Structure-properties relationships in small pi-conjugated molecules : electrochromism, photovoltaic conversion and mechano-fluorochromism

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molecules : electrochromism, photovoltaic conversion and mechano-fluorochromism

Yue Jiang

To cite this version:

Yue Jiang. Structure-properties relationships in small pi-conjugated molecules : electrochromism,

photovoltaic conversion and mechano-fluorochromism. Organic chemistry. Université d’Angers; South

China University of Technology, 2015. English. �NNT : 2015ANGE0026�. �tel-01482331�

Yue JIANG

Mémoire présenté en vue de l’obtention du grade de Docteur de l’Université d’Angers sous le label de L’Université Nantes Angers Le Mans

École doctorale: 3MPL

Discipline: Chime

Spécialité:Materiaux fonctionnels Unité de recherche:UMR6200

Soutenue le 8 Sep 2015 Thèse N: 107852

Structure-properties relationships in small pi- conjugated molecules: electrochromism,

photovoltaic conversion and mechanofluorochromism

JURY

Rapporteurs : Jiang LU, Professor, Sun Yat-sen University,China

Junwu CHEN, Professor, South China University of technology, China

Examinateurs : Huanfeng JIANG, Professor, South China University of technology, China Zhen TONG, Professor, South China University of technology, China Clement Cabanetos,, Charge de Recherche, University of Angers Directeur de Thèse : Jean RONCALI, Directeur de Recherche, University of Angers Co-directeur de Thèse : Ping LIU, Professor, South China University of technology, China

This thesis is co-directed between University of Angers and

South China University of Technology

Table of contents

Introduction

1. New X-shaped oligothiophenes as electrochromic material.

1.1. Introduction

1.2. Synthesis, structure and stability of X-shaped oligothiophenes.

1.3. Optical and electrochemical properties.

1.4. Electrochromic devices and performance.

1.5. Conclusion.

2. Generalities on organic materials for organic photovoltaics (OPV) 2.1. Brief history of OPV.

2.2. Operation of organic solar cells.

2.3. Characterization of organic solar cells.

2.4. Active materials for OPV.

2.4.1. Conjugated polymers.

2.4.2. Molecular donors for OPV.

2.4.3.Towards small molecular donors.

2.4.4. Acceptor materials for OPV.

3. Molecular acceptors for OPV based on benzodithiophene.

3.1. Synthesis.

3.2. Structure and Stability.

3.3. Optical and electrical property.

3.4. Photovoltaic performances.

3.5. Conclusion.

4. Small push-pull molecules based on triarylamines 4.1. Synthesis

4.3. Structure

4.4. Optical and electrical properties.

4.5. Photovoltaic performances.

5. Small push-pull molecules based on diarylamines substituted by aliphatic chains

4.1. Synthesis

4.3. Structure

4.4. Optical and electrical properties.

4.5. Photovoltaic performances.

6. New self-organized materials derived from small push-pull molecules 6.1. Introduction

6.2. Synthesis

6.3. Optical and electrical properties 6.4. Photovoltaic performances 3.3.9 Conclusion

6. Conclusion and perspectives

7. Experimental annexes

Introduction

The interaction of light with organic matter is at the origin of some of the most important processes involved in all kinds of life on earth. Thus, the photosynthesis process is at the same time a source of food, combustible and thus energy and artificial light.

The production of artificial light starts from combustion of wood, oil, and latter fossil energy such as coal or petrol. A major step has been the invention of incandescent light bulb at the end of the 19

century, while the invention of the fluorescent tube marks the first step towards the production of light without resorting to the conversion of thermal energy. The most and latest step of this long history has been the invention of solid-state light-emitting devices in which injected electrical charges directly interact with an active solid light-emitting material. While widely developed with inorganic active materials, organic LEDS has been triggered in 1986 by a seminal paper of Tang

and boosted some years later by the first OLEDs based on conjugated polymers by Friend and coworkers in 1990.

While a LED basically converts electrical energy into light, the reverse process namely the photovoltaic conversion of solar light into electricity has acquired a growing importance over the years.

Since their early development for space application at the end of the 50’s, silicon solar cells has undergone a tremendous development and they ensure today the major part of the production of photovoltaic electricity.

This spectacular development is due to the previsible exhaustion of fossil energy ressources combined with the increasing awareness of environmental and global warming problems.

However, in spite of their efficiency and of a continuous decrease of prices, photovoltaic conversion based on silicon remains an expensive technology, while environmental impact of the metallurgy of crystalline silicon and the fabrication of silicon wafers and then solar cells remain high.

During the past decades several kinds of alternative photovoltaic technologies have been proposed and investigated in order to reduce the cost and environmental impact of silicon solar cells.

These have included purely inorganic materials such as CdTe or CIGS, hybrid devices such as dye-sensitized solar cells (DSSCs) based on wide band gap inorganic semiconductors (essentially titanium dioxide) sensitized by organic dyes that can absorb a large part of the visible part of the solar spectrum, and purely organic solar cells (OPV).

Early research on OPV has been initiated at the end of the seventies using as active materials dyes

and pigments such as merocyanines, squaraines, phthalocyanines that had been initially developed for

other applications. In 1986 Tang described the first example of OPV cell based on the creation of a

planar heterojunction between an electron-donor material and an electron-acceptor,

this publication

is generally considered as the starting point of the modern research on OPV. However, the actual

widespread research on OPV started in the mid-nineties triggered by several factors. The first was a

drastic change of the point of view on conducting polymers with a decline of attention for bulk applications of the doped materials (essentially oxidized) and an increasing interest for the optical and semiconducting properties of the neutral state. Thus, the initial work on OLEDs and field-effect transistors based on conjugated polymers was rapidly followed by the first reports on the use of conjugated polymers as active materials for OPV.

