Optimization of CdSe layer on modified ZnO hierarchical spheres by spin-SILAR for efficient CdS/CdSe co-sensitized solar cells

In this study, after CdS quantum dots sensitized ZnO hierarchical spheres (ZnO HS), we used a simple process to deposit CdSe QDs on ZnO by spin-coating-based SILAR, and applied to photoanodes of quantum dots-sensitized solar cells. Before CdS and CdSe QDs deposition, the ZnO HS photoanodes were modified by Zn(CH₃COO)₂·2H₂O methanol solution to further enhance the open-circuit voltage and power conversion efficiency (PCE). The program of modifying photoanodes and the number of CdSe spin-SILAR cycles are evaluated on the optical and electrochemical properties of the cells. As a result, a high energy conversion efficiency of 2.49 % was obtained by using modified ZnO HS/CdS photoanode under AM 1.5 illumination of 100 mW cm^?2. And further decorated by the CdSe QDs, the ZnO HS/CdS/CdSe cell achieved a PCE of 5.36 % due to the modification of ZnO HS nanostructure, the enhanced absorption in the visible region, the lower recombination reaction and higher electron lifetime.     

Vertically aligned and ordered ZnO/CdS nanowire arrays for self-powered UV–visible photosensing

Photodetectors based on photoconductivity effect are usually driven by an external power source. A self-powered photodetector can be powered by incident light using the photovoltaic effect. Here, photoelectrochemical cells with periodically aligned ZnO/CdS nanowire arrays as photoanodes were fabricated and investigated for detecting UV and visible light. At zero bias, this self-powered UV–visible photodetector showed high responsivities of 35.4 and 23.2 mA/W for UV and visible light, a fast rise time of 0.18 s, and a decay time of 0.32 s. The spectral responses of the self-powered photodetectors based on ZnO/CdS nanowire arrays exhibited superior photoresponse in both UV and visible regions in comparison with ZnO nanowire film and ZnO nanowire arrays. The high photosensing performance originates from the excellent light trapping ability at broadband wavelengths and the high charge collection efficiency of the highly ordered ZnO/CdS nanowire arrays. The results indicate that the ZnO/CdS heterojunctions with periodic nanostructures provide a facile frame for UV–visible detecting applications.     

Effect of inorganic shells on luminescence properties of ZnS:Ag nanoparticles

The hydrothermally synthesized Ag-doped ZnS (ZnS:Ag) nanoparticles have been coated with inorganic shells by a chemical precipitation method. The ZnS:Ag/ZnS, ZnS:Ag/CdS, and ZnS:Ag/ZnO core–shell nanoparticles with different thickness of ZnS, CdS, and ZnO shells have been prepared. The effects of shells on the luminescence properties of ZnS:Ag cores have been investigated through the photoluminescence (PL) spectra and luminescence stabilities of products. In the core–shell nanoparticles involved here, the ZnO shell can most significantly enhance the luminescence of ZnS:Ag cores. The 450 nm emission intensity of ZnS:Ag/ZnO nanoparticles is up to 125 % of that of ZnS:Ag nanoparticles. However, the ZnO shell can hardly influence the luminescence stability under ultraviolet irradiation. The ZnS shell can only increase the luminescence of ZnS:Ag cores to some extent, but it can improve the luminescence stability under ultraviolet irradiation. Although the CdS shell can improve the luminescence stability to some extent, it quenches the luminescence of ZnS:Ag nanoparticles dramatically.     

ZnO@CdS core–shell thin film: fabrication and enhancement of exciton life time by CdS nanoparticle

In the present study photoluminescence behavior of ZnO and ZnO@CdS core–shell nanorods film has been reported. ZnO nanorods were grown on the glass coated indium tin oxide (ITO) surface by seeding ZnO particle followed with nanorods growth. These nanorods were coated with CdS by chemical bath deposition techniques to have ZnO@CdS thin film and further annealed at 200 °C for their adherence to the ITO surface. The coating was characterized for surface morphology using SEM and optical behavior using UV–visible spectrophotometer. Energy dispersive X-ray (EDX) was used for compositional analysis and time resolve photoluminescence decay for excitons life time measurement. The absorption spectrum reveals that the absorption edge of ZnO@CdS core–shell heterostructure shifted to 480 nm in the visible region whereas ZnO nanorods have absorption maxima at 360 nm. The excitons lifetime of ZnO@CdS was found to be increased with the thickness of the CdS layer on ZnO nanorod. These ZnO@CdS core–shell nanostructures will be of great use in the field of photovoltaic cell and photocatalysis in a UV–visible region.     

