Photovoltaic characteristics of DSSCs assembled with the quasi solid-state electrolytes containing different KI contents under 100 mW cm-2.
1.1. Dye-sensitized solar cells (DSSCs)
The increasing global need for energy coupled with the depletion of easily accessible, hence cheap, fossil fuel reserves, poses a serious threat to the human global economy in the near future . Considering in addition the harmful ecological impact of conventional energy sources, it becomes obvious that development of clean alternative energy sources is a necessity [2, 3]. Best renewable energy options must rely on a reliable input of energy onto the earth. Since the sun is our only external energy source, harnessing its energy, which is clean, non-hazardous and infinite, satisfies the main objectives of all alternative energy strategies. Mastering the conversion of sunlight to electricity or to a nonfossil fuel like hydrogen is without any doubt the most promising solution to the energy challenge. It is remarkable that a mere 10 min of solar irradiation onto the Earth’s surface is equal to the total yearly human energy consumption . Therefore, solar power is considered to be one of the best sustainable energies for future generations. To date photovoltaics has been dominated by solid-state junction devices, usually in silicon, crystalline or amorphous, and profiting from the experience and materials availability resulting from the semiconductor industry. However, the expensive and energy-intensive high-temperature and high-vacuum processes is needed for the silicon based solar cells. Therefore, the dominance of the photovoltatic field by such kind of inorganic solid-state junction devices is now being challenged by the emergence of a third generation solar cell based on interpenetrating network structures, such as dye-sensitized solar cells (DSSCs) .
Since Professor M. Grätzel in EPFL introduced the nanoporous films into dye-derived wideband semiconductor research and made the breakthrough in the photoelectric conversion efficiency of DSSCs, academic and commercial interests have been focused on DSSCs for their high efficiency, potential low-cost and simple assembly technology. This became especially noticeable when the first cell with a certified efficiency of greater than 10% was demonstrated [6-10]. By incorporating the novel YD2-o-C8 dye and cosensitizing with Y123 dye, the DSSC with a traditional liquid electrolyte has achieved a 12.3% efficiency record , encouraging the surge to explore new organic materials for the conversion of solar to electric power.
The DSSC device is composed of three adjacent thin layers such as a high band-gap nanocrystalline semiconductor-based mesoporous film adsorbed with a dye sensitizer on the working electrode for the absorption in the visible region, a platinized counter electrode for the collection of electrons and a redox electrolyte, sandwiched between the two electrodes. The usual choice for the semiconductor material is titanium dioxide (TiO2), whereas ruthenium bipyridyl derivatives (N3, N719, Z907 and black dye etc.) are for the dye sensitizer. The electrolyte mostly contains I-/I3- redox couple, which was obtained by the mixing of iodine (I2) and inorganic or organic iodides in suitable non-aqueous solvents. Upon absorption of light, an electron is injected from a metal-to-ligand charge transfer excited state of the dye into the conduction band of the metal oxide. The rate of this electron injection reaction is ultrafast, typically occurring on the order of hundreds of femtoseconds to tens of picoseconds. The injected electron percolates through the TiO2 film, and is thought to move by a “hopping” mechanism and is driven by a chemical diffusion gradient (rather than an electric field), and is collected at a transparent conductive substrate of fluorine doped tin oxide glass (SnO2: F), on which the TiO2 film is formed. After passing through an external circuit, the electron is reintroduced into the solar cell at the platinum counter electrode, where triiodide is reduced to iodide. The iodide then regenerates the oxidized dye, thereby completing the circuit with no net chemical change.
1.2. Ionic liquids (ILs)
Ionic liquids (ILs) are low-temperature molten salts with melting points below 100 oC, that is, liquids composed of ions only. The salts are characterized by weak interactions, owing to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and dissymmetry (cation). ILs are basically composed of organic ions that may undergo almost unlimited structural variations because of the easy preparation of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulphonyl)imide, bis(fluorosulphonyl) imide and hexafluorophosphate as anions.
