D-π-A organic dyes with carbazole as donor for dye-sensitized solar cells

Two donor-π-acceptor (D-π-A) organic dyes with carbazole as donor, phenylethynyl, thienylethynyl as π-spacers and cyanoacrylic acid as acceptor, have been synthesized and characterized. These dyes exhibit charge transfer character in the ground and excited states as supported by the UV–vis and fluorescence studies. They also show interesting electrochemical properties. DFT and TDDFT studies reveal that large intramolecular charge transfer takes place from the HOMO to LUMO, though the donor carbazole is twisted (～51°) with respect to the π-conjugated spacer and acceptors with an idea of testing the dyes as sensitizers for DSSC. The DSSC devices were fabricated with these dyes by using redox electrolyte in a nonvolatile methoxypropionitrile solvent. The efficiency of the cells, short circuit current density, J_sc, and open circuit photovoltage, V_oc, and fill factor, FF, has been obtained for the two molecules. Calculations based on DFT plane wave method reveal the strong binding of the dyes on the surface of TiO₂ (1 0 1) surface. It is concluded that these dyes can play the role of sensitizers in DSSC.     

Effects of anchoring groups in multi-anchoring organic dyes with thiophene bridge for dye-sensitized solar cells

Organic photo-sensitizers were designed and synthesized based on a phenothiazine framework containing single- and double-electron acceptors that were bridged with thiophene for the dye sensitized solar cells (DSSCs). The optimized geometries were determined with density functional theory (DFT) calculations to estimate the photovoltaic properties of the dyes in the design stage. The organic dye with the double electron acceptors exhibited a better light absorption at long wavelength and an effective electron extraction pathway from the electron donor to the TiO₂ surface, leading to an improved short-circuit current (11.6 mA cm^?2), compared with that of the dye with the single electron acceptor (10.2 mA cm^?2) and the conventional N3 Ru-dye (10.4 mA cm^?2). Contrarily, the open-circuit voltage of the organic dye with the double electron acceptors decreased because the additional protonated carboxylic groups caused a positive shift in the Fermi level of TiO₂.     

Effect of donor moiety in organic sensitizer on spectral response,electrochemical and photovoltaic properties

This paper describes the effects of donating ability in a D (donor)–π–A (acceptor) structured organic dye on the HOMO and LUMO energy levels and photovoltaic performance in dye-sensitized solar cell. Two different dyes were synthesized based on the cyanoacetic acid as an acceptor and the phenyl-thiophene as a π-conjugated bridge, where one contains a diethyl amino group (P2) and the other has an ethoxy group (P5) as the donor. The HOMO–LUMO energy gap was significantly influenced by donor group, where change of ethoxy donor with stronger amino one in donating ability decreased the energy gap from 2.84 eV to 2.21 eV. Moreover, it was also found that HOMO energy level was more sensitive to change in donor group than LUMO one. Photovoltaic performances of P2- and P5-dye-sensitized solar cells were systematically investigated in terms of the effects of co-adsorbent concentration, TiO₂ film thickness and TiCl₄ post-treatment. Under the best condition, the P5 dye showed the conversion efficiency of 2.62%, while the P2 dye exhibited much higher efficiency of 5.89% due to broader absorption.     

Properties of composite films of titania nanofibers and Safranin O dye

Titania (TiO₂) nanofibers and composite thin films of titania nanofibers and Safranin O dye (SAF) were studied. TiO₂ nanofibers were prepared by electrospinning technique from titanium tetra-isopropoxide precursor solution in ethanol. Surface topology of the nanofibers was observed using scanning electron microscopy (SEM), their crystal structure was studied by X-ray diffraction (XRD) and the chemical composition by X-ray photoelectron spectroscopy (XPS). Properties of the TiO₂ nanofibers were studied in dependence on the values of relative air humidity in the range from 15% to 55%. It was necessary to maintain the relative humidity lower than 30% during electrospinning in order to obtain high quality nanofiber films. The average minimum diameter of the as-prepared TiO₂ nanofibers was found to be around 100 nm. Nanofiber diameter diminishes to about 50 nm after annealing at 420 °C for 1 h. The as-prepared titania nanofiber films were completely amorphous while anatase crystal phase was detected in the films after annealing. In order to prepare the composite films, solution of SAF dye with poly(vinylpyrrolidone) in ethanol/water was dropped off on the prepared titania nanofibers surface. Opto-electrical properties of SAF dye and the resulting nanocomposite films were studied by UV–Vis spectroscopy and current–voltage characteristics. Safranin O is characterized by two strong absorption peaks; one at 274 nm and a wide band with splitting between 420 nm and 600 nm. The optical energy band gap of titania nanofibers was estimated from the UV–Vis measurements to be 3.4 eV. The charge transport in the composite films is influenced by the space charge limited currents due to the very high resistance of the materials.     

