Importance of ionic liquid-doped solid polymer (PVPI) electrolyte

The aim of the present work is to synthesize a porous nanocrystalline TiO₂ electrode and to use a new kind of solid polymer electrolyte, poly(N-methyl 4-vinylpyridine iodide), doped with an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulphonyl)imide, for dye-sensitized solar cell (DSSC) applications. The fabrication process of nanoporous TiO₂ electrode and electrical, structural and photoelectrochemical properties of polymer electrolytes are presented in detail. A novel DSSC has also been fabricated, which shows overall efficiency of 0.56% at one sun condition.     

3D graphene from CO2 and K as an excellent counter electrode for dye‐sensitized solar cells

3D graphene, which was synthesized directly from CO₂ via its exothermic reaction with liquid K, exhibited excellent performance as a counter electrode for a dye‐sensitized solar cell (DSSC). The DSSC has achieved a high power conversion efficiency of 8.25%, which is 10 times larger than that (0.74%) of a DSSC with a counter electrode of the regular graphene synthesized via chemical exfoliation of graphite. The efficiency is even higher than that (7.73%) of a dye‐sensitized solar cell with an expensive standard Pt counter electrode. This work provides a novel approach to utilize a greenhouse gas for DSSCs.     

A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells

A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et₂) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface roughness and I⁻/I₃⁻ redox reaction behaviors based on PProDOT-Et₂, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et₂ counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective surface area and good catalytic properties for I₃⁻ reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et₂ layer, but the aggregation of PProDOT-Et₂ in thicker layers (>80 mC cm⁻²) resulted in decreases in J_SC and the cell conversion efficiency. The charge transfer resistances (R_ct1) of the PProDOT-Et₂ counter electrodes had the lowest value of ∼18 Ω at a deposition charge capacity of 40 mC cm⁻². These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I₃⁻ reduction can potentially be used as the counter electrode in a high-performance DSSC.     

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.     

Review of recent progress in solid-state dye-sensitized solar cells

The dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs) provide a promising alternative concept to conventional p–n junction photovoltaic devices. However, liquid-state DSSCs possess the problem of low stability since a volatile liquid electrolyte is utilized. An effective approach to solve such a problem is by replacing the volatile liquid electrolyte with solid-state or quasi solid-state hole conductor, such as p-type semiconductors, ionic liquid electrolyte and polymer electrolyte. In this paper, the recent progress on the selection and utilization of these hole conductors are mainly discussed. Research on mechanisms of solid-state DSSCs was also summarized here including the hole transfer process at dye/hole conductor interface, ionic transportation inside hole conductor media and the factors which depress the efficiency of solid-state cells. With a thorough analysis of the problems of solid-state DSSCs, several ways towards higher efficiency and lower cost are suggested.     

Enhanced performance of a quasi-solid-state dye-sensitized solar cell with aluminum nitride in its gel polymer electrolyte

The effects of incorporation of aluminum nitride (AlN) in the gel polymer electrolyte (GPE) of a quasi-solid-state dye-sensitized solar cell (DSSC) were studied in terms of performance of the cell. The electrolyte, consisting of lithium iodide (LiI), iodine (I₂), and 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN), was solidified with poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The 0.05, 0.1, 0.3, and 0.5 wt% of AlN were added to the electrolyte for this study. XRD analysis showed a reduction of crystallinity in the polymer PVDF-HFP for all the additions of AlN. The DSSC fabricated with a GPE containing 0.1 wt% AlN showed a short-circuit current density (J_SC) and power-conversion efficiency (η) of 12.92±0.54 mA/cm² and 5.27±0.23%, respectively, at 100 mW/cm² illumination, in contrast to the corresponding values of 11.52±0.21 mA/cm² and 4.75±0.08% for a cell without AlN. The increases both in J_SC and in η of the promoted DSSC are attributed to the higher apparent diffusion coefficient of I⁻ in its electrolyte (3.52×10⁻⁶ cm²/s), compared to that in the electrolyte without AlN of a DSSC (2.97×10⁻⁶ cm²/s). At-rest stability of the quasi-solid-state DSSC with 0.1 wt% of AlN was found to decrease hardly by 5% and 7% at room temperature and at 40 °C, respectively, after 1000 h duration. The DSSC with a liquid electrolyte showed a decrease of about 40% at room temperature, while it virtually lost its performance in about 150 h at 40 °C. Explanations are further substantiated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and by porosity measurements.     

