4.2 illustrates the

smooth, crack-free films obtained using our 2 layer spin-coating technique for n-type and


p-type films.

Figure 4.1: Cross-sectional SEM image of a 30 nm quantum dot film on a silicon substratecovered with 200 nm of thermal oxide SiO2.

(a) (b)

Figure 4.2: Top view SEM of A) n-type tetrabutylammonium iodide (TBAI) PbS filmand B) p-type Ag doped tetramethylammonium hydroxide (TMAOH) PbS film.

4.3 Hysteresis

Figure 4.3 illustrates the type of hysteresis seen in our halide treated quantum dot films,

the reverse scan (on to off) current is less than the forward scan current (off to on). First,

we considered the presence of free unbound halide ions in the PbS films contributing to


the hysteresis. Mobile ions of the same polarity as majority carriers in the semiconductor

- 1 0 - 5 0 5 1 0 1 5 2 0

0 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

1 . 2

1 . 4

- 1 0 - 5 0 5 1 0 1 5 2 0

0 . 0 0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0 . 1 0

Id (µA

)

V g ( V )

0 m s d e l a y 1 0 m s d e l a y 1 0 0 m s d e l a y

V d = 5 V

Id (µA

)

V g ( V )

0 m s d e l a y 1 0 m s d e l a y 1 0 0 m s d e l a y

A

B

Figure 4.3: Hysteresis in transfer curves for A) as-is 10 mg/mL TBAI treated PbS withdifferent delay times and B) after 15 minutes of methanol soaking.

would move slowly into the channel and decrease the number of majority carriers at a

given gate voltage. This would explain the lower reverse scan current and larger hysteresis

with slower scan speeds. In an attempt to remove all unbound halide a 15 minute

methanol soaking period was applied after film fabrication. This lead to drastically lower

conductivity and mobility, see figure 4.3, while no significant change in hysteresis was

observed. We suspect that the lower mobility indicates removal of bound iodide which

should be more difficult to remove than unbound iodide. Consequently, the presence of

hysteresis after methanol soaking rules out the presence of mobile unbound halide anions

causing hysteresis in CQD TFTs.


The hysteresis seen in our transfer curves is a direct results of the transient drain

current. Figure 4.4 shows the decay and rise of the drain current while the gate voltage

is increased and decreased respectively in a step function. We attribute the decay and

0 1 0 2 0 3 0 4 0 5 0

0 . 0 6

0 . 0 8

0 . 1 0

0 . 1 2

0 . 1 4Id

(µA)

t i m e ( s )

A

0 2 4 6 8 1 0

0 . 0 8 0

0 . 0 8 5

0 . 0 9 0

0 . 0 9 5

0 . 1 0 0

Id (µA

)

V g ( V )

0 1 0 2 0 3 0 4 0 5 002468

1 0

Vg (V

)

t i m e ( s )

B

C

Figure 4.4: (A) ID as a function of time when successive VG steps are applied, (B) VG

steps as a function of time and (C) resulting ID vs VG graph from combining results of(A) and (B) where the current is taken at the last point of the voltage step.

rise of the drain current to trapping and de-trapping of electrons respectively. As the gate

voltage is increased to higher positive values electron traps begin to fill and this results

in a decay of the measured source-drain current. As the gate voltage is then reduced

in the backwards scan electrons become de-trapped and there is a corresponding rise in

the source-drain current. We observe the same effect for negative voltages indicating the

presence of hole traps.

In order to rule out the role of electron traps from hydroxyl groups on the surface of

SiO2, surface passivation using hexamethyldisilazane (HMDS) was implemented. HMDS

treated substrates resulted in the same transient source/drain current and resulting hys-

teresis, see figure 4.5. Thus it is most likely bulk trap states in the CQD film that are

responsible for the hysteresis and transient currents in TFT devices.

In order to reduce hysteresis effects we employ the gate-pulse method. Figure 4.6

depicts the need for a sufficiently long zero time in order for the film to recover trapped


0 1 0 2 0 3 0 4 00 . 0 0 00 . 0 0 20 . 0 0 40 . 0 0 60 . 0 0 80 . 0 1 00 . 0 1 2

0 1 0 2 0 3 0 4 005

1 01 52 0

Id (µA

)

T i m e ( s )

Vg (V

)

T i m e ( s )

A

B

Figure 4.5: Source/drain current transient for HMDS treated SiO2 substrates. A) ID asa function of time when successive VG steps are applied, B) VG steps as a function oftime.

electrons, if a short zero voltage time is applied the drain current will not recover to

the same value. Figure 4.7 illustrates the reduced hysteresis when using this technique.

