Hiroshi Shimizu (Invited Senior Researcher), Yongjin Li (Researcher), and Liping Zhao (AIST Postdoctoral Researcher), the Nanosystem Research Institute (Director: Kiyoshi Yase) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), have developed a ternary material with a core-shell structure consisting of multi-walled carbon nanotubes, an ionic liquid, and a conducting polymer. It was found that, when used as the counter electrode of dye-sensitized solar cells, the material exhibits photoelectric conversion efficiency as high as that of platinum.

Dye-sensitized solar cells (DSCs) are in the development stage and platinum, one of rare metals, is considered to be a promising material of counter electrodes. However, because of the rapidly increasing use of platinum as catalysts in vehicles and fuel cells, there is concern that the supply and demand balance of platinum may be affected. If the new ternary material, which is produced by using simple processes, can replace platinum, then its use would help reduce the consumption of platinum. It would also enable a cost reduction and an increase in the area of DSCs.

Details of this technology were published in the November 9, 2010 issue of Chemistry of Materials and were presented at the 13th Tokyo Industry Exhibition, held at Tokyo Big Sight from November 10 to 12, 2010.

Figure 1 The developed ternary counter electrode material with a core-shell structure

In recent years, photovoltaic power generation has attracted attention as a renewable energy source. Silicon-based solar cells are widely used, but there is concern about shortages in the supply of silicon as a raw material. In this context, the development of non-silicon-based solar cells has been actively pursued in recent years. For example, organic material-based solar cells can be produced by simple processes at low cost; allow a reduction in weight and an increase in size; and allow flexibility to be added. Because of these characteristics, such cells are attracting significant attention. Organic solar cells include DSCs, organic thin-film solar cells, and quantum-dot solar cells, which are in the research stage.

Generally, the production process of a DSC involves forming a porous titanium dioxide film on a glass substrate with a transparent electrode; adsorbing dye molecules to the porous film; supplying an electrolyte into the gap between the glass substrates by using a platinum-coated glass substrate as the counter electrode; and sealing the substrates with a sealant.

Platinum used in the counter electrode is a rare metal. Its total production since the beginning of human history has been only 4000 tons—only one-thirtieth of the production of gold. Only 3 g of platinum can be extracted from 1 ton of ore, making it more expensive than gold. In recent years, although global demand for platinum as catalysts in vehicles and fuel cells has rapidly increased, production of platinum has remained at 180 ton/year, raising concern that the supply and demand balance may be affected. To reduce the consumption of platinum and production costs, counter electrode materials to replace platinum need to be developed.

We focused on carbon nanotubes which have excellent electrical conductivity and can now be mass-produced, as an alternative to the platinum counter electrode, and we began research and development using multi-walled nanotubes (MWNTs). Being powders and difficult to form, MWNTs cannot be made into solar cell counter electrodes unless they are dispersed into a matrix polymer. However, MWNTs tend to aggregate and are difficult to disperse, even with the use of organic solvents.

AIST has led the research and development of new composite materials through nano-blending of polymers and nano-dispersion of various particles or fillers of nano-sizes into polymers. By using the technologies developed in this research and development, we attempted to produce a polymer material in which MWNTs are dispersed and to use this material as a counter electrode of a solar cell.

This study was funded by the research acceleration budget to promote innovation.

Because MWNTs have a strong affinity for ionic liquids (ILs), we used an IL to modify the surfaces of MWNTs. We selected an imidazole-based IL containing two hydroxy groups and added it to MWNTs. When the MWNTs were mechanically mixed with the IL, gelation occurred and the aggregates of the MWNTs were broken, resulting in the dispersion of isolated MWNTs. The improved dispersibility likely occurred because the gelation made the MWNTs hydrophilic. However, when only this binary material (IL-MWNT) was used as the counter electrode of a dye-sensitized solar cell, the photoelectric conversion efficiency was lower than that of the solar cell with a platinum counter electrode.

To further improve the electrical conductivity, we mixed IL-MWNT with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a conducting polymer that has a thiophene skeleton and exhibits hydrophilicity in a pair with sulfonate. First, IL-MWNT was added to an aqueous solution of PEDOT:PSS and dispersed using ultrasound. The mixture was then centrifuged, and a ternary conducting material (IL-MWNT/PEDOT:PSS) was obtained. Analysis of the structure of IL-MWNT/PEDOT:PSS showed that a core-shell structure was formed in which the MWNT with IL molecules attached to the surface was the core and PEDOT:PSS was the shell. Figure 2 shows the scheme of the IL-MWNT/PEDOT:PSS production.

Figure 2 Production scheme of the new conducting ternary material

Figure 3 (a to c) shows transmission electron microscope (TEM) images of each structure formed in the scheme shown in Fig. 2. The MWNTs are mixed and kneaded with the IL and IL molecules are adsorbed onto the wall of the MWNT, forming IL-MWNT (Fig. 3a). As a result of addition of the PEDOT:PSS aqueous solution to IL-MWNT, followed by ultrasonic dispersion and centrifugal separation, PEDOT:PSS shell is formed on the outside of IL-MWNT, exhibiting a core-shell structure (Fig. 3b). When the MWNTs are directly mixed with the PEDOT:PSS aqueous solution, PEDOT:PSS is not adsorbed onto the outer surface of the MWNT, but is dispersed as particles around the MWNTs (Fig. 3c). This means that, in a binary system such as this, the dispersibility is poor, making it difficult to form a stable electrode material and to increase the electrode’s area. However, the addition of the IL increases the affinity of the MWNT for PEDOT:PSS, and dispersibility and formability are substantially improved in the core-shell ternary system.

(a) (b) (c)

Figure 3 : (a) TEM image of IL-MWNT; (b) TEM image of IL-MWNT/PEDOT:PSS, the ternary material forming the core-shell structure; (c) TEM image of the MWNT/PEDOT:PSS system

We have fabricated DSCs using IL-MWNT/PEDOT:PSS with a core-shell structure and other materials as the counter electrode, and measured the characteristics. The photovoltaic parameters of these devices, i.e. the short-circuit current (J_sc), the open-circuit voltage (V_oc), the fill factor (FF), and the energy conversion efficiency (η), are summarized in Table 1. As shown in Table 1, characteristics nearly identical to those of a platinum electrode were obtained. A significant improvement in photoelectric conversion efficiency was observed; this had not been achieved with the binary materials.

The next step in this study is to increase the area of the counter electrode. In addition, uses other than as a counter electrode will be explored as part of our active pursuit of commercialization of the new material. We plan to set up a new venture business next year in cooperation with Division for Start-ups, Research and Innovation Promotion Headquarters, AIST.