Pristine ZnO Films Research Articles

Efficient Quantum Dot Infrared Photovoltaic with Enhanced Charge Extraction via Applying Gradient Electron Transport Layers

AbstractPbS quantum dot (QD) infrared (IR) solar cells that can absorb low‐energy photons are promising photovoltaic devices to improve utilization of sunlight energy by broadening absorption range of the sunlight spectrum to short‐wave infrared region. For PbS QD photovoltaics, depleted heterojunction established between photoactive layer and ZnO electron transport layer (ETL) is critical to determine the performance of devices. However, undesired defects in ZnO films and mismatched energy levels have limited the improvement of device properties. Herein, a novel and simple gradient ZnO ETL is developed for achieving high‐performance QD IR solar cells by depositing a Cs‐doped ZnO thin layer on the pristine ZnO film. The resulting gradient ETL exhibits significantly suppressive trap states and a much smoother surface, efficiently reducing the trap‐assisted nonradiative recombination at the interfaces. Meanwhile, realigned energy levels of the gradient ZnO ETL facilitate the transport and extraction of photogenerated carriers. As a result, the champion device shows a high IR open circuit voltage (VOC) of 0.446 V and IR power conversion efficiency (PCE) of 1.27% under 1100 nm filtered illumination. The VOC and PCE are 0.507 V and 11.04% under AM1.5 illumination, respectively. These results demonstrate a promising strategy for exploiting high‐performance infrared optoelectronic devices.

Degradation of Paracetamol in Distilled and Drinking Water via Ag/ZnO Photocatalysis under UV and Natural Sunlight

The present study demonstrates the synthesis and application of Ag/ZnO powder films (thickness of 4 μm) as photocatalysts for natural sunlight and ultraviolet (UV, 315–400 nm) irradiation. The synthesis procedure is simple and eco-friendly, based on the photo-fixation of silver ions onto commercial ZnO powder via UV illumination for the first time. The photocatalytic efficiency of the newly developed films is evaluated through degradation of paracetamol in distilled and drinking water. Our experimental evidences show that the Ag/ZnO nanostructure films are more active than pristine ZnO films in the photodegradation process. Namely, the photocatalytic efficiency of the films modified with 10−2 M concentration of silver ions achieve the highest degradation (D) percentages for paracetamol in both types of water (Ddistilled = 80.97%, Ddrinking = 82.5%) under natural sunlight. Under UV exposure, the degradation percentages are slightly lower but still higher than those achieved by pure ZnO films (Ddistilled = 53.13%, Ddrinking = 61.87%). It is found that the photocatalytic activity grows in direct proportion to the concentration of Ag+ ions: ZnO < Ag 10−4/ZnO < Ag 10−3/ZnO < Ag 10−2/ZnO. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance and photoluminescence spectroscopy are used to characterize the as-prepared ZnO and Ag/ZnO nanostructures. The improved photocatalytic performance of the Ag/ZnO films is mostly attributed to the combination of excited electron transfer from ZnO to Ag and the inhibition of photogenerated electron–hole pair recombination. Furthermore, Ag/ZnO nanostructure films can retain their photocatalytic activity after three cycles of use, highlighting their potential practical application for the treatment of pharmaceutical wastewater in real-world scenarios where natural sunlight is often more readily available than artificial UV light.

Room temperature excitonic emission in highly aligned ZnO nanostructures prepared by glancing angle Xe+ ion irradiation

