Sol-gel ZnO Research Articles

Design, Synthesis, and Postvapor Treatment of Neutral Fulleropyrrolidine Electron-Collecting Interlayers for High-Efficiency Inverted Polymer Solar Cells

A novel fullerene derivative (FBPNOH) grafted with bilateral polar pendants was explored as an efficient electron-collecting layer (ECL) for inverted polymer solar cells (i-PSCs). Using PTB7-Th:PC71BM as the photoactive material, the devices based on FBPNOH achieved power conversion efficiencies (PCEs) exceeding 8.85%, which are higher than those of the reference devices based on sol–gel ZnO. The impact of postsolvent treatment on the interfacial properties and charge transport ability of interlayers and the corresponding optoelectronic characteristics have been systematically investigated in comparison with those of its unilateral counterpart (FPNOH). The results show that the post-solvent annealing process is beneficial for enhancing the carrier transport and extraction characteristics of fullerene-based ECLs for PSCs. Meanwhile, according to ultraviolet photoelectron spectroscopy, dimethylformamide solvent annealing can further reduce the work function of indium tin oxide. The electron-selective device...

Studies on sol-gel dip-coated nanostructured ZnO thin films

Nanostructured ZnO thin films were prepared by sol-gel dip coating technique. Zinc acetate and ammonium hydroxide were used as precursors and ethanol was as solvent. Ammonium hydroxide (NH4OH) solution was added drop-wise under vigorous stirring to obtain the sol-gel of different pH (varying from 6.9 to 7.2). ZnO thin films were obtained by dipping the glass substrates for few seconds and then dried in air at room temperature. This process was repeated for different number of coats for the typical sol. Different numbers of coating cycle was employed to obtain the films with varying thicknesses. These films were annealed at 5000C and were characterized by x-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis of x-rays (EDAX). ZnO thin films obtained from sol-gel dip-coating technique were observed to nanostructured. Average particle size was observed to be smaller than 50 nm. The most of the particles were observed to be spherical in shape. ZnO films were observed to be nonstoichiometric (Zinc deficient) in nature. The results were discussed and interpreted.

Morphology Controlled Synthesis of ZnO Nanorods for Glucose Enzymatic Biosensor

The use of zinc oxide nanorods is a promising option to increase the specific area for efficient glucose oxidase immobilization which further enhance the signal performance of glucose detection. In this work ZnO nanorods were prepared via hydrothermal process on sol-gel ZnO seed layer. The effects of different temperature of annealed seed layer on the morphology and properties of grow ZnO nanorods were investigated. The ZnO seed layers were annealed at various temperatures ranging from 300 to 600 °C for 2 h. The study demonstrated that the process condition of ZnO seed layer had a strong influence on the morphology and crystallinity of ZnO nanorods grow. X-ray diffraction analysis and scanning electronic microscopy were employed to characterize the crystal structure and morphology of the prepared ZnO seed layer and grow ZnO nanorods. The performance of different aspect ratio of ZnO nanorods on glucose detection were measured by using electrochemical analysis. With optimized ZnO nanorods, the glucose biosensor exhibited an enhanced in signal performance with a high sensitivity of 11.36 μA mM-1 cm-2 in the range of 0.05-1 mM. Such high performance was due to more immobilization on the well-aligned ZnO nanorods array and direct electron conduction between the nanorods and the electrodes

Defect Passivation of Low-Temperature Processed ZnO Electron Transport Layer with Polyethylenimine for PbS Quantum Dot Photovoltaics

Lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) present the distinctive ability to utilize short-wave infrared light, good ambient stability, and convenient solution-based fabrication processes and thus attract much attention in the photovoltaic research field. The performance of CQDSCs has been improved by constructing the ZnO/PbS heterojunction, due to suitable band levels and electron mobility of ZnO electron transfer layer (ETL). However, the huge number of defects in low-temperature processed ZnO cause an unbalanced carrier-related processes, which restrict further performance enhancement and flexible production of CQDSCs. Here, we described a facile method to passivate defects in low-temperature sol–gel ZnO by introducing polyethylenimine (PEI) into the precursor solution. Versus the original ZnO film, the composite ZnO:PEI films exhibit better crystallization because of the Zn–N interaction. A series of electronic analyses have shown that the addition of PEI reduces the work function ...