Given the low hysteresis seen in IV curves for our quantum dot photovoltaic devices

and stable maximum power point measurements we attribute the transient current and

consequential hysteresis seen in TFT devices to charging of deep traps caused by the

charging nature of TFTs and high applied electric fields. Thus the measured mobility

and doping density using the gate-pulse method are a good reflectance of values in an

actual PV device.

4.4 Conclusion

In conclusion, we demonstrate a method to create thin (30 nm), crack-free quantum dot

films for TFTs using a 2 layer spin-coating process. Transient currents are evident in

our TFTs and cause hysteresis in output and transfer curves. This transient current is

attributed to bulk trap states in the quantum dot film. Trapping on the oxide surface is

ruled out by HMDS treatment. When OH- electron traps are passivated with HMDS the


0 1 0 2 00 . 0 00 . 0 20 . 0 40 . 0 60 . 0 80 . 1 00 . 1 20 . 1 40 . 1 60 . 1 80 . 2 00 . 2 2 I d

V g

Id (µA

)

t i m e ( s )

0

2

4

6

8

1 0

Vg (V

)

Figure 4.6: ID decay with constant "on" gate voltage and recovery of ID by zeroing thegate voltage between measurements.

- 1 0 - 5 0 5 1 0 1 5 2 00 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

0 . 7 f o r w a r d r e v e r s e

Id (µA

)

V g ( V )

Figure 4.7: TFT transfer curve of TBAI treated PbS film, in the linear regime (Vd=5V),using the gate pulse method with delay time of 100 ms and zeroing time of 1000 ms.

transient current and hysteresis are still evident. Thus to minimize hysteresis and gain

accurate mobility and doping density values from our transfer curves we implement the

gate pulse method in which the gate voltage is held at zero volts between measurements

to release any trapped electrons.

Chapter 5

N-type Doping of Colloidal Quantum

Dot Films

5.1 Doping with Halide Ions

When doping using surface ligands it is important to ensure the surface of the quantum

dot is properly passivated in order to avoid trap states from dangling bonds. Recent

experimental results indicate that lead chalcogenide colloidal quantum dots are non-

stoichiometric, consisting of a stoichiometric core surrounded by a Pb-rich surface [4, 15].

Thus we concentrate on passivation of Pb2+ surface cations. For n-type doping, halogen

ions (chloride, bromide and iodide) are used to replace bulky organic ligands and passivate

the Pb sites on the quantum dot surface. All n-type doping was done in a nitrogen filled

glove box to avoid oxidation. Using the shortest possible ligand (only 1 atom) in an

oxygen free environment we can achieve an electron mobility as high as 0.04 cm2/Vs.

Various halide salts were tested, see figure 5.1, these include hexadecyltrimethylam-

monium chloride (HTAC), cetyltrimethylammonium bromide (CTAB), tetrabutylammo-

nium chloride (TBAC), tetrabutylammonium bromide (TBAB) and tetrabutylammo-

nium iodide (TBAI). Mobility and doping density were extracted from TFT curves and

31

Chapter 5. N-type Doping of Colloidal Quantum Dot Films 32

Figure 5.1: Structure and name of salts used for halide doping.

the results are shown in figure 5.2 for halide salt treatments with equimolar halide con-

centration (TBAI 10 mg/mL, TBAB 8.7 mg/mL, TBAC 6.9 mg/mL). When we take into

consideration the cation, and keep it consistent, there is a clear trend for doping Cl>Br>I

and mobility I>Br>Cl. From X-ray photoelectron spectroscopy (XPS) data, see table

5.1, there is no trend in Halide:Pb ratio to explain the increase in doping (Cl>Br>I);

the doping density trend cannot be explained the amount of bound halide. Further-

more, from XPS data it is clear that there is on the order of hundreds of halide anions

per quantum dot which would lead to doping density values of over 1x1021cm−3 if each

halide anion donated 1 electron, assuming a quantum dot concentration of 1x1019cm−3.

This is vastly higher than the measured doping density values. We thus hypothesize that

halide anions also form traps that lie below the Fermi level such that not all donated

electrons contribute to the free carrier density.

If we compare CTAB with TBAB using equimolar concentrations it is clear that the

cation does play a role in the resulting doping of the film. A more bulky cation (CTAB)

results in less doping and lower conductivity.


