有機材料及無機奈米粒子之氫鍵超分子作用於光電材料之應用

國 立 交 通 大 學

材料科學與工程研究所

博 士 論 文

有機材料及無機奈米粒子之氫鍵超分子作用

於光電材料之應用

H-Bonded Supramolecular Interactions of Organic

Materials and Inorganic Nanoparticles for

Applications of Electro-Optical Materials

研 究 生 : 方曉萍

指導教授 : 林宏洲 博士

中華民國一 o 二年 一 月

有機材料及無機奈米粒子之氫鍵超分子作用

於光電材料之應用

H-Bonded Supramolecular Interactions of Organic

Materials and Inorganic Nanoparticles for

Applications of Electro-Optical Materials

研 究 生 : 方曉萍 Student: Hsiao-Ping Fang

指導教授 : 林宏洲 Advisor: Prof. Hong-Cheu Lin

國立交通大學

材料科學與工程學系

博士論文

A Thesis

Submitted to Department of Materials Science and Engineering

College of Engineering

National Chiao Tung University

In Partial Fulfillment of the Requirement

for the Degree of Doctor of Philosophy of Science

In Materials Science and Engineering

January 2013

Hsinchu, Taiwan

中華民國一 o 二年 一月

H-Bonded Supramolecular Interactions of Organic

Materials and Inorganic Nanoparticles for

Applications of Electro-Optical Materials

Student: Hsiao-Ping Fang Advisor: Prof. Hong-Cheu Lin

Department of Materials Science and Engineering National Chiao Tung

University

Abstract

First, Two kinds of novel conjugated polymers containing 2,7-carbazole, thiophene, and fused-dithienothiophene rings as backbones bearing acid-protected and benzoic acid pendants (PCA and PCB, respectively) were utilized for organic solar cell applications. The absorption spectra of these polymers (in both solutions and solid films) showed an absorption range at 300–580 nm. Furthermore, ZnO nanoparticles were synthesized and surface-modified with pyridyl surfactants (ZnOpy) to be ca. 3–4 nm. The pyridyl surfactants of ZnOpy nanoparticles (as electron acceptors to partially replace expensive electron acceptor PCBM) not only induce supramolecular interactions with benzoic acid pendants of polymer PCB via H-bonds, but also enhance the homogeneous dispersions of ZnOpy nanoparticles in polymer PCB. Thus, the ternary systems of PCA,PCB/ZnOpy/PCBM in weight ratios of 1:0.05:1 and 1:0.1:1 were investigated in bulk heterojunction polymer solar cells (PSCs). Under the

standard illumination of AM 1.5, 100 mW/cm2, the best power conversion efficiency (PCE) of the PSC cell containing a polymer blend of PCB/ZnOpy/PCBM=1:0.05:1 reached PCE=0.55%, with Jsc=2.11 mA/cm2, Voc=0.88 V, and FF=29.4%.

Second, Three kinds of dithienothiophene/carbazole-based conjugated polymers (PCA, PCB, PCC), which bear acid-protected and benzoic acid pendants in PCA and

PCB, respectively, were synthesized via Suzuki coupling reaction. Interestingly, PCA, PCB, and PCC exhibited reversible electrochromism during the oxidation processes

of cyclic voltammogram studies, and PCB (with H-bonds) revealed the best electrochromic property with the most noticeable color change. According to powder X-ray diffraction (XRD) analysis, these polymers exhibited obvious diffraction features indicating bilayered packings between polymer backbones and π - π

stacking between layers in the solid state. Compared with the XRD data of PCA (without H-bands), H-bonds of PCB induced a higher crystallinity in the small-angle region (corresponding to a higher ordered bilayered packings between polymer backbones), but with a similar crystallinity in the wide angle region indicating a comparable π - π stacking distance between layers. Moreover, based on the

preliminary photovoltaic properties of PSC devices (PCA, PCB, and PCC blended individually with PCBM acceptor in the weight ratio of 1:1), PCB (with H-bonds) possessed the highest power conversion efficiency of 0.61% (with Jsc = 2.26 mA/cm2,

FF = 29.8%, and Voc = 0.9 V). In contrast to PCA (without H-bands), the thermal

stability, crystallinity, and electrochromic along with photovoltaic properties of PCB were generally enhanced due to its H-bonded effects.

Third,Four novel metallo-polymers (P1-P4) containing aryl-imidazo-phenanthrolines (AIP) ligands (incorporated with phenyl and fused-thiophene cores) were synthesized and characterized. Interestingly, P1-P4 exhibited electrochromism during the

oxidation processes of cyclic voltammogram studies. In addition, P1-P4 were blended with surface-modified pyridyl-ZnO nanoparticles (ZnOpy as proton acceptors) to form nanocomposites, where P3-P4 were functionalized with carboxylic acid pendants (as proton donors) on the polymer backbones to study for the H-bonded effects on surface-modified ZnOpy nanoparticles. In order to investigate the nanocomposites containing metallo-polymers P1-P4 and surface-modified ZnOpy nanoparticles, nanocomposites P1-P4/ZnOpy were characterized by UV-visible (UV) absorption spectra, Fourier transform infrared (FTIR), photoluminescence (PL) spectra, time-resolved photoluminescence decays, X-ray diffraction (XRD) measurements, and transmission electron microscopy (TEM) analyses. In contrast to nanocomposites P1/ZnOpy and P2/ZnOpy, higher crystallinities with a distinct layered-structure of H-bonded nanocomposites P3/ZnOpy and P4/ZnOpy in XRD measurements were induced by the introduction of surface-modified ZnOpy nanoparticles to metallo-polymers P3 and P4, correspondingly. Furthermore, due to the supramolecular interactions of surface-modified ZnOpy nanoparticles with metallo-polymers P3-P4, TEM images verified that ZnOpy nanoparticles were more homogeneously distributed in nanocomposites P3-P4/ZnOpy (with H-bonds) than those in P1-P2/ZnOpy (without H-bonds), respectively.

Finally, In this study we synthesized three metal-free organic dyes (Cpd11, Cpd16, and Cpd22) featuring 3,4,5-tris(dodecyloxy)phenyl and cyanoacrylic acid moieties as electron-donor and electron-acceptor/anchoring units, respectively, linked through various dithienothiophenyl conjugated spacers. Cpd16 exhibits mesomorphic properties, confirmed through polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD), due to the appropriate ratio of the lengths of its flexible chain to its rigid core. Molecular modeling of Cpd16, and its d-spacing

determined from XRD data, verified the existence of a tilt angle in the SmC phase. Among these metal-free organic dyes, a dye-sensitized solar cell incorporating

Cpd16 exhibited the best performance, presumably because of its better packing and

its mesomorphic properties; the power conversion efficiency was 3.72% (Voc = 0.58

V; Jsc = 9.98 mA cm–2; FF = 0.65) under simulated AM 1.5 irradiation (100 mW

摘 要

本論文研究方向為探討一系列包含有機材料及無機奈米粒子之氫鍵超分子作用

在有機光電材料為研究主軸。

第一部分，兩種主鏈包含 2,7-carbazole 和 fused-dithienothiophene 環的共軛高分 子在側邊懸掛 acid-protected 和 benzoic acid 官能基 (PCA 和 PCB)，此系列高 分子的吸收範圍約在 300 nm-580 nm， 以作為太陽能電池的應用。此外也合成 氧化鋅奈米粒子, 和表面改質含有吡啶官能基(ZnOpy)，大小約 3-4 奈米。

ZnOpy 奈米粒子當電子接受者取代部分 PCBM，不但透過氫鍵與高分子 PCB 的

benzoic acid 產生超分子作用力，而且增加 ZnOpy 奈米粒子在高分子 PCB 均勻 的分散性。因此，在複合系統中， PCB/ ZnOpy/PCBM 在重量 1:0.05:1 混摻時，

