Rubbery copolymer electrolytes containing polymerized ionic liquid for dye-sensitized solar cells

A novel type of random copolymer comprised of a polymerized ionic liquid, poly(1-((4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII), and amorphous rubbery poly(oxyethylene methacrylate) (POEM) was synthesized and employed as a solid electrolyte in an I₂-free dye-sensitized solar cell (DSSC). The copolymer electrolytes deeply infiltrated into the nanopores of mesoporous TiO2 films, resulting in improved interfacial contact of electrode/electrolyte. The glass transition temperature (T _g) of the PEBII–POEM (?23 °C) was lower than that of PEBII homopolymer (?4 °C), indicating greater chain flexibility in the former. However, the DSSC efficiency of PEBII–POEM (4.5 % at 100 mW/cm²) was lower than that of PEBII (5.9 %), indicating that ion concentration is more important than chain flexibility. Interestingly, upon the introduction of ionic liquid, i.e., 1-methyl-3 propylimidazolium iodide, the efficiency of PEBII remained almost constant (5.8 %), whereas that of PEBII–POEM was significantly improved up to 7.0 % due to increased I^? ion concentration, which is one of the highest values for I₂-free DSSCs.     

Liquid electrolytes for dye-sensitized solar cells

The present review offers a survey of liquid electrolytes used in dye-sensitized solar cells from the beginning of photoelectrochemical cell research. It handles both the solvents employed, and the prerequisites identified for an ideal liquid solvent, as well as the various effects of electrolyte solutes in terms of redox systems and additives. The conclusions of the present review call for more detailed molecular insight into the electrolyte-electrode interface reactions and structures.     

Ionic liquid electrolytes for dye-sensitized solar cells

The potential of room-temperature molten salts (ionic liquids) as solvents for electrolytes for dye-sensitized solar cells has been investigated during the last decade. The non-volatility, good solvent properties and high electrochemical stability of ionic liquids make them attractive solvents in contrast to volatile organic solvents. Despite this, the relatively high viscosity of ionic liquids leads to mass-transport limitations. Here we review recent developments in the application of different ionic liquids as solvents or components of liquid and quasi-solid electrolytes for dye-sensitized solar cells.     

Recent research progress on quasi-solid-state electrolytes for dye-sensitized solar cells

Dye-sensitized solar cells(DSSCs) are the most promising, low cost and most extensively investigated solar cells. They are famous for their clean and efficient solar energy conversion. Nevertheless this, long-time stability is still to be acquired. In recent years research on solid and quasi-solid state electrolytes is extensively increased. Various quasi-solid electrolytes, including composites polymer electrolytes, ionic liquid electrolytes,thermoplastic polymer electrolytes and thermosetting polymer electrolytes have been used. Performance and stability of a quasi-solid state electrolyte are between liquid and solid electrolytes. High photovoltaic performances of QS-DSSCs along better long-term stability can be obtained by designing and optimizing quasi-solid electrolytes. It is a prospective candidate for highly efficient and stable DSSCs.     

Composite electrolytes based on poly(ethylene oxide) and binary ionic liquids for dye-sensitized solar cells

The sunlight is the largest single available source of clean and renewable energy to ensure human society’s sustainable development. Owing to their low production cost and high energy conversion efficiency, dye-sensitized solar cells (DSSCs) have been regarded as good alternatives to conventional photovoltaic devices. Herein, a series of composite electrolytes based on poly(ethylene oxide) (PEO) and the binary ionic liquids 1-propyl-3-methy-imidazolium iodide ([PMIm]I) and 1-ethyl-3-methylimidazolium thiocyanate ([EMIm][SCN]) were prepared and then applied to fabricate six DSSCs. The composite electrolytes were characterized by fourier transform infrared spectroscopy (FTIS), X-ray diffraction (XRD), and electrochemical impedance spectra (EIS). It was shown that the addition of binary ionic liquids would reduce the degree of crystallinity of PEO, thus improving the ionic conductivities of the electrolytes by about 2 orders of magnitude. Investigation on the photovoltaic performances of these DSSCs showed that the fill factor (FF) could reach up to 0.67 and energy conversion efficiency (η) could reach up to 4.04% under AM 1.5 full sunlight (100 mW/cm2).     

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.     

