Chemical conversion synthesis of ZnS shell on ZnO nanowire arrays: morphology evolution and its effect on dye-sensitized solar cell

Heterostructured ZnO/ZnS core/shell nanowire arrays have been successfully fabricated to serve as photoanode for the dye-sensitized solar cells (DSSCs) by a facile two-step approach, combining hydrothermal deposition and liquid-phase chemical conversion process. The morphology evolution of the ZnS coated on the ZnO nanowires and its effect on the performance of the DSSCs were systematically investigated by varying the reaction time during the chemical conversion process. The results show that the compact ZnS shell can effectively promote the photogenerated electrons transfer from the excited dye molecules to the conduction band of the ZnO, simultaneously suppress the recombination for the injected elelctrons from the dye and the redox electrolyte. As reaction time goes by, the surface of the nanowires becomes coarse because of the newly formed ZnS nanoparticles, which will enhance the dye loading, resulting in increment of the short-circuit current density (J(SC)) . Open-circuit photovoltage decay measurements also show that the electron lifetime (τ(n)) in the ZnO/ZnS core/shell nanostructures can be significantly prolonged because of the lower surface trap density in the ZnO after ZnS coating. For the ZnO/ZnS core/shell nanostructures, the J(SC) and η can reach a maximum of 8.38 mA/cm(2) and 1.92% after 6 h conversion time, corresponding to 12- and 16-fold increments of as-synthesized ZnO, respectively.     

Enhanced photoelectrochemical performance of bridged ZnO nanorod arrays grown on V-grooved structure

Bridged ZnO nanorod arrays on a V-grooved Si(100) substrate were used as the photoanode of a photoelectrochemical (PEC) cell for water splitting. Photolithography followed by reactive ion etching was employed to create a V-grooved structure on a Si substrate. ZnO nanorod arrays were grown via a hydrothermal method. The light trapping and PEC properties are greatly enhanced using the bridged ZnO nanorod arrays on a V-grooved Si substrate compared with those on a flat one. Increased short circuit photocurrent density (J(SC), 0.73?mA?cm(-2)) and half-life time (1500?s) are achieved. This improved J(SC) and half-life time are 4 times and 10 times, respectively, higher than those of the ZnO nanorod arrays grown on a flat substrate. The overall PEC cell performance improvement for the V-groove grown ZnO array is attributed to the reduced light reflection and enhanced light trapping effect. Moreover, V-groove ZnO showed stronger adhesion between ZnO nanorod arrays and the substrate.     

Effect of surface modifications on ZnO nanorod arrays electrode for dye-sensitized solar cells

High quality, large area and well-oriented ZnO nanorod arrays electrodes were successfully synthesized on conductive transparent oxide substrates by low-temperature hydrothermal methods for dye-sensitized solar cells. Aiming at getting further enhancement and study the effect of the surface modification on cell performance, ZnO thin film and ZnO nanoparticles are carried out to modify the as-grown ZnO nanorod arrays. The morphology, structure and photoluminescence property of the modified ZnO electrodes are characterized in detail. Furthermore, the I-V characterization result shows that these modification methods have distinct influences on the performance of the cell based on ZnO nanorod arrays electrode. The overall conversion efficiency can be optimized by choosing the suitable modification route.     

Solid-state dye-sensitized solar cells based on ordered ZnO nanowire arrays

A solid-state dye-sensitized solar cell (DSC) is fabricated by using arrays of 11-12 μm long, vertically oriented ZnO nanowires as the anode and CuSCN as the solid hole-transport material. The fabricated DSC yields a remarkably higher photocurrent density (J(SC) = 8 mA cm(-2)) compared to previously reported data for solid-state DSCs based on either one-dimensional nanostructures (J(SC) = 0.34 mA cm(-2)) or nanoporous nanocrystalline structures (J(SC) = 4.5 mA cm(-2)) of ZnO. A power conversion efficiency of 1.7% under an irradiation of AM 1.5 G simulated sunlight is reported.     

Synthesis and characterization of flower like ZnO nanorods for dye-sensitized solar cells

Flower like ZnO nanorods have been prepared by chemical bath deposition method. X-ray diffraction result shows that flower like ZnO nanorods exhibit hexagonal structure. Dye sensitized solar cells have been assembled by using ZnO nanorod film photoelectrode sensitized using natural dye extracted from daucus carota as sensitizer. The flower like ZnO nanorods have been used as photo-anode material to fabricate the dye sensitized solar cell which exhibited an overall light to electricity conversion efficiency of 0.78 % with a fill factor of 0.39, short-circuit current density of 3.70 mA/cm² and open-circuit voltage of 0.26 V.     

