Synthesis and characterization of trivalent metal porphyrin with NCS ligand for application in dye-sensitized solar cells

Two novel trivalent metal porphyrin dyes, P_Mn-HT-SCN and P_Ga-HT-SCN, were designed, synthesized and firstly applied in dye-sensitized solar cells (DSSCs). These two dyes possess porphyrin donor modified with manganese (III) and gallium (III) as coordination metal and NCS as the second ligand, cyanoacrylic acid as electron-accepting moiety and 4-hexylthiophene as π-spacers. Each of the porphyrin showed different adsorption behavior and saturated coverage on the TiO₂ surface. Between the two dyes, the P_Mn-HT-SCN-based DSSCs afforded the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 4.32 mA/cm², an open-circuit photovoltage (V_oc) of 0.61 V and a fill factor (FF) of 0.58, corresponding to a solar-to-electricity conversion efficiency of 1.53% under 100 mW/cm² irradiation.     

Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells

A new class of triphenylamine substituted methanofullerene derivatives, bis(4'-(diphenylamino)biphenyl-4-yl)methanofullerene (1) and the bisadduct (2), were synthesized. The incident photon to current efficiency (IPCE) studies revealed that the diphenylamino components have contribution to the photocurrent that expands the light harvesting window around 400 nm. When being blended with poly (3-hexylthiophene) (P3HT) to fabricate the solar cell, the device of P3HT:1 (1:0.7) shows high open circuit voltage (V_oc) of 0.69 V under the illumination of AM 1.5, 100 mW/cm² with high power conversion efficiency (PCE) of 3.16%, which is about 0.1 V higher than that of the corresponding [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) devices. This indicates that the arylamine substituents on 1 have played some special roles on the high V_oc performance. Similar effects are also observed for 2. The device of P3HT:2 (1:1) shows even higher V_oc of 0.87 V with the PCE of 1.83%. These results indicate that 1 and 2 are alternative high performance acceptors.     

Novel symmetric squaraine chromophore containing triphenylamine for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, SQ-bis[HP-HT2-TPA] and SQ-bis[HP-HT2-BT] incorporating as novel derivative of squaraine-pyrrole framework and π-extended thiophene with triphenyamine (TPA) and benzothiophene (BT) end group, have been synthesized and characterized. The effects of TPA moiety were investigated. Compared to SQ-bis[HP-HT2-BT], SQ-bis[HP-HT2-TPA] exhibited three times improved transporting property of hole carrier and four times enhanced absorptivity by more efficient intermolecular π−π interaction for high-efficiency bulk heterojunction (BHJ) device, suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices fabricated with SQ-bis[HP-HT2-TPA]/C₇₁-PCBM BHJ film had an average power-conversion efficiency of 1.83%(±0.12) under 100 mW/cm², with a short-circuit current (J_sc) of 9.32 mA/cm², fill factor (FF) of 0.30, and open-circuit voltage (V_oc) of 0.65 V, which has ∼42% higher efficiency compared to SQ-bis[HP-HT2-BT]/C₇₁-PCBM BHJ films.     

Low band gap dyes based on 2-styryl-5-phenylazo-pyrrole: Synthesis and application for efficient dye-sensitized solar cells

A new series of low band gap dyes, C1, C2 and S, based on 2-styryl-5-phenylazo-pyrrole was synthesized. These dyes contain one carboxy, two carboxy and one sulfonic acid anchoring groups, respectively. They were soluble in common organic solvents, showed long-wavelength absorption maximum at ∼620 nm and optical band gap of 1.66-1.68 eV. The photophysical and electrochemical properties of these dyes were investigated and found to be suitable as photosensitizers for dye sensitized solar cells (DSSCs). The quasi solid state DSSCs with dye S showed a maximum monochromatic incident photon to current efficiency (IPCE) of 78% and an overall power conversion efficiency (PCE) of 4.17% under illumination intensity of 100 mW cm⁻² (1.5 AM), which is higher than the other dyes (3.26% for C2 and 2.59% for C1). Even though dye S contains one sulfonic acid anchoring group, the higher PCE for the DSSCs based on this dye has been attributed to the higher dye loading at the TiO₂ surface and enhanced electron lifetime in the device, as indicated by absorption spectra and electrochemical impedance spectra measurements. Finally, by increasing the molecular weight of poly(ethylene oxide) (PEO) in electrolyte, the PCE also increases up to 4.8% for the electrolyte with PEO molecular weight of 2.0 × 10⁶. This improvement has been attributed to the enhancement in iodide ions diffusion due to the increase in free volume of polymer gel electrolyte.     