Aqueous synthesis of CdSe and CdSe/CdS quantum dots with controllable introduction of Se and S sources

A new and convenient route is developed to synthesize CdSe and core–shell CdSe/CdS quantum dots (QDs) in aqueous solution. CdSe QDs are prepared by introducing H₂Se gas into the aqueous medium containing Cd²⁺ ions. The synthesized CdSe QDs are further capped with CdS to form core–shell CdSe/CdS QDs by reacting with H₂S gas. The gaseous precursors, H₂Se and H₂S, are generated on-line by reducing SeO₃ ^2? with NaBH₄ and the reaction between Na₂S and H₂SO₄, and introduced sequentially into the solution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield (3 and 20 %) and narrow full-width-at-half-maximum (43 and 38 nm). The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals. The relatively standard deviation of maxima fluorescence intensity is only 2.1 % (n = 7) for CdSe and 3.6 % (n = 7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and can be readily extended to the large-scale aqueous synthesis of QDs.     

Close-space sublimation grown CdS window layers for CdS/CdTe thin-film solar cells

We report on the synthesis and characterization of CdS window layers grown by close-space sublimation (CSS) method for CdS/CdTe thin-film solar cells. Comparing with CdS window layers grown by other methods such as sputtering and chemical bath deposition, CSS-grown CdS layers can facilitate the consumption of CdS layers and suppress the diffusion of Te into CdS window layers. CSS-grown CdS layers exhibit much larger grains with faceted morphology. Due to large grains, CSS CdS layers must be grown thick enough to minimize the effects of pin-holes. The use of thicker CdS layer causes reduced blue response, resulting in current loss. Therefore, the thickness of CSS CdS window layer must be carefully optimized to achieve high efficiency. Our best small area dot cell using a CSS CdS window layer has exhibited a cell efficiency of about 14.2 % with an open circuit voltage (V_OC) of 806 mV, a short circuit current (J_SC) of 25.2 mA/cm², and a fill factor (FF) of 69.8 % under AM1.5 illumination and without an antireflection coating, slightly lower than our best reference cell using a sputtered CdS window layer (V_OC = 845 mV, J_SC = 24.5 mA/cm², FF = 76.8 %, and efficiency = 15.8 %).     

Dependence of photovoltaic performance of solvothermally prepared CdS/CdTe solar cells on morphology and thickness of window and absorber layers

In the present work a new strategy for straightforward fabrication of CdS/CdTe solar cells, containing CdS nanowires and nanoparticles as a window layer and CdTe nanoparticles and microparticles as an absorber layer, are reported. CdS and CdTe nanostructures were synthesized by solvothermal method. X-ray diffraction analysis revealed that highly pure and crystallized CdS nanowires and nanoparticles with hexagonal structure and CdTe nanoparticles with cubic structure were obtained. Atomic force microscope and field emission scanning electron microscope images showed that CdS nanowires with length of several μm and average diameter of 35 nm, CdS nanoparticles with average particle size of 32 nm and CdTe nanoparticles with average particle size of 43 nm, were uniformly coated on the substrate by the homemade formulated pastes. Based on ultraviolet–visible absorption spectra, the band gap energies of CdS nanowires, CdS nanoparticles and CdTe nanoparticles were calculated 2.80, 2.65 and 1.64 eV, respectively. It was found that, the photovoltaic performance of the solar cells depends on thickness of CdTe and CdS films, reaching a maximum at a specific value of 6 μm and 225 nm, respectively. For such cell made of CdS nanowires and CdTe nanoparticles the V_OC, J_SC, fill factor and power conversion efficiency were calculated 0.62 V, 6.82 mA/cm², 59.7 and 2.53 %, respectively. Moreover, photovoltaic characteristics of the solar cells were dependent on CdTe and CdS morphologies. CdS/CdTe solar cell made of CdTe and CdS nanoparticles had the highest cell efficiency (i.e., 2.73 %) amongst all fabricated solar cells. The presented strategy would open up new concept for fabrication of low-cost CdS/CdTe solar cells due to employment of a simple chemical route rather than the vapor phase methods.     

