Thienopyrazine‐Based Low‐Bandgap Poly(heteroaryleneethynylene)s for Photovoltaic Devices

Summary: Low‐bandgap π‐conjugated polymers that consist of alkyl thiophene/alkoxy phenylene and 2,3‐diphenylthieno[3,4‐b]pyrazine units have been prepared in high yields by a Sonogashira polycondensation. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. Thin films of the polymers P1 , P2 , and P3 exhibit an optical bandgap of ≈1.57–1.60 eV. Under simulated AM 1.5 conditions P2/PCBM devices on polyester foil provide a short circuit current of I_SC = 10.72 mA · cm⁻², an open circuit voltage of V_oc = 0.67 V, and a power conversion efficiency of 2.37%.

Schematic of the photovoltaic device made from the polymers synthesized here.     

Rational Design of High‐Performance Wide‐Bandgap (≈2 eV) Polymer Semiconductors as Electron Donors in Organic Photovoltaics Exhibiting High Open Circuit Voltages (≈1 V)

Systematic optimization of the chemical structure of wide‐bandgap (≈2.0 eV) “donor–acceptor” copolymers consisting of indacenodithiophene or indacenodithieno[3,2‐b ]thiophene as the electron‐rich unit and thieno[3,4‐c ]pyrrole‐4,6‐dione as the electron‐deficient moiety in terms of alkyl side chain engineering and distance of the electron‐rich and electron‐deficient monomers within the repeat unit of the polymer chain results in high‐performance electron donor materials for organic photovoltaics. Specifically, preliminary results demonstrate extremely high open circuit voltages (V _ocs) of ≈1.0 V, reasonable short circuit current density (J _sc) of around 11 mA cm⁻², and moderate fill factors resulting in efficiencies close to 6%. All the devices are fabricated in an inverted architecture with the photoactive layer processed by doctor blade equipment, showing the compatibility with roll‐to‐roll large‐scale manufacturing processes. From the correlation of the chemical structure—optoelectronic properties—photovoltaic performance, a rational guide toward further optimization of the chemical structure in this family of copolymers, has been achieved.

     

Shape‐Persistent Oligothienylene–Ethynylene‐Based Dendrimers: Synthesis,Spectroscopy and Electrochemical Characterization

A series of novel structurally well‐defined oligothienylene–ethynylene‐based dendritic macromolecules up to the 3rd generation (G3) were successfully synthesized by a combination of Pd‐catalyzed Sonogashira‐type cross‐coupling and oxidative homocoupling steps. Oxidative homocoupling of dendrons successfully afforded dendrimers up to the 2nd generation (G2). In contrast, the G3 dendrimer was effectively prepared by a four‐fold Sonogashira‐type cross‐coupling reaction. All compounds showed broad and structureless absorption and emission spectra arising from the presence of different π‐conjugated chromophores. With increasing generation, a bathochromic shift of the π–π* absorption band and an increase of the absorption coefficient were observed. The insertion of ethynylene groups into the conjugated dendrimer backbone resulted in a hypsochromic shift compared to all‐thiophene dendrimers reported earlier by our group. All dendritic compounds are fluorescent and showed moderate quantum efficiencies due to an effective intramolecular charge‐transfer (ICT) process. Cyclic voltammetry measurements also revealed the presence of multiple π‐conjugative pathways that show very broad oxidation waves for higher generations. HOMO–LUMO energy levels of these dendrons and dendrimers were estimated from optical and redox measurements and the calculated band gaps were within the range of 3.3 to 2.4 eV, typical for oligo‐ and polythiophenes. Electrochemical polymerizations of several desilylated compounds were performed and characterization of the films is reported. Preliminary bulk heterojunction solar cells that utilise these ethynylated dendrimers as the donor and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM[60]) as the acceptor showed moderate efficiencies ranging from 0.18–0.64 %.     

Anthracene‐Containing Wide‐Band‐Gap Conjugated Polymers for High‐Open‐Circuit‐Voltage Polymer Solar Cells

The synthesis, characterization, and photophysical and photovoltaic properties of two anthracene‐containing wide‐band‐gap donor and acceptor (D–A) alternating conjugated polymers ( P1 and P2 ) are described. These two polymers absorb in the range of 300–600 nm with a band gap of about 2.12 eV. Polymer solar cells with P1 :PC₇₁BM as the active layer demonstrate a power conversion efficiency (PCE) of 2.23% with a high V_oc of 0.96 V, a J_sc of 4.4 mA cm⁻², and a comparable fill factor (FF) of 0.53 under simulated solar illumination of AM 1.5 G (100 mW cm⁻²). In addition, P2 :PC₇₁BM blend‐based solar cells exhibit a PCE of 1.42% with a comparable V_oc of 0.89 V, a J_sc of 3.0 mA cm⁻², and an FF of 0.53.

