Optimization of ZnO for front and rear contacts in a-Si solar cells

The enhancement of the reflection from the rear contact of p-i-n a-Si solar cells using ZnO combined with metals (Ag/Al) as a back reflector was demonstrated theoretically and experimentally. Futhermore, the incorporation of unreacted H₂O as source gas in the ZnO films was clearly observed through the thermal evolution measurement, suggesting the need for employing the pre-annealing technique for ZnO films before using them as a front contact in p-i-n a-Si solar cells. By using these approaches, the a-Si solar cells with glass/annealed-ZnO/delta-doped p/buffer/i/n/ZnO/metals(Ag/Al) structure were successfully fabricated and a conversion efficiency of 12.1% (AM-1.5, area 3×3 mm²) was obtained. Moreover, the solar cells with a structure of AR coated glass/SnO₂/delta-doped/p/buffer/i/n/ZnO/metals(Ag/Al) were also fabricated and by optimizing the use of the ZnO layer at the rear contact, a conversion efficiency of 12.6% was obtained. To make the ZnO films more appropriate for solar cells application, the growth rate of the ZnO films was increased by increasing the flow rate of diethylzinc used as a source gas.     

Shape-dependent localized surface plasmon enhanced photocatalytic effect of ZnO nanorods decorated with Ag

We report shape-dependent localized surface plasmon enhanced photocatalytic effect of ZnO nanorods decorated with Ag nanostructures. The plasmonic ZnO photoelectrode modified by Ag nanoprisms exhibits a significant enhancement in photoelectric conversion with a maximum photoconversion efficiency of 1.45%. The photocurrent intensity (at 0.5 V vs. Ag/AgCl) of ZnO–Ag nanoprisms is 3.1 and 10 times greater than that of ZnO–Ag nanoparticles and as-grown ZnO nanorods, respectively. Moreover, ZnO–Ag nanoprisms showed a fast photoresponse due to the fast transport of photogenerated charge carriers in ZnO nanorods with a low recombination rate. It is suggested that the mechanism of photocatalytic enhancement by Ag nanoprisms is mainly ascribed to the significantly enhanced plasmonic ‘hotspots’ in sharp tips of nanoprisms. Such shape-dependent localized surface plasmon effect is further confirmed by FDTD simulation, which revealed that Ag nanoprisms showed stronger electromagnetic field intensity than that of Ag nanoparticles.     

Zinc oxide nanorods based catalysts for hydrogen production by steam reforming of methanol

Zinc oxide (ZnO) nanorods were epitaxially grown on porous cordierite support by a hydrothermal process and utilized for catalyzing methanol steam reforming (MSR) reaction. Catalytic activity of ZnO nanorods for MSR process was correlated to the terminated surfaces of ZnO crystallites. Copper (Cu), palladium (Pd) and gold (Au) nanoparticles infused ZnO nanorods were prepared by in-situ precipitation of the metals on the nanorods. 28% hydrogen selectivity was observed with Cu/ZnO nanorods (Cu/10Zn), while Pd/ZnO nanorods and (Pd/10Zn) showed slightly lower activity. Higher catalytic activity of copper and palladium impregnated ZnO nanorods can be attributed to the synergistic combination of bimetallic oxides. In contrast, Au/ZnO nanorods (Au/10Zn) showed very high activity for methanol dehydrogenation and higher than 97% methanol conversion was achieved for operating temperatures as low as 200 °C.     

Electrochemically grown ZnO nanorods for hybrid solar cell applications

A hybrid solar cell is designed and proposed as a feasible and reasonable alternative, according to acquired efficiency with the employment of zinc oxide (ZnO) nanorods and ZnO thin films at the same time. Both of these ZnO structures were grown electrochemically and poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester; (P3HT:PCBM) was used as an active polymer blend, which was found to be compatible to prepared indium-tin-oxide (ITO) substrate base. This ITO base was introduced with mentioned ZnO structure in such a way that, the most efficient configuration was optimized to be ITO/ZnO film/ZnO nanorod/P3HT: PCBM/Ag. Efficiency of this optimized device is found to be 2.44%. All ZnO works were carried out electrochemically, that is indeed for the first time and at relatively lower temperatures.     

