Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties

Synthesis of zinc oxide (ZnO) nanowires (NWs) grown via vapor-liquid-solid (VLS) process using Gold (Au) as a catalyst metal on aluminum-doped zinc oxide (AZO) seed layer is reported in the present work. During the growth procedure, the nucleation process helps us to obtain ZnO nanowires with Au on the tip, confirming the VLS growth mechanism. Different morphologies were obtained after the variation in the growth parameters in the VLS process, and further, their role in the photocatalytic performance was studied. Changes in the structural properties of nanowires allowed us to modify the aspect ratio and surface area of the nanostructures. X-ray diffraction (XRD) showed that the principal orientation of the nanowires was (002) in the present case. Scanning electron microscopy (SEM) showed the structural properties of 1-D nanostructures (nanowires), and statistical analysis revealed that the average diameter in the present case was found to be varied from 57 to 85 nm. Scanning transmission electron microscopy (STEM) technique revealed the different elements present on the surface of ZnO NWs. Further, the compositional profile of nanostructures was cross-verified using Energy dispersive Spectroscopy (EDS). Photoluminescence (PL) and UV Visible studies were employed to study the optical properties of nanowires. UV–Vis measurements showed the role of different structural properties of nanowires on the absorption spectra, especially in the visible region. The ZnO nanowires were tested as photocatalysts for hydrogen production from water splitting reaction, and it was found in particular nanowires with random orientation with optimal diameter distribution show the stable and highest photocatalytic performance.     

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.     

Effects of ITO precursor thickness on transparent conductive Al doped ZnO film for solar cell applications

Al doped ZnO (AZO) film was continuously deposited on ITO precursor on glass substrate by d.c. magnetron sputtering. The thickness of ITO was varied from 30 to 120 nm in order to investigate the effect of ITO thickness on crystallinity of AZO film. X-ray diffraction measurement shows that AZO film grown on ITO has an enhanced (0 0 2) preferred orientation as the ITO thickness was increased. The crystalline structure improvement of AZO film with an increase of ITO precursor thickness is due to the near-epitaxial growth of AZO on ITO precursor. As the ITO thickness was increased, mobility of AZO film by the Hall measurement was significantly increased from 5.4 cm²/V s (no ITO) to 23.6 cm²/V s (ITO 120 nm), and resistivity was about 81.7% improved from 1.99×10⁻³ to 3.63×10⁻⁴ Ω cm. The AZO films with ITO revealed excellent average transmission of visible (90.0%) and NIR (89.6%) regions, whereas those of AZO film without ITO were 82.1% and 88.1%, respectively. The haze values of AZO film with ITO of 90 and 120 nm are similar or higher than those of AZO film without ITO. The surface textured AZO film with ITO precursor is promising for optoelectronic applications such as the front TCO of thin film solar cells.     

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.     

Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells

Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO₂ and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO_x EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance.     

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.     

Fabrication of flexible CdTe solar modules with monolithic cell interconnection

CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl₂ activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF₂ on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF₂ coating, respectively.Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm² were developed by interconnection of 11 solar cells in series.     

Effects of graphene counter electrode and CdSe quantum dots in TiO2 and ZnO on dye‐sensitized solar cell performance

Fabrication and performance study of dye‐sensitized solar cells using different counter electrodes and photoanodes is reported. Spin coated, E‐beam coated platinum, and graphene electrodes were used as counter electrodes. Different combinations of TiO₂ nanoparticle and ZnO nanorods (NRs) with CdSe quantum dots were prepared and used as photoanodes. The photoanodes comprising of both ZnO NRs and TiO₂ nanoparticles have shown improved performances in short‐circuit current density and open‐circuit voltage comparing the devices fabricated using only ZnO NR or TiO₂ nanoparticles. The inclusion of CdSe quantum dots has been found to increase the performance of dye‐sensitized solar cell for all the photoanodes. In case of counter electrodes, the cells fabricated with graphene showed improved performance. Copyright © 2014 John Wiley & Sons, Ltd.     

电纺AZO NWs电子传输层提高钙钛矿太阳电池性能的研究

研究通过静电纺丝技术制备不同掺铝比例的氧化锌纳米线(AZO NWs),并将其作为电子传输层来提高钙钛矿太阳电池效率.首先使用静电纺丝和煅烧工艺制备出表面光滑连续、形貌均匀、长径比达～105的AZO NWs,然后将AZO NWs作为电子传输层应用于钙钛矿太阳电池.与单层SnO2电子传输层的电池相比,加入AZO NWs后可...     

Ultra-thin metal layer passivation of the transparent conductive anode in organic solar cells

Efficiency of organic solar cells shows a strong improvement when the transparent conductive anode (indium tin oxide—ITO, aluminium-doped zinc oxide—AZO, fluorine-doped tin oxide—FTO), is covered with an ultra-thin metallic film. It is shown that the best results are achieved with a gold film (0.5 nm). The efficiency of the solar cells using AZO or FTO is improved up to one order of magnitude, while in the case of ITO it is at least 50%. It is shown that if the matching between the work function of the anode and the highest occupied molecular orbital (HOMO) of the organic electron donor is the most important factor limiting the hole transfer efficiency, others factors such as transparent conductive oxide (TCO) surface roughness and adhesion of the organic layer are also key factors.     

