Poly(p-phenylenevinylene) derivatives with conjugated thiophene side chains: Synthesis,photophysics and photovoltaics

Three novel poly(p-phenylenevinylene) (PPV) derivatives with conjugated thiophene side chains, P1, P2 and P3, were designed and synthesized for application in polymer solar cells (PSCs). The effects of the conjugated side chains on the thermal, photophysical, electrochemical and photovoltaic properties of these polymers were investigated. The polymers exhibited good thermal stability and film-forming ability. The absorption spectra indicated that the short conjugated side chains have slight influence on the UV-region spectra of PPVs; whereas with increasing the length of conjugated side chains, the absorption of the UV-region red-shifted. The photoluminescence spectra reveal that complete exciton energy transfer occur from the conjugated side chains to the main chains of the polymers. The polymers emitted yellow-orange light with the emission maximum peaks in the region of 525–550 nm in chloroform solution and 611–616 nm in thin films. Cyclic voltammograms displayed that the band gaps were reduced effectively by the attachment of the conjugated thiophene side chains. The bulk heterojunction solar cells were fabricated based on the blend of the polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in a 1:4 weight ratio. The maximum power conversion efficiency (PCE = 0.53%) was obtained by using P3 as the electron donor under the illumination of AM 1.5, 100 mW/cm².     

Solution processable organic electro-phosphorescent iridium complex based on a benzothiazole derivative

We have synthesized a new solution processable iridium complex, di[2-(4′-octyloxyphenyl) benzothiazole]iridium(III)acetoacetone, [(OPBT)₂Ir(acac)], based on benzothiazole derivative for organic electro-phosphorescent devices. The synthesized molecule was identified by ¹H NMR and ¹³C NMR, and readily soluble in common organic solvents such as chlorobenzene. The UV–visible absorption and photoluminescence properties of pristine [(OPBT)₂Ir(acac) thin film as well as poly(N-vinylcarbzole) (PVK) thin film doped with the iridium complex were studied. The maximum UV–visible absorption and photoluminescence (PL) spectra are found to be at 337 nm and 547 nm, respectively. We have fabricated phosphorescent organic light-emitting devices using the ITO/PEDOT:PSS (40 nm)/PVK:(OPBT)₂Ir(acac) (40 nm)/Balq (40 nm)/LiF (1 nm)/Al (80 nm) configuration with the iridium complex as a triplet emissive dopant in poly(N-vinylcarbazole) (PVK) host. The electroluminescence (EL) devices showed greenish yellow light emission with maximum peak at 551 nm. Especially, the maximum external quantum and current efficiency of 1 mol% doped device were 1.74% and 4.89 cd/A, respectively.     

Synthesis and light-emitting properties of fluorene-based copolymers: Bearing diphenyl imidazole chromophore

New type of alternating fluorene-based copolymers containing electron-deficient aryl-substituted imidazole units in the main chains were designed, synthesized and characterized. The resulting copolymers were amorphous and showed excellent solubility in common organic solvents, such as dichloromethane, chloroform, 1,2-dichloroethane, chlorobenzene, toluene, and THF. These polymers also possessed good thermal stability with glass transition temperatures of 96 and 82 °C and decomposition onset temperatures of 365 and 379 °C, respectively. They exhibited good blue photoluminescence properties with high photoluminescence efficiencies. The long-wavelength emission of polyfluorenes had been effectively reduced by introduction of nonlinear structure aryl-substituted imidazole units. Light-emitting devices with an ITO/PEDOT/PVK/copolymer/LiF/Al configuration could emit strong blue light with high external quantum efficiencies of 1.49 and 2.05%.     

Photoluminescence of MEH-PPV with ultraviolet excitation

The basic optical properties of PPV-based polymers have been extensively studied due to their potential technological applications. However, a detailed investigation of electronic processes following photoexcitation in the ultraviolet is still lacking. We report photoluminescence measurements on poly(1-methoxy-4-ethylhexyloxy-paraphenylenevinylene) – MEH-PPV in the 2.0–5.6 eV range, with excitation up to 5.6 eV. The photoluminescence spectra lineshape is independent of excitation energy. The photoluminescence efficiency is high for energies well below the absorption maximum due to near-resonant excitation of the longest conjugated segments which are responsible for the PL. It decreases strongly for excitation energies in the range 2.1–2.5 eV (up to the absorption maximum) and slightly from 2.5 to 5.6 eV. The results indicate that states excited in the ultraviolet rapidly relax nonradiatively to the lowest state, from where the usual luminescence occurs.     

