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.     

Improved performance of dense TiO2/CdSe coupled thin films by low temperature process

Dense TiO₂ and TiO₂/CdSe coupled nanocrystalline thin films were synthesized onto ITO coated glass substrate by chemical route at relatively low temperature (≤100 °C). TiO₂ films were nanocrystalline and crystallinity disappears after CdSe deposition as evidenced by X-ray powder diffraction. Surface morphology and physical appearance of films were studied from SEM and actual photo-images, reveals dense nature of TiO₂ (10-12 nm spherical grains, faint violet) and CdSe (80-90 nm spherical grains, deep brown), respectively. Presence of two absorption edges in UV spectra implies existence of separate phases rather than composite formation. TiO₂ film was found to have higher water contact angle (71°) than TiO₂/CdSe (61°) and CdSe (56°). I-V and stability tests of photo-electrochemical cells were performed with TiO₂ and TiO₂/CdSe film electrodes (under light of illumination intensity 80 mW/cm²) in lithium iodide as an electrolyte using two-electrode system.     

Synthesis and characterization of mesoporous TiO2 nanostructured films prepared by a modified sol-gel method for application in dye solar cells

Anatase TiO₂ colloidal dispersions were obtained by hydrothermal synthesis at 200 °C from titanium isopropoxide gels modified with acetic acid in the presence of a non-ionic surfactant. Absolute ethanol, anhydrous terpineol and ethyl cellulose were added to this anatase dispersion resulting in a 23 wt% TiO₂ paste. Mesoporous films for application as working electrodes in dye-sensitized solar cells were prepared by the screen-printing method, yielding reproducible films with thicknesses about 10 μm and desired porosity levels in a single printing operation. An average energy conversion efficiency of 5.2%, and a fill factor of 0.66 were achieved with anatase particle sizes ranging between 15 and 20 nm. The reproducibility of the results was confirmed by electrochemical impedance spectroscopy analysis.     

Enhancement of photoelectric conversion by high-voltage electric field assisted crystallization of a novel ternary-encapsulated spherical TiO2 aggregate for solar cells

A novel ternary-encapsulated spherical TiO₂ aggregate (TES-TiO₂) with submicron particle sizes was formed by blending commercial P25 TiO₂ and two different sizes of TiO₂ particles (synthesized by modified sol-gel and hydrothermal methods). A double-layered TiO₂ electrode for dye-sensitized solar cells (DSSCs) was fabricated by depositing TES-TiO₂ particles onto nanocrystalline mesoporous TiO₂ (Meso-TiO₂)-coated FTO glass by a cathodic electrophoresis technique and then calcined at 450 °C for 30 min. Compared to double-layered Meso-TiO₂/P25 electrodes, the energy conversion efficiency (η) of DSSCs from the obtained Meso-TiO₂/TES-TiO₂ electrode was improved by 9.3%, from 5.94% to 6.49%. When the prepared double-layered Meso-TiO₂/TES-TiO₂ electrode was calcined at high temperature, a high-voltage electric field (HVEF) was introduced to assist crystallization. As a result, η was further enhanced by 8.6%, from 6.49% to 7.05%. Notably, compared to typical 20 nm TiO₂ nanocrystallites applied in the active layer of DSSCs, the prepared loosely porous TES-TiO₂ with submicron size increased the light-scattering effect and promoted dye molecule adsorption and the diffusion of electrolytes. In addition, introduction of the HVEF provided better connection among TiO₂ particles, which facilitated electron transport and avoided charge recombination.     

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.     

