Optical designing of infrared photodetectors based on ...lib.ugent.be/fulltxt/RUG01/002/367/258/RUG01-002367258_2017_0001... · Optical designing of infrared photodetectors based

Mehedi Mamun

quantum dots

Optical designing of infrared photodetectors based on

Academic year 2016-2017Faculty of Engineering and ArchitectureChair: Prof. dr. Isabel Van DriesscheVakgroep Anorganische en Fysische Chemie

Master of Science in Photonics EngineeringMaster's dissertation submitted in order to obtain the academic degree of

Supervisors: Prof. dr. Zeger Hens, Dr. David Cheyns (imec)

i

Acknowledgements

First of all, I would like to thank almighty Allah.

I would really like to express my gratitude to my IMEC supervisor - Dr. DavidCheyns, for giving me the opportunity to be part of his research group and forhis valuable time and guidance throughout the whole project. I would also liketo thank my promoter - Prof. Dr. Zeger Hens at Ghent University, for his guid-ance and feedback.

I am grateful for having the opportunity to work in such an innovative projectand be part of the amazing working environment that IMEC provides, in whichI met many brilliant people. I am especially thankful to Dr. Pawel Malinowskifor the various comments and feedback he provided during my research.

I would also like to thank Epimitheas Georgitzikis, for his suggestions, supportand the countless discussions during the project. Moreover, I am thankful toJorick Maes from Ghent University for supply of quantum dots and Kuo Haofor providing training on device fabrication.

Finally, I am extremely grateful to my family and especially my wife and par-ents, whose support has made this project as well as the entirety of my studiespossible.

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ABSTRACT

The current technology for infrared detectors makes use of binary and ternarysemiconductor compounds.Device fabrication based on those materials requiresexpensive processes and equipment, resulting in high cost for unit area of sen-sor; furthermore, hybrid integration with CMOS readout circuit is often manda-tory, a fact that reduces performances and increases cost. Colloidal quantumdots, nanometer-scale semiconductor crystals capped with surfactants and dis-persed in a solvent, promise to solve those issues through solution processabil-ity: this allows to reduce the manufacturing cost and opens possibilities for di-rect deposition on different substrates, from silicon readout circuits to exiblematerials.

One of the main aims of this thesis is to optimize the device stack of colloidalquantum dot infrared photodetector through optical simulation and experiments.Designing and implementation of new semi transparent top contact is anotherimportant goal of this thesis.

The work in this thesis was divided in three parts - optical modeling , fabricationand characterization through experiments. Optical modeling worked as a refer-ence point for experiments. Optical transfer matrix method was used to carryout optical modeling.

Bottom illuminated device stack was first optimized through simulation andthen experimentally analyzed. After optimizing device stack for bottom illumi-nated devices, the best device stack exhibits EQE - 18% , dark current density -106 A/cm2 and detectivity - 1.4x1011 Jones. The newly designed semi trans-parent top contact shows transmittance of 70% for wavelength of 1400 -1700.After optimizing device stack for top illuminated devices, the best device stackexhibits 13.2% and dark current density of 106 A/cm2. This device stack canbe used for the realization of monolithic infrared image sensor.

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Contents

Acknowledgements i

1 Introduction 1

1.1 Importance of new materials for infrared detectors . . . . . . . . . 1

1.2 Contribution of this thesis . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theory and Literature review 4

2.1 Photodetector Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Semiconductor physics of photodectors . . . . . . . . . . . 5

2.1.2 Figures of Merit . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Infrared Photodetectors . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.1 The Infrared Spectrum . . . . . . . . . . . . . . . . . . . . . 12

2.2.2 Applications of NIR Detectors . . . . . . . . . . . . . . . . 12

2.2.3 Traditional IR Detectors . . . . . . . . . . . . . . . . . . . . 14

2.3 Colloidal Quantum Dot Photodetectors . . . . . . . . . . . . . . . 15

2.4 Theory of Multilayer Thin Films . . . . . . . . . . . . . . . . . . . 17

2.4.1 Transfer Matrix . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4.2 Anti Reflecting Coating . . . . . . . . . . . . . . . . . . . . 19

2.5 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.5.1 Colloidal quantum dot photodetector . . . . . . . . . . . . 21

2.5.2 Transparent top contact . . . . . . . . . . . . . . . . . . . . 23

3 Experimental Procedures 25

3.1 Device Description . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2 Fabrication Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.1 Solution preparation . . . . . . . . . . . . . . . . . . . . . . 29

3.2.2 Substrate cleaning . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.3 Electron Transport Layer(ETL) deposition . . . . . . . . . . 30

3.2.4 Active Layer deposition . . . . . . . . . . . . . . . . . . . . 31

3.2.5 Hole Transport Layer (HTL) deposition . . . . . . . . . . . 31

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3.2.6 Top contact deposition: . . . . . . . . . . . . . . . . . . . . 31

3.3 Device Characterization . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3.1 Microscopy Analysis . . . . . . . . . . . . . . . . . . . . . . 33

3.3.2 Thickness Measurement . . . . . . . . . . . . . . . . . . . . 33

3.3.3 I-V / J-V Measurement . . . . . . . . . . . . . . . . . . . . . 33

3.3.4 Spectral Response . . . . . . . . . . . . . . . . . . . . . . . 34

4 Bottom Illumination 35

4.1 Calculation of n and k values for optical simulation . . . . . . . . 36

4.2 Optical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.2.1 ETL thickness optimization . . . . . . . . . . . . . . . . . . 40

4.2.2 Trend for ETL thickness test . . . . . . . . . . . . . . . . . . 42

4.2.3 Active layer thickness Test . . . . . . . . . . . . . . . . . . . 42

4.3 Experimental results & comparison with optical simulation . . . . 43

4.3.1 Active layer thickness experiment . . . . . . . . . . . . . . 43

4.3.2 Trend comparison . . . . . . . . . . . . . . . . . . . . . . . 44

4.3.3 Dark and photo current comparison . . . . . . . . . . . . . 45

4.3.4 Detectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5 Top Illumination 47

5.1 Top contact design . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2 Performance analysis of new semi transparent top contact . . . . 52

5.3 Top illumination device performance analysis . . . . . . . . . . . 54

5.4 Top illumination device simulations . . . . . . . . . . . . . . . . . 55

5.4.1 Active layer thickness test . . . . . . . . . . . . . . . . . . 55

5.4.2 ETL thickness test . . . . . . . . . . . . . . . . . . . . . . . . 56

5.4.3 Bottom contact influence . . . . . . . . . . . . . . . . . . . . 57

5.5 Experimental analysis of Top illumination devices . . . . . . . . . 58

6 Conclusion 62

6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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List of Figures

1.1 Comparison between different infrared chip technologies. a) Hy-brid chip currently used, consisting of a part sensitive to IR lightand a part able to interpret the electrical signal. b) New design,with active layer deposited directly on the readout circuit. . . . . 2

2.1 Working principle of a detector . . . . . . . . . . . . . . . . . . . . 5

2.2 Energy diagram of different classes of materials . . . . . . . . . . 6

2.3 Effect of impurities on the energy levels of semiconductors. . . . . 7

2.4 Representation of the depletion zone in a P-N junction . . . . . . 7

2.5 Shallow and deep traps effects on electrons. Shallow traps onlyconfine electrons, while deep traps can act as recombination cen-ters, coupling effectively with electrons and holes . . . . . . . . . 8

2.6 Visible and NIR images comparison. a) San Francisco Bay Bridgein foggy condition, as seen in the visible range. b) Image of thesame scene taken with a NIR camera, able to see through the fog . 13

2.7 The suns spectrum reaching the earth, with indication of the ele-ments causing absorption bands . . . . . . . . . . . . . . . . . . . 13

2.8 Quantum size effect tunability of the absorption spectrum in PbScolloidal quantum dots. The quantum dot size varies from 10 nm(red) to 3 nm (black) in diameter . . . . . . . . . . . . . . . . . . . 15

2.9 PbS colloidal quantum dots. a) Representation of a CQD with itsorganic ligands. b) HRTEM picture of PbS QCD with an excitonabsorption at 1440 nm. c) Close-up with a single dot, showing theatoms arranged in the crystalline structure . . . . . . . . . . . . . 16

2.10 Quantities influencing the energy levels of quantum dots. a) Ef-fect of the size . b) Effect of the different ligands . . . . . . . . . . 17

2.11 A general multilayer structure having m layers between a semi-infinite transparent ambient and a semi-infinite substrate. Theoptical electric field at any point in layer j is represented by twocomponents: one propagating in the positive direction and one inthe negative x direction, E+j and E

j , respectively. . . . . . . . . . . 18

2.12 Single layer anti reflective coating . . . . . . . . . . . . . . . . . . . 19

2.13 Three layer anti reflective coating . . . . . . . . . . . . . . . . . . . 20

2.14 Structure of the photodiode realized by Sargent group . . . . . . . 21

2.15 Structures used in the work of Pal et al. to build photodiodes. a)Standard structure. b)Inverted structure . . . . . . . . . . . . . . . 22

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2.16 Detectivity curves for the devices of Pal et al. for standard struc-ture (Device 1) and inverted structure (Device 2)(ref) . . . . . . . . 22