A second major step has been the invention of the bulk heterojunction (BHJ) OPV cells.

Compared to Tang’s planar heterojunction BHJ cells present several distinct advantages. The first is the possibility to fabricate the cells by solution-process and the second is that the creation of a blend with phase-segregated material donor and acceptor material leads to a huge increase of the interfacial area between the donor and the acceptor thus allowing the dissociation of a much larger number of excitons and hence a large improvement of conversion efficiency.

During the past two decades OPV cells have generate a considerable multi-disciplinary research effort focused on both the elucidation of the fundamental mechanisms involved in the operation of the devices as well as on the optimization of their efficiency. This huge amount of work has produced spectacular progress leading to an tenfold increase of the conversion efficiency from ca 1.0 % to more than 10.0 %.

Examination of the active materials Thus inThis latter topic has been dealing with both the technology of the cells

was based on remained confidential for almost two decades.

he design and of organic semiconductors has triggered a strong renewal of interest for OPV as illustrated by the considerable huge increase of exponential increase of the number of publications on this topic. (Figure?).

Besides the production of light or the photovoltaic conversion, a third topic deals wxitht the control of the intensity and distribution of light . Electrochromic devices like those found in rear mirrors can be seen as the optoeelectrical equivalent of a curtain. Electrochromic devices pare also at the basis of information technology witht the frabrication of non-emissive display devices.

While for more than two decades these various topics has been developed by phycisits who worked with avaialable mactive materials that had not been specifically designed for this purpose. The past decade has seen the rapid emergence of synthetic chemistry focused on the development of active materials specifically desgined for these different aspects topics related to light and of the interactions of light and organic material.

A

In this work various aspects of these conjunctions of electric charges transport serve to modulate optical function .

After a shoft have been considred through the synthesis of various classes of new materials designed as active materials for essentially two types of application.

A fitst chapter the synthess of materials for EC will be described. In rthe second and largest part of this manuscript deals with the synthyesis characterization of active materials for OPV. Some first attempst to develop electron acceptors matrerials based on small molecular structures will be described first and then the largest part of the work will be described concerning several series of molecular donors materials derived from arylamines. Finally a fourth chapter will discuss the new oportunities offered by a new class of chromophores that combined unusited self-organization behavior with agreegation induced linear and nonlinear optical properties.

1 D. D. C. B. J.H.urroughes, A.R.Brown, R.N.Marks, K.Mackey, R.H.Friend, P.L.Burns, A.B.Holmes, Nature 1990, 347, 539.

2 C. W. Tang, Appl. Phys. Lett. 1986, 48, 183.

3 G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789.

4 C. A. W. J.J.M.Halls, N.C.Greenham, E.A.Marseglla, R.H.Friend, S.C.Morattl, A.B.Holmes, nature 1995, 376, 498.

5 Z. C. He, B. Xiao, F. Liu, H. B. Wu, Y. L. Yang, S. Xiao, C. Wang, T. P. Russell, Y. Cao, Nat. Photonics 2015, 9, 174; Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu, Y. Cao, Nat. Photonics 2012, 6, 591; C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Frechet, M. D. McGehee, P. M. Beaujuge, J. Am.

Chem. Soc. 2013, 135, 4656.

1. New X-shaped oligothiophenes as electrochromic material.

1.1. Introduction.

Electrochomism (EC) refers to the reversible color change of a substance when submitted to electrical stimulation. In 1969, Deb first demonstrated that an EC device could be fabricated with a thin film of tungsten trioxide (WO

).

WO

+ x(Li

+ e

)  Li

W

(1-x)W

O

Transparent  Blue

WO

is transparent as a thin film whereas on electrochemical reduction, W

sites from W

are generated to give the electrochromic effect. The mechanism is generally accepted that the injection and extraction of electrons and metal cations (Li

, H

, …) play a key role. In the case of Li

cation the reaction can be written as Equ.1.1. At low x the films have an intense blue colour caused by intervalence charge transfer (IV-CT) between adjacent W

and W

sites. At higher x, insertion irreversibly forms a metallic ‘bronze’ which is red or golden in colour.

During the following fifty years many electrochromic devices (ECD) have been proposed and investigated which can be classified into two main categories. In liquid phase systems such as some early examples of viologen-based systems, the active material is dissolved in a solvent. While in the most widely developed technology, the active material is deposited onto an optically transparent electrode, usually ITO, which is then immersed in an electrolytic medium which can be either a liquid or a quasi-solid gel and the device is completed by a counter electrode (ITO for example) (Fig. 1.1).

The liquid medium can help to achieve fast response time, but encapsulation can be a source of major stability problem due to possible leak of electrolyte. Thus though slow, the quasi-solid architecture is appropriate to practical applications.

Fig. 1.1. Electrochromic devices with liquid electrolyte (left) and as quasi-solid state device (right).

Electrochromism basically involves the emergence or bleaching of absorption bands in different region of the visible spectrum by means of electrochemical oxidation or reduction reaction of the active material. An EC device is typically characterized by different parameters. The first is defined by the change of transmittance at a given wavelength (∆T, %). The coloration efficiency is expressed by ratio of the modification of the optical absorbance at a given wavelength (∆A) divided by the density of positive (or negative) charge electrochemically injected in the device to induce a full switch (Q

).

Another important parameter is the cyclicity which corresponds to the number of effective switches that the system can perform before it loses 50% of its initial contrast. EC materials have been developed for various applications such as display devices but until now the most widely developed applications are as car rear-view mirrors and sunglasses.

