Clarification of Catalytic Effect on Large Stretchable and Compressible Rubber Dye-Sensitized Solar Cells

Abstract

Rubber involving magnetic compound fluid (MCF) and TiO₂ is effective in dye-sensitized solar cells (DSSCs) to create large efficacy. Wearable and portable solar cells made of MCF rubber are the most desirable as soft materials in robots or flexible devices, and they are further desirable because they have self-generated power and power supply with sensing. Therefore, we investigated the effect of TiO₂ catalysts on the photovoltaic effect of MCF rubber DSSCs under large tension and compression. The characteristics of the built-in electricity and photoelectricity were clarified experimentally. The experimental results were explained by a chemical–photovoltaic mechanism involving the behavior of dye, electrolytes, water, and rubber molecules, as well as a catalytic effect of the metal component of the MCF on Ni, Fe₃O₄, and TiO₂. Once we are able to produce solar cells that have large tension and compression, the present experimental results and the model of the chemical–photovoltaic mechanism will be of great interest.

1. Introduction

The problem of global energy consumption is being addressed in order to resolve the continuously ascending consumption of energy that boosts global warming and acid rain. Therefore, solar cells are receiving remarkable attention as a renewable energy technology. Organic solar cells have become expectable, including various types structured with organic or inorganic materials [1,2,3] or with organic or inorganic materials including liquids [4] such as ionic liquid [5,6,7,8,9,10] and dielectric liquid [11]. Above all, dye-sensitized solar cells (DSSCs) [12,13] such as Gratzel-type solar cells [14] and Perovskite-type solar cells [15] are well known to be lightweight low-cost cells. These organic solar cells are viable materials in the creation of flexible engineering instruments that lead to wearable and portable electronic devices, as opposed to rigid-type solar cells made of solid semiconductors such as silicon and metal glass. The wearable and portable devices may include self-powered supply or health monitors [16] and electronic devices embedded in clothes, watches, or glasses [17]. In order to achieve flexibility, not only polymers [18,19,20,21,22] but also thin films [23,24] have been commonly utilized, and fibers [25,26], cloth, or textiles [27] through which electrolytes or dye may be percolated have been proposed. The utilization of rubber is another effective method. However, there have been few studies on the use of rubber, and investigations of silicone rubber (Q) sheets are rare [28,29]. Q in solar cells has been utilized as a substrate of layered materials for photovoltaics, in which case it is not metamorphosed to bring about the photovoltaics [28]. Neither the effects of tension and compression on photovoltaic properties nor experimental data on their correlation have been elucidated [29], due in large part to the lack of clarity regarding the chemical–photovoltaic mechanism and the lack of a high-efficiency profile. In contrast, Shimada proposed suitable solar cells made of rubber involving DSSCs and addressed the cardinal chemical–photovoltaic mechanism [30,31,32,33]. By applying a magnetic compound fluid (MCF), which is a magnetic-responsive fluid compounded with 10 nm sphere magnetite (Fe₃O₄) particles coated with oleic acid and 1 μm metal particles such as Ni or Fe to the rubber, and with electrolytic polymerization under the application of a magnetic field, the MCF rubber is solidified, producing many needle-like magnetic clusters that are aligned along the magnetic field lines, which have the same direction as the electric field [34,35,36,37,38,39]. The magnetic clusters efficiently induce an exchange of ions and electrons between the Fe₃O₄ metal particles and rubber molecules because they have a bulk hetero-type structure inside the rubber. Although DSSCs have dye and electrolytes, even if the MCF rubber does not involve dye and electrolytes, the photovoltaic effect can be created. The rubber must be water-soluble rubber with C=C bonds in order for electrolytic polymerization. Therefore, natural rubber (NR) and chloroprene rubber (CR), which are categorized as diene rubber involving C=C bonds, are optimal. If the rubber is non-diene rubber or a water-insoluble rubber such as Q, the MCF rubber needs to include polyvinyl alcohol (PVA) and a water-soluble rubber such as NR or CR [40,41,42]. For example, if we use Q, which is structured as the basis of dimethylpolysiloxane (PDMS), PVA combines PDMS and isoprene molecules of NR or CR via emulsion polymerization so that the non-diene rubber or water-insoluble rubber can be electrolytically polymerized. The photovoltaic effect is created because Fe₃O₄ metal particles, rubber, and water molecules have p-type and n-type semiconductor-like roles in the form of ionized A⁻ (A is the acceptor) and D⁺ (D is the donor) [30,31]. Naturally, when the MCF rubber involves dye and electrolytes to be electrolytically polymerized, the photovoltaic effect can also manifest as materialized DSSCs. The MCF rubber not only has a photovoltaic effect, but also has sensing capabilities [30,31,32,33] and serves as a capacitor [32]. In addition, the brilliant peculiarity of MCF rubber is that it is largely stretchable and compressible [31]. Regarding previous studies on flexible solar cells utilizing polymers or thin films, the solar cells bend but do not stretch and are, therefore, considered solid films [23,24]. Alternatively, the deformation of the solar cells is so small that the tensile and compressive rates are not comparatively very large. However, MCF rubber can be stretched to twice its original length or compressed to half its initial size. Therefore, wearable and portable solar cells made of MCF rubber are highly promising for use as soft materials in robots and flexible devices, especially because of their self-generated power and power supply with sensing.

