Analysis of the electron transport properties in dye-sensitized solar cells using highly ordered TiO2 nanotubes and TiO2 nanoparticles

This study uses TiO2 nanoparticles and highly ordered anatase TiO2 nanotubes (AOTnt) as thin film photoanodes for dye-sensitized solar cells (DSSCs). DSSCs are assembled by single-layer and double-layer films of photoanodes and their electron transfer performance is compared. TiO2 nanoparticles were fabricated by the sol-gel method, and AOTnts were grown on titanium foil. This study uses TiO2 nanoparticles or AOTnts to prepare single-layer photoanodes and TiO2 nanoparticles coated on an AOTnt film to fabricate double-layer photoanodes. These three different photoanodes are soaked in dye and assembled into DSSCs, and their open-loop voltage recession, electrochemical impedance, lifetime, life cycle, and effective diffusion coefficient are measured. Electron transfer efficiency of the photoanodes and light harvesting efficiency are further analyzed. The results show that the electron transfer efficiency, open-loop voltage recession, lifetime, life cycle, and effective diffusion coefficient of the DSSCs assembled using double-layer photoanodes (AOTnt-TiO2) are superior to those of single-layer photoanodes (TiO2 or AOTnt).     

Enhanced electron transport in mesoporous TiO2 films modified by sol-gel necking for dye-sensitized solar cells

Mesoporous TiO2 films modified via sol-gel necking were fabricated by dispersing Ti tetra-isopropoxide (TTIP; 8 to 16 wt% over TiO2) with TiO2 nanoparticles in isopropyl alcohol. The dye-sensitized solar cells (DSSCs) with 13 wt% TTIP-modified TiO2 film exhibited significantly improved overall energy conversion efficiency, despite having less adsorbed dye when compared with DSSCs with untreated and TiCl4 post-treated TiO2 films. The improvement can be attributed to the sol-gel necking (or interconnection) between the nanoparticles which leads to a much faster electron transport and a suppression of the recombination (or back electron transfer) between the TiO2 and electrolyte.     

Enhancement of photovoltaic performance in dye-sensitized solar cells with the spin-coated TiO2 blocking layer

The TiO2 thin film layers were introduced with the spin-coating method between FTO electrode and TiO2 photoanode in dye sensitized solar cell (DSSC) to prevent electron back migration from the FTO electrode to electrolyte. The DSSC containg different thickness of TiO2 thin film (10-30, 40-60 and 120-150 nm) were prepared and photovoltaic performances were analysed with /-Vcurves and electrochemical impedance spectroscopy. The maximum cell performance was observed in DSSC with 10-30 nm of TiO2 thin film thickness (11.92 mA/cm2, 0.74 V, 64%, and 5.62%) to compare with that of pristine DSSC (11.09 mA/cm2, 0.65 V, 62%, and 4.43%). The variation of photoelectric conversion efficiency of the DSSCs with different TiO2 thin film thickness was discussed with the analysis of crystallographic and microstructural properties of TiO2 thin films.     

Self-assembled TiO₂ with increased photoelectron production, and improved conduction and transfer: enhancing photovoltaic performance of dye-sensitized solar cells

Highly crystalline mesoporous anatase TiO(2) is prepared through supramolecular self-assembly and by utilizing cetyltrimethylammonium bromide (CTAB) as templating material. Photoanodes of dye-sensitized solar cells (DSSCs) made from these TiO(2) nanoparticles are found to have a high specific surface area of 153 m(2)/g and high surface roughness. Optical absorption spectroscopy studies reveal that the photoanode films adsorb four times more dye than films made of commercial P25 TiO(2). Mercury porosimetry and field emission scanning electron microscope (FESEM) studies show hierarchical macro- and meso-porosity of the photoanode films leading to better dye and electrolyte percolation, combined with improved electron conduction pathways compared to P25 films. Electrochemical impedance studies confirm lower impedance and higher electron lifetime in the synthesized mesoporous TiO(2) films compared to P25 films. Higher photovoltaic efficiency was recorded of cells made from the synthesized mesoporous TiO(2) in comparison to the corresponding cells made from P25. Incident-photon-to-current efficiency data provided critical understanding of recombination kinetics, and provided proof of Mie scattering by the self-assembled submicrometer sized TiO(2) aggregates and the macropores in their structure. The scattering phenomenon was further corroborated by diffused reflectance studies. An in-depth analysis of CTAB-templated mesoporous TiO(2) has been conducted to show how it can be a good candidate photoanode material for enhancing the performance of DSSCs.     

