Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Electrochemical properties of TiO2 nanoparticle/nanorod composite photoanode for dye-sensitized solar cells

A unique composite of TiO₂ nanoparticles (NPs) and nanorods (NRs) has been used to fabricate a photoelectrode for developing dye-sensitized solar cells (DSSCs) with higher sensitivity. The TiO₂ nanorods were synthesized using a mechanical process, in which electrospun TiO₂ nanofibers was grinded in a controlled way to obtain uniform size distribution. The characteristics of electron transport, recombination lifetime and charge collection were investigated by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). Photoelectrodes prepared with the composites of NRs and NPs showed significant improvements in electron transportation compared to only NP photoelectrodes, which would enhance the photovoltaic performance of DSSCs. IMPS and IMVS measurements show that fast electron transport and slightly decreased recombination lifetime resulted in the improvement of efficiency. The highest energy conversion efficiency obtained from the photoelectrodes fabricated with the as-prepared rutile TiO₂ nanofibers at 5 wt% NR content was up to 6.1% under AM1.5G solar illumination. The results demonstrate that the composite nanostructure can take advantage of both the fast electron transport of the nanorods and the high surface area of the nanoparticles.     

ZnO/TiO2 core–shell nanowire arrays for enhanced dye-sensitized solar cell efficiency

We present a new method of synthesizing ZnO/TiO₂ core–shell nanowire (NW) arrays for the fabrication of dye-sensitized solar cells (DSSCs). Vertically aligned ZnO NW arrays were obtained on Si substrates, and modified by a TiO₂ shell in order to solve the recombination problems via a cost-effective spin-coating method. The structure of the ZnO/TiO₂ composite NW arrays was characterized. The experimental results indicate that the TiO₂ shell enhances the performance of the DSSCs, through improving the stability of the ZnO NWs and decreasing the recombination of photogenerated electrons on the NW surface. The highest overall conversion efficiency of the cell reaches about 3.0 %.     

Electron transport in dye-sensitized solar cells based on TiO2 nanowires

Anatase titanium dioxide nanowire arrays were prepared by hydrothermally oxidizing titanium foils in aqueous alkali and transferred onto fluorinated tin oxide(FTO)glass for use as the photoanodes of front side illuminated dye-sensitized solar cells(DSCs).Electrochemical impedance spectroscopy(EIS)measurement was applied to compare the electron transport and recombination properties of DSCs using TiO2nanowire films and TiO2nanoparticle films as photoanodes.It was found that the nanowire array films possess smaller electron transport resistance(Rt)and larger electron diffusion length(Le)in the photoanodes,suggesting that the nanowire arrays can enhance the electron transport rate and have a potential to improve the charge collection efficiency of DSCs.     

Influence of ZnO growth temperature on the performance of dye-sensitized solar cell utilizing TiO2-ZnO composite film photoanode

Ionics - TiO2 nanoflower (NF)-ZnO composite films were synthesized by using liquid phase deposition (LPD) and dip coating method. At the lower growth temperature below 50 °C, the...     

Oriented ZnO nanotubes arrays decorated with TiO2 nanoparticles for dye-sensitized solar cell applications

Dye-sensitized solar cells (DSSCs) based on a novel composite photoanode of TiO₂ nanoparticles coating on electrodeposited ZnO nanotube arrays are fabricated and characterized. An efficiency of 3.94 % is achieved for the composite cell, increasing 86.7 % than 2.11 % of the ZnO nanotubes cell. The short-circuit current (J _sc) and open-circuit voltage (V _oc) are also enhancing 52.9 % and 25.3 %, respectively. The improvements are because of the high surface area of TiO₂ nanoparticles, as well as fast electron transport and light scattering effect of ZnO nanotubes.     

Hybrid photoanode films based on sparse ZnO rod array-TiO2 nanoparticles in dye-sensitized solar cells

ZnO-TiO2 hybrid photoanodes were fabricated via the doctor-blade method by integrating vertically-grown sparse ZnO arrays with hydrothermal TiO2 nanoparticles. A special surface-coating technique was developed to deposit a thin TiO2 layer on the surface of ZnO rods. Microstructure, optical and photoelectrochemical performance of the hybrid photoanodes were investigated. The denser ZnO array exhibited bad filling behavior of nanoparticles in the interspace of ZnO rods, strong scattering and low conversion efficiency (0.27%). The sparser array showed a much better integrated microstructure, improved transmittance and high conversion efficiency (2.68%). The surface modification of ZnO rods by the TiO2 thin layer was found useful in improving the interfacial microstructure between the ZnO rod and the TiO2 bulk film, and the total conversion efficiency of 3.01% was achieved, higher than that of the pure TiO2 nanoparticle cell (2.93%). The increased scattering effects on the incident light, the enhanced electron transportation at TiO2/dye/electrolyte interface, and the inhabited recombination were responsible for this improvement.     

