KR20230130868A - rGO-Cu2S counter electrode and solar cell, and method for preparing them - Google Patents

Abstract

The present invention relates to an rGO-Cu_2S counter electrode, a solar cell, and a manufacturing method thereof. The present invention provides a method of manufacturing a counter electrode to form reduced graphene oxide (rGO)-Cu_2S by a one-step electrochemical co-reduction process resulting in simultaneous reduction of graphene oxide (GO) and Cu_2+ followed by a sulfurization process. Therefore, it is possible to manufacture a novel and environmentally friendly counter electrode easily.

Description

rGO-Cu2S counter electrode and solar cell, and method for preparing them {rGO-Cu2S counter electrode and solar cell, and method for preparing them}

The present invention relates to a rGO-Cu ₂ S counter electrode, solar cell, and method for manufacturing the same.

Renewable energy has recently emerged as the best alternative energy source with regard to the development of green air. Quantum dot-sensitized solar cells (QDSSCs) have been considered as a type of third-generation solar cells and have been widely adopted due to their outstanding properties such as economical properties, easy manufacturing, and high theoretical power conversion efficiency (PCE). Quantum dots (QDs) have a tunable band gap and can also generate multiple electron/hole pairs, acting as sensitizers in QDSSCs and thus have attracted considerable attention as photovoltaic systems. Typically, a QDSSC consists of a photoanode (e.g., TiO ₂ ) sensitized with QDs (e.g., PbS, ZnS, CdS, CdSe, and SnSe), a counter electrode (CE), and a redox couple (e.g. , S ^2- /S _x ^2- ).

Despite the theoretical advantages of QDSSC, improving the low efficiency of PCE is a challenge. To increase the PCE of QDSSC, the enhancement of the catalytic activity of CE showed promising results. Therefore, CE, which catalyzes the redox reaction that occurs inside the cell, is an important component in QDSSC. Traditionally, noble metal Pt films have been mainly used as efficient CE for quantum dot dye-sensitized solar cells (QDSSC). However, Pt is not suitable as CE for QDSSC with polysulfide electrolyte because it exhibits low electrocatalytic activity due to surface poisoning with adsorbed sulfur ions. To overcome this problem, various alternative electrocatalysts have been explored, such as Cu ₂ S, CoS, NiS, and carbon materials. Among these alternatives, Cu ₂ S has been widely used due to its low cost and high electrocatalytic ability. However, the mechanical instability of Cu ₂ S as an electrocatalyst is a major limiting factor during its preparation.

To solve this problem and further improve the electrocatalytic properties of Cu ₂ S CE, several studies have introduced reduced graphene oxide (rGO) as a binding material. The large theoretical specific surface area, high thermal conductivity, large intrinsic electron mobility, and good mechanical strength made rGO an effective material to modify the CE of QDSSC. Reportedly, rGO on the surface of Cu ₂ S improved both the electrocatalytic properties of CE and the overall efficiency of QDSSC. For example, Yuan et al. investigated (rGO)-Cu ₂ S composite prepared by solvothermal process followed by spin coating and used it as CE for QDSSC; An efficiency of 4.76% was obtained. Additionally, Ye et al. prepared (rGO)-Cu ₂ S CE by a doctor blade method followed by a solvothermal procedure and obtained an efficiency of 3.85%. Similarly, Cu _x S-rGO CE was prepared by Hessein et al., and an optimal efficiency of 2.36% was obtained. However, in all these reported studies, the electrode modification of Cu ₂ S by rGO was typically performed in multiple steps using physical methods such as hydrothermal or solvothermal methods followed by doctor blending or spin coating. All these reported methods require multiple time-consuming steps, expensive temperature systems for thermal processing, and additional binding reagents, and they are also sensitive to process manipulations. Therefore, the manufacturing method of rGO-Cu ₂ S CE needs to be carefully redesigned to be more scientific, economical, fast, and easy.

The purpose of the present invention is to provide a new rGO-Cu ₂ S counter electrode, solar cell, and method for manufacturing the same.

In order to achieve the above-described object, the present invention: produces reduced graphene oxide (GO) by a one-step electrochemical co-reduction process followed by a sulfurization process resulting in simultaneous reduction of graphene oxide (GO) and Cu ²⁺ A method for manufacturing a counter electrode forming rGO)-Cu ₂ S is provided.

In the present invention, the electrochemical co-reduction process can be performed using 4 to 6 cycles of cyclic voltammetry.

In the present invention, the electrochemical co-reduction process may be an electrochemical co-deposition process using an electropotential electrodeposition method in a potential range of -1.6 to 0.8 V.

Additionally, the present invention provides a counter electrode containing rGO-Cu ₂ S prepared according to the above-described method.

In the present invention, rGO-Cu ₂ S may include Cu ₂ S microspheres completely wrapped by the rGO surface.

In the present invention, rGO-Cu ₂ S may have an amorphous structure.

In the present invention, rGO-Cu ₂ S may include 36 to 42 atomic% of Cu, 22 to 28 atomic% of C, 13 to 19 atomic% of O, and 17 to 23 atomic% of S.

In the present invention, the optical band gap of rGO-Cu ₂ S may be 1.1 to 1.4 eV.

In addition, the present invention includes the above-described counter electrode; photoanode; and a redox electrolyte disposed between the counter electrode and the photoanode.

The solar cell according to the present invention may be a quantum dot-sensitized solar cell.

In the present invention, the photoanode may include a TiO ₂ film deposited on a substrate, CdS and CdSe quantum dots deposited on the surface of the TiO ₂ film, and a ZnS layer coated on the quantum dots.

In the present invention, the redox electrolyte may be polysulfide containing S, Na ₂ S, and KCl.

The power conversion efficiency (PCE) of the solar cell according to the present invention is 3.9 to 4.8%, the short-circuit current density (J _sc ) is 16.3 to 18.4 mA/cm2, the open circuit voltage (V _oc ) is 0.55 to 0.6 V, and the charging rate is (FF) may be 43 to 48%.