     

Modification of Electrode Surfaces by Self‐Assembled Monolayers of Thiol‐Terminated Oligo(Phenyleneethynylene)s

The wire‐like properties of four S‐(4‐{2‐[4‐(2‐phenylethynyl)phenyl]ethynyl}phenyl) thioacetate derivatives, PhC≡CC₆H₄C≡CC₆H₄SAc ( 1 ), H₂NC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 2 ), PhC≡CC₆H₂(OMe)₂C≡CC₆H₄SAc ( 3 ) and AcSC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 4 ) (Figure 1 ), all of which possess a high degree of conjugation along the oligo(phenyleneethynylene) (OPE) backbone, were investigated as self‐assembled monolayers (SAMs) on gold and platinum electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The redox probe [Fe(CN)₆]₄^? was used in both the CV and impedance experiments. The results indicate that the thiolates derived from thioacetate‐protected precursor molecules 1 and 2 form well‐ordered monolayers on a gold electrode, whereas SAMs derived from 3 and 4 exhibit randomly distributed pinholes. The electron tunnelling resistance and fractional coverage of SAMs of all four compounds were examined using electron tunnelling theory. The analysis of the results reveal that the well‐ordered SAMs of 1 and 2 exhibit higher charge‐transfer resistance in comparison to the defect‐ridden SAMs of 3 and 4 . The additional steric bulk offered by the methoxy groups in 3 is likely to prevent efficient packing within the SAM, leading to a microelectrode behaviour, when assembled on a gold electrode surface. The protected dithiol derivative 4 probably binds to the surface through both terminal groups which prevents dense packing and leads to the formation of a monolayer with randomly distributed pinholes. Atomic force microscopy (AFM) was used to examine the morphology of the monolayers, and height images gave root‐mean‐square (RMS) roughness′s which are in agreement with the proposed SAM structures.     

Synthesis and Photovoltaic Properties of the Copolymers Based on Carbazole with Tetrathiophene Porphyrin Side Chains Linked by a Flexible Alkyl‐interval

Four copolymers (XP10, XP11, XP12 and XP13) based on diketopyrrolopyrrole (DPP) and carbazole units with tetrathiophene porphyrin (TTP) side chains linked by a flexible alkyl‐interval were designed and synthesized. The effects of the introduction of TTP on the optical and electrochemical properties, the morphology, the mobility and the photovoltaic performance of copolymers were systematically studied. The results revealed that XP11 with a TTP/DPP ratio of 2/8 possessed the best performances, i.e., broad absorption spectra, balanced hole/electron mobility and suitable microphase separation. After optimizing via solution vapor annealing, the organic solar cell devices based on XP11 and PC₇₁BM showed the best power conversion efficiency of 5.11% with a short‐circuit current density (J_sc) of 10.36 mA·cm^–2, an open‐circuit voltage (V_oc) of 0.77 V, and a fill factor (FF) of 0.64.     

Poly(phenylene vinylene)‐based copolymers containing 3,7‐phenothiazylene and 2,6‐pyridylene chromophores: Fluorescence sensors for acids,metal ions,and oxidation

A novel copolymer, poly(N‐hexyl‐3,7‐phenothiazylene‐1,2‐ethenylene‐2,6‐pyridylene‐1,2‐ethenylene) ( P3 ), containing N‐hexyl‐3,7‐phenothiazylene and 2,6‐pyridylene chromophores was synthesized to investigate the effect of protonation, metal complexation, and chemical oxidation on its absorption and photoluminescence (PL). Poly(N‐hexyl‐3,8‐iminodibenzyl‐1,2‐ethenylene‐1,3‐phenylene‐1,2‐ethenylene) and poly(N‐hexyl‐3,7‐phenothiazylene‐1,2‐ethenylene‐1,3‐phenylene‐1,2‐ethenylene) ( P2 ), consisting of 1,3‐divinylbenzene alternated with N‐hexyl‐3,8‐iminodibenzyl and N‐hexyl‐3,7‐phenothiazylene, respectively, were also prepared for comparison. Electrochemical investigations revealed that P3 exhibited lower band gaps (2.34 eV) due to alternating donor and acceptor conjugated units (push–pull structure). The absorption and PL spectral variations of P3 were easily manipulated by protonation, metal chelation, and chemical oxidation. P3 displayed significant bathochromic shifts when protonated with trifluoroacetic acid in chloroform. The complexation of P3 with Fe³⁺ led to a significant absorption change and fluorescence quenching, and this implied the coordination of ferric ions with the 2,6‐pyridylene groups in the backbone. Moreover, both phenothiazylene‐containing P2 and P3 showed conspicuous PL quenching with a slight redshift when oxidized with NOBF₄. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1272–1284, 2004     

