The Application of Dye-Sensitized Solar Cell Using rGO and MBs in Series-Parallel Under Low Illumination

The advantage of the dye-sensitized solar cell (DSSC) is utilized indoor fluorescent light illumination, but the output power of DSSC is not high enough. In this study, we provided a structure for DSSCs, and characterized the photovoltaic performances under air mass 1.5 global and indoor fluorescent light illumination. The photoanode of DSSC is based on titanium dioxide (TiO2) – magnet beads (MBs) – reduced graphene oxide (rGO) composited photoanode (TMGP), which was fabricated by hydrothermal method, spin coating, and doctor blade. According to the experimental results, adding MBs and rGO to DSSCs can enhance the charge transfer ability, reducing the occurrence of charge recombination, thereby improving the photovoltaic performance. DSSCs with TMGP photoanode can maintain a photovoltaic conversion efficiency of around 14 % under indoor light.


I. INTRODUCTION
The demand in energy supply has accelerated fossil fuel depletion. It is predicted that the reverses of fossil fuel can only last forty years, sixty years for natural gas, and two hundred for coal [1]. The renewable energy has been considered in recent years, such as photovoltaic, wind power, geothermal heat, hydropower, and biomass energy. Among all renewable energy technologies, the photovoltaic technology is the particularly promising technology for direct conversion sunlight into electricity energy [2], [3], [4], [5], [6]. The developments of the solar cells are able to be divided into four generations: (1) the silicon-based solar cells (single, polycrystalline, and amorphous silicon); (2) the thin-film cells (CdTe, CIGS, and CIS); (3) the organic matter and The associate editor coordinating the review of this manuscript and approving it for publication was Huamin Li . nanotechnology cells. However, silicon-based solar cells are high production costs and confine to application in terrestrial photovoltaic. In comparison with silicon-based solar cells and thin-film cells, the dye-sensitized solar cell (DSSC) has low manufacturing costs, high-temperature environment, transparency, flexibility, and generate electricity with indoor light sources [7], [8], [9], [10], [11]. Due to the lower illumination application, the DSSC can work efficiently in dark conditions, for example, dawn or dusk. Hence, DSSC can be the most promising photovoltaic device. The DSSC consists of three parts with the sensitized working layer (photoanode), electrolyte, and the counter electrode. In general, the photoanode material is titanium dioxide (TiO 2 ), because the TiO 2 has good compatible with dye molecules. Moreover, the photoanode of DSSC is acted as a very important role, which takes responsibility for transforming sunlight into electric power [12]. Therefore, the photovoltaic conversion efficiency VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ of DSSC is usually improved by modifying photoanode. Moreover, the topic of our research group is the enhancement of photovoltaic conversion which is ameliorated by modifying the nanostructure of active layer (semiconductor layer) or retardation of dark reaction at interface. For instance, Chou et al. in [13], the indium gallium zinc oxide (IGZO), the indium gallium zinc oxide (IGZO), IGZO barrier layer is based on the photoanode with DSSC via sputtering. It can retard dark reaction and back reaction to improve the photovoltaic conversion efficiency. Furthermore, Chou et al. in [14], graphene oxide (GO) and zinc oxide (ZnO) have been applied to photoanode to increase the amount of dye-loading and reduce electron transfer impedance, respectively. In addition, Chou et al. in [15], GO and magnetic beads (MBs) have also introduced into counter electrode for photocatalytic activity and fast electrocatalyst of triiodide reduction. Furthermore, Chou et al. in [16], the Fe 3 O 4 has been applied to active layer for the decrease in electron recombination. The advantage of the dye-sensitized solar cell (DSSC) is utilized indoor fluorescent light illumination, but the output power of DSSC is not high enough. Therefore, we propose the titanium dioxide (TiO 2 ) -magnet beads (MBs) -reduced graphene oxide (rGO) composited photoanode (TMGP) photoanode to enhance the photovoltaic conversion efficiency under low Illumination. It under the indoor light condition can maintain photovoltaic conversion efficiency (PCE) by about 14 %.