The photovoltaic behaviors of PPV- and PPE-type conjugated polymers featured with diketopyrrolopyrrole (DPP) units

The photovoltaic behaviors of diketopyrrolopyrrole (DPP) embedded poly(phenylenevinylene) (C8-DPP-PPV) and poly(phenyleneethynylene) (C10-DPP-PPE) were investigated. The two polymers exhibited good solubility in common organic solvents, high thermal stability and broad UV/visible absorption ranging from 300 to 600 nm in films. Their optical band gaps were found to be 1.94 eV for C8-DPP-PPV and 2.05 eV for C10-DPP-PPE. Moreover, their HOMO energy levels were relatively low at around ?5.6 to ?5.8 eV as estimated by electrochemical measurements. The polymer/PCBM bulk heterojunction solar cells exhibited a power conversion efficiency of 0.16% for C10-DPP-PPE. In addition, all-polymer solar cells consisting of C10-DPP-PPE/P3HT were also fabricated in view of the potential of C10-DPP-PPE as a polymeric acceptor, revealing an efficiency of 0.01%.     

Synthesis and characterization of soluble narrow band gap conducting polymers based on diketopyrrolopyrrole and propylenedioxythiophenes

A series of novel electro-active conjugated polymers containing 2,5-dialkyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (DPPs) and 3,4-dihydro-3,3-dialkyl-2H-thieno[3,4-b][1,4]dioxepines (dialkyl-ProDOTs) were synthesized using Stille coupling reaction in presence of CuO. The molecular weights of the synthesized polymers were found to be in the range of 18,000–45,000. Incorporation of the electron deficient DPP units and the electron rich dialkyl-ProDOT units in the conjugated backbone leads to low band gap polymers. All the polymers were found to be highly soluble in most chlorinated organic solvents as well THF and toluene with excellent film forming properties. From the UV–vis spectra, the band gap of the polymers was determined as 1.40–1.42 eV which is lower than the poly(dialkylProDOT)s. From the electrochemical study, highest occupied molecular orbital (HOMO) energy levels of the synthesized polymers were found to be in the range of 5.54–5.51 eV. Because of such high HOMO level, the resulting polymers were found to be more oxidatively stable. Polymers are thermally stable till 325–346 °C with only 5% weight loss which was confirmed from thermogravimetric analysis (TGA). The polymers were found to be moderately conducting with maximum conductivity up to 0.2–6.0 S/cm.     

Synthesis and characterization of low bandgap poly(dithienosilole vinylene) derivatives

Two low bandgap conjugated polymers containing vinylene unit on their main chains, poly(dithienosilole vinylene) derivatives P1 and P2, were synthesized by Pd-catalyzed Stille-coupling method. The two polymers show good solubility in common organic solvents, broad absorption bands from 500 nm to 700 nm and lower energy bandgap of 1.72 eV for P1 and 1.77 eV for P2. The electronic energy levels of the polymers were measured by electrochemical cyclic voltammetry. The HOMO and LUMO energy levels of P1 and P2 are at ?5.03 eV, ?5.18 eV and ?3.39 eV, ?3.43 eV, respectively. The polymers P1 and P2 containing dithienosilole units show lower HOMO and LUMO energy levels, in comparison with other poly(thienylene vinylene) derivatives.     

Synthesis and optical properties of quinoxaline-containing poly(aryl ether)s

Three novel poly(aryl ether)s were synthesized from the reaction of three bisphenols with 2,3-bis(4-fluorophenyl)-quinoxaline via nucleophilic aromatic substitution. The polymers contained electron transporting and emissive moieties separated by ether linkages. Quinoxaline was used as electron transporting segment, while p-distyrylbenzene, 2,6-distyrylpyridine and p-quinquephenyl were used as emissive segments. The low reactivity 2,3-bis(4-fluorophenyl)-quinoxaline towards nucleophilic aromatic substitution results in polymers with limited molecular weights. The polymers were soluble in polar organic solvents and strong organic acids. In THF solutions the polymers showed absorption maxima at 349–354 nm and emission maxima at 417–454 nm, with quantum yields of 22–41%. In solid state the polymers showed absorption maxima at 350–353 nm and emission maxima at 427–461 nm. The optical properties of polymers were influenced by intrachain interactions in solution, while interchain interactions were important in solid state.     