2.17 Structure used in the work of Manders et al. (ref) . . . . . . . . . . 22

2.18 Structure of Anti reflection transparent conductor realized by thegroup of Pruneri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1 Stack of Standard Structure (left) vs Inverted Structure (right) . . 26

3.2 Top view of (a) silicon substrate,(b) glass substrate with pre-patternedITO lines and (c) glass substrate . . . . . . . . . . . . . . . . . . . . 27

3.3 Top view of Ag- top contact on ITO substrate . . . . . . . . . . . . 27

3.4 Poly-TPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.5 Absorption spectrum of PbS quantum dots . . . . . . . . . . . . . 29

3.6 Structure of the device before and after metalization. a) Sampleready for metalization, exposing ITO lines. b) Final look of thesample, with the indication of the 12 devices. . . . . . . . . . . . . 32

4.1 Standard substrate used for transmission, reflection and absorp-tion measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2 Samples used for T.R.A. measurement . . . . . . . . . . . . . . . . 36

4.3 (a)Transmission comparison (b)reflection comparison of glass, glass+ITO,glass+TiOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.4 (a)Transmission comparison, (b)reflection comparison of sample2,3,4,5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.5 Absorption comparison of device 2,3,4,5 . . . . . . . . . . . . . . . 38

4.6 Standard stack for bottom illumination device . . . . . . . . . . . 39

4.7 (a)Device stack , (b)EQE of 5 layers of PbS QD . . . . . . . . . . . 39

4.8 EQE of 5 different devices with ETL 20 nm, 40 nm, 60 nm, 80 nm,100 nm and active layer of 120 nm . . . . . . . . . . . . . . . . . . 40

4.9 EQE of (a) 7 layers, (b) 8 layers of PbS QD with ETL of 27 ,41 and52 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.10 EQE of (a) 9 layers, (b) 10 layers of PbS QD with ETL of 27 ,41 and52 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.11 EQE of (a) 11 layers, (b) 12 layers of PbS QD with ETL of 27 ,41and 52 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.12 EQE of (a) EQE of 8 layers (120 nm) of QDs with ETL thickness of27 , 41 and 52 nm (b) zoomed image of (a) . . . . . . . . . . . . . . 42

4.13 EQE of (a) 5 - 11 layers, (b) 12 - 17 layers of PbS QD with ETL 41 nm 43

4.14 (a) Device stack (b) EQE of devices with active layer thickness 75nm to 150 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

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4.15 (a) EQE of devices with active layer thickness- 75 nm to 150 nm(b) simulated EQE of devices with active layer thickness- 75 nm to165 nm (c) simulated EQE of devices with active layer thickness-180 nm to 255 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.16 (a) EQE of devices with active layer thickness- 120 nm (8 layers ofQDs)and ETL thickness of 27 nm , 41 nm and 52 nm (b) simulatedEQE of the same device stacks (c) Trend . . . . . . . . . . . . . . . 45

4.17 (a) Dark current comparison (b) Photo current comparison . . . . 45

4.18 Detectivity of 8 layers (120 nm) of PbS QDs with ETL 41 nm . . . 46

5.1 (Left) top contact stack , (right) sample used for this experiment . 47

5.2 Transmission, reflection and absorption of Ag (15 nm) . . . . . . . 48

5.3 Simulated and experimentally realized- Top contacts . . . . . . . 48

5.4 Transmission comparison of different top contacts . . . . . . . . . 48

5.5 Resistivity measurement procedure . . . . . . . . . . . . . . . . . . 49

5.6 Resistivity and transmission comparison of different top contacts 49

5.7 comparison of simulation and experimental results of differenttop contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.8 Different top contacts of Gold and their resistivity . . . . . . . . . 50

5.9 Transmission comparison of top contacts of gold . . . . . . . . . . 51

5.10 Reference device with 5 layers (70 nm - 75 nm) QDs and spectralresponse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.11 device stack with 5 & 7 layers of QDs and spectral response . . . 52

5.12 Spectral response comparison for bottom illumination . . . . . . . 53

5.13 Spectral response comparison for top illumination . . . . . . . . . 53

5.14 (a) Device stack with new top contact (b) device stack with Ag 10nm - top contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.15 Dark current comparison . . . . . . . . . . . . . . . . . . . . . . . 54

5.16 Photo current comparison . . . . . . . . . . . . . . . . . . . . . . . 55

5.17 dark current comparison . . . . . . . . . . . . . . . . . . . . . . . 55


5.19 EQE comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.20 EQE comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.21 ETL thickness test with 120 nm of active layer . . . . . . . . . . . 57



5.24 Bottom contact influence test-1 . . . . . . . . . . . . . . . . . . . . 59

5.25 Bottom contact influence test-2 . . . . . . . . . . . . . . . . . . . . 59

5.26 Device stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

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5.27 Dark current comparison . . . . . . . . . . . . . . . . . . . . . . . 60


5.29 EQE comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.30 Detectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

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List of Tables

3.1 Properties of PbS dots . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.1 n and k value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.1 Comparison between newly designed top contact & conventionaltop contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

1

Chapter 1

Introduction

The journey of Infrared detection started through the discovery of infrared radi-ation by Frederick William Herschel on February 11th 1800 [1].

However, the important advances in the near infrared range were made in the1980s through the introduction of the first Ge-on-Si detector ( Luryi, 1984 ) andthe development of InGaAs (1985) [2]. InGaAs is the most used material for NIRapplications at present. Although Ge is regarded as more economical alterna-tive, the performance of Ge is not as good as InGaAs - most notably in uncooledconditions.

The development of infrared detectors has been mostly linked to military ap-plications over the period of time. However, at present, the civil market is alsohighly interested in the technologies based on infrared detection [3]. This hasbecome a driving force towards new developments in this field. But, the mostimportant factor that is preventing the widespread use of IR detection technol-ogy is the cost of detectors based on current technology.

1.1 Importance of new materials for infrared detectors

The main aim of this research work is to realize infrared photodetectors thatwill facilitate cost effective infrared cameras. Now a days, the cost of a visiblecamera based on CCD or CMOS technology is about 10 euros per mega pixel andthe NIR camera based on InGaAs costs around 35000 euros per megapixel [4].This massive contrast in price is due to the materials used for infrared detection.

The materials that are currently used for NIR detection can not be grown intocrystals anywhere near the size of modern silicon crystals [4]. Moreover, theresulting wafers do not have the uniformity of silicon wafers. InGaAs wafersare only 100 mm in diameter (compared to the 300 mm of silicon), and theirproduction volume is only some thousands per year (compared to hundreds ofmillion of silicon) [4].

Focal plane arrays are the other key contributor in the cost of NIR cameras. Gen-erally, infrared sensors (pixels) used for imaging applications are called as focalplane array [4]. The materials used for the construction of the arrays of IR sen-sitive pixels are not similar to the ones used for the ROIC chip. The readoutintegrated circuit (ROIC), is mostly fabricated in silicon using standard CMOStechnology. This ROIC chip is an important component as it is used for thetransportation of the resulting charge, voltage and resistance of each pixel to themeasurement circuitry. Therefore, the final device is obtained typically through

2 Chapter 1. Introduction

indium pumping bonding This not only increases the cost but also reduces theperformance of the device [4].

The comparison of the cost between commercially available visible cameras basedon CCD or CMOS technology and a InGaAs near infrared cameras gives a goodindication on the importance of finding new materials to reduce the cost of NIRcamera and increase its widespread usage.

Figure 1.1: Comparison between different infrared chip technologies. a) Hybridchip currently used, consisting of a part sensitive to IR light and a part able tointerpret the electrical signal. b) New design, with active layer deposited directly

on the readout circuit.

Colloidal quantum dots have been recently identified as a promising materialfor near infrared photodetectors. The key advantages in using Colloidal quan-tum dots arise from their processing conditions and solution processibility [5].Device fabrication based on conventional single crystalline semiconductors re-quires high vacuum and high-temperature processing conditions, resulting inhigh equipment cost [5]. But using colloidal quantum dots ensures low cost astheir processing conditions do not require high temperatures and high vacuumvalues.

Moreover, using solution processed colloidal quantum dots solves the integra-tion problem. It has been reported that thin film photodetectors based on quan-tum dots can be integrated into imagers [6]. Since it can be deposited also overa wide range of substrates, it is possible to realize wearable and flexible devices.Moreover, the band gap of colloidal quantum dots can be easily tuned due totheir unique electrical structure.

1.2 Contribution of this thesis

The goal of this thesis is to optimize the device stack of infrared photodetectorsbased on PbS colloidal quantum dots through optical simulations and experi-ments. Optical simulations through transfer matrix method play an importantrole to enhance the performance of thin film photodiodes. Because in thin filmphotodiodes, a number of layers are stacked on top of each other creating a mul-tilayer structure [7]. The distribution of the incident light inside the thin filmphotodiodes is affected by reflections at the interfaces of the multilayer stack.Therefore, optical simulations were carried out to understand the optical prop-erties of the device.

1.2. Contribution of this thesis 3

One of the important contribution of this thesis is the designing and implemen-tation of new top contact which is semi transparent in near infrared spectrum -focusing on the wavelengths of 1400 nm - 1450 nm .