Controlled synthesis of TiO2-B nanowires and nanoparticles for dye-sensitized solar cells

Controllable synthesis of the TiO₂-B nanowires (NWs) and nanoparticles (NPs) had been achieved via a facile hydrothermal route, respectively, only by tuning the solution volume. The dye-sensitized solar cells prototypes had been fabricated using TiO₂-B NW and NP electrodes, respectively. The TiO₂-B NP cells had higher photocurrent and photoelectrical conversion efficiency than the TiO₂-B NW cells though the latter exhibited larger photovoltage compared to the former. The key factors such as the photogenerated electron injection drive force, surface defects and the interfacial charge transfer, which determined the photoelectrical properties, had been systematically researched with the surface photovoltage spectra (SPS) and the electrochemical impedance spectra (EIS). The SPS proved that there was larger photoelectron injection drive force in TiO₂-B NP photoelectrode than that in NW photoelectrode. And the electrochemical impedance spectra (EIS) revealed that TiO₂-B NP cells had faster interface charge transfer compared to TiO₂-B NW cells. Both proved that NP cells had the higher photocurrents.     

Photovoltaic properties of ZnO photoanode incorporating with CNTs for dye-sensitized solar cell application

Zinc oxide carbon nanotube (ZnO-CNTs) thin films were prepared by a chemical bath deposition (CBD) method and immersed in N719 dye for 24 h. The structure and surface morphology of the samples was captured by X-ray diffraction (XRD) and field effect scanning electron microscopy (FESEM) unit, respectively. The photovoltaic properties of ZnO- and ZnO-CNT-based dye-sensitized solar cells (DSSCs) were measured by considering the power conversion efficiency (η), photocurrent density (J _sc), open-circuit voltage (V _oc), and fill factor (FF). The cell's efficiency doped with single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) reached 0.65 and 0.28 %, respectively. ZnO-based DSSC generated only η?=?0.003 %. The electrochemical impedance spectroscopy (EIS) unit was employed to investigate the electron transport properties such as effective electron lifetime (τ _eff), effective electron chemical diffusion coefficient (D _eff), and effective electron diffusion length (L _n ). The addition of CNTs has enhanced the photovoltaic properties of the DSSCs and reduced the recombination effect inside the solar cell.     

PEO electrolytes containing dioctyl phthalate (DOP) for dye-sensitized nanocrystalline TiO2 solar cells

In this paper, we aim to prepare polymer electrolytes consisting of NaI and I₂ dissolved in poly(ethylene oxide) (PEO) and dioctyl phthalate (DOP) as an additive and apply the electrolytes to dye-sensitized solar cells (DSSC). Upon the incorporation of salt, the phthalic-stretching C=O bands of DOP in Fourier transform infrared spectra shifted to a lower wave number (Δf = 93 cm⁻¹), confirming the unusual strong complex formation between sodium ions and phthalic oxygen. Coordinative interactions and structural changes of PEO/NaI/I₂/DOP electrolytes have also been characterized by wide angle X-ray scattering, presenting an almost amorphous structure of the polymer electrolytes. The ionic conductivity of the polymer electrolytes reached ∼10^–4 S/cm at room temperature at the mole ratio of [EO]:[Na]:[DOP] = 10:1:0.5, as determined by the four-probe method. DSSC using the polymer electrolytes and conductive indium tin oxide glasses exhibited 2.9% of overall energy conversion efficiency (=P _max/P _in × 100) at one sun condition (100 mW/cm²). The good interfacial contact between the electrolytes and the dye-attached nanocrystalline TiO₂ layers were verified by field-emission scanning electron microscopy.     