Influence of electrolyte on the photovoltaic performance of a dye-sensitized TiO2 solar cell based on a Ru(II) terpyridyl complex photosensitizer

We have investigated the influence of electrolyte composition on the photovoltaic performance of a dye-sensitized nanocrystalline TiO₂ solar cell (DSSC) based on a Ru(II) terpyridyl complex photosensitizer (the black dye). We have also spectroscopically investigated the interaction between the electrolyte components and the adsorbed dye. The absorption peaks attributed to the metal-to-ligand charge transfer transitions of the black dye in solution and adsorbed on a TiO₂ film, were red-shifted in the presence of Li cations, which led to an expansion of the spectral response of the solar cell toward the near-IR region. The photovoltaic performance of the DSSC based on the black dye depended remarkably on the electrolyte composition. We developed a novel efficient organic liquid electrolyte containing an imidazolium iodide such as 1,2-dimethyl-3-n-propylimidazolium iodide or 1-ethyl-3-methylimidazolium iodide (EMImI) for a DSSC based on the black dye. A high solar energy-to-electricity conversion efficiency of 9.2% (J_sc=19.0 mA cm⁻², V_oc=0.67 V, and FF=0.72) was attained under AM 1.5 irradiation (100 mW cm⁻²) using a novel electrolyte consisting of 1.5 M EMImI, 0.05 M iodine, and acetonitrile as a solvent with an antireflection film.     

Synthesis of carbon nanomaterials for dye‐sensitized solar cells

In this paper, it was demonstrated that Na₂O can react with CO to produce carbon nanofibers at 500 °C and carbon nanosheets at 550 °C. Furthermore, the nanosheets exhibited excellent performance as a counter electrode for a dye‐sensitized solar cell (DSSC), leading to a high power conversion efficiency of 7.57%. The efficiency is larger than that (4.72%) of a DSSC with the carbon nanofiber counter electrode and even comparable with that of an expensive Pt‐based DSSC. Copyright © 2015 John Wiley & Sons, Ltd.     

电聚合PANI作为低成本离子液体型DSSC对电极

用原位电聚合的方法在FTO导电玻璃上制备出了SO42-杂化的聚苯胺(PANI)(厚度10μm),并用循环伏安表征了其电聚合过程。通过扫描电镜表征,这种聚苯胺具有粗糙多孔的结构,有利于形成高比表面积的对电极。制备了以PMII和[EMIm]BF4、[EMIm]NTf2、[EMIm]N(CN)2不同组成比例的二元离子液体作为电解液的染料敏化太阳电池(DSSC)并研究其光伏行为,发现:使用[EMIm]N(CN)2和PMII(4∶6)电解液能得到最佳光电参数:短路电流密度Jsc(10.4mA.cm-2),开路电压Voc(0.655V),填充因子FF(0.521)和光电转换效率η(3.56%)。通过研究3种电解液的电导率I,3-和I-离子扩散系数,发现[EMIm]N(CN)2和PMII(4∶6)电解液具有最高的电导率和离子扩散系数。     

Preparation of polymer film of micro-porous or island-like structure and its application in dye-sensitized solar cell

A polystyrene (PS) film of micro-porous or island-like structure has been prepared by spin-coating the PS–PMMA (polymethyl-methacrylate) blend on sputtered platinum electrode, followed by selectively dissolving PMMA out of the blend. The film forms either micro-porous or island-like structure depending on the blending proportion of PS to PMMA. After soaking with liquid electrolyte, this film can be utilized as gel polymer electrolyte for dye-sensitized solar cell to improve its durability because the evaporation rate of liquid electrolyte can be significantly reduced. As to the convenience of cell assembly, this polymer film can also serve as a spacer and effectively separate the photo-anode and counter electrode. In addition, we also find that the cell performance is affected significantly by the polymer blend concentration. An energy conversion efficiency of 4.58% can be obtained with 2 wt% PS–2 wt% PMMA blend proportion under AM 1.5, 1 Sun illumination.     

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.     