As a general technique that has been seen in the case of ordinary solar cells, involving a catalyst is an effective way of enhancing the high efficiency of solar cells. In water-soluble solar cells such as DSSCs, a number of different effective catalysts have been proposed, such as graphite, carbon black, SnO₂ [43], Pt [44], MoS_x [45], MnWO₄, ZnWO₄, and CuWO₄ [46]. TiO₂ is one of many useful catalysts and is effective enough to enhance the photovoltaic effect. Because of the relevant reaction of water on photovoltaics, the chemical–photovoltaic mechanism can be explained by the Honda–Fujishima effect [47]. MCF rubber involving TiO₂ is effective in the type of DSSCs that contain MCF in order to achieve large efficacy. However, the effect of TiO₂ on DSSCs with MCF remains to be clarified, and the chemical–photovoltaic mechanism involving the catalytic effect has not been dealt with.

In the present study, we investigated the effect of TiO₂ as a catalyst on the photovoltaic effect of MCF rubber DSSCs under tension and compression. The experimental results were investigated by using a chemical–photovoltaic mechanism that induces the Honda–Fujishima effect.

2. Effect of TiO₂ on Photovoltaics

2.1. Experimental Procedure

The experimental procedure of fabricating the MCF rubber DSSCs is shown in Figure A1 in Appendix A. As the first step, we fabricated the MCF rubber. The MCF rubber included the n-type semiconductor TiO₂ (titanium with a 10–50 nm particle size, anatase form, Fujifilm Wako Pure Chemical Co., Ltd., Osaka, Japan), 6 g of carbonyl Ni powder with µm ordered and pimple-shaped particles (No. 123, Yamaishi Co., Ltd., Noda, Japan), 4.5 g of water-based magnetic fluid (MF) with 40 wt.% Fe₃O₄ (W-40 with 10 nm ordered sphere particles, Ichinen Chemicals Co., Ltd., Shibaura, Japan), and 9 g of NR latex (Rejitex Co., Ltd., Atsugi, Japan), mixed using a supersonic stirrer (UR-20P, Tomy Seiko Co., Ltd., Tokyo, Japan) for 5 min and with air evacuated for 15 min. During electrolytic polymerization, an electric field with constant 6 V and 2.7 A was applied between stainless-steel plates with a 1 mm gap for 10 min under atmospheric conditions under the application of a constant 188 mT magnetic field created by neodymium permanent magnets between the electrodes. Therefore, the directions of the electric and magnetic fields were the same. The electrolytically polymerized MCF rubbers had an area of almost 20 mm × 23 mm and a thickness of 1 mm. Next, we constructed the MCF rubber DSSCs. First, 0.17 g of electrolytes and 0.06 g of liquid dye were poured onto the surface of the MCF rubber. The electrolytes took the form of a KI + I₂ solution, a mixture of potassium iodide (KI) and iodine (I₂) (both from Fujifilm Wako). KI contains 3.3 g of iodine I₂ in a solution of 40 g of potassium iodide KI and 60 g of water. The dye is based on ruthenium complexes PEC-TOM-P04 (Peccell Technologies Co., Ltd., Yokohama, Japan). The use of ruthenium dye is ordinary in DSSCs [48]. As shown in Figure A1 (Appendix A), the electrolyte solution was deposited on one side of the MCF rubber at the cathode during electrolytic polymerization (a in Figure A1, Appendix A), and the dye on the other side was deposited at the anode (b in Figure A1, Appendix A). The surface of side a in Figure A1 (Appendix A) was in contact with a transparent glass to serve as an anode of the solar cells, and the surface of side b in Figure A1 (Appendix A) was in contact with a transparent glass coated with TiO₂ as a cathode of the solar cell; the MCF rubber was sandwiched between the transparent glasses (20 mm × 30 mm), as shown in Figure 1. The transparent electrode coated with TiO₂ was illuminated by visible light (238 Lux) or ultraviolet light (227 Lux). The voltage, electric current, and electrical resistance between the electrodes were measured using a digital multimeter (PC710, Sanwa Supply Co., Ltd., Okayama, Japan).

The surface of the electrolytically polymerized MCF rubber is shown in Figure 2. The cathode side (Figure 2b,d) is typical and shows that the more TiO₂ the MCF rubber has, the harder it is to compound TiO₂. The compounded condition of TiO₂ affects the photovoltaics, as shown in the later figures in the next section.

