A novel high-quality electrochromic material from 3,4-ethylenedioxythiophene bis-substituted fluorene

Poly(2,7-bis (2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9H-fluorene) (P(EDOT-FE)), a novel electrochromic material obtained from 3,4-ethylenedioxythiophene (EDOT) bis-substituted fluorene (FE) in CH₂Cl₂ solution, and its applications in electrochromic devices (ECD) are discussed. The external EDOT units will not only function as donor groups but also lower the oxidation potential. Fluorescent spectral studies indicate that P(EDOT-FE) with high fluorescence quantum yields and photochemical stability is a novel green-light-emitter. P(EDOT-FE) is switched between brown in the reduced state and blue in the oxidized state. ECD based on P(EDOT-FE) and poly(3,4-ethylenedioxythiophene) (PEDOT) was also fabricated and showed a good electrochromic performance. The ECD constructed by P(EDOT-FE) and PEDOT has good optical contrast (36% at 625 nm), high coloration efficiency (784 cm² C⁻¹), fast response time (0.5 s at 625 nm), better optical memory and long-term stability. Clear change from dark red (neutral) to dark blue color (oxidized) of ECD is demonstrated with robust cycle life. These results provide an avenue for applications of PEDOT family in electrochromic devices.     

New triphenylamine-based poly(amine-imide)s with carbazole-substituents for electrochromic applications

A new carbazole-derived triphenylamine-containing diimide-diacid monomer (5), 4,4′-bis(trimellitimido)-4″-N-carbazolytriphenylamine, is prepared by the condensation of 4,4′-diamino-4″-N-carbazolytriphenylamine (4) and two molar equivalents of trimellitic anhydride (TMA). A series of new poly(amide-imide)s (PAIs) 7a–7d with carbazole-substituted triphenylamine units are prepared by direct polymerization from the new diimide-diacid (5) and various aromatic diamines (6a–6d). The PAIs shows high glass transition temperature between 269 and 297 °C, and high 5% weight loss temperature between 526 and 561 °C under nitrogen environment. Cyclic voltammograms of the PAIs films, which are casted onto the indium–tin oxide (ITO)-coated glass substrate, exhibit two reversible oxidation redox couples at 1.05–1.08 V and 1.38–1.46 V under an anodic sweep. The PAI-7a film reveals excellent stability of electrochromic characteristics for the radical cations generated, changing color from original pale yellowish neutral form to the green and then to dark blue oxidized forms. Furthermore, the anodically coloring of PAI-7a shows high coloration efficiency (CE = 205 cm²/C), high contrast of optical transmittance change (ΔT = 80% at 776 nm) and long-term redox reversibility.     

Novel electrochromic and infrared emissivity modulation films based on poly(carbazoyltriphenylamine) and poly(carbazoyltriphenylamine-thiophene)

Synthesis and characterizations of novel conducting polymer film based on poly(carbazoyltriphenylamine) (poly(CBZ-TPA)) were studied. Additionally, copolymer film based on CBZ-TPA and thiophene (Th) was also obtained by electrochemical polymerization. Electrochromic properties of poly(CBZ-TPA) and poly(CBZ-TPA-Th) films were investigated by cyclic voltammetry and double step potential chronoamperometry. Optical properties of the two films were characterized by UV–vis spectrophotometer and FT-IR spectrometer. Different colors of poly(CBZ-TPA) film were achieved, which exhibits as camel gray (at −0.5 V), light gray (at 0 V) and army green (at 1.6 V). The maximum UV–vis absorption difference of poly(CBZ-TPA) film is about 15.83% in the visible region when applied with the voltage between 0 V and 1.6 V, and the coloring and bleaching time are 1.8 s and 1.0 s, respectively. The biggest different average IR-emissivity modulation of poly(CBZ-TPA) film is 0.37 in the wavelength regions of 8–14 μm. The copolymerization film of thiophene with CBZ-TPA has a distinct effect on the electrochromic performance of poly(CBZ-TPA) film. Compared with poly(CBZ-TPA) film, poly(CBZ-TPA-Th) film shows different colors, which exhibits as maize-yellow (at −0.5 V), milk white (at 0 V) and dark green (at 1.4 V). The maximum UV–vis absorption difference of poly(CBZ-TPA-Th) film is about 23.30% in the visible region between 0 V and 1.4 V, but it has a slower response speed (coloring and bleaching time are 3.0 s and 4.0 s, respectively). However, the biggest different average IR-emissivity modulation of poly(CBZ-TPA-Th) film reaches up to 0.41 in the wavelength regions of 8–14 μm compared to that of poly(CBZ-TPA) film.     