Influence of Au Photodeposition and Doping in CdS Nanorods: Optical and Photocatalytic Study

Au-modified CdS nanorods (100–200 nm × 5–10 nm) are synthesized via two different techniques, namely photodeposition and doping. The prepared samples are characterized by x-ray powder diffraction, transmission electron microscopy (TEM), and UV–vis and fluorescence spectroscopy. X-ray diffraction study confirmed the hexagonal phase of bare and Au-CdS samples, whereas, 5 wt% Au³⁺ doping into CdS resulted in a slight distortion in the crystal structure toward higher degree side. TEM images revealed the fine distribution of Au nanodeposits of size in the range of 2.5–4.5 nm on to the CdS surface in the photodeposited sample. The optical spectrum shows a significant red-shift in absorption onset (485 nm → 515 nm) and band-edge emission (505 nm → 512 nm) of CdS nanorods with the replacement of certain Cd²⁺ ions with Au³⁺. The influence of Au photodeposition and doping in CdS nanorods was comparatively tested by photooxidation of RhB (50 ppm) dye aqueous solution under direct sunlight irradiation (35–40 mWcm^?2). Our results point out that 5 wt% Au³⁺ doping into CdS nanorods remarkably improved its activity and stability due to homogeneous dispersion of charge throughout the crystal, quick Fermi level equilibration, and an improvement in ionic bond formation.     

Application of ultra-thin CdS film as buffer layer in non-doped blue organic light-emitting diodes

The device inserted 0.5 nm thick cadmium sulfide (CdS) as buffer layer, prepared by vacuum thermal evaporation method, has been studied on the non-doped blue organic light-emitting diode. Compared to the device without ultra-thin CdS film, the maximum luminance of the device with ultra-thin CdS film was 11,370 cd/m² at 11 V, and the maximum current efficiency reached 3.10 cd/A, increased 1.5 times and 1.2 times, respectively. In the optimized devices with the structure of ITO/MoO₃ (10 nm)/TPABBI (35 nm)/Bphen (40 nm)/CdS (0, 0.1, 0.3 and 0.5 nm)/LiF (0.5 nm)/Al (100 nm), the effects of CdS layer on the photoelectric performance of the devices were investigated in detail. When the CdS thickness was 0.3 nm, the maximum luminance was 13,590 cd/m² at 9 V and the turn on voltage was only 3 V. The maximum current efficiency of 3.42 cd/A was obtained. It is indicated that the simple structure of the device with inserted ultra-thin CdS film, cheap and stable inorganic photoelectric material, may be a promising way to fabricate hybrid organic–inorganic LEDs with high performances.     

Photocatalytic performance of CdS nanomaterials for photodegradation of organic azo dyes under artificial visible light and natural solar light irradiation

The CdS semiconductors have been prepared at low temperature via catalyst-free chemical precipitation method without using any surfactant or capping agent. Either water or ethylene glycol, as a solvent, provides spherical CdS nanostructures with a comparable size of 117–121 nm. The molar ratio of Cd/S plays an important role in determining phase structure, morphology and photocatalytic performance of the prepared CdS nanostructures. Increasing molar ratio of S^2? results in not only mixed cubic-hexagonal phases but also low photocatalytic performance. CdS nanoparticles with good dispersibility prepared at Cd/S molar ratio of 1:1 shows high photocatytic efficiency of 95% toward photodegradation of reactive red azo dye (RR141) under visible light irradiation up to 240 min. The degradation efficiency of CdS nanoparticles also reaches 48% under natural solar light irradiation for 80 min. This work demonstrates the promising potential of CdS nanomaterials as photocatalysts for environmental remediation.     