4

Chapter 2

Theory and Literature review

The background theory behind this work is discussed in this chapter. First, thegeneral theory of photodetectors, with the indication of the different figures ofmerit for detectors and noise sources have been discussed. Then, a focus on theinfrared detectors is presented: the starting point is an overview of the infraredradiation, followed by some examples of applications in this spectral range. Thedifferent categories of IR detectors are also introduced. The theory of transfermatrix method and anti reflective coating are also discussed in brief. Finally, sci-entific literature regarding colloidal quantum dot phtodetectors and transparenttop contacts related to this work are reported.

2.1. Photodetector Theory 5

2.1 Photodetector Theory

Generally we define a detector as a device which can detect a certain kind of sig-nal and converts it to another one. The general working principle of a detector isshown in figure 2.1 [8]. Detectors can detect various kinds of input signals likemechanical vibrations, electromagnetic radiation, chemical variations etc. anda detector is generally classified according to the kind of signal it detects as aninput signal.

Figure 2.1: Working principle of a detector

Photodetector is defined as a device which detects optical signal as an input sig-nal and convert this optical signal to electrical signal as output. In general terms,photodetectors are optoelectronic devices that are used to detect or measure por-tions of electromagnetic spectrum mostly visible light, ultraviolet and infraredradiation [1].

2.1.1 Semiconductor physics of photodectors

Semiconductor materials are the main building block of most photodetectorsbecause of their distinctive electronic properties. Hence, the basic concepts re-garding the interaction between light and matter is discussed in this subsection.

A detectors ability to detect a photon is dependent on the energy of that photonand the energy level of the active material of the detector. Large number ofclosely spaced energy levels which are known as energy bands exist in solidstate materials. The energy bands can be considered as the collection of theindividual energy levels of electrons surrounding each atom [9]. The highestoccupied energy level at room temperature or 0 k is defined as Fermi level EFand the zones of forbidden energy levels in the energy band structure is knownas energy gap [9]. There are three classes of materials based on the relationbetween the band structure and fermi level- metal, insulator and semiconductor.

There are no forbidden energy gaps in metals. Therefore, small amount of en-ergy can cause the electrons to jump over the fermi energy level and fill theunoccupied energy levels. The occupation probability for the energy levels ofmetals is described by the Fermi-Dirac distribution function [10] -

P (En) =1

e(EnEF )/KT(2.1)

where En is the energy of the nth state, k is the Boltzmann constant, T is theabsolute temperature.

However, insulators have relatively large energy gaps. Electrons cannot be ex-cited to conduction band at room temperature by thermal energy due to thelarge width of the energy band gap.

6 Chapter 2. Theory and Literature review

There is another class of material that has lower conductivity than metals buthigher conductivity than insulators. This material class is known as semicon-ductors. The energy gap of a semiconductor is smaller than insulators and elec-trons can be excited to conduction band at the temperature of interest. Generally,the energy band gap of a semiconductor is less than 4eV [11].

The energy band diagram of metals, insulators and semiconductors is presentedin figure 2.2

Figure 2.2: Energy diagram of different classes of materials

The excitation of electrons to conduction band in photodetectors is not based onthermal energy. In photodetectors, electrons are excited to conduction band dueto the absorption of photons that have higher energy than the energy gap of thesemiconductor.

hv qEg (2.2)

where h is the Planck constant, v is the photon wavelength, q is the charge ofthe excited carrier.

Materials property like conductivity changes upon irradiation. The conductiv-ity of a semiconductor increases if more carriers are excited to conduction banddue to the irradiation on the material. This mechanism is also knowns as photo-conductivity and mostly used in photoconductive detectors .

Intrinsic semiconductors often show low conductivity values that are not suit-able for real world applications. Because of this, impurities are added to createextrinsic semiconductors in order to enhance the number of free carriers. Thenumber of valence electrons of these impurities are different to the bulk material.When the impurities have more valence electrons (donor) than bulk material- Ntype semiconductor is formed while P-type semiconductor is formed when theimpurities have less electrons (acceptors). New electronic states are created inthe energy gap and the position of the fermi energy level also shifts due to theaddition of these impurities as presented in figure 2.3. The number of free carri-ers also increase due to this, meaning more conductivity.


Figure 2.3: Effect of impurities on the energy levels of semiconductors.

The p-n junction is formed when a p-type semiconductor and n-type semicon-ductor are placed in contact with each other [12]. A depletion region is alsoformed near the p-n junction as free electron diffuse across to p side and com-bine with holes to form negative ions and leave behind positive ions at the donorimpurity sites shown in figure 2.4 [13]

Figure 2.4: Representation of the depletion zone in a P-N junction

All these opposite polarity of charges create a built in field (E) and potential (V).This potential barrier makes the p-n junction to be a non linear circuit element:diode. We can increase the depletion region and potential barrier by applyingreverse bias and no current will flow through the circuit. However, current willflow through the the circuit if forward bias is applied as it will decrease thedepletion region and potential barrier [12].

The electronics states in a semiconductor belong to conduction and valancebands. However, there can be also some states within the energy gap of a semi-conductor. These states are are usually localized and associated to defects in thecrystal [14]. If a carrier falls into one these states, it gets trapped and can notcome out from these confined condition. But if a specific amount of energy isprovided, the trapped carrier will come out. These type of states are known astrap states or trap centers. Two kinds of trap states have been determined basedon locations- bulk (point defects, impurities and local stresses) and surface (ad-sorbed molecules, dangling bonds, misfit induced stresses) [14]. The majority oftrap states in a bulk crystal are bulk. However, surface traps make the biggestimpact in nanocrystals because of their higher surface/volume ratio.

Trap centers can be also distinguished depending on their position inside theenergy band gap. Generally the following two trap centers are associated withelectron trapping :

Shallow traps:


These trap centers are located close to conduction band 2.8 [4]. The energy re-quired to free a carrier from a trap is similar to KBT . Shallow trap centers cap-ture the electrons very quickly after excitation. Since the energy level of con-duction band and shallow traps are very close- the trapped electrons return toconduction band after a certain period. When the electrons are trapped, they cannot move and consequently fail to recombine with holes as the trapping centeris far away from the valance band- this increases carrier life time and photocon-ductors generally utilize this to increase gain [15] [16] [17][18].

Deep traps:

The position of these trapping centers is close to the middle of the band gap. Thispositioning enables efficient coupling with both conduction and valence bands2.8 [4]. If an electron is captured by one of these trap centers, it can fall back tothe valence band and lost as free carrier [4].Holes can be excited to deep trapcenters from the valence band and can recombine with the already trapped elec-tron. Therefore, these deep trap states can also act as recombination centers. Thisrecombination mechanism is called Shockley-Read-Hall recombination, and isusually detrimental in most devices (like photodetectors and photovoltaic cells)[15] [17] [18] [19] [9].

Figure 2.5: Shallow and deep traps effects on electrons. Shallow traps only con-fine electrons, while deep traps can act as recombination centers, coupling effec-

tively with electrons and holes

2.1.2 Figures of Merit

Photodetectors are generally characterized by using various parameters. Theseparameters are also known as figures of merit. It is very important to know andunderstand the figures of merit because these are very useful to compare theperformances of different photodetectors.

Spectral Responsivity One of the most fundamental figures of merit is thespectral responsivity. This is defined as the ratio between output currentof the photodetector Iphoto and input radiant power Poptical ():

R =Iphoto

Poptical()(2.3)

The unit of responsivity is amperes/watt (A/W). If a photodetector hashigh R value, we can assume that it is a good detector.

Quantum Efficiency Quantum efficiency is another important figures ofmerit of photodetectors. Quantum efficiency is expressed in percentage.


Two types of quantum efficiencies are used : external quantum efficiency(EQE) and internal quantum efficiency (IQE).

EQE is defined as the ratio between the number of electrons generated andthe number of incident photons.

EQE =photoelectrons/s

incidentphotons/s(2.4)

For an example- if 2000 photons are incident on a photodetector and 200electrons are generated, the EQE of the photodetector is 10%.

On the other hand, IQE is defined as the ratio between the number of gen-erated electrons and absorbed photons. IQE exclude the contributions ofthe incident photons which are transmitted and reflected.

IQE =photoelectrons/s

absorbedphotons/s(2.5)

The highest possible theoretical EQE is 100% for a photodiode. However,photoconductor can have EQE more than 100% due to its photoconduc-tivity gain.

Cut-off Wavelength Each semiconductor material has a different energyband gap. Therefore, the maximum wavelength possible to detect by aphotodiode is also different and it is called as the cut-off wavelength c ofthe photodiode.

c =1.24

Eg(2.6)

Here, Eg is energy bandgap in eV. Cut-off wavelegth is generally expressedby c. From the above equation, it is clear that if we want to detect longerwavelengths, we need to use semiconductor material with small energyband gap.

Response Time The speed of a photodetector is characterized by a param-eter called response time. The response time of a photodetector determineshow fast or slow it is in response to a light pulse. The response time of aphotodetector is described by the rise and fall time. Rise time is defined asthe time required for output signal to rise from 10% to 90% , whereas falltime is defined as the time required for the output signal to fall from 90%to 10%. The response time of a photodiode is faster than a photoconduc-tor. There are many application like image sensors where a fast responseis crucial and photodiodes are favorite candidate for such applications.