Multifunctional Conjugated Polymers with Main‐Chain Donors and Side‐Chain Acceptors for Dye Sensitized Solar Cells (DSSCs) and Organic Photovoltaic Cells (OPVs)

A novel multifunctional conjugated polymer (RCP‐1) composed of an electron‐donating backbone (carbazole) and an electron‐accepting side chain (cyanoacetic acid) connected through conjugated vinylene and terthiophene has been synthesized and tested as a photosensitizer in two major molecule‐based solar cells, namely dye sensitized solar cells (DSSCs) and organic photovoltaic cells (OPVs). Promising initial results on overall power conversion efficiencies of 4.11% and 1.04% are obtained from the basic structure of DSSCs and OPVs based on RCP‐1, respectively. The well‐defined donor (D)‐acceptor (A) structure of RCP‐1 has made it possible, for the first time, to reach over 4% of power conversion efficiency in DSSCs with an organic polymer sensitizer and good operation stability.     

Direct Observation of α‐Chloro‐p‐quinodimethane as the Real Monomer in the Gilch Polymerization Leading to Poly(p‐phenylene vinylene)s

It is the general consensus that in Gilch polymerizations the 1,4‐bis(chloromethylene)benzene starting material first changes into p‐quinodimethane intermediates which then act as the real monomers. However, direct observation of these intermediates has not been possible so far. This is because usually the p‐quinodimethane auto‐initiates its rapid radical polymerization instantaneously, keeping its concentration extremely low throughout the whole process. Here it is shown that, when the reaction is carried out at very low temperatures, the formation of p‐quinodimethane still proceeds but chain growth is suppressed. Hence, the concentration of the active monomer reaches a level sufficient for NMR analysis.

     

Properties of single‐walled carbon nanotube‐based poly(phenylene vinylene) electroluminescent nanocomposites

Devices with varying concentrations of single‐walled carbon nanotubes (SWNTs) dispersed in three derivatives of poly(p‐phenylene vinylene) are prepared, and their electroluminescent properties evaluated. Increasing the concentration of SWNTs improves the electrical conductivity of the nanocomposites. However, an undesired increase in the electroluminescence (EL) turn‐on voltage is observed for the hybrids, possibly due to photoluminescence quenching of excitons by the SWNTs. At relatively low concentrations of SWNTs, there is an increase in the EL lifetime; in contrast, at relatively high concentrations of SWNTs, due to photoluminescence quenching by the nanotubes, significant reduction in brightness and faster degradation of the EL performance of the devices is observed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Computer Simulation of Long Side‐Chain Substituted Poly(phenylene vinylene) Polymers

A molecular dynamics (MD) simulation was employed to investigate structure features and segment orientation of four poly(phenylene vinylene) (PPV) derivatives with long, flexible side chains at room temperature. In the simulations, the main chains of the polymers were found to be semirigid and exhibit a tendency to coil into ellipsoidal helices or form zigzag conformations of limited regularity. The simulations show that continuous quasi‐coplanar segments along the backbone are in a range of ≈2–4 repeat units. The ordered orientation and coupling distance of interchain aromatic rings can be correlated with optical properties of materials. A simplified quantum‐mechanical method was developed to investigate optical properties based on MD trajectories. The method was tested to simulate the absorption spectra of four PPV derivatives. The absorption maxima of the calculated spectra are in reasonable agreement with experimental data. This work implies that long‐range electron transfer along the backbones of these polymers may not occur, but may be mediated by interchain interactions.     

