Electrodes For Dye-sensitized Solar Cells Research Articles

Efficient Dye-Sensitized Solar Cell with Affordable Alternative to Platinum free Counter Electrode

The study focuses on MnCO3/reduced graphene oxide (rGO) composite to investigate its capability against reduction of triiodide (I3⁻), and hence to utilise its potential as a platinum free electrode in dye-sensitized solar cells (DSSCs). The MnCO3/rGO composites were prepared with varying weight percentages of rGO to identify the optimal configuration to attain enhanced catalytic performance. The MnCO3/rGO 75wt% composite exhibited the optimal catalytic reaction kinetics for converting triiodide (I3⁻) to iodide (I⁻), achieving a photon to electron conversion efficiency (PCE) of 6.62%, with a photocurrent density (Jsc) of 13.29mAcm⁻², voltage (Voc) of 0.76V, and a fill factor (FF) of 0.65 at without load. This output performance is on par with that of to that of a conventional platinum (Pt) Counter Electrode, which achieved a PCE of 6.98%. The findings indicate that MnCO3/rGO 75wt% composite shows significant promise, cost-effective alternative to Pt, offering efficient performance and improved material sustainability.

Next-generation counter electrodes for dye-sensitized solar cells: A comprehensive overview

Next-generation counter electrodes for dye-sensitized solar cells: A comprehensive overview

Enhanced Photovoltaic Performance of Poly(3,4-Ethylenedioxythiophene)Poly(N-Alkylcarbazole) Copolymer-Based Counter Electrode in Dye-Sensitized Solar Cells

Conducting polymers are emerging as promising alternatives to rare and expensive platinum for counter electrodes in dye-sensitized solar cells; due to their ease of synthesis, they can be chemically tuned and are suitable for roll-to-roll production. Among these, poly (3,4-ethylenedioxythiophene) (PEDOT)-based counter electrodes have shown leading photovoltaic performance. However, certain conductivity issues remain that affect the effectiveness of these counter electrodes. In this study, we present an electropolymerized PEDOT and poly(N-alkylated-carbazole) copolymer as an efficient electrocatalyst for the reduction in I3− in dye-sensitized solar cells. Copolymerization with N-alkylated carbazoles significantly increases the conductivity of the polymer film and facilitates rapid charge transport at the interface between the polymer electrode and the electrolyte. The length of the alkyl substituents also plays a crucial role in this improvement. Electrochemical analysis showed a reduction in charge transport resistance from 3.31 Ω·cm2 for PEDOT to 2.26 Ω·cm2 for the PEDOT:poly(N-octylcarbazole) copolymer, which is almost half the resistance of a platinum-based counter electrode (4.12 Ω·cm2). Photovoltaic measurements showed that the solar cell with the PEDOT:poly(N-octylcarbazole) counter electrode achieved an efficiency of 8.88%, outperforming both PEDOT (7.90%) and platinum-based devices (7.57%).

Effect of Tungsten Doping on the Properties of Titanium Dioxide Dye-Sensitized Solar Cells

Tungsten-doped TiO2 thin films were prepared by sol–gel method on fluorine-doped tin oxide-coated substrates as working electrodes of dye-sensitized solar cells. The influences of different W doping (0, 2, 4, 6, and 8 at%) on the microstructure, optical, and photovoltaic properties of the W-TiO2 thin-film DSSCs were studied by the measurement of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, and electrochemical impedance spectroscopy (EIS). An optimal DSSCs performance was observed with a 6 at% W-doped TiO2 thin film, resulting in a Voc of 0.68 V, a Jsc of 20.2 mA/cm2, an FF of 68.6%, and an efficiency (η) of 9.42%. The efficiency of DSSCs with 6 at% W-doped TiO2 photoanode improved by 75%. This is because the 6 at% W-doped TiO2 thin film increases the specific surface area and electron transfer rate.

