Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications

With 4.2 nm quantum-dots (QDs) as seeds on TiO₂ film, a highly efficient TiO₂ photoelectrode was prepared by a seed-growing process using chemical bath deposition technique, followed by a covering process with ZnS layer, and a post-sintering process at 400 °C. The assembled solar cells presented IPCE peak values of 73% and power conversion efficiency of 3.21% under AM 1.5 G irradiation.     

Sea urchin TiO2-nanoparticle hybrid composite photoelectrodes for CdS/CdSe/ZnS quantum-dot-sensitized solar cells

The sea urchin TiO(2) (SU TiO(2)) particles composed of radially aligned rutile TiO(2) nanowires are successfully synthesized through the simple solvothermal process. SU TiO(2) was incorporated into the TiO(2) nanoparticle (NP) network to construct the SU-NP composite film, and applied to the CdS/CdSe/ZnS quantum-dot-sensitized solar cells (QDSSCs). A conversion efficiency of 4.2% was achieved with a short-circuit photocurrent density of 18.2 mA cm(-2) and an open-circuit voltage of 531 mV, which corresponds to ～20% improvement as compared with the values obtained from the reference cell made of the NP film. We attribute this extraordinary result to the light scattering effect and efficient charge collection.     

Highly efficient plastic substrate dye-sensitized solar cells using a compression method for preparation of TiO(2) photoelectrodes

The efficiency of a plastic-substrate dye-sensitized solar cell was much improved by a new method consisting of a press method without heat treatment, light confinement effect of TiO(2) film and water-based TiO(2) paste; this device shows the highest light-to-electrical energy conversion efficiency based on plastic-substrate dye-sensitized solar cells, 7.4% under 100 mW cm(-2) (1 sun) AM1.5 illumination.     

Highly efficient inorganic-organic heterojunction solar cells based on SnS-sensitized spherical TiO2 electrodes

All-solid-state inorganic-organic heterojunction solar cells (HSCs) were designed and fabricated using earth-abundant element, non-toxic, low-cost SnS-sensitized mesoporous spherical TiO(2) films under ambient conditions using a solution-processable, simple, and convenient fabrication technique. SnS-HSCs show a promising photovoltaic performance, with an efficiency of 2.8% and a significantly high V(OC) of 0.85 V.     

Dye-sensitized solar cells based on TiO2-B nanobelt/TiO2 nanoparticle sandwich-type photoelectrodes with controllable nanobelt length

The TiO(2)-B nanobelt (NB)/TiO(2) nanoparticle (NP) sandwich-type structure photoelectrode, with controllable nanobelt length, has been used to fabricate high-efficiency dye-sensitized solar cells (DSSCs), which combine the advantages of the rapid electron transport in TiO(2)-B NBs and the high surface area of TiO(2) NPs. The results indicate that the sandwich-type photoelectrode achieves higher photoelectrical conversion efficiency when compared with the TiO(2) nanoparticulate electrode. Increasing the length of TiO(2)-B NBs has been demonstrated to improve the photoelectric conversion efficiency (η). DSSCs with the longest (10 μm) TiO(2)-B NBs yield the highest η of 7.94%. The interfacial electron transport of DSSCs with different lengths of TiO(2)-B NBs has been quantitatively investigated using the photovoltage transient and the electrochemical impedance spectra, which demonstrates that the DSSCs with longest TiO(2)-B NBs display the highest electron collection efficiency and the fastest interfacial electron transfer.     

Highly efficient CdS quantum dot-sensitized solar cells based on a modified polysulfide electrolyte

A modified polysulfide redox couple, [(CH(3))(4)N](2)S/[(CH(3))(4)N](2)S(n), in an organic solvent (3-methoxypropionitrile) was employed in CdS quantum dot (QD)-sensitized solar cells (QDSSCs), and an unprecedented energy conversion efficiency of up to 3.2% was obtained under AM 1.5 G illumination. The QDs were linked to nanoporous TiO(2) via covalent bonds by using thioglycolic acid, and chemical bath deposition in an organic solvent was then used to prepare the QDSSCs, facilitating high wettability and superior penetration capability of the TiO(2) films. A very high fill factor of 0.89 was observed with the optimized QDSSCs.     

