pi-Conjugated polymers for photovoltaics
Citation for published version (APA):
Zoombelt, A. P. (2009). pi-Conjugated polymers for photovoltaics. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR642867
DOI:
10.6100/IR642867
Document status and date: Published: 01/01/2009 Document Version:
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π
π
π
π
-Conjugated Polymers for Photovoltaics
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit
Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor
een commissie aangewezen door het College voor Promoties in het openbaar te
verdedigen op dinsdag 30 juni om 16.00 uur
door
Arjan Pieter Zoombelt
prof.dr.ir. R.A.J. Janssen
Cover Design: Arjan Zoombelt
Printing: Gildeprint Drukkerijen B.V. in Enschede
A catalogue record is available from the Eindhoven University of Technology Library
ISBN: 978-90-386-1841-8
Chapter 1 Introduction
1.1 Background 2
1.2 Organic solar cells 3
1.3 Aim and scope of the thesis 5
1.4 Device characteristics 5
1.5 Polymer requirements 6
1.6 Development in recent years 9
1.7 Outline of the thesis 13
1.8 References 14
Chapter 2 The synthesis and photovoltaic performance of regioregular poly[3-(n-butoxymethyl)thiophene] 2.1 Introduction 20
2.2 Results and discussion 20
2.3 Conclusions 26
2.4 Experimental 26
2.5 References 29
Chapter 3 Synthesis and photovoltaic performance of a series of small band gap polymers 3.1 Introduction 32
3.2 Results and discussion 33
3.3 Conclusions 43
3.4 Experimental 43
3.5 References 49
Chapter 4 Photovoltaic performance of an ultra small band gap polymer 4.1 Introduction 54
4.2 Results and discussion 55
4.3 Conclusions 60
4.4 Experimental 60
The influence of side chains on solubility and photovoltaic performance of dithiophene-thienopyrazine small band gap copolymers
5.1 Introduction 68
5.2 Results and discussion 69
5.3 Conclusions 77
5.4 Experimental 78
5.5 References 83
Chapter 6 Effect of extended thiophene segments in small band gap polymers with thienopyrazine 6.1 Introduction 86
6.2 Results and discussion 87
6.3 Conclusions 96
6.4 Experimental 96
6.5 References 101
Chapter 7 Small band gap polymers based on diketopyrrolopyrrole 7.1 Introduction 104
7.2 Results and discussion 105
7.3 Conclusions 120 7.4 Experimental 121 7.5 References 125 Summary Samenvatting Curriculum vitae List of publications Dankwoord
1
1.1
Background
Renewable energy sources are of major interest to the world today. Providing energy without producing vast amounts of greenhouse gasses such as CO2, is required to prevent global warming
which might induce irreversible climate changes.1,2 Converting sunlight into electrical energy,
photovoltaics (PV), is a fast growing technology and is very likely to play an important role in renewable energy-supply in future years.3
The photovoltaic effect was first discovered by Becquerel in 1839,4 who found that certain
materials produced an electrical current in an electrolytic cell when exposed to sunlight. However, the true potential of PV was not known until 1954 when Chapin et al.5 made the first silicon p-n junction
solar cell that converted solar radiation into electricity with 6% efficiency. Since then a lot of effort has been directed towards efficiency improvement and lowering production costs. Crystalline silicon solar cells have nowadays reached efficiencies up to 25%,6 but material requirements are high making them
rather expensive. The indirect band gap of Si necessitates thick active layers to ensure sufficient light absorption.7 To avoid recombination losses during the diffusion controlled transport of charge carriers
to the electrodes high purity Si is required. Second generation solar cells focus on cutting back production costs by applying cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous and nanocrystalline silicon (a-Si, µc-Si) as thin films. A successful way to fabricate PV cells from µc-Si is to use a p-i-n type device configuration. An intrinsic (i) layer is sandwiched between a n- and p-doped layer creating a field and hence charge carrier collection is considered to be more efficient since it is drift instead of diffusion controlled. This enables charge carriers to overcome defects and grain boudaries.7 Efficiencies of 9.5 and 10.1% have been reported for a-Si and µc-Si,
respectively.6 The direct band gap of materials like CdTe and CIGS ensures adequate light absorption
and allows the fabrication of more cost-efficient thin film solar cells with efficiencies of 16.7 and 19.4%, correspondingly.6 The power conversion efficiency of the inorganic solar cells can be further enhanced
by constructing multi-junction devices. The most efficient tandem cells are metamorphic triple-junction GaInP/GaInAs/Ge cells that convert 32% of the sunlight into electricity.8 Metamorphic
semiconductors allow facile manipulation of band gaps, resulting in a broad absorption by the various layers in the GaInP/GaInAs/Ge cell, covering a large part of the solar emission spectrum. Solar concentrators further increase the efficiencies of these multi-junction cells to 40.8% due to enhanced absorption.9 Although efficient, material availability might become a significant problem for these
1.2
Organic solar cells
Organic solar cells are considered a low cost alternative for their more expensive inorganic counterpart. The field of organic photovoltaics can be divided into three classes: small molecule,10-13
dye-sensitized14-17 and polymer based solar cells.18,19 In this thesis we focus on the latter. Next to the
chemical and synthetic versatility of organic materials, the prospect of high throughput roll-to-roll production of large area PV cells has stimulated research in this area. Although recent advances are encouraging, these thin film organic solar cells have not yet reached efficiencies that match those of inorganic PVs. Further improvements in efficiency and stability under operational conditions are required to make industrial production of organic solar cells economically feasible.20
The discovery of Shirakawa, MacDiarmid and Heeger in 1977 that polyacetylene can be made conducting,21 showed the potential of π-conjugated polymers. Since then the attention has shifted from
conducting to semiconducting properties which led to the development of electronic applications like light-emitting diodes (LEDs), field-effect transistors (FETs) and photovoltaics (PV). Concerning photovoltaics, there is an important difference between organic and inorganic semiconductor materials with respect to photogeneration of charge carriers. Whereas light absorption in inorganic semiconductors readily leads to free charge carriers throughout the bulk, the relatively low dielectric constant of organic materials results in a Coulombically bound electron-hole pair (exciton).22-24 The
Coulomb barrier needs to be overcome in an organic semiconductor to give free charges. A successful strategy, first applied by Tang in 1986,25 is to facilitate charge dissociation by creating a heterojunction.