在 AM 1.5 的標準太陽光照射下，最佳元件效率可達到 0.55%，Jsc=2.11 mA/cm2，

Voc=0.88 V，FF=29.4 %。

第二部分，我們利用 Suzuki coupling reaction 合成了以 2,7-carbazole 及 fused dithienothiophene 單元為分子主鏈的共軛型高分子(PCA, PCB, 和 PCC ) ，此系 列高分子的吸收範圍約在 300 nm-550 nm。其中 PCB 主鏈側邊修飾有酸的保護 基,而 PCB 的主鏈側邊修飾有酸的官能基,相互作為研究氫鍵效應的對照組,而 且 PCA, PCB, 和 PCC 在電化學實驗氧化過程中有電致變色的性質，PCB 有明 顯的顏色變化(由橘變黑) ， X 光繞射光譜儀(XRD)也可觀察到 PCB 的氫鍵結 構有較高的結晶度，在小角度繞射區段可觀察到高分子主鏈層的層間距。且當與 PCBM 重量比 1：1 混摻時，在 AM 1.5 的標準太陽光照射下，最佳元件效率可 達到 0.61%，Jsc= 2.26 mA/cm2，Voc=0.9 V，FF=29.8%。相較於高分子 PCA 無氫 鍵形成， PCB 含有氫鍵結構其熱穩定性，結晶度，電致變色及太陽能電池轉化 效率都因為 PCB 含有氫鍵結構而提高其性質。 第三部分,合成鑑定四個高分子(P1-P4)包含aryl-imidazo-phenanthrolines (AIP) 敖合基，其中修飾含有phenyl 和 fused-thiophene。高分子(P1-P4)在電化學氧化 過程中顯示有電致變色的性質，另外，四個高分子(P1-P4) ，其中P3-P4高分子 側邊修飾含有carboxylic acid的官能基和表面改質吡啶的氧化鋅奈米粒子混摻以

形成奈米複合材料加以討論氫鍵效應。利用紫外光光譜儀(UV) ，螢光光譜儀 (PL) ， 時間解析光激螢光光譜儀(TRPL) ， X光繞射光譜儀(XRD) ， 穿透 式電子顯微鏡(TEM)加以分析氫鍵的效應。P3/ZnOpy, P4/ZnOpy 奈米複合材 料因為氫鍵結構顯示有較高的結晶度，此外，由於氧化鋅和高分子的超分子作用 力，從TEM圖可得知含有氫鍵的奈米複合材料(P3/ZnOpy, P4/ZnOpy)其分散性 較好。 最後一個部份，本研究中擬開發新的非金屬系(metal-free)有機光敏化染料，所設 計的染料分子是以 tris-dodecyloxyphenyl-與 cyanoacrylic acid 為電子予體與受 體，而主要是合成修飾不同的電子予體或電子予體與電子受體間之共軛橋樑 (conjugated spacer)。在共軛橋樑設計上 ，主結構是以 dithieno[3,3-b:2’,3’-d] thiophene，其中再分別含有 dithiophene 以及 bithiazole 來延長共軛系統長度，以 提升電荷轉移的能力和加強吸收光譜的強度和範圍。由於含有液晶性質的分子， 具有較良好的分子排列性，藉由引入一般運用在液晶上的片段液晶分子結構 trioxyphenyl-，加上共平面性較好的融合環 dithieno[3,3-b:2’,3’-d] thiophene，來探 討所設計的分子在染料敏化型太陽能電池上的應用與特性。其中所設計的染料分 子化合物 16 其光轉換效率為 3.72% (Voc= 0.58V, Jsc=9.98 mA/cm2, FF =0.65)， 在相同條件下，已可達 N719 效率(7.04%)的 53%，雖然效率不及以往文獻 triarylamine 為電子予體的效率，但成功的開發出新一系列不同於以往的電子 予體染料分子，未來可在分子的設計上，對於電子予體上做取代基的轉換，以利 於光轉換效率的提升。

ACKNOWLEDGEMENTS

本論文首先感謝林宏洲老師這些年來對我的照顧及鼓勵，老師對

於研究上的辛苦用心及待人處世的教導，使我一路成長，如今順利完

成博士學業，老師指導的恩惠，學生將永記於心。感謝韋光華老師、

韓建中老師、林建村老師、徐新光、許千樹老師於百忙之中審核論文

並給予寶貴的建議及指正。

博士班近五年半的時光使我獲益良多，很幸運也很快樂地在這實

驗室度過這些日子，在此特別感謝實驗室的學長們在實驗上的教導與

幫助，並感謝實驗室的衆多的同學及學弟妹在實驗上的協助，使我的

實驗得以順利完成，還有張立實驗室、呂志鵬實驗室以及許許多多材

料所的學長、同學、學弟妹們在這些日子的陪伴，使我的交大生活更

增添的許多歡樂。謝謝所有真心愛我、支持我的人，我會努力成為更

好的人，以達謝大家對我的支持與愛護。

最後要特別由衷地感謝一直栽培我的父母親、支持我的老公、妹

妹、弟弟和所有家人，謝謝你們一路上的支持與鼓勵，讓我能在無後

顧之憂下求學並完成博士學位。

Table of Contents

Abstract...II Acknowledgements ...V III Table of Contents ... I IX Table Lists... XIIII Figure Lists ...XIII III

Chapter 1 Introduction ... 1

1.1 Polymeric Solar Cells (PSCs)………..1

1.2. Hybrid Polymer-Inorganic Solar Cells ... 2

1.3 Supramolecular H-Bond Polymers for Organic Solar Cells ... 4

1.4 Dye-Sensitized Solar Cells (DSSCs) ... 5

1.4.1 Metal-Free Organic Dye in DSSCs... 8

1.5 Metallopolymer ... 10

1.5.1 Nanocomposite Systems Base on Metallpolymer and Nanoparticles ... 13

Chapter 2 Applications of novel dithienothiophene- and 2,7-carbazole-based conjugated polymers with surface-modified ZnO nanoparticles for organic photovoltaic cells………... 14

2.1 Introduction ... 15

2.2 Experimental... 20

2.2.1 Materials ... 20

2.2.2 Measurements and characterization ... 20

2.2.3 Device fabrication and characterization of polymer solar cells... 22

2.2.4 Synthesis ... 23

2.3.1 Thermal properties ... 25

2.3.2 Optical properties ... 25

2.3.3 Electrochemical characterization ... 29

2.3.4 Morphology ... 32

2.3.5 Polymeric photovoltaic cell properties... 34

2.4 Conclusions ... 36

Chapter 3 Synthesis of Novel Dithienothiophene- and 2,7-Carbazole-Based Conjugated Polymers and H-Bonded Effects on Electrochromic and Photovoltaic Properties ... 37

3.1 Introduction ... 38

3.2 Experimental Section ... 41

3.2.1 Materials ... 41

3.3 Results and Discussion... 53

3.3.1 Syntheses and Chemical Characterization... 53

3.3.2 Optical Properties... 55

3.3.3 Electrochemical Properties ... 55

3.3.4 X-Ray Diffraction (XRD) Analyses... 64

3.3.5 Morphology ... 66

3.4 Conclusions ... 68

Chapter 4 Synthesis and Study of Novel Supramolecular Nanocomposites Containing Aryl-Imidazo-Phenanthroline-Based Metallo-Polymers (H-Donors) and Surface-Modified ZnO Nanoparticles (H-Acceptors)………... ….70

