TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance

The effect of co-sensitization and ZnS passivation on the photovoltaic performance of CdS quantum dot sensitized solar cells (QDSSCs) were investigated. The deposition of CdS, CdSe quantum dots (QD) and ZnS passivation on TiO₂ photoanode was carried out by successive ionic layer adsorption and reaction (SILAR) method. CdS/CdSe co-sensitization developed two staggered type-II heterojunctions at TiO₂/CdS and CdS/CdSe interfaces and resulted a cascade energy band structure. This suitable band alignment facilitated the double charge transfer mechanism at each heterojunction and transported the electrons easily into the photoanode. The narrow bandgap sensitizers CdS and CdSe significantly improved the potential utilization of solar spectrum with more charge carrier generation. ZnS passivation on QD surface suppressed electrode/electrolyte interfacial charge recombination and facilitated more electron injection from QDs into TiO₂ photoanode. The EDAX elemental mapping results inferred that CdS, CdSe and ZnS have efficiently covered the TiO₂ surface. TiO₂/CdS and CdS/CdSe interfaces and the amorphous nature of ZnS could be verified with HRTEM images. Hence, the co-sensitization and surface passivation played a significant role to enhance the PCE of CdS QDSSC from 1.9% to 4.05%.     

Efficient PbS/CdS co-sensitized solar cells based on TiO2 nanorod arrays

Narrow bandgap PbS nanoparticles, which may expand the light absorption range to the near-infrared region, were deposited on TiO₂ nanorod arrays by successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The thicknesses of PbS nanoparticles were optimized to enhance the photovoltaic performance of PbS QDSCs. A uniform CdS layer was directly coated on previously grown PbS-TiO₂ photoanode to protect the PbS from the chemical attack of polysulfide electrolytes. A remarkable short-circuit photocurrent density (approximately 10.4 mA/cm²) for PbS/CdS co-sensitized solar cell was recorded while the photocurrent density of only PbS-sensitized solar cells was lower than 3 mA/cm². The power conversion efficiency of the PbS/CdS co-sensitized solar cell reached 1.3%, which was beyond the arithmetic addition of the efficiencies of single constituents (PbS and CdS). These results indicate that the synergistic combination of PbS with CdS may provide a stable and effective sensitizer for practical solar cell applications.     

CdS quantum dot-sensitized solar cells based on nano-branched TiO2 arrays

Nano-branched rutile TiO₂ nanorod arrays were grown on F:SnO₂ conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO₂ arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO₂ solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm² and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO₂ arrays, indicating an increase of 138% compared to those based on unbranched TiO₂ nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO₂ nanorod trunks.     

Quasi solid state dye-sensitized solar cells with modified TiO2 photoelectrodes and triphenylamine-based dye

Quasi solid state dye-sensitized solar cells (DSSCs) have been fabricated with organic sol or TiCl₄ modified TiO₂ and porous TiO₂ photoanode and a triphenylamine-based dye (TPAR3) used as photosensitizer. Dark current measurements suggested that both modified TiO₂ photoelectrodes had significantly reduced the recombination rate of photoelectrons due to the reduced bare FTO surface in comparison to porous photoelectrode. The DSSC based on modified TiO₂ photoelectrodes showed improved photovoltaic parameters compared to the porous TiO₂ photoelectrode. The overall power conversion efficiency (PCE) is 3.27%, 4.73% and 6.8% for porous, TiCl₄ modified and sol modified TiO₂ photoelectrodes, respectively. The improved PCE with modified TiO₂ electrodes was attributed to the formation of a compact layer. This effectively improves adherence of TiO₂ to FTO surface, providing a larger TiO₂/FTO contact area and reducing the electron recombination by blocking the direct contact between redox electrolyte and the conductive FTO surface and enhances the electron collection efficiency.     

