Synthesis and characterization of dithienothiophene vinylene based co-polymer for bulk heterojunction photovoltaic cells

An electron-donor–acceptor type co-polymer, PDTTBBO, composed of 3,5-dihexyldithieno [3,2-b:2′3′-d] dithiophene as donor and 2,6-dimethyl benzo[1,2-d; 5,4-d′] bisoxazole as an acceptor unit was synthesized by Horner–Wadsworth–Emmons (HWE) olefination reaction via multi-step procedure. The UV–vis absorption and photoluminescence emission peaks of the co-polymer in chlorobenzene were observed in a range from UV to near 700 nm. The lowest unoccupied molecular orbital (LUMO) and highest unoccupied molecular orbital (HOMO) levels of the co-polymer were estimated at ?5.71 eV and ?3.46 eV, respectively, corresponding to a band-gap of 2.25 eV. Bulk heterojunction photovoltaic cells were fabricated using a blend of PDTTBBO and PCBM with a ratio of 1:1. The short-circuit current density (J_sc), open-circuit voltage (V_oc) and fill factor (FF) of the device were estimated to be 1.306 mA/cm², 0.628 and 0.37, respectively, corresponding to power conversion efficiency (PCE) of 0.31% under AM 1.5 illumination.     

Synthesis,optical and electroluminescent properties of an alternating copolymer of triphenylamine and fumaronitrile

A new conjugated alternating copolymer of triphenylamine and fumaronitrile, TPA–FN, was synthesized via Heck coupling reaction and characterized by GPC, UV–vis, ¹H NMR and elemental analysis. The polymer film showed two absorption peaks at ca. 370 nm and 500 nm, respectively, and a narrow bandgap of 1.72 eV. The photoluminescence spectrum of TPA–FN film displayed a main peak at ca. 724 nm and a shoulder peak at ca. 808 nm, indicating that the polymer could be a near-infrared luminescent polymer. Cyclic voltammetry measurements revealed a lower LUMO energy level of TPA–FN at −3.85 eV. The lower LUMO level could be ascribed to the introduction of the electron-withdrawing fumaronitrile with two cyano groups attached to trans-vinylene, and it favors the electron injection for the application in the polymer light-emitting diodes (PLEDs). A single-layer PLED based on TPA–FN was fabricated and the device showed the turn-on voltage of 6 V and the maximum luminance of 136 cd/m² at 11.5 V.     

Synthesis,characterization and photovoltaic properties of poly{[1′,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene}

A new conjugated polymer, poly{[1′,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene} (PTVMEH-PPV) was synthesized via Grim polymerization. The polymer is soluble in common organic solvents such as chloroform and tetrahydrofuran, and possesses adequate thermal stability (T_d > 246 °C). The absorption spectrum of PTVMEH-PPV film shows a broader absorption peak covering the wavelength range from 380 nm to 620 nm, which is red-shifted and broadened in comparison with that of MEH-PPV. The onset oxidation potential of the polymer is 0.12 V versus Ag/Ag⁺, ca. 0.2 V lower than that of MEH-PPV. The band gap of the polymer measured by cyclic voltammetry is 1.82 eV, which basically agrees with that obtained from the onset wavelength of the absorption spectra. Polymer solar cell was fabricated based on the blend of PTVMEH-PPV and PCBM with a weight ratio of 1:1. The device shows the maximum external quantum efficiency of 14% at ca. 520 nm, an open circuit voltage of 0.67 V and a power conversion efficiency of 0.32% under the illumination of AM1.5, 80 mW/cm².     

Synthesis and photovoltaic property of a novel low band gap conjugated donor–acceptor copolymer consisting of 2,7-carbazole and (bithiophenevinyl)-(2-pyran-4-ylidenemalononitrile) (TVM)