Two-dimensional regioregular polythiophenes with conjugated side chains for use in organic solar cells

We have prepared two two-dimensional polythiophenes (2D-PTs; P1 and P2) possessing alkyl-thiophene side chains by Stille coupling reactions. Optical measurements indicate that the bandgaps of P1 and P2 being 1.98 and 1.77 eV, respectively. P2 displayed a red-shift in its absorption spectrum because of the longer length of its conjugated side chains. Desirable highest occupied molecular orbital (HUMO) and lowest unoccupied molecular orbital (LUMO) energy levels were obtained from electrochemical studies, which suggested that these systems would exhibit high open-circuit voltages when blended with fullerene as electron acceptors. The hole mobility (thin film transistor (TFT) measurement) of P1 and P2 are 3.5×10⁻⁴ and 4.6×10⁻³ cm² V⁻¹ s⁻¹, respectively. A power conversion efficiency of 2.5% is obtained under simulated solar illumination (AM 1.5G, 100 mW cm⁻²) from a polymer solar cell comprising an active layer containing 25 wt% P1 and 75 wt% [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM).     

Dimethyl-2H-benzimidazole based small molecules as donor materials for organic photovoltaics

Utilization of 2,2-dimethyl-2H-benzimidazole has received strong attention as the electron-deficient unit for the generation of electron donor material for organic photovoltaic cells (OPVs). This paper reports the first small organic molecules based on dimethyl-2H-benzimidazole, which can produce intramolecular charge transfer. Two soluble small organic molecules, MMM and OMO, with dimethyl-2H-benzimidazole unit were synthesized by Suzuki coupling reaction with Pd(0)-catalyst. The spectra of MMM and OMO in the solid thin films show absorption bands with maximum peaks at 374, 598 and 373, 588 nm, and the absorption onsets at 678 and 673 nm, corresponding to band gaps of 1.83 and 1.84 eV, respectively. The devices comprising MMM with PC61BM (1:3) showed a V_OC of 0.66 V, a J_SC of 2.03 mA/cm², and a fill factor (FF) of 0.27, giving a power-conversion efficiency of 0.37%.     

Ni and metal aluminate mixtures for solid oxide fuel cell anode supports

Mixtures of nickel and metal aluminate (Ni–MAl₂O₄ [M = Fe, Co, Ni and Cu]) were fabricated, and their electrical conductivities, microstructures and thermal expansions were measured. During the sintering of these mixtures, MAl₂O₄ reacts with NiO to form NiAl₂O₄ and MO_x which are thought to be the reasons for the differences in the microstructures and electrical properties. Except for FeAl₂O₄, Ni–MAl₂O₄ mixtures show metallic conductivity behavior and their electrical conductivities are sufficient for cell operation. Their thermal expansion coefficients are much lower than conventional Ni-YSZ mixtures and closer to the 8YSZ electrolyte. The peak power densities of single cells supported with Ni–NiAl₂O₄ and Ni–CoAl₂O₄ are 410 and 440 mW cm⁻² at 850 °C, respectively, which are lower than 490 mW cm⁻² of Ni-YSZ. This is due to the polarization resistances of functional anode layer. The Ni–CuAl₂O₄-supported cell has no electrical performance because of Cu migration and segregation.     

Stable dye-sensitized solar cells based on organic chromophores and ionic liquid electrolyte

A series of polyene-diphenylaniline based organic dyes (coded as D5, D7, D9 and D11) have been reported for the application in ionic liquid electrolyte based dye-sensitized solar cells. The effects of substitution of organic dyes on the photovoltaic performance have been investigated, which show addition of methoxy groups on the triphenylamine donor group increases short-circuit current, open-circuit voltage and photovoltaic performance. A power conversion efficiency of 6.5% under AM 1.5 sunlight at 100 mW/cm² has been obtained with D11 dye in combination with a binary ionic liquid electrolyte, which when subjected to accelerated testing under one sun light soaking at 60 °C, the efficiency remained 90% of initial efficiency.     

