Alternating oligo(p-phenylene vinylene)--perylene bisimide copolymers: synthesis,photophysics, and photovoltaic properties of a new class of donor--acceptor materials

A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores. The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units. Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical calculations and in terms of conformational preferences and folding of the two polymers. In solution, the lifetime of the charge-separated state is longer than in the films where geminate recombination is much faster. However, in the films some charges are able to escape from geminate recombination and diffuse away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.     

Oligo(phenylene vinylene)-poly(methylstyrene) hybrids: controlled step-wise molecular wiring of oligo(phenylene vinylene)

Well defined oligo(phenylene vinylene) grafted polymers known as oligo(phenylene vinylene)-poly(methylstyrene) hybrids have been developed using a step-wise synthetic protocol, where the length of the OPV can be controlled systematically to achieve specific optoelectronic properties. The process allows the structural modification of attached OPV at a molecular level either by varying the chain length or by changing functionalities. The step-wise generation of OPV chains on the backbone of a highly soluble polymer ensures solubilization in a variety of solvents and also the exhibition of interesting optical properties.     

New selenophene‐based low‐band gap conjugated polymers for organic photovoltaics

Low‐band gap selenophene‐based polymers were synthesized. Their optoelectronic and photovoltaic properties and space‐charge limited currents were compared with those of the related thiophene‐based polymers. The band gaps of the Se‐based derivatives were approximately 0.05–0.12 eV lower than those of their thiophene counterparts. Organic photovoltaic (OPV) devices based on the blends of these polymers and 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐C₇₁ (PC₇₁BM) were fabricated, and the maximum power conversion efficiency of the OPV device based on PSPSBT and PC₇₁BM was 3.1%—with a short‐circuit current density (J_sc) of 9.3 mA cm^?2, an open‐circuit voltage (V_oc) of 0.79 V, and a fill factor of 0.42—under AM 1.5 G illumination (100 mW cm^?2). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4550–4557     

Fullerene Derivatives Functionalized with Diethylamino‐Substituted Conjugated Oligomers: Synthesis and Photoinduced Electron Transfer

Diethylamino‐substituted oligophenylenevinylene (OPV) building blocks have been prepared and used for the synthesis of two [60]fullerene–OPV dyads, F‐D1 and F‐D2 , which exhibit different conjugation length of the OPV fragments. The electrochemical properties of these acceptor–donor dyads have been studied by cyclic voltammetry. The first reduction is always assigned to the fullerene moiety and the first oxidation centered on the diethylaniline groups of the OPV rods, thus making these systems suitable candidates for photoinduced electron transfer. Both the OPV and the fullerene‐centered fluorescence bands are quenched in toluene and benzonitrile, which suggests the occurrence of photoinduced electron transfer from the amino‐substituted OPVs to the carbon sphere in the dyads in both solvents. By means of bimolecular quenching experiments, transient absorption spectral fingerprints of the radical cationic species are detected in the visible (670 nm) and near‐IR (1300–1500 nm) regions, along with the much weaker fullerene anion band at λ_max=1030 nm. Definitive evidence for photoinduced electron transfer in F‐D1 and F‐D2 comes from transient absorption measurements. A charge‐separated state is formed within 100 ps and decays in less than 5 ns.     

Fluorophores as Optical Sensors for Local Forces

The main aim of this study is to investigate correlations between the impact of an external mechanical force on the molecular framework of fluorophores and the resultant changes in their fluorescence properties. Taking into account previous theoretical studies, we designed a suitable custom‐tailored oligoparaphenylenevinylene derivative (OPV5) with a twisted molecular backbone. Thin foils made of PVC doped with 100 nM OPV were prepared. By applying uniaxial force, the foils were stretched and three major optical effects were observed simultaneously. First, the fluorescence anisotropy increased, which indicates a reorientation of the fluorophores within the matrix. Second, the fluorescence lifetime decreased by approximately 2.5 % (25 ps). Finally, we observed an increase in the emission energy of about 0.2 % (corresponding to a blue‐shift of 1.2 nm). In addition, analogous measurements with Rhodamine 123 as an inert reference dye showed only minor effects, which can be attributed to matrix effects due to refractive index changes. To relate the observed spectroscopic changes to the underlying changes in molecular properties, quantum‐chemical calculations were also performed. Semiempirical methods had to be used because of the size of the OPV5 chromophore. Two conformers of OPV5 (C₂ and C_i symmetry) were considered and both gave very similar results. Both the observed blue‐shift of fluorescence and the reduced lifetime of OPV5 under tensile stress are consistent with the results of the semiempirical calculations. Our study proves the feasibility of fluorescence‐based local force probes for polymers under tension. Improved optical sensors of this type should in principle be able to monitor local mechanical stress in transparent samples down to the single‐molecule level, which harbors promising applications in polymer science and nanotechnology.     