Correlating titania morphology and chemical composition with dye-sensitized solar cell performance

We have investigated the use of various morphologies, including nanoparticles, nanowires, and sea-urchins of TiO(2) as the semiconducting material used as components of dye-sensitized solar cells (DSSCs). Analysis of the solar cells under AM 1.5 solar irradiation reveals the superior performance of hydrothermally derived nanoparticles, by comparison with two readily available commercial nanoparticle materials, within the DSSC architecture. The sub-structural morphology of films of these nanostructured materials has been directly characterized using SEM and indirectly probed using dye desorption. Furthermore, the surfaces of these nanomaterials were studied using TEM in order to visualize their structure, prior to their application within DSSCs. Surface areas of the materials have been quantitatively analyzed by collecting BET adsorption and dye desorption data. Additional investigation using open circuit voltage decay measurements reveals the efficiency of electron conduction through each TiO(2) material. Moreover, the utilization of various chemically distinctive titanate materials within the DSSCs has also been investigated, demonstrating the deficiencies of using these particular chemical compositions within traditional DSSCs.     

Fabrication and characterization of photoelectrode thin films with different morphologies of TiO2 nanoparticles for dye-sensitized solar cells

This study deals with the fabrication of three different morphologies of TiO2 nanoparticles to fabricate two-layer photoelectrode thin film for dye-sensitized solar cells (DSSC). The four different TiO2 morphologies are titania nanotubes (Tnt), TiO2 nanoparticles (H220), TiO2 nanoparticle (SP) and commercial DP-25 nanoparticles (P-25). To prepare the thin films of the photoelectrodes, the first layer is coated by H220 TiO2 nanoparticles, and the second is coated by 3 kinds of materials optimally proportionally mixed - P25, SP and Tnt. The photoelectric conversion efficiency of DSSCs with photoelectrodes fabricated using H220 reached 6.31%. Finally, the TiO2 nanaomaterials with four different morphologies were used to prepare a two layer photoelectrode with the structure of H220/P25-Tnt-SP which was combined with a Pt counter electrode to assemble DSSCs. These DSSCs had photoelectric conversion efficiencies of as high as 7.47%.     

Performance and electron transport properties of TiO(2) nanocomposite dye-sensitized solar cells

TiO(2) nanowire (NW)/nanoparticle (NP) composite films have been fabricated by hybridizing various ratios of hydrothermal anatase NWs and TiO(2) NPs for use in dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) images reveal that uniform NW/NP composite films were formed on fluorine-doped tin oxide?(FTO) substrates by the dip-coating method. The NWs are randomly but neither vertically nor horizontally oriented within the composite film. The TiO(2) NP DSSC possesses superior performance to those of the NW/NP composite and the pure NW cells, and the efficiency of the NW/NP composite DSSC increases on increasing the NP/NW ratio in the composite anode. All types of DSSC possess the same dependence of performance on the anode thickness that the efficiency increases with the anode thickness to a maximum value, then it decreases when the anode is thickened further. Electrochemical impedance spectroscopy analyses reveal that the NP DSSCs possess larger effective electron diffusion coefficients (D(eff)) in the photoanodes and smaller diffusion resistances of I(3)(-) in electrolytes compared to those in the NW/NP and the NW DSSCs. D(eff) decreases when NWs are added into the photoanode. These results suggest that the vertical feature of the NWs within the anodes is crucial for achieving a high electron transport rate in the anode.     

