Head-to-tail regioregularity of poly(3-hexylthiophene) in oxidative coupling polymerization with FeCl3

We investigated a head-to-tail regioregularity of poly(3-alkylthiophenes) from 3-alkylthiophene by an oxidative coupling polymerization, which is the simplest and easiest way for the synthesis of polythiophenes. The polymerizations were conducted using ferric chloride (III) as an oxidant in chloroform. Investigating the polymerization conditions, a lower temperature and a lower concentration were effective for increasing the head-to-tail (HT) content. The best HT content of 88% was obtained when the temperature was −45°C and the initial monomer concentration was 0.02 mol L⁻¹. Washing the resulting polymer by n-hexane further increased the content to 91%. Thus, it was found that the high regioselectivity can be achieved by the simple polymerization and the simple operation such as washing. The polymerization mechanism causing the regularity is also discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1943–1948, 1999     

Large open-circuit voltage polymer solar cells by poly(3-hexylthiophene) with multi-adducts fullerenes

Polymer solar cells (PSCs) made by poly(3-hexylthiophene) (P3HT) with multi-adducts fullerenes, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC61BM-bisadduct (bisPC61BM) and PC61BM-trisadduct (trisPC61BM), were reported. Electrochemistry studies indicated that PC61BM, bisPC61BM and trisPC61BM had step-up distributional lowest unoccupied molecular orbital (LUMO) energy. PSCs made by P3HT with above PC61BMs show a trend of enlarged open-circuit voltages, which is in good agreement with the energy difference between the LUMO of PC61BMs and the HOMO of P3HT. On the contrary, reduced short-circuit currents (Jsc) were observed. The investigation of photo responsibility, dynamics analysis based on photo-induced absorption of composite films, P3HT:PC61BMs and n-channel thin film field-effect transistors of PC61BMs suggested that the short polaron lifetimes and low carrier mobilities were response for reduced Jsc. All these results demonstrated that it was important to develop an electron acceptor which has both high carrier mobility, and good compatibility with the electron donor conjugated polymer for approaching high performance PSCs.     

Regioregularity and solar cell device performance of poly(3‐dodecylthienylenevinylene)

Influence of side chain regioregularity on photovoltaic performance has been investigated in bulk heterojunction solar cells based on a series of poly(3‐dodecylthienylenevinylene)s (C₁₂‐PTV) and 6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM). Performance of each C₁₂‐PTV is optimized for fair comparison. It is found that regiorandomness has no detrimental effect on device performance, in sharp contrast to poly(3‐hexylthiophene) (P3HT). Fully regioregular C₁₂‐PTV performs slightly poorer than less regioregular ones mainly due to its fast crystallization behavior. The results suggest that introduction of side chain regiorandomness is an effective strategy to enhance processability of certain types of polymers without a reduction in photovoltaic performance. The better polymer:PCBM weight ratio, found to be 3:7 for all C₁₂‐PTVs, and improved device performance, as compared with the literature work on the same polymer synthesized by a different method, demonstrate again the importance of the integrity of polymer main chain structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

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.     

Air-tolerant poly(3-hexylthiophene) synthesis via catalyst-transfer polymerization

The discovery of catalyst-transfer polymerization and its further developments have led to unprecedented control over the length and sequence of conjugated polymers. However, the methods themselves are technically challenging to perform due to the air- and moisture-sensitivities of the monomers and catalysts. Herein, we report a catalyst-transfer polymerization method that affords poly(3-hexylthiophene) in high yields without using an inert atmosphere. The synthesis capitalizes on a rapid Negishi cross-coupling using a moisture-tolerant organozinc monomer mediated by an air-stable Pd precatalyst. This simple method should make conjugated polymer synthesis more accessible to a broader range of researchers and may be generalizable to other monomer scaffolds.     

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.     