The present invention provides a novel and environmentally friendly counter electrode manufacturing method by a facile one-step electrochemical co-reduction process followed by facile sulfurization. The method according to the invention is more economical, scientific, easy to operate and less time-consuming. A solar cell including a counter electrode according to the present invention exhibits excellent efficiency.

Figure 1 shows CV curves obtained from (a) electrochemical reduction of Cu ²⁺ and (b) electrochemical co-reduction of Cu ²⁺ and GO.
Figure 2 shows (a) Cu ₂ S (5), (b) rGO-Cu ₂ S (3), (c) rGO-Cu ₂ S (5), and (d) rGO-Cu ₂ S (7) films. Displays a floor plan image. The insets in (ad) show enlarged images of each film.
Figure 3 shows typical (a) TEM and (b) and (c) HRTEM images of rGO-Cu ₂ S (5); The inscribed circle in (b) represents the image of Cu ₂ S, and the inset in (c) represents the SAED pattern for rGO-Cu ₂ S (5). Additionally, EDX mappings of (d) C, (e) Cu, and (f) S are shown.
Figure 4 shows the XPS spectra of (a) GO and (b) C1s of rGO-Cu ₂ S (5) and (C) Cu 2p and (d) S 2p spectra of rGO-Cu ₂ S (5).
Figure 5 shows the CE/electrolyte/CE composition for Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5) and rGO-Cu ₂ S (7) CE, respectively. Shows the Nyquist plot of a symmetric cell. The inset shows the Nyquist plot for Pt CE and the electrical equivalent circuit model used for data fitting.
Figure 6 shows the symmetry of Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5) and rGO-Cu ₂ S (7) CEs with CE/electrolyte/CE. The Tafel polarization curve of the cell is shown.
Figure 7 shows (a) photovoltaic JV of QDSSC with Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5) and rGO-Cu ₂ S (7) CE, respectively. Performance and (b) IPCE spectrum are shown.

Hereinafter, the present invention will be described in detail.

The counter electrode manufacturing method according to the present invention is a one-step electrochemical co-reduction process that results in simultaneous reduction of graphene oxide (GO) and Cu ²⁺ , followed by a sulfurization process to produce reduced graphene oxide (rGO). -Cu ₂ S is formed.

The electrochemical co-reduction process can be carried out using cyclic voltammetry, where the number of cycles is for example 3 to 7 cycles, preferably 4 to 6 cycles, most preferably 5 cycles. You can. Solar cells (QDSSC) with rGO-Cu ₂ S (5) prepared by 5-cycle cyclic voltammetry show excellent efficiency.

Additionally, the electrochemical co-reduction process may be an electrochemical co-deposition process using an electropotential electrodeposition method in a potential range of -1.6 to 0.8 V.

Graphene oxide (GO) can be prepared from graphite powder by the modified Hummers method.

The rGO-Cu ₂ S counter electrode (CE) can be prepared by an electrochemical co-deposition method. rGO-Cu ₂ S CE can be deposited on a substrate, and fluorine-doped tin oxide (FTO), etc. can be used as the substrate, and prior to deposition, the substrate is soaked in acetone, ethanol, and deionized (DI) under ultrasonic treatment. It can be washed with water.

To disperse GO before preparing the deposition mixture, GO can be sonicated. The deposition mixture may be an aqueous solution containing a copper compound and well-dispersed GO. Copper compounds such as copper sulfate and copper nitrate can be used, and the concentration can be, for example, 5±3mM. The concentration of GO may be, for example, 0.1 ± 0.05 mg/mL. This aqueous solution can be used in an electrochemical co-deposition process using an electropotential electrodeposition method in a potential range of -1.6 to 0.8 V. The film can be dried after the co-deposition process.

Afterwards, the sulfurization process can be performed by immersing the dried rGO-Cu ²⁺ film obtained after electrochemical co-deposition into an aqueous polysulfide solution containing S and Na ₂ S. The concentrations of S and Na ₂ S may each independently be, for example, 1 ± 0.5 M. The soaking time may be, for example, 10 to 60 seconds. After the sulfurization process, the film is washed and dried to finally obtain a counter electrode thin film.

Additionally, the present invention provides a counter electrode containing rGO-Cu ₂ S prepared according to the above-described method.

rGO-Cu ₂ S may contain Cu ₂ S microspheres that are completely wrapped by the rGO surface. Cu ₂ S can have a microsphere structure, Cu ₂ S microspheres can be formed by nanoflakes, and Cu ₂ S microspheres can be completely wrapped by the rGO surface.

The thickness of rGO-Cu ₂ S may be, for example, 0.48 to 0.68 ㎛, preferably 0.53 to 0.63 ㎛, more preferably 0.56 to 0.6 ㎛. Additionally, rGO-Cu ₂ S may have an amorphous structure.

rGO-Cu ₂ S may include 36 to 42 atomic % Cu, 22 to 28 atomic % C, 13 to 19 atomic % O, and 17 to 23 atomic % S. Preferably, rGO-Cu ₂ S may comprise 37 to 41 atomic % Cu, 23 to 27 atomic % C, 14 to 18 atomic % O, and 18 to 22 atomic % S. More preferably, rGO-Cu ₂ S may include 38 to 40 atomic% Cu, 24 to 26 atomic% C, 15 to 17 atomic% O, and 19 to 21 atomic% S.

On the rGO-Cu ₂ S surface, as Cu ₂ S, the oxidation state of Cu may be +1. Additionally, in Cu ₂ S, Cu ions and S ions can form a coordination bond.

The presence of rGO can enhance the absorption near the near-infrared (NIR) region and change the optical band gap. The optical band gap of rGO-Cu ₂ S may be 1.1 to 1.4 eV, preferably 1.2 to 1.3 eV.