Characteristics of indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrode grown by dual target DC sputtering at room temperature for low-cost organic photovoltaics

We compared the electrical, optical, structural and surface properties of indium-free Ga-doped ZnO (GZO)/Ag/GZO and Al-doped ZnO (AZO)/Ag/AZO multilayer electrodes deposited by dual target direct current sputtering at room temperature for low-cost organic photovoltaics. It was shown that the electrical and optical properties of the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes could be improved by the insertion of an Ag layer with optimized thickness between oxide layers, due to its very low resistivity and surface plasmon effect. In addition, the Auger electron spectroscopy depth profile results for the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes showed no interfacial reaction between the Ag layer and GZO or AZO layer, due to the low preparation temperature and the stability of the Ag layer. Moreover, the bulk heterojunction organic solar cell fabricated on the multilayer electrodes exhibited higher power conversion efficiency than the organic solar cells fabricated on the single GZO or AZO layer, due to much lower sheet resistance of the multilayer electrode. This indicates that indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes are a promising low-cost and low-temperature processing electrode scheme for low-cost organic photovoltaics.     

Hydrogen sensing properties of ZnO nanorods: Effects of annealing,temperature and electrode structure

In this study, the hydrogen (H₂) sensing properties of vertically aligned zinc oxide (ZnO) nanorods were investigated depending on annealing, Pd coating, temperature and electrode structure. ZnO nanorods were fabricated by using hydrothermal method on a glass substrate and an indium tin oxide (ITO) coated glass substrate. In order to determine the effects of annealing on the H₂ sensor performance, the nanorods were heated at 500 °C in dry air. H₂ sensing measurements were done in the temperature range of 25–200 °C. It was found that, the sensor response of Pd coated ZnO nanorods were much higher than the un-coated nanorods due to the catalytic effect of Pd thin film. Moreover, the un-annealed samples showed better sensor response than the annealed samples due to the number of oxygen deficiency. In addition, the lateral electrode structure showed higher sensor response than the sandwich electrode structure.     

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.     

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%.     

Recent advances in photo‐anode for dye‐sensitized solar cells: a review

Dye‐sensitized solar cell (DSSC) attracts immense interest in the last few decades due to its various attractive features such as low production cost, ease of fabrication and relatively high conversion efficiency, which make it a strong competitor to the conventional silicon‐based solar cell. In DSSC, photo‐anode performs two important functions, viz. governs the collection and transportation of photo‐excited electrons from dye to external circuit as well as acts as a scaffold layer for dye adsorption. The photo‐anode usually consists of wide band gap semiconducting metal oxides such as titanium dioxide (TiO₂) and zinc oxide (ZnO) deposited on the transparent conducting oxide substrates. The morphology and composition of the semiconductor oxides have significant impact on the DSSC photovoltaic performance. Therefore, enormous research efforts have been undertaken to investigate the influences of photo‐anode modifications on DSSC performance. The modifications can be classified into three categories, namely interfacial modification through the introduction of blocking and scattering layer, doping with non‐metallic anions and metallic cations and replacing the conventional mesoporous semiconducting metal oxide films with one‐dimensional or two‐dimensional nanostructures. In the present review, the previously mentioned modifications on photo‐anode are summarized based on the recent findings, with particular emphasis given to published works for the past 5 years. Copyright © 2017 John Wiley & Sons, Ltd.     

Deposition of Ag-based Al-doped ZnO multilayer coatings for the transparent conductive electrodes by electron beam evaporation

Silver-based Al-doped ZnO (AZO) multilayer coatings were prepared on glass by e-beam evaporation techniques. Optimization of the deposition conditions of both AZO and Ag layers were performed for better electrical and optical properties. The properties of the multilayers were affected by the deposition process of both AZO and Ag layers. The best multilayer coatings exhibit low sheet resistance of 5.34 Ω/sq and transmittance of more than 85%. The coatings have satisfactory properties of low resistance and high transmittance for application as transparent conductive electrodes.     

LPCVD ZnO-based intermediate reflector for micromorph tandem solar cells

The use of zinc oxide (ZnO) based intermediate reflector (ZIR) in micromorph solar cells using low pressure chemical vapor deposition (LPCVD) was investigated. The influences of deposition temperature and dopant gas concentration on grain size and lateral electrical conductivity measurements are presented. Further ZIR deposition conditions were then directly evaluated in micromorph solar cell devices. Their electrical performances were compared to reference cells and cells incorporating silicon oxide based intermediate reflector. It is shown that both reduced ZIR deposition temperature and increased total flow rate allow for better performing devices with increased shunt resistance, as further supported by lock-in thermography shunt imaging. Relative micromorph efficiency increase of above 7% is shown with thin ZnO layers, along with absence of loss or even small increase of total current in the whole structure compared to cells without intermediate reflector.     

Si/ZnO heterostructures for efficient diode and water-splitting applications

We have developed thin zinc oxide (ZnO) layers protected highly conductive p-type silicon (Si) electrodes and investigated their diode and photoanode characteristics. ZnO layers have been deposited on the glass as well as p-Si substrates at a temperature of 400 °C by pulsed spray pyrolysis method. The crystal structure, surface morphology, and phase purity of the layers along with electrical characteristics of the heterostructures were investigated. Finally, the photocatalytic water oxidation performance of the ZnO/Si structures was studied in an alkaline electrolyte solution (pH = 10). The as-grown devices exhibited excellent diode characteristics with a turn-on voltage of 4.5 V, and applied bias-voltage dependent carrier transport mechanisms. As compared to bare Si, ZnO coated Si-based PEC devices showed good stability and durability along with very low onset potential of 0.07 V versus Ag/AgCl.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Vertically aligned ZnO nanowire arrays in Rose Bengal-based dye-sensitized solar cells

We fabricate dye-sensitized solar cells (DSSC) using vertically oriented, high density, and crystalline array of ZnO nanowires, which can be a suitable alternative to titanium dioxide nanoparticle films. The vertical nanowires provide fast routes or channels for electron transport to the substrate electrode. As an alternative to conventional ruthenium complex, we introduce Rose Bengal dye, which acts as a photosensitizer in the dye-sensitized solar cells. The dye energetically matches the ZnO with usual KII₂ redox couple for dye-sensitized solar cell applications.