The effect of ZnO-coating on the performance of a dye-sensitized solar cell

This study investigates the effect of a ZnO-coated TiO₂ working electrode on the power conversion efficiency of a dye-sensitized solar cell (DSSC). This electrode was designed and fabricated by dipping the TiO₂ electrode with the TiCl₄ treatment in a solution of zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] and ethanol. The effects of the concentration of Zn(CH₃COO)₂·2H₂O and the duration of dipping on the band gap of a working electrode and the power conversion efficiency of a DSSC were also examined. The band gap of the working electrode increases to 3.75 eV [TiO₂ electrode dipped in 0.05 M Zn(CH₃COO)₂·2H₂O) for 3 min] from 3.22 eV (TiO₂ electrode). Interestingly, the power conversion efficiency of the DSSC with a Zn-coated TiO₂ electrode (6.7%) substantially exceeds that of the conventional DSSC with a TiO₂ electrode (5.9%), and it may be originated from an increased energy barrier between ZnO and TiO₂ that reduces the electron recombination rate.     

Efficiency enhancement in DSSC using metal nanoparticles: A size dependent study

The photoelectrode of Eosin-Y sensitised DSSC was modified by incorporating Au-nanoparticles to enhance the power conversion efficiency via scattering from surface plasmon polaritons. Size dependence of Au nanoparticle on conversion efficiency was performed in DSSC for the first time by varying the particle size from 20 to 94 nm. It was found that, the conversion efficiency is highly dependent on the size of the Au nanoparticles. For larger particles (>50 nm), the efficiency was found to be increased due to constructive interference between the transmitted and scattered waves from the Au nanoparticle while for smaller particles, the efficiency decreases due to destructive interference. Also a reduction in the V_oc was observed in general, due to the negative shifting of the TiO₂ Fermi level on the adsorption of Au nanoparticle. This shift was negligible for larger particles. When 94 nm size particles were employed the conversion efficiency was doubled from 0.74% to 1.52%. This study points towards the application of the scattering effect of metal nanoparticle to enhance the conversion efficiency in DSSCs.     

Physical properties of the CNT:TiO2 thin films prepared by sol–gel dip coating

Uniform and highly adherent thin films of CNT:TiO₂ were synthesized by sol–gel dip coating method. Both TiO₂ and CNT:TiO₂ films showed very identical structural characteristics and no significant changes in the lattice values were observed. The crystalline size decreased from 20 nm for TiO₂ film to 17 nm for the 4%CNT:TiO₂ film. The film surface was very smooth and compact, as indicated by the roughness data obtained from AFM measurements; the root mean square (rms) average of the roughness was as low as 3 nm. The HRTEM showed that the CNTs are embedded in the matrix of TiO₂ indicating the formation of a composite. In Raman spectra the characteristic vibrations of the TiO₂ are identified, the increase in the FWHM of main anatase peak (144 cm^?1) in the case of the 4%CNT:TiO₂ film is interpreted as due to the incorporation of CNTs in the film. At the wavelength of 600 nm the refractive index of pure TiO₂ was 2.07 and the 4%CNT:TiO₂ showed a value of 2.29. The photoresponse curves showed typical features of charge trapping centers in the band gap of the films.     

Hybrid poly (3-hexylthiophene)/titanium dioxide nanorods material for solar cell applications

We conducted an extensive study on poly(3-hexylthiophene) (P3HT) in combination with titanium dioxide (TiO₂) nanorods hybrid material for polymer solar cell applications. The device performance critically depends on the morphology of the hybrid film that will be determined by the molecular weight of P3HT, the solvent type, the hybrid compositions, the surface ligand on the TiO₂ nanorods, film thickness, process conditions, and so on. The current–voltage characteristic of the device fabricated in air has shown a power conversion efficiency of 0.83% under air mass (AM) 1.5 illumination using high molecular weight (65,000 D) P3HT, high boiling point solvent trichlorobenzene, and pyridine-modified TiO₂ nanorods with a film thickness of about 100 nm. The Kelvin probe force microscopy (KPFM) study of hybrid films shows large-scale phase separation with domain size greater than 10 nm, which may be the main factor limiting device performance.     