Improved performance in dye-sensitized solar cells employing TiO2 photoelectrodes coated with metal hydroxides

The performance of dye-sensitized solar cells (DSCs) was compared before and after processing the TiO(2) electrodes by minute-order electrochemical reactions with metal nitrates, where the metals were Mg, Zn, Al, and La, in 2-propanol. An overcoating of metal hydroxide was formed without the need for a sintering process, and magnesium hydroxide was found to give the largest improvement in photovoltage, fill factor, and eventually overall conversion efficiency of the DSCs. To analyze the nature of the improvement, the diffusion coefficient (D) and electron lifetime (tau) were determined. While little influence of overcoating on D was seen, a correlation between the increase in tau and V(oc) was observed for the metals examined here. The remarkable improvement in the electron lifetime of the DSCs suggests that an overcoating with magnesium hydroxide species function as the blocking layers at the fluorine-doped tin oxide and TiO(2) interfaces, thus contributing to the suppression of electron leakage, i.e., recombination processes between unidirectional transporting electrons and poly-iodides such as tri-iodide in the processed TiO(2) photoelectrode systems. The increase in V(oc) can be explained by the increased electron density caused by the increase in electron lifetime.     

Double-layer coating of SrCO3/TiO2 on nanoporous TiO2 for efficient dye-sensitized solar cells

Surface modification plays a crucial role in improving the efficiency of dye-sensitized solar cells (DSSCs), but the reported surface treatments are in general superior to the untreated TiO(2) but inferior to the typical TiCl(4)-treated TiO(2) in terms of solar cell performance. This work demonstrates a two-step treatment of the nanoporous titania surface with strontium acetate [Sr(OAc)(2)] and TiCl(4) in order, each step followed by sintering. An electronically insulating layer of SrCO(3) is formed on the TiO(2) surface via the Sr(OAc)(2) treatment and then a fresh TiO(2) layer is deposited on top of the SrCO(3) layer via the TiCl(4) treatment, corresponding to a double layer of Sr(OAc)(2)/TiO(2) coated on the TiO(2) surface. As compared to the typical TiCl(4)-treated DSSC, the Sr(OAc)(2)-TiCl(4) treated DSSC improves short-circuit photocurrent (J(sc)) by 17%, open-circuit photovoltage (V(oc)) by 2%, and power conversion efficiency by 20%. These results indicate that the Sr(OAc)(2)-TiCl(4) treatment is better than the often used TiCl(4) treatment for fabrication of efficient DSSCs. Charge density at open circuit and controlled intensity modulated photocurrent/photovoltage spectroscopy reveal that the two electrodes show almost same conduction band level but different electron diffusion coefficient and charge recombination rate constant. Owing to the blocking effect of the SrCO(3) layer on electron recombination with I(3)(-) ions, the charge recombination rate constant of the Sr(OAc)(2)-TiCl(4) treated DSSC is half that of the TiCl(4)-treated DSSC, accounting well for the difference of their V(oc). The improved J(sc) is also attributed to the middle SrCO(3) layer, which increases dye adsorption and may improve charge separation efficiency due to the blocking effect of SrCO(3) on charge recombination.     

Room temperature fabrication of porous ZnO photoelectrodes for flexible dye-sensitized solar cells

We fabricated ZnO photoelectrodes at room temperature by doctor-blading ZnO gel; the adequate interparticle connection and the effective ammonia activation process improved the flexible DSC's efficiency to 3.8% (under 100 mW cm(-2)).     

Spatial arrangement of carbon nanotubes in TiO2 photoelectrodes to enhance the efficiency of dye-sensitized solar cells

Three electrode structures with different spatial arrangements of carbon nanotubes (CNTs) in the mesoporous TiO(2) layer were employed in dye-sensitized solar cells to study the effect of surface states at the interface formed by the incorporation of CNTs. It was found that the decay of open circuit voltage (V(oc)) was significantly minimized by avoiding the direct contact of nanotubes to the conducting substrate by introducing a thin buffer layer of TiO(2) while maintaining the superior electron collection efficiency from the incorporation of nanotubes.     

Microstructure design of nanoporous TiO2 photoelectrodes for dye-sensitized solar cell modules

The optimization of dye-sensitized solar cells, especially the design of nanoporous TiO2 film microstructure, is an urgent problem for high efficiency and future commercial applications. However, up to now, little attention has been focused on the design of nanoporous TiO2 microstructure for a high efficiency of dye-sensitized solar cell modules. The optimization and design of TiO2 photoelectrode microstructure are discussed in this paper. TiO2 photoelectrodes with three different layers, including layers of small pore size films, larger pore size films, and light-scattering particles on the conducting glass with the desirable thickness, were designed and investigated. Moreover, the photovoltaic properties showed that the different porosities, pore size distribution, and BET surface area of each layer have a dramatic influence on short-circuit current, open-circuit voltage, and fill factor of the modules. The optimization and design of TiO2 photoelectrode microstructure contribute a high efficiency of DSC modules. The photoelectric conversion efficiency around 6% with 15 x 20 cm2 modules under illumination of simulated AM1.5 sunlight (100 mW/cm2) and 40 x 60 cm2 panels with the same performance tested outdoor have been achieved by our group.     

Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes: Preparation,characterization, and application to photoelectrochemical cells

A novel electrodeposited CdS nanoparticle-modified highly-ordered TiO₂ nanotube-array photoelectrode and its application to photoelectrochemical cells is reported. Results show formation of a thin, nanoparticulate CdS layer, comprised of sphere-like 10–20 nm diameter nanoparticles, on the anodic synthesized TiO₂ nanotube-array (inner diameter of 70 nm, wall thickness 25 nm and ca. 400 nm length) electrode. The resulting CdS–TiO₂ photoelectrode has an as-fabricated bandgap of 2.53, and 2.41 eV bandgap after sintering at 350 °C in N₂ ambient. Photoelectrochemical properties are described in detail.     

CdSe/TiO2 nanocrystalline solar cells

CdSe sensitized TiO₂ nanocrystalline solar cells were made with the participation of silicotungstic acid (STA) during the deposition of CdSe, the resulting Voc and Isc were 0.23 V cm^-2 and 10 mA cm^-2, respectively. The doping, time and microporous membrane effects were also discussed.     

Investigation of the structural properties of annealed CdIn2Te4/CdS thin film solar cells produced by the electron-beam evaporation (e-beam) technique

Thin film CdIn₂Te₄/CdS solar cells were deposited onto the ITO-coated glass substrate by electron beam evaporation (e-beam) technique, and the the effect of annealing on their structural properties is studied. The annealing was performed under nitrogen atmosphere for 1 h. The manufactured solar cells were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) analysis. Crystallite size (D), inter-planer distance (d) and lattice constant (a) values were calculated for the thin film solar cell from XRD data. Annealed samples display well defined XRD patterns with three diffraction peaks. We observed increased peak intensity in the annealed films. EDAX analysis showed that only CdIn₂Te₄ is present in absorber layer and CdS is found in the window layer, but no impurity atoms are present the structure. It is observed that surface roughness of the annealed films incresed, according to SEM images. The I–V characteristics show that the current is increased for annealed thin films solar cells.

     

Effect of the rutile content on the photovoltaic performance of the dye-sensitized solar cells composed of mixed-phase TiO2 photoelectrodes

The effect of the rutile content on the photovoltaic performance of dye-sensitized solar cells (DSSCs) composed of mixed-phase TiO(2) photoelectrode has been investigated. The mixed-phase TiO(2) particles with varied amounts of rutile, relative to anatase phase, are synthesized by an in situ method where the concentration of sulfate ion is used as a phase-controlling parameter in the formation of TiO(2) using TiCl(4) hydrolysis. The surface area (S(BET)) varies from 33 (pure rutile) to 165 (pure anatase) m(2) g(-1). Generally, both the current density (J(sc)) and photo-conversion efficiency (η) decrease as the rutile content increases. The incorporation of rod-shaped rutile particles causes low uptake of dye due to the reduced surface area, as well as slow electron transport in less efficiently-stacked structure. However, maximum J(sc) (14.63 mA cm(-2)) and η (8.69%) appear when relatively low rutile content (16%) is employed. The reported synergistic effect by the efficient interparticle electron transport from rutile to anatase seems to overbalance the decrease of surface area when small amount of rutile particles is incorporated.     

Highly efficient visible-light-induced photocatalytic activity of nanostructured AgI/TiO2 photocatalyst

Nanostructured AgI/TiO(2) photocatalyst was synthesized by a feasible approach with AgNO(3), LiI, and Ti(OBu)(4) and characterized by X-ray diffraction, transmission electron microscopy, angle-dependent X-ray photoelectron spectroscopy, diffusive reflectance UV-vis spectroscopy, Raman spectroscopy, photoluminescence, and the Brunauer-Emmett-Teller technique. The results of characterization reveal that the nanostructured AgI/TiO(2) has a novel core/shell/shell nanostructure of AgI/Ag-I(2)/TiO(2). Compared with TiO(2) (P25) supported AgI, the formation of the nanostructure results in substantial shifting of the absorption edge of AgI to red, enhancement of the absorption intensity, and the appearance of a strong tail absorption above 490 nm, which is assigned to the absorption of I(2) and Ag. Photocatalytic tests show that the nanostructured AgI/TiO(2) photocatalyst exhibited very high visible-light-induced photocatalytic activity for the photodegradation of crystal violet and 4-chlorophenol, which is 4 and 6 times higher than that of P25 titania supported AgI, respectively. The highly efficient visible-light-induced photocatalytic activity of the nanostructured AgI/TiO(2) is attributed to its strong absorption in the visible region and low recombination rate of the electron-hole pair due to the synergetic effect among the components of AgI, Ag, I(20, and TiO(2) in the nanostructured AgI/TiO(2).     