The MCF rubber involving TiO₂ has larger stretchability than commercial pressure-sensitive electrically conductive rubber (PSECR), as shown in our previous study [31]. DSSCs made of MCF rubber involving TiO₂ are the most desirable as soft materials in robots or flexible devices, enough to be wearable and portable.

2.2. Experimental Photovolatics

Figure 3 shows the voltage, electric current, and electrical resistance as a function of the amount of TiO₂. In the images, “on” with a solid arrow indicates the onset of illumination and “off” with a dotted arrow indicates the termination of illumination. Voltage, electric current, and electrical resistance were measured separately. However, the tendencies of their correlation were obtained and were found to be larger with ultraviolet light than with visible light; that is, MCF rubber DSSCs reacted largely to ultraviolet light.

Under illumination, the electrical resistance increased at the same time that the voltage and electric current increased as a photovoltaic effect. When the illumination was turned off, the electrical resistance decreased at the same time that the voltage and electric current decreased. The electrical resistance under illumination is relevant to microscopic behavior; extraneous electrons cannot be transferred among ionized particles and molecules. Therefore, when photoelectricity was enhanced by illumination, the electrical resistance increased. This behavior is further discussed below.

In some cases, when the sample was first illuminated and the illumination was then turned off, it can be seen that the electric current changed rapidly and then returned smoothly to constant values; this occurred, for example, with the 3 g TiO₂ sample under visible light and the 4 g TiO₂ sample under ultraviolet light. This was due to dark current or leak current of the solar cells. The incidence of this phenomenon is subject to the varied conditions of the surface of the transparent glass and the MCF rubber during the fabrication of the solar cells, the ambience during fabrication, etc. Furthermore, it is impossible to predict the incidence; thus, certain precautionary measures must be considered. However, since this problem is often seen in ordinary solar cells, the same preemptive methods might be carried out.

Incidentally, the surface of MCF rubber is dark brown, as shown in Figure 1, so that it can absorb light more effectively.

Figure 4 shows the relationship between the electric current I and voltage V measured using a potentiostat (HA-151B, Hokuto Denko Co., Ltd., Tokyo, Japan) at 50 mHz scan rates in the potential domain of −1.5 to 1.5 V. As TiO₂ increased, the area surrounded by I and V decreases, and I in particular became larger at the largest V. This means that the MCF rubber solar cell was close to the I–V curve of a photodiode [49,50]. On the other hand, our previous study [32] clarified the characteristics of the photodiode under greater tension or compression. MCF rubber solar cells can typically be used as photodiodes. In order to make the characteristics of MCF rubber solar cells more similar to those of photodiodes, more TiO₂ might be used, as shown in Figure 4.

The area of the I–V curve connotes the excited-charge regeneration. We can guess the effect of TiO₂ on the rate of excited-charge regeneration in the MCF rubber DSSCs. The capacity of the excited-charge regeneration might have an optimal amount of TiO₂.

Regarding the general aspects of ordinary photodiodes and solar cells, photodiodes have a longer response time to light scattering than solar cells. In addition, photodiodes might involve dark current. Therefore, dark current also occurs in MCF rubber, as indicated for the 3 g TiO₂ in Figure 3b and the 2–5 g TiO₂ in Figure 3e.

As shown in Figure 5, focusing on the correlation among the particles of Fe₃O₄, Ni, and TiO₂, and the molecules of rubber, water, and a mixture of potassium iodide (KI) and iodine (I₂) electrons were transferred so that the particles and molecules were ionized, corresponding to A⁻ and D⁺ [30,31]. The electrons and holes induced by the transfer of the electrons were mobile such that photoelectricity (photocurrent and photovoltage) was generated by a short circuit created by the photocurrent induced by the transfer of the electrons and holes and the photovoltage induced between the electrons and holes [30,31,34]. On the other hand, the ionized particles and molecules were static and without a mobile state, with the result that the built-in electricity (built-in voltage and built-in current) was created; the voltage between them induced the built-in voltage, while the apparent moving distance between them induced the built-in current. Therefore, MCF rubber also exhibited sensing as a piezo element whose excitement was generated by the built-in voltage reacting to pressure, such that it was a hybrid with both sensor material and solar cell material [31].

