Thesis

Celdas solares de películas delgadas de Ag-Sb-S

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Abstract

In the present work novel materials and solar cells based in silver antimony chalcogenides are developed by all-chemical deposition. A chemical formulation for the deposition of amorphous Ag-Sb-S thin films is presented. When this film is annealed, the formation of AgSbS2 is seen, which presents Eg of 1.79 eV, σlight = 1.6 × 10−5 Ω−1cm−1 with cubic crystalline structure similar to mineral cuboargyrite (a = 5.6520 Å). The heat treatment applied to AgSbS2 promotes a better crystallinity of the film but satisfactory grain growth for solar cell application is not achieved. The optical, electrical and structural parameters of the films are related to the heating process. With the AgSbS2 thin film, some solar cells in superstrate configuration TCO/CdS/AgSbS2/C/Ag were developed. The main heterojunction is formed between cubic-CdS/cubic-AgSbS2, which has a lattice mismatch of 2.6 %. The best solar cell shows Voc = 0.625 V, Jsc = 1.35mA/cm2, FF = 0.64 and η = 0.54%. The problem of this solar cell is the lack of Jsc; for this reason the incorporation of Se into AgSbS2 is considered to form solid solutions which would enhance the optical absorption of the device. The solid solution AgSbS1.3Se0.7 is obtained when an amorphous Ag-Sb-S film is heat-treated with a Se source (selenization). This film shows Eg = 1.47 eV and ∆σ = 1.6 × 10−5 Ω−1cm−1. The solar cell with the solid solution is designed in superstrate configuration where η = 0.65 % is obtained with the Voc = 0.527 V, Jsc = 2.07 mA/cm2 and FF = 0.60. The problem with this solar cells is still the lack of Jsc. To solve Se diffusion into CdS, a proof-of-concept solar cell in substrate configuration is presented: Mo/AgSb(SxSe1 – x)2AgSbS2/CdS/ZnO:Al. Finally, an analysis of the heterojunction CdS/Absorber is made to estimate the length of the depletion region in equilibrium and under steady state illumination. With the parameters obtained during the analysis and experimental development of silver antimony chalcogenides, a simulation of solar cells is made using the software SCAPS- 1D. In this simulation the modification of film thickness for improve solar cell design is predicted to prevent optical losses. Further, some strategies for modify the back contact of solar cells to improve the performance are also presented.

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In this work, we study the structural, electronic and optical properties of AgSbS2, using full-potential linearized augmented plane wave and the pseudopotential plane wave scheme in the frame of generalized gradient approximation. Features such as the lattice constant, bulk modulus and its pressure derivative are reported. Our results suggest a phase transition from AF-IIb phase to rocksalt (B1) phase under high pressure. The calculated band structure and density of states show that the material under load has an indirect energy band gap X→(LГ) for AF-IIb phase (semiconductor) and a negative band gap W→(ГX) for B1 phase (semimetal). The optical properties are analyzed and the origin of some peaks in the spectra is discussed. Besides, the dielectric function, refractive index and extinction coefficient for radiation up to 14 eV have also been reported and discussed.
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We present a ternary semiconductor nanoparticle sensitizer – AgSbS2 – for solar cells. AgSbS2 nanoparticles were grown using a two-stage successive ionic layer adsorption and reaction process. First, Ag2S nanoparticles were grown on the surface of a nanoporous TiO2 electrode. Secondly, a Sb–S film was coated on top of the Ag2S. The double-layered structure was transformed into AgSbS2 nanoparticles ~ 40 nm in diameter, after post-deposition heating at 350 °C. The AgSbS2-sensitized TiO2 electrodes were fabricated into liquid-junction solar cells. The best cell yielded a power conversion efficiency of 0.34% at 1 sun and 0.42% at 0.1 sun. The external quantum efficiency (EQE) spectrum covered the range of 380–680 nm with a maximal EQE of 10.5% at λ = 470 nm. The method can be applied to grow other systems of ternary semiconductor nanoparticles for solar absorbers.