T B A I T B A B C T A B T B A C H T A C

1 E - 4

1 E - 3

0 . 0 1

Mobili

ty (cm

2 /Vs)

H a l i d e S a l t

T B A I T B A B C T A B T B A C H T A C

1 E 1 6

1 E 1 7

1 E 1 8

Dopin

g Den

sity (c

m-3 )

H a l i d e S a l t

T B A I T B A B C T A B T B A C H T A C0 . 00 . 20 . 40 . 60 . 81 . 01 . 21 . 41 . 61 . 8

Cond

uctivi

ty (10

-4 Ω cm

)

H a l i d e S a l t

A

B

C

Figure 5.2: Mobility and doping density comparison for PbS films treated with equimolarconcentrations of TBAI, TBAB, CTAB, TBAC and HTAC.

Fourier transform infrared (FTIR) spectroscopy shows the removal of original oleic

acid capping ligands with halide treatment, figure 5.3. This is shown by the removal

of vibrations at 2922 cm−1 (asymmetric C-H), 2852 cm−1 (symmetric C-H), 1545 cm−1

(asymmetric COO−1 ) and 1403 cm−1 (symmetric COO−1). Moreover, halide ligand ex-

change retains quantum confinement as indicated by the exciton peak in the transmission


Table 5.1: XPS measurements of atomic percentages of Pb, S and halides in quantumdot films treated with TBAC, TBAB and TBAI

Sample % Pb % S % X S:Pb X:PbPbS-OA 62.2 37.8 - 0.61 -

TBAC (X=Cl) 48.2 37.9 13.9 0.79 0.29TBAB (X=Br) 47.0 36.6 16.4 0.78 0.35TBAI (X=I) 47.7 35.0 17.3 0.73 0.36

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 00 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 0

Trans

mitta

nce

W a v e n u m b e r ( c m - 1 )

C o n t r o l T B A I T B A B T B A C

Figure 5.3: FTIR spectra of PbS films treated with equimolar concentrations of TBAI,TBAB and TBAC. After halide treatment the removal of vibrations at 2922 cm−1 (asym-metric C-H), 2852 cm−1 (symmetric C-H), 1545 cm−1 (asymmetric COO−1 ) and 1403cm−1 (symmetric COO−1) indicates the removal of original oleate ligands.

spectrum, figure 5.4. The noise present in transmission spectrum is due to the change of

gratings that occurs around 800nm.

5.2 Controlled Doping by Ion Concentration

By changing the concentration of iodide present during ligand exchange we can control

the amount of doping in our PbS CQD films. Figure 5.5 shows that with increase in

TBAI concentration there is a resulting increase in the doping density. From XPS results

we can conclude that this dependence is due to the incorporation of more iodide. There is


4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 00

2 0

4 0

6 0

8 0

1 0 0

C o n t r o l T B A I T B A B T B A C

T (%)

W a v e l e n g t h ( n m )

Figure 5.4: Transmittance spectra of PbS films treated with equimolar TBAI, TBABand TBAC measured inside an integrating sphere.

a definite increase in I:Pb ratio with increased TBAI concentration. There is also a time

dependence of doping density with treatment where a longer treatment time (1 minute)

results in a higher doping density and higher I:Pb ratio compared to a shorter (10 second)

treatment time. Controlling the doping density using increased concentrations of halide

ions further supports the hypothesis that it is the donor states created by halide anions

that cause n-type doping in quantum dot films. Thus we can increase the amount of

bound I- by introducing more into the film (higher concentration or longer the treatment

time).

5.3 Pb:S Ratio

The Pb:S ratio in PbS quantum dots is controlled by the amount of sulfur precursor, bis-

(trimethylsilyl)sulfide (TMS), added during synthesis. Three batches of PbS quantum


0 5 1 0 1 5 2 01 0 - 3

1 0 - 2

1 0 - 1

0 5 1 0 1 5 2 01 E 1 5

1 E 1 6

1 E 1 7

1 0 s 1 m i n

Linea

r Mob

ility (c

m2 /Vs)

T B A I C o n c e n t r a t i o n ( m g / m L )

1 0 s 1 m i n

Dopin

g (cm

-3 )

T B A I C o n c e n t r a t i o n ( m g / m L )

Figure 5.5: Controlled doping of PbS quantum dot films by iodide concentration.

Table 5.2: XPS measurements of atomic percentages of lead (Pb), and iodine (I) inquantum dot films treated with different concentrations of TBAI

TBAI concentration treatment time atomic % Pb atomic % I I:Pb ratio1 mg/mL 10 s 80.5 19.5 8 0.2425 mg/mL 10 s 75.3 24.7 8 0.32810 mg/mL 10 s 74.7 25.3 8 0.33920 mg/mL 10 s 73.3 26.7 8 0.3641 mg/mL 1 min 76.1 23.9 0.31410 mg/mL 1 min 72.53 27.47 0.37920 mg/mL 1 min 72.34 27.66 0.382

dots with different TMS amounts were synthesized. Figure 5.6 shows the close absorption

peaks of the three samples indicating similar band gaps. Using TBAC we were able to

successfully control the level of doping which is inversely proportional to the TMS amount