The built-in electricity is relevant to the electrical resistance inasmuch as extraneous electrons cannot transfer among ionized particles and molecules via the application of voltage from an outer power supply. In general, in the case of the application of extraneous voltage, electricity as voltage and electric current is categorized in the field of piezo-resistivity, as is built-in electricity. We investigated the relationships among photoelectricity, built-in electricity, and piezo-resistivity in order to clarify the relationship between sensor and solar cells in MCF rubber. Figure 6a shows the changes in electrical resistance by pressure, which corresponds to piezo-resistivity. The state at the initial zero pressure corresponds to built-in electricity. As pressure increases, the electrical resistance decreases because electrons transfer easily between the ionized particles and molecules by decreasing the distance between them. Under the greatest pressure, the electrical resistance is saturated and constant because the distance has reached its finite limit. The electrical resistance values at the initial zero pressure and at the greatest pressure, as well as their difference, are shown in Figure 6b. The experimental apparatus was the same as that in our previous studies [34,35,36,37]. The voltage under the electrical resistance of 1.8 kΩ was measured under the application of a 10 V electric field via the power supply to the electric circuit with the MCF rubber solar cells connected to the 1.8 kΩ resistance. The MCF rubber solar cells were pushed between the two 7 mm stainless-steel square plates. The upper plate was moved to touch the lower plate via an actuator at a pressing speed of 10 mm/min. The actuator was operated by a small SL-6002 automatic measuring tensile testing machine (IMADA-SS Co., Ltd., Toyohashi, Japan). As TiO₂ increased, the electrical resistance at the initial zero pressure decreased. However, the electrical resistance under the greatest pressure increased. Therefore, their difference decreased. Thus, the electrical resistance depended on the amount of TiO₂.

In Figure 6c,d, the built-in electricity and photoelectricity are those shown in Figure 3a. As TiO₂ increased, the built-in electricity decreased and the photoelectricity increased. The electrical resistance values of both the built-in electricity and the photoelectricity are shown in Figure 6e. As TiO₂ increased, the electrical resistance of the built-in electricity decreased, while that of the photoelectricity first decreased and then increased. Thus, the catalyst TiO₂ affected photoelectricity.

Lastly, we compared the MCF rubber to PSECRs that have been used in the fields of robot sensing and sensors and were the same as those used in our previous studies [31,34]. These rubbers were made of NR and had an area of almost 28 mm × 21 mm and a thickness of 1 mm. The KI + I₂ electrolyte solution and PEC-TOM-P04 dye were poured onto the surface of the rubbers following the same procedure as that described for the MCF rubber (Figure 1). In Figure 7, as in Figure 3, the solid and dotted arrows indicate the onset and termination of illumination, respectively. In contrast to the present study, there was a unique investigation of DSSCs with Q compounded by carbon black or graphite powder [29]; however, it presented only experimental data on the I–V relationship without clarifying the photovoltaic characteristics.

The voltage, electric current, and electrical resistance in the case of PSECR were smaller than those in the case of MCF rubber under both visible and ultraviolet light. This indicates that the photovoltaic effect of MCF rubber was larger than that of PSECR.

2.3. Chemical–Photovoltaic Mechanism

2.3.1. Case of Photovoltaic Mechanism by Dye and Electrolyte

The ordinary chemical–photovoltaic mechanism of DSSCs is well known and can be expressed as Equations (1)–(5).

At the cathode, the dye (Dye) is ionized to Dye⁺ with TiO₂ coated on the transparent glass by illumination. Electrons from the dye enter the cathode through TiO₂, which serves as an electron transport body.

At the anode, the electrolyte reacts as shown in Equation (1) by obtaining electrons from the anode. A part of $I_3^{-}$ reacts as shown in Equation (2).

$$3I_2+2e^{-}\to 2I_3^{-}$$

(1)

$$2I_3^{-}+2e^{-}\to 3I^{-}.$$

(2)

$I_3^{-}$ on the right side of Equation (1) transfers to the cathode and reduces the ionized Dye⁺ at the cathode as shown in Equation (3). The dye on the right side of Equation (3) is ionized to Dye⁺ at the cathode again as above. I₂ on the right side of Equation (3) transfers to the anode and reacts as shown in Equation (1).

$$2Dy{e}^{+}+2I_3^{-}\to 2Dye+3I_2.$$

(3)

On the other hand, at the cathode, I⁻ on the right side of Equation (2) reacts via photoexcitation as shown in Equations (4) and (5) such that the electrons enter the cathode. I₂ on the right hand of Equation (5) transfers to the anode and reacts as shown in Equation (1).

$$3I^{-}\to I_3^{-}+ 2e^{-}.$$

(4)

$$2I_3^{-}\to 3I_2+2e^{-}.$$

(5)

The above chemical–photovoltaic mechanism can be applied to ordinary DSSCs which are wet-type solar cells with liquid electrolyte and dye without any solid material. However, we can obtain the photovoltaic effect even if we use a conventional rubber sandwiched between the dye and the electrolyte solution, as shown in Figure 7 and in a previous study by Muller [29]. As a liquid, in general, cannot be percolated through a rubber, the creation of the photovoltaics might be due to the electrons and ions transferring over the rubber surface. In contrast, in the case of the MCF rubber DSSCs, a liquid can be percolated through MCF rubber as shown in Figure A2 in Appendix A, as presented in our previous study [41]. Therefore, the above chemical–photovoltaic mechanism occurs within the MCF rubber.

