Effect of the purity of CdTe starting material on the impurity profile in CdTe/CdS solar cell structures

We have investigated CdTe/CdS/In₂O₃:F/glass solar cell structures using quantitative SIMS for profiling the impurity distribution from the CdTe free surface through to the glass substrate. Ion implanted CdTe standards were used. The effect of the purity of the CdTe starting material was determined by studying two structures grown from 7N and 5N source materials. Particular emphasis was placed on the potentially electrically active impurities that may originate from the CdTe starting material, and are likely to affect the CdTe/CdS solar cell performance. It was shown that Cu, Zn, Sn, Sb and Pb profiles had the same level and shape in the CdTe layer regardless of the purity of the starting material used, and were therefore not originating from the starting material. Cl, O, Na and Si showed higher levels for structures grown using 5N purity CdTe compared to those from 7N, and may, at least in part, be due to the CdTe starting material used. It was also postulated that at least some impurities (in addition to Cl) may partially come from the CdCl₂ treatment, and/or from the TCO (In) and glass (for Si and Na). Te and S interdiffusion at the CdTe/CdS interface was also shown to be enhanced when 5N CdTe source material is used as compared to 7N.     

Microwave Synthesis of Cu/Cu<Subscript>2</Subscript>O/SnO<Subscript>2</Subscript> Composite with Improved Photocatalytic Ability Using SnCl<Subscript>4</Subscript> as a Protector

Cu/Cu₂O/SnO₂ composites were successfully prepared with a facile microwave synthesis method. The structure of Cu/Cu₂O/SnO₂ composite was studied by morphology characterizations, such as X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy, which showed that the size of the Cu/Cu₂O/SnO₂ particles is 20–50 nm. The synthesis mechanism revealed that SnCl₄ obstructed between Cu(OH) and ethylene glycol, preventing Cu(OH) being reduced into Cu at high temperature. The photocatalytic property of Cu/Cu₂O/SnO₂ composite was investigated by degrading the mixed dyestuff under the irradiation of visible light at room temperature. Benefiting from the effect of electron transfer, the photocatalytic performance of the microwave-prepared Cu/Cu₂O/SnO₂ composite was much better than that of pure Cu₂O. The possible photocatalytic mechanism of the Cu/Cu₂O/SnO₂ composite catalysts was proposed and elaborated in this study. This synthesis of Cu/Cu₂O/SnO₂ composite may provide a method for other Cu₂O/semiconductor composites microwave preparation.     

Effects of CdCl2 treatment and annealing on CdS/SnO2/glass heterostructures for solar cells

Effects of CdCl₂ post-growth treatments and annealing under different conditions on the surface and interface properties of CdS/SnO₂/glass heterostructure were studied. CdS thin films were grown on SnO₂-coated glass substrates for CdS/CdTe heterojunction solar cells by the solution growth technique. It was found that CdCl₂ post-growth treatments and annealing enhanced the CdS-related XRD peaks, narrowed the CdS characteristic Raman bands, removed or depressed the disorder related Raman features, and improved the CdS film crystalline quality significantly, which are advantageous to the application in solar cells as a window layer material.     

High-performance ammonia gas sensor based on bimetallic oxide Zn2SnO4 decorated with reduced graphene oxide

The coupling effect and synergistic effect between the two metal elements of the bimetallic oxide make it has unique electrical characteristics and gas-sensitive properties, but it has the limitation of low conductivity. In this paper, the bimetallic oxide Zn₂SnO₄ was decorated with reduced graphene oxide (rGO) to increase its electrical conductivity and promote charge transfer during gas adsorption, which enhances the response and shortens the response time of the bimetallic oxide gas sensor. The high-performance ammonia sensor based on Zn₂SnO₄/rGO nanocomposite material was prepared by environmentally friendly hydrothermal method and spin coating technology. The structure and properties of composite materials were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The ammonia sensing performance of Zn₂SnO₄/rGO nanocomposite sensor was tested at room temperature, including the dynamic response, response/recovery time, selectivity, repeatability, long-term stability. It showed a good sensing response to ammonia (22.94 for 100 ppm), and a fast response/recovery time (20 s/27 s). Finally, the response mechanism of Zn₂SnO₄/rGO nanocomposite sensor is explained. The enhanced ammonia sensing properties of Zn₂SnO₄/rGO nanocomposite sensor were ascribed to the synergistic effect and p–n heterojunction between Zn₂SnO₄ and rGO.