4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 00 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

Abso

rbanc

e


3 0 0 µL T M S 1 8 0 µL T M S 1 4 0 µL T M S

Figure 5.6: Absorption spectra for PbS quantum dot solutions synthesized with differentTMS amounts.

used during synthesis of the quantum dots, see figure 5.7. Less TMS results in an increase

in the Pb:S ratio and therefore more Pb for chloride atoms to bind to and an increase

in the doping density; however, using TBAI the effect is not as strong. Perhaps this is

because of the amount of traps introduced. Although there are more available Pb atoms

to bind to, since I- introduces more trap states there is less change in the doping density


1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 3 0 0 3 2 01 0 - 4

1 0 - 3

1 0 - 2

1 0 - 1

1 4 0 2 1 0 2 8 01 0 1 5

1 0 1 6

1 0 1 7

1 0 1 8

1 0 1 9

T B A I T B A C

Linea

r Mob

ility (c

m2 /Vs)

T M S a m o u n t ( µL )

T B A I T B A C

Dopin

g Den

sity (c

m-3 )

T M S a m o u n t ( µL )

Figure 5.7: Effect of increase TMS amount used in PbS synthesis on majority carriermobility and doping density values.

5.4 Air Exposure of N-type Films

As stated previously, all n-type films were fabricated and tested in the glove box under

an N2 atmosphere. When fabricated in air the films are p-type. We then sought to

determine if our n-type films would turn p-type after air exposure. N-type TBAI TFTs

were fabricated using bottom gold contacts to probe hole conductivity. Although gold

has a deep work function (5.1eV), we were still able to obtain electron modulation as

shown in figure 5.8A and the film is confirmed to be n-type. After air exposure (5-10

minutes) there is no more electron modulation and instead the film behaves as a p-type


0 5 1 0 1 5 2 00 . 00 . 51 . 01 . 52 . 0

- 2 0 - 1 0 0 1 0 2 00 . 00 . 10 . 20 . 30 . 40 . 50 . 6

- 0 . 1 6- 0 . 1 2- 0 . 0 8- 0 . 0 40 . 0 0 - 2 0 - 1 5 - 1 0 - 5 0

- 0 . 0 8- 0 . 0 6- 0 . 0 4- 0 . 0 20 . 0 0

- 2 0 - 1 0 0 1 0 2 0

- 0 . 0 8- 0 . 0 6- 0 . 0 4- 0 . 0 20 . 0 0 - 2 0 - 1 5 - 1 0 - 5 0

- 0 . 0 4- 0 . 0 3- 0 . 0 2- 0 . 0 10 . 0 0 - 2 0 - 1 0 0 1 0 2 0

V g = 0 V g = 1 0 V g = 2 0 V g = 3 0

Id (µA

)

V d ( V )

Id (µA

)

V g ( V ) V d ( V )

Id (µA

)

V g = 0 V g = - 1 0 V g = - 2 0 V g = - 3 0

V g ( V )

Id (µA

)

V d ( V )

Id (µA

)

V g = 0 V g = - 1 0 V g = - 2 0 V g = - 3 0

V g ( V )

Id (µA

)

A

B

C

Figure 5.8: TFT output and transfer curves for n-type PbS-TBAI films fabricated underN2 in a glove box using gold bottom contacts. A) Tested under N2 in a glove box afterfabrication, B) Tested in air and C) Tested back in the glove box under N2.

material. The device was then pumped back into the glove box and re-tested under a

N2 environment and again it is still p-type. The composition of air is mainly nitrogen,

oxygen and argon, along with trace amounts of water vapour. It was shown that nitrogen

does not change the doping of air exposed p-type PbS CQD films and when fabricated

under nitrogen PbS CQD halide films are n-type. Therefore, we believe that nitrogen

acts as an inert atmosphere. Argon as well is an inert gas and therefore we do not expect

it to have much effect on the doping of PbS CQD films. Therefore we are left with

oxygen or water vapour which could play a role in the p-type doping of our PbS CQD


films. This indicates the importance of encapsulation in order to maintain n-type doping

of halide treated PbS CQD films and further confirms the role of oxygen or water as a

p-type dopant.

5.5 Conclusion

In conclusion, we have found that the n-type doping density increases with electroneg-

ativity of the halide Cl>Br>I, whereas the mobility follows the opposite trend. We

attribute this to introduced trap states for different halides. We also find that the n-type

doping increases with TBAI concentration and with decrease TMS amount, this is due

to the increase in bound halide anions. Furthermore we show that n-type films are doped

p-type after air exposure, indicating that oxygen or water vapour is an acceptor.