Highly aligned ZnO nanostructures have been prepared using glancing angle ion irradiation technique. ZnO thin films were deposited on Si substrates using pulsed laser deposition method. These films were irradiated with 100 keV Xe+ ions of different fluencies ranging from 1 × 1016 to 1 × 1017 ions/cm2 at a glancing angle of 70° with respect to the sample normal. The scanning electron and atomic force microscopy studies reveal a randomly oriented flower petal like structure in the pristine ZnO film. However after ion irradiation, these petals are aligned in the beam direction. Further, with the increase of the ion fluence the petals are transformed to the nanostripe, nanowire like structures and at highest fluence wrinkled structure are formed. Interestingly, on closer examination, these wrinkles consist of nanopillar like structures aligned in the beam direction. The X-ray diffraction analysis reveals that the formed ZnO nanostructures are of hexagonal wurtzite structure with preferred orientation along the c-axis. Further, the ion beam modified nanostructures exhibit interesting photoluminescence emissions in the UV regime. The exciton emission of these ion beam modified ZnO nanostructures has been discussed with the aid of X-ray absorption near edge spectroscopy at O K-edge.

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.

Coupling modification of Fermi level, band flattening and lattice defects to approach outstanding thermoelectric performance of ZnO films via tuning In and Ga incorporation

Band structure modification is found one of the state-of-the-art strategies to control electrical, thermal transport properties, and optimize performance of thermoelectric (TE) materials. Despite being a high-temperature n-type TE material, low-dimensional ZnO structures, specifically thin films normally possess a poor performance due to instability, low electrical, and high thermal conductivities. Herein, it is found that the trade-offs between TE parameters of ZnO films can be simultaneously abated by co-doping In and Ga. The dual incorporation of In and Ga enhances crystallinity, thermal stability, and TE properties up to 573 K. Tuning In and Ga contents not only optimizes carrier concentration associating with Fermi level modification, but also engineers lattice defects contributing to scattering transport mechanisms. Furthermore, the In3+ compensation at substitutional sites, specifically at high temperature, significantly increases density-of-state effective mass due to conduction band flattening. As a result, the coupling modification of Fermi level, band flattening, and lattice defects leads to maintaining electrical conductivity at a medium degree, increasing Seebeck coefficient, and decreasing thermal conductivity, respectively. The TE dimensionless figure of merit (ZT) and power factor (PF) of the films deposited from the In0.01Ga0.04Zn99.95O compound significantly boost to 0.2 and 745.2 µW/mK2 at 573 K, respectively. These values are 1328 and 536% higher than pristine ZnO film; 400 and 258% greater than Ga single-doped ZnO film, respectively. It proposes that our ZnO-based thin films approach the high ZT region (ZT ≥ 0.2) of advanced nanostructured ZnO-based bulks.

Fabricating High-Conduction and High-Transparency Tungsten-Doped Zinc Oxide Films by Pulse Laser Deposition Technique

Utilising a pulse laser deposition technique, 1.0 wt.% tungsten-doped zinc oxide (WZO) films were fabricated under different growth temperatures (200–400 °C), and their structural, optical, morphological, and electrical properties were discussed. The crystalline structures of the WZO target and films were examined by X-ray diffraction (XRD) analysis, and preferred orientations along the strong c-axis (002) were strongly observed for all growth temperatures. All WZO films demonstrated transparencies above 75%, along with a wide spectral range (400–700 nm). Their bandgap values ranged between 3.21 and 3.35 eV and their optimised resistivity, which was significantly influenced by the growth temperature, was measured as 1.97 × 10−3 Ω cm. Further, the electrical characteristics of the WZO films were investigated under different W-doping amounts (1.0–9.0 wt.%) and a constant growth temperature (300 °C), and the results indicated that the carrier mobility showed an opposite tendency to the W-doping percentage. In addition, the elemental compositions of the WZO films and pristine ZnO films were comparatively studied in terms of Zn, O, and W contents, via X-ray photoelectron spectroscopy (XPS) analysis.