Synthesis of new low band gap dyes with BF2-azopyrrole complex and their use for dye-sensitized solar cells

The diazonium salt derived from 4-aminobenzoic acid, 4-aminophenol or 2-aminophenol reacted with half equivalent of pyrrole to afford symmetrical 2,5-bisazopyrroles. They reacted subsequently with boron trifluoride in the presence of triethylamine to afford the corresponding BF₂-azopyrrole complexes D1, D2 and D3 respectively. They were soluble and stable in nonprotic solvents such as chloroform, dichloromethane and tetrahydrofuran but unstable in protic solvents such as ethanol. Their absorption spectra were broad with optical band gap of 1.49-1.70 eV. Among these dyes D2 displayed the broader absorption spectrum with low band gap of 1.49 eV. We have utilized these complexes as photosensitizers for quasi solid state dye-sensitized solar cells (DSSCs) and achieved power conversion efficiency in the range of 4.0-6.0%. We have also found that the co-adsorption of citric acid hindered the formation of dye aggregates and might improve the electron injection efficiency leading to an enhancement in short circuit photocurrent. This work suggests that metal-free dyes based on BF₂-azopyrrole complex are promising candidates for improvement of the DSSC performance.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

Synthesis and organic photovoltaic (OPV) properties of triphenylamine derivatives based on a hexafluorocyclopentene “core”

A series of new “D-A-D” chromophores containing hexafluorocyclopentene thiophene as an acceptor and a triphenylamine unit as a donor, called TP-G1, TP-G2, TPB-G1 and TPB-G2, were designed and synthesized. Heterojunction organic photovoltaic (OPV) cells containing these chromophores were fabricated, and device 1, with the structure of ITO/PEDOT:PSS/TP-G1:P3HT/LiF/Al, displayed an open-circuit voltage (V_oc), short-circuit current (J_sc) and power-conversion efficiency (η) of 0.74 V, 1.178 mA/cm² and 0.22%, respectively. The triphenylamine group could effectively induce the open-ring isomer to close because the 4- and 4′- positions of the benzene rings were substituted by an electron-donating group and the value of the quantum yields of the closed-ring isomers increased. As a result, the closed-ring isomer facilitated intramolecular π-electron delocalization and exhibited a broad absorption band ranging from 200 to 850 nm. Due to the fluorine substitution of hexafluorocylopentene at the molecular center and the hole-transport characteristics of the triphenylamine moiety on the periphery, our chromophores showed obvious dual semiconductor properties, i.e., n- and p-type, which demonstrated a potential application for OPV devices.     

Syntheses and photovoltaic properties of polymeric sensitizers using thiophene-based copolymer derivatives for dye-sensitized solar cells

We synthesized the thiophene-based copolymers (P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n) using two different kinds of thiophene monomers, (N-(3-thienylmethylene)-2-aminofluorene and 3-thiophene acetic acid), as sensitizers on the DSSCs. P(3TAF-co-3TAa)-A-n (n=1, 2, 3) was synthesized with different molar ratios (3TAF:3TAa=1:5, 1:10, 1:20) of monomers at room temperature, respectively. Also, P(3TAF-co-3TAa)-B-n (n=1, 2, 3) was synthesized with above molar ratios of monomers at 0 °C, respectively. The DSSCs devices were fabricated using the thiophene-based copolymers as sensitizers and their photovoltaic performances were measured by using a solar simulator under AM 1.5. In the DSSCs devices using polymeric sensitizers, V_oc is 0.53-0.60 V, J_sc is 1.9-4.5 mA/cm², FF is 0.51-0.63 and the power conversion efficiency is 0.63-1.53%, respectively.     

