Layer In Dye-sensitized Solar Cells Research Articles

Study of Two Facile Methods for Preparation of Titanium Dioxide/Graphene Nanocomposite for DSSC’s Photoanode

Titanium dioxide (TiO2) is a well-known promising photocatalyst that has been used as the photoanode in dye sensitized solar cells (DSSC). Since graphene has good electrical, mechanical and chemical properties, its use is supposed to enhance the photocatalytic activity of TiO2, the absorption of dye and enhance the mechanical strength of the layers of DSSC. There are several methods of preparing TiO2/graphene composite using complicated process and high-tech instruments. In this study, TiO2/graphene nanocomposite was prepared using two facile methods, which is achieved by mixing graphene oxide (GO) sheets with commercially available TiO2paste and the other method was based on thermal reaction of mixed TiO2and GO before incorporating it to the working electrode of DSSC. The quality of GO reduction in the process of making the composite was characterized by using FTIR spectra and Raman spectroscopy.

Characterization of the photoelectron behavior of working electrodes modified with a titanium-dioxide window layer in dye-sensitized solar cells

Titanium dioxide (TiO2) is currently used as a working electrode in dye-sensitized solar cells (DSSCs). The conventional working electrode is comprised of an absorption layer (with a TiO2 particle size of ~20 nm) and a scattering layer (with a TiO2 particle size of ~300 nm) on a fluorinedoped tin-oxide (FTO) substrate. In this study, we inserted a window layer with a 10-nm TiO2 particle size between the FTO substrate and the absorption layer in order to increase both the transmittance and the specific surface area of the TiO2. Electrochemical impedance spectroscopic analysis was conducted to characterize the electronic behavior of the working electrode. The Bode phase plot and the Nyquist plot were interpreted to determine the impact of the insertion of the window layer on the internal resistance of the DSSC and on the carrier lifetime. The photocurrent from the cell increased due to an increase in the specific surface area, which was a result of the addition of the window layer with a smaller particle size (10 nm). Further, the insertion of the window layer in the working electrode was found to lead to an effective increase in the transmission of the incident light. Therefore, The insertion of the window layer it was concluded to contribute to an increased conversion efficiency in the DSSCs.

Flower-like TiO2 with highly exposed {001} facets used as scattering layers for dye-sensitized solar cells

The TiO2 crystals of flower-like shapes (FF-TiO2) were prepared by a one-step hydrothermal reaction from the metallic titanium powders. The synthesized TiO2 particles exhibit truncated tetragonal bipyramidal morphologies, which has ∼30% of the surface enclosed by the {001} facets. The FF-TiO2 was employed as back-illumination photoanodes in fabricating the dye-sensitized solar cells (DSSCs). The power-conversion efficiency of the DSSCs was found to increase from 5.35% to 7.65% after incorporation of the FF-TiO2 into the P25 fabricated photoanodes. The specific structure and the dopant fluorine were proven to have great impacts on the lifetime (τn) for the photoelectrons to stay in the conduction band of TiO2 and the recombination time (τr) of the photogenerated electrons that would be quenched by the I3− ions. Reasons for the improvement in the photovoltaic properties have been clarified as follows: (1) the microsized particles were prepared within the optimum size region for Mie scattering; (2) the flower-shaped structured FF-TiO2 enhanced the saturated capacity for adsorption of dye molecules; and (3) the {001} facets with adsorbed fluorine (F−) at the surface could remove the trap states, and restrain the photoelectrons recombination between the photoanode and electrolyte.

Sandwich-like design of TiO2 electrodes containing multiple light scattering layers for dye-sensitized solar cells applications

This study comes up with a new architecture of multi-layered photoanode electrodes containing three thick layers (i.e., 4μm) of nanocrystalline TiO2 particles and three thin layers (i.e., 1μm) of uniform TiO2 aggregates, which are alternately deposited. The aggregates layers are deposited by a straightforward gel process, developed for the preparation of uniform and sponge-like light scattering layer for dye-sensitized solar cells (DSCs) applications. The aggregates layers are composed of uniform spherical particles with average diameter of 2μm, containing small nanoparticles with the average grain size of 20nm. The nanocrystalline layers contain 20-nm-diameter TiO2 nanoparticles. X-ray diffraction (XRD) reveals that the nanocrystalline layers have a pure anatase phase, whereas the aggregates layers show a mixture of anatase and rutile phases. Diffuse reflectance spectroscopy (DRS) demonstrates that the multi-layered electrode enjoys better light scattering ability than that of mono-layered electrode due to the incorporation of a thin light scattering layer into the nanocrystalline film. The multi-layered DSC shows the highest power conversion efficiency of 7.85% as a result of higher light harvesting and less recombination which is demonstrated by electrochemical impedance spectroscopy (EIS). From IPCE measurement, the external quantum efficiency of the multi-layered cell at 530nm is equal to 89%, which is higher than that of mono-layered cell (i.e., 78%).