Electrochromic Window Based on Conducting Poly(3,4‐ethylenedioxythiophene)–Poly(styrene sulfonate)

A short survey of technological aspects of electrochromism with various electroactive species is given. Different approaches with inorganic and organic materials have been pursued in the past. So far widespread usage of this technology for large area applications has not been achieved. Nevertheless one major technical product, self‐darkening rear‐view mirrors for cars, is already well established. This article reviews some research results on electroactive polythiophenes, especially poly(3,4‐alkylenedioxythiophenes). Some promising results with the commercially available electrically conducting polymer Baytron P (PEDT/PSS) are presented. It is demonstrated that an all solid‐state electrochromic multilayer assembly based on a polymeric electrochromic material might be close to technical realization. The coating of large area substrates with aqueous poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) dispersion can be a way to an economically viable product.     

Effect of electrode surface on the electrochromic properties of electropolymerized poly(3,4-ethylenedioxythiophene) thin films

Electrochromism or electrochromic contrast in electropolymerized thin films of dioxythiophenes based conjugated polymers is known to be sensitive to the structure of monomer and the polymerizing conditions. However in our studies we found that it is also sensitive to the electrode surface wherein a significantly high electrochromic contrast is observed in the electropolymerized thin films of poly(3,4-ethylenedioxythiophene), PEDOT deposited on platinum (71%) as compared to that on indium tin oxide, ITO, coated glass surface (54%). This is attributed to the formation of more conjugated polymer on the metallic surfaces as confirmed by narrow and red shifted absorption peak for PEDOT on platinum compared to broad and blue shifted peak on ITO in the UV–vis absorption spectra. This difference in the electrochromic properties of electropolymerized PEDOT thin films on the two surfaces is investigated by studying their electrochemical growth using UV–vis absorption, Raman spectroscopy and atomic force microscopy techniques. These results suggest the deposition of more conjugated polymer in the initial stages of growth (≤3 mC/cm²) on both the substrates whereas it continues the same way into the intermediate stages (up to ∼15 mC/cm²) only on platinum, thereby, resulting in higher electrochromic contrast on platinum. The coloration efficiency of PEDOT thin film was also found to be improved on platinum (465 cm²/C) compared to that on ITO (230 cm²/C). Moreover, we observed that the EC contrast of electropolymerized PEDOT thin films on platinum was found to be insensitive to polymerizing solvent that is generally not the case when polymerized on ITO. The cyclic stability of PEDOT-Pt films are better compared to that of PEDOT-ITO which is attributed to the improved reversibility of these films with respect to potential switching.     

Enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene) thin films treated with H2SO4

Poly(3,4-ethylenedioxythiophene)–tosylate–polyethylene glycol–polypropylene glycol–polyethylene glycol (PEDOT–Tos–PPP) films were prepared via a vapor phase polymerization (VPP) method. The thermoelectric (TE) properties of the films before and after treated with H₂SO₄ at different concentrations were measured at 295 K. The TE properties of the films have been significantly improved by the H₂SO₄ treatment. For example, after treated with H₂SO₄ at 1 M, the electrical conductivity of the film has increased remarkably from 944 to 1750 S cm⁻¹, the Seebeck coefficient of the film reduced slightly from 16.5 to 14.6 μV K⁻¹, and the thermal conductivity decreased from 0.495 to 0.474 W/mK. Hence, the ZT value at 295 K has increased from 0.016 to 0.024. The electrical conductivity (Seebeck coefficient) of the untreated and 1 M H₂SO₄ treated PEDOT-Tos-PPP films decreases (increases) with increasing temperature from 295 to 375 K. And the power factor of the films monotonically increases with temperature. The power factor at 375 K of the 1 M H₂SO₄ treated film is almost twice as high as that at 295 K. Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) analyses, and the thickness measurement of the films indicate that the tri-block copolymer PPP have been removed from the PEDOT-Tos-PPP films after the H₂SO₄ treatment, and the UV–Vis-NIR absorption spectroscopy and Raman spectroscopy analyses reveal an increasing in the doping level in the PEDOT chains after the H₂SO₄ treatment. Therefore, the TE properties enhancement may be attributed to the combined effects of the removal of the insulating PPP from the PEDOT-Tos-PPP film, increase the doping level and conformational change of the PEDOT chains resulted from the H₂SO₄ treatment.     

