Elastic and extensible solar cells are highly desirable in robots with installed solar cells and a sensor operating in a nuclear reactor building or in space for extravehicular activity. In the future, robots will substitute for humans more and more to carry out activities in hazardous circumstances. The robot should have more than two types of sensing, minimally with photovoltaics and built-in electricity. In addition, the sensor or solar cells must be elastic and extensible. The robot needs to be able to sense force, temperature, etc. Photovoltaics allow for free activity with a battery, making the robot independent of any external power source or electric wires. Independent sensors run on an external power source such as built-in electricity can function as piezo-materials. In order to achieve both elasticity and extensibility, rubber is the best material. Thus, rubber should be used to develop materials or elements for sensing, photovoltaics and built-in electricity. We propose elastic and extensible solar cells with sensing capabilities for force and γ-ray irradiation, and with built-in electricity, built-in voltage and built-in current [1]. By applying both DC electric and magnetic fields to a water-soluble rubber latex such as natural rubber (NR) or chloroprene rubber (CR) compounded with our developed magnetic responsive fluid, a magnetic compound fluid (MCF), the MCF rubber liquid can be electrolytically polymerized to form a solid [2]. A bulk heterostructure of magnetic clusters is formed in the MCF rubber. The magnetic clusters involved in the magnetic and metal particles of the MCF are many thin-rod like shape [3]. MCF has nm-ordered magnetite (Fe₃O₄) particles and μm-ordered metal particles such as Ni, Fe, etc. [4]. The electrolytically polymerized MCF rubber has the properties of both piezo-resistivity and piezo-electricity [5]. As for the latter, the induced voltage is related to piezo-electricity that is evaluated as built-in electricity, built-in voltage and current. The built-in electricity occurs due to ionized molecules and particles of the rubber latex, oleic acid coated around Fe₃O₄ of magnetic fluid (MF), and water, all of which together play the roles of an acceptor-like p-type semiconductor (corresponding to A⁻) and of a donor-like n-type semiconductor (corresponding to D⁺), as discussed in our previous studies [6] [7]. Given its semiconductor-like role, the MCF rubber is also photovoltaic [6]. In addition, due to the elasticity and extensibility of NR or CR, the MCF rubber satisfies the above-mentioned prerequisites and we can therefore expect it to be suitable for use in a nuclear reactor building or in space for extravehicular activity.

Regarding ordinary solar cells used in space, solar cells have seen considerable use in power sources installed on telecommunications satellites, in armaments, and in equipment used for meteorological or scientific research. Space environments are especially severe because of radiation exposure, which may induce lattice defects in solar cells or generate transient effects depending on the extent of the enhancement of electron-hole (e-h) pairs [8] [9]. These defects and effects due to irradiation lead to the degradation or metamorphosis of semiconductors. Semiconductors are basic not only to solar cells but also to photodetectors, metal semiconductors (MSs), metal insulator semiconductors (MISs), and metal oxide semiconductors (MOSs). Recent research has examined the effects of irradiation on solar cells [10], MIS solar cells [11], photodetectors [12], MS Schottky diodes [13], MIS Schottky diodes [14], MOS capacitors [15] [16], and MOS diodes [17]. In space, irradiation is defined by a linear energy transfer (LET) in which energy is transferred to a material by ionization when radiation passes through the material during unit length. Types of radiation with low LET include X-rays, γ-rays and electron beams, while those with high LET include proton beams and heavy particle beams. Low-LET radiation induces a total ionizing dose (TID) effect generated by ionization under excessive incident radiation so that the threshold level of voltage changes and mutual conductance decreases, while high-LET radiation induces a single-event (SE) effect generated by the creation of high density electric charges under ionization at the incident single particle so that an overcurrent is generated in the electric circuit, causing a malfunction.

