Conduction Band Of Zinc Oxide Research Articles

Magnetically recyclable ZnO coated Fe3O4 nanocomposite for MO dye degradation under UV-light irradiation

We describe the synthesis of zinc oxide (ZnO) coated magnetite (Fe3O4) [ZnO/Fe3O4] nanocomposite and its photocatalytic application towards the methyl orange (MO) dye degradation. ZnO/Fe3O4 nanocomposite was prepared by opting the ex-situ mechanical stirring process of ZnO powder and solvothermally prepared Fe3O4 nanoparticles. The photocatalytic properties of the ZnO/Fe3O4 nanocomposite were investigated, and the degradation of methyl orange (MO) dye was found to be 84% in 120 min under UV light. The electron transfer mechanism might explain the still debatable MO dye degradation mechanism in the present case. The photogenerated electrons transfer to the conduction band (CB) of Fe3O4 from the CB of ZnO under UV light exposure. This results in the creation of defect levels (i.e., Fe2+ or Fe3+) in ZnO, lowering the CB of ZnO. The reusability and efficient magnetic separability of ZnO/Fe3O4 photocatalyst was established by 60% degradation after four cycles under UV light irradiation. The nanocomposite's XRD pattern before and after photocatalytic degradation exhibited no structural change, demonstrating the photocatalyst's outstanding stability. Our investigations demonstrated the potential for MO dye degradation by ZnO/Fe3O4 nanocomposite under UV light irradiation.

Influence of Ag size and shape in dye photodegradation using silver nanoparticle/ZnO nanohybrids and polychromatic light

In this work, we propose zinc oxide (ZnO) surface functionalization with plasmonic silver nanoparticles (AgNP) of different sizes and shapes (spheres, prisms, and rods) creating ZnO/AgNP nanohybrids. These were characterized by UV-Vis spectroscopy, X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, diffuse reflectance spectroscopy, and photoluminescence spectroscopy. Surface functionalization with AgNP improved photocatalyst electronic properties, its visible light absorption, and slow electron/hole recombination on the ZnO surface. Photocatalysis assays performed with a polychromatic Hg lamp degraded methyl orange, a model of persistent organic pollutant in water. A systematic study showed that the photodegradation kinetics of the nanohybrids are significantly more efficient than pure ZnO (up to 18 times) and that AgNP size and especially its shape are important in dye degradation. Mechanistic studies revealed that degradation occurred by direct dye reduction on the ZnO surface holes, ZnO electron transfer to Ag followed by •O2- formation, and direct injection of AgNP hot electrons in the ZnO conduction band. The last effect was stronger for anisotropic AgNP, which explains their high kinetic degradation rates. Therefore, the rational design in ZnO/AgNP nanohybrid engineering and a systematic approach used in this manuscript allowed a detailed description of photodegradation process that occur at ZnO/AgNP interface. Our results are not conclusive about AgNP size; on the other hand, it clearly demonstrates that anisotropic nanoparticles (as Ag rods and prims) present superior photodegradation efficiency and are promising particles for further large-scale use of solar-irradiated nanohybrids.

Fermi level shifts of gold nanospheres on ZnO film upon UV irradiation.

Here, Fermi level shifts of gold nanospheres on zinc oxide (ZnO) film upon UV irradiation are studied. These Fermi level shifts are obtained by recording extinction spectra of gold nanospheres characterized by their plasmon resonance and are due to the presence of additional electrons stored in gold nanospheres and coming from ZnO film upon UV irradiation. It is demonstrated that the blueshift of the plasmon resonance of the Au nanospheres upon UV irradiation points out the transfer of electrons from ZnO film to Au nanospheres. Moreover, it is reported that the Fermi level shifts vary with UV irradiation time by approaching the conduction band of ZnO, and are estimated by an analogous model to the electronic structure of a semiconductor, which can be expressed as a function of the plasmon resonance blueshift of the Au nanospheres. Henceforward, these stored electrons and these Fermi level shifts can allow the improvement of the properties of photocatalytic reduction for the gold-semiconductor structures and can also be used for photo-induced enhanced Raman spectroscopy.