In addition, the present invention includes the above-described counter electrode; photoanode; and a redox electrolyte disposed between the counter electrode and the photoanode. The solar cell may be a quantum dot-sensitized solar cell (QDSSC).

The photoanode may include a TiO ₂ film deposited on a substrate, CdS and CdSe quantum dots deposited on the surface of the TiO ₂ film, and a ZnS layer coated on the quantum dots. As a substrate, an FTO substrate or the like can be used.

TiO ₂ film can be formed by coating using a doctor blading method and then sintering. The TiO ₂ film may be a mesoporous coating layer and may have a thickness of 10 ± 5 ㎛.

CdS and CdSe quantum dots (QDs) can be deposited on the surface of TiO ₂ films using the continuous ion layer adsorption and reaction (SILAR) method. In the case of CdS QDs, for example, they can be formed by sequentially immersing a TiO ₂ film in a Cd(NO ₃ ) ₂ solution and a Na ₂ S solution. The concentrations of the Cd(NO ₃ ) ₂ solution and the Na ₂ S solution may each independently be, for example, 0.1 ± 0.05 M, and the immersion time may each independently be, for example, 1 ± 0.5 minutes. The SILAR cycle can be repeated 6 ± 2 times.

For CdSe QDs. For example, it can be formed using a Cd(NO ₃ ) ₂ solution and a Na ₂ Se solution, and the concentration of each solution can be, for example, 30 ± 10 mM. Na ₂ Se solution can be prepared using 30±10mM SeO ₂ and 60±20mM NaBH ₄ solution. The SILAR process may involve dipping the TiO ₂ /CdS-QD film in a Cd(NO ₃ ) ₂ solution and then in a Na ₂ Se solution, with the immersion times each independently being, for example, 1 ± 0.5 minutes. , the SILAR cycle can be repeated 7 ± 2 times.

A ZnS passivation layer can be coated on the obtained TiO ₂ /CdS/CdSe-QD film by the SILAR process. For example, Zn(NO ₃ ) ₂ solution and Na ₂ S solution can be used, the concentration of each solution can be, for example, 0.1 ± 0.05 M, and the SILAR process can be repeated for 2 ± 1 cycles.

The redox electrolyte can be injected as a photoanode after sandwiching CdS/CdSe-QD-assembled TiO ₂ film and rGO-Cu ₂ S CE using a sealing material with a thickness of 60 ± 20 ㎛. The redox electrolyte may be a polysulfide containing S, Na ₂ S, and KCl. The concentrations of S and Na ₂ S may each independently be, for example, 1 ± 0.5 M, and the concentration of KCl may be, for example, 0.2 ± 0.1 M.

The charge transfer resistance (R _ct ) of the solar cell may be, for example, 4 to 5 Ω, preferably 4.5 to 4.9 Ω, and more preferably 4.6 to 4.8 Ω. The series resistance (R _s ) between the CE of the solar cell and the electrolyte may be, for example, 6 to 6.7 Ω, preferably 6.2 to 6.6 Ω, and more preferably 6.4 to 6.6 Ω. The constant phase element (CPE) of the solar cell is 2.91 × 10 ^-3 to 3.3 × 10 ^-3 ㎌, preferably 3 × 10 ^-3 to 3.2 × 10 ^-3 ㎌, more preferably 3.05 × 10 ^-3 to 3.15. It may be × 10 ^-3 ㎌.

The low R _ct of rGO-Cu ₂ S may be due to the introduction of rGO and rGO nanosheets on the surface of Cu ₂ S microspheres, and the nanosheets could create an efficient conductive framework in the CE film and provide mechanical strength. In addition, it can facilitate electron movement. A surface with low R _ct can not only increase the regeneration rate of the electrolyte, but also lower energy loss during electron transfer, thereby improving the overall photovoltaic performance of the solar cell.

The larger the CPE, the better the surface roughness and active surface area. The effective surface area of rGO-Cu ₂ S may be, for example, 0.085 to 0.12 cm2, preferably 0.09 to 0.115 cm2, more preferably 0.095 to 0.11 cm2.

The power conversion efficiency (PCE) of the solar cell may be, for example, 3.9 to 4.8%, preferably 4 to 4.6%, and more preferably 4.1 to 4.4%. The short-circuit current density (J _sc ) of the solar cell may be, for example, 16.3 to 18.4 mA/cm2, preferably 16.5 to 18 mA/cm2, more preferably 17 to 17.5 mA/cm2. The open circuit voltage (V _oc ) of the solar cell may be, for example, 0.55 to 0.6 V, preferably 0.56 to 0.58 V. The charge factor (FF) of the solar cell may be, for example, 43 to 48%, preferably 43.3 to 46%, and more preferably 43.5 to 45%.

The improved J _sc and FF of rGO-Cu ₂ S can be attributed to the lower R _ct and R _s values, respectively, resulting in improved efficiency of rGO-Cu ₂ S. Deposition of rGO with Cu ₂ S in rGO-Cu ₂ S CE can exhibit feasible charge transfer, improved electrical contact with the substrate surface, and enhanced active surface area. Due to its easy fabrication, low cost, and less time-consuming electrodeposition method, rGO-Cu ₂ S is an efficient electrical device for a variety of applications, including QDSSC, DSSC, (photo)electrochemical oxygen and hydrogen generation cells, and supercapacitors. It may be promising as a catalyst.

[Example]

In the present invention, we report a novel and environmentally friendly preparation method of rGO-Cu ₂ S CE by a facile one-step electrochemical co-reduction process resulting in simultaneous reduction of GO and Cu ²⁺ followed by facile sulfurization. . This method is more economical, scientific, easy to operate, and less time-consuming. Cyclic voltammetry was used for deposition without the use of additional binding reagents, followed by a facile sulfurization process to prepare rGO-Cu ₂ S CE. As a result, the QDSSC with rGO-Cu ₂ S (5) prepared by 5 cycles of cyclic voltammetry showed a maximum efficiency of 4.26%, which was better than that of Cu ₂ S (3.77%).

