Recent research progress on polymer electrolytes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have attracted large attention due to their easy fabrication, low cost and high conversion efficiency. Electrolytes play an important role in the photovoltaic performance of the DSSCs and many efforts have been contributed to study different kinds of electrolytes with various characteristics such as liquid electrolytes, polymer electrolytes and so on. Because the leakage and the volatilization of liquid electrolytes hinder their practical applications in the DSSCs, polymer electrolytes with high ionic conductivity, excellent thermal stability and long-term stability of the DSSCs based on them are alternatives to liquid electrolytes especially for the quasi-solid-state DSSCs. In this paper, research on solid polymer electrolytes is summarized and the influence of salt concentration on ionic conductivity of solid polymer electrolytes is described and thoroughly explained. On the other hand, the advantages of gel polymer electrolytes (GPEs) are introduced. The factors affecting the ionic conductivity of GPE and the performance of their DSSCs, consisting of the polymer concentration and type, the iodide salts, the solvents and the temperature are discussed. The tendency and the reasons of their influence are expounded in detail. To enhance the properties of GPE, many strategies are taken such as the fabrication of porous structure in the GPE membrane, the incorporation of proton donors, the introduction of inorganic nanoparticles and the addition of pyridine derivatives. Their improving effect and the causes for the enhancement are set forth.     

Quasi-solid-state dye-sensitized solar cells employing ternary component polymer-gel electrolytes

Ternary component polymer-gel electrolytes are designed to facilitate ion transport in a highly viscous medium and utilize incident light more efficiently in dye-sensitized solar cells (DSSCs). Polyethers with multiple molecular size distributions are employed as solvents and TiO₂ nanoparticles as a filler to prepare the polymer-gel electrolytes. The ion transport properties of the electrolytes are systematically investigated using electrochemical analyses such as ion conductivity and diffusion coefficient measurements. The influences of the electrolyte components on the electron transport in photoanodes are also investigated by measuring the laser-induced photovoltage and photocurrent transient response, incident photon-to-current efficiency (IPCE), and current–voltage (J–V) curves. The optimized polymer-gel electrolyte results in greatly enhanced energy conversion efficiency (i.e., 7.2% at 1 sun) due to the significantly improved ion transport and good light-scattering effect of the nanofillers.     

Molten phosphonium iodides as electrolytes in dye-sensitized nanocrystalline solar cells

We have synthesized new room temperature ionic liquids and test them as new potential electrolytes for dye-sensitized nanocrystalline solar cells (DNSCs) of Grätzel type based on asymmetric tetraalkylphosphonium iodides. A systematic study on the conductivity behavior of BH₃PI, iBH₃PI, OH₃PI and iBO₃PI molten salts and the effect of the addition of low concentrations of iodine with and without 3-methoxypropionitrile is presented. Solar cells using iBH₃PI-based electrolyte reached an overall light-to-electricity efficiency of 5.7% under moderate light intensity (27 000 lx) conditions and those results are compared to similar systems using alternative ionic liquids as electrolytes.     

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.     

Dye-sensitized solar cells based on electrospun polymer blends as electrolytes

We prepared electrospun polymer nanofibers by the electrospinning method and investigated about their applications to dye-sensitized solar cells (DSSCs). Electrospun poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) and PVDF-HFP/polystyrene (PS) blend nanofibers were prepared and examined the uptake, the ionic conductivity, and the porosity by impedance measurement and Scanning Electron Microscope (SEM). The best results of V_oc, J_sc, FF, and efficiency of the DSSC devices using the electrospun PVDF-HFP/PS(3:1) blend nanofibers were 0.76 V, 11.8 mA/cm², 0.66, and 5.75% under AM 1.5.     

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.     

Application of PEO based gel network polymer electrolytes in dye-sensitized photoelectrochemical cells

Poly(ethyleneoxide) (PEO) based gel network polymer electrolytes prepared from crosslinking reaction were applied in fabricating quasi-solid-state dye-sensitized TiO₂ photoelectrochemical cells. Incident photon-to-current conversion efficiencies up to 48% and 40%, overall energy conversion efficiencies up to 3.6% and 2.9% have been achieved respectively for the resulting cells containing PEO₂₀₀₀ and PEO₁₅₀₀ segments.     