Role of interfacial contact between 2D materials and preselected nanostructures in the degradation of toxic dyes: Multifunctional facets of graphene

Designing intimate interfacial contact between nanostructures and two-dimensional (2D) materials is highly desirable to influence the movement of generated charge carriers. Nanostructured zinc oxide (ZnO) is a fascinating material with unique optical and electrical properties. 2D reduced graphene oxide (rGO) exhibits semiconductor behaviour with tunable catalytic activity and excellent biocompatibility. Hence, we have designed a hybrid material by selecting nanostructures of an oxide semiconductor (ZnO) with reduced graphene oxide (rGO) using a hard integration technique followed by a low-temperature hydrothermal route. The good encapsulation of rGO over the ZnO nanorods was confirmed by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The photocatalytic activities of ZnO, rGO, and ZnO/rGO were studied under visible-light irradiation using three different toxic dyes, methylene blue (MB), methyl orange (MO), and Congo red (CR). The composite materials exhibited excellent efficiencies of 100, 95, and 90% for the degradation of MB, MO, and CR, respectively. Moreover, the degradation of the dye was found to follow first-order kinetics. The enhanced efficiencies are attributed to the adsorption and efficient charge transfer from rGO to the conduction band of ZnO. The role of the multifunctional facets of graphene was presented to elucidate the visible-light activity of the composite materials for enhanced efficiency. The main reactive species (e−) of the reduction reaction were confirmed through a radical trapping experiment, which showed the generation of highly reactive •OH radicals that decompose the toxic dye. The results provide a perspective for developing graphene-based composite materials with desired preselected nanostructures for solar energy utilisation.

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

Nitrogen-doped carbon-ZnO heterojunction derived from ZIF-8: a photocatalytic antibacterial strategy for scaffold

Antibacterial property is urgently needed in scaffold-induced bone regeneration. Zinc oxide (ZnO) as a photocatalyst enables to produce photoexcited electron-hole pairs to catalyze reactive oxygen species (ROS) generation, which triggers the peroxidation and consequently apoptosis of bacteria. However, the fast recombination of electron-hole pairs and the wide band gap of ZnO restrict the full play of its antibacterial property. Herein, a nitrogen-doped carbon-ZnO (NC-ZnO) heterojunction derived from metal organic frameworks was synthesized by direct carbonization. On one hand, the N-doped C in NC-ZnO acted as electron acceptors to trap photoexcited electrons in conduction band of ZnO, which improved the separation efficiency of electron-hole pairs. On the other hand, the N-doped C with high absorption coefficient enabled NC-ZnO to possess a relatively narrow band gap and wide light absorption range compared with ZnO. The obtained NC-ZnO heterojunction was then introduced into poly-l-lactic acid (PLLA) scaffold to construct a photodynamic antibacterial platform. The results showed that NC-ZnO endowed the scaffold with excellent photocatalytic activity and greatly stimulated the production of ROS. The antibacterial tests suggested that the PLLA/NC-ZnO scaffold possessed robust antibacterial activities with 99.9% antibacterial efficiency toward Escherichia coli, which was increased by 113.9% compared with PLLA/ZnO scaffold.

Structural and Optical Properties of Bismuth-doped ZnO Nanoparticles Synthesized by Co-precipitation

Bismuth-doped zinc oxide (ZnO) nanoparticles can serve as efficient photocatalysts for various reactions. Herein, we synthesized and discussed the growth mechanisms of Bi-doped ZnO nanoflakes using co-precipitation with Bi concentrations ranging from 0 to 3 %. The resulting ZnO were hexagonal nanosheets with diameters ranging from 80 nm (ZnO) to 200 nm (ZnO: Bi 3%). The dominant crystal structure matches hexagonal wurtzite with a small presence of Bi2O3 diffraction peaks. The estimated crystallite sizes range from ~ 33 nm to ~ 45 nm, indicating multiple crystalline regions in each nanoflake. Nevertheless, as sheet resistance monotonically decreases with the Bi concentration, the higher number of grain boundaries likely has a lower effect on the conductivity compared to an increase in free carriers and larger grain size in the samples with higher Bi concentration. The bandgap decreases from ~ 3.13 eV to ~ 2.96 eV, likely due to the shrinkage effect from electron-electron or electron-impurity interaction that lowers the conduction band of ZnO.

