Preparation and characterization of core-shell electrodes for application in gel electrolyte-based dye-sensitized solar cells

Core-shell electrodes based on TiO₂ covered with different oxides were prepared and characterized. These electrodes were applied in gel electrolyte-based dye-sensitized solar cells (DSSC). The TiO₂ electrodes were prepared from TiO₂ powder (P25 Degussa) and coated with thin layers of Al₂O₃, MgO, Nb₂O₅, and SrTiO₃ prepared by the sol-gel method. The core-shell electrodes were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscopy measurements. J-V curves in the dark and under standard AM 1.5 conditions and photovoltage decay measurements under open-circuit conditions were carried out in order to evaluate the influence of the oxide layer on the charge recombination dynamics and on the device's performance. The results indicated an improvement in the conversion efficiency as a result of an increase in the open circuit voltage. The photovoltage decay curves under open-circuit conditions showed that the core-shell electrodes provide longer electron lifetime values compared to uncoated TiO₂ electrodes, corroborating with a minimization in the recombination losses at the nanoparticle surface/electrolyte interface. This is the first time that a study has been applied to DSSC based on gel polymer electrolyte. The optimum performance was achieved by solar cells based on TiO₂/MgO core-shell electrodes: fill factor of ∼0.60, short-circuit current density J_sc of 12 mA cm⁻², open-circuit voltage V_oc of 0.78 V and overall energy conversion efficiency of ∼5% (under illumination of 100 mW cm⁻²).     

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.     

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.     

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.     

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.     

Polymer–metal complex as gel electrolyte for quasi-solid-state dye-sensitized solar cells

A kind of polymer–metal complex gel electrolyte is successfully prepared and is used in dye-sensitized solar cells. Raman and X-ray photoelectron spectroscopy confirm the structure of this complex and is found that the metal ion reacts with nitrogen in the polymer. This novel electrolyte shows apparent diffusion coefficient of iodide of 8.37 × 10⁻⁷ cm² s⁻¹ and the energy conversion efficiency of 6.10% when the amount of ZnI₂ is 0.04 M. By studying the dissociation active energy of the inorganic salt in electrolytes, we find that the metal salts can dissociate more easily after reacting with polymer and as a result can provide extra free iodide ion. The cell maintains ca. 93% of its initial efficiency after 20 d without further sealing, which shows good long-time stability.     

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.     

聚噻吩类化合物在染料敏化太阳能电池中的应用研究进展

阐述了聚噻吩类化合物作为染料敏化剂和电解质在染料敏化太阳能电池中的应用研究进展。     

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.     

A quasi solid-state dye-sensitized solar cell made of polypyrrole counter electrodes

Quasi-solid state dye-sensitized solar cells have been constructed using nanocrystalline titania, a Ureasil-based nanocomposite gel electrolyte and polypyrrole-functionalized counter electrode. Polypyrrole was synthesized by potentiostatic electrodeposition using pyrrole monomer as precursor by a simple procedure in aqueous solution. The thus obtained polypyrrole films were very robust. They were characterized by FE-SEM microscopy and electrochemical impedance spectroscopy and they were used for the construction of solar cells. The employment of polypyrrole electrocatalyst was judged satisfactory for the present application since it was only 30% less efficient than the corresponding counter electrodes functionalized with Pt.     

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.     

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.     

Carbon nanotubes-polyethylene oxide composite electrolyte for solid-state dye-sensitized solar cells

Novel carbon nanotubes (CNTs)-polyethylene oxide (PEO) composite electrolyte for dye-sensitized solar cell (DSSC) was prepared and characterized for the first time. The strong bonding and interaction between CNTs and PEO in CNTs-PEO composites was observed by the characterization of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectra. The introduction of CNTs into PEO matrix significantly improved the electrolyte properties of DSSC such as roughness, amorphicity and ionic conductivity. The solid-state DSSC fabricated with the optimum composite electrolyte (added 1% CNTs in PEO matrix, 1%CNT-PEO) achieved maximum conversion efficiency of 3.5%, an open circuit voltage (V_OC) of 0.589 V, short circuit current density (J_SC) of 10.64 mA/cm² and fill factor (FF) of 56%. The highest IPCE in the DSSC fabricated with 1%CNT-PEO electrolyte is ascribed to the improved ionic conductivity of composite electrolytes and enhanced interfacial contact between electrode and electrolyte.     

Effect of Cr-doping on the physicochemical properties of blue TiO2 and its application in dye-sensitized solar cells via low-temperature fabrication process

