Chemical reactions in TiO2/SnO2/TiCl4 hybrid electrodes and their impacts to power conversion efficiency of dye-sensitized solar cells

This study examined the applicability of TiO₂/SnO₂/TiCl₄ hybrid electrodes in dye-sensitized solar cells (DSSCs) by combining chemical modeling with experimentation. The interfacial chemical reactions in a TiO₂/SnO₂/TiCl₄ system were simulated using a thermochemistry software package, which led to the design and testing of hybrid working electrodes. Chemical thermodynamic modeling proved that TiCl₄ is an effective agent in removing Tiⁿ⁺ (n<4) and Sn^m+ (m<4) ion impurities from dry-mixed TiO₂/SnO₂ composite particles. Our results demonstrate that the power conversion efficiency of DSSC with a TiO₂/SnO₂/TiCl₄ hybrid electrode exceeds that of the conventional DSSC with a TiO₂ electrode due to the effects of light-scattering and the formation of additional absorbance (SnCl₂), which is an unexpected side effect of TiCl₄ treatment enabling the absorption of visible light. The proposed approach is ideally suited to establishing relationships between chemistry theory and the structure and performance of advanced DSSCs as well as photo-electro-chemical systems.     

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.     

SnO$lt;sub$gt;2$lt;/sub$gt;纳米晶体的制备、结构与发光性质

使用软化学方法在碱性溶液中制备出了颗粒尺寸分布均匀的SnO₂纳米颗粒,使用透射电子显微镜（TEM）、X射线衍射（XRD）、光致发光谱（PL）和光吸收谱等方法分析与表征了SnO₂纳米颗粒的结构和光学性能.实验中通过表面活性剂的加入来控制纳米颗粒的结晶与凝聚.XRD,TEM的结果表明,原始制备出的SnO₂纳米颗粒的平均粒径小于4 nm,为完好的晶体状态.纳米颗粒经过400—1000 ℃退火后晶粒尺寸进一步增大.光吸收谱表明,相对于体材料,纳米颗粒的禁带宽度展宽并随颗粒尺寸增大而红移.光致发光谱测试表明,不同温度下退火的SnO₂纳米颗粒在350—750 nm有较强的发光,研究表明这是来源于颗粒表面的氧空位缺陷发光. 关键词： 氧化锡 表面活性剂 纳米颗粒 光致发光     

Highly transparent Zn1−xMgxO/ITO multilayer for window of thin film solar cells

Texture-etched Zn_1−xMg_xO films were fabricated and applied as front transparent electrodes for superstrate type thin film solar cells. The Zn_0.65Mg_0.35O film (x = 0.35) showed optical transparency better than commercially available Asahi VU and double-textured ZnO (WT-ZnO) substrates. To provide pertinent conductivity, ITO film was coated on the texture-etched Zn_0.65Mg_0.35O. By employing the Zn_0.65Mg_0.35O/ITO substrate instead of the SnO₂, we demonstrated an enhancement of quantum efficiency for amorphous silicon thin film solar cell devices, resulted in efficiency improvement from 8.92 to 9.56%. We also examined effectiveness of the Zn_0.65Mg_0.35O/ITO substrate for the microcrystalline silicon solar cells which delivered an efficiency of 9.73% with proper anti-reflection coating. Our experimental results suggest that the Zn_0.65Mg_0.35O/ITO multilayer front contact can be beneficial for reinforcing performances of silicon-based thin film solar cell devices.     

