Enhanced efficiency in dye-sensitized solar cells based on TiO2 nanocrystal/nanotube double-layered films

A new bilayer-structured film with TiO₂ nanocrystals as underlayer and TiO₂ nanotubes as overlayer was fabricated. The resultant double-layer TiO₂ (DL-TiO₂) film could significantly improve the efficiency of dye-sensitized solar cells (DSSCs) owing to its synergic effects, i.e. effective dye adsorption mainly originated from TiO₂ nanocrystal layer and rapid electron transport in one-dimensional TiO₂ nanotube layer. The overall energy-conversion efficiency (η) of 6.15% was achieved by the formation of DL-TiO₂ film, which is 44.7% higher than that formed by pure nanocrystalline TiO₂ (NC-TiO₂) film and far larger than that formed by nanotube TiO₂ (NT-TiO₂) film (η = 0.37%). The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that DL-TiO₂ film-based cell possessed the lowest transfer resistance and the longest electron lifetime. The incident-photon-to-current efficiency spectra indicated that the broad bands covered almost the entire visible spectrum from 400 to 700 nm with the maxima of 57.3%, 40.3%, and 2.2% at a wavelength of ∼530 nm for DL-TiO₂-, NC-TiO₂-, and NT-TiO₂-based solar cells, respectively. It is expected that the double-layer film electrode can be extended to other composite films with different layer structures and morphologies for enhancing the efficiencies of DSSCs.     

An approach to laminated flexible Dye sensitized solar cells

We have built TiO₂ Dye sensitized solar cells (DSSCs) that combined flexible TiO₂ photoanodes coated on ITO/PET substrates with a gel electrolyte based on PVDF-HFP-SiO₂ films. Titanium isopropoxide (TiP₄) was used as additive to TiO₂ nanoparticles for increasing power conversion efficiency in Dye sensitized solar cell electrodes prepared at low-temperature (130 °C). An efficiency η_AM1.5G = 3.55% on ITO/PET substrates is obtained at 48 mW/cm² illumination with a standard liquid electrolyte based on methoxypropionitrile. Among several solvents forming gels with PVDF-HFP-SiO₂, N-methyl (pyrrolidone) (NMP) was found to enable the most stable devices. A power conversion efficiency η_AM1.5G = 2% was obtained under 10 mW/cm² with flexible TiO₂-ITO-PET photoanodes and the PVDF-HFP-SiO₂ + NMP gel electrolyte.     

A series of LiI/acetamide phase transition electrolytes and their applications in dye-sensitized solar cells

A new series of electrolytes composed of LiI and acetamide have been investigated in dye-sensitized solar cells (DSSCs). These electrolytes melt at about 50 °C and their ionic conductivities vary drastically below and above the melting points (T_m). They tend to form large crystals at low temperature, leading to poor penetration and contact within porous TiO₂ anode film. This shortage is improved by introducing nano-SiO₂ particles into the electrolyte. A total conversion efficiencies (η) of 0.3% at 35 °C and 4.2% at 75 °C are achieved respectively under AM 1.5 simulated solar light illumination when a LiI/acetamide (1:16) electrolyte with 8 wt% nano-SiO₂ is used. It is expected that the DSSC using phase transition electrolyte could show high efficiency for operation at high temperature and high stability for storage at low temperature.     

Enhanced performance of a dye-sensitized solar cell with a modified poly(3,4-ethylenedioxythiophene)/TiO2/FTO counter electrode

ClO₄⁻-poly(3,4-ethylenedioxythiophene)/TiO₂/FTO (ClO₄⁻-PEDOT/TiO₂/FTO) counter electrode (CE) in dye-sensitized solar cells (DSSCs) is fabricated by using an electrochemical deposition method. Comparing with the DSSCs with ClO₄⁻-PEDOT/FTO counter electrode, the photocurrent-voltage (I-V) measurement reveals that the photocurrent conversion efficiency (η), fill factor (FF) and short-circuit current density (J_SC) of DSSCs with a ClO₄⁻-PEDOT/TiO₂/FTO CE increase. The enhanced performances of the DSSCs are attributed to the higher J_SC arising from the increase of active surface area of ClO₄⁻-PEDOT/TiO₂/FTO CE. Electrochemical impedance spectra (EIS) also indicate that the charge-transfer resistance on the ClO₄⁻-PEDOT/electrolyte interface decreases. Cyclic voltammetry results indicate that the ClO₄⁻-PEDOT/TiO₂/FTO electrode shows higher activity towards I₃⁻/I⁻ redox reaction than that of ClO₄⁻-PEDOT/FTO electrode.     