This concept involves two semiconductor materials with different electron affinity (Figure 1.1),
E
-+
hν Donor Acceptor HOMO LUMO HOMO LUMOFigure 1.1. Schematic energy diagram of two semiconductors with different LUMO levels. The material with highest LUMO level is called donor and the semiconductor with the highest electron affinity the acceptor. After excitation, the electron is transferred from the donor to the acceptor. Although not shown here, excitation of the acceptor can lead to the same charge separated state, when electron transfer from the HOMO level of the donor to
i.e. different LUMO (lowest unoccupied molecular orbital) levels, sandwiched between two metal electrodes. When the LUMOs are appropriately matched, absorption can lead to charge transfer and subsequent dissociation when the electron and hole can escape from their Coulomb attraction.22,23
Using this donor-acceptor approach, with a phthalocyanine as a donor (p-type) and a perylene derivative as an acceptor (n-type) sandwiched between a transparent conducting oxide and a semitransparent metal electrode, Tang reported a power conversion efficiency of nearly 1%.25 The
drawback of this bilayer (Figure 1.2) is the limited exciton diffusion length, generally considered to be in the 5-15 nm range for π-conjugated polymers,26-29 which only allows excitons created within this
distance of the donor-acceptor interface to contribute to free charge carrier generation. The invention of the bulk heterojunction (Figure 1.2),30-33 which entails the intimate mixing of the donor and acceptor
material, led to a major increase in photogenerated charge carriers. Ideally, a nanoscopic phase separation between the donor and acceptor ensures that every exciton can reach the interface where
Figure 1.2. (left) Schematic device build-up of a solar cell with a glass substrate covered with a transparent indiumtinoxide (ITO) electrode. Subsequently, PEDOT:PSS and the active layer are spin coated on top, followed by vacuum deposition of the lithium fluoride/aluminium as back electrode. (middle) Bilayer device configuration in which charge dissociation can only take place in close proximity of the heterojunction. (right) Bulk heterojunction configuration where a nanoscopic phase separation ensures quantitative exciton dissociation.
it can dissociate into free charges. Percolating pathways facilitate the hole and electron transport to the corresponding electrodes. At present, most bulk heterojunction (BHJ) solar cells combine a p-type semiconducting polymer as a donor with acceptors in the form of a small molecule, an n-type semiconducting polymer or an inorganic material.20,34 The best performance has been achieved with a
soluble buckminsterfullerene C60 (or C70) derivative, [6,6]-phenyl-C61-butyric acid methyl ester
([60]PCBM or [70]PCBM),33 as electron acceptors (Figure 1.3). The ultrafast photoinduced electron
transfer, in the sub picosecond timescale,35 from the excited state of a polymer to the fullerene and its
good electron transport properties36 make these fullerenes excellent acceptor materials. Efficiencies
Al
Glas s PEDOT:PSSActive layer
Li F SMU + -Illumination ITO Glass LiF + -n-type
p-type
well over 5% have recently been reported,37,38 while 6.5% has been achieved with tandem solar cells.39
Calculations have shown that further material optimization should lead to efficiencies up to 10% for single junction cells and even 15% for tandem solar cells.40-42
O O O
O
[60]PCBM [70]PCBM
Figure 1.3. The structure of [6,6]-phenyl-C61-butyric acid methyl ester [60]PCBM and [6,6]-phenyl-C71-butyric acid
methyl ester [70]PCBM.
1.3
Aim and scope of the thesis
The research described in this thesis focuses on synthesizing and characterizing new p-type semiconducting polymers for photovoltaic applications having properties tuned to improve power conversion efficiency when combined with PCBM as n-type material. Appropriately matching the energy levels of the polymer to PCBM should allow a high open circuit voltage (Voc) while
maintaining efficient electron transfer from the LUMO level of the polymer to the LUMO level of the PCBM. Furthermore, reducing the band gap of the π-conjugated polymer, allocating a good overlap between the polymer absorption and the solar emission spectrum, potentially increases the number of absorbed photons and hence photovoltaic performance. By varying the chemical nature of the building blocks we aim to control the position and separation of the energy levels, intended to lead to efficient bulk heterojunction solar cells.
1.4
Device characteristics
Typical current-voltage characteristics of a solar cell are shown in Figure 1.4. The maximum power point (MPP) of a solar cell is characterized by its open circuit voltage (Voc), short circuit current
sc oc
J
V
J
V
FF
×
×
=
max max (2)where Vmax and Jmax are the voltage and current at the maximum power point. The estimated efficiency
(ηest) can then be calculated by:
in est
P
MPP
=
η
(3)where Pin is the incident light power density.
0.0 0.5 1.0 -2 -1 0 1 MPP C ur ren t de ns ity (m A /c m 2) Voltage (V) V oc J sc V max J max FF
Figure 1.4. Current-Voltage (J-V) plot of a solar cell in which the open circuit voltage (Voc) and the short circuit
current (Jsc) are characterized by the intersection of the abscissa and ordinate, respectively. The maximum power
output is reflected by Vmax × Jmax in the maximum power point (MPP).
1.5
Polymer requirements
Absorption The first process in a solar cell is the absorption of light by the active layer. Since PCBM possesses a relatively low extinction coefficient, in particular at longer wavelengths, most of the solar radiation needs to be absorbed by the π-conjugated polymer. To absorb as much light as possible, a good overlap between the polymer absorption spectrum and the solar emission, which peaks around 1.77 eV, is essential. Absorption by a semiconductor essentially involves the transition of an electron from the valence band (VB) to the conduction band (CB), and hence the energy separation between the VB and CB, or band gap, determines at which wavelength light is absorbed. For π-conjugated polymers, in which single and double bonds are alternating in the polymer chain, the band gap originates from the overlap of π-orbitals of the repeating units.43 Here, the electron can
be promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) creating an exciton. Since light absorption in π-conjugated polymers often
involves a spin- and dipole-allowed π-π* transition, from the ground state (S0) to the singlet excited
state (S1), these materials generally have high extinction coefficients.
The first polymers that were extensively studied in BHJ solar cells, poly(2-methoxy-5-{3’,7’-dimethyloctyloxy}-p-phenylenevinylene) (MDMO-PPV) and poly(3-hexylthiophene) (P3HT), give efficiencies of 2-3% and 4-5%, respectively.44-47 The main disadvantage of these polymers is their
relatively large band gap of about 2 eV, and thus do not utilize a large part of the solar spectrum. This signifies the potential of small band gap polymers for application in BHJ solar cells. Band gap reduction can be achieved by minimizing bond length alternation, i.e. reducing the energy difference between the aromatic and the quinoidal resonance structures. That is why polyisothianaphthene (PITN) has a band gap of only ~1 eV compared to the ~2 eV for polythiophene (Figure 1.5). The fused six-membered ring in PITN gains aromaticity and therefore stabilizes the quinoidal structure, resulting in a smaller band gap.48 Another strategy to reduce the band gap involves the alternation of
donor and acceptor units in a polymer chain.18,43,49 This decreases bond length alternation by charge
stabilization (D-A ↔ D+=A–). Hybridization of the donor and acceptor energy levels and extending
conjugation by polymerization affords a small band gap, illustrated in Figure 1.5. The size of the gap can be controlled by the strength of the donor and acceptor units.