Carbon-nanofiber counter electrodes for quasi-solid state dye-sensitized solar cells

Carbon-nanofibers (CNFs) with antler and herringbone structures are studied as a tri-iodide (I₃⁻) reduction electrocatalyst in combination with the liquid electrolyte or an alternative stable quasi-solid state electrolyte. The catalytic properties of the counter electrode (CE) are characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The doctor bladed low temperature CNFs-CE has faster I₃⁻ reduction rate and low charge transfer resistance (R_CT) of ∼0.5 Ω cm² than platinum (Pt) (∼2.3 Ω cm²) due to the nanofiber stacking morphology. Its herringbone and antler structures with graphitic layers lead to defect rich edge planes and larger diameter of CNFs facilitate the electron transfer kinetics. The cells with CNF counter electrodes are showing promising energy conversion efficiency greater than 7.0% for the glass based devices and 5.0% for the flexible cells filled with the quasi-solid state electrolyte, which is similar to Pt performance. Application of CNFs-CE in flexible and quasi-solid state electrolyte increases the possibility of roll to roll process, low cost and stable dye-sensitized solar cells (DSCs).     

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 TiO₂/dye/electrolyte interface was observed and resulted from the poor penetration of the electrolyte into the porous TiO₂. 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.     

Quasi solid state polymer electrolyte with binary iodide salts for photo-electrochemical solar cells

Quasi-solid-state polymer electrolytes can be used in dye sensitized solar cells (DSSCs) in order to overcome various problems associated with liquid electrolytes. Prior to fabricating commercially viable solar cells, the efficiency of quasi solid state DSSCs needs to be improved. Using electrolytes with a binary iodide mixture is a novel technique used to obtain such efficiency enhancement. In this work we report both conductivity and solar cell performance enhancements due to incorporation of a mixture containing LiI and tetrahexylammonium iodide in a quasi-solid-state electrolyte. The conductivity of the electrolyte increases with added amounts of LiI and thus the highest conductivity, 3.15 × 10⁻³ S cm⁻¹ at 25 °C, is obtained for the electrolyte 100 wt% LiI. The predominantly ionic behavior of the electrolytes was established from dc polarization measurements. The iodide ion conductivity, measured using iodine pellet electrodes decreased somewhat with increasing amount of LiI even though the overall conductivity increased. However, the highest efficiency was obtained for the DSSC containing a polymer electrolyte with Hex₄N⁺I^¯:LiI = 1:2 mass ratio. This cell had the largest short circuit current density of about 13 mA cm⁻² and more than 4% overall energy conversion efficiency. The results thus show that electrolytes with Hex₄N⁺I^¯/LiI mixed iodide system show better DSSC performance than single iodide systems.     

High-performance and low platinum loading Pt/Carbon black counter electrode for dye-sensitized solar cells

Pt/Carbon black counter electrode for dye-sensitized solar cells (DSSCs) was prepared by reducing H₂PtCl₆ with NaBH₄ in carbon black. The Pt/Carbon black electrode had a high electrocatalytic activity for iodide/triiodide redox reaction. Using the Pt/Carbon black counter electrode, DSSC achieved 6.72% energy conversion efficiency under one sun illumination. Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which makes it available in DSSCs practical applications.     

Portable,parallel grid dye-sensitized solar cell module prepared by screen printing

Screen-printing technology is used to fabricate large dye-sensitized solar cells (DSSCs). The high series-resistance associated with transparent conductive oxide glass substrates causes poor performance in large DSSCs especially at an exposure of 1 sun. The DSSC design has an embedded silver grid; a fluorine-doped tin oxide (FTO) glass substrate and stripe type titanium dioxide (TiO₂) active layers introduced by screen-printing. The counter electrode is prepared from a screen printable paste based on hexachloro platinic acid. A DSSC module, which consists of five stripe-type working electrodes on a 5 cm × 5 cm, embedded silver grid FTO glass substrate, shows stable performance with an energy conversion efficiency of 5.45% under standard test conditions.     

Photovoltaic performance of solid-state solar cells based on ZnO nanosheets sensitized with low-cost metal-free organic dye

In this study, the fabrication of photoanode of dye-sensitized solar cell (DSSC) using two-dimensional ZnO nanosheets (ZnONSs) and low-cost metal-free photosensitizer, evans blue, and evaluation of its photovoltaic performance in the solid-state DSSC with TiO₂ nanotubes (TNTs) modified poly(ethylene oxide) (PEO) polymer electrolyte is described. The ZnONSs are synthesized via hydrothermal method and are characterized by high resolution scanning electron microscopy (HR-SEM), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PLS) and X-ray diffraction (XRD) analysis. The photovoltaic performance of the cells is evaluated under standard air mass 1.5 global simulated illumination (100 mW cm⁻²). The current-voltage (I-V) and photocurrent-time (I-T) curves proved effective collection of electrons in the solid-state DSSCs with the ZnONSs photoanode. The solar to electrical energy conversion efficiency of the ZnONSs based DSSC with TNTs modified PEO electrolyte is 0.12%, which is about 1.5 times higher than that of the ZnO nanoparticles based DSSC, due to fast electron diffusion within the nanosheets.     