A comparative study of the structural and optical properties of transition metals (M = Fe, Co, Mn, Ni) doped ZnO films deposited by spray-pyrolysis technique for optoelectronic applications

A comparative study of the functional properties of transition metals (M = Fe, Co, Mn and Ni) doped ZnO nanostructured films has been performed for applications in optoelectronics and optical devices. Pristine and M-doped ZnO (M = Fe, Co, Mn, Ni) nanostructured films were prepared by spray pyrolysis procedure. The surfaces' morphology, elemental analysis, bonds’ vibrations, structural and optical properties of the samples are studied by a scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), FT-IR spectrophotometry, X-ray diffraction and UV–visible–NIR spectrophotometry, respectively. The structural analysis shows that all samples possess hexagonal wurtzite structures. The Rietveld refinement results reveal noticeable variations in the lattice parameters of the M-doped ZnO nanostructures as compared with those of the pristine one. Due to M-doping, clear deviations in the crystallite size, micro-strain and dislocation density as well as the crystallinity nature are noticed in the host ZnO nanostructures. The UV–visible–NIR spectra analysis reveals that the optical bandgap of the pristine ZnO films varies from 3.36 eV to 3.13 eV, 3.20 eV, 3.30 eV and 3.58 eV due to doping with Fe, Co, Mn and Ni respectively. The effect of M-doping on the Urbach energy and refractive index of the ZnO nanostructures has been explored. To the best of our knowledge, this is the first time that a comparative study on the functional properties of four sprayed transition metals doped in ZnO nanostructures has been reported. These unique findings introduce the sprayed M-doped ZnO nanostructures for new applications in optoelectronics and optical devices.

Modification in the physical properties of nanocrystalline ZnO thin films by Sn/Ni co-doping for transparent conductive oxide applications

In this paper, the modification in the physical properties of spin-coated ZnO thin films upon Sn/Ni co-doping has been presented. All the films possess the hexagonal wurtzite structure of ZnO with varying surface morphologies. Among all the films, the optical transmittance beyond 550 nm wavelength is maximum for the 1 at% Ni-doped ZnO (1NZO) and (1 at% Sn + 1 at% Ni) co-doped ZnO (1T1NZO) films. The optical band gap for the pristine ZnO film is 3.26 eV, which decreases upon doping and co-doping. The PL spectra of the films exhibit the UV emission corresponding to excitonic transitions and the blue and green emissions related to native defects in the ZnO structure. The room-temperature electrical conductivity is found maximum in the 1T1NZO film. The existence of 1T1NZO film with improved optical transmittance and high electrical conductivity supports its use in transparent conductive oxide applications.

Study of front panel electrode coatings for combined visible and short wavelength infrared photodetectors

ZnO-Ga2O3 composite oxide films were RF sputtered and studied how the optical and electrical film properties are affected by the sputtering conditions and film thickness. The aim was to achieve low resistivity and at the same time, high transparency for the visible and short wavelength infrared (SWIR) electromagnetic range for the needs of simple, transparent and indium-free electrodes. A comparison was made with pristine ZnO films. We found that plasma voltage, respectively plasma power plays a crucial role in determining the optoelectronic properties of the deposited films for the fabrication of combined photodetectors. The effective surface roughness of the ZnO-Ga2O3 films varied between 12.9 nm and 16.7 nm when plasma power density varied between 35 and 54 W/cm2, respectively. The increase of the sputtering power narrowed the transmission band to the longer wavelength from the visible spectrum. The transparency in the SWIR range varied between 83 and 96% and decreased with the plasma power increase, while the corresponding transparency in the visible range varied between 60 and 82%. The sheet resistance for the films under the best parameters demonstrating the highest transmission in both electromagnetic ranges showed relatively low sheet resistance of 48 Ω/sq. The potential application of the prepared films is for VIS-IR detectors used in robotics for spatial information gain, gaming and virtual reality and autonomous car navigation, where the day and the night visions are equally important.

