The modification of the characteristics of ZnO nanofibers by TCNQ doping content

In this study, the electrical properties of an Al/p-Si metal/semiconductor photodiodes with Tetracyanoquinodimethane–Polyvinyl chloride (TCNQ–PVC) and PVC–TCNQ:ZnO interfacial layers were investigated. Growing of the interfacial layers on p-Si were fulfilled using electrospinning method as a fiber form. Al metallic and ohmic contacts were deposited via physical vapor deposition method. Scanning electron microscopy (SEM) pictures of the devices were captured to examine the morphology of the structure. Within the scope of electrical characterization, I–V measurements of the Al/PVC–TCNQ/p-Si and Al/PVC–TCNQ:ZnO/p-Si devices were accomplished both in the dark and under illumination conditions. Various device parameters, such as ideality factor and barrier height values were determined from I–V characteristics. Although the ideality factor values were obtained as 8.47 and 6.85 for undoped and ZnO-doped Al/PVC–TCNQ/p-Si diodes, the barrier height values were calculated as 0.84 for both devices. When a comparison was made between ZnO doped and undoped Al/PVC–TCNQ/p-Si diodes, it was evaluated that the rectification and photoresponse properties of the heterojunction diode was improved with ZnO dopant.

     

Properties of PEDOT:PEG/ZnO/p-Si heterojunction diode

The zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene) bis-poly(ethyleneglycol) (PEDOT:PEG) films were deposited on p-Si substrate by sputter and spin coating methods, respectively. An organic/inorganic heterojunction diode having PEDOT:PEG/ZnO on p-Si substrate was fabricated. The barrier height (BH) and the ideality factor values for the device were found to be 0.82 ± 0.01 eV and 1.9 ± 0.01, respectively. It has been seen that the value of BH is significantly larger than those of conventional Au/p-Si metal–semiconductor contacts. The PEDOT:PEG/ZnO/p-Si heterostructure exhibits a non-ideal I–V behavior with the ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. The modified Norde's function combined with conventional forward I–V method was used to extract the parameters including the barrier height and series resistance. At the same time, the physical properties of ZnO and PEDOT:PEG films deposited by sputter and spin coating technique, respectively, were investigated at room temperature. The obtained results indicate that the electrical parameters of the diode are affected by structural properties of ZnO film and PEDOT:PEG organic film.     

Growth and temperature dependent characterization of pulsed laser deposited Ag/n-ZnO/p-Si/Al heterojunction

The Ag/n-ZnO/p-Si(100)/Al heterojunction diodes were fabricated by pulsed laser deposition of zinc oxide (ZnO) thin films on p-type silicon. The X-ray diffraction analysis shows the formation of ZnO thin film with hexagonal structure having strong (002) plane as preferred orientation. The energy band gap of ZnO films simultaneously deposited on quartz substrate was calculated from the measured UV–Visible transmittance spectra. High purity vacuum evaporated silver and aluminum thin films were used to make contacts to the n-ZnO and p-silicon, respectively. The current–voltage and capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterostructures were measured over the temperature range of 80–300 K. The Schottky barrier height and ideality factor were determined by fitting of the measured current–voltage data into thermionic emission diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior. This decrease in barrier height and increase in ideality factor at low temperature are attributed to the occurrence Gaussian distribution of barrier heights. The capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterojunction diode were also studied over the wide temperature range. Capacitance–voltage data are used to estimate the barrier height and impurity concentration in n-type ZnO.     

Improved efficiency of organic/inorganic photovoltaic devices by electrospun ZnO nanofibers

The efficiency of the photovoltaic (PV) device based on P3HT and PCBM bulk heterojunction is improved by introducing small-diameter electrospun ZnO diffused nanofibers network. Diameter, diffusion and melting of nanofibers are controlled by calcination temperature. The thickness of the active layer is optimized for efficient PV devices by varying electrospinning (ES) time. Increased nanofiber's mat thickness by an increase in electrospinning time beyond a certain optimum value reduces the device performance due to increased series resistance, increased traps and reduced blend infiltration through the nanofiber pores. ES time suggests optimized active area for energy absorption and exciton dissociation. In this study, we report the improvement in power conversion efficiency (PCE) from 0.9% to 2.23%, for optimum ES time (∼300 s).     

