Surface Defects Of ZnO Research Articles

Passivating ZnO with a naphthalimide-Schiff base as electron transport layer for inverted polymer solar cells

Abstract Optimizing electron transport layer (ETL) is a key issue to guarantee the performance of polymer solar cells (PSCs). In this work, by passivating ZnO with a naphthalimide-Schiff base (NS) as ETL, the inverted PSCs with polythieno[3,4-b]- thiophene-co-benzodithiophene (PTB7): [6, 6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the light harvesting layer (LHL) exhibited enhanced device performance. Studies demonstrated that the NS introduction passivates the surface defects of ZnO thus suppressing exciton quenching, resulting in enhanced exciton dissociation, efficient charge carrier collection and reduced loss of charge recombination, and thereby giving high power conversion efficiency (PCE). While the PCE of the inverted PSCs with pure ZnO ETL is 7.34%, that of the devices with NS passivated ZnO (NS-ZnO) ETL shows an enhanced PCE of 8.20% (∼11.7% improvement). More importantly, the NS-ZnO can be facilely obtained by adding NS into ZnO precursor, suggesting a simple and efficient way to enhance the PSCs performance.

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%.

Structural, Optical, and Photocatalytic Activities of Ag-Doped and Mn-Doped ZnO Nanoparticles

We report the photocatalytic activities of ZnO, Ag-doped ZnO, and Mn-doped ZnO nanoparticles (NPs). Ag-doped and Mn-doped ZnO samples were synthesized using a coprecipitation method and calcined at 600°C. XRD, SEM, EDX, and UV-vis spectroscopy techniques were employed for characterization of the synthesized samples. The photocatalytic activities of the samples were evaluated by measuring the photocatalytic decolorization of methyl violet with sunlight being the source of energy. XRD patterns of the samples confirmed the wurtzite structure without change which was indicative of the absence of Mn- and Ag-related secondary phases for the doped ZnO. The UV-vis spectra indicated the band gap energy of ZnO, Ag-doped ZnO, and Mn-doped ZnO to be 2.98, 2.80, and 2.64 eV, respectively. Photocatalytic decolorization of methyl violet for the synthesized samples was found to be favorable at a pH of 9.0, catalyst dose of 1 g/L, and initial dye concentration of 4.5 × 10−4 g/L. Mn-doped ZnO and Ag-doped ZnO photocatalytic decolorization efficiency was significantly higher than undoped ZnO. Incorporation of Mn and Ag enhanced the visible-light photocatalytic activity of ZnO; this could be due to the ability of these metals to increase the surface defects of ZnO which in turn shift their optical absorption towards the visible region.

Low temperature-processed ZnO nanorods-TiO2 nanoparticles composite as electron transporting layer for perovskite solar cells

Electron transport is one of the most crucial processes that determine charge collection efficiency in perovskite solar cells. Herein, a low temperature-processed ZnO-TiO2 nanocomposite is developed as an electron transporting layer for perovskite solar cells. Highly-crystalline ZnO nanorods were deposited electrochemically which served as the scaffold for spin-coated TiO2 nanoparticles. This ZnO-TiO2 nanocomposite is designed to integrate the fast electron transport along the nanorods and the additional surface area provided by the nanoparticles for enhanced electronic contact between the electron-transporting layer and the perovskite layer. A weak photoluminescence quenching behavior was observed for ZnO nanorods after TiO2 nanoparticle coating which signifies a reduction in ZnO surface defects. Steady-state photoluminescence and optical absorption measurements indicated improved charge transfer and higher absorption of light, respectively, when ZnO-TiO2 nanocomposite is contacted with the CH3NH3PbI3 layer. However, the clustering of the nanoparticles caused inefficient charge transfer from TiO2 to ZnO.