Poly(alkylthio‐p‐phenylenevinylene): Synthesis and electroluminescent and photovoltaic properties

Two alkylthio‐substituted poly(p‐phenylenevinylene) (AT–PPV) derivatives, poly(2‐octylthio‐p‐phenylenevinylene) (OT–PPV) and poly[5‐methoxy‐2‐(2′‐ethyl‐hexylthio)‐p‐phenylenevinylene] (MEHT–PPV), were synthesized by a Heck coupling reaction for the investigation of the effect of alkylthio groups on the optoelectronic properties of poly(p‐phenylenevinylene) derivatives. The absorption peaks of OT–PPV and MEHT–PPV solutions were located at 431 and 438 nm, respectively. As for solid films, an OT–PPV film showed an absorption maximum wavelength at 444 nm, 13 nm redshifted in comparison with its solution value, whereas an MEHT–PPV film displayed the same absorption peak position as its dilute solution; this indicated that there was no interchain interaction in the MEHT–PPV film. Polymeric light‐emitting diodes (PLEDs) and polymer solar cells (PSCs) based on OT–PPV and MEHT–PPV were fabricated and characterized. Very narrow bandwidths of the electroluminescence (EL) spectra of the two AT–PPVs were found, with the full width at half‐maximum of the emission being 40 and 47 nm for OT–PPV and MEHT–PPV, respectively. The maximum EL efficiency of the single‐layer PLED based on MEHT–PPV with Al as a cathode reached 1.49 cd/A. The PSC based on a blend of OT–PPV and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) showed the power conversion efficiency of 1.4% under the illumination of AM1.5 (80 mW/cm²). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1279–1290, 2006     

Poly((2‐alkylbenzo[1,2,3]triazole‐4,7‐diyl)vinylene)s for organic solar cells

Poly((2‐Alkylbenzo[1,2,3]triazole‐4,7‐diyl)vinylene)s (pBTzVs) synthesized by Stille coupling show different absorption spectra, solid‐state morphology, and photovoltaic performance, depending on straight‐chain versus branched‐chain (pBTzV12 and pBTzV20) pendant substitution. Periodic boundary condition density functional computations show limited alkyl pendant effects on isolated chain electronic properties; however, pendants could influence polymer backbone conjugative planarity and polymer solid film packing. The polymers are electronically ambipolar, with best performance by pBTzV12 with hole and electron transport mobilities of 4.86 × 10^?6 and 1.96 × 10^?6 cm² V^?1 s^?1, respectively. pBTzV12 gives a smooth film morphology, whereas pBTzV20 gives a very different fibrillar morphology. For ITO/PEDOT:PSS/(1:1 w/w polymer:PC₇₁BM)/LiF/Al devices, pBTzV12 gives power conversion efficiency (PCE) up to 2.87%, and pBTzV20 gives up to PCE = 1.40%; both have open‐circuit voltages of V_OC = 0.6–0.7 V. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1539–1545     

Approach to High Open‐Circuit Voltage in Organic Solar Cells Utilizing a Structural Change of the Oxazolino‐C70 Derivative

Reactions of 2,5‐Bn₂C₇₀ (Bn=CH₂Ph) with hydroxide and ArCN (Ar=Ph, m‐ClPh) followed by quenching with I₂ and BnBr afforded dibenzylated and tetrabenzylated oxazolino[70]fullerenes, respectively. The latter has a novel structural configuration, in which the addends are positioned from the polar to the transequatorial region. A key structural feature of this compound is that the oxygen atom of the oxazoline ring is bound to the equatorial belt region of C₇₀, giving structural change in its reduced state. This enables stabilization of the reduced state, suppressing charge recombination dynamics in organic solar cells to give a high open‐circuit voltage (0.85, 0.93, and 1.11 V in devices using P3HT, PTB7, and DPP(TBFu)₂, respectively).     

Electroconductive Hydrogels: Electrical and Electrochemical Properties of Polypyrrole‐Poly(HEMA) Composites

