The role of TiO2 addition in ZnO nanocrystalline thin films: Variation of photoelectrochemical responsivity

In this study, the effects of TiO₂ addition on the physical and photoelectrochemical properties of ZnO thin films have been investigated. The (TiO₂)_x–(ZnO)_1−x nanocomposite thin films were dip-coated on both glass and indium tin oxide (ITO)-coated conducting glass substrates with various values of x, specifically 0, 0.05, 0.1, 0.25 and 0.5. Optical properties of the samples were studied by UV–vis spectrophotometry in the range of 300–1100 nm. The optical spectra of the nanocomposite thin films showed high transparency in the visible region. The optical bandgap energy of the (TiO₂)_x–(ZnO)_1−x films increased slightly with increasing values of x. The crystalline structure of the nanocomposite films was investigated by X-ray diffraction, which indicated the formation of ZnO nanocrystals in the thin films with x < 0.5. Moreover, the crystallinity of the films decreased with increasing values of x. The surface chemical composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS), which revealed stoichiometric ZnO and TiO₂ on the surfaces of the films. The photoelectrochemical properties of the samples were also characterized using a high-pressure xenon light source and KOH electrolyte. The addition of 10 mol% (x = 0.1) TiO₂ to the ZnO thin films resulted in the best photoresponse in the visible region of the solar spectrum. In addition, the effect of TiO₂ concentration on the electrical properties and the flat-band potential of the (TiO₂)_x–(ZnO)_1−x system was studied by impedance spectroscopy; x = 0.1 exhibited the highest donor density and charge-transfer resistance.     

Influence of ZnO buffer layer on AZO film properties by radio frequency magnetron sputtering

Transparent conductive films of Al-doped zinc oxide (AZO) were deposited on glass substrates under various ZnO buffer layer deposition conditions (radio frequency (r.f.) power, sputtering pressure, thickness, and annealing) using r.f. magnetron sputtering at room temperature. This work investigates the influence of ZnO buffer layer on structural, electrical, and optical properties of AZO films. The use of grey-based Taguchi method to determine the ZnO buffer layer deposition processing parameters by considering multiple performance characteristics has been reported. Findings show that the ZnO buffer layer improves the optoelectronic performances of AZO films. The AZO films deposited on the 150-nm thick ZnO buffer layer exhibit a very smooth surface with excellent optical properties. Highly c-axis-orientated AZO/ZnO/glass films were grown. Under the optimized ZnO buffer layer deposition conditions, the AZO films show lowest electrical resistivity of 6.75 × 10⁻⁴ Ω cm, about 85% optical transmittance in the visible region, and the best surface roughness of R_a = 0.933 nm.     

Study of Pb-based and Pb-free perovskite solar cells using Cu-doped Ni1-xO thin films as hole transport material

SCAPS solar cell simulation program was applied to model an inverted structure of perovskite solar cells using Cu-doped Ni_1-xO thin films as hole transport layer. The Cu-doped Ni_1-xO film were made by co-sputtering deposition under different deposition conditions. By increasing the amount of the Cu-dopant, the film crystallinity enhanced whereas the bandgap energy decreased. The transmittance of the thin films decreased significantly by increasing the sputtering power of copper. High quality, uniform, compact, and pin-hole free films with low surface roughness were achieved. The structural, chemical, surface morphology, optical, electrical, and electronic properties of the Cu doped Ni_1-xO films were used as input parameters in the simulation of Pb-based (MAPbI_3-xCl_x) and Pb-free (MAGeI₃) perovskite solar cells. Simulation results showed that the performance of both Pb-based and Pb-free perovskite solar cell devices significantly enhanced with Cu-doped Ni_1-xO film. The highest power conversion efficiency (PCE) for the Pb-free perovskite solar cell is 8.9% which is lower than the highest PCE of 17.5% for the Pb-based perovskite solar cell.     

Nucleant layer effect on nanocolumnar ZnO films grown by electrodeposition

Different ZnO nanostructured films were electrochemically grown, using an aqueous solution based on ZnCl₂, on three types of transparent conductive oxides grow on commercial ITO (In₂O₃:Sn)-covered glass substrates: (1) ZnO prepared by spin coating, (2) ZnO prepared by direct current magnetron sputtering, and (3) commercial ITO-covered glass substrates. Although thin, these primary oxide layers play an important role on the properties of the nanostructured films grown on top of them. Additionally, these primary oxide layers prevent direct hole combination when used in optoelectronic devices. Structural and optical characterizations were carried out by scanning electron microscopy, atomic force microscopy, and optical transmission spectroscopy. We show that the properties of the ZnO nanostructured films depend strongly on the type of primary oxide-covered substrate used. Previous studies on different electrodeposition methods for nucleation and growth are considered in the final discussion.     

