Balance between the explored Pt counter electrode in an electrolyte medium and the photoanode for highly efficient liquid-junction photovoltaic devices

r e ng .A Faculty of Biotechnology, Chemistry and Environmental Engineering, Phenikaa University, Hanoi, 10000, Viet Nam Sohag University, Sohag, 82524, Egypt Phenikaa University, Hanoi, 10000, Viet Nam have extensively been researched and are commonly used [1e11]. Among solar cells, DSC has the largest potential for a wide range of gh efficiency and al DSC consists of trode (CE). In gen- alline TiO2, coated SnO2:F, where the is based on a so- usually selected to and excellent cat- ponents affect the important role in So far, several methods have been employed to fabricate the Pt CE such as thermal decomposition, sputtering, electro-deposition or plasma reduction techniques [12e15]. For example, Jeong et al. fabricated periodically aligned Pt nanocups CEs which resulted in an increase of the short-circuit current density (Jsc), the Fill Factor (FF) and the power conversion efficiency (PCE) of the DSC compared to the cell with Pt-sputtered CE [12]. Different shapes of * Corresponding author. Faculty of Biotechnology, Chemistry and Environmental Engineering, Phenikaa University, Hanoi, 10000, Viet Nam. E-mail address: duong.daovan@phenikaa-uni.edu.vn (V.-D. Dao). Contents lists available at ScienceDirect Journal of Science: Advanc .e l Journal of Science: Advanced Materials and Devices 5 (2020) 180e184Peer review under responsibility of Vietnam National University, Hanoi.(DSCs), quantum-dot-sensitized solar cells, perovskite solar cells, solar-driven evaporation generation devices, and photocatalysis the device performance, while the CE is the key to regenerating iodide ions from tri-iodide ions.Explored Pt area 1. Introduction Nowadays, the human being is facing both energy and climate problems. Therefore, researches on renewable energy have received a lot of attention. Several renewable energy sources such as solar power, hydroelectric power, tidal power, wind power, etc. have been exploited. Among them, solar power has numerous ad- vantages compared to other sources, and it has a great potential for the development of solar cells. Currently, in the solar energy field, dye-sensitized solar cells applications due to its cost-effectiveness, hi environmentally friendly fabrication [1]. A typic a photoanode, an electrolyte and a counter elec eral, the photoanode is fabricated by nano-cryst on a transparent conducting oxide (TCO) such as TiO2 layer absorbs the N719 dye. The electrolyte lution consisting of the I
/I3
redox couple. Pt is be the CE due to its high electrical conductivity alytic activity [5]. It is well known that all com performance of DSCs. The photoanode plays anPhotoanode Counter electrode This is an open access article under the CC BY license ( of Material Science and Engineering, a r t i c l e i n f o Article history: Received 2 January 2020 Received in revised form 23 March 2020 Accepted 26 March 2020 Available online 1 April 2020 Keywords: Dye-sensitized solar cellshttps://doi.org/10.1016/j.jsamd.2020.03.002 2468-2179/© 2020 The Authors. Publishing services b ( b s t r a c t This work investigates the effect of the ratio of the explored Pt area in the electrolyte medium and the photoanode area (REP) on the performance of dye-sensitized solar cells (DSCs). It is found that the power conversion efficiency of DSCs increases by the ascending REP. The highest power conversion efficiency, which was obtained for the cell with the REP of 64/49, was 8.40%. Furthermore, a relationship between the efficiency and fabrication cost is analyzed in terms of reducing or enhancing the surface area of CE compared to the photoanode's surface area. These findings may provide a way for the development of efficient and large-scale DSCs. © 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.Faculty of Environmental Sciences, University d Department of Chemistry, Faculty of Science, eb Phenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang Ngan, Hanoi, 10000, Viet Nam c of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet NamOriginal Article Balance between the explored Pt counte medium and the photoanode for highly photovoltaic devices Hai-Linh Thi Dang c, Van-Duong Dao a, b, *, Ngoc Hu Hong Ha Thi Vu a, b, Thi Hanh Nguyen c, Ibrahim M Doan Anh Vu e, Pham Anh Tuan e a journal homepage: wwwy Elsevier B.V. on behalf of Vietnamelectrode in an electrolyte fficient liquid-junction Vu a, b, Dang Viet Quang a, b, . Mohamed d, Xuan-Co Hoang c, ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license ncedFig. 