Composites of inherently conductive polypyrrole (PPy) within highly hydrophilic poly(2‐hydroxyethyl methacrylate)‐based hydrogels (p(HEMA)) have been fabricated and their electrochemical properties investigated. The electrochemical characteristics observed by cyclic voltammetry suggest less facile reduction of PPy within the composite hydrogel compared to electropolymerized PPy, as shown by the shift in the reduction peak potential from ?472 mV for electropolymerized polypyrrole to ?636 mV for the electroconductive composite gel. The network impedance magnitude for the electroconductive hydrogel remains quite low, ca. 100 Ω, even upon approach to DC, over all frequencies and at all offset potentials suggesting retained electronic (bipolaronic) conductivity within the composite. In contrast, sustained application of +0.7 V (vs. Ag/AgCl, 3 M Cl^?) for typically 100 min. (conditioning) to reduce the background amperometric current to <1.0 μA, resulted in complete loss of electroactivity. Nyquist plots suggest that sustained application of such a modest potential to the composite hydrogel results in impedance characteristics that resembles p(HEMA) without evidence of the conducting polymer component. PPy composite gels supported a larger ferrocene monocarboxylate diffusivity (D_appt=7.97×10^?5 cm² s^?1) compared to electropolymerized PPy (D_appt=5.56×10^?5 cm² s^?1), however a marked reduction in diffusivity (D_appt=1.01×10^?5 cm² s^?1) was observed with the conditioned hydrogel composite. Cyclic voltammograms in buffer containing H₂O₂ showed an absence of redox peaks for electrodes coated with PPy‐containing membranes, suggesting possible chemical oxidation of polypyrrole by the oxidant     

Electrochemical Properties of Oxidized Carbon Nano‐Onions: DRIFTS‐FTIR and Raman Spectroscopic Analyses

The electrochemical reactions of carboxylic and lactone groups on carbon nano‐onions (CNOs) in aqueous solutions result in non‐Kolbe products: alcohols, ketones, ethers and epoxides. The anodic/cathodic conversion of ox‐CNOs was assessed by Boehm titrations and by Raman and DRIFTS‐FTIR (diffuse reflectance infrared Fourier transform spectroscopy). The electrochemical properties of oxidized carbon nano‐onions were investigated by cyclic voltammetry in aqueous solutions. The ox‐CNOs are electrochemically active as a result of the reduction of the oxygen‐containing groups.     

Modification and Electrochemical Properties of Poly(phenylazomethine) Derivatives

Summary: Oligo(phenylazomethine)s (OPAs) and aniline-capped OPAs (OPA's) are used as model compounds of polyphenylazomethine (PPA), and their fundamental properties and their modification methods are investigated. Cyclic voltammograms of bis(diphenylmethyleneamino)benzene (OPA2′) showed irreversible redox response in the presence of trifluoroacetic acid. A selective synthesis of oligophenanthridine was achieved through the photocyclization of OPA2′ in concentrated sulfuric acid. Stepwise complexation behavior in dendritic poly(phenylazomethine)s (DPAs) was supported by the shell-selective reduction of the imines. Using the shell-selective reduction method and the terminal-modification method of the DPAs, the core and terminals of DPAs were functionalized by ferrocene units, respectively.     

Effect of Poly(amine)s on the Redox Properties of Carboxymethylcellulose and Ferrocene‐modified Poly(amine) Layer‐by‐Layer Films

Multilayer films consisting of carboxymethylcellulose (CMC) and ferrocene‐modified poly(ethyleneimine) (Fc‐PEI) or poly(allylamine hydrochloride) (Fc‐PAH) were successfully prepared on a gold electrode to examine their redox properties. The redox current of (Fc‐PEI/CMC)_n film‐coated electrodes increased with the number of layers, while the (Fc‐PAH/CMC)_n film‐coated electrodes exhibited increased response only for the first eight bilayers. The (Fc‐PEI/CMC)_n and (Fc‐PAH/CMC)_n films deposited on the surface of Fc‐free multilayer film‐coated electrodes also showed a redox response. The (PEI/CMC)₅ film‐coated electrode showed redox responses in Fc‐PEI and Fc‐PAH solutions, confirming the uptake of the Fc‐polymers in the inner film. In contrast, the uptake of the Fc‐polymers in the (PAH/CMC)₅ film was severely suppressed, suggesting that different permeability of (PEI/CMC)₅ and (PAH/CMC)₅ films.     

The thermal conversion of the tetrahydrothiophene‐precursor polymer to poly(p‐phenylene vinylene)

This paper reports the thermal conversion of the tetrahydrothiophene (THT)‐precursor to poly(p‐phenylene vinylene) (PPV). Detailed investigations of the conversion process show that the leaving groups THT and HCl do not eliminate simultaneously. Moderate temperatures (≤125 °C) are sufficient to eliminate the THT while a higher temperature of ≈150 °C is necessary for the leaving group HCl. Furthermore, the THT groups split off at two characteristic temperatures. Our investigations have shown that a consistent picture of the reaction mechanism can only be obtained if the configuration of the polymer chain is considered. For the total reaction of the THT‐precursor to PPV a reaction mechanism is suggested that consists of at least four steps. Copyright © 1999 John Wiley & Sons, Ltd.