EIS analysis on low temperature fabrication of TiO2 porous films for dye-sensitized solar cells

A low temperature (<150 °C) fabrication method for preparation of TiO₂ porous films with high efficiency in dye-sensitized solar cells (DSSCs) has been developed. The Ti(IV) tetraisopropoxide (TTIP) was added to the paste of TiO₂ nanoparticles to interconnect the TiO₂ particles. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) and electron lifetime in the TiO₂ film (τ_e) under different molar ratios of TTIP/TiO₂ and also at various TiO₂ thicknesses. It was found that the R_ct2 decreased as the molar ratio increased from 0.02 to 0.08, however, it increased at a molar ratio of 0.2 due to the reduction in surface area for dye adsorption. In addition, the characteristic frequency peak shifted to lower frequency at a molar ratio of 0.08, indicating the longer electron lifetime. As for the thickness effect, TiO₂ film with a thickness around 17 μm achieved the best cell efficiency. EIS study also confirmed that, under illumination, the smallest R_ct2 was associated with a TiO₂ thickness of 17 μm, with the R_ct2 increased as the thickness of TiO₂ film increased. In the Bode plots, the characteristic frequency peaks shifted to higher frequency when the thickness of TiO₂ increased from 17.2 to 48.2 μm, indicating the electron recombination increases as the thickness of the TiO₂ electrode increases.Finally, to make better use of longer wavelength light, 30 wt% of larger TiO₂ particle (300 nm) was mixed with P25 TiO₂ as light scattering particles. It effectively increased the short-circuit current density and cell conversion efficiency from 7.44 to 8.80 mA cm⁻² and 3.75 to 4.20%, respectively.     

Dye-sensitized solar cells based on double-layered TiO2 composite films and enhanced photovoltaic performance

Dye-sensitized solar cells (DSSCs) are fabricated based on double-layered composite films of TiO₂ nanoparticles and hollow spheres. The photoelectric conversion performances of DSSCs based on nanoparticles/nanoparticles (PP), hollow spheres/hollow spheres (HH), hollow spheres/nanoparticles (HP), and nanoparticles/hollow spheres (PH) double-layered films are investigated, and their photo-electric conversion efficiencies are 4.33, 4.72, 4.93 and 5.28%, respectively. The enhanced performance of TiO₂ nanoparticles/hollow spheres double-layered composite film solar cells can be attributed to the combined effect of following factors. The light scattering of overlayer hollow spheres enhances harvesting light of the DSSCs and the underlayer TiO₂ nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and pore volume of TiO₂ hollow spheres are respectively beneficial to adsorption of dye molecules and transfer of electrolyte solution.     

Sol–gel hydrothermal synthesis of bismuth–TiO2 nanocubes for dye-sensitized solar cell

Bismuth–TiO₂ nanocubes were synthesized via a facile sol–gel hydrothermal method with titanium tetraisopropoxide as the precursor. The influence of the bismuth on the size, morphology, crystallinity and optical behavior of TiO₂ nanocubes were investigated. The samples were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy (UV–vis). Photovoltaic behavior of dye-sensitized solar cells (DSSCs) fabricated using Bi–TiO₂ nanocubes was studied. The DSSCs had an open-circuit voltage (V_oc) of 590 mV, a short-circuit current density (J_sc) of 7.71 mA/cm², and the conversion efficiency (η) of 2.11% under AM 1.5 illumination, a 77% increment as compared to pure TiO₂ nanocubes.     