Dual-functional zinc oxide aggregates with reaction time-dependent morphology as the dye-adsorption layer for dye-sensitized solar cells

Abstract High surface area and light scattering abilities are dispensable for a well-performed photoanode of dye-sensitized solar cells (DSSCs) to respectively provide abundant active sites for dye adsorption and enhance the light path for electron excitation. In this study, an energy-saving and cost-effective low-temperature (80 °C) aqueous solution method is applied to synthesize dual-functional ZnO aggregates which are composed of small nanocrystallites with the diameter of 20 nm. The reaction time plays a significant role in controlling the morphology of ZnO nanostructures. The ZnO aggregates are randomly oriented at the early stage and the flower-like morphology gradually forms when the reaction time reaches 4 h, while the well-defined structure is further destroyed when the reaction time is increased to 8 h. The growth mechanism is proposed to discuss the formation of ZnO nanoflower. The ZnO nanoflower-based DSSC achieves a light-to-electricity conversion efficiency (η) of 4.41%, which is higher than that for the cell with commercial ZnO nanoparticles on its photoanode (3.42%) under AM 1.5G simulated sunlight with an intensity of 100 mW/cm2. The electrochemical impedance spectroscopy (EIS) is also applied to analyze the electron transport parameters to understand the kinetics of electron transport in the ZnO films. The low-temperature (150 °C) fabrication process for preparing the highly-performed ZnO film also enables the future applications of flexible DSSCs with polymeric substrates.

Ordered Mesoporous Particles in Titania Films with Hierarchical Structure as Scattering Layers in Dye-Sensitized Solar Cells

This work aimed to understand the relationship between the physical properties of scattering particle layers in dye-sensitized solar cells (DSSCs) and their performance, to assist optimization of this component of the DSSC. Highly ordered anatase 2D-hexagonal mesoporous titania (meso-TiO2) nanoparticles with a high surface area and large pore size were fabricated. Meso-TiO2 was used as scattering particles and mixed with titania nanocrystallites at weight proportions ranging from 0 to 100%. Films made from the composites were used as scattering layers in DSSCs. The influence of meso-TiO2 proportion on the structure, morphology, and optical properties of the films were investigated. The results show that the films became more porous, with a larger surface roughness, and had higher surface areas and greater light-scattering effects when meso-TiO2 was incorporated. The performance of these scattering layers in relatively large, 1 cm2 area, DSSCs was studied to link cell performance to the detailed physical p...

Multi-layered architecture of electrodes containing uniform TiO2 aggregates layers for improving the light scattering efficiency of dye-sensitized solar cells

This study comes up with a new architecture of multi-layered photoanode electrodes containing two thick layers (i.e., 6 µm) of nanocrystalline TiO2 particles and two thin layers (i.e., 1 µm) of uniform TiO2 aggregates, which are alternately deposited. The aggregates layers are deposited by a straightforward gel process, developed for the preparation of uniform and sponge-like light scattering layer for dye-sensitized solar cells (DSSCs) applications. The aggregates layers are composed of uniform spherical particles with average diameter of 2 µm, containing small nanoparticles with the average grain size of 20 nm. The nanocrystalline layers contain 20-nm-diameter TiO2 nanoparticles. X-ray diffraction reveals that the nanocrystalline layers have a pure anatase phase, whereas the aggregates layers show a mixture of anatase and rutile phases. Diffuse reflectance spectroscopy demonstrates that the multi-layered electrode enjoys better light scattering ability than that of mono-layered electrode due to the incorporation of a thin light scattering layer into the nanocrystalline film. The multi-layered DSSC shows the highest power conversion efficiency of 7.69 % as a result of higher light harvesting and less recombination which is demonstrated by electrochemical impedance spectroscopy. From IPCE measurement, the external quantum efficiency of the multi-layered cell is equal 88 %, which is higher than that of mono-layered cell (i.e., 78 %).