1. Experiment

1.1. ingredient

Graphite powder, copper nitrate pentahydrate (CuSO ₄ ·5H ₂ O), sodium sulfide nonahydrate (Na ₂ S·9H ₂ O), boric acid (H ₃ BO ₃ ), sulfur (S), cadmium nitrate tetrahydrate (Cd( NO ₃ ) ₂ ·4H ₂ O), selenium dioxide (SeO ₂ ), sodium borohydride (NaBH ₄ ), and zinc nitrate hexahydrate (Zn(NO ₃ ) ₂ ·6H ₂ O), K ₃ [Fe(CN) ₆ ] was purchased from Sigma-Aldrich. Solvents (ethanol, methanol, and acetone) were purchased from Daejeong, Korea. All chemicals were used without further purification.

1.2. Preparation of graphene oxide

Graphene oxide (GO) was prepared from graphite powder by a modified Hummers method. First, 1 mg of graphite powder was mixed with a mixture containing 23 mL of H ₂ SO ₄ (98%) and 0.5 mg of NaNO ₃ . The entire mixture was continuously stirred and cooled with an ice bath. Under vigorous stirring, 3 mg of KMnO ₄ was added in portions to the mixture within 2 hours. The reaction mixture was then allowed to reach room temperature for 4 hours and then heated to 35° C. for 30 minutes. Afterwards, the entire mixture was poured into a flask containing 50 mL of deionized (DI) water and heated again at 70°C for 15 min. Again, the entire mixture was decanted into a flask containing 250 mL of DI water, 3% H ₂ O ₂ (added to remove unreacted KMnO ₄ ). Afterwards, it was allowed to sink for a long time. Additionally, the obtained GO was purified by repeated washing and decanting for one week. Finally, the synthesized GO was oven-dried at 60°C for 48 hours.

1.3. Preparation of rGO-Cu ₂ S CE

rGO-Cu ₂ S CE was prepared by electrochemical co-deposition method. Prior to deposition, the FTO substrate was washed with acetone, ethanol, and DI water respectively under sonication for 5 min. For dispersion of GO before preparing the deposition mixture, 0.1 mg/mL GO was sonicated in DI water for 1 h. Finally, an aqueous solution containing 5 mM CuSO ₄ and well-dispersed GO was used in an electrochemical co-deposition process using an electropotential electrodeposition method in the potential range of -1.6 to 0.8 V. The reduction peak began to appear at -0.4 V due to the reduction of Cu ²⁺ ions (see Figures 1a and 1b); Additionally, a large reduction peak appeared at -1.1 V due to the reduction of various oxygen functional groups of GO (see Figure 1b) and continued to increase with increasing number of cycles. The enhanced peak current with increased cycle number may be due to the modification of the film with the rGO surface, which exhibits increased electrochemically active sites. And this also indicated the successful reduction of rGO on the FTO surface. Afterwards, the obtained film was washed with DI water and dried in a dryer. For comparative analysis, two other similar complexes with cycle numbers of 3 and 7 were prepared.

Afterwards, the dried rGO-Cu ²⁺ film (5) obtained after electrochemical co-deposition was immersed in an aqueous polysulfide solution containing 1 MS and 1 M Na ₂ S for 30 s, followed by washing and drying. Finally, the obtained thin film was labeled with rGO-Cu ₂ S (5). A similar process was followed to prepare rGO-Cu ₂ S (3) and rGO-Cu ₂ S (7) CE. Meanwhile, Cu ₂ S CE was also prepared by an electrodeposition method with 5 cycles using a solution containing 5 mM CuSO ₄ followed by sulfurization process, which was labeled as Cu ₂ S (5). Pt CE was also prepared by doctor blading method using Pt-nanocluster-containing Pt paste (PT-1, Dyesol, Ltd.) on FTO substrate followed by calcination at 450°C for 30 min.

1.4. Fabrication of FTO/TiO ₂ /CdS/CdSe/ZnS QD photoanode

A compact blocking layer of TiO ₂ was deposited on the FTO substrate by immersing the FTO substrate in an aqueous solution of 40 mM TiCl ₄ and heating at 70° C. for 30 minutes. A mesoporous layer of TiO ₂ was coated using the doctor blading method. TiO ₂ paste (Ti-Nanoxide T/SP, Solaronix SA) was used, dried at 70°C, and then sintered at 450°C for 30 minutes. . Afterwards, a 10 μm thick TiO ₂ film with a measured active area of 0.16 cm 2 was obtained. CdS and CdSe QDs were deposited on the surface of TiO ₂ film using the continuous ion layer adsorption and reaction (SILAR) method. This method involves soaking the FTO/TiO ₂ film in a 0.1 M Cd(NO ₃ ) ₂ ethanol solution for 1 min, washing with ethanol and drying, and further soaking in 0.1 M Na ₂ S methanol/DI water (1: 1 volume) followed by soaking in the solution, followed by washing again with ethanol and drying. These two soak steps included one SILAR cycle and were repeated six times. Afterwards, CdSe QDs were added to the obtained FTO/TiO ₂ /CdS-QD film electrode by a similar SILAR process. For this process, CdSe QDs were obtained using 30 mM Cd(NO ₃ ) ₂ ethanol solution and Na ₂ Se ethanol solution, where Na ₂ Se solution was prepared using 30 mM SeO ₂ and 60 mM NaBH ₄ solution. did. The SILAR process involves dipping the FTO/TiO ₂ /CdS-QD film in a Cd(NO ₃ ) ₂ solution for 1 minute, washing with ethanol and drying with a dryer, further immersing in a Na ₂ Se solution, and again with ethanol. It involved washing and drying steps. As before, the two-step soaking process included one SILAR cycle and was repeated seven times. Finally, a ZnS passivation layer was coated on the obtained FTO/TiO ₂ /CdS/CdSe-QD film by the SILAR process, where the solution was 0.1 M Zn(NO ₃ ) ₂ ethanol solution and 0.1 M Na ₂ S methanol/DI water. (1:1 volume) solution. The SILAR process was repeated for 2 cycles.