Carrier transport mechanism on ZnO nanorods/p-Si heterojunction diodes with various atmospheres annealing hydrothermal seed-layer

Annealing in various atmospheres (vacuum, N₂, and O₂) was employed for a hydrothermal seed-layer. The influence on ZnO nanorods (NRs) and carrier transport of ZnO NRs/p-Si heterojunction diodes (HJDs) was investigated. In this work, a hydrothermal method was employed to prepare a seed-layer on a Si substrate, and then annealing at 450 °C in various atmospheres was carried out to improve the subsequent growth of ZnO NRs according to the same method. Observations indicated that ZnO NRs with an O₂-annealed seed-layer have a higher nucleation density and absorb fewer OH groups or O₂⁻ ions, and hence they have fewer defect-level centres. This leads to a very large rectification ratio of 1.9 × 10⁵ in the ZnO NRs/p-Si HJDs because oxygen atoms compensate for the oxygen vacancy-related defects. More band-gap states are present at the ZnO/p-Si interface for the vacuum annealing sample, and this enables recombination-tunnelling transport with a rather large ideality factor of 7 at forward voltage less than 0.7 V. In contrast, diffusion-recombination transport was obtained in the N₂- and O₂-annealed samples with ideality factors as low as 2.4 and 2.2, respectively.     

A novel poly(triazine imide) hollow tube/ZnO heterojunction for tetracycline hydrochloride degradation under visible light irradiation

A novel poly(triazine imide) hollow tube (PTI)/ZnO heterojunction was prepared by a molten salts method. The photocatalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy, respectively. The results showed the ZnO nanoparticles were successfully coupled into the PTI hollow tube to form PTI/ZnO heterojunction. The degradation of tetracycline hydrochloride (TC) under visible light irradiation was used to evaluate the activity of photocatalyst. The PTI/ZnO-6?wt% heterojunction exhibits the highest photocatalytic activity, which can degrade almost all TC within 90?min. The kinetic constant of degradation reaction with PTI/ZnO-6?wt% heterojunction (0.034?min^?1) is about 5 times as high as that of the PTI (0.0070?min^?1). A possible photocatalytic mechanism for heterojunction according to the energy-band theory was proposed.     

Preparation of ZnO/graphene heterojunction via high temperature and its photocatalytic property

This paper introduces a novel electrochemical route for preparing the ZnO/graphene heterojunction composite via high temperature. This process includes: (1) depositing the electrochemically reduced graphene oxide (ERGO) on ITO glass via cyclic voltammetry; (2) pulse plating a zinc (Zn) layer on the ERGO; (3) thermally treating the Zn/ERGO composite and “in situ” to obtain the ZnO/ERGO composite. SEM characterizations revealed that the Zinc Oxide (ZnO) particles were homogenously distributed on the surface of graphene sheets. XRD and Raman spectra found a ZnCO₃ phase in the ZnO/ERGO composite, which demonstrated that when the Zn film transformed into ZnO particles during thermal treatment, Zn also reacted with graphene and formed a ZnCO₃ intermediate layer at the interface between ZnO and ERGO via short-range diffusion. Compared with the heterojunction formed from regular chemical route, the present process provided a tight contact and combination between ZnO and ERGO, which eventually led to a heterojunction between ZnO and graphene sheets. This heterojunction exhibited great improvement for separation efficiency of photo-generate electron–hole pairs. Experimental results of ultraviolet–visible (UV–Vis) light catalysis demonstrated that the photocatalytic activity of the ZnO/ERGO composite had been greatly improved, and exhibited a value of three times higher than that of pure ZnO.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