New π-conjugated polymers constituted of dialkoxybenzodithiophene units: synthesis and electronic properties

New stable and conductive polymers constituted of a new fused thiophene unit, 4,8-dialkoxybenzodithiophene, and related copolymers with thiophene were prepared by organometallic polycondensation in high yields. They showed electrical conductivity of 10⁻²–5 S cm⁻¹ on doping with iodine. Homopolymers were prepared by Ni-promoted dehalogenation polycondensation, and their molecular structures were confirmed by data from IR spectroscopy and elemental analysis. They were partly soluble in organic solvents, and the soluble part gave number average molecular weights, M_n, of 3000–3600 with M_w/M_n value (M_w=weight average molecular weight) of 1.1–2.4 in the GPC analysis; the M_n value corresponds to a degree of polymerization of about eight. Alternative copolymers consisting of the 4,8-dimethoxybenzodithiophene and thiophene or bithiophene units were synthesized by a Pd catalyzed polycondensation.Soluble organic model compounds for the homopolymer and copolymers were synthesized via analogous organometallic coupling reaction, and optical and electrochemical properties of the model compounds were compared with those of the polymers. UV–VIS spectrum of the oligomeric model compounds showed a large bathochromic shift by >50 nm from the monomeric compound. The p-doping of the polymers was reversible, and the iodine-doped polymers were neutralized by hydrazine. Packing structures of the polymers are discussed based on their XRD patterns and density.     

Self-assembling into interconnected nanoribbons in thin films of hairy rod poly(9,9-di(2-ethylhexyl)fluorene): Effects of concentration,substrate and solvent

This study utilizes atomic force microscopy to investigate the self-assembling behaviors from dilute solution into thin film of a well-known conjugated polymer, poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6). We have found that the structures of nanoscale aggregates depend on various experimental parameters including concentration, substrate and solvent. The self-assembling of PF2/6 from 0.05 mg/mL solution in toluene onto SiO_x/Si substrate results in the formation of interconnected nanoribbons with thickness and width of about 20 and 150 nm, respectively. Varying polymer concentration and type of substrate (SiO_x/Si or mica) significantly affects the nanoscale structures. The change of solvent to chlorobenzene, a slightly more polar solvent with slower evaporation rate, causes the growth of ribbon width to micron size with slight increase of the thickness. When the solvents with higher evaporation rate (i.e. chloroform and dichloromethane) are used, densely packed nanoribbons are obtained. Its width also grows to micron size. The measurements of UV/vis absorption and photoluminescence spectra detect some discrepancies in pattern, reflecting the variation of local chain organization within thin films prepared by using different solvents.     

Synthesis and preliminary characterization of novel naphthalene bisimide based copolymers

Here we report the synthesis and preliminary characterization of three novel conjugated 2,6-(naphthalene-1,4,5,8-tetracarboxylic-N,N′-dialkyldiimide) based copolymers with π-conjugation throughout the main chain. Two (PNATF and PNTTOF) copolymers were polymerized through Suzuki type cross-coupling reactions, the other (PNATT3) via a Stille-type reaction. The modifications of the side chain on the naphthalene bisimide unit and the thiophene contents in the polymer backbone were investigated. The structures of these polymers were confirmed by NMR. PNATF and PNTTOF could be dissolved in common solvents such as toluene, CHCl₃ and 1,2-dichlorobenzene. The optical spectra of the three copolymers revealed that they had two strong absorption bands at 386–394 and 600–670 nm. The photoluminescence spectra of PNATF and PNTTOF showed a weak red emission. The band gap values of the copolymer films determined from electrochemical and optical measurements were in good agreement, ranging between 1.58–1.88 and 1.53–1.69 eV, respectively. From these data we suggest these three copolymers are worthy of further investigation as potential candidates for applications in electronic devices.     

Detection of microwaves using the organic semiconductor poly(p-phenylenevinylene)

We report a new procedure to convert the polymer precursor poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using microwave irradiation. Spin-coated PTHT films were irradiated at room temperature under ambient conditions in a commercial microwave oven, with varying power from 20 W to 100 W. Complete conversion was reached within only 5 min of irradiation for powers above 50 W, yielding PPV films with absorption and photoluminescence spectra that are practically indistinguishable from the spectra of thermally converted PPV films, which require ca. 2 h of a high temperature (～200 °C) thermal treatment. In addition to a much faster conversion procedure, the irradiation with microwaves led to a red shift in the absorption spectrum of a PTHT film, which varied linearly with the time of irradiation. These films can then be used as low-cost, easy-to-use detectors of microwaves.     

