Synthesis of a novel imidazolium-based electrolytes and application for dye-sensitized solar cells

A series of new imidazolium-based oligomers with different length of a poly(ethylene glycol) moiety as a linker were synthesized and studied as electrolytes for dye-sensitized solar cell (DSSC). These oligomeric molecules are expected to have an intra- or inter-molecular hydrogen bonding interaction through its urethane and urea bonds. They can be used to prepare the liquid-type electrolytes for DSSC by dissolving them into conventional solvent system or to develop solvent-free electrolytes by incorporating an extra redox mediator and other functional materials together as additives. It was found that these oligomers could replace the cationic component of the conventional electrolytes and became the source of redox species when iodine is added. The photocurrent-voltage characteristics of DSSCs with the electrolytes containing these oligomers demonstrated that they can successfully replace the conventional ionic liquid-type electrolytes such as 1-methyl-3-propyl imidazolium iodide (PMII) in 3-methoxypropionitrile (MPN) if the length of the linker is optimized.     

Gel polymer electrolytes based on a novel quaternary ammonium salt for dye-sensitized solar cells

Gel polymer electrolytes were prepared with polyacrylonitrile (PAN) and solutions of a novel quaternary ammonium salt, polysiloxane with quaternary ammonium side groups (PSQAS), in a mixture of ethylene carbonate (EC) and propylene carbonate (PC). The influences of PAN content and salt concentration on the ionic conductivity have been investigated. The ionic conductivity can be further improved with the use of the mixtures of KI and PSQAS, which can be expected as inorganic-organic salts. The gel polymer electrolytes were used in the fabrication of the dye-sensitized solar cells with a nanoporous TiO₂ working electrode, cis-di(thiocyanato)-N,N-bis(2,2-bipyridyl-4,4-dicarboxylic acid) ruthenium(II) complex dye and a counter electrode based on platinized conducting glass. The cells showed open-circuit voltages (V _oc) around 0.6 V and short-circuit current densities (J _sc) larger than 7.5 mA cm^–2 under 60 mW cm^–2 irradiation. The fill factors (FF) and energy conversion efficiencies () of the cells were calculated to be higher than 0.56 and 4.4%, respectively.     

Plastic-polymer composite electrolytes for solid state dye-sensitized solar cells

Three types of alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a solid plasticizer to develop a series of novel solvent-free plastic-polymer composite electrolytes. All these electrolytes appeared as a soft solid at room temperature and became sticky gel state at high temperature of 100 °C. Among the as-prepared plastic-polymer electrolytes, the SCN-PMVII (succinonitrile-poly(1-vinyl-3-methylimidazolium) iodide) electrolytes with a SCN content of 40-60 wt.% showed a room temperature conductivity of 1.0-1.6 mS cm⁻¹and a photoconversion efficiency of >4.1%, which are comparable to those observed from liquid organic carbonate electrolyte and the DSSCs using the liquid electrolyte at the same experimental conditions. Also, the DSSCs assembled with the SCN-PMVII electrolytes maintained their photoconversion efficiency very steadily during aging test of 50 days despite of being placed at 40 °C under 1 sun illumination or stored at 60 °C in an oven. Since these plastic-polymer electrolytes are solvent-free, highly conductive and electrochemically compatible, it is possible to use this type electrolyte for development of practical DSSCs.     

Effect of imidazole based polymer blend electrolytes for dye-sensitized solar cells in energy harvesting window glass applications

The exploration of polymer electrolyte in the field of dye sensitized solar cell (DSSC) can contribute to increase the invention of renewable energy applications. In the present work, the influence of imidazole on the poly (vinylidene fluoride) (PVDF)-poly (methyl methacrylate) (PMMA)-Ethylene carbonate (EC)-KI-I₂ polymer blend electrolytes has been evaluated. The different weight percentages of imidazole added into polymer blend electrolytes have been prepared by solution casting. The prepared films were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), UV-visible spectra, photoluminescence spectra and impedance spectroscopy. The surface roughness texture of the film was analyzed by atomic force microscopy (AFM). The ionic conductivity of the optimized polymer blend electrolyte was determined by impedance measurement, which is 1.95×10^-3 S·cm^-1 at room temperature. The polymer electrolyte containing 40 wt% of imidazole content exhibits the highest photo-conversion efficiency of 3.04% under the illumination of 100 mW·cm^-2. Moreover, a considerable enhancement in the stability of the DSSC device was demonstrated.     