Electrical characteristics and stability of gold and palladium Schottky contacts on ZnO nanorods

The electrical characteristics and stability of Pd and Au Schottky contacts on ZnO nanorods grown on glass substrate have been investigated. The nanorods were grown using the aqueous chemical growth method. The nanorods were characterized with scanning electron microscopy (SEM), x-ray diffraction (XRD) and photoluminescence (PL). Prior to the metal contact deposition, an insulating PMMA layer was deposited between the nanorods. The best-produced Schottky contact was an as-deposited Pd/ZnO contact with an ideality factor of 1.74 ± 0.43 and a barrier height of 0.67 ± 0.09?eV. The relatively high ideality factor indicates that the current transport cannot be described by pure thermionic transport. The presence of surface states due to the high evaporation pressure is probably the reason for the high ideality factor. Post metal deposition annealing at 150?°C for 30?min in air lowered the barrier height and decreased the Au/ZnO ideality factor but increased it for Pd/ZnO. The current follows ohmic behavior when the applied forward bias, V(forward), is lower than 0.1?V, whereas for V(forward) between 0.1 and 0.45?V the current follows I～exp(cV), and at higher forward biases the current-voltage characteristics follow the relation I～V(2), indicating that the space-charge current-limiting mechanism is dominating the current transport.     

ZnO纳米棒/ZnCo2O4纳米片异质结的制备及其光电探测性能

ZnO纳米材料异质结是构筑高性能紫外光电探测器的有力候选之一。本工作中, 设计并制备了一种新型ZnO纳米棒/ZnCo₂O₄纳米片异质结, 研究了其电学性能及光电探测性能。使用油水界面自组装, 将ZnCo₂O₄纳米片在ITO玻璃上组装为均匀的薄膜; 通过调控ZnO种子层厚度, 在ZnCo₂O₄纳米片薄膜上水热生长了取向一致、密度适中的ZnO纳米棒阵列, 获得了高质量的ZnO纳米棒/ZnCo₂O₄纳米片异质结。该异质结具有优良的整流特性, 整流比达到673.7; 其工作在反偏状态时, 光暗电流比超过2个量级, 紫外-可见判别比为29.4, 在光电探测中有良好的波长选择特性。研究表明, 该异质结有潜力应用于构筑高性能紫外光电探测器。     

Combination of surface texturing and nanostructure coating for reduction of light reflection in ZnO/Si heterojunction thin film solar cell

In this study, we report a single heterojunction solar cell based on n-type zinc oxide/p-type silicon. Three different solar cells were fabricated based on ZnO thin film on Si substrate, ZnO nanorods on Si substrate, and ZnO nanorods on micro-pyramidal structure of Si substrate. The comparison between these three kinds of solar cells was studied. Pyramidal structure of silicon was fabricated using chemical etching technique of p-type Si (100). The chemical solution consists of NaOH, isopropyl alcohol and hydrazine hydrate. The results showed that Si micro-pyramids can enhance optical absorption of Si substrates by increasing surface area and entrapping of incident light. For fabrication of uniform ZnO nanorods, a seed layer of ZnO was deposited on Si substrates via radio frequency magnetron sputtering technique. This layer can be used as an active n-type material in heterojunction solar cells as well. ZnO nanostructures can increase light absorption due to their high specific surface area. The combination of ZnO nanorods and Si micro-pyramids can enhance light trapping effect and increase the efficiency of solar cells. The structural and morphology of samples were studied using field-emission scanning electron microscopy, atomic force microscopy and X-ray diffractometry while the optical properties were investigated using photoluminescence and reflectance spectrometry. The efficiency and fill factor of solar cells were obtained from current–voltage characteristics using a solar simulator and a source-meter. The results showed that the efficiency of solar cell based on nanostructures of ZnO/micropyramids of Si is highly increased due to high anti-reflective behavior of this sample.     