Luminscent graphene quantum dots for organic photovoltaic devices

Recent research in organic photovoltaic (OPV) is largely focused on developing low cost OPV materials such as graphene. However, graphene sheets (GSs) blended conjugated polymers are known to show inferior OPV characteristics as compared to fullerene adduct blended with conjugated polymer. Here, we demonstrate that graphene quantum dots blended with regioregular poly(3-hexylthiophene-2,5-diyl) or poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene) polymer results in a significant improvement in the OPV characteristics as compared to GSs blended conjugated polymers. This work has implications for inexpensive and efficient solar cells as well as organic light emitting diodes.     

Polyurethane–oligo(phenylenevinylene) random copolymers: π‐Conjugated pores,vesicles, and nanospheres via solvent‐induced self‐organization

We report a new series of polyurethane–oligo(phenylenevinylene) (OPV) random copolymers and their self‐assembled nanomaterials such as pores, vesicles, and luminescent spheres. The polymers were synthesized through melt transurethane process by reacting a hydroxyl‐functionalized OPV with diurethane monomer and diol under solvent‐free and nonisocyanate conditions. The amount of OPV was varied up to 50 mol % in the feed to incorporate various amounts of π‐conjugated segments in the polyurethane backbone. The π‐conjugated segmented polymers were subjected to solvent induced self‐organization in THF or THF+water to produce variety of morphologies ranging from pores (500 nm to 1 μm) to spheres (100 nm to 2 μm). Upon shining 370‐nm light, the dark solid nanospheres of the copolymers transformed into blue luminescent nanoballs under fluorescence microscope. The mechanistic aspects of the self‐organization process were studied using solution FTIR and photophysical techniques such as absorption and emission to trace the factors which control the morphology. FTIR studies revealed that the hydrogen bonding plays a significant role in the copolymers with lower amount of OPV units. Time resolved fluorescent decay measurements of copolymers revealed that molecular aggregation via π‐conjugated segments play a major role in the samples with higher OPV content in the random block polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 46: 5897–5915, 2008     

Suppressing the environmental dependence of the open‐circuit voltage in inverted polymer solar cells through a radical polymer anodic modifier

Developing stable, readily‐synthesized, and solution‐processable transparent conducting polymers for interfacial modifying layers in organic photovoltaic (OPV) devices has become of great importance. Here, the radical polymer, poly(2,2,6,6‐tetramethylpiperidinyloxy methacrylate (PTMA), is shown to not affect the absorption of the well‐studied poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) active layer when incorporated into inverted OPV devices, as it is highly transparent in the visible spectrum due to the non‐conjugated nature of the PTMA backbone. The inclusion of this radical polymer as an anode‐modifying layer enhanced the open‐circuit voltage and short‐circuit current density values over devices that did not contain an anodic modifier. Importantly, devices fabricated with the PTMA interlayer had performance metrics that were time‐independent over the entire course of multiples days of testing after exposing the OPV devices to ambient conditions. Furthermore, these high performance values were independent of the metal used as the top electrode contact in the inverted OPV devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 311–316     

A facile one‐pot reactive solution blending approach for main‐chain donor–acceptor polymeric materials

A high‐temperature solution blending process has been used to synthesize a series of copolymers incorporating varying mole ratios of perylenebisimide (PBI) into the backbone of an engineering thermoplastic polyester [poly(1,4‐cyclohexylenedimethylene‐1,4‐cyclohexanedicarboxylate)] (PCCD). A random donor–acceptor copolymer incorporating oligo(p‐phenylene vinylene) (OPV) and PBI was also synthesized. The chemical incorporation of these chromophores into PCCD was confirmed by carrying out the melt condensation using 1,4‐cyclohexanedimethanol and 1,4‐dimethylcyclohexane dicarboxylate with hydroxyl‐functionalized PBI and OPV derivatives. Higher extent of incorporation of PBI (35 mol %) could be achieved using the blending approach retaining solubility, film‐forming ability, and higher molecular weights. The PBI polymers produced using the two different approaches exhibited structural variations. The polymers formed from the solution blending approach had a semicrystalline nature with blocks of PCCD separating the PBI units, whereas those produced using the melt condensation route were amorphous polymers. This structural variation was reflected in their photophysical properties also with the reactive solution‐blended polymers exhibiting higher fluorescence quantum yields. These results demonstrate the easy incorporation of suitably functionalized donor and acceptor moieties into a completely aliphatic polyester backbone to produce free‐standing films of hitherto nonprocessable polymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Self‐assembly of oligo(p‐phenylenevinylene)‐block‐poly(ethylene oxide) in polar media and solubilization of an oligo(p‐phenylenevinylene) homooligomer inside the assembly