     

A microstructural study on the surface and interface of CdTe/CdS solar cells

The surface and interface properties of CdTe/CdS solar cells, including interfacial mixing, surface and interface geometrical morphology, CdTe grain size and preferential crystal orientation of CdTe layers were studied using Auger electron spectroscopy (AES) depth profiling, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, optical reflectance (OR) and X-ray diffraction (XRD) techniques. The correlation between the surface and interface properties and CdTe/CdS solar cell performance was also investigated. AES depth profiling was used to analyse the interdiffusion between the CdTe and CdS layers. Atomic force microscopy (AFM) suggests that the interfacial geometrical morphology has a significant influence on the photovoltaic property of CdTe/CdS solar cells. Rough interfaces tend to increase the photovoltaic conversion efficiency of solar cells because of multiple reflections. X-ray diffraction shows that polycrystalline CdTe/CdS solar cells with higher efficiencies appear to be orientated with more (1 1 1) planes of CdTe parallel to the macrosurface, but CdTe single crystals with differently indexed surface planes show almost the same reflection behaviour. Further theoretical and experimental analyses are therefore needed to clarify this observation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis, characterization and photocatalytic property of nano-sized Zn2SnO4

Nano-sized Zn₂SnO₄ materials have been synthesized using the coprecipitation method. The synthetic conditions and the calcination behaviors of nano-sized Zn₂SnO₄ materials have been studied. The nano-sized Zn₂SnO₄ materials have been characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry and differential thermal analysis (TG-DTA) and specific surface area. As a result, the kinetic grain growth equation for nano-sized Zn₂SnO₄ can be expressed as D ^4.78 = 9.12 × 10²³ t exp(–40.6 × 10³/T), with an activation energy for grain growth of Q = 337.9 KJ/mol. The nano-sized Zn₂SnO₄ materials have been used as photocatalysts to decompose benzene in water solution. The results show that Zn₂SnO₄ can photocatalytically decompose benzene, and the photocatalytic capacity for Zn₂SnO₄ relates to the grain size, which is discussed in terms of the surface effect and the quantum size effect.     

Photoluminesence studies of CdTe/SnO2 and CdTe/CdS heterojunctions: The influence of oxygen and the CdCl2 heat treatment

The influence of oxygen and annealing in the presence of CdCl₂ on the photoluminescence (PL) spectra of CdTe, component of SnO₂/CdTe heterojunction (HJ), has been studied in a temperature range of 17-100 K. The changes in the photoluminescence spectra were studied as a function of excitation intensity. Analysis of the PL spectra was carried out with considerations of spectra obtained from CdS/CdTe heterojunctions. CdTe side PL (SnO₂/CdTe HJ) consisted of 1.450 eV-DA defect band and 1.243 eV band (17 K). Annealing resulted in the disappearance of 1.243 eV band in oxygen containing samples. Interface PL for the unannealed samples consisted of mainly the 1.264 eV and a trace of the defect band. The CdCl₂ treatment is responsible for an almost symmetrical 1.416 eV band.     

Intermetallic compounds formed at the interface between Cu substrate and an Sn-9Zn-0.5Ag lead-free solder

The intermetallic compounds (IMCs) formed at the interface between Cu substrate and an Sn-9Zn-0.5Ag lead-free solder alloy have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction (ED). The XRD patterns show that the main IMCs formed at the interface of Sn-9Zn-0.5Ag/Cu are γ-Cu₅Zn₈ and η′-Cu₆Sn₅. The Ag₃Sn IMC with orthorhombic structure was also observed at the Sn-9Zn-0.5Ag/Cu interface by TEM and ED analyses. The interfacial adhesion strength between the Cu substrate and Sn-9Zn-0.5Ag lead-free solder alloy is higher than that of the Sn-9Zn alloy due to the formation of Ag₃Sn IMC at the interface.     

