Carbon nanotube/metal-sulfide composite flexible electrodes for high-performance quantum dot-sensitized solar cells and supercapacitors

Carbon nanotubes (CNT) and metal sulfides have attracted considerable attention owing to their outstanding properties and multiple application areas, such as electrochemical energy conversion and energy storage. Here we describes a cost-effective and facile solution approach to the preparation of metal sulfides (PbS, CuS, CoS, and NiS) grown directly on CNTs, such as CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS flexible electrodes for quantum dot-sensitized solar cells (QDSSCs) and supercapacitors (SCs). X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy confirmed that the CNT network was covered with high-purity metal sulfide compounds. QDSSCs equipped with the CNT/NiS counter electrode (CE) showed an impressive energy conversion efficiency (η) of 6.41% and remarkable stability. Interestingly, the assembled symmetric CNT/NiS-based polysulfide SC device exhibited a maximal energy density of 35.39 W h kg−1 and superior cycling durability with 98.39% retention after 1,000 cycles compared to the other CNT/metal-sulfides. The elevated performance of the composites was attributed mainly to the good conductivity, high surface area with mesoporous structures and stability of the CNTs and the high electrocatalytic activity of the metal sulfides. Overall, the designed composite CNT/metal-sulfide electrodes offer an important guideline for the development of next level energy conversion and energy storage devices.


Results and Discussion
The morphology of the carbonaceous materials with metal sulfides (CNT/PbS, CNT/CuS, CNT/CoS, and CNT/ NiS) on Ni-foam were confirmed by FESEM. As shown in Figure 1(a,a1), the CNTs were randomly entangled with an outer diameter of approximately 10 nm. Figure 1(b,b1) presents the surface morphology of the CNT/PbS composite. During the deposition of PbS on CNT, the PbS nanocubes were deposited uniformly on the surface  Figure 1(c,c1) and 1(d,d1) shows the nanoparticle morphology of CuS and CoS deposited on the CNT surface. The diameters of the CuS and CoS nanoparticles on CNT's were in the range of 38-63 nm and 36-94 nm, respectively. On the other hand, uniform agglomerated NiS nanoparticles were observed on the CNT surface ( Fig. 1(e,e1)). A uniform dispersion of NiS aggregates on the CNTs increased the number of reactive sites for the reaction. As a result, the void spaces between the CNTs were filled with metal sulfides that formed a network wrapping the CNTs. This resulted in improved electronic conductivity between the metal sulfides and CNT network, yielding more defects and an enhancement of the charge transport process compared to the bare CNT material. Furthermore, EDX and elemental mapping ( Fig. 1(e2)) was used to identify the uniformity of the elemental distribution in CNT/NiS sample, where Ni and S are decorated uniformly and densely on the surface of the CNTs, denoting that the Ni and S atoms are well deposited on the CNT structure, the resulting C content is lower than that of the other elements. Figure 2(a) presents a schematic representation of the CNT/metal-sulfides. The electrons in CNT/metal-sulfides find the shortest path to accelerate charge transport and facilitate reduction of the polysulfide electrolyte compared to the bare CNT network, which encourages the enhanced electrocatalytic activity of the composite electrode material to enable higher QDSSC and SC performance ( Fig. 2(b)). The CNT/metal-sulfide composites were characterized further by XPS. Figure 3(b-f) depicts the XPS survey spectra of CNT, CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS, respectively. The survey spectra of all the electrodes affirm the existence of C and the presence of trace amounts of O in all films is correlated with moisture or oxygen molecules. The high resolution XPS spectra of C, Pb, Cu, Co, Ni and S elements in all electrodes are shown in Fig. S1 †. Figure 3(c) shows the two main peaks of Pb 4 f located at 137.4 and 142.3 eV, which were assigned to Pb 4f 7/2 and Pb 4f 