## 2 Principle of a Dry-Type MCF Rubber Solar Cell  
A compound MF, consisting of other metal particles such as Ni, of size of the order of 1 \(\upmu\)m is an MCF, one of the intelligent fluids responsive to a magnetic field. The MCF in the present study is a colloidal fluid composed of 10 nm Fe\({}_{3}\)O\({}_{4}\) particles coated with oleic acid having 5 nm thickness for uniform dispersion because MF consists of Fe\({}_{3}\)O\({}_{4}\) particles coated with oleic acid. Each Fe\({}_{3}\)O\({}_{4}\) particle coated by oleic acid is dispersed in a solvent of water. The configuration of the oleic acid is the same as the one of ordinary magnetic fluid (MF). Due to the hydrophilic group of the oleic acid, whose molecule is easily wetted by water, it is effective for water diffusion. Therefore, MCF rubber liquid has water-based solvent so that the oleic acid is stable in the solvent. We can consider three parts of the MCF rubber solar cell as shown in Figure 1a-c. The configuration of cross-linked polyisoprene on the electrolytically polymerized MCF rubber, compounded with natural rubber (NR-latex), as shown in Figure 1b [7], was elucidated in a previous study [14]. NR-latex is ordinary rubber with greater elasticity and compressibility, so that the H-Skin conforms to the purpose of the present study [7]. It must be understood that the reactions shown in Figure 1a-c occur at the microscopic level among the particles of Fe\({}_{3}\)O\({}_{4}\) and Ni, and the molecules of  
Figure 1: Model of electrolytically polymerized MCF solar cell rubber vulcanized in a magnetic field: (**a**) response at the area facing anode side with transit from (**a-1**) to (**a-2**) which is generated by irradiation; (**b**) response at the area between anode and cathode sides which is shifted from (**b-1**) to (**b-3**) as built-in voltage and current; (**c**) response at the area facing cathode side with transit (**c-1**) which is generated by irradiation. polyisoprene, which are dispersed in a water solvent. When a magnetic field is applied, the Fe\({}_{3}\)O\({}_{4}\) particles play a bonding role among the metal particles of Ni, causing numerous Ni and Fe\({}_{3}\)O\({}_{4}\) particles to aggregate into a magnetic cluster of needle-like shapes as shown in Figure 1b [7], which can be ascertained by using the technique of extraction of magnetic clusters from magnetic responsive intelligent fluids, devised in 2003 [15]. The existence of magnetic clusters in the MCF rubber facilitates the development of a solar cell with a bulk-heterojunction of alternating junctions of multi-layered donor and acceptor materials for higher solar cell efficiency, unlike the case of ordinary organic thin-film solar cells in which it is difficult to obtain the configuration. When the NR-latex compounded with MCF is electrolytically polymerized in a magnetic field applied in the same direction as the electric field lines, the polyisoprene NR-latex molecules align along the magnetic field lines and crosslink each other. This is due to the induced electrochemical reaction occurring around the anode, given by Equation (1), analogous to the chemical analysis by X-ray photoelectron spectroscopy (XPS) and so on, and the electrochemical analysis by oxidation-reduction potential (ORP) and pH [14]. On the other hand, the oleic acid and polyisoprene are crosslinked, as shown in Figure 1b, by the electrochemical reaction given by Equation (2), which occurs around the cathode. Three types of dry-type MCF rubber solar cell were investigated: (A) MCF rubber without photosensitized dye molecules, TiO\({}_{2}\) as an electron transport material or KI + I\({}_{2}\) (which is mixed by potassium iodide (KI) and iodine (I\({}_{2}\))) as an electrolyte, that are commonly used in ordinary dye-sensitized solar cells [11; 12]; (B) MCF rubber with just TiO\({}_{2}\); and (C) MCF rubber with dye and KI + I\({}_{2}\) as well as TiO\({}_{2}\).