A novel photoelectrode of NiO@ZnO nanocomposite prepared by Pechini method coupled with PLD for efficiency enhancement in DSSCs

AbstractThe dye-sensitized solar cells made of NiO@ZnO nanoparticles were synthesized by a novel Pechini route using different NiO molar concentration ratios. The thermal, structural morphological, optical and electrical properties of the prepared samples were investigated using thermal gravimetric analysis and differential scanning calorimetery (TGA/DSC), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), FT-IR and Raman spectroscopy, UV-diffuse reflectance (UV-DRS), photoluminescence (PL) and current-voltage (I-V) measurements. The success of doping process was confirmed by the XRD patterns, which revealed the existence of new peak at 43.2° corresponding to secondary phase NiO. UV spectra exhibited red shifts in NiO doped ZnO NCs and PL spectra showed strong emission band at 355 nm. The doping of ZnO with NiO was intended to enhance the surface defects of ZnO. The current-voltage measurements showed an improvement of the short circuit photocurrent (Jsc) and fill factor (FF) and a decrease in the open circuit voltage (VOC) for dye-sensitized solar cell (DSSC) based on NiO-ZnO NCs. A clear enhancement in efficiency of DSSC from 1.26±0.10 % for pure ZnO to 3.01±0.25 % for NiO-ZnO NCs at the optimum doping with 1.5 mol% of NiO to ZnO (ZN1.5) was observed. The obtained material can be a suitable candidate for photovoltaic applications.

Solution-Processable ZnO/Carbon Quantum Dots Electron Extraction Layer for Highly Efficient Polymer Solar Cells.

In this work, we report the effort to develop high-efficiency inverted polymer solar cells (PSCs) by applying a solution-processable bilayer ZnO/carbon quantum dots (C-QDs) electron extraction layer (EEL). It is shown that the use of the bilayer EEL helps to suppress the exciton quenching by passivating the ZnO surface defects in the EEL, leading to an enhanced exciton dissociation, reduced charge recombination and more efficient charge extraction probability, and thereby achieving high power conversion efficiency (PCE). The inverted PSCs, based on the blend of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} and [6,6]-phenyl C71-butyric acid methyl ester, possess a significant improvement in PCE of ∼9.64%, which is >27% higher than that of a control cell (∼7.59%). The use of a bilayer ZnO/C-QD EEL offers a promising approach for attaining high-efficiency inverted PSCs.

Passivating ZnO Surface States by C60 Pyrrolidine Tris-Acid for Hybrid Solar Cells Based on Poly(3-hexylthiophene)/ZnO Nanorod Arrays.

Construction of ordered electron acceptors is a feasible way to solve the issue of phase separation in polymer solar cells by using vertically-aligned ZnO nanorod arrays (NRAs). However, the inert charge transfer between conducting polymer and ZnO limits the performance enhancement of this type of hybrid solar cells. In this work, a fullerene derivative named C60 pyrrolidine tris-acid is used to modify the interface of ZnO/poly(3-hexylthiophene) (P3HT). Results indicate that the C60 modification passivates the surface defects of ZnO and improves its intrinsic fluorescence. The quenching efficiency of P3HT photoluminescence is enhanced upon C60 functionalization, suggesting a more efficient charge transfer occurs across the modified P3HT/ZnO interface. Furthermore, the fullerene modified hybrid solar cell based on P3HT/ZnO NRAs displays substantially-enhanced performance as compared to the unmodified one and the devices with other modifiers, which is contributed to retarded recombination and enhanced exciton separation as evidenced by electrochemical impedance spectra. Therefore, fullerene passivation is a promising method to ameliorate the connection between conjugated polymers and metal oxides, and is applicable in diverse areas, such as solar cells, transistors, and light-emitting dioxides.

Decreased Charge Transport Barrier and Recombination of Organic Solar Cells by Constructing Interfacial Nanojunction with Annealing-Free ZnO and Al Layers.