Influences of water in bis-benzimidazole-derivative electrolyte additives to the degradation of the dye-sensitized solar cells

The chemical stability of dye-sensitized solar cells (DSSC) determines both the cell performance and the cell life-time. The presence of water in the solar cell from surrounding leakage through the imperfection packaging sealants causes the decrease in life-time of photogenerated electrons on the working electrodes, which induces the occurrence of the dark current to the electrolytes and thus leakage current significantly deteriorated the life-time of the DSSC. Reliable electrolyte additives diminishing the influences of water to the DSSCs degradation process becomes a critical issue in maintaining an acceptable cell life-time.In this work, the effects of four benzimidazole derivatives a-d as the electrolyte additives in the presence of water were comprehensively examined by time-dependent photovoltaic performance of the cells. As a result, open-circuit voltage (V_oc), short-circuit current (J_sc), efficiency (η), and fill factor (FF) collected from I-V curves were studied. In addition, electrochemical impedance spectroscopy (EIS) technique was implemented to evaluate the effects of the charge-transfer resistance (R_ct) at the interfaces between TiO₂/dye/electrolyte. Results showed that the bis-benzimidazole derivative c gives significant improvement in the long-term stability due to the effective protection of the ligands between dye and working electrodes from the attack by environmental water molecules.     

A nickel-complex sensitiser for dye-sensitised solar cells

We report the first example of a Ni(II) complex that demonstrates sensitiser function in a Dye-Sensitised Solar Cell (DSSC). Complexes [Ni(dcbpy)(qdt)] (1), [Ni(decbpy)(qdt)] (2) and [Ni(decbpy)Cl₂] (3) (where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine; decbpy = 4,4′-di(CO₂Et)-2,2′-bipyridine; and qdt = quinoxaline-2,3-dithiolate) have been prepared. Characterisation was carried out using electrochemical, spectroscopic and computational techniques. Intensive visible transitions of 1 and 2 have been assigned predominantly to Ligand-to-Ligand Charge Transfer (LLCT) from the qdt to the diimine ligand, suggesting appropriate charge separation for application in a photoelectrochemical device. TiO₂ sensitised with 2, following charge injection, processes a recombination time significantly long for photovoltaic function. In a DSSC, using redox electrolyte, photocurrents and photovoltages of 0.293 mA and 521 mV were observed, with optimum values requiring TiCl₄ post-treatment of TiO₂ and co-adsorption of Chenodeoxycholic acid (Cheno). Although photovoltaic function was observed, the low photocurrent is attributed to a short-lived excited state lifetime resulting in poor charge injection from the Ni(II) sensitiser.     

Gas and vapor adsorption in octacyanometallate-based frameworks Mn2[M(CN)8] (M = W,Mo) with exposed Mn sites

Dehydration of the isostructural three-dimensional (3D) octacyanometallate-based materials Mn₂M(CN)₈·7H₂O (M = Mo, 1·7H₂O; W, 2·7H₂O) generates robust porous frameworks (1 and 2). In the structure, the [M(CN)₈]⁴⁻ units are linked via octahedral Mn²⁺ centers to form an open 3D framework with 1D channels, in which the non-coordinated and coordinated water molecules are involved. The permanent porosities have been confirmed by thermogravimetric analysis, variable-temperature X-ray diffraction and Raman spectra, and adsorption (H₂O, N₂ and H₂) measurements. H₂ adsorption at 1.1 bar and 77 K was 0.60 wt% for 1 and 0.49 wt% for 2. At initial loading ΔH_ads has the value of ca. 10.0 kJ mol⁻¹ for both materials, which represents the highest value reported for any cyanide-based assemblies. The high enthalpy can be attributed to the presence of coordinatively-unsaturated Mn²⁺ sites left exposed by the removal of coordinated water molecules in the structure.     

New compounds in the potassium-aluminium-hydrogen system observed during release and uptake of hydrogen

Three new compounds are observed in the potassium-aluminium-hydrogen system, which are characterised using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis (TG and DSC) and Siverts measurements (PCT). All three new compounds (denoted 1, 2 and 3) are observed during release and uptake of hydrogen in the potassium-aluminium-hydrogen system and may be new intermediates. All three compounds were indexed and the following unit cells were found, 1: cubic, a = 17.0248(9) Å, 2: cubic, a = 14.2746(4) Å and 3: orthorhombic, a = 10.46(1), b = 6.661(6) and c = 6.173(5) Å. Formation and observation of 1, 2 and 3 depends on the mechano-chemical sample preparation (ball milling), temperature, (heating rate), and hydrogen pressure (and temperature for pressure change). Compound 1 is often observed in the temperature range 55-150 °C for a medium ball-milled sample heated under a constant hydrogen pressure of 50 bar 1 is clearly an intermediate for formation of KAlH₄ and may have the composition K_yAlH_x with 1 ≤ y ≤ 3 and 4 ≤ x ≤ 6. Applying hydrogen pressure abruptly at elevated temperatures leads to faster hydrogenation, which can then be performed at lower hydrogen pressures. In the latter case, 1 is only observed shortly in a few PXD patterns. Compounds 2 and 3 are mainly observed during dehydrogenation of KAlH₄ in the temperature ranges of ca. 200 to 350 °C and 200 to 390 °C respectively, as relatively weak Bragg diffraction peaks.     