NoiseAny unwanted signal in the system is known as noise. The signal thatneeds to be detected is masked by the noise and consequently the perfor-mance of the detector is severely affected by this undesired signal-noise.There are many noise sources such as Johnson noise, shot noise, 1/f noiseassociated with Electric noise in photodetectors [20]. But the most impor-tant noise source in photodetectors are dark current. Therefore, dark cur-rent of a photodetector should be reduced as much as possible to main-tain and improve the performance related aspects like low consumptionof power and increasing the potency of read out process [21] .

Signal to noise ratio or SNR is the simplest metric for noise detection. SNRis defined as [22] :

SNR =IphotoInoise

=IphotoIdark

(2.7)

The noise equivalent power is another important metric for noise detec-tion in photodetectors. It is defined as the minimum input optical powerrequired to achieve an output signal (voltage or current) equal to the out-put noise of the detector. NEP can be also stated as the minimum inputpower that generates SNR value equal to 1.

NEP =Snoise.

f

R(2.8)

Here, Snoise - is the spectral noise density in A/Hz.

f is the noise bandwidth in Hz.

Detectivity

D is the reciprocal of the noise equivalent power. Detectivity depends onthe area of the device and bandwidth noise. However, normalized andspecific detectivity D is often used to make comparison among differentphotodetectors (25 themis).

D =

f.A

NEP(2.9)

As the main noise source is the shot noise coming from the dark current,Snoise becomes:

Snoise =

2q.Idark (2.10)

Now, combining the equation 2.8 and 2.10, the equation for D becomes:

D =R.

A

2q.Idark=

R2q.Jdark

(2.11)

Here,Idark is the dark current of the circuit when there is no light incident.Jdark is the normalized dark current over the area in absence of illumina-tion.


Dark CurrentDark current is the most important noise source in a photodetector. As wehave stated earlier that dark current should be as low as possible speciallyfor image sensing applications.

Though, there are not enough studies that have addressed the origin ofdark current despite its important role in the noise mechanism for pho-todetectors; crucial steps have been taken by researchers to combat thisproblem. According to [23] [24], injection of charges from the metal con-tacts into semiconductor material has been identified as one of the keysources of dark current. It has been demonstrated in literature that lowdark current can be achieved by increasing the distance of electrodes. Thisis done by using a thicker active layer. But there is a trade off in this situa-tion as if we increase the thickness of the active layer, we will have largerdiffusion length causing in quick carrier recombination and consequentlylower EQE. Therefore it is important to make a compromise according tothe performance desired from the photodetector.


2.2 Infrared Photodetectors

Infrared Photodetector is a detector that responds to infrared radiation. The de-velopment of IR photon detectors began during the mid of 20th century. At thebeginning, the development was mostly focused on military based applications.However, the discovery of the variable band gap HgCdTe ternary alloy by Law-son was the turning point in the development of IR photon detectors [1] . Thisnot only opened the doors of new applications for IR photon detectors but alsomotivated researchers to find new materials and design techniques.

Materials like- InGaAs, Quantum Well superlattices (or QWIP) and QuantumDots arrays (or QDIP) are the outcomes due to the pursuit of finding functioningmaterials to develop new applications and new design approach [4].

2.2.1 The Infrared Spectrum

The complete range of electromagnetic (EM) radiation based on frequency orwavelength is known as electromagnetic (EM) spectrum. The spectrum consistsof radiations such as gamma rays, x-rays, ultraviolet, visible, infrared and radio[25]. The visible part of the EM spectrum is sensitive to human eye and it isin the range of 400-700 nm. The infrared(IR) radiation generally starts from theedge of the red part( = 700 nm) of the visible spectrum and spreads till thebeginning of radio spectrum ( = 1 mm)

The entire IR spectrum is divided into various sections. The exact wavelengthrange of these three sections varies to slight extent based on the application field[4].

2.2.2 Applications of NIR Detectors

Near infrared region ( = 750-2000 nm) of eletromagnetic spectrum is the focalpoint of this work. Therefore, applications focused on this region of the EMspectrum are discussed in this section.

Night-time vision:Vision during night time and in dark places is very important, specially formilitary and surveillance purposes. Near infrared detectors from 1500 nmand cameras are useful for these applications. These are also less heavierthan their thermal counterpart. The images are also more alike to visibleimages due to the presence of contrasts and shadows [4].

NIR cameras are also very effective for night time imaging in outdoor be-cause ambient star light and background radiance (night-glow) are naturalemitters of NIR [4]. Moreover, NIR imagers can be very useful to solve theimaging problem to a large extend during cloudy and foggy weather con-ditions as the longer wavelengths suffer less scattering.

Medical imaging:The transparency windows of tissues at 800 nm and 1100 nm [26]. can beutilized to detect tumors or to probe other biological processes [27] [28].Infrared tags and detectors can be used in these applications. Moreover,

2.2. Infrared Photodetectors 13

Figure 2.6: Visible and NIR images comparison. a) San Francisco Bay Bridge infoggy condition, as seen in the visible range. b) Image of the same scene taken

with a NIR camera, able to see through the fog

these IR tags and detectors can be based on quantum dots. Identifyingvein location for administrating blood transfusions (specially for infantpatients and elderly people) is an emerging bio-medical applilaction forNIR cameras [29].

Sensing:Remote sensing is a procedure in which information is obtained about anobject without any form of physical contact. IR detection systems are usedfor remote sensing in various applications [30] There are some chemicalbonds shown in following figure [4]. which demonstrate strong absorption

Figure 2.7: The suns spectrum reaching the earth, with indication of the elementscausing absorption bands

characteristic in IR spectrum. This has facilitated the detection of manysubstances. Therefore, NIR imaging arrays are used in applications likesorting of recycled plastics, monitoring incoming sources of raw agricul-tural products to groom out contamination by dirt, stones or packagingdebris[29].NIR cameras are also used in applications like monitoring temperature,uniformity, critical dimensions and process end points for hot processes ofglass bottles, steel rolling, and metal smelting [4].

Environment monitoring:Infrared radiation contributes for approximatively 53% of the sun irradi-ance reaching earths surface, according to the Reference Solar SpectralIrradiance AM 1.5. It is evident from figure 2.7 that there are also sometransparency windows through earths atmosphere in the NIR region andthis attribute has been utilized to monitor the earth in various applicationslike identification of fires, control of deforestation and polar ice develop-ment [4]. Furthermore, IR detectors play an important role in astronomy.


IR detectors can probe through galactic dust clouds. This has enabled theopportunity to investigate the universe from earth. Astronomers can nowsee stars beyond the galactic center of our own galaxy [4].

2.2.3 Traditional IR Detectors

Traditional IR detectors are built on the basis of usual technologies of semicon-ductor industry and often made of binary or ternary crystalline compounds [1].These are used due to the fact that different band gaps can be obtained by us-ing various alloys or compositions. Quantum effects are also not utilized in thiscase. However, one of the most important limitations of these systems is theirhigh maintenance cost. The systems need to cool down to a very low tempera-ture to repress the noise originated mainly due to thermal excitation [31] [32].

Indium Antimonide:The energy band gap of InSb was the lowest among the semiconductor ma-terials during 1950s [4]. It was used in middle wavelength infrared detec-tor applications.Standard manufacturing techniques for semiconductors isused to fabricate these devices.

Lead chalcogenides:One of the first class of materials to be used as IR detectors are the leadchalcogenides, and in particular PbS (lead sulphide), PbSe (lead selenide)and PbTe (lead telluride) [4]. These materials have small band gaps andcan be used in crystalline form. The class of Lead chalcogenides are widelyused as photoconductors in many applications.

Mercury Cadmium Telluride:HgCdTe is currently the most used material for IR photodetectors. One ofthe main advantages of HgCdTe is the tunability of its energy band gapas this allows to build IR detectors which can perform over the whole IRspectrum. However, the complicated deposition technique due to the thehigh vapor pressure of Hg is the main downside of this material [4].

Indium Gallium Arsenide:InxGa1xAs is also a highly used material in NIR range. InxGa1xAshas demonstrated better performance than HgCdTe at room temperature[1][33]. However, the main drawback of using this material is the highmanufacturing cost associated with it. The cost per unit area of sensorbecomes very high due to the fact that molecular beam epitaxy(MBE) isrequired for the growth of InGaAs.

Germanium:Germanium (Ge)is another material used in NIR range. The band gap ofGe can be tuned to absorb in the range 1300 nm - 1600 nm [34] [32].

The performance of Ge is better in combination with Si because the latticemismatch of Ge with Si is low. This makes it easier to grow Ge epitaxiallyon Si substrates for easier integration. However, the performace of Ge islower than InGaAs at room temperature [32].

2.3. Colloidal Quantum Dot Photodetectors 15

Silicon:Silicon (Si) is also sometimes used as a material for NIR detectors. Themain reason to use Si is that the integration on electronics readout circuitsis easier with Si. It suffers with some major limitations such as: Si can onlyabsorb wavelengths below 1100 nm and the absorbance of Si is low overthe whole NIR spectrum [35] .

2.3 Colloidal Quantum Dot Photodetectors

Colloidal quantum dots (CQDs) are nanometer-scale semiconductor crystals cappedwith surfactants and dispersed in a solvent [36] [37]. The energy gap of quan-tum confined systems is dimension dependent [38]. It is possible to tune theemission and absorption spectrum of CQDs by synthesizing them in differentdimensions through the quantum size effect [38][39][40] .