Early work was essentially focused on EC active materials based on transition metal oxide such as WO

, NiO

, TiO

or V

O

WO

which can be switched between colorless and blue state by application of negative potential had been commercialized for a long time. However, based on their lightness, flexibility, low-cost and modulable electronic properties organic materials have rapidly attracted interest as potential active materials for EC devices.

Viologen derivatives were among the first systems investigated and in particular dimethyl viologen (MV).

Electrochromism of MV is based on the contrast between the colored radical cation in which the delocalized positive charge leads to an intramolecular charge transfer and the colorless dication

(scheme 1.1).

Scheme 1.1 Electrochromic mechanism of MV between colorless and purple.

Conducting polymers such as polypyrrole (1.1) and polythiophene (1.2) have been widely

investigated as possible active materials for EC devices. More recent work has also considered

poly(3,4- (ethylenedioxy)thiophene) (PEDOT) (1.3), polytriphenylamines (1.6), polyaniline (1.5) and

some low band gap polymers. A major merit of these materials is that they can be directly

electrochemically deposited on the transparent conducting glass electrode.

Poly(pyrrole),

is blue-violet ( 

670 nm) in the oxidized state and yellow-green ( 

420 nm) in the reduced state.

However, films of poly(pyrrole) rapidly degrade during electrochemical cycling. Polythiophene (PT) was first introduced as EC material by Garnier and coworkers in 1983.

Electrochemically polymerized PT is blue in the oxidized state and red in the neutral state.

Following this early work, many of 3-substituted or 3,4-disubstituted PTs have been investigated in order to modify the color of the active layer.

3,4-disubstituted PTs present the advantage to prevent parasitic electrochemical side reactions involving free positionsThis led to the research of 3,4-dimethyl or 3,4-dialkoxy-thiophenes.

Poly(3,4-(ethylenedioxy)thiophene) (PEDOT) was developed during the late 1980s at Bayer company. This polymer combines a narrow optical band-gap of about 1.60 eV and an particularly low oxidation potential compared to ~ 0.80 V for unsubstituted PT (1.2).

Oxidized PEDOT, namely the stable state is quasi-transparent sky blue and becomes deep-blue upon reduction to the neutral state as shown by Inganäs et al. who described the first electrochemically prepared PEDOT-based ECDs.

Reynolds and coworkers have extensively developed derivatives of 3,4-propylenedioxythiophene (ProDOT) (1.4).

First reported in 2002, butyl substituted ProDOT can be switched between dark purple and transmissive/sky blue states.

Furthermore, the application of the donor-acceptor (D-A) approach led to the control of the HOMO and LUMO energy level and thus to the development of a whole color palette. In 2012, two water soluble polymers with four ester groups per donor moiety were described.

Another class of organic electrochromic materials is based on polytriphenylamine (polyTPA) which also combines stable neutral state and oxidized state with fast response time due to high charge-transport rate.

Liou et al. have reported the electrochromism on TPA-based polymers that the color is mainly green for the radical-cation and blue for bis-radical cation.

More recently, parent small molecules have also shown similar EC behavior with high transparency in visible region.

The electrochromic property of polyaniline (6) depends not only on its oxidation state, but also on its protonation state, and hence the pH of the electrolytic medium.

Moreover its electrochromic process involves three states, namely transparent yellow to green to dark blue to black,

The yellow to green transition is durable under repetitive switching.

Among the three basic colors namely red, green and blue, green is always the most difficult to obtain because it requires two absorption bands in the red and blue region.

Using 2,3-di(thien-3-yl)-5,7-di(thien-2-yl)thieno[3,4-b]pyrazine (DDTP) and thiophene block Wudl obtained green neutral and transparent oxidized states with copolymers poly(DDTP) (1.7).

Stability tests revealed 10 000 double potential cycles without significant current loss with a contrast of 42% and 27%

at the peak of the red and blue region.

Based on an approach - alternation of donor and acceptor groups, many low band gap polymers

have been designed for EC devices.

For example, Amb et al. reported a series of blue-to-transparent

copolymers consisting of alternating ProDOT and 2,1,3-benzothiadiazole (BTD) units (1.8), with contrast ratios up to 52%, and sub-second switching speed.

The color could be tuned by changing the D/A ratio.

Scheme 1.2 Chemical structure of electrochromic polymers 1.1-1.8.

1.2. Synthesis, structure and stability of X-shaped oligothiophenes.

Although many EC materials based on linear thiophenic systems have been developed branched systems have not been investigated for this application. As a first step in this direction, three highly branched X-shaped oligothiophenes (V3T, X5T, X7T) were designed and synthesized and further studied as EC materials in quasi-solid state EC devices.

V3T S

S S

X5T S

S S

S S

X7T S

S S

S S

S S

Scheme 1.3. Structures of the target compounds V3T, X5T, X7T.

S B OH OH

ii)

i)

i) Br₂

1.11 V3T

S B

OH OH

S

S S

S

S S

Br Br

Br Br

S B

OH OH

S

S S

S S

S

S S

S S

S S

i)

iii) S

Br Br

S

Br Br

Br Br

NBS, CH₃COOH

X5T X7T

1.9

1.10

1.12

Scheme 1.4. Synthesis route of the target compounds V3T, X5T, X7T. i) Pd(PPh3)2Cl2, v:v, THF: 2 M K2CO3 aqueous solution = 5: 4, 63^oC, 48h; ii) CH2Cl2, 1 week, without light; iii) stir for 1 hour.

The synthesis of target compounds V3T, X5T, X7T is shown in Scheme 1.4. Suzuki coupling of 3,4-dibromothiophene with 2-thiophenylboric acid gave V3T in 87% yield. Bromination of V3T with NBS in dichloromethane gave 1.11 which led to X7T in 52% yield by Suzuki coupling. The same reaction applied to tetrabromothiophene 1.12 gave X5T in 48% yield.

