Synthesis and properties of polythiophenes with conjugated side‐chains containing carbon–carbon double and triple bonds

Two soluble side‐chain conjugated polythiophenes, poly{3‐[2‐(4‐octyloxy‐phenyl)‐vinyl]‐thiophene} (P3OPVT) and poly{3‐(4‐octyloxy‐phenylethynyl)‐thiophene} (P3OPET) have been synthesized successfully. In P3OPVT and P3OPET, substituted benzene rings are connected with the polythiophene backbone through trans carbon–carbon double bond and carbon–carbon triple bond, respectively. Absorption spectra of the P3OPVT and P3OPET both show two absorption peaks located in UV and visible region, respectively. The results of optical and electrochemical measurements indicate that the conjugated side‐chains can reduce the bandgap effectively. This type of side‐chain conjugated polythiophenes may be promising for the applications in polymer photovoltaic cells and field effect transistors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2206–2214, 2006     

HREELS to identify electronic structures of organic thin films

The electronic structure of alpha-oligothiophene (alphanT) thin films has been investigated for increasing chain lengths of n= 4-8 thiophene units with high resolution electron energy loss spectroscopy (HREELS) in the specular reflection geometry at a primary energy of 15 eV. The great advantage of this technique in contrast to UV/VIS absorption spectroscopy results from the fact, that the impact scattering mechanism of HREELS makes it possible to also detect optically forbidden electronic transitions. On the other hand, the electrons used as probes in HREELS have a wavelength which is two orders of magnitudes smaller if compared to those of photons used in UV/VIS absorption spectroscopy. Therefore individual molecules are excited by HREELS independent from each other and hence the excitation of collective excitons is not possible. As a result, information about the orientation of the molecules cannot be achieved with HREELS, which, however, is possible in polarization-dependent UV/VIS spectroscopy.     

Side chain engineering and conjugation enhancement of benzodithiophene and phenanthrenequnioxaline based conjugated polymers for photovoltaic devices

A series of donor‐acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy chains substituted phenanthrenequnioxaline unit in the backbone, serious steric hindrance resulted in quite low molecular weight. The implementation of thiophene ring spacer in polymer P2 greatly suppressed the interannular twisting to extend the effective conjugation length and consequently gave rise to improved absorption property and device performance. In addition, utilizing the alkylthienyl side chains to replace the alkyl side chains at BDT unit in polymer P3 further enhanced the photovoltaic performance due to the increased conjugation length. For polymer P4, translating the alkoxy side chains at the phenanthrenequnioxaline ring into the alkyl side chains at thiophene linker group enhanced molecular planarity and strengthened π?π stacking. Consequently improved absorption property and increased hole mobility were achieved for polymer P4. Our results indicated that side chain engineering not only can influence the solubility of polymer but also can determine the polymer backbone planarity and hence the photovoltaic properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1915–1926     

Structural hierarchy in melt-processed poly(3-hexyl thiophene)-montmorillonite clay nanocomposites: novel physical, mechanical, optical, and conductivity properties

Poly(3-hexyl thiophene) (P3HT) organically modified montmorillonite (om-MMT) polymer nanocomposites (PNCs) are prepared in the melt-cooled state. Hierarchical structures up to third order, namely, side chain mesomorph formation followed by the interchain lamellar structure of P3HT and finally its intercalation within the clay tactoids are observed. The structures are supported by transmission electron microscopy (TEM) and wide-angle X-ray scattering (WAXS) experiments. The TGA curves show two-stage degradation corresponding to those of the side chain and main chain of P3HT, and both temperatures decrease with an increase in clay concentration in the PNCs. The melting points of PNCs have increased by 2-3 degrees C higher than that of P3HT. The glass-transition temperature (Tg) and beta-transition temperature (Tbeta), measured by DMA, increase with an increase in clay concentration. The storage modulus (G) of PNCs has also increased more dramatically than that of P3HT. The UV-vis spectra of the PNCs show a blue shift in the pi-pi* absorption peak of the conjugated chain, but the photoluminescence spectra showed a red shift with an increase in the clay concentration. The quantum yield of the photoluminescence process also increases in the melt-cooled PNCs, and this is in sharp contrast to that of solvent cast PNCs where photoluminescence quenching was observed. Fibrillar network structure of the solvent cast PNCs promotes energy transfer of the charge carriers, but its absence in the melt-cooled films inhibits such energy transfer, increasing the quantum yield. The room-temperature dc conductivity of the PNCs decreased by an order compared to that of P3HT in both the doped and undoped states. The I-V characteristic curve shows semiconducting behavior, and it slowly transforms into insulator with increasing clay concentration.     