Figure 2.8: Quantum size effect tunability of the absorption spectrum in PbS col-loidal quantum dots. The quantum dot size varies from 10 nm (red) to 3 nm

(black) in diameter

SynthesisColloidal quantum dots are obtained with synthetic wet routes. CQDs pro-vide narrow size distribution, high stability and higher shape uniformitythan other quantum confned systems.

Brus and Henglin conducted the first studies on the synthesis of colloidalquantum dots during 1980s [41] [42]. The recent synthesis techniques havebeen evolved over time and currently based on hot injection method whichwas presented in 2003 [43]. PbS quantum dots are produced mostly byfollowing this method. The diameter of PbS quantum dots obtained usingthis method can be in the range of 2.6 - 7.2 nm . Consequently, this allowsto absorb over a spectral range from 825 nm - 1750 nm [44] [45]. Longorganic ligands (like oleic acid) are generally used to cap the synthesizedCQDs [46]. PbS quantum dots are presented in figure 2.9 [47]

Physical ProcessingThere is a physical process involved in order to obtain the final CQDs film.This is a fundamental step and carried out by transforming the solutioninto solid. This physical processing is achieved by the deposition of asmall amount of CQDs solution and subsequent solvent evaporation. Thesolvent with a low boiling point is generally preferred because it will notonly help to achieve fast drying but also ensure good quality and repro-ducible films [4].


Figure 2.9: PbS colloidal quantum dots. a) Representation of a CQD with its or-ganic ligands. b) HRTEM picture of PbS QCD with an exciton absorption at 1440nm. c) Close-up with a single dot, showing the atoms arranged in the crystalline

structure

The various techniques available for this process differ only in terms of thedeposition of the initial solution [44]. Drop-casting, dip-coating and spin-coating are the three most followed techniques. However, spin coating isthe most used and popular technique as it also ensures a good quality filmmorphology [48] [33][49][50].

Chemical ProcessingLong ligands are used to cap the deposited quantum dot films to ensurereliable storing. These long ligands have two negative effects: the quan-tum dots become soluble in organic solvents- therefore successive layerswill dissolve the previous one and the conductivity of the film decreasessignificantly.

However, it is possible to increase the conductivity of the film by reducingthe inter-dot distance as it improves the coupling between dots [44][51][52].This conduction mechanism is not yet completely understood. But mod-els such as direct tunneling or phonon-assisted hopping have been used toanalyze this mechanism [51].

There is a process called solid state ligand exchange that is generally fol-lowed in order to reduce the distance between dots. This ligand exchangeprocedure is carried out in the following way- the solution that containsthe short ligand is dropped on the freshly deposited quantum dots. Thisenables the exchange of the long ligands with the new shorter ligands. Theresulted quantum dot layer is formed in crystalline manner. The volumeis changed due to this process and it creates some cracks in the film. Butas long as the cracks are small, this will not have a negative impact assubsequent layers will fill the void [4].

Furthermore, this solid state ligand exchange process helps to remove thedangling bonds on the surface of the dots. These dangling bonds can actas trapping centers [44][52].

The molecules that are generally used for this ligand exchange process arethiols [53][54] [55] or halides. Thiols are used for legand exchange in thisstudy due to the following reasons:

As it is a bidentate ligand, it has strong capabilities to link differentdots together. This attribute also ensures the enhancement of the elec-tronic properties of the material [55][56].

It has delocalized molecular orbitals; this helps to increase the car-rier mobility and consequently improves the interaction between dots[55].

2.4. Theory of Multilayer Thin Films 17

Electronic propertiesIt has been already mentioned that the energy gap of quantum dots can betuned through the quantum size effect. The following equation is derivedto establish the direct relation between the energy gap of the dots and theparticle diameter, specially PbS quantum dots [57] .

Eg = 0.41 +1

0.0252d2 + 0.283d

here d is the diameter of the nanoparticle. The following figure [40] [58][59] is a very good representation of the size dependent characteristic ofPbS quantum dots capped with oleic acid.

Figure 2.10: Quantities influencing the energy levels of quantum dots. a) Effectof the size . b) Effect of the different ligands

Using different kinds of ligands during the deposition of the film can alsocause shift in the energy levels of the quantum dots. It generally happenssince alteration in the chemical properties of the binding atom and dipoleof the ligand inflicts a change in the quantum dots surface and this resultsinto a shift in the position of the valence and conduction bands [59]. Thisshift is also evident from the above figure.

2.4 Theory of Multilayer Thin Films

2.4.1 Transfer Matrix

Transfer matrix method is one of the most elegant approaches to obtain trans-mission and reflection coefficient of the electromagnetic field inside multilayerstructures. Multilayer structures with isotropic and homogeneous media andparallel-plane interfaces can be described by 2 X 2 matrices due to the fact thatthe equations governing the propagation of the electric field are linear and thatthe tangential component of the electric field is continuous [7].

2.11 shows a plane wave is incident from left at a general multilayer structure.The multilayer structure consists of m number of layers. These layers are in be-tween a semi-infinite transparent ambient and a semi-infinite substrate as pre-sented in figure 2.17. The thickness of each layer j ( j= 1,2,...,m ) is dj. Complexindex of refraction nj = nj+iKj is used to describe the optical properties of eachlayer. The complex index of refraction is a useful parameter as it is a functionof wavelength of incident light. Two components represent the optical electricfield at any point within the system: One component is propagating in positive


Figure 2.11: A general multilayer structure having m layers between a semiinfi-nite transparent ambient and a semi-infinite substrate. The optical electric fieldat any point in layer j is represented by two components: one propagating in thepositive direction and one in the negative x direction, E+

jand E

j, respectively.

x direction and the other component is propagating in negative x direction. Bothof these are denoted at a position x in layer j by E+j (x)and E

j (x). The interfacematrix shown in equation 2.12 describes each interface in the structure

Ijk =1

tjk

[

1 rjkrjk 1

]

(2.12)

where tjk and rjk are the Fresnel complex transmission and reflection. For TEwaves (when electric field of light is perpendicular to plane of incidence) theFresnel complex transmission and reflection coefficients are defined by

tjk =2qj

qj + qk(2.13)

rjk =qj qkqj + qk

(2.14)

and for TM waves ( when electric field of light is parallel to plane of incidence)the Fresnel complex transmission and reflection coefficients are defined by

tjk =2njnkqj

nk2qj + nj2qk(2.15)

rjk =nk

2qj nj2qknk2qj + nj2qk

(2.16)

Here,qj = njcosj (2.17)

The wave experiences a phase change while it traverses layer j. This is definedby jdj , which is also the layer phase thickness.

Lj =

[

eijdj 00 eijdj

]

(2.18)

Now combining the interface matrix and the layer matrix, the total system trans-fer matrix can be written as the following [7].

[

E0+

E0

]

= S

[

Em+1+

Em+1

]

(2.19)

2.4. Theory of Multilayer Thin Films 19

where,

S =

[

S11 S12S21 S22

]

(2.20)

This transfer matrix also relates the electric field at ambient side and substrateside.

2.4.2 Anti Reflecting Coating

Optical properties of a material like increasing or decreasing reflection can bemodified by using multilayer stacks as coatings.

Many applications require Minimal reflection or maximum transmission andanti reflective coating (ARC) is a very useful way to achieve this target. A sin-gle layer of ARC coating is a thin layer of dielectric material with a particularthickness. The most important aspect of designing an anti reflective coating is tocreate a destructive interference between the reflection at the front of the coatingwith the reflection at the back of the coating.

Assuming an anti reflective coating with thickness d and refractive index n2 infigure 2.12

Figure 2.12: Single layer anti reflective coating

Here, n1 is the refractive index of air and n3 is the material that is highly reflec-tive and the aim is to reduce the reflection of this material using anti reflectivecoating.

If n1 < n2 < n3 , the two waves will have destructive interference if the thicknessof ARC layer is:

d =1.o4n2

(2.21)

Hence, it is also known quarter wave layer where o is the wavelength of theincoming wave.

The equation for the refractive index of the ARC layer to achieve zero reflectionis derived as the following [60]

n2 =n1n3 (2.22)

However, it is not always possible to find a material with such a refractive indexfor a particular application. In spite of this, reflection can be reduced a signif-icant amount by using a material with refractive index close to that given byequation 2.22. According to equation 2.22, a material for single layer ARC withrefractive index between n1 and n3 will always reduce reflection irrespective


regardless of the ARC layer thickness. Consequently, the reflection is alwayshigher for an uncoated surface than a coated surface, given that the refractiveindex of the coating is between that of two media [61].

Figure 2.13: Three layer anti reflective coating

Low reflection at a broader spectral range can be obtained by using a three ormore layers coating presented in figure 2.13. A zero reflection can be achievedif the thickness of each layer is 4 and the refractive index satisfy the followingequation:

n1n3n2

=non4 (2.23)

where no and n4 are the refractive indices of air and the reflective material thatneeds to be coated.

2.5. Literature Review 21

2.5 Literature Review

2.5.1 Colloidal quantum dot photodetector

Colloidal quantum dot photodetector is a relatively new research area. The mainaim of this section is to present the progress made in QDIP research landscape.

