Nanocomposite gel electrolyte with large enhanced charge transport properties of an I3/I redox couple for quasi-solid-state dye-sensitized solar cells

TiO₂ nanoparticles was introduced into quasi-solid-state Poly(vinylidenefluoride-co-hexafluoropropylene) (P(VDF-HFP)) based gel electrolyte to form nanocomposite gel electrolyte for quasi-solid-state dye-sensitized solar cells. The steady-state voltammograms revealed that the diffusion performance of the triiodide and iodide in the quasi-solid-state P(VDF-HFP) based gel electrolyte was greatly enhanced after the addition of TiO₂ nanoparticles. Especially, the apparent diffusion coefficient of I₃⁻ increased from 0.76×10⁻¹⁰ m²/s to 4.42×10⁻¹⁰ m²/s, reached the level of the liquid electrolyte (4.04×10⁻¹⁰ m²/s). By introducing TiO₂ nanoparticles, the photoelectric conversion efficiency of the gel based device increased from 5.72% to 7.18%, which reached the level of the liquid electrolytes based device (7.01%). The electrical impedance spectrum revealed that the addition of TiO₂ nanoparticles could reduce the charge recombination at the interface of dyed TiO₂ electrode/electrolyte. The results of the accelerated aging tests showed that the nano-TiO₂ composite gel electrolytes based devices could maintain 90% of their initial value after heating at 60 °C for 1000 h, which indicated that they had better thermostability than the corresponding normal gel electrolyte based devices and liquid electrolyte based devices.     

ZnO-doped BaZr0.85Y0.15O3−δ proton-conducting electrolytes: Characterization and fabrication of thin films

ZnO-doped BaZr_0.85Y_0.15O_3−δ perovskite oxide sintered at 1500 °C has bulk conductivity of the order of 10⁻² S cm⁻¹ above 650 °C, which makes it an attractive proton-conducting electrolyte for intermediate-temperature solid oxide fuel cells. The structure, morphology and electrical conductivity of the electrolyte vary with sintering temperature. Optimal electrochemical performance is achieved when the sintering temperature is about 1500 °C. Cathode-supported electrolyte assemblies were prepared using spin coating technique. Thin film electrolytes were shown to be dense using SEM and EDX analyses.     

Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

Binary room-temperature ionic liquids based electrolytes solidified with SiO2 nanoparticles for dye-sensitized solar cells

In this study, binary ionic liquids (bi-IL) of imidazolium salts containing cations with different carbon side chain lengths (C = 2, 4, 6, 8) and anions such as iodide (I⁻), tetrafluoroborate (BF₄⁻), hexafluorophosphate (PF₆⁻) and trifluoromethansulfonate (SO₃CF₃⁻) were used as electrolytes in dye-sensitized solar cells (DSSCs). On increasing the side chain length of imidazolinium salts, the diffusion coefficients of I₃⁻ and the cell conversion efficiencies decreased; however, the electron lifetimes in TiO₂ electrode increased. As for different anions, the cell which contains 1-butyl-3-methyl imidazolium trifluoromethansulfonate (BMISO₃CF₃) electrolyte has better performance than those containing BMIBF₄ and BMIPF₆. From the impedance measurement, the cell containing BMISO₃CF₃ electrolyte has a small charge transfer resistance (R_ct2) at the TiO₂/dye/electrolyte interface. Moreover, the characteristic frequency peak for TiO₂ in the cell based on BMISO₃CF₃ is less than that of BMIBF₄ and BMIPF₆, indicating the cell with bi-IL electrolyte based on BMISO₃CF₃ has higher electron lifetime in TiO₂ electrode. Finally, the solid-state composite was introduced to form solid-state electrolytes for highly efficient DSSCs with a conversion efficiency of 4.83% under illumination of 100 mW cm⁻². The long-term stability of DSSCs with a solidified bi-IL electrolyte containing SiO₂ nanoparticles, which is superior to that of a bi-IL electrolyte alone, was also presented.     

All solid-state electrochromic devices with gelatin-based electrolyte

6×8 cm² electrochromic devices (ECDs) with the configuration K-glass/EC-layer/electrolyte/ion-storage (IS) layer/K-glass, have been assembled using Nb₂O₅:Mo EC layers, a (CeO₂)_0.81–TiO₂ IS-layer and a new gelatin electrolyte containing Li⁺ ions. The structure of the electrolyte is X-ray amorphous. Its ionic conductivity passed by a maximum of 1.5×10⁻⁵ S/cm for a lithium concentration of 0.3 g/15 ml. The value increases with temperature and follows an Arrhenius law with an activation energy of 49.5 kJ/mol. All solid-state devices show a reversible gray coloration, a long-term stability of more than 25,000 switching cycles (±2.0 V/90 s), a transmission change at 550 nm between 60% (bleached state) and 40% (colored state) corresponding to a change of the optical density (ΔOD=0.15) with a coloration efficiency increasing from 10 cm²/C (initial cycle) to 23 cm²/C (25,000th cycle).     