5/2 , respectively. In the case of the XP spectrum of CNT/CuS (Fig. 3(d)), the Cu 2p 3/2 , and Cu 2p 1/2 peaks were detected at 932.4 and 952.7 eV, respectively, and the CNT/CoS spectrum in Fig. 3(e) clearly reveals the intense Co 2p 3/2 and Co 2p 1/2 peaks at 779.5 eV and 794.6 eV, respectively. As shown in Fig. 3(f), the two main peaks separated by 18.2 eV at 856.1 eV and 874.3 eV correspond to Ni 2p 3/2 and Ni 2p 1/2 , respectively. All samples in Fig. 3(b-f) exhibiting a peak of S 2p at 162.3 eV were indexed to sulfide. Table S1 (Supporting Information) lists the amounts of elements that exist in all the CNT/metal-sulfide samples. The C content decreased with the further deposition of metal sulfides on the CNT surface. Interestingly, a lower C content was observed in the CNT/NiS sample, suggesting that the residual NiS was well enclosed inside and on the surface of the CNT matrix to enable higher electrocatalytic activity. In addition, the higher sulfur content in CNT/NiS will lead to more defects in the form of metal vacancies, resulting in an increase in the electrical conductivity and catalytic activity of CE. The XPS results were unambiguous and corroborated that the metal sulfides had been coated successfully on the CNT surface. Detailed structural and morphological studies of the CNT/metal-sulfides were examined by FE-TEM and the corresponding low-magnified and high-magnified TEM images are shown in Fig. 4. In addition, the crystal structures of the CNT/metal-sulfides were crosschecked by measuring the lattice spacing. TEM showed well-dispersed CNTs with a diameter of 20 nm in all CNT/metal-sulfides. Figure 4(a,a1) shows that the PbS nanocubes were  spread over the CNT surface with lattice fringes with a d spacing of 0.2916 nm ( Fig. 4(a2)), which was indexed to the (200) plane of PbS. The CuS/CNT composite ( Fig. 4(b,b1)) showed a conductive CNT network coated with CuS nanoparticles. Figure 4(b2) shows that the lattice spacing's are 0.2809 nm and 0.336 nm, corresponding to the (103) and (002) crystal planes of the CuS and CNT phase, respectively. The nanoparticle-like CoS structures were assembled on surface of the CNTs in the CNT/CuS composite ( Fig. 4(c,c1)) and the d-spacing extracted from the lattice fringes were 0.1934 nm and 0.33361 nm ( Fig. 4(c2)), which were indexed to the (102) and (002) planes of the CoS and CNT phases, respectively. Figure 4(d,d1) shows the agglomerated NiS nanoparticles with CNTs and the size corresponds approximately to that indicated by the SEM images. The high resolution TEM image of CNT/ NiS in Fig. 4(d2) shows an interplanar spacing of 0.3361 nm and 0.1947 nm corresponding to the (002) and (102) planes of the CNT and NiS phase, respectively. Therefore the TEM images of CNT/metal-sulfides are consistent with the SEM and XRD analyses.
Furthermore, the specific surface area and porosity of the CNT/metal-sulfides were measured by nitrogen adsorption/desorption analysis. CNT/NiS film exhibits a higher Brunauer-Emmett-Teller (BET) surface area (222.5 m 2 g −1 ) than the CNT/CoS (147.2 m 2 g −1 ), CNT/CuS (106.3 m 2 g −1 ), CNT/PbS (29.6 m 2 g −1 ) and CNT (12.5 m 2 g −1 ) films, respectively ( Fig. 5(a)). Moreover, CNT/NiS has the pore size distribution of 12.5 and 16.1 nm, To examine the mechanism for the charge transfer behavior from CE to the electrolyte, EIS was conducted in CNT/metal-sulfide based symmetrical cells. The Nyquist plots in Fig. 5(b) show two semicircles illustrating the impedance behavior of the symmetric cell. The series resistance (R s ) was extracted at the high frequency intercept on the real axis of the plot, while the first semicircle in the middle region originated from the charge transfer resistance (R ct ) and the corresponding chemical capacitance (C μ ) at the interface of CE/electrolyte. The linear portion of the small circle in the low-frequency region is related to the Warburg diffusion impedance (Z W ) with in the electrolyte 45,46 . The Nyquist plots were fitted by the equivalent circuit demonstrated in the inset of Fig. 5(b), and the corresponding parameters are listed in Table 1.