The leading research group in QDIP research is the group spearheaded by E.Sargent and G.Konstantatos. This group was the first to develop photodiodesbased on colloidal quantum dots. S.McDonald alongside E.Sargent used col-loidal quantum dots for Infrared detection and reported photo conductivity inMEH-PPV and PbS composite in 2004 [62].

G.Konstantatos et al. presented an ultrasensitive photoconductor based on PbSquantum dots in 2006. This photoconductor demonstrated better performancethan InGaAs photoconductors regarding detectivity. G.Konstantatos et al used avery simple architecture utilizing the active material between two gold contacts[33].

The Bawendi group demonstrated first photodiodes based CdSe colloidal quan-tum dots in 2005. It was sensitive in visible. Despite displaying fast response, itsuffered with poor detectivity [63].

In 2008, Sargent group demonstrated photodiode that had detecting ability inNIR. The dectectivity at 1100 and 1450 nm was above 1011 Jones. It was thebest device in terms of detectivity during that time. This device used a standardstructure (section 3.1.1) and realized through a junction between PbS quantumdots and aluminum [64].

Figure 2.14: Structure of the photodiode realized by Sargent group

Sarasqueta et al introduced electron transport layer and hole transport layer instandard structure between active layer and the electrodes. The active layermaterial was PbSe. The peak detectivity was around 1011 Jones in infrared [65].

The inverted device structure was first used by Pal et al.in 2012. They also pre-sented a comparison with standard structure [48]. The two devices used in thiswork are presented in figure 2.15


Figure 2.15: Structures used in the work of Pal et al. to build photodiodes. a)Standard structure. b)Inverted structure

The working principle of both devices is based on the formation of depletion re-gion between N-type oxide and P-type quantum dot film. This type of structureis also known as depleted hetero-junction.

Figure 2.16: Detectivity curves for the devices of Palet al. for standard structure (Device 1) and inverted

structure (Device 2)(ref)

Both devices showed similar perfor-mance regarding current. However,the detectivity was lower for the in-verted structure device than standardstructure device in NIR. The stan-dard device displayed better detectiv-ity than the device realized by Sargentgroup at 1000 nm.

The photodiodes of Z. Jiang and J.Xus group used a tandem config-uration of PbSe quantum dots andachieve detectivity of 1.4X1012 Joneson a rigid substrate and 8.8X1011 Jones on a flexible substrate in the infraredduring 2014-2015. However, the detectivity of single cells forming the devicewas around 1010 Jones [66] [67].

The group of F.So demonstrated photodiodes with very low dark current (noise)and better detectivity in the range of 1.4X1012 Jones compared to past perfor-mances. They used standard structure(figure 2.17) and oxide transport layers toobtain such a performance [68].

Figure 2.17: Structure used in the work of Manders et al. (ref)

2.5. Literature Review 23

Moreover, the F.So group presented a photodiode in 2015 using standard struc-ture (figure ) with a very promising detectivity 2.6X1013 Jones at 1200 nm. How-ever, the detectivity at 1200 nm was 1.2X1011 Jones. They attained this perfor-mance by utilizing gain through electron tunneling injection from the electrontransport layer [69].

2.5.2 Transparent top contact

Transparent top contact electrode (TTCE) is a very important building block forwide range of optoelectronic devices like solar cells, smart windows, light emit-ting diodes, liquid crystal displays [70] [71][72] The main aim of this subsectionis to highlight some of the recent progress made in the research of transparenttop contact electrode.

Indium tin oxide ( ITO ) is the most widely used transparent top contact elec-trode for its high transmission in visible spectrum, low sheet resistance, highchemical and environmental stability [73]. But there are some disadvantages ofITO like lack of mechanical flexibility, high temperature processing and mostimportantly - the high cost of raw material that increases the fabrication cost.Hence, lots of efforts are put into finding a replacement of ITO which will havethe merits of ITO but also comes with low cost [74].

Many alternative materials have been considered like Al- doped ZnO ( AZO), carbon nanotube, metal nanowires, ultrathin metals, conducting polymers.Moreover, graphene has also been investigated recently as an alternative to ITO[74].

Different oxides that exhibit high transmission in visible, have also been widelyinvestigated like TiO2, Nb2O5, SnO2 [75].

Besides, thin metal films sandwiched by transparent oxides - multi layer stackof O/M/O (oxide/metal/oxide) have also been explored. Researchers haveused Ag because thin Ag ( less than 20 nm ) displays low resistivity and goodtransmission in visible[75]. Many oxide combinations have been investigatedby researchers to form O/thin Ag/O multilayer stack, such as, ZnO [76][77],SnO2[78] , TilnZnO [79] , MoO3[80], Al2O3 [81], ZrON [82] and ZnSnO [83].

Many researchers have also studied TiO2 / Ag / TiO2 multi layer stack. TiO2have been widely investigated due to its high transmission of 90% in visible andgood chemical stability [84].

The effect of Ag thickness in a TiO2 / Ag / TiO2 multi layer stack was investi-gated by Dhar and Alford. The thickness of TiO2 was fixed at 30 nm and the Agthickness was only varied. They reported transmission of 90% at 590 nm andsheet resistance of 5.7 sq1 with 9.5 nm Ag [85].

Jia et al reported a multi layer stack TiO2 (10 nm) / Ag (8 nm) / TiO2 (10 nm)that exhibited transmission of 90% at 500-700 nm with a sheet resistance of 30sq1 [86].

Schubert et al reported highly transparent electrode based on thermally evapo-rated calcium:silver blend thinfilms in the visible range. The transmittance ofthe electrode was 93.6 % for 400-700 nm with a sheet resistance of 27.3 sq1

[87]


There is an important thing to note in the recent research carried out in thisdomain that most of these alternatives can solve some of the problems like me-chanical flexibility, high temperature processing and cost of ITO, but still sufferfrom demerits like poor adhesion and large surface roughness. Most impor-tantly, they sometimes fail to obtain a competitive trade off between high trans-mittance and low sheet resistance.

Maniyara , Pruneri et al recently presented a paper in which they reported atransparent conductor using anti reflection coating. They utilized anti reflectionundercoat and overcoat layer with thin silver in the middle. This transparentconductor exhibited transmittance of 98% in visible range with a low sheet re-sistance of < 6 sq1 . This transparent conductor exhibits best electro opticalperformance so far. Moreover, it is flexible and processed in room temperature[74].

Figure 2.18: Structure of Anti reflection transparent conductor realized by thegroup of Pruneri

The other important factor to notice is that a relatively very small amount oftransparent conductor being reported in literature for near infrared wavelengthrange.

Chunxiong Bao et al reported highly conductive metal nanowire networks withhigh transmittance ( 91.1% ) and low sheet resistance ( 2.2 sq1 ) from deep-ultraviolet to near-infrared. Although it can work as an alternative to trans-parent conductive oxides due to its electro optical properties. But the biggestdemerits of this is that it is based on complex and high cost fabrication technique[88].

Kwang-Hyuk Choi et al also recently presented a transparent conductor - Agnanowire (NW) network embedded ITO films that can be utilized in near in-frared. Moreover, it can be also used as transparent and flexible electrodes inflexible organic solar cells (FOSCs). The flexible ITO/Ag NW/ITO multilayerelectrode exhibits low sheet resistance of 11.58 O/sq and a high diffusive trans-mittance of 84.78% . However, the drawback of this is that it suffers from pooradhesion and irregular morphology for FOSCs [89].

25

Chapter 3

Experimental Procedures

In this chapter, we review the experimental procedures followed for the fab-rication and characterization of colloidal quantum dot infrared photodetector(QDIP) devices. This chapter is divided into three parts. The device structureand the materials which are used to fabricate the devices are discussed in thefirst part. The complete fabrication process of QDIPs are explained in the sec-ond part. Finally, a brief explanation on the techniques used to characterize theQDIPs is given.

26 Chapter 3. Experimental Procedures

3.1 Device Description

3.1.1 Structure

The basic structure of the device consists of four important building blocks:Electrodes, active layer, electron transport layer (ETL) and hole transport layer(HTL).

There are two electrodes in the device. One electrode is on top and theother electrode is on the bottom of the device. One electrode is generallytransparent to illuminate the active layer and the other electrode is usuallymetallic.

The active layer of the device consists of colloidal quantum dots.

Electron transport layer (ETL) and hole transport layer (HTL) have beenused for enhancing the performances of the device.

These four building blocks lead to two kinds of device structures dependingon how the layers are ordered in the device. These two structures are calledstandard and inverted structure.

The standard structure is one of the most commonly used and it is composed of aHTL layer deposited on top of a transparent electrode like - ITO, followed by theactive layer. There is also a metal electrode with low working function (Ca or Al)deposited on top of the active layer. But the main drawbacks of this structureare fast degradation and poor device performance due to the top electrodesoxidization in contact with air [90].

The inverted structure is realized by changing the polarity of the device by sub-stituting the low work function top electrode with high work function top elec-trode like Ag or Au [21]. Generally, the inverted structure is made of by theITO, followed by ETL, active layer and HTL. The high work function electrodeis deposited on top of the HTL. The top contact is not sensitive to air in thiscase. As a result of this, the inverted structure improves lifetime of the deviceand solves the problem of degradation and stability faced by the standard struc-ture. Moreover, this structure is very useful for integration with readout circuitin imaging applications as the top contact electrode can be directly connectedwith the circuit itself [91].