Influences of poly(ether urethane) introduction on poly(ethylene oxide) based polymer electrolyte for solvent-free dye-sensitized solar cells

A poly(ether urethane) (PEUR)/poly(ethylene oxide) (PEO)/SiO₂ based nanocomposite polymer is prepared and employed in the construction of high efficiency all-solid-state dye-sensitized nanocrystalline solar cells. The introduction of low-molecular weight PEUR prepolymer into PEO electrolyte has greatly enhance the electrolyte performance by both improving the interfacial contact properties of electrode/electrolyte and decreasing the PEO crystallization, which were confirmed by XRD and SEM characteristics. The effects of polymer composition, nano SiO₂ content on the ionic conductivity and I₃⁻ ions diffusion of polymer-blend electrolyte are investigated. The optimized composition yields an energy conversion efficiency of 3.71% under irradiation by white light (100 mW cm⁻²).     

The use of poly(vinylpyridine-co-acrylonitrile) in polymer electrolytes for quasi-solid dye-sensitized solar cells

Poly(vinylpyridine-co-acrylonitrile) (P(VP-co-AN)) was used to form polymer electrolytes for dye-sensitized solar cells (DSSCs). The effects of P(VP-co-AN) on the photovoltaic performances of DSSCs have been investigated with nonaqueous electrolytes containing alkali-iodide and iodine. It was found that the effect of P(VP-co-AN) on V_oc closely related to its amount in the electrolyte. Lower amount of P(VP-co-AN) was benefit for the construction of a solar cell containing P(VP-co-AN) with higher energy conversion efficiency. Chemically crosslinking solidification with backbone polymer P(VP-co-AN) amount of 3% fabricated quasi-solid DSSCs with 10% increased conversion efficiencies with relative to that of the initial liquid DSSCs.     

Studies on isomeric effects of 2- and 4-Mercapto pyridine as dopants in polymer electrolyte in dye-sensitized solar cells

In this present work, isomers like 2- and 4-Mercapto pyridine were used as dopants (additives) in Poly (ethylene oxide) based polymer electrolyte and their effects in dye-sensitized solar cells (DSC) have been investigated. Due to the coordinating and plasticizing effects of Mercapto pyridine, enhanced ionic conductivity and reduced crystallinity of PEO polymer electrolyte accompanied by a better penetration of the same into the dye coated nanocrystalline TiO₂ in order to have better performances were achieved. The 2-Mercapto pyridine doped PEO (E) shows comparatively better performance than 4-Mercapto pyridine doped one (F), is due to the fact that the π-electron donicity of 2-Mercapto pyridine is greater. These results suggests that the electron donating capacity of 2-Mercapto pyridine and 4-Mercapto pyridine would influence the interaction of nanocrystalline TiO₂ electrode and I⁻/I₃⁻ redox couple leading to radical changes in the cell performance.     

Effect of modified montmorillonites on the ionic conductivity of (PEO)16LiClO4 electrolytes

Three kinds of modified montmorillonites were prepared by ion exchange method, and added into (PEO)₁₆LiClO₄ matrix to study the effect on the ionic conductivity of (PEO)₁₆LiClO₄ electrolytes. The structure of the modified montmorillonites and polymer composites were characterized by wide-angle X-ray diffraction. HP 4192A was used to measure the ionic conductivity of the polymer electrolytes. The results show that the addition of optimum content of 250-Li-mont enhances the ionic conductivity of the PEO based electrolyte by nearly 30 times more than the plain system and that is much higher than the other two modified montmorillonites. The difference of enhancement in conductivity caused by adding these three montmorillonites can be attributed to the difference in structure of the samples as characterized by wide-angle X-ray diffraction.     

A novel CuI-based iodine-free gel electrolyte for dye-sensitized solar cells

A novel CuI-based iodine-free gel electrolyte using polyethylene oxide (PEO, MW = 100,000) as plasticizer and lithium perchlorate (LiClO₄) as salt additive was developed for dye-sensitized solar cells (DSSCs). Such CuI-based gel electrolyte can avoid the problems caused by liquid iodine electrolyte and has relative high conductivity and stability. The effects of PEO and LiClO₄ concentrations on the viscosity and ionic conductivity of the mentioned iodine-free electrolyte, as well as the performance of the corresponding quasi solid-state DSSCs were investigated comparatively. Experimental results indicate that the performance of DSSCs can be dramatically improved by adding LiClO₄ and PEO, and there are interactions (Li⁺–O coordination) between LiClO₄ and PEO, these Li⁺–O coordination interactions have important influence on the structure, morphology and ionic conductivity of the present CuI-based electrolyte. Addition of PEO into the electrolyte can inhibit the rapid crystal growth of CuI, and enhance the ion and hole transportation property owing to its long helix chain structure. The optimal efficiency (2.81%) was obtained for the quasi solid-state DSSC fabricated with CuI-based electrolyte containing 3 wt% LiClO₄ and 20 wt% PEO under AM 1.5 G (1 sun) light illumination, with a 116.2% improvement in the efficiency compared with the cell without addition of LiClO₄, indicating the promising application in solar cells of the present CuI-based iodine-free electrolyte.     

