Investigation of PEO-imidazole ionic liquid oligomer electrolytes for dye-sensitized solar cells

Ionic liquid oligomers prepared by incorporating imidazolium ionic liquid with PEO oligomers were investigated as electrolytes for dye-sensitized solar cells (DSSCs). The influences of PEO molecular weight and imidazolium group of the ionic liquid oligomers on the ionic conductivity, apparent diffusion coefficient of the redox species in the electrolytes and the performance of solar cells were examined. The structural effects of the ionic liquid oligomers on the kinetic behaviors of dye regeneration and triiodide reduction reactions taken place at nanocrystalline TiO₂ electrode and Pt counter-electrode, respectively, were further studied by cyclic-voltammetry and electrochemical impedance spectroscopy measurements. The increase of the PEO molecular weight of the ionic liquid oligomers results in the faster dye regeneration rate and lower charge transfer resistance of triiodide reduction leading to the improvement of cell performance effectively.     

A quasi-solid-state dye-sensitized solar cell based on the stable polymer-grafted nanoparticle composite electrolyte

A novel gel electrolyte was prepared by dispersing the polymer-grafted ZnO nanoparticle into liquid electrolyte. This gel electrolyte behaves long-term stability as the poly(ethylene glycol methyl ether) molecules are strongly connected to ZnO nanoparticles with covalent bond in polymer-grafted ZnO nanoparticle. A quasi-solid-state dye-sensitized solar cell (DSC) based on this gel electrolyte yields the energy transfer efficiency of 3.1% at AM 1.5 direct irradiation of 75 mW cm⁻² light intensity. Addition of 4-tert-butylpyridine into the electrolyte results in dramatically improved short circuit current density I_sc, and the overall efficiency is also improved to 5.0%, while the open circuit voltage (V_oc) and fill factor (ff) are insensitive to the presence of 4-tert-butylpyridine. DSC fabricated with this novel gel electrolyte displays better thermal stability than those solidified with the conventional nanoparticle ZnO(Ac).     

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.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).     

The effect of temperature on the performance of dye-sensitized solar cells based on a propyl-methyl-imidazolium iodide electrolyte

The performance of dye-sensitized solar cells (DSSC), based on an ionic liquid (propyl-methyl-imidazolium iodide) electrolyte, has been evaluated at varying iodine concentrations and cell temperatures (5–50 °C) for two irradiancies (0.1 and 1 sun). At 1 sun and at lower temperatures, the short-circuit current (J_SC) is limited by the diffusion of tri-iodide, while at higher temperatures, the J_SC decreases due to more pronounced recombinations. Also, the conversion efficiency of a DSSC resembles the J_SC behaviour. At 0.1 sun irradiance, the efficiency monotonically decreases with increasing temperature, while at 1 sun, a five-fold increase in efficiency is observed.     

Novel hydrophobic ionic liquids electrolyte based on cyclic sulfonium used in dye-sensitized solar cells

A novel series of hydrophobic room temperature ionic liquids based on six cyclic sulfonium cations were first time synthesized and applied in dye-sensitized solar cells as pure solvents for electrolyte system. The chronoamperograms result showed that the length of substituent on sulfonium cations could inhibit the diffusion and the five-ring structure of sulfonium was benefit for fast triiodide ion diffusion. The electrochemical impendence spectra measurement of dye-sensitized solar cells with these ionic liquid electrolytes was carried out and the result indicated that the cations’ structure had indeed influence on the cells’ performance especially for the fill factor, which was further proved by the measurement result of I-V curves of these dye-sensitized solar cells. The conclusion was obtained that the electron exchange reaction on Pt counter electrode/electrolyte interface dominated the cells’ performance for these ionic liquid electrolyte-based DSCs.     

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.     