B. PREPARATION OF THE COLLOIDAL PASTE
The MBs-TiO 2 spin paste had consisted of 1.5 g P25 TiO 2 powder, 0.1 ml acetic acid, 3 ml deionized water (D.I. water) and 0.125 mM MBs. The rGO-TiO 2 doctor blade paste was prepared as follows: Firstly, the 5 mg rGO powder put in 4 ml D.I. water that the solution into an ultrasonic oscillator for one hour. The purpose was dispersed rGO powder evenly in the solution. After that, 2 g TiO 2 powder and 0.4mL absolute alcohol were added into the rGO solution. Finally, the rGO-TiO 2 solution mixture was stirred for one day to obtain rGO-TiO 2 colloid paste. Besides, we prepared pure TiO 2 colloid paste without MBs and rGO powder [17].

C. FABRICATION OF THE PHOTOANODE FOR DYE-SENSITIZED SOLAR CELL
The fluorine-doped tin oxide (FTO) glass (7 /cm 2 ) was cleaned by acetone, ethanol, and D.I. water, respectively. The TiCl 4 stock solution was diluted with D.I. water to the 40 mM concentration TiCl 4 treatment solution via the ice bath method. Then the cleaned FTO was immersed into 40 mM TiCl 4 solution at ∼ 70 • C for the half-hour to make the TiO 2 compact layer then annealing at 450 • C for the half-hour at the furnace. The working layer was based on the TiO 2 compact layer by using the spin method and doctor blade method that the photoanode annealing at 450 • C for the half-hour. Finally, the annealed photoanode used TiCl 4 treatment and annealing for a half-hour. Annealing the photoanode film at an ambient temperature of 450 • C enables the removal of organic impurities in the photoanode film and strengthens the contact between nanoparticles within the film.

D. INSTRUMENTATION
The photocurrent-voltage curve measurements were conducted under Xe lamp solar simulator (MFS-PV-Basic-HMT, Taiwan) with the sunlight intensity of 100 mW/cm 2 and indoor fluorescent light source (T5) and light decay filter. Besides, we used light filter 80%, 50%, 30%, and 10% to investigate the DSSCs performance of different sunlight intensities. The Nyquist plot was used to measure the frequency range of 1 MHz to 50 Hz in the dark at a potential of 0.7 V.