Novel chemically cross-linked solid state electrolyte for dye-sensitized solar cells

Poly(vinylpyridine-co-ethylene glycol methyl ether methacrylate) (P(VP-co-MEOMA)) and α,ω-diiodo poly(ethylene oxide-co-propylene oxide) (I[(EO)_0.8-co-(PO)_0.2]_yI) were synthesized and used as chemically cross-linked precursors of the electrolyte for dye-sensitized solar cells. Meanwhile, α-iodo poly(ethylene oxide-co-propylene oxide) methyl ether (CH₃O[(EO)_0.8-co-(PO)_0.2]_xI) was synthesized and added into the electrolyte as an internal plasticizer. Novel polymer electrolyte resulting from chemically cross-linked precursors was obtained by the quaterisation at 90 °C for 30 min. The characteristics for this kind of electrolyte were investigated by means of ionic conductivity, thermogravimetric and photocurrent-voltage. The ambient ionic conductivity was significantly enhanced to 2.3 × 10⁻⁴ S cm⁻¹ after introducing plasticizer, modified-ionic liquid. The weight loss of the solid state electrolyte at 200 °C was 1.8%, and its decomposition temperature was 287 °C. Solid state dye-sensitized solar cell based on chemically cross-linked electrolyte presented an overall conversion efficiency of 2.35% under AM1.5 irradiation (100 mW cm⁻²). The as-fabricated device maintained 88% of its initial performance at room temperature even without sealing for 30 days, showing a good stability.     

Structure and conductive properties of poly(ethylene oxide)/layered double hydroxide nanocomposite polymer electrolytes

The oligo(ethylene oxide) modified layered double hydroxide (LDH) prepared by template method was added as a nanoscale nucleating agent into poly(ethylene oxide) (PEO) to form PEO/OLDH nanocomposite electrolytes. The effects of OLDH addition on morphology and conductivities of nanocomposite electrolytes were studied using wide-angle X-ray diffractometer, polarized optical microscopy, differential scanning calorimetry and ionic conductivity measurement. The results show that the exfoliated morphology of nanocomposites is formed due to the surface modification of LDH layers with PEO matrix compatible oligo(ethylene oxide)s. The nanoscale dispersed OLDH layers inhibit the crystal growth of PEO crystallites and result in a plenty amount of intercrystalline grain boundary within PEO/OLDH nanocomposites. The ionic conductivities of nanocomposite electrolytes are enhanced by three orders of magnitude compared to the pure PEO polymer electrolytes at ambient temperature. It can be attributed to the ease transport of Li⁺ along intercrystalline amorphous phase. This novel nanocomposite electrolytes system with high conductivities will be benefited to fabricate the thin-film type of Li-polymer secondary battery.     

Influence of molecular weight of PEG on the property of polymer gel electrolyte and performance of quasi-solid-state dye-sensitized solar cells

A new kind of polymer gel electrolyte based on poly(acrylic acid)-poly(ethylene glycol) (PAA-PEG) hybrid was synthesized. The factor of molecular weight of PEG in the hybrid plays an important role in determining the liquid electrolyte absorbency of the hybrid and ionic conductivity of the polymer gel electrolyte, sequentially affects the photovoltaic performance of quasi-solid-state dye-sensitized solar cells. Using the hybrid with PEG molecular weight of 20,000, a polymer gel electrolyte with liquid electrolyte absorbency of 6.9 g g⁻¹ and ionic conductivity of 5.35 mS cm⁻¹ was obtained. Based on the polymer gel electrolyte, a quasi-solid-state dye-sensitized solar cell with conversion efficiency of 5.25% was achieved under irradiation of AM 1.5, 100 mW cm⁻².     