Poly(aniline)-multiwall carbon nanotube (PANI@MWCNT) composite as high-cost Pt free counter electrode for dye-sensitized solar cells

In this study, we synthesized a poly (aniline)-multiwall carbon nanotube (PANI@MWCNT) composite for use as a counter electrode (CE) in dye-sensitized solar cells. Moreover, FE-SEM and HR-TEM images of PANI@MWCNT revealed carbon nanotube/ropes embedded in the polymer matrix. An X-ray diffraction pattern confirmed the amorphous nature of the composite. Further, Electrochemical impedance spectroscopy studies indicated a charge transport resistance value (Rct) of 4.36 KΩ for PANI@MWCNT. CV studies demonstrated improved electrocatalytic performance and faster redox behavior in the PANI@MWCNT composite. BET analysis measured the pore size of the as-prepared composite to be 15–50 nm. Subsequently, the as-synthesized PANI, PANT@MWNCT, pristine MWCNT, and platinum were evaluated as CEs in DSSCs using a polyethylene oxide polymer-based liquid electrolyte and a TiO2 nanocrystalline photoanode. Among these CEs, the PANI@MWCNT CE exhibited an efficiency of 8.07 %, and the stability test indicated that the as-prepared composite CE retained 7.81 % efficiency after 15 days.

Density functional theory study of cobalt sulfide as a counter electrode material for dye-sensitized solar cells

Abstract Dye-sensitized solar cells (DSSCs) are viewed as potential substitutes for traditional silicon-based solar cells due to their affordability, impressive performance in low-light conditions, eco-friendly energy production, and adaptability in solar product integration. The use of noble metals such as platinum as counter electrodes in DSSCs was initially limited by their high cost; however, cobalt sulfide has been recognized as a viable alternative due to its abundance, non-toxic properties, and cost-effectiveness. In this work, the crystal structure, electronic, optical characteristics and electrocatalytic activity of the tetragonal phases of cobalt sulfide are investigated through density functional theory with a quantum espresso package. The results obtained for the lattice parameter are a = b = 3.53 Å and c = 4.80 Å. The generalized gradient approximation and hybrid exchange correlation function yield bandgaps of 1.63 and 1.69 eV, respectively, which are consistent with the reported experimental values. An analysis of the density of states and the projected density was carried out to validate the accuracy of the calculated band gaps. Additionally, significant information was obtained from the optical properties through the calculation of the dielectric function. The findings reveal real and imaginary static dielectric constants of 11.85 and 0.13, respectively. Furthermore, the measured absorption and conductivity spectra exhibit promising attributes in the UV–visible range and good electrical conductivity. Moreover, the electrocatalytic activity was studied to analyze the adsorption energy of CoS and the electrolyte. Generally, the calculated electronic and optical properties of CoS crystals indicate their potential application as counter electrodes in dye-sensitized solar cells.

Honeycomb shaped carbon dopped 1T-2H-MoS2 heterojunction counter electrode with high short current for dye sensitized solar cells

Recent years have witnessed energy crises in various regions and the utilization of new energy is crucial. Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic cells that are expected to be more cost-effective than other types of solar cells. In this work, we report the synthesis of 1T-2H-MoS2/honeycomb shaped carbon (HSC) heterojunction composite counter electrodes for DSSCs via adding porous carbon in the facile hydrothermal method. The effects of HSC on the morphology, structure, electrochemical and photoelectric conversion properties of the 1T-2H-MoS2/HSC composites were investigated systematically. Both the results of electrochemical and photoelectric measurement suggest the excellent performance of the reduction of I3- and the regeneration of dye. The whole cell assembled with the MoS2/HSC-5.0 counter electrode achieved a short current density 22.24 mA∙cm−2 and a power conversion efficiency (PCE) of 8.42 %, which is significantly higher than that of the pure MoS2 electrode (5.89 %) and Platinum electrode (6.76 %), implying the MoS2/HSC-5.0 composite is a promising candidate for counter electrodes catalyst of DSSCs.