X-ray diffraction analysis of single crystals of V3T and X5T reveal that the terthienyl segment is fundamentally coplanar in X5T, while the thiophenes in -positions are almost perpendicular to the central thiophene ring, (Fig.1.2). Unfortunately no single crystal of X7T could be obtained, presumably because of the many rotational isomers.

Fig. 1.2. Crystallographic structure of V3T (left) and X5T (right).

Consistent with crystallographic results the data of differential scanning calorimetry (DSC) from

room temperature to 260

C suggests that material V3T and X5T are crystalline showing huge

endothermic melting peak at 69

C to 193

C whereas X7T is amorphous, with a small endothermic

peak corresponding to the glass transition at 114

C and a melting peak at 228

C, (Fig. 1.3).

0 50 100 150 200 250 -6

-5 -4 -3 -2 -1 0

DSC(mW/mg)

Temperature(^o_C) V3T X5T X7T

Fig. 1.3. DSC curves of V3T, X5T, X7T, scan rate 10 K/min.

1.3. Optical and electrochemical properties of X-shaped oligothiophenes

The UV absorption spectra of all compounds were recorded in dichloromethane, shown in Fig.

1.4. A first peak at 250 nm belongs to the absorption of thiophene ring. This first band is followed by a shoulder at 280 nm for V3T assigned to  linked conjugation and by a second band with a maximum at 327 nm for X7T and 358 nm of X5T respectively which corresponds to the longest conjugated segment in the molecule. The blue shift of the maximum for X7T vs X5T suggests that the longest  linked terthienyl is more distorted in X7T due to the steric hindrance of the bithienyl branches attached at the -positions.

250 300 350 400 450 500

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Absorbance

Wavelength [nm]

V3T X5T X7T

Fig. 1.4 UV-Vis absorption spectra of compounds V3T, X5T, X7T in CH2Cl2

Table 1.1 Data of UV-Vis absorption spectroscopy in CH2Cl2 and cyclic voltammetry in 0.10M Bu4NClO4/CH2Cl2, scan rate 50 mV s^-1, Pt electrodes, reference Ag/Ag⁺.

Compd 

(M

cm

)



(nm) E

[V]

E

[V]

E

[eV]

E

[eV]

V3T 16600 249,280 1.90 -1.70 6.02 3.99 X5T 80200 249,358 1.34 -1.60 5.83 3.95 X7T 73200 250,327 1.21 -1.34 5.76 3.79

(a)

Using E

with an offset of -4.72 eV for Ag/Ag

versus the vacuum level.

Fig. 1.5 shows the cyclic voltammograms (CV) of compounds V3T, X5T and X7T in CH

Cl

in the presence of tetrabutylammonium perchlorate (Bu

NClO

) as the supporting electrolyte. The CV of all compounds shows a quasi-reversible oxidation process and an irreversible reduction wave. The extension of the conjugated system leads to a negative shift of the anodic peak potential (E

) and to a positive shift of the cathodic peak potential (E

) (Table 1.1).

E [V vs Ag/Ag⁺]

-3 -2 -1 0 1 2 3 4

V3T X5T X7T

10A

Fig. 1.5 Cyclic voltammograms of V3T, X5T, X7T (c= 1 mM) in 0.10 M Bu4NClO4/CH2Cl2, scan rate 50 mV s^-1, Pt working electrodes.

1.4. Electrochromic devices and performance

Electrochromic devices were prepared sequentially as follows. At first, films ca. 80-100 nm

thickness on ITO glass electrodes were vacuum deposited. Then ingredients of the gel electrolyte, that

is Bu

NClO

, acetonitrile, PMMA (M

550 000) and propylene carbonate, were mixed together

according to the weight percentage w%: 3:70:7:20. After stirring at 70

C for 2 hours, the mixture was

cooled down to room temperature and degassed with nitrogen for 30 min. Another ITO glass as

counter-electrode was covered with this gel by blade coating, and the final device was assembled by overlapping two substrates face to face.

For V3T application of a positive voltage of 2.10 V across the two electrodes leads to a color change from colorless to blue while the inversion of polarity leads to a yellowish brown color. All states of the films are highly transparent. The UV-Vis absorption spectra of the oxidized and reduced states are shown in Fig. 1.6. A new absorption band emerges at 440 nm which can be attributed to the cation radical while the peak at 280 nm shows a red shift and a broadening suggesting some rearrangement or electrochemical reaction within the film.

200 300 400 500

- - - doping(+2.1V) dedoping(-2.1V) V3T

Wavelength [nm]

Absorbance [a.u]

Fig. 1.6 Photographs (left) and normalized UV-Vis absorption spectra (right) for a film of compound V3T at 2.1 V.

The possible mechanism associated with the oxidation process is shown in Scheme 1.5. The coloration of the devices is associated with the oxidation of the compound into a delocalized radical cation. The irreversible process leading from colorless to colored states can be attributed to the dimerization or few couplings of the radical cation which could explain the red shift and broadening of absorption band in the spectrum of the undoped state.

Scheme. 1.5. Electrochromic mechanism of V3T.

Electrochemistry was also conducted in a solution of 5 mM V3T in 0.1 M Bu

NClO

/acetonitrile with an Ag/Ag

reference electrode and ITO working electrode, 50 mV s

as scan rate between -2.1 to 2.1 V. Application of repetitive potential scans leads to a darkening of the film possibly due to the electrochemical deposition of the cation radical of V3T on the surface of ITO.

Fig. 1.7. Photographs of the process of electrochemical deposition of V3T (c= 5 mM) in 0.10 M Bu4NClO4/acetonitrile, scan rate 50 mV s^-1, ITO working electrodes.