4.1 Introduction ... 71

4.2 Experimental Section ... 74

4.2.2 Synthesis ... 75

4.2.3 General Synthetic Procedure for Metallo-Polymers P1-P4………... 76

4.3 Results and Discussion... 82

4.3.1 Syntheses and chemical characterization………...84

4.3.2 UV spectroscopic studies... 85

4.3.3 Electrochemical characterization………87

4.3.4 FT-IR spectroscopic studies………89

4.3.5 Time-resolved PL spectroscopic studies……….…91

4.3.6 X-ray diffraction (XRD) analyses………..……….…....94

4.3.7 Transmission electron microscopy studies………...96

4.4 Conclusions ... 98

Chapter 5 Synthesis of Metal-Free Organic Dyes Containing Tris(dodecyloxy)phenyl and Dithienothiophenyl Units and a Study of Their Mesomorphic and Photovoltaic Properties………100

5.1 Introduction ...101

5.2 Experimental...101

5.2.1 Materials ...104

5.3 Results and Discussion...104

5.3.1 Optical properties ...115 5.3.2 Electrochemical properties ...120 5.3.3 Mesomorphic properties ...122 5.3.4 Photovoltaic properties of DSSCs……….…127 5.4 Conclusions ...115 Chapter 6 Conclusion...131 References...135

Table Lists

Table 2.1 Molecular weights, yields, and thermal data of polymers PCA, and

PCB………..26 Table 2.2 Photophysical data of polymers PCA, and PCB in chloroform solutions

and solid films... 27

Table 2.3 Electrochemical potentials, energy levels, and band gap energies of

polymers PCA, and PCB ... 30

Table 2.4 Photovoltaic properties of PSC devices containing an active layer of

Polymer:ZnOpy:PCBM (w/w) with a device configuration of

ITO/PEDOT:PSS/Polymer:ZnOpy:PCBM/Ca/Al ………..33

Table 3.1 Molecular Weights, Yields, and Thermal Data of Polymers PCC, PCA, and PCB…...55 Table 3.2 Photophysical Data in THF Solutions and Solid Films, Optical Band Gaps,

Electrochemical Potentials, Energy Levels and Band Gap Energies of Polymers PCC,

PCA, and PCB………... …..57 Table 4.1 Photophysical Data in DMF Solutions and Solid Films and Electrochemical

Potentials, Energy Levels, and Band Gap Energies of Metallo-Polymers P1-P4 ... 87

Table 4.2 Fluorescence Lifetimes of Metallo-Polymers P1-P4 and its

Nanocomposites………. …..94

Table 5.1 Absorption, emission, and electrochemical properties of

dyes………...118

Table 5.2 Phase transition temperatures and enthalpies of the dyes Cpd11, Cpd16,

and Cpd22……….………….124

Table 5.3 Cell performance of Cpd11, Cpd16, Cpd22, and N719-sensitized solar

Figure Lists

Figure 1.2.1 The energy levels of in the ternary system solar cell showing the HOMO

and LUMO levels of the materials and work function of the electrodes. The arrows indicate the expected charge transfer and charge transport processes……... 3

Figure 1.3.1 Superstructure of self-assembly of [60]fullerene derivative 1 with

perylene bisimide 5 by H-bonding………. 5

Figure 1.4.1 a) Fundamental processes in a dye-sensitized solar cell. b) Energy-level

diagram of a DSSC. TCO=transparent conducting oxide……….. 7

Figure 1.5.1 Structural diversity of metal-containing polymers………... …..12 Figure 1.5.2 Characterization of photo- and electro-active noncovalent assemblies

based on hydrogen bond……… …. 13

Figure 2.1 Chemical structures of polymers PCA and PCB.. ... 19 Figure 2.2 Thermogravimetric curves of ZnO nanoparticles before and after pyridyl

surface-modification………...25

Figure 2.3 Normalized optical absorption spectra of polymers PCA and PCB in

solutions(chlorobenzene) (10−6 M), and solid films (spin-coating from chlorobenzene solutions)... 28

Figure 2.4 Normalized photoluminescence (PL) spectra of polymers PCA and PCB

in solutions (chlorobenzene) (10−6 M), and solid films (spin-coating from

chlorobenzene solutions)………... 28

Figure 2.5 Cyclic voltammograms of polymers PCA and PCB (solid films) at a scan

rate of 100 mV/s………...31

Figure 2.6 Energy band diagram of HOMO/LUMO levels for electron donor

polymers PCA and PCB, electron acceptors ZnOpy and PCBM, and the work functions of ITO and Al... 31

Figure 2.7 The AFM images obtained for films of PCA/ZnOpy/PCBM and PCB/ZnOpy/PCBM containing different amounts of nanoparticles and fullerene. .. 33 Figure 2.8 I–V curves of solar cells under simulated AM 1.5 solar irradiation with an

active layer of (a) PCA:ZnOpy:PCBM (with different weight ratios of ZnOpy) and (b) PCB:ZnOpy:PCBM (with different weight ratios of ZnOpy)………..…35

Figure 3.1 Normalized optical absorption spectra of polymers PCC, PCA, and PCB

in (a) solutions (THF) (10-6 M) and (b) solid films (spin-coating from THF solutions)………..58

Figure 3.2 Normalized photoluminescence (PL) spectra of polymers PCC, PCA, and PCB in (a) solutions (THF) (10-6 M), and (b) solid films (spin-coating from THF solutions)……….…59

Figure 3.3 (a) Cyclic voltammograms of polymers PCC, PCA, and PCB (in solid

films) at a scan rate of 100 mV/s, (b) absorption spectra and optical images of PCB on ITO at various applied potentials (0 V–1.3 V), (c) CIE chromaticity diagram of

PCB at ‘‘off’’ (0 V) and ‘‘on’’ (1.3 V) states………... 62 Figure 3.4 Energy band diagram with HOMO/LUMO levels of donor polymers

P1-P3 in relation to the work functions of ITO and Al. ... 63

Figure 3.5 Powder X-ray diffraction (XRD) spectra of (a) PCC and (b) PCA and PCB; and schematic representations of proposed three-dimensional layered and π-π

stacked arrangements of (c) PCC and (d) PCA in their XRD measurements (PCB is similar to PCA)……….……….……..66

Figure 3.6 AFM images obtained from solid films of PCC, PCA, and PCB/PCBM

(1:1w/w)………..………..…....…..68

Figure 4.1Chemical structures of metallo-polymers P1-P4………77 Figure 4.2 1H NMR spectra (aromatic region) of ligand L1 and metallo-polymers

P1 and P3 in DMSO-d6………...80

Figure 4.3 1H NMR spectra (aromatic region) of ligand L2 and metallo-polymers

P2 and P4 in DMSO-d6………...80

Figure 4.4 Supramolecular structures of H-bonded nanocomposites P3/ZnOpy and P4/ZnOpy………... 82 Figure 4.5 Normalized UV-vis spectra of ligands L1-L2 and metallo-polymers P1-P4

in (a) DMF solutions and (b) solid films………... 86

Figure 4.6 Cyclic voltammagrams and electrochromic photos of metallo-polymers P1-P4 in solid films………... …..89 Figure 4.7 FTIR spectra of metallo-polymers (P3 and P4) and nanocomposites

(P3/ZnOpy and P4/ZnOpy)………90

Figure 4.8 PL spectra of metallo-polymers P1-P4 and nanoparticle ZnOpy.