Analysis of CdS/CdTe devices incorporating a ZnTe:Cu/Ti Contact

High-performance CdS/CdTe photovoltaic devices can be produced using a ZnTe:Cu/Ti back contact deposited onto the CdTe layer. We observe that prolonged exposure of the ZnTe:Cu and Ti sputtering targets to an oxygen-containing plasma significantly reduces device open-circuit voltage and fill factor. High-resolution compositional analysis of these devices reveals that Cu concentration in the CdTe and CdS layers is lower for devices with poor performance. Capacitance-voltage analysis and related numerical simulations indicate that the net acceptor concentration in the CdTe is also lower for devices with poor performance. Photoluminescence analyses of the junction region reveal that the intensity of a luminescent peak associated with a defect complex involving interstitial Cu (Cu_i) and oxygen on Te (O_Te) is reduced in devices with poor performance. Combined with thermodynamic considerations, these results suggest that oxygen incorporation into the ZnTe:Cu sputtering target reduces the ability of sputtered ZnTe:Cu film to diffuse Cu into the CdTe.     

Effect of interdiffusion and impurities on thin film CdTe/CdS photovoltaic junctions

We have used low temperature photoluminescence (PL) to study thin film CdTe/CdS solar cell structures. The devices were produced by close space sublimation (CSS) and have undergone a post-growth treatment, a vital step in increasing device efficiency. The treatment consisted of evaporating a thin layer of CdCl₂ onto the back CdTe surface and heat treating in air at 400 °C for between 10 and 120 min. This produced a range of device efficiencies from 2% to 9%. The efficiency improvements are the result of a complex interaction between the CdCl₂, impurities and sulfur interdiffusion. The structures were prepared for PL by a chemical bevel etching technique which allows the luminescence emission to be studied as a function of depth throughout the sample. The main features in the PL spectra have been identified as being due to the Cl-A center and the Te-dislocation-related Y luminescence band. Using PL we have quantified the S diffusion into the CdTe which has a maximum of 20% at the interface in the most efficient samples. We have also obtained the profiles of recombination and non-radiative recombination centers in the device. We observe correlations between impurity centers and device efficiency which can help explain the effects of the CdCl₂ treatment on the optoelectronic properties of the CdTe/CdS junction.     

Interfacial microstructure and defect analysis in Cu(In,Ga)Se([sub]2)-based multilayered film by analytical transmission electron microscopy and focused ion beam

Interfacial microstructures of Cu(In,Ga)Se₂(CIGS)-based multilayered film are closely characterized by TEM (transmission electron microscopy), SEM (scanning electron microscopy) and FIB (focused ion beam). A cross-sectional TEM, energy dispersive X-ray spectroscopy and energy-filtered TEM reveal that a pronounced Cu diffusion occurs across the interface of the CdS/CIGS, which leads to a large amount of Cu rich in the CdS layer and a Cu-deficient sub-surface in the CIGS layer as well as a rough interfacial structure. TEM studies further reveal that the interface microstructures in the multilayered film are dissimilar, both ZnO/CdS and CdS/CIGS interfaces are strongly bonded whereas the CIGS/Mo interface is weakly bonded and interface separation occasionally occurs. Mo back contact layer shows a well adhesion to glass substrate.Detailed observation on defects in the CIGS-based multilayered film is carried out by 3D (3-dimensional) FIB and SEM techniques. Sequential 2D (2-demensional) cross-sectioning shows that dominant growth-defects in the CIGS and top SiO₂ layers are micro-scale crack, appearing as diversified morphologies. The micro-scale crack in the CIGS layer is possibly released by propagating into the adjacent layer while the crack in the SiO₂ layer is relieved usually by forming a small particle behind. It is noted that in the multilayered film the interface frequently acts as crack initiation sites due to distinct thermal expansion coefficients.     

