3. Synthesis and characterization of end-functional Polythiophene building blocks

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CONTENTS 1. Introduction

………..1

1.1 New materials for energy conversion with solar cells…...……….……….1

1.2 Solar Cell Concepts ………..5

1.2.1 Overview of π-conjugated polymers as electron donor………...5

1.2.2 Overview of quantum dots as electron acceptor………..8

1.3 Organic Solar Cells and Resources ….……….………...………....…10

1.3.1 Organic Solar Cells Working Principle………...………..…………11

i) Light Harvesting and Exciton Formation………12

ii) Exciton Dissociation and Charge Separation………..………13

iii) Charge Transport……….…13

1.3.2 Various Devices and active layers……….17

1.3.2.1 Single Layer ………...…17

1.3.2.2 Bilayer Heterojunction………....18

1.3.2.3 Bulk Heterojunction………20

1.3.2.4 Diffuse Bilayer Heterojunction………...20

1.4 Hybrid Solar Cells………..21

1.4.1 Solid state dye-sensitized solar cells………..22

1.4.2 Nanoparticle sensitized TiOx solar cells………24

1.4.3 Extremely thin absorber (ETA) solar cells………25

1.4.4 Hybrid solar cells based on bulk heterojunction concept………..25

1.5 Role of Carbon nanotube in solar Cells………...……….27

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1.6 Scope of Work……….29

2. Materials and Methods

……….………..39

2.1 Nanoparticles………..……….39

2.2 Reagents ………..39

2.3 Instruments and techniques ……….………….41

3. Synthesis and characterization of end-functional Polythiophene building blocks

...44

3.1 Synthesis of asymmetrically allyl/alkynyl terminated rr-P3HT……….………47

3.1.1 Synthesis of 2,5-dibromo-3-hexylthiophene……….47

3.1.2 Synthesis of asymmetrically allyl/Br end-capped rr- P3HT………...47

3.1.3 Synthesis of allyl/alkynyl terminated rr-P3HT by Sonogoshira Coupling reaction ………48

3.2 Asymmetrically end capped rr-P3HT chain end modification by phosphonic group.49 3.2.1 Synthesis of diethyl 4-bromobenzene phosphonate………..49

3.2.2 Synthesis of diethyl 4- acetylenebenzene phosphonate………50

3.2.3 Synthesis phosphonic acid terminated rr-P3HT………...50

3.3 Results and Discussion………...51

4. Synthesis and characterization of Poly(thiophene-b-acrylonitrile)

………68

4.1 ATRP polymerization of AN to obtaine azide group at one chain end………..69

4.1.1 Synthesis of 3-azido-1-propanal………...……….69

4.1.2 Synthesis of 3-azidopropyl-2-bromoisobutyrate………69

4.1.3 Polymerization of acrylonitrile by ATRP ……….70

4.2 Click reaction between alkynyl terminated P3HT and azide terminated PAN……70

4.3 Results and discussion………71

5. Polymer grafted Multiwall Carbon Nanotube

...84

5.1 Experimental part/details………..85

5.1.1 Functionalization of MWNT with PAN by grafting “from” technique…..85

i) Functionalization of carbon nanotubes with carboxyl groups………...85

ii) Synthesis of 2-hydroxyethyl-2′-bromoisobutyrate………...86

iii) Carboxyl functionalized MWNT modified by bromoisobutyrate groups (MWNT-Br)………..…………86

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iv) Functionalization of MWNT-Br with a dithio benzoic ester RAFT agent

(MWNT-RAFT)………87

v) Grafting “from” RAFT polymerization of acrylonitrile (MWNT-g-PAN)...87

5.1.2 Functionalization of MWNT with PAN by Grafting “onto” technique…..88

i) Synthesis of (2-ethoxycarbonyl−prop-2-yl dithiobenzoate) (5)...…………..88

ii) RAFT polymerization of acrylonitrile in the presence of MWNT−COOH..88

5.2 Result and Discussion……….89

6. Polymer Stabilized CdSe Nanocrystals

……….106

6.1 Controlled growth of CdSe Nanoparticles onto MWNT-g-PAN………..……108

6.1.1 Synthetic procedures………109

6.1.2 Results and discussion……….109

6.2 Synthesis of CdSe nanoparticles in the presence of poly(thiophene-b- acrylonitrile)………..118

6.2.1 Synthetic procedures………119

6.3 Preparation of asymmetric rr-P3HT polymer covered CdSe nanoparticles……..127

6.3.1 Synthesis of TOPO capped CdSe Nanoparticles……….128

6.3.2 Ligand exchange with pyridine………128

6.3.3 CdSe NPs stabilization with phosphonic acid-terminated rr-P3HT ……...…129

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Acknowledgements Acknowledgements Acknowledgements Acknowledgements