1.5. cell fabrication

CdS/CdSe-QD-assembled TiO ₂ film and Pt, Cu ₂ S, rGO-Cu ₂ S (3), rGO-Cu ₂ S (5), and rGO-Cu ₂ S (7) CE as photoanode. It was sandwiched using a 60 ㎛ thick sealing material (Surlyn). Polysulfide containing 1 MS, 1 M Na ₂ S, and 0.2 M KCl in methanol/DI water (7:3 volume ratio) was used as the redox electrolyte.

1.6. Characteristic evaluation

The surface morphology of CE was observed by field-emission scanning electron microscopy (FE-SEM) using an S-4800 (Hitachi, Japan) system operating at an acceleration voltage of 10 kV and a working distance of 7 mm. The morphology, structure, and elemental composition of rGO-Cu ₂ S (5) were studied using high-resolution transmission electron microscopy (HR-TEM), limited-field electron diffraction (SAED), and elemental mapping.

The crystalline properties of the samples were analyzed by X-ray diffraction (XRD; PANalytical X'Pert). The reduction of GO and the chemical composition of the rGO-Cu ₂ S film were confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS, omicron Nanotechnology ESCA probe system). The obtained measurements and core-level spectra were processed using Casa XPS software. The photovoltaic current-voltage (JV) characteristics of the QDSSC were measured under 1 solar irradiance (100 mW/cm2, AM 1.5G) using a xenon lamp solar simulator (PEC-L12, Peccell Ltd., Japan), which is based on the AIST- This was confirmed using a calibrated Si-solar cell. Additionally, the incident-photon-to-current efficiency (IPCE) spectrum of QDSSC was analyzed. Electrochemical impedance spectroscopy (EIS) and Tafel polarization were performed using an electrochemical workstation (Metrohm PGSTAT302N, AUTOLAB, Netherlands) using a two-electrode symmetric cell (CE/electrolyte/CE) with an active area of 0.96 cm2. . Under dark conditions, a frequency range between 10 ^-1 and 10 ⁵ Hz with an amplitude of 10 mV was used for EIS measurements. EIS results were obtained at 10 points per decade and fitted by non-linear least squares using Z-View 2 software. Using cyclic voltammetry, the electrochemically active area of the prepared films was evaluated in the potential range of -0.2 to 0.6 V at various scan rates. Tafel analysis of symmetric cells was performed at a scan rate of 5 mV/s. All electrochemical measurements were performed at room temperature.

2. Results and discussion

2.1. physical properties

The surface morphologies of rGO-Cu ₂ S (5), Cu ₂ S (5), rGO-Cu ₂ S (3), and rGO-Cu ₂ S (7) CE were investigated using FE-SEM. Figure 2a shows the microsphere structure of Cu ₂ S (5) before deposition of rGO, containing numerous petal-shaped bodies. After electrochemical co-deposition, the obtained rGO-Cu ₂ S (3) exhibited a large-flowered rGO structure, which anchored the Cu ₂ S microspheres inside (see Figure 2b). Additionally, rGO-Cu ₂ S (5) exhibited Cu ₂ S microspheres, which were completely wrapped by the rGO surface (see Figure 2c); Additionally, the density of the film components increased with the number of cycles. Another composite, rGO-Cu ₂ S (7), showed significantly denser and more irregular wrapping of Cu ₂ S on the rGO surface after 7 cycles of deposition (see Figure 2d). All three CE surfaces showed that the deposition of rGO and Cu improved with an increase in the number of deposition cycles. Some of the oxygen functional groups present on the rGO surface may interact with Cu ions during the deposition of the film, which may be important in enhancing the deposition as well as the growth of Cu ₂ S in all three CEs. Additionally, cross-sectional SEM images of all electrodeposited CEs were also examined by FE-SEM. Both Cu ₂ S (5) and rGO-Cu ₂ S (5) showed similar thicknesses of 0.55 ㎛ and 0.58 ㎛, respectively. However, the rGO surface thickness varied depending on the extent of the reduction process. The other two CEs, rGO-Cu ₂ S (3) and rGO-Cu ₂ S (7) CE, had film thicknesses of 0.48 μm and 0.68 μm, respectively. Notably, differences in the thickness of different films may be due to the different number of cycles used in the deposition process.

The detailed structure of rGO-Cu ₂ S (5) was observed through transmission electron microscopy (TEM) (see Figure 3). As revealed by closer inspection, Cu ₂ S microspheres were formed by nanoflakes (see Figure 3a), and these microspheres were covered with a thin layer of rGO in the rGO-Cu ₂ S structure, as shown by HR-TEM images. was (see Figures 3b and 3c). In all three images, the dark areas on the Cu ₂ S nanoflakes are visible due to the stacking of rGO nanostructures with some of the oxygen functional groups, while the most highly transparent and thin films are rGO formed by electrochemical co-reduction. Indicates the layer. The inset (see Figure 3c) shows the SAED pattern for the rGO-Cu ₂ S (5) surface. It exhibits two characteristic halo ring patterns, indicating that the deposited component has an amorphous nature in the produced film. Double rings were obtained from the multilayer rGO surface, and they corresponded to (110) and (100). The crystalline properties of rGO-Cu ₂ S (5) were further analyzed by XRD. The absence of diffraction peaks from both rGO and Cu ₂ S in the XRD pattern indicates that the deposited film has an amorphous structure as confirmed by the SAED image. It was also concluded that the amorphous nature of Cu ₂ S was formed after the sulfurization process, and this result is consistent with previous reports. EDX mapping revealed a uniform distribution of C, Cu, and S (see Figures 3d, 3e _, and 3f) in the as-prepared rGO-Cu 2 S, which further suggests the synthesis of rGO-Cu ₂ S by electrochemical co-reduction method. Confirm with .