... 92

Figure 4.9 Normalized time-resolved photoluminescence decays of (a)

metallo-polymers P1-P4 and (b) nanocomposites P1/ZnOpy, P2/ZnOpy, P3/ZnOpy, and P4/ZnOpy……….93

P2/ZnOpy, (c) P3/ZnOpy, and (d) P4/ZnOpy………...96 Figure 4.11 Schematic illustrations of possible structural arrangements for polymer

chains in nanocomposites by powder X-ray diffractions………96

Figure 4.12 TEM images of nanocomposites (a) P1/ZnOpy, (b) P2/ZnOpy, (c) P3/ZnOpy, and (d) P4/ZnOpy………98 Figure 5.1 Chemical structures of dyes Cpd11, Cpd16, andCpd22………....118 Figure 5.2 a) UV-vis absorption spectra of metal-free organic dyes in THF solutions

(10-5 M) and (b) Normalized photoluminescence (PL) spectra of metal-free organic

dyes in THF solutions (10-5 M)………..119

Figure 5.3 Cyclic voltammograms of dyes (in THF) at a scan rate of 100 mV/s.

...122

Figure 5.4 (a) Optical texture of the nematic phase in dye Cpd16 observed by POM

at 225 °C (cooling) and (b) XRD intensity against angle profiles obtained from dye

Cpd16 at 225 °C………125 Figure 5.5 Molecular modeling of dye Cpd16……….126 Figure 5.6 (a) IPCE plots of DSSCs fabricated using dyes Cpd11, Cpd16, Cpd22,

and N719. (b) I-V curves of DSSCs based on dyes Cpd11, Cpd16, Cpd22, and N719. ...129

Chapter 1

Introduction

The need to develop inexpensive renewable energy sources stimulates scientific

research for efficient, low-cost photovoltaic devices. The organic polymer-based

photovoltaic elements have introduced at least the potential of obtaining cheap and

easy methods to produce energy from light. The possibility of chemically

manipulating the material properties of polymers (plastics) combined with a variety of

easy and cheap processing techniques has made polymer-based materials present in

almost every aspect of modern society. Organic semiconductors have several

advantages: (a) low cost synthesis, and (b) easy manufacture of thin film devices by

vacuum evaporation/sublimation or solution cast or printing technologies.1

1.1 Polymeric Solar Cells (PSCs)

Novel materials are developed for organic optoelectronic devices, such as

polymeric solar cells (PSCs), which is a popular research topic in recent decades,

because they are low cost and green materials for sustainable resources to reduce

consumptions of fossil energy and nuclear power.2 In particularly, bulk

heterojunction (BHJ) solar cells consisting of electron-donating conjugated

films.3 Up to now, regio-regular poly[2-methoxy-5-(3’,7’-

dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV)4 and

poly(3-hexylthiophene) (P3HT)5 as electron donors blended with

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor

approached high power conversion efficiency (PCE) values of 5.0% in PSCs. More

recently, the PCE values of BHJ solar cells using new low-band gap conjugated

polymers have reached 6 to 8%.6,7 The PCE values of BHJ solar cells were affected by,

for example, the energy band gaps of polymers, which is related to the chemical

structure of the conjugated polymers.

1.2 H

These hybrid polymer-inorganic solar cells utilize the high electron mobility of the

inorganic phase to overcome charge-transport limitations associatedwith organic

materials. The efficient BHJ solar cells made of ZnO nanoparticles and a conjugated

polymer have been reported previously.8 The ZnO nanoparticles were blended with

poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMOPPV) to

possess a highest power conversion efficiency (PCE) approaching 1.6% in PSCs.9

Hybrid solar cells based on CdSe nanoparticles and a PPV-type polymer containing

very low photocurrent and fill factor values, which were attributed to the poor charge

transport in the trioctylphosphine oxide (TOPO)-capped CdSe nanoparticle phase.

Thus, ternary systems based on mixtures of PFT/CdSe and the fullerene derivative

[6,6]-phenyl C61 butyric acid methyl ester (PCBM) were investigated. It was

observed that for the optimized composition of 20 wt.% PFT+40 wt.% CdSe+40 wt.%

PCBM the devices presented higher photocurrents and efficiencies.(Figure 1.2.1) The

use of inorganic nanoparticles, such as TiO2, ZnO, CuInS2, PbSe, CdSe, and CdTe,

have some advantages, related to the versatility of these materials, which often can be

easily synthesized in a great variety of sizes and shapes, according to the desired

properties.10

Figure 1.2.1 The energy levels of in the ternary system solar cell showing the HOMO

and LUMO levels of the materials and work function of the electrodes. The arrows indicate the expected charge transfer and charge transport processes.

1.3 Supramolecular H-bond Polymers for Organic Solar Cells

Nanostructured materials with tailor-made properties and functions can be developed

by exploiting the supramolecular approach through molecular recognition.In fact, the

hierarchical self-assembly of multivalent molecular modules through the concerted

action of multiple noncovalent interactions represents a very powerful approach as it

makes possible the simultaneous organization of various molecular systems into

intrinsically defect-free 2D architecture featuring a long-range order.11

Hydrogen bonds (H-bonds) are ideal noncovalent interactions to form self-assembled

architectures due to their selectivity and directionality. A numerous advantages of

H-bonded polymers, such as stronger light absorptions, lower HOMO levels, higher

Voc values, higher hole mobilities, and higher crystallinities, were utilized for organic

solar cells.12 Therefore, great efforts have been taken toward the preparation and

characterization of photo- and electroactive noncovalent assemblies based on

hydrogen bonds (H-bonds). Wurthner,12(a,b) El-ghayoury et al., and Jonkheijm et

al.12(c,d) reported H-bonded assemblies of perylene bisimide and melamine derivatives.

In addition, El-ghayoury et al. reported a PCE value of 0.39% for PSCs by utilizing a

H-bonded polymer containing oligo(p-phenylene vinyene) and ureido-pyrimidinone

units.12(c) Because of several advantages in polymers, including low cost, easy

processing, and tunable chemical properties, the conjugated polymers consisting of

electrochromic behavior as well as photovoltaic properties.

Moreover, connecting the electrooptical properties in organic devices have been

established through the supramolecular interactions, e.g. H-bonds, in organic,

dendritic, and polymeric H-bonded complex systems. This was illustrated by a recent

report on a triple hydrogen-bonded triad consisting of a central perylene that was

connected to two C60 chromophores (Figure 1.3.1).13

Figure 1.3.1 Superstructure of self-assembly of [60]fullerene derivative 1 with

perylene bisimide 5 by H-bonding.

1.4 Dye-Sensitized Solar Cells (DSSCs)

Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years

as they offer the possibility of low-cost conversion of photovoltaic energy. In this

context, dye-sensitized solar cells (DSSC) have attracted considerable attention in

recent years. Efforts in the synthesis of sensitizers for DSSCs can be grouped into two

broad areas: 1) Functional ruthenium(II)–polypyridyl complexes such as N3,15 N719,

former class of compounds contains expensive ruthenium metal and requires careful

synthesis and tricky purification steps. On the other hand, the second class can be

prepared rather inexpensively by following established design strategies.

Conventional DSSCs typically contain five components: 1) a photoanode, 2) a

mesoporous semiconductor metal oxide film, 3) a sensitizer (dye), 4) an

electrolyte/hole transporter, and 5) a counter electrode. In DSSCs, the incoming light

is absorbed by the sensitizer, which is anchored to the surface of semiconducting TiO2

nanocrystals. Charge separation takes place at the interface through photoinduced

electron injection from the excited dye into the conduction band of the TiO2. Holes

are created at the dye ground state, which is further regenerated through reduction by

the hole-transport material (HTM), which itself is regenerated at the counterelectrode

by electrons through an external circuit. In principle, for efficient DSSCs the

regeneration of the sensitizer by a hole transporter should be much faster than the

recombination of the conduction band electrons with the oxidized sensitizer.