Low-temperature growth of well-aligned ZnO nanowire arrays by chemical bath deposition for hybrid solar cell application

Well-aligned ZnO nanowire arrays were grown on indium tin oxide coated glass substrates by a facile chemical bath deposition technique. Morphologies, crystalline structure and optical transmission were investigated by field-emission scanning electron microscope, X-ray diffraction and UV–visible transmission spectrum, respectively. The results showed that ZnO nanowires were aligned in a dense array approximately perpendicular to substrate surface, they were wurtzite-structured (hexagonal) ZnO. In addition, the nanowire arrays exhibited high optical transmission (>85 %) in the visible region. Furthermore, an inverted inorganic/polymer hybrid solar cell was built using as-grown well-aligned ZnO nanowire arrays as inorganic layer, under the AM 1.5 illumination with a light intensity of 80 mW/cm², the device showed an open circuit voltage (V_oc) of 0.44 V, a short circuit current (J_sc) of 3.23 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.68 %.     

P-type conductivity in doped and codoped ZnO thin films synthesized by RF magnetron sputtering

N-doped and Al–N codoped ZnO thin films with different volume ratios of N₂ reactive gas were deposited on plane glass substrates using the radio frequency magnetron sputtering method. The phase transition temperature and absorption edge of the ZnO powder were studied by differential scanning calorimetry at different heating rates and with Fourier transform infrared spectroscopy, respectively. The target used for the sputtering was synthesized using a palletize machine. It was sintered at 450 °C for 5 h. The X-ray diffraction results confirm that the thin films have wurtzite hexagonal structures with a very small distortion. The results indicate that the ZnO thin films have obviously enhanced transmittance of up to 80% on an average in the visible region. The Al–N codoped ZnO thin films exhibited the best p-type conductivity with a resistivity of 0.825 Ω-cm, a hole concentration of 6.55 × 10¹⁹ cm^?3, and a Hall mobility of 1.25 cm²/Vs. The p-type conductivity was observed after doping and codoping of the ZnO thin film.     

Structural and optical characterization of CdS and CdHgTe thin films for the fabrication of CdHgTe/CdS structure

This article presents the deposition and characterization of CdS and CdHgTe thin films for the fabrication of CdHgTe/CdS structure. The growth of CdS and CdHgTe thin films on FTO-coated conducting glass substrates have been performed by chemical bath deposition (CBD) and electrodeposition methods, respectively. The deposition conditions have been optimized for getting better quality layers of CdS and CdHgTe. The grown layers of both CdS and CdHgTe have been characterized by photoelectrochemical cell (PEC) measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis spectrophotometer. Annealing effect of the deposited films has also been investigated. Finally the fabrication of CdHgTe/CdS structure has been performed and investigated by I–V characteristics. PEC, XRD, SEM and UV–vis spectrophotometer studies reveal that chemically deposited CdS layers are n-type with band gap values vary from 2.29 to 2.41 eV and cubic with (111) preferential orientation, and have spherical grain distributed over the surface. However, electrodeposited CdHgTe layers are p-type with band gap values varying from 1.50 to 1.53 eV and cubic with highly oriented CdHgTe crystallites with the (111) planes parallel to the substrate, and have uniform distribution of granular grains over the surface. The fabricated CdHgTe/CdS structure gave an open-circuit photovoltage and a short-circuit photocurrent of 510 mV and 13 mA/cm² respectively, under AM 1.5 illumination.     

Effect of electron irradiation on the photopleochroism of ZnO/CdS/Cu(In,Ga)Se<Subscript>2</Subscript> solar cells

The effect of irradiation with 1-MeV electrons to various doses on the photosensitivity of ZnO/CdS/Cu(In, Ga)Se₂ solar cells and related CdS/Cu(In,Ga)Se₂ and ZnO/Cu(In,Ga)Se₂ heterostructures has been studied. Both the photoconversion efficiency and the coefficient of induced photopleochroism of ZnO/CdS/CIGS solar cells remained practically unchanged upon irradiation up to a total dose of 10⁻¹⁷ cm⁻². It is suggested that the method of polarization photoelectric spectroscopy can be used for evaluating the effect of electron irradiation on the photosensitivity of semiconductor photoconverters.     