Soft‐template synthesis of high surface area mesoporous titanium dioxide for dye‐sensitized solar cells

In the present work, 10 to 14 nm titania nanoparticles with high‐packing density are synthesized by the soft‐template method using a range of cationic surfactants including cetyl trimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS), and dodecyl trimethylammonium bromide (DTAB). The synthesized nanoparticles are used as a photoanode material in dye solar cells. Density functional theory (DFT) simulations reproduce our experimental results of charge transfer and strong interaction between the TiO₂ and N719. N719‐TiO₂ complex establishes strong electrostatic bonding through H of the dye with the O of TiO₂ surface. Solar cell efficiency of 6.08% with 12.63 mA/cm², 793 mV, and 48.5% for short circuit current density, open circuit voltage, and fill factor, respectively, are obtained under 1 sun illumination for the dye‐sensitized solar cell (DSSC) using a film of mesoporous TiO₂ synthesized from the SDS surfactant. On the other hand, the 21 nm commercial TiO₂ powder (P25) device results in 4.60% efficiency under similar conditions. Electrochemical impedance spectroscopic studies show that the SDS device has lesser charge transport resistance than the other devices because of its higher surface area, packing density, and dye loading capacity. Our results show that employing high packing density‐based TiO₂ nanoparticles represents a commercially viable approach for highly beneficial photoanode development for future DSSC applications.     

TiO2–Au plasmonic nanocomposite for enhanced dye-sensitized solar cell (DSSC) performance

Anatase TiO₂ nanoparticles dressed with gold nanoparticles were synthesized by hydrothermal process by using mixed precursor and controlled conditions. Diffused Reflectance Spectra (DRS) reveal that in addition to the expected TiO₂ interband absorption below 360 nm gold surface plasmon feature occurs near 564 nm. It is shown that the dye sensitized solar cells made using TiO₂–Au plasmonic nanocomposite yield superior performance with conversion efficiency (CE) of ～6% (no light harvesting), current density (J_SC) of ～13.2 mA/cm², open circuit voltage (V_oc) of ～0.74 V and fill factor (FF) 0.61; considerably better than that with only TiO₂ nanoparticles (CE ～ 5%, J_SC ～ 12.6 mA/cm², V_oc ～ 0.70 V, FF ～ 0.56).     

Perylene imide dyes for solid-state dye-sensitized solar cells: Spectroscopy,energy levels and photovoltaic performance

We report the solid-state dye-sensitized solar cell performances of perylene imide using nanoporous TiO₂ electrodes. Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD). The experimental E_LUMO levels of perylene imide dyes are found to be 3.75 and 3.77 eV, respectively. Therefore, perylene imide dyes can inject electrons to the conduction band of titanium dioxide in organic dye-sensitized solar cells. TiO₂ thin films of about 2 μm in thicknesses were prepared. Both preparation and thickness of the compact TiO₂ layer were optimized using spray pyrolysis. The studies revealed that an optimum film thickness of 130–150 nm of compact TiO₂ yielded the best rectifying behavior and SDSC performance. In this work, our goal was to investigate the performance of perylene sensitizers in connection with spiro-OMeTAD hole transport material. Short-circuit current densities, open circuit voltages and overall conversion efficiencies of the solar cell with 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-OMeTAD) as a hole conductor and perylene imide as sensitizer on mesoporous TiO₂ were investigated.     

Synthesis of lithium insertion material Li4Ti5O12 from rutile TiO2 via surface activation

The synthesis of spinel-type lithium titanate, Li₄Ti₅O₁₂, a promising anode material of secondary lithium-ion battery, from “inert” rutile TiO₂, is investigated. On the purpose of increasing the reactivity of rutile TiO₂, it is treated by concentrated HNO₃. By applying such activated rutile TiO₂ as the titanium source in combination with the cellulose-assisted combustion synthesis, phase-pure Li₄Ti₅O₁₂ is successfully synthesized at 800 °C, at least 150 °C lower than that based on solid-state reaction. The resulted oxide shows a reversible discharge capacity of ～175 mAh g^?1 at 1 C rate, near the theoretical value. The resulted oxide also shows promising high rate performance with a discharge capacity of ～100 mAh g^?1 at 10 C rate and high cycling stability.     