Effect of molecular weight of oligomer on ionic diffusion in oligomer electrolytes and its implication for dye-sensitized solar cells

This study measures the diffusion coefficients of I⁻ and I₃⁻ in oligomer electrolytes as a function of the molecular weight of oligomers and investigates their effect on the performance of dye-sensitized solar cells (DSSCs). The high-diffusion coefficients of ions in an oligomer electrolyte with a lower molecular weight can help to promote the redox mechanism in DSSCs and thereby increase the short-circuit current density. They can also cause a decrease in the open-circuit voltage since a high-diffusion coefficient of I₃⁻ is capable of reducing the lifetime of electrons in TiO₂ electrodes. To offset these effects, N-methyl-benzimidazole is added to the oligomer electrolytes, thereby improving the open-circuit voltage and fill factor and, consequently, the overall energy-conversion efficiency, which increases to over 5%. A further test involving storage at a high temperature of 75 °C demonstrates that DSSCs employing the oligomer electrolytes show excellent thermal stability over 200 h.     

Effect of solvents in liquid electrolyte on the photovoltaic performance of dye-sensitized solar cells

The effect of solvents in liquid electrolyte on the photovoltaic performance of dye-sensitized solar cells was investigated. The solvents with large donor number enhanced the open-circuit voltage but reduced the short-circuit current density. By mixing 30 vol.% NMP with 70 vol.% GBL, the open-circuit voltage increased from 0.55 to 0.632 V and the fill factor increased from 0.607 to 0.613 while the short-circuit current density decreased little. The further addition of 0.4 M pyridine into the above mixed solvent caused a huge increase of overall conversion efficiency from 5.73 to 6.70% under irradiation of 100 mW cm⁻².     

Ionic liquid electrolytes based on 1-vinyl-3-alkylimidazolium iodides for dye-sensitized solar cells

Five new ionic liquids of 1-vinyl-3-alkylimidazolium iodide were synthesized to develop novel electrolytes for dye-sensitized solar cells. The effects of photovoltaic characteristics of the cell and the ionic liquid features such as viscosity and ionic conductivity were described. The 1-vinyl-3-alkylimidazolium cation volume was calculated by quantum chemistry method. The linear dependence of photon-to-current conversion efficiency on the non-solvated cation volume was revealed. After lithium iodide was added to 1-vinyl-3-alkylimidazolium salts as electrolytes, except the photovoltage, the photocurrent, fill factor and photon-to-current efficiency were improved correspondingly.     

Enhancing the performance of dye-sensitized solar cells by incorporating nanosilicate platelets in gel electrolyte

Two kinds of gel-type dye-sensitized solar cells (DSSCs), composed of two types of electrolytes, were constructed and the respective cell performance was evaluated in this study. One electrolyte, TEOS-Triton X-100 gel, was based on a hybrid organic/inorganic gel electrolyte made by the sol–gel method and the other was based on poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer. TEOS-Triton X-100 gel was based on the reticulate structure of silica, formed by hydrolysis, and condensation of tetraethoxysilane (TEOS), while its organic subphase was a mixture of surfactant (Triton X-100) and ionic liquid electrolytes. Both DSSC gel-type electrolytes were composed of iodine, 1-propy-3-methyl-imidazolium iodide, and 3-methoxypropionitrile to create the redox couple of I₃⁻/I⁻. Based on the results obtained from the I–V characteristics, it was found that the optimal iodine concentrations for the TEOS-Triton X-100 gel electrolyte and PVDF-HFP gel electrolyte are 0.05 M and 0.1 M, respectively. Although the increase in the iodine concentration could enhance the short-circuit current density (J_SC), a further increase in the iodine concentration would reduce the J_SC due to increased dark current. Therefore, the concentration of I₂ is a significant factor in determining the performance of DSSCs.In order to enhance cell performance, the addition of nanosilicate platelets (NSPs) in the above-mentioned gel electrolytes was investigated. By incorporating NSP-Triton X-100 into the electrolytes, the J_SC of the cells increased due to the decrease of diffusion resistance, while the open circuit voltage (V_OC) remained almost the same. As the loading of the NSP-Triton X-100 in the TEOS-Triton X-100 gel electrolyte increased to 0.5 wt%, the J_SC and the conversion efficiency increased from 8.5 to 12 mA/cm² and from 3.6% to 4.7%, respectively. However, the J_SC decreased as the loading of NSP-Triton X-100 exceeded 0.5 wt%. At higher NSP-Triton X-100 loading, NSPs acted as a barrier interface between the electrolyte and the dye molecules, hindering electron transfer, hence, reducing the cell's photocurrent density. The same behavior was also observed in the PVDF-HFP gel electrolyte DSSC system.     