An IL, triethylammonium nitrate (a pure low-melting salt), was firstly identified more than a century ago. In the 1930s, a patent application described cellulose dissolution using a molten pyridinium salt above 130 °C. It was the need for a sturdy medium for nuclear fuel reprocessing that prompted the study of low-melting-point chloroaluminates. Among the onium cations with positive nitrogen(s), those derived from the imidazolium ring proved to be the best choice in terms of melting points and electrochemical stability . At the same time, the need for new anions for organic polymer electrolytes based on polyethylene oxide led to the concept of a plasticizing anion, that is, an anion having a delocalized charge and multiple conformations differing only marginally in energy. The archetype of such anions is the bis(trifluoromethylsulphonyl) amide (CF3SO2-N-SO2CF3) ion, also known as NTf2, in which the extremely electron-withdrawing CF3SO2-groups are conjugated and linked by flexible S-N-S bonds. When combined with an imidazolium cation, such as the ethylmethylimidazolium cation, this anion produces a fluid IL (melting point: -15 oC) with an ion conductivity comparable to that of the best organic electrolyte solutions; it shows no vapour pressure or no decomposition up to ~300-400 °C . It is not miscible with water (~1,000 p.p.m. in equilibrium with liquid H2O), and thus defies the conventional wisdom that states polarity is synonymous with hydrophilicity. ILs then developed rapidly, with a reinvestigation of ions, for example quaternary ammonium cations, that had been avoided previously by organic chemists because of unsymmetrical shapes that hindered easy purification through crystallization. The organic chemistry community had earlier engaged in research of media with controllable Lewis acidity (chloroaluminate ILs), but the modern era of ILs has produced numerous neutral ILs, that is, those based on ions which are unreactive towards acids or bases, be they Lewis or Bronsted. As a result, it is now difficult to name an organic reaction that has not been performed successfully in these potentially green solvents, which can be recycled almost indefinitely with no or minimal use of volatile organic compounds. Most products made in ILs can be distilled off, in the case of small molecules, or extracted with water or hydrocarbon solvents, at least one of which is usually immiscible with the ionic liquid.
It is this unique solvent potential that makes ILs key materials for the development of a range of emerging technologies. The advent of ILs has made viable processes that fail, or are even impossible, with conventional solvents. Water sensitive metals or semiconductors that previously could not be deposited from conventional water baths can now, by turning to ILs, be directly electroplated. Energy devices, such as the quasi-solid/all-solid-state DSSCs, polymer-electrolyte-membrane fuel cells, lithium batteries and supercapacitors presently under development to address the challenges of increasing energy costs and global warming, may greatly benefit from a switch to low-vapour-pressure, non-flammable, ILs-based electrolytes.
1.3. ILs as the electrolyte for DSSCs
The role of the electrolyte in DSSCs is very important as it provides the necessary ionic conductivity in the bulk of the solution and sets the potential barrier necessary for the energy conversion. In addition, it offers a reduction reaction at the counter electrode and helps for the dye regeneration by the charge-transfer reaction with the dye molecule . Usually, the conventional inorganic and organic iodide electrolyte salts are lithium and tetra-alkyl ammonium iodides, respectively. Besides this, several molten salts, particularly ionic liquid based imidazolinium salts, have also been used for improving the performance of the DSSCs [15-19]. In these works, cations play an important role in determining the conversion efficiency of the DSSCs. For example, the interaction of Li+ with TiO2 enhances the electron transfer from the sensitized dye to the TiO2 and from I- to the oxidized dye, leading to high photocurrent [20-22]. In the case of imidazolinium cations, the increase in the concentration of imidazolinium cations leads to the decrease of recombination at the working electrode due to the multilayer adsorption; thus improving the DSSC performance . Kubo
The highest efficiency record of DSSC was obtained based on the highly volatile organic solvent electrolyte due to the efficient infiltration of organic electrolyte in nanocrystalline films. However, commercialization of the cells with organic liquid electrolytes was impeded owing to technological problems related to hermetic sealing, precipitation of salts at low temperature and evaporation of liquids at high temperature; long-term stability is thus a major problem for these types of cells. Therefore, p-type inorganic semiconductors [24-26], organic hole conducting materials [27-30], ionic gel electrolytes having a polymer or a gelator [15, 31-33], and ionic liquid (IL) based electrolytes (or IL based electrolyte containing dispersed nano-components) [18, 34-40] were recently investigated for preparing the electrolytes. In these cases, imperfect filling of the dye-coated porous TiO2 film by p-type inorganic semiconductors or polymers has resulted in poor efficiency for the cells. Another weakness of the inorganic p-type materials is the decided chemical structure, resulting in the limitedly adjustable chemical/physical properties in the application of solid-state electrolytes. Meanwhile, the inorganic p-type material derived all-solid-state DSSCs shows no stability. This should be ascribed to inorganic p-type materials tending to be oxidized under continuous illumination and the worsening of the interfacial contact between dye-sensitized TiO2 and electrolyte along with the growth of age . Moreover, the carrier diffusion length was limited in the case of conducting polymers due to their low conductivity. Thereby, ILs based electrolytes were considered to be most attractive for replacing the organic solvents; they are preferred because of their negligible vapor pressure, high thermal stability, wide electrochemical window, and high ionic conductivity [42-46]. However, most ILs based electrolytes are liquid at room temperature [36, 43, 45-48]. Therefore, the fluidity and potential leakage of ILs based electrolytes during long-term operation is still unavoidable, which limits their wide application in DSSCs. To overcome this problem, the solid-state ILs have been applied as solid-state electrolytes for DSSCs recently.
