1 organic small molecule gel electrolyte

The organic small-molecule gel used in the dye-sensitized solar cell mainly includes a sugar derivative, an amino acid compound, an amido (urea) compound, a biphenyl compound, and the like. Compared with high molecular polymers, the molecular weight of small molecule gelling agents is relatively small, generally below 1000. Small molecule gels generally contain polar groups such as amide bonds, hydroxyl groups, amine groups or long aliphatic chains. In an organic solvent, a gel electrolyte is gelled by hydrogen bonds, π-π bonds, electrostatic attractive forces, van der Waals forces, and hydrophobic interactions between gel molecules. The specific operation is to first add a small molecule gelling agent to the liquid electrolyte, heat it to a certain temperature to keep the electrolyte in a liquid state, and then inject it into the two electrodes while hot, so as to ensure the electrolyte and the nano TiO2 film. Full contact, after the temperature drops to room temperature, the liquid electrolyte becomes quasi-solid. It is particularly mentioned that the liquid electrolyte is gelled with a small molecule gel, and the efficiency of the battery after gelation does not decrease significantly.

The DSSC made by curing the liquid electrolyte with an organic small molecule gel has good photoelectric properties. Japan's Wataru Kubo et al. first began to use small molecule gels in the preparation of quasi-solid electrolytes. In 1998, they used amino acid compounds to gel liquid electrolytes, and the DSSC produced at 100 mW/cm 2 was used. The lower photoelectric conversion efficiency exceeds 3%. In 2001, they used four small-molecule gels to cure liquid electrolytes. The composition of the liquid electrolyte was: 016 mol/L1, 2-dimethyl-3-propylimidazolium, 011 Mol / L I2, 011 mol / L LiI, 1 mol / L 4 - tert-butylpyridine, the solvent is 3-methoxypropionitrile. The four gelling agents used are shown in Figure 1, and the DSSC performance is shown. As can be seen from Table 1, the electrolyte did not degrade various properties after solidification. Under the light intensity of 100 mW/cm2 (AM115), the short-circuit current density J sc, open circuit voltage Voc and photoelectric conversion efficiency η were 1218 mA/cm 2, 0167V, 5191%, respectively. In 2004, researchers in Germany and Switzerland used liquid sorbitol derivatives to cure liquid electrolytes, and achieved good results. The gelling agent they used was bis(3,4-dimethyl-dibenzylidene sorbitol), the English abbreviation is DMDBS, and the structure is as shown. The composition of the liquid electrolyte to be solidified is: 016 mol / L 1 , 2 - dimethyl - 3 - propyl imidazolium iodide (DMPII), 011 mol / LI 2, 015 mol / L N - methyl benzimidazole, solvent is A Oxypropionitrile (MPN). The liquid electrolyte was cured by adding a 115% (Wt%) gel, and the DSSC produced had good photoelectric properties, and the performance of the battery did not decrease after curing. At 115 AM light intensity, J sc, Voc, FF, and η were 12145 mA/cm 2, 718 mV, 0169, and 611 %, respectively, and the battery also had good thermal stability.

1 Performance of quasi-solid DSSC made with four gels

Electrolyte V oc / VJ sc / mA cm - 2 FFη / % liquid 0. 622 11. 0 0. 674 4. 66 gel 1 0. 625 10. 9 0. 658 4. 46 gel 2 0. 632 11. 1 0 658 4. 62 gel 3 0. 640 11. 1 0. 634 4. 49 gel 4 0. 623 11. 2 0. 664 4. 67

2 bis(3,4-dimethyl-dibenzylidene sorbitol) materials, chemically used DSSCs made from organic small molecule gel electrolytes do have good optoelectronic properties, but later researchers found this quasi-solid state The electrolyte is not very stable, and the solvent in the electrolyte system will volatilize over time, so people begin to look at the ionic liquid electrolyte. The ionic liquid electrolyte is non-volatile and has a large temperature stability range, good chemical stability, and a wide electrochemical window. In 2003, Kubo et al. used the first gelling agent to cure the ionic liquid electrolyte. It was found that the ionic liquid electrolyte had the same photoelectric conversion efficiency before and after curing, and Jsc, Voc and filling factor were obtained under the intensity of 100mW/cm 2 (AM115). FF and η are 1118 mA/cm2, 0164 V, 0167, 510% DSSC.

Although solar cells made using small molecule gels are highly efficient, small molecule gel molecules rely only on weaker intermolecular forces to form unstable physical crosslinks. Therefore, such electrolytes tend to have poor mechanical properties, and such quasi-solid electrolytes are thermally reversible and become liquid electrolytes at relatively high temperatures. As a result, the stability of the battery will decrease and the life will be reduced.