Article
Silver antimony selenide (AgSbSe2) thin films were prepared by heating sequentially deposited multilayers of antimony sulphide (Sb2S3), silver selenide (Ag2Se), selenium (Se) and silver (Ag). Sb2S3 thin film was prepared from a chemical bath containing SbCl3 and Na2S2O3, Ag2Se from a solution containing AgNO3 and Na2SeSO3 and Se thin films from an acidified solution of Na2SeSO3, at room temperature on glass substrates. Ag thin film was deposited by thermal evaporation. The annealing temperature was 350 °C in vacuum (10−3 Torr) for 1 h. X-ray diffraction analysis showed that the thin films formed were polycrystalline AgSbSe2 or AgSb(S,Se)2 depending on selenium content in the precursor films. Morphology and elemental analysis of these films were done using scanning electron microscopy and energy dispersive X-ray spectroscopy. Optical band gap was evaluated from the UV–visible absorption spectra of these films. Electrical characterizations were done using Hall effect and photocurrent measurements. A photovoltaic structure: glass/ITO/CdS/AgSbSe2/Al was formed, in which CdS was deposited by chemical bath deposition. J–V characteristics of this structure showed Voc = 435 mV and Jsc = 0.08 mA/cm2 under illumination using a tungsten halogen lamp. Preparation of a photovoltaic structure using AgSbSe2 as an absorber material by a non-toxic selenization process is achieved.
Article
This work focuses on demonstrating the suitability of various high resistivity transparent (HRT) layers prepared by magnetron sputtering for sputtered CdS/CdTe cells. HRT buffer layers added between the transparent conducting oxide (TCO), and the CdS layer are important for reducing the effects of non-uniformities and shunts in large-area thin-film devices. CdS/CdTe cells were fabricated on Pilkington TEC 7 glass coated with a sputter deposited HRT layer of ZnO:Al or SnO2. In some cases O2 was added to the Ar sputter gas to increase the resistivity of the HRT buffer layers. Film properties were optimized for HRT performance by adjustments in the substrate deposition temperature, sputter gas pressure, and RF power. Best results have been obtained with reactively sputtered ZnO:Al with 2 % O2 in Ar.
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Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration Na–Nd, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10−10–10−9 s and too thin (3–5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28–30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼0.1 Ω·cm, ≥20–30 µm, ≥1016 cm−3, and ≥10−6 s, respectively.
Article
The reaction kinetics of the MoSe2 formation have been investigated by selenizing Mo layers in Se vapor at different temperatures and for different durations. The samples were characterized by means of Rutherford backscattering spectrometry, X-ray diffraction (XRD), electron diffraction, and by bright-field and high-resolution transmission electron microscopy. It was found that in all samples, a homogeneous MoSe2 layer is formed on top of the Mo layer. The temperature dependence shows that the MoSe2 layer thickness increases strongly for temperatures higher than ca. 550 °C. At a substrate temperature of 450 °C, no difference in the MoSe2 thickness was detected for samples selenized for different durations. The diffusion constant of Se in MoSe2 is estimated from the selenization duration dependence of MoSe2 layer thicknesses at 580 °C. Finally, Cu(In,Ga)Se2 (CIGS) solar cells in substrate configuration were developed on indium tin oxide (ITO) transparent back contacts. An intentionally grown MoSe2 intermediate layer on ITO, prior to CIGS deposition, causes a significant efficiency improvement, suggesting that MoSe2 can facilitate a quasi-ohmic contact. Solar cell efficiencies of up to 11.8% are obtained using an ITO/MoSe2 back contact.
Article
Bulk AgSbSe2 was prepared by melting the constituent elements in stoichiometric proportions. The ternary composition thus obtained was found to possess an NaCl-type structure. X-ray diffraction and electron microscopy techniques were used to obtain insight into the structural information of the sample. AgSbSe2 films were prepared by thermal evaporation under vacuum (10−5 Torr) on clean glass substrates. The room temperature deposited films were amorphous in nature and an amorphous-to-crystalline transition could be obtained by thermal annealing at 423 K. The degree of crystallinity increased with increasing temperature and film thickness. When thick films of this composition were evaporated thermally in vacuum two-phase films with AgSbSe2 as a major phase were formed. The effect of thermal annealing was studied using electron microscopy diffraction. The electrical resistivity, the carrier concentration and the temperature coefficient of resistance of AgSbSe2 deposited onto glass substrates have been studied as a function of thickness.
Article
Thin films of AgSbSe2 were prepared by direct thermal evaporation of the bulk compound. The films were deposited onto various substrates, namely cleaved surfaces of NaCl, KCl, KBr, KI and NaNO3, and amorphous carbon films backed by NaCl surfaces, maintained at different temperatures. The growth characteristics of the deposits were studied using transmission electron microscopy and the selected area electron diffraction technique. It was observed that the low temperature deposits were amorphous whilst those deposited at moderate temperatures were polycrystalline or epitaxial depending on the substrate material and temperature. Neither any thermal dissociation of the compound nor the presence of any extra phases was observed.