This chapter mainly reviewed the recent researches on the topic of solid-state ILs-based electrolytes for DSSCs. Here the solid-state ILs employed in the electrolytes of DSSCs can be classified as follows: (a) ILs crystals (system A), (b) ILs polymers (system B), and (c) ILs conductors (system C).
2. Solid-state ILs-based electrolytes for DSSCs
2.1. ILs crystals (system A)
In year 2005, Yamanaka
Table 2 is a partial list of the all-solid-state DSSCs with ILs crystals-based electrolytes, which were obtained from the literatures.
|Yamanaka ||C12MImI||C12MImI; I2||N. A.||~7.00||~520||~0.63||~2.30||N. A.|
at-rest at 25 oC,
|EMII||EMII; PMII; SWCNTs||N719||8.07||716||0.61||3.49||1,000 h|
at-rest at 25 oC,
|EMII; EMIBF4; SWCNTs||9.74||620||0.66||4.01||1,000 h|
at-rest at 25 oC,
|Armel ||C1mpyrN(CN)2||C1mpyrNCN)2; EMII; LiI; I2; NMB||N719||8.60||775||0.77||5.10||N. A.|
|Chen ||DMPII||DMPII; KI; PEO||N719||14.11||710||0.59||5.87||N. A.|
|P12TFSI||P12TFSI; PMII; LiI; NBB||Z907||12.45||588||0.65||4.78||50 days|
at-rest at 25 oC,
|Cao-Cen ||C4BImBr||EMII; I2; C4BImBr; PMII||Z907||12.39||609||0.67||5.07||1,000 h|
light soaking at 25 oC, decay 5%
2.2. ILs polymers (system B)
Table 3 is a partial list of the all-solid-state DSSCs with ILs polymers-based electrolytes, which were obtained from the literatures.
|Wang ||PEAII||PEAII||N3||9.75||838||0.65||5.29||1,000 h at-rest|
at 25 oC, decay 15%
|Fang ||P-HI||P-HI; HMII; AMII; NMB; GuNCS||N3||15.10||643||0.72||6.95||N. A.|
|Chen ||Poly[BVIm] [HIm][TFSI]||Poly[BVIm] [HIm][TFSI]; EMII; PMII; EMISCN); I2; GuSCN; NBB||N719||12.92||676||0.68||5.92||1,200 h|
light soaking at 60 oC, decay 4%
|Chang ||Poly(AMImI)||Poly(AMImI); NMB; GuSCN; I2; MWCNT- poly(AMImI)||N3||8.51||646||0.64||3.55||N. A.|
2.3. ILs conductors (system C)
|Midya ||SD2||SD2; I2; Li[(CF3SO2)2N]; tBP; EMIB(CN)4||N719||6.23||718||0.64||2.85||N. A.|
3. Summary and future prospects
ILs are organic salts, composed mostly of organic ions that may undergo almost unlimited structural variations. Recently, the ILs act as useful electrolyte materials in DSSCs due to their negligible vapor pressure, high thermal stability, high ionic conductivity, and wide electrochemical window properties. This chapter mainly deals with the topic of novel ILs based electrolytes for solid-state DSSCs. The novel ILs based electrolytes include the IL-crystals (system A), IL-polymers (system B) and IL-conductors (system C).
Among system A, Chen
Recently, the stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), was demonstrated to be a promising redox system for DSSCs [80, 81], offering an alternative to the widely used iodide/triiodide couple. In the future, we can synthesize a novel IL with TEMPO-imidazole complex [82-84] for an iodine (I2)-free mediator system, and make application on solid-state DSSCs. This kind of TEMPO-imidazole complex containing TEMPO-redox radical and iodide-redox anion could potentially provide dual channels for charge transportation within the DSSCs.
This work was supported in part by the National Science Council of Taiwan under grant numbers NSC 100-2923-E-002-004-MY3 and NSC 100-2221-E-002-242-MY2.