Preparation of sol–gel TiO2/purified Na-bentonite composites and their photovoltaic application for natural dye-sensitized solar cells

The sol–gel TiO₂/purified natural clay electrodes having Ti:Si molar ratios of 95:5 and 90:10 were initially prepared, sensitized with natural red cabbage dye, and compared to the sol–gel TiO₂ electrode in terms of physicochemical characteristics and solar cell efficiency. The results showed that the increase in purified Na-bentonite content greatly increased the specific surface area and total pore volume of the prepared sol–gel TiO₂/purified Na-bentonite composites because the clay platelets prevented TiO₂ particle agglomeration. The sol–gel TiO₂/5 mol% Si purified Na-bentonite and sol–gel TiO₂/10 mol% Si purified Na-bentonite composites could increase the film thickness of solar cells without cracking when they were coated as a scattering layer on the TiO₂ semiconductor-based film, leading to increasing the efficiency of the natural dye-sensitized solar cells in this work.     

Dye-sensitized solar cells based on a single layer deposition of TiO2 from a new formulation paste and their photovoltaic performance

A new strategy for enhancing the efficiency and reducing the production cost of TiO₂ solar cells by design of a new formulated TiO₂ paste with tailored crystal structure and morphology is reported. The conventional three- or four-fold layer deposition process was eliminated and replaced by a single layer deposition of TiO₂ compound. Different TiO₂ pastes with various crystal structures, morphologies and crystallite sizes were prepared by an aqueous particulate sol–gel process. Based on simultaneous differential thermal (SDT) analysis the minimum annealing temperature to obtain organic-free TiO₂ paste was determined at 400 °C, being one of the lowest crystallization temperatures of TiO₂ photoanode electrodes for solar cell application. Photovoltaic measurements showed that TiO₂ solar cell with pure anatase crystal structure had higher power conversion efficiency (PCE) than that made of pure rutile-TiO₂. However, the PCE of solar cells depends on the anatase to rutile weight ratio, reaching a maximum at a specific value due to the synergic effect between anatase and rutile TiO₂ nanoparticles. Moreover, it was found that the PCE of solar cells made of crystalline TiO₂ powders was much higher, increasing in the range 32–84% depending on anatase to rutile weight ratio, than that of prepared by amorphous powders. TiO₂ solar cell with the morphology of mixtures of nanoparticles and microparticles had higher PCE than the solar cell with the same phase composition containing TiO₂ nanoparticles due to the role of TiO₂ microparticles as light scattering particles. The presented strategy would open up new insight into fabrication and structural design of low-cost TiO₂ solar cells with high power conversion efficiency.     

Nano-structured spherical porous SnO2 anodes for lithium-ion batteries

Spherical porous tin oxide was fabricated via a spray pyrolysis technique. TEM revealed that the primary SnO₂ crystals had an average size of about 5 nm. The electrochemical measurements showed that the spherical porous SnO₂ samples have excellent cyclability, which can deliver a reversible capacity of 410 mAh g⁻¹ up to 50 cycles as a negative electrode for lithium batteries. The second step of the study was to thermal treat the initial tin oxide for 3 h at 600, 800, 1000 and 1200 °C, respectively, in order to identify the particle size effect on the electrochemical performance toward lithium. It was found that the morphologies of these spherical clusters could be maintained even after thermal treatment at 1200 °C. It was also proved that finer the size of the tin oxide particles the better the electrochemical performance.     

Tuning photocurrent response through size control of CdTe quantum dots sensitized solar cells

The photovoltaic performance of CdTe quantum dots (QDs) sensitized solar cells (QDSSCs) as a function of tuning the band gap of CdTe QDs size is studied. The tuning of band gap was carried out through controlling the size of QDs. Presynthesized CdTe QDs of radii from 2.1 nm to 2.5 nm) were deposited by direct adsorption (DA) technique onto a layer of TiO₂ nanoparticles (NPs) to serve as sensitizers for the solar cells. The characteristic parameters of the assembled QDSSCs were measured under AM 1.5 sun illuminations. The values of current density (J_sc) and overall efficiency (η) increase with decreasing CdTe QDs size, since the lowest unoccupied molecular orbital (LUMO) levels shifts closer to vacuum level, which causes an increase in the driving force. Consequently the electrons’ injections to the conduction band (CB) of TiO₂ NPs become faster. The maximum values of J_sc (1105 μA/cm²) and η (0.190%) were obtained for the smallest CdTe QDs size (2.10 nm). The open circuit voltages (V_oc) varies slightly with the size of the CdTe QDs, however it is only dictated by the CB level of TiO₂ NPs and the VB of the electrolyte. Furthermore, the photocurrent response of the assembled cells to ON–OFF cycles of the illumination indicates the prompt generation of anodic current.     