Extraction and Characterization of Silicon Dioxide from Coal Fly Ash as Counter Electrode Material in Dye-Sensitized Solar Cells (DSSCs)

The counter electrode in DSSCs must be made of a material with various advantageous chemical and physical characteristics to guarantee the cell’s efficient functioning and cost efficiency. Coal fly ash is recognized for its substantial silicon dioxide (SiO2) content and other minerals and metals. This paper provides a detailed analysis and description of extracting and characterizing SiO2 from coal fly ash. This extraction aims to utilize SiO2 as a material for the counter electrode in DSSCs. The extraction procedure of SiO2 from coal fly ash involves a multi-step process that entailed the utilization of acid leaching using hydrochloric acid 1 M at 90°C for 4 h, which was subsequently followed by precipitation using NaOH 3 M at 90°C for 4 h to separate SiO2. The SiO2 gel was cleaned of contaminants with hot distilled water and dried at 110°C for 12 h. The SiO2 that was obtained was analyzed utilizing a range of analytical techniques to evaluate its structural, morphological, chemical, and optical properties. The X-ray diffractometer (XRD) examination revealed that the crystal structure of coal fly ash consisted of quartz, corundum, hematite, lime, and periclase. The SiO2 that was obtained exhibited a crystal structure that was both cubic and triclinic. The morphology is visualized using a scanning electron microscope (SEM). The study using atomic absorption spectroscopy revealed that the coal fly ash contained 52.91% SiO2, whereas the extracted SiO2 had a purity of 91.20%. The UV-Vis spectrophotometry investigation revealed that the SiO2 exhibited absorbance spectra with a wide band gap of 4.17 eV, whereas the coal fly ash had absorbance spectra of 3.37 eV.

W-based carbide derived from PW12@ZIF-8 as Pt-free catalyst on counter electrode of dye-sensitized solar cells for triiodide reduction

The preparation of counter electrodes (CEs) catalysts at different pyrolysis temperatures will directly affect the composition, morphology, and performance of the catalysts in dye-sensitized solar cells (DSSCs) applications. Herein, four W-based carbides catalysts were derived from H3PW12O40-based ZIF-8 metal–organic frameworks (PW12@ZIF-8) by in-situ pyrolysis at 600, 700, 800, and 900 ℃ under N2 atmosphere. The pyrolysis mechanism and structure characteristics of the prepared W-based carbides with temperature variation were confirmed by TG-DTG-DSC simultaneous, XRD, XPS, TEM, SEM and N2 adsorption/desorption, which exhibits the gradual transformation of PW12@ZIF–8 → WC-C→WC→W1-xC→W2C. Furthermore, the photovoltaic performance of four different W-based carbides as CEs catalysts for regenerating I3−/I− shuttles in DSSCs were identified by photocurrent-voltage (J-V) measurement, which achieved PCEs of 4.63, 4.91, 5.05 and 5.78 %, respectively. As a result, W2C (900 ℃) provide more vacancies and defects on the catalyst surface as abundant catalytic active sites, as well as promote the appearance of more pores and voids in the morphology to diffuse and permeate electrolyte due to the highest specific surface area, and the PCE value of 5.78 % was also superior to 5.51 % of Pt CE.

N, S co-doped lignin graded porous carbon as efficient Pt-free counter electrodes for dye-sensitized solar cells

N, S co-doped lignin graded porous carbon as efficient Pt-free counter electrodes for dye-sensitized solar cells

2D/2D-Graphene Functionalized MXene as an Alternative Counter Electrode for Dye-Sensitized Solar Cells

2D/2D-Graphene Functionalized MXene as an Alternative Counter Electrode for Dye-Sensitized Solar Cells

Effects of ALD Deposition Cycles of Al2O3 on the Morphology and Performance of FTO-Based Dye-Sensitized Solar Cells

In dye-sensitized solar cells (DSSCs), materials classified as Transparent Conducting Oxides (TCOs) have the capacity to conduct electricity and transmit light at the same time. Their exceptional blend of optical transparency and electrical conductivity makes them popular choices for transparent electrodes in DSSCs. Fluorine Tin Oxide (FTO) was utilized in this experiment. The optical and electrical characteristics of TCOs may be negatively impacted by their frequent exposure to hostile environments and potential for deterioration. TCOs are coated with passivating layers to increase their performance, stability, and defense against environmental elements including oxygen, moisture, and chemical pollutants. Because of its superior dielectric qualities, strong chemical stability, and suitability with TCO materials, aluminum oxide (Al2O3) was utilized as a passivating layer for the FTO. In this research work, Al2O3 was deposited via atomic layer deposition (ALD) to form thin mesoporous layers as a passivator in the photoanode (working electrode). The work focuses on finding an appropriate thickness of Al2O3 for optimum performance of the dye-sensitized solar cells. The solar simulation and sheet resistance analysis clearly showed 200 cycles of Al2O3 to exhibit an efficiency of 4.31%, which was the most efficient performance. The surface morphology and topography of all samples were discussed and analyzed.