Cr-doped blue TiO₂ (Cr-BTiO₂) nanoparticles were fabricated at room temperature using lithium-ethylenediamine (Li-EDA) as reducing agent. The addition of Li-EDA promotes the selective reduction of the rutile phase of TiO₂ into the amorphous phase keeping anatase phase unaltered. Hence, the phase-selective reduction of TiO₂ leads to the formation of blue TiO₂ nanoparticles. Synthesized samples were characterized by equipment fitted with modern technology. The shifting of (101) peak to a lower angle (2θ) in Cr-BTiO₂ in X-ray diffraction (XRD) pattern suggests the successful doping of chromium into TiO₂ lattices. In Raman spectra, the shifting of the active Eg peak of Cr-BTiO₂ nanoparticles to higher wavenumber also suggests the successful substitution of Ti by Cr. The blue TiO₂ and Cr-BTiO₂ show increased absorption of light in the visible region compared to TiO₂ (P25). The modified TiO₂ samples have improved electron-hole separation tendency as predicted by the photoluminescence spectra (PL). Also, doping of Cr- into TiO₂ lattice results the formation of oxygen vacancy as detected by X-ray photoelectron spectroscopy (XPS). Among all samples, Cr-BTiO₂ demonstrated improvement in J_sc and overall incident photon to current conversion efficiency. Therefore, the synthetic effect is thus responsible for the enhancement in efficiency of Cr-BTiO₂ towards the dye-sensitized solar cell (DSSC) by 2.5 and 1.5 times higher than the P25 and blue TiO₂, respectively.     

Effect of pyridine in electrolyte on the current-voltage characteristics in dye-sensitized solar cells

This study aimed to investigate the effect of pyridine in electrolyte on the photocurrent density-photovoltage (J-V) characteristics in dye-sensitized solar cells. When the concentration of pyridine (c_p) in an electrolyte (0.1 M LiI and 0.05 M I₂ in acetonitrile) is increased from 0 to 0.4 M, the open-circuit voltage (V_oc) increased by 0.14 V, whereas the short-circuit photocurrent density (J_sc) decreased by 3.0 mA cm⁻². The optimum c_p for the maximum cell performance was around 0.1-0.2 M. The model calculation predicted that by increasing c_p from 0 to 0.4 M, the electron diffusion coefficient (D) decreases by more than one order of magnitude and the lifetime of conduction band-free electrons (τ) increases by more than one order of magnitude, implying that the electron diffusion length () increases by 31%, whereas the electron injection flux from dye molecules to TiO₂ nanoporous films (Φ) decreases by 28%.     

Composite electrolyte of heteropolyacid (HPA) and polyethylene oxide (PEO) for solid-state dye-sensitized solar cell

A novel heteropolyacid (HPA) and polyethylene oxide (PEO) composite electrolyte was prepared and applied as the solid electrolyte for solid-state dye-sensitized solar cell. Advanced HPA-PEO composite electrolytes could be prepared by the optimized mixture solvent of chloroform and methanol (ChMe), which possessed the enhanced morphological properties, ionic conductivity and amorphicity. It is due to the presence of well-dispersed HPAs on the texture of PEO with the strong interaction between them. DSSCs fabricated with HPA-PEO/ChMe electrolyte showed significantly high photovoltaic (PV) performance with the overall conversion efficiency of 3.1%, open circuit voltage of 0.524 V and a short circuit current of 9.7 mA/cm². HPA in the composite electrolytes may act as an electron acceptor to prohibit the photo-reduction of I⁻, resulting in the advanced photocurrent density and stability without significant decline of the PV performance for 7 days.     

Influence of iodine concentration on the photoelectrochemical performance of dye-sensitized solar cells containing non-volatile electrolyte

The effect of iodine concentration in the electrolyte with non-volatile solvent of dye-sensitized solar cells (DSCs) on photovoltaic performance was studied. The electron transport and interfacial recombination kinetics were also systematically investigated by electron impedance spectroscopy (EIS). With the iodine concentration increased from 0.025 to 0.1 M, open-circuit voltage (V_oc) and photocurrent density (J_sc) decreased while fill factor (ff) increased significantly. The decline of the V_oc and J_sc was mainly ascribed to increased electron recombination with tri-iodide ions (I₃⁻). The increased fill factor was primarily brought by a decrease in the total resistance. From impedance spectra of the solar cells, it can be concluded that increasing the iodine concentration in electrolytes could decrease charge transfer resistance (R_ct) and the chemical capacitance (C_μ), increase the electron transport resistance (R_t), and hence decrease the electron lifetime (τ) and the effective diffusion coefficient (D_n) of electrons in the TiO₂ semiconductor. With optimum iodine concentration, device showed a photocurrent density of 16.19 mA cm⁻², an open-circuit voltage of 0.765 V, a fill factor of 0.66, and an overall photo-energy conversion efficiency of 8.15% at standard AM 1.5 simulated sunlight (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.     

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.     

Influence of ionic additives NaI/I2 on the properties of polymer gel electrolyte and performance of quasi-solid-state dye-sensitized solar cells

The ionic additives NaI/I₂ in polymer gel electrolyte not only provide cations, but also affect the liquid electrolyte absorbency of the poly(acrylic acid)-poly(ethylene glycol) hybrid, which results in the change of ionic conductivity of polymer gel electrolyte and the photovoltaic performance of quasi-solid-state dye-sensitized solar cell. With the optimized components of liquid electrolyte containing 0.5 M NaI, 0.05 M I₂, 0.4 M pyridine, 70 vol.% γ-butyrolactone and 30 vol.% N-methylpyrrolidone, a 4.74% power conversion efficiency of quasi-solid-state dye-sensitized solar cell was obtained under 100 mW cm⁻² (AM 1.5) irradiation.