TiO2-grafted multi-walled carbon nanotubes for dye-sensitized solar cells

Dye-Sensitized Solar Cells (DSSCs) comprised of TiO2 porous films with multi-walled carbon nanotubes (MWNT) were prepared at low temperature (150 degrees C). MWNT were incorporated to facilitate the fast electron transport resulting from metallic properties of carbon nanotubes. In order to enhance the effect of MWNT incorporation, TiO2-grafted MWNT (TiO2-MWNT) was synthesized which can increase the electron transport rate further due to proximity of TiO2 to MWNT The presence of TiO2 nanoparticles on the surface of MWNT was confirmed by electron microscopy and energy dispersive X-ray spectroscopy. As in the DSSCs prepared through high temperature sintering of photoanodes, the maximum content of MWNT incorporated into TiO2 was limited to 0.01 wt% relative to TiO2. TiO2 photoanodes including TiO2-grafted MWNT (TiO2-MWNT/P25) enhanced the cell efficiencies by ca. 28% and 14%, relative to TiO2 photoanodes without and with MWNT respectively, reaching the efficiency of 5.0%. Electrochemical impedance spectroscopy (EIS) was utilized to examine the effect of incorporation of TiO2 nanoparticles grafted to MWNT on the cell performance.     

Improvement in performances of dye-sensitized solar cell with SiO2-coated TiO2 photoelectrode

The pure TiO2 and the nano-porous SiO2-coated TiO2 (STO) films were deposited on the FTO substrates by spray technique for the application of dye-sensitized solar cells (DSSCs). XRD pattern shows the pure TiO2 and STO films exhibits the same structure. We found that there is no much difference in dye absorption between the STO and the pure TiO2 films. The electrochemical impedance spectra reveal that insulating nature of the porous SiO2 increases surface resistance of the TiO2 film and supresses back transfer of the photogenerated electrons to the electrolyte. The field-emission scanning electron microscopy (FE-SEM) and energy dispersion X-ray spectroscopy (EDS) reveal that the surface morphology and the existence of SiO2 layer on the surface of the TiO2 films, respectively. The photoelectrochemical results show that the short-circuit photocurrent (J(SC)) increased from 16.73 mA cm(-2) to 18.31 mA cm(-2) and the open-circuit voltage (V(OC)) value changed from 0.71 V to 0.74 V for the STO films. The efficiency of cell has been greatly improved from 8.25 to 9.3%.     

Fabrication of dye-sensitized solar cells using Nb2O5 blocking layer made by sol-gel method

In this study, nanocrystalline Nb2O5 thin film has been prepared via sol-gel process using niobium ethoxide as a precursor. Sol-gel films using various ratios of H2O/Nb have been prepared on fluorinated tin oxide (FTO) glass substrate, and used as electron-blocking layer of dye-sensitized solar cell (DSSC). The Nb2O5 film as deposited was amorphous, but became crystalline with hexagonal phase after heat treatment at 600 degrees C. With higher H2O/Nb molar ratio, denser and more uniform Nb2O5 film surface was obtained. DSSCs with the structure of FTO/Nb2O5/TiO2/Dye/EL/Pt/FTO have been prepared, and their solar-cell performance was evaluated. By introduction of Nb2O5 sol-gel film between FTO and TiO2 layer in DSSCs, energy conversion efficiency could be improved.     

Strong electronic coupling and ultrafast electron transfer between PbS quantum dots and TiO2 nanocrystalline films

Hot carrier and multiple exciton extractions from lead salt quantum dots (QDs) to TiO(2) single crystals have been reported. Implementing these ideas on practical solar cells likely requires the use of nanocrystalline TiO(2) thin films to enhance the light harvesting efficiency. Here, we report 6.4 ± 0.4 fs electron transfer time from PbS QDs to TiO(2) nanocrystalline thin films, suggesting the possibility of extracting hot carriers and multiple excitons in solar cells based on these materials.     

Intertwined aligned carbon nanotube fiber based dye-sensitized solar cells

Metal wires suffer from corrosion in fiber-shaped dye-sensitized solar cells (DSSCs). We report herein that stable, ultrastrong, and highly flexible aligned carbon nanotube fibers can be used not only as catalytic counter electrodes but also as conductive materials to support dye-loaded TiO(2) nanoparticles in DSSCs. The power conversion efficiency of this fiber solar cell can achieve 2.94%. These solar power fibers, exhibiting power conversion efficiency independent of incident light angle and cell length, can be woven into textiles via a convenient weaving technology.     