The R s value of CNT/NiS CE (6.11 Ω) was much smaller than the CNT (10.06 Ω), CNT/PbS (8.51 Ω), CNT/CuS (7.72 Ω), and CNT/CoS (7.17 Ω), respectively. The smallest R s value of CNT/NiS was assigned to the improved conductivity of Ni-foam with CNT/NiS than other CEs. The reduced R s value could lead to an improved FF of the QDDSCs 47 . The difference in electrocatalytic activity among the CEs is associated mainly with the R ct and Z w values. The sample of CNT/NiS exhibited smaller R ct (13.85 Ω) and Z w (5.02 Ω) values than the samples of CNT (R ct = 59.05 Ω, Z w = 10.05 Ω), CNT/PbS (R ct = 47.52 Ω, Z w = 9.78 Ω), CNT/CuS (R ct = 31.49 Ω, Z w = 7.84 Ω), and CNT/CoS (R ct = 26.02 Ω, Z w = 5.02 Ω), showing that the CNT/NiS electrode exhibits enhanced electrocatalytic activity toward polysulfide reduction. This activity might have been due to the higher surface area because of the uniform agglomerated NiS nanoparticle distribution on the CNT network, as well as to the large reduction activity of NiS. This indicates that the CNT/NiS electrode can effectively catalyze the reduction of the polysulfide electrolyte due to the low R ct at the interface of the CE/electrolyte, leading to the high performance of the QDSSC with CNT/NiS CEs.
To reconfirm the electrocatalytic activity of the CNT/metal-sulfide composite catalysts, Tafel polarization curves were measured using symmetrical cells. Figure 5(c) shows the Tafel curves with the logarithmic current density as a function of the voltage in the polysulfide electrolyte. The exchange current density (J 0 ) can be extracted from the extrapolated intercepts of the anodic and cathodic branches of the related Tafel curves, thus, the slope of the cathodic and anodic branches indicates J 0 . The slope of the composite CEs varied in the following order: CNT/NiS > CNT/CoS > CNT/CuS > CNT/PbS > CNT, indicating a higher and lower J 0 for the CNT/NiS and CNT electrode surface. The variation of J 0 values is consistent with the R ct values obtained in the EIS measurements and both values are related to the following equation (1): where T is the absolute temperature, R is the gas constant, F is the Faraday constant, n is the number of electrons participating in the electrochemical reduction reaction, and R ct is the charge transfer resistance extracted from the EIS results. This result confirms the higher catalytic activity of CNT/NiS CE than the other CEs. Another important factor inferred from the Tafel curve at a high potential is the limiting diffusion current density (J lim ). The J lim value is connected directly to the diffusion coefficient (D) of the polysulfide electrolyte using equation (2): where D is the diffusion coefficient of the polysulfide, C is the concentration of S n 2− , l is the spacer thickness, and n and F have their usual meanings. The J lim for the CNT/NiS CE is higher than that for CNT and other CNT/ metal-sulfide CEs, which demonstrates that CNT/NiS has a larger diffusion velocity of S 2− /S n 2− in the polysulfide redox couple, which is a magnificent advantage for the electrocatalytic activity of CE. The high J 0 and J lim values of CNT/NiS and CNT/CoS CEs would favor efficient electron transfer at the CE/electrolyte interface, and result in higher J SC , FF, and photovoltaic performance in the QDSSCs.
QDSSCs were characterized under the standard simulated AM 1.5 illumination with an intensity of 100 mW cm −2 . Figure S3 † presents the photovoltaic performance of the QDSSCs assembled with bare metal sulfide CEs, and Table S2 lists the corresponding performance parameters. The QDSSC with the NiS CE shows a higher power conversion efficiency (η ) of 3.06% than PbS (2.13%), CuS (2.44%), and CoS (2.80%). To further enhance the performance of QDSSCs, metal sulfides were attached to the surface of the CNTs. As a result,  This remarkable increase in the FF and η of QDSSCs with CNT/NiS CE is apparently due to the higher electrical conductivity, the best electrocatalytic ability toward S n 2− reduction and the improved charge transfer ability. To show the reproducibility, more than five cells assembled with CNT/metal-sulfide CEs were fabricated and the results are shown in Table S3. To the best of our knowledge, it is worth denoting that the results of CNT/CoS and CNT/NiS are superior to that of the recent reports of carbon based composite CEs ( Table 2). The photocurrent response to the incident light for QDSSCs with various CEs was characterized by IPCE analysis, as shown in Fig. 5(e). The IPCE values of the QDSSCs with the composite CEs containing CNT, CNT/PbS, CNT/CuS, CNT/ CoS, and CNT/NiS over a frequency range of 400-650 nm were approximately 58%, 64%, 73%, 78%, and 84%, respectively. The significant IPCE improvement of the CNT/NiS CE-based QDSSCs also suggests that CNT/NiS possesses super-electrocatalytic activity in reducing S n 2− to nS 2− . The stability of a QDSSC is important factor in real applications. The stability of sealed QDSSCs with composite CEs was investigated under continuous illumination with AM 1.5 G simulated sun light for over a 50 h period. The power conversion efficiencies were observed over a five hour interval, as shown in Fig. 5(f). The η of all CEs showed a steady increase in the first few hours, which was attributed to the capillary effect of a slow permeation of the electrolyte solution into the pores of TiO 2 , and enhanced ionic transport due to heating of the electrolyte 48 . After continuous observations for 50 h, the η of QDSSCs based on the CNT, CNT/PbS, CNT/CuS and CNT/ CoS CEs retained 78.57%, 94.14%, 88.11%, and 96.71% of its initial value, whereas the CNT/NiS showed a 4.83% increase compared to its initial value (6.41% to 6.72%). This shows that the CNT/NiS-based QDSSC has high chemical stability and superior corrosion resistance compared to that of CNT, CNT/PbS, CNT/CuS, and CNT/ CoS based QDSSCs.