Synthesis and characterization of conjugated copolymer containing 2,5-bis(3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole and 2,6-[(1,5-didecyloxy)naphthalene for polymer solar cells

A new alternating donor–acceptor conjugated copolymer, poly{(2,5-bis(3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl)-alt-(2,6-[(1,5-didecyloxy)naphthalene])} (PNTzTz) based on thiazolothiazole and didecyloxynaphthalene units was synthesized using a typical Suzuki coupling polymerization method and was explored as a donor polymer in bulk heterojunction (BHJ) solar cells. The copolymer was characterized using differential scanning calorimetry, thermogravimetric analysis, UV–Vis absorption and cyclic voltammetry. It possesses moderate molecular weights and excellent thermal properties with a 5% weight loss around 340 °C. It also exhibits good optical absorption with an absorption peak at 475 nm and an absorption onset at 560 nm in the film state. Photovoltaic properties of the copolymer were investigated blended with electron acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The optimized photovoltaic device with a device structure of ITO/PEDOT:PSS/PNTzTz:PC₆₁BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 0.99% with a short circuit current density of 3.3 mA cm^?2, an open circuit voltage of 0.96 V and a fill factor of 0.33 under the illumination of AM 1.5, 100 mW cm^?2.     

Molecular design of donor–acceptor-type cruciform dyes for efficient dyes-sensitized solar cells

Novel donor–acceptor (D–A) type cruciform dyes were synthesized for use in a dye-sensitized solar cell. The geometries and electronic structures of the dyes were designed by the results of density functional theory (DFT) calculations. Two phenothiazine units in these dyes act as electron donors, while two cyanoacrylic acid groups act as electron acceptors. The solubility of these compounds is increased because of the presence of the hexyl groups in the phenothiazine rings. The photovoltaic properties of 14D-25A and 12A-45D were measured to identify the effects of the dye having two electron donors and two electron acceptors on the DSSC performance. The 12A-45D dye showed a power conversion efficiency of 4.5% under AM 1.5 illumination (100 mW cm^?2) in an photoactive area of 0.42 cm², short circuit current density of 10.8 mA cm^?2, open circuit photovoltage of 0.65 V and a fill factor of 64%.     

Synthesis of a side chain conjugated polythiophene copolymer and its photovoltaic property

A new soluble polythiophene copolymer with phenylene–vinylene conjugated side chain (PEHOPVTh-Th) was synthesized. Polymer photovoltaic solar cells using PEHOPVTh-Th as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as acceptor was demonstrated. The absorption spectrum of the PEHOPVTh-Th showed strong absorption in the visible region and a 55 nm red-shift in comparison with that of its homopolymer (PEHOPVTh). By adjusting the ratio of PEHOPVTh-Th to PC₆₁BM, the polymer solar cells based on PEHOPVTh-Th showed a power conversion efficiency of 0.37% under 100 mW/cm² AM 1.5G simulated sunlight when the weight ratio of the polymer to PC₆₁BM equals to 1:3.     

Low band gap copolymer containing naphthalene-1,4,5,8-tetracarboxylic bisimide: Synthesis,properties and organic solar cell applications

A new alternating donor–acceptor copolymer, poly{[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-4,7-(2,1,3-benzothiadiazole)]-co-[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracarboxylic-N,N′-di(2-ethylhexyl)imide]} (PFTBTN), was synthesized for the use of photovoltaic cells. The copolymer containing fluorene, as the donor segment and naphthalene bisimide, dithienylbenzothiadiazole, as the acceptor segment was polymerized via Suzuki couplings to achieve a polymer with a narrow band gap. The band gap values of the copolymer film determined from optical and electrochemical measurements were 1.69 and 2.06 eV, respectively. The optical absorption spectrum revealed two broad bands in the range of 300–750 nm. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high open-circuit voltage when fullerene derivative ([6,6]-phenyl-C₆₁ butyric acid methyl ester) was used as an electron acceptor. Bulk heterojunction photovoltaic cells were fabricated in the device configuration of ITO/PEDOT:PSS/PFTBTN:PCBM/TiO_x/Al. Open-circuit voltage reached 0.74 V with the maximum energy conversion efficiency of 0.40% under the illumination of AM 1.5 (100 mW/cm²).     

Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

Synthesis and third order optical nonlinearity studies of the donor–acceptor conjugated polymer,poly(2-[3,4-didodecyloxy-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-5-phenyl-1,3,4-oxadiazole) and a polymer/TiO2 nanocomposite

A donor–acceptor conjugated polymer (PDOThPOD) containing 3,4-didodecyloxythiophene and (1,3,4-oxadiazolyl)benzene units is synthesized by using precursor polyhydrazide route. Cyclic voltammetry studies reveal that the polymer possesses a low-lying LUMO energy level (?3.57 eV). A nanocomposite of polymer PDOThPOD and nano TiO₂ is also prepared. The linear optical properties of PDOThPOD and the nanocomposite are studied using UV–vis absorption spectroscopy and fluorescence emission spectroscopy. z-scan technique is used to study the third order nonlinear optical properties of the polymer and nanocomposite films. Both polymer and nanocomposite films show a strong optical limiting behavior. The value of nonlinear absorption coefficient (β) is found to be of the order of 10^?7 m/W, indicating that these materials are potentially suited for fabricating optical limiters.     

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 and characterization of pentacene derivative by Wittig–Horner reagent and its composite with nano-sized TiO2

Wittig–Horner reagent was firstly used to prepare a pentacene derivative which was designated as 6-phenylidene-13,13-dihydropentacene (PHP). The newly prepared derivative was more soluble than pentacene due to the introduction of substituted group. UV–vis spectra showed that it displayed a wide absorption in the range of 470–650 nm in CHCl₃, and with a maximum absorption at 555 nm. In cyclic voltammetry (CV) test, it was found that its forbidden band gap was as small as 1.34 eV. During the photo-oxidation test, the as-prepared pentacene was sensitive to ambient light but sluggish to air in solution. Moreover, a composite of PHP–TiO₂ was also constructed. A series of characterizations proved that an interaction between PHP and TiO₂ most probably had occurred, leading to generation of some novel properties which were different from both PHP and TiO₂. The most interesting result is that the band gap of the composite material is smaller than that of PHP and TiO₂, with the value of 1.18 eV.     

A cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer for organic solar cells

A new cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer (P1) was synthesized using a microwave-assisted Stille coupling procedure and was compared to a known benzodithiophene-based copolymer using the same thienopyrroledione acceptor monomer (PBDTTPD). Cyclopentadithiophene-based copolymers have been known to exhibit lower band gaps than their corresponding benzodithiophene-based counterparts. The polymer showed excellent solubility at room temperature in chlorinated solvents. The absorption onset for P1 is close to 740 nm as compared with ～685 nm for PBDTTPD, corresponding to an optical band gap of 1.67 eV, which is 0.15 eV lower than PBDTTPD. The photovoltaic characteristics of the polymer were determined under AM1.5 illumination. The P1:PCBM BHJ device showed a high V_oc (0.92 V) and J_sc (8.02 mA/cm²) as well as a good PCE (2.43%), while the best device with 2% solvent additive gave a PCE of 3.47%.     

Conducting polymer embedded with nanoferrite and titanium dioxide nanoparticles for microwave absorption

The present paper deals with the synthesis of conducting ferrimagnetic polyaniline nanocomposite embedded with γ-Fe₂O₃ (9–12 nm) and titanium dioxide (70–90 nm) nanoparticles via a micro-emulsion polymerization. The microwave absorption properties of nanocomposite in 12.4–18 GHz (Ku-band) frequency range shows shielding effectiveness due to absorption (SE_A) value of ?45 dB, which is much higher than polyaniline composite with iron oxide and polyaniline–TiO₂ composites. The higher EMI shielding is mainly arising due to combined effect of γ-Fe₂O₃ and TiO₂ that leads to more dielectric and magnetic losses which consequently contributed to higher values of shielding effectiveness. XRD analysis of the nanocomposite reveals the incorporation of nanoparticles in the conducting polymer matrix while the thermal gravimetric analysis (TGA) demonstrates that the nanocomposite is stable up to 250 °C.     

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.     