Enhancement of performance of Ga incorporated ZnO UV photodetectors prepared by simplified two step chemical solution process

The present study establishes the UV photodetector capability of Zinc Oxide (ZnO) and Gallium incorporated ZnO (GZO) thin films prepared by a simplified two-step chemical bath deposition technique. The prepared UV detectors were characterized by tools such as XRD, SEM, UV, PL and Photoresponse to investigate their properties. The structural study revealed the enhancement of film crystallinity with respect to Ga doping level. The optical studies confirm the higher absorption at UV range for 3% Ga doped ZnO film. From PL study, the green emission observed at 522 nm is associated with the defects from oxygen vacancies (Vo+). Once the UV light is irradiated on the GZO film surface, electrons and holes are generated as the illumination energy is larger than the ZnO bandgap energy, known as above-band energy illumination. The photo-generated holes then neutralize the chemisorbed ions causing the release of electrons in the conduction band of ZnO, this could increase the photocurrent. In this study, the calculated photo-detectivity (D*), responsivity (R) and external quantum efficiency (EQE) of the prepared UV detectors are 1.24 ×1010 jones, 0.38 AW-1, and 125.10% respectively for 3% Ga doped ZnO film. These parameters are noticeable and this study proves that GZO films in fact work as good sensitive UV detectors.

Fabrication of nanohybrids toward improving therapeutic potential of a NIR photo-sensitizer: An optical spectroscopic and computational study

Implementation of near infrared radiation (NIR) active drugs in photodynamic therapy (PDT) is considered as a highly promising tool in therapeutic application. Because of its strong NIR absorbance, indocyanine green (ICG) has become an attractive choice in emerging photo-theranostics. ICG generates singlet oxygen as well as shows photothermal effect under NIR irradiation. However, ICG is unable to produce sufficient amount of others types of reactive oxygen species (ROS) such as superoxide, hydroxyl radical. But, its ability of singlet oxygen generation under NIR irradiation is arrested by its tendency to aggregate in aqueous medium. Because of this limitation, it shows better effectiveness in photothermal therapy compare to it’s action as a photodynamic agent. Herein, we have addressed to overcome this limitation via the fabrication of zinc oxide (ZnO) based ICG-ZnO nanohybrid which provides lesser H-aggregation between ICG molecules on its surface and yield significantly greater amount of ROS relative to ICG upon photoexcitation, due to excited state electron transfer from ICG to ZnO. Also, this surface functionalization prevents aggregation of ICG in water significantly. Classical MD simulation shows the dimeric structure of ICG breakes down on ZnO surface which corroborates results from the aggregation study in water. Density functional theory (DFT) along with time-dependent DFT studies elucidate that upon photoexcitation electron transfer takes place from the higher energy orbital of ICG to the conduction band of ZnO. The greater efficiency in ROS generation by the ICG-ZnO nanohybrid than ICG or ZnO demonstrates remarkable antimicrobial activity against gram-negative bacteria E. coli. Overall, the present study highlights the scope of developing ICG-ZnO nanohybrid as a highly efficient NIR agent for use in antibacterial photodynamic therapy.

Enhanced UV Photoresponsivity of ZnO Nanorods Decorated with Ag2S/ZnS Nanoparticles by Successive Ionic Layer Adsorption and Reaction Method.

Recently, different kinds of energy band structures have been utilized to improve the photoelectric properties of zinc oxide (ZnO). In this work, ZnO nanorods were prepared by the hydrothermal method and then decorated with silver sulfide (Ag2S)/zinc sulfide (ZnS) via two-step successive ionic layer adsorption and reaction method. The photoelectric properties of nanocomposites are investigated. The results show that ZnO decorated with Ag2S/ZnS can improve the photocurrent of photodetectors from 0.34 to 0.56 A at bias of 9 V. With the immersion time increasing from 15 to 60 minutes, the photocurrent of photodetectors increases by 0.22 A. The holes in the valence band of ZnO can be transferred to the valence band of ZnS and Ag2S, which promotes the separation and suppresses the recombination of hole-electron pairs generated in ZnO. Moreover, electrons excited by ultraviolet (UV) light in Ag2S can also be injected into the conduction band of ZnO, which causes the photocurrent to increase more than the ZnO photodetector.