Photovoltaic activity in a ZnTe/PEO–chitosan blend electrolyte junction

A ZnTe/polymer junction has been fabricated and the photovoltaic properties studied. The polymer is a blend of 50 wt% chitosan and 50 wt% polyethylene oxide (PEO). The polymer blend was complexed with ammonium iodide (NH₄I) and some iodine crystals were added to the polymer–NH₄I solution to provide the I⁻/I³⁻ redox couple. The ionic conductivity of the polymer electrolyte is 4.32×10⁻⁶ S cm⁻¹ at room temperature. ZnTe was electrodeposited on ITO conducting glass. The polymer film was sandwiched between the ZnTe semiconductor and an ITO glass to form a ZnTe/polymer electrolyte/ITO photovoltaic cell. The open circuit voltage (V_oc) of the fabricated cells ranges between 300 and 400 mV and the short circuit current between 2 and 5 μA.     

Effect of electrolytes on the photovoltaic performance of a hybrid dye sensitized ZnO solar cell

The efficiency of dye sensitized solar cell depends on the number of factors such as impedance due to anions in the electrolytes, oxidation–reduction process of anions and size of cations of the electrolyte. This paper reports the effect of electrolytes on the photovoltaic performance of hybrid dye sensitized ZnO solar cells based on Eosin Y dye. The size of the cations has been varied by choosing different electrolytes such as LiBr+Br₂, LiI+I₂, tetrapropylammonium iodide +I₂ in mixed solvent of acetronitrile and ethylene carbonate. The impedance of anions has been determined by electrochemical impedance spectra. It is observed that Br⁻/Br₃⁻ offers high impedance as compared to I⁻/I₃⁻ couple. The oxidation–reduction reactions of electrolytes are measured by linear sweep voltammogram. It is found that Br⁻/Br₃⁻ is more suitable than an I⁻/I₃⁻ couple in dye sensitized solar cell (DSSC) in terms of higher open-circuit photovoltage production and higher overall energy conversion efficiency. This is attributed to more positive potential of the dye sensitizer than that of Br⁻/Br₃⁻. The gain in V_oc was due to the enlarged energy level difference between the redox potential of the electrolyte and the Fermi level (E_f) of ZnO and the suppressed charge recombination as well.     

Quasi-solid-state dye-sensitized solar cells based on a sol–gel organic–inorganic composite electrolyte containing an organic iodide salt

An organic–inorganic composite gel electrolyte based on TiO₂ gel, γ-butyrolactone (γ-BL) and N-methyl pyridine iodide was prepared by the sol–gel method. This gel electrolyte shows high ambient ionic conductivity of 7.63 mS cm⁻¹, which is close to the data of liquid electrolyte with the same organic iodide salt and γ-butyrolactone. Based on the gel electrolyte, a quasi-solid-state dye-sensitized solar cell was fabricated and the highest overall energy conversion efficiency of light-to-electricity of 3.06% was achieved under irradiation of 60 mW cm⁻².     

A polymer gel electrolyte composed of a poly(ethylene oxide) copolymer and the influence of its composition on the dynamics and performance of dye-sensitized solar cells

A polymer gel electrolyte composed of a poly(ethylene oxide) derivative, poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl ether), mixed with gamma-butyrolactone (GBL), LiI and I₂ is employed in dye sensitized solar cells (DSSC). The electrolyte is characterized by conductivity experiments, Raman spectroscopy and thermal analysis. The influence of the electrolyte composition on the kinetics of DSSC is also investigated by transient absorption spectroscopy (TAS). The electrolyte containing 70 wt.% of GBL and 20 wt.% of LiI presents the highest conductivity (1.9 × 10⁻³ S cm⁻¹). An efficiency of 4.4% is achieved using this composition. The increase in I_SC as a function of GBL can be attributed an increase in the mobility of the iodide (polyiodide) species. The increase in the yield of the intermediate species, I₂⁻, originating in the regeneration reaction, is confirmed by TAS. However, the charge recombination process is faster at this composition and a decrease in the V_oc is observed. Photovoltage decay experiments confirm an acceleration in charge recombination for the DSSC assembled with the electrolyte containing more GBL. Raman investigations show that in this electrolyte the I₅⁻/I₃⁻ ratio is higher. Theoretical calculations also indicate that the I₅⁻ species is a better electron acceptor.     