A novel soluble conjugated donor–acceptor (D–A) copolymer, poly(2-(2-((E)-2(3,4-dihexyl-5-(7-methyl-9-octyl-9H-carbazole-2-yl)thiophene-2-yl)vinyl)-6((E)-2-(3,4-dihexyl-5-methylthiophen-2yl)vinyl-4H-pyran-4-ylidene)malononitrile) (CZPM), consisting of alternating 2,7-carbazole and (bithiophenevinyl)-(2-pyran-4-ylidenemalononitrile) (TVM) was synthesized by Suzuki coupling polymerization. The photophysical and photovoltaic properties of the copolymer were investigated. The UV–vis absorption spectrum and cyclic voltammetry measurements of CZPM film showed that the copolymer has a relatively low band gap (1.81 eV) and a deep highest occupied molecular orbital (HOMO) level (?5.55 eV). Bulk heterojunction photovoltaic devices were fabricated using CZPM as donor and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) as acceptor with the weight blend ratio ranging from 1:1 to 1:4. The device with the blend ratio of 1:2 between CZPM and PCBM exhibited an open-circuit voltage (V_oc) of 0.74 V and a power conversion efficiency (PCE) of 0.1% under simulated AM1.5 solar irradiation (100 mW/cm²). The relatively low conversion efficiency of the device is attributed to the ineffective exciton dissociation at the CZPM/PCBM interface and the imperfect film-forming property of the polymer resulting from the low molecular weight.     

Synthesis,electrochemical, and spectroelectrochemical properties of conductive poly-[2,5-di-(2-thienyl)-1H-pyrrole-1-(p-benzoic acid)]

Poly(ter-heteroaromatic(thiophene-pyrrole-thiophene)), PDPB, was electrochemically prepared from the 2,5-di(2-thienyl)-1H-pyrrole-1-(p-benzoic acid) (DPB) monomer using the Paal-Knorr pyrrole condensation reaction. The structure of the monomer was confirmed using ¹H-, ¹³C NMR, FT-IR and mass spectroscopy. The maximum UV–visible absorption and PL emission bands of DPB were observed at 330 nm and 500 nm, respectively. The cyclic voltammograms (CVs) recorded for the electrochemically polymerized DPB revealed a set of redox peaks at 0.65/0.53 V. The conductivity monotonically increased with respect to the applied potential from 0.0 V to 1.0 V, exhibiting a maximum conductivity of 0.18 S/cm at +0.80 V. The in situ UV–visible spectroelectrochemical analysis of PDPB revealed electronic transitions at 420 nm, 654 nm, and 870 nm corresponding to the π–π* transition, polaron, and bipolaron states, respectively. The optical band-gap (E_g) of PDPB was 2.16 eV. The color of the PDPB film transitioned yellow (at 0.0 V) to blue (at 1.0 V) when the potential was switched between the reduced and oxidized states with a good electrochromic response time (0.95 s).     

Synthesis,optical and electrochemical properties of novel hole-blocking poly(biphenylene-1,3,4-oxadiazole) containing electron-withdrawing trifluoromethyl groups

New poly(biphenylene-1,3,4-oxadiazole) P1 and poly(biphenylene hydrazide) P2 containing electron-withdrawing trifluoromethyl group at the 2 and 2′ positions of biphenyl moiety were synthesized. The biphenyl is forced to adapt a non-coplanar conformation due to the bulky trifluoromethyl group. High quality polymer P1 thin film can be easily obtained by thermal cyclodehydration from its soluble polyhydrazide precursor P2. The polymer P1 exhibited good thermal stability with glass transition temperature of 234 °C and 5% decomposition temperature of 469 °C. The optical and electrochemical properties were investigated by UV–vis spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. For polymer P1 film, two absorption peaks at 370 and 414 nm were observed. It also exhibited a photoluminescent peak at 555 nm when excited by 414 nm light. The cyclic voltammetric studies revealed that polymer P1 had extremely low HOMO (?6.85 eV) and LUMO (?3.71 eV) energy levels due to the presence of strong electron-withdrawing trifluoromethyl group. It also exhibited a large energy gap (3.14 eV) which is an indication of short conjugation length resulted from non-coplanar biphenyl structure. Its HOMO energy was even lower than that of widely used hole-blocking material, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole PBD (?6.30 eV). The low LUMO energy of polymer P1 could allow the easier electron injection from air-stable cathode such as aluminum. The good electron transfer ability was also shown by measuring the current density of electron-only devices with the structures of Al/P1 (150 nm)/Alq₃ (50 nm)/Al and Al/Alq₃ (200 nm)/Al. Combined with high thermal stability and amorphous morphology, polymer P1 would be a promising candidate as the hole-blocking material for organic light-emitting diodes.     