A similar electrochromic behavior was observed for X5T and X7T respectively as confirmed by the photographs and UV-Vis absorption spectra shown in Fig. 1.8. X5T exhibits a blue colored film at 2.5 V and a yellow-orange color at reversed voltage. In addition a large broadening of the absorption spectrum is observed, indicative of the occurrence of an electrochemical reaction in the film.

As shown by the photographs, at an applied voltage of 2.7 V X7T is oxidized into a bluish green film and turns to yellowish brown when a voltage of -2.7 V is applied. Again the UV-Vis absorption spectrum shows a red shift of the maximum from 335 to 350 nm with a broadening of the absorption band (Fig. 1.8).

200 300 400 500 600 700 800 Wavelength [nm]

Absorbance [a.u]

doping(+2.5V) dedopoing(-2.5V) X5T

200 300 400 500 600 700 800

X7T

Absorbance [a.u]

Wavelength [nm]

doping(+2.7V) dedoping(-2.7V)

Fig. 1.8 Photographs (top) and UV-Vis absorption spectra (bottom) of X5T (left) and X7T (right) as solid state electrochromic devices at positive and negative voltage.

-3 -2 -1 0 1 2 3 cycle1

Current

E [V vs SCE]

X5T ^cycle5

-3 -2 -1 0 1 2 3

X7T

Current

E [V vs SCE]

cycle1 cycle5

Fig. 1.9. Cyclic voltammograms of X5T and X7T, (c= 5 mM) in 0.10 M Bu4NClO4/acetonitrile, scan rate 50 mV s^-1, ITO working electrodes.

The cyclic voltammetry of X5T and X7T was performed in the same conditions as for V3T except that a saturated calomel electrode (SCE) was used as reference electrode instead of Ag/Ag

(Fig.

1.9).

The CV shows that repetitive cycling produces a small increase of the peak currents but no emergence of a new oxidation wave at lower potential. This allows to rule out of the significant occurrence of radical cation coupling and appears consistent with an electrochemical deposition of a cation-radical salt.

Comparison of the electrochromic processes of the three compounds shows that the compounds with the longer conjugated branches require application of a higher voltage to obtain coloration from 2.10 V of V3T to 2.50 V of X5T and 2.70 V of X7T. This result is in contradiction with the electrochemical data in Table 1.1 which shows a negative shift of E

and a positive shift of E

for the larger molecules.

A possible explanation could involve a kind of mutual neutralization of the anodic and cathodic coloration processes since it is likely that the application of a voltage across the cell in a two-electrode configuration can induce both processes simultaneously due to the absence of internal reference electrode. Further detailed electrochemical and spectro-electrochemical studies are needed to clarify this point.

1.5. Conclusion

Three X-shaped oligothiophenes have been synthesized and qualitatively evaluated as active material in electrochromic devices. Although preliminary results suggests the possibility to develop electrochromic materials on the basis of these structures further work devoted to the molecular engineering of new systems combining improved contrast and cyclicity is necessary to progress in that direction.

2 3

Current

E (V vs SCE)

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2. New active materials for organic photovoltaics (OPV)

2.1. Background

The photovoltaic effect was first observed by Becquerel on platinum electrodes covered with silver halide in aqueous solution.

Then the famous silicon solar cells were developed from Bell Lab in 1950s, initially for space applications and progressively entered terrestrial applications in particular for isolated sites. Today solar cells based on mono- or poly-crystalline represent by far the largest part of the market of photovoltaic devices. Commercial silicon cells typically reach power conversion efficiencies (PCE) of 15-20% while record PCE over 40% has been measured in laboratories for prototype champion cells.

In spite of the advantages and wide dissemination of silicon cells, the electrical energy produced by these devices remains much more expensive than that produced by more conventional sources such as coal, oil or nuclear power. In the past two decades, the emergence of environmental and climatic concerns together with the previsible exhaustion of fossil energy resources has generated an increasing interest for renewable energy sources such as hydroelectricity, wind and photovoltaics (PV). In this context, PV electricity occupies a privileged position due to the possibility to develop individual power sources in almost any location. However, the cost and environmental impact of solar cells based on crystalline silicon still pose problems. Although silicon is a highly abundant material, its metallurgy, purification, crystallization and sawing into wafers remains an expensive process which consumes much energy and generates chemical wastes. It is generally admitted that the pay-back time of a silicon solar cell (the time need for a cell to produce the amount of energy needed for its fabrication) is 2-3 years.

In order to solve these problems alternative cost-effective PV technologies have been investigated.

These include for example amorphous silicon, thin film PV cells based on copper indium gallium selenide solar cell (CIGS cell or CIGS) while the past two years have been marked by the spectacular emergence of solar cells based on perovskites.

Besides these inorganic materials, PV cells based on organic materials have been widely investigated as well.

Dye-sensitized solar cells (DSSC) represent a first type of hybrid PV cells in which organic

compounds serve as active material. DSSCs basically involve a layer of nano-porous titanium dioxide

(TiO

), a wide band gap inorganic semiconductor on which is grafted a monolayer of a dye capable to

absorb a large part of the solar spectrum and to efficiently inject electron in the conduction band of

TiO

. This electrode is immersed in an electrolytic medium containing a redox couple capable to

transport the hole photo-generated on the dye molecule to the opposite electrode. DSSC have reached

PCE better than 12%. However, despite these good results, the development of DSSCs is still

hampered by the presence of a liquid electrolyte.

All-organic PV cells (OPV) form a second broad class of devices. Although the origin of this topic can be tracked back in the early seventies, it is essentially during the last decade that this research area has witnessed its greatest development.