S n S n S n S n aromatic quinoid PITN PT E HOMO LUMO HOMO LUMO
Donor D-A Acceptor Eg
Figure 1.5. (left) Aromatic and quinoidal resonance structures of polythiophene (PT) and polyisothianaphthene (PITN). (right) Hybridization of the donor and acceptor energy levels leading to band gap reduction.
Energy levels After absorption of light by the semiconducting polymer, the exciton needs to be dissociated into free charges. This charge generation is suggested to be initiated by the difference in LUMO levels of the donor and acceptor.22,23,40,41,50 Therefore, certain guidelines need to be taken into
account when designing small band gap polymers for PV. An offset between the LUMO of the p-type polymer and the LUMO of the fullerene acceptor is required to facilitate charge (electron) transfer from the polymer to the fullerene acceptor. Since photoinduced electron transfer occurs over relatively
disputed. For example, Mihailetchi et al.52 explain the dissociation of bound charge pairs by an
external field. Their model, based on the Onsager-Braun theory,53 excellently describes the device
performance of polymer:PCBM solar cells using charge carrier mobilities determined by space charge limited current measurements. However, another important parameter in this model is the geminate charge recombination rate (kF). Recombination rates in the order of microseconds are required for the
model to fit the experimental J-V curves,54-57 while other studies have shown that charge
recombination in comparable donor-acceptor blends is a lot faster.58-60 Instead of using the charge
carrier mobility in the bulk, Veldman et al.51 draw on an Onsager-Braun model which emphasizes on a
high local charge mobility driving charge separation at the interface. They argue that high electron mobility in PCBM clusters facilitates charge dissociation, and hence plays an important role in the charge generation process.
The position of the HOMO level of the polymer has a direct influence on the power output of a bulk heterojunction solar cell. The open circuit voltage (Voc) is determined by the energy difference
between the HOMO of the donor and the LUMO of the acceptor, provided there is ohmic contact with the electrodes.61-63 Band bending, as a result of charge accumulation, induces a ~0.2 V (at room
temperature) loss at each electrode. The maximum attainable Voc is related to the polymer oxidation
potential according to:
V
D
E
A
E
V
oc=
−
RED(
)
−
OX(
)
−
0
.
4
(4)where Ered(A) is the reduction potential of the PCBM and Eox(D) is the oxidation potential of the
polymer. Reducing the band gap of a polymer by decreasing the oxidation potential, i.e. raising its HOMO level, will thus result in a direct loss in Voc. Hence, judicious positioning of energy levels,
ensuring sufficient driving force for electron transfer while keeping the Voc as high as possible, is of
critical importance when designing small band gap polymers.
Charge recombination is a loss mechanism in BHJ devices and should be kept to a minimum.64,65 It has been reported that triplet formation in polymer BHJ solar cells can be an
important decay pathway and may affect charge carrier generation efficiency. If the polymer (or PCBM)66 triplet excited state is positioned lower in energy than the charge separated state (CSS),
charge carrier recombination into the triplet state can reduce geminate charge pair dissociation and thus lower PV performance.66-68 This implies that ideally the polymer should be designed to have a
Charge transport After exciton dissociation the free electron and hole are transported, drift or diffusion controlled, and collected at the electrodes. The effectiveness by which the photogenerated charges are extracted has a large influence on the power output of a solar cell. Two factors mainly govern the efficiency of charge collection; the carrier lifetime τ and the charge carrier mobility µ.
Basically, the free charge carrier needs to be extracted before non-geminate recombination occurs. It has been shown that increasing µhole in P3HT upon annealing decreases recombination and results in a
higher Jsc and thus enhances PV performance.65,69 Hence, it is important to design a polymer with good
charge transport properties, i.e. make a polymer that exhibits mesoscopic order and crystallinity in the solid state.70,71 Furthermore, the electron and hole mobility should be balanced in a BHJ solar cell.
Unbalanced and slow charge transport has been shown to lead to space charge effects that lower Jsc
and FF consequently reducing device performance.72-74
Processability Ideally, all of the requirements mentioned above are incorporated into a polymer that can be processed from solution. Large scale production of plastic solar cells by processing techniques like inkjet or screen printing requires the active blend to be soluble in common solvents or solvent mixtures. For buckminsterfullerene, functionalization with a solubilizing butyric acid methyl ester group facilitates processing from solution.33 Consequently, also the semiconducting
polymer needs to be equipped with solubilizing side chains. However, these substituents generally do not absorb light and do not contribute to the photocurrent and hence should be kept to a minimum. The chemical nature, position and size of the side chains do have a pronounced effect on the (photo)physical properties. Electron donating or withdrawing side chains influence the redox potential, while their position and bulkiness can affect planarity or solid state packing. Furthermore, solubility is a key factor that influences the maximum attainable molecular weight upon polymerization. The molecular weight of a polymer is an important parameter since it may have a profound effect on material properties.75,76
1.6
Development in recent years
It is beyond the scope of this thesis to give an extensive overview of all new π-conjugated polymers that have been designed and synthesized in recent years for application in BHJ solar cells. For a comprehensive summary, see Bundgaard et al.18 and Kroon et al.19 Instead, a small selection of
Although P3HT and MDMO-PPV (poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylene vinylene]) were the state-of-the-art polymers in BHJ solar cells for many years, their poor energy level alignment with respect to PCBM and relatively large band gap leaves room for improvement. Two strategies can be utilized to enhance power conversion efficiencies. The first strives to attain a polymer with a better energy level matching while keeping the band gap the same. Ideally, this would increase the maximum attainable photovoltage and thus PV performance. Figure 1.6 and Table 1.1 summarize three examples of such materials. Polymer 1, Figure 1.6, was first developed by Svensson et al.77 and
combines fluorene and dithiophene as donors with a benzothiadiazole acceptor. This copolymer has a relatively high oxidation potential affording an open circuit voltage of ~1 V and an estimated efficiency of 2.2% when blended in a solar cell with PCBM. Slooff et al.78 recently further optimized
this polymer, by using different side chains, increasing overall device performance up to 4.2%. The acetylene bond in polymer 2 was incorporated to increase the oxidation potential, i.e. lower the HOMO level, compared to P3HT. This resulted in a Voc of approximately 1 V,79 a 0.4 V increase from
the 0.6 V for P3HT. The lack of aggregation of 2 in a blend with PCBM was suggested to limit the short circuit current and fill factor affording η = 1.1%. A fairly recent interesting development is the
S S N S N R R n S S n H13C6 H13C6 1 2 S S S S R R S R S S R S S S S S S S R R S R S S R S S S S S R S S R S S R S S R S R S R S S S S S R S R S S S S S S
Figure 1.6. Semiconducting polymers designed to provide a high open circuit voltage in a BHJ solar cell with PCBM.
use of conjugated dendrimers as p-type materials in BHJ solar cells.80,81 These are well defined
monodisperse structures that are soluble in common organic solvents due to their hyperbranched structure, alleviating the need for solubilizing side chains. The twisted structure in solution and the absence of electron donating alkyl chains results in a higher oxidation potential for dendrimer 3 than P3HT. Furthermore, the lack of aggregate formation, mainly influencing the oxidation potential of a
polymer, allowed a relatively high Voc of ~ 1 V, but probably limits Jsc and FF. Together with a Jsc of 4.2
mA/cm2 and a FF of 0.42 an estimated efficiency of 1.7% was reported.82
Table 1.1. Photovoltaic performance some ‘high voltage’ semiconducting materials.