High-stability Ti4+ precursor for the TiO2 compact layer of dye-sensitized solar cells

A compact layer (blocking layer) can effectively block the direct contact between the fluorine-doped tin oxide (FTO) glass substrate and electrolyte in dye-sensitized solar cells (DSSCs). The TiCl4 hydrolysis has been widely adopted for preparing the TiO2 compact layer (H-TiO2). However, the TiCl4 aqueous solution is unstable for its high reactivity. To improve the chemical stability of TiCl4 aqueous solution, the Ti4+ is encapsulated by the polymer, polyethyleneimine (PEI). Experimentals show that the Ti-PEI precursor solution can maintain their initial performances for several months. The resulting TiO2 film (P-TiO2) grown by the Ti-PEI precursor is dense, smooth and uniform without any visible and detectable cracks or voids. The P-TiO2 compact layer is even denser than the H-TiO2 compact layer, suggesting reducing the electron recombination and prolonging the electron lifetime in dye-sensitized solar cells. Indeed, the electron lifetime of the DSSC based on the P-TiO2 is 13.15ms, which is longer than the 10.83ms based on H-TiO2. Meanwhile, the power conversion efficiency of the DSSC based on P-TiO2 compact film is about 12.5% higher than that based on H-TiO2. Therefore, this encapsulation technology can not only improve the stability of the metal ions solution but also meet a large-scale fabrication demand of the TiO2 compact layer in future DSSCs.

Improving Electron Transfer from Dye to TiO2 by Using CdTe Nanostructure Layers in Dye-Sensitized Solar Cells

In this work, TiO2 P25 was deposited successfully on the FTO glass by electrophoresis method. Different chemical methods were served for deposition of nanosized CdTe such as successive ion layer adsorption and reaction (SILAR) and drop-cast. Dye-sensitized solar cells were fabricated from prepared electrodes, Pt as a counter electrode, dye solution, and electrolyte. The effects of chemical deposition methods were investigated on the surface quality, optical properties, and solar cell efficiency. It was observed that deposition method has an important role on the solar cell performance. It was also seen that deposition method affects directly on surface thickness and the amount of dye adsorption. In fact, each deposition method creates different surfaces, and hence, they act variously in electron transfer across the electrode surface. Among different deposition methods that were used in this experimental work, SILAR method showed the best performance and the surface that was created by this method could transfer the electrons across the electrode faster than the other ones. But this chemical method cannot improve solar cell efficiency due to some different reasons that we mentioned in this paper.

Tunable synthesis of single-crystalline-like TiO2 mesocrystals and their application as effective scattering layer in dye-sensitized solar cells

Single-crystalline-like TiO2 mesocrystals (TMCs) with spherical and spindle-like shapes were selectively prepared in acetic acid system using benzoic acid as structural directing regent. It was found that the intermediate butyl-benzoic acid could interval the oriented assembly of the primary nanoparticles as porogen and shape regulator, which results in spherical TMCs with greater pore size distribution compared with the spindle-like TMCs. When used as scattering layer in dye sensitized solar cells (DSSCs), the spherical TMCs with long-range ordered stacking pattern results in characteristic photonic reflection, which enhances the scattering effect of the photoanode and leads to a high short circuit current density of 16.6mAcm−2. Therefore, Cell-spherical TMCs demonstrated a high power conversion efficiencies of 8.10%, indicating substantial improvements compared with Cell-spindle-like TMCs (7.58%) and Cell-nanoparticle (6.59%).

Improving Performance via Blocking Layers in Dye-Sensitized Solar Cells Based on Nanowire Photoanodes.

Electron recombination in dye-sensitized solar cells (DSSCs) results in significant electron loss and performance degradation. However, the reduction of electron recombination via blocking layers in nanowire-based DSSCs has rarely been investigated. In this study, HfO2 or TiO2 blocking layers are deposited on nanowire surfaces via atomic layer deposition (ALD) to reduce electron recombination in nanowire-based DSSCs. The control cell consisting of ITO nanowires coated with a porous shell of TiO2 by TiCl4 treatment yields an efficiency of 2.82%. The efficiency increases dramatically to 5.38% upon the insertion of a 1.3 nm TiO2 compact layer between the nanowire surface and porous TiO2 shell. This efficiency enhancement implies that porous sol-gel coatings on nanowires (e.g., via TiCl4 treatment) result in significant electron recombination in nanowire-based DSSCs, while compact coatings formed by ALD are more advantageous because of their ability to act as a blocking layer. By comparing nanowire-based DSSCs with their nanoparticle-based counterparts, we find that the nanowire-based DSSCs suffer more severe electron recombination from ITO due to the much higher surface area exposed to the electrolyte. While the insertion of a high band gap compact layer of HfO2 between the interface of the conductive nanowire and TiO2 shell improves performance, a comparison of the cell performance between TiO2 and HfO2 compact layers indicates that charge collection is suppressed by the difference in energy states. Consequently, the use of high band gap materials at the interface of conductive nanowires and TiO2 is not recommended.