Properties of sol–gel synthesized n-ZnO/n-GaN (0001) isotype heterojunction

The electronic and optical properties of sol–gel synthesized n-ZnO/n-GaN (0001) isotype heterojunction were reported. By incorporating ZnO–GaN with the same wurtzite structure and the similar lattice constant, a heterojunction was fabricated. The junction properties were evaluated by measuring the electrical characteristics. The n-ZnO/n-GaN heterostructure exhibits a non-ideal I–V behavior with the ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. The forward turn on voltage is about 0.7 V and the reverse breakdown voltage is more than 2 V. The optical band gaps of the ZnO film and GaN using optical absorption method were found to be 3.272 eV and 3.309 eV, respectively. The fundamental absorption edge in the film is formed by the direct allowed transitions.     

Characteristics of n-type ZnO nanorods on top of p-type poly(3-hexylthiophene) heterojunction by solution-based growth

We report that ZnO nanorods (NRs) are grown on an organic layer of poly(3-hexylthiophene) (P3HT) using a modified seeding layer. Thus, ZnO NRs/P3HT heterojunction light-emitting diodes could be fabricated using the hydrothermal method, in which ZnO acts as an n-type material and P3HT as a p-type material. The ZnO NRs improve the electron transportation in the devices. A three-fold enhancement of current density of the device is observed due to the NRs formed on the P3HT. The electroluminescence (EL) of the optimized ZnO-based device is 1.5 times larger than that without NRs. The influence of the P3HT thickness for the EL spectrum is also discussed.     

ZnO nanoparticles and polyaniline blend as an active layer for bulk heterojunction solar cell applications

In this work, a facile and straightforward procedure was introduced to prepare a blend as an active layer for hybrid solar cell applications. The active layer consisting of a blend of ZnO nanoparticles (NPs) and polyaniline (PANI) dispersions was deposited by spin coating on ITO covered glasses. The current density–voltage characteristics were studied under AM1.5G standard illumination, without any encapsulation process. Also, the samples were studied using UV–Vis spectroscopy, energy dispersive X-ray spectroscopy (EDS) and field emission-scanning electron microscopy. The investigation is limited just to the active layer, so the cells were fabricated without any interlayer. The effect of various volume ratios of ZnO–NPs:PANI solutions, thickness and the annealing temperature of the active layer on the open circuit voltage and the short circuit current density of the cells were investigated. Moreover, the blending time of ZnO–NPs:PANI dispersions as a significant factor for achieving the optimum results were studied.     

High density Si/ZnO core/shell nanowire arrays for photoelectrochemical water splitting

Si/ZnO core/shell nanowire (NW) arrays were fabricated using atomic layer deposition of ZnO shell on n-Si NW arrays prepared by metal assisted electroless etching method. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were utilized to characterize the core/shell structures. Water splitting performance of the core/shell structures was preliminarily studied. The Si/ZnO core/shell NW arrays yielded significantly higher photocurrent density than the planar Si/ZnO structure due to their low reflectance and high surface area. The photoelectrochemical efficiency was found to be 0.035 and 0.002 % for 10 μm-long Si/ZnO NW array and planar Si/ZnO sample, respectively. These results suggested that core/shell structure is superior to planar heterojunction for PEC electrode design. We demonstrated the dependence of photocurrent density on the length of the core/shell array, and analyzed the reasons why longer NW arrays could produce higher photocurrent density. The relationship between the thickness of ZnO shell and the photoconversion efficiency of Si/ZnO NW arrays was also discussed. By applying the core/shell structure in electrode design, one may be able to improve the photoelectrochemical efficiency and photovoltaic device performance.     