2 polymer gel electrolyte

Polymers are the most commonly used materials for the preparation of quasi-solid electrolytes. In general, such polymers include high molecular weight polymers and low molecular weight polymers, each of which has advantages and disadvantages. The spatial network structure formed by the polymer is relatively stable and the mechanical strength is relatively good. However, the viscosity of the electrolyte system is large, the conductivity is poor, and the affinity between the electrolyte and the TiO2 film is not good, resulting in the impedance between the electrolyte and the TiO2 film. Raise. Although the quasi-solid electrolyte formed by the oligomer is slightly inferior in mechanical properties, such an electrolyte tends to have a high electrical conductivity, and the resulting battery has a high photoelectric conversion efficiency. When a high molecular polymer is used to prepare a quasi-solid electrolyte, it is usually necessary to add a small molecule that acts as a crosslink or plasticizer. The currently used high molecular polymers are mainly polyethylene oxide (PEO), polyvinyl pyridine, polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), copolymer of vinylidene fluoride and hexafluoropropylene (P) VDF - HFP), polypropylene oxide, and the like. However, in order to improve the conductivity and mechanical properties of the polymer, two or more methods of polymer copolymerization are generally employed. The polymer itself is a long-chain structure. In the quasi-solid electrolyte, a three-dimensional network structure is formed between the polymer chains, and the supporting force between the structures is an atomic covalent bond, so the structure is smaller than that. Molecular gels form much more stable structures, and such quasi-solid electrolytes tend to be thermally irreversible.

There are generally two methods for preparing quasi-solid electrolytes using polymers: one is to add a polymer to a liquid electrolyte to react as a small-molecule gel to produce cross-linking to form a quasi-solid electrolyte; The film is formed, and then the liquid electrolyte is absorbed to become a quasi-solid electrolyte.

In the field of DSSC, researchers in various countries are competing to use high molecular polymers on quasi-solid electrolytes. At first, although the researchers added some plasticized small molecules to the polymer, the efficiency of the battery is always not good. The reason is because the migration of conductive ions in this electrolyte is not like in the liquid state. The electrolyte is transported through a free volume, but is carried out by means of a Lewis acid-base action or a movement of a polymer chain by conductive ions and some atoms on the polymer chain, and the conductivity of the electrolyte having such a conductivity is low. Can not meet the actual needs. Therefore, in later studies, researchers generally used a copolymerization method or a low molecular weight polymer as a gelling agent, which increased the free space in the quasi-solid electrolyte system, thereby improving the conductivity of the electrolyte. .

In foreign countries, Japan's Sakaguchi combines high molecular weight polymer with small molecular crosslinkers to achieve a good gel effect without increasing the amount of polymer. In 2004, South Korea's Dong2Won Kim and others first copolymerized acrylonitrile (AN) and methyl acrylate (MMA) into a film, and then immersed in a liquid electrolyte. The final DSSC was Jsc at a light intensity of 100 mW/cm 2 . Voc and η are respectively 6127 mA cm - 2, 0172 V, 214 %.

In 2003, Japan's Ryoichi Komiya et al. used an oligomer in a quasi-solid electrolyte, and the structure of this polymer is as shown.

In the production of such a DSSC, a polymer film is first formed on the electrode after the dye is adsorbed, and then the electrode and the film are immersed together in the liquid electrolyte. The DSSC made with this polymer has a Jsc, Voc, and FF of 1418 mA/cm 2, 0178V, and 0170 at a light intensity of 100 mW/cm2, respectively, and the photoelectric conversion efficiency is as high as 811%.