Novel agarose polymer electrolyte for quasi-solid state dye-sensitized solar cell

Quasi-solid state dye-sensitized solar cells (DSSCs) are fabricated with a novel polysaccharide gel electrolyte composed of agarose in 1-methyl-2-pyrrolidinone (NMP) as polymer matrix, lithium iodide (LiI)/iodine (I₂) as redox couple and titania nanoparticles as fillers. The polysaccharide electrolyte with different agarose concentrations (1-5 wt%) and various inorganic filler TiO₂ concentrations (0-10 wt%) are studied systematically by differential scanning calorimetry (DSC) and the AC impedance spectra. The electrochemical and photoelectric performances of DSSCs with these electrolytes are also investigated. It is found that increasing agarose and inorganic filler concentration leads to a decrease in T_g in the range of 1-2 wt% for agarose and 0-2.5 wt% for TiO₂ changed electrolytes, which results in high conductivity in these electrolytes. From the electrochemical analysis, it is observed that the electron lifetime in TiO₂ of DSSCs increases with agarose, while decreases with inorganic filler contents. The prolonged electron lifetime in DSSCs is advantageous to improve open-circuit voltage (V_oc). Based on these results, the cell with the electrolyte of 2 wt% agarose shows the optimized energy conversion efficiency of 4.14%. The optimized efficiency of the DSSC with added titania is 4.74% at 2.5 wt% titania concentration.     

Using modified poly(3,4-ethylene dioxythiophene): Poly(styrene sulfonate) film as a counter electrode in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs), assembling with nano-crystalline TiO₂ adsorbed cis-Ru(dcb)₂(NCS)₂ dye (known as N3) using polar solvent-treated poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) coating on a conductive glass (fluorine-doped tin oxide, FTO) as a counter electrode, were studied. The conductivity of a bare PEDOT:PSS film was only 2±0.05 S/cm. However, the conductivities of PEDOT:PSS films treated with dimethyl sulfoxide (DMSO), N,N-dimethyl acetamide (DMAc), N,N-dimethyl formamide (DMF), and dichloromethane (DMC) reached 85±15, 45±10, 36±7, and 20±6 S/cm, respectively. In addition, carbon blacks (0.02, 0.1, 0.5, 1.0, 2.0 wt% with respect to PEDOT:PSS aqueous solution) were added into the DMSO-treated PEDOT:PSS solution (denoted as DMSO-PEDOT:PSS) to enhance the conductivity. Atomic force microscopy (AFM) images of PEDOT:PSS and various DMSO-PEDOT:PSS films coated on the FTO glasses were examined. The topographical images reveal that the increased surface roughness is responsible for the enhanced electrochemical property of the DMSO-PEDOT:PSS films. AC impedance technique was also employed to analyze the kinetics at the electrolyte/counter electrode interface. The DSSC using carbon black (0.1 wt%)-modified DMSO-PEDOT:PSS conductive coating as a counter electrode reached a cell efficiency of 5.81% under 100 mW/cm². This efficiency is higher than a DSSC using Pt as a counter electrode (5.66%).     

Study and development of non-aqueous silicon-air battery

Silicon-air battery utilizing a single-crystal heavily doped n-type silicon wafer anode and an air cathode is reported in this paper. The battery employs hydrophilic 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI·(HF)_2.3F] room temperature ionic liquid electrolyte. Electrochemical studies, including polarization and galvanostatic experiments, performed on various silicon types reveal the predominance performance of heavily doped n-type. Cell discharging at constant current densities of 10, 50, 100 and 300 μA cm⁻² in ambient atmosphere, shows working voltages of 1.1-0.8 V. The study shows that as discharge advances, the moist interface of the air electrode is covered by discharge products, which prevent a continuous diffusion of oxygen to the electrode-electrolyte interface. The oxygen suffocation, governed by the settlement of the cell reaction products, is the main factor for an early failure of the cells. Based on the results obtained from scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy studies, we propose a series of reactions governing the discharge process in silicon-air batteries, as well as a detailed mechanism for silicon oxide deposition on the air electrode porous carbon.