Solvent-free Ionic Liquid Electrolyte Research Articles

Multi-component eutectic salts to enhance the conductivity of solvent-free ionic liquid electrolytes for dye-sensitized solar cells

Ionic liquids (ILs) possess intrinsic nonvolatility, low flammability, high ionic conductivity, high thermal and chemical stability, rendering themselves as excellent candidates for next generation dye-sensitized solar cells (DSSCs). Further improving the ionic conductivity of ionic liquid electrolytes is critical to the performance enhancement of DSSCs. Here, multi-component eutectic salts by mixing four ILs are proposed to achieve higher ionic conductivity compared to the individual one. In addition, the effect of mixing strategy on the eutectic salts was systematically investigated by diverse structural, thermal, electrochemical and photovoltaic characterization in detailed. It revealed that mixing strategy could not only tune the solid-state property of individual ILs to liquid-state eutectic salts, but also greatly improve the ionic conductivity of final multi-component eutectic salts. Thus, based on the multi-component eutectic salt, a power conversion efficiency (PCE) of 4.81% could be obtained by the resultant device. Moreover, the DSSCs with multi-component eutectic salt also display better stability than the referenced device with volatile organic solvent. The mixing eutectic salts offer us a feasible strategy to design high ionic conductive electrolytes for high performance dye-sensitized solar cells.

Phenanthroline-based ruthenium complexes for enhanced charge transportation in solvent-free ionic liquid electrolyte

In the present work, we synthesized a series of 1,10-phenanthroline-based ruthenium complexes (PRCs). The PRCs were characterized by 1H and 13C NMR, UV–Vis and fluorescence spectroscopic analyses. Further, we used these PRCs complexes as effective additives in ionic liquid (IL) electrolyte to enhance current conduction in electrochemical cells. Anodic peak current in cyclic voltammetry response of PRCs showed functional group dependence on the nature of ruthenium complex. Mono-propenyl-functionalized PRC additive in IL electrolyte exhibited relatively higher current density, lower charge transfer resistance and decreased impedance compared to the tri-propenyl-functionalized PRC as well as without functional groups on 1,10-phenanthroline ligand in Ru complex. Finally, the PRCs as additives in IL electrolyte were tested in dye-sensitized solar cell which showed an increase of 65% in the efficiency when mono-propenyl-functionalized PRC complex was used as an additive.

Electrochemically Reduced Graphene Oxide Multilayer Films as Efficient Counter Electrode for Dye-Sensitized Solar Cells

We report on a new counter electrode for dye-sensitized solar cells (DSCs), which is prepared using layer-by-layer assembly of negatively charged graphene oxide and positively charged poly (diallyldimethylammonium chloride) followed by an electrochemical reduction procedure. The DSC devises using the heteroleptic Ru complex C106TBA as sensitizer and this new counter electrode reach power conversion efficiencies of 9.5% and 7.6% in conjunction with low volatility and solvent free ionic liquid electrolytes, respectively. The new counter electrode exhibits good durability (60°C for 1000 h in a solar simulator, 100 mW cm−2) during the accelerated tests when used in combination with an ionic liquid electrolyte. This work identifies a new class of electro-catalysts with potential for low cost photovoltaic devices.

Dye solar cells combining a TiO2 surface-blocking organic sensitizer and solvent-free ionic liquid-based redox electrolyte

A surface-blocking triphenylamine-based organic dye (named as D35) is utilized for the fabrication of efficient dye solar cells (DSCs) employing solvent-free ionic liquid electrolytes. The prepared DSCs present power conversion efficiencies of 5.2% under 1 sun AM 1.5G illumination conditions, a performance very similar to solar cells using a reference electrolyte with identical amount (0.2 M) of iodine dissolved in a standard acetonitrile–valeronitrile solvent mixture. Despite the high concentration of recombining species inside the electrolyte, the IL-based cells are able to afford a photopotential (Voc) of 0.76 V. Electrochemical Impedance Spectroscopy (EIS) has explored the recombination dynamics and conduction band edge shifts, proving the role of the dye's molecular structure on attaining such a high value of Voc. The obtained short-circuit photocurrent (Jsc) values are discussed and justified in terms of mass-transport limitation processes in the viscous ionic liquid electrolyte.

Solvent-free ionic liquid electrolytes without elemental iodine for dye-sensitized solar cells

A new type of electrolyte with a sulfide/polysulfide redox couple and I(-) was prepared as a solvent-free ionic liquid for application in dye-sensitized solar cells, reaching efficiencies of 5.2-6.4% under AM 1.5G, 100 mW cm(-2) light illumination, and 6.6% efficiency was obtained under 0.1 sun irradiation.