Chapter 6

Control of Dopant Type Using Surface

Ligands

In this section we aim to control the majority carrier type of PbS quantum dot films by

simple exchange of the surface ligand that passivates the PbS quantum dots. By doing

so we can extend our understanding of the doping mechanism of surface ligands.

6.1 Amines

First we confirm that amines are indeed p-type dopants as shown previously for PbSe

quantum dot films [8]. Several amines (n-butylamine, sec-butylamine, tert-butylamine

and ethylenediamine) were tested at 1% volume concentration in methanol and all were

found to give p-type PbS films based on TFT results (fabricated in N2). Furthermore,

FTIR and absorbance spectra indicate the removal of oleic acid and retention of quantum

confinement, see figures 6.1 and 6.2.

41

Chapter 6. Control of Dopant Type Using Surface Ligands 42

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 00 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 0

Trans

mitta

nce

W a v e n u m b e r ( c m - 1 )

c o n t r o l n - b u t y l a m i n e

Figure 6.1: FTIR spectra of PbS films treated with 1% n-butylamine. After treatmentthe removal of vibrations at 1545 cm−1 (asymmetric COO−1 ) and 1403 cm−1 (symmetricCOO−1) indicates the removal of original oleate ligands.

4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 00

2 0

4 0

6 0

8 0

1 0 0

T (%)


C o n t r o l n - b u t y l a m i n e

Figure 6.2: Transmittance spectra of PbS films treated with 1% n-butylamine measuredinside an integrating sphere.

6.2 Tuning of Majority Carrier Type

From the previous section it is known that TBAI treated PbS films are doped n-type

by the replacement of oleic acid with iodide anions. Next, we showed that by starting


with an n-type TBAI film we can change the doping to p-type by treating the film with

a 1% volume solution of n-butylamine for 1 minute and then back to n-type again by

treating with a 10 mg/mL solution of TBAI for 2 minutes, see figure 6.3. Furthermore,

the reverse is achieved by treating n-butylamine PbS films with TBAI (10 mg/mL for 2

minutes) to change from a p-type to n-type film, see figure 6.4.

0 5 1 0 1 5 2 00 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

- 2 0 - 1 5 - 1 0 - 5 0- 0 . 1 4- 0 . 1 2- 0 . 1 0- 0 . 0 8- 0 . 0 6- 0 . 0 4- 0 . 0 20 . 0 0

- 2 0 - 1 0 0 1 0 2 0 3 00 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 8

- 3 0 - 2 0 - 1 0 0 1 0 2 0- 0 . 0 8- 0 . 0 7- 0 . 0 6- 0 . 0 5- 0 . 0 4- 0 . 0 3- 0 . 0 2- 0 . 0 10 . 0 0

0 5 1 0 1 5 2 00 . 0 0

0 . 0 5

0 . 1 0

0 . 1 5

0 . 2 0

- 2 0 - 1 0 0 1 0 2 0 3 00 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

a m i n e

a m i n e

Id (µA

)

V d ( V )

V g = 0 V g = 1 0 V g = 2 0 V g = 3 0

T B A I

A B

ED F

C

T B A I

Id (µA

)

V d ( V )

V g = 0 V g = - 1 0 V g = - 2 0 V g = - 3 0

Id (µA

)

V g ( V )

V d = 1 0 V

Id (µA

)

V g ( V )

V d = - 1 0 V

Id (µA

)

V d ( V )

V g = 0 V g = 1 0 V g = 2 0 V g = 3 0

Id (µA

)V g ( V )

V d = 1 0 V

Figure 6.3: Output curves for A) original n-type TBAI treated PbS films, B) after n-butylamine treatment which turns the film to p-type and C) after TBAI treatment whichturns the film back to n-type. Transfer curves for D) original n-type TBAI treated PbSfilm, E) after n-butylamine treatment which turns the film p-type and F) after TBAItreatment which turns the film back to n-type.

Possible theories that could explain the mechanism behind doping quantum dot films

by surface ligands are:

1. Doping rules follow bulk case- If doping via surface ligands follows the same rules as

doping in bulk semiconductors than we would expect halogens to be n-type dopants

and amines and hydroxides to be p-type dopants as Cl, Br and I are donors and O

and N are acceptors in bulk PbS.