Rod–coil amphiphilic diblock copolymers, consisting of oligo(p‐phenylenevinylene) (OPV) as a rod and hydrophobic block and poly(ethylene oxide) (PEO) as a coil and hydrophilic block, were synthesized by a convergent method. The aggregation behavior of the block copolymers in a selective solvent (tetrahydrofuran/H₂O) was probed with the absorption and emission of the OPV block. With increasing H₂O concentration, the absorption maximum was blueshifted, the emission from the molecularly dissolved OPV decreased, and that from the aggregated OPV increased. This indicated that the OPV blocks formed H‐type aggregates in which the OPV blocks aligned in a parallel orientation with one another. The transmission electron microscopy observation revealed that the block copolymers with PEO weight fractions of 41 and 62 wt % formed cylindrical aggregates with a diameter of 6–8 nm and a length of several hundreds nanometers, whereas the block copolymer with 79 wt % PEO formed distorted spherical aggregates with an average diameter of 13 nm. Furthermore, the solubilization of an OPV homooligomer with the block copolymer was studied. When the total polymer concentration was less than 0.1 wt %, the block copolymer solubilized OPV with a 50 mol % concentration. The structure of the aggregates was a cylinder with a relatively large diameter distribution. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1569–1578, 2005     

Synthesis of novel thieno[3,2-b]thienobis(silolothiophene) based low bandgap polymers for organic photovoltaics

Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices.     

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes

A new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C₆₀ fullerene monomers into ionene polymers. The power of these novel “C₆₀‐ionenes” for interface modification enables the use of numerous high work‐function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C₆₀‐ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non‐fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C₆₀‐ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard‐soft materials interface to the benefit of organic electronics.     

Structure‐Dependent Photoinduced Electron Transfer in Fullerodendrimers with Light‐Harvesting Oligophenylenevinylene Terminals

Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C₆₀ by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C₆₀G1 – C₆₀G4 , reaching a 99:1 ratio for C₆₀G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C₆₀G1 – C₆₀G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C₆₀ singlet energy transfer. In CH₂Cl₂, the OPV→C₆₀ electron transfer from the lowest fullerene singlet level (¹C₆₀*) is slightly exergonic (ΔG_CS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C₆₀G2 – C₆₀G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C₆₀ electron transfer observed for C₆₀G3 and C₆₀G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer.     

Electrochemical deposition of poly[ethylene-dioxythiophene] (PEDOT) films on ITO electrodes for organic photovoltaic cells: control of morphology,thickness, and electronic properties

In this article, controlled changes on morphology, thickness, and band gap of poly[ethylenedioxythiophene] (PEDOT) polymer films fabricated by electrochemical polymerization (potentiostatically) are analyzed. Electropolymerization of the monomer ethylenedioxythiophene (EDOT) was carried out on indium tin oxide (ITO) electrodes, in different dry organic electrolytic media, such as acetonitrile, acetonitrile–dichloromethane, and toluene–acetonitrile mixtures. It was found that electropolymerization kinetics can be controlled by changing the polarity of the electrolytic media, and kinetics is slower for those with low polarity. This fact combined with an accurate control of EDOT monomer concentration and electropolymerization at E_peak/2 potential, allows to control the morphology and thickness of the electropolymerized PEDOT films (E-PEDOT:ClO₄); toluene/ACN (4:1, v/v) and [EDOT]?=?0.3 mM gave the best films for application in organic photovoltaic (OPV) cells. The performance of the E-PEDOT:ClO₄ films was tested on ITO electrodes as anode buffer layer in OPV cells with the configuration ITO/E-PEDOT:ClO₄/P3HT:PC₆₁BM/Field’s metal, where Field’s metal (cathode) is a eutectic alloy that lets to fabricate OPV devices easily and in a fast and economical way at free vacuum conditions. The performance of these devices was compared with an OPV device constructed with a buffer layer anode, prepared using the classical spin coating of PEDOT:PSS on ITO. Results showed that OPV cells fabricated with E-PEDOT:ClO₄ have a slightly increased PV performance.     