Additionally, the highest occupied molecular orbital (HOMO) of the dye should lie

below the energy level of the hole transporter, so that the oxidized dyes formed after

electron injection into the conduction band of TiO2 can be effectively regenerated by

accepting electrons from the HTM. The general operating principle of a

DSSCs is expanding very rapidly and attracting scientist from different disciplines: 1)

Chemists to design and synthesize suitable donor–acceptor dyes and study

structure–property relationships; 2) physicists to build solar cell devices with the

novel materials, to characterize and optimize their performances, and to understand

the fundamental photophysical processes; and 3) engineers to develop new device

architectures. The synergy between all the disciplines will play a major role for future

advancements in this area.

Figure 1.4.1 a) Fundamental processes in a dye-sensitized solar cell. b) Energy-level

1.4.1 Metal-Free Organic Dyes in

DSSCs

The development of novel materials for use in organic optoelectronic devices, such as

dye-sensitized solar cells (DSSCs),16 has become a popular research topic in the quest

for low-cost, green materials for sustainable use and a decrease in demand for fossil

fuels and nuclear power. DSSCs based on Ru-photosensitizers,17-18 such as

cis-bis(isothiocyanato)bis(2,2´-bipyridyl-4,4´-dicarboxylato)-ruthenium(II) (N3)15and

related derivatives, have been applied very successfully with high power conversion

efficiencies (PCEs) of 9–12%.15,19-23 Recently, it has been demonstrated that DSSCs

can also be constructed from metal-free organic dyes.14 Because of the high cost of

rare Ru metal and the relatively low molar extinction coefficients and tedious

purification of Ru-photosensitizes,19 metal-free organic sensitizers have become

increasingly attractive and widely developed.24,25 Nevertheless, the ability to reach

higher efficiencies when using metal-free organic dyes remains a challenge, although

great progress has been made in this field.26-29 The key characteristics for a dye to be

used in a DSSC are high absorption over a wide range of the solar spectrum with high

molar extinction coefficients, efficient charge separation, redox stability, andsuitable

functional groups to interact with the electron sink (TiO2). Metal-free organic dyes

featuring a donor/acceptor structural design were synthesized have particularly wide

Some general principles to construct an efficient dye and efficient DSSCs are as

follows: 1) The absorption range of the dye should cover the whole visible and some

of the near-infrared region, and its molar extinction coefficient must be as high as

possible to enable efficient light harvesting with thinner TiO2 layers (panchromatic

absorption). 2) For efficient electron injection into the anode, the lowest unoccupied

molecular orbital (LUMO) of the dye should be localized near the anchoring group

(usually a carboxylic or phosphonic acid) and above the conduction band edge of the

semiconductor electrode (typically TiO2). 3) The HOMO of the dye should lie below

the energy level of the redox mediator to allow efficient regeneration of the oxidized

dye. 4) To minimize charge recombination between the injected electrons and the

resulting oxidized dye, the positive charge resulting after electron injection should be

localized on the donor part, which is further away from the TiO2 surface. 5) The

periphery of the dye should be hydrophobic to minimize direct contact between the

electrolyte and the anode to prevent water-induced desorption of the dye from the

TiO2 surface and consequently enhance the longterm stability. 6) The dye should not

aggregate on the surface to avoid nonradiative decay of the excited state to the ground

1.5 Metalloplymer

The rapid growth of supramolecular chemistry since the 1970s has led to many new

opportunities to take advantage of reversible interactions. A major contribution to the

diversity of the field of metal-containing polymers has involved the development of

metallosupramolecular polymers, in which the metal ions are bound by non-covalent

coordination interactions that allow for reversible, ‘dynamic’, binding analogous to

hydrogen bonding (figure panel b).34-35 The recent developments are illustrated by the

formation of metallosupramolecular polymers that involve labile multidentate ligation

and metallophilic interactions. As another key contributor to structural diversity,

metal containing polymers can contain a variety of metal centres, from

transition-metal ions and main-group metals through to lanthanides and actinides. In

addition, the metal centres can be located either in the polymer main chain or in the

side-chain structure (figure panels c and d) (Figure 1.5.1). As examples of further

subdivisions, metallopolymers can be linear, star-shaped, highly branched or

dendritic36-42 (figure panels e, f and g) (Figure 1.5.1). Significantly, with all of these

materials the typical classical polymer processing possibilities, such as spin coating,

inkjet printing, extrusion, compounding and film blowing, are maintained.

Metal-ligand coordination seems to be particularly attractive in past few decades

supramolecular metallo-polymers applied to electro-optical materials have been

commonly conducted, because the advantages of these materials, such as easy

processability, cheap fabrication, rapid coordination, and tunability of the optical band

gap, can promote long-range electrons or energy transfers.43 Supramolecular

metallo-architecture is formed with coordination ability of transition metal ions and

chelating ligands because of their self-recognition and self-assembly.44-46 Moreover,

metal-ligand complexes realized ideal conditions from self-assembly to form the

kinetically labile but nevertheless thermodynamically stable bonds.47 In the

meanwhile, metallo-polymers are also good candidates to study for their

electrochromic properties during the redox processes.48-49

2,2’:6’,2’’-Terpyridine(terpy) and bipyrdine (bpy) derivatives have been utilized

recently for multinuclear supramolecular interactions.50-52 The transport of energy and

electrons within nanoscale ordered materials is significant to optoelectronics. It needs

to control over both of their physical and chemical properties in the self-assembled

1.5.1 Nanocomposite Systems Base on Metallopolymer and

Nanoparticles

Nanocomposite systems are that combine the favorable features of, for example,

fullerenes and porphyrins as electron acceptors and donors, respectively.49 They have

received interest in the areas of light-induced electron-transfer chemistry and solar

energy conversion.53 Common electron donor-acceptor systems are based on covalent

linkages. However, much less is, known about noncovalent electron donor-acceptor

nanocomposites and the function of the intervening spacers.54 Compared with other

intermolecular forces, such as van der Waals, π-π stacking, or Coulombic interactions,

hydrogen bonds are particularly attractive as they are directional and do not possess

electronic energy levels that interfere with those in materials for organic

electro-optical applications.55-56 Therefore, that great efforts have been expended

toward the preparation and characterization of photo- and electro-active noncovalent

assemblies based on hydrogen bonds (H-bonds). (Figure 1.5.2)

Figure 1.5.2 Characterization of photo- and electro-active noncovalent assemblies

Chapter 2

Applications of novel dithienothiophene- and

2,7-carbazole-based conjugated

polymers with surface-modified ZnO nanoparticles

for organic photovoltaic cells

Two kinds of novel conjugated polymers containing 2,7-carbazole, thiophene, and

fused-dithienothiophene rings as backbones bearing acid-protected and benzoic acid

pendants (PCA and PCB, respectively) were utilized for organic solar cell

applications. The absorption spectra of these polymers (in both solutions and solid

films) showed an absorption range at 300–580 nm. Furthermore, ZnO nanoparticles

were synthesized and surface-modified with pyridyl surfactants (ZnOpy) to be ca.

3–4 nm. The pyridyl surfactants of ZnOpy nanoparticles (as electron acceptors to

partially replace expensive electron acceptor PCBM) not only induce supramolecular

interactions with benzoic acid pendants of polymer PCB via H-bonds, but also

enhance the homogeneous dispersions of ZnOpy nanoparticles in polymer PCB. Thus,

the ternary systems of PCA,PCB/ZnOpy/PCBM in weight ratios of 1:0.05:1 and

1:0.1:1 were investigated in bulk heterojunction polymer solar cells (PSCs). Under

the standard illumination of AM 1.5, 100 mW/cm2, the best power conversion

PCB/ZnOpy/PCBM=1:0.05:1 reached PCE=0.55%, with Jsc=2.11 mA/cm2,

Voc=0.88 V, and FF=29.4%.

2.1 Introduction

The developments of new materials to be used in organic optoelectronic devices such

as polymeric solar cells (PSCs) have become dramatically attractive because they

represent a green and renewable energy alternative to fossil energy and nuclear power.