In addition to applications for the CE in QDSSCs, CNT/metal-sulfides can also be applicable for energy storage devices. To assess the CNT and CNT/metal-sulfides as supercapacitor electrodes, a symmetrical two-electrode cell configuration was used in a polysulfide electrolyte. The CNT/metal-sulfides were deposited on a piece of Ni-foam (1 × 3 cm 2 ). For the cyclic voltammetry (CV) and charge-discharge measurements, two electrode symmetrical cells were prepared with a positive and negative electrode containing an equal amount of active material. The mass loading of active material on the CNT, CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS electrodes was 5.0, 5.5, 5.5, 5.5, and 5.5 mg, respectively. The two electrodes were separated by cellulose paper soaked with electrolyte (polysulfide) and pressed, then wrapped with parafilm. The schematic of the flexible symmetric supercapacitor is shown in Fig. 6(a). The specific capacitance (C s ), energy density (E, W h kg −1 ), and power density (P, W kg −1 ) were obtained from charge-discharge analysis using the following equations: where I (A), t (s), ∆V (V), and m (g) are the discharge current, discharge time, potential window, and mass of the active material, respectively. CV was performed over the potential range, -0.2 to + 0.6 V, at different scanning rates. The galvanostatic charge-discharge tests were conducted at various constant currents at voltages between − 0.2 and + 0.6 V. Figure 6(b) shows the typical CV scans of the CNT and CNT/metal-sulfide samples at a scan rate of 100 mV s −1 over the potential range of − 0.2 to 0.6 V. The CV areas of the electrodes increased with increasing scan rate, indicating good capacitance retention (Fig. S4 †, Supporting Information). The CV curves in the current work showed no obvious redox peaks, denoting that the present supercapacitors are charged and discharged at a pseudoconstant rate 49 . Therefore, the two-electrode symmetric supercapacitors are considered to be mainly nonfaradaic within their respecting voltage range 50 , which has been observed in many symmetric supercapacitors 50,51 . The CNT supercapacitor showed tilted CVs with a low area, illustrating inferior capacitive behavior. The supercapacitors, fabricated using CNT/PbS, CNT/CuS, and CNT/CoS showed reasonable CV areas, indicating sufficient capacitance. The electrodes prepared using CNT/NiS showed a larger CV area, suggesting a higher capacitance. The high performance of the CNT/NiS electrode was attributed to the material with a high specific surface area and high porosity, which improves the transport of ions to the active sites of the electrodes 52 .