Effect of electrodeposition and annealing of ZnO on optical and photovoltaic properties of the p-Cu2O/n-ZnO solar cells

Cu₂O/ZnO p–n heterojunction solar cells were fabricated by rf sputtering deposition of n-ZnO layer, followed by electrodeposition of p-Cu₂O layer. The different electrodeposition potentials were applied to deposit Cu₂O on ZnO. The particle size, crystal faces, crystallinity of Cu₂O is important factor which determine the p–n junction interface and consequently their effect on the performance of the heterojunction solar cell. It is observed that at −0.6 V, p-Cu₂O film generates fewer surface states in the interband region due to the termination of [1 1 0] resulting in higher efficiency (0.24%) with maximum particle size (53 nm). The bandgap of Cu₂O at this potential is found to be 2.17 eV. Furthermore, annealing of ZnO film was performed to get rid of deteriorating one and two dimensional defects, which always reduce the performance of solar cell significantly. We found that the solar cell performance efficiency is nearly doubled by increasing the annealing temperature of ZnO thin films due to increasing electrical conductance and electron mobility. Doping studies and fine tuning of the junction morphology will be necessary to further improve the performance of Cu₂O/ZnO heterojunction solar cells.     

Improved Crystal Quality of Transparent Conductive Ga‐doped ZnO Films by Magnesium Doping Through Radio‐Frequency Magnetron Sputtering Preparation

This study investigates the enhanced structural, and optoelectronic properties of transparent conductive Ga‐doped Mg_xZn_{1 ? x}O (GMZO) thin films with a varied magnesium (Mg) composition of 2% and 8%, respectively. The X‐ray diffraction (XRD) measurements revealed that GMZO with an 8% Mg composition shows a stronger (002) diffraction intensity and narrower linewidth than that with a 2% Mg composition. Improved crystallinity and enlarged grain size in the postgrowth thermal annealed GMZO thin films were also observed in XRD and morphological measurements by atomic force microscopy. Photoluminescence measurements were conducted to investigate the improved GMZO thin‐film quality, and the oxygen vacancy signal was found to decrease with increased Mg content, consistent with X‐ray photoelectron spectroscopy measurements. This study also shows high optical transmittance over 98%, and a low resistivity of 5.7 × 10^?4 Ω·cm in Ga‐doped Mg_xZn_{1 ? x}O (x = 0.02) thin film, which indicates the highly promising candidate for use in optoelectronic devices.     

The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes

High performance is expected in dye-sensitized solar cells (DSSCs) that utilize one-dimensional (1-D) TiO₂ nanostructures owing to the effective electron transport. However, due to the low dye adsorption, mainly because of their smooth surfaces, 1-D TiO₂ DSSCs show relatively lower efficiencies than nanoparticle-based ones. Herein, we demonstrate a very simple approach using thick TiO₂ electrospun nanofiber films as photoanodes to obtain high conversion efficiency. To improve the performance of the DSCCs, anatase-rutile mixed-phase TiO₂ nanofibers are achieved by increasing sintering temperature above 500°C, and very thin ZnO films are deposited by atomic layer deposition (ALD) method as blocking layers. With approximately 40-μm-thick mixed-phase (approximately 15.6?wt.% rutile) TiO₂ nanofiber as photoanode and 15-nm-thick compact ZnO film as a blocking layer in DSSC, the photoelectric conversion efficiency and short-circuit current are measured as 8.01% and 17.3?mA?cm^?2, respectively. Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy measurements reveal that extremely large electron diffusion length is the key point to support the usage of thick TiO₂ nanofibers as photoanodes with very thin ZnO blocking layers to obtain high photocurrents and high conversion efficiencies.     

Extremely Thin Al2O3 Surface‐Passivated Nanocrystalline ZnO Thin‐Film Transistors Designed for Low Process Temperature