Synthesis of a benzothiadiazole/thiophene-based oligomer for bulk heterojunction photovoltaic cells

An electron-donor–acceptor-type oligomer, oligo(B–EDOT₂–HT₃), composed of 2,1,3-benzothiadiazole (B), 3,4-ethylenedioxythiophene (EDOT) and 3-hexylthiophene (HT) units was synthesized via a multi-step procedure. The UV–vis absorption and photoluminescence emission peaks of the material in chloroform were observed at 510 and 660 nm, respectively. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of the material were estimated as ?3.47 and ?5.49 eV, respectively, corresponding to a band-gap of 1.97 eV. Bulk heterojunction photovoltaic cells were fabricated, using oligo(B–EDOT₂–HT₃) and PCBM, in the device configuration of ITO/PEDOT:PSS:6% glycerol/oligo(B–EDOT₂–HT₃):PCBM/LiF/Ag. The short-circuit current density, open-circuit voltage and fill factor of the device were estimated to be 1.44 mA/cm², 0.33 V and 0.36, respectively, with a 1:4 ratio of oligo(B–EDOT₂–HT₃) to PCBM, corresponding to an energy conversion efficiency of 0.17% under AM 1.5 illumination. One main reason for the low efficiency of the device was attributed to the low absorptivity of oligo(B–EDOT₂–HT₃) in the UV–vis spectral range.     

The influence of electron-deficient comonomer on chain alignment and OTFT characteristics of polythiophenes

The structure–property relations of polythiophene derivatives have been the subject of research for both materials and electronic device applications. We have designed and prepared two kinds of polythiophene derivatives with similar chemical structures but different electronic properties by Stille polymerization. One is composed of only electron-rich thiophenes (PQT2T), while the other is composed of both electron-rich thiophenes and electron-deficient thiazoles (PTZT2T). The number-average molecular weights of PQT2T and PTZT2T were 23,000 (PDI = 1.5) and 13,000 (PDI = 1.7), respectively. PTZT2T showed a large bathochromic shift of absorption maximum from solution to film by 57 nm. Further investigation by differential scanning calorimetry and X-ray diffraction revealed the formation of well-ordered interdigitated structure of PTZT2T in the solid state (d₁ = 20.7 Å). From absorption onset, optical band-gap of PQT2T and PTZT2T was calculated to be 2.22 eV and 1.92 eV, respectively. A mobility of 1.1 × 10^?3 cm²/Vs, an on/off ratio of 4.7 × 10⁴ have been obtained from organic thin-film transistors (OTFTs) using PTZT2T as a channel. By comparison with PQT2T, the mobility was 18 times and the on/off ratio was 52 times higher for PTZT2T, which shows the usefulness of electron-deficient thiazoles as comonomer units of polythiophene derivatives for OTFT applications.     

Electrosynthesis of 2,7-linked polycarbazole derivatives to realize low-bandgap electroactive polymers

Poly(2,7-carbazole) derivatives have been synthesized because they have more extended conjugation lengths and lower energy bandgaps than poly(3,6-carbazole)s; however, few studies regarding electrochemical synthesis of poly(2,7-carbazole)s have been reported. Here, a series of N-alkylated-2,7-di(2-furyl)carbazoles, N-butyl-2,7-di(2-thienyl)carbazole, and N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole, were synthesized to obtain electroactive films of poly(2,7-carbazole) derivatives by electrochemical polymerization. Cyclic voltammetry revealed that these monomers have excellent polymerization activity, due to their low oxidation potentials (<0.57 V vs. ferrocene (Fc/Fc⁺)), and the corresponding polymers exhibit good redox properties. The energy bandgaps of the polymers obtained from optical absorption spectra range from 2.1 to 2.3 eV. Among the polymers, poly[N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole] shows the lowest bandgap energy of 2.1 eV, which is lower than that of previously reported poly(3,6-carbazole) analogues (2.4 eV). This polymer exhibits a significant color change from red in the oxidized state to blue in the reduced state during an electrochemical redox process. The electrochemical and optical properties of the monomers are dependent on external heteroaromatic rings attached to the 2 and 7 positions of the carbazole unit.     

Spectroscopic investigation of the interactions between emeraldine base polyaniline and Eu(III) ions