Theoretical and experimental study of low band gap polymers for organic solar cells

A combined theoretical and experimental investigation of the electronic structure and optical properties of poly(3-hexylthiophene) (P3HT), poly[3-(4-octylphenyl)thiophene] (POPT) and poly[3-(4-octylphenoxy)thiophene] (POPOT) is reported. In comparison with P3HT, POPT and POPOT exhibit better stabilities and the presence of an oxygen atom and/or a phenyl ring in the side chains enhances conjugation. Quantum chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT to the new derivative POPOT. The knowledge of the structure of these polymers is of utmost importance in understanding their optical properties in different phases (solution and condensed phase). The calculations indicate that, in opposition to P3HT and POPT polymers where the introduction of alkyl chains and the pendant phenyl disturbs the planarity of the backbone of the conjugated segment, POPOT has a better degree of organization in both states: the conjugated chain remains planar even in the presence of the phenoxy groups. Finally, the exciton binding energy is evaluated for these polymers and allows us to conclude that the POPOT is a promising polymer for photovoltaic applications when compared to P3HT and POPT.     

Photoactive Polythiophene:Titania Hybrids with Excellent Miscibility for Use in Polymer Photovoltaic Cells

A series of poly{(3‐hexylthiophene)‐co‐[3‐(6‐hydroxyhexyl)thiophene]}:titania (P3HT‐OH:TiO₂) hybrids were synthesized via the in situ polycondensation of titanium (IV) n‐butoxide in the presence of P3HT‐OH. Introducing a hydroxyl moiety onto the side‐chain of poly(3‐hexylthiophene) (P3HT) significantly promotes the polymer‐titania interaction, resulting in the formation of homogeneous hybrid colloids. The UV‐vis spectra of P3HT‐OH:TiO₂ films demonstrate that TiO₂ markedly affects the stacking structure and the chain conformation of P3HT‐OH. The maximum absorption wavelength of these hybrid materials can be tailor‐made by merely varying the weight percentage of TiO₂. Moreover, P3HT‐OH:TiO₂ can be further utilized as an efficient compatibilizer in preparing photoactive P3HT:P3HT‐OH:TiO₂ films with excellent miscibility. The photovoltaic cell based on such a hybrid exhibited a 2.4‐fold higher value of power‐conversion efficiency compared to the cell based on P3HT:TiO₂.

     

Ozonization of electronic conducting polymers: II. Degradation or doping

The ozonization of poly[3-pentylthiophene], poly[3-heptylthiophene] and poly[3-nonylthiophene] was carried out in room temperature. The DC conductivity changes during the ozonization have been recorded, revealing significant conductivity increase (up to two orders of magnitude). Moreover, an analysis of electron microscopy images has shown that ozone related swelling of polymer bundles caused the granular structure of the samples to fade away. The analysis of FTIR spectra indicated the presence of physisorbed ozone as well as oxidative degradation products in the ozonized polymer samples.     

Optoelectronic Properties of Dicyanofluorene‐Based n‐Type Polymers

Three new donor–acceptor‐type copolymers ( P1 , P2 , P3 ) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV‐visible absorption and fluorescence spectroscopy, cyclic voltammetry, space‐charge‐limited current (SCLC), flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV‐visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n‐type nature of the materials. By using FP‐TRMC, the intrapolymer charge‐carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron‐accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3‐hexylthiophene) (P3HT (donor)), as evident from the FP‐TRMC analysis. The P3 polymer exhibited the highest FP‐TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n‐type materials in all‐polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron‐transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.     