Fabrication and photoelectrochemical properties of TiO2 films on Ti substrate for flexible dye-sensitized solar cells

The dye-sensitized solar cells (DSSCs) using Ti foil supporting substrate for fabricating nanocrystalline TiO₂ flexible film electrodes were developed, intending to improve the photoelectrochemical properties of flexible substrate-based DSSCs. The obtained cells were characterized by electrochemical impedance spectra (EIS), open circuit voltage decay (OCVD) measurement and Tafel plots. The experimental results indicate that the most important advantage of a Ti foil-based TiO₂ flexible electrode over a FTO glass-based electrode lies in its reduced sheet resistance, electron traps, and the retarded back reaction of electrons with tri-iodine ions in DSSCs. All above characteristics for the Ti substrate TiO₂ films are beneficial for decreasing the charge recombination in the TiO₂ electrode and prolonging the electron lifetimes for the DSSCs, as well as improvement of the overall solar conversion efficiency. The photocurrent of the cell fabricated with the Ti foil-based flexible electrode increased significantly, leading to a much higher overall solar conversion efficiency of 5.45% at 100 mW/cm² than the cell made with FTO glass-based TiO₂ electrodes. Above results demonstrate that Ti foil is a potential alternative to the conventional FTO glass substrate for the DSSCs.     

Dye-sensitized solar cells based on hollow anatase TiO2 spheres prepared by self-transformation method

Dye-sensitized solar cells (DSSCs) are fabricated based on hollow anatase TiO₂ (HA-TiO₂) spheres synthesized by a chemically induced self-transformation (CIST) strategy using urea as a base catalyst, whose walls are composed of anatase nanocrystals and exhibit hierarchical porosity. TiO₂ hollow structured materials not only have low density, high specific surface areas, and hierarchically porous structures, but also exhibit high light-collection efficiency and fast motion of charge carriers. Effects of calcination temperatures on the performance of HA-TiO₂ solar cells are investigated and discussed. With increasing calcination temperatures, the light-electricity conversion efficiencies (η) increase. At 600 °C, HA-TiO₂ cells reach the highest efficiency. The performances of HA-TiO₂ cells are also compared with Degussa P-25 (P25) TiO₂ nanoparticle cells at the same film thickness, and their optimal efficiencies at 600 °C are 4.82 and 4.35%, respectively. The enhanced performance of HA-TiO₂ cells is due to their high surface area and hierarchically nanoporous structures when compared with the nonporous TiO₂ nanoparticles (P25).     

Improvement of performances of complex non-platinum electrode materials for hydrogen evolution

Structural and electrochemical characteristics of hypo-hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied and modified in order to improve their performances. All studied materials were of general composition 10% Ni + 18% TiO₂ + C.All materials were prepared of amorphous or crystalline TiO₂, crystalline Ni or NiCo (10-20 nm) and Vulcan XC-72, by sol-gel procedure.Both, material's intrinsic catalytic activity and surface area were affected by applied modifications. As a result, the electrocatalytic activity was improved, e.g. transformation of TiO₂ into anatase form lowers the HER overpotential for 60 mV. Introduction of MWCNTs was even more effective, lowering η for 120 mV. Co addition to metallic phase lowers η for utmost 195 mV.Combined modification of TiO₂ and carbon substrate lowers η for 145 mV, while the complete modification of all three catalyst's components was the most effective with 230 mV decrease of overpotential.     

Photovoltaic performance of solid-state DSSCs sensitized with organic isophorone dyes: Effect of dye-loaded amount and dipole moment

Two isophorone sensitizers (S4 and D-3) were utilized in solid-state dye-sensitized solar cells (DSSCs) using spiro-OMeTAD as hole-transporting material. The dye-loaded amount of D-3 was almost 1.5 times as that of S4 which lead to higher light harvesting efficiency than S4. Moreover, the larger dipole moment along the direction for D-3 could cause more negative charges located close to the TiO₂ surface than that of S4, resulting in a larger conduction band (CB) upshift of TiO₂ for D-3 which was beneficial to an increase of V_oc. Promising results sensitized by D-3 in solid-state DSSCs were achieved with a short-circuit photocurrent density (J_sc) of 3.4 mA cm⁻², an open-circuit photovoltage (V_oc) of 760 mV, a fill factor (FF) of 0.71, and an overall efficiency (η) of 1.92% while ruthenium dye N3 produced a η of 2.55% under the same conditions (AM 1.5, 100 mW cm⁻²).     

