## 1 Introduction  
Ordinary solar cells are very difficult to bend, squash by compression, or extend by tensile strength. This is because they are solid-state devices, generally made of silicon systems including nanocrystalline or non-crystalline chemical compounds, or organic polymer films, such as plastic polymers. Recently, studies on flexible solar power generators have attracted attention [1; 2; 3]; however, they have remained within the realm of solid-state materials, distinct from flexible materials, such as rubber. For example, they have been used to fabricate a thin solid film, even though they have no particular propensity for expansion or compaction in order to be characterized as bending materials; the amount of research on rubber-type solar cells remains limited [4; 5]. The effect of rubber properties on simultaneously appearing photovoltaics and piezoelectricity are not yet reported. These effects have been investigated separately. If solar cell materials could be developed with elastic, flexible, and extensible properties, and could also exhibit piezoelectricity, they would be extremely effective for use as artificial skin installed over human-like robots or humanoids. The casing could generate electric power from solar energy and perceive all force or temperature changes. Other varied engineering applications would also become feasible. In addition,such hybrid properties of both photovoltaics and piezoelectricity, or piezoresistivity would not require a power supply or batteries. This new type of artificial skin, having all the above properties, has been named \"hybrid skin (H-Skin)\". Human skin, humanoid skin, or outer layer of robot skin made of artificial material integrated with multiple functionalities of flexibility and sensing modalities to force, temperature and so on, have been termed sensitive skin, smart skin, or electronic skin (E-Skin) [6]. The difference between H-skin and E-Skin is that the former has the functionality of photovoltaics, while the latter does not. Since H-skin also has piezoelectric properties, it can use solar power for self-sensing by utilizing the generated Piezo effect. The novel artificial skin is effective in futuristic applications for humanoid and the outer layer of robot skin. Its fabrication and properties are detailed below. In the process of development of an elastic solar cell, a new method was used for vulcanizing natural rubber by electrolytic polymerization, together with a configuration of magnetic clusters of metal particles incorporated into the rubber by the application of a magnetic field [7]. This material is named magnetic compound fluid (MCF) rubber, which was reported in a previous study. As for MCF, many engineering applications in dampers and polishing machines have been proposed with MCF as other applications [8], since it is an effective magnetic-responsive intelligent fluid, unlike magnetic fluid (MF) and magneto-rheological fluid (MRF), which were devised in 2001. MCF rubber is not only elastic, flexible, and extensible, but it is also sensitized to piezoelectricity and piezoresistivity [9], where the piezoelectricity is due to the built-in voltage generated in the material due to change in the distance between the ions in the material under compression. The piezoresistivity is attributed to the resistance of the material caused by the change in electric current due to the application of voltage under compression. This mechanism results from the conductivity of the material. The production methods and power-generation principles of ordinary solid-state solar cells, like photoexcitation based on p- and n-type semiconductors, photosensitized dye molecules and so on, are relevant and effective for the preparation of an elastic and extensible solar cell with rubber. It is not intrinsic semiconductors, but doped semiconductors, which are used for preparing the solar cell. By the doping process, the p- and n-type semiconductors are ionized.