Polymer Acceptor Eg (opt.) Voc (V) Jsc (mA/cm2) FF (-) η (%) 1 [60]PCBM 1.9 1.0 7.7 0.54 4.2 2 [60]PCBM 2.1 1.0 3.1 0.36 1.1 3 [60]PCBM 2.2 0.97 4.2 0.42 1.7
A second approach towards improved photovoltaic performance of π-conjugated polymers in solar cells is to reduce the band gap of the semiconducting polymer, allocating a better overlap of the polymer absorption with the emission spectrum of the sun. In addition, as was discussed in paragraph 1.5, judiciously positioning the energy levels is crucial to minimize losses and maximize performance. All small band gap polymers shown in Figure 1.7 are based on alternating donor and acceptor units in a polymer chain and possess band gaps in the range of 1.2-1.6 eV (Table 1.2). Their highest reported solar cell performance is listed in Table 1.2. Polymer 4 (Figure 1.7), reported by Dhanabalan et al.,83
was one of the first small band gap polymers (1.6 eV) to show relatively efficient power conversion with η = 1%.84 An even further red shifted photoresponse up to 1 µm was achieved with polymer 5.
Incorporating the strong thiadiazoloquinoxaline acceptor into a copolymer with fluorene afforded an optical band gap of ~1.2 eV that, when blended with a C70 fullerene derivative, gave Jsc = 3.4 mA/cm2,
Voc = 0.58 V and FF = 0.34, providing an estimated efficiency of 0.7%.85 The fact that a polymer with a
band gap of 1.2 eV afforded a similar Voc as P3HT (with Eg = 1.9 eV) indicates a much better energy
level alignment. However, the main limiting factor was the relatively low Jsc. After optimizing the side
chains, improved short circuit currents were obtained with polymer 6, depicted in Figure 1.7. Zhang et al.86 reported a Jsc of 8.9 mA/cm2 for 6, using ethylhexyloxy side chains (see Table 1.2), resulting in an
overall efficiency of 2.2%. Although promising results, higher efficiencies are required for organic PV to play an important role in future energy supply. A huge improvement was made in recent years by using the planar cyclopentadithiophene (CPDT) as a donor unit in small band gap polymers. The copolymer of benzothiadiazole and CPDT (polymer 7 in Figure 1.7) has an optical band gap of 1.5 eV. The first solar cells made with [70]PCBM were promising with η = 3.2%,87 while optimizing processing
S S N S N R R n N N H17C8 C8H17 S S S N N R R n N S N S S n N S N S S N C12H25 n N S N Si S S n S S COOC12H25 S S OC8H17 OC8H17 n S S S S N N O O C12H25 H25C12 n S S N S N N S S n 4 5 6 7 8 9 10 11
Figure 1.7. Small band gap polymers for photovoltaics developed in recent years.
of an alkanedithiol allowed the polymer to aggregate in the blend and afforded a power conversion efficiency of 5.5%.37,76 The main improvement is in the fill factor and short circuit current which was
reported to be 16.2 mA/cm2; still among the highest to date. Derivatives of CPDT, either silole88 (8) or
pyrrole89 (9) based, have been synthesized and applied in BHJ solar cells and also exhibit good PV
performance (Table 1.2). The latest developments involve polymers 10 and 11, shown in Figure 1.7. Polymer 10 combines quaterthiophene as a donor and a diketopyrrolopyrrole as a novel acceptor resulting in an optical band gap of 1.4 eV.90 When applied in a solar cell with [70]PCBM, it provides an
estimated efficiency of 4.0%. Benzodithiophene and thieno[3,4-b]thiophene are used as the donor and acceptor unit, respectively, in polymer 11. The best cells of 11 made with [70]PCBM gave Jsc = 15.6
mA/cm2, Voc = 0.56 V and FF = 0.63, resulting in an estimated efficiency of 5.5%.38
In summary, the polymers presented here are a selection of some important and most recent advances in the field of organic photovoltaics. Considerable research efforts are nowadays directed towards optimizing the properties of the semiconducting polymer with respect to PCBM and have afforded encouraging efficiencies of over 5%. However, further improvements are needed for polymer solar cells to ensure there use in commercial applications.
Table 1.2. Photovoltaic performance of some small band gap polymers. Polymer Acceptor Eg (opt.) (eV) Voc (V) Jsc (mA/cm2) FF (-) η (%) 4 [60]PCBM 1.6 0.72 3.1 0.37 1.0 5 BTPF70 1.2 0.58 3.4 0.35 0.7 6a 6b 6c 7 8 9 10 11 [60]PCBM [60]PCBM [60]PCBM [70]PCBM [70]PCBM [60]PCBM [70]PCBM [70]PCBM 1.6 1.6 1.6 1.5 1.5 1.5 1.4 1.6 0.78 0.59 0.64 0.62 0.68 0.52 0.61 0.56 3.0 8.9 3.9 16.2 12.7 9.5 11.3 15.6 0.4 0.42 0.58 0.55 0.55 0.44 0.58 0.63 0.9 2.2 1.4 5.5 4.7d 2.2 4.0 5.5
a R = H, b R = ethylhexyloxy, c R = octyl. d is an average value, their best cell gave 5.1%.
1.7
Outline of the thesis
The aim of the thesis is to tune the properties of the π-conjugated polymer with respect to PCBM to improve power conversion efficiency. The employed strategies and their effect on polymer properties and photovoltaic performance are summarized in Chapters 2-7.