Solar module using dye-sensitized solar cells with a polymer electrolyte

Dye-sensitized TiO₂ solar cells assembled with a polymer electrolyte were investigated, aiming at the construction of an 8 V solar module. The individual solar cells were assembled with 4.5 cm² active area and were characterized under outdoor conditions, exhibiting an average efficiency of 0.9% per cell (at 12:00 noon). The solar module was built by connecting 13 cells in series. The integrated average daily power was estimated to be 183 mW. The present paper discusses the performance of the individual cells and the module.     

Novel thixotropic gel electrolytes based on dicationic bis-imidazolium salts for quasi-solid-state dye-sensitized solar cells

Novel thixotropic gel electrolytes have been successfully prepared by utilizing oligomeric poly(ethylene oxide) (PEO)-based bis-imidazolium diiodide salts and hydrophilic silica nanoparticles for application in quasi-solid-state dye-sensitized solar cells (DSSCs). The thixotropic gel-state of the ionic liquid-based composite electrolytes is confirmed by observing the typical hysteresis loop and temporary hydrogen bonding. On using the PEO-based composite electrolyte, a quasi-solid-state DSSC exhibited highly improved properties such as easy penetration of the electrolyte into the cell without leakage, long-term stability, high open-circuit voltage without the use of 4-tert-butylpyridine, and a high energy-conversion efficiency of 5.25% under AM 1.5 illumination (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.     

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.     

Heteropolyacid-impregnated PVDF as a solid polymer electrolyte for dye-sensitized solar cells

Heteropolyacid (HPA)-impregnated polyvinylidene fluoride (PVDF) with iodine/iodide was prepared as a new polymer electrolyte for bio-mimicking natural photosynthesis. With this new polymer electrolyte, dye-sensitized solar cell was fabricated using N3 dye-adsorbed over TiO₂ nanoparticles (photoanode) and conducting carbon cement coated on conducting glass (photocathode). The fabricated cell generates high open circuit voltage (V_OC 426 mV) and short circuit current (I_SC 3.90 mA) upon illumination with visible light. It is also demonstrated that the polymer electrolytes prevent the back-electron transfer reactions taking place in dye-sensitized hetero-junctions and are highly promising for solar energy conversion to electricity.     

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.     

Mono-ion transport electrolyte based on ionic liquid polymer for all-solid-state dye-sensitized solar cells

Ionic liquid polymers, poly(1-alkyl-3-(acryloyloxy)hexylimidazolium iodide), are synthesized and used as mono-ion transport electrolytes for all-solid-state dye-sensitized solar cells. For these ionic liquid polymers, imidazolium cations are tethered on polymer main chain and only iodide species is mobile. Such a mono-ion transport feature is favorable as solid-state electrolyte in dye-sensitized solar cells. High thermal stability up to 200 °C for these ionic liquid polymers is confirmed by thermogravimetric analysis. Among these ionic liquid polymers, poly(1-ethyl-3-(acryloyloxy)hexylimidazolium iodide) (PEI) exhibits the highest ionic conductivity (3.63 × 10^?4 S cm^?1) at room temperature. The dye-sensitized solar cell based on PEI electrolyte without the addition of iodine exhibits the open-circuit voltage of 838 mV, the short-circuit current density of 9.75 mA cm^?2 and the conversion efficiency of 5.29%, measured at AM 1.5 illumination (100 mW cm^?2). Incorporating iodine into PEI electrolyte results in the decrease of both the open-circuit voltage and the photocurrent density due to the visible light adsorption by iodine and the enhancement of the recombination between conduction band electrons and the triiodide.     