Effect of TiO2 on thermal,structural and third-order nonlinear optical properties of Ca–La–B–O glass system

A series of calcium lanthanum metaborate glasses in the composition (wt%) of 23.88CaO–28.33La₂O₃–47.79B₂O₃ modified with TiO₂ up to 20 wt% are prepared by a melt quenching technique to study the influence of TiO₂ on their thermal, structural, linear and nonlinear optical properties. The differential thermal analysis (DTA) studies have demonstrated significant effects due to the presence of TiO₂ on the glass forming ability and crystallization situations. The glass with 15 wt% TiO₂ has achieved a eutectic composition and also exhibited a better glass forming ability among the glasses studied. The FT-IR spectra of these glasses show mainly vibration modes corresponding to stretching of BO₃ trigonal, BO₄ tetrahedral units and of B–O–B bending bonds. At higher concentrations of TiO₂, development of vibration band around 400 cm^?1 has indicated the formation of TiO₆ structural units in the glass network. The red shift of optical absorption edge (UV cutoff) shows a monotonous decrease in direct and indirect optical band gap energies (E_opt) with an increase of TiO₂ content in the glasses based on their absorption spectra. The optical transparency of these glasses is found to be varied from 64 to 87% within the wavelength range 450–1100 nm depending on the TiO₂ content. Besides these studies, linear refractive indices, the nonlinear optical properties of these glasses have also been evaluated.     

Synthesis and photovoltaic application of donor–acceptor type copolymers containing 9-cyanomethylenefluorene derivatives with chemically tunable electronic properties

Four 9-cyanomethylenefluorene derivatives were synthesized as a new class of electron-deficient building blocks for donor–acceptor (D–A) type alternating copolymers. UV–vis absorption spectroscopy, cyclic voltammetry and density functional theory calculations show that the substituent X at the cyanomethylene unit, depending on its electron-withdrawing ability (X = –H, –CONMe₂, –COOiBu and –CN), can continuously control the LUMO energy level of the monomer. D–A type copolymers were synthesized using the derivatives and an oligothiophene as the electron-donating block. The photovoltaic performance of D–A copolymer:PCBM bulk heterojunction devices shows the importance of LUMO level alignment to achieve efficient charge separation at the donor/acceptor interface. After optimization, the power conversion efficiency of the device with X = –H reached 1.14% with V_OC of 0.61 V, J_SC of 3.24 mA cm^?2 and FF of 0.58 under the irradiation of AM1.5 simulated solar light. The electronic properties of the copolymers correlated well with the electron-withdrawing ability of the monomers while the structure of the π-conjugated backbone was preserved, demonstrating the advantage of the chemically tunable monomer design.     

Novel donor–acceptor molecules as donors for bulk heterojunction solar cells

Three novel donor–acceptor molecules (BCPD, BPPD and PDHC) have been investigated as donors for the bulk heterojunction solar cells. BCPD and BPPD have the same acceptor group of TPO while BCPD and PDHC have the similar donor group of carbazole. The absorption data indicates that the introduction of either strong electron-donating group or strong electron-withdrawing group will induce the red shift of ICT transition absorption. Electrochemistry measurement shows that the three compounds present reversible or partly reversible oxidation and reduction processes. When the donor moiety was changed from carbazole to phenothiazine, the oxidation potentials shifted from 1.10 V to 0.72 V. While the acceptor moiety was changed from TPO to CEM, the reduction potentials shifted from −1.42 V to −0.92 V. The bulk heterojunction solar cells using the three compounds as donors and PCBM as acceptor were fabricated. The cell based on PDHC containing strong electron-accepting group shows a relatively high energy conversion efficiency of 0.51% under the illumination of white light at 100 mW cm⁻², which is almost two times of the magnitude of the cells based on BCPD and BPPD.     

Application of near-infrared absorbing heptamethine cyanine dyes as sensitizers for zinc oxide solar cell

Novel near-infrared absorbing heptamethine cyanine dyes derived from indole, benzoxazole, benzothiazole, and quinoline were synthesized and examined as sensitizers for a zinc oxide solar cell. A 2-carboxyphenylthio-substituted indolium heptamethine cyanine dye showed incident photon-to-current efficiency (IPCE) of 4.17% at 804 nm.     

Photoluminescence and electroluminescence of 3-methyl-8-dimethylaminophenazine

A new phenazine dye—3-methyl-8-dimethylaminophenazine (MDAP) with intramolecular charge transfer (ICT) property was synthesized. The photoluminescence and electroluminescence of were investigated. The device with a configuration of ITO/TPD (30 nm)/TPD:MDAP (30 nm)/Alq₃:MDAP (35 nm)/Alq₃ (30 nm)/Mg:Ag (200 nm) showed a good performance with a brightness of 21650 cd/m² at 250 mA/cm², a maximum luminous efficiency of 9.97 cd/A and a yellow emission peaked at about 564–586 nm.