Optimal engineering of CdS/PbS co-sensitized TiO2 nanotube arrays for enhanced photoelectrochemical performance

TiO₂ nanotube arrays (NTAs) are decorated with CdS/PbS nano-sensitizers by successive ionic layer adsorption and reaction (SILAR) method. The uniform growth of the CdS and PbS nanoparticles on the surface and inner side of TiO₂ Nanotube Arrays (NTAs) has been confirmed by Transmission Electron microscopy measurements. The impact of the CdS and PbS semiconductor quantum dots (SQDs) on the photoelectrochemical performance (PEC) of TiO₂ NTAs was systematically investigated, and the optimal decoration of the CdS and PbS SQDs on the TiO₂ NTAs was obtained. CdS/PbS co-sensitized TiO₂ NTA photoanode films show excellent response to visible light (with absorption extended to 825 nm) and enhanced PEC performance. The best performing device showed an enhanced photocurrent density under the 0.62V vs SCE up to 8.2 mA/cm², and high photoconversion efficiency up to 5.35%, which is 16.7 times higher than the pure TiO₂ NTAs. The enhanced PEC performance of TiO₂ NTAs is attributed to the co-sensitization, heterojunction formation and electron “pool” effect imparted on the NTAs by the coupling of CdS and PbS SQDs.     

Effect of an electrodeposited TiO<Subscript>2</Subscript> blocking layer on efficiency improvement of dye-sensitized solar cell

A TiO₂ blocking layer in DSSC provides good adhesion between the fluorinated tin oxide (FTO) and an active TiO₂ layer, and represses the electron back transport between electrolyte and FTO by blocking direct contact. In addition, it offers a more uniform layer than bare FTO glass. In this study, a dense TiO₂ layer is prepared by electrodeposition technique onto an FTO substrate, and it is further used for efficiency measurement of dye-sensitized solar cell (DSSC). The thickness of TiO₂ blocking layers is controlled by applied voltage and deposition time. The morphology and crystalline structure of TiO₂ blocking layers are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The effect of thickness of TiO₂ blocking layers on transmittance is also examined by UV-vis spectrophotometer. For the best performance of the cell efficiency, the optimum blocking layer thickness is about 450 nm fabricated at 0.7 V for 20 min. The conversion efficiency from the DSSC including the optimum blocking layer is 59.34% improved compared to the reference cell from 2.41% to 3.84%. It demonstrates that the electrodeposition is a useful method to produce TiO₂ blocking layer for DSSC applications.     

CdS Quantum Dot‐Sensitized Vertical TiO2 Nanorod Arrays by a Simple Linker‐Assisted SILAR Method

The homogeneously deposition of quantum dots (QDs) on the mesoporous or one‐dimensional TiO₂ film with a well‐covered layer has become one major challenge in quantum dot‐sensitized solar cell (QDSSC). In this study, one‐dimensional TiO₂ nanorod arrays based photoanode prepared by a simple linker‐assisted SILAR method is studied. The influence of the short organic ligand (TGA) on the homogeneity of QDs on TiO₂ nanorod arrays is discussed and the photoelectrochemical performance of QDSSC is evaluated by UV–vis absorbance spectroscopy, current–voltage performance and electrochemical impedance spectroscopy. The results show that the TGA ligand can act as an efficient bridge between CdS QDs and TiO₂, whereas the content and particle size of the QDs can be easily tuned by controlling the dipping time and SILAR cycle. Owing to the presence of TGA, the charge‐transfer rate at CdS QDs and TiO₂ interfacial region is noticeable enhanced.     

Bipolar resistive switching behavior and conduction mechanisms of composite nanostructured TiO2/ZrO2 thin film

In this work, TiO₂/ZrO₂ bilayer thin film was prepared on fluorine doped tin oxide (FTO)/glass substrates by using a simple and low-cost chemical solution deposition method. Reproducible bipolar resistive switching (RS) characteristics in Au/TiO₂/ZrO₂/FTO/glass devices are reported in this work. TiO₂/ZrO₂ bilayer thin films prepared in this work shows reversible bipolar resistive switching and unidirectional conduction performances under applying voltage and these special performances of TiO₂/ZrO₂ bilayer thin films was first reported. Obvious resistive switching performance can be observed after setting a compliance current, the ratio of high/low resistance reached about 100 at a read voltage of +0.1V and −0.1V and the RS properties showed no obvious degradation after 100 successive cycles tests. The resistive switching characteristics of Au/TiO₂/ZrO₂/FTO/glass device can be explained by electron trapping/detrapping related with the vacancy oxygen defects in TiO₂/ZrO₂ bilayer thin film layer. According to slope fitting, the main conduction mechanisms of the sample are Ohmic and Space charge limited current mechanism.     