Regenerative effects by temperature variations in dye-sensitized solar cells

The effect of repeated temperature variations on the performance of both fresh and aged dye-sensitized solar cells with liquid and semi-solid electrolytes has been studied. The cell performance was characterized with IV-curves obtained at different cell operating temperatures and electrochemical impedance spectroscopy measurements before and after the temperature treatments. Consecutive temperature rampings of the aged cells did regenerate the cell function, so that the total efficiency drop over the observation period was on average 18%/unit less for the temperature-treated cells than for reference cells aged at constant temperature.     

Polymer electrolytes for dye-sensitized solar cells prepared by photopolymerization of PEG-based oligomers

We report on the preparation and characterization of novel polymer electrolyte membranes for quasi-solid dye-sensitized solar cells. New methacrylic–acrylic gel-polymer electrolytes were prepared by photo-polymerization of mono/di-functional monomers. The crosslinked films were self standing, transparent and flexible. They were swelled by an iodine–iodide solution, obtaining a stable gel, where the polymeric network acts as a cage to retain the liquid, preventing its evaporation. Such a system combines the cohesive property of a solid with the high ionic conductivity of a liquid. The evaluation of the structural and physical-chemical characteristics of the polymer, combined with the electrical characterization of the membranes by means of the electrochemical impedance spectroscopy, allowed us to investigate the structure/property relationship of the material. The electric characterizations of the solar harvester based on the gel-polymer electrolyte showed a maximum photovoltaic conversion efficiency of 4.41%. Moreover, a significant improvement in the durability of the device was demonstrated with respect to the liquid electrolyte-based counterpart.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

On the modeling of the dye-sensitized solar cell

A simplified electric model of the dye-sensitized electrochemical solar cell (DSC) is presented. It permits the calculation of internal steady-state cell characteristics like particle density distributions or the electric field as a function of the (measured) external current I_ext. The cell is modeled as an one-dimensional pseudo-homogeneous medium of thickness L, where all the electroactive particles involved in the current supporting process move according to different effective transport coefficients (i.e. effective diffusivities D and effective mobilities μ). The electroactive particles are the electrons e⁻ injected into the nanoporous TiO₂ layer after light absorption by the dye, the reduced and the oxidized counterpart of the redox electrolyte El_Red and El_Ox, and the positively charged cation Kat⁺ being brought into the cell together with the electrolyte. By applying the continuity equation, the transport-equation and Poisson's equation to all the electroactive species involved (e⁻, El_Red, El_Ox and Kat⁺) and by assuming a linear Boltzmann relaxation approximation for the back reaction, a system of differential equations is derived, describing particle densities, particle currents and the electric field within the cell. The underlying simplifying assumptions as well as the resulting limits of the model are stated, and some possible extensions are given. This paper aims to outline the general ideas and limitations of the proposed electric modeling, numerical calculations have been successfully implemented, but will be presented in a future paper.     