ZnO纳米四脚状阵列电极染料敏化太阳能电池

采用化学气相沉积(CVD)法制备了不同尺寸的四脚状纳米ZnO和ZnO纳米棒。采用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)对纳米ZnO的晶型结构和形貌进行表征,研究结果表明CVD法制备的四脚状纳米ZnO具有三维空间结构,其最小平均臂宽约为70nm,臂长约300nm,制备的纳米棒直径约为84nm,长约2μm,且都为六方纤锌矿晶型结构。将ZnO纳米四脚状及纳米棒利用滚涂法在FTO导电玻璃上形成ZnO光阳极,经N719染料敏化后组装成染料敏化太阳能电池,光电性能结果表明,染料敏化小尺寸的四脚状纳米ZnO太阳能电池光电转换效率(η=1.88%)高于染料敏化大尺寸的四脚状纳米ZnO太阳能电池光电转换效率(η=1.18%),远高于染料敏化ZnO纳米棒太阳能电池的光电转换效率(η=0.7%)。     

Thickness dependence of the MoO(3) blocking layers on ZnO nanorod-inverted organic photovoltaic devices

Organic solar cells based on vertically aligned zinc oxide nanorod arrays (ZNR) in an inverted structure of indium tin oxide (ITO)∕ZNR∕poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester(P3HT:PCBM)∕MoO(3)∕aluminum(Al) were studied. We found that the optimum MoO(3) layer thickness condition of 20 nm, the MoO(3) can effectively decrease the probability of bimolecular recombination either at the Al interface or within the active layer itself. For this optimum condition we get a power conversion efficiency of 2.15%, a short-circuit current density of 9.02 mA∕cm(2), an open-circuit voltage of 0.55V, and a fill factor of 0.44 under 100 mW∕cm(2) irradiation. Our investigations also show that the highly crystallized ZNR can create short and continuous pathways for electron transport and increase the contact area between the ZNR and the organic materials.     

Modification of ZnO nanorods through Au nanoparticles surface coating for dye-sensitized solar cells applications

Dye-sensitized solar cells (DSSC) based on ZnO nanorods were fabricated and modified through the addition of Au nanoparticles. The as-synthesized ZnO nanorods were well-dispersed and of high crystallinity quality leading to a high cell efficiency of 5.2%. On the other hand, thick layer of Au nanoparticles aggregation may have led to distortion of plasmonic effect. Also, the addition of Au nanoparticles have effectively decreased the surface area of ZnO nanorods with direct contact to the dye molecules, resulting in a lower amount of adhered dye molecules to convert sunlight. The electrons generated by the photo-absorption through thick aggregated Au nanoparticles layer may have a lower injection rate to ZnO nanorods as compared to those absorbed by the dye.     

Solution-processable zinc oxide for the polymer solar cell based on P3HT:PCBM

The device performance of polymer solar cells with zinc oxide (ZnO) nanoparticles inserted as an electron injection layer between the poly(3-hexylthiopene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) active layer and the Al electrode was studied. The polymer solar cell consists of molybdenum-oxide (MoO3) as a hole injection layer, P3HT:PCBM bulk heterojunction as an active layer, and ZnO NPs as an electron injection layer. The ZnO layer was formed from a precursor solution on the top part of the P3HT:PCBM film (1:0.8 weight ratio) via sol-gel spin-coating, and was annealed at a low temperature (150 degrees C). The crystallinity, the atomic ratio of Zn and O, the absorption spectra, and the surface morphology of the ZnO thin films were studied. The device with a ZnO layer showed 9-11% higher J(SC) and 8-9% higher PCE compared to the devices without a ZnO layer. These improved device properties are attributed to the efficient electron extraction and the decreased reflectivity owing to the use of a ZnO layer.     

Poly(3-hexylthiophene)/hexamine modified ZnO hybrid nanocomposite: structural,optical, thermal and electrical transport studies

This paper reports the synthesis of poly(3-hexylthiophene) (P3HT)/HA@ZnO nanocomposite by in situ polymerization and demonstrates their thermal, morphological and optoelectronic properties. Zinc oxide (ZnO) nanoparticles were prepared by the simple approach of co- precipitation method using zinc acetate dihydrate as precursor modified by hexamine (HA) acting as a capping agent. Structural and photo physical studies shows that conjugated polymer chains intimately contact with the inorganic semiconductor. ZnO has wurtzite structure with average crystallite size of 40 nm. The emission spectra indicate that modified ZnO nanoparticles results in more efficient photo induced charge transfer than that of the simple nanocomposite of P3HT/ZnO. The morphological studies revealed that the transformation of granular morphology of P3HT to the clusters in P3HT/HA@ZnO hybrid nanocomposites. Cyclic voltammeter elucidates the electrochemical behavior and the HOMO–LUMO energy levels of the nanocomposites. The results indicate that the P3HT/HA@ZnO nanocomposite has energy gap of 0.72 eV, indicating this composite has potential for the fabricating hybrid organic–inorganic solid state solar cells. A solar to electric energy conversion efficiency of 0.1238 % was attained with the system.     