The interactions of emeraldine base form of polyaniline (EB-PANI) and Eu(III) ions in 1-methyl-2-pyrrolidinone (NMP) solution and in films have been investigated by UV–vis–NIR, resonance Raman, luminescence and electron paramagnetic resonance (EPR) spectroscopies. These spectroscopic techniques allowed to characterize quinone and semiquinone segments in the polymeric chains, and the oxidation state of europium ions in Eu-PANI samples. For high values of Eu(III)/N molar ratio (24/1) the presence of a weak polaronic absorption band at 980 nm in UV–vis–NIR spectrum and the observation of bands at 1330 and 1378 (ν_C–N+) cm^?1 due to emeraldine salt in the Raman spectrum at 1064 nm indicate a low doping degree. Oxidation of EB-PANI to pernigraniline base (PB-PANI) occurs in diluted solutions. The experimental data showed that the solvent plays an important role on the nature of formed species. The narrow EPR signal at g = 2.006 (line width 8G) confirms the presence of PANI radical cations in Eu-PANI film. The absence of broad signal characteristic of Eu(II) in EPR spectrum suggested that europium ions are primarily at Eu(III) oxidation state. The luminescence spectra of Eu-PANI film presented emission bands at 405 and 418 nm assigned to PANI moieties and bands at 594, 615 and 701 nm assigned to ⁵D₀ → ⁷F_J (J = 1, 2 and 4, respectively) transitions of Eu(III). EPR and photoluminescence data confirm that europium ions are mainly in Eu(III) oxidation state in Eu(III)/PANI films.     

Luminescence and photovoltaic cells of benzoselenadiazole-containing polyfluorenes

In this work, light-emitting and photovoltaic properties of new low band-gap copolymers (PFO-DBSe) based on 9,9-dioctylfluorene and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole (DBSe) were investigated. The peak positions of photoluminescence and electroluminescence (EL) of the copolymers reach 758 and 727 nm, respectively, and locate in near-infrared light region. At a current of ∼5 mA, EL emission with peak position of 701 nm and a maximum external quantum efficiency of 1.62% was achieved from EL device with poly(9,9-dioctylfluorene) and the copolymer with DBSe content of 5% as the blend-type emitter, by utilizing of a configuration of ITO/PEDOT/PVK/emitter/CsF/Al. This is a high efficiency for deep red EL emission. The copolymer with DBSe content of 35% possesses good solar light spectral coverage according to its absorption spectrum, and a bulk-heterojunction photovoltaic cell with the copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor displayed an energy conversion efficiency of 0.91% under an AM1.5 solar simulator at 100 mW/cm².     

Design and synthesis of a new thiophene based donor–acceptor type conjugated polymer with large third order nonlinear optical activity

In this communication we describe the design and synthesis of a new conjugated polymer (P) carrying 3,4-dialkoxythiophene, 1,3,4-oxadiazole and thienylene-vinylene units, from its monomers using Wittig condensation method. The structure of newly synthesized polymer was confirmed by FT-IR, ¹H NMR, UV–vis spectral, elemental analysis and gel permeation chromatographic techniques. The polymer exhibited good thermal stability with the onset decomposition temperature around 320 °C in nitrogen atmosphere. Further, its electrochemical, linear and nonlinear optical properties have been investigated. The optical and electrochemical band gap was found to be 2.21 eV. Its third-order nonlinear optical property was investigated by Z-scan and DFWM techniques, using a Q-switched, frequency doubled Nd:YAG laser producing 7 ns laser pulses at 532 nm. Z-scan results reveal that the polymer exhibits self-defocusing nonlinearity. The operating mechanism involves reverse saturable absorption. The polymer shows strong optical limiting behavior due to effective two-photon absorption (2PA). The value of 2PA coefficient was found to be 3.0 × 10^?11 mW, which is comparable to that of good optical limiting materials. The fluorescence quantum yield of the polymer in solution was determined using quinine sulfate as standard and it was found to be 35%.     

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 properties of a new two-photon absorbing chromophore

This paper presents of a new, conjugated and symmetric multi-branched two-photon absorption chromophore 1,2,4,5-tetrakis(4-N-methyl-N-hydroxyethylamino)vinylphenyl benzene (TKAVPB) with D-π-D structure. The new dye built on 1,2,4,5-benzene core with four amino-branching units, which allows efficient π-electron delocalization, exhibits moderate two-photon absorption (TPA) value in the femtoseconds regime (TPA cross-section as 97 × 10⁻⁵⁰ cm⁴ s photon⁻¹ molecule⁻¹ with 200 fs laser pulses). Linear absorption, single and two-photon fluorescence spectra were experimentally studied. When pumped with 760 nm laser irradiation, TKAVPB shows strong two-photon up-conversion fluorescence with peak at 527 nm.     

