Long-Term Activity of Thermoplastic Gel Electrolyte in a Photo-Electrochemical Assembly Involving Poly Bithiophene (PBTh) as Photoactive Working Electrode

Evidence for the long period of a sustainable function of a thermoplastic gel electrolyte (TPGE) consists of polyethylene glycol (PEG)/I2/I- in propylene carbonate (PC) was recorded. The studied photoactive assembly consists of PBTH/FTO/TPGE I2/I-/Platinized FTO. The study showed that the assembly regenerates the expected photoelectrochemical (PEC) quantities such as photocurrent, and other dielectric properties with infrequent use through an elapsed period of 18 months. The behavior of PBTh/occluded with CdS was mentored during this period and showed a similar result. PEC studies indicated the presence of p-p type hole accumulations interface, evident from the initial sharp rise in photocurrent. The change of open circuit potential (dVoc) indicates that the shortest electron lifetime is 100 ms. The behavioral outcome of the assemblies within the period of study refracts stability of the electrode and the long life cycle of the electrolyte.


Introduction
The phenomena of corrosion and evaporation represent disadvantages that limit the sustainable use of liquid electrolytes used in dye-sensitized solar cells (DSSC).
The use of polymer gel electrolytes is a potential alternative solution to these disadvantages. Several studies were focused on fabrication, working principle, and the up-to-date status of DSSCs and batteries using polymer electrolytes [1] [2] How to cite this paper: Kasem paration properties with temperature changes as electrolytes was investigated to follow up with the thermal loss of electrochemical (EC) storage devices such as supercapacitors and lithium-ion batteries [9] [10] [11] [12].
The physical state of the electrolyte, as well as the importance of the redox system used in these electrolytes, are both important. This is because the redox system can affect the electrochemical potential at the counter electrode. This consequently affects the photovoltage outcome of the solar cell. For these reasons, the choice of the redox system in photoelectrochemical (PEC) cells is a very important step towards improving conversion efficiency. Many redox systems were used, but the issues of solubility, light absorption, and stabilities as well as low efficiency [13] [14] were concerns that limited their use. Further, gel-state electrolytes, especially thermoplastic gel electrolytes (TPGE) may have several advantages over liquid state electrolytes such as longer-term stability (life cycles), wide ranges of temperature change tolerance, no loss in the electrolyte contents, and non-flammable electrode reaction products.
I − possesses several desirable properties [15] [16] [17], such as good solubility, low absorption of light, a suitable redox potential, fast dye generation, and very slow recombination with some inorganic semiconductors such as TiO 2, therefore I − / 3 I − attracts attention for use in electrochemical and DSSC devices. A study [18] showed that a balance I − / 3 I − is required to generate maximum electrochromic effects. The I − / 3 I − was used as redox-active material in the proposed TPGE.
In this study, we explore the sustainability of both the TPGE and the photoactive organic film of poly bithiophene (PBTh) assembled in PEC cells for 18 months.
The effect of doping of the photoactive film on the overall cell activity was also explored.
Deionized (DI) water was used to prepare aqueous electrolytes.

Preparation of Thermoplastic Gel Electrolyte (TPGE)
Thermoplastic gel electrolyte (TPGE) wAs-prepared following the published pro-

Instrumentation
Electro-polymerization was performed in a 20 cm 3 three-electrode cell, consisting of a Pt flag as a counter electrode, an Ag/AgCl as a reference electrode, and FTO with a surface area of 2.0 cm 2 as the working electrode [20]. The generated film thickness was ≈ 1 μm. Photoelectrochemical studies of the thin solid films were performed using the experimental setup as described in Figure 1

Electrochemical Studies on FTO/PBTH/TPGE
The EC studies on FTO/modified with PBTh were carried out by cycling the po-

Hole Accumulation Phenomena
The observed photocurrent spears in Figure 3 can be explained on a basis of the existence of hole accumulation in the mixed phases of the organic polymers, as PBTh is considered to be a p-type organic semiconductor. We assume that more than one phase of PBTh is formed, and p-p type heterojunction is created. This allows hole accumulation to take place, as was evident from the appearance of the sharp rise in the photocurrent as shown in Figure 3 The plot of lnR vs time generates a straight line with slope = 1/τ. The reciprocal of the slope determines the value of τ, in seconds. The greater I i and the smaller I st make R-value depends on I t . In the sharper spear, I t is large, consequently larger R, which means larger slope and smaller τ.

Change in the Open Circuit Potential d(Voc)
Electron lifetime (τ n ) contributes to the photoactivity outcome of the studied assemblies. The equation relates τ n with the open circuit potential (V oc ) decay [22]: where k B is the Boltzmann constant, e is the electron charge, T is the temperature in K, and V oc is the open circuit potential in Volts. The plot of V oc for As-prepared PBTh/I − / 3 I − gel electrolytes, is displayed in   The observed greater photocurrent than dark current indicates that films of PBTh/CdS/Gel electrolyte assembly offered a charge separation and small charge recombination. Although this assembly generated less photocurrent after 18 months, the magnitude of the generated photocurrent reflects that the photoactivity was maintained. This is added evidence for the long-time sustainable activity of the gel electrolyte. Figure 6 shows the photocurrent-time plot at −1.0 V. As PBTh/CdS is a p-type organic semiconductor, the generated assembly show lesser hole accumulation than the PBTh only. The smaller photocurrent spear shown in Figure   6, compared to that displayed in Figure 3, is direct evidence for the effect of CdS nanoparticles on the host polymer PBTh. Figure 6(A) and Figure 6(B) indicate that PBTh/CdS/TPGE assembly maintained its activity for a long period.

Electrochemical Impedance Spectroscopy
Electrochemical impedance spectra of the studied assemblies were measured in a frequency range between 10 5 -10 −2 Hz at −0.8 V. Impedance complexes (Nyquist plot) generated from the studied PBTh/I − / 3 I − TPGE assembly on FTO substrate in the dark and under illumination, are displayed in Figure 7     frequency range. This evident from the presence of un-concentered semicircle at high frequencies and the existence of Warburg impedance which reflects the film porosity [24]. Figure 7(B) and Figure 8(B), display the measured AC conductivities (σ ac ) as dielectric behavior at 25˚C. AC conductivities (σ ac ) were calculated adopting the following equation [25]: where L is film thickness, and a, is the electrode surface area (2.0 cm 2 ). conductivity can also increase due to the hopping of charge carriers at high frequency. The behavior of the assembly, however, was different under dark, where the conductivity of the As-prepared assembly decreased as frequency increased.
In Figure 7(B)a the plot indicates that as the frequency decreases, the corresponding ac conductivity increases. Such behavior was seen in absence of illumination. Leaving the assembly for 18 months in the dark at room temperature causes certain structural changes in the gel electrolyte. Such changes generated structural conditions that caused decreasing the capacitive reactance as frequency increases. The AC conductivity therefore increases.

Conclusion
The activities of both the TPGE and the photoactive organic film of poly bithiophene (PBTh) assembled in PEC cells were investigated in 18 months. The reproducible results showed that such assemblies sustained the photoactivity functions and other dielectric properties with infrequent use through an elapsed period of 18 months. The TPGE based Polyethylene glycol/I − / 3 I − is safe, stable with a long life cycle, and can provide a liquid-like electrochemical outcome.