The first prototypes of OPV cells were based on pigments derived from merocyanines (2.1) and squaraines (2.2) implemented in single layer (Schottky type) devices in which the organic layer was sandwiched between a metal and an optically transparent electrode. Although a PCE of 0.70% has been claimed, in most cases this first generation of OPV cell gave in general very low PCEs due essentially to the important quenching of excitons by metal electrodes.

Scheme 2.1 Chemical structure of compound 2.1-2.4.

A major breakthrough occurred in 1986, when Tang reported the first example of donor-acceptor (D-A) planar heterojunction (PHJ) OPV cell consisting of a bilayer device of copper phthalocyanine (2.3) as D and a perylene diimide (PDI) (2.4) as A. This cell which gave a PCE of ca 1.0 % is generally considered as the starting point of the modern era of research on OPV.

Another major step was in the mid-nineties when the groups of Heeger

and Friend

reported

almost simultaneously the first examples of bulk heterojunction (BHJ) OPV cells. Instead of the

separate layers of donor and acceptor of a PHJ solar cell, a BHJ is formed by mixing D and A

materials in a common solvent in order to cast films in which the D and A will form interpenetrated

networks in segregated phases. This morphology results in a considerable extension of the area of the

heterojunction interface. Furthermore, since in organic materials the diffusion length of excitons is

generally limited to 10-20 nm, an appropriate control of the morphology of the D/A interface is to let

most of the photogenerated excitons have a high probability to reach a D/A interface and hence to

dissociate into positive and negative charges. Finally, BHJ presents the major advantage to be

solution-processible and thus offers the possibility to produce large area of (eventually flexible) substrates by low energy-demanding technologies such as printing or roll-to-roll process.

2.2. Mechanisms of operation of organic solar cells

In a basic conventional OPV cell the photoactive layers are deposited between a layer of high work function anode, usually Indium tin oxide (ITO) coated glass, and a layer of low work function cathode, usually Al. The active layers usually comprise of p-type semiconductor donor and n-type semiconductor acceptor. The absorption of an incident photon by a molecule produces an exciton which can be viewed as hole/electron pair bound by coulombic attraction. The low dieclectric constant of organic material does not allow the immediate dissociation of excitons which must diffuse to the D/A interface where they dissociate into electrons and holes by the interfacial electric field, Fig. 2.1.

Fig. 2.1 Mechanism of transfer from optical energy into electrical energy.

2.3. Characterization of organic solar cells

OPV cells are characterized using two main techniques. The first consists in recording the current density vs voltage curve in the dark and under simulated solar illumination (Fig. 2.2). The curve recorded in the dark should ideally exhibit a rectifying behavior. From the curve recorded under simulated solar illumination three main quantities can be obtained.

The short-circuit current density (J

) is the intercept of J-V curve with x-axis representing the maximum photo-current that the cell can produce.

The open circuit-voltage (V

), the intercept of J-V curve with y-axis, is the maximum voltage

developed by the cell at zero current. As for any electrical generator, the product J

V

defines the

maximum output power of the cell. However, because of the non-ideal character of the cell the actual

output power is defined by the rectangle of maximum area which can be inscribed inside the J vs V

curve (Fig. 2.2). The ratio between the area of these two rectangles defines the filling factor (FF)

which is a measure of the non-ideality of the device. Taking FF into account, the power conversion

efficiency (PCE) of the device is given by J

V

 FF divided by the incident light power (P

).

Fig. 2.2. Current-density vs Voltage curves (left) and external quantum efficiency spectrum (right) of an OPV cell.

Another important characteristic of a PV cell is the external quantum efficiency (EQE) defined as the ratio of the number of photo-generated charges to the number of incident photons. This quantity is also often called the incident-photon to current efficiency (IPCE). This spectral response is obtained by recording the photo-current delivered by the device as a function of wavelength when irradiated with monochromatic light. (Figure 2.2)

2.4. Design and synthesis of donor materials for OPV

The advantages of BHJ in terms of simplicity of solution process and possibility of applications on flexible substrates have triggered an intense research activity on the design of new donor materials.

Initial prototype of BHJ cells were based on conjugated polymers of the poly(p-phenylenevinylene) (PPV) family like for example poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) (2.5) and fullerene C

derivatives or electron acceptor polymers cyano-polyphenylene vinylene (CN-PPV) (2.6).

Fig. 2.3; Basic architecture of organic solar cells.

After reaching a maximum PCE of ca 2.50% in 2003, PPVs were supplanted by

poly(3-hexylthiophene) (P3HT) (2.7) ,

it was considered to be the best standard donor giving up to

5% efficiency.

Because of its band gap E

of ~ 2.0 eV, P3HT absorbs only ca one third of solar

photons vs ~ 80% for silicon (E

~ 1.10 eV). This problem has triggered the synthesis of low band gap

CPs specifically designed for OPV at the turn of the millennium.

However it was only in 2008 that

low band gap CPs surpassing P3HT were synthesized.

Following this trend, in the past few years new classes of donors have generated impressive progress and best efficiency ~10% has been achieved.

2.4.1 Low band gap conjugated polymers as donor material for OPV

Low band gap conjugated polymers have been developed for many years.

However the first ones specifically designed for OPV were reported only around 2000.

In 2007, Leclerc and coworkers reported a fully aromatic polymer 2.8 combining solubility, air stability and a conversion efficiency of 3.6 % when mixed with PC

BM ((6,6)-phenyl-C61-butyric acid methyl ester) in BHJ cells.

In 2009, the PCE was pushed up to 6.1 % using PC

BM ((6,6)-phenyl-C71-butyric acid methyl ester) as acceptor.

In 2011, the insertion of M

O

as anode interfacial layer further improved the PCE to 7.20 % in conventional architecture

and 6% in inverted cells.