To overcome drawbacks of the electron transport layer, such as complex surface defects and unmatched energy levels, we successfully employed a smart semiconductor-metal interfacial nanojunciton in organic solar cells by evaporating an ultrathin Al interlayer onto annealing-free ZnO electron transport layer, resulting in a high fill factor of 73.68% and power conversion efficiency of 9.81%. The construction of ZnO-Al nanojunction could effectively fill the surface defects of ZnO and reduce its work function because of the electron transfer from Al to ZnO by Fermi level equilibrium. The filling of surface defects decreased the interfacial carrier recombination in midgap trap states. The reduced surface work function of ZnO-Al remodulated the interfacial characteristics between ZnO and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), decreasing or even eliminating the interfacial barrier against the electron transport, which is beneficial to improve the electron extraction capacity. The filled surface defects and reduced interfacial barrier were realistically observed by photoluminescence measurements of ZnO film and the performance of electron injection devices, respectively. This work provides a simple and effective method to simultaneously solve the problems of surface defects and unmatched energy level for the annealing-free ZnO or other metal oxide semiconductors, paving a way for the future popularization in photovoltaic devices.

A Review of the Synthesis and Photoluminescence Properties of Hybrid ZnO and Carbon Nanomaterials

Photoluminescent ZnO carbon nanomaterials are an emerging class of nanomaterials with unique optical properties. They each, ZnO and carbon nanomaterials, have an advantage of being nontoxic and environmentally friendly. Their cost-effective production methods along with simple synthesis routes are also of interest. Moreover, ZnO presents photoluminescence emission in the UV and visible region depending on the synthesis routes, shape, size, deep level, and surface defects. When combined with carbon nanomaterials, modification of surface defects in ZnO allows tuning of these photoluminescence properties to produce, for example, white light. Moreover, efficient energy transfer from the ZnO to carbon nanostructures makes them suitable candidates not only in energy harvesting applications but also in biosensors, photodetectors, and low temperature thermal imaging. This work reviews the synthesis and photoluminescence properties of 3 carbon allotropes: carbon quantum or nanodots, graphene, and carbon nanotubes when hybridized with ZnO nanostructures. Various synthesis routes for the hybrid materials with different morphologies of ZnO are presented. Moreover, differences in photoluminescence emission when combining ZnO with each of the three different allotropes are analysed.

CdSe tetrapods-modified ZnO cathode buffer layers for enhancement of power conversion efficiency in inverted polymer solar cells

CdSe tetrapods (TPs)-modified ZnO served as cathode buffer layer in the inverted polymer solar cell, could not only passivate the surface defects of ZnO, but also fabricate electron percolation pathways and reduce the band offset at the interface between cathode and active layer, which could promote effective carrier transfer and reduce the rate of surface recombination, and thus boost the device efficiency. The improved crystal quality of ZnO and photon absorption are in favor of the increasing of the short-circuit current density (J sc). The decreased series resistance (R s) and increased shunt resistance (R sh) of the entire devices, are beneficial to the fill factor (FF) enhancement. By optimizing the content of CdSe TPs, the power conversion efficiency of the inverted device based on ZnO/CdSe-2 has been greatly improved to 2.91 % with high J sc of 8.03 mA/cm2 and FF of 61.3 %.

Enhanced hydrogen selectivity via photo-engineered surface defects for methanol steam reformation using zinc oxide–copper nanocomposite catalysts

Methanol steam reformation (MSR) to produce hydrogen (H2) gas using copper on zinc oxide (Cu/ZnO) supported catalysts is attractive due to the simple and low cost preparation process of the catalyst. H2 yield from MSR is proportional to total catalyst loading which can be tuned during catalyst preparation. By creating UV-c light induced surface defects on ZnO nanorods, we have shown improved copper (Cu) nano-particle distribution on the ZnO nanorods leading to better H2 yield. Increase in Cu nanoparticle adsorption is achieved by in situ reduction of Cu ions by photo-generated electrons, facilitated by ZnO surface defects that act as high energy sites favorable for Cu ion adsorption and their subsequent growth into nanoparticles. The modulated Cu/ZnO catalyst increases H2 selectivity by 57% along with a corresponding increase in CO content, which can be controlled by adjusting H2O:MeOH ratio in the precursor solution.

Effect of Grinding on the Photocatalytic Activity of Commercial ZnO Powder

The effect of grinding on the photocatalytic activity of commercial ZnO powder was explored using methyl orange as a model pollutant and the corresponding mechanism was discussed tentatively. The results indicate that the photocatalytic activity of the ZnO powder grinded decreases obviously in comprison with that of the ZnO powder untreated. This phoenomenon might be ascribed to the decrease of surface defects of ZnO induced by grinding, suggesting that the further treatment leading to the change of surface states of ZnO powder as a photocatalyst ought to be avoided.