Columnar structure growth of Mn-doped ZnO (MZO) thin films by radio frequency co-sputtering and studies on films properties

ABSTRACT The present investigation deals with the columnar structured growth of Mn doped ZnO (MZO) thin films by radio frequency sputtering technique. The X-ray diffraction and the processed electron diffraction pattern confirmed the hexagonal wurtzite structure and the film growth direction along (101) plane. The structure of grown film was compared with the pristine ZnO film and the JCPDS card No. 00-36-1451. The microstructures of the pristine ZnO and MZO films were studied by scanning electron microscopy and atomic force microscopy. The columnar structure growth of MZO was confirmed by transmission electron microscopy using TEM cross-section sample. The presence of constituent elements and their concentrations in the films were recorded by energy dispersive X-ray diffraction spectroscopy. The optical transmittance, absorption edge and band gap of the material were studied by UV-Visible transmission spectroscopy. The field dependent magnetization at room-temperature was analysed by using vibrating sample magnetometer and magneto-optic Kerr effect.

Correlation between defect and magnetism of low energy 800 keV Ar4+ irradiated ZnO thin films

Present work demonstrates ion beam irradiation induced modification in physical properties of ZnO thin films prepared by sol-gel spin coating technique.Different fluenceviz: 5x1014, 1015, 5x1015 and 1016 ions/cm2 were chosen for irradiation.Measurements of structural, optical and magnetic properties of these films have been performed. XRD patterns of the films exhibits wurzite structure ZnO. Optical transmittances of these films were recorded in the wavelength range of 250-1000 nm, and band gap of the respective films have been estimated. Irradiated films did not show significant degradation in the crystalline structure however a noticeable shift of band edge with fluence has been observed and explained. Pristine ZnO film exhibits diamagnetic behaviour at room temperature. However prominent hysteresis has been observed for lowest fluence (5x1014 ions/cm2) irradiated films. The correlation between defects and ferromagnetism has been established in this work.

Evolution of microstructure and photoluminescence of ZnO thin films irradiated with 3 MeV alpha particles

The effects of irradiation with 3 MeV alpha particles on the microstructure and photoluminescence (PL) emission of ZnO thin films were investigated as a function of exposure time between 100 s and 8 h. The analysis of atomic force microscopy (AFM) data reveals modifications of the surface morphology when the irradiation periods exceed 500 s. The roughness of the films exposed for 500 s and 1000 s slightly decreases. The surface becomes almost flat after 8 h irradiation, showing a small density of randomly distributed large aggregates, increased crystallite size and reduced lattice strain and residual stress. The PL spectra of pristine ZnO films present the main characteristic emission bands at 3.25 eV and 2.75 eV. The emission band at 2.75 eV is quenched after irradiation for 100 s, but recovers constantly for irradiation times increasing to 500 s and 1000 s. New emission bands appear at 2.80 eV in the PL spectra of the films exposed to radiation for 500 s, and at 3.10 eV, 3.00 eV, 2.66 eV and 2.64 eV, when the irradiation time increases to 1000 s. The time evolution of radiation induced defects is discussed in relationship with the PL data, the morphology of the films and details of first principles electronic structure of defective models.

Investigations on silver incorporated RF sputtered ZnO thin films

In this work, ZnO thin films were fabricated using RF Magnetron Sputtering technique on ITO substrates. The operating parameters time of deposition, Argon working pressure, distance between target and substrate, oxygen pressure, RF power etc. were optimized so as to get good quality ZnO thin films. The prepared films were characterised using various techniques like XPS, SEM, EDS etc. It was observed that the sheet resistance of the pristine ZnO film was around 200 MΩcm−1 even when coated on ITO substrates. For device level application the resistance of the film must be low. Therefore silver was incorporated by sputtering, and it served the purpose of reducing the sheet resistance of pristine ZnO film. The Ag incorporated films were also characterised by the above mentioned techniques. Detailed compositional analysis was done using depth profiling method of XPS. Resistance measurements were also done in these samples. The sheet resistance decreased (from ∼ 200 MΩcm−1) to a range of about 500 MΩcm−1 by silver incorporation.