Article
Measurements of the electrical conductivity and thermoelectric power of the AgSbSe2 semiconductor in the solid and liquid states were carried out from 350 to 975 °C. The activation energy in solid and liquid states was EσS = 0.343 eV and EσL = 0.813 eV, respectively. In the liquid state, the data was analyzed in terms of a model developed for the density of states and electrical transport in solid amorphous semiconductors by Mott. Negative thermoelectric power in the liquid state suggests a large predominance of electrons in electrical transport. Moreover, the coefficient of the linear decrease of energy gap with temperature was found to be 2.265 ± 10−4 eV K−1.
Article
Selenium thin films of thickness similar to 300 nm were deposited from a solution of sodium selenosulfate of pH 4.5. These films are amorphous, but they are crystalline and photoconductive through annealing for 15 min at 150-200 degrees C. In this paper we present the properties of these films and their use as a planar source of selenium vapor (1.7 x 10(-6) mol/cm(2) of the film) to react with metal films to form metal selenide layers. For this, metal films of Ag, Sn, In, Cu, Sb, etc., deposited by thermal evaporation, are kept in contact with the Se-thin film and are heated at temperatures typically < 350 degrees C in nitrogen. This process leads to the formation of thin films of Ag2Se, SnSe2, In2Se3, CuSe/Cu2-xSe, Sb2Se3, etc. Se-metal reaction also takes place under microwave heating. In the case of an evaporated Ag film on a chemically deposited thin film of Sb2S3, AgSbSe2 is produced through heating at 200-300 degrees C. Photovoltaic structures SnO2: F-CdS-Sb2S3-AgSbSe2 fabricated this way show open-circuit voltage > 500 mV and short-circuit current density of 2-5 mA/cm(2). (c) 2006 The Electrochemical Society.
Article
A photovoltaic structure, glass/ Sn O2: Fn-CdS Sb2 S3 p-AgSb Se2 Ag -print, showing Voc 550 mV and Jsc 2.3 mA cm2 under 1 kW m2 (tungsten halogen) intensity has been developed on commercial Sn O2: F -coated glass from chemically deposited thin films of CdS (80 nm), Sb2 S3 (450 nm), and Ag2 Se (150 nm), and subsequently heating the layer at 200-300°C in contact with a chemically deposited Se thin film. The reaction between Sb2 S3 and Ag2 Se in the Se vapor forms p-type AgSb Se2 thin film with a bandgap ≈1 eV. The methodology for preparing all-chemically deposited photovoltaic structures at process temperatures 300°C or below presented here is compatible with solar cell technology.
Article
The defect structure and conductivity type in CdTe layers formed with CdCl2 flux are investigated. It is stated that chlorine outdiffusion combined with the slow cooling thermal treatment under tellurium pressure enables one to get high stable p-conductivity in CdTe layers recrystallized with CdCl2. p-type resistivity under 100 Ω · cm and a dark-to-light resistance ratio under 1.02 have been achieved. It is supposed that high p-type conductivity is originated from a shallow acceptor complex (Te2−iCl+Te)− or/and (V2−CdCl+Te)−. Interstitial tellurium is proposed as the 1.4 eV photoluminescence band killer in CdTe.
Article
One important application area of chalcogenide materials is rewritable optical data storage. This technology is based on a reversible phase transition between the crystalline and the amorphous state and vice versa. Currently dominant materials for rewritable optical recording are Ge–Sb–Te and Ag–In–Sb–Te alloys. Material research still continues due to the need for increasing storage capacity and data rates. Polycrystalline bulks of AgSbS2 were prepared by melt-quench technique. Composition and homogeneity of these bulks were checked by scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), crystallinity was studied by X-ray diffraction (XRD). Targets for RF magnetron sputtering were prepared from pulverized bulks by hot-pressing technique. Targets were characterized the same way as bulks. Thin Ag–Sb–S films were prepared by RF magnetron sputtering as potential candidates for rewritable optical data storage films. Composition and homogeneity of prepared thin films were characterized by SEM-EDX, Rutherford Back Scattering (RBS) and Elastic Recoil Detection Analysis (ERDA); character (amorphous/crystalline) was traced by XRD. Optical properties (spectral dependence of refractive index) were evaluated on the basis of UV–Vis–NIR spectroscopy and variable angle spectral ellipsometry (VASE). Crystallization abilities were studied by the measurement of thermal dependence of the prepared thin films optical transmission.