A new photosensitive coordination compound [RuL(bpy)2](PF6)2 and its application in photocatalytic H2 production under the irradiation of visible light

A new organic–inorganic photosensitive coordination compound [RuL(bpy)₂](PF₆)₂ (to represent by TM1) had been synthesized by reaction of L (L = 2-hydroxyl-5-(imidazo-[4,5-f]-1,10-phenanthrolin) benzoic acid) with bipyridyl ruthenium, and further characterized by UV–vis, IR, NMR MS and CV. The target photocatalyst 6 wt% TM1-0.5 wt% Pt-TiO₂ (Ⅰ) was obtained by sensitization of Pt-loaded TiO₂ with TM1. The H₂ production activity of target photocatalyst Ⅰ was systematically evaluated by the reaction of photocatalytic H₂ production from water under visible light irradiation. The maximum H₂ evolution of 386.7 μmol in irradiation 3 h and H₂ production rate of 2578 μmol · h⁻¹ · g⁻¹ was detected under the optimal conditions with pH 5, target photocatalyst Ⅰ 50 mg and 5% sacrificial reagent TEOA (v/v).     

Solution processable quinacridone based materials as acceptor for organic heterojunction solar cells

Two series of novel quinacridone (QA) based materials that combined a strong absorption over a broad range in visible region with good electrical characteristics, which were used as the new electron-accepting materials for organic solar cells, are explored. Unique cyclic compounds 1-6 are synthesized by incorporating electron withdrawing groups (CN, COOH) at carbonyl position of alkyl substituted quinacridones, which lead to the compounds possessing the characteristics of solution-processed and being suitable for photovoltaic applications. Heterojunction solar cells with simple device configuration using these soluble materials as acceptor and effective donor poly (3-hexyl thiophene) (P3HT) were fabricated. The maximum power conversion efficiency (PCE) achieved in the solar cell based on compound 5 is 0.42% under simulated AM 1.5 solar irradiation with J_sc=1.80 mA cm⁻², V_oc=0.50 V and FF=47%. Although the aimed devices just exhibit moderate PCE, our results clearly suggest that the new-type electron-accepting materials different from fullerene have great potential as acceptor in heterojunction solar cell due to many advantages of the QA derivatives such as relatively inexpensive, good electrochemical stability and could be readily modified.     

Novel zinc porphyrin with phenylenevinylene meso-substituents: Synthesis and application in dye-sensitized solar cells

A novel zinc porphyrin, P, with phenylenevinylene segments at two opposite meso-positions and carboxyphenyl at the other two meso-positions of the porphyrin ring, was synthesized and characterized. The phenylenevinylene substituents were terminated with electron-accepting 4-nitro-α-cyanostilbene units. Elongation of the π-conjugation enhanced the solubility of P as well as broadened and strengthened the absorption spectrum. We have investigated the application of P in quasi solid state dye-sensitized solar cells (DSSCs). Under illumination intensity of 100 mW cm⁻², a power conversion efficiency of 2.90% was obtained for the DSSC based on P as sensitizer, which was significantly improved to 4.22% upon addition of deoxycholic acid (DCA) into the P solution for TiO₂ sensitization. Coadsorption of DCA decreased the dye adsorption, but significantly improved both short circuit current (J_sc) and open circuit voltage (V_oc). The breakup of π stacked aggregates might improve the electron injection yield and thus J_sc. The electrochemical impedance data indicate that the electron lifetime was improved by the coadsorption of DCA, which was attributed to the improvement in both V_oc and J_sc. The increase in J_sc has also been attributed to the reduction of the back reaction i.e., the recombination of electrons with tri-iodide ions.