High performance black-to-transmissive electrochromic device with panchromatic absorption based on TiO2-supported viologen and triphenylamine derivatives

A novel black-to-transmissive electrochromic device based on TiO₂-supported viologen and triphenylamine derivatives was designed and constructed via the absorption-complementary approach. In the device, cathodically coloring electrochromic material 1,4-bis[((N-phosphono-2-ethyl)-4,4′-bipyridinium)-methyl]-benzene tetrachloride acted as working electrode and novel anodically coloring electrochromic material (4-((4-(dimethylamino)-phenyl)(4-methoxyphenyl)-amino)-benzyl) phosphonic acid acted as counter electrode. The assembled electrochromic device achieved panchromatic absorption over entire visible spectrum with almost zero transmittance in colored state. The optical contrast (ΔT) of the device realized in this work was comparable to the highest value (60%) among all reported black-to-transmissive ECDs. Furthermore, excellent cycling stability was achieved, which maintained almost 80% of the initial ΔT value at 570 nm after continuous 100,000 switchings. These outstanding comprehensive electrochromic performances potentially make this device a promising candidate for electrochromic device applications.     

Durable electrochromic coatings prepared from electronically conductive poly(3HT-co-3TPP)-silica hybrid materials

In this paper, a series of organic-inorganic hybrid materials, consisting of heterocyclic conjugated poly(3-hexylthiophene), P3HT, network, and silica particles, were successfully prepared for electrochromic studies. First, the heterocyclic co-polymer of poly[3-hexylthiophene-co-N-(3-trimethoxysilylpropyl) pyrrole], P(3HT-co-3TPP), containing trimethoxysilyl functional groups in the co-polymer backbone as the sol-gel precursor were prepared by conventional oxidative polymerization. Subsequently, P(3HT-co-3TPP)-silica hybrid sol-gel materials in the form of coatings were prepared by baking the microslides and ITO-coated electrodes that had been cast with homogeneous blending solutions containing co-polymer, acid-dopant, tetraethyl orthosilicate (TEOS), and a few drops of water. The microstructures of silica particles formed in the P(3HT-co-3TPP)-silica hybrid materials were investigated by transmission electron microscopy (TEM). The as-prepared hybrid coatings had improved adhesion capability on inorganic glass substrates relative to the pure P3HT on the basis of electrochemical cyclic voltammetric studies and Scotch tape test evaluations. During potential cycling, the film color of P(3HT-co-3TPP)-silica hybrid materials and P3HT coated on ITO electrode changed from orange yellow (i.e., reduced form) to dark blue (i.e., oxidized form) as the redox reactions proceeded. Effects of the material composition of P3HT along with hybrid materials on the electrochemistry, spectroelectrochemistry, thermal stability, and electrical conductivity were also studied.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Donor–π-bridge–acceptor fluorescent polymers based on thiophene and triphenylamine derivatives as solution processable electrochromic materials

A series of novel donor–π-bridge–acceptor type conjugated polymers bearing 3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and triphenylamine (TPA) derivatives are synthesized via Stille coupling. The optical, fluorescent, electrochemical, spectroelectrochemical and electrochromic switching properties are characterized. The polymers display excellent solubility in low-boiling point organic solvents. Different electron-withdrawing pendent groups attached on TPA exert great influence on polymer’s fluorescent and electrochromic properties. All polymers show large transmittance changes in near-infrared region and reveal fast switching from one second to several seconds.     

A Highly Stable,New Electrochromic Polymer: Poly(1,4‐bis(2‐(3′,4′‐ethylenedioxy) thienyl)‐2‐methoxy‐5‐2″‐ethylhexyloxybenzene)

A highly stable new electrochromic polymer, poly(1,4‐bis(2‐(3′,4′‐ethylenedioxy)thienyl)‐2‐methoxy‐5‐2″‐ethylhexyloxybenzene) (P(BEDOT‐MEHB)) was synthesized and its electrochemical and electrochromic properties are reported. P(BEDOT‐MEHB) showed a very well defined electrochemistry with a relatively low oxidation potential of the monomer at + 0.44 V versus Ag/Ag⁺, E_1/2 at – 0.35 V versus Ag/Ag⁺ and stability to long‐term switching up to 5000 cycles. A high level of stability to over‐oxidation has also been observed as this material shows limited degradation of its electroactivity at potentials 1.4 V above its half‐wave potential. Spectroelectrochemistry showed that the absorbance of the π–π* transition in the neutral state is blue‐shifted compared to PEDOT, displaying a maximum at 538 nm (onset at 640 nm), thus giving an almost colorless, highly transparent oxidized polymer with a bandgap of 1.95 eV. Different colors observed at different oxidation levels and strong absorption in the near‐IR make this polymer a good candidate for several applications.     