Hierarchical structured TiO2 photoanodes for dye-sensitized solar cells

A novel approach has been developed to fabricate hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs). The appropriately aggregated TiO2 clusters in the photoanode layer could cause stronger light scattering and higher dye loading that increases the efficiency of photovoltaic device. For detailed light-harvesting study, different molecular weights of polyvinyl alcohol (PVA) were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. A series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices, were measured and investigated. An optimized device had energy conversion efficiency of 4.47% having a higher dye loading and good light harvesting, achieving a 23% increase of short-circuit current J(sc) in DSSCs.     

Carbon nanotubes on fluorine-doped tin oxide for fabrication of dye-sensitized solar cells at low temperature condition

The multi-walled carbon nanotubes (MWCNTs), electrophoretically deposited on fluorine-doped tin oxide (FTO), were employed as charge-collecting channels in the TiO2 photoelectrode of dye-sensitized solar cells (DSSCs) fabricated at 200 degrees C. The CNT-networks at the conducting substrate increased the charge collection efficiency of the porous TiO2 film, while the short circuit current increased up to ca. 43% under optimized condition. However, the significant decrease in the open-circuit voltage (Voc) up to ca. 132 mV resulted in the failure of the overall cell efficiency improvement. Findings reveal that the transfer process for the back electron is mainly responsible for the significant Voc drop when the MWCNTs were deposited at the electron-collecting substrate of the photoelectrode. The study demonstrates that electrophoretic deposition of MWCNTs on charge collecting substrate would be applicable to introduce an effective charge-collecting channel for the fabrication of flexible DSSCs under low temperature sintering condition.     

Preparation of nanorod-like anatase TiO2 nanocrystals and their photovoltaic properties

Anatase TiO2 nanocrystals with the high specific surface area were prepared by the hydrothermal treatment of anatase TiO2 sols at the temperature of 150 degrees C and above. When TiO2 sols with a lower content of TiO2 and at a relatively high pH value were hydrothermal treated, the dispersible and nanorod-like TiO2 nanocrystals were formed via the oriented attachment. The nanorod-like TiO2 nanocrystals with an aspect ratio of larger than 5 and a mean diameter of less than 7 nm were obtained in the absence of organic compounds. The as-prepared TiO2 nanocrystals were characterized with X-ray diffraction, transmission electron microscopy and BET surface area techniques. The TiO2 nanostructures were deposited on the FTO conductive glass as the anodic electrode for the dye-sensitized solar cells (DSSCs) and assembled into solar cells. The derived solar cells showed a conversion efficiency of 6.12% under 1 sun illumination of simulated sunlight and external quantum efficiency (EQE) of more than 60% at the wavelength of 550 nm. The DSSCs from the anatase nanorods has a higher open circuit voltage compared to the spherical nanocrystals.     

Preparation and properties of a carbon nanotube-based nanocomposite photoanode for dye-sensitized solar cells

This study fabricates dye-sensitized solar cells (DSSCs) based on TiO(2)/multi-walled carbon nanotube (MWCNT) nanocomposite photoanodes obtained by the modified acid-catalyzed sol-gel procedure. Results show that incorporating MWCNTs into a TiO(2)-based electrode efficiently improves the physicochemical properties of the solar cell. The results of dye adsorption and cell performance measurements indicate that introducing MWCNTs would improve the roughness factor (from?834 to?1267) of the electrode and the charge recombination of electron/hole (e(-)/h(+)) pairs. These significant changes could lead to higher adsorbed dye quantities, photocurrent and DSSC cell performance. Nevertheless, a higher loading of MWCNTs causes light-harvesting competition that affects the light adsorption of the dye-sensitizer, and consequently reduces the cell efficiency. This study suggests an optimum MWCNT loading in the electrode of 0.3?wt%, and proposes a sol-gel synthesis procedure as a promising method of preparing the TiO(2)-based nanocomposite.     