Synthesis of carboxylated chlorophyll derivatives and their activities in dye-sensitized solar cells

Chlorophyll-a derivatives possessing a carboxy group in the substituent at the 3-position were prepared by chemical modification of methyl pyropheophorbide-d bearing the 3-formyl group via a Wittig, Barbier-type, or Knoevenagel reaction. The synthetic carboxylated chlorophyll pigments were employed as dye sensitizers for solar cells and their performances were compared in a conventional device based on a mesoporous titanium dioxide electrode and a liquid electrolyte. The solar power conversion efficiency was suppressed with an increase in the length of the oligomethylene moiety between the chlorin π-system and the carboxy group, while a corresponding π-linked ethenylene spacer enhanced the efficiency.     

Synthesis of new truxene based organic sensitizers for iodine-free dye-sensitized solar cells

Developing arylamine photosensitizers with high extinction coefficients, proper electronic structures, and steric properties is warranted for the dye-sensitized solar cells (DSCs) employing iodine-free redox shuttles. Two new organic sensitizers (M21 and M22) featuring unsymmetrical truxene-based triarylamine donor have been synthesized and compared to its reference sensitizer M4. The effects of unsymmetrical truxene-based triarylamine donors were investigated by their absorption spectra, electrochemical and photovoltaic properties. The incorporation of strong electron donor unit (i.e., dipropylfluorene and 4-methoxybiphenyl) has resulted in an improved light harvesting capacity, and thus photocurrent as well as efficiency of cells. M22 sensitized DSCs employing the Co(II/III)tris(1,10-phenanthroline)-based redox electrolyte affords a short circuit photocurrent of 13.1 mA cm⁻², an open circuit voltage of 861 mV, and a fill factor of 0.70, corresponding to an overall conversion efficiency of 7.89% under standard AM 1.5 sunlight.     

Synthesis of new di-anchoring organic sensitizer based on quinoxaline acceptor for dye-sensitized solar cells

New quinoxaline-based organic sensitizer bearing di-anchoring group for dye-sensitized solar cells (DSSCs) was synthesized from diethyl 4,5-diaminophthaltate, in which was prepared under mild condition by using Takehito’s method. The synthesized sensitizer was compared with mono-anchoring sensitizer through absorption spectra, emission spectra, J-V curve, and IPCE spectra, indicating the di-anchoring group leads to a noticeable improvement of J_sc value owing to more efficient intramolecular charge transfer and channel number increment.     

Characteristics of high efficiency dye-sensitized solar cells

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.     

Gelation of ionic liquid-based electrolytes with silica nanoparticles for quasi-solid-state dye-sensitized solar cells

For the first time silica nanoparticles were used to solidify ionic liquids. These ionic liquid-based quasi-solid-state electrolytes were successfully employed for regenerative photoelectrochemical cells that yielded 7% efficiency at AM 1.5 sunlight in combination with an amphiphilic ruthenium polypyridyl photosensitizer.     

Solidifying liquid electrolytes with fluorine polymer and silica nanoparticles for quasi-solid dye-sensitized solar cells

Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) and silica nanoparticles were employed to solidify 3-methoxypropionitrile based liquid electrolytes containing an ionic liquid, 1-methyl-3-propylimidazolium iodide, as iodide resource. These new quasi-solid-state electrolytes were successfully used for regenerative phtoelectrochemical cells that yielded about 6.7% efficiency under simulated full sunlight (air mass 1.5, 100 mW cm⁻²) in combination with an amphiphilic ruthenium polypyridyl photosensitizer. The as-fabricated device showed a good thermostability at 80 °C for 30 days, maintaining higher than 90% of its initial performance.     

Y-shaped dyes based on triphenylamine for efficient dye-sensitized solar cells

Three new triphenylamine-based dyes with Y-shaped conformation bearing triphenylamino-vinyl, 10-octyl-10H-phenothiazine-vinyl and 9-octyl-9H-carbazole-vinyl as arms (TT, TP, and TC) have been synthesized. From electrochemical investigations it is found that they can be employed in DSSCs due to the balanced HOMO and LUMO energy levels. Notably, the photo-to-electrical conversion efficiency of the DSSCs sensitized with branched TT, TP, and TC reach 5.12%, 4.84%, and 3.63%, which are higher than that sensitized with T (2.79%), and the DSSC sensitized with TT shows higher IPCE response and better photovoltaic performances (J_sc=12.37 mA/cm², V_oc=0.72 V and ff=0.58) than others. These results reveal that the introduction of branched Y-shaped extended π-conjugated donors to D-π-A dyes cannot only enlarge the spectral response range, but also suppress the molecular aggregation on TiO₂ films to a certain extent, which would enhance the performance of DSSCs.     