Preparation of sulfonated poly(ether ether ketone)s containing amino groups/epoxy resin composite membranes and their in situ crosslinking for application in fuel cells

A series of amino-containing sulfonated poly(aryl ether ketone)/4,4′-diglycidyl(biphenyl) epoxy resin (DGBP) composite membranes for proton exchange membranes fuel cells (PEMFCs) are prepared by solution blending and casting. The reaction kinetics and the effects of introduction of DGBP content on the properties of the composite membranes are thoroughly investigated. The crosslinked composite membranes after treatment at either 120 °C or 200 °C have improved oxidative and dimensional stability than those without crosslinking. Despite the fact that crosslinked membranes generally have lower proton conductivity in comparison with the original ones, the proton conductivities of the membranes treated at 120 °C are above 2.22 × 10⁻² S cm⁻¹ at room temperature and 9.42 × 10⁻² S cm⁻¹ at 100 °C. Even for the samples treated at 200 °C, their proton conductivities are still higher than 1.26 × 10⁻² S cm⁻¹ at room temperature and higher than 8.67 × 10⁻² S cm⁻¹ at 100 °C, which are well satisfied with elementary requirement of fuel cells. In addition, all the evaluated membranes have low methanol permeability. For example, the methanol permeability of AP6FSPEEK/DGBP1 cured at 200 °C is 0.33 × 10⁻⁶ cm² s⁻¹, which is an order magnitude lower than Nafion 117. Therefore, these novel crosslinked composite membranes could be potential usage in fuel cells.     

On the use of triethylamine hydroiodide as a supporting electrolyte in dye-sensitized solar cells

For the first time, the application of a molten salt, triethylamine hydroiodide (THI), as a supporting electrolyte was investigated for the dye-sensitized solar cells (DSSCs). Titanium dioxide (TiO₂) electrode was modified by incorporation of high- and low-molecular weight poly(ethylene glycol) along with TiO₂ nanoparticles of two different sizes (300 nm (30 wt%) and 20 nm (70 wt%)). The highest apparent diffusion coefficient (D) of 8.12×10⁻⁶ cm² s⁻¹ was obtained for I⁻ (0.5 M of THI) from linear sweep voltammetry (LSV). Short-circuit current density (J_sc) increases with the concentration of THI whereas open-circuit potential (V_oc) remains the same. Optimum J_sc (19.28 mA cm⁻²) and V_oc (0.7 V) with a highest conversion efficiency (η) of 8.45% were obtained for the DSSC containing 0.5 M of THI/0.05 M I₂/0.5 M TBP in CH₃CN. It is also observed that the J_sc and η of the DSSC mainly relates with the D values of I⁻ and charge-transfer resistances such as R_ct1 and R_ct2 operating along Pt/TiO₂ electrolyte interface, obtained from LSV and electrochemical impedance spectroscopy (EIS). For comparison, tetraethylammonium iodide (TEAI) and LiI were also selected as supporting electrolytes. Though both the THI and TEAI have similar structures, replacement of one methyl group by hydrogen improves the efficiency of the DSSC containing the former electrolyte. Further, the DSSC containing THI exhibits higher J_sc and η than LiI (7.70%), from which it is concluded that THI may be used as an efficient and alternative candidate to replace LiI in the current research of DSSCs.     

Electronic and photovoltaic properties of Al/p-Si/copper phthalocyanine photodiode junction barrier

Al/p-Si/copper phthalocyanine photovoltaic device has been fabricated and characterised by current–voltage and capacitance–voltage measurements. Electrical properties of the device were determined by current–voltage characterizations under dark and illumination conditions. The density distribution of the interface states of the photodiode was found to vary from 8.88×10¹² eV⁻¹ cm⁻² in E_ss-0.54 eV to 4.51×10¹² eV⁻¹ cm⁻² in E_ss-0.61 eV. The device shows a photovoltaic behaviour with a maximum open circuit voltage Voc of 0.16 V and short-circuits current I_sc of 0.45 μA under 3500 lux light intensity.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Preparation of dense and uniform La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) films for fundamental studies of SOFC cathodes