Molecular Copper(I)-Copper(II) Photosensitizer-Catalyst Photoelectrode for Water Oxidation.

Copper(II)-based electrocatalysts for water oxidation in aqueous solution have been studied previously, but photodriving these systems still remains a challenge. In this work, a bis(diimine)copper(I)-based donor-chromophore-acceptor system is synthesized and applied as the light-harvesting component of a photoanode. This molecular assembly was integrated onto a zinc oxide nanowire surface, and upon photoexcitation, chronoamperometric studies reveal that the integrated triad can inject electrons directly into the conduction band of zinc oxide, generating oxidizing equivalents that are then transferred to a copper(II) water oxidation catalyst in aqueous solution, yielding O2 from water with a Faradaic efficiency of 76%.

Photo sensitizing and electrochemical performance analysis of mixed natural dye and nanostructured ZnO based DSSC

The paper highlights the enactment of the natural dyes, like purple cabbage, spinach, turmeric and their mixture as a photo-sensitizer, with nanostructured zinc oxide (ZnO) as a photo-anode, based dye-sensitized solar cell (DSSC). The field emission scanning electron microscopic image of ZnO, prepared by chemical bath deposition process, proclaims two different types of morphology, nano-wire and nano-plate shape. The photo sensitizing properties of the natural dyes and their mixed part are explored through FTIR spectra and UV-vis light absorbance characteristics. The FTIR spectra explore the presence of different anchoring chemical groups which confirm the strong anchoring bond with ZnO and enhancement of electron mobility. The diffused reflectance spectra (DRS) of dye-loaded ZnO films incisive the absorption of dye on the mesoporous ZnO surface. The relative energy band positions of individual and mixed dye, yield stable execution of the cell that has been performed through the cyclic voltammeter. The driving force energy requirement for effortless transport of electron from the mixed dye to the conduction band of ZnO is revealed lowers (0.34 eV) as compared to individual dyes. Electrochemical impedance spectroscopy (EIS) analysis has been executed to find the charge transfer resistance, total bulk resistance, recombination loss through Nyquist plots and Bode plots. These characteristics pretense as the mixed dye has an eminent rate of electron transportation and lower recombination loss with longer electron lifetime. The photon to electrical power conversion efficiencies of purple cabbage, spinach, turmeric and their mix dyes are explored as 0.1015%, 0.1312%, 0.3045%, and 0.602%, respectively under same simulated light condition. The mixed dye reveals the stable performance of the cell with the highest conversion efficiency due to the absorption of an extensive range of the solar spectrum and well-suited electrochemical responses.

Channel Defect Profiling and Passivation for ZnO Thin-Film Transistors.

The electrical characteristics of Zinc oxide (ZnO) thin-film transistors are analyzed to apprehend the effects of oxygen vacancies after vacuum treatment. The energy level of the oxygen vacancies was found to be located near the conduction band of ZnO, which contributed to the increase in drain current (ID) via trap-assisted tunneling when the gate voltage (VG) is lower than the specific voltage associated with the trap level. The oxygen vacancies were successfully passivated after the annealing of ZnO in oxygen ambient. We determined that the trap-induced Schottky barrier lowering reduced a drain barrier when the drain was subjected to negative bias stress. Consequentially, the field effect mobility increased from 8.5 m2 V−1·s−1 to 8.9 m2 V−1·s−1 and on-current increased by ~13%.