一维取代聚噻吩的电子性能

报道了3种取代聚噻吩,3-己基聚噻吩(P3HT)、3,4-二戊基聚噻吩(P34PT)、3-辛氧基聚噻吩(P3OOT)的合成方法1、H-NMR测试结果及UV-Vis吸收光谱和荧光光谱分析结果。用密度泛函方法计算了无取代噻吩、3-乙基噻吩、3,4-二乙基噻吩、3-乙氧基噻吩二聚体的电子性能。随聚合度的提高,聚合物能隙变窄。无取代噻吩二聚体的能隙为4.216 eV,重复单元长度为0.392 7 nm;乙基取代噻吩二聚体的能隙为4.733 eV,重复单元长度为0.393 9 nm;乙氧基取代噻吩二聚体的能隙为3.890 eV,重复单元长度为0.390 8 nm;双乙基取代噻吩二聚体的能隙为5.168 eV,重复单元长度为0.392 5 nm。理论变化规律与实验结果基本一致。     

Interfacial properties of free-standing poly(3-hexylthiophene) films

Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of different parts of the P3HT chain and the surface tensions of the films were calculated in a temperature range of 540 K-600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexyl groups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilt toward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excess compared to the methylene groups or thiophene rings. The surface tension of P3HT in its melt state shows similar temperature dependence to that of polymers that have long alkyl side chains. The surface tension values are comparable to those polymers that expose methyl or methylene groups on the surface. The surface tension values determined for the melt state are lower than the experimental reported values for crystalline P3HT films, as expected.     

Durability of a starch-based biodegradable polymer

The photo-oxidation and the photo-stabilization of a commercial biodegradable polymer have been investigated in order to establish the possibility of using this polymer as raw material for films for agriculture. The degradation has been followed by measuring the mechanical properties as a function of photo-oxidation time and in particular by following the elongation at break. The virgin polymer, made from maize starch and a synthetic biodegradable polyester, shows poor resistance to the UV irradiation as observed by the fast decay of the elongation at break, but the presence of small amounts of conventional UV stabilizers strongly improves the durability of this polymer. The UV stabilizers remarkably extend the induction time without modifying the photo-oxidation kinetics.Among the investigated stabilizers, the benzophenone compound seems to work slightly better than the benzotriazoles, than the triazine and than the sterically hindered amine. This behaviour has been attributed to the larger absorbance in the UV range of the biodegradable polymer.     

HREELS to identify electronic structures of organic thin films

The electronic structure of -oligothiophene (nT) thin films has been investigated for increasing chain lengths of n=4–8 thiophene units with high resolution electron energy loss spectroscopy (HREELS) in the specular reflection geometry at a primary energy of 15 eV. The great advantage of this technique in contrast to UV/VIS absorption spectroscopy results from the fact, that the impact scattering mechanism of HREELS makes it possible to also detect optically forbidden electronic transitions. On the other hand, the electrons used as probes in HREELS have a wavelength which is two orders of magnitudes smaller if compared to those of photons used in UV/VIS absorption spectroscopy. Therefore individual molecules are excited by HREELS independent from each other and hence the excitation of collective excitons is not possible. As a result, information about the orientation of the molecules cannot be achieved with HREELS, which, however, is possible in polarization-dependent UV/VIS spectroscopy.     

Design and synthesis of pyromellitic diimide‐based donor–acceptor conjugated polymers for photovoltaic application

Three of conjugated polymers based on pyromellitic diimide (PMDI) as the acceptor unit and thienothiophene (TT) as the donor unit were successfully synthesized by Stille coupling. The effect of the side chain length and thiophene π‐bridge on the polymers' optical and electrochemical properties was investigated. Electrochemical characterization indicated that these polymers have deep highest occupied molecular orbital energy levels between ?5.7 and ?5.8 eV. Polymer solar cells were fabricated by using these PMDI‐based polymers as the donor and [6,6]‐phenyl‐C61‐butyric acid methyl ester as the acceptor. The polymer P1 whose PMDI unit was functionalized with 2‐ethylhexyl side chain shows the higher short‐circuit current (J_sc) and fill factor (FF) compared with that of P2 with a 2‐octyldodecyl side chain on the PMDI unit. The results also illustrate that the insertion of a thiophene π‐bridge between PMDI and TT (the polymer P3) leads to the broader absorption and better photovoltaic performance. The best performance was obtained from the cell based on the polymer P3 with a power conversion efficiency of 0.43% under the illumination of AM 1.5 G, 100 mW/cm². Copyright © 2014 John Wiley & Sons, Ltd.     