Highly dispersed phase of SnO2 on TiO2 nanoparticles synthesized by polyol-mediated route: Photocatalytic activity for hydrogen generation

TiO₂–SnO₂ mixed oxides (Ti:Sn = 98:2 (TS2), 95:5 (TS5) and 90:10 (TS10) by atomic weight) of large surface area and small particle size, in which SnO₂ is in a dispersed phase on TiO₂, have been synthesized by a polyol-mediated route. Characterization by various techniques has shown that a highly dispersed phase of SnO₂ on anatase TiO₂ is formed in TS2 sample. No separate discernible phases corresponding to cassiterite SnO₂ or rutile TiO₂ is seen in TS2 sample, whereas rutile TiO₂ and SnO₂ are observed besides the anatase phase of TiO₂ in TS5 and TS10 samples. The average particle size of the mixed oxide samples is ～20 nm. All samples absorb visible light and the onset of absorption was ～425 nm. These mixed oxides show emission from defect levels arising due to the anion vacancies present in TiO₂. The visible light absorption of these samples is attributed to the presence of defect levels in the bandgap of TiO₂. Photocatalytic activity of these samples for hydrogen generation from water using methanol as sacrificial reagent was studied under sunlight type radiation. The results indicate that mixed oxides have better activity compared to pure TiO₂ synthesized by the same method and the activity decreases with increasing SnO₂ concentration in TiO₂. The enhanced activity of TS2 sample is ascribed to the efficient charge separation from TiO₂ to SnO₂ owing to the high dispersion of SnO₂ in TiO₂. The decreased photocatalytic activity with increased SnO₂ concentration is due to the aggregation of SnO₂ on TiO₂, which results in relatively poor dispersion of SnO₂ and decreased charge transfer efficiency, but still maintains better photocatalytic activity compared to TiO₂. In addition loading Pd co-catalyst produces a pronounced increase in the hydrogen yield due to the accumulation of electrons in the metal from the TiO₂ and SnO₂ semiconductors and the increased reductive power of the Pd loaded mixed oxide nanoparticles.     

In situ polyol-assisted synthesis of nano-SnO2/carbon composite materials as anodes for lithium-ion batteries

Nano-SnO₂/carbon composite materials were synthesized in situ using the polyol method by oxidizing SnCl₂·2H₂O in the presence of a carbon matrix. All the as-synthesized composites consisted of SnO₂ nanoparticles (5–10 nm) uniformly embedded into the carbon matrix as evidenced by TEM. XRD confirmed the presence of nano-sized SnO₂ particles that are crystallized in a rutile structure and XPS revealed a tin oxidation state of +4. Cyclic voltammetry of the composites showed an irreversible peak at 1.4 V in the first cycle and a typical alloying/de-alloying process at 0.1–0.5 V. The best composite (“composite I”, 15 wt% SnO₂) showed an improved lithium storage capacity of 370 mAh g^?1 at 200 mA g^?1 (～C/2) which correspond to 32% improvement and lower capacity fade compared to commercial SnO₂ (50 nm). We have also investigated the effect of the heating method and we found that the use of a microwave was beneficial in not only shortening reaction time but also in producing smaller SnO₂ particles that are also better dispersed within the carbon matrix which also resulted in higher lithium storage capacity.     

Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi₂S₃), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi₂S₃ compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi₂S₃ film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi₂S₃/P3OT/Au structure, Bi₂S₃ was established as an electron acceptor. To produce ITO/Bi₂S₃/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi₂S₃ film and a gold film was thermally evaporated. Under 100 mW/cm² tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi₂S₃ film with thickness of 50 nm has the highest open circuit voltage (V_oc) of 440 mV and short-circuit current density (J_sc) of 0.022 mA/cm². The addition of a CdS thin film (90 nm) between ITO and B₂S₃ would increase V_oc to 480 mV, and J_sc to 0.035 mA/cm². The role of surface morphology and optoelectronic properties of the Bi₂S₃ film in the photovoltaic performance of the junction is discussed.     

