Solid-state dye-sensitized solar cells
The use of liquid electrolyte causes several problems such as evaporation or leak of the electrolyte due to difficulty in hermetic sealing. As far as liquid electrolytes are employed, low temperatures may induce crystallization of the iodide salts and high temperatures may enhance the leak of the volatile organic solvent and iodine. Replacing the liquid electrolyte with solid-state hole-transport materials is an essential research subject for improving the long-time stability of the cells. Several attempts have been made to replace the liquid electrolyte with p-type semiconductors or organic hole transport materials (HTMs).
An interpenetrating network of two conducting solids can so easily be established that an immobilized molecule (the sensitizing dye) at their interface can exchange charge carriers with both .
However, the conversion efficiency of these devices was relatively low particularly under high light irradiance when compared to the liquid electrolyte systems. This may be due to high frequencies of charge recombination to HTMs or poor electronic contact between dye molecules and the HTMs caused by incomplete penetration of solid HTMs in the void of the mesoporous TiO2 electrodes .
A schematic presentation of the structure of solid-state DSSCs is given in Fig.1. At the heart of the system is a mesoporous TiO2 film, which is placed in contact with a solid-state hole conductor. Attached to the surface of the nanocrystalline TiO2 film is a monolayer of a charge transfer dye. Photo-excitation of the dye results in the injection of an electron into the conduction band of the TiO2. The original state of the dye is subsequently restored by electron donation from the hole conductor. The regeneration of the sensitizer by the hole conductor intercepts the recapture of the conduction band electron by the oxidized dye. The hole conductor is regenerated in turn at the counter-electrode, and the circuit is completed via electron migration through the external load .
Fig.1. Structure of solid-state dye-sensitized nanocrystalline TiO2 solar cells.
The hole conductors employed in solid-state DSSCs can be classified as p-type semiconductors, ionic liquid electrolytes and polymer electrolytes .
The most common approach to fabricate solid-state DSSCs is by using p-type semiconductors. Several aspects are essential for any p-type semiconductor in a DSSC:
- It must be able to transfer holes from the sensitizing dye after the dye has injected electrons into the TiO2; that is, the upper edge of the valence band of p-type semiconductors must be located above the ground state level of the dye.
- It must be able to be deposited within the porous nanocrystalline layer.
- A method must be available for depositing the p-type semiconductors without dissolving or degrading the monolayer of dye on TiO2 nanocrystallites.
- It must be transparent in the visible spectrum, or, if it absorbs light, it must be as efficient in electron injection as the dye .
Inorganic p-type semiconductors
Many inorganic p-type semiconductors satisfy several of the above requirements; however, the familiar large-band gap p-type semiconductors such as SiC and GaN are not suitable for use in DSSCs since the high-temperature deposition techniques for these materials will certainly degrade the dye. After extensive experimentation, a type of inorganic p-type semiconductor based on copper compounds such as CuI, CuBr, or CuSCN, was found to meet all of these requirements. These copper-based materials can be cast from solution or vacuum deposition to form a complete hole-transporting layer, and CuI and CuSCN share good conductivity, which facilitates their hole conducting ability .
DSSCs utilizing CuI are not very stable: deterioration of CuI-based solid-state DSSCs was very rapid, even much faster than that of liquid-state DSSCs. One of the reasons responsible for the degradation is that the excessive iodine in the CuI film strongly decreased the photocurrent of the cell. Also, CuI tends to be oxidized under continuous illumination. For this reason solid-state DSSCs were fabricated with a MgO-coated TiO2 nanoporous film and a CuI layer. The solar cell as fabricated showed both improved conversion efficiency and stability. They attributed the improved lifetime to the suppression of the photooxidation capability of TiO2 by blocking the transfer of photogenerated holes to the CuI layer. Another important factor for the degradation of CuI-based DSSCs is the loosening of the contact between the dye-sensitized TiO2 surface and CuI crystallites. CuI deposited from the acetonitrile solution produces large (~10 μm) crystallites that do not penetrate into the pores of the nanocrystalline matrix and form loose contacts with TiO2. The stability of the CuI-based DSSC can be greatly improved by incorporation of a small quantity of 1-methyl-3-ethylimidazolium thiocyanate (MEISCN) in the coating solution. In this case, MEISCN acted as a CuI crystal growth inhibitor and at the same time the thin film of this compound remaining at the CuI grain boundaries or the interface between CuI and the dyed TiO2 seemed to allow hole conduction. However, MEISCN is fairly expensive as its purification involves a chromatographic separation .
One alternative to using CuI is employing CuSCN, which does not decompose to SCN- and there is no indication that stoichiometrically excessive SCN- creates surface traps in CuSCN. This is consistent with the observation that solid-state DSSCs based on CuSCN have more stable performance. The energy conversion efficiency of cells using electrodeposited CuSCN is 1.5%. Nevertheless, the performance of cells made by CuSCN is still lower than that of cells utilizing CuI, probably due to the relatively lower hole conductance .
Organic p-type semiconductors
Compared with inorganic p-type semiconductors, organic p-type semiconductors (i.e. organic hole-transport materials) possess the advantages of having plentiful sources, easy film formation and low cost.
Some organic p-type semiconductors are shown in the Fig.2 .