New diketo-pyrrolo-pyrrole (DPP) sensitizer containing a furan moiety for efficient and stable dye-sensitized solar cells

A new metal-free organic sensitizer containing a furan moiety as the π-spacer based on the diketo-pyrrolo-pyrrole unit was synthesized through simple synthetic routes and with low cost for the application of dye-sensitized solar cells. Two corresponding dyes with benzene and thiophene spacers were also synthesized for the purpose of comparison. On the basis of optimized DSSC test conditions, the sensitizer containing the furan shows prominent solar energy conversion efficiency ( η) of 5.65% ( J sc = 15.96 mA cm −2, V oc = 541 mV, ff = 0.65) under simulated full sunlight irradiation. The dyes were also tested in a solvent-free ionic liquid electrolyte devices and the stability of devices was performed over 2000 h at full sunlight. The sensitizer containing the furan moiety exhibited good stability and better photovoltaic performance of up to 4.41% power conversion efficiency.

Anion-correlated conduction band edge shifts and charge transfer kinetics in dye-sensitized solar cells with ionic liquid electrolytes

In solvent-free ionic liquid electrolytes for efficient dye-sensitized solar cells, iodide and non-iodide melts are ordinarily blended in order to attain a high dynamic fluidity and thereby meet a fast mass transport of electroactive species. This common tactic could bring forth a prominent impact of various anions on cell efficiency by altering photocurrent and/or photovoltage. Herein we report evident effects of the dicyanamide versus tetracyanoborate anion on the energetics of titania conduction band edge and the kinetics of multi-channel charge-transfer reactions in cells employing a high absorption coefficient ruthenium sensitizer C106. A slightly shorter photoluminescence lifetime of C106 grafted on alumina is probed for the tetracyanoborate-based reference cell with respect to the dicyanamide counterpart. However, owing to a more favourable thermodynamic driving force of ∼90 meV, the tetracyanoborate anion prompts an almost 3-fold faster electron injection from the excited-state dye to titania than dicyanamide, leading to a higher charge separation yield, which is in good agreement with an almost indistinguishable ratio of external quantum efficiency enhancement in the whole spectral response region. Compared to tetracyanoborate, the presence of dicyanamide at the titania/electrolyte interface evokes a 27-fold smaller interfacial electron exchange rate (K) with triiodide, accounting for the open-circuit photovoltage variation observed in current-voltage measurements.

Anion Effects in Organic Dye-Sensitized Mesoscopic Solar Cells with Ionic Liquid Electrolytes: Tetracyanoborate vs Dicyanamide

We compare the impacts of tetracyanoborate and dicyanamide anions in solvent-free ionic liquid electrolytes upon the optoelectronic features of dye-sensitized solar cells based on an organic push−pull chromophore. With respect to dicyanamide, tetracyanoborate confers a bathochromic and higher photocurrent response upon a dye-coated mesoporous titania film. Numerical simulations on electrical impedance spectroscopies disclose that tetracyanoborate anions evoke a downward displacement of the conduction-band edge of an electrolyte-sinking nanocrystalline film in comparison with dicyanamide, generating a more favorable energy-offset at the titania/dye interface and therefore a better exciton dissociation yield, as proved via the transient emission measurements. An effective recombination reaction rate constant U0k is used to depict the charge-transfer behavior at the titania/electrolyte interface in dye-sensitized solar cells. The negative impact of an about 4 times larger U0k of the dicyanamide cell is overwhelmed by the noticeably higher conduction band edge, leading to the experimentally observed anion-dependent open-circuit photovoltage.

Effects of Polyaniline Additive in Solvent-Free Ionic Liquid Electrolyte for Dye-Sensitized Solar Cell

Department of Green Energy & Research Center for Convergence Technology, Hoseo University, Chungnam 336-795, Korea. *E-mail: jklee@hoseo.edu Department of Advanced Technology Fusion, Konkuk University, Seoul 143-701, Korea. *E-mail: jjlee@kku.ac.kr Probiond Co. Ltd, Konkuk University, Seoul 143-701, Korea Department of Applied Chemistry, Konkuk University, Chungju 380-701, Korea Received September 4, 2010, Accepted October 20, 2010

Hierarchical TiO2 Photoanode for Dye-Sensitized Solar Cells

Hierarchical or one-dimensional architectures are among the most exciting developments in material science these recent years. We present a nanostructured TiO(2) assembly combining these two concepts and resembling a forest composed of individual, high aspect-ratio, treelike nanostructures. We propose to use these structures for the photoanode in dye-sensitized solar cells, and we achieved 4.9% conversion efficiency in combination with C101 dye. We demonstrate this morphology beneficial to hamper the electron recombination and also mass transport control in the mesopores when solvent-free ionic liquid electrolyte is used.