Nanocrystalline ZnO (nc‐ZnO) thin‐film transistors (TFTs) exhibit inherent instability under bias/photo stresses, which originates from the oxygen molecules adsorbed on the surface of the crystal grains. The space charge region at nanocrystal surfaces that is induced by adsorbed oxygen molecules produces a high electrical potential barrier and significantly interrupts charge transport between the source and drain in nc‐ZnO TFTs. In this article, we developed high‐performance TFTs via the continuous deposition of an extremely thin Al₂O₃ layer on a nc‐ZnO channel. These devices were fabricated by atomic layer deposition at an extremely low process temperature of 150°C, including both the deposition and postannealing temperatures. The nc‐ZnO TFT with an extremely thin Al₂O₃ layer (1.8 nm) showed a significantly higher mobility (25 cm²/Vs) compared to devices without an Al₂O₃ layer (3.6 cm²/Vs). This dramatic difference was ascribed to the suppression of the chemisorption of oxygen molecules at the nanocrystal surface during thermal annealing (reducing the potential barrier width/height between adjacent nanocrystals). Furthermore, ultrathin Al₂O₃‐covered nc‐ZnO TFTs exhibited considerably enhanced electrical/photo stability due to the reduction in adsorption/desorption events of oxygen molecules on the nanocrystal surfaces (with no change in the depletion width after illumination) under gate bias or illumination stress.     

The effect of compression on electron transport and recombination in plastic TiO2 photoanodes

The compression method was applied for the preparation of plastic TiO₂ porous films on a conductive indium–tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrate at low temperature for the generation of high-efficiency plastic dye-sensitized solar cells (DSCs). The compression parameters, including pressure and time, were varied in order to determine their effect on the photovoltaic performance of the plastic DSCs. The results from electrochemical impedance spectroscopy (EIS) showed that charge transport resistance in the porous TiO₂ films (R_t) gradually decreased when the applied pressure was increased from 0 MPa to 150 MPa, which indicated a better connection between the TiO₂ nanoparticles and electron transport in the TiO₂ films. In addition, a longer press time led to an increased resistance of electron recombination (R_ct) and an increased charge-collection efficiency. After optimization of the compression parameters, the efficiency of energy conversion was increased by approximately 81.6%. In addition, the efficiency of energy conversion was increased by an additional 4.65% under AM1.5 illumination.     

ZnO Nanorods as Antireflective Coatings for Industrial‐Scale Single‐Crystalline Silicon Solar Cells

In this work, both planar and textured, industrial scale (156 mm × 156 mm) single‐crystalline silicon (Si) solar cells have been fabricated using zinc oxide (ZnO) nanorods as antireflection coating (ARC). ZnO nanorods were grown in a few minutes via hydrothermal method within a commercially available microwave oven. Relative improvement in excess of 65% in the reflectivity was observed for both planar and textured Si surfaces. Through ZnO nanorods, effective lifetime (τ_eff) measurements were presented to investigate the surface passivation property of such an ARC layer. ZnO nanorods increased the τ_eff from 9 to 71 μs at a carrier injection level of 10¹⁵ cm^?3. Increased carrier lifetime revealed the passivation effect of the ZnO nanorods in addition to their ARC property. 33% and 16% enhancement in the photovoltaic conversion efficiency was obtained in planar and textured single‐crystalline solar cells, respectively. Our results reveal the potential of ZnO nanorods as ARC that can be deposited through simple solution‐based methods and the method investigated herein can be simply adapted to industrial scale fabrication.     

Electrical and Optical Properties of Transparent Conducting p‐Type SrTiO3 Thin Films

We report a systematic study of the electrical and optical properties of epitaxial perovskite p‐type In‐doped SrTiO₃ thin films (SrIn_xTi_1?xO₃, 0 ≤ x ≤ 0.15) grown on single‐crystal (100)‐oriented LaAlO₃ substrates using a hybrid method which combines pulsed laser deposition and molecular beam epitaxy in a range of deposition conditions. X‐ray diffraction analysis confirms the epitaxial growth of high crystal quality films. Four‐point probe and Hall Effect measurements demonstrate that the films are p‐type semiconductors with a low resistivity of ~10^?2 Ω·cm and a high carrier concentration of ~10¹⁹ cm^?3. The optical transmittance spectra reveal that the films are highly transparent (?70%) in the visible region.     

In situ TEM observation on the ferroelectric-antiferroelectric transition in Pb(Nb,Zr,Sn,Ti)O3/ZnO