Enhancing the performance of dye-sensitized solar cells based on an organic dye by incorporating TiO2 nanotube in a TiO2 nanoparticle film

The photoelectrochemical properties of a high molar extinction coefficient charge transfer organic dye containing thienylfluorene segment called FL, and the effect of incorporating TiO₂ nanotube (TiNT) in TiO₂ nanoparticle film along with the above dye on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were investigated. The influence of soaking time of the TiO₂ electrode in dye solution and the effect of varying its concentration, on the solar cell efficiency was also studied. Cyclic voltammetric (CV) analysis revealed the linear relationship between the anodic peak current and the scan rate, indicating a surface-confined diffusion process.The surface morphology of TiNT was characterized using SEM, TEM and XRD. The open-circuit voltage (V_OC) of the DSSC increased with the increase in the wt% of TiNT and shows optimal value at about 5 wt%, which is correlated with the suppression of the electron recombination as found out from the electron lifetime studies.The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance (R_ct) and electron lifetime under different ratios of the TiNT/nanoparticle. The electron lifetimes of the DSSCs based on FL and N3 dye were very close to one another and the DSSC based on the FL showed respectable photovoltaic performance of ca. 7.8% under the light intensity of 100 mW cm⁻² (AM 1.5G).     

Organic dyes incorporating the cyclopentadithiophene moiety for efficient dye-sensitized solar cells

Three triarylamine organic dyes (XS28-30) containing a cyclopentadithiophene unit as the conjugated bridge have been designed and synthesized for a potential application in dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties have been investigated. The incorporation of ethyl-substituted cyclopentadithiophene is highly beneficial to light-harvesting and preventing close π-π aggregation, thus favorably generating high efficiency. For a typical device, a solar energy conversion efficiency (η) of 5.8% based on XS29 was achieved under simulated AM 1.5 solar irradiation (100 mW cm⁻²) with a short-circuit photocurrent density (J_SC) of 14.4 mA cm⁻², an open-circuit voltage (V_OC) of 601 mV, and a fill factor (ff) of 0.68. These results suggest that the functionalized cyclopentadithiophene unit is a promising candidate for DSSCs.     

Developing the properties of new blue phosphors: TiO2-doped Zn2SiO4

2ZnO + SiO₂ + X mol% TiO₂ (Zn₂SiO₄-X-TiO₂, 1 ≤ X ≤ 3) and 2ZnO + SiO₂ + 3 mol% MnO₂ (Zn₂SiO₄-3-TiO₂) compositions were prepared using nanoscale ZnO, SiO₂, TiO₂, and MnO₂ particles. The mixing powders were calcined between 1000 °C and 1300 °C in a N₂ atmosphere. Zn₂SiO₄ was the only phase in the calcined Zn₂SiO₄-X-TiO₂ phosphors. We found that the photoluminescence (PL) properties of synthesized Zn₂SiO₄-X-TiO₂ phosphors revealed these to be blue rather than green. The effects of TiO₂ content and calcining temperature on the PL properties of Zn₂SiO₄-X-TiO₂ phosphors were rigorously investigated.     

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO₂ aerogel film electrodes were fabricated. TiO₂ aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO₂) drying. First, TiO₂ wet gels were obtained by sol-gel method. Then, the solvents in the TiO₂ wet gels were replaced by acetone. The TiO₂ aerogels were obtained by using sc-CO₂ drying from the TiO₂ wet gels. The conditions of sc-CO₂ drying were at 313, 323 K and 7.8–15.5 MPa. The electrodes with TiO₂ aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO₂ aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO₂ films with sc-CO₂ drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO₂ films increased with increasing surface area of the TiO₂ film. DSSCs were assembled by using the TiO₂ aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313 K and 15.5 MPa showed the best power performance (J_sc = 7.30 mA/cm², V_oc = 772 mV, η = 3.28%).     

Synthesis of a novel alkylimidazolium iodide containing an amide group for electrolyte of dye-sensitized solar cells

A novel alkylimidazolium iodide containing an amide group, 1-(2-hexanamidoethyl)-3-methylimidazol-3-ium iodide (amido-ImI), was synthesized to act as the quasi-solid-state electrolyte of dye-sensitized solar cells (DSSCs). The DSSC with the amido-ImI electrolyte exhibited short-circuit photocurrent density (J_sc) and overall energy conversion efficiency (η) that were improved by 7.2% and 10.2%, respectively, compared to those obtained with the cell containing 1-hexyl-2,3-dimethylimidazolium iodide, a commonly used liquid electrolyte, at 100 mW cm⁻². Furthermore, the stability of the DSSC was enhanced by the presence of amido-ImI.     