Antimicrobial surfaces with self-cleaning properties functionalized by photocatalytic ZnO electrosprayed coatings.

Photoactive coatings of sol-gel ZnO suspensions were electrosprayed on glass substrates to produce self-cleaning antimicrobial functionalized surfaces. ZnO-functionalized materials exhibited a uniform external surface consisting of a pattern of microspheres with diameters in the 100-300 nm range. Electrospray allowed surface densities up to 0.30 mg cm-2 that displayed considerable hydrophilicity. Water contact angle decreased with UV irradiation to values below 10°. Two different UV doses were tested by adjusting the irradiation time to simulate Summer-Spring and Winter-Fall conditions. The functionalized coatings showed excellent photocatalytic properties towards the photodegradation of Methylene blue. The electrosprayed surfaces also displayed antibacterial activity against Staphylococcus aureus, with >99.5% reduction in the number of culturable cells. The biocidal activity is attributed to the photogenerated reactive oxygen species on the surface of ZnO coatings and the bioavailable zinc ions produced from ZnO dissolution. The photoactive coatings kept surfaces free from bacterial colonization and biofilm formation.

High-Working-Pressure Sputtering of ZnO for Stable and Efficient Perovskite Solar Cells

Owing to its high mobility and low sintering temperature, ZnO is a promising electron-transporting layer for flexible and tandem applications of perovskite solar cells. However, ZnO inevitably triggers the degradation of perovskite materials. Such degradation can be inhibited when ZnO films with improved stoichiometry and minimized defects are used. In this work, a high efficiency with substantial stability of ZnO-based perovskite solar cells is achieved using a high-working-pressure sputtering technique. The high-working-pressure sputtering process can produce higher quality ZnO films with fewer surface defects compared with conventional sputtering or sol–gel ZnO solution processes. A power conversion efficiency of 17.3% is recorded. In addition, the stability of these devices is significantly higher than that of the conventional ZnO-based perovskite solar cells. This work showcases that ZnO can be a good candidate for the electron-transporting layer in perovskite solar cells, particularly for flexible and tandem applications when the right sputtering conditions are used.

Low temperature processed efficient and stable perovskite solar cell

MA0.6FA0.4PbI3 material based efficient and stable perovskite solar cells (PSCs) are fabricated by electron transport layer (ETL) interfacial modification. The highest power conversion efficiency (PCE) of device was ~ 17%. Cesium acetate and cesium carbonate were used with low temperature processed sol-gel ZnO ETL for interface modifications. Low leakage current and enhanced dark injection current are observed from dark current-voltage measurement. From the electrochemical impedance spectroscopy (EIS) measurement higher recombination resistance and lower interfacial contact resistance are observed in the PSC devices. Mott-Schottky analysis also shows the higher flat-band potential and enhanced device performance with cesium acetate ETL. Cesium acetate related ZnO ETL has large grain size which leads to reduce the device series resistance and contact resistance in PSC compared to cesium carbonate ETL related device. Perovskite film on cesium acetate ETL has better surface morphology, topography and hydrophobicity characterization compared to perovskite film grown on cesium carbonate ETL film. The material work function and electron injection barrier are also investigated by X-Ray photoelectron spectroscopy (XPS) measurement and ultraviolet photoelectron spectroscopy (UPS). From electrochemical impedance spectroscopy measurements the charge transport behaviour and trap-assisted carrier recombination are estimated. Fabricated PSCs device stability has been measured for a month-long degradation study. The PSC device stability is observed four times higher with cesium acetate PSCs compared to cesium carbonate ETL related PSCs. The overall device PCE is around 82% higher with cesium acetate compared to cesium carbonate devices.