Therefore, the inverted structure is chosen and used in this research work. Theschematics of standard and inverted structures used in this work are presentedbelow:

Figure 3.1: Stack of Standard Structure (left) vs Inverted Structure (right)

3.1. Device Description 27

3.1.2 Materials

SubstrateTwo types of substrates were used to fabricate the devices. 3x3 cm glasssubstrates with pre-patterned ITO stripes as bottom contact were used forboth bottom illumination and top illumination test. 3x3 cm silicon sub-strates were used only for top illumination test. However, 3x3 cm glasssubstrates were used to experimentally verify the simulation results of thenewly designed top contact. Top view of the substrates is shown in thefollowing figure:

Figure 3.2: Top view of (a) silicon substrate,(b) glass substrate with pre-patternedITO lines and (c) glass substrate

ElectrodeElectrode is a very important component in our inverted device structure.The electrodes used in this work are the followings:

Indium Tin Oxide(ITO): ITO is one of the most popular choices in organicphotodetectors due to its electrical and optical properties. ITO exhibitslow resistivity (


about this top contact electrode is discussed in chapter on "Top Illumina-tion".

Electron Transport LayerGenerally, wide bandgap materials with deep HOMO level are used forelectron transport layer [4].

Titanium oxide (TiOx) has been used as ETL in this work. It is highlytransparent. TiOx is also one of the most widely used ETL materials fororganic photo diodes and solar cells based on inverted structure. Trans-porting electrons efficiently from the active layer material and blockingholes towards cathode are the main aims behind using this electron trans-port layer [21].

Hole Transport LayerThe main function of HTL is opposite to ETL. HTL ensures effective trans-portation of holes from the active layer material while blocking electrons.Poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine (Poly-TPD) apolymeric material has been used as HTL in this work. This is also widebandgap material but with shallow conduction band edge or LOMO level.Figure 3.4 [21] shows the structure of Poly-TPD material.

Figure 3.4: Poly-TPD

Active Layer MaterialPbS colloidal quantum dots from Ghent university have been used as ac-tive layer material. The dots were capped with oleic acid. A brief summaryof some properties of the active material is presented in 3.1.

Table 3.1: Properties of PbS dots

Property Value

wavelength exciton peak 1417 nmdiameter 5.49 nm

weight PbS QDs 1.2gsolvent n-octane

These data were provided by the supplier. The absorption spectrum of thePbS colloidal quantum dots is shown in figure 3.5 .

Ligand: Thiols were used for solid state ligand exchange in this work toreduce inter dot distance. This material was provided by Sigma-Aldrich.

3.2. Fabrication Process 29

Figure 3.5: Absorption spectrum of PbS quantum dots

3.2 Fabrication Process

The fabrication of the device was performed according to the following steps:

Solution preparation, including quantum dots and ligand exchange solu-tion

Cleaning of the substrate.

Deposition of the electron transport layer.

Anneal the substrate in ambient atmosphere at a fixed temperature.

Deposition of the active layer.

Solution exchange treatment.

Deposition of the hole transport layer.

Anneal the device inside the nitrogen glovebox at a fixed temperature.

Deposition of the metal contacts.The device was then stored in a nitrogen glovebox with controlled O2 andH2O levels.

3.2.1 Solution preparation

TiOx solutionTiOx solution was provided by thin film PV group of IMEC. It was storedin glove box. The color of the TiOx solution becomes yellowish when it isexposed to air.

Quantum dots solutionThe PbS quantum dots were dispersed into octane. Octane was also usedas solvent to prepare the quantum dots solution. Octane was added ac-cording to desired concentration. The solution was prepared inside nitro-gen glovebox.


Using octane as a solvent improves the quality of the film. It also helps toimprove the drying during the spin coating process.

Poly-TPD solutionChlorobenzene was used as solvent to prepare the Poly TPD solution.Chlorobenzene was added according to desired concentration. The solu-tion was prepared inside nitrogen glovebox.

All solutions were filtered before using for spin coating process.

3.2.2 Substrate cleaning

The glass/ITO substrates and silicon substrates were cleaned in an ultrasonicbath with different solvents at a fixed temperature in the following order:

Glass/ITO substrate cleaning procedure

Soap water.

Deionized water.

Recycled Acetone.

Fresh acetone.

Recycled isopropyl alcohol.

Fresh isopropyl alcohol.

Silicon substrate cleaning procedure

Recycled Acetone.

Fresh acetone.

Recycled isopropyl alcohol.

Fresh isopropyl alcohol.

All the substrates were kept in each solvent for approximately 5 minutes,and at the end were dried with a nitrogen gun.

3.2.3 Electron Transport Layer(ETL) deposition

The electron transport layer was deposited from solution using a spin-coatingprocess. The composition of the TiOx solution is confidential and we received aready made solution for the spin coating process.

Different thicknesses of ETL were tested in this work. These thicknesseswere achieved using different spin coating parameters.

The substrates were annealed after finishing the spin coating process.

3.2. Fabrication Process 31

3.2.4 Active Layer deposition

The active layer deposition process consisted of three parts. In the first part, wedeposited the PbS quantum dots , we did the ligand exchange treatment in thesecond part and rinsing was done by acetone nitrile (ACN) to remove the excessligands in the third part.

The spin coating process consisted of a two step spin coating program. The firststep was used to obtain a good spread of the solution on the substrate and thesecond step ensures the total evaporation of the solvent.

Part 1 - Deposition of PbS quantum dots

The deposition of PbS quantum dots was carried out by spin coating. Thisspin coating was a two step process as mentioned above.

100l is used for one layer. The thickness for one layer is around 15-20 nm.

Part 2 - Ligand exchange treatment

Ligand exchange step was carried out by dropping the ligand solution tocover the whole substrate on top of the QD layer. We waited for a fixedamount of time for the reaction to occur and after that the substrate wasspun again using the same spin coating settings used for Part 1.

Part 3 - Rinsing

In order to remove the excess ligands, two rinse steps were conducted us-ing acetonitrile (ACN). After dropping ACN each time, we spin coated itusing the settings of Part 1. We repeated this process twice.

These three steps were repeated in a layer-by-layer approach to obtain the de-sired thickness.

3.2.5 Hole Transport Layer (HTL) deposition

Similar to the electron transport layer, the hole transport layer was also de-posited by spin-coating process.

The HTL deposition is also a two step spin coating process. The thicknessfor HTL layer is around 10 nm.

The substrates were annealed at a fixed temperature after finishing thespin coating process.

3.2.6 Top contact deposition:

The top contact electrode was deposited on top after the deposition of all layers.The sample was prepared for top contact deposition by performing the follow-ing step:


A portion of material from the top and the bottom edge of the device wasremoved. This removal was done by scratching with twitzer in order toexpose the ITO lines. This results into a suitable contact for measurement.The above mentioned step was followed for top contact deposition of bothbottom and top illuminated devices.

However, the top contact deposition process for bottom illumination device wasdifferent from top illumination device. The two procedures are discussed indetails below:

Top contact deposition for bottom illumination

100 nm of silver(Ag)was deposited at a rate of 1/s

16 different metal pads were deposited in this process using the fol-lowing mask shown in figure 3.6.There are 4 pads for the bottom electrode (in this case it is ITO) andthe real detector devices are formed by the remaining 12 pads. Thereare two rows of devices and each row contains 6 devices (figure 3.6).Devices from each row also share a common bottom contact (ITO).

Figure 3.6: Structure of the device before and after metalization. a) Sample readyfor metalization, exposing ITO lines. b) Final look of the sample, with the indica-

tion of the 12 devices.

The deposition process was carried out by using Angstrom Engineer-ing PVD tool through thermal evaporation.

Top contact deposition for top illumination

Deposition of 10 nm silver using the mask shown in figure at a rateof 1/s.

Deposition of anti reflective coating material of 120 nm using at a rateof 3/s.

Deposition of 100 nm silver at a rate of 1/s.

The deposition process was carried out using Angstrom EngineeringPVD tool through thermal evaporation.

3.3. Device Characterization 33

3.3 Device Characterization

3.3.1 Microscopy Analysis

Optical microscopy and Atomic force microscopy techniques were used to char-acterize the quality of the film. Specifically,

Optical microscopy technique was used to investigate the surface qualityof the final film (presence or absence of cracks or particles). This techniquewas also used to monitor the surface quality after depositing each layerin order to have a better understanding and comparison with final film.5x,10x,50x- magnification options were generally used to carry out thistechnique.

The surface morphology of the film was analyzed by using atomic forcemicroscopy technique. AFM was used to investigate the crystalline ar-rangement.

3.3.2 Thickness Measurement

The profile analysis tool- "Vecco Dektak 150" was used to measure the thicknessof each layer of the stack. This process was carried out in two following steps:

Single scratch was created on the sample surface at five different places byusing a sharp tweezer.

The profile analysis tool then was used to measure thickness as it allowsaccurate measurement of hills and valleys which were created on the sam-ple surface due to the deposition of layers.The measurement length was 800m with stylus force of 3 mg. The reso-lution of the measurement was 0.267m .