Soft chemistry based sponge-like indium tin oxide (ITO) — a prospective component of photoanode for solar cell application

Previously we reported the synthesis of novel organic-inorganic composite indium tin oxide (ITO) foam precursor leading to the formation of “sponge-like” ITO by burning away the organics. This newly made sponge-like ITO possesses relatively high electrical conductivity due to phonon confinement with reasonable pore structure and may have potential application as functional materials in semiconducting dye absorbing layer in dye-sensitized solar cell (DSSC) and also as the receptor of electrons injected from the quantum dots (QDs) of organic-inorganic hybrid QD based solar cell. This report is a short review of “sponge-like” ITO described as a lecture note on its future use as an alternative new prospective material for photoanode of solar cell in the domain of sustainable energy.

Synthesis of dispersed long single-crystalline TiO2 paste and its application in DSSC as a scattering layer

TiO2 nanowire (NW) is one of the potential scattering layer materials in dye-sensitized solar cells (DSSCs) owing to its fast electron conductivity and excellent light scattering property resulting from its one-dimensional (1D) morphology. However, TiO2 NWs used as scattering layers in previous works were either aggregated or shortened into shuttles that cannot use their unique 1D properties. In this paper, we present the preparation of a well-dispersed long NW paste (exceeding 1 mm) by a mild method and used as a scattering layer in DSSC. The paste achieved a photoconversion efficiency of 5.73% and an efficiency enhancement of 12% compared with commercial scattering layer (P200 paste). Compared with the DSSC without a scattering layer, an efficiency enhancement of 54.9% was achieved. Also, the largest efficiency of 6.89% was obtained after optimization of photoanode thickness. The photoanodes were investigated through dye desorbed experiments and transmission spectra, which suggested that P25 nanoparticles with the as-prepared NW scattering layer loaded more dye than those with P200 paste. These results indicate that well-dispersed long NW paste has a potential application in scattering layers.

Bifunctional Zinc Oxide Nanoburger Aggregates as the Dye-Adsorption and Light-Scattering Layer for Dye-Sensitized Solar Cells

A novel burger-like aggregate nanostructure assembled from ZnO nanocrystals was synthesized by using a simple aqueous solution method at a low temperature (80°C). The burger-shaped nanostructure consists of two bun-like exterior layers joined together via a disk-shaped middle layer. Each exterior layer has a thickness of 200 to 300nm and a diameter of 400 to 600nm, while the relatively small middle layer has a thickness of 20 to 50nm and a diameter approximately 80nm smaller than the exterior layer. The nanoburger-based dye-sensitized solar cells (DSSCs) achieved a higher light-to-electricity conversion efficiency of 4.03% than those made of commercial ZnO nanoparticles (3.07%). The superior photovoltaic performance of the nanoburger-based DSSCs can be ascribed primarily to the enhanced dye adsorption ability of the aggregates. Because of their submicron sizes, the burger-like aggregates may also generate prominent light scattering, thereby improving the photoanode's optical absorption. Furthermore, the low-temperature heat-treatment process (150°C) used to fabricate the photoanode film provides opportunities to utilize polymeric substrates in flexible DSSCs.

Nitrogen-doped submicron-size TiO2 particles as bifunctional light scatterers in dye-sensitized solar cells

The structural, electrical, optical, and photovoltaic properties of aggregated submicron nitrogen-doped TiO2 particles (NTiO2) and the influence of utilizing them, in comparison with undoped ones, as the light-scattering layer of dye-sensitized solar cells were investigated. Field emission scanning electron microscope, X-ray diffraction, and diffuse reflectance spectra showed that both type samples have similar morphology, crystal phase, and scattering feature. Moreover, photoluminescence, Mott–Schottkey, and photovoltaic characteristics such as IMPS/IMVS and charge extraction analysis indicated that the NTiO2 layer is an efficient scatterer in two aspects: enhancement of light-harvesting efficiency by having submicron-size centers and modification of the electrical properties such as charge collection efficiency in photoanode. As a result, the overall conversion efficiency reached 7.34 % upon employing NTiO2 as the light-scattering layer, which is 13 % higher than undoped one. This improvement is a consequence of trap density reduction, electrons transfer enhancement in the interface of photoactive/scattering layer, and shunt resistance increment at photoelectrode/electrolyte interface.