Synthesize of gadolinium-doped ZnO nano particles for energy applications by enhance its optoelectronic properties

In recent research, the zinc oxide (ZnO) semiconductor nano particles have received a lot of attention due to its unique physical, chemical, magnetic and optoelectronic properties. The present study focuses on the photo physical and optoelectronic properties of ZnO nanostructures. To attain better optical and optoelectronic properties for light emitting diodes, solar cells and photo detectors, the rare earth transition metal ions, specifically gadolinium (Gd) were doped into ZnO nano material. Gadolinium-doped ZnO nano materials have been synthesized by a simple chemical deposition method. The structural, morphological, optical properties of synthesized samples have been investigated by various techniques, including x-ray diffraction (XRD), UV-absorption spectroscopy, and photo luminescence spectroscopy. X-ray diffraction analysis confirmed the formation of pure single phase with a particle size in the range of 30–42 nm. Optical absorption studies demonstrates that the absorption edge of the Gd:ZnO appeared to shift towards the longer wavelength due to the charge transfer between the ZnO valence or conduction band and rare earth ion 4f energy level. The band gap of the doped ZnO samples was found to be decreases and it varies from 3.2 eV to 3.1 eV due to the quantum confinement of the charge carriers in ZnO. These results provide a new approach to design ZnO nano particles with enhanced photochemical properties, light emitting electronics and solar cells.

Copper zinc oxide heterostructure nanoflowers for hydrogen evolution

The nanoflowers copper-zinc oxide (CZO) heterostructure has been prepared using the hydrothermal method. As it exhibits a low band gap energy the material may absorb large amounts of natural sunlight. Under visible light irradiation, CZO showed signs of activity towards hydrogen production in the two conditions, which are: i) the condition of photocatalytic reaction with light as the only energy source, in the presence of a sacrificial electron donor and; ii) the photoelectrochemical conditions when the material was deposited onto biased conductive support, in pure water. The improved activity and stability of CZO results from a mutual charge transfer between copper oxide and zinc oxide. Therefore we can suggest that under illumination photogenerated electrons from the conduction band of copper oxide migrate to conduction band of zinc oxide, while holes migrate in the opposite direction, thus improving the charge separation and lowering the rate of unfavourable recombination.

Enhanced Electron Transportation by Dye Doping in Very Low-Temperature (

A perylene bisimide functionalized with four 4-carboxyphenoxy substituents at bay area (PBI-COOH) was embedded in sol-gel-derived zinc oxide (ZnO) to fabricate organic-inorganic hybrid photoconductive cathode interlayers (ZnO:PBI-COOH) that can be annealed at a rather low temperature of 120-130 °C as desired for plastic substrates for flexible devices. For these interlayers, the structural defects including oxygen vacancy and residual hydroxy groups are reduced that leads to increased electron mobility, and a photoinduced electron transfer from the organic dopant into the conduction band of ZnO endows the hybrid thin film with relatively higher conductivity when compared to the undoped ZnO thin film. The low-temperature-processed hybrid thin films were applied on indium tin oxide electrodes to produce inverted organic solar cells (OSCs) with power conversion efficiencies of 11.68 and 13.48% when using J71:ITIC and PBDB-T-2Cl:IT4F as active layers, respectively. Finally, flexible OSCs are fabricated on poly(ethylene terephthalate) substrates that maintained stability with relatively high performance after 100 times bending.

Quasi-metallic behavior of ZnO grown by atomic layer deposition: The role of hydrogen

Zinc oxide (ZnO) fabricated by atomic layer deposition (ALD) is intrinsically well-conductive (∼5 mΩ cm), in contrast to the single-crystalline bulk material or sputtered ZnO thin films. There are generally three groups of candidates for the intrinsic n-type conductivity: intrinsic point defects, elemental impurities other than hydrogen, and incorporated hydrogen itself. In this study, we assess the different candidates concerning their impact on conductivity. In the presence of free electron densities of up to 5 × 1019 cm−3, impurities other than hydrogen are ruled out due to their ultra-low concentrations in the ppm range. Intrinsic point defects are also considered unlikely since the evolution of conductivity with deposition temperature is not reproduced in the Zn/O ratio as measured by Rutherford backscattering spectrometry. Hence, the most promising candidate is hydrogen with a concentration of ∼1 at. %, i.e., more than sufficient to account for the free electron density. In addition, we find a correlation between the deposition-temperature dependence of the carrier concentration and the hydrogen concentration. The formation energy of the conductive, hydrogen-related state is determined to be ∼40 meV. Hall measurements down to liquid helium temperatures revealed that the electron densities are constant over the whole temperature range. This constitutes a quasi-metallic behavior of ALD-ZnO for deposition temperatures of ≥150 °C. We propose that the very high concentration of hydrogen-induced donor states causes a vanishing ionization energy so that the donor band merges energetically with the ZnO conduction band. This model is supported by ultraviolet photoelectron spectroscopy measurements.