Thin films of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) were deposited on (1 0 0) silicon and on GDC electrolyte substrates by rf-magnetron sputtering using a single-phase oxide target of LSCF. The conditions for sputtering were systematically studied to get dense and uniform films, including substrate temperature (23–600 °C) background pressure (1.2 × 10⁻² to 3.0 × 10⁻² mbar), power, and deposition time. Results indicate that to produce a dense, uniform, and crack-free LSCF film, the best substrate temperature is 23 °C and the argon pressure is 2.5 × 10⁻² mbar. Further, the electrochemical properties of a dense LSCF film were also determined in a cell consisting of a dense LSCF film (as working electrode), a GDC electrolyte membrane, and a porous LSCF counter electrode. Successful fabrication of high quality (dense and uniform) LSCF films with control of thickness, morphology, and crystallinity is vital to fundamental studies of cathode materials for solid oxide fuel cells.     

Bulk p-CuInSe2 photo-electrochemical solar cells

Dense CuInSe₂ of high quality, prepared by the fusion technique in evacuated quartz ampoule from stoichiometric melt, crystallizes in the chalcopyrite structure. Compositional analysis carried out by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy (EDS) indicates a uniform distribution of elements through the depth and a composition close to the stoichiometry. The diffuse reflectance spectrum gives a band gap at 0.94 eV. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polarons hopping. Above 320 °C, CuInSe₂ undergoes an irreversible oxidation. The thermal variation of the thermopower indicates p-type behavior attributed to copper deficiency and a hole mobility μ_300 K of 0.133 cm² V⁻¹ s⁻¹, thermally activated. In KCl media, the compound exhibits an excellent chemical stability with a corrosion rate of 8 μmol cm⁻² month⁻¹. The photo-electrochemical properties, investigated for the first time on the ingots, confirm the p-type conductivity. From the capacitance measurements, the flat band potential (V_fb=−0.62V_SCE) and the holes density (N_A=4×10¹⁷ cm⁻³) were determined. The valence band, located at 4.43 eV below vacuum, is made up of mainly Se orbital with little admixture of Cu character. The change of the electrolyte causes a variation in the potential V_fb (dV_fb/dpH=−0.058 V pH⁻¹) indicating strong OH⁻ adsorption. The fill factor in S²⁻ media was found to be 0.54; such result was corroborated by semi-logarithmic plots.     

Fabrication of cathode supported solid oxide fuel cell by multi-layer tape casting and co-firing method

La_0.3Sr_0.7FeO_3-δ (LSF)/CeO₂ cathode supported Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte was prepared by a simple multilayer tape casting and co-firing method. SDC electrolyte slurry and LSF/CeO₂ cathode slurry were optimized and the green bi-layer tapes were co-fired at different temperature. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and Scan Electronic Microscopy (SEM). No additional phase peak line was observed in electrolyte and cathode support when the sintering temperature lower was than 1400 °C. The electrolytes were extremely dense with the thickness of about 20 μm. The cathode support was porous with electrical conductivity of about 4.21 S/cm at 750 °C. With Ni/SDC as anode, Open Current Voltage and maximum power density reached 0.61 V and 233 mW cm⁻² at 750 °C, respectively.     

Quasi-solid state dye-sensitized solar cells based on gel polymer electrolyte with poly(acrylonitrile-co-styrene)/NaI+I2

Gel polymer electrolyte based on poly(acrylonitrile-co-styrene)/NaI+I₂ and binary solvent mixture was prepared. When the system contains 0.5 M NaI and 0.05 M I₂, the maximum ionic conductivity (at 30 °C) of 2.37 mS cm⁻¹ was achieved. Based on a gel polymer electrolyte with 0.5 M NaI, 0.05 M I₂ and 0.5 M 4-tert-butylpyridine, a quasi-solid state dye-sensitized solar cell was fabricated and its overall energy conversion efficiency of light-to-electricity of 2.75% was achieved under irradiation of 60 mW cm⁻².     

A preliminary study of the electro-oxidation of l-ascorbic acid on polycrystalline silver in alkaline solution

Electrochemical oxidation of l-ascorbic acid on polycrystalline silver in alkaline aqueous solutions is studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (IS). The anodic electro-oxidation starts at −500 mV versus SCE and shows continued anodic oxidation in the cathodic half cycle in the CV regime signifying slowly oxidizing adsorbates. Diffusion coefficient of ascorbate ion measured under both voltammetric regimes is around 1.4 × 10⁻⁵ cm² s⁻¹. Impedance spectroscopy measures the capacitances associated with double layer and adsorption around 50 μF cm⁻² and 4 mF cm⁻² as well as the adsorption and decomposition resistances (rates).     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.