Tuning photoluminescence properties of ZnO nanorods via surface modification

Zinc oxide (ZnO) is a versatile material that has been used in photocatalysis, solar cells, chemical sensors, and piezoelectric transducers. All these are directly related to its surface properties. Here ZnO nanorod arrays were successfully synthesized by electrochemical deposition method, the surface of which was modified by dopamine, a robust anchor. Compared with pristine ZnO sample, the surface modification can greatly enhance the ultraviolet and visible-light photoluminescence. This is due to the formation of polydopamine on the nanorod surface, which may act as a dye that can be photoexcited. The resultant photogenerated electrons can inject into the conduction band of ZnO and take part in the luminescent process. These results may provide a foundation for real applications of ZnO nanomaterials in optoelectronic devices and, especially, for the applications in biological field as both the dopamine and ZnO are biocompatible materials.     

Controlled synthesis of ZnO branched nanorod arrays by hierarchical solution growth and application in dye-sensitized solar cells

We demonstrate the controlled synthesis of ZnO branched nanorod arrays on fluorine-doped SnO₂-coated glass substrates by the hierarchical solution growth method. In the secondary growth, the concentration of Zn(NO₃)₂/hexamethylenetetramine plays an important role in controlling the morphology of the branched nanorod arrays, besides that of diaminopropane used as a structure-directing agent to induce the growth of branches. The population density and morphology of the branched nanorod arrays depend on those of the nanorod arrays obtained from the primary growth, which can be modulated though the concentration of Zn(NO₃)₂/hexamethylenetetramine in the primary growth solution. The dye-sensitized ZnO branched nanorod arrays exhibit much stronger optical absorption as compared with its corresponding primary nanorod arrays, suggesting that the addition of the branches improves light harvesting. The dye-sensitized solar cell based on the optimized ZnO branched nanorod array reaches a conversion efficiency of 1.66% under the light radiation of 1000 W/m². The branched nanorod arrays can also be applied in other application fields of ZnO.     

ZnO纳米棒阵列膜的制备及其光电化学性能研究

采用化学溶液沉积法,在ZnO纳米颗粒膜修饰的FTO导电玻璃基底上,制备了ZnO纳米棒阵列。用X射线衍射仪（XRD）、场发射扫描电子显微镜（FESEM）、透射电子显微镜（TEM）对样品进行表征。研究结果表明所制备的ZnO纳米棒为六方纤锌矿相单晶结构,沿c轴择优取向生长,平均直径约为40nm,长度约为900nm;ZnO纳米棒阵列生长致密,取向性较一致。以曙红Y敏化的ZnO纳米棒阵列膜为光阳极制作了染料敏化太阳能电池原型器件,在光照强度为100mW/cm2下,其开路电压为0.418V,短路电流为0.889mA/cm2,总的光电转换效率为0.133%。     

Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells

This paper reports additive-free, reproducible, low-temperature solution-based process for the preparation of crystalline ZnO nanorods by homogeneous precipitation from zinc acetate. Also, ZnO nanorod structured dye sensitized solar cells using ruthenium dye (Z907) have been fabricated and characterized. The formation and growth of zinc oxide nanorods are successfully achieved. We analyzed three different synthesis method using solution phase, autoclave and microwave. The calcination effects on the morphology of ZnO nanorods are also investigated. Analysis of ZnO nanorods shows that calcination at lower temperature is resulted in a nanorod growth. Additive-free, well-aligned ZnO nanorods are obtained with the length of 330–558 nm and diameters of 14–36 nm. The XRD, SEM, and PL spectra have been provided for the characterization of ZnO nanorods. Microwave-assisted ZnO nanostructured dye sensitized solar cell devices yielded a short-circuit photocurrent density of 6.60 mA/cm², an open-circuit voltage of 600 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 2.35% under standard AM 1.5 sun light.     

General route to vertical ZnO nanowire arrays using textured ZnO seeds

A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A approximately 10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200-350 degrees C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod-polymer solar cells.     