In particular, the so-called bulk heterojunction (BHJ) concept2 has been established in

thin films of organic solar cell devices utilizing electron-donating conjugated

polymers blended with electron-accepting species, such as fullerenes,3 dicyano-based

polymers,4,57 or n-type nanoparticles.58 These hybrid polymer-inorganic solar cells

utilize the high electron mobility of the inorganic phase to overcome charge-transport

limitations associated with organic materials. The efficient BHJ solar cells made of

ZnO nanoparticles and a conjugated polymer have been reported previously.8 The

ZnO nanoparticles were blended with poly[2-

methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMOPPV) to possess a

highest power conversion efficiency (PCE) approaching 1.6% in PSCs.9 Hybrid solar

cells based on CdSe nanoparticles and a PPV-type polymer containing fluorene and

thiophene units (PFT) wereinvestigated. The CdSe/PFT devices showed very low

in the trioctylphosphine oxide (TOPO)-capped CdSe nanoparticle phase. Thus,

ternary systems based on mixtures of PFT/CdSe and the fullerene derivative

[6,6]-phenyl C61 butyric acid methyl ester (PCBM) were investigated. It was

observed that for the optimized composition of 20 wt.% PFT+40 wt.% CdSe+40 wt.%

PCBM the devices presented higher photocurrents and efficiencies. The use of

inorganic nanoparticles, such as TiO2, ZnO, CuInS2, PbSe, CdSe, and CdTe, have

some advantages, related to the versatility of these materials, which often can be

easily synthesized in a great variety of sizes and shapes, according to the desired

properties.10 In parallel, oligo- and poly(2,7-carbazole)61 derivatives have been

successfully used in polymer light emitting diodes (PLEDs)62 and organic field-effect

transistors (OFETs)62-63, demonstrating good p-type transport properties.60 Recently,

Müllen and co-workers61 have reported solar cells consisting of

poly(N-alkyl-2,7-carbazole)with a PCE value of 0.6%. Moreover, in contrast to the

fluorene unit the carbazole moiety is fully aromatic, providing a better chemical and

environmental stability. Taking all of these results into account, the development of

new copolymers based on carbazoles should therefore lead to interesting features for

photovoltaic applications. A class of polymers that have to date received little

attention as p-type materials for use in solar cells is polycarbazoles. Carbazole is a

candidates as p-type materials for solar cells.64 Dithieno[3,2-b:2′,3′-d]thiophene (DTT)

is a sulfur rich (three-S atoms) and electron rich segment, and serves as an important

building block of a wide variety of materials for electronic and optical applications,

such as electroluminescence, two photon absorptions, nonlinear optics,

photochromism, OFETs, and OPVs.65 Besides, the fused aromatic rings can make the

polymer backbones more rigid and coplanar, therefore enhancing effective

π-conjugation lengths, lowering band gaps, and extending absorption lengths. Powder

X-ray diffraction (XRD) analyses suggested that these copolymers formed

self-assembled π–π stacking and pseudo-bilayered structures.66 Molecules containing

fused ring systems intend to maximize the π-orbital overlaps by restricting

intramolecular rotation in these systems and possibly to induce face-to-face π–π

stackings, facilitating intermolecular hoppings and charge transports.65 In order to

increase the solubility in poly(DTT) without causing any additional twisting of the

repeating units in the resulting polymers, alkylsubstituted thiophene units were

incorporated into the polymer backbones as copolymers to fabricate OPVs12 and

OFETs.67 Based on this concept, two different moieties, i.e., fused dithienothiophene

and carbazole, were utilized as donor monomers to synthesize fused

dithienothiophene-based polymers PCA and PCB (see Figure 2.1). In order to

pyridyl-surface-modified ZnO nanoparticles (ZnOpy) were synthesized according to

Scheme 2.1. Compared with those reported fused dithienothiophene-based polymers,

polymers PCA and PCB showed much improved open circuit voltage (Voc) values

with a highest open-circuit voltage of up to 0.88 V (in PCB) as well as suitable

electronic energy levels and good processabilities for PSC applications. So far, the

preliminary PSC performance of these structurally related copolymers showed the

best PCE value of up to 0.55% while blended with ZnOpy and PCBM in a weight

ratio of 1:0.05:1, with a short circuit current density (Isc) of 2.11 mA/cm2, an open

circuit voltage (Voc) of 0.88 V, and a fill factor (FF) of 0.29 under the solar simulator

adjusted to give 100 mW/cm2 of AM 1.5 G irradiation. Although the results for the

PCE values of these nonoptimized PSCs are not sufficiently high enough, this

research affords a new concept to incorporate electron donor polymers and electron

2.2 Experimental

2.2.1 Materials

All chemicals and solvents were used as received.

2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole,60 2,7-dibromo-carbazole,60 and

3,5-didecanyldithieno[3,2-b:2′3′-d]thiophene)68-69 were synthesized according to the

literature procedures. The detailed synthetic routes of polymers PCA and PCB will be

published somewhere later. The synthetic routes of surface-modified ZnO

nanoparticles (ZnOpy) are shown in Scheme 1. ZnO nanoparticles were synthesized

by following the literature procedures.70 Chemicals and solvents were reagent grades

and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Toluene,

tetrahydrofuran, and diethyl ether were distilled to keep anhydrous before use.

2.2.2 Measurements and characterization

1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using

CDCl3 solvents. Elemental analyseswere performed on a HERAEUS CHN-OS

RAPID elemental analyzer. Transition temperature were determined by differential

scanning calorimetry (DSC, Perkin-Elmer Pyris 7) with a heating and cooling rate of

10 °C/min. Thermogravimetric analyses (TGA) were conducted with a TA instrument

Q500 at a heating rate of 10 °C/min under nitrogen. Gel permeation chromatography

polystyrene as a standard and THF as an eluent. UV–visible absorption and

photoluminescence (PL) spectrawere recorded in dilute chlorobenzene solutions (10−6

M) on a HP G1103A and Hitachi F-4500 spectrophotometer, respectively. Solid films

of UV–vis and PL measurements were spin-coated on quartz substrates from

chlorobenzene solutions with a concentration of 10 mg/mL. Cyclic voltammetry (CV)

measurements were performed using a BAS 100 electrochemical analyzer with a

standard threeelectrode electrochemical cell in a 0.1M tetrabutylammonium

hexafluorophosphate (TBAPF6) solution (in chorobenzene) at room temperature

with a scanning rate of 50mV/s. In each case, a carbon working electrode coated with

a thin layer of these copolymers, a platinum wire as the counter electrode, and a silver

wire as the quasireference electrode were used. Ag/AgCl (3MKCl) electrode was

served as a reference electrode for all potentials quoted herein. During the CV

measurements, the solutions were purged with nitrogen for 30 s, and the redox couple

ferrocene/ferrocenium ion (Fc/Fc+) was used as an external standard. The

corresponding HOMO and LUMO levels in copolymer films of PCA, PCB and

ZnOpy were calculated from Eox/onset and Ered/onset values of the electrochemical

experiments. The LUMO value of PCBM69 was in accordance with the literature data.

Each onset potential in the CV measurements was defined by the intersection of two

morphology were determined using a Veeco Nanoscope DI 3100 AFM microscope

operating in the tapping mode. The actual resolution of AFM measurements is 50 nm.