2.3.2. Case of Electrical–Chemical Mechanism before Electrolytic Polymerization and without Light Scattering

From our water-soluble rubber in the form of NR or CR structured by polyisoprene molecules in the MCF rubber, one unit of isoprene molecules is defined as SH with presenting S, as shown in Equation (6).

.

(6)

Without electrolytic polymerization, SH is anionized to S⁻, as shown in Equation (7), via the Honda–Fujishima effect [30,47].

$$SH\leftrightarrow {\left(SH\right)}^{*}\leftrightarrow {\left(S^{-}{H}^{+}\right)}^{*}\leftrightarrow \left(S^{-}{H}^{+}\right)\leftrightarrow S^{-}+H^{+}.$$

(7)

S⁻ generates a radical ^● as shown in Equation (8) via some influences such as illumination.

$$S^{-}\to S^{●}+e^{-}.$$

(8)

Water molecules are ionized as shown in Equation (9) [51].

$$H_{2}O\to H^{+}+O{H}^{-}.$$

(9)

The generated radical OH⁻ becomes a hydroxyl radical OH^● as shown in Equation (10) due to certain influences.

$$O{H}^{-}\to O{H}^{●}+e^{-}.$$

(10)

2.3.3. Case of Electrical–Chemical Mechanism by Electrolytic Polymerization and without Light Scattering

The reaction occurs as shown in Equation (11) at the anode via electrolytic polymerization [38].

$$SH+S^{-}\to SS+H^{●}+e^{-}.$$

(11)

SS denotes cross-linking between isoprene molecules. The reaction occurs as shown in Equation (12) at the cathode, when the oleic acid coating around Fe₃O₄ is defined by P as shown in Equation (13) [38].

$$P+H^{+}+S^{-}+e^{-}\to SPH+e^{-}.$$

(12)

(13)

The electrons on the right side of Equations (10) and (11) transfer into the anode. The electron on the left side of Equation (12) transfers from the cathode and that on the right side transfers into the anode. Their transfers are delineated as shown in Figure 8a, which shows the electrical–chemical model of electrolytic polymerization. SPH denotes cross-linking between isoprene and oleic acid molecules. The hydroxyl radical OH^● is unstable enough to have a powerful oxidizing effect and, therefore, reacts with radical hydrogen H^●, as shown in Equation (14).

$$O{H}^{●}+H^{●}\to H_{2}O.$$

(14)

2.3.4. Case of Photovoltaic Mechanism after Electrolytic Polymerization and with Light Scattering

In the case of solar cells under scattered light, reverse reactions of Equations (11) and (12) are generated as shown in Equations (15) and (16) under the outer-sphere electron transfer reaction (OSETR), which means that the structural coordination of molecules is not deformed and only the electrons are transferred by the tunneling effect [30].

$$SS+H^{●}+e^{-}\to SH+S^{-}.$$

(15)

$$SPH+e^{-}\to P+H^{+}+S^{-}+e^{-}.$$

(16)

The electron on the left side of Equation (15) transfers from the anode. The electron on the left side of Equation (16) transfers from the anode and that on the right transfers into the cathode. S⁻ on the right side of Equations (15) and (16) generates the reaction shown in Equation (8), and then the electron on the right side of Equation (8) transfers into the cathode. These transfers are delineated as shown in Figure 8b, which depicts the chemical–photovoltaic mechanism of solar cells.

2.3.5. Case of Photovoltaic Mechanism by Catalyst Effect of TiO₂, Ni, and Fe₃O₄.

Next, let us consider the catalyst effect of TiO₂, Ni, and Fe₃O₄. Due to these catalysts, Equations (17) and (18) are created.

$$4H^{+}+H_{2}O\to 2H_2+O_2+4H^{+}.$$

(17)

$$2S^{-}+2H_{2}O\to 2S+H_2+2O{H}^{-}.$$

(18)

H⁺ on the right side of Equation (17) creates Equation (19) and then Equation (14). The electron on the left side of Equation (19) transfers from the anode. OH⁻ on the right side of Equation (18) creates Equation (10).

$$H^{+}+e^{-}\to H^{●}.$$

(19)

Additionally, upon electrolytic polymerization, the hydroxyl group of the oleic acid reacts with Ni as shown in Equations (20) and (21).

.

(20)

.

(21)

SPH is created from the right side of Equation (21) through Equation (12), and Ni²⁺ on the right side of Equation (20) is generated as shown in Equation (22) [52].

$$2N{i}^{2+}+2H_{2}O\to 2Ni+O_2+4H^{+}.$$

(22)

H⁺ on the right side of Equation (22) creates Equation (19) and then Equation (14); the electron on the left side of Equation (19) transfers from the cathode via electrolytic polymerization and without light scattering; the electron on the left side of Equation (19) transfers from the anode via electrolytic polymerization and with light scattering.