Improvement in long-term stability of dye-sensitized solar cell for outdoor use

To improve the intrinsic stability of the component of dye-sensitized solar cells (DSCs), we have fabricated the unit cell using solvent-free ionic liquid electrolyte. The degradation in the continuous 1 sun light soaking test at 60 °C over 15,000 h was effectively suppressed, compared with the cell using γ-butyrolactone electrolyte. The lifetime for outdoor use was estimated over 15 years from acceleration factor based on the outdoor exposure test. To confirm the stability of the DSC under practical outdoor use, we fabricated the solar light using the DSC modules, rechargeable batteries and bright light emitting diode (LED). The solar lights have been emitting a bright white light at night using the electricity from batteries charged by the DSC modules during the daytime in any weather condition for a half year.

ILHYPOS Ionic Liquid-Based Supercapacitors

This paper presents work carried out within a European Union project which led to the development of 3.7 V and 340 F pre-series electrochemical double-layer capacitor cell featuring an asymmetric configuration with positive and negative carbon electrodes of different weight and solvent-free ionic liquid electrolyte. Such supercapacitor design with nonvolatile and safe electrolyte provides a successful response to the requirement for power assist application in hybrid electric vehicles (HEVs).

High Molar Extinction Coefficient Organic Sensitizers for Efficient Dye‐Sensitized Solar Cells

We have designed and synthesized highly efficient organic sensitizers with a planar thienothiophene-vinylene-thienothiophene linker. Under standard global AM 1.5 solar conditions, the JK-113-sensitized cell gave a short circuit photocurrent density (J(sc)) of 17.61 mA cm(-2), an open-circuit voltage (V(oc)) of 0.71 V, and a fill factor (FF) of 72%, corresponding to an overall conversion efficiency (eta) of 9.1%. The incident monochromatic photo-to-current conversion efficiency (IPCE) of JK-113 exceeds 80% over the spectral region from 400 to 640 nm, reaching its maximum of 93% at 475 nm. The band tails off toward 770 nm, contributing to the broad spectral light harvesting. Solar-cell devices based on the sensitizer JK-113 in conjunction with a volatile electrolyte and a solvent-free ionic liquid electrolyte gave high conversion efficiencies of 9.1% and 7.9%, respectively. The JK-113-based solar cell fabricated using a solvent-free ionic liquid electrolyte showed excellent stability under light soaking at 60 degrees C for 1000 h.

CoS Supersedes Pt as Efficient Electrocatalyst for Triiodide Reduction in Dye-Sensitized Solar Cells

We report an efficient nonplatinized flexible counter electrode for dye-sensitized solar cells. In combination with a solvent-free ionic liquid electrolyte, we have demonstrated a approximately 6.5% cell with an amphiphilic ruthenium polypyridyl photosensitizer showing excellent stability measured under prolonged light soaking at 60 degrees C. Compared to the Pt deposited PEN film, the CoS deposited PEN film shows higher electrocatalytic activity for the reduction of triiodide. This is expected to have an important practical consequence on the production of flexible low-cost and lightweight thin film DSC devices based on the plastic matrix.

On the addition of conducting ceramic nanoparticles in solvent-free ionic liquid electrolyte for dye-sensitized solar cells

Titanium carbide (TiC) is an extremely hard conducting ceramic material often used as a coating for titanium alloys as well as steel and aluminum components to improve their surface properties. In this study, conducting ceramic nanoparticles (CCNPs) have been used, for the first time, in dye-sensitized solar cells (DSSCs), and the incorporation of TiC nanoparticles in a binary ionic liquid electrolyte on the cell performance has been investigated. Cell conversion efficiency with 0.6 wt% TiC reached 1.68%, which was higher than that without adding TiC (1.18%); however, cell efficiency decreased when the TiC content reached 1.0 wt%. The electrochemical impedance spectroscopy (EIS) technique was employed to analyze the interfacial resistance in DSSCs, and it was found that the resistance of the charge-transfer process at the Pt counter electrode ( R ct1) decreased when up to 1.0 wt% TiC was added. Presumably, this was due to the formation of the extended electron transfer surface (EETS) which facilitates electron transfer to the bulk electrolyte, resulting in a decrease of the dark current, whereby the open-circuit potential ( V OC) could be improved. Furthermore, a significant increase in the fill factor (FF) for all TiC additions was related to the decrease in the series resistance ( R S) of the DSSCs. However, at 1.0 wt% TiC, the largest charge-transfer resistance at the TiO 2/dye/electrolyte interface was observed and resulted from the poor penetration of the electrolyte into the porous TiO 2. The long-term stability of DSSCs with a binary ionic liquid electrolyte, which is superior to that of an organic solvent-based electrolyte, was also studied.