Poly(3‐hexylthiophene) aggregate formation in binary solvent mixtures: An excitonic coupling analysis

The aggregation behavior of P3HT [M_n ≈ 28.2 kDa, regioregularity >96%, PDI ≈ 1.3] in 96 solvent mixtures is studied using UV–vis absorption spectroscopy. Hansen solubility parameters and Spano excitonic coupling analyses are used to identify correlations between the properties of the solvent mixtures and the extent of structural order of the aggregates. It is clear that the identity of the poor solvent used to drive aggregation has a significant impact on the excitonic coupling behavior and, hence, the structural order of the P3HT aggregates. However, solubility parameter theory does not account nor provide a predictive theory for the observed trends. Instead, qualitative arguments based on the nature of the interactions between the solvents and the polythiophene and hexyl side chain motifs are used to rationalize the kinetics of formation and the observed excitonic coupling characteristics of the P3HT aggregates. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 526–538     

Transport properties of CdS nanowire embedded poly(3-hexyl thiophene) nanocomposite

Electrical transport properties of CdS nanowire embedded regioregular poly(3-hexyl thiophene) (P3HT) nanocomposite are investigated and the results are compared with those of the pure P3HT polymer. Both dc and ac conductivities of P3HT and CdS-P3HT nanocomposites show semiconducting nature and conductivity of CdS-P3HT is much less than that of the pure P3HT conducting polymer. The difference in the electrical behavior is attributed to the formation of nanowires in the CdS-P3HT nanocomposite. The dc conductivity data at high and low temperatures are qualitatively consistent with the optical-photon-assisted hopping and variable-range hopping models, respectively. However, the multiphonon-assisted hopping models of small polarons in nonadiabatic regime, which consider strong interactions with electrons with the optical as well as acoustical phonons, are the best to interpret quantitatively the experimental data in the entire temperature range for both P3HT and CdS-P3HT nanocomposites. We observe that the quantum tunneling model of small polarons is consistent with almost every respect of the loss data of P3HT conducting polymer, while for CdS-P3HT nanocomposite the ac conductivity varies with temperature in a different manner, which is explained in terms of simple quantum tunneling model of electrons in contrast to that of pure P3HT conducting polymer.     

Photo-oxidative degradation of bisphenol A polycarbonate and its possible initiation processes

During UV degradation of bisphenol A polycarbonate (BPA-PC), photo-Fries rearrangements and photo-oxidation reactions take place, however, in outdoor exposure conditions the photo-oxidation reaction is the most dominant one. To initiate this autocatalytic oxidation process, an initiating radical is required. In this research two possible sources for photo-initiation are explored, viz. hydroperoxides and peroxides formed by thermo-oxidation and charge transfer complexes between the polymer and oxygen. A comparison of the photodegradation rate of thermo-oxidized and undegraded polycarbonate samples was made. It was shown that the formed thermo-oxidation products in BPA-PC did not affect the photo-oxidation rate. Charge transfer complexes (CTCs) between oxygen and polycarbonate were studied by UV absorption spectroscopy at different air pressures. The concentration of CTCs increased with oxygen pressures. These CTCs absorb wavelengths in the region of terrestrial sunlight and could cause the initiation of the photo-oxidation. The influence of oxygen pressure on the photodegradation of BPA-PC was studied by irradiating the samples with a by the polymer absorbing wavelength, viz. 250 nm. The absorption of this wavelength leads to photo-Fries products. It was shown that at higher oxygen pressures the rate of the photo-Fries reaction is reduced, which was explained by quenching of the photo-Fries reaction by oxygen.     