Semicrystalline morphology in thin films of poly(3-hexylthiophene)

The thermodynamic phase behavior and the morphology in thin films of poly(3-hexylthiophene) (P3HT) has been studied using calorimetry, X-ray scattering, and scanning force microscopy (AFM). Around 225 °C a phase transition from the crystalline state to a layered, liquid crystalline structure occurs in regioregular P3HT, while the regiorandom counterpart material is disordered at all temperatures and displays a glass transition temperature T_g–3 °C. Regioregular P3HT is semicrystalline and forms needle or plate like crystallites which in solution cast thin films are oriented with respect to the substrate. Films produced by spin coating display a non-equilibrium structure with reduced order and orientation. Annealing of these films in the liquid crystalline state leads to the formation of a morphology similar to the one observed in solution cast films.

Surface-induced conformational changes in poly(3-hexylthiophene) monolayer films

With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system).     

Uniaxial alignment of poly(3-hexylthiophene) nanofibers by zone-casting approach

The preparation of large area coverage of films with uniaxially aligned poly(3-hexylthiophene)(P3HT) nanofibers by using zone-casting approach is reported.The length and the orientation of the nanofibers are defined by the solubility of the solvent,the P3HT molecular weight and the substrate temperature.The length of the oriented nanofibers could be increased from 1 μm to more than 10 μm by adding poor solvent into the P3HT solution.It is found that for P3HT of relatively low molecular weight,a solvent with relatively low solubility has to be chosen to get the oriented film.While for the high molecular weight P3HT,the solvent with a relatively high solubility has to be used.The well-aligned film could be obtained because of the solute concentration gradient in the region where the critical concentration is reached during the zone-casting process.Particularly,the solvent evaporation rate and crystallization rate must be chosen properly to satisfy the stationary conditions above,which were controlled by an appropriate choice of solvent and substrate temperature.The film prepared by zone-casting approach had microcrystalline P3HT domains with more inter-chain order than spin-coating film.Meanwhile,the P3HT π-π stacking direction was parallel to the alignment direction of the nanofibers.     

Influence of the microstructure on the photooxidative degradation of poly(3-hexylthiophene)

The influence of the microstructure of poly(3-hexylthiophene) (P3HT) on its photodegradation upon exposure to UV-visible light was studied. High-regioregular (>98%) and low-regioregular (<95%) P3HTs with different molecular weights, dispersities and purities were submitted to irradiation under accelerated artificial ageing conditions. While the molecular weight had no effect on the photodegradation rate, a linear relationship was observed between the photodegradation rate and the regioregularity. The higher photo-instability of low-regioregular P3HTs, which have low crystalline phase content, could be due to the fact that the radical chain oxidation occurs predominantly in the amorphous phase of the polymer. Low-regioregular P3HTs also have a higher number of impurities, especially Fe residues, which are well known for their photocatalytic effect. The results also show that, at low concentrations, oxidation products are able to quench the singlet state of P3HT by acting as electron traps, and that the shortest wavelengths of solar light are the most harmful. Finally, the photooxidation of P3HT leads to the formation of low-molecular-weight carboxylic acids that can easily diffuse and migrate out of low-regioregular P3HT thin films. These results are therefore relevant to the preparation, storage and lifetimes of P3HT-based organic solar cells.     

Structure of friction-transferred highly oriented poly(3-hexylthiophene) thin films

Oriented thin films of P3HT were obtained by a friction-transfer technique. The morphology and structure of the film were studied by means of optical microscopy, atomic force microscopy and transmission electron microscopy. Optical microscopy observation indicates that large size well-ordered P3HT thin films can be produced by a friction-transfer technique. Highly ordered lamellae were observed in P3HT friction-transferred films by electron microscopy. Electron diffraction results confirm the existence of high orientation with the a- and c-axes of P3HT crystals aligned in the film plane while the c-axis parallel to the friction-transfer direction. The atomic force microscopy observation of the as-prepared P3HT thin film shows, however, a featureless top surface morphology, indicating the structure inhomogeneity of the obtained film. To get highly oriented P3HT thin films with homogenous structure, high temperature annealing, solvent vapor annealing and self-seeding recrystallization of the friction-transferred film were performed. It is confirmed that solvent vapor annealing and self-seeding recrystallization methods are efficient in improving the surface morphology and structure of the frictiontransferred P3HT thin film. Highly oriented P3HT films with unique structure can be obtained through friction-transfer with subsequent solvent vapor annealing and self-seeding recrystallization.     