Improvement of ZnO nanorod-based dye-sensitized solar cell efficiency by Al-doping

The current study investigates the performance of dye-sensitized solar cells (DSSCs) based on Al-doped and undoped ZnO nanorod arrays synthesized by a simple hydrothermal method. Current density-voltage (J-V) characterizations indicate that Al-doping in ZnO crystal structure can significantly improve current densities and the energy conversion efficiency (η) of ZnO nanorod-based DSSCs. The maximum η, 1.34%, was achieved in DSSC when Al-doped ZnO nanorod arrays were grown in 0.04 M zinc acetate dihydrate solution with 5 mM aluminum nitrate nonahydrate. This result represents a large increase of η in Al-doped ZnO nanorod-based DSSCs as compared to undoped (0.05%). The improved DSSC photovoltaic performance can be attributed to two main factors: (1) increased light harvesting efficiency due to a large amount of N719 adsorbed on the large surface area of Al-doped ZnO nanorod arrays, and (2) increased electrical conductivity due to A1³⁺ ion doped into the ZnO lattice at the divalent Zn²⁺ site, allowing electrons to move easily into the Al-doped ZnO conduction band.     

Superstrate p-i-n a-Si:H solar cells on textured ZnO:Al front transparent conduction oxide

Superstrate p-i-n amorphous silicon thin-film (a-Si:H) solar cells are prepared on SnO₂:F and ZnO:Al transparent conducting oxides (TCOs) in order to see the effect of TCO/p-layers on a-Si:H solar cell operation. The solar cells prepared on textured ZnO:Al have higher open circuit voltage V_oc than cells prepared on SnO₂:F. The presence of a thin microcrystalline p-type silicon layer (μc-Si:H) between ZnO:Al and p a-SiC:H plays a major role by causing an improvement in the fill factor as well as in V_oc of a-Si:H solar cells prepared on ZnO:Al TCO. Without any treatment of the p-i interface, we could obtain a high V_oc of 994 mV while keeping the fill factor (72.7%) and short circuit current density J_sc at the same level as for the cells on SnO₂:F TCO. This high V_oc value can be attributed to modification in the current transport in this region due to creation of a potential barrier.     

A novel TiO2 nanoparticles/nanowires composite core with ZrO2 nanoparticles shell coating photoanode for high-performance dye-sensitized solar cell based on different electrolytes

The novel TiO₂ nanopartilces/nanowires (TNPWs) composite with ZrO₂ nanoparticles (ZNPs) shell-coated photoanodes were prepared to fabricate high-performance dye-sensitized solar cell (DSSC) based on different types of electrolytes. Hafnium oxide (HfO₂) is a new and efficient blocking layer material applied over the TNPWs-ZNPs core-shell photoanode film. TiO₂ nanoparticles (TNPs) and TiO₂ nanowires (TNWs) were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). DSSCs were fabricated using the novel photoanodes with an organic sensitizer D149 dye and different types of electrolytes namely liquid electrolyte, ionic liquid electrolyte, solid-state electrolyte, and quasi-solid-state electrolyte. The DSSC-4 made through the novel core-shell photoanode using quasi-solid-state electrolyte showed better photocurrent efficiency (PCE) as compared to the other DSSCs. It has such photocurrent-voltage characteristics: short circuit photocurrent (Jsc)?=?19 mA/cm², the open circuit voltage (Voc)?=?650 mV, fill factor (FF)?=?65 %, and PCE (η)?=?8.03 %. The improved performance of DSSC-4 is ascribed to the core-shell with blocking layer photoanode could increased electron transport and suppressed recombination of charge carriers at the TNPWs-ZNPs/dye/electrolyte interface.     

Minimizing structural deformation of gold nanorods in plasmon-enhanced dye-sensitized solar cells