High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

The device characteristics of top-gate field-effect transistors (FETs) based on typical polymer semiconductor regioregular poly(3-alkylthiophenes) (P3ATs) with different alkyl chain lengths are investigated. High field-effect mobilities of ∼10⁻² cm²/Vs are obtained irrespective of alkyl chain length even when polymer gate insulators with different dielectric constants (2.1–3.9) are used. This is attributed to the spontaneous formation of highly ordered edge-on lamellar structures at the surface of P3AT thin films that are the channel regions in top-gate FETs. In addition, top-gate P3AT FETs containing different gate insulators exhibit high operational stability, with low threshold voltage shifts of <0.5 V following prolonged gate bias stress, which is comparable to that of hydrogenated amorphous silicon thin film transistors.     

Poly(3,4-ethylenedioxythiophene)/graphene/carbon nanotube ternary composites with improved thermoelectric performance

Polymer based ternary thermoelectric composites have been studied. Here, poly(3,4-ethylenedioxythiophene)/graphene/carbon nanotube (PEDOT/graphene/CNT) ternary composites are prepared by in situ polymerization and subsequent physical mixing. Then, the morphology is directly observed by scanning electron microscopy. Finally, the thermoelectric performances are measured and discussed, where the effect of acid-treatment is investigated and comparison with those of the neat PEDOT and the binary PEDOT/graphene composite is conducted.     

Thermally Induced Transient Absorption of Light by Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) Films: A Way to Probe Charge‐Carrier Thermalization Processes

Infrared‐induced transient absorptions in the millisecond and sub‐picosecond time domains have been used to study the dynamics of charge carriers of a conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed infrared light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of infrared‐induced changes in the resistivity of the material. Near 1.5 eV, the infrared‐induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalization processes using short laser pulses. Pump‐probe spectroscopy on a sub‐picosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time constant of less than 500 fs, i.e., faster than for noble metals.     

A transparent honeycomb-like poly(3,4-ethylenedioxythiophene)/multi-wall carbon nanotube counter electrode for bifacial dye-sensitized solar cells

A novel transparent honeycomb-like poly(3,4-ethylenedioxythiophene)/multi-wall carbon nanotube (PEDOT/MWCNT) electrode served as a counter electrode (CE) for the bifacial dye-sensitized solar cell (DSSC) has been fabricated on the fluorine-doped tin oxide (FTO) glass using a sacrificial template of poly(methyl methacrylate) for the first time. Cyclic voltammetry and electrochemical impedance spectroscopy measurements indicate that the honeycomb-like PEDOT/MWCNT CE has a higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction and a smaller charge transfer resistance than those of the flat PEDOT CE. Ultraviolet–visible spectrophotometer measurement indicates that the PEDOT/MWCNT CE with honeycomb-like nanostructure demonstrates high transparency for the backside illumination. The bifacial DSSC based on this honeycomb-like PEDOT/MWCNT CE shows conversion efficiencies of 9.07% and 5.62% from front and rear side illumination, respectively, which are higher than those of the bifacial DSSC based on the flat PEDOT CE (7.51% and 3.49% respectively).     

Towards a Transparent,Highly Conductive Poly(3,4‐ethylenedioxythiophene)

A detailed investigation of the processing parameters influencing the oxidative polymerization of 3,4‐ethylenedioxythiophene (EDOT) and a methanol‐substituted derivative (EDOT–CH₂OH) was performed with the goal of maximizing the conductivity of the polymer. We show that the conductivity can be significantly enhanced by varying the monomer, oxidant (iron(III ) p‐toluenesulfonate (Fe(OTs)₃)), weak base (imidazole (Im)), solvent (various alcohols), and solution concentrations. The effect of each variable on the final materials properties is investigated, and the parameters have been optimized to achieve conductivities as high as 900 S cm^–1. Surface resistance below 150 Ω/□ for 80–90 nm thick films with visible‐spectrum transparency exceeding 80 % is achieved. The combination of these properties makes the films highly suitable for numerous device applications.     