Amphiphilic poly(vinyl chloride)-g-poly(oxyethylene methacrylate) graft polymer electrolytes: Interactions, nanostructures and applications to dye-sensitized solar cells

An amphiphilic graft copolymer, i.e. poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) comprised of a PVC backbone and POEM side chains was synthesized via atom transfer radical polymerization (ATRP) and complexed with a salt for dye-sensitized solar cell (DSSC) applications. The coordinative interactions and structural changes of polymer electrolytes were investigated using FT-IR spectroscopy, wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed that the d-spacing between PVC domains was significantly increased upon the introduction of metal salt, ionic liquid and oligomer, indicating their selective confinement in the hydrophilic POEM domains. The ion-conducting POEM domains were well interconnected, resulting in high ionic conductivity (∼10⁻⁴ S/cm at 25 °C) and energy conversion efficiency (∼5.0% at 100 mW/cm²) in the solid-state.     

Influence of crystalline complexes on electrical properties of PEO:LiTFSI electrolyte

Polymer electrolytes of poly(ethylene oxide) matrix with lithium imide salt LiN(CF₃SO₂)₂ were prepared by casting from solution. Thin films with compositions corresponding to molar ratios 6:1, 3:1 and 2:1 EO:Li were investigated by impedance spectroscopy, impedance spectroscopy simultaneous with polarizing microscope observation, X-ray diffraction and differential scanning calorimetry. The presence of PEO:LiTFSI stoichiometric complexes was found to significantly decrease conductivity at temperature of crystallization, which indicates that those complexes should be regarded as poorly conductive. Changes of properties of amorphous phase related to crystallization were also observed. Crystallization induced phase segregation, which in some cases caused considerable shift of the glass transition temperature of amorphous phase remaining in a semicrystalline system. For PEO:LiTFSI electrolyte with molar ratio of 3:1 EO:Li this effect was found to be responsible for enhancement of conductivity of semicrystalline sample in respect to the amorphous one, which was observed at low temperatures. Phase separation involving precipitation of LiTFSI salt was also found to be a likely explanation for significant enhancement of conductivity for PEO:LiTFSI 2:1 electrolyte subjected to rapid cooling below the glass transition temperature.     

Novel silane-substituted benzimidazolium iodide as gel electrolyte for dye-sensitized solar cells

A novel ionic electrolyte, 3-(iodohexyl)-1-(3-(triethoxysilyl)propylcarbamoyl)-1H-benzo[d]imidazol-3-ium iodide (SSBI) was prepared through the reaction of N-[3-(triethoxy-4-silyl)propyl]-1H-benzimidazole-1-carboxamide with 1,6-diiodohexane for the quasi-solidification of the electrolytic solution of a dye-sensitized solar cell (DSSC). The SSB-containing electrolyte, the sol electrolyte, in a DSSC was converted to a gel by heating it at 60 °C for 30 min in an oven. The DSSC consisting of the gel electrolyte showed a solar-to-electricity conversion efficiency comparable to that of the cell with the reference liquid electrolyte, and consistently higher stability than that of the reference cell. The performances of the DSSCs containing the reference, sol and gel electrolytes were discussed by scanning electron microscopy (SEM), impedance and chronoamperometric measurements, ionic conductivity, and UV-vis absorption spectroscopy.     

Preparation and characteristics of natural rubber/poly(ethylene oxide) salt hybrid mixtures as novel polymer electrolytes

Poly(ethylene oxide) (PEO) of molecular weight 1000 (PEO₁₀₀₀) containing lithium benzenesulfonate (LiBs) (PEO₁₀₀₀/LiBs), PEO derivatives having benzenesulfonate groups on both chain ends (PEO₁₀₀₀–(BSLi)₂), or 1-ethyl-2,3-dimethylimidazolium bromide (ImB), were each blended with natural rubber (NR). The ionic conductivity was measured from AC impedance values. The ionic conductivity of the mixture of NR and PEO₁₀₀₀/LiBs (40 wt%) was about 10⁻⁶ S cm⁻¹ at 50°C; this mixture retained rubbery physical characteristics. At NR content of 10 wt%, the ionic conductivity of the mixture (NR/PEO₁₀₀₀/LiBs) was 2.7×10⁻⁵ Scm⁻¹ at 50°C, approximately 10 times higher than that of the bulk PEO/LiBs mixture. For mixtures of NR and PEO₁₀₀₀–(BSLi)₂, no improvement in ionic conductivity by mixing was found. The ionic conductivity of the mixture of NR and ImB was about 10 times higher than for the bulk of PEO₁₀₀₀–(BSLi)₂ at a NR content of 10 wt%. We propose that the ionic conductivity of the mixture increases when an ion conducting matrix containing simple salt is added. On the other hand, the DSC curve for NR/PEO derivatives showed two T_gs based on the separate components, suggesting phase separation of the PEO derivative in the NR phase.     