Plasmonic metal nanoparticles have shown great promise in enhancing the light absorption of organic dyes and thus improving the performance of dye-sensitized solar cells (DSSCs). However, as the plasmon resonance of spherical nanoparticles is limited to a single wavelength maximum (e.g., ~ 520 nm for Au nanoparticles), we have here utilized silica-coated gold nanorods (Au@SiO₂ NRs) to improve the performance at higher wavelengths as well. By adjusting the aspect ratio of the Au@SiO₂ NRs, we can shift their absorption maxima to better match the absorption spectrum of the utilized dye (here we targeted the 600–800 nm range). The main challenge in utilizing anisotropic nanoparticles in DSSCs is their deformation during the heating step required to sinter the mesoporous TiO₂ photoanode and we show that the Au@SiO₂ NRs start to deform already at temperatures as low as 200 °C. In order to circumvent this problem, we incorporated the Au@SiO₂ NRs in a TiO₂ nanoparticle suspension that does not need high sintering temperatures to produce a functional photoanode. With various characterization methods, we observed that adding the plasmonic particles also affected the structure of the produced films. Nonetheless, utilizing this low-temperature processing protocol, we were able to minimize the structural deformation of the gold nanorods and preserve their characteristic plasmon peaks. This allowed us to see a clear redshift of the maximum in the incident photon-to-current efficiency spectra of the plasmonic devices (Δλ ~ 14 nm), which further proves the great potential of utilizing Au@SiO₂ NRs in DSSCs.

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Graphical Abstract Undeformed gold nanorods provide an enhanced performance of dye-sensitized solar cells at high wavelengths

     

Mixed cation effect and iodide ion conductivity in electrolytes for dye sensitized solar cells

Dye-sensitized solar cells (DSSCs) offer an alternative to conventional silicon solar cell because of low cost and easy fabrication. However, one major drawback in DSSCs is their low efficiency. This paper reports the effect of using a binary iodide salt mixture with different size cations on the efficiency enhancement in dye sensitized solar cells based on polymer gel electrolytes. Several different polymers, such as polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), poly (vinylidenefluoride (PVdF)), and polyethylene oxide (PEO) have been used as host polymers. A binary iodide mixture consisting of an alkaline iodide salt (small cation) and a quaternary ammonium iodide salt such as tetrapropyl ammonium iodide (Pr₄NI) (large cation) has been used as the iodide ion source. In some of these systems, efficiency enhancement of more than 25% has been reached due to the “mixed cation effect”. From these studies, it was established that the variation of the power conversion efficiency with the concentration ratio of the two iodide salts follows the same trend as the short circuit current density (J _sc) and goes through a maximum at a particular salt concentration ratio. The maximum efficiency was found to be higher than the efficiencies of the DSSCs with only a single iodide salt in the electrolyte. The J _sc in these DSSCs appears to be governed by the iodide ion conductivity of the gel electrolyte. The observed efficiency enhancement has been explained on the basis of the electrode effects as well as electrolyte effects where the cations play a dominant role.

     

Effects of Gd<Superscript>3+</Superscript> modifications on the photoelectrochemical properties of TiO<Subscript>2</Subscript>-based dye-sensitized solar cells

In this paper, TiO₂ particles (~30 nm) modified with Gd₂O₃-coating layer (~2 nm) for dye-sensitized solar cells (DSSCs) were fabricated via the hydrothermal method. Among the solar cells based on the Gd³⁺-doped TiO₂ photoanodes, the optimal conversion efficiency was obtained from the 0.025Gd³⁺-modified TiO₂-based cell, with a 17.7% improvement in the efficiency as compared to the unmodified one (7.18%). This enhancement was probably due to the improved UV radiation harvesting via a down-conversion luminescence process by Gd³⁺ ions, enhancement of visible light absorption and improved dye loading capacity. In addition, after Gd modification, a thin coating could be formed on the TiO₂ nanoparticles, which worked as an energy barrier and resulted in a lower charge recombination.     

Single-crystalline Zn2SnO4 hexangular microprisms: Fabrication, characterization and optical properties

Large-scale Zn₂SnO₄ hexangular microprisms were successfully synthesized through a simple thermal evaporation method by heating metal Zn and Sn powders under varying temperatures. The synthesized microprisms are single-crystalline, tens of micrometers in length. And their surfaces have many nano-scale skewed steps along the axial direction. Structurally, we supposed that the hexangular prism could be described as a row of inlaid octahedron of Zn₂SnO₄ crystals. A broad asymmetrical emission band was observed in the PL spectrum of these Zn₂SnO₄ microprisms, which was discussed in detail in the paper.     