As in the case of solar cells used in space, the above-mentioned irradiation effects must also be taken into account in the case of solar cells used in a nuclear reactor building. Therefore, in the present study, we investigated γ-ray irradiation effects on elastic and extensible MCF rubber solar cells with an eye to their use in elastic and extensible MCF rubber sensors proposed for engineering production. MCF rubber solar cells are produced by a consummate fabrication process, which includes our proposed novel adhesion technique to solve the problem of adhesion between rubber and metal electrodes [18]. The current commercial conductive adhesive provides electrical insulation with degrading conductivity between rubber and electrodes. In contrast, under our proposed novel adhesion technique, MCF rubber with a metal complex hydrate is electrolytically polymerized under the application of a magnetic field so that electrodes can be adhered to the rubber. The hybrid MCF rubber used in sensors and solar cells discussed in our previous study was produced using only water-soluble NR or CR. To the best of our knowledge, MCF rubber made of water-insoluble materials has not yet been investigated. It is necessary to drop dye and electrolytes on each MCF rubber surface so that the photovoltaics degrade due to aridity. Therefore, in order to resolve their aridity, in the present consummate fabrication process, we have adopted a method in which water-soluble and water-insoluble rubbers are mixed to be electrolytically polymerized by an emulsifier such as polyvinyl alcohol (PVA) [19], and dye and electrolytes are involved in the MCF rubber beforehand [20]. We also investigated the photovoltaic and γ-irradiation effects on the MCF rubber using knowledge previously obtained on MS, MIS and MOS [8] - [17].

We investigated the γ-irradiation effect of the MCF rubber solar cells produced by our proposed consummate fabrication process at the irradiation facility at the Laboratory for Advanced Nuclear Energy of the Tokyo Institute of Technology (Tokyo, Japan) using the set-up shown in Figure 11.

The γ-rays were irradiated from a ⁶⁰Co source and the absorbed dose rate was 230 Gy/h. The voltage and electric current of the MCF rubber solar cells induced by the irradiation were measured both with and without 900-Lux ultraviolet illumination. The ultraviolet light was delivered to the MCF rubber solar cells by flare. The voltage V and electric current I induced by γ-irradiation and ultraviolet light illumination are shown in Figure 12. Without irradiation, the flare could be turned on and off in the γ-irradiation chamber by a human being, but this is not possible with irradiation. Measurement therefore began after the flare was turned on. Without γ-irradiation, the induced voltage and current increase due to ultraviolet light illumination (“a” in Figure 12), while with γ-irradiation, they decrease with ultraviolet light illumination (“b” in Figure 12). Without ultraviolet light illumination, the voltage and current induced by γ-irradiation decrease (“c” in Figure 12), while with ultraviolet light illumination, they decrease (“d” in Figure 12).

Cyclic voltammograms of the MCF rubber solar cells in the γ-irradiation facility were measured using a potentiostat at a scan rate of 50 mHz in the potential domain of −1.5 - 1.5 V (Figure 13). Whether or not γ-rays are irradiated, the

area of the hysteresis voltammogram curve increases due to ultraviolet light illumination. However, whether or not ultraviolet light is irradiated, the area of the hysteresis voltammogram curve decreases due to γ-irradiation.

We examined our obtained results in the light of existing knowledge about MSs, MISs and MOSs as follows.

As with the well-known consequence that γ-irradiation generates e-h pairs in the interfacial insulator of solar cells and semiconductors [23], e-h pairs are generated by γ-rays in the interfacial insulator of PDMS, NR molecules, oleic acid and PVA in MCF rubber solar cells. Simultaneously, A⁻ corresponding to the role of acceptor-like p-type semiconductors and D⁺ corresponding to the role of donor-like n-type semiconductors are generated from the molecules of TiO₂, Fe₃O₄, Ni, Na₂WO₄∙2H₂O, PDMS, NR, oleic acid, PVA and water, and the depletion layer is enhanced. The mechanism is induced by the following process.