Self-assembled CdS quantum dots-sensitized TiO2 nanospheroidal solar cells: Structural and charge transport analysis

A nanospheroidal TiO₂ mesoporous layer combined with cadmium sulfide (CdS) quantum dots (QDs) as a sensitizer was firstly utilized for solar cell applications, resulting in an efficiency of 1.2% at a 1 sun condition. CdS quantum dots (18 nm) were attached to the TiO₂ nanospheroidal electrode by using a chemical bath deposition technique. The influence of surface treatment using dimethyl formamide on the interconnectivity of the TiO₂ nanospheroidal electrodes was investigated. The charge transport of TiO₂/CdS QDs/electrolyte sandwich-type cells was characterized by electrochemical impedance spectroscopy and the device performance was compared with conventional nanospherical TiO₂ (Degauusa P25) electrodes. The electrodes with nanospheroidal morphology showed better device performance than the P25 nanoparticle electrodes primarily due to both better connectivity among nanospheroidal TiO₂ particles and larger mesopores, resulting in deeper penetration of the electrolyte in QD-sensitized solar cells.     

An efficient and low-cost TiO2 compact layer for performance improvement of dye-sensitized solar cells

A TiO₂ organic sol was synthesised for the preparation of a compact TiO₂ layer on fluorine-doped tin oxide (FTO) glass by a dip-coating technique. The resultant thin film was used for the fabrication of dye-sensitized solar cells (DSSCs). The compact layer typically has a thickness of ca. 110 nm as indicated by its SEM, and consists of anatase as confirmed by the XRD pattern. Compared with the traditional DSSCs without this compact layer, the solar energy-to-electricity conversion efficiency, short-circuit current and open-circuit potential of the DSSCs with the compact layer were improved by 33.3%, 20.3%, and 10.2%, respectively. This can be attributed to the merits brought by the compact layer. It can effectively improve adherence of TiO₂ to FTO surface, provide a larger TiO₂/FTO contact area, and reduce the electron recombination by blocking the direct contact between the redox electrolyte and the conductive FTO surface.     

Tertiary hierarchically structured TiO2 for CdS quantum-dot-sensitized solar cells

A tertiary hierarchically structured mesoporous spherical TiO₂ (with a diameter of 1190 ± 60 nm) was synthesized by combining the sol–gel and the subsequent solvothermal treatment, and applied to CdS quantum-dot-sensitized solar cells (QDSSCs). This mesoporous spherical (MS) TiO₂ offers a high surface area (76.02 m² g⁻¹), a high internal reflectance in the visible region and a pore accessibility. A conversion efficiency of 1.9% was achieved by CdS QDSSCs composed of the MS TiO₂ photoanode, which corresponds to ∼58% improvement as compared with the values obtained from the conventional devices made with 20-nm-sized nanocrystalline TiO₂ under AM 1.5 illumination of 100 mW cm⁻². Thus, the MS TiO₂ can be a promising candidate for the photoanode material of QDSSCs.     

Synthesis and photoelectrochemical response of CdS quantum dot-sensitized TiO2 nanorod array photoelectrodes

A continuous and compact CdS quantum dot-sensitive layer was synthesized on TiO₂ nanorods by successive ionic layer adsorption and reaction (SILAR) and subsequent thermal annealing. The thickness of the CdS quantum dot layer was tuned by SILAR cycles, which was found to be closely related to light absorption and carrier transformation. The CdS quantum dot-sensitized TiO₂ nanorod array photoelectrodes were characterized by scanning electron microscopy, X-ray diffraction, ultraviolet–visible absorption spectroscopy, and photoelectrochemical property measurement. The optimum sample was fabricated by SILAR in 70 cycles and then annealed at 400°C for 1 h in air atmosphere. A TiO₂/CdS core-shell structure was formed with a diameter of 35 nm, which presented an improvement in light harvesting. Finally, a saturated photocurrent of 3.6 mA/cm² was produced under the irradiation of AM1.5G simulated sunlight at 100 mW/cm². In particular, the saturated current density maintained a fixed value of approximately 3 mA/cm² without decadence as time passed under the light conditions, indicating the steady photoelectronic property of the photoanode.     