Dye-Sensitized Solar Cells Based on Organic Sensitizers with Different Conjugated Linkers: Furan, Bifuran, Thiophene, Bithiophene, Selenophene, and Biselenophene

Six organic dyes with different conjugated linkers such as furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene have been prepared in combination with the dihexyloxy-substituted triphenylamine donor and the cyanoacrylic acid acceptor. In conjunction with an acetonitrile-based electrolyte and a solvent-free ionic liquid electrolyte, these dyes exhibit 6.88−7.77% and 6.39−7.00% efficiencies, respectively. We have demonstrated that furan and selenophene can be employed as building blocks of sensitizers in stable solar cells for the first time. We have also studied the influence of heteroatoms on photocurrents and photovoltages with the aid of quantum calculations and transient photoelectrical decay measurements. Temperature-dependent electrical impedance experiments have shown that a relatively low external quantum efficiency of the dye with biselenophene linker is not related to the charge collection yield in the case of an acetonitrile electrolyte.

High‐Performance Liquid and Solid Dye‐Sensitized Solar Cells Based on a Novel Metal‐Free Organic Sensitizer

A new metal-free organic sensitizer (see figure) for high-performance and applicable dye-sensitized solar cells is presented. In combination with a solvent-free ionic liquid electrolyte, a ∼ 7% cell made with this sensitizer shows an excellent stability measured under thermal and light-soaking dual stress. For the first time a 4.8% efficiency is reached for all-solid-state dye-sensitized solar cells based on an organic dye. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

We report a high molar extinction coefficient metal-free sensitizer composed of a triarylamine donor in combination with the 2-(2,2′-bithiophen-5-yl)acrylonitrile conjugation unit and cyanoacrylic acid as an acceptor. In conjugation with a volatile acetonitrile-based electrolyte or a solvent-free ionic liquid electrolyte, we have fabricated efficient dye-sensitized solar cells showing a corresponding 7.5% or 6.1% efficiency measured under the air mass 1.5 global sunlight. The ionic liquid cell exhibits excellent stability during a 1000 h accelerated test under the light-soaking and thermal dual stress. Intensity-modulated photocurrent and photovolatge spectroscopies were employed along with the transient photoelectrical decay measurements to detail the electron transport in the mesoporous titania films filled with these two electrolytes.

Safe, high-energy supercapacitors based on solvent-free ionic liquid electrolytes

Safety is the main concern for energy storage-system application in hybrid-electrical vehicles (HEVs) and ionic liquids (ILs) of low vapor pressure and high thermal stability represent a strategy to meet this key requisite. The use of solvent-free ILs in supercapacitors enables the high cell voltages required for increasing supercapacitor energy up to the values for power-assist application in HEVs. In order to exploit the wide electrochemical stability window of ILs, tailored electrode materials and cell configurations have to be used. The performance of asymmetric double-layer carbon supercapacitors (AEDLCs) and carbon/poly(3-methylthiophene) hybrid supercapacitors operating with different pyrrolidinium-based ILs are reported and compared. This study demonstrates that a design-optimized AEDLC operating with safe, solvent-free IL electrolyte meets cycling stability and the energy and power requisites for power-assisted HEVs at the investigated temperatures.

Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

We systematically studied the temperature-dependent physicochemical properties, such as density, conductivity, and fluidity, of 1,3-dialkylimidazolium iodides. In combination with the amphiphilic Z907Na sensitizer, we have found that it is important to use low-viscosity iodide melts with small cations to achieve high-efficiency dye-sensitized solar cells. By employing high-fluidity eutectic-based melts the device efficiencies considerably increased compared to those for cells with the corresponding state of the art ionic liquid electrolytes. We propose a modified Stokes−Einstein equation by correlating ion mobility and fluidity to quantitatively depict the triiodide transport in ionic liquid electrolytes. These studies reveal that the viscosity-dependent transport of triiodide in ionic liquid electrolytes with high iodide concentration can be explained by two parallel processes. Apart from the normal physical diffusion, the coupling process of physical diffusion and bond exchange is responsible for the observed abnormally high diffusion coefficients. This work has provided useful insight for further improvement of solvent-free electrolytes based on rational design of their constituents, facilitating the large-scale practical application of lightweight, flexible dye-sensitized solar cells.