A comparison of the morphological and electrical properties of sol-gel dip coating and atomic layer deposition of ZnO on 3D nanospring mats

We report on the morphological and electrical properties, with (light) and without (dark) UV illumination, of conformal coatings of ZnO on silica nanosprings deposited by sol-gel and atomic layer deposition (ALD) for the first time. Field Emission Scanning Electron Microscopy (FESEM) imaging showed that both methods produce conformal coatings of ZnO on the nanosprings. The surface of the sol-gel coatings exhibited cracks at higher numbers of dipping/sintering cycles, while the morphology of ALD ZnO films were always smooth and devoid of cracks. The effective photoconductivity of the sol-gel ZnO coated nanospring mats increased nonlinearly with increasing coating thickness. The corresponding dark effective conductivity of the sol-gel ZnO coated nanospring mats also increased within the same thickness range. Alternatively, the effective photoconductivity of the ALD ZnO coated nanospring mats increased linearly with increasing coating thickness. The corresponding dark effective conductivity also increased within the same thickness range. The superior effective conductivity and photoconductivity of the ALD ZnO coated nanospring mats is attributed to the uniformity of the coating and the absence of cracks, which are observed for the thicker sol-gel ZnO coatings.

Surface nanosheets evolution and enhanced photoluminescence properties of Al-doped ZnO films induced by excessive doping concentration

Previous reports found Al-doped ZnO nanosheets were formed at a certain preparation condition, but the formation mechanism was vague. In this work, we prepare sol-gel Al-doped ZnO films, and find that films show the obvious nanorods structure on surface of pure ZnO, but the nanorods changes to nanosheets gradually with rising doping concentration. The formation mechanism of nanosheets is clarified, and the doping concentration dependence of morphology is discussed in this paper. In addition, the photoluminescence spectra of all films appear a series of emissions including the near band edge emission (NBE) at 388 nm and several defect emissions (DE) at 396–499 nm. Previous reports proposed that NBE and DE would rise monotonously or decline monotonously with doping. However, this work reveals that the NBE decreases firstly in the range of low doping concentrations and then improves upon excessive doping concentration, due to crystalline quality and optical energy gap --- this reason is different from traditional explanations. Also, DE at different positions show the different variation trends, without the same increase or the same decrease. At excessive doping concentration, both NBE and DE enhance, illustrating the potential application for the enhanced ultraviolet and white light emitting devices.

Low-temperature-processed ZnO thin films as electron transporting layer to achieve stable perovskite solar cells

Morphology and surface property of ZnO thin films as electron transporting layer in perovskite solar cells are crucial for obtaining high-efficient and stable perovskite solar cells. In this work, two different preparation methods of ZnO thin films were carried out and the photovoltaic performances of the subsequent perovskite solar cells were investigated. ZnO thin film prepared by sol–gel method was homogenous but provided high series resistance in solar cells, leading to low short circuit current density. Lower series resistance of solar cell was obtained from homogeneous ZnO thin film from spin-coating of colloidal ZnO nanoparticles (synthesized by hydrolysis–condensation) in a mixture of 1-butanol, chloroform and methanol. The perovskite solar cells using this film achieved the highest power conversion efficiency (PCE) of 4.79% when poly(3-hexylthiophene) was used as a hole transporting layer. In addition, the stability of perovskite solar cells was also examined by measuring the photovoltaic characteristic for six consecutive weeks with the interval of 2 weeks. It was found that using double layers of the sol–gel ZnO and ZnO nanoparticles provided better stability with no degradation of PCE in 10 weeks. Therefore, this work provides a simple method for preparing homogeneous ZnO thin films in order to achieve stable perovskite solar cells, also for controlling their surface properties which help better understand the characteristics of perovskite solar cells.

Electrospun 3D composite nano-flowers for high performance triple-cation perovskite solar cells