γ-irradiation emits electrons, e_aq, which are generated as shown in Equation (1) and then bring about a radiation-chemical reaction by attacking metal, M, as shown in Equation (2) [24]. In addition, at the level of composite iron oxide nanoparticles, metal forms on iron oxide, as shown in Equation (3), and noble metals form in the aqueous phase and are then immobilized on iron oxide, as shown in Equation (4) [25].

H 2 O → γ -irradiation e aq , H 3 O + , OH − (1)

e aq + M n + → M ( n − 1 ) + (2)

M n + ( onoxide ) → M 0 ( onoxide ) (3)

M n + ( aq . ) → M 0 ( aq . ) → M 0 ( onoxide ) (4)

TiO₂ in the form of the Mⁿ⁺, TiO 4 + , makes the polyisoprene molecules of NR

radical, as shown in Equation (5) [7], which means that it functions as a catalyst, with the result that the radical polyisoprene molecules become oxidants.

Concerning the reverse-bias saturation current I_S, we can deduce as follows. The Schottky barrier generated between the energy potential of the Mⁿ⁺ of TiO₂, Fe₃O₄, Ni, and Na₂WO₄∙2H₂O, and that of the H⁺, radical polyisoprene, is increased by γ-irradiation. There is also the emission of electrons from TiO₂ by γ-irradiation, identified in our previous study [1] because as the electron falls into the inner shell, it causes an avalanche of outer-shell electrons. As a result, the diffusion potential V_D increases. The MCF rubber solar cells were analyzed by an equivalent electric circuit using Equation (6) [26], where I is the electric current of solar cells, I_ph is the photocurrent, I_d is the diode current, I_sh is the shunt current, e is the elementary charge of electrons, V is the voltage of solar cells, R_s is the series resistance, N is the diode ideality factor, k is the Boltzmann constant, T is the absolute temperature, and R_sh is the shunt resistance.

I = I p h − I d − I s h = I p h − I D [ exp { e ( V + R s I ) N k T } − 1 ] − V + R s I R s h (6)

I_s is shown in Equation (7) [10], where A is the effective diode area and A^* is the effective Richardson constant. Then, as V_D increases, I_s is enhanced. I_s stems from the enhancement of current leakage between A⁻ and D⁺ in the MCF rubber.

I s = A A * T 2 exp ( − e V D k T ) (7)

On the other hand, we can deduce the tendency of N as follows. N is equal to V_f/(V_f − V_i), where V_f is the forward-bias voltage and V_i is the voltage drop at the thin isolated layer. Recognizing the consequences clarified in other studies on solar cells with γ-irradiation [10] [27], we investigated changes in N that also arise as a result of the enhancement of current leakage between A⁻ and D⁺ in MCF rubber as follows. The method as presented in the study [10] is effective to evaluate N from the relation between electric current and voltage obtained by cyclic voltammetry as shown in Figure 13.

By applying Equation (8), which is derived from Equation (6), with deriving from Figure 13, we can obtain R_s as gradient of Equation (8) and NkT/e as a constant of Equation (8) to the variable I, as shown in Table 4. “Decreasing vias voltage” and “Increasing bias voltage” mean decreasing and increasing voltage of Figure 13 respectively. Therefore, with respect to R_s and NkT/e, “decreasing” and “increasing” are different because the capacity of discharge and the capacity of charge in the MCF rubber are different. Then the resistance among molecules of TiO₂, Fe₃O₄, Ni, Na₂WO₄∙2H₂O, PDMS, NR, oleic acid, PVA and water changes diversely according to the ultraviolet light illumination and γ-irradiation conditions. Regarding NkT/e, the quantitative relation between V_i and V_f can be seen from NkT/e, as shown in Figure 14. The difference in the absolute value of the gradient of the linear line demonstrates the magnitude correlation between V_i and V_f. In cases of ultraviolet light illumination, γ-irradiation, or synchronizing ultraviolet light illumination and γ-irradiation, N increases. V_f is increased by γ-irradiation regardless of ultraviolet light illumination. In the case of ultraviolet light illumination, V_f is larger than V_i regardless of γ-irradiation.

d V d ln I = R s I + N k T e (8)

Thus, as a result, γ-irradiation causes current leakage so that I_s and N increase. We can consider PDMS, NR, oleic acid and PVA as the insulator. When there exists very thin insulator on the surface of the semiconductor, Equation (6) is approximated to Equation (9). As a result, I increases.