Fig.2. Several organic p-type semiconductors utilized in solid-state DSSCs: 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (OMeTAD); (HTM-TEG1) and (HTM-TEG2) based on an arylamine TPD (TDP=N,N'-diphenyl-N,N'-(m-tolyl)-benzidine) structure comprising a tetraethylene glycol (TEG) group on the fluorene unit.
However, the conversion efficiencies of most of the solid-state DSSCs employing organic p-type semiconductors are relatively low particularly under high light irradiation. This is due to the low intrinsic conductivities of organic HTMs, the high frequencies of charge recombination from TiO2 to HTMs, the poor electronic contact between dye molecules and HTMs caused by incomplete penetration of solid HTMs in the pores of the mesoporous TiO2 electrodes. Among all the disadvantages of organic HTMs, the poor pore filling is considered as the most important factor .
In order to improve the pore filling of organic p-type semiconductors, we can use in situ photo-electrochemically polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) as the hole transport phase in the pores of the dye-sensitized mesoporous TiO2 films.
The in situ polymerization method solves the difficulty in accurate filling of the voids with the semiconductor hole-transport layer, because PEDOT can be photo-polymerized on the labyrinthine surface of the TiO2 electrode with the help of the oxidizing ability of the photoexcited dye molecules on the surface .
However, the performance of these cells was poor in that the polypyrrole itself absorbs visible. The thiophene ligands of PEDOT improve charge transport at the interface of the sensitizing dye molecules and the HTMs, so they improve the photovoltaic performance of the solid DSSCs . PEDOT also has high transparency in the visible range, high conductivity and remarkable stability at room temperature. By this means, good pore filling and an overall efficiency of 0.53% at AM 1.5 illumination (100mWcm-2) was achieved .
IONIC LIQUID ELECTROLYTES
Room-temperature ionic liquids (RTILs) have good chemical and thermal stability, negligible vapor pressure, nonflammability, and high ionic conductivity. When incorporated into DSSCs, they can be both the source of iodide and the solvent themselves.
Some ionic liquid electrolytes are:
- methyl-hexyl-imidazolium iodide (MHImI);
- EMIm-F...2.3HF (obtained by the reaction of 1-ethyl-3-methyl imidazolium chloride (EMIC) and hydrogen fluoride);
- an ionic liquid electrolyte composed of 1-methyl-3-propylimidazolium iodide (PMII), 1-methyl-3-ethylimidazolium dicyanamide (EMIDCN), and lithium iodide combined with an amphiphilic polypyridyl ruthenium sensitizer called Z-907 ((Ru(H2dcbpy)(dnbpy)(NCS)2, where H2dcbpy is 4,4'-dicarboxylic acid-2,2'-bipyridine and dnbpy is 4,4'-dinonyl-2,2'-bipyridine));
- imidazolium compounds such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4, 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6),1-ethyl-3-methylimidazolium selenocyanate (EMISeCN)
The very high density of ions present in these RTILs appears to facilitate charge separation although the mechanism by which the screening is achieved remains obscure to date .
Since the efficiencies of solid-state DSSCs utilizing p-type semiconductors were found to be unsatisfactory compared with those using organic liquid-phase redox electrolytes and the RTIL-based DSSCs still have the disadvantage of being fluid, the polymer electrolytes have been taken into account recently. It is of great importance that polymer electrolytes have the advantages of relatively high ionic conductivity and easy solidification. Using a gel network polymer electrolyte is the usual way to improve the contact between solid electrolytes and nanoporous TiO2 layers. Actually, quasi solid-state DSSCs containing physical gels have almost the same performance as those containing liquid gel precursors, which remain in a liquid state before becoming gelated to a solid-state gel. The process for the fabrication of quasi solid-state DSSCs is given in Fig.3 .
Fig.3. Process for fabrication of quasi solid-state DSSCs based on gel electrolyte.
Gel electrolyte precursors containing liquid electrolytes and gelators are injected into cells that are already set. Gelation is usually carried out in the cell by heating the cell. Gel electrolyte precursors are liquid at first, which makes it possible for the precursors to be impregnated into nanoscale pores of TiO2 films. By heating, gelators propagate along a three-dimensional polymer network in the cell.
Several conditions are necessary for gel electrolytes:
- Polymerization must occur in the presence of iodine.
- Polymerization must occur at a temperature below which the dye would not decompose.
- Polymerization should be able to be initiated and completed even in the presence of some impurities such as oxygen, water, ions and so on.
- Polymerization has to be completed without generating byproducts that could decrease the photovoltaic performance.
- Polymerization can proceed without an initiator because the resultant decomposition products of the initiator may decrease the photovoltaic performance .
Some of the gel polymer precursors are shown in Fig.4.
Fig.4. Structures of some of the gel polymer precursors.
MECHANISM IN SOLID-STATE DSSCs
The charge-transfer reactions at the dye-sensitized nanocrystalline TiO2/holetransporting material (HTM) interface play a key role in the determination of the overall efficiency of solar cell devices. A schematic diagram of the solid-state device it is shown in Fig.5 .
Fig.5. Schematic diagram of charge transporting in solid-state DSSCs with organic HTMs.
In conclusion, the dye regeneration by HTM proceeds faster than the dye regeneration process involving iodine/iodide electrolytes .