2. Doping is related to stoichiometry, i.e. Pb-rich leads to n-type and S-rich leads to


- 2 0 - 1 5 - 1 0 - 5 0- 0 . 1 8

- 0 . 1 6

- 0 . 1 4

- 0 . 1 2

- 0 . 1 0

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0 . 0 0

0 5 1 0 1 5 2 00 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

1 . 2

1 . 4

1 . 6

1 . 8

- 3 0 - 2 0 - 1 0 0 1 0 2 0

- 0 . 0 7

- 0 . 0 6

- 0 . 0 5

- 0 . 0 4

- 0 . 0 3

- 0 . 0 2

- 0 . 0 1

0 . 0 0

- 2 0 - 1 0 0 1 0 2 0 3 00 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

1 . 2

1 . 4

Id (µA

)

V d ( V )

V g = 0 V V g = - 1 0 V V g = - 2 0 V V g = - 3 0 V

Id (µΑ

)

V d ( V )

V g = 0 V V g = 1 0 V V g = 2 0 V V g = 2 5 V V g = 3 0 V

Id (µA

)

V g ( V )

V d = - 1 0 V

T B A I

Id (µA

)V g ( V )

V d = 5 V

T B A I

A B

C D

Figure 6.4: Output curves for A) original p-type n-butylamine treated PbS film and B)after TBAI treatment which turns the film to n-type. Transfer curves for C) originalp-type n-butylamine treated PbS film and D) after TBAI treatment which turns the filmn-type.

p-type PbS films

By successfully changing the dopant type from p-type to n-type and vice versa by

simple ligand exchange we conclude that the doping mechanism is similar to the case

for bulk PbS. Etching of PbS dots to obtain Pb-rich n-type films and S-rich p-type films

can be ruled out due to the complete reversibility of dopant type from n to p and back

to n. Furthermore, from the previous section it was shown that air exposure leads to

p-type doping and since oxygen gas or water vapour cannot remove Pb atoms from the

quantum dot film it must be the results of the acceptor states created by the physisorption

of oxygen. We thus conclude that the doping mechanism for PbS CQD films using surface

ligands is the same as in bulk PbS.


6.3 Conclusion

In conclusion, using amines we can change the dopant type from initially n-type, by using

TBAI, to p-type. The reverse is also possible if we start with amine PbS films and treat

with halide anions to go from p-type to n-type. The reversibility of the dopant type

indicates that it is the ligand, the replacement of iodide with amines or vice-versa, which

controls the doping.

Chapter 7

P-type Doping of Colloidal Quantum

Dot Films

7.1 Doping with Oxygen

In order to create a stable p-n junction we seek a p-type doping technique that is not

changed by halide treatment. In this case we cannot use amines since subsequent halide

treatment will change the doping of the film. P-type doping is also achievable by solid

state exchange of organic ligands in air. In this case oxygen is responsible for p-type

doping. Because of the low mobility of p-type films we use the p-type material to act as

a "window layer" in our PV devices. Thus we need a highly doped p-type material in

order to obtain high a Voc in our devices. Table 7.1 summarizes the various ligands used

for solid state exchange of oleic acid ligands to create p-type materials in air.

46

Chapter 7. P-type Doping of Colloidal Quantum Dot Films 47

Table 7.1: Mobility and doping density of p-type PbS (950 nm) CQD films for differentsolid state exchanges performed in air

Solid state exchange Mobility Doping(cm2/Vs) (cm−3)

Cetyl trimethylammonium bromide 10−5 1017(CTAB) 10 mg/mLSodium Hydroxide 10−4 1017(NaOH) 5 mg/mL

Tetramethylammonium hydroxide 10−5 1017(TMAOH) 5 mg/mL

7.2 Intrinsic Doping with Metals

In order to achieve higher doping levels we have implemented a similar technique to

Banin et al. and introduced silver impurities to obtain highly doped PbS quantum dots.

The mechanism for doping using Ag impurities in InAs quantum dots is caused by the

substitution of indium for Ag in the InAs lattice [14]. Similarly, Ag can be used as a

dopant for PbS quantum dots since Pb also has more valence electrons than Ag and

therefore a substitution of Pb for Ag in the PbS lattice would results in hole doping.

We also switched to smaller PbS CQDs with absorption peaks of around 815 nm (1.52

eV band gap). Using a larger band gap for the p-type film will allow for more effective

blocking of electrons, from moving from the n-type to the p-type side when making

a p-n homojunction PV device, and more absorption in the active n-type layer. By

introducing Ag dopants during PbS synthesis combined with post-synthesis solid state

ligand exchange using tetramethylammonium hydroxide(TMAOH) we are able to get

highly doped PbS films on the order of 1019cm−3.

