Improvement of composition of CdTe thin films during heat treatment in the presence of CdCl<Subscript>2</Subscript>

CdCl₂ treatment is a crucial step in development of CdS/CdTe solar cells. Although this processing step has been used over a period of three decades, full understanding is not yet achieved. This paper reports the experimental evidence for improvement of composition of CdTe layers during CdCl₂ treatment. This investigation makes use of four selected analytical techniques; Photo-electro-chemical (PEC) cell, X-ray diffraction (XRD), Raman spectroscopy and Scanning electron microscopy (SEM). CdTe layers used were electroplated using three Cd precursors; CdSO₄, Cd(NO₃)₂ and CdCl₂. Results show the improvement of stoichiometry of CdTe layers during CdCl₂ treatment through chemical reaction between Cd from CdCl₂ and elemental Te that usually precipitate during CdTe growth, due to its natural behaviour. XRD and SEM results show that the low-temperature (~85 °C) electroplated CdTe layers consist of ~(20–60) nm size crystallites, but after CdCl₂ treatment, the layers show drastic recrystallisation with grains becoming a few microns in size. These CdCl₂ treated layers are then comparable to high temperature grown CdTe layers by the size of grains.     

Structural and optical properties of CdS thin films prepared by chemical bath deposition at different ammonia concentration and S/Cd molar ratios

CdS thin films were prepared by chemical bath deposition technique using the precursors of SC(NH₂)₂, CdCl₂, NH₄Cl, NH₃·H₂O and deionized water. The obtained thin films were characterized by scanning electron microscopy, X-ray diffraction, energy dispersive spectrometer, UV–VIS specrophotometry and photoluminescence spectroscopy. The morphology, structural and optical properties of CdS thin films were investigated as a function of ammonia concentration and S/Cd molar ratios in precursors. The results reveal that morphology of CdS films change from flake like into spherical particle like, crystal structure from wurtzite structure to zinc blende structure, S/Cd atom ratios in CdS thin films increase and optical band gap E _g decrease with increasing ammonia concentration in precursors. The room temperature photoluminescence spectrum of CdS thin films shows a strong peak at about 500 nm and a weak peak at about 675 nm.     

CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells

In this work, a systematic research on CdS annealing treatments under various atmospheres had been done to understand their effects on CdS/CdTe solar cells. CdS films were prepared by a standard CBD method and annealed under various atmospheres, including Ar, Ar+H₂, O₂, Ar+S and Ar+CdCl₂. Morphological, structural, optical and chemical properties were investigated using Atom force microscope (AFM), X-ray diffraction (XRD), UV–VIS spectroscopy and X-ray photoelectron spectroscopy (XPS). Annealing treatments enhanced modifications of morphology, structure and electrical properties of CdS films. AFM showed different surface morphologies and roughnesses of CdS films annealed under various atmospheres. XRD indicated the transition of CdS films from metastable cubic structure to stable hexagonal structure after annealing treatment, especially annealed in Ar+CdCl₂. From XPS analysis, Fermi levels of CdS films shifted closer to conduction band after annealing under O₂ and Ar+CdCl₂, while the levels shifted away from conduction band under Ar+H₂ and Ar+S. The relationships between those modifications by annealing treatments and effects on the performance of solar cells were discussed. Solar cell based on CdS annealed with Ar+CdCl₂ had the best performance due to the high n-doping of CdS layer introduced by annealing process.     

Unusual formation of nano-particles of CdO and Cd(OH)2 from the reaction of dimethyl cadmium with DMF

This paper presents generation of CdO and Cd(OH)₂ nano-particles from Dimethyl Cadmium in DMF. The CdO nano-particles were obtained instead of CdSe, even when the reaction was done in presence of 1,2,3-selenadiazole (the source of selenium) with Me₂Cd in DMF (product-I). The direct reaction of Me₂Cd in DMF also leads to formation of CdO (product-II). However, Cd(OH)₂ nano-particles were obtained when Me₂Cd was refluxed in DMF for a few hours followed by reaction of H₂S gas (product-III). The formation of Cd(OH)₂ was also established via decomposition of Me₂Cd:Et₂O adduct (product-IV). Nano-particles of CdO and Cd(OH)₂ (product-I to product-IV) were characterized by X-ray powder diffraction (XRD), TEM and SEM/EDS measurements, FTIR, thermal analysis (TGA) and XPS analysis. The particle size of all the products as calculated by XRD patterns were in the range of about 20 nm. TEM images showed that the products are agglomerated clusters with the particles in the nano-meter regime. The synthesis however, is understood to be unusual as the reactions with selenium source and sulfur source should have generated the CdSe and CdS however, the end products were always found to be the product-I to product-IV.     