Synergistic effect in ZIF-67/MoS2 entrenched MWCNT photo (electro) catalyst for degradation of tetracycline and alternative to Pt counter electrode in dye-sensitized solar cells

Photocatalytic water purification is an effective environmental protection approach for removing hazardous chemicals in water, besides solar energy conversion is a trustworthy and sustainable choice for the future due to rising energy demands. The present research focus is on synthesis of Metal-Organic Framework (MOF) based composite, Zeolitic Imidazolate Framework-67/Molybdenum Disulfide (ZIF-67/MoS2) entrenched Multi-Walled Carbon Nanotubes (MWCNT) as a catalyst for photodegradation of tetracycline (TC), as well as substitute for Pt counter electrode in Dye-Sensitized Solar Cell (DSSC). The crystalline structure, optical response, shape, porosity, quantum states, and chemical configuration were determined using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-DRS), Photoluminescence spectroscopy (PL), Transmission Electron Microscopy (TEM), Brunauer-Emmett-Teller (BET), and X-ray Photoelectron Spectroscopy (XPS), respectively. The degradation efficiency of ZIF-67/MoS2/MWCNT catalyst under effect of operational parameters namely, photocatalyst dosage, solution pH, coexisting ions, TC concentrations and irradiation time were studied consecutively. ZIF-67/MoS2/MWCNT composite achieved 96.1 % degradation efficiency of TC under white light irradiation. Interestingly, when utilized as a counter electrode in DSSCs, this composite-based electrode delivered the highest Power Conversion Efficiency (PCE) of 3.86 % with a short-circuit current density (JSC) of 7.54 mA cm−2 and open-circuit voltage (VOC) of 0.79 V. The observed enhancement is attributed to lesser peak-to-peak separation (EPP), lower electron lifetime, lower charge transfer resistance (RCT) and higher exchange current density (J0). Based on these findings, ZIF-67 inclusion in MoS2/MWCNT composite is not only an effective catalyst for water treatment but also a good catalyst for I3− reduction in DSSCs.

Enhanced efficiency and stability of dye-sensitized solar cells utilizing FeCo2S4 nanowires as Pt-free counter electrodes

Replacing traditional Pt-based counter electrodes with low-cost and highly stable materials is crucial for the development of commercially viable dye-sensitized solar cells (DSSCs). In this study, we synthesized a ternary Pt-free FeCo2S4 nanowire (NW)-based sulfide electrocatalyst by a three-step solvothermal method. This material was then used as a counter electrode in DSSCs to facilitate the reduction of triiodide species. The formation of FeCo2S4 NW was confirmed by various characterization techniques such as X-ray diffraction, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Scanning electron microscopy analysis revealed the structure of the nanowires. Electrochemical studies, which included cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization methods, revealed the excellent stability, superior electrocatalytic activity and remarkable kinetics of FeCo2S4 NW in the reduction of triiodide to iodide. Photovoltaic measurements of the fabricated DSSCs yielded a power conversion efficiency (PCE) of 7.88 % for the FeCo2S4-based devices, outperforming the control device made of Pt (PCE=7.45 %). This improvement was primarily due to the increase in short-circuit current density (JSC), thanks to the lower charge transfer resistance (RCT) of FeCo2S4 NW (JSC=15.23 mA/cm2; RCT=5.54 Ω cm2) compared to Pt (JSC=14.12 mA/cm2; RCT=7.07 Ω cm2). In addition, the FeCo2S4 NW-based solar cells exhibited excellent stability and maintained their high PCE value even after 10 days of aging under ambient conditions, while Pt-based DSSCs showed a 3 % decrease from their initial PCE value. This FeCo2S4 NW counter electrode proves to be an excellent alternative to Pt, and the presented results provide valuable insights for the development of cost-effective and highly stable DSSCs.