The research described in chapter two involves the modification of P3HT. One of the limiting factors of P3HT:[60]PCBM solar cells is the relatively low open circuit voltage of 0.6 V,46,47 while the
polymer absorbs light with a much higher energy of approximately 1.9 eV. This voltage loss originates from the large energy level offset between P3HT and PCBM. The energy difference between the LUMO levels is about 0.9 eV, which drives charge transfer but limits the Voc to 0.6 V according to
equation 4. In this chapter the n-hexyl side chain in P3HT is substituted with n-butoxymethyl to lower the energy levels. The inductive electron withdrawing effect of n-butoxymethyl should increase the polymer oxidation potential and consequently enhance the open circuit voltage, while ideally not altering any of the other properties. The photovoltaic performance with [60]PCBM is evaluated.
Chapter three deals with the synthesis and (photo)physical properties of small band gap polymers. Electron-rich and electron-deficient units are incorporated, as donor and acceptor respectively, to reduce the band gap. Dithiophene is used as a donor, while the acceptor is either
and fine-tune material properties. The polymers are tested in BHJ solar cells with [60]PCBM and [70]PCBM.
The polymer described in chapter four involves the alternation of dithiophene as a donor and the highly electron-deficient thiadiazoloquinoxaline. To potentially harvest the low energy part of the solar spectrum, this strong acceptor was used to further reduce the band gap into the near infrared. This chapter deals with the synthesis and properties of the polymer and its solar cell performance with [60]PCBM and [84]PCBM.
Since the solubilizing side chains generally do not contribute to the photocurrent, the number of side chains is reduced for the polymers described in chapter five. The synthesis of small band gap polymers consisting out of dithiophene as a donor and thienopyrazine as an acceptor is described. The effect of solubility, by incorporating either n-octyl, ethylhexyl or n-butoxymethyl side chains, on material properties and PV performance was investigated.
In chapter six the donor acceptor ratio is increased and its influence on optical and electrochemical properties studied. Thienopyrazine, used as an electron-deficient unit, is combined with either quaterthiophene or quinquethiophene functioning as a donor. In addition, the effect of planarity, by introducing head-to-head coupling, on the PV device performance of the polymer is studied. The polymers were blended with [60]PCBM and [70]PCBM in BHJ solar cells.
The research described in the last chapter involves the utilization of diketopyrrolopyrrole (DPP) as a new type of acceptor. The monomer based on DPP, 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione, is polymerized via Yamamoto coupling or combined with various donor units like fluorene, cyclopentadithiophene and thiophene via Suzuki polymerization. The strength of the donor is varied to tune the band gap and energy levels for optimal PV performance. All polymers were applied and tested in FETs and BHJ solar cells.
1.8
References
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2
The synthesis and photovoltaic performance of
regioregular
poly[3-(n-butoxymethyl)thiophene]
*Abstract. The synthesis and photovoltaic performance of poly[3-(n-butoxymethyl)thiophene] (P3BMT) is reported. Incorporation of a n-butoxymethyl side chain, instead of an alkyl chain as in poly(3-hexylthiophene) (P3HT), may increase the oxidation potential via an inductive effect leading to an increase in open-circuit voltage (Voc) in bulk heterojunction solar cells. Applying the Grignard
metathesis polymerization route afforded highly regioregular P3BMT with >95% HT coupling. In solution, the optical and electrochemical properties of P3BMT are nearly identical to those of P3HT. However, the absorption data in the film point to a lesser degree of 3D ordering. The ether functionality appears to prevent a close packing of the chains. The best bulk heterojunction solar cells made from P3BMT with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor had Voc = 0.71
V, which is indeed ~0.1 V higher than that of P3HT:PCBM cells. As a result of the lower 3D ordering, the short-circuit current (Jsc = 4.16 mA/cm2) is less. In combination with a fill factor of 0.57, a maximum
power conversion efficiency of 1.7% was obtained.
2.1
Introduction
Currently, state-of-the-art bulk heterojunction solar cells made from P3HT:PCBM blends reach efficiencies up to 4-5%.1-6 The main improvements in the last few years have resulted from
advances in processing, using thermal annealing or slow solvent evaporation in film formation. One of the factors limiting the efficiency of P3HT:PCBM solar cells is the relatively low open-circuit voltage (Voc). P3HT absorbs light with photon energies of >1.9 eV, yet the Voc obtained from the P3HT:PCBM
solar cells is only around 0.6 V. In bulk heterojunction cells, the Voc is mainly determined by the
energy difference between the HOMO of the donor and the LUMO of the acceptor.7,8 While the large
offset that exists between the LUMO of P3HT and the LUMO of PCBM (estimated to be ~0.9 V) is beneficial for efficient exciton dissociation and charge generation at the donor-acceptor interface, it implies a significant reduction of the maximum Voc.9,10 By reducing the energy offset of the LUMOs
while keeping the same band gap, the Voc and thereby the overall efficiency might be increased.11 We
note, however, that a minimum offset of ~0.4 eV is required to ensure efficient charge separation.12,13
Here we report the synthesis of poly[3-(n-butoxymethyl)thiophene] (P3BMT, Figure 2.1) and its photovoltaic performance in bulk heterojunction solar cells. The ether functionalized side chain is expected to increase the oxidation potential via inductive electronic effects with respect to that of P3HT and thus enhance Voc.
O
S n
S n
P3HT P3BMT
Figure 2.1. Structures of poly(3-hexylthiophene) (P3HT) and poly[3-(n-butoxymethyl)thiophene] (P3BMT).
2.2
Results and discussion
Synthesis Two related synthesis routes towards P3BMT are depicted in Scheme 2.1. Starting from 3-thiophenemethanol, a Williamson-ether synthesis leads to 3-(n-butoxymethyl)thiophene in 61% yield. Subsequent dibromination, by adding 2 equivalents of N-bromosuccinimide (NBS) in chloroform/acetic acid (1:1 v/v) affords monomer 2 that is polymerized using the Grignard metathesis (GRIM) reaction employing Ni(dppp)Cl2 as a catalyst. When applied to
coupled poly(3-akylthiophenes).14-16 However, for 2 the GRIM method results in regioirregular
P3BMT. The four signals at δ 3.5 and 4.5 in the 1H-NMR spectrum (Figure 2.2, top curve), originating
from the OCH2 protons, give clear evidence of two different configurations of the side chains in the
polymer in an almost 50:50 ratio. A probable explanation for the irregular polymerization is the observed (by 1H-NMR) lack in selectivity of the metalation of 2 upon treatment with the Grignard
reagent. In this case the two regioisomers, 2-bromo-5-bromomagnesio[3-(n-butoxymethyl)thiophene] and 2-bromomagnesio-5-bromo[3-(n-butoxymethyl)thiophene], are obtained in a 50:50 ratio. We note that 3-alkylthiophenes show a ratio of 85:15, with a preference for the metalation at the 5-position.