In order to deduce the main mechanism for p-type doping we compared solid state

exchange in the glove box (N2 environment) versus in air (O2 environment). The results

are shown in figure 7.2. Fabrication of TFTs under N2 results in lower doping levels

due to the absence of atmospheric oxygen; however, after annealing overnight in air the

doping level of the film is similar to those fabricated in air. Thus we conclude that it is


Ag

S

S

SS

Pb Pb

PbPb

Figure 7.1: Lewis chemical structure diagrams illustrating doping of PbS quantum dotsusing Ag. The black lines and paired dots (color indicates the atom that it belongs to)represent bonding electrons.

N i t r o g e n A i r1 E - 7

1 E - 6

1 E - 5

Linea

r Mob

ility (c

m2 /Vs)

F a b r i c a t i o n A t m o s p h e r e

a f t e r f a b r i c a t i o n a f t e r o v e r n i g h t a n n e a l

A

B

N i t r o g e n A i r1 E 1 7

1 E 1 8

1 E 1 9

Dopin

g Den

sity (c

m-3 )

F a b r i c a t i o n A t m o s p h e r e

Figure 7.2: A) Linear electron mobility and B) doping density for PbS Ag-doped (815nm) films fabricated in a nitrogen vs air environment and the effect of annealing overnightin air.

the role of oxygen which is mainly responsible for the high doping density levels.


7.3 P-N Compatibility

Since the architecture of our homojunction PV device is p+/n we need to verify that

subsequent n-doping treatments for top layers will not affect underlying p layers. Figure

7.3 illustrates the retention of p-type doping after various halide treatments (used for

n-type layers).

T B A I C T A B H T A C1 E - 7

1 E - 6

1 E - 5Lin

ear M

obility

(cm2 /Vs

)

P o s t H a l i d e T r e a t m e n t

b e f o r e a f t e r

A

B

T B A I C T A B H T A C1 E 1 7

1 E 1 8

1 E 1 9

Dopin

g Den

sity (c

m-3 )

P o s t H a l i d e T r e a t m e n t

Figure 7.3: A) Linear electron mobility and B) doping density for PbS Ag-doped filmstreated with TMAOH pre- and post-halide salt treatment.

Furthermore, combining Ag-doped TMAOH PbS p-type films and halide n-type films,

a stable homojunction PV device can be fabricated. All PV performance results are

courtesy of Dr. Huan Liu. The architecture for the homojunction PV device consists of

an ITO substrate, which provides an ohmic contact to the thin p-type PbS film on top,

followed by a thick n-type PbS films and an AZO top ohmic contact. The best PCE for


this device was found to be 6 % with Jsc of 23 mA cm−2 and Voc of 0.52V, see figure 7.5.

Figure 7.4: Homojunction PV device architecture.

Figure 7.5: Static measurements of Voc, Jsc and MPP and J-V curve of homojunctionPV device.

7.4 Conclusion

Doping by incorporating Ag impurities into PbS and subsequent solid state ligand ex-

change using TMAOH leads to highly doped (1019cm−3) CQD films; however, comparing

fabrication in the N2 versus air leads to almost 2 orders of magnitude higher doping


density for films fabricated in air indicating that oxygen is the major dopant responsible

for such high doping density values. Moreover, Ag-doped TMAOH treated PbS films

are shown to be stable and no change in doping type or density after subsequent halide

treatment is observed. Finally, a stable homojunction PV device can be fabricated out

of TMAOH treated Ag-PbS p-type films and TBAI treated PbS n-type films.

Chapter 8

Conclusion

8.1 Thesis Findings and Conclusions

Colloidal quantum dot photovoltaics have reached efficiencies of up to 6% [20]. This

technology holds great promise due to low solution processing costs and the ability to

tune the band gap by changing the size of the quantum dot. Although there have been

major strides in CQD photovoltaics, little is known about the doping mechanism of

these materials and controlling the doping of CQD films remains a challenge. Major

contributions to the field are as follows:

1. It was found herein that oxygen or water vapour has a large impact in doping of

PbS CQD films. Comparing the fabrication of halide passivated PbS films in air

and in nitrogen we find p-type and n-type PbS films respectively. Upon exposure

of n-type films to air the films are doped p-type within minutes. Thus we conclude

the role of oxygen or water vapour as an acceptor in PbS CQD films.

2. This work presents for the first time a controlled method of doping PbS CQD films

through a range of doping densities. Low, near intrinsic levels (1015cm−3) and

highly doped (1018cm−3) PbS films are achieved by using different halide anions for

ligand exchange. Furthermore we show that we can alter the doping by changing

52

Chapter 8. Conclusion 53

the amount of iodide introduced into the film.

3. A method for the fabrication of highly doped p-type PbS films was developed.

Combing air fabrication with Ag-doped PbS quantum dots and TMAOH ligand

exchange we achieve highly doped p-type films of 1019cm−3 which is ideal for ob-

taining high Voc in CQD PV devices. These films retain the doping level after

n-type treatments indicating the compatibility needed to make a stable p-n homo-

junctions.