Three dimensional (3-D) flower-shaped SnO2-TiO2 nano-structure has been synthesized by electro-spinning and incorporated on top of sol-gel ZnO ETL to fabricate highly efficient (highest efficiency: 17.25%) triple-cation (methyl ammonium, formamidinium and rubidium cations) based perovskite solar cell (PSC). The flower-based PSCs demonstrate superior photovoltaic performance compared to control ZnO or one-dimensional (1-D) fiber-shaped nano-structure ETL based devices. Nano-structured ETLs passivate the interstitial trap sites in pristine ZnO by intercalation of metal atoms in host ZnO lattice matrix and increase the n-type conductivity of the (nano-structured) ETL films by reducing the functional groups on ZnO surface. The accumulated ions at the perovskite/ETL interface are also well-distributed and hence the accumulation capacitance is significantly reduced in nano-structured ETL based PSCs, due to the branch-structured ETL network. Moreover, the nano-flower based PSC demonstrates superior charge transfer property, compared to nano-fiber based PSC owing to enhanced material crystallinity and higher effective surface area of 3-D nano-flower network, with respect to 1-D nano-fiber structure. The photo-current hysteretic phenomena are also most suppressed in nano-flower based PSC, due to mitigated electrode polarization mechanism in it. Adding to the merits, PSCs incorporating nano-flower ETL demonstrate enhanced device stability compared to the control devices, retaining about 92% of its initial efficiency even after a month. The enhanced device stability with nano-flower based PSC is contributed by the lower hydrophilicity, lower extent of functional surface hydroxyl group and lower content of vacant interstitial trap sites of the respective ETL film.

Changes in the growth orientation, morphological and optical properties of sol-gel nanocrystalline ZnO thin films coated with different thickness

ABSTRACTNanocrystalline ZnO thin films with the varied thickness were deposited on glass substrate using sol-gel spin coating method, and the different thickness of samples was achieved via changing the number of layers. X-ray diffraction patterns of samples showed the preferential orientation at (002) planeupon coating with 2 layers and the gradual increase in random orientation with coating up to 6 layers. Scanning electron microscopeimages appeared a few layer interfaces for 4 and 6 layers sample, and atomic force microscope images revealed the larger particle size and rougher surfacewith increasing thickness, which showed the obvious effects on optical properties of samples too. The increased absorption and reduced energy gap were found through deducing transmittance curves of samples. Further, photoluminescence characteristic was measured and revealed that there are the best intrinsic emission in ultraviolet region and lowest defect emissions in visible region for the sample coated with 4 layers.

Efficient inverted polymer solar cells via pyridine-based organic molecules as interfacial modification layer on sol-gel zinc oxide surface

Abstract Pyridine-based organic molecules (POMs), 4-phenylpyridine (4-Py) and 4-tert-butylpyridine (4-tbp) were used to modify the sol-gel ZnO surface for inverted polymer solar cells (PSCs). X-ray photoelectron spectroscopic (XPS) results showed the formation of N Zn bonding and the reduction of oxygen vacancies (Vo), depicting that POMs chemisorbed on Vo sites of ZnO surface to passivate O defects. From ultraviolet photoelectron spectroscopic (UPS) data, the work function (WF) of ZnO slightly decreased due to interfacial dipole, enhancing the electron extraction from ZnO surface. ZnO surface modified with POMs (ZnO/POMs) possessed smoother morphology and more hydrophobicity than the pristine ZnO surface, providing good interfacial contact with polymeric active layer. The improved surface properties benefited power conversion efficiency (PCE) of bulk-heterojunction (BHJ) PSCs and their stability. As a result, PSCs with ZnO/4-tbp and ZnO/4-Py achieved 9.29% and 9.07% respectively, while 8.16% with the pristine ZnO. Especially, PSCs with ZnO/4-tbp possessed the best PCE because of its intramolecular dipole from electron donating group. This study demonstrates the benefits of POMs modifying the sol-gol ZnO surface for inverted PSCs.

Enhanced electron transport enables over 12% efficiency by interface engineering of non-fullerene organic solar cells

Organic solar cells have attracted much attention in the recent years due to their many intrinsic advantages, such as, light weight, flexibility, low-cost, solution processing, and facile device fabrication. In this study, effective interface engineering was employed to improve all the photovoltaic parameters of an organic solar cell simultaneously by incorporating ZnO nanoparticle (ZnO-NP) interlayer in between the indium tin oxide cathode and sol-gel processed ZnO electron transport layer. The significance of incorporating a ZnO-NP/ZnO bilayer as the electron transport layer in the bulk heterojunction organic solar cells was demonstrated via systematic study, employing a high efficiency photoactive layer system (PBDB-T:ITIC). The bilayer electron transport layer demonstrated reduced work function compared to the sol-gel ZnO, which enabled effective electron transfer from the active layer to the electron transport layer. In addition, improved bilayer surface morphology, via reduction of ZnO-NP layer roughness, and better crystallinity compared to sol-gel ZnO facilitated charge separation and transmission between electron transport layer and active layer. Consequently, the devices with bilayer interlayer exhibited an enhancement of > 13% in power conversion efficiency compared to the control devices with sol-gel only ZnO as electron transport layer. The mechanisms behind the improvement in device performance were analysed using the ultraviolet and X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The champion bilayer device exhibited 12.24% efficiency which is much higher than the efficiency of 10.69% for the control device.