OH S NaH Br-Bu THF O S NBS CH3COOH CH3Cl O S Br Br THF S n Ni(dppp)Cl2 O NBS CH3COOH O S Br O S n THF Ni(dppp)Cl2 PhI(OAc)2 I2 CHCl3 O S Br I 1 2 irr-P3BMT 1) 2) 3 rr-P3BMT 1) 2) 1 eq 4 t-BuMgCl t-BuMgCl 61% 74% 52% 87% 72% 54% a b
Scheme 2.1. Synthetic routes for P3BMT.
Figure 2.2. 1H-NMR spectrum of regioirregular (top) and regioregular P3BMT (bottom).
To prepare regioregular P3BMT, 2-bromo-5-iodo[3-(n-butoxymethyl)thiophene] (4) was used as monomer (Scheme 2.1). The iodine, being more susceptible to metalation, allowed 100% selective formation of the 2-bromo-5-bromomagnesio[3-(n-butoxymethyl)thiophene] isomer. Subsequent addition of Ni(dppp)Cl2 afforded regioregular P3BMT. The 1H-NMR spectrum displays only two
δ(ppm) 3.50 4.00 4.50 δ(ppm) 3.50 4.00 4.50 a b
The molecular weight as determined by GPC of rr-P3BMT (Mn = 7 kg/mol, Mw = 12 kg/mol) was less
than that of irr-P3BMT (Mn = 14 kg/mol, Mw = 24 kg/mol).
Optical properties The UV-Vis spectra of rr-P3BMT and irr-P3BMT dissolved in o-dichlorobenzene are compared to that of (regioregular) P3HT prepared by the GRIM method in Figure 2.3a. The absorption maximum of irr-P3BMT (λmax = 438 nm, Table 2.1) is blue-shifted by 30 nm
compared to P3HT. This blue shift is attributed to a less co-planar conformation of adjacent thiophene rings, induced by the HH coupling present in the polymer that reduces the effective conjugation length. For rr-P3BMT the maximum absorption (λmax = 462 nm) is close to that of P3HT (λmax = 468 nm).
In the solid state, the absorption maximum of P3HT exhibits a larger red shift of 103 nm and 62 nm compared to irr-P3BMT and rr-P3BMT, respectively (Figure 2.3b). Additionally, the absorption spectrum of P3HT exhibits a characteristic vibronic fine structure, originating from coupling of the electronic π→π* transition to the C=C stretch vibration. This fine structure is only observed under conditions where highly ordered, semi-crystalline domains are present.17 The absence of vibronic fine
structure for both rr-P3BMT and irr-P3BMT clearly indicates a significant lesser degree of 3D ordering in these films. Apparently, replacing the second methylene group in the side chain by oxygen prohibits a close packing of the chains. In contrast, replacing the third methylene group by oxygen, does allow a high degree of ordering of the polymer in film.18
300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 a N or m al iz ed a bs or pt io n λ (nm) P3HT rrP3BMT irrP3BMT 300 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 b N or m al iz ed a bs or pt io n λ (nm) P3HT rrP3BMT irrP3BMT
Figure 2.3. UV-Vis absorption of P3HT, irr-P3BMT, and rr-P3BMT in (a) o-dichlorobenzene solution and (b) as thin film on glass.
Table 2.1. UV-Vis absorption data recorded in o-dichlorobenzene solution and in film. Solution Film Polymer λmax (nm) λonset (nm) Eg (eV) λmax (nm) λonset (nm) Eg (eV) P3HT irr-P3BMT rr-P3BMT 468 438 462 557 527 553 2.23 2.36 2.25 562 459 500 655 569 609 1.90 2.18 2.04
Annealing of P3BMT films was attempted to possibly enhance the degree of ordering. However, up to 80 oC no change in the absorption was observed, while at temperatures of 100 oC and
above the absorption maximum blue shifted by about 15 nm. No vibronic fine structure was induced by the thermal treatment. It is thus clear that the n-butoxymethyl side chain does not allow the formation of highly ordered films.
Electrochemistry The aim of using P3BMT was to increase the oxidation potential (Eox) of
P3HT by chemical modification to enhance the Voc and the efficiency of the corresponding solar cells.
Cyclic voltammetry revealed that the reduction potential remains unaffected by the introduction of the n-butoxymethyl side chain or the degree of HT coupling (Table 2.2). Hence, the LUMO levels of all three polymers are almost equal. In contrast, the oxidation potential is influenced, but predominantly by the amount of regularity in the polymer chain. While the HOMO level of rr-P3BMT is only 30 meV lower than that of P3HT, the HOMO level of irr-P3BMT is lower by 170 meV. We note that, in solution, the optical band gap and electrochemical band gap are in fair agreement (compare Tables 2.1 and 2.2).
Table 2.2 Electrochemical data (vs. Fc/Fc+) in o-dichlorobenzene solution containing 0.1 M TBAPF6 (scan rate = 100
mV/s, concentration 2 × 10-3 M based on monomer units).
Polymer ox onset
E
(V) red onsetE
(V) cv gE
(eV) P3HT 0.020 -2.20 2.22 irr-P3BMT 0.193 -2.20 2.40 rr-P3BMT 0.050 -2.19 2.24Accordingly, the incorporation of an electron withdrawing n-butoxymethyl side chain has a small influence on the optical and electrochemical properties in solution. The reduction and, more importantly, the oxidation potential of P3HT and rr-P3BMT are nearly identical. The crucial difference between the two polymers is expressed in the solid state properties (Figure 2.3b).
Solar cells Bulk heterojunction solar cells were fabricated by spin coating a mixture of P3BMT with [60]PCBM onto an indium tin oxide (ITO) covered glass substrate covered by a 60 nm film of PEDOT:PSS (Bayer AG). After drying in air, 1 nm LiF and 100 nm Al were thermally evaporated as back electrode. Different P3BMT:PCBM weight ratios were used to investigate the photovoltaic performance (Table 2.3).
Table 2.3 Photovoltaic performance of irr-P3BMT and rr-P3BMT with [60]PCBM.
Polymer P3BMT:PCBM (w/w) Layer thickness (nm) Voc (V) Jsc(SR) (mA/cm2) FF (-) η (%) irr-P3BMT rr-P3BMT 1:1 1:1 1:2 2:1.2 120 114 140 91 0.64 0.70 0.66 0.71 1.78 2.65 1.64 4.16 0.27 0.52 0.31 0.57 0.31 0.96 0.34 1.68
For irr-P3BMT:PCBM (1:1, by wt.) active layers were spin coated from chlorobenzene using a polymer concentration of 20 mg/mL. Under 110 mW/cm2 white light illumination from a tungsten-halogen
lamp, the best cells give Voc = 0.64 V, Jsc = 1.19 mA/cm2, and a fill factor FF = 0.27 (Figure 2.4a).