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.     

Hydrothermal synthesis and optical characteristics of Eu in Zn2SnO4 nanocrystals

Zn₂SnO₄:Eu³⁺ nanocrystals were one-step synthesized by hydrothermal method for the first time. All the products were systematically characterized by powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), electron probe X-ray microanalyzer (EPMA), photoluminescence (PL) and photoluminescent excitation (PLE). The characteristic peak of Eu³⁺-doped in Zn₂SnO₄ nanocrystals was also detected. The luminescent properties of blank and Eu³⁺-doped Zn₂SnO₄ nanocrystals were reported.     

Influence of TiO2 nanotube morphology and TiCl4 treatment on the charge transfer in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ nanoparticles (NPs), TiO₂ nanotube arrays (NTAs), and surface-modified NTAs with a TiCl₄ treatment. The photovoltaic efficiencies of the DSSCs using TiO₂ NP, NTA, and TiCl₄-treated NTA electrodes are 4.25, 4.74, and 7.47 %, respectively. The highest performance was observed with a TiCl₄-treated TiO₂ NTA photoanode, although in the case of the latter two electrodes, the amounts of N719 dye adsorbed were similar and 68 % of that of the NP electrode. Electrochemical impedance measurements show that the overall resistance, including the charge–transfer resistance, was smaller with NTA morphologies than with NP morphologies. We suggest that a different electron transfer mechanism along the one-dimensional nanostructure of the TiO₂ NTAs contributes to the smaller charge–transfer resistance, resulting in a higher short circuit current (J _sc), even at lower dye adsorption. Furthermore, the TiCl₄-treated NTAs showed even smaller charge–transfer resistance, resulting in the highest J _sc value, because the downward shift in the conduction band edge improves the electron injection efficiency from the excited dye into the TiCl₄-treated TiO₂ electrodes.     

One-pot,large-scale synthesis silica-encapsulated NIR alloy quantum dots CdSeTe within short time

Hydrothermal process has been employed to synthesize titanium oxide (TiO₂) bottle brush. The nanostructured bottle brushes with tetragonal nanorods of ~75 nm diameter have been synthesized by changing the nature of the precursors and hydrothermal processing parameters. The morphological features and structural properties of TiO₂ films were investigated by field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron spectroscopy, Fourier transform Raman spectroscopy, and X-ray photoelectron spectroscopy. The influence of such nanostructures on the performance of dye-sensitized solar cells (DSSCs) is investigated in detail. The interface and transient properties of these nanorods and bottle brush-based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopic measurements in order to understand the critical factors contributing to such high power conversion efficiency. Surface area of sample was recorded using Brunauer–Emmett–Teller measurements. It is found that bottle brush provides effective large surface area 89.34 m² g^?1 which is much higher than TiO₂ nanorods 63.7 m² g^?1. Such effective surface area can facilitate the effective light harvesting, and hence improves the dye adsorption and the photovoltaic performance of DSSCs, typically in short-circuit photocurrent and power conversion efficiency. A best power conversion efficiency of 6.63 % has been achieved. We believe that the present device performance would have wide interests in dye-sensitized solar cell research.     

Effects of hydroxyls on the structural and room temperature ferromagnetic properties of Co doped SnO2 nanoparticles

Co doped SnO₂ nanoparticles have been prepared via a wet chemical method with different precipitation processes. The structure and morphology of Co doped SnO₂ nanoparticles demonstrate that the nanoparticles are in a rutile single phase and uniform, respectively. X-ray photoelectron spectroscopy shows that the Co dopants are in 2+ oxidation valence state and doped ∼2 atm% in SnO₂ nanoparticles. Moreover, Raman spectroscopy further confirms that Co doped SnO₂ nanoparticles have single phase crystallinity without forming any extra modes related to secondary phases. The magnetic measurements reveal that all nanoparticles exhibit room temperature ferromagnetism (RTFM) due to the presence of disorders and defects introduced by hydroxyls in the crystal structure. In addition, it has been clearly observed that the saturated magnetic moments are strongly affected by the precipitation processes which control the incorporation of hydroxyls into the lattice.     