TiO2/TNO homojunction introduced in a dye‐sensitized solar cell with a novel TNO transparent conductive oxide film

In this study, niobium‐doped titanium oxide (TNO) was employed for a novel transparent conductive oxide (TCO) film to construct a porous‐TiO₂/TNO homojunction in a dye‐sensitized solar cell (DSSC). However, considering a balance between the electrical and optical properties of the TCO film, the sheet resistance in TNO was tuned to be higher than that in a typical fluorine‐doped tin oxide (FTO). The photovoltaic performance of the cell with the TNO film (TNO cell) was optimized to be almost comparable to that with a conventional FTO film (FTO cell) by coating the surface of the porous‐TiO₂ layer with a thin alumina or magnesia film to block a back reaction within the cell. An electrochemical impedance measurement was conducted to determine the detailed photovoltaic performance from the viewpoint of electron transportation in the cell. R₁, the real part of ω₁, indicated that electron transportation at the porous‐TiO₂/TNO interface was more favorable than that at the porous‐TiO₂/FTO interface, which was supported by AC phase change in the cell at a high‐frequency range. We found that the homojunction newly introduced in the cell is one of the key concepts for developing a DSSC into a high‐performance photovoltaic device.     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

A portable photocatalytic fuel cell based on TiO2 nanorod photoanode for wastewater treatment and efficient electricity generation

A portable photocatalytic fuel cell (P-PFC) has been proposed in the present study, which has a simple structure and high performance. The photoanode of the P-PFC was composed of F-doped tin oxide (FTO)/TiO₂ seed layer/TiO₂ nanorods, and this nanostructure could expand the absorption range of the solar light. The cathode was a silver paste that was printed on a waterproof and breathable layer. It could effectively absorb oxygen from the air and avoid adding additional oxygen supply equipment. Two electrodes were encapsulated in a hot-melt film to form a small package with a quick-release port. This packaging model enabled a fuel replacement, and this P-PFC was universal to all types of fuels. When 1 mol/L methanol was used as fuel, the photocurrent could reach 2.8 mA/cm² under a UV light of 30 mW/cm², and the power density was 0.49 mW/cm². When the power density of the simulated solar light was as low as 2 mW/cm², the system could still work normally. The degradation performance of the P-PFC was also evaluated, and its maximum degradation efficiency for methylene blue (MB) reached 74.3%. The results indicated that the P-PFC had great potential in simultaneously degrading organic pollutants and generating electricity.     

Improving the efficiency of cadmium sulfide-sensitized titanium dioxide/indium tin oxide glass photoelectrodes using silver sulfide as an energy barrier layer and a light absorber

Cadmium sulfide (CdS) and silver sulfide (Ag₂S) nanocrystals are deposited on the titanium dioxide (TiO₂) nanocrystalline film on indium tin oxide (ITO) substrate to prepare CdS/Ag₂S/TiO₂/ITO photoelectrodes through a new method known as the molecular precursor decomposition method. The Ag₂S is interposed between the TiO₂ nanocrystal film and CdS nanocrystals as an energy barrier layer and a light absorber. As a consequence, the energy conversion efficiency of the CdS/Ag₂S/TiO₂/ITO electrodes is significantly improved. Under AM 1.5 G sunlight irradiation, the maximum efficiency achieved for the CdS(4)/Ag₂S/TiO₂/ITO electrode is 3.46%, corresponding to an increase of about 150% as compared to the CdS(4)/TiO₂/ITO electrode without the Ag₂S layer. Our experimental results show that the improved efficiency is mainly due to the formation of Ag₂S layer that may increase the light absorbance and reduce the recombination of photogenerated electrons with redox ions from the electrolyte.     