The improvement of electron transport rate of TiO2 dye-sensitized solar cells using mixed nanostructures with different phase compositions

Dye-sensitized solar cells (DSCCs) in the form of mixed nanostructures containing TiO₂ nanoparticles and nanowires with different weight ratios and phase compositions are reported. X-ray diffraction and field emission scanning electron microscopy analyses revealed that the synthesized TiO₂ nanoparticles had average crystallite size in the range 21–39 nm, whereas TiO₂ nanowires showed diameter in the range 20–50 nm. The indirect optical band gap energy of TiO₂ nanowires, anatase- and rutile-TiO₂ nanoparticles was calculated to be 3.35, 3.28 and 3.17 eV, respectively. The power conversion efficiency of the solar cells changed with nanowire to nanoparticle weight ratio, reaching a maximum at a specific value. An increase of 4.3% in cell efficiency was achieved by introducing 10 wt% nanowire into the as-synthesized TiO₂ nanoparticles (WP1 cell). Furthermore, an increase of 27.6% in cell efficiency was achieved by using crystalline anatase-TiO₂ nanoparticles rather than as-synthesized TiO₂ nanoparticles in WP1 solar cell. It was found that the power conversion efficiency and short circuit current of WP1 cell were decreased down to around 30.8% and 39.1%, respectively using rutile nanoparticles rather than anatase nanoparticles. The improvement of cell efficiency was related to rapid electron transport and less recombination of photogenerated electrons, as confirmed by electrochemical impedance spectroscopy.     

Efficiency improvement of dye-sensitized solar cells based on electrodeposited TiO2 films by low temperature post-treatment

Porous crystalline TiO₂ films can be prepared at low temperatures (80 °C) by surfactant-assisted electrodeposition from TiCl₃ solution. Nevertheless, up to now calcination at high temperatures (typically 450 °C) was still necessary to establish a good performance of these films in dye-sensitized solar cells (DSSC). With this study we report that water vapour treatment at much lower temperatures (150 °C) for 1 week improves the performance of the films in DSSC to the same degree as calcination although the overall crystallinity remains lower. Reason for the good efficiency is that the porous structure stays intact and thus the dye molecules can be better adsorbed. Avoiding high temperatures during the preparation process of TiO₂ films for the application in DSSC enables the use of polymer substrates for the fabrication of flexible solar cells.     

Effects of annealing temperature on the photocatalytic activity of N-doped TiO2 thin films

The effects of annealing temperature on the photocatalytic activity of nitrogen-doped (N-doped) titanium oxide (TiO₂) thin films deposited on soda-lime-silica slide glass by radio frequency (RF) magnetron sputtering have been studied. Glancing incident X-ray diffraction (GIAXRD), Raman spectrum, scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectra were utilized to characterize the N-doped TiO₂ thin films with and without annealing treatment. GIAXRD and Raman results show as-deposited N-doped TiO₂ thin films to be nearly amorphous and that the rutile and anatase phases coexisted when the N-doped TiO₂ thin films were annealed at 623 and 823 K for 1 h, respectively. SEM microstructure shows uniformly close packed and nearly round particles with a size of about 10 nm which are on the slide glass surface for TiO₂ thin films annealed at 623 K for 1 h. AFM image shows the lowest surface roughness for the N-doped TiO₂ thin films annealed at 623 K for 1 h. The N-doped TiO₂ thin films annealed at 623 K for 1 h exhibit the best photocatalytic activity, with a rate constant (k_a) of about 0.0034 h⁻¹.     

Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells

Spherical pure anatase TiO₂ spheres with a mesoporous structure and high surface area of up to 116.5 m² g⁻¹ were prepared by a simple urea-assisted hydrothermal process and investigated as dye-sensitized solar-cell electrodes. Although the particle diameters of the prepared TiO₂ spheres ranged from 0.4 to 1.3 μm, due to the high specific surface area, mesoporous TiO₂ sphere electrode was obtained with enhanced light harvesting and a larger amount of dye loading. An overall light conversion efficiency of 7.54% under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) was achieved using the mesoporous TiO₂ spheres electrode, which was significantly higher than a commercial Degussa P25 TiO₂ nanocrystals electrode (5.69%).     