ZIF-L-derived porous C-doped ZnO/CdS graded nanorods with Z-scheme heterojunctions for enhanced photocatalytic hydrogen evolution

This study presents a novel approach for synthesizing C–ZnO/CdS graded nanorods derived from metal–organic frameworks (MOFs) that can be applied as a catalyst for photocatalytic hydrogen evolution from pure water. Porous C-doped ZnO was prepared by a self-template method using imidazole-like metal–organic backbone (ZIF-L) as a precursor through a two-step calcination method. CdS nanoparticles were deposited on ZIF-L surface by chemical deposition. The two-step calcination method introduced elemental C, and the unique architecture of ZIF-L played an essential role in forming the hierarchical structure of the porous ZnO nanorods. Compared with other ZnO/CdS catalysts, the C-doped ZnO/CdS graded nanorods exhibited remarkable photocatalytic activity for hydrogen production. The highest hydrogen production rate of 20.25 mmol g^?1 h^?1 with an apparent quantum yield (AQY) of 24.7% at 365 nm obtained over C–ZnO/CdS with Pt as co-catalyst, which was 24.4 and 65.3 times higher than that over CdS (0.83 mmol g^?1 h^?1) and ZnO (0.31 mmol g^?1 h^?1), respectively. This outcome was attributed to (i) the formation of Z-scheme heterojunction that significantly promoted the separation and migration of photogenerated electron–hole pairs; (ii) C doping that reduced the bandgap of ZnO and broadened its spectral response range; and (iii) the ordered arrangement of porous nanorods that effectively reduced the recombination rate of the electron–hole pairs.     

Construction of ZnO@NiO heterostructure photoelectrodes for improved photoelectrochemical performance

In this report, a p-n junction has been constructed using ZnO/NiO heterostructured photoelectrode by spin coating NiO layers over vertically aligned ZnO nanorod arrays to demonstrate its potential in water splitting applications. Before investigating their PEC performance, we thoroughly studied the introduction of NiO layers on the structure, morphology and light absorption property of ZnO nanorods. 9 layered NiO coated ZnO nanorods exhibited optimum photocurrent density of 0.251 mA/cm² at 0.8 V vs. Ag/AgCl which is attributed to its high absorbance and better charge transfer as recorded from UV–Vis and EIS data. Furthermore, we also studied the effect of (cation (Mg) and anion (Cl)) doping in PEC performance of ZnO nanorods on this optimized sample. Cl_ZnO/NiO showed high J_ph of 1.282 mA/cm² at 1.2 V vs. Ag/AgCl under visible light illumination. The reason behind better photoresponse is its enhanced absorption and well-defined p-n heterojunction between Cl_ZnO and NiO which favoured the separation and transfer of the photocarriers. The results displayed in this work provides a suitable approach of building p-n junction for high performance PEC water oxidation.     

Enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route

A series of ZnO/ZnS core/shell nanorods with different ZnS/ZnO molar ratios was synthesized via a new water bath route. The nanorods have a diameter of about 100 nm and a length ranging from a few hundred nanometers to several micrometers. They are formed by coating ZnO nanorod with a layer of porous ZnS shell mainly consisting of crystals which are about 12 nm in diameter. The results showed that the deposition thickness of the ZnS shell layer strongly affected the morphologies, surface area, structure, photo absorption and photocatalytic performance of the ZnO/ZnS core/shell nanorods. The as-prepared ZnO/ZnS core/shell nanorods exhibited a higher photocatalytic activity for H₂ evolution from the glycerol/water mixtures compared with the ZnO nanorods under the same conditions. The maximum H₂ production was 2608.7 and 388.4 μmol h⁻¹ gcat⁻¹ under UV and solar-simulated light irradiation and the corresponding quantum efficiencies were 22% and 13%, respectively. The deposition thickness of the ZnS shell and the interaction between the ZnO rod and ZnS shell and the core/shell structure with n-p heterojunction substantially influence the optical and catalytic performance of the ZnO/ZnS core/shell nanorods.     

PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties

The gradient PEC electrodes of ZnO nanorods sensitized by CdS with different size were fabricated via successive ionic layer adsorption and reaction (SILAR) method and applied in photochemical water splitting. The concentrations of reaction solution and SILAR cycles were investigated in the synthetic process and the working mechanism of the gradient PEC electrode was suggested. The results showed that the hydrogen generation efficiency of 4.88% was achieved for the ZnO/CdS gradient PEC electrode constructed by decreasing of the CdS quantum dots size on ZnO nanorods due to the improved absorption and appropriate energy gap structure, which was confirmed by enhanced absorption spectrum. The expected products have potential application in photoelectrochemical water splitting.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Nanostructured solar cell based on spray pyrolysis deposited ZnO nanorod array

In this paper we present a realization of an extremely thin absorber (ETA) layer solar cell by the chemical spray pyrolysis method. CuInS₂ absorber was deposited onto a blocking layer coated ZnO nanorods grown on a transparent conductive oxide. Layers and cells were characterized by optical and Raman spectroscopy, and scanning electron microscopy. Current–voltage, spectral response and electron beam induced current measurements were applied to solar cells. ZnO nanorod cell showed twice higher short circuit current density than the flat reference. ETA cells with efficiency of 2.2% (j=12 mA/cm², V_oc=425 mV, FF=43%) and of 2.5% were prepared using TiO₂-anatase and an indium sulfide blocking layer, respectively.     

Fabrication and opto-electric properties of ITO/ZnO bilayer films on polyethersulfone substrates by ion beam-assisted evaporation

Transparent conducting oxides bilayer films stacked by one 130-nm-thick indium tin oxide (ITO) top layer and one 75-nm-thick zinc oxide (ZnO) buffer layer were grown onto polyethersulfone (PES) substrates by ion beam-assisted evaporation. The effects of ion energy and ZnO buffer layers on the structural and opto-electric properties of ITO films were initially investigated. The as-deposited ZnO buffer layers show wurtzite (0 0 2) preferred orientation on the PES substrates with ion beam assistance. The results of X-ray diffraction reveal a marked increase in the crystallinity of the ITO films which use ZnO as a buffer layer material. A drop of ∼60% in electrical resistivity of the ITO film on the PES can be achieved by using ZnO buffer layer. The transmittance of the ITO/ZnO bilayer was not deteriorated due to the insertion of ZnO layer. The lowest electrical resistivity of 6.552×10⁻⁴ Ω-cm associated with the transmittance of ∼80% at the wavelength of 550 nm can be obtained for the ITO film on the ZnO-coated PES at ion energy of 60 eV. The ITO films on the ZnO-buffered PES with moderate control of ion energy have a promising future for the application of the contact layers for flexible solar cells.     

ZnS/ZnO heterojunction as photoelectrode: Type II band alignment towards enhanced photoelectrochemical performance

Heterojunction structures are attracting lots of attention for enhancing the electron injection across the interface. The ZnS/ZnO one-dimensional heterojunction film was firstly prepared via a chemical sulfidization following hydrothermal reaction. The heterostructure was characterized as ZnS(blende)/ZnO(wurtzite) shell–core nanorods via XRD, SEM and TEM. A type II band alignment structure of ZnS/ZnO composite was synthesized via a temperate condition proved by PLS and XPS. The values for valence band offset (VBO) and conduction band offset (CBO) were calculated to be 0.96 eV and 1.25 eV, respectively. The special electron structure in the heterojunction helped reduce the energy barrier height at the interface and enhance the separation of photo-generated carriers. Thus, the photoelectrochemical performance was highly improved, and a photocurrent density of 380 μA/cm² at 0.9 V (vs. Ag/AgCl) was obtained.     