I = I s exp ( − e V f N k T ) (9)

Regarding photocurrent, the conductivity of PVA increases by doping Ni in the PVA so that the photocurrent also increases by γ-irradiation, as discussed in the previous study [23]. The conductivity of Fe₃O₄ [28] and TiO₂ [29] is also increased by γ-irradiation.

The above results together mean that these tendencies induce the enhancement of photovoltage and photocurrent by illumination, which is consistent not only with the trends indicated as “a” and “b” in Figure 12 but also with the enhancement of the area of the hysteresis voltammogram curve by ultraviolet light illumination shown in Figure 13.

In contrast, transmittance through the TiO₂-coated transparent glass, which is caused by fluorine-doped tin oxide (FTO), decreases and the dye-sensitized solar cells is degraded by γ-rays, as demonstrated in the previous study [30].

In addition, it is important to also take into consideration that the effect of the attenuation of photon energy is larger than the effect of the creation of e-h pairs generated by γ-irradiation. In general, the photon energy of γ-rays attenuates through a material by the electron pair effect, the Compton effect, and the photoelectric effect, the first two of which are created at high photon energy, and the third of which is created at low photon energy. Because Fe₃O₄, Na₂WO₄∙2H₂O and magnetic cluster are involved in MCF rubber solar cells, all of these effects are created to a greater or lesser extent, as seen from the following.

From Equation (3) and Equation (4), we see that Fe forms on Fe₃O₄. The attenuation of photon energy has been clarified by the collision of γ-rays with Fe, Pb and W [31]. As a result, the photon energy is attenuated at Fe₃O₄.

Regarding W, it has been clarified that the photon energy attenuates at the oxide of W particles as well as at W metal [32]. Therefore, the photon energy also attenuates due to W of Na₂WO₄∙2H₂O.

As for the electron pair effect, an electromagnetic field is needed. In the case of MCF rubber, magnetic cluster is magnetized and the magnetic field line is closed in the magnetic cluster so that an internal electromagnetic field exists [3]. Therefore, the electron pair effect could be created in MCF rubber solar cells and photon energy might also attenuate at the magnetic cluster.

As a result, the photovoltage and photocurrent under ultraviolet light illumination decrease by γ-irradiation, which demonstrates not only the trends indicated as “c” and “d” in Figure 12 but also the decrease in the area of the hysteresis voltammogram curve by γ-irradiation in Figure 13.

I_S and N resulting from the enhancement of the diffusion potential V_D are increased not only by the e-h pairs generated by γ-rays in the interfacial insulator of PDMS, NR molecules, oleic acid and PVA in MCF rubber solar cells, but also by the p- and n-type semiconductor-like roles of the molecules of TiO₂, Fe₃O₄, Ni, Na₂WO₄∙2H₂O, PDMS, NR, oleic acid, PVA and water. Additionally, the photon energy of γ-irradiation attenuates owing to the Fe₃O₄, Na₂WO₄∙2H₂O and magnetic cluster involved in MCF rubber.

MCF rubber solar cells are made of rubber so that they will be elastic and extensible. Provided that the transparent glass used in fabrication is made of a flexible material such as a thin transparent polymer plate, the MCF rubber solar cells could also be flexible. To achieve flexibility and an effective photovoltaic effect, the constitution of the MCF rubber solar cells requires both soluble and insoluble rubbers, Fe₃O₄, TiO₂, Na₂WO₄∙2H₂O, etc. On the basis of this required constitution, we propose a consummate fabrication process for MCF rubber solar cells by synthesizing: 1) a novel adhesion technique, 2) electrolytic polymerization of rubber mixed with water-soluble and water-insoluble rubbers, and 3) the permeation of liquid to rubber.