Polythiophene derivative with the simplest conjugated-side-chain of alkenyl: synthesis and applications in polymer solar cells and field-effect transistors

A polythiophene derivative with the simplest conjugated side chain, poly(3-hexy-1-enylthiophene) (P3HET), was synthesized by Stille self-coupling reaction. A comparative study of the newly synthesized polymer with poly(3-hexylthiophene) (P3HT), one of the most widely investigated optoelectronic materials, is presented. The effect of double bond (C=C) on the side chain toward thermal stability and optical and electronic properties was fully characterized by TGA, UV-vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The hole mobility of P3HET determined by the space-charge-limited current (SCLC) model is 6.7 x 10(-3) cm(2)/V s, which is comparable to P3HT with similar molecular weight and regularity and 1 order of magnitude higher than most conjugated-side-chain polythiophene derivatives. Polymer solar cells (PSCs) and field effect transistors (FETs) were fabricated respectively to exploit its potential applications in optoelectronic devices.     

Crystallization mechanism of regioregular poly(3‐alkyl thiophene)s

The crystallization properties of three regioregular poly(3‐alkyl thiophene)s (P3ATs) are studied: poly(3‐hexyl thiophene) (P3HT), poly(3‐octyl thiophene) (P3OT), and poly(3‐dodecyl thiophene) (P3DDT). The morphology of the isothermally crystallized samples is a whisker type. The values of the enthalpy of fusion of ideal crystals (ΔH), determined from the melting point depression in the polymer–diluent system, are 99, 73.6, and 52 J/g for P3HT, P3OT, and P3DDT, respectively. The values of the equilibrium melting point (T), determined from the Hoffman–Weeks extrapolation procedure, are 300, 230, and 180 °C for P3HT, P3OT, and P3DDT, respectively. From the linear extrapolation of the P3AT data, the T and ΔH values of unsubstituted polythiophene are predicted to be 400 °C and 139 J/g, respectively. The crystallization kinetics of these polymers are studied with differential scanning calorimetry, and the Avrami exponents vary between 0.6 and 1.4, indicating one‐dimensional heterogeneous nucleation with linear growth. As the P3AT whiskers are produced from the chain‐folding process, the Lauritzen–Hoffman growth rate theory is applied to analyze the temperature coefficient of the crystallization rate data. Graphical plots indicate a transition from regime I to regime II during isothermal crystallization for all the P3ATs studied. The fold surface energy and the work of chain folding calculated from the slopes of the graphical plots decrease with an increase in the number of carbon atoms of the side chain. The primary crystallization process of the side‐chain crystallization is very fast and is attributed to the zipping effect of the main‐chain crystals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2073–2085, 2002     

The mechanism of photo- and thermooxidation of poly(3-hexylthiophene) (P3HT) reconsidered

Poly(3-hexylthiophene) (P3HT) has been the focus of great interest as it is widely used in organic solar cells. However, P3HT has relatively poor photochemical and thermal stability under ambient atmosphere, which leads to a reduced lifetime of the solar cells. It was therefore necessary to study the photo- and thermooxidation of P3HT. Thin P3HT films were exposed to UV-visible light irradiation and thermal ageing, both in the presence of air. Changes in the infrared spectra of the aged samples were recorded, and the oxidation products were identified. A degradation mechanism that accounted for the modifications in the infrared spectra was then developed. This mechanism confirmed that singlet oxygen plays no decisive role, as previously reported. Oxidation was shown to involve the radical oxidation of the n-hexyl side-chains and the subsequent degradation of the thiophene rings. The breaking of the macromolecular backbone resulted in a loss of π-conjugation, provoking the bleaching of the sample.     

Thiophene π bridge effect on bulky side‐chained benzodithiophene‐based photovoltaic polymers

Recently, we have used terthiophene side chain to modify benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to form novel building block for BDT polymers. In this paper, this building block is used to copolymerized with thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) and thieno[3,4‐b]thiophene (TT). This building block and TPD‐ or TT‐based polymers (P1 and P3) show high open circuit voltage (V_OC) (ca. 0.9–0.95 V) and low energy loss (E_g–eV_OC) in solar cells devices compared with similar polymers without bulky side chain. We further introduce thiophene π bridge into these polymers backbone to form two other polymers (P2 and P4). We find this thiophene π bridge does contribute to this bulky side chained benzodithiophene polymer photovoltaic performances, especially for power conversion efficiencies (PCEs). The polymer solar cells (PSCs) performances are moderate in this article due to the serious aggregation in the PSCs active layer. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1615–1622