Side chain engineering of quinoxaline-based small molecular nonfullerene acceptors for high-performance poly(3-hexylthiophene)-based organic solar cells

Poly(3-hexylthiophene)(P3HT)is one of the most used semiconducting polymers for organic photovoltaics because it has potential for commercialization due to its easy synthesis and stability.Although the rapid development of the small molecular non-fullerene acceptors(NFAs)have largely improved the power conversion efficiency(PCE)of organic solar cells(OSCs)based on other complicated p-type polymers,the PCE of P3HT-based OSCs is still low.In addition,the design principle and structure-properties correlation for the NFAs matching well with P3HTare still unclear and need to be investigated in depth.Here we designed a series of NFAs comprised of acceptor(A)and donor(D)units with an A2-A1-D-A1-A2 configuration.These NFAs are abbreviated as Qx3,Qx3 b and Qx3c,where indaceno[1,2-b:5,6-b′]dithiophene(IDT),quinoxaline(Qx)and 2-(1,1-dicyanomethylene)rhodanine serve as the middle D,bridged A1 and the end group A2,respectively.By subtracting the phenyl side groups appended on both IDT and Qx skeletons,the absorption spectra,energy levels and crystallinity could be regularly modulated.When paired with P3 HT,three NFAs show totally different photovoltaic performance with PCEs of 3.37%(Qx3),6.37%(Qx3b)and 0.03%(Qx3 c),respectively.From Qx3 to Qx3b,the removing of phenyl side chain in the middle IDT unit results in the increase of crystallinity and electron mobility.However,after subtracting all the grafted phenyl side groups on both IDT and Qx units,the final molecule Qx3 c exhibits the lowest PCE of only 0.03%,which is mainly attributed to the serious phase-separation of the blend film.These results demonstrate that optimizing the substituted position of phenyl side groups for A2-A1-D-A1-A2 type NFAs is vital to regulate the optoelectronic property of molecule and morphological property of active layer for high performance P3HT-based OSCs.     

Solvent evaporation induced liquid crystalline phase in poly(3-hexylthiophene)

We report on the evolution of the chain orientation of a representative π-conjugated polymer, poly(3-hexylthiophene) (P3HT), during the solution-casting process, as monitored using polarized Raman spectroscopy. These measurements point to the formation of a liquid-crystalline phase of P3HT solutions within a specific time period during solvent evaporation, which leads to a conducting channel. These conclusions are based on the angular dependence of polarized Raman scattering peaks, the anisotropy in the fluorescence background signal, analysis of the scattering-peak shape, and direct observations of the three-phase contact line in an optical microscope under crossed polarizers. These results shed new light on the evolution of chain alignment and thus materials nanostructure, specifically in solution-processed P3HT and more generally in π-conjugated systems. They may further enable the design of improved materials and processes for this important class of polymers.     

Time-temperature superposition for viscoelastic properties of regioregular poly(3-hexylthiophene) films

Shear moduli were determined for chemically polymerized and solvent cast regioregular poly(3-hexylthiophene) films, using thickness shear mode acoustic wave resonators. The results are strikingly different to those for electropolymerized regiorandom poly(3-hexylthiophene) films. The time scale of the measurement was varied directly by use of higher harmonics of the acoustic wave resonator and indirectly via temperature. The significant variations in shear modulus with effective time scale can be "normalized"onto a stress master relaxation curve by using the concept of time-temperature superposition; this is the first time this has been demonstrated for electroactive films. The shift factors required to effect this normalization do not follow the classical Williams-Landel-Ferry (WLF) equation developed for long-range backbone motions of bulk polymers. Instead, they follow an Arrhenius-like behavior, commonly used to describe secondary motions of polymer side-chains. The activation enthalpy associated with this is independent of applied potential, is the same as for as cast (undoped) films, and is similar to that for rotation about a carbon-carbon single bond. These all point to the hexyl side-chains as the origins of the observed phenomena, consistent with the "melting point" separating two temperature-dependent phases and with the different molecular packing arrangements that would necessarily apply to regioregular and regiorandom materials.     