Synthesis and characterization of low-bandgap copolymers based on dihexyl-2H-benzimidazole and terthiophene

A series of new semiconducting polymers with 3-(hexyloxy)thiophene, 2,2-dihexyl-2H-benzimidazole (HBI) and thiophene units was synthesized using Stille polymerization. These random copolymers show good solubility at room temperature in organic solvents owing to the long alkyl chain in new acceptor, 2,2-dihexyl-2H-benzimidazole. In HBI, the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was replaced with the carbon to make a highly soluble electron deficient moiety while keeping the 1,2-quinoid form of BT unit. The spectra of the solid films show absorption bands with maximum peaks at about 408–526 nm and the absorption onsets at 550–692 nm, corresponding to band gaps of 1.79–2.25 eV. The onset wavelengths of the absorption spectra in thin films exhibit a gradual red-shift with decreased amount of dihexyl-2H-benzimidazol unit, that is, from 550 nm with PHOTDTHBI-9 to 692 nm with PHOTDTHBI-1. Under white light illumination (AM 1.5 G, 100 mW/cm²), the device with PHOTDTHBI-3:PCBM blend demonstrated a V_OC value of 0.36 V, a J_SC value of 1.20 mA/cm², and a FF of 0.37, leading to the efficiency of 0.16%.     

Synthesis and characterization of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene polymers

Two novel diketopyrrolopyrrole (DPP) and p-phenylenevinylene alternating copolymers, poly(1,4-(2,5-dicyano)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(1,4-(2,5-diethoxy)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P2), were synthesized through Wittig reaction in good yields, and were characterized by FTIR, NMR. Two polymers possessed moderate molecular weights and polydispersities, well-defined structures, and were readily soluble in common organic solvents. In particular, P1 and P2 exhibited excellent thermal stability with T_d = 393 and 398 °C at 5% weight loss, respectively. Both P1 and P2 in solution and in thin films exhibited strong red photoluminescence. Both electroluminescence devices using ITO/PEDOT/polymer/Ba/Al configuration with P1 and P2 as the emitting layer showed nearly pure red emission with the emission peaks at 646 nm for P1 and 648 nm for P2, and exhibited low turn-on voltages of 4.5 and 3.4 V, respectively. The results show that P1 and P2 are promising candidates for red emissive materials in polymer light-emitting diodes.     

A polythiophene derivative with octyloxyl triphenylamine-vinylene conjugated side chain: Synthesis and its applications in field-effect transistor and polymer solar cell

A new polythiophene derivative with octyloxyl triphenylamine-vinylene (OTPAV) conjugated side chain, OTPAV-PT, was synthesized according to the Stille coupling method, and characterized by ¹H NMR, elemental analysis, GPC, TGA, UV–vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The polymer possesses excellent solubility in common organic solvents and good thermal stability with 5% weight loss temperature of 413 °C. The weight-average molecular weight of OTPAV-PT was 1.04 × 10⁴ with the polydispersity index of 1.45. Polymer solar cell with the configuration of ITO/PEDOT:PSS/OTPAV-PT:PCBM/Al was fabricated, and the power conversion efficiency of the device was 0.21% under the illumination of AM1.5, 100 mW/cm². The field effect hole mobility of the polymer reached 1.6 × 10^?4 cm² V^?1 s^?1.     

A comparative investigation between poly(1-ethynylpyrene) and poly(1,6-(3-ethynylpyrenylene)): Influence of the structure on the thermal,optical, electrochemical properties and conductivity

A comparative investigation between trans-poly(1-ethynylpyrene) (trans-PEP) obtained chemically and poly(1,6-(3-ethynylpyrenylene) (E-PEP) prepared electrochemically was carried out. Thermal and optical properties of the polymers were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and absorption spectroscopy. Electrochemical properties were evaluated by cyclic voltamperometry, in a 0.1 M Et₄NClO₄/THF solution at 10 mV/s, using a Pt disc as working electrode and Ag/AgCl as reference electrode. On the other hand, the conductivity of both polymers was measured in pressed pellet. Trans-PEP (T₁₀ = 369 °C) showed a higher thermal stability than its homologue E-PEP (T₁₀ = 256 °C). DSC of the polymers showed that trans-PEP exhibits a softening point at 330 °C, whereas E-PEP does it at 117 °C. Absorption spectra of the polymers revealed that trans-PEP exhibits two absorption bands at λ = 336 nm and λ = 580 nm due to the pyrene moieties and the highly conjugated polyacetylene main chain, respectively. By contrast, E-PEP showed only an absorption band at λ = 358 nm followed by a tail, which reveals that this polymer possesses a lower degree of conjugation. Molecular modelling performed in short segments of these polymers confirmed this hypothesis. Regarding the electrochemical properties, trans-PEP showed an anodic peak at 1500 mV and a conductivity value σ = 2.7 × 10^?2 S/cm, whereas E-PEP exhibited an anodic oxidation peak at 1670 mV and σ = 8.4 × 10^?2 S/cm.     

