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. The former is an acceptor, A, which is charged negatively by accepting an electron (denoted by A
−); the latter is donor, D, which is charged positively by giving an electron (denoted by D
+). As the MCF rubber has A
− and D
+ ions by electrolytic polymerization, it attains photovoltaic properties, as shown by the electrolytic polymerization reaction in the following section. The photosensitized dye molecules yield two types of MCF rubber solar cells: The dye displays photoexcitation or photoactivity when it is compounded with MCF rubber, which on vulcanization results in arid MCF rubber solar cell (dry type); when the dye seeps into the electrolytically polymerized MCF rubber, the photovoltaics are generated by a redox reaction, then the MCF rubber solar cell is wet (wet type). The former is closer to the concept of a perovskite-type solar cell [
10] and is dealt with in this report, while the latter refers to a novel dye-sensitized solar cell [
11,
12] or Gratzel-type solar cell [
13] and will be discussed later. These principles and concepts are useful for the production of the MCF rubber solar cell suggested in this study.
In the present study, the method of production of soft solar MCF rubber having photovoltaic and piezoelectric properties and using a chemical-physical model, was investigated. In the beginning, the principle of generation of photovoltaics in MCF rubber solar cell is described. Next, the photovoltage and current due to photoexcitation, based on p- and n-type semiconductors resulting from the electrolytic polymerization of MCF rubber, by doping or due to the dye, were measured. The effect of compression on the properties of the dry-type MCF rubber solar cell was studied. The photovoltage, photocurrent density, and piezoelectric sensing were measured to ascertain the photovoltaics due to irradiation by visible light under compression.
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 μm 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
3O
4 particles coated with oleic acid having 5 nm thickness for uniform dispersion because MF consists of Fe
3O
4 particles coated with oleic acid. Each Fe
3O
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
3O
4 and Ni, and the molecules of polyisoprene, which are dispersed in a water solvent. When a magnetic field is applied, the Fe
3O
4 particles play a bonding role among the metal particles of Ni, causing numerous Ni and Fe
3O
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. These are based on the following concepts: (A) polyisoprene exhibits photovoltaic effect because C=C bonds display photoexcitation; (B) TiO
2 has the role of an n-type semiconductor as a donor, and that the photovoltaic effect of (B) is the one with which the one of (A) is added; (C) the dye undergoes photoexcitation or photoactivity, K
+ of KI has the role of an n-type semiconductor as a donor, and iodide/triiodide (I
−/I
3−) of KI + I
2 plays the role of a p-type semiconductor as an acceptor, and that the photovoltaic effect of (C) is the one with which the ones of (A) and (B) are added. In each of the types (B) and (C), the dye, TiO
2, and KI + I
2 are compounded in the MCF rubber and electrolytically polymerized to be dried and vulcanized in slightly wetter conditions. In all the types (A), (B), and (C), the solar cell has a cathode of transparent glass coated with a thin TiO
2 layer on which the light is incident, similar to an ordinary dye-sensitized solar cell.
Owing to the incident light at the cathode, a reaction is generated as shown in Equations (3a) and (4), respectively. The former is shown in (a-1) at the anode and the latter in (c-1) of
Figure 1 at the cathode: at (c-1), the electron is given to the cathode and at (a-1), from the anode. As shown in (a-2) of
Figure 1, electrons move away from A
− and pass through the MCF rubber, corresponding to Equation (3b). The terms in Equations (3) and (4) correspond to those in Equations (1) and (2), respectively: D
+Py
− in Equation (4) corresponds to the right-hand side (RHS) of Equation (2), P, to the first and the fourth terms on the left-hand side (LHS) of Equation (2), and D
+, to the second term of the LHS of Equation (2); Py
+Ay
− in Equation (3) corresponds to the first term of the RHS in Equation (1), P, to the first term of the LHS in Equation (1), and A
−, to the second term of the LHS in Equation (1).
In type (A), A− is the negatively ionized polyisoprene P, and polyisoprene is an anion with a radical reaction in the normal state of NR-latex, as shown by Equation (5). In types (B) and (C), A− corresponds to P− and iodide/triiodide (I−/I3−); D+ to the ionized TiO2 and K+.
It can be understood from Equations (1), (2), and (5) that the water molecule is a significant component. NR-latex is a water-soluble rubber and an aqueous medium is used for the solar cell with a transparent glass electrode coated with TiO
2; hence, the effect of water molecule on a photovoltaics is generated in the MCF rubber solar cell, called the Honda–Fujishima effect [
16] designated by Equation (6), with D and A under the catalyst of anionic polyisoprene (as the first term of the RHS in Equation (5)) and radical oleic acid (as Equation (7)). This reaction has been discussed in the field of macromolecular complexes, and because MCF contains metal particles (Ni in the present study), it is similar to a polymer-metal complex. Therefore, the physical interpretation of macromolecular complexes can be assumed according to which the photovoltaic effect of polyisoprene is due to the sequential potential field along C=C bonds, which can be found in other polymer-metal complexes [
17]: the path and orientation of the electron-transfer reaction gives rise to the photoexcitation and is defined by a sequential potential field. The electron-transfer reaction is also caused by other sequential potential fields between D and A, as shown by Equations (3), (4), and (6)—the electron can move easily from higher to lower potential.