Effects of ZnO buffer layer on the optoelectronic performances of GZO films

Gallium-doped zinc oxide (ZnO:Ga=97:3 wt%, GZO) transparent conducting films have been deposited on glass substrates (Corning 1737F), with and without ZnO buffer layers by radio-frequency (r.f.) magnetron sputtering. The effect of ZnO buffer layer deposition parameters on electrical, structural, morphological and optical properties of GZO films (GZO/ZnO/glass) was investigated. The optimization of coating process parameters (r.f. power, sputtering pressure, thickness, annealing) on ZnO buffer layer with multiple qualities based on the orthogonal array has been studied. The electrical resistivity and the average transmittance of the GZO/ZnO/glass films were improved by annealing in vacuum ambient of the ZnO buffer layer. Findings based on the grey relational analysis show that the lowest electrical resistivity of GZO/ZnO/glass films to be about 9.45×10⁻⁴ Ω cm, and visible range transmittance about 85%.     

Visible light driven photosplitting of water using one dimensional Mg doped ZnO nanorod arrays

In the present work, we have introduced Mg doped ZnO nanorods based photoanodes for photoelectrochemical water splitting applications. Vertically aligned Mg doped ZnO nanorods were fabricated by sol-gel and hydrothermal technique. The as-prepared nanorod samples exhibited hexagonal wurtzite structure as confirmed from XRD measurements. We achieved a photocurrent density of 0.35 mA/cm² at 1.5 V vs. Ag/AgCl for 10% Mg doped ZnO photoanode which is 9 times higher than that of undoped ZnO nanorods (0.03 mA/cm²). Incorporation of Mg resulted in faster charge transport and longer life time of electrons with reduced recombination rate. Mg dopant tuned the optical band gap of ZnO and increased the carrier concentration boosting the PEC performance of the photoanodes. Since seawater is one of the most abundant natural resource on earth, we further carried out seawater splitting of 10MgZ under visible light illumination which indicated its high photostability in natural seawater for 5 h of continuous illumination.     

Effective conversion efficiency enhancement of solar cell using ZnO/PS antireflection coating layers

Zinc oxide (ZnO) film was deposited on a porous silicon (PS) layer using a radio frequency sputtering system while the PS layer was prepared by a photoelectrochemical etching method. The ZnO/PS layers were found to be an excellent antireflection coating (ARC), exhibiting exceptional light trapping at wavelengths ranging from 400 to 1000 nm because of their lowest effective reflectance. This, in turn, leads to increase the efficiency of solar cell to 18.15%. The ZnO film was highly oriented with the c-axis perpendicular to the PS layer. The average crystallite size of the PS and ZnO/PS layers were 17.06 and 17.94 nm, respectively. Photoluminescence emission peaks proved the nanocrystalline characteristic of the PS layer and the ZnO film. Raman measurements of the ZnO/PS layers were determined at room temperature and indicate that a high-quality ZnO nanocrystalline film was formed. In the current paper, ZnO/PS ARC layers are attractive and offer a promising technique to produce high-efficiency, low-cost solar cells.     

Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol

Ag promoted ZnO/Al₂O₃ catalysts were prepared by using the incipient wetness impregnation method. The catalytic properties of steam reforming reaction for hydrogen production on the prepared catalysts were evaluated with H₂O:C₂H₅OH molar ratios of 3:1 at 450 °C and atmospheric pressure. Ag promoted ZnO/Al₂O₃ catalysts show higher SRE catalytic activity than ZnO/Al₂O₃ catalysts. H₂ and CH₃CHO are the major products on Ag promoted catalysts, and C₂H₄ is also produced probably due to acid sites on Al₂O₃. SRE mechanism on Ag promoted ZnO/Al₂O₃ catalysts, which contains C-C scission, is different from that on ZnO/Al₂O₃ catalysts. A method based on thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al₂O₃ catalysts show two different types of coking, and suffer higher coke deposition during the steam reforming reaction.     

Spontaneous formation of nanoripples on the surface of ZnO thin films as hole-blocking layer of inverted organic solar cells

A simple method for spontaneous formation of nanoripples on ZnO thin films was developed, and these nanostructured ZnO films were used as hole-blocking layer in inverted organic solar cells. Moreover, the size (height) of nanoripples on ZnO surface could be controlled in the range of several tens of nanometers. Among various ZnO films, surface structures with ∼70 nm-high nanoripples resulted in the best photovoltaic performance of the organic solar cell consisting of a stack of indium tin oxide/ZnO/ regioregular poly (3-hexyl thiophene), phenyl-C₆₁-butyric acid methyl ester/Ag. The power conversion efficiency of inverted organic solar cells consisting of with 70 nm-high ZnO nanoripples (∼3.2%) was higher than that of a relatively flat ZnO surface by a factor of ∼2. Existence of nanoripples on ZnO results in a higher contact area between ZnO and active layer, leading to an enhanced photovoltaic performance.     