I am greatly indebted to my research supervisor Prof. Valter Castelvetro for their guidance and encouragement while I carry out this research work and study for my PhD. I experienced the support of a supervisor beyond responsibility under difficult circumstances. I would like to thank for showed immense reserves of patience all along my research and allowed a great freedom in the lab as well as in the time and leisure management throughout the course of my study.

I would like to take this opportunity to thank Dr. Sabrina Bianchi and other lab mates for them friendly kind support, suggestions and discussions.

I am thankful to Prof Mauro Lucchesi, Daniele Prevosto and Dr. Lucia Conzatti for the valuable discussion for AFM and TEM and technical assistance to characterize the samples.

Most importantly, I wish to express my gratitude to my parents, brothers (Pradeep & Kuldeep), my sister Jyoti and friends for all their love, support and encouragement. Although I have been away from my parents for many years, I always bear them in mind, and they are my hopes in my entire life. I hope that the completion of degree will make them happy.

Last but certainly not least, I gratefully acknowledge for the financial support by Galileo Galilei Graduate School.

Amit Kumar Tevtia Amit Kumar TevtiaAmit Kumar Tevtia Amit Kumar Tevtia

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LIST OF FIGURES

Figure 1.1 (a) The device structure and TEM cross-sectional image of the polymer tandem solar cell (b) Energy-level diagram showing the HOMO and LUMO energies of each of the component materials.

Figure 1.2 Structures of several common conjugated polymers Figure 1.3 Structures of different nanocrystals

Figure 1.4 Comparison of absorption spectra with the solar photon flux of poly[2-methoxy-5-(3′,7′- dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), poly(3-hexylthiophene) (P3HT), 1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 (PCBM) and Zn- phthalocyanine (ZnPc)

Figure 1.5 Solar cells working pirinciple

Figure 1.6 I−V characteristic of a solar cell showing the parameters, i.e., the open-circuit voltage (VOC), the short-circuit current (ISC) and the square ImaxVmax used to calculate the PCE Figure 1.7 Illustration of the consecutive processes leading to a photocurrent within nanoparticle–

polymer PVs

Figure 1.8 p-type Schottky device

Figure 1.9 Bilayer configuration in organic PV devices

Figure 1.10 Bulk heterojunction configuration in organic solar cells.

Figure 1.11 Bilayer and bulk hetrojuncation layer difference (a, b) Figure 1.12 Solid state dye-sensitized solar cell

Figure 1.13 Energy diagram for an efficient charge transfer between solid state DSSC components Figure 1.14 Hybrid polymer solar cells prepared by blending CdSe QDs with rr−P3HT

Figure 1.15 Single-walled nanotubes (SWNT) and multi−walled nanotubes (MWNT)

Figure 2.1 Schematic drawings of nanoparticles studied in this thesis: (a) CdSe NPs growth on MWNT-g-PAN (b) CdSe NPs growth on P3HT-b-PAN (c) TOPO capped CdSe NPs (d) rr-P3HT Capped CdSe NPs

Figure 3.1 GC spectrum of purified 2, 5-dibromo-3-hexylthiophene Figure 3.2 ¹H NMR spectrum of 2, 5-dibromo-3-hexylthiophene Figure 3.3 FTIR spectrum of 2,5-dibromo-3-hexylthiophene

Figure 3.4 GPC curve of different molecular weight of P3HT with respect to catalyst time Figure 3.5 ¹H NMR spectrum of allyl/Br terminated poly(3-hexylthiophene)

Figure 3.6 ¹H NMR spectrum of allyl/alkynyl asymmetrically terminatedrr-P3HT

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Figure 3.7 FTIR spectrum of allyl/alkynyl asymmetrically terminated rr-P3HT recorded on KBr window