Galvanostatic charge-discharge (GCD) is the most accurate technique for capacitance measurements, hence, GCD measurements were conducted for the CNT and CNT/metal-sulfides samples. Figure S5 † shows the typical GCD measurements of CNT and CNT/metal-sulfide samples in a two-electrode configuration at various current densities ranging from 1 to 10 mA cm −2 with voltages between − 0.2 and 0.6 V. Figure 6(c) presents the GCD curves for the CNT, CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS at a current density of 1 mA cm −2 . The CNT/NiS exhibited the longest discharge duration compared to the CNT, CNT/PbS, CNT/CuS, and CNT/ CoS, reflecting the substantially superior performance of CNT/NiS. The specific capacitance of the CNT/NiS device was 398.16 F g −1 at 1 mA cm −2 from GCD curve, whereas specific capacitances of 288.43, 176.20, 101.94, and 41.87 F g −1 were observed in the CNT/CoS, CNT/CuS, CNT/PbS, and CNT devices, respectively (Fig. 6(d)). These results show that the specific surface area and mesoporous structures with a uniform morphology are crucial factors for obtaining high supercapacitor performance. In addition, the stability of the symmetric CNT/ metal-sulfide cell was examined by repeated charge-discharge testing at a current density of 4 mA cm −2 for 1000 cycles (Fig. 6(e)). The cycling stability of the CNT/NiS symmetric cell shows that the capacitance retention is approximately 98.39% of its initial value after 1000 cycles, which is better than the CNT/CoS (95.3%), CNT/CuS (78.57%), CNT/PbS (69.49%), and CNT (48%) cells. Furthermore, Fig. 6(f) shows a Ragone plot of the specific energy vs. the specific power, which was used to estimate the precise performance of the supercapacitors. . This confirms that CNT/ NiS is the best supercapacitor cell among the other supercapacitor cells fabricated from CNT/metal-sulfides. The CV and GCD curves of CNT/NiS symmetric supercapacitor extracted at various bending angles show nearly the same capacitive behavior (Fig. S6 †), demonstrating no apparent change of electrochemical behavior at different bending angles. These results denote that the CNT/NiS device is highly flexible.

Conclusion
In conclusion, novel composite CNT/metal-sulfides as electrodes were designed for energy conversion (QDSSCs) and energy storage (SCs) applications. CNT/metal-sulfide composites were first synthesized by grinding a mixture of organic binders and CNTs. The resulting slurry was then pasted onto Ni-foam. Metal-sulfides were then deposited on the surface of the CNTs by a facile solution approach. The roles of the metal sulfides were to provide active sites for the reduction of the polysulfide redox couple, while the CNT network facilitates the electron pathways to metal sulfides. As a result, a new catalyst of the CNT/NiS composite CE achieved a power-conversion efficiency of up to 6.41% for the QDSSCs, indicating a 90.8%, 50.1%, 27.0%, and 10.9% increase in the η value compared to the devices prepared using the CNT, CNT/PbS, CNT/CuS, and CNT/CoS CEs, respectively. The QDSSC-based on CNT/NiS CE delivered excellent stability. The EIS and Tafel polarization measurements supported the excellent electrocatalytic activity of the CNT/NiS composite CE. In addition, CNT/metal-sulfide composites were applied to develop a symmetric supercapacitor using a polysulfide electrolyte. The CNT/NiS symmetrical supercapacitor exhibited a high specific capacitance and energy density of 398.16 F g −1 and 35.39 W h kg −1 at 1 mA cm −2 , and enhanced cycling stability (98.39% retention after 1000 cycles at 4 mA cm −2 ). Overall, these results indicate that the CNT/metal-sulfide composites provide a new path for the development of similar advanced electrochemical electrode materials for a range of applications. Fourth, the flexible property of the supercapacitor is highly desired for actual applications.
Synthesis of CNT/metal sulfide composites. Prior to synthesis, the Ni foam was etched ultrasonically with a 2 M HCl solution for 30 min and then cleaned sequentially with acetone, ethanol, and deionized water (DI) for 10 min each. The counter electrode was prepared by mixing 0.05 g of MWCNT powder and 0.1 g of polyvinylidene fluoride (PVDF) in a solvent of 2 ml N-Methyl-2-pyrrolidone (NMP). The mixture was first well mixed with a mortar and pestle to form a slurry and coated onto a piece of Ni foam (1.3 × 1.6 cm 2 ). The coated foam was then sintered at 150 °C for 30 min, the fabricated thin film is denoted as the CNT electrode.