2.2.3

The photovoltaic cell (PVC) device structure used in this study was a sandwich

configuration of ITO/PEDOT:PSS/active layer/Ca/Al, where the active layer was

made of electron donor polymers PCA and PCB mixed with both electron acceptors

ZnOpy and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in the weight ratios

of polymer:ZnOpy:PCBM=1:0.05:1 and 1:0.1:1. The PVC devices were fabricated

according to the procedures similar to those of EL devices. The ITO coated glass

substrates were pre-cleaned and treated with oxygen plasma prior to use. A thin layer

(~50 nm) of PEDOT:PSS was spincoated on an ITO substrate and heated at 130 °C

for 1 h. Subsequently, the preliminary active layer was prepared by spin coating from

composite solutions of PCA, PCB:ZnOpy:PCBM in chlorobenzene (10 mg/mL) on

the top of PEDOT:PSS layer. The spin rate was about 800 rpm, and the thickness of

the active layer was typically ranged at 100–160 nm, unless the detailed thickness is

specified. The PVC devices were completed by deposition with 1 nm of Ca and 120

nm of Al. The film thicknesses were measured by a profilometer (Dektak3,

Veeco/Sloan Instruments Inc., USA). For photovoltaic measurements, I–V curves

300W xenon lamp (Oriel, #6258) with AM 1.5 filter (Oriel, #81080 kit) was used as

the white light source, and the optical power shone on the sample was 100 mW/cm2

detected by Oriel thermopile 71964. The I-V characteristics were measured using a

CHI 650B potentiostat/galvanostat. The external quantum efficiency (EQE) was

measured using a CHI 650B coupled with Oriel Cornerstone 260 monochromator. All

PVC devices were prepared and measured under ambient conditions.

2.2.4

The synthetic routes of surfactant and pyridyl-modified ZnO (ZnOpy) are shown in

Scheme 2.1.

(Pyridine-4-yl) methyl-3-(trethoxysily)propylcarbamate

4-Pyridinylmethanol (628 mg, 5.76 mmol) and 3-(triethoxysilyl) propyl isocyanate

(2.8 g, 11.48 mmol) were dissolved in dry THF (40 mL) and stirred in a flask. Besides,

dibutyltin dilaurate (36 mg, 0.058 mmol) was added dropwise. The mixture was

refluxed overnight. Solvent was removed under vacuum, and the crude product was

purified by flash column chromatography using hexane/ethyl acetate=1:2, v/v as

eluent. Subsequently, the pure compound was obtained as a yellow powder. Yield:

1.84 g (66%). 1H NMR (300 MHz, CDCl3): δ (ppm) 8.68 (dd, 2H), 8.05 (d, 1H), 7.52

(dd, 2H), 5.34 (s, 2H), 3.82 (quart, 6H), 3.18 (m, 2H), 1.87 (quint, 2H), 1.22

356. Element analysis calculation for C16H28N2O5Si: N, 7.86; C, 53.91; and H, 7.92.

Found: N, 7.90; C, 53.94; and H, 7.81.

Synthesis of pyridyl-modified ZnO nanoparticles (ZnOpy)

ZnO nanoparticles70 were dissolved and stirred in dry toluene (10 mL). Then,

(pyridine-4-yl) methyl-3-(trethoxysily)propylcarbamate (1 g, 0.058 mmol) was added

dropwise. The mixture was stirred to react overnight at 100 °C. The resulting

precipitates were isolated by centrifugation along with decantation, and then were

rewashed several times to remove all residues. The resulting product was

subsequently collected and dried under vacuum. The amount of pyridyl

surface-modifiers attached to ZnO nanoparticle surface can be estimated by TGA

analysis and was found ca. 5 wt.% in Figure 2.2. In addition, the sizes of ZnO

nanoparticles surface-modified with pyridyl surfactants (ZnOpy) were ca. 3-4 nm.

Figure 2.2 Thermogravimetric curves of ZnO nanoparticles before and after pyridyl

surface- modification.

2.3 Results and Discussion

2.3.1

The thermal stabilities and phase transition properties of polymers PCA and PCB

were characterized by thermogravimetric analyses (TGA) and differential scanning

calorimetry (DSC) measurements under nitrogen atmosphere, and the thermal

decomposition temperatures (Td) and melting points (Tm) are summarized in Table 2.1.

It is apparent that all copolymers exhibited good thermal stabilities, which showed

less than 5% weight loss upon heating to 366–408 °C. Regarding DSC experiments,

samples (weighted 1-5 mg) sealed in an aluminum pan were operated at 30-250 °C

under N2 atmosphere with a scan rate of 10 °C/min. These polymers showed

melting of the polymer backbones, and two polymers exhibited the glass transition (Tg)

temperatures at 135 °C and 143 °C for PCA and PCB, respectively. The Tg and Td

values of PCB are higher than that of PCA, implying that the polymer networks

formed by H-bonds (due to acid groups) of PCB make the structure more rigid.

2.3.2

The optical absorption spectra of polymers PCA and PCB in chlorobenzene solutions

(10−6 M) and solid films are shown in Figure 2.3, and their photophysical properties

are demonstrated in Table 2.2. As can be seen, the absorption spectra of polymers

PCA and PCB covered broad wavelength ranges for both solutions and solid films.

Similar maximum absorption wavelengths (442 and 441 nm) of PCA and PCB in

chlorobenzene solutions were observed. These donor polymers (PCA and PCB)

achieved the absorption spectra in the visible range of 350–580 nm (with tailing up to

around 650 nm) in solid films. Due to the π–π stacking of these polymer chains in

465 nm in polymers PCA and PCB. In addition, the long tailing around 650 nm in the

absorption spectra of PCA and PCB in both solutions and solid films were observed.

As shown in Table 2.2, the optical band gaps (Eg,opt) of 2.25 eV in polymers PCA and

PCB can be determined by the cut off of the absorption spectra in solid films. The

photoluminescence (PL) spectra of polymers PCA and PCB in chlorobenzene

solutions and solid films excited at incident wavelengths of 465 nm are shown in

Figure 2.4, respectively. The PL emission spectra of the polymers in the film were

dramatically quenched. Interestingly, in contrast to polymer PCA in Figure 2.4, the

PL spectra of PCB containing acid moieties were completely quenched in solid films.

The corresponding optical properties of these copolymers in solid films, including the

broad and strong optical absorptions, proposed their potential applications in the

Figure 2.3 Normalized optical absorption spectra of polymers PCA–PCB in solutions

(chlorobenzene) (10−6 M), and solid films (spin-coating from chlorobenzene solutions).

Figure 2.4 Normalized photoluminescence (PL) spectra of polymers PCA–PCB in

solutions (chlorobenzene) (10−6 M), and solid films (spin-coating from chlorobenzene solutions).

2.3.3 Electrochemical characterization

The electronic states, i.e. highest occupied molecular orbital (HOMO) and lowest

unoccupied molecular orbital (LUMO) levels, of the polymers were investigated by

cyclic voltammetry (CV) in order to understand the charge injection processes in

these polymers and their PSC devices. The oxidation and reduction cyclic

voltammograms of polymers PCA and PCB in solid films are displayed in Figure 2.5.