Optical and electrical characterization of CdS thin films

Thin CdS films have been grown by chemical bath (CdCl₂, thiourea, ammonia) deposition (CBD) on SnO₂ (TO)-coated glass substrate for use as window materials in CdS/CdTe solar cells. High-resolution transmission electron microscopy revealed grains with an average size of 10 nm. The structure was predominantly hexagonal with a high density of stacking faults. The film crystallinity improved with annealing in air. Annealing in a CdCl₂ flux increased the grain size considerably and reduced the density of stacking faults. The optical transmission of the as-deposited films indicated a band gap energy of 2.41 eV. Annealing in air reduced the band gap by 0.1 eV. Annealing in CdCl₂ led to a sharper optical absorption edge that remained at 2.41 eV. Similar band gap values were obtained by photocurrent spectroscopy and electroabsorption spectroscopy (EEA) using an electrolyte contact. EEA spectra were broad for the as-deposited and air-annealed samples, but narrower for the CdCl₂-annealed films, reflecting the reduction in stacking fault density. Donor densities of ca. 10¹⁷ cm ^–3 were derived from the film/electrolyte junction capacitance.     

Properties of hydrothermally synthesized Zn2SnO4 nanoparticles using Na2CO3 as a novel mineralizer

Zn₂SnO₄ nanoparticles with a band gap of 2.96–3.36 eV were synthesized via a simple hydrothermal method using Na₂CO₃ as a new mineralizer. The effects of the concentration of Na₂CO₃ on the formation, morphology and optical properties of the Zn₂SnO₄ nanoparticles are discussed. Also, the reaction parameters, such as the reaction temperature and reaction time, were optimized. The as-synthesized Zn₂SnO₄ nanoparticles exhibit no impurity phases such as SnO₂ and ZnO throughout the entire process, as confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. Also, the formation of impurity free Zn₂SnO₄ nanoparticles in the presence of Na₂CO₃ is explained by a unique hydrothermal reaction sequence. The average particle size of the highly crystalline Zn₂SnO₄ nanoparticles was controllable by adjusting the mineralizer concentration, which enabled the facile tuning of the bandgap. From the photoluminescence (PL) study, two emission bands could be found after peak analysis, which are probably due to the presence of oxygen or cation vacancies generated during the hydrothermal process. These bands were shifted to lower frequencies as the concentration of the Na₂CO₃ mineralizer increased.     

Tantalum and niobium as a diffusion barrier between copper and silicon

The efficiency of Ta and Nb films as diffusion barriers between thin Cu film and Si substrate has been studied using Auger electron spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and sheet resistance measurements. Two kinds of system were prepared by electron-beam evaporation: Cu/Ta (or Nb)/Si and Cu/Ta (or Mb) SiO₂/Si. The samples were annealed at temperatures from 400 to 800C in a vacuum of 1 × 10^–7 torr (13 Pa) for 30 min. In the Cu/Ta (or Nb)/Si system, the thermal stability was determined by interdiffusion at local sites, forming suicides, whereas the Cu/Ta (or Nb)/SiO₂/Si system degraded by interdiffusion at the interface between Ta (or Nb) and Cu. It appears that Ta is a more effective diffusion barrier than Nb for both kinds of system. This difference in the barrier effect of the transition metals is attributed to differences between oxygen segregation at grain boundaries of barrier layers and differences between diffusion coefficients through barrier layers. It is suggested that the driving force for interdiffusion may play a major role in the reaction that determines the thermal stability of a given contact system; this suggestion is based on the fact that the interdiffusion in Cu/barrier/Si systems is suppressed by interposing an SiO₂ layer in the Si substrate.     

SnO2/α-Fe2O3 hierarchical nanostructure: Hydrothermal preparation and formation mechanism

In this paper, we reported a simple solution method to assemble SnO₂ nanorods hierarchically on the surface of α-Fe₂O₃ nanosheets using Fe₃O₄ nanosheets as precursor. The product was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Our experimental results show that the lattice mismatch at the interface of SnO₂ nanorods with α-Fe₂O₃ nanosheets played an important role in determining the growth direction of SnO₂ nanorods. The interface prefers to take the least lattice mismatch and thus the preferential growth direction of SnO₂ nanorods was along [1 0 1] direction. The result may have important impact on the understanding of the nucleation growth process in a heterogeneous system.     