3.3.3 I-V / J-V Measurement

The main aim of I-V/ J-V measurements was to obtain and understand the elec-trical characteristic of the device. These measurements are also important torealize the performance of the device and investigate certain parameters rolein the device performance. The bottom and top illumination devices fabricatedon ITO substrates were analyzed in glovebox using a four-probe measurementsystem. This procedure was carried out in the exact way for measurements inthe dark and under illumination. A solar simulator (Abet Technologies) with anAM1.5 reference spectrum was used for illuminating the device.

However, a two probe measurement system of "SUSS MicroTec" was used forthe top illumination devices that fabricated on silicon substrate. These top il-luminated devices on silicon substrates were analyed using "Agilent PrecisionSemiconductor Parameter Analyzer" with a double voltage sweep. The deviceswere measured in N2 glovebox.


3.3.4 Spectral Response

A Bentham PV300 Spectral Response system was used in order to measure thespectral response of the fabricated devices. External quantum efficiency (EQE)and spectral responsivity (R) were measured using this system. This spectralresponse system was also used to measure reflection and transmission of differ-ent samples used in this work. Internal quantum efficiency can be also obtainedusing this spectral system due to its ability to measure reflection and transmis-sion.

Light from a Xenon/Quartz halogen source was coupled into a monochromator.This provides a spectral range of 300-2500nm. However, the target wavelengthrange for the measurements was 1100 - 1700 nm in this work. A germanium(Ge)photodiode was used to calibrate the light intensity.

Moreover, after chopping the incident light on the device, a lock-in amplifierwas used for the detection of modulated current signal.

The EQE measurements were carried out at 2V applied bias for devices with ITOsubstrates. However, 3V applied bias was used for silicon substrate devices.

35

Chapter 4

Bottom Illumination

In this chapter, we have discussed the optimization of bottom illumination de-vice stack. The optimization was first done with simulation using optical trans-fer matrix method. The n and k values of the materials used in this work werefirst calculated and subsequently used in the simulation. After that, the devicestack was optimized by altering ETL and active layer thickness. Different combi-nations of ETL and active layer thickness were used to understand the influenceof these two layers on device performance. Finally, the experimental results areanalyzed and compared with simulations.

36 Chapter 4. Bottom Illumination

4.1 Calculation of n and k values for optical simulation

Transmission, reflection and absorption measurements were carried out on dif-ferent samples to calculate n and k values of the materials that were used tofabricate the final device. Absorption was calculated using the equation 4.1

Absorption = Noise transmissionRefelection (4.1)

Here noise is the reference measurement which was almost always 100% .

The standard substrate which is shown in figure 4.1 was used for this experi-ment.

Figure 4.1: Standard substrate used for transmission, reflection and absorptionmeasurement

Five different samples were fabricated to carry out the transmission, reflectionand absorption measurement. The stack of the samples are presented in figure4.1. Here, 1 layer of QDs means that the QD solution is spin coated only once.The thickness of 4 layers is around 100-105 nm.

Figure 4.2: Samples used for T.R.A. measurement

4.1. Calculation of n and k values for optical simulation 37

Transmission measurements of sample 1 and 2 are shown in figure 4.3

Figure 4.3: (a)Transmission comparison (b)reflection comparison of glass,glass+ITO, glass+TiOx

It can be analyzed from of figure 4.3- (a) that glass and glass+TiOx transmit al-most 90% of the incident light ( = 1100nm1700nm). However, ITO transmitsonly around 57% at our target wavelength of 1420 nm. It can be seen from figure4.3- (b) that glass and glass+TiOx reflect around 5% light whereas the reflectionof ITO is around 15% at 1420 nm. This also means that the absorption of ITOis around 25 - 30% at 1420 nm. Moreover, glass and TiOx absorbs very little. Itis also evident from figure 4.3 that the transmission of ITO decreases as we godeep into the infrared region.

Light was incident only on ITO strip, glass and between the two ITO strips dur-ing the (T.R.) measurements of ITO, glass and glass+TiOx samples respectivelyof figure 4.1.

The comparison of transmission and reflection for sample 2, 3, 4, 5 (figure 4.2) ispresented in figure 4.4 to have a better understanding of their optical character-istic.

Figure 4.4: (a)Transmission comparison, (b)reflection comparison of sample2,3,4,5

The highest transmission is for sample 1. The transmission of 1 layer of QDs isalso high. As the number of layers increase, the transmission decreases. Only62% of light passes through 4 layers of QDs.


From figure 4.4- (b), it can be seen that TiOx has the lowest reflection. The re-flection goes up as we increase the number of layers of QDs. More layers meanincreased number of internal reflection among the layers. As a result of this, thetotal reflection of the device becomes high. It is clear from the figure 4.4, as 4layers of QDs have higher reflection than 2 layers of QDs.

The absorption comparison among sample 2, 3, 4, 5 is presented in figure 4.5 -

Figure 4.5: Absorption comparison of device 2,3,4,5

Figure 4.5 shows that TiOx hardly absorbs anything which is around 2%. How-ever the absorption of 4 layers of QDs is 15%.

All these T.R.A measurement values were used in elipsometry to get the finalvalue of refractive index (n) and extinction coefficient (k). The values are pre-sented in table 4.1

Table 4.1: n and k value

Material Refractive index (n) Extinction coefficient (k)

ITO 1.58031 0.21065TiOx 1.49774 0.04342

PbS QDs 2.35619 0.06868Silver 0.41832 9.74622

The n and k values mentioned in table 4.1 were used for optical simulation.

4.2. Optical Simulation 39

4.2 Optical Simulation

Different stacks were simulated to obtain an understanding about optical char-acteristic of the colloidal quantum dot photodetector devices. The simulationsalso worked as a reference point for experimental results. The standard stack ofa bottom illumination device is presented in the following figure:

Figure 4.6: Standard stack for bottom illumination device

The bottom illumination device stack that is shown in figure 4.6 was simulatedusing different thickness of ETL and active layer (QD layer). The thickness ofthe top contact and the bottom contact was always fixed. The main aim forthese simulations was to optimize the device stack before the fabrication of thedevices and make a comparison between simulation and experimental results.

Figure 4.7 presents a device stack with 5 layers of QDs. The approximate thick-ness for 5 layers of QD is around 75 nm. We used the thickness of ETL and HTLas 27 nm and 10 nm respectively to carry out this simulation. The starting pointof this ETL and HTL thickness was based on the previous work done at imecon quantum dot photodetectors. According to the simulation result presentedin figure 4.7 - around 75 nm of QDs is needed to obtain a minimum EQE of 6%. We used this simulation result and stack parameters as a reference point tooptimize the final device stack.

Figure 4.7: (a)Device stack , (b)EQE of 5 layers of PbS QD


4.2.1 ETL thickness optimization

The simulation results of using different thickness of ETL will be discussed inthis section. Different thickness of ETL were tested to understand how the thick-ness of ETL plays a role in the device performance.

The thickness of ETL layer is important as the photodiode will be realized byforming a depletion region between n-type ETL and p-type QD layer. Five dif-ferent thicknesses of TiOx were used with active layer thickness of 120 nm (8layers of QDs) for this investigation. The optical simulation results of this in-vestigation is shown in figure 4.8 . The right side of this figure is the zoomedversion of left side.

Figure 4.8: EQE of 5 different devices with ETL 20 nm, 40 nm, 60 nm, 80 nm, 100nm and active layer of 120 nm

Figure 4.8 shows that the EQE is around 17 % with ETL thickness of 20 nmand the EQE reaches to almost 19 % when the ETL thickness is 100 nm. Fromthis, it can be said that increasing the ETL thickness enhances the EQE but theenhancement is very small.

After the above mentioned ETL thickness test through simulation, we chose 27 ,41 and 52 nm as ETL thickness for further investigation since we could achieveonly these three thicknesses on a consistent basis through spin coating of ourTiOx solution. The thickness of the active layer was also varied during the fur-ther investigation of ETL thickness to realize the relation between ETL thicknessand active layer thickness.

The EQE of 7 layers (105 nm) and 8 layers (120 nm) of QDs is presented in figure4.9 (a) and (b) respectively. The device stack used for this two devices is similarto the stack shown in figure 4.6.

4.2. Optical Simulation 41

Figure 4.9: EQE of (a) 7 layers, (b) 8 layers of PbS QD with ETL of 27 ,41 and 52nm

We can see from figure 4.9 that the EQE of 8 layers of QDs is larger than 7 layersof QDs. It can be also realized from this simulation that the change of ETL thick-ness from 27 nm to 52 nm with a fixed active layer thickness of 105 nm (7 layersof QDs) caused a small increase in EQE as we have previously seen in figure 4.8.The difference of EQE for three different ETL thickness is very small. This trendis also visible in figure 4.10, 4.11 for 9 layers (135 nm), 10 layers (150 nm), 11layers (165 nm), 12 layers (180 nm)of QDs respectively.

Figure 4.10: EQE of (a) 9 layers, (b) 10 layers of PbS QD with ETL of 27 ,41 and52 nm

If we give a closer look in figure 4.9(a), a small but broader peak is visible aroundthe wavelength of 900 nm. This peak becomes smaller and shifts towards higherwavelength by increasing the thickness of the active layer and ETL. Figure 4.10-(a) shows us that the combination of ETL thickness of 52 nm and active layerthickness of 165 nm flattens out the small peak and generates a constructiveinterference with the

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