Iron-nickel-cobalt selenide nanoparticles as an efficient and transparent counter electrode for dye-sensitized solar cells

In this study, we developed a quaternary metal selenide (Fe0.35Ni0.11Co0.19Se) counter electrode (CE) material, grown onto fluorine-doped tin oxide, for dye-sensitized solar cells (DSSCs) using the potential-reversal electrochemical deposition technique at ambient conditions. Under optimized conditions, the Fe0.35Ni0.11Co0.19Se CE exhibited excellent electrocatalytic activity in the I-/I3- redox reaction with the average transmittance of 78.85 %. The additional valence states of Fe in the Ni0.3Co0.3Se electrode material further enhanced the catalytic activity during the electrochemical redox process. The DSSC employing this electrocatalytically active CE realized a power conversion efficiency of 7.73 %, surpassing the efficiency of both Pt and Ni0.3Co0.3Se CEs-based DSSCs (7.23 % and 7.23 %, respectively). This electro-preparation method offers a simpler and more cost-effective fabrication process for CE design, demonstrating a good potential for replacing expensive platinum CEs in DSSCs.

Electrodeposited polyaniline modified CNT fiber as efficient counter electrode in flexible dye-sensitized solar cells

Carbon-based electrodes played a significant role toward the development of wearable and flexible photovoltaic energy devices in an efficient and affordable manners. Here, all-carbon-based electrodes composed of carbon nanotubes fiber (CNTF) have been used to fabricate a flexible wire shaped dye-sensitized solar cell (DSSC). A facile electrodeposited approach has been adopted to modify a CNTF with polyaniline (PANI) under acidic conditions. Both photoanode (TiO2@CNTs) and counter electrode (PANI@CNTs) twisted around each other, acted as electrons and hole collectors in the presence of redox couple (iodide/triiodide) as electrolyte. Scanning electron microscopy (SEM) images demonstrated the successful growth of TiO2 nanoparticles and conducting polyaniline (PANI) over the surface of CNTs fibers, as photoanode and counter electrode, respectively. Cyclic voltammetric (CV) measurements and electrochemical impedance spectroscopy (EIS), examined the reduction of triiodide and resistance of charge transfer. While the current-voltage measurements were carried out to check the performance of fabricated device when illuminated under simulated light source. With APNI modified counter electrode, device exhibited higher photoelectric conversion efficiency (3.57 %) under optimal conditions as compared to pristine CNTs fiber-based device with lower efficiency (2.26 %). The improved performance of modified counter electrode further confirmed by the lower peak separation (ΔEp= 0.42 V) and charge transfer resistance (RCE =19.28 Ω) in cyclic voltammetry and impedance spectroscopy, respectively. These fabricated electrodes could be promising alternate for metal-based electrodes in dye-sensitized solar cells due to its facile fabrication process, low cost, and higher chemical stability.

Nanocomposites of Co-NiS/GO as a Versatile Catalyst: Enabling Platinum-Free DSSC Counter Electrodes and Enhancing Organic Dye Degradation

This study presents the synthesis of a nanocomposite intended to serve as a counter electrode in dye-sensitized solar cells (DSSCs), replacing platinum electrodes, as well as functioning as a nanocatalyst for organic dye degradation. Graphene oxide was synthesized using a modified Hummers method, and cobalt-doped nickel sulfide on graphene oxide (Co-NiS/GO) was prepared via hydrothermal synthesis. The samples underwent characterization through various testing methods. X-ray diffraction analysis revealed a hexagonal structure with a crystallite size of 30 nm. Field-emission scanning electron microscopy/energy-dispersive X-ray images showed a cornflake-like structure, with elements such as cobalt, nickel, sulfur, carbon, and oxygen present. Chemical valence states were confirmed through X-ray photoelectron specteroscopy analysis. The power conversion efficiency of the Co-NiS/GO counter electrode in DSSCs was investigated, with parameters such as open-circuit voltage, short-circuit current density, fill factor, and power conversion efficiency calculated to be 8.6032 mV, 0.5484 mA cm−2, 61, and 2.83%, respectively, based on I-V studies. Furthermore, the developed Co-NiS/GO nanocomposite was assessed for its photo catalytic dye degradation capabilities using malachite green (MG), achieving a degradation rate of approximately 96% within 180 min.