Surfactant-Enriched ZnO Surface via Sol-Gel Process for the Efficient Inverted Polymer Solar Cell.

In this study, we demonstrate that the top surface is enriched by surfactants, tetraoctylammonium bromide, and cetylpyridinium bromide (CPB), in the sol-gel ZnO, being evidenced by the Br depth profile of electron spectroscopy for chemical analysis data. X-ray photoelectron spectroscopy results showed the formation of Zn-Br bonding due to the oxygen defects occupied by Br at the surfactant-enriched ZnO surface. The surfactant-enriched ZnO surface possessed a smoother surface and more hydrophobicity than the pristine ZnO from the experimental results of atomic force microscopy and contact angle, respectively. On the basis of ultraviolet photoelectron spectroscopy data, the work function slightly reduced due to the dipole built-up by the electrostatic force between Br- and N+ to enhance the electron extraction ability. The improved properties benefited the power conversion efficiency (PCE) of bulk-heterojunction polymer solar cells (PSCs) by spin-coating the active layer on the surfactant-enriched ZnO surface. The inverted PSCs with the surfactant-enriched ZnO surface showed the highest PCE of 9.55% for the CPB case, in comparison with the pristine ZnO surface (8.08% PCE). This study discloses that turning the ZnO surface is easily achieved by the addition of surfactants with different molecular structures in the sol-gel ZnO for high performance polymer solar cells.

Enhanced Lifetime and Efficiency of Red Quantum Dot Light‐Emitting Diodes with Y‐Doped ZnO Sol–Gel Electron‐Transport Layers by Reducing Excess Electron Injection

AbstractCharge balance between electrons and holes, which are injected into the quantum dot (QD) emission layer (EML), is critical for realizing stable and efficient QD light‐emitting diodes (QLEDs). ZnO has been widely used as an electron‐transport layer (ETL) because of its superior performance compared to other metal oxides. However, nearly barrier‐free electron injection into the QD EML leads to spontaneous charge transfer and excess electron injection, resulting in reduced device performance. Here, to adjust electron–hole balance and thereby improve the lifetime and efficiency of QLEDs, we introduce an yttrium (Y)‐doped ZnO (YZO) ETL into QLEDs. The sol–gel processed YZO film, with a mobility that could be simply tuned by varying the Y concentration, provides enhanced charge‐transport balance by suppressing excess electron flow to the QD EML. Furthermore, YZO helps suppress QD charging and smooth the surface morphology. Applying the YZO ETL into the inverted‐structure QLED enables us to achieve color‐saturated red emission, an improved external quantum efficiency of up to 8.6%, and an eight times longer lifetime compared to the device with undoped ZnO. This result provides a simple method for enhancing the performance of QLEDs by easily controlling metal doping concentration in the sol–gel processed ZnO ETL.

Structural and morphological properties of sol-gel ZnO:Ni films

Ni doping induces modifications and considerable changes in the optical, electrical and magnetic properties of ZnO films. In this work, the influence is discussed of Ni-doping (two nickel concentrations) and annealing temperature (ranging from 300 ° C to 800 °C) on the structural and optical properties of sol-gel derived ZnO:Ni films. Uniform and smooth films were obtained by spin-coating on quartz and Si substrates. The ZnO:Ni films were crystallized in wurtzite phase with no impurity phases found for annealing temperatures up to 600 °C. The size of the crystallites was strongly affected by the Ni content and the heat treatment. Furthermore, the Ni doping improved the optical transparency of the sol-gel films, while the AFM studies showed that the film morphology and the roughness were influenced by the nickel doping.