Effect of ZnO buffer layer on AZO film properties and photovoltaic applications

Aluminum-doped ZnO (AZO) transparent conducting films were deposited on glass substrates with and without intrinsic ZnO (i-ZnO) buffer layers by a home made and low cost radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient and under a low vacuum level. The films were examined and characterized for electrical, optical, and structural properties for the application of CIGS solar cells. The influence of sputter power, deposition pressure, film thickness and residual pressure on electrical and optical properties of layered films of AZO, i-ZnO and AZO/i-ZnO was investigated. The optimization of coating process parameters (RF power, sputtering pressure, thickness) was carried out. The effects of i-ZnO buffer layer on AZO films were investigated. By inserting thin i-ZnO layers with a thickness not greater than 125 nm under the AZO layers, both the carrier concentration and Hall mobility were increased. The resistivity of these layered films was lower than that of single layered AZO films. The related mechanisms and plasma physics were discussed. Copper indium gallium selenide (CIGS) thin film solar cells were fabricated by incorporating bi-layer ZnO films on CdS/CIGS/Mo/glass substrates. Efficiencies of the order of 7–8% were achieved for the manufactured CIGS solar cells (4–5 cm² in size) without antireflective films. The results demonstrated that RF sputtered layered AZO/i-ZnO films are suitable for application in low cost CIGS solar cells as transparent conductive electrodes.     

Effect of rapid thermal annealing on Zn/ZnO layers

Zn/ZnO layers were deposited on SiO₂/Si substrate by magnetron sputtering at room temperature, and then these layers were annealed at various temperatures from 200 to 400 °C in nitrogen atmosphere for 1 min. The structural and electrical properties of the Zn/ZnO layers before and after annealing are systematically investigated by X-ray diffraction, scanning electron microscopy, current–voltage measurement system, and Auger electron spectroscopy. Current–voltage measurements show that the Zn/ZnO layers exhibit an Ohmic contact behavior. It is shown that, initially, the specific contact resistivity decreases with the increase of the annealing temperature and reaches a minimum value of 9.76 × 10^?5 Ω cm² at an annealing temperature of 300 °C. However, with a further increase of the annealing temperature, the Ohmic contact behavior degrades. This phenomenon can be explained by considering the diffusion of zinc interstitials and oxygen vacancies. It is also shown that Zn-rich ZnO thin films can be obtained by annealing Zn on the surface of ZnO film and that good Ohmic contact between Zn and ZnO layers can be observed when the annealing temperature was 300 °C.     

Characterization of CBD–CdS nanocrystals doped with Co<Superscript>2+</Superscript>

Co-doped CdS nanofilms are synthesised by chemical bath deposition growth technique at the temperature of 60?±?2 °C. The cobalt molar fraction was ranged from 0 ≤ x ≤ 5.47, which was determined by energy-dispersive X-ray spectroscopy. The X-ray diffraction shows that the nanofilms are of CoS–CdS nanocomposites with individual CdS and CoS crystalline planes. The Co-doped CdS crystalline phase was zinc-blende that was determined by X-ray diffraction and confirmed by Raman spectroscopy. The average grain size of the CdS films was ranged from 2.56 to 1.67 nm that was determined by Debye–Scherrer equation from ZB (111) direction and it was confirmed by Wang equation and high resolution transmission electron microscopy (HRTEM). Raman scattering shows that the CdS lattice dynamics is characteristic of a bimodal behaviour, in which the first optical longitudinal mode denotes the characteristic peak at 305 cm^?1 of the CdS nanocrystals that is associated with the cobalt incorporation. Nanofilms present two main bandgaps at ~?2.56 and 3.80 eV, which are attributed to single CdS and quantum-confinement due to nanocrystals size. The increase in band gap with increase in cobalt concentration suggests intermetallic compound of CoS (E_g = 1.60 eV) with CdS (E_g = 2.44 eV). The CdS nanocrystals size was ranged from 2.46 to 1.81 nm that was determined from ZB (111) direction by Debye–Scherrer equation and confirmed by the Wang equation. The room-temperature photoluminescence of the Co-doped CdS presents well-resolved radiative bands associated to structural defects and with the quantum-confinement. For the Co-doped CdS the photoluminescence intensity increases indicate a high-passivation of the nanocrystals.     