Enhanced performance of organic photovoltaics by TiO2-interlayer with precisely controlled thickness between ZnO electron collecting and active layers

Organic photovoltaics (OPVs) consisting of ZnO ripple structures used as electron-collecting layer were fabricated without and with additional TiO2 layers prepared by atomic layer deposition (ALD). We show that TiO2 layer with a thickness of less than 1nm can enhance OPV performance, since additional TiO2 can heal surface defects of ZnO, which can act as recombination center of electron and hole created in the active layer by photon-absorption. In contrast, a thicker TiO2 film (>2nm) showed a lower OPV performance than that of bare sample without TiO2, most likely due to a lower conductivity of TiO2 than that of ZnO. We show that precise control of oxide thin layer using ALD can be beneficial for fabricating high-performing OPV.

Fine dispersion and self-assembly of ZnO nanoparticles driven by P3HT-b-PEO diblocks for improvement of hybrid solar cells performance

The rod-coil conjugated diblock copolymers of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO), acting as an electron donor, were blended with ZnO nanoparticles to fabricate the hybrid bulk-heterojunction (BHJ) solar cells. According to the ultraviolet-visible (UV-vis) absorption spectroscopy, the intensity at 610 nm, derived from a strong inter-molecular interaction of π–π stacking and the high crystallizability of the P3HT main chains, was higher in P3HT-b-PEO/ZnO blend films than that in P3HT/ZnO blend films especially after thermal treatment, revealing that PEO segments could make P3HT form more densely stacked and orderly structured. Due to the nature of block copolymers and the interaction between the oxygen atoms of the PEO chains and the ZnO polar surface, the surface defects of ZnO were passivated and the fine dispersion and self-assembly of ZnO in the polymer matrix driven by the P3HT-b-PEO diblock copolymer was obtained, leading to the improvement of device power conversion efficiency and thermal stability. Overall, this work demonstrated that the application of conjugated block copolymers in hybrid BHJ solar cells was a promising approach to improve the device performance and thermal stability.

Superior photocatalytic properties of phosphorous-doped ZnO nanocombs

We demonstrated that phosphorous (P)-doped ZnO nanocombs exhibited extremely high photocatalytic efficiency, which was evaluated by photodegrading methylene blue dyes in aqueous solutions. The photocatalyst takes advantage of the large surface to volume ratio (S/V) and abundant surface defect states of ZnO nanostructures. To maximize the S/V for more efficient light irradiation, the morphologies of the P-doped ZnO nanostructures were tailored by controlling the ratio of argon and oxygen in the carrier gas. P-doping caused a bunch of surface defects in ZnO, which could effectively restrain the recombination of photogenerated carriers and improve the photocatalytic behavior. Superior photocatalytic behaviors of P-doped ZnO nanocombs, as well as the availability of catalyst-free large scale synthesis, provide a new paradigm for rational synthesis of high efficient photocatalysts.

Influence of surface modification by 2-aminothiophenol on optoelectronics properties of ZnO nanoparticles

In this study, a precipitation method was used to synthesise ZnO nanoparticles using suitable precursors. An efficient surface modification method was proposed in order to reduce the agglomeration among synthesised small sized ZnO nanoparticles using 2-aminothiophenol as a capping agent. This article briefly investigated the effects of capping agent like 2-aminothiophenol on the optoelectronic properties of ZnO nanoparticles. The modified effectivity of 2-aminothiophenol has been examined on the nanosized ZnO nanoparticle for fluorescence and UV–visible (UV–vis) studies. The mechanism was studied for ZnO nanoparticles light emitting capability under different conditions. By facilitating the capping of ZnO with 2-aminothiophenol, fluorescence emission of the surface defects vanishes and ultraviolet (UV) emission increases. Surface capping by 2-aminothiophenol effectively covers most of the surface defects of ZnO and results in quenching of the visible region. The UV–vis absorption spectra of modified ZnO nanoparticles has been influenced by modified ZnO nanoparticles as a result of surface modification; where marked blue shift in absorption edge results. By surface modification of ZnO nanoparticles, change in optoelectronics properties has opened the new scope and possibilities to explore and fine tune the optical character of the modified ZnO for various optoelectronics applications such as UV laser.