Direct arylation synthesis of thienoisoindigo‐based low‐band‐gap polymer from asymmetric donor–acceptor monomer

Thienoisoindigo (TIG) moiety has been paid numerous attentions as an excellent acceptor building block in low‐band‐gap polymers. Herein, a new TIG‐dithiophene alternating copolymer (PTIG2T) was successfully synthesized from an asymmetric TIG‐based donor–acceptor (D‐A) monomer via the self‐condensation‐type direct arylation polymerization. PTIG2T exhibited the light absorption over 1000 nm owing to the intramolecular charge transfer in the thin film state, which corresponded to an optical band gap of 1.24 eV. The HOMO and LUMO levels of PTIG2T were determined to be −5.08 and −3.60 eV, respectively. Furthermore, the organic photovoltaic (OPV) with a PTIG2T/PC₆₁BM active layer achieved a power conversion efficiency (PCE) of 3.19%, which is one of the highest PEC achieved by OPVs with TIG‐based materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 430–436     

Enhancing Device Performance of Small Molecular Organic Photovoltaic Cells by Controlling the Deposition Rate of Fullerene

To improve the power conversion efficiency (PCE) of small molecular weight organic photovoltaic cells (OPVs) it is proposed to use a simple fabrication method by controlling the deposition rate of the acceptor material fullerene (C₆₀) in planar heterojunction OPV structures of ITO/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/copper phthalocyanine (CuPc)/C₆₀/bathocuproine (BCP)/Ag. In our optimised device, the highest PCE of 1.7% was obtained through a deposition rate of C60 equal to 0.3 nm/s due to the superior charge balance in the CuPc/C₆₀ heterojunction. Such a charge balance condition increased the fill factor from 52.4% to 56.1% by reducing the carrier accumulation in the OPV device. The electron only device was fabricated with the purpose of analysing the electron mobility of C₆₀ as a function of the deposition rate. In addition, the effects of the deposition rates on the performance of planar OPV devices were exhaustively analysed by examining the absorption properties and the surface morphologies.     

An Azulene‐Containing Low Bandgap Small Molecule for Organic Photovoltaics with High Open‐Circuit Voltage

A simple azulene‐containing squaraine dye ( AzUSQ ) showing bandgap of 1.38 eV and hole mobility up to 1.25×10^?4 cm² V^?1 s^?1 was synthesized. With its low bandgap, an organic photovoltaic (OPV) device based on it has been made that exhibits an impressive open‐circuit voltages (V_oc) of 0.80 V. Hence, azulene might be a promising structural unit to construct OPV materials with simultaneous low bandgap, high hole mobility and high V_oc.     

Anionic polymerization initiators containing protected functional groups and functionally terminated diene polymers

Organolithium reagents substituted with hydroxyl-carrying mixed acetals (i.e., tetra-hydropyranyl and α-ethoxyethyl ethers) have been prepared in high yields and used to polymerize 1,3-butadiene to various acetal-terminated polybutadiene polymers. A method is described for converting acetal-containing polymers into hydroxyl-containing polymers. The polybutadienes have been characterized with regard to endgroup types, quantitative functionalities, molecular weights, molecular weight distributions, and microstructures. Dihydroxyl terminated polymers are prepared anionically in the absence of gel. Such materials are more suitable from the standpoint of f (OH) and M?_w/M?_n for chain extension studies than are prepolymers prepared by radical methods.     

Thermal behaviors of vinyl esters of long-chain fatty acids and their comblike polymers

Phase transition behaviors of vinyl esters of long-chain fatty acids (C₁₂–C₁₈) and their comblike polymers have been investigated by the thermal analysis combining with X-ray diffraction and infrared spectroscopy. Effect of the length of hydrocarbon chain on the thermal behaviors of both monomers and polymers have been elucidated. Vinyl stearate exhibits three crystalline modifications, α (hexagonal), β₁ (monoclinic, M_∥) and β₂ (monoclinic, O⊥) forms. With shortening of the alkyl chain the polymorphic behaviors become simpler. The thermal behaviors of the resultant polymers are influenced by the packing mode of monomer molecules and the polymerization temperature.     

Excitation energy migration in oligo(p-phenylenevinylene) based organogels: structure-property relationship and FRET efficiency

Excitation energy migration (EM) and assisted energy transfer (ET) properties of a few oligo(p-phenylenevinylene) (OPV) based organogelators with different end functional groups have been studied using picosecond time-resolved emission spectroscopy (TRES). EM was found to be more efficient in OPV gelators with small end functional groups (OPV3-4) when compared to that of the gelators with bulky end groups (OPV1-2) in the gel state. TRES studies at elevated temperature and in chloroform solution highlight the role of the self-assembled scaffolds in assisting the EM and ET processes. Increase in temperature and solvent polarity leads to the aggregate breaking and hence adversely affects the EM and ET efficiencies. The effect of EM efficiency on the fluorescence resonance energy transfer (FRET) properties of the OPV gels was studied by using OPV1 and OPV3 as the donors and OPV5 as the acceptor. Better transfer of excitation energy was observed in the donor system (OPV3) having higher EM efficiency even at very low concentration (3.1 mol%) of the acceptor molecules, whereas ET efficiency was lower in the donor system (OPV1) with low EM efficiency.