Study of structural and electrochemical characteristics of Co-based hypo-hyper d-electrocatalysts for hydrogen evolution

Structural and electrochemical characteristics of hypo-hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied. The basic catalytic material was prepared of 10% amorphous Co (grain size <2 nm), 18% amorphous TiO₂ and Vulcan XC-72, by sol-gel procedure. A number of modifications were applied aimed at improving the materials performances: (i) TiO₂ was transformed into anatase by heating at 480 °C for 1 h, (ii) multiwalled carbon nanotubes (MWCNT) were used as a catalyst support instead of Vulcan XC-72 and (iii) Mo was added to Co phase in a quantity of 25 at.% (Mo:Co = 1:3).Both, material's intrinsic catalytic activity and surface area were affected by these modifications. As a result, the electrocatalytic activity for hydrogen evolution was improved, e.g. transformation of TiO₂ into anatase form lowers the HER overpotential (η) for 15 mV at 60 mA cm⁻². Introduction of MWCNTs lowered η for 30 mV, while addition of Mo to metallic phase for 40 mV.The complete modification of all three catalyst's components (10% MoCo₃ + 18% anatase + MWCNTs) was the most effective with 60 mV decrease of overpotential.Characterization was made by XRD, SEM, IR and XPS methods. Surface area was measured by means of cyclic voltammetry.     

The photovoltaic efficiency of dye sensitized solar cell assembled using carbon capsulated TiO2 electrode

Carbon-encapsulated TiO₂ was synthesized and its applications to dye-sensitized solar cells (DSSCs) were evaluated. When carbon-encapsulated TiO₂ was applied to DSSCs as a scattering material, the efficiency was enhanced considerably compared to that using only double-layered, nanometer-sized pure TiO₂. The photovoltaic efficiency of the carbon-encapsulated TiO₂-DSSCs was approximately 3.59%, while it was 2.87% for the pure TiO₂-DSSCs. The quantum efficiencies evaluated from IPCE spectroscopy were better on the carbon-encapsulated TiO₂-DSSCs compared to of a TiO₂-DSSCs. PL spectroscopy showed that the rate of recombination between the holes and electrons on the carbon-encapsulated TiO₂ were lower than that on pure TiO₂.     

Chlorophyll assembled electrode for photovoltaic conversion device

Chlorophyll-a (Chl-a) assembled in hydrophobic domain by fatty acid with long alkyl hydrocarbon chain such as myristic acid (Myr), stearic acid (Ste) and cholic acid (Cho) modified onto nanocrystalline TiO₂ electrode is prepared and the photovoltaic properties of the nanocrystalline TiO₂ film by Chl-a are studied. Incident photon to current efficiency (IPCE) value at 660 nm in photocurrent action spectrum of Chl-a/Ste-TiO₂, Chl-a/Myr-TiO₂ and Chl-a/Cho-TiO₂ electrodes are 5.0%, 4.1% and 4.1%, respectively. Thus, the IPCE is maximum using Chl-a/Ste-TiO₂ electrode. From the results of photocurrent responses with light intensity of 100 mW cm⁻² irradiation or monochromatic light with 660 nm, generated photocurrent increases using Chl-a/Ste-TiO₂ electrode compared with the other Chl-a assembled TiO₂ electrodes. These results show that the hydrophobic domain formed by stearic acid with long alkyl hydrocarbon chain is suitable for fixation of Chl-a onto TiO₂ film electrodes and photovoltaic performance is improved using Chl-a onto Ste-TiO₂ film electrode.     

Performance improvement of dye-sensitizing solar cell by semi-rigid triarylamine-based donors

Novel organic dyes (IDB and ISB dyes), which contain 5-phenyl-iminodibenzyl (IDB) and 5-phenyl-iminostilbene (ISB) as electron donors and a cyanoacrylic acid moiety as an electron acceptor and an anchoring group, connected with a thiophene as a π-conjugated system, have been synthesized and used as the sensitizers for dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the dyes were investigated by absorption spectrometry, cyclic voltammetry and density functional theory calculations. As demonstrated, the IDB and ISB unit exhibited stronger electron-donating ability and broader absorption spectra when coated onto TiO₂. The DSSC based on ISB-2 consisting of ISB unit produced 5.83% of η (J_sc = 13.14 mA cm⁻², V_oc = 0.64 V, and ff = 0.68) under 100 mW cm⁻² simulated AM 1.5 G solar irradiation.