A facile solution approach of chemical bath deposition (CBD) was used to deposit metal sulfides on the Ni-foam based CNT electrode. The CuS, NiS, PbS, CoS deposition solutions were prepared with 0. After deposition, the electrodes were removed from the oven and cleaned with DI water and ethanol. Finally, the prepared materials were dried overnight at 100 °C prior to use. The samples denoted as CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS were prepared from PbS, CuS, CoS, and NiS solutions, respectively. TiO 2 /CdS/CdSe/ZnS photoanode fabrication. Mesoporous TiO 2 films were prepared by doctor-blading a TiO 2 paste onto the fluorine-doped tin oxide (FTO) substrate with an active area of 0.27 cm 2 , followed by sintering at 450 °C for 30 min to remove impurities and improve the crystallinity. CdS seed layer was deposited by SILAR process on surface of TiO 2 to facilitate the subsequent CdSe growth. The TiO 2 film was first immersed into a 0.1 M Cd(CH 3 COO) 2 .2H 2 O aqueous solution for 2 min and rinsed with DI water and ethanol, after the film was immersed into a 0.1 M Na 2 S aqueous solution for another 2 min, followed by rinsing with DI water and ethanol and dry with drier. This process was repeated five times. The as prepared samples names as TiO 2 /CdS.
CdSe QDs was deposited on the surface of TiO 2 /CdS film through a CBD procedure. TiO 2 /CdS films dipped into a solution containing mixture of 80 mM CdSO 4 ·8/3H 2 O, 90 mM N(CH 2 CO 2 H) 3 , and 80 mM Na 2 SeSO 3 at 40 °C for 135 min. The electrodes were annealed at 300 °C for 1 h, cooled naturally. The as-prepared electrodes are named as TiO 2 /CdS/CdSe. After CdSe deposition, 3 cycles of ZnS was deposited by a SILAR method through dipping the TiO 2 /CdS/CdSe film in an aqueous solution containing 0.1 M Zn(CH 3 COO) 2 .2H 2 O and 0.1 M Na 2 S for 1 min. Finally the photoanode is denoted as TiO 2 /CdS/CdSe/ZnS. QDSSC device fabrication. The preparation process for the TiO 2 /CdS/CdSe/ZnS photoanode is shown in the supporting information. The QDSSCs were assembled as a sandwich structure with a photoanode and a CE by a sealant (SX 1170-60, Solaronix) at 100 °C. The polysulfide electrolyte (1 M Na 2 S, 2 M S, and 0.1 M KCl in methanol: water is 7:3) was used to fill the space between the electrodes. The back of the porous Ni foam-based CNT/metal-sulfides were made up with a piece of glass sheet, so electrolyte leakage did not occur through the porous CNT/metal-sulfide CE.

Fabrication of symmetric cells for Tafel polarization and EIS measurements.
Tafel polarization measurements and EIS experiments were conducted in the dark using a symmetrical dummy cell (CE/electrolyte/ CE) configuration of a symmetrical cell with an active area of 0.27 cm 2 . The EIS measurements were performed over the frequency range, 500 kHz-0.1 Hz, at zero bias with a 10 mV AC amplitude. The Tafel polarization measurements were conducted at a scan rate of 10 mV s −1 .
Characteriations. The surface morphology and structure of the resulting samples were studied by field emission scanning electron microscopy (FE-SEM, S-2400, Hitachi) with energy-dispersive X-ray spectroscopy (EDX, 15 kV). The crystal structure was examined by X-ray diffraction (XRD) analysis (D8 ADVANCE with a DAVINCI diffractometer (Bruker AXS)) with Cu Kα radiation operated at 40 kV and 40 mA. The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB 250). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a CJ111 high-resolution electron microscope with an acceleration voltage of 200 kV. Brunauer-Emmett-Teller (BET) analysis was used to measure the specific surface area of the samples using a BELsorp-Max (BEL, Japan) instrument at 77 K.
The current-voltage (J-V) curves of the QDSSCs were derived using an ABET Technologies (USA) solar simulator under one sun illumination (AM 1.5 G, 100 mW cm −2 ). The incident photon-to-current conversion efficiency (IPCE) spectra of the QDSSCs were examined using an Oriel ® IQE-200 ™ . Electrochemical impedance spectroscopy (EIS) and Tafel polarization were executed on symmetrical cells using a BioLogic potentiostat/ galvanostat/EIS analyzer (SP-150, France). CV was carried out in a symmetrical supercapacitor using a BioLogic electrochemical analyzer. Supercapacitor charging-discharging measurements were carried out using galvanostatic charge-discharge (GCD, BioLogic analyzer, SP-150, France) tests.