Ceramic composites of (1-x)Pb_0.99{Nb_0.02[(Zr_0.57Sn_0.43)_0.937Ti_0.063]_0.98}O₃ (PNZST)/xZnO were recently reported to exhibit exceptionally high pyroelectric coefficients near human body temperature due to the ferroelectric-antiferroelectric transition of the matrix grains. In the present work, a comparative study is conducted on two composites of x = 0.1 and 0.4 with in situ heating transmission electron microscopy (TEM). The results verify the presence of strain field in the PNZST grain adjacent to a ZnO particle and the stabilized ferroelectric phase at room temperature in the composite of x = 0.1. During heating, the ferroelectric matrix grain transforms to the antiferroelectric phase, contributing to the pyroelectric effect. In the composite of x = 0.4, high-angle annular dark-field imaging combined with energy-dispersive X-ray spectroscopy reveal the existence of both ZnO and Zn₂SnO₄. The formation of Zn₂SnO₄ indicates that Sn in the PNZST matrix grain is selectively extracted, and decomposition of the perovskite phase has taken place. The decomposition products in the form of fine particles are observed to facilitate the nucleation of the antiferroelectric phase and restrict the motion of the phase boundary during heating. The larger amount of ZnO and Zn₂SnO₄ and the decomposition of the PNZST perovskite phase are suggested to be responsible for the much lower pyroelectric coefficient in the x = 0.4 composite.     

Ga-doped ZnO transparent electrodes with TiO<Subscript>2</Subscript> blocking layer/nanoparticles for dye-sensitized solar cells

Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs]. The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO₂ blocking layer was deposited on the GZO surface. Then, TiO₂ nanoparticles were coated on the blocking layer, and dye was attached for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO₂ glasses showed an enhanced conversion efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge recombination and series resistance.     

Solution-processed nickel oxide hole transport layer for highly efficient perovskite-based photovoltaics

Solution processed NiO_x is one of the promising hole transport layer (HTL) for planar perovskite solar cells, which can replace hygroscopic poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) HTL. In this study, we investigated effects of ethylenediamine (EDA) additive in NiO_x precursor solution (nickel nitrate hexahydrate dissolved in ethyleneglycol) on optoelectronic and surface morphological properties of resultant solution processed NiO_x films. By varying EDA content (0–10.0?v/v %) in the precursor, we could find out that adequate EDA additive (~5.0%) provide much reduced electrical resistivity and enhanced optical transmission compared with control NiO_x film (No EDA) by suppressing formation of byproducts (i.e. nickel hydroxide). In addition, AFM surface topography showed much compact and dense deposition of NiO_x film on ITO electrode. This contributed to improve charge transport properties and suppress charge recombination loss at ITO/perovskite interface, which provided strong enhancement in fill factor from 0.599 to 0.714 in the perovskite solar cells. As a result, a power conversion efficiency (PCE) was strongly increased from 13.9 (No EDA) to 16.7% (EDA 5.0%). This also outperformed the performance (14.3%) of device using PEDOT: PSS, which indicates that the adequate control of EDA additive for NiO_x HTL could offer much promising photovoltaic performance.     

Preparation and properties of AZO/PS/AZO tri-layer transparent conductive film with a light-trapping structure

The light-trapping structure is an effective method to increase solar light capture efficiency in the solar cells. In this study, Al-doped ZnO (AZO)/polystyrene (PS)/AZO tri-layer transparent conductive film with light-trapping structure was fabricated by magnetron sputtering and liquid phase methods. The structural, optical and electrical properties of the AZO films could be controlled by different growth conditions. When the sputtering pressure of the under-layer AZO film was 0.2 Pa, the discharge voltage was around 80 V, which was within the optimal process window for obtaining AZO film with high crystallinity. The optimal under-layer AZO film had a large surface roughness and a very low static water contact angle of 75.71°, promoting the relatively uniform distribution of PS spheres. Under this sputtering condition, the prepared AZO/PS/AZO tri-layer film had the highest crystallinity and least point defects. The highest carrier concentration and Hall mobility are 3.0 × 10²¹ cm^-3and 5.39 cm² V^-1 s^-1, respectively. Additionally, a transparent conductive film with the lowest resistivity value (3.88 × 10^-4 Ω cm) and the highest average haze value (26.5%) was obtained by optimizing the process parameters. These properties were comparable to or exceed the reported values of surface-textured SnO₂-based as well as ZnO-based TCOs films, making our films suitable for transparent electrode applications, especially in thin-film solar cells.     

Impact of Stoichiometry on Structural and Optical Properties of Sputter Deposited Multicomponent Tellurite Glass Films

Multicomponent TeO₂–Bi₂O₃–ZnO (TBZ) glass thin films were prepared using RF magnetron sputtering under different oxygen flow rates. The influences of oxygen flow rate on the structural and optical properties of the resulting thin films were investigated. We observed that thin films sputtered in an oxygen‐rich environment are optically transparent while those sputtered in an oxygen‐deficient environment exhibit broadband absorption. The structural origin of the optical property variation was studied using X‐ray diffraction, X‐ray photoelectron spectroscopy, Raman Spectroscopy, and transmission electron microscopy which revealed that the presence of under‐coordinated Te leads to the observed optical absorption in oxygen‐deficient films.     