Importantly, the effective reduction of GO after the electrochemical co-deposition process in rGO-Cu ₂ S (5) film was confirmed by FTIR and XPS. In the FTIR spectrum, GO showed characteristic peaks for various oxygen functional groups. Strong characteristic peaks due to stretching vibrations of C=O and CO groups at 1017 and 1798 cm ^-1 , respectively, were observed in the GO sample prepared by the Hummers method. Additionally, the peak at 1671 cm ^-1 was obtained due to the remaining C=C stretching vibration, while the peak around 3000 cm ^-1 was obtained due to the stretching vibration of OH from both alcohol (-OH) and carboxyl functional group (-COOH). represents. However, in the CE prepared after electrochemical co-deposition, rGO-Cu ₂ S (5) showed a strong peak at 1652 cm ^-1 due to C=C stretching vibration, which became stronger after the reduction process. Additionally, the characteristic peaks due to oxygen functional groups C=O and CO (at 1017 and 1798 cm ^-1 , respectively) almost disappeared in rGO-Cu ₂ S (5). Therefore, as these results show, GO was successfully synthesized and reduced to rGO after electrochemical co-reduction.

The XPS results are shown in Figure 4. The chemical composition of the rGO-Cu ₂ S composite was 39.2% Cu, 24.8% C, 15.6% O, and 20.4% S. The obtained elemental composition further confirmed the uniform distribution of all elements and the synthesis of rGO-Cu ₂ S CE. C1s spectra of GO (see Figure 4a) and rGO-Cu ₂ S (5) (see Figure 4b) were observed; Characteristic peaks around 284.8, 286.7, and 288.2 eV confirmed the presence of CC, epoxy, and alkoxy CO groups, and carboxyl -COOH groups, respectively, in both samples. The main XPS peaks were observed at 932.1 and 952.1 eV on the rGO-Cu ₂ S (5) surface due to Cu 2p _3/2 and Cu 2p _1/2 , respectively (see Figure 4c). Additionally, no satellite peaks were observed. These results confirm the +1 oxidation state of Cu, as Cu ₂ S, on the prepared rGO-Cu ₂ S (5) surface. The XPS spectrum in Figure 4d shows two peaks at binding energies of 161.4 and 162.4 eV for S 2p _3/2 and S 2p _1/2 , respectively; These peaks can be attributed to S ions coordinated with Cu ions in Cu ₂ S.

In addition, ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy was performed to investigate the optical band gaps of Cu ₂ S (5) and rGO-Cu ₂ S (5) films. According to the extinction spectra of Cu ₂ S (5) and rGO-Cu ₂ S (5), the presence of rGO enhanced the absorption near the NIR region. The band gap of Cu ₂ S with and without rGO was estimated using a Tauc plot and the following equation.

[Equation 1]

(αhν) ^1/n = A(hν - E _g )

α represents the extinction coefficient, h is Planck's constant, ν is the photon frequency, A is a constant, and E _g is the optical bandgap. The value of n is 1/2 and 2 for direct and indirect bandgap materials, respectively. E _g can be calculated by plotting (αhν) ² versus hν and extrapolating the linear portion to the X-axis intercept. The optical band gap was estimated to be 1.41 eV and 1.27 eV for Cu ₂ S (5) and rGO-Cu ₂ S (5), respectively, indicating that the presence of rGO in the rGO-Cu ₂ S film changed the optical band gap. represents. The energy band diagram based on the obtained E _g level and Tauc plot shows the charge transfer between rGO-Cu ₂ S and S ^2- /S _x ^2- . Some of the oxygen functional groups on the rGO surface act as defect sites, providing sufficient light absorption to form excess electron-hole (eh) pairs. Additionally, this facilitates electron transfer towards the electrode by reducing band-to-band recombination.

2.2. Electrochemical properties

Figure 5 shows EIS measurements of various symmetric cells containing Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5) and rGO-Cu ₂ S (7) CE, respectively. The obtained Nyquist plot is shown; The inset diagram shows the corresponding equivalent electrical circuit. R _ct is the charge transfer resistance, R _s is the series resistance between CE and the polysulfide redox couple, and CPE is the constant phase factor. All fitted EIS parameters are shown in Table 1. Table 1 shows the Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5), and rGO-Cu ₂ S (7) estimates from the Nyquist plot of the symmetric cell for CE. Indicates EIS characteristics.

Sample R _s /Ω R _ct /Ω CPE (㎌) Pt 11.9 1132.5 1.49 × 10 ^-3 Cu ₂ S (5) 12.5 5.05 2.66 × 10 ^-3 rGO-Cu ₂ S (3) 7.5 12.4 2.88 × 10 ^-3 rGO- _Cu2S (5) 6.5 4.7 3.08 × 10 ^-3 rGO- _Cu2S (7) 6.8 6.2 2.90 × 10 ^-3