On the other hand, upon electrolytic polymerization, Fe₃O₄ is generated as shown in Equations (23)–(27), where Fe (II) is FeO, Fe (III) is Fe₂O₃, and O₂^●− indicates superoxide radicals [53].

$$Fe\left(\mathit{II}\right)+H_2{O}_2\to Fe\left(\mathit{III}\right)+O{H}^{●}+O{H}^{-}.$$

(23)

$$Fe\left(\mathit{III}\right)+H_2{O}_2\to F{e}^{-}{\left(OOH\right)}^{2+}+H^{+}.$$

(24)

$$Fe\left(\mathit{III}\right)+H_{2}O\to {\left[FeOH\right]}^{2+}+H^{+}.$$

(25)

$$Fe\left(\mathit{II}\right)+O{H}^{●}\to Fe\left(\mathit{III}\right)+O{H}^{-}.$$

(26)

$$Fe\left(\mathit{II}\right)+O_2\to Fe\left(\mathit{III}\right)+O_2^{●-}.$$

(27)

In cases in which ultraviolet light is applied, Equation (28) is generated, followed by Equation (25). Then, Equation (26) is generated.

$${\left[FeOH\right]}^{2+}\to Fe\left(\mathit{II}\right)+O{H}^{●}.$$

(28)

OH⁻ in Equations (23) and (26) creates Equation (14) through Equation (10); the electron on the right side of Equation (10) transfers into the anode via electrolytic polymerization and without light scattering; the electron on the left side of Equation (19) transfers into the cathode via electrolytic polymerization and with light scattering. H⁺ in Equations (24) and (25) creates Equations (12) and (19). H^● generated by Equation (19) creates Equation (14); the electron on the left side of Equation (19) transfers from the cathode via electrolytic polymerization and without light scattering; the electron on the left side of Equation (19) transfers from the anode via electrolytic polymerization and with light scattering.

Regarding other aspects, when the light is irradiated, the reactions below are generated with the TiO₂ catalyst [54,55,56,57,58,59]. First, hole h⁺ and an electron are created from TiO₂ as shown in Equation (29) [60].

$$Ti{O}_2\to h^{+}+e^{-}.$$

(29)

This electron is scavenged by oxygen, and then superoxide radicals O₂^●− are generated as shown in Equation (30) [61,62].

$$e^{-}+O_2\to O_2^{●-}.$$

(30)

Hole h⁺ oxidizes molecules such as rubber or oleic acid, which are defined as R or R-H, as shown in Equation (31), and reacts with water as shown in Equations (32) and (33).

$$h^{+}+ R\to R^{+●}.$$

(31)

$$4h^{+}+H_{2}O\to \frac{1}{2}{O}_2+2H^{+}.$$

(32)

$$h^{+}+H_{2}O\to O{H}^{●}+H^{+}.$$

(33)

Hydroxyl radical OH^● in Equation (33) oxidizes R-H and reacts with H^● as shown in Equation (14). As a result, molecules such as rubber or oleic acid are oxidized as shown in Equations (31) and (34) [63].

$$O{H}^{●}+ R -H \to {R^{\prime }}^{●}+H_{2}O.$$

(34)

The superoxide radicals O₂^●− on the right side of Equation (30) are protonated, which means that proton H⁺ is ionized, thereby generating hydroperoxyl radicals HO₂^● as shown in Equation (35).

$$O_2^{●-}+H^{+}\leftrightarrow H{O}_2^{●}.$$

(35)

Hydrogen peroxide H₂O₂ is generated as shown in Equation (36) from the hydroperoxyl radicals HO₂^● on the right side of Equation (35).

$$2H{O}_2^{●}\to H_2{O}_2+O_2.$$

(36)

In addition, hydrogen peroxide H₂O₂ is generated as shown in Equation (37) from the hydroperoxyl radicals O₂^●− on the right side of Equation (30); the electron in Equation (37) is scavenged by O₂^●−.

$$2N{i}^{2+}+2H_{2}O\to 2Ni+O_2+4H^{+}.$$

(37)

Summarizing from the above electrochemical reactions in Equations (4) and (5), in the case of light scattering, the creation of the photocurrent can be considered to correspond to the generating electrons as follows:

(a)

As shown in Equations (15) and (16), electrons transfer. Electrons are generated not only as shown in Equation (8) through S⁻ in Equations (15) and (16) but also as shown in Equation (19) through H⁺ in Equation (16). This chemical–photovoltaic mechanism is irrelevant to the behavior of the rubber molecule.