Atomic layer deposition of ZnO/TiO2 nanolaminates as ultra-long life anode material for lithium-ion batteries

In this work, we designed ZnO/TiO2 nanolaminates by atomic layer deposition (ALD) as anode material for lithium ion batteries. ZnO/TiO2 nanolaminates were fabricated on copper foil by depositing unit of 26 cycles ZnO/26 cycles TiO2 repeatedly using ALD. ZnO/TiO2 nanolaminates are much more stable than pristine ZnO films during electrochemical cycling process. Therefore, ZnO/TiO2 nanolaminates exhibit excellent lithium storage performance with an improved cycling performance and superior rate capability compared to pristine ZnO films. Moreover, coulombic efficiency (CE) of ZnO/TiO2 nanolaminates is above 99%, which is much higher than the value of pristine ZnO films. Excellent ultralong-life performance is gained for ZnO/TiO2 nanolaminates, retaining a reversible capacity of ~667 mAh g−1 within cut-off voltage of 0.05-2.5 V after 1200 cycles of charge-discharge at 500 mA g−1. Constructing nanolaminates structures via ALD might open up new opportunities for improving the performance of anode materials with large volume expansion in lithium ion batteries.

Effect of implantation of Sm+ ions into RF sputtered ZnO thin film

The effects of implantation of Samarium ions (Sm+), a rare earth ion (RE) on the properties of ZnO films grown on Si (001) substrate by RF sputtering system are presented. The structural properties of the virgin and Sm–implanted ZnO thin films were investigated by Atomic force microscopy, Rutherford backscattering spectroscopy and Raman spectroscopy. Local lattice softening caused by the incorporation of highly mismatched Sm+ (ionic radii 0.096 nm and 0.113 nm for Sm3+ and Sm2+ respectively) into Zn antisites was detected as a red shift in E2 (high) mode likely caused by reduction in the crystallinity of the ZnO film. Photoluminescence on the pristine ZnO film showed a strong near band gap (NBE) emission and an intrinsic defect related blue, green-orange emission. The NBE is suppressed after implantation of Sm+ while the blue, green – orange emission intensities are enhanced as a result of increased structural defects with mismatched charge states. Moreover the effect of varying the concentration of Sm+ ions is presented and compared with predictions made from Stopping and Range of Ions in Matter (SRIM) calculation.

High-Efficiency Air-Stable Colloidal Quantum Dot Solar Cells Based on a Potassium-Doped ZnO Electron-Accepting Layer.

High-efficiency colloidal quantum dot (CQD) solar cells (CQDSCs) with improved air stability were developed by employing potassium-modified ZnO as an electron-accepting layer (EAL). The effective potassium modification was achievable by a simple treatment with a KOH solution of pristine ZnO films prepared by a low-temperature solution process. The resulting K-doped ZnO (ZnO-K) exhibited EAL properties superior to those of a pristine ZnO-EAL. The Fermi energy level of ZnO was upshifted, which increased the internal electric field and amplified the depletion region (i.e., charge drift) of the devices. The surface defects of ZnO were effectively passivated by K modification, which considerably suppressed interfacial charge recombination. The CQDSC based on ZnO-K achieved improved power conversion efficiency (PCE) of ≈10.75% ( VOC of 0.67 V, JSC of 23.89 mA cm-2, and fill factor of 0.68), whereas the CQDSC based on pristine ZnO showed PCE of 9.97%. Moreover, the suppressed surface defects of ZnO-K substantially improved long-term stability under air. The device using ZnO-K exhibited superior long-term air storage stability (96% retention after 90 days) compared to that using pristine ZnO (88% retention after 90 days). The ZnO-K-based device also exhibited improved photostability under air. Under continuous light illumination for 600 min, the ZnO-K-based device retained 96% of its initial PCE, whereas the pristine ZnO-based device retained only 67%.