Effects of environmentally benign solvents in the agarose gel electrolytes on dye-sensitized solar cells

The ionic agarose gel electrolytes are prepared by using environmental benign solvents and co-solvents to improve the agarose solubility and capacities of the additives for dye-sensitized solar cells. The effects of single solvents (dimethyl sulfoxide (DMSO), propylene carbonate (PC), propylene glycol, triethylene glycol, and tetraethylene glycol) and DMSO-based co-solvents are examined on the conductivities, diffusion coefficients of triiodide, and energy conversion efficiencies. The highest conductivity, 14.2 mS cm⁻¹, and the highest diffusion coefficient of triiodide, 2.7 × 10⁻⁶ cm² s⁻¹, are achieved for the electrolyte containing the co-solvent of 80 vol.% PC and 20 vol.% DMSO. The environmental benign co-solvent such as DMSO/PC can significantly increase the conversion efficiency to 3.4% with agarose compared to pure MPII with agarose (1.4%), while retaining ∼80% of the energy conversion efficiencies of the reference cell without agarose under the illumination at AM 1.5, 100 mW cm⁻².     

Fabrication of hole-patterned TiO2 photoelectrodes for solid-state dye-sensitized solar cells

We suggest a simple process to fabricate a hole-patterned TiO₂ electrode for a solid-state dye-sensitized solar cell (DSSC) to enhance cell performance through interfacial properties of the electrode with the electrolyte with minimum dye loading. The method involves prepatterning of SU-8 photoresist on a conducting glass, followed by the deposition of a nanocrystalline TiO₂ layer, calcination at 450 °C and characterization using scanning electron microscopy (SEM). Hole-patterned TiO₂ photoelectrodes yielded better solar energy conversion efficiency per dye loading compared to a conventional non-patterned photoelectrode. For example, a 50 μm hole-patterned DSSC exhibited 4.50% conversion efficiency in the solid state, which is comparable to an unpatterned flat TiO₂ photoelectrode (4.57%) however the efficiency per dye loading of the former (0.986%/g) was much greater than that of the latter (0.898%/g). The improvement was attributed to improved transmittance through the electrode as well as better interfacial properties between the electrolyte and electrode, as confirmed by UV-visible spectroscopy and electrochemical impedance (EIS) analysis.     

Improved photovoltage and performance by aminosilane-modified PEO/P(VDF-HFP) composite polymer electrolyte dye-sensitized solar cells

A PEO/P(VDF-HFP) composite polymer electrolyte was modified by different amounts of NH₂-end functional silane (3-amonopropyltriethoxysilane, APTS). Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were carried out to examine the configuration changes of the polymer electrolyte. The newly formed Si-O-Si network and interactions influenced the ionic conductivity of the APTS-modified polymer electrolyte and also enhanced the connection of the polymer electrolyte with the electrodes of the dye sensitized solar cells (DSSCs). The cyclic voltammograms and electrochemical impedance measurements indicated that the APTS deprotonated the TiO₂ photoanode surface and negatively changed the Fermi energy level and the conduction band edge to the vacuum level. This effectively reduced the interface recombination in the DSSC and improved the open circuit voltage. With moderate APTS content (0.1 M) modification, the DSSC exhibited a 58 mV improvement of photovoltage and an improved performance of 5.08% compared with 3.74% of the original DSSC.     

The application of P(MMA-co-MAA)/PEG polyblend gel electrolyte in quasi-solid state dye-sensitized solar cell at higher temperature

A poly(methyl methacrylate-co-methacrylate acid)/poly(ethylene glycol) [P(MMA-co-MAA)/PEG] polyblend with viscoelasticity was synthesized by a copolymerizing reaction between methyl methacrylate (MMA) and methacrylate acid (MAA) using azobisisobutyronitrile (AIBN) as initiator in poly(ethylene glycol) (PEG) methanol solution. Then, a polyblend gel electrolyte was prepared by adding KI and I₂ to P(MMA-co-MAA)/PEG system. The influence of compositions of the polyblend gel electrolyte on the ionic conductivity and the effect of temperature on photoelectronic performance of quasi-solid state dye-sensitized solar cell (QS-DSSC) were discussed. It was found that the polyblend gel electrolyte was a good candidate as high-temperature electrolyte for QS-DSSCs. Under an optimized condition, the highest conductivity of the polyblend gel electrolyte was 2.70 mS/cm² at 30 °C. Based on the polyblend gel electrolyte, a light-to-electricity conversion efficiency of 4.85% for QS-DSSC was achieved under AM 1.5 simulated solar light illumination at 60 °C.