High luminance of new green emitting phosphor, Mg2SnO4:Mn

Mg₂SnO₄, which has an inverse spinel structure, was adopted as the host material of a new green emitting phosphor. Luminescence properties of the manganese-doped magnesium tin oxide prepared by the solid state reaction were investigated under vacuum ultraviolet (VUV) ray and low-voltage electron excitation. The Mg₂SnO₄:Mn phosphor exhibited green luminescence with the emission spectrum centered at 500 nm due to spin flip transition of the d-orbital electron associated with the Mn²⁺ ion. Optimum Mn concentration of Mg₂SnO₄:Mn under VUV excitation with 147 nm wavelength and electron beam excitation with 800 V excitation voltage are 0.25 and 0.6 mol%, respectively. The emission intensities of Mg₂SnO₄:Mn phosphors under the two excitation sources are higher than those of Zn₂SiO₄:Mn and ZnGa₂O₄:Mn phosphors. At 0.25 mol% of Mn concentration, on the other hand, the decay time is shorter than 10 ms.     

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.

     

Preparation of SnO2–graphene from SnS–graphene oxide for enhanced reversible lithium ion storage

SnO₂–graphene nanocomposites (SnO₂–GNS) have been prepared through a simple hydrothermal reaction with SnS–graphene oxide composites as the precursor. The composite material as prepared was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller analysis, and thermogravimetric analysis. The results indicate that SnO₂ nanoparticles possess a good dispersion on the surface of graphene. Electrochemical tests demonstrate the high reversible lithium ion storage properties of SnO₂–GNS. The nanocomposites retained a reversible capacity of 503 mAh?g^?1 after 40 cycles. Moreover, the composite material exhibited higher capacity and better cyclic performance compared to free SnO₂ nanoparticles physically mixed with graphene in the relative weight ratio. The results suggest that the combination of SnO₂ and graphene leads to synergistic performance, which enhances lithium ion storage properties of the overall system.     

A facile low temperature synthesis of TiO2 nanorods for high efficiency dye sensitized solar cells

Titania (TiO₂) nanorods have been synthesized with controlled size for dye-sensitized solar cells (DSSCs) via hydrothermal route at low hydrothermal temperature of 100 °C for 24 h. The titania nanorods were characterized using XRD, SEM, TEM/HRTEM, UV-vis Spectroscopy, FTIR and BET specific surface area (S _BET), as well as pore-size distribution by BJH. The results indicated that the bulk traps and the surface states within the TiO₂ nanorods films have enhanced the efficiency of DSSCs. The size of the titania nanorods was 6.7 nm in width and 22 nm in length. The high surface area can provide more sites for dye adsorption, while the fast photoelectron-transfer channel can enhance the photogenerated electron transfer to complete the circuit. The specific surface area S _BET was 77.14 m²?g^?1 at the synthesis conditions. However, the band gap energy of the obtained titania nanorods was 3.2 eV. The oriented nanorods with appropriate lengths are beneficial in improving the electron transport property and thus leading to the increase of photocurrent, together enhancing the power conversion efficiency. A nearly quantitative absorbed photon-to-electrical current conversion achieved upon excitation at wave length of 550 nm and the power efficiency was enhanced from 5.6 % for commercial TiO₂ nanoparticles Degussa (P25) cells to 7.2 % for TiO₂ nanorods cells under AM 1.5 illumination (100 mW?cm^?2). The TiO₂ cells performance was improved due to their high surface area, hierarchically mesoporous structures and fast electron-transfer rate compared with the Degussa (P25).