Surface modification of ZnO nanorods with CdS quantum dots for application in inverted organic solar cells: effect of deposition duration

Incorporating cadmium sulfide quantum dots (CdS QDs) onto ZnO nanorod (ZNRs) has been investigated to be an efficient approach to enhance the photovoltaic performance of the inverted organic solar cell (IOSC) devices based on ZNRs/poly (3-hexylthiophene) (P3HT). To synthesize CdS/ZNRs, different durations of deposition per cycle from 1 to 9 min were used to deposit CdS via SILAR technique onto ZNRs surface grown via hydrothermal method at low temperature on FTO substrate. In typical procedures, P3HT as donor polymer were spun-coating onto CdS/ZNRs to fabricate IOSC devices, followed by Ag deposition as anode by magnetron sputtering technique. Incorporation of CdS QDs has modified the morphological, structural, and optical properties of ZNRs. Incorporation of CdS QDs onto ZNRs also led to higher open circuit voltage (V_oc) and short circuit current density (J_sc) of optimum ZNRs/CdS QDs devices due to the increased interfacial area between ZNRs and P3HT for more efficient exciton dissociation, reduced interfacial charge carrier recombination as a result of lower number of oxygen defects which act as electron traps in ZnO and prolonged carrier recombination lifetime. Therefore, the ZNRs/CdS QDs/P3HT device exhibited threefold higher PCE (0.55%) at 5 min in comparison to pristine ZNR constructed device (0.16%). Overall, our study highlights the potential of ZNRs/CdS QDs to be excellent electron acceptors for high efficiency hybrid optoelectronic devices.     

Tuning of optical,thermal and antimicrobial capabilities of CdS nanoparticles with incorporated Mn prepared by chemical method

Cadmium sulfide (CdS) nanoparticles with different amounts of incorporated Manganese (Mn: 10, 15 and 20 mol%) has been prepared by chemical method. In optical properties the UV–Vis–NIR absorption spectra of all samples showed blue shift compared with the bulk CdS and decrease in band gap with Mn concentration. The presence of functional groups was identified by fourier transform infrared spectroscopy. It confirmed presence of metal sulfur bonding and weak interaction between anions (S^2?) and cations (Mn²⁺). The Photoluminescence spectra showed two emission peaks at 397 and 541 nm corresponding to the electron-hole recombination of CdS and surface trap induced emission respectively. In thermal properties thermogravimetric curve indicated increase in weight loss with Mn incorporation suggesting that pure CdS nanoparticles are thermally more stable than Mn incorporated CdS nanoparticles. The antimicrobial activities of Mn incorporated CdS were studied against Gram-positive and Gram-negative bacteria as test microorganisms using agar plating-spot inoculation method.     

Sol–gel derived Ag-doped ZnO thin film for UV photodetector with enhanced response

Ultraviolet (UV) photodetectors based on pure zinc oxide (ZnO) and Ag-doped ZnO (Ag:ZnO) thin films with different Ag doping contents (0.05, 0.15, 0.65, 1.30 and 2.20 %) have been prepared by sol–gel technique. Photoresponse characteristics of the prepared detectors have been studied for UV radiation of λ = 365 nm and intensity = 24 μW/cm². The Ag:ZnO thin film-based photodetector having an optimum amount of 0.15 at. wt% Ag dopant exhibits a high photoconductive gain (K = 1.32 × 10³) with relatively fast recovery (T _37 % = 600 ms) and minimal persistence in comparison to other prepared photodetectors. The incorporation of Ag dopant (≤0.15 %) at Zn lattice sites (Ag_zn) in ZnO creates acceptor levels in the forbidden gap, thereby reducing the value of dark current. Upon illumination with UV radiation, the photogenerated holes recombine with the captured electrons at the Ag_zn sites. The photogenerated electrons increase the concentration of conduction electrons, thereby giving an enhanced photoresponse for Ag:ZnO photodetector (0.15 % Ag). At higher dopant concentration (≥0.65 %), Ag incorporated at the interstitial sites of ZnO leads to the formation of deep energy levels below the conduction band along with increase in oxygen-related defects, thereby giving higher values of dark current. The incorporation of Ag at interstitial sites results in degradation of photoresponse along with the appearance of persistence in recovery of the photodetector in the absence of UV radiation.