High mobility organic field-effect transistors based on defect-free regioregular poly(3-hexylthiophene-2,5-diyl)

We report on organic field-effect transistors (OFETs) prepared using defect free (100% regioregular) poly(3-hexylthiophene-2,5-diyl) (DF-P3HT) as semiconductor and cross-linked poly(vinyl alcohol) (cr-PVA) as gate insulator. High field-effect mobility (μ_FET) of 1.2 cm² V⁻¹ s⁻¹ is obtained and attributed to the absence of regioregularity defects. These transistors have transconductance of 0.35 μS and the DF-P3HT film shows larger crystallites (∼80 Å) than a highly regioregular (>98%) material (∼32 Å). Devices with increased μ_FET (2.8 cm² V⁻¹ s⁻¹) could be obtained at the expense of the On-Off current ratio, which was reduced by one order of magnitude, when poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) treatment was applied to the dielectric surface. Our results suggest that the interaction of charged sites at the dielectric surface with regioregularity defects of the P3HT is an important factor degrading μ_FET even at very low concentration of regioregularity defects.     

Synthesis and characterizations of poly(3,6-thienophenanthrene) and poly(2,7-thienophenanthrene) and their applications in polymer light-emitting devices and solar cells

Here we report the synthesis of two novel phenylene-based polymers-poly(3,6-thienophenanthrene) (PTP36) and poly(2,7-thienophenanthrene) (PTP27) via base-free Suzuki–Miyaura reaction. The structure and electroluminescent properties of the meta-linked PTP36 and para-linked PTP27 are fully characterized. The obtained polymers were found to be liquid-crystalline, with broad band gap of 2.72 eV and 2.49 eV, respectively, which are much smaller than those of corresponding polyphenanthrenes. On the basis of PTP36 and PTP27, copolymers of 2,7-thienophenanthrene and 3,6-thienophenanthrene with 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzothiadiazole (DBT), namely PTP36-DBT and PTP27-DBT were prepared and be investigated as a potential donor material for polymer solar cells. The preliminary data show that the maximal power conversion efficiencies (PCEs) of the PTP27-DBT- and PTP36-DBT-based polymer solar cells are 3.5% and 0.9%, respectively.     

A dual electrochromic film based on nanocomposite of aniline and o-toluidine copolymer with tungsten oxide nanoparticles

A dual electrochromic (EC) film was prepared based on composite of tungsten oxide nanoparticles and copolymer of aniline and o-toluidine, by electrochemical polymerization method. In this method, aniline and o-toluidine monomers were dispersed in the solution containing tungsten oxide (WO₃) nanoparticles and the final mixture was used for electrodeposition of film on fluorine doped tin oxide (FTO) coated glass. WO₃ nanoparticles were incorporated in the copolymer matrix structure, and a dual EC film was constructed. EC properties of WO₃, copolymer and WO₃-copolymer nanocomposite films were evaluated by cyclic voltammetry (CV) and the UV–Vis spectrophotometry. Also, the optical response and coloration efficiency (CE) of samples were investigated. The composition of organic-inorganic EC materials have improved properties in application of the EC film such as significant optical modulation (37.35% at 633 nm) and high switching speed and high coloration efficiency (116.6 cm² C⁻¹ at 633 nm). These achievements were better than WO₃ or copolymer films. The improved EC properties were appertained to the mixture of prominences of both materials.     

The interplay of electronic structure,molecular orientation and charge transport in organic semiconductors: Poly(thiophene) and poly(bithiophene)

Ultrathin films of poly(thiophene) (PT) and poly(bithiophene) (PBT) were prepared by electrochemical route using ionic liquid (BFEE) as medium and electrolyte. Distinct morphologies and electrical properties were observed in these materials. To evaluate its response in photovoltaics, these films were used as active layer in bilayer geometry solar cells with the electron acceptor molecule C₆₀. The best performance was observed for PT films. In order to probe the differences in molecular dynamics and structural order, ultrafast electron dynamics in the low-femtosecond regime was evaluated by resonant Auger spectroscopy using the core–hole clock method at the sulfur K absorption edge. Electron delocalization times for the different polymeric films were derived as a function of the excitation energy. Photoabsorption measurements were conducted and molecular orientation derived. These results corroborated with the morphology found for these films and thus the performance of PT and PBT in the devices, and with the proposed conduction mechanism.