Impedance spectroscopy using microscopic reference electrodes to analyze different rate-determining steps in aqueous dye-sensitized solar cells using nitroxide radicals as redox mediators

For new components in dye-sensitized solar cells (DSSCs), identification and quantification of rate-limiting steps is needed to evaluate their applicability. This is particularly important if fundamental changes are studied. In this work, the use of a micro-reference electrode in DSSCs is proposed to increase the significance of electrochemical impedance spectroscopy. The recombination of charge carriers at the photoanode and the regeneration of redox mediators at the counter electrode, which typically occur on similar timescales, could be studied separately but simultaneously in cells under operating conditions. This is particularly useful in the study of water-based DSSCs. Here, cells with 2,2,6,6-Tetramethylpiperidinoxyl (TEMPO) or 4-Hydroxy-TEMPO (OH-TEMPO) as redox mediators using additives like 1-Methylbenzimidazole (MBI) are discussed. It was revealed that the charge transfer resistance (RCE) for the reduction of the oxidized redox mediator at the counter electrode (CE) limits the fill factor (FF) of such DSSCs. TEMPO/MBI electrolytes yielded low RCE and high FF, whereas OH-TEMPO in otherwise identical cells resulted in large RCE, low FF, and low conversion efficiencies. This indicates that the interface between the CE and the electrolyte significantly influences the DSSC performance of these cells and strongly depends on the electrolyte composition. Important optimization strategies could be discussed based on the present results.

Analysis of nickel oxide as a counter electrode for dye-sensitized solar cells using OghmaNano software

Dye-sensitized solar cells (DSSCs), a promising green technology, convert solar energy into electricity more cost-effectively than traditional solar cells. While platinum (Pt) is commonly used in DSSCs, its high cost and toxicity limit practical applications. Recent research aims to develop low-cost counter electrodes with high efficiency. Nickel oxide (NiO), a p-type semiconductor with a wide bandgap, good transmittance, and suitable work function, emerges as a potential alternative for counter electrode of DSSCs. In this work, DSSCs with NiO of thicknesses varying from 100 nm to 1000 nm were simulated to determine its influence on photovoltaic performance using OghmaNano software. The structure of simulated solar cells consists of NiO as counter electrode, zinc oxide (ZnO) as photoanode, N719 as dyes, electrolyte as charge carrier transport, and fluorine-doped tin oxide (FTO) as a contact layer. There are five data of NiO used as an active layer. From the simulation results, NiO-doped gold exhibits the highest power conversion efficiency (PCE) of 15.95% at a thickness of 700 nm, while pure NiO shows the lowest PCE with 4.53% at a thickness of 600 nm. These results have demonstrated that NiO can replace Pt as a counter electrode for DSSCs and doping plays a vital role in increasing efficiency.

Design and construction of MoS2/Ni3S2 nanosheets decorated on multi-walled carbon nanotubes as a sustainable counter electrode for dye-sensitized solar cells: An experimental and computational investigation

To achieve a satisfactory photovoltaic performance in dye-sensitized solar cells (DSSCs), counter electrode (CE) catalysts with low cost, superior catalytic activity and excellent conductivity should be developed. Herein, we successfully prepared the MoS2/Ni3S2 heterostructures via the hydrothermal method and decorated on multi-walled carbon nanotubes (MWCNTs) by sonication method. The performance of the prepared CEs was investigated by various electrochemical techniques. Based on the electrochemical tests, the MoS2/Ni3S2/MWCNTs ternary nanocomposite displayed high electron transfer ability, remarkable catalytic activities, and good stability in the I3−/I− electrolyte. The power conversion efficiency (PCE) of DSSCs based on MoS2/Ni3S2/MWCNTs CE reached 8.27 %, which was even higher than that of the DSSC with Pt CE (7.23 %). The excellent performance of the catalyst is related to the structural properties, synergistic effect of MoS2/Ni3S2 and MWCNTs, and high porosity of MoS2/Ni3S2/MWCNTs composite. The results of calculations indicated that by decorating the MoS2/Ni3S2 on carbon nanotubes (CNTs), the bandgap decreased to 2.79 eV, which was smaller than the individual CNTs (6.27 eV). This finding depicted that synergistic effect between MoS2/Ni3S2 and MWCNTs can reduce the bandgap of MoS2/Ni3S2/MWCNTs composite. The lower bandgap mostly results in higher electrical conductivity and lower charge-transfer resistance, which plays an important role in improving the efficiency of DSSCs.