Effect of CdS contents on H<Subscript>2</Subscript> production from Pt-(CdS/TiO<Subscript>2</Subscript>) film-typed photocatalysts

Pt-(CdS/TiO₂) film-typed photocatalysts are prepared with a doctor-blade method followed by a chemical bath deposition (CBD) process, and the films are characterized by UV-vis spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy. The film-typed structure is composed of photocatalysts and Pt metal part on a FTO substrate without additional electric device, so it is relatively simpler than the conventional photoelectrochemical cell. CdS quantum dots are introduced as a sensitizer for visible light response. Amounts of CdS quantum dots on TiO₂ surface are increased with increasing CBD cycles, but they start to aggregate after certain CdS concentration due to oversaturation phenomenon. This high CdS content induces high electron losses, and therefore it reduces amounts of hydrogen production. As a result, there is a saturation point of CdS content at Cd/Ti ratio of 0.197, and the amounts of evolved hydrogen are 5.407 μmol/cm²·h at this photocatalyst formulation.     

A novel hierarchical Pt- and FTO-free counter electrode for dye-sensitized solar cell

A novel hierarchical Pt- and FTO-free counter electrode (CE) for the dye-sensitized solar cell (DSSC) was prepared by spin coating the mixture of TiO₂ nanoparticles and poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) solution onto the glass substrate. Compared with traditional Pt/FTO CE, the cost of the new CE is dramatically reduced by the application of bilayer TiO₂-PEDOT:PSS/PEDOT:PSS film and the glass substrate. The sheet resistance of this composite film is 35 Ω sq⁻¹ and is low enough to be used as an electrode. The surface morphologies of TiO₂-PEDOT:PSS layer and modified PEDOT:PSS layer were characterized by scanning electron microscope, which shows that the former had larger surface areas than the latter. Electrochemical impedance spectra and Tafel polarization curves prove that the catalytic activity of TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE is higher than that of PEDOT:PSS/FTO CE and is similar to Pt/FTO CE''s. This new fabricated device with TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE achieves a high power conversion efficiency (PCE) of 4.67%, reaching 91.39% of DSSC with Pt/FTO CE (5.11%).     

Absorption boost of TiO2 nanotubes by doping with N and sensitization with CdS quantum dots

A process of obtaining N-doped TiO₂ nanotubes sensitized by CdS nanoparticles is presented, including detailed characterizations performed along the synthesis. Transparent TiO₂ films consisting of nanotubes, 2.5 µm long and of ~60 nm inner diameter, were obtained after anodization of a titanium film deposited onto FTO glass substrate. N-doping was achieved by annealing of TiO₂ film in ammonia. X-ray Photoelectron Spectroscopy measurements showed that nitrogen was substitutionally incorporated in the TiO₂ matrix, with the N:Ti concentration ratio of 1:100. The doping changed the optical properties of the material in such a way that the absorption edge was shifted from 380 nm to 507 nm, as observed from diffuse reflectance spectra. The influence of the microwave (MW) irradiation on the synthesized CdS quantum dots and their optical properties was investigated. It was shown that the diameter of CdS nanoparticles was increased due to releasing of S^2- ions from dimethyl sulfoxide (DMSO) as a consequence of the MW treatment. The (N)TiO₂ films were then used as substrates for matrix assisted pulsed laser deposition of the CdS quantum dots with DMSO as a matrix. The laser parameters for the deposition were optimized in order to preserve the nanotubular structure open, the latter being an important feature of this type of photoanode. The structure obtained under optimized conditions has an additional absorption edge shift, reaching 603 nm.     

A highly efficient ZnS/CdS@TiO2 photoelectrode for photogenerated cathodic protection of metals

Shell-core nanostructured ZnS/CdS quantum dots (QDs) were assembled uniformly on the surface of TiO₂ nanotube arrays by sequence chemical bath deposition (CBD) of CdS and ZnS in alcohol solution system. The morphology and chemical composition of the obtained composite thin films were characterized by scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The effect of solvent and immersion cycles for the photoanode preparation on the photoelectrochemical activity and photogenerated cathodic protection property was investigated. It is found that the nanostructured CdS QDs (20 cycles) coated on TiO₂ nanotube arrays show a remarkably enhanced photoelectrochemical activity. The coating of ZnS QD shells (5 cycles) is able to improve the stability of the CdS@TiO₂ photoanode under white-light irradiation. After the irradiation light is turned off, the photogenerated cathodic protection of 403 stainless steel (403SS) can be remained for several hours.