(b)

The operations expressed in Equations (19) and (21) are facilitated by the catalyst effect of TiO₂, Ni, and Fe₃O₄ through the operations shown in Equations (17), (20), and (22). The ones expressed in Equation (10) are also facilitated through the one shown in Equation (18). This chemical–photovoltaic mechanism is irrelevant to the catalyst behavior of TiO₂, Ni, and Fe₃O₄.

(c)

Equations (30) and (37) show that electrons are scavenged. This induces the reduction of photovoltaic current. In addition, not only Equations (17) and (18) but also Equations (9), (10), (14), and (19) show that the electrons are generated by water with an aiding catalyst. These summarized results provide the complicated changes in photovoltaic current. This chemical–photovoltaic mechanism is relevant to the behavior of correlation between TiO₂ catalyst and H₂O.

Thus, the photocurrent is created by summarizing the abovementioned operations (a–c) so that the photoelectricity is created as shown in the experimental data (e.g., Figure 7).

Incidentally, the photovoltaics of MCF rubber DSSCs are related to the integrated correlation as shown in many equations. The behavior of redox reactions of iodide and triiodide as shown in Equations (1) and (2) with respect to the overall reactions can be guessed via the comparison between Figure 3d,e and the results in our previous study [30] without using dye and electrolyte, as shown in Figure A3 in Appendix A. The role of iodide and the triiodide redox reaction is not diminutive.

Furthermore, concerning the chemical–photovoltaic mechanism, we could proceed with the photovoltaic theorem by using the theoretical formula of an equivalent electric circuit and tunneling theory as shown in our previous study [32] with conjugating Equations (1)–(37) in order to clarify the catalyst effect of TiO₂, Ni, and Fe₃O₄, although we do not present that here.

3. Effect of Tension and Compression

3.1. Experimental Procedure under Tension and Compression

We used the same experimental apparatus as in our previous studies [31] and as shown in Figure A4 in Appendix A. This apparatus provides tension and compression of MCF rubber DSSCs, as shown in Figure A1 (Appendix A), with 0.17 g of poured electrolyte solution and 0.06 g if poured liquid dye. The MCF rubber was elongated vertically by a small SL-6002 automatic measuring tensile testing machine (IMADA-SS) and, at the same time, was compressed by a thickness gauge transverse to the rubber, which was sandwiched by pieces of transparent glass (20 mm × 30 mm). For the compression, the irradiated glass coated with TiO₂ was compressed by a cylinder with a φ 6.5 mm connected to the thickness gauge. Therefore, the irradiated area of the MCF rubber was the area that remained outside its circular area. The irradiation light was ultraviolet (40 Lux). The voltage and electric current between the electrodes were measured using a PC710 digital multimeter (Sanwa). The set-up is depicted in Figure 9. In the experiment, the MCF rubber was elongated not more than twice by tension and not more than half as many times by compression.

3.2. Results and Discussion

First of all, we clarified the length of conjugated double bond of MCF in our previous studies [34,38] as shown in Figure A5 in Appendix A, which is an example of electric current and tension. The length of the conjugated double bond lengthens via electrolytic polymerization and the application of a magnetic field. Therefore, through these factors, the photovoltaics are enhanced.

The effects of compression and tension on the built-in electricity and photoelectricity (for voltage and current density, respectively) and the electrical resistance corresponding to the built-in electricity and photoelectricity are shown in Figure 10, Figure 11 and Figure 12. These effects are presented as changes from the initial state (with neither compression nor tension) due to compression or tension. Here, the MCF rubber contained 3 g of TiO₂, 6 g of Ni, 4.5 g of MF, and 9 g of NR latex. Where the compressive strain changed, the tension or tensile strain remained constant under the compression. Where the tensile strain changed, the pressure remained steady under the tension. If we consider the effect of either compression or tension individually, the case at 0 mm tensile stress or 0 Pa pressure is indicated in the figures. In the cases of voltage and electric current, the compressive strain–pressure relationship and the tensile strain–stress relationship are also shown (Figure 10 and Figure 11). While the former was only a linear relation, the latter had linear elastic and plastic regions.

Built-in voltage and current under increasing compression and tension tended to first increase and then decrease and remain constant. The enhancement occurred with the smallest compression or tension, and, in the case of tension, it occurred in the inner linear elastic region. On the other hand, while photovoltage and current under increasing tension showed the same tendency, under increasing compression, they showed a different tendency, without the initial increase. In contrast, regardless of condition, electrical resistance tended to show the first pattern, that is, an initial increase, followed by a decrease, after which values held constant. Figure 13 shows a microscopic model that explains these results, with electrons and holes bringing about photoelectricity and ions bringing about built-in electricity (Figure 5). The behaviors of the particles of Fe₃O₄, Ni, and TiO₂, and the molecules of rubber, water, a mixture of potassium iodide (KI) and iodine (I₂), and electrons as shown in Figure 5 are presented in Figure 13, and their configurations are different from those of ceramic and metal. Although the rubber prevented the particles and the molecules in Figure 13 from moving, the position of the particles and the molecules could move elastically in accordance with the rubber’s deformation via compression or tension. In the case of compression, as shown in Figure 13a, because the distance between the electron and the hole or between ions was narrowed, the voltage and electric current were increased. Under sequentially greater compression, the electron and hole became contiguous such that the voltage and electric current were zero, and, at further compression, the situation of the electron and hole was inverse such that the voltage and electric current attained negative values. On the other hand, in the case of tension (Figure 13b), because the distance between the electron and the hole or between ions was widened, the voltage and electric current decreased. However, the opposite situation could be considered to exist; that is, an adjacent electron and hole (“A” in Figure 13b) moved closer to each other due to reduced deformation transverse to the direction of tension. Therefore, the voltage and electric current potentially increased once.