Improved efficiency of photovoltaics based on CdSe nanorods and poly(3-hexylthiophene) nanofibers

We present photovoltaic devices based on a blend of the conjugated polymer poly(3-hexylthiophene) (P3HT) with cadmium selenide nanorods, where the solvent for film deposition has been carefully chosen to optimize the film morphology. Using 1,2,4-trichlorobenzene (TCB), which has a high boiling point, as solvent for P3HT it is possible to obtain a fibrilar morphology, providing extended pathways for hole transport. Blend devices fabricated using this solvent gave solar power conversion efficiencies of 2.6%. This indicates that efficient transport of electrons and holes is achieved in these films, allowing them to operate effectively at solar illumination intensities.     

电场极化调控聚(3-己基噻吩)薄膜润湿性

本文首次采用电场极化技术精确控制共轭聚合物P₃HT薄膜表面的润湿性,通过调节极化条件,成功实现了对P₃HT薄膜表面润湿性的精确控制,薄膜表面水接触角可以实现从疏水性到亲水性的转变.通过光谱学、形貌学及接触角等表征手段,详细研究了电场极化作用下共轭聚合物分子取向聚集形态及作用机理.该工作不但扩展了共轭聚合物薄膜材料的应用范围,也为分子形态学的研究奠定了基础.     

Impact of the anion on electrochemically doped regioregular and regiorandom poly(3-hexylthiophene)

Chemical and electrochemical doping of π-conjugated polymers is an important aspect in determining the performance and enabling the operation of many organic electronic devices, from organic light emitting diodes and thermoelectrics to organic electrochemical transistors. In both chemical doping and electrochemical doping an ionized dopant or counterion is present along with the doped π-conjugated polymer. This dopant or counterion is not a benign spectator, rather, its presence can significantly impact the optical, electronic, and thermoelectric properties of the resulting material. Here, we investigate how counterion structure impacts the electrochemical doping ability, oxidation potential, ionization energy, and polaron absorbance of regioregular (rr) and regiorandom (rra) P3HT. We find that in most cases the anion has a small effect on the polymer oxidation potential, except for in the case of rr-P3HT with the large tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion. We propose that this large anion is excluded from the crystalline regions and thus the oxidation potential is similar to that of rra-P3HT. The anions also result in significant differences in polaron absorbance and ionization energies, thereby emphasizing the important role of the counterion in determining the optical and electronic properties of doped π-conjugated polymers.     

Synthesis,electrolyte properties and solar cell performance of hyperbranched poly(aryl-ether-urea)s

Hyperbranched poly(aryl-ether-urea)s with phenyl, N,N-dimethylamino ethyl and polyethylene oxide end-groups linked through urethane group – HBPEU-1, HBPEU-2 and HBPEU-3 respectively – were synthesized from an AB₂-type blocked isocyanate monomer and characterized by FT-IR, ¹H-NMR, SEC-MALLS, TGA and DSC techniques. The molecular weight of the polymers were found to be ranged from 4.9 × 10³ ? 1.96 × 10⁴ g/mol. The TGA results showed that the polymers decompose between 175°C – 220°C. In the DSC curves, HBPEU-1 and HBPEU-3 showed T_g at 160°C and 53°C respectively, whereas HBPEU-2 did not showed clear T_g. All the three polymers were converted into polymer electrolytes by doping with LiI/I₂. The doped polymers showed remarkably high ionic conductivity, up to 222 – 277 times compared to the un-doped polymers and the highest conductivity was observed with doped HBPEU-2. The TiO₂ based dye-sensitized solar cells (DSSCs) were fabricated using the doped polymer electrolytes and their performance was tested; HBPEU-2 showed good performance by yielding energy conversion efficiency (η) of 4.5%.