Structural and antireflective properties of ZnO nanorods synthesized using the sputtered ZnO seed layer for solar cell applications

We report the structural and antireflective properties of ZnO nanorod arrays (NRAs) on silicon (Si) substrate by wet chemical growth using the sputtered ZnO seed layer for solar cell applications. The size, height, shape, and number of ZnO nanorods depend strongly on the ZnO seed layer thickness as well as the molar zinc nitrate concentration. Clearly, the ZnO nanorods are of wurzite crystal structure from the X-ray diffraction analysis. To achieve the low reflectance over a wide wavelength range, the ZnO seed layer thickness, molar concentration, and growth time are optimized. It is found that the specular reflection spectrum of ZnO NRAs is closely related to the ZnO seed layer thickness. The solar weighted reflectance, Rw, of ZnO NRAs as antireflection coatings for Si solar cells is estimated under AM1.5 g illumination. For ZnO NRAs with 50 nm ZnO seed layer in 10 mM aqueous solution for 12 hours, the low specular reflectance (i.e., <7%) is obtained at wavelengths of 300-1200 nm, indicating a low Rw of 3.86%.     

Chemical bath deposition of ZnO nanorods for dye sensitized solar cell applications

ZnO nanorods using various molar concentrations have been synthesized through the chemical bath deposition method. X-ray diffraction result shows that the ZnO nanorods are of hexagonal structure. The morphology of the ZnO nanorods has been examined by scanning electron microscopy. The ZnO nanorods have diameters ranging from 100 to 200 nm and length of 1–3 μm. Dye-sensitized solar cells have been assembled by using ZnO nanorod film photoelectrode sensitized using natural dye extracted from lantana camara as sensitizer. The ZnO nanorods have been used as electrode material to fabricate dye sensitized solar cells which exhibited an efficiency of 0.71 %, the maximum efficiency was obtained for films deposited for 0.07 M concentration.     

ZnO nanostructures as efficient antireflection layers in solar cells

An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.     

Achievement of 4.7% conversion efficiency in ZnO dye-sensitized solar cells fabricated by spray deposition using hydrothermally synthesized nanoparticles

ZnO-based dye-sensitized solar cells (DSSCs) have been fabricated using hydrothermally synthesized ZnO nanoparticles by spray deposition. The effect of self-assembled nanostructures in ZnO photoelectrodes, due to the electric field during spray deposition, has been studied. Thickness of the photoelectrode is found to play a role on the cell performance, the cell with nanocrystalline film thickness of ～4.3?μm yielding an efficiency of ～2.8% for a cell area of ～3.2?cm(2). On the other hand, the cell with ZnO nanostructures is found to yield an efficiency of 4.7% (enhancement of ～60%) which is highest for the cell with area>1?cm(2) having a photoelectrode thickness of ～4.5?μm. Increased surface area due to the presence of ZnO nanostructures in the photoelectrode film helps in the adsorption of more dye molecules to the ZnO surface, which contributes to the better cell performance. The improved dye-sensitized solar cell performance is also explained with the help of light scattering by the ZnO nanostructures through extending the distance traveled by light so as to increase the light-harvesting efficiency of the photoelectrode film.     

Approach to a block polymer precursor from poly(3-hexylthiophene) nitroxide-mediated in situ polymerization for stabilization of poly(3-hexylthiophene)/ZnO hybrid solar cells

A cross-linked block copolymer poly(3-hexylthiophene)-b-poly(zinc dimethacrylate) (P3HT-b-PZn(MA)₂), which acted as precursor for the preparation of poly(3-hexylthiophene)/ZnO (P3HT/ZnO) hybrid film by in-situ hydrolysis, was rationally designed and synthesized via nitroxide-mediated in-situ polymerization of zinc methacrylate (Zn(MA)₂) using poly(3-hexylthiophene) alkoxyamine (P3HT-TIPNO) as macroinitiator for the purpose of stabilizing the P3HT/ZnO hybrid solar cells. The cross-linking was confirmed by the insolubility of the film in organic solvents and Fourier-transform infrared experiment. With the function of the cross-linked template, the diffusion of ZnO nanoparticles prepared by in-situ hydrolysis could be lowered to suppress the formation of large aggregations, which favored the formation of a better and more stable interpenetrating network and provided more heterojunction interfaces for exciton dissociation. As a result, the inverted device based on cross-linked P3HT/ZnO hybrid film obtained by in situ hydrolyzing P3HT-b-PZn(MA)₂ block copolymer yielded a power conversion efficiency of 0.45% under AM 1.5G illumination from a calibrated solar simulator with an intensity of 100 mW/cm², and the deterioration of the photoconversion performance was suppressed in the hybrid solar cells with the cross-linked P3HT/ZnO compared to cells with non-cross-linked P3HT/ZnO obtained by in situ hydrolyzing P3HT-TIPNO/Zn(MA)₂ blend film.