Next, we investigated the effects of a TiO₂ catalyst on built-in electricity and photoelectricity under compression and tension, as shown in Figure 14, Figure 15, Figure 16 and Figure 17. Here, the MCF rubber contained 2, 4, or 5 g of TiO₂, 6 g of Ni, 4.5 g of MF, and 9 g of NR latex. As in Figure 10, Figure 11 and Figure 12, the tension remained constant under compression, and the pressure remained constant under tension. Changes due to compression or tension are shown as differences from the initial state (with neither compression nor tension).

Under compression, the built-in current tended to show values that changed from positive to negative as TiO₂ increased, while, in the case of photocurrent, negative values tended to decrease as TiO₂ increased. These results indicate that the enhancement of TiO₂ reduced the built-in current and photocurrent by compression. This can be explained by the model in Figure 13a, where ionized particles increased with increasing TiO₂, and the adjacent ionized particles approached each other and became contiguous such that the voltage and electric current became zero due to compression.

On the other hand, under tension, built-in current tended to show values that changed from negative to positive as TiO₂ increased, while positive photocurrent values tended to decrease to zero with increasing TiO₂. These results indicate that the enhancement of TiO₂ increased built-in current values but decreased photocurrent values due to tension. This can be explained by the model in Figure 13b, where the adjacent ionized electron and hole (“A” of built-in electricity in Figure 13b) approached each other due to the reduced deformation under tension such that the voltage and electric current were enhanced; however, the adjacent electrons and holes that emerged from neutral particles and molecules (“A” of photoelectricity in Figure 13b) approached each other such that the voltage and electric current became zero due to tension.

4. Conclusions

As TiO₂ increases, the area surrounded by the I–V curve becomes small and MCF rubber DSSCs serve almost as photodiodes. Electrical resistance depends on the amount of TiO₂. As TiO₂ increases, the electrical resistance as piezo-resistivity at the initial zero pressure decreases; however, the electrical resistance under the greatest pressure increases and, therefore, the difference decreases.

Built-in voltage and current under increasing compression and tension tend to first increase, then decrease, and then remain steady. The increase occurs under the smallest amount of compression or tension and, in the case of tension, occurs in the linear elastic region. Photo voltage and current under increasing tension show the same tendency as above; however, under increasing compression, they do not show the initial increase in values. In contrast, regardless of condition, electrical resistance tends to first increase, then decrease, and then remain constant. Regarding compression, increasing TiO₂ reduces the changes in built-in current and photocurrent, while, in the case of tension, increasing TiO₂ increases the change in built-in current but reduces the change in photocurrent.

The particles and molecules in MCF rubber DSSCs are ionized, corresponding to A⁻ and D⁺. In addition, the present experimental results regarding the effect of TiO₂ on photovoltaics can be explained by the chemical–photovoltaic mechanism involving the behavior of dye, electrolyte solution, water, and rubber molecules and the catalytic effect of Ni and Fe₃O₄ on the MCF, TiO₂, and K, together with the Honda–Fujishima effect. To summarize the chemical reactions in the case of light scattering, the creation of the photocurrent comes about by generating electrons; the chemical–photovoltaic mechanism is irrelevant to the behavior of rubber molecules where the operations of electrons are facilitated by the catalyst of TiO₂, Ni, and Fe₃O₄.

Because ordinary solar cells do not have the properties of large tension or compression, they are rigid and not very flexible, and solar cells having these properties are currently novel enough to be almost unknown. However, MCF rubber DSSCs typically have large tension and compression, and their production opens the door to a number of practical devices and instruments. MCF rubber solar cells are suitable for a great variety of engineering applications. Once they are ready to go into production, the present experimental results and the model of the chemical–photovoltaic mechanism will be of great interest. For further development of the MCF rubber DSSCs, large stretchable transparent glass needs to be developed because large stretchable MCF rubber DSSCs integrated with stretchable transparent glass bring about more practical and convenient devices and instruments. The study to develop the large stretchable transparent glass will be addressed in another investigation.