Effects of electric fields on the conduction of polyvinyl alcohol (PVA)/ZnO films by photoluminescence analysis

We verified the electrical properties of zinc oxide (ZnO) and hydrated polyvinyl alcohol (PVA)/ZnO films under the influence of electric field through photoluminescence (PL) analysis. The ZnO films were deposited by sputtering on A-N-B substrates that had nonconductive areas (N) between A and B electrodes. The PVA/ZnO films were prepared by spin-coating PVA gel on top of the ZnO film. Current–voltage (I–V) data analysis showed that the conductivity values of the ZnO films significantly increased in the presence of PVA gel. Large amounts of free electrons and oxygen vacancies (Vo) were generated by the chemisorption reaction of PVA gel with ZnO and detected by PL spectra produced without the presence of an electric field (0V). The Fermi-energy level for a PVA/ZnO film approaches the conduction band of ZnO because of many free electrons in the film. The variations of both the free electron and Vo densities for ZnO and PVA/ZnO films were analyzed using PL spectra in the presence of various electric fields (>0V). The conduction mechanism of PVA/ZnO was explained and related to the variations of both the free electron and Vo densities as revealed by the PL spectral analysis.

A Comparative Study of Nanostructured TiO2, ZnO and Bilayer TiO2/ZnO Dye-Sensitized Solar Cells

Titanium dioxide (TiO2), Zinc oxide (ZnO) and bilayer TiO2/ZnO (TZO) based cells have been developed and sensitized with five organic dyes and one cocktail dye composed of five dyes. Photovoltaic performance of TiO2 and ZnO solar cell sensitized with six dyes is compared to that of bilayer TZO cells. The forward current is found to increase with applied voltage in the range V ≤ 0.4 V, which is dominated by thermionic emission, whereas in 0.4 ≤ V ≤ 0.7 V, the current transport is due to space charge-limited current controlled by exponential trap distribution in all devices. The combined properties of the materials enhance the efficiency of composite TZO cells. TiO2 permits the formation of an energy barrier at the ZnO electrode/electrolyte interface, which reduces the back electron transfer from the conduction band of ZnO to I3 − in the electrolyte. Also, due to the TiO2 layer on the ZnO, the latter forms a compact layer between flourine-doped tin oxide (FTO)/TiO2 which benefits the fast electron transfer from TiO2 to ZnO to FTO glass. This reduces the charge recombination occurring at the ZnO/FTO interface leading to higher open circuit voltage (V oc), higher short circuit current (J sc), lower series resistance (R s), and in turn higher efficiency in TZO solar cells as compared to ZnO cells. Among the six dyes, Eosin-Y and Rose Bengal dye gave the best performance as sensitizers with TZO.

Carrier concentration dependent optical and electrical properties of Ga doped ZnO hexagonal nanocrystals.

Colloidal trivalent gallium (Ga) doped zinc oxide (ZnO) hexagonal nanocrystals have been prepared to introduce more carrier concentration into the wide band gap of ZnO. The dopant (Ga) modifies the morphology and size of ZnO nanocrystals. Low content of Ga enhances the optical band gap of ZnO due to excess carrier concentration in the conduction band of ZnO. The interaction among free carriers arising from higher concentration of Ga gives rise to narrowing of the band gap. Surface plasmon resonance absorption appears in the infrared region due to excessive carrier concentration. A broad emission band consists of blue, yellow and green colors associated with different native defects of ZnO. Intrinsic defects and extrinsic dopant Ga control the defect related emission spectrum in the visible region. Replacement of Zn by Ga induces a room temperature metallic state in a degenerate semiconductor. Cationic disorder leads to metal-semiconductor transition at low temperature strongly dependent on the concentration of Ga. Pure semiconducting behavior up to about 80 K is observed for the highest amount of Ga. Temperature dependent metal-semiconductor transition has been interpreted by localization of charge carriers due to disorder arising from random Ga substitution.