Figure 3.8 ¹H NMR spectrum of 4-bromobenzenephosphonate Figure 3.9 ¹H NMR spectrum of phenyl acetylene phosphonate

Figure 3.10 FTIR spectrum of diethyl 4- acetylenebenzene phosphonate Figure 3.11 ¹H NMR spectrum of phosphonic acid terminated P3HT

Figure 3.12¹³C NMR spectrum of allyl/phosphonic acid terminated rr-P3HT

Figure 3.13 FTIR of (a) allyl/phosphonate and (b) allyl/phosphonic acid terminated rr-P3HT Figure 4.1 ¹HNMR of 3-azidopropyl 2-bromoisobutyrate

Figure 4.2 FTIR spectrum of 3-azidopropyl 2-bromoisobutyrate

Figure 4.3 SEC traces of PAN samples obtained at different reaction times from the same reaction mixture composition elution in DMF

Figure 4.4 ¹H NMR spectrum of azide terminated polyacrylonitrile

Figure 4.5 FTIR monitoring of the “Click” reaction steps: a) azide-PAN; b) mixture of alkynyl- P3HT and azide-PAN at time zero; c) same mixture as in (b) after 5 days reaction.

Figure 4.6 ¹H NMR spectrum of block copolymer of P3HT-b-PAN

Figure 4.7 DSC curve of P3HT-b-PAN. (Inset the magnified region of heating scan between 30 and 95 °C with marks for the two Tg)

Figure 4.8 TGA thermogram of PAN (--), P3HT (--) and P3HT-b-PAN (--) (air, at 10 °C/min heating rate). The PAN-N3 and allyl/alkynyl-terminated P3HT were used as the reference homopolymers.

Figure 5.1 ¹H NMR spectrum of 2-hydroxyethyl-2-bromoisobutyrate Figure 5.2 FTIR spectrum of 2-hydroxyethyl-2′-bromoisobutyrate

Figure 5.3 FTIR of (a) pristine MWNT, (b) MWNT-COOH, (c) MWNT−RAFT

Figure 5.4 Weight loss curves recorded by TGA from: a) pristine MWNT; b) MWNT-Br; c) MWNT-RAFT (10 °C/min heating rate, 100-600 °C scan with nitrogen purge, followed by further heating to 900 °C in air as the purge gas). In the inset is reported an expansion of the TGA curves from 100-600 °C.

Figure 5.5 FTIR spectrum of MWNT-g-PAN

Figure 5.6 ¹H NMR of MWNT-g-PAN recorded in DMSO solvent

Figure 5.7 ¹H NMR spectrum of 2-(ethoxycarbonyl)-prop-2-yl dithiobenzoate (RAFT-CTA) (5) Figure 5.8 FTIR spectrum of RAFT-CTA agent (5)

Figure 5.9 Dispersibility behavior of a) MWNT-COOH with RAFT polymerized PAN; b) MWNT- g-PAN in DMF (image taken 48 hr after stirring was stopped)

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Figure 5.10 TGA and DTGA curves of the MWNT-g-PAN obtained by “grafting from” technique (nitrogen purge 10°C/min heating rate)

Figure 5.11 TGA and DTGA curves of the MWNT-g-PAN obtained by “grafting onto” technique (nitrogen purge, 10°C/min heating rate)

Figure 5.12 TEM images of MWNT-g-PAN at scale 200 nm (a) 50 nm (b)

Figure 6.0 Schematic representation of the charge transfer in hybrid composites of inorganic semiconductor nanoparticles and polymer on the example of the system CdSe/P3HT.

Figure 6.1 The FT-IR spectra of (a) MWNT-COOH (b) MWNT-g-PAN (c) CdSe/MWNT-g-PAN Figure 6.2 SEM images of MWNT-g-PAN (a), and CdSe/ MWNT-g-PAN (b)

Figure 6.3 EDS spectrum of MWNT-g-PAN (a) and CdSe/ MWNT-g-PAN (b)

Figure 6.4 TEM images of MWNT based hybrid materials MWNT-g-PAN (a, b) MWNT-g-PAN- CdSe NPs (c, d)

Figure 6.5 Tapping mode AFM images of the surface of MWNT-g-PAN/CdSe grown on glass substrate by dip coating and annealed under vacuum at 80 °C (a) 5X5 micron (b, c) The magnified view (1X1 micron) of height and phase respectively.

