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 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 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.     

Energy level diagrams of C60/pentacene/Au and pentacene/C60/Au

The electronic structures of pentacene and C₆₀ interfaces were investigated using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The magnitudes of measured interface dipole were 0.11 eV and 0.07 eV for the C₆₀ deposited on pentacene (C₆₀/pentacene) and the pentacene deposited on C₆₀ (pentacene/C₆₀), respectively. The obtained C 1s spectra on these samples show that no significant chemical bonds at the interface. The offsets of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the C₆₀-pentacene interface were 1.29 eV and 0.89 eV for C₆₀/pentacene/Au, while for pentacene/C₆₀/Au they were 1.5 eV and 1.1 eV. In this paper we present the complete energy level diagrams of C₆₀/pentacene/Au and pentacene/C₆₀/Au.     

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%.     

Graphene doping of P3HT:PCBM photovoltaic devices

We have introduced solution-processable functionalized graphene material (SPFGraphene) in P3HT/PCBM photovoltaic devices. The best results were obtained with a P3HT/PCBM (1:1) mixture with 10 wt% of graphene is open-circuit voltage (V_oc) of 0.64 V, a short-circuit current density (J_sc) of 5.3 mA/cm² and a FF of 0.41 which led to a power conversion efficiency of 1.4% at illumination at 100 mW/cm² AM1.5. This device takes advantage of the electron-accepting feature of fullerenes and the high electron transport capability of graphene. The results indicate that the addition of graphene enhances the performance of polymer photovoltaic cells.     

A study of thermal,optical and electrical properties of new branched triphenylamine-based polyazomethines

A series of branched aromatic polyazomethines have been obtained by high temperature solution polycondensation of 4,4′,4″-triformyltriphenylamine with 3,3′-dimethoxybenzidine with different feed molar ratio. For three polymers additional condensation of chain end groups with monofunctional monomers such as 4-formyltriphenylamine or 2-naphthylamine was carried out. Moreover, two model compounds were prepared and investigated for comparison with branched polymers. The structures of polymers and models were characterized by means FTIR, ¹H, ¹³C NMR spectroscopy, elemental analysis and gel permeation chromatography (GPC). UV–vis properties of the thin films of the polymers and compounds were investigated on the glass substrate. Eg of the branched polymers was found about 2.47 eV. UV–vis and FTIR spectroscopy for iodine doped compounds were investigated. Doping decreased the value of Eg of the branched polyazomethines to about 1.71 eV. Refractive index (n) for branched polyazomethines was found about 1.97, while for the doped compounds was a little higher (～2.48). Absorption (UV–vis) properties of the doped with iodine branched imines were investigated additionally after heating in different temperatures from 50 to 200 °C. Intensity of photoluminescence of branched imines in relation to 9,10-diphenylanthracene was found in the range 0.2–1.0% and 2.7–43.7% in dependence on the excitation wavelengths. Current–voltage (I–V) measurements were performed on ITO/TiO₂/polymer/Al, ITO/polymer/Alq₃/Al and ITO/TiO₂/polymer/Alq₃/Al devices in the dark and during irradiation with light (under illumination 1000 W/m²). The sol–gel technique was applied to prepared TiO₂ layer. TiO₂ layers and devices were investigated by Atomic Force Microscopy (AFM). Moreover, properties of these branched polymers were compared with the linear polyazomethine based on 3,3′-dimethoxybenzidine and 4,4′-diformyltriphenylamine.     

Simple solvothermal routes to synthesize nanocrystalline Bi2MoO6 photocatalysts with different morphologies

Nanocrystalline Bi₂MoO₆ photocatalysts were successfully synthesized by conventional solvothermal and microwave–solvothermal routes, respectively. The prepared samples were characterized by X-ray diffraction, BET surface area analysis, UV–vis diffuse reflectance spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The samples have high surface areas in the range of 10–32 m² g⁻¹. Their average crystallite sizes are in the range of 16–35 nm. The absorption edge of the samples is at ∼491 nm, corresponding to a band gap energy of about 2.53 eV. Different morphologies with nanosheets and nanorods were also observed. The photocatalytic activities of Bi₂MoO₆ photocatalysts were evaluated by the decomposition of Rhodamine B under visible-light irradiation (λ > 420 nm). Nanocrystalline Bi₂MoO₆ samples obtained via different conditions exhibited different photocatalytic performances. The effects of the crystallinity, specific surface area and morphology of the samples on the photocatalytic activities are also discussed.     