Vanadium oxide as new charge recombination blocking layer for high efficiency dye-sensitized solar cells

Vanadium pentoxide (V2O5) was used as a novel blocking layer in dye-sensitized solar cells (DSCs), leading to a significant efficiency increase from 8.78% to 9.65%. The addition of V2O5 layer to nanocrystalline (nc)-TiO2 increased peak external quantum efficiency (EQE) from~80% to ~88–89%. Cyclic Voltammetry analysis indicated a positive shift of Fermi-level in case of TiO2/V2O5 based cells supported by an increase of its capacitance comparing to bare TiO2 based cells. Electrochemical impedance spectroscopy (EIS) results exhibited a ~5 times higher charge recombination resistance (RCT) in V2O5 layer modified DSCs than conventional cells, which indicated that back charge transfer from TiO2 to tri-iodide in the electrolyte was substantially suppressed. Transient photovoltage measurements on conventional and V2O5 layer modified cells were conducted and their decays were fitted to calculate the electron recombination lifetime (τn), which increased by a factor of ~3 in V2O5-based DSCs. This indicated that V2O5 significantly reduced the recombination rate at TiO2/electrolyte interface, further supporting that V2O5 functioned as a new effective surface passivation layer.

Performance enhancement of dye-sensitized solar cell with a TiCl4-treated TiO2 compact layer

We here show that an effective blocking layer for dye-sensitized solar cells (DSSCs) can be formed by spin coating a commercial TiO2 paste onto a conducting glass substrate. The spin-coated TiO2 layer was made more compact than the main absorption layer by TiCl4 treatment. DSSCs employing a compact layer exhibited an average current density and an efficiency of 19.09 mA/cm2 and 9.10%, respectively, while 16.91 mA/cm2 and 8.33% were obtained from unblocked reference cells. The enhanced DSSC performance is attributed to the increased electron lifetime. Intensity-modulated photovoltage spectroscopy and open-circuit voltage decay analysis showed that a TiCl4-treated compact layer substantially suppresses the charge recombination at the TiO2/substrate interface, thereby increasing the electron lifetime.

Fabrication and integration of quasi-one-dimensional hierarchical TiO2nanotubes for dye-sensitized solar cells

We present a novel, rational design of three-dimensional (3D) TiO2-based photoanodes using anatase hierarchical TiO2 nanotube arrays as the bottom layer and P25 nanoparticles as the top layer for dye-sensitized solar cells. Owing to favorable electron transport, large surface area and pronounced light harvesting, the 3D nanoarchitecture-based solar cell shows a significantly enhanced efficiency of 7.24% compared to the monolayer counterparts (~5%).

Template-free TiO2 hollow submicrospheres embedded with SnO2 nanobeans as a versatile scattering layer for dye-sensitized solar cells.

Nanobean SnO2-embedded TiO2 hollow submicrospheres are presented as a scattering layer for the first time in dye-sensitized solar cells. This designed mesoporous submicrostructure simultaneously promotes dye adsorption, light harvesting, and electron transport, leading to 28% improvement in the conversion efficiency compared to film-based SnO2.

Ultrafine dice-like anatase TiO2 for highly efficient dye-sensitized solar cells

TiO2 is the preferred photoanode material for dye-sensitized solar cells owing to its unique structural and photoelectric characteristics. Because of the weak light scattering, ultrafine TiO2 nanoparticles were regarded as unpromising materials for light scattering layer in dye-sensitized solar cells. We successfully developed a novel nanostructured anatase TiO2 (below 20nm), i.e., ultrafine dice-like TiO2 with nano-concavity and facet-to-facet joints. Compared with spherical nanostructures with point-to-point joints, the dice-like TiO2 film as photoanode displays some surprising characteristics such as superior light scattering, highly photoelectric conversion performance and inhibiting multilayer dye aggregation. We further fabricate ultrafine–ultrafine anatase TiO2 combination of dice-like TiO2 as scattering layer and commercial TiO2 (P-18) as highly transparent active layer. Because of the bilayer TiO2 combination, the energy conversion efficiency could be enhanced significantly to 8.66%, 36% higher than that of the device with commercial TiO2 nanospheres (200nm) as scattering layer. The well-balanced combination is a novel practicable strategy to improve the performance of dye-sensitized solar cells.