Among all measured CE, rGO-Cu ₂ S (5) showed the lowest R _ct and R _s (4.7 and 6.5 Ω) values; The R _ct and R _s values of the different CEs measured were: Pt (1132.5 and 11.9 Ω), Cu ₂ S (5.05 and 12.5 Ω), rGO-Cu ₂ S (3) (12.4 and 7.5 Ω), and rGO-Cu ₂ S (7) (6.2 and 6.8 Ω). Obviously, rGO-Cu ₂ S (5) showed lower R _ct than Cu ₂ S (5), probably due to the introduction of rGO. The lowest R _ct in the case of rGO-Cu ₂ S (5) can be attributed to the rGO nanosheets on the surface of Cu ₂ S microspheres; The nanosheet not only provided mechanical strength but also facilitated electron transfer by creating an efficient conductive framework in the CE film. As is well known, the influence of CE on the performance of QDSSC is mainly due to its conductivity and electrocatalytic activity. A surface with a minimum R _ct can not only increase the regeneration rate of the electrolyte in the cell but also lower the energy loss during electron transport, which will consequently improve the overall photovoltaic performance of the QDSSC cell. However, R _s decreased after the introduction of rGO in the composite compared to Cu ₂ S alone, probably due to the enhancement of electrical contact between rGO-Cu ₂ S CE and FTO substrate. This also indicates an easier mode of electron transfer on the rGO-Cu ₂ S (5) surface, and the corresponding QDSSC improved photovoltaic performance. In the case of rGO-Cu ₂ S (7), the compact morphology obtained from the deposition process could influence the charge transfer process, resulting in a slightly higher R _ct ; However, the extremely high R _ct obtained for Pt CE may be due to the low electrocatalytic properties in the polysulfide electrolyte. Sulfide ions adsorbed on the Pt surface have a poisoning effect and can ultimately inhibit the reduction process Sn ^2- +2e (CE) = Sn _-1 ^2- + S ^2- . Additionally, the CPE value (see Table 1) was evaluated, which is directly correlated to the surface area and porosity of the material. A slightly larger CPE was obtained for rGO-Cu ₂ S CE than for Cu ₂ S, probably due to the enhancement of surface unevenness after the introduction of rGO, which could further improve the roughness. In most cases, CPE is assumed to be proportional to the active surface area. This indicates that rGO and Cu ₂ S were rough and had a large surface area in the synthesized rGO-Cu ₂ S CE.

Peak anode current-scan rate square root curves were obtained from cyclic voltammetry curves measured in 0.1 M [Fe(CN)] ^3- solution. The electroactive surface area of the electrode material can be estimated from these curves using the Randles-Sevcik equation.

[Equation 2]

I _p = (268, 600)AD ^1/2 n ^3/2 υ ^1/2 C

Here, I _p is the peak anode current (A), n is the number of electrons participating in the redox process, A is the effective surface area (㎠), and D is the diffusion coefficient of [Fe(CN) ₆ ] ^3- (6.7 × 10 ^-6 cm2/s), C is the concentration of electrolyte (mol/L), and υ is the scan speed (V/s). rGO-Cu ₂ S (5) showed the highest effective surface area (0.102 cm2), while Cu ₂ S (5) showed a surface area of 0.083 cm2. rGO-Cu ₂ S (3) and rGO-Cu ₂ S (7) had surface areas of 0.077 cm 2 and 0.051 cm 2, respectively, which increased the electrochemically active sites of the films with an appropriate amount of rGO co-deposited, according to Nyquist. As shown in the plot, it shows improved charge transfer kinetics. Despite a similar thickness to Cu ₂ S (5), rGO-Cu ₂ S (5) provided significantly reduced R _ct and R _s and increased electroactive surface area, which suggests that the improved performance is comparable to that of rGO in Cu ₂ S. This indicates that it is due to the introduction of .

To evaluate the catalytic properties of all prepared CEs, Tafel polarization technique, an effective electrochemical method, was used. Figure 6 shows the Tafel properties of Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5), and rGO-Cu ₂ S (7). For Tafel measurements, a symmetric cell (CE/electrolyte/CE) was prepared and the qualitative relationship between J ₀ and R _ct was observed using the following equation.

[Equation 3]

J ₀ = RT/nFR _ct

where J ₀ is the exchange current density, R is the gas constant, T is the temperature, F is the Faraday constant, n is the number of electrons involved in the redox process of the polysulfide electrolyte on the surface of the CE, and R _ct is the charge transfer resistance. represents. In the Tafel plot, a large slope of the curve indicates high J ₀ and good electrocatalytic activity of the corresponding CE. The slope of rGO-Cu ₂ S (5) was the steepest among all other CEs, showing the highest J ₀ and highest electrocatalytic properties; This result is also consistent with that obtained from the Nyquist plot, where rGO-Cu ₂ S has the lowest R _ct . In the case of Cu ₂ S-5, the slope was less steep than that of rGO-Cu ₂ S (5), indicating poor catalytic properties. Therefore, as Tafel results show, rGO-Cu ₂ S (5) has the highest catalytic activity among all other CEs. However, with increasing and decreasing number of deposition cycles, rGO-Cu ₂ S (7) and rGO-Cu ₂ S (3) showed relatively poor performance.

JV characteristics of QDSSC devices composed of Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5), and rGO-Cu ₂ S (7) CE, respectively, under 1 solar irradiance. Measured (see Figure 7a). All four most important parameters of JV characteristics, namely open circuit voltage (V _oc ), short-circuit current density (J _sc ), charging factor (FF), and PCE (η), were recorded and presented in Table 2. Table 2 shows the photovoltaic JV characteristics of QDSSCs with Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5) and rGO-Cu ₂ S (7) CE, respectively. .

Sample J _sc (mA/㎠) V _oc (V) FF(%) η (%) Pt 12.04 ± 0.05 0.47 ± 0.005 42.3 ± 0.4 2.39 ± 0.01 Cu ₂ S (5) 16.2 ± 0.03 0.56 ± 0.007 41.4 ± 0.3 3.77 ± 0.01 rGO-Cu ₂ S (3) 14.6 ± 0.09 0.54 ± 0.014 42.8 ± 0.3 3.38 ± 0.09 rGO- _Cu2S (5) 17.2 ± 0.07 0.56 ± 0.007 44±0.3 4.26 ± 0.02 rGO- _Cu2S (7) 16±0.09 0.55 ± 0.02 43.2 ± 0.2 3.83 ± 0.06