Alternating phenylenevinylene and thienylenevinylene copolymers with cyano groups: Synthesis,photophysics and photovoltaics

Two alternating conjugated copolymers TTP and TTT were synthesized by Heck coupling of 2,3-bis-(5-bromothiophene-2-yl)acrylonitrile with 1,4-dihexyloxy-2,5-divinylbenzene and 3-hexyl-2,5-divinylthiophene, respectively. The absorption spectra of the copolymers in THF solution showed three maxima around 270, 370 and 460 nm with optical band gaps of 2.30–2.34 eV. The electrochemically estimated band gaps of copolymers were 2.04–2.10 eV. The thin film absorption spectra were broad and extended about from 250 to 600 nm with a long wavelength maximum near 470 nm. The copolymers emitted yellow-orange light with maximum at 528–551 nm in THF solution and 567–610 nm in thin film. TTP showed the most red shifted emission maximum between the copolymers. The performance of the photovoltaic cells which were fabricated using blends of the copolymers with 6,6-phenyl C61-butyric acid methyl ester (PCBM) (1:1 and 1:4, w/w) as well as pure copolymers was investigated.     

Porphyrin dye-sensitization of polythiophene in a conjugated polymer/TiO2 p–n hetero-junction solar cell

In the blended solid of poly(3-hexylthiophene-2,5-diyl) (P3HT) and porphyrin (TPP)/TiO₂ p–n hetero-junction solar cells, a photo-induced charge transfer between P3HT and TPP accelerated the charge separation in the depletion layer formed at the P3HT + TPP/TiO₂ interface, enhancing the photovoltaic properties. For the blended cell containing zinc porphyrin as TPP, the energy conversion yield of 0.26% was obtained under the illumination of solar simulated light AM1.5–100 mW/cm².     

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 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.     

Growth and characterization of epitaxial Ba(Zn1/3Ta2/3)O3 (1 0 0) thin films

We have synthesized and characterized epitaxial and stoichiometric Ba(Zn_1/3Ta_2/3)O₃ (1 0 0) dielectric thin films grown on MgO (1 0 0) substrates by pulsed laser deposition. Advanced electronic structure calculations were used to guide the interpretation of the experimental data. Zn-enriched targets and high oxygen pressures were used to compensate for Zn loss during film growth. The Ba(Zn_1/3Ta_2/3)O₃ films had an indirect optical band gap of ～3.0 eV and a refractive index of 1.91 in the visible spectral range. Zn–Ta B-site ordering was not observed in the Ba(Zn_1/3Ta_2/3)O₃ thin film X-ray diffraction data. A dielectric constant of 25 and dissipation factor of 0.0025 at 100 kHz were measured using the interdigital capacitor method. The Ba(Zn_1/3Ta_2/3)O₃ films exhibited a small thermally activated ohmic leakage current at high fields (<250 kV cm^–1) and high temperatures (<200 °C) with an activation energy of 0.85 eV.     

Synthesis and characterization of novel 2,5-diphenyl-1,3,4-oxadiazole derivatives of anthracene and its application as electron transporting blue emitters in OLEDs

With a general aim to make anthracene derivatives multifunctional (n-type emitter) and also study their suitability as electron transport layers for organic light emitting diodes (OLED), we report the synthesis and characterization of five novel molecules in which the 9 and 10 positions of anthracene have been directly substituted by 2,5-diphenyl-1,3,4-oxadiazole groups. We have carried out detailed characterization of these molecules which include photophysical, electrochemical, thermal, electroluminescent and computational studies. The electron affinity is very high, around 3.7 eV, and the ionization potential is around 6.7–6.8 eV, which is relatively higher than the most commonly used electron transport electroluminescent layer Alq₃. The studies reveal that the new molecules being reported by us, in addition to the high thermal stability, are quite efficient in a two layer unoptimized nondoped device with the device structure ITO/α-NPD/10a–11b/LiF/Al and have an emission in pure blue. They also show very high efficiency as electron transport layer in device structure ITO(120 nm)/α-NPD(30 nm)/Ir(ppy)₃ doped CBP(35 nm)/BCP(6 nm)/10a(28 nm)/LiF(1 nm)/Al(150 nm). From these studies we conclude that these anthracene derivatives also have considerable potential as multifunctional layers and as electron transport layers in OLED.     