Impedance Spectroscopy Characterization in Bipolar Ta/MnOx/Pt Resistive Switching Thin Films

Impedance spectroscopy was applied to MnO_x‐based thin films prepared in symmetric and asymmetric electrode configurations, i.e., Pt/MnO_x/Pt and Ta/MnO_x/Pt, respectively. Equivalent circuit analysis suggests the presence of higher resistance surface layers adjacent the electrodes, in addition to a higher conductivity component at central portions of the MnO_x thin films. The asymmetric configuration enables the Ta/MnO_x interfacial layer to facilitate the redox transport of oxygen ions, where significant changes in resistance with the electric field are responsible for the higher on/off resistance ratio in Ta/MnO_x/Pt. The higher dielectric constant and bias‐dependent capacitance and resistance support the coexistence of both oxidized surfaces and interfacial layers.     

Thermochromic properties of ZnO/VO2/ZnO films on soda lime silicate glass deposited by RF magnetron sputtering

A smart window based on VO₂ is a promising thermochromic (TC) glass that can regulate heat flow through windows by solar modulation near room temperature. TC glasses with high visible-light transmittance and large difference in infrared transmittance between high- and low-temperature VO₂ phases are required to save large amounts of energy in buildings. VO₂-based multilayer films with a buffer layer and/or an anti-reflective (AR) layer are used when the films are deposited by sputtering. In this study, VO₂-based multilayer films were prepared on soda lime glass using ZnO as both the buffer and the AR layers. The structure of the multilayer film was simulated using the optical constants measured from the deposited films. The effect of buffer and AR layers on the TC properties of VO₂-based multilayer films prepared by sputtering was investigated by simulation of the multilayer structure and deposition of the films with the simulated structure. The TC properties were measured and compared with the calculated properties. Improved TC properties (luminous transmittance (T_lum) of ～50%/46% (30 °C/80 °C) and solar modulation ability (ΔT_sol) of ～14%), compared to those without the buffer and AR layer, were obtained from the ZnO/VO₂/ZnO film deposited on glass. The calculated transmittances agree better with the measured ones when the optical constants measured directly from the deposited films are used and the roughnesses of the surface/interface of the multilayer films are considered in the calculation of the optical constants.     

Mechanism of Nanovoid Formation at the ZnO–Glass Interface in Planar Multilayered Structures of AlOx–ZnO–Glass

Functional porous materials require easy fabrication methods with controllability of a wide range of pore size and its density for practical applications including optical devices. The Kirkendall effect based on unbalanced material diffusion provides such a possibility in conjunction with material configurations of multilayers. This study reports a formation of nanoscale pores within ZnO films in planar multilayered structures of Al₂O₃–ZnO‐aluminosilicate glass and demonstrates the mechanism of forming relatively large nanopores in ZnO near the ZnO–glass interface via stress‐promoted Kirkendall diffusion. Experimental characterizations supported by atomic simulation reveal that an enhanced in‐plane tensile stress in the ZnO films with increasing the thickness of the neighboring Al₂O₃ films can promote the diffusivity of the Zn atoms and the pore growth in the ZnO films. The pore size and location in the intermediate ZnO layer of the Al₂O₃–ZnO–glass is alterable by simply selecting the thickness of the Al₂O₃ layer. Promoted diffusion of the Zn atoms enables to fabricate porous planar ZnO films with pore sizes up to a few hundred nm with an enhanced light scattering ability. These findings offer a promising route to produce porous planar films through in‐depth understanding of diffusivity enhancement in glass–metal oxide couples.     

Effect of oxygen pressure on the p-type conductivity of Ga,P co-doped ZnO thin film grown by pulsed laser deposition

The effect of the oxygen partial pressure on the conductivity of (Ga, P) co-doped ZnO thin films (ZnO:Ga_0.01P_0.02, ZnO:Ga_0.01P_0.04) was investigated. The thin films were grown by using the pulsed laser deposition (PLD) method. As the oxygen partial pressure increased from 1 mTorr to 200 mTorr, the electron carrier concentration of the ZnO:Ga_0.01P_0.04 thin films decreased. Above 200 mTorr, however, the electron carrier concentration increased and a transition from n-type to p-type conductivity was observed. On the other hand, in the case of the ZnO:Ga_0.01P_0.02 thin films, their electron carrier concentration continuously decreased as the oxygen partial pressure increased from 1 to 500 mTorr, showing the typical n-type semi-conductive characteristics. The X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) analyses were used to characterize the n-type to p-type conductivity transitions with increasing oxygen partial pressure.