Figure 6.6 UV-VIS absorption spectra of (a) PAN-CdSe (b) MWNT-g-PAN (c) MWNT-g- PAN/CdSe

Figure 6.7 FTIR spectra of the P3HT-b-PAN copolymer prepared by “Click reaction from the respective end-functional alkynyl-P3HT and azide-PAN; a) reagents at time 0; b) block copolymer after 5 days reaction; c) hybrid obtained by growth of CdSe NPs in the presence of templating P3HT-b-PAN.

Figure 6.8 UV-VIS (left) and photoluminescence (right, excitation wavelength 390 nm) spectra of:

a) P3HT-b-PAN; b) P3HT-b-PAN/CdSe hybrid in 1/3 DMF/THF mixed solvent ().

Spectra are corrected for the background scattering absorption.

Figure 6.9 Fluorescence spectra of P3HT-b-PAN/CdSe nanocomposite under increasingly exciton wavelength (a−e) 390, 380, 370, 360, 300 nm.

Figure 6.10 TEM images of a) P3HT-b-PAN/CdSe NPs; b), enlarge view of the surface within the squared box

Figure 6.11 AFM images of P3HT-b-PAN/ CdSe hybrid recorded as height (left), phase (middle) and height profile (in nm scale, right). The different magnifications correspond to a scanned surface of 3×3 µm (a) and 1×1 µm (b). (Film obtained by casting and evaporating one drop of dispersion of the hybrid containing a 1.2×10^-3 mol·L^-1 concentration of CdSe as calculated by UV-Vis absorption). The height profile to the right was recorded along the highlighted scanning line drawn on the height image (left).

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Figure 6.12 TOPO capped CdSe NPs size determination by DLS

Figure 6.13 FTIR of TOPO (a), TOPO capped CdSe NPs (b), rr-P3HT ligand (c), rr-P3HT capped CdSe NPs (d)

Figure 6.14 ¹H NMR spectra of; a) TOPO; b) TOPO capped CdSe NPs; c) Pyridine capped CdSe NPs; d) P3HT-PA capped CdSe NPs (* assigned to the TOPO CH₃ proton).

Figure 6.15 ³¹P NMR spectra of; a) P3HT-PA capped CdSe NPs; b) P3HT-PA; c) TOPO capped CdSe NPs; d) TOPO and TDPA solution mixture in CDCl3.(Chemical shift referred to trimethyl phosphine oxide as an internal reference with chemical shift at δ=39 ppm Figure 6.16 UV-VIS (left) and photoluminescence (right) spectra of; a) TOPO capped CdSe NPs; b)

P3HT-PA; c) P3HT-PA/ CdSe hybrids and in inset the fluorescent image of d) P3HT- PA/CdSe; e) TOPO/CdSe.

Figure 6.17 TEM images of; a) TOPO−capped CdSe NPs; b) Pyridine−capped CdSe NPs; c) P3HT-PA−capped CdSe NPs; d) enlarged selected area from (∼5 ×10^-3 M concentration).

Figure 6.18 Tapping mode AFM images of ; a) the TOPO-CdSe NPs; b) Pyridine-CdSe NPs; (Left) height image, (Middle) Phase image, (Right) Cross-section profile.(5.2×10^-3 M concentration of CdSe NPs calculated by UV-VIS maximum).

Figure 6.19 Tapping mode AFM images of the phosphonic acid terminated rr-P3HT (a) height (b) phase (c) potential fluctuations (d) Cross-section profile of kelvin potential.

Figure 6.20 Tapping mode AFM images of the rr-P3HT capped CdSe NPs (a) height (b) phase (c) potential fluctuations (d) Cross-section profile of kelvin potential.