Effect of CdCl2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl₂ coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl₂-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO₄ on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl₂ layer protected it from oxidation during annealing in air and promoted formation of Cl_S and V_Cd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the Cl_S, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl₂ heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.     

Preparation and photo-induced charge transfer of the composites based on branched CdS nanocrystals and PCPDTBT

Rich branched CdS nanocrystals were synthesized by a facile hydrothermal treatment from Cd(NO₃)₂, thiourea and hexamethylenetetramine [(CH₂)₆N₄, HMT], where HMT acted as a capping agent. The morphology, structure and phase composition of CdS nanostructures were examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and high-resolution TEM. The composites based on CdS nanocrystals and Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b’]dithiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) have been prepared by mixing of the two components in chloroform. The optical properties of the composites are investigated using ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) spectroscopies. A significant fluorescence quenching of PCPDTBT in the composites is observed at high CdS nanocrystals/PCPDTBT ratios, indicating that the photo-induced charge transfer occurred due to the energy level offset between the donor PCPDTBT and the acceptor CdS nanocrystals.     

Reaction of Cd(NO3)2·4H2O with 4,4'–bipyridine (bpy) in MeOH solvent: synthesis and characterization of T-shaped [Cd(bpy)1.5(NO3)2]·3H2O,square grid [Cd(bpy)2(H2O)2](NO3)2·4H2O and linear polymeric [Cd(bpy)(H2O)2(NO3)2]

The influence of concentration of water and metal salt in the reaction between Cd(NO₃)₂·4H₂O and 4,4′–bipyridine in MeOH has been studied and three compounds namely, T-shaped [Cd(bpy)_1.5(NO₃)₂]·3H₂O, 1 square grid [Cd(bpy)₂(H₂O)₂](NO₃)₂ 4H₂O, 2 and one dimensional linear polymer, [Cd(bpy)(H₂O)₂(NO₃)₂], 3 were isolated quantitatively in this process. Compound 1 forms in MeOH at high dilution of the metal salt (5.0 mg/mL or less) and for the metal-to-ligand ratio 1:(1.5–2.0). Compound 2 forms exclusively in the concentration range, 17–33% for water in MeOH by volume and 12–28 mg/mL for the metal salt of the solution. Outside these limits, mixtures of 2 and 3 were isolated. For 1:1 ratio of metal salt to bpy, the linear polymer, 3 was obtained in major quantity and its formation was found to be independent of concentration of water or the metal salt. Compounds 1 and 2 have been characterized by X-ray crystallography. On heating all the compounds decompose through a common intermediate [Cd(bpy)(NO₃)₂] and finally to CdO as monitored by TG.     

Effects of CdCl2 treatment on the properties of CdS films prepared by r.f. magnetron sputtering

Effects of the thickness of CdCl₂ layer and the annealing on structural and optical properties of sputter-deposited CdS films were investigated. The annealing process of evaporated CdCl₂ was carried out by heating the sample in air at 350-500 °C for 20 min. As the thickness of the CdCl₂ increases, the (002) peak of CdS becomes weak and the other peaks of CdS increases. Especially, for 200 nm CdCl₂, the preferential orientation of the (002) plane disappears and the c-axis of the CdS film tends to orient parallel to the substrate. As the CdCl₂ layer is thicker, the grains are enlarged significantly. The improvement of optical properties of CdS films with thicker CdCl₂ layer might be successfully employed in achieving better conversion efficiencies in solar cells.     

Effects of CdCl2 in CdTe on the properties of sintered CdS/CdTe solar cells

Sintered CdS films on glass substrates with low electrical resistivity and high optical transmittance have been prepared by a coating and sintering method. All-polycrystalline CdS/CdTe solar cells with different microstructures and properties of the CdTe layer were fabricated by coating a number of CdTe slurries, which consisted of cadmium and tellurium powders, an appropriate amount of propylene glycol and various amounts of CdCl₂, on the sintered CdS films and by sintering the glass-CdS-(Cd + Te) composites at various temperatures. The presence of more than 5 wt% of CdCl₂ in the (Cd + Te) layer enhances the sintering of the CdTe film and the junction formation by a liquid-phase sintering mechanism. A low sintering temperature results in poor densification of the CdTe layer and the CdS-CdTe interface, whereas a high sintering temperature results in a deeply buried homojunction. The optimum temperature for the sintering of the CdTe layer and for junction formation decreases with increasing amount of CdCl₂. All-polycrystalline CdS/CdTe solar cells with an efficiency of 10.2% under solar irradiation have been fabricated by a coating and sintering method using cadmium and tellurium powders for the CdTe layer.     