1. Configurations of three CEs in the DSC structures: a), b), c) show the cross- sectional views of DSCs with different CEs; d), e), and f) show the top-views of Pt CEs with different active areas, respectively.H.-L.T. Dang et al. / Journal of Science: AdvaPt have also been synthesized such as Pt hollow spheres, Pt nano- urchins, microporous Pt, mesh-shaped nanopatterning of Pt and Pt honeycomb structures to apply in CEs of DSCs [12,16e21]. Shi et al. demonstrated the use of FeOx-supported single Pt atoms as CEs in DSCs [22]. Recently, Pt-based alloys have been developed as the efficient CEs of DSCs [23e29]. Previous researches mainly focused on improving the specific active area, i.e. the catalytic activity of catalysts in CEs. However, neither the explored area of CE in the electrolyte medium nor the relationship between the photoanode area and the CE area has been investigated yet. In this research, the influence of the explored Pt active area in the electrolyte medium on the PCE of the DSCs is investigated. Moreover, in this work, we present the effect of the ratio of the explored Pt area in the electrolyte medium and that in the photo- anode area (herein after referred to REP) on the performance of DSCs. In general, the surface area of CE is fabricated together with the photoanode. Given that the surface area of the photoanode is 7  7 mm2 (or 49 mm2), the CE's surface area is also 49 mm2. We also discuss the relationship between efficiency and fabrication cost by reducing or enhancing the surface area of CE compared to the photoanode's surface area. In this regard, we fabricated three Pt electrodes by a sputtering method with three different explored Pt areas in the electrolyte medium (Fig. 1). The catalytic activity of the developed electrodes was carefully assessed through cyclic vol- tammogram (CV), electrochemical impedance spectroscopy (EIS), Bode analysis and photocurrent-voltage (J-V) measurements. 2. Experimental 2.1. Preparation of CEs Pt layers with different Pt areas of 5  5 mm2, 7  7 mm2, and 8 8 mm2 were sputtered on 2  2 cm2 FTO glass substrates (~8 U/ paste and after annealing, the blocking layers were fabricated as in our previous work [16]. For dye adsorption, the TiO2-coated FTO glass substrates were immersed into 0.3 mM N719 dye solution (Solaronix, Switzerland) of acetonitrile (SigmaeAldrich) and tert- butyl alcohol (Aldrich) with the volume ratio of 1:1 for 24 h at the temperature of 25 C.,, Pilkington, USA) by DC-sputtering (Young Hi-Tech) of Pt at 10mA and 2 10
3 torr for 5 min. Accordingly, 100 nm thickness of Pt is coated on FTO glass substrates which were measured in a previous work [16]. We denoted the electrodes as 5  5 mm, 7  7 mm and 8  8 mm (Fig. 1 def). 2.2. Preparation of the photoanode The photoanodes are fabricated by the doctor-blade method [10]. 11 mm thickness of TiO2 (Solaronix, Switzerland) were coated on a 2  2 cm2 FTO glass substrates. Note that the photoanode area is 7 7mm2. Then, it was sequentially annealed at 325 C for 5min, at 375 C for 5 min, at 450 C for 15 min, and finally at 500 C for 15 min under ambient condition. Note that before coating TiO2Fig. 2. IeV curves of DSCs with different CEs.Materials and Devices 5 (2020) 180e184 1812.3. Assembly of DSCs The photoanodes were matched with CEs into a sandwich-type cell. They were sealed at 120 C for 5 min with a Surlyn polymer (DuPont) of 60 mm thickness. 2.4. Measurements The J-V under 1 sun, CV and EIS measurements were carried out as in our previous work [18]. The J-V characteristics of the DSCs were measured under one sun illumination from a Sun 3000 solar simulator composed of a 1000 W mercury-based Xe arc lamp and AM 1.5 G filters. A CV was used to measure the electrochemical catalysis of the electrodes. The experiments were performed in a three-electrode cell with a potentiostat (IVIUMSTAT) as in a pre- vious study. The EIS of the DSCs was measured with constant light illumination (100 mWcm