Light-trapping enhancement based on Ga2O3 nano-islands coated glass substrate

Nano-size textured topography is important to gain high absorption ratio of the incoming light for thin film solar cells. In this paper, a Ga₂O₃ nano-islands coated glass substrate is introduced by annealing Ga nano-islands in air. The aspect ratio of the islands can be easily controlled by tuning the average thickness of Ga film. Based on the proposed substrate, with average horizontal size of 500 nm and root-mean-square roughness of 80 nm of the nano-islands, the average reflectivity under AM 1.5 illumination spectrum can be limited at 6.6% when Si absorber layer is only 480 nm thick.     

Study on dye-sensitized solar cell using novel infrared dye

The performances of infrared-dye-sensitized solar cells fabricated using two novel cyanine dyes (light absorption edge: 900 nm) were investigated. The performance of the cell using NK6037 dye was superior to that using NK4432; hence, an intimate interaction between the dye and the TiO₂ photoelectrode via the functional group is essential for efficient electron injection. The efficiency reached 1.9% by optimizing the light-confining effect of the TiO₂ photoelectrode and TiO₂ film thickness, and reached 2.3% by adjusting the concentration of deoxycholic acid (DCA) in the dye solution. The roles of the dye and DCA were clarified by photochemical and electrochemical characterization.     

Applying porous polyimide films in fibrous dye-sensitized solar cells

Fibrous dye-sensitized solar cells (DSSCs) were fabricated using porous polyimide films prepared by the phase inversion technique and their photovoltaic characteristics were investigated. A titanium wire was used as the photoelectrode substrate and a platinum wire as the counter electrode. The platinum wire was coated with a porous polyimide film to prevent direct contact between the two electrodes while facilitating electrolyte penetration through the pores. The pore size and pore size distribution of the porous polyimide films were controlled by varying the immersion time in nonsolvent. With increased immersion time, the pore size distribution gradually broadened as the pore size increased, enhancing electrolyte penetration. Increased electron transport owing to the enhanced electrolyte penetration resulted in enhanced photovoltaic performances of the fibrous DSSC. The fibrous DSSC with 60 min immersion time had a short-circuit current density (J_sc) of 0.0372 mA/cm², an open-circuit voltage (V_oc) of 0.182 V, and a fill factor (FF) of 0.33.     

Photocatalytic colour and COD removal in the distillery effluent by solar radiation

The photodegradation of distillery effluent has been studied for removal of colour and COD reduction in the presence of solar radiation. The influence of experimental parameters such as H₂O₂ concentration dosage, effluent COD concentration, TiO₂ catalyst and pH on colour and COD removal efficiency through solar photochemical process has been investigated. Maximum colour removal of the distillery effluent achieved was 79% at an H₂O₂ concentration of 0.3 M, pH 6, effluent COD concentration of 500 ppm and catalyst dosage of 0.1 g/L. The TiO₂/H₂O₂ system seems to be more efficient in comparison to the synergetic action that appears when using H₂O₂ and TiO₂. The photocatalytic degradation process using solar light as an irradiation source showed potential application for the colour removal of the distillery effluent treatment. Solar radiation can be an considered as an alternative, effective and economic energy carrier for the treatment of industrial effluent.     

Nanocrystalline TiO2 (anatase) for Li-ion batteries

Nanocrystalline TiO₂ (anatase) was synthesized successfully by the direct conversion of TiO₂-sol at 85 °C. The as-prepared TiO₂ at 85 °C were calcined at different temperatures and time in order to optimize the system with best electrochemical performance. The particle sizes of the synthesized materials were found to be in the range of 15–20 nm as revealed by the HR-TEM studies. Commercial TiO₂ anatase (micron size) was also studied for its Li-insertion and deinsertion properties in order to compare with the nanocrystalline TiO₂. The full cell studies were performed with LiCoO₂ cathode with the best performing nano-TiO₂ as anode. The specific capacity of the nanocrystalline TiO₂ synthesized at 500 °C/2 h in a half-cell configuration was 169 mAh g⁻¹ while for the cell with LiCoO₂ cathode, it was 95 mAh g⁻¹ in the 2 V region. The specific reversible capacity and the cycling performance of the synthesized nano-TiO₂ anode in full cell configuration across LiCoO₂ cathode are superior to that reported in the literature. Cyclic voltammetry measurements showed a larger peak separation for the micro-TiO₂ than the nano-TiO₂, clearly indicating the influence of nano-particle size on the electrochemical performance.