Crystal formation involving 1-methylbenzimidazole in iodide/triiodide electrolytes for dye-sensitized solar cells

Nitrogen heterocyclic compounds, such as N-methylbenzimidazole (MBI), are commonly used as additives to electrolytes for dye-sensitized solar cells (DSCs), but the chemical transformation of additives in electrolyte solutions remains poorly understood. Solid crystalline compound (MBI)₆(MBI-H⁺)₂(I⁻)(I₃⁻) (1) was isolated from different electrolytes for DSCs containing MBI as additive. The crystal structure of 1 was determined by single-crystal X-ray diffraction. In the crystal structure, 1 contains neutral and protonated MBI fragments; iodide and triiodide anions form infinite chains along the crystallographic a-axis. The role of the solvent and additives in the crystallization process in electrolytes is discussed.     

Efficient hole collection by introducing ultra-thin UV-ozone treated Au in polymer solar cells

Efficient hole collection in polymer solar cells (PSCs) has been achieved by introducing an ultra-thin UV-ozone (UVO) treated Au on indium tin oxide (ITO) to substitute poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Through optimizing the Au thickness and the duration of UVO treatment, it is shown that the ITO/Au (1 nm) treated by UVO for 15 s improves the fill factor significantly to 67.2% and power conversion efficiency (PCE) to 3.47%, which is competitive to that of the PEDOT:PSS-based PSCs with PCE of 3.38%. The results of ultraviolet photoemission spectroscopy and the analysis of the current conduction mechanism show that the UVO-treated Au offers favorable band alignment at metal/polymer interface of the anode for efficient hole collection. Meanwhile, the series resistance of the device decreases drastically.     

Improved stability quasi-solid-state dye-sensitized solar cell based on polyether framework gel electrolytes

We report three improved stability dye-sensitized TiO₂ photoelectrochemical cells using quasi-solid polymer electrolytes containing poly(propylene oxide) (PPO), poly(ethylene oxide) (PEO) or poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (P₁₂₃). After introducing the polyether into the liquid electrolyte, the parameters of these quasi-solid-state solar cells are 90% comparable to that of the liquid photochemical cells, although the conductivities of these polyether framework gel electrolytes are lower than that of the bulk liquid electrolyte. The different morphologies of dried liquid electrolyte and the polyether gel electrolytes are characterized with an atomic force microscope (AFM) to explain the better stability exhibited by the polyether gel electrolytes.     

Solid-state dye-sensitized solar cells based on spiro-MeOTAD

Dye-sensitized solar cells (DSC) could become low cost alternatives to inorganic photovoltaic devices. The DSC studied here use the hole transporting material 2,2′7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD). The effect of t-butyl pyridine (t-BP) on the current–voltage characteristics of thin layer cells were investigated using complete cells and cells consisting only of the TiO₂ blocking layer. In addition, absorption spectroscopy was used to study the effect t-BP has on the doping density of spiro-MeOTAD. The results are correlated with device performance.     

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cells

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cell (DSSC) has been investigated to optimize the device geometry for reliable energy conversion efficiency assessment. Mesoporous TiO₂ layers with an identical active area (0.40 cm²) and different dimension are prepared on FTO glass substrate by the screen printing method and used as photoanodes for DSSCs. Under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), both the open-circuit voltage and the short-circuit current density are independent of electrode geometry whereas the fill factor and hence energy conversion efficiency show strong dependency. Electrochemical impedance spectroscopy analysis indicates that the distance between active layer and ohmic contact directly contributes to internal series resistance and influence photovoltaic performance.     

Molten and solid metal-iodide-doped trialkylsulphonium iodides and polyiodides as electrolytes in dye-sensitized nanocrystalline solar cells

The conductivity and solar cell performance of metal-iodide-doped trialkylsulphonium iodides and polyiodides have been investigated as electrolytes in dye-sensitized nanocrystalline solar cells (DNSCs). Nine different metal-iodide-containing (R₂R′S)I with additional iodine provided overall solar-to-electric energy conversion efficiencies of over 2%, while used as electrolytes in DNSCs in simulated AM 1.5 solar light at the light intensity of 100 W m⁻². The highest overall conversion efficiency, 3.1%, was achieved by using the electrolyte (Bu₂MeS)I:AgI:I₂ in the proportions (1:0.03:0.05). The effects from 4-tert-butylpyridine treatment of the electrodes were studied. The effects of metal-iodide doping were also investigated with respect to speciation in the electrolytes and potential influence on electrochemical conductivity.