Comparison of the performances of dye-sensitized solar cells based on different TiO2 electrode nanostructures

This article reports on the performances of dye-sensitized solar cells based on three different working electrode structures, i.e., (i) sintered TiO₂ nanoparticles (20–40 nm diameters), (ii) ordered arrays of TiO₂ nanotubules (150 nm external diameters and 80 nm internal diameters), and (iii) ordered arrays of TiO₂ nanorods (150 nm diameters). Even though the highest short-circuit current density was achieved with systems based on TiO₂ nanotubules, the most efficient cells were those based on ordered arrays of TiO₂ nanorods. This is probably due to higher open-circuit photovoltage values attained with TiO₂ nanorods than with TiO₂ nanotubules. The nanorods are thicker than the nanotubules and therefore the injected electrons, stored in the trap states of the inner TiO₂ molecules, are shielded from recombination with holes in the redox electrolyte at open circuit. The high short-circuit photocurrent densities seen in the ordered TiO₂ systems can be explained by the fact that, as opposed to the sintered nanoparticles, the parallel and vertical orientation of the ordered nanostructures provide well-defined electrons percolation paths thus significantly reduce the diffusion distance and time constant.     

Dependence of photovoltaic parameters on the size of cations adsorbed by TiO2 photoanode in dye-sensitized solar cells

The effects of “pre-adsorbed cations” in photoanodes on the performances of dye-sensitized solar cells (DSSCs) were studied using two different size cations (K⁺ and guanidine cation (G⁺)). While the DSSCs with optimized K⁺ ions pre-adsorbed photoanodes showed a maximum efficiency of 7.04%, the DSSCs with G⁺ ions pre-adsorbed photoanodes showed an efficiency of 6.73%. DSSCs fabricated with conventional photanodes (without pre-cation adsorption) showed an efficiency of 6.21%. Differences in efficiencies are very likely due to the cation pre-adsorption effects and could be due to a higher number of K⁺ cation adsorption per unit area of TiO₂ surface of the photoanode compared to a smaller number of G⁺ cation adsorption in TiO₂, due to their difference in sizes. This pre-cation adsorption technique can be used to improve the overall efficiency of a DSSC by about 14% fold over the conventional photoanodes use in DSSCs, specially using smaller cations.

     

Electron transport properties of TiO_2 shell on Al_2O_3 core in dye-sensitized solar cells

The performance of dye-sensitized solar cells(DSSCs)is strongly affected by the properties of semiconductor nanoparticles.In this work,we used TiO_2 particles prepared by TiCl_4 hydrolysis n times on Al_2O_3 films(A/T(n)),and investigated morphology,photoelectric,and electron transport properties of A/T(n).The TiO_2 shell was composed of 10-20 nm nanoparticles and the number of nanoparticles increased with increasing TiCl_4 treatment times.The highest photoelectric conversion efficiency of 3.23% was obtained as A/T(4).IMPS results indicated that electron transport rate was high enough to conduct current,and was not the dominating effect to limit the Jsc.Jsc was mainly determined by dye loading on TiO_2 and the interconnection of TiO_2.These may provide a new strategy for preparing semiconductor working electrodes for DSSC.     

SiC/SiO2 core–shell nanowires with different shapes: Synthesis and field emission properties

Three different shapes of SiC/SiO₂ core–shell nanowires were synthesized on Si substrates through a reaction between methane and silica using iron as catalyst. Analysis of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results indicated that catalyst morphology was the key factor for the formation of these three different products. The field emission properties of these three nanowires were investigated. Comparing the field emission results of these three nanowires, we can obtain a conclusion that a vertically well-aligned orientation to the substrate played a very significant role in improving the field emission properties when the emitters are up to a considerable number.     

Interface modification of MoS2 counter electrode/electrolyte in dye-sensitized solar cells by incorporating TiO2 nanoparticles

To achieve the high efficiency in dye-sensitized solar cells (DSSCs), the interface modification of MoS₂ counter electrode (CE)/electrolyte should be carried out. Making the modified MoS₂ CE by incorporating TiO₂ nanoparticles provides possibilities to enhance electrocatalytic activity. The DSSCs with the MoS₂/TiO₂ CE show enhanced performance compared with DSSCs with the MoS₂ CE. The experimental results revealed that the MoS₂/TiO₂ nanocomposite influences on the power conversion efficiency by enhancing electrocatalytic activity and increasing the active surface area that serve to increase the short circuit current. This understanding can provide guidance for the development of highly efficient DSSCs with platinum-free CEs.     

Hybrid solar cell based on polythiophene and GaN nanoparticles composite

Hybrid solar cells based on poly(3-hexylthiophene)(P3HT)and Galium nitride(GaN)nanoparticle bulk heterojunction are fabricated and analyzed.The GaN nanocrystal is synthesized by means of a combination of sol–gel process with high temperature ammoniation using Ga(OC2H5)as a precursor.Their characteristics are determined by X-ray diffraction,X-ray photoelectron spectroscopy,and scanning electron microscopy.With the addition of GaN nanoparticle to P3HT,the device performance is greatly enhanced.