Highly crystalline bilayer electron transport layer for efficient conjugated polymer solar cells

Solution processed solar cells are a promising renewable energy technology due to the low fabrication costs. The most commonly used electron transport layer for solution processed organic solar cells is ZnO. However, sol-gel derived ZnO is amorphous, which limits interfacial charge transport. In this study, we demonstrate a ZnO bilayer, composed of a nanoparticle ZnO and sol-gel derived ZnO layer, as the electron transport layer in polymer solar cells incorporating the novel polymer poly [(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di (2-octyldodecyl)-2,2′; 5′,2″; 5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD). Compared with the single layer sol-gel ZnO, the bilayer displayed enhanced crystallinity. Consequently, the interfacial transport from the active layer was improved, as evidenced by dark J-V and PL spectroscopy measurements. Solar cells incorporating this bilayer ZnO layer achieved PCE values exceeding 10%, a relative improvement of 25% compared to the sol-gel ZnO devices.

Fabrication of a Combustion-Reacted High-Performance ZnO Electron Transport Layer with Silver Nanowire Electrodes for Organic Solar Cells.

Herein, a new methodology for solution-processed ZnO fabrication on Ag nanowire network electrode via combustion reaction is reported, where the amount of heat emitted during combustion was minimized by controlling the reaction temperature to avoid damaging the underlying Ag nanowires. The degree of participation of acetylacetones, which are volatile fuels in the combustion reaction, was found to vary with the reaction temperature, as revealed by thermogravimetric and compositional analyses. An optimized processing temperature of 180 °C was chosen to successfully fabricate a combustion-reacted ZnO and Ag nanowire hybrid electrode with a sheet resistance of 30 Ω/sq and transmittance of 87%. A combustion-reacted ZnO on Ag nanowire hybrid structure was demonstrated as an efficient transparent electrode and electron transport layer for the PTB7-Th-based polymer solar cells. The superior electrical conductivity of combustion-reacted ZnO, compared to that of conventional sol-gel ZnO, increased the external quantum efficiency over the entire absorption range, whereas a unique light scattering effect due to the presence of nanopores in the combustion-derived ZnO further enhanced the external quantum efficiency in the 450-550 nm wavelength range. A power conversion efficiency of 8.48% was demonstrated for the PTB7-Th-based polymer solar cell with the use of a combustion-reacted ZnO/Ag NW hybrid transparent electrode.

High sensitivity and fast response sol-gel ZnO electrode buffer layer based organic photodetectors with large linear dynamic range at low operating voltage

Abstract In this work, we demonstrate high sensitive organic photodetectors (OPDs) by incorporating the common sol-gel processed ZnO film between the ITO electrode and the polymer:fullerene active layer. The device requires a low negative working voltage to exhibit fast response and broad linear dynamic range (LDR), and the external quantum efficiency (EQE) of the devices can exceed 100%. The sol-gel ZnO layer can efficiently block the external charge injection from the ITO electrode to the active layer, which dramatically reduces the dark current density. Two kind devices (ITO/ZnO/P3HT:PC61BM/Al and ITO/ZnO/PTB7:PC71BM/Ag) are demonstrated and their dark current densities are effectively reduced from 0.07 mA/cm2 to 8.16 × 10−5 mA/cm2 and from 0.75 mA/cm2 to 6.50 × 10−5 mA/cm2 at −0.5 V, respectively. Although the two kind devices show the comparable characteristics, the transient photocurrent and capacitance–voltage (C-V) measurements indicate the two different working mechanisms related. Under illumination, in the P3HT:PC61BM based devices, some surface defect states of ZnO can be occupied and the photogenerated charges accumulate around the interface between the ZnO layer and the active layer, which contribute a tunneling injection under the applied reverse bias. While in the PTB7:PC71BM based devices, the photogenerated charges rapidly transport to the corresponding electrodes and then be collected under reverse bias. The different working mechanisms lead to the various device performances. The OPDs based on P3HT:PC61BM exhibit a higher EQE of more than 200% (460 nm - 600 nm) at −0.5 V, while the PTB7:PC71BM based devices show a more than 1.0 × 1015 Jones detectivity (390 nm - 730 nm) and achieve the over 110 dB linear dynamic range (LDR) under the different monochromatic illuminations at −0.5 V.