4. This work also shows the ability to make a p-n homojunction photovoltaic device

where both the p and n region is comprised of PbS quantum dots. This homojunc-

tion device shows superior stability for over 60 hours of continuous light soaking at

maximum power point and power conversion efficiencies of up to 6%

8.2 Future Work

The future work that needs to be done is as follows:

1. Improve transport in p-type PbS films. Currently, our p-type PbS films have poor

mobility which limits the Jsc in our devices. By optimizing p-type PbS doping

towards an oxygen free fabrication we can avoid PbSO3 and PbSO4 traps and

improve transport in our films.

2. Optimize device architecture for p-n PbS CQD photovoltaics. With doping control

of n and p-type PbS films new device configurations such as p/i/n or p+/p/n/n+

can be explored in order to take advantage of this new doping technique.

Appendices

54

55

.1 TFT Measurement

TFT measurements were performed using a homemade setup inside the glove box. The

set up consists of three probes (source, drain and gate) connected to two Keithley 2400

source-meters. One Keithley is used to apply drain-source voltage and measure the

current while the other is used to apply gate-source voltage and acquire the current. The

gate-pulse method was used to reduce hysteresis with a zero-voltage time of 1s and delay

time of 100 ms.

.2 P-N Homojunction CQD PV Device Fabrication

P-type films were spin coated on clean ITO in air. The film is built up layer by layer, first

spinning a film of oleic acid capped Ag-PbS quantum dots from a solution of 25 mg/mL

in octane and then treating with TMAOH and rinsing three times with methanol. PbS

CQDs doped with Ag and with exciton peak ∼830 nm were used for p layers. For

TMAOH treatment, a concentration of 10 mg/mL in methanol for 10 seconds was used.

This process was repeated to create a 4 layer thin p-type film. The film was then annealed

overnight at 50C in air. Next, the n-type film was fabricated on top of the p-type film.

All film fabrication for the n-type film was done in the glove box in an N2 environment.

Films were made by spinning oleic acid capped PbS quantum dots (∼ 980 nm) from a

solution of 50 mg/mL in octane, then treating twice with 10 mg/mL of TBAI in methanol

for 1 minute and rinsing three times with methanol. This process was repeated to create

a 10 layer thick n-type film. Finally, 30 nm of AZO and 200 nm of Ag were deposited as

top contacts.

56

.3 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra were obtained using a Burker Tensor 27 infrared spectrometer. Infrared

spectroscopy is used to probe the vibrational states of a molecule and allows for the

identification of chemical functional groups in a sample. Samples were prepared on clean

FTO by either drop-casting for as is oleic acid capped PbS or spin-coating a 6 layer-by-

layer film for amine and halide treated films.

.4 Absorption Spectrum

Absorption measurements are preformed using a Varian CARY 500 UV-Vis-NIR spec-

trometer. For solution absorption measurements a dilute solution of PbS quantum dots

is loaded into a quartz cuvette. The 100% transmission (cuvette with pure solvent) and

0% transmission (block the light completely by a metal plate) baselines are collected be-

fore the sample measurement. For film absorption measurements, an integrating sphere

is used and again baseline (100% and 0% transmission) measurements are first taken.

Samples were prepared on clean ITO by either drop-casting for as is oleic acid capped

PbS or spin-coating a 6 layer-by-layer film for amine and halide treated films.

.5 Scanning Electron Microscopy (SEM)

In SEM a high-energy beam of electrons is used to image a sample by scanning in a raster

pattern and collecting signals produced by the sample. SEM images were acquired at the

chemistry department of the University of Toronto using the Hitachi S-5200 transmis-

sion electron microscope under accelerating voltage of 10kV. TFT devices were directly

imaged.

57

.6 X-ray Photoelectron Spectroscopy (XPS)

XPS is a quantitative spectroscopic technique that measures the elemental composition

of the top surface (1-10 nm) of a sample. A beam of X-rays is used to eject electrons from

the surface. The binding energy (Ebinding) and thus the element present in the sample

can be determined using the equation,

Ebinding = Ephoton − (Ekinetic + φ) (1)

where Ephoton is the energy of the incoming x-ray photons, Ekinetic is the kinetic

energy of the measured electron and φ is the work function of the spectrometer. XPS

measurements were obtained by Mark Greiner in the department of material science and

engineering at the University of Toronto using the PHI 5500 analytical chamber. Samples

were made by David Zhitomirsky by spin-coating on ITO using the same recipe as for

TFTs.

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