A. PHOTOVOLTAIC PERFORMANCES OF THE DSSC WITH TiO 2 AND TMGP
The current-voltage curves (J-V) of DSSC based on TiO 2 and TMGP, the photovoltaic conversion efficiency (PCE) is enhanced by 31.75 % from 5.20 % to 6.85 %, as shown in Fig. 1. This improvement in photovoltaic conversion efficiency is due to the enhancement of short-circuit current density (J SC ) by magnetic beads (MBs) and reduced graphene oxide (rGO), First, the MBs provides another charge transfer path, which can improve the charge transfer characteristics in the TiO 2 film and reduce the electron recombination opportunity. Besides, the rGO is acted as a bridge to accelerate the excited-electron from the conduction band (CB) of titanium dioxide (TiO 2 ) to the CB of FTO. This role of rGO can reduce the dark reaction between the excited-electron and oxidizeddye molecule [18], [19], [20], [21], [22], [23].
Moreover, the equivalent circuit for DSSC in this study is exhibited in Fig. 2, and it would not exhibit in electrochemical Impedance Spectroscopy (EIS) measurement as below. It composes of R S , R 1 , R 2 , C 1 , and C 2 . First, the R S indicates the resistance between FTO substrate and wire. Besides, Z 1 indicates high-frequency impedance of interface between electrolyte and counter electrode, which is the  first semicircle at the left-hand side. An equivalent circuit, Z 1 indicates the parallel connection between R 1 and C 1 , and R 1 is the interface resistance between the counter electrode and electrolyte. Additionally, Z 2 is the frequency impedance of interface between the electrolyte and the active layer (photoanode), which is the second semicircle at the righthand side. An equivalent circuit, the Z 2 is the parallel connection between R 2 and C 2 , and R 2 is the interface resistance between the active layer (photoanode) and electrolyte. Additionally, the DSSC is operated in direct current, that the capacitance can be ignored.
It can be seen from Fig. 2 that R 2 is reduced from 11.4 to 6.8 because MBs provide another charge transfer path for the photoanode for better electron transfer, and the rGO can increase the amount of dye in the photoanode and reduce charge recombination. Because of the above properties, MBs  and rGO enable more excited electrons to be transported to external circuits. Finally, the J SC current can be enhanced [24], [25]. Furthermore, the details of MBs and rGO were shown in our previous research [17].     of electrolyte. Besides, the amount of excited-electrons are reduced with decline of light intensity. Similarly, because the amount of excited-electron is decreased, the J SC current is also reduced. The enhancement in photovoltaic conversion efficiency is due to the increase in fill factor (F.F.). The fill factor is increased from 59.33 % to 72.29 %. However, the better PCE of DSSC can be obtained under lower illumination than indoor fluorescent, which are due to fewer amount of excited-electrons. Moreover, the higher improvement in photovoltaic conversion efficiency under low illumination can be obtained from DSSC based on TMGP. Because the amount of excited-electrons is little, the decrease of dark reaction is helpful for the improvement of photovoltaic conversion efficiency of DSSC under low illumination.
In the result with Table 1 and Table 2, the PCE of DSSC based on TiO 2 decreased from 11.39 % to 5.50 % as the light intensity was decreased from 1.75 mW/cm 2 to 0.22 mW/cm 2 . In addition, the J SC dropped from 462.5 µA/cm 2 to 46.8 µA/cm 2 as the light intensity was decreased from 1.75 mW/cm 2 to 0.22 mW/cm 2 . Moreover, the V OC of DSSC based on TiO 2 decreased from 0.62 V to 0.49 V as the light intensity was decreased from 1.75 mW/cm 2 to 0.22 mW/cm 2 . Because the dye molecule has a better response with indoor fluorescent lamp, the V OC of 1.75 mW/cm 2 was higher than that of 10 mW/cm 2 . In addition, the F.F. was decreased from 69.49% to 52.74%, while the light intensity was decreased from 1.75 mW/cm 2 to 0.22 mW/cm 2 . Moreover, the result in the PCE decreased from 11.39 % to 5.50 %. Photovoltaic conversion efficiency decreased from 11.39 % to 5.50 % while light intensity was decreased from 1.75 mW/cm 2 to 0.22 mW/cm 2 .
In Table 3 According to previous literature [26], [27], the reduced dark reaction for photoanode of DSSC is a key point to enhance photovoltaic performances, but there are a few literature to apply the reduced dark reaction on DSSC. Moreover, DSSCs in series-parallel has not been investigated, and it has the potential to develop to drive the device.

IV. CONCLUSION
In summary, the properties of DSSCs with MBs and rGO in series-parallel have been investigated. The DSSCs with MBs and rGO can obtain higher photovoltaic conversion efficiency under low illumination, which is due to the retardation of dark reaction. Because the amount of excited-electrons is few under low illumination, a decrease in reverse recombination is a key point to increase the photovoltaic conversion. MBs provides another charge transfer path, which can improve the charge transfer. Besides, the rGO is acted as a bridge to accelerate the excited-electrons from the conduction band of FTO to the conduction band of titanium dioxide (TiO 2 ). In other words, rGO can retard the dark reaction between the excited-electrons and oxidized-dye molecules. The advantage of the dye-sensitized solar cell (DSSC) is utilized indoor fluorescent light illumination, but the output power of DSSC is not high enough under indoor fluorescent light illumination. Moreover, the output power can be enhanced by series and parallel.  PO-FENG CHENG was born in Nantou, Taiwan, on July 1999. He received the bachelor's degree from the Department of Electrical Engineering, National Kaohsiung University of Science and Technology. He is currently pursuing the master's degree with the Graduate School of Electronic Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan. His current research interest includes dye-sensitized solar cells. VOLUME 10, 2022