水溶性Cu∶CdTe/CdS核壳量子点的合成、表征及细胞毒性检测

采用水相法合成了Cu掺杂CdTe量子点,并用CdS壳层进行包覆,得到了Cu∶CdTe/CdS核壳结构量子点。采用荧光发射光谱(FL)、紫外可见吸收光谱(UV-Vis)、透射电镜(TEM)以及能谱仪(EDS)等手段对CdTe量子点和Cu∶CdTe/CdS核壳量子点进行了表征。研究了不同Cu掺杂浓度、CdS壳层生长时间以及Cd/硫脲物质的量比对Cu∶CdTe掺杂量子点光学性能的影响,并采用人成骨肉瘤细胞(MG-63细胞)对样品做了细胞毒性分析。研究结果表明:通过掺杂和包壳的步骤,合成的Cu∶CdTe/CdS核壳量子点在CdTe量子点的基础上实现了荧光发射红移,荧光强度提高,以及细胞毒性降低。     

Sensing behaviour of nanosized zinc-tin composite oxide towards liquefied petroleum gas and ethanol

A chemical route has been used to synthesize composite oxides of zinc and tin. An ammonia solution was added to equal amounts of zinc and tin chloride solutions of same molarities to obtain precipitates. Three portions of these precipitates were annealed at 400, 600 and 800 °C, respectively. Results of X-ray diffraction and transmission electron microscopy clearly depicted coexistence of phases of nano-sized SnO₂, ZnO, Zn₂SnO₄ and ZnSnO₃. The effect of annealing on structure, morphology and sensing has been observed as well. It has been observed that annealing promoted growth of Zn₂SnO₄ and ZnSnO₃ at the expense of zinc. The sensing response of fabricated sensors from these materials to 250 ppm LPG and ethanol has been investigated. The sensor fabricated from powder annealed at 400 °C responded better to LPG than ethanol.     

Interface-level thermodynamic stability diagram for in situ internal oxidation of Ag(SnO<Subscript>2</Subscript>)<Subscript>p</Subscript> composites

We present a combined experimental and theoretical study on interfacial structure and thermodynamics in internally oxidized Ag(SnO₂)_p composites. The orientation relation between in situ formed SnO₂ particles and the silver matrix was characterized using high-resolution transmission electron microscopy techniques. First-principles energetic calculations were then performed to predict the equilibrium interface structures and corresponding adhesion properties as functions of temperature and oxygen partial pressure. All results were combined to construct the interface-level thermodynamic stability diagram, for guiding the optimal design of internal oxidation parameters.     

Structure and optical properties of polycrystalline Cd1−xZnxTe thin films prepared by simultaneous evaporation

Cd_1–xZn_xTe films were deposited by simultaneous evaporation of CdTe and ZnTe. These Cd_1–xZn_xTe films were of cubic phase, and strongly (1 1 1) oriented as deposited. Predominant direct optical transitions were observed and the band gap varied with zinc content in a non-linear way. A structure development of CdS/CdTe/ZnTe : Cu solar cells with a Cd_1–xZn_xTe buffer layer was proposed.     

The copper influence on the PL spectra of CdTe thin film as a component of the CdS/CdTe heterojunction

The influence of annealing in the presence of CdCl₂ and a thin copper layer deposited onto CdTe on the photoluminescence spectra of CdTe, as a component of CdS/CdTe heterojunction, has been studied for two excitation wavelengths: 0.337 μm and 0.6328 μm. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. At 0.6328 μm excitation, the interface PL consists of a known 1.43X band, and the chloride annealing enhances radiative transitions at 1.536 eV. The intensity of the 1.536 eV transitions increases when Cu is present. The PL of as-deposited CdTe films prepared in the presence of oxygen has the 1.45X band attenuated when excited with 0.337 μm excitation wavelength.