Among all QDDSC cells, QDSSC with rGO-Cu ₂ S (5) CE showed the best performance with the highest efficiency of 4.26%, J _sc of 17.2 mA/cm2, V _oc of 0.58 V, and FF of 44%. However, QDSSC cells with Pt, Cu ₂ S, rGO-Cu ₂ S (3), and rGO-Cu ₂ S (7) CE showed efficiencies of 2.45%, 3.77%, 3.01%, and 3.83%, respectively. The improved J _sc and FF of rGO-Cu ₂ S (5) can be attributed to the lowest R _ct and R _s values, respectively, while the lower performance of Pt CE can be attributed to the highest R _ct . The electronic conductivity and electrocatalytic activity of CE significantly affect the performance of QDSSC. The reduced R _ct improves the regeneration rate of the redox-coupled electrolyte, suppressing the back-electron recombination rate at the photoanode/electrolyte interface, thereby improving J _sc . Reduced R _s indicates reduced internal resistance and improves the FF of the solar cell. Based on these facts, the introduction of rGO together with Cu ₂ S improved the FF value from 41.4% to 44% in the case of Cu ₂ S (5) and rGO-Cu ₂ S (5); This is likely due to an almost 50% reduction in R _s . Therefore, the excellent electrochemical results obtained have the lowest R _ct and R _s values with the measured symmetric cell, which is due to the highest J _sc and FF, resulting in the maximum efficiency of rGO-Cu ₂ S (5). The efficiency of this QDSSC cell containing rGO-Cu ₂ S (5) CE is similar to and also superior to some of the previously reported results based on different fabrication methods (see Table 3). Table 3 compares the results between the obtained photovoltaic parameters and those obtained from previously reported similar counter electrodes.

C.E. method QD J _sc
(mA/㎠) V _oc
(V) FF
(%) η
(%) rGO/ _Cu2S solvothermal heat CdS/CdSe 17.11 0.58 48 4.76 rGO/ _Cu2S solvothermal heat CdS/CdSe 15.85 0.55 44 3.85 rGO/ _Cu2S spin coating CdS/CdSe 18.4 0.51 46 4.40 _Cu2S @rGO sequence CdS/ZnS 8.67 0.57 47 2.36 rGO- _Cu2S (5) Electrochemical co-reduction CdS/CdSe/ZnS 17.2 0.56 44 4.26

Finally, the IPCE spectra of QDSSCs with Pt, Cu ₂ S (5), rGO-Cu ₂ S (3), rGO-Cu ₂ S (5), and rGO-Cu ₂ S (7) CE, respectively, were analyzed by wavelength ( observed as a function of nm) (see Figure 7b). QDSSCs with as-prepared rGO-Cu ₂ S (5) CE showed the highest IPCE among those prepared with all other CEs, a result that is also consistent with the JV results. Overall, the introduction of rGO together with Cu ₂ S in the prepared rGO-Cu ₂ S (5) CE significantly improved the reactivity of the surface by lowering both R _ct and R _s values, thereby improving the photovoltaic performance of QDSSC. I ordered it.

3. Conclusion

A new electrochemical co-reduction method was introduced for the preparation of rGO-Cu ₂ S CE. The fabricated electrode showed a remarkable improvement in catalytic properties for the reduction of polysulfide redox couples (S _n ^{2 -} to S ^{2 -} ), as evidenced by Nyquist plots and Tafel polarization curves. Fabricated rGO-Cu ₂ S (5) The deposition of rGO with Cu ₂ S in CE showed feasible charge transfer, improved electrical contact with the FTO surface, and enhanced active surface area. As a result, QDSSC based on rGO-Cu ₂ S (5) CE showed the highest PCE of 4.26%, which was due to the enhancement of FF and J _sc compared to Cu ₂ S CE (3.77%). Due to its easy fabrication, low cost, and less time-consuming electrodeposition method, rGO-Cu ₂ S is an efficient electrical device for a variety of applications, including QDSSC, DSSC, (photo)electrochemical oxygen and hydrogen generation cells, and supercapacitors. It may be a promising platform as a catalyst.

Claims (13)

Counter electrode forming reduced graphene oxide (rGO)-Cu ₂ S by a one-step electrochemical co-reduction process resulting in simultaneous reduction of graphene oxide (GO) and Cu ²⁺ followed by a sulfurization process. Manufacturing method. According to paragraph 1,
A counter electrode manufacturing method in which the electrochemical co-reduction process is performed using 4 to 6 cycles of cyclic voltammetry. According to paragraph 1,
The electrochemical co-reduction process is a counter electrode manufacturing method that is an electrochemical co-deposition process using an electropotential electrodeposition method in a potential range of -1.6 to 0.8 V. A counter electrode containing rGO-Cu ₂ S prepared according to claim 1. According to paragraph 4,
rGO-Cu ₂ S is a counter electrode containing Cu ₂ S microspheres completely wrapped by the rGO surface. According to paragraph 4,
rGO-Cu ₂ S is a counter electrode with an amorphous structure. According to paragraph 4,
rGO-Cu ₂ S is a counter electrode containing 36 to 42 atomic% of Cu, 22 to 28 atomic% of C, 13 to 19 atomic% of O, and 17 to 23 atomic% of S. According to paragraph 4,
The optical band gap of rGO-Cu ₂ S is a counter electrode of 1.1 to 1.4 eV. Counter electrode according to clause 4;
photoanode; and
A solar cell containing a redox electrolyte disposed between a counter electrode and a photoanode. According to clause 9,
The solar cell is a quantum dot-sensitized solar cell. According to clause 9,
The photoanode is a solar cell comprising a TiO ₂ film deposited on a substrate, CdS and CdSe quantum dots deposited on the surface of the TiO ₂ film, and a ZnS layer coated on the quantum dots. According to clause 9,
A solar cell in which the redox electrolyte is a polysulfide containing S, Na ₂ S, and KCl. According to clause 9,
The power conversion efficiency (PCE) of the solar cell is 3.9 to 4.8%, the short-circuit current density (J _sc ) is 16.3 to 18.4 mA/cm2, the open circuit voltage (V _oc ) is 0.55 to 0.6 V, and the charging rate (FF) is Solar cells at 43 to 48%.