碳球模板剂对TiO2光阳极微结构及其光电性能影响研究

以葡萄糖为原料水热合成碳球作为模板剂,将其与TiO2纳米晶共混制备纳米多孔TiO2光阳极。采用场发射电子扫描电镜(SEM)、台阶仪、紫外-可见分光光度计(UV-Vis)等对TiO2薄膜的表面形貌、厚度和散射能力进行表征。研究发现,随着碳球含量的增加,光阳极单位体积内的表面积先增加后减小;薄膜对光的散射能力也呈现同样趋势。采用所制备的光阳极组装染料敏化太阳能电池,性能测试结果表明,随着碳球含量的增加,电池短路电流密度先增加,后减小。当碳球加入量为TiO2纳米晶质量的3%时,电池光电转换效率达到最佳为5.15%。     

Influence of niobium doping in hierarchically organized titania nanostructure on performance of dye-sensitized solar cells

Niobium doped hierarchically organized TiO2 nanostructures composed of 20 nm size anatase nanocrystals were synthesized using pulsed laser deposition (PLD). The Nb doping concentration could be facilely controlled by adjusting the concentration of Nb in target materials. We could investigate the influence of Nb doping in the TiO2 photoelectrode on the cell performance of dye-sensitized solar cells (DSSCs) by the exclusion of morphological effects using the prepared Nb-doped TiO2 anostructures. We found no significant change in short circuit current density (Jsc) as a function of Nb doping concentration. However, open circuit voltage (Voc) and fill factor (FF) monotonously decrease with increasing Nb concentration. Dark current characteristics of the DSSCs reveal that the decrease in Voc and FF is attributed to the decrease in shunt resistance due to the increase in conductivity TiO2 by Nb doping. However, electrochemical impedance spectra (EIS) analysis at open circuit condition under illumination showed that the resistance at the TiO2/dye/electrolyte interface increases with Nb concentration, revealing that Nb doping suppress the charge recombination at the interface. In addition, electron life time obtained using characteristic frequency in Bode plot increases from 14 msec to 56 msec with increasing Nb concentration from 0 to 1.2 at%. This implies that the improved light harvesting can be achieved by increasing diffusion length through Nb-doping in the conventional TiO2 photoelectrode.     

Electrospinning processed nanofibrous TiO(2) membranes for photovoltaic applications

We have recently fabricated dye-sensitized solar cells (DSSCs) comprising nanofibrous TiO(2) membranes as electrode materials. A thin TiO(2) film was pre-deposited on fluorine doped tin oxide (FTO) coated conducting glass substrate by immersion in TiF(4) aqueous solution to reduce the electron back-transfer from FTO to the electrolyte. The composite polyvinyl acetate (PVac)/titania nanofibrous membranes can be deposited on the pre-deposited thin TiO(2) film coated FTO by electrospinning of a mixture of PVac and titanium isopropoxide in N,N-dimethylformamide (DMF). The nanofibrous TiO(2) membranes were obtained by calcining the electrospun composite nanofibres of PVac/titania as the precursor. Spectral sensitization of the nanofibrous TiO(2) membranes was carried out with a ruthenium (II) complex, cis-dithiocyanate-N,N(')-bis(2,2(')-bipyridyl-4,4(')-dicarboxylic acid) ruthenium (II) dihydrate. The results indicated that the photocurrent and conversion efficiency of electrodes can be increased with the addition of the pre-deposited TiO(2) film and the adhesion treatment using DMF. Additionally, the dye loading, photocurrent, and efficiency of the electrodes were gradually increased by increasing the average thickness of the nanofibrous TiO(2) membranes. The efficiency of the fibrous TiO(2) photoelectrode with the average membrane thickness of 3.9?μm has a maximum value of 4.14%.     

Enhancement in dye-sensitized solar cells based on MgO-coated TiO(2) electrodes by reactive DC magnetron sputtering

A surface modification method was carried out by reactive DC magnetron sputtering to fabricate TiO(2) electrodes coated with insulating MgO for dye-sensitized solar cells. The MgO-coated TiO(2) electrode had been characterized by x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), UV-vis spectrophotometer, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The study results revealed that the TiO(2) modification increases dye adsorption, decreases trap states and suppresses interfacial recombination. The effects of sputtering MgO for different times on the performance of DSSCs were investigated. It indicated that sputtering MgO for 3?min on TiO(2) increases all cell parameters, resulting in increasing efficiency from 6.45% to?7.57%.