The oligomer structure used in China, in 2004, the research team led by Dai Songyuan of the Chinese Academy of Sciences plasma used a vinylidene fluoride and hexafluoropropylene copolymer P (VDF - HFP) gel liquid electrolyte, the quasi-solid DSSC obtained 6161% photoelectric conversion efficiency, J sc, Voc, FF reached 15152 mA / cm 2, 0170 V, 0162. The short-circuit current density is 013 ~ 014 mA / cm 2 lower than the liquid electrolyte, when the amount of gel added is At 10%, the battery efficiency is only about 016% lower than that of liquid electrolyte. The research team led by Lin Yuan, Institute of Chemistry, Chinese Academy of Sciences, used long-chain polymer materials to react with polysiloxanes containing quaternary ammonium salt side chains (PSQAS) to prepare quasi-solid electrolytes. . In 2003, their solar energy (solar photovoltaic soliton research) battery made from this quasi-solid electrolyte had a photoelectric conversion efficiency of 1139% at a light intensity of 60 mW/cm2. Although the photoelectric conversion efficiency of such a battery is not high, it provides a new method after all. In 2004, they used polyacrylonitrile (PAN) to make quasi-solid DSSCs. They added 5% by mass of PAN to liquid electrolytes (liquid electrolytes containing PC, EC, I2, and PSQAS) to form quasi-solid electrolytes. Finally, the Jsc, Voc, and FF of the battery were 7 mA/cm2, 01565 V, and 0156 respectively. The photoelectric conversion efficiency was 413% at a light intensity of 60 mW/cm2, and the photoelectric conversion efficiency was 100 mW/cm2. Up to 217%. In 2005, they gelled the liquid electrolyte by reacting a derivative of a polyamidoamine (PAMAM) dendrimer with a halogenated compound of PEO in a quaternary ammonium salt. Morphological observation by atomic force microscopy (AFM) revealed that the polymer was in the form of a necklace (such as 4) in the quasi-solid electrolyte system, in which the bead-like substance was a derivative of polyamidoamine (PAMAM) dendrimer. The biggest feature of this derivative is that it has a well-developed dendritic structure, and the line used to wear the beads is the PEO chain. The final DSSC photoelectric performance is very good. Under the light intensity of 100 mW/cm 2 , Jsc, Voc and FF are 1711 mA/cm 2, 700 mV and 0164 respectively, and the photoelectric conversion efficiency is as high as 7172%. This is currently domestic. The highest efficiency of the quasi-solid-state DSSC. In the past, when a polymer gelling agent was used, a small cross-linking molecule was added to form a chaotic cross-linking structure between the polymer chains, and a polymer chain was often cross-linked with many other polymer chains. In this way, the resistance of the conductive ions increases during the migration process, so that the efficiency of the battery is generally not high. Different from the past, this cross-linking structure is quite special. Basically, only two ends of each polymer PEO chain are cross-linked with other PEO chains through derivatives of polyamidoamine (PA2 MAM) dendrimers. Such a structure can be electrically conductive. The migration of ions provides more free space, which increases the conductivity of the electrolyte and increases the efficiency of the battery.

In this example, we have seen that this method takes advantage of the respective advantages of high molecular weight polymers and small molecular polymers. The electrolyte after gelation has both a stable chemical cross-linking structure and a high electrical conductivity. It provides a good reference for our future research.

All of the above are polymer electrolyte liquid electrolytes, and the liquid electrolytes herein are composed of I2, LiI, additives, and organic solvents. Some researchers have used polymer gel ionic liquid electrolytes, Wang et al. used a copolymer of vinylidene fluoride and hexafluoropropylene to gel 1-methyl-3-propylimidazolium iodide (MPII) electrolyte, and finally made it with Z907 dye. Quasi-solid-state DSSC, at a light intensity of 100 mW/cm2, the Jsc, Voc, and FF of the battery are 11129 mA/cm2, 01665 V, and 01712, respectively, and the photoelectric conversion efficiency is 513%. The ionic liquid reference solid electrolyte is used. The photoelectric conversion efficiency of DSSC is very close to that of liquid DSSC, and the ionic liquid reference solid electrolyte system is relatively stable. The efficiency of the battery does not decrease significantly after long-time irradiation, which is very important for the practical use of DSSC.

In summary, the polymer gel electrolyte is structurally stable compared to the small molecule gel electrolyte, and the mechanical properties are greatly improved. The DSSC efficiency by copolymerization or use of the oligomer is not inferior to that of the liquid electrolyte DSSC. Therefore, polymer gel electrolytes are a class of highly potential electrolytes.

3 adding nanoparticle gel electrolyte

In order to make the quasi-solid electrolyte have good electrical conductivity, we often use a small molecule gel or a small amount of polymer gel to prepare a quasi-solid electrolyte. As a result, the mechanical properties of the electrolyte system are often poor, and can not meet the requirements of use, which makes us begin to think about how to improve the mechanical properties of the electrolyte without reducing the photoelectric performance of the battery. To achieve the above objectives, researchers have added nano-materials to the electrolyte system. Such materials rely on their own unique properties to form a quasi-solid electrolyte together with a small molecule gelling agent or a polymeric gelling agent by chemical or physical crosslinking in an electrolyte system. It should be mentioned that such nano-materials can also gel the liquid electrolyte, so the method of using the nanoparticles as a gel is different. Some researchers use nano-materials as gels alone, while others use gels alone. The nanomaterial is combined with other gelling agents to form a gel liquid electrolyte. The addition of nano-materials not only improves the mechanical properties of the electrolyte system, but also improves the electrical conductivity of materials and chemical electrolyte systems. Because of the addition of nano-inorganic particles, the inorganic nanoparticles can form more pores in the gel system. These channels are filled with liquid electrolyte and can greatly improve the conductivity of the electrolyte. Researchers from DSSCs around the world have done a lot of research on nano-based gels in recent years. Currently, nano-type gels used in quasi-solid-state solar cells include nano-SiO2, nano-TiO2, carbon black, carbon nanotubes, etc. .