2) under open-circuit conditions. The frequency range used in the experiments was from 100 kHz to 100 mHz with a perturbation amplitude of 10 mV. The obtained spectra were fitted using Z-view software with reference to the proposed equivalent circuit. nceTable 1 Photovoltaic parameters of DSCs with different CEs. Counter electrode Jsc (mAcm
2) Voc(V) FF h (%) 5  5 mm 14.61 0.785 67.74 7.77 (±0.09) 7  7 mm 13.96 0.815 70.51 8.02 (±0.13) 8  8 mm 14.37 0.815 71.77 8.40 (±0.10) Fig. 3. Nyquist plots of DSCs with different CEs. The top image shows an equivalent circuit diagram used to fit the observed impedance spectra in this figure. The solid lines are the fitted curves. H.-L.T. Dang et al. / Journal of Science: Adva1823. Results and discussion To compare the effect of the catalytic activity on the perfor- mance of DSCs, we first conducted photovoltaic (PV) performances. The obtained results are presented in Fig. 2 and Table 1. As can be seen in Table 1, the change in Jsc value is not significant compared to that of the Voc, FF and PCE values. The Jsc value for a cell with a 5  5 mm CE was lower than those of cells with 7  7 mm and 8  8 mm CEs. It is well-known that the value of Jsc is influenced by the light-harvesting and charge collection efficiencies, the effi- ciency of the electron injection from the excited dye into the TiO2, and the efficiency of the dye regeneration. It should be noted that in this work, identical compounds were used to fabricate all devices, except for the CEs. Thus, all factors can be excluded except the last one, which is the efficiency of the dye regeneration. Therefore, the change in Jsc can be attributed to the Rct1 value, which is further confirmed by the EIS analyses. Specifically, the DSC fabricated with a 5  5 mm CE which has the REP of 25/49, showed Jsc, Voc, FF, and PCE values of 14.61 mA cm
2, 0.785 V, 67.74% and 7.77%, respec- tively. With REP of 49/49, the PV parameters were 13.96 mA cm
2 for Jsc, 0.815 V for Voc, 70.51% for FF and 8.04% for PCE. Interestingly, increasing REP to 64/49 for the DSC with a 8  8 mm CE resulted in a higher PCE value of 8.40%. Therefore, the REP is considered as an important factor to control the PCE of DSC. As a result, the ascending surface area of CE from 49mm2 to 64mm2 resulted in an increase of PCE from 8.02% to 8.41%. On the other hand, while the surface area decreasing by 48.97%, the efficiency of the device reduced by 0.35%. The decline of the surface area of CE from 49 mm2 to 25 mm2 lead to a fall in PCE from 8.02% to 7.77%. Accordingly, at increasing the surface area of CE by 30.61%, 0.39% efficiency is lost. The obtained results suggest a relationship be- tween fabrication cost and device efficiency. Note that this is an important key for designing and fabricating DSC.To study the effect of the explored Pt area in the electrolyte medium on the reduction of tri-iodide ions, we conducted EIS (Fig. 3) and Bode (Fig. 4) measurements. The EIS measurements were performed with the same DSC under 1 sun. The charge- transfer resistances of both the CE (Rct1) and the photoanode (Rct2) and also the CPE parameters for the interfaces of both cathode and photoanode were estimated through fitting Nyquist plots and were summarized in Table 2. As seen in Table 2, the Rct1 values of the 7  7 mm electrode and the 8  8 mm electrode were as small as 2.95 U and 2.46 U, respectively, while that of the 5  5 mm one was higher with 3.64 U. This difference is due to the larger surface-active area of the 7  7 mm and 8  8 mm electrodes compared to that of the 5  5 mm CE. It should be noted that a lower Rct at the CE/electrolyte interface indicates a higher reaction rate for the reduction of tri-iodide ions. On the other hand, a lower Rct1 value means a higher catalytic activity of CE. As a result, the highest catalytic activity belongs to CE (8  8 mm) while the catalytic activity of the 7  7 mm CE is higher than that of the 5  5 mm CE. The constant-phase element (CPE1¼(CPE1-T)
1(jw)-(CPE1