Fabrication of hole-patterned TiO2 photoelectrodes for solid-state dye-sensitized solar cells

We suggest a simple process to fabricate a hole-patterned TiO₂ electrode for a solid-state dye-sensitized solar cell (DSSC) to enhance cell performance through interfacial properties of the electrode with the electrolyte with minimum dye loading. The method involves prepatterning of SU-8 photoresist on a conducting glass, followed by the deposition of a nanocrystalline TiO₂ layer, calcination at 450 °C and characterization using scanning electron microscopy (SEM). Hole-patterned TiO₂ photoelectrodes yielded better solar energy conversion efficiency per dye loading compared to a conventional non-patterned photoelectrode. For example, a 50 μm hole-patterned DSSC exhibited 4.50% conversion efficiency in the solid state, which is comparable to an unpatterned flat TiO₂ photoelectrode (4.57%) however the efficiency per dye loading of the former (0.986%/g) was much greater than that of the latter (0.898%/g). The improvement was attributed to improved transmittance through the electrode as well as better interfacial properties between the electrolyte and electrode, as confirmed by UV-visible spectroscopy and electrochemical impedance (EIS) analysis.     

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.     

TiO2 films prepared by micro-plasma oxidation method for dye-sensitized solar cell

Nanocrystalline TiO₂ films are widely investigated as the electrodes of dye-sensitized solar cell(s) with different preparation methods. In this paper, thin titanium dioxide films have been prepared on titanium plates by the micro-plasma oxidation method in the sulfuric acid solution. The thin TiO₂ films were sensitized with a cis-RuL₂(SCN)₂·2H₂O (L = cis-2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium complex and implemented into a dye-sensitized solar cell configuration. The influence of reaction current density (10, 15, 20, 25 and 30 A dm⁻²) on the structural and the surface morphology of the films was investigated by X-ray diffraction, scanning electron microscopy, atom force microscopy and X-ray photoelectricity spectroscopy. Impedance analysis for dye-sensitized solar cells was carried out by electrochemical impedance spectroscopy. The results show that the rise of current density leads to the increase in the amount of rutile and the thickness of the TiO₂ film, which makes the TiO₂ films have different photovoltages and photocurrents. The relatively higher photoelectricity properties were obtained in the TiO₂ films prepared at a current density of 20 A dm⁻². The open-circuit voltage and the short-circuit current are 605 mV and 165 μA cm⁻², respectively.     

Photosensitization of TiO2 nanorods with CdS quantum dots for photovoltaic devices

CdS quantum dots (QDs) coated TiO₂ nanorod arrays have been prepared via a two-step method. TiO₂ nanorod arrays were synthesized by a facile hydrothermal method, and CdS QDs were deposited on the nanorods by a sequential chemical bath deposition (S-CBD) technique. The surface morphology, structure, optical and photoelectrochemical behaviors of the core-shell nanorod array films are considered. A photocurrent of 2.5 mA/cm², an open circuit photovoltage of 1.10 V, and a conversion efficiency of 1.91% were obtained under an illumination of 100 mW/cm², when the CdS QDs deposited on TiO₂ nanorods film for about 7 cycles. The results demonstrate that the composite films are of excellence with respect to photovoltaic conversion.     

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.     

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.     

Viewing nanocrystalline TiO2 photoelectrodes as three-dimensional electrodes: Effect of the electrolyte upon the photocurrent efficiency

Nanocrystalline TiO₂ films are widely investigated both as photoelectrodes to carry out photocatalytic reactions under band-gap near-UV illumination or, alternatively, as an electronically conducting matrix of dye-sensitised electrodes as a part of a liquid-junction solar photovoltaic cell. The main features of these photoelectrodes are: (i) significant thickness to allow effective absorption of incident light—the films consist typically of 100-1000 superimposed layers of individual nanoparticles; (ii) large porosity—ca. 50% of the film volume is normally filled with electrolyte; (iii) large surface-to-volume ratio resulting from the small size of TiO₂ nanoparticles (with diameters in the range of 10-30 nm) forming the film. This work re-examines the question of charge and mass transport in nanocrystalline titanium dioxide electrodes which are among the major factors affecting the efficiency of TiO₂-based dye-sensitised solar cells and photocatalytic devices. In contrast to most of the reports published in the literature, which analyse of the behaviour of such films under transient conditions, we focus here on the steady-state operation of nanocrystalline TiO₂ electrodes, directly relevant to the efficiency of cells used in real-world applications. Understanding of the charge and mass transport across these nanocrystalline films is greatly facilitated by considering them as three-dimensional electrodes including two coincidental, superimposed continua, i.e., the solid matrix that conducts electrons towards the back contact and the electrolyte—an ionic conductor carrying another part of the current through the pores of the nanostructured film. It is shown that the composition and conductivity of the chosen electrolyte and, in particular, the nature of the ionic species which act as hole scavengers or redox mediators largely affect the current distribution within the film and thus the final photon-to-current conversion efficiency of the photoelectrodes.     

Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells

In this study, dye-sensitized solar cells (DSSCs) were fabricated using nanocrystalline titanium dioxide (TiO₂) nanoparticles as photoanode. Photoanode thin films were prepared by doctor blading method with 420 kg/cm² of mechanical compression process and heat treatment in the air at 500°C for 30 min. The optimal thickness of the TiO₂ NP photoanode is 26.6 μm with an efficiency of 9.01% under AM 1.5G illumination at 100 mW/cm². The efficiency is around two times higher than that of conventional DSSCs with an uncompressed photoanode. The open-circuit voltage of DSSCs decreases as the thickness increases. One DSSC (sample D) has the highest conversion efficiency while it has the maximum short-circuit current density. The results indicate that the short-circuit current density is a compromise between two conflict factors: enlargement of the surface area by increasing photoanode thickness and extension of the electron diffusion length to the electrode as the thickness increases.     

Effects of tungsten thickness and annealing temperature on the electrical properties of W-TiO2 thin films

TiO₂ thin films were prepared by RF magnetron sputtering onto glass substrates and tungsten was deposited onto these thin films (deposition time 15-60 s) to form W-TiO₂ bi-layer thin films. The crystal structure, morphology, and transmittance of these TiO₂ and W-TiO₂ bi-layer thin films were investigated. Amorphous, rutile, and anatase TiO₂ phases were observed in the TiO₂ and W-TiO₂ bi-layer thin films. Tungsten thickness and annealing temperature had large effects on the transmittance of the W-TiO₂ thin films. The W-TiO₂ bi-layer thin films with a tungsten deposition time of 60 s were annealed at 200 °C-400 °C. The band gap energies of the TiO₂ and the non-annealed and annealed W-TiO₂ bi-layer thin films were evaluated using (αhν)^1/2 versus energy plots, showing that tungsten thickness and annealing temperature had major effects on the transmittance and band gap energy of W-TiO₂ bi-layer thin films.     

Dye-sensitized solar cell based on optically transparent TiO2 nanocrystalline electrode prepared by atomized spray pyrolysis technique

Preparation of crack-free thin films of interconnected and non-agglomerated TiO₂ nanoparticles on electronically conducting fluorine doped tin oxide surfaces is instrumental in designing and developing transparent dye-sensitized solar cells (DSCs). A novel technique called “Atomized Spray Pyrolysis” (ASP) has been designed and developed to achieve such perfectly transparent thin films. Optical transmittance of TiO₂ films produced on FTO surface by this ASP method has been compared with those obtained by doctor-blading and by hand spray methods and found that the atomized spray pyrolysis technique give films with high transparency. Dye adsorption per gram of TiO₂ is 2.16 times higher in the sample produced by the ASP method when compared to the film produced by the hand spray method and is 1.60 times higher than that produced by the doctor-blading method using a commercially available TiO₂ nanocrystalline paste. SEM studies show the presence of interconnected discrete particles in the film produced by the ASP method. The fill factor (ff) remains almost constant for the cells with thickness from 6 μm to 13 μm but the highest photovoltage and photocurrent were found in ∼10 μm film based DSC which gave 8.2% conversion efficiency at AM 1.5 irradiation for cells of 0.25 cm² active area.