Magneto-optical studies of 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61; PCBM

In the quest for better power conversion efficiency of organic photovoltaic cells (OPVC) various architectures have been considered. Bulk heterojunction type OPVC proved to be the most successful, where the polymer as electron donor is blended with fullerene molecule as electron acceptor. With the goal of improving power conversion efficiency of OPVC we studied 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as a film and dispersed in polystyrene matrix. We used a variety of continuous wave (cw) optical probes including photoinduced absorption (PA), photoluminescence (PL), and optically detected magnetic resonance (ODMR). The steady state photophysics of PCBM is dominated by triplet exciton. For both PCBM dispersed in polystyrene and PCBM film, the triplet exciton specie is characterized by a PA band at about 1.7 eV and spin 1 PA detected magnetic resonance (PADMR) powder pattern resonance around g ～ 2. However, in PCBM film the occurrence of a polaron PA band at 1.2 eV and spin 1/2 PADMR are also noticed.     

Kinetic effect of boron on the thermal stability of Si–(B–)C–N polymer-derived ceramics

The isothermal mass loss of two polymer-derived ceramics with compositions SiC_1.4N_0.9 and SiC_1.5N_1.0B_0.05 were measured as a function of time using thermal gravimetric analysis at various temperatures ranging between 1580 and 1720 °C. The process of mass loss, attributed to the reaction Si₃N₄ + 3C → 3SiC + 2N₂^↑, takes substantially more time for the boron-containing ceramic compared with the boron-free one. The continuous formation of SiC crystallites as the product of the reaction between Si₃N₄ and C was revealed through X-ray diffraction (XRD) measurements during the course of the reaction. The kinetics of this reaction was studied using a generalized model for the analysis of chemical reaction kinetics. Consequently, the effective activation energies for the Si₃N₄ degradation were estimated to be 11.6 ± 0.5 eV and 17.1 ± 0.7 eV for the Si–C–N and Si–B–C–N ceramics, respectively. Moreover, the results obtained indicate that the dominant mechanisms of the Si₃N₄ degradation are strongly influenced by the presence of boron. For the Si–C–N ceramic, the chemical reaction at interfaces of the reactants and the crystallization of SiC as the reaction product are proposed to be the main probable stages controlling the progress of the investigated reaction. However, the local diffusion of C out of BNC_x turbostratic layers surrounding the Si₃N₄ nanocrystals and the gas (N₂) release from the reaction zone are suggested to be the most plausible processes limiting the progress of Si₃N₄ degradation for the Si–B–C–N ceramic.     

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).     

Different solvents effect on the performance of the solar cells based on poly(3-hexylthiophene):methanofullerenes

This paper reports the effect on the performance of the solar cells based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with different casting solvents. These blend films are characterized by UV–vis absorption spectra, photoluminescence spectra, charge-transport dark J–V curve, X-ray diffraction pattern curve, and AFM images. The results indicate that high boiling point solvent leads to an enhanced self-organization of P3HT in the active layer, which causes an increased charge transport. Increased incident light absorption and higher carrier mobility in the active layer contribute to the enhancement in the device performance, the power conversion efficiency of 3.69% and fill factor up to 65.3% are achieved with 1,2,4-trichlorobenzene as casting solvent without further heat treatment under Air Mass 1.5, 100 mW/cm².     

Third-order nonlinear optical property of poly(2,5-dihexyloxy)-p-phenylene vinylene modified by N+ ion implantation

A soluble poly(2,5-dihexyloxy)-p-phenylene vinylene (PDHOPV) was prepared via dehydrochlorination reaction from hydroquinone and n-hexyl bromide as raw materials. The PDHOPV films implanted by 20 keV nitrogen ions (N⁺) in the dose range of 3.8 × 10¹⁵–9.6 × 10¹⁶ ions/cm² were characterized by UV–vis absorption spectra, which showed the optical absorption was raised gradually in the visible region followed by a red shift of optical absorption threshold and the band gap (E_g) was reduced from 2.12 eV to 1.71 eV with the increase of ion dose. The third-order nonlinear optical (NLO) susceptibilities (χ⁽³⁾) were measured by degenerate four-wave mixing (DFWM) technique at 532 nm. The results demonstrated that the χ⁽³⁾ value of PDHOPV film could be enhanced effectively by means of N⁺ ion implantation. When the ion dose was 3.8 × 10¹⁶ ions/cm², the χ⁽³⁾ value of PDHOPV film was maximized to 1.06 × 10⁻⁸ esu which was almost eleven times larger than that of the pristine film. Moreover, the mechanism responsible for the variation of χ⁽³⁾ value of PDHOPV film with N⁺ ion dose was also discussed tentatively.     

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.