P)) of the CE can also be used to es- Fig. 4. Bode phase plots of DSCs with different CEs. d Materials and Devices 5 (2020) 180e184timate the surface-active area of CEs via the CPE1-T values. Gratzel et al. proposed that a higher value of CPE1-T means a larger active surface area [30]. As shown in Table 2, the CPE1-T values for the 5  5 mm electrode, 7  7 mm electrode and 8  8 mm electrode were 10.4 mF, 16.0 mF, and 18.5 mF, respec- tively. The obtained results indicated that the active surface areas of the 7  7 mm CE and 8  8 mm CE are larger than that of the 5  5 mm CE. The effect of the explored Pt area on the catalytic activity for the reduction of tri-iodide ions to iodide ions at CEs was further confirmed by the electron lifetime (t). From Bode plots, we found the fmax values for 5  5 mm, 7  7 mm and 8  8 mm CE being 1.000 104 Hz, 1.259 104 Hz and 1.585 104 Hz, respectively. t is calculated by the expression: t ¼ 1/2pfmax, where fmax is the peak frequency in the Bode plots. As calculated results, the t values for the 5  5 mm CE, 7  7 mm CE and 8  8 mm CE were 4.42 ns, 3.52 ns and 2.79 ns, respectively. It is known that a shorter lifetime indicates a faster electron transfer across the CE/electrolyte inter- face [12]. The obtained results illustrated that the speed of electron transfer across the CE/electrolyte interface of the 8  8 mm CE was higher than that of the 7  7 mm CE, whereas the lowest one was for the 5  5 mm CE. The achieved results showed that a larger explored Pt area in the electrolyte medium leads to an ectr 8  8 mm 5.29 2.46 18.5 0.94 ncedenhancement of the rate of electron transfer across the CE/elec- trolyte interface. To further confirm the catalytic activity of CEs, we conducted CV Fig. 5. CV curves of three CEs with different active areas.Table 2 Impedance parameters of DSCs with different CEs estimated from the impedance sp Counter electrode Rh (U) Rct1 (U) CPE1-T (mF) CPE1-P 5  5 mm 5.21 3.64 10.4 0.94 7  7 mm 5.29 2.95 16.0 0.91 H.-L.T. Dang et al. / Journal of Science: Advameasurements as shown in Fig. 5. For this purpose, we determined the absolute value of the redox current peak (|Jred|) and the peak-to- peak separations (DE). We found that the |Jred| value increased associated with a rising trend of the explored Pt area in the elec- trolyte medium. Specifically, the |Jred| values for the 5  5 mm, 7  7 mm, and 8  8 mm electrodes were 0.854 mA, 0.908 mA, and 0.955 mA, respectively. It was reported that a high |Jred| value re- flects a high diffusion current of ions [31], a larger active surface area [12] and more active reaction kinetics [32,33]. Furthermore, the rate-limiting step of the regeneration process of iodide ions from tri-iodide ions is estimated by the rate of the iodide ions desorption. The desorption energy, CE surface structure, surface morphology, CE roughness factor, double-layer thickness, and CE work function should be considered as the impact factors on the rate of iodide ions desorption [34e38]. This was further verified by the change in diffusion impedance (Zw) in EIS measurements (as shown in Table 2). Fig. 5 and Table 2 also show DE for different electrodes. As can be seen from the chart and the table, DE of 370 mV was obtained for the 5  5 mm CE, which is higher than those of 360 mV for the 7 7 mm CE and 8 8 mm CE. The change in DE supported the explanation of the change in Voc values for DSCs with different CEs [16]. 4. Conclusion We investigated the impact of the REP on the performance of DSCs. The obtained results indicated that REP could be used to control the PCE of devices. The PCE value increases with the in- crease of REP. On the other hand, PCE also increases while increasing the surface area of CEs or the catalytic activity of CEs. The REPs of 25/49, 49/49 and 64/49 resulted in the PCE values of 7.77%,8.02%, and 8.40%, respectively. We also found a relation between the fabrication cost and device efficiency. Our study may openways to further improve the PCE of DSCs, to reduce the cost of devices, and to the large-scale application of solar cells in the future. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgments This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2018.27. References [1] B. O'Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye- sensitized colloidal TiO2 films, Nature 353 (1991) 737e740. [2] W. Tress, K. Domanski, B. Carlsen, A. Agarwalla, E.A. Alharbi, M. Graetzel, A. 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