Structural and optical properties of chemically deposited Cd(S-Se): CdCl<Subscript>2</Subscript>, Sm films

Results of SEM and XRD studies, optical absorption and photoluminescence (PL) emission spectra and photoconductivity (PC), rise and decay studies are reported for Cd(S-Se): CdCl₂, Sm films prepared by chemical deposition method on glass substrates at 60°C in a water bath. SEM studies show ball-type structures along with voids which are related to layered growth. XRD studies show prominent diffraction lines of CdS and CdSe along with some peaks of CdCl₂ and impurity Sm. The values of strain (ɛ), grain size (D) and dislocation density (δ) are evaluated from XRD studies and the nature of crystallinity of the films are discussed. Optical absorption spectra also show the presence of Sm in the lattice. From the results of optical absorption spectra, the band gaps are determined. PL emission spectra of Cd(S-Se) consist of two peaks which are related to the edge emission of CdS and CdSe involving excitons. In Sm-doped emissions corresponding to transitions ⁴ G _5/2 to ⁶ H _5/2, ⁶ H _7/2 and ⁶ H _9/2 are observed. Sufficiently high photo current (I _pc) to dark current (I _dc) ratios with a maximum value of the order of 10⁶ are also obtained in some special cases. This high photosensitization is related to increase in mobility and life time of carriers due to photo excitation.     

High aspect ratio CdS nanowires synthesized in microemulsion system

CdS nanowires with typical length more than 8 μm and width of 30 nm on average have been successfully synthesized through Cd(NO₃)₂ reacting with CS₂ and ethylenediamine in microemulsion system of sodium dodecylbenzene sulfonate (SBDS). The microstructures of the as-synthesized CdS nanowires were characterized using XRD, transmission electron microscopy (TEM) and HRTEM. The possible formation mechanism was discussed. The morphologies of CdS sample strongly depend on the concentration of surfactant in solutions.     

Electrical properties of semiconductor CdS studied by adsorption spectroscopy

The electrical conductivity of semiconductor CdS films (d ≤ 1 μm) having (0001)S and (0001)Cd polar faces has been measured as a function of temperature at different degrees of adsorption of O₂, NO₂, and N₂O molecules. Adsorption spectroscopy has been used to identify bulk and surface electronic centers in a series of states with a wide range of ionization energies: E _t = 0.14–2.21 eV. In contrast to the bulk centers, which have a quasi-continuous energy spectrum characteristic of CdS, the surface adsorption electronic centers have a discrete energy spectrum. The ionization energy of the electronic centers of adsorption origin in CdS + O₂, CdS + NO₂, and CdS + N₂O structures has been measured for the first time at both polarities of the CdS faces and has been shown to depend on the chemistry of the adsorbate and the polarity of the film face.     

Formation of CdS films by spray pyrolysis

In this paper the formation of CdS films by spray pyrolysis of neutral aqueous solutions of CdCl₂ and SC(NH₂)₂ is described. It was established that the process passes through the stage of the intermediate complex compound [Cd(SCN₂H₄)₂]Cl₂ for various molar ratios of the initial components. The complex compounds are characterized by IR spectroscopy, X-ray diffraction and chemical analysis. The thermal decomposition of [Cd(SCN₂H₄)₂]Cl₂ to cadmium sulphide was studied.     

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.     

Preparation and characteristics of CdS thin films by dip-coating method using its nanocrystal ink

CdS nanocrystals were synthesized by hot injection method using EG as solvent, polyvinylpyrrolidone as dispersant, triethanolamine as stabilizing agent, Cd(NO₃)₂·4H₂O and H₂NCSNH₂ as cadmium and sulfur sources respectively. The synthesized nanocrystals were washed and ultrasonically dispersed with absolute ethanol to prepare nanocrystal ink. CdS thin films were deposited by dip-coating glass substrates with the nanocrystal ink and annealed at 450 °C in Ar atmosphere. Crystalline phase, morphology and element stoichiometry of the CdS nanocrystals derived from different synthesis temperatures were investigated by XRD, TEM and EDS. Surface morphology, crystalline phase and optical absorption spectrum of the CdS films were characterized by SEM, XRD and UV-Vis.     