Convolution of the spectral response (SR) (Figure 2.4b) with the AM1.5G spectrum afforded an estimate of Jsc(SR) = 1.78 mA/cm2 under standard solar light conditions (AM1.5G, 100 mW/cm2),
providing an estimated energy conversion efficiency of η = 0.31%. The Voc of the best cell did not show
a large increase compared to the generally obtained Voc ≈ 0.6 V for the state-of-the-art P3HT:[60]PCBM devices. We note that in contrast to our expectation, the increase in oxidation potential of irr-P3BMT is not reflected in an increase of Voc. However, devices spin coated at lower spin speeds give a Voc above
-1.0 -0.5 0.0 0.5 1.0 -4 -2 0 2 4 a C ur re nt d en si ty J [m A /c m 2 ] Voltage (V) irrP3BMT:PCBM 1:1 rrP3BMT:PCBM 1:2 rrP3BMT:PCBM 1:1 rrP3BMT:PCBM 2:1.2 400 450 500 550 600 650 700 0.0 0.1 0.2 0.3 0.4 0.5 b irrP3BMT:PCBM (1:1) rrP3BMT:PCBM (1:1) rrP3BMT:PCBM (2:1.2) E Q E (-) λ (nm)
Figure 2.4. (a) J-V curves of P3BMT:PCBM solar cells under 110 mW/cm2 white light illumination. (b)
Monochromatic EQE of P3BMT:PCBM solar cells.
The active layers based on rr-P3BMT:PCBM with a weight ratio of 2:1.2, spin coated from chlorobenzene with a polymer concentration of 20 mg/mL, show the best performance (Figure 2.4a, Table 2.3). These cells exhibit Voc = 0.71 V, Jsc = 2.97 mA/cm2 [estimated AM1.5G Jsc(SR) = 4.16 mA/cm2],
and FF = 0.57 under 110 mW/cm2 white light illumination, providing an estimated energy conversion
efficiency of η = 1.68%. The Voc of the best rr-P3BMT:PCBM solar cell is increased by approximately
100 mV compared to P3HT:PCBM cells. However, the relatively low current still limits the photovoltaic performance of P3BMT:PCBM films. The reduced 3D order is responsible for the absence of a red shift in the absorption spectrum in thin films and likely lowers the charge carrier mobility of P3BMT compared to P3HT. Hence less charge carriers are created and collected.
The spectrally resolved external quantum efficiency (EQE) (Figure 2.4b) extends roughly to 600-650 nm and shows a maximum around 450 nm for both irr-P3BMT and rr-P3BMT. However, for rr-P3BMT a broader plateau exists between 400 and 500 nm. A maximum of 25% in EQE for the irr-P3BMT is located at 430 nm. The EQE of the devices made from rr-irr-P3BMT:PCBM with a weight ratio of 2:1.2 reaches 45% in the 450-500 nm region. For a weight ratio of 1:1 the maximum EQE is limited to approximately 28%. The spectral response of the cells made from rr-P3BMT:PCBM in a ratio of 1:2 was not measured.
Morphology The morphology of the blend was studied using atomic force microscopy (AFM). Figure 2.5 shows the height image of the rr-P3BMT:PCBM (2:1.2, by wt.) blend. The AFM image reveals a rather uniform phase separation with relatively small features. The morphology proved to be virtually independent of spin speed. Where large phases may result into a loss of short circuit current due to ineffective exciton dissociation,5,19,20 this seems not to be a limiting factor here.
2x2 µm 0 15 2x2 µm 0 15
Figure 2.5. AFM height image of a rr-P3BMT:[60]PCBM (2:1.2, by wt.) thin film.
2.3
Conclusions
Applying the GRIM method to 2-bromo-5-iodo[3-(n-butoxymethyl)thiophene] leads to highly regioregular P3BMT. In solution, the differences in optical and electrochemical properties of P3HT and rr-P3BMT are very small. There is a minor increase in oxidation potential of P3BMT. However, the solid state properties of P3BMT are significantly different from P3HT. In contrast to P3HT, the absorption maximum of P3BMT does not exhibit a strong red shift going from solution to a thin film and the absence of vibronic fine structure in the film spectrum indicates a low degree of 3D ordering. Apparently, the presence of oxygen in the side chain, placed after the first methylene group, prohibits a close packing of the chains.
The solar cells show that the incorporation of an ether side chain increases the Voc from ~0.6 V
to ~0.7 V. Nevertheless, the overall device efficiency of rr-P3BMT:[60]PCBM (1.7%) cells is not improved with respect to the best P3HT:[60]PCBM (4-5%) solar cells, because the photocurrent is limited by lesser absorption and reduced charge carrier mobility.
2.4
Experimental
General Methods 1H NMR and 13C NMR spectra were recorded on a Varian Mercury 400 MHz NMR
spectrometer. Molecular weights were determined using size exclusion chromatography in HPLC-grade chloroform against polystyrene standards on a Shimadzu GPC consisting of a LC-10AD liquid chromatograph, a SPD-10AV UV-Vis detector (420 nm), and a SCL-10A system controller. A 300 × 7.5 mm ResiPore column was used, together with a pre-column. The flow rate was 1 mL/min and the injection volume 20 μl.
UV-Vis absorption spectra were measured with a Perkin-Elmer Lambda 900 spectrometer. Cyclic voltammetry (scan rate = 100 mV/s) was performed on an Autolab PGSTAT30 potentiostat in a three-electrode single-compartment cell using o-dichlorobenzene containing 0.1 M TBAPF6 (Fluka) as electrolyte. The working
electrode was platinum disk, the counter electrode a silver rod, and the reference electrode Ag/AgCl. Potentials are relative to Fc/Fc+.
Photovoltaic devices were made by spin coating EL-grade PEDOT:PSS onto pre-cleaned, patterned indium tin oxide (ITO) substrates (14 Ω per square). The photoactive layer was deposited by spin coating from the appropriate solvent. The counter electrode of LiF (1 nm) and aluminum (100 nm) was deposited by vacuum evaporation at 5 × 10-6 mbar. The active area of the cells was 0.17 cm2. Monochromatic spectral response was
measured on devices kept behind a quartz window in a nitrogen filled container with a Keithley 2400 source meter, using light from a tungsten halogen lamp, dispersed by an Oriel Cornerstone 130 monochromator. A calibrated Si cell was used as reference. J-V characteristics were measured under ~110 mW/cm2 white light from a
tungsten-halogen lamp filtered using Schott GG385 UV filter and a 300 nm ITO on glass near-IR filter, using a Keithley 2400 source meter. The Jsc obtained from the J-V curves is an overestimate due to the spectral mismatch
and relatively high intensity (110 mW/cm2) of the tungsten-halogen lamp compared to the solar spectrum. Short
circuit currents under standard solar illumination conditions (Jsc(SR)) were obtained from convolution of the
spectral response with the AM1.5G (100 mW/cm2) solar spectrum. The estimated power conversion efficiency was
determined by combining Jsc(SR) with Voc and FF from the J-V measurements. We have reported previously that,
for similar devices, this procedure affords estimated conversion efficiencies that are virtually identical to those obtained with a solar simulator and correcting for spectral mismatch.21
Tapping mode AFM was measured in a NanoScope Dimension 3100 microscope (Veeco, Digital Instruments) using PPP-NCHR probes (Nanosensors).