Structures and optical properties of Zn1−x Ni x O nanoparticles by coprecipitation method

Nanocrystals of undoped and nickel-doped zinc oxide (Zn1−xNixO, where x = 0.00–0.05) were synthesized by the coprecipitation method. Crystalline size, morphology, and optical absorption of prepared samples were determined by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and UV–visible spectrometer. XRD and SEM studies revealed that Ni-doped ZnO crystallized in hexagonal wurtzite structure. Doping of ZnO with Ni2+ was intended to enhance the surface defects of ZnO. The incorporation of Ni2+ in place of Zn2+ provoked an increase in the size of nanocrystals as compared to undoped ZnO. Crystalline size of nanocrystals varied from 10 to 40 nm as the calcination temperature increased. Enhancement in the optical absorption of Ni-doped ZnO indicated that it can be used as an efficient photocatalyst under visible light irradiation. Optical absorption measurements indicated a red shift in the absorption band edge upon Ni doping. The band gap value of prepared undoped and Ni-doped ZnO nanoparticles decreased as annealing temperature was increased up to 800 °C.

Enhanced UV Photon-response of Tin Nano Cluster Loaded- laser Irradiated ZnO Thin film detector

ABSTRACTZinc Oxide (ZnO), II-VI compound semiconductor, is a promising material for ultraviolet (UV) photon sensor applications due to its attractive properties such as good photoconductivity, ease processing at low temperatures and excellent radiation hardness. The rf magnetron sputtering is a suitable deposition technique due to better control over stoichiometry and deposition of uniform film. Studies have shown that the presence of surface defects in ZnO and subsequently their passivation are crucial for enhanced photo-response characteristics, and to obtain the fast response speed. Worldwide efforts are continuing to develop good quality ZnO thin films with novel design structures for realization of an efficient UV photon sensor. In the present work, UV photon sensor is fabricated using a ZnO thin films deposited by rf magnetron sputtering on the corning glass substrate. Photo-response, (Ion/Ioff) of as-grown ZnO film of thickness 100 nm is found to be 3×103 with response time of 90 ms for UV intensity of 140 μW/cm2 (λ = 365 nm). With irradiation on ZnO thin film by pulsed Nd:YAG laser (forth harmonics 266 nm), the sensitivity of the UV sensor is found to enhance. The photo-response increases after laser irradiation to 4x104 with a fast response speed of 35 ms and attributed to the change in surface states and the native defects in the ZnO thin film. Further, enhancement in the ultraviolet (UV) photo-response (8×104) of detector was observed after integrating the nano-scale islands of Sn metal on the surface of laser irradiated ZnO thin film.

Open-Circuit Voltage Improvement in Hybrid ZnO–Polymer Photovoltaic Devices With Oxide Engineering

We present strategies to improve low open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> variability is the ZnO bandgap (E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> by 0.14 eV and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> by depositing conformal amorphous TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> coatings monotonically increased V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> from 0.4 to 0.8 V for devices with increasing TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> coating significantly decreases the reverse-bias current density, leading to an improvement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> , in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.

Highly efficient luminescence from hybrid structures of ZnO/multi-walled carbon nanotubesfor high performance display applications

We report an interesting observation on strong enhancement in green luminescence fromhybrid ZnO/multi-walled carbon nanotubes (MWCNTs). The hybrid structures weresynthesized via a high temperature sintering method. The strong green emission at 510 nmhas been attributed to surface defects of ZnO, originating from interactions betweenZnO and the MWCNT surface, which has been confirmed by high resolutiontransmission electron microscopy and x-ray photoelectron spectroscopy. Furthermore, thetwo-dimensional (2D) layer of this hybrid material shows a high degree of homogeneity and82% transparency. Time resolved emission spectroscopy measurement shows aphotoluminescence decay time in microseconds, which is suitable for making optoelectronicdevices.