自供能ZnO/ZnS异质结紫外探测器的性能研究

用化学浴法在ZnO纳米棒表面沉积ZnS制备出ZnO/ZnS核壳纳米棒阵列,使用SEM、XRD和XPS等手段表征了样品的形貌、结构和成分。结果表明,ZnO/ZnS核壳纳米棒阵列表面粗糙,生长致密、分布均匀,其平均直径约为150 nm。以Pt为对电极组装的自供能ZnO/ZnS异质结紫外探测器,对紫外光具有很好的探测性能,能循环工作且性能稳定。这种探测器对微弱的紫外光也有较强的响应和较高的光敏性,且随着光强度的提高光电流密度线性增大。与自供能ZnO纳米棒紫外探测器相比,ZnO/ZnS异质结紫外探测器具有更高的响应速度,上升时间和下降时间分别提高到0.02 s和0.03 s。     

Investigation of conductive and transparent Al-doped ZnO/Ag/Al-doped ZnO multilayer coatings by electron beam evaporation

Multilayer coatings consisting of thin silver layer sandwiched between layers of Al-doped ZnO (AZO) were prepared by electron beam evaporation. The optical and electrical performances of AZO/Ag/AZO multilayers were investigated. Optimization of the multilayer coatings resulted with low sheet resistance of 7.7 Ω/sq and transmittance of 85%. The influence of thickness of each layer on the optic and electrical performance was analyzed. The sheet resistance of the multilayer was reduced to 5.34 Ω/sq. and the average transmittance was improved to 90% by the thermal treatment. The coatings had satisfactory properties of low resistance, high transmittance and thermal stability.     

Fabrication and electrical characterization of p-Cu2O/n-ZnO heterojunction

The conducting metal oxide (ZnO, Cu2O) films were used for fabrication of p-n heterojunction by rf sputtering and electrodeposition techniques respectively. The as synthesized films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), UV spectroscopy and electrical techniques. The electrical properties of the p-Cu2O/n-ZnO heterojunction were examined using the current-voltage measurements. The current-voltage (I-V) result showed that potential barrier was higher than the turn-on voltage, which was attributed to the presence of the interface defect states. The PN junction parameters such as ideality factor, barrier height, and series resistance were determined using conventional forward bias current-voltage characteristics. The annealing of Cu2O increase the crystallinity size and which enhance the photo current from 1.6 mA/cm2 to 3.7 mA/cm2. The annealing of respective film resulted in a decrease of these parameters with an increase in efficiency of solar cell from 0.14% to 0.3% at 350 degrees C.     

Controlled growth of ZnO nanomaterials via hydrothermal method: effect of buffer layer

We report a facile solution-based method for the controlled growth of ZnO nanomaterials on an AIN/Si substrate. A ZnO buffer layer was coated on the substrate before growing the ZnO nano-materials. The shape of the ZnO nanomaterials changed from nanosheet to nanorod as the thickness of the ZnO buffer layer increased. Doping of the buffer layer with Ga decreased the average grain size of the ZnO buffer layer, which resulted in the growth of longer and thinner ZnO nanorods on the buffer layer. The UV sensing results of the ZnO nanorod-based device revealed that the aspect ratio of the ZnO nanorods is crucial for enhancing the performance of the device.     

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The effect of ZnO film depositions using various film deposition methods such as magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) on the photovoltaic properties of ZnO–Cu₂O heterojunction solar cells is described in this report. In addition, the relationship between the resulting photovoltaic properties and the film deposition conditions such as supply power and substrate arrangement was investigated in Al-doped ZnO (AZO)–Cu₂O heterojunction devices fabricated using AZO thin films prepared by d.c. magnetron sputtering (d.c.MSP) or r.f. magnetron sputtering (r.f.MSP). The results showed that the measured photovoltaic properties of devices fabricated with films deposited on substrates oriented perpendicular to the target were better than those of devices fabricated with films deposited on substrates oriented parallel to the target. It was also found that ZnO film depositions under conditions where a relatively weaker oxidizing atmosphere was used yield better properties than films derived from MSP, which utilizes a high-density and high-energy plasma. Using VAPE and PLD, for example, high efficiencies of 1.52 and 1.42%, respectively, were obtained under AM2 solar illumination in devices fabricated at a substrate temperature around 200 °C.