In order to obtain the solid films with an equal thickness, the concentrations in the

THF solutions and film forming conditions were kept constant (ca. 5 mg/mL). The

electrochemical measurements of the formal potentials, onset potentials, and band

gaps, along with the estimated positions of the upper edges of the valence band

(HOMO levels) and the lower edges of the conduction band (LUMO levels) are

summarized in Table 2.3. On the contrary, all polymers PCA and PCB exhibited one

quasi-reversible oxidation peaks as evident from the areas and close proximity of the

anodic and cathodic scans in Figure 2.5, which are a good sign for high structural

stability in the charged state. As illustrated in Table 2.3, the formal oxidation

potentials of these polymers were in the range of 0.7–1.1 V.The moderate onset

oxidation potentials of polymers PCA and PCB occurred between 0.7 and 1.1 V

fromwhich the estimated HOMO levels of −5.60 eV and LUMO levels of ca. −3.35

eV were acquired according to the following equation:72-73 EHOMO/LUMO=[−(Eonset (vs

ferrocene below the vacuum level and Eonset (Fc/Fc+ vs Ag/AgCl)=0.4 eV. In contrast, the

electrochemical reductions of polymers PCA and PCB showed similar LUMO energy

levels at ca. −3.35 eV, which represent to possess high electron affinities and also

make these polymers suitable electron donors for electron injection and transporting

to ZnOpy and PCBM acceptors (with 0.4 eV offsets in LUMO levels regarding

PCBM with a LUMO level of −3.75 eV,70 as shown in Fig. 6 for the polymeric bulk

heterojunction solar cell devices.71 Interestingly, the energy band gaps Eg, measured

directly from CV (Eg,=Eox/onset−Ered/onset, where Eg, values are 2.25 eV) are close to the

Figure 2.5. Cyclic voltammograms of polymers PCA and PCB (solid films) at a scan

rate of 100 mV/s.

Figure 2.6. Energy band diagram of HOMO/LUMO levels for electron donor

polymers PCA and PCB, electron acceptors ZnOpy and PCBM, and the work functions of ITO and Al.

2.3.4

The AFM topographies by the tappingmode of polymer blends

(PCA,PCB:ZnOpy:PCBM=1:0.05:1 and 1:0.1:1 in wt. ratios) were investigated via

casting films of dichlorobenzene solutions as shown in Figure 2.7, where the bumps

on the surface views are possibly attributed to the aggregation of ZnOnanoparticles

(ZnOpy) and polymers (PCA and PCB). The results show that the morphologies of

blends PCA/ZnOpy/PCBM have larger roughnesses (34 and 19 nmin Figure 2.7(a)

and (b), respectively) than those (10 and 11 nmin Figure 2.7(c) and (d), respectively)

of blends PCB/ZnOpy/PCBM. PCA/ZnOpy/PCBM films exhibited larger roughness

variations than PCB/ ZnOpy/PCBM. The roughness and phase separation must be

controlled/ optimized in order to improve the efficiency of devices. 74-76 It is worthy to

mention that the solid films of blended PCA:ZnOpy:PCBM in different ratios of

1:0.05:1 and 1:0.1:1w/w showed rougher surfaces, but the larger values of rms

roughnesses (34 and 19 nm, respectively) were contributed from the aggregation of

nanoparticles due to no interaction between polymer and nanoparticles, which

reduced the interfaces between donor (polymers) and acceptor (ZnOpy:PCBM)

significantly. Owing to the unfavorable morphologies for charge transport offered by

the aggregation of nanoparticles, the PSC devices based on PCA gave relatively low

Figure 2.7 The AFM images obtained for films of PCA/ZnOpy/PCBM and

2.3.5

The motivation for the design and syntheses of the conjugated polymers is to look for

new polymers for the application of PSCs. To investigate the potential use of

polymers PCA and PCB in PSCs, bulk heterojunction devices were fabricated from

an active layer in which polymers PCA and PCB were blended with the ZnOpy and

PCBM. The PSC devices with a configuration of ITO/PEDOT:PSS/PCA,

PCB:ZnOpy: PCBM(w/w)/Ca/Al were fabricated by depositing a thin layer (ca. 50

nm) of PEDOT:PSS onto patterned ITO slides. The active layer (ca. 100-160 nm)

consisting of PCA, PCB, ZnOpy and PCBM (w/w) was then deposited from a

solution (10 mg/mL in dichlorobenzene) by a spin rate of 500 rpm on the

PEDOT:PSS film, and followed by the deposition of Ca (ca. 50 nm) and aluminum

(100 nm) back electrodes.

The PSC devices were measured under AM 1.5 illuminations for a calibrated solar

simulator with an intensity of 100 mW/cm2. The preliminarily obtained properties are

summarized in Table 2.4, and the typical I-V characteristics of all PSC devices are

shown in Figure 2.8. The PSC device containing polymer PCB blended with ZnOpy

and PCBM acceptors had the highest PCE value of 0.55% with the values of Isc=2.11

Figure 2.8 I-V curves of solar cells under simulated AM 1.5 solar irradiation with an

active layer of (a) PCA:ZnOpy:PCBM (with different weight ratios of ZnOpy) and (b) PCB: ZnOpy:PCBM (with different weight ratios of ZnOpy).

2.4 Conclusions

The concept of supermolecular interactions, such as H-bonds formed between

conjugated polymers (PCA and PCB) and surface-modified nanoparticles ZnO

(ZnOpy), has been introduced by the syntheses of ZnOpy nanoparticles and two

fused dithienothiophene/carbazole-based polymers. The band gaps and the

HOMO/LUMO energy levels of these resulting copolymers can be finely tuned as

demonstrated by the investigation of optical absorption properties and electrochemical

studies. The pyridyl surfactants of ZnOpy nanoparticles (as electron acceptors to

partially replace expensive electron acceptor PCBM) not only induce supramolecular

interactions with benzoic acid pendants of polymer PCB via H-bonds, but also

enhance the homogeneous dispersions of ZnOpy nanoparticles in polymer PCB.

Thus, the PSC device containing ternary components of polymer PCB blended with

ZnOpy and PCBM acceptors (PCB:ZnOpy:PCBM=1:0.05:1) had the power

conversion efficiency of up to 0.55%, which gave the best performance with the

Chapter 3

Synthesis

of

Novel

Dithienothiophene-

and

2,7-Carbazole-Based

Conjugated

Polymers

and

H-Bonded Effects on Electrochromic and

Photovoltaic Properties

Three kinds of dithienothiophene/carbazole-based conjugated polymers (PCC, PCA

and PCB), which bear acid-protected and benzoic acid pendants in PCA and PCB,

respectively, were synthesized via Suzuki coupling reaction. Interestingly, PCC, PCA

and PCB exhibited reversible electrochromism during the oxidation processes of

cyclic voltammogram studies, and PCB (with H-bonds) revealed the best

electrochromic property with the most noticeable color change. According to powder

X-ray diffraction (XRD) analysis, these polymers exhibited obvious diffraction

features indicating bilayered packings between polymer backbones and π–π stacking

between layers in the solid state. Compared with the XRD data of PCA (without

H-bands), H-bonds of PCB induced a higher crystallinity in the small-angle region

(corresponding to a higher ordered bilayered packings between polymer backbones),

butwith a similar crystallinity in the wide angle region indicating a comparable π–π

stacking distance between layers. Moreover, based on the preliminary photovoltaic

(PCC, PCA and PCB blended individually with PCBM acceptor in the weight ratio of

1:1), PCB (with H-bonds) possessed the highest power conversion efficiency of 0.61%

(with Jsc =2.26 mA/cm2, FF=29.8%, and Voc = 0.9 V). In contrast to PCA (without

H-bands), the thermal stability, crystallinity, and electrochromic along with

photovoltaic properties of PCB were generally enhanced due to its H-bonded effects.

3.1 Introduction

Novel materials are developed for organic optoelectronic devices, such as

polymeric solar cells (PSCs), which is a popular research topic in recent decades,

because they are low cost and green materials for sustainable resources to reduce

consumptions of fossil energy and nuclear power.2 In particularly, bulk

heterojunction (BHJ) solar cells consisting of electron-donating conjugated

polymers blended with electron-accepting fullerenes are fabricated in solid thin

films.3 Up to now, regio-regular poly[2-methoxy-5-(3’,7’-

dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV)4 and

poly(3-hexylthiophene) (P3HT)5 as electron donors blended with

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor

approached high power conversion efficiency (PCE) values of 5.0% in PSCs. More

recently, the PCE values of BHJ solar cells using new low-band gap conjugated