This can be explained by using the electron transfer theory in the field of macromolecular complexes. After the dispersion of the particles of Fe
3O
4 and Ni, and molecules of polyisoprene, the interaction among these is varied. When the distance between them is large, the interaction is comparatively weak and the photovoltaic reaction of Equations (3) and (4) can be considered as an outer-sphere electron transfer reaction (OSETR), which means the structural coordination of molecules is not deformed and only the electrons are transferred by the tunneling effect. In contrast, there can be an inter-sphere electron transfer reaction (ISETR) in the case of a comparatively strong interaction among them due to the following reasons: There is an inherent reaction, as shown by Equation (8) indwelling in Equation (1), where Br is given by Equation (9) and RH by the LHS in Equation (5) [
14]. From Equation (8), the anionic isoprene, R, can be considered a bridging ligand and Equation (1) can be considered to be an ISETR, generated on the basis of the interaction of the reactants and mediation of the bridging ligand. On the other hand, Equation (2) is divided into Equations (10) and (11), where anionic oleic acid can be considered a bridging ligand; hence, Equation (2) can be an estimated ISETR. Whether OSETR or ISETR is generated depends on the probability of the distance among the particles of Fe
3O
4 and Ni, and molecules of polyisoprene—OSETR takes place in the case of long distances and ISETR for smaller ones. As numerous particles and molecules are dispersed in a solvent, many such OSETRs or ISETRs must be taken into account.
In type (C), the dye is excited by the incident light and the electron is emitted by the cathode. The ionized dye is reduced by an electron as shown in Equation (12), with sequential reaction as shown in Equation (13). As a result, a photocurrent is generated.
Meanwhile, before illumination, when a microscopic part of the adjacent particles or molecules, as shown in (b-2) of
Figure 1, is viewed, D and A appear to be ionized. A depletion layer area is developed due to the neutralization of the electrons and holes that originated from D and A, respectively; a voltage is generated between D
+ and A
− as built-in voltage and an electric current by the remnants of electrons and holes as built-in current, which is the same phenomenon as that occurring in the case of common piezoelectricity, as shown in (b-3) of
Figure 1. The numerous particles of Ni, Fe
3O
4, and NR-latex, and the molecules of water, acceptor A, and donor D are dispersed in a jumbled state as shown in (b-1) of
Figure 1. Finally, the photocurrents, built-in voltage, and current generated by the above-mentioned reactions were measured indicating the possibility of acquiring both photovoltaics and piezo-sensing. This is shown in the following section on experimental results.
3. Materials and Experimental Methods
Figure 2 illustrates the schematics of the production procedure for an MCF rubber solar cell. A 0.2 g n-type semiconductor TiO
2 and 0.02 g dye sensitizer MK2 (metal-free solar cell dye, C
58H
70N
2O
2S
4, Sigma-Aldrich Japan Co. LLC., Shimomeguro, Japan) were compounded with MCF rubber, which consisted of 0.6 g carbonyl Ni powder, with particles on the order of micrometers and bumps on the surface (No. 123, Yamaishi Co., Ltd., Noda, Japan), 0.45 g water-based MF with 40 wt % Fe
3O
4 (W-40, Ichinen-Chemicals Co., Ltd., Shibaura, Japan), and 1.8 g NR-latex (Rejitex Co., Ltd., Atsugi, Japan). These components were mixed as stated for each type of (A), (B), and (C) viz. A constant electric field was applied at 6 V, and an electric current of 2.7 A was passed between stainless-steel plates with a 1 mm gap for 10 min under atmospheric conditions and application of a 188 mT magnetic field across the liquid. While using MK2, two cases of electrolytic polymerization were considered viz. After electrolytic polymerization without KI + I
2, 0.17 g solution KI + I
2, compounded with 3.3 g I
2 in a solution of 40 g potassium iodide KI, and 60 g water, was poured on one side of the electrolytically polymerized MCF rubber as an electrolyte, and the rubber was electrolytically polymerized again (twice polymerization); MCF rubber was once electrolytically polymerized with KI + I
2 (once polymerization).
The final electrolytically polymerized MCF rubber was around 20 mm × 23 mm × 1 mm in size. It was sandwiched between a transparent glass electrode and a TiO
2-coated one. Visible light (238 Lux) and ultraviolet light (227 Lux) were scattered on the transparent electrode coated with TiO
2. As shown in the bottom left of
Figure 2, a weight was applied on one side of the electrode under the incident light. The rod with a diameter of φ8 mm installed the underneath of the weight was attached to the electrode. Voltage and electric current between the electrodes were measured using a digital multi-meter (PC710, Sanwa Co. Ltd., Fukuoka, Japan). The MCF rubber produced using the present procedure of electrolytic polymerization was of dry type because of dehydration to some extent owing to the heat of the electrolytic polymerization, in contrast to an ordinary dye-sensitized solar cell [
10,
11], where the dye sensitizer and electrolyte are involved in the MCF rubber. Therefore, the present MCF rubber solar cell was different from that in a fluidic state due to the dye and electrolyte being poured on it.