A random copolymer based on dithienothiophene and diketopyrrolopyrrole units for high performance organic solar cells

A random donor-acceptor semiconducting copolymer based on diketopyrrolopyrrole as the acceptor unit and dithienothiophene as the donor unit has been synthesized and characterized. Preliminary studies of BHJ solar cells based on this polymer with PC(71)BM showed a high PCE of above 5% under 100 mW cm(-2) AM1.5 solar illumination.     

A solvent-free, SeCN-/(SeCN)3- based ionic liquid electrolyte for high-efficiency dye-sensitized nanocrystalline solar cells

A solvent-free ionic liquid electrolyte based on the SeCN-/(SeCN)3- redox couple has been employed for high-efficiency dye-sensitized nanocrystalline solar cells. For the first time an alternative redox couple has been identified to rival and even exceed the performance of the iodide/triiodide couple even at full sunlight. Unprecedented 7.5-8.3% power conversion efficiencies under AM 1.5 sunlight have been achieved for photovoltaic devices with solvent-free ionic liquid electrolytes.     

Starburst triarylamine based dyes for efficient dye-sensitized solar cells

We report here on the synthesis and photophysical/electrochemical properties of a series of novel starburst triarylamine-based organic dyes (S1, S2, S3, and S4) as well as their application in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). For the four designed dyes, the starburst triarylamine group and the cyanoacetic acid take the role of electron donor and electron acceptor, respectively. It was found that the introduction of starburst triarylamine group to form the D-D-pi-A configuration brought about superior performance over the simple D-pi-A configuration, in terms of bathochromically extended absorption spectra, enhanced molar extinction coefficients and better thermo-stability. Moreover, the HOMO and LUMO energy levels tuning can be conveniently accomplished by alternating the donor moiety, which was confirmed by electrochemical measurements and theoretical calculations. The DSSCs based on the dye S4 showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 85%, a short-circuit photocurrent density (J(sc)) of 13.8 mA cm(-2), an open-circuit photovoltage (V(oc)) of 0.63 V, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 6.02% under 100 mW cm(-2) irradiation. This work suggests that the dyes based on starburst triphenylamine donor are promising candidates for improvement of the performance of the DSSCs.     

Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free organic dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br(-)/Br(3)(-) and I(-)/I(3)(-). The effects of additive Li(+) ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addition of Li(+) ions in electrolytes can broaden the absorption spectra of the dyes on TiO(2) films and shift both the LUMO levels of the dyes and the conduction band of TiO(2), thus leading to the increase of J(sc) and the decrease of V(oc). Upon using Br(-)/Br(3)(-) instead of I(-)/I(3)(-), a large increase of V(oc) is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO(2), as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochemical impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φ(r)) depend on the potential difference (the driving forces) between the oxidized dyes and the Br(-)/Br(3)(-) redox couple. For the dyes for which the HOMO levels are more positive than the redox potential of Br(-)/Br(3)(-) sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br(-)/Br(3)(-), insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.     

Electrolytes for dye-sensitized solar cells based on interhalogen ionic salts and liquids

In this paper, we report on the preparation of interhalogen ionic liquids of the general formula [K+]XY2-, where K+=1,3-dialkylimidazolium, 1,2,3-trialkylimidazolium, or N-alkylpyridinium; XY2-=IBr2- or I2Br-. These compounds were characterized in solution and the solid state by NMR, IR, Raman, and mass spectroscopy. The crystal structure of the compound [Me2BuIm]IBr2 (7) shows that the IBr2- anion has a linear Br-I-Br structure. Indications of an equilibrium between different forms of XY2- anions in solution are observed. Interhalogen ionic salts and liquids were used as electrolyte components for encapsulated monolithic dye-sensitized solar cells. Overall light-to-electricity conversion efficiencies up to 6.4%, 5.0%, and 2.4% at 1000 W/m2 were achieved by using electrolytes based on interhalogen ionic salts and gamma-butyrolactone, glutaronitrile, or native ionic liquids as solvents, respectively. Moreover, in terms of stability, the cell performance lost 9-14% of the initial performance after 1000 h illumination at 350 W/m2.     

Multichromophoric dye-sensitized solar cells based on supramolecular zinc-porphyrinperylene-imide dyads

Multichromophoric dye-sensitized solar cells (DSCs) based on self-assembled zinc-porphyrinperyleneimide dyads on TiO(2) films display more efficient light-to-electrical energy conversion than DSCs based on individual dyes. Higher efficiency of multichromophoric dyes can be attributed to co-sensitization as well as vectorial electron transfer that lead to better electron-hole separation in the device.