3-(n-Butoxymethyl)thiophene (1)
To a suspension of sodium hydride (60% in mineral oil) (350 mg, 8.75 mmol) in 20 mL THF at 0 °C, 3-thiophenemethanol (516 mg, 4.52 mmol) was added dropwise in 5 min. The mixture was stirred at 0 °C for 30 min, after which bromobutane (1.17 g, 8.57 mmol) was added dropwise. After stirring at reflux for 3 h, extra 1-bromobutane (0.38 g, 2.79 mmol) was added. After refluxing for 16 h, the mixture was quenched with demineralized water, extracted with CH2Cl2 (3 × 50 mL) and the organic phase dried with Na2SO4. The solvent
was removed under reduced pressure and purification by column chromatography on silica (CH2Cl2/heptane, 1:1)
yielded 470 mg of a colorless oil (61% yield). 1H-NMR (400 MHz, CDCl3): δ7.28 (dd, 1H, J = 4.7Hz, J = 3.2Hz), 7.20
(dd, 1H, J = 1.8Hz, J = 0.9Hz), 7.07 (dd, 1H, J = 4.9Hz, J = 1.2Hz), 4.50 (s, 2H), 3.46 (t, 2H, J = 6.6Hz), 1.64-1.53 (m, 2H), 1.45-1.32 (m, 2H), 0.91 (t, 3H, J = 7.2Hz). 13C-NMR (400 MHz, CDCl3): δ139.90, 127.30, 125.83, 122.46, 70.14,
68.11, 31.82, 19.37, 13.92.
2,5-Dibromo-3-(n-butoxymethyl)thiophene (2)
To a solution of 3-(n-butoxymethyl)thiophene (1) (2.49 g, 14.6 mmol) in distilled THF (50 mL), N-bromosuccinimide (NBS) (5.22 g, 29.3 mmol) was added and left to stir for 16 h at room temperature (in the dark). Subsequently, the reaction was quenched with demineralized water (50 mL) and the mixture extracted with CH2Cl2 (3 × 75 mL). The organic phase was (gently) washed with 0.1 M NaOH (2 × 75 mL), demineralized water
(s, 2H), 3.45 (t, 2H, J = 6.6Hz), 1.62-1.54 (m, 2H), 1.43-1.33 (m, 2H), 0.92 (t, 3H, J = 7.2Hz). 13C NMR (400 MHz,
CDCl3): δ 139.67, 130.81, 111.17, 109.64, 70.40, 66.59, 31.72, 19.31, 13.89.
Poly[3-(n-butoxymethyl)thiophene] (irr-P3BMT)
A solution of 2,5-dibromo-3-(n-butoxymethyl)thiophene (2) (0.5 g, 1.5 mmol) in distilled THF (15 mL) was prepared. After addition of t-BuMgCl in THF (1.35 mL, 1.30 M), the mixture was stirred at 40 °C for 2 h. Subsequently, another aliquot of t-BuMgCl (0.25 mL, 1.30 M) was added. Following an additional 90 min of stirring at 40 °C, Ni(dppp)Cl2 (2.9 mg, 6.8 × 10-3 mmol) was added to the reaction mixture. After stirring at room
temperature for 16 h, the mixture was heated to 50 °C during 1 h, whereafter the polymer was precipitated in methanol. The polymer was fractionated by Soxhlet extraction using methanol, hexane and chloroform. The chloroform fraction contained 123 mg of a red solid (52% yield). 1H NMR (400 MHz, CDCl3): δ 7.30-7.20 (m, 1H),
4.61-4.33 (m, 2H), 3.60-3.38 (m, 2H), 1.72-1.50 (m, 2H), 1.50-1.34 (m, 2H), 0.99-0.83 (m, 3H). 13C NMR (400 MHz,
CDCl3): δ 139.41, 139.33, 136.03, 128.98, 125.23, 70.17, 66.18, 31.56, 19.63, 19.10.GPC (CHCl3): Mn = 14 kg/mol, Mw
= 24 kg/mol.
2-Bromo-3-(n-butoxymethyl)thiophene (3)
To a solution of 3-(n-butoxymethyl)thiophene (1) (370 mg, 2.17 mmol) in 5 mL glacial acetic acid, N-bromosuccinimide (378 mg, 2.12 mmol) was added, while the mixture was stirred at room temperature in absence of light. After stirring for 16 h, demineralized water (10 mL) was added and the product was extracted with CH2Cl2 (3 × 50 mL). The organic phase was washed with 1 M NaOH (2 × 50 mL), demineralized water (2 × 50 mL)
and dried with Na2SO4. The solvent was removed under reduced pressure and purification by column
chromatography on silica (CH2Cl2/heptane, 1:4) afforded 470 mg of 11 as a colorless oil (87% yield). 1H-NMR (400
MHz, CDCl3): δ7.24 (d, 1H, J = 5.6Hz), 6.99 (d, 1H, J = 5.6Hz), 4.45 (s, 2H), 3.47 (t, 2H, J = 6.5Hz), 1.62-1.53 (m, 2H),
1.42-1.33 (m, 2H), 0.91 (t, 3H, J = 7.3Hz). 13C-NMR (400 MHz, CDCl3): δ138.58, 128.17, 125.82, 110.81, 70.21, 66.75,
31.74, 19.32, 13.88.GC-MS (Mw = 249.16): m/z = 248.05 [M+].
2-Bromo-3-(n-butoxymethyl)-5-iodothiophene (4)
To a solution of 2-bromo-3-(n-butoxymethyl)thiophene (3) (470 mg, 2.76 mmol) in anhydrous chloroform (10 mL), iodine (392 mg, 1.54 mmol) and iodobenzene-diacetate (540 mg, 1.68 mmol) were added. After stirring for 2 h at room temperature (in the dark), the mixture was poured out in an aqueous solution of sodium thiosulfate. The aqueous phase was extracted with diethyl ether (3 × 50 mL). The combined organic layers were dried with Na2SO4 and the excess solvent was removed under reduced pressure. Flash column chromatography
(CH2Cl2/heptane, 1:4) afforded 590 mg of 4 (72% yield). 1H NMR (400 MHz, CDCl3): δ 7.16 (s, 1H), 4.39 (s, 2H),