Highly efficient dye-sensitized SnO2 solar cells having sufficient electron diffusion length

Dye-sensitized solar cells (DSCs) were fabricated from mesoporous SnO₂ electrodes, which were prepared from nano-sized SnO₂ particles. Current–voltage characteristics of the DSCs were compared with DSCs prepared from conventional TiO₂ electrodes, which have similar amount of adsorbed dye with the SnO₂. As a result, short-circuit current of the SnO₂DSC were comparable with that of the TiO₂DSCs, and more than 15 mA/cm² was obtained with the SnO₂ at the thickness of 10 μm under one sun conditions. Electron diffusion coefficients and lifetimes in the SnO₂ and TiO₂ electrodes were measured, showing slower diffusion and longer lifetime in the SnO₂DSC than in the TiO₂. The results imply that the electron transport and transfer dynamics in such electrodes is dominated by the influence of intra-band charge traps, and the control of the trap conditions would be the key strategy to employ various metal oxides for such solar cells.     

Highly ordered combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles prepared by a novel route for dye-sensitized solar cells

Combined structure of anodic TiO₂ nanotubes and TiO₂ nanoparticles (TiNTs-TiNPs) has been synthesized by a facile combination of hydrothermal and chemical vapor deposition methods. Ordered TiO₂ nanotubes with smooth walls were fabricated by two step anodization method in ethylene glycol containing NH₄F at 50 V. This nanotubular array after annealing at 450 °C was subjected to the hydrothermally produced gaseous environment in an autoclave with diluted TiCl₄ solution at its bottom. Vapors of TiCl₄ were allowed to react chemically with water vapors for predefined time durations at 180 °C that resulted in the deposition of TiO₂ nanoparticles on tubes’ surface and side walls. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that for one hour reaction duration, nanoparticles were evenly coated on the walls of nanotubes, whereas, longer durations tend to deteriorate the tubular structure. Consequently, the ordered TiNTs-TiNPs array produced after one hour coating has shown better performance for dye-sensitized solar cell DSSC) in back illumination mode with 130% increase in efficiency as compared to the device based on bare TiO₂ nanotubes. The same photoanode has higher reflective properties with higher scattering ability. The solar cell based on this photoanode exhibits higher external quantum efficiency and effective charge transport properties. This study shows that porous ordered 1D structures based on TiO₂ are of crucial importance for the high performance of DSSCs.     

Na-doping-induced modification of the Cu2ZnSn(S,Se)4/CdS heterojunction towards efficient solar cells

It is very important to understand why a small amount of alkali metal doping in Cu2ZnSn(S,Se)4 (CZTSSe)solar cells can improve the conversion efficiency.In this...     

The performance of coupled (CdS:CdSe) quantum dot-sensitized TiO2 nanofibrous solar cells

Highly porous networks and reduced grain boundaries with one-dimensional (1-D) nanofibrous morphology offer enhanced charge transport in solar cells applications. Quantum dot (QDs) decorated TiO₂ nanofibrous electrodes, unlike organic dye sensitizers, can yield multiple carrier generations due to the quantum confinement effect. This paper describes the first attempt to combine these two novel approaches, in which CdS (～18 nm) and CdSe (～8 nm) QDs are sensitized onto electrospun TiO₂ nanofibrous (diameter ～80–100 nm) electrodes. The photovoltaic performances of single (CdS and CdSe) and coupled (CdS/CdSe) QDs-sensitized TiO₂ fibrous electrodes are demonstrated in sandwich-type solar cells using polysulfide electrolyte. The observed difficulties in charge injection and lesser spectral coverage of single QDs-sensitizers are solved by coupling (CdS:CdSe) two QDs-sensitizers, resulting in a enhanced open-circuit voltage (0.64 V) with 2.69% efficiency. These results suggest the versatility of fibrous electrodes in QDs-sensitized solar cell applications.