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LIST OF SCHEMES

Scheme 3.1 The Rieke method producing either regiorandom or HT-P3ATs depending on the type of catalyst

Scheme 3.2 Transmetalation reaction in the Grignard metathesis (GRIM) procedure Scheme 3.3 Synthesis of 2,5-dibromo-3-hexylthiophene

Scheme 3.4 Synthesis of allyl/Br terminated poly(3-hexylthiophene)

Scheme 3.5 Mechanism for the synthesis of mono capped rr-P3HT by GRIM method Scheme 3.6 Synthesis of allyl/alkynyl terminated P3HT

Scheme 3.7 Synthesis of allyl/phosphonic acid terminated rr-P3HT Scheme 4.1 Synthesis of 3-azidopropyl 2-bromoisobutyrate

Scheme 4.2 “Click” reaction for P3HT-b-PAN

Scheme 5.1 Synthesis of 2-hydroxyethyl 2′-bromoisobutyrate

Scheme 5.2 Functionalization of MWNT with dithioester RAFT agent Scheme 5.3Polymerization of Acrylonitrile on MWNT-RAFT

Scheme 5.4 Synthesis of free RAFT-CTA agent

Scheme 5.5 polymerization of acrylonitrile by grafting “onto” technique in presence of acid modified MWNTs

Scheme 6.1 CdSe NPs growth on modified MWNTs

Scheme 6.2 Controlled growth of CdSe NPs on P3HT-b-PAN

Scheme 6.3 Ligand exchange multistep procedure to prepare CdSe NPs stabilized with end- functional P3HT-PA

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LIST OF TABLES

Table 1.1 Selected parameters for conducting polymers used in hybrid polymer–semiconductor PV cells

Table 1.2 Material parameters for several common inorganic semiconductors Table 1.3 Performance of typical carbon-based photovoltaic cells

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LIST OF KEY ABBREVIATIONS

AFM Atomic force microscope Ag/AgCl (sat.) Silver-silver chloride electrode

AM Air mass

°C Degrees centigrade

CV Cyclic voltmeter

C60 Fullerene

δ Chemical shift

DCM Dichloro methane DMF Dimethylformamide DMSO Dimethyl sulfoxide DPn Degree of polymerization dppe Bis-diphenylphosphinoethane dppm Bis-diphenylphosphinomethane EQE External Quantum Efficiency

FF Fill Factor

FTIR Fourier transformer Infrared GPC Gel permeation chromatography

hr Hour

HOMO Highest occupied molecular orbital

Hz Hertz

Isc Short-circuit current ITO Indium tin oxide

J Coupling constant

KFM Kelvin probe force microscopy

λ Wavelength

LiClO4 Lithium perchlorate

LUMO Lowest unoccupied molecular orbital

MEH-PPV Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]

mL Milliliter

mmol Milimole

Mn Number-average molecular weight MPP Maximum Power Point

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ν Frequency

NMR Nuclear magnetic resonance OLEDs Organic light emitting diodes PAN Poly(acrylonitrile)

P3HT Poly(3-hexylthiophene)

PCBM 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61

PCE or η Power Conversion Efficiency PEDOT Poly(ethylene-dioxythiophene)

P3HT-PA mono phosphonic acid terminated regioregular poly(3-hexylthiophene)

Ph Phenyl

PMe3 Trimethylphosphine

PPE-PPVs Poly(phenylene-ethynylene)-(para-phenylenevinylenes) ppm Parts per million

PPV Poly(para-phenylenevinylene)

PS Polystyrene

PVK Poly(N-vinyl-carbazole) PTFE poly(tetrafluoroethylene)

r.t Room temperature

SEM Scanning electron microscope TEM Transmission electron microscopy

TBAPF6 Tetrabutylammonium hexafluorophosphate

t-Bu Tertiary butyl

THF Tetrahydrofuran

v/v Volume-to-volume ratio Voc Open-circuit voltage

3. Synthesis and characterization of end-functional Polythiophene building blocks

Table of Contents

CONTENTS 1. Introduction

2. Materials and Methods

3. Synthesis and characterization of end-functional Polythiophene building blocks

4. Synthesis and characterization of Poly(thiophene-b-acrylonitrile)

5. Polymer grafted Multiwall Carbon Nanotube

6. Polymer Stabilized CdSe Nanocrystals

LIST OF FIGURES

LIST OF SCHEMES

LIST OF TABLES

LIST OF KEY ABBREVIATIONS