Hydrogen production by steam reforming of methanol in a micro-channel reactor coated with Cu/ZnO/ZrO2/Al2O3 catalyst

Hydrogen production by steam reforming of methanol is studied over Cu/Zn-based catalysts (Cu/ZnO, Cu/ZnO/Al₂O₃, Cu/ZnO/ZrO₂/Al₂O₃). Cu/Zn-based catalysts are derived from hydrotalcite-like precursors prepared by a co-precipitation method. The catalysts are characterized by N₂O chemisorption, XRD, and BET surface area measurements. ZrO₂ added to the Cu/Zn-based catalyst enhances copper dispersion on the catalyst surface. Among the catalysts tested, Cu/ZnO/ZrO₂/Al₂O₃ exhibits the highest methanol conversion and the lowest CO concentration in the outlet gas. A micro-channel reactor coated with a Cu/ZnO/ZrO₂/Al₂O₃ catalyst in the presence of an undercoated Al₂O₃ buffer layer exhibits higher methanol conversion and lower CO concentration in the outlet gas than in the absence of an undercoated Al₂O₃ buffer layer. The micro-channel reactor with a undercoated Al₂O₃ buffer layer produces large amounts of hydrogen compared with one without a buffer layer. The undercoated Al₂O₃ buffer layer enhances the adhesion between catalysts and micro-channel walls, which leads to improvement in reactor performance.     

Light trapping and optical losses in microcrystalline silicon pin solar cells deposited on surface-textured glass/ZnO substrates

Influence of front TCO thickness, surface texture and different back reflectors on short-circuit current density and fill factor of thin film silicon solar cells were investigated. For the front TCO studies, we used ZnO layers of different thickness and applied wet chemical etching in diluted HCl. This approach allowed us to adjust ZnO texture and thickness almost independently. Additionally, we used optical modeling to calculate optical absorption losses in every layer. Results show that texture and thickness reduction of front ZnO increase quantum efficiency over the whole spectral range. The major gain is in the red/IR region. However, the higher sheet resistance of the thin ZnO causes a reduction in fill factor. In the back reflector studies, we compared four different back reflectors: ZnO/Ag, Ag, ZnO/Al and Al. ZnO/Ag yielded the best, Al the worst light trapping properties. Furthermore, the Ag back contact turned out to be superior to ZnO/Al for microcrystalline cells. Finally, the smooth ZnO/Ag back contact showed a higher reflectivity than the rough one. We prepared pin cells with rough and smooth ZnO/Ag interface, leaving the roughness of all other interfaces unchanged.     

Microcrystalline silicon solar cell using p-a-Si:H window layer deposited by photo-CVD method

A p-a-Si:H layer, deposited by a photo-assisted chemical vapor deposition (photo-CVD) method, was adopted as the window layer of a hydrogenated microcrystalline silicon (μc-Si:H) solar cell instead of the conventional p-μc-Si:H layer. We verified the usefulness of p-a-Si:H for the p-layer of the μc-Si:H solar cell by applying it to SnO₂-coated glass substrate. It was found that the quantum efficiency (QE) characteristics and solar cell performance strongly depend on the p-a-Si:H layer thicknesses. We applied boron-doped nanocrystalline silion (nc-Si:H) p/i buffer layers to μc-Si:H solar cells and investigated the correlation of the p/i buffer layer B₂H₆ flow rate and solar cell performance. When the B₂H₆ flow rate was 0.2 sccm, there was a little improvement in fill factor (FF), but the other parameters became poor as the B₂H₆ flow rate increased. This is because the conductivity of the buffer layer decreases as the B₂H₆ flow rate increases above appropriate values. A μc-Si:H single-junction solar cell with ZnO/Ag back reflector with an efficiency of 7.76% has been prepared.