In 2002, Wang et al. first used a nanoparticle gel ionic liquid electrolyte to produce a solar cell with a photoelectric conversion efficiency of 710% at a light intensity of 100 mW/cm 2 . In 2003, the Yanagida research team in Japan applied various inorganic nanoparticles to the quasi-solid DSSC. These nanomaterials include single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon black (CB), nano-TiO2, graphite, and carbon fibers (CFs). The DSSC performance obtained is as follows: 2. They mix various nanoparticles with an ionic liquid electrolyte, grind it, and then centrifuge to separate the solid-liquid to obtain a quasi-solid electrolyte. Later, some researchers treated the nanoparticles and prepared the quasi-solid DSSCs with the treated nanoparticles. Particularly noteworthy is the mesoporous material, which itself has a pore structure, so that mesoporous nanoparticle gels have been used. The liquid electrolyte forms more transport channels throughout the electrolyte system, which increases the conductivity of the electrolyte, thereby increasing the photoelectric conversion efficiency of the battery. In 2005, Yang Hong et al. of Fudan University gelled liquid electrolyte with mesoporous SiO2 nanoparticles, and the DSSC obtained by DSSC had a photoelectric conversion efficiency of 4134% at a light intensity of 100 mW/cm 2 . Wang et al. used P(VDF - HFP) and nano-SiO 2 to cure liquid solution, and the DSSC achieved an efficiency of 617% at 100 mW/cm2.

Moreover, after 30 days of thermal stability test at 80 ° C, the solar cell can still achieve an efficiency of 90%, showing good thermal stability. In the same year, Kato et al. of Japan used a nano-TiO2 gel liquid electrolyte. Unlike before, they modified TiO2 before they were attached to the nano-TiO2 particles via Ti-O-CO and long alkyl chains. It was found that when unmodified nano-TiO2 was added to the electrolyte system, the photoelectric performance of the battery decreased sharply with the increase of the amount of TiO2. However, if the modified nano-TiO2 particles are added to the liquid electrolyte system, although the photoelectric performance of the battery will decrease, the downward trend becomes very slow. Finally, they found that the length of the alkyl chain attached to TiO2 has a great influence on this downward trend. The longer the alkyl chain, the slower the battery performance declines. When the alkyl chain length is 12, the performance of the quasi-solid battery is basically Same as a liquid battery. Later, the researchers combined the nanoparticles with the high molecular polymer gel, and the DSSC produced also had high photoelectric conversion efficiency.

2 Various nanogels quasi-solid DSSC performance <36 > Electrolyte V oc (mV) J sc (mA / cm 2) η (%) FF lonic liquid 648 11. 57 4. 21 0. 56 MWCNTs composite gel 706 12 02 4. 79 0. 57 CB composite gel 672 11. 02 4. 83 0. 65 TiO 2 composite gel 675 11. 45 5. 00 0. 65 SWCVTs composite gel 695 10. 78 4. 60 0. 62 CFs composite Gel 688 11. 11 4. 97 0. 65 Graphic composite gel 681 10. 60 4. 570. 63

Although the addition of nanoparticles can improve the overall performance of the battery, the nano-material itself has a large specific surface area, and agglomeration occurs over a long period of time, which degrades the stability of the entire battery. Therefore, if a dispersant is added to the electrolyte system to prevent agglomeration between the nanoparticles, the stability of the battery is improved, so finding an effective dispersant to stabilize the gel is one of the key issues that should be noted.

4 prospects and prospects

The use of quasi-solid electrolytes solves some of the key issues in the practical application of dye-sensitized solar cells. However, other problems have been exposed in the research, such as the problem that the small molecule gel electrolyte is not stable enough, and the polymer electrolyte gel electrolyte has low conductivity. Some of these problems have been well resolved through the continuous efforts of scientists from various countries. But we should see that for now, we are still some distance away from the practical use of dye-sensitized solar cells. Therefore, we believe that we should do more work in the following aspects: (1) Looking for some highly polar, multi-functional macromolecules or oligomers as gelling agents, so as to ensure higher conductivity and Have better mechanical properties and facilitate battery assembly; (2) use a variety of gelling agents in an electrolyte system by co-mixing, which can reduce the crystallinity inside the electrolyte and improve the conductivity; (3) Try to The liquid electrolyte is replenished into the quasi-solid electrolyte to make an electrolyte system that can be continuously updated; (4) The nano-substance added to the electrolyte system is modified.

In summary, quasi-solid electrolytes have seen the prospect of practical application of dye-sensitized solar cells with their unique advantages. It is believed that through the continuous efforts and attempts of researchers all over the world, in the near future, quasi-solid dye sensitization Solar cells must be able to be widely promoted.

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