For the consummate fabrication process, we clarified that there exist photovoltaic effects of emulsifier PVA, insoluble rubber Q, and metal complex hydrate Na₂WO₄∙2H₂O, which appear to be the same as the photovoltaic effects of water-soluble rubber. We clarified the photovoltaic effects of porous MCF rubber permeated by any liquid and of adhesive MCF rubber liquids. We also verified the effectiveness of our production procedure, in which MCF rubber that is electrolytically polymerized in advance is attached to TiO₂-coated transparent glass and again electrolytically polymerized.

We investigated the γ-irradiation effect of the MCF rubber solar cells produced by our proposed consummate fabrication process. Whether or not γ-rays are irradiated, the photovoltage and current increase and the area of the hysteresis voltammogram curve are enhanced by ultraviolet light illumination. However, whether or not ultraviolet light is irradiated, the photovoltage and current decrease and the area of the hysteresis voltammogram curve are decreased by γ-irradiation. These tendencies might be deduced by synthesizing existing knowledge of the enhancement of the reverse-bias saturation current, the diode ideality factor, and the attenuation of photon energy.

This work was supported in part by JSPS KAKENHI Grant Number JP 18K04040 and by the Fukushima Innovation Coast Framework project “Namie Town Creation with Revitalizics using Risk Communication Engineering: for Realization of Innovation Coast Framework” in Japan. The authors would like to thank I. Yoda, member of the technical staff of the Cobalt-60 Irradiation Facility of the Laboratory for Advanced Nuclear Energy at the Tokyo Institute of Technology (Tokyo, Japan) for irradiation support.

The authors declare no conflicts of interest regarding the publication of this paper.

Shimada, K., Kato, R., Ikeda, R., Kikura, H. and Takahashi, H. (2020) γ-Ray Irradiation Effect on MCF Rubber Solar Cells with both Photovoltaics and Sensing Involving Semiconductors Fabricated under Magnetic and Electric Fields. World Journal of Mechanics, 10, 95-119. https://doi.org/10.4236/wjm.2020.108008

The completed sensors shown in FigureA1(c) and FigureA1(d) consisted of MCF rubber liquid with complex hydrates as 1 g Ni powder, 0.75 g MF (W-40), 3 g NR-latex, 3 g CR-latex (671A), and 0.5 g complex hydrates, and MCF rubber without complex hydrates as 3 g Ni powder, 0.75 g MF (W-40), 3 g NR-latex, and 3 g CR-latex (671A), with a 1-mm metal plate gap, a constant electric field at 6 V, 2.7 A and 5 min under atmosphere, and a 312-mT magnetic field created by permanent magnets as paired opposites at each electrolytic polymerization [18].

Although ordinary solid rubber cannot be doped by any reagents directly because a solid rubber is generally impermeable, any liquid can permeate the electrolytically polymerized MCF rubber, as shown in FigureA2 [20]. Here, KF96 has 1000 cSt, MF has a W40. 312-mT magnetic field, and an electric field (30 V, 2.7 A, 5 min) is applied with a 1-mm electrode gap. FigureA2 shows water-soluble and combination water-soluble and water-insoluble MCF rubbers. In FigureA2(b) and FigureA2(d), water can be seen as a reflection on the rubber surface after permeation.

Whether the MCF rubber is made of water-soluble rubber or a combination of water-soluble and water-insoluble rubbers, by combining it with either metal complex hydrate Na₂WO₄∙2H₂O or water, or by combining it with a mixture of the two, the MCF rubber becomes highly porous [20] (FigureA3).