Cost-effective fabrication of ZnO/g-C3N4 composite film coated stainless steel meshes for visible light responsive photocatalysis

The ZnO and ZnO/g-C3N4 composite films were deposited on stainless steel meshes having different apertures (370, 500, 780 and 1000μm) using a simple cost-effective automated tubular nebulizer spray pyrolysis technique for photocatalytic degradation of organic pollutants. Photocatalytic studies were done for the degradation of methylene blue (MB) and malachite green (MG). The photocatalytic studies reveal that the ZnO/g-C3N4 composite film having smallest aperture exhibits superior photocatalytic activity compared with other meshes. The degradation efficiency of this composite film for MB and MG dyes are 97% (irradiation time-75min) and 95% (irradiation time-45min), whereas for pristine ZnO film the corresponding efficiencies are only 88% and 87%. The aperture size related photocatalytic efficiency variations and the effect of g-C3N4 addition on the photocatalytic efficiency of ZnO films have been addressed with appropriate supporting experimental evidences and reports.

Effect of seed layers (Al, Ti) on optical and morphology of Fe-doped ZnO thin film nanowires grown on Si substrate via electron beam evaporation

The effects of Al and Ti seed layers were studied for undoped and Fe-doped ZnO thin films deposited on n-type Si substrates by electron beam (e-beam) evaporation. The films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The films grown on seed layers showed wurtzite hexagonal crystal nanorod and nanowire structures. A higher angle phase shift was observed in the doped thin films compared to the pristine ZnO films. Microstructural studies confirmed the growth of nanorods and nanowires with average widths of ~32nm and ~8–29nm, respectively. The nanostructures were denser and more crystalline on the Al seed layer than on the Ti seed layer for the doped thin films. However, in the undoped thin films, a more crystalline nature was observed on the Ti seeded layer than the Al seeded layer.

Perovskite solar cells for roll-to-roll fabrication

Perovskite solar cell (PSCs) is considered as the game changer in emerging photovoltaics technology. The highest certified efficiency is 22% with high temperature processed (∼500 °C) TiO2 based electron transport layer (ETL). High temperature process is a rudimentary hindrance towards roll-to-roll processing of PSCs on flexible substrates. Low temperature solution process ( . The mixed organic perovskite (MA0.6 FA0.4 PbI3 ) devices with Al doped ZnO (AZO) ETL demonstrate average cell efficiency over 16%, which is the highest ever reported efficiency for this device configuration. The energy level alignment and related interfacial charge transport dynamics at the interface of ZnO and perovskite films and the adjacent charge transport layers are investigated. Significantly improved device stability, hysteresis free device photocurrent have been observed in MA0.6 FA0.4 PbI3 cells. A systematic electrochemical impedance spectroscopy, frequency dependent capacitance spectra, surface morphology and topography characterization have been conducted to understand the role of interfacial electronic properties between perovskite and neighbouring layers in perovskite device. A standardized degradation study, interfacial electronic property and capacitive spectra analysis of aged device, have been measured to understand the enhanced device stability in mixed MA0.6 FA0.4 PbI3 cells. Slow perovskite material decomposition rate and augmented device lifetime with AZO based devices have been found to be correlated with the more hydrophobic and acidic nature of AZO surface compared to pristine ZnO film.

Surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film

A surface acoustic wave (SAW) resonator with ZnO/SiO2 (ZS) composite film was used as an ammonia sensor in this study. ZS composite films were deposited on the surface of SAW devices using the sol–gel method, and were characterized using SEM, AFM, and XRD. The performance of the sensors under ammonia gas was optimized by adjusting the molar ratio of ZnO:SiO2 to 1:1, 1:2 and 1:3, and the sensor with the ratio of ZnO to SiO2 equaling to 1:2 was found to have the best performance. The response of sensor was 1.132kHz under 10ppm NH3, which was much higher than that of the sensor based on a pristine ZnO film. Moreover, the sensor has good selectivity, reversibility and stability at room temperature. These can be attributed to the enhanced absorption of ammonia and unique surface reaction on composite films due to the existence of silica.