Scheme 2.2 Chemical structure of fullerene acceptors-C60, PC61BM, PC71BM.

Other efficient polymers, such as 2.9 and 2.10 gave PCE of ~ 5.30-5.40%.

In 2007, Yu and coworkers introduced a new family of low gap polymers combining benzodithiophene as donor block and substituted thienothiophene as acceptor 2.11,

and reported a PCE of 7.4 % for cells based on polymers 2.12 or 2.13a and PC

BM.

.

Cao et al. reported PCE of 8.37% in 2011

and 9.2% in 2012

for inverted cells based on 2.13a and containing a layer of alcohol/water-soluble polyelectrolyte as the cathode interlayer.

Indacenodithiophene (IDT) has also been used in a D-A copolymer 2.14 leading to a PCE of 7.03%

with a large V

of 0.95 V.

More recently, Heeger et al. reported a PCE of 9.40 % for a cell based on polymer 2.15 and PC

BM mixture with high J

of 20.07 mA cm

and FF of 0.70. This high efficiency results from maximized light absorption by thick active layer.

On the other hand, it is noteworthy mentioning that the above results and materials deal with single junction solar cells and even higher PCEs have been obtained with tandem solar cells.

In 2013, using polymer 2.16 two junction solar cells with 10.2% efficiency was achieved,

which was surpassed by 10.6% in the same year with three junction among 2.16, 2.9a and 2.9b.

At

the same time, group Heeger also investigated tandem organic solar cells incorporating a DPP-based

low band gap polymer 2.17 and best PCE is as high as 8.58%.

More recently, the same group also reported until now the best efficiency of 11.3% based on polymer 2.18.

Scheme 2.3 Chemical structure of low band gap donor polymers for OPV.

2.4.2 Soluble molecular donors for BHJ solar cells.

Since the pioneering work of the seventies, insoluble dyes and pigments have been the only class of donor materials for OPV cells fabricated by vacuum deposition.

In fact, this technology is still widely investigated and porphyrins (2.19), squaraines (2.2) or oligothiophenes (2.20) are still intensively investigated.

In 2005, Roncali and coworkers for the first time proposed to replace polymers by soluble molecular donors. Initial work reported 3D molecules based on a silicon node (2.21) or a twisted bithiophene unit (2.22). Although initial results showed low PCE of 0.20-0.30 %, they were nevertheless high enough to demonstrate the validity of the concept.

Shortly after the same group introduced donors based on triphenylamine (TPA) and showed the creation of an internal charge transfer by attaching electron acceptor groups to a TPA-based system (2.23) led to a red shift of the absorption onset with a large increase of PCE,

while field-effect hole-mobility up to 1.1×10

cm

V

s

was reported.

Research on molecular donors has rapidly expanded and many classes of chromophores have been investigated including

3D oligothiophenes (2.21),

triphenylamines,

diketopyrrolopyrroles (DPP) (2.24),

borondipyrromethenes (Bodipys) (2.25),

indigos (2.26),

and many other tailored π-conjugated molecules.

In 2012, Bazan et al. reported that a hybrid molecule containing two benzothiadiazole groups (2.27) could reach a PCE of 6.70 % when combined with PC

BM.

PCE was then raised higher to 7.88 % by introduction of fluorine in the structure (2.28).

In recent years the group of Chen at Nankai University has synthesized different series of molecular systems based on oligothiophene with electron acceptor end groups (2.29) and fabricated solution-procssible BHJ cells with high efficiency. In particular very recently they reached the symbolic value of ca 10% PCE (2.30).

On the other hand the group of Cao has obtained a PCE of ca 8.08 % with a molecular system

combining porphyrins and DPP (2.31).

Scheme 2.4 Chemical structure of molecular donors for OPV.

Scheme 2.5 Chemical structure of molecular donors for OPV.

Examination of compounds that led to record PCE shows that they have in common a complex

structure of high molecular weight, they are prepared by multi-step syntheses with low overall yield

involving expensive and toxic organometallic reagents and catalysts. It is therefore difficult to envision

a large scale production at low cost and low environmental impact especially when one considers that

some reagents or solvents used at the laboratory scale are prohibited in industry. All these factors

contribute to make these advanced materials expensive. For example one of the most efficient low band gap polymer is sold by Solamer at the price of 3 000 USD per gram.

On the other hand the record devices generally combine full optimization and a small active area (few mm

). Consequently, in spite of their scientific interest, the results obtained on such devices are of limited utility as far as the development of low-cost large scale production is considered.

2.4.3 Towards small molecular donors

In 2011, Wong and his coworkers presented a series of D-A TPA-based small molecules, among which molecule 2.32 presented the highest PCE ca. 3.82%.

Then, the insertion of an acceptor block into the molecule to form a D-A-A system 2.33 leads to the bathochromic shift of absorption and the increase of PCE to 6.6%.

At the same time, merocyanine dye 2.34 also proved to be a highly efficient and truly small molecule with maximum PCE 6.1%.

The group of Würthner also reported compound 2.35 which gave a quite promising 4.5-5.1% PCE.

Though squaraines have been used as active material in some of the earliest prototypes of OPV cells, they have recently received a renewed interest. Solution-processed BHJ cells fabricated with donor 2.36 and PC

BM as acceptor presented a PCE of 5,2%.

Further, Yang et al. pushed the PCE to 4.0%.

Another class of small molecular donor material is based on 3,4-ethylenedioxythiophene (EDOT) (2.37). EDOT was initially incorporated in active OPV materials in 2006 and revisited more recently.

However, the strong electron-donor effect of EDOT tends to raise the HOMO level and hence to limit the V

of the cells in spite of the increase of hole mobility.

Scheme 2.6. Chemical structures of small molecular donors for OPV.