CdS magic-size clusters exhibiting one sharp ultraviolet absorption singlet peaking at 361 nm

We report, for the first time, the synthesis of CdS magic-size clusters (MSCs) which exhibit a single sharp absorption peaking at ∼ 361 nm, along with sharp band edge photoemission at ∼ 377 nm and broad trap emission peaking at ∼ 490 nm. These MSCs are produced in a single-ensemble form without the contamination of conventional quantum dots (QDs) and/or other-bandgap clusters. They are denoted as MSC-361. We present the details of several controlled syntheses done in oleylamine (OLA), using Cd(NO₃)₂ or Cd(OAc)₂ as a Cd source and thioacetamide (TAA) or elementary sulfur (S) as a S source. A high synthetic reproducibility of the reaction of Cd(NO₃)₂ and TAA to single-ensemble MSC-361 is achieved, the product of which is not contaminated by other bandgap clusters and/or QDs. In some cases, the reaction product exhibits an additional absorption peak at ∼ 322 nm. We demonstrate that the two peaks, at 361 and 322 nm, do not evolve synchronously. Therefore, the 322 nm peak is not a higher order electronic transition of MSC-361, but due to the presence of another ensemble, namely MSC-322. The present study suggests that there is an outstanding need for the development of a physical model to narrow the knowledge gap regarding the electronic structure in these colloidal semiconductor CdS MSCs.

     

One step synthesis of silica-coated nanoparticles at room temperature and their luminescence properties

A novel one-step preparation method for core-shell CdS/SiO₂ nanoparticles was proposed using 3-mercaptopropyl-trimethoxysilane (MPS) as stabilizing agent in MeOH, in which CdCl₂·2.5H₂O and Na₂S·9H₂O were used as the source of Cd and S, respectively. And the photostability of the nanoparticles was further investigated. The morphology and structure of products was characterized by scanning probe microscope and FT-IR spectra, respectively. Furthermore, the influences of Zn and Cu doped in the products, individually, on the fluorescence property were discussed. The results showed that the products were ball-shaped and silica-coated with a uniform size, the spectra of Zn-doped was blue shifted, however, the spectra of Cu-doped was red shifted, the core-shell CdS/SiO₂ nanoparticles was more steady compared with thioglycolic acid-capped CdS nanoparticles.     

Towards understanding the nanomaterials characteristics of vapor transported CdS in an open end tube

In this study, the evaporation condensation of CdS was carried out in a quartz tube with an open end. The synthesis was carried out on Au coated Si(100) and quartz substrates. The Au coated Si and quartz substrates were put approximately 1 cm and up to 25 cm away from the alumina boat. It was observed that the materials deposited on the substrates in the reactor tube formed with different colors in different regions. The regions were found to be 6–14 cm, 14–18 cm and 18–24 cm away from the center of the alumina boat and were labeled B, C and D, respectively. The morphology, structural and chemical composition of the obtained materials at different regions were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive analysis of X-ray (EDAX), respectively. EDAX analysis revealed the presence of Cd and S with nearly stoichiometric CdS at region B. At region C a pronounced peak of oxygen was observed together with the peaks of Cd and S. At region D the S peak was diminished. XRD examinations revealed the formation of single crystalline phase of hexagonal CdS in region B. Mixed crystalline phases of hexagonal CdS, monoclinic cadmium sulfite, CdSO₃, and cubic CdO were formed in region C. Cubic CdO was formed in region D. The SEM examinations showed that the morphology of CdS was nanowires (NWs) in shape. The morphology of the mixed oxysulfide phase was a mixture between NWs and nanoparticles (NPs). The morphology of CdO was NPs in shape. The optical constants, the thickness and the surface roughness of the prepared nanostructured films were determined by spectroscopic ellipsometry measurements. A model consisted of two layers was used to fit the calculated data to the experimental ellipsometric spectra. The obtained optical constants were compared with those of CdS and CdO obtained by other preparation methods.     

Deposition and properties of CBD and CSS CdS thin films for solar cell application

We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl₂ vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl₂ treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl₂ treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.     

Template-free non-aqueous electrochemical growth of CdO nanorods

The direct template-free synthesis and characterization of CdO nanorods (NRs) obtained through electrochemical reduction process of molecular oxygen (O₂) in a dimethylsulfoxide solution containing CdCl₂are reported. X-ray diffraction characterization of the NRs shows that the CdO phase was obtained without the presence of any other secondary phases. In agreement with the aspect ratio of a typical nanorod structure, CdO NRs presented average diameters of ca. 170 nm and average lengths of ca. 920 nm. Current results also demonstrate that the employed electrochemical synthesis route favors the growth of NRs with a preferred crystallographic orientation along the [200] axis direction. The synthesized CdO NRs exhibited a n-type semiconductor character with a donor carrier concentration of N_D = 4.3 × 10¹⁹ cm^− 3.