Influence of polymer concentration and dyes on photovoltaic performance of dye-sensitized solar cell with P(VdF-HFP)-based gel polymer electrolyte

A series of gel polymer electrolytes (GPEs) is synthesized using Poly(vinylidenefluoride-hexafluoropropylene) P(VdF-HFP) as the host matrix and propylene carbonate (PC)–diethyl carbonate (DEC) as plasticizers to fabricate dye-sensitized solar cells. Equal amounts of PC and DEC are used to comprehend high dielectric constant and low viscosity of the electrolyte. The as-prepared GPEs are characterized by XRD, FTIR and SEM. Their thermal properties and ionic conductivities are investigated by TGA/DSC analyses and AC impedance measurements, respectively. The optimized gel polymer electrolyte gives a maximum ionic conductivity of 5.25 × 10⁻³ S cm⁻¹ at room temperature. The formation of porous structure in the electrolyte film supports the entrapment of large volumes of liquid electrolyte inside its cavities. The role of N3 and N719 dyes are also investigated for better photovoltaic performance of DSSC. The overall light-to-electrical-energy conversion efficiencies of 3.95% and 4.41% are obtained for N3 and N719 dyes, respectively, under 100 mW cm⁻² irradiation, which are comparable to those obtained from the corresponding liquid electrolyte cell.     

High-performance carbon counter electrode for dye-sensitized solar cells

Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode of dye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I₃⁻/I⁻ couple.     

Pyridinium molten salts as co-adsorbents in dye-sensitized solar cells

The influence of using pyridinium molten salts as co-adsorbents to modify the monolayer of a TiO₂ semiconductor on the performance of a dye-sensitized solar cell is studied. The current-voltage characteristics are measured under AM 1.5 (100 mW cm⁻²). The pyridinium molten salts significantly enhance the open-circuit photovoltage (V_oc), the short circuit photocurrent density (J_sc) as well as the solar energy conversion efficiency (η). 1-Ethyl-3-carboxypyridinium iodide ([ECP][I]) is applied successfully to prepare an insulating molecular layer with N719, and achieve high energy conversion efficiency as high as 4.49% at 100 mW cm⁻² and AM 1.5. The resulting efficiency is 20% higher than that of a non-additive device. This enhancement of conversion efficiency is attributed to the negative shift of the conduction band (CB) edge and the abundant concentration of I⁻ on the surface of the electrode when using [ECP][I] as the co-adsorbent.     

High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells

Polypyrrole (PPy) nanoparticle was synthesized and coated on a conducting FTO glass to construct PPy counter electrode used in dye-sensitized solar cell (DSSC). Scanning electron microscope images show that PPy with porous and particle diameter in 40–60 nm is covered on the FTO glass uniformly and tightly. Cyclic voltammograms of I₂/I⁻ system measurement reveals that the PPy electrode has smaller charge-transfer resistance and higher electrocatalytic activity for the I₂/I⁻ redox reaction than that Pt electrode does. Overall energy conversion efficiency of the DSSC with the PPy counter electrode reaches 7.66%, which is higher (11%) than that of the DSSC with Pt counter electrode. The excellent photoelectric properties, simple preparation procedure and inexpensive cost allow the PPy electrode to be a credible alternative used in DSSCs.     

Quasi-solid-state dye-sensitized solar cells with cyanoacrylate as electrolyte matrix

A quasi-solid-state dye-sensitized solar cells (DSSCs) employing a commercial glue (“SuperGlue^®”) as electrolyte matrix was fabricated. The cyano groups of the cyanoacrylate can form a supramolecular complex with tetrapropylammonium cations. This immobilizes the cations and therefore might lead to a favored anionic charge transport necessary for a good performance of the iodide/triiodide electrolytic conductor. Obtaining energy conversion efficiencies of more than 4% under 100 mW/cm² of simulated A.M. 1.5 illumination, the cyanoacrylate quasi-solid-state electrolyte is an ordinary and low-cost compound which has fast drying property and offers significant advantages in the fabrication of solar cells and modules as it is in itself is a very good laminating agent. The influences of different porous layer thicknesses of titanium oxide and various kinds of cations on DSSC performance and long-term stability are presented.