A large part of the present activity of the Angers groups on OPV is focused on the design of very

small molecular donors of low molecular weight in order to develop materials that can be obtained in

good yield with clean and scalable syntheses. Thus, the push-pull compound 2.38 in which a TPA

donor block is connected to a dicyanovinyl acceptor through a thienyl conjugating spacer has been

selected as starting platform to develop different kinds of structural variations.

These involve for example the search of the minimal molecular size but still able to produce a significant PV conversion (2.39), the bridging of the acceptor group (2.40, 2.41), the introduction of naphtyl groups (2.42) or the search for cleaner synthesis by formation of an hydrazone group instead of using classical meta-catalyzed coupling reactions (2.43). In this context an important part of this work will be focused on the development of new families of small molecular donors based on triarylamine.

Scheme 2.7. Chemical structure of TPA-based small donor molecules developed in Angers.

2.5. Molecular acceptors for OPV

Although the first prototypes of planar D-A heterojunction used molecules such as PDI (2.4) as acceptor,

the discovery of photo-induced electron-transfer from CPs to fullerene-C

for evaporation-deposition and PCBM (PC

BM

and PC

BM

) for solution-processed BHJ cells, have contributed to make fullerene derivatives the most widely used acceptor materials in OPV.

In fact, fullerenes present several obvious advantages such as electron acceptor properties, good electron mobility and isotropic charge-transport associated with their spherical or ovoid shape, a property particularly interesting for BHJ cells. Although CPs/fullerene solar cells can achieve impressive PCE (9.20 % for single-junction cells),

fullerenes present several important drawbacks.

Due to their wide band gap fullerenes do not absorb the visible and NIR part of the solar spectrum. As

fullerenes often account for at least 50% of the volume of a CP:fullerene blends, the use of fullerenes

significantly undercuts the absorption strength of OPVs. Furthermore, CP:fullerene blends are

metastable materials and the progressive phase separation of donor and acceptor leads to a decrease of PCE.

Whereas most of mono-substituted fullerenes have a LUMO level of about -4.0 eV, along with the fact that their poor tunability of energy levels because their substitution has only a limited impact on these levels. This nearly fixed energy level imposes additional constraints to the design of donor materials since the V

and hence PCE of OPVs depend on the HOMO-LUMO offset between D and A,

. Besides, as underlined in a recent review article,

the embodied energy of electronic grade PCBMs production and recycling ~ 65 and 90GJ Kg

for C

and C

respectively is one or two order of magnitude higher than a standard polymer donor such as regioregular P3HT. In short words, despite some interesting properties fullerenes remain expensive materials with high environmental impact.

There are two options of non-fullerene acceptors, namely, polymeric and molecular acceptors.

Polymeric acceptors such as CN-PPV

(2.6), and benzothiadiazole-based polymers

(2.44) have been used in OPVs since 1995. However, these two types of acceptor polymers exhibit low electron mobilities that result in low FF for the organic solar cells (OSCs).

Scheme 2.8. Chemical structure of polymeric acceptors.

In 2009, Yan and co-workers reported the first high mobility n-type polymeric semiconductor based on combinations of naphthalenediimide (NDI) or PDI and thiophene (2.45).

. This polymer soon became a widely studied acceptor material for OSCs. Relatively low initial PCE ~1.0% has been recently improved up to 4.0 %.

In fact a major problem with 2.45 is an excessive propensity to crystallize which favours the formation of domains of excessive size (up to ~1 m). However, optimized hot processing procedures have permitted to limit the domain size and to raise PCE to 4.0 %.

This result shows that 2.45 has a high potential for OPV but requires structural modifications in order

to better adapt its physical properties for this application.

As for molecular donors, replacing polymeric acceptors by small molecules forms another main branch with intense attention. In recent years several groups have reported molecular acceptors based on various structures such as vinazene (2.46),

small bridged systems associated with dicyanovinylbenzothiadiazole (2.47a,b),

oligothiophene-S,S–dioxides (2.48),

or DPP (2.49).

Scheme 2.9. Chemical structure of molecular acceptors

One example of efficient molecular acceptors is based on bifluorenylidene which is the substructure of the fullerene shell. The core is particularly effective at stabilizing a negative charge because of both steric and electronic effects. A preliminary study of methoxy derivative (2.50) revealed a PCE as high as 1.7%.

Typically used as p-type organic semiconductor, pentacene was also applied into acceptors.

Through adding electron withdrawing groups, the LUMO level was systematically adjusted.

Monocyano derivative (2.51) was proved to be the best with PCE as high as 1.29%.

Among the small molecular acceptors reported so far, the most successful family is based on PDI derivatives. Although PDI (2.4) present optical and electrical properties appropriate for PV conversion,

its high propensity to crystallize poses a problem for the fabrication of solution-processed BHJ cells. Various chemical approaches have been developed in order to address this question. In the earlier investigations of PDI, all the work was concentrated on the modification of alkyl substituent,

so that it is convinced that core-alkylated PDI derivatives could improve the performance than the N-alkylated PDI derivatives.

In 2013, Yao et al. linked two PDI molecules at their bay position with thiophene (2.52) resulting

4.03% of best PCE when mixing with conjugated polymer and 5% of co-solvent.

This decent

outcome was attributed to the reduction of aggregation due to its highly twisted dimeric backbone. The

efficiency was further pushed up to 6% by Jen, that the mixture of 2.53 with CP in the inverted

architecture gave better light absorption and the addition of co-solvent raised FF as a result of

optimized morphology.

More recently Pei et al. have synthesized a series of PDI dimers linked by

various aromatic linkers and obtained PCE of 2.30% for acceptors containing spiro-bifluorene linkers

(2.54) and P3HT as donor.