Near infrared electrochromism of lutetium phthalocyanine

The near infrared (NIR) electrochromism of lutetium phthalocyanine (LuPc₂) is studied both in its solution and vacuum deposited thin film. The LuPc₂ in neutral state has near infrared absorption in the region from 1100 nm to 1600 nm. In electrochemical oxidized state (+1.0 V), it has an absorption peak at 880 nm. In electrochemical reduced state (−1.0 V), it shows no absorption in the near IR region. The attenuation at 1300 nm of electrochromic (EC) devices based on solution and thin film of LuPC₂ was also studied. Attenuation of 0.31 dB and 0.28 dB was achieved respectively and the response time was 24 s and 1 s, respectively. These results demonstrate that LuPc₂ is a kind of NIR EC material and it has potential application for variable optical attenuator.     

Synthesis,crystal structure and two-photon absorption of a cadmium complex constructed by DMIT ligand

The cadmium complex [(CH₃)₄N][Cd(C₃S₅)₂]·H₂O constructed by 1,3-dithiole-2-thione-4,5-dithiolato (DMIT) ligand has been synthesized and its structure has been determined by means of X-ray single crystal diffraction. Its two-photon absorption (TPA) has been studied in acetonitrile solution using open-aperture Z-scan technique with 20 picosecond (ps) pulses at wavelength 1064 nm. Its TPA coefficient β was determined to be 4.475 × 10^?10 m/W, corresponding the TPA cross-section σ₂ is 3.95 × 10^?53 (m⁴ s/photon molecule).     

PEDOT/PAMPS: An electrically conductive polymer composite with electrochromic and cation exchange properties

Poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propane sulfonate) (PAMPS) composite films were electrochemically prepared from a mixture of water and N,N-dimethylformamide (DMF) containing 3,4-ethylenedioxythiophene (EDOT) and the polyelectrolyte, PAMPS. The presence of PAMPS in the PEDOT matrix was confirmed by spectroscopic and electrochemical methods. Depending on the current density, the conductivity of PEDOT/PAMPS free standing composite films reached values of 80 S/cm. Spectroelectrochemistry of neutralized PEDOT/PAMPS composite films showed a maximum absorption at 2.0 eV (615 nm) and a band gap of 1.65 eV, as calculated from the onset of the π–π* transition. Thin PEDOT/PAMPS composite films were found to switch rapidly between oxidized and reduced states in less than 0.4 s with an initial optical contrast of 76% at λ_max: 615 nm. The morphology of the polymer composites demonstrates a cauliflower structure. Despite the high molecular weight of the polyelectrolyte, the film density was found to be similar to classical PEDOT (i.e., ca. 1.4 g/cm³), while the surface roughness averaged below 5%. As expected with the use of a sulfonated polyelectrolyte as dopant, cation exchange properties were observed with hexaammineruthenium(III) chloride as an active electrolyte.     

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.     

Dynamics of space charge distributions in side-chain PPV LEDs

A single wavelength electro-optical reflection technique was used to study the creation of space charge distributions (SCDs) in an Al/PPV/ITO LED by monitoring the third order NLO response of the electro-luminescent layer, at a wavelength far from the main absorption band (λ=632.8 nm). The polymer is a PPV derivative with oxidiazole groups grafted as side chains. The measurements show the presence of SCD given by the sum of a static contribution close to the ITO, and of a slowly changing and bias dependent one due to mobile charges injection. Taking into account Debye–Hückel screening in the polymer, we estimate the density of the fixed charges to be ρ₀=1.5×10²⁴ m⁻³, for a uniform distribution in a 110 nm thick layer. The dynamics of the mobile charges was investigated by time-dependent measurements. The samples were biased at a voltage V_b for a time interval and the time dependence of the signal was studied after switching off V_b. The measurements were performed for several temperatures between 60 and 110°C, indicating complex dynamics of the signal. Measurements performed at low temperature (88 K) show the features of dispersive transport. The dynamic measurements were investigated by the electro-absorption technique also, with wavelengths laying in the main absorption band of PPV.     

Ablation of PEDOT/PSS with excimer lasers for micro structuring of organic electronic devices

In this paper we present a potentially fast method for high resolution micro structuring of organic electronics via laser patterning. An investigation of the absorption spectrum in the UV/VIS regime of poly (3,4-ethylene dioxythiophene) poly (styrene-sulfonate) (PEDOT/PSS) has shown that UV-laser radiation should be used for optimal laser ablation of the material. Hence, the ablation characteristics of PEDOT/PSS with two different excimer lasers are compared with each other. The optimal fluence for the ablation of the material has been determined. The lasers used in this study are ArF (λ = 193 nm) and KrF (λ = 248 nm) excimer lasers.