Synthesis and voltage-controlled multicolor electroluminescent property of copolyfluorenes containing a pendent anthraquinone-2,3-dicarboxylic imide

A new series of four fluorene-thiophene copolymers, PF-co-2%TAQI, PF-co-5%TAQI, PF-co-10%TAQI and PF-co-15%TAQI, were synthesized by the Suzuki cross-coupling polymerization of 9,9-dihexylfluorene-2,7-dibromofluorene and N-butyl 6-[3-(2,5-dibromothienyl)]anthraquinone-2,3-dicarboxylic imide. The polymers have good solubility in common organic solvents and can form thin films by spin coating or casting. The UV–vis and photoluminescence (PL) spectra for the copolymers in dilute solution are similar to those of polyfluorene, i.e. maximum absorption at 381 nm and 417 nm and PL at 439 nm. The UV–vis spectra of the polymer films also exhibit a single peak at 378 nm, while an extra emission peak was found at 612 nm, 618 nm, 628 nm and 626 nm for PF-co-2%TAQI, PF-co-5%TAQI, PF-co-10%TAQI and PF-co-15%TAQI, respectively, presumably resulting from the excimer of anthraquinone imide groups. The electroluminescent properties of the copolymers were investigated using single-layer (ITO/PEDOT/Polymer/LiF/Al) and double-layer (ITO/PEDOT/Polymer/TPBi/LiF/Al) (TPBi, 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene) LED devices. The double-layer devices based on copolymers emit multicolors under different voltages. The Commission Internationale de l’Eclairage (CIE) coordinates of PF-co-2%TAQI are (0.356, 0.249), (0.285, 0.210), (0.264, 0.277) and (0.240, 0.250) under 12 V, 14 V, 16 V and 18 V, respectively.     

Synthesis of random copolymers based on 3-hexylthiophene and quinoxaline derivative: Influence between the intramolecular charge transfer (ICT) effect and π-conjugation length for their photovoltaic properties

A series of low band gap, donor–acceptor polymers composed of regioregular 3-hexylthiophene segments and quinoxaline derivative units were synthesized by Stille coupling polymerization. The polymers had relatively low optical band gaps ranging from 1.61 to 1.83 eV. A bulk-heterojunction structure of glass/indium–tin oxide (ITO)/PEDOT:PSS/polymer-PCBM (1:3)/BaF₂/Ba/Al was fabricated to examine the photovoltaic properties. 1-(3-Methoxycarbonyl)propyl-1-phenyl-[6,6]-C-71 (PC₇₁BM) was used as the acceptor material owing to its increased absorption property in the visible region compared to 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PC₆₁BM). Among these polymers, P1 showed the best device performance with a PCE of 0.88%. These results provided an effective strategy for the design and synthesis of low band gap conjugated polymers with a broad range of absorption.     

Syntheses and investigation of polymers containing 1-triazene-1,3-diyl and 1,4-phenylene group

Triazene-group-containing polymers have been synthesized from p-phenylenediamine precursors by diazotization and azo-coupling methods. For this purpose, two pathways have been used. The effect of monomer/sodium nitrite ratio on the solubility of the polymers, as well as the dependence of properties such as electrical conductivity and ESR upon the main chain structure, have been investigated. Solubility tests showed that the lower the sodium nitrite/p-phenylenediamine molar ratio (<1), the higher the solubility of the resulting polytriazene. ¹H NMR, IR, and UV/vis absorption spectroscopies have been used to elucidate the chemical structures of the polymers obtained. Electrical conductivity studies and ESR measurements have been conducted on both as-synthesized and doped polymer samples. The room temperature electrical conductivity of the obtained polymers was found to increase from 10^?11–10^?9 to 7 × 10^?3–2 × 10^?2 S/m upon doping with iodine. According to ESR spectroscopy data, this increase in conductivity may be attributed to improved charge carrier (polaron) mobility. An important feature of these polymers is that their conductivity may be increased up to 10^?4 S/m by doping with hydrochloric and sulfuric acids.     

Characterization of poly(N-alkylanilines) by Raman spectroscopy

Thin films of poly(N-alkylaniline) were synthesized in acidic aqueous solution and in mixtures of aqueous and organic solvents. The polymer films (alkyl = methyl, ethyl, propyl and butyl) were characterized by Raman spectroscopy with the excitation wavelengths of 514.5, 632.8 and 780 nm. The main Raman bands have been characterized for the leucoemeraldine, emeraldine and pernigraniline oxidation states between −0.2 and 0.8 V (vs. Ag|AgCl). This fundamental study shows that the structure of the half-oxidized emeraldine form contains quinoid units, which supports the commonly accepted oxidation and reduction scheme of poly(N-alkylanilines).