Close-spaced Vapor Transport Research Articles

Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu–Au was used as the back contact in all the solar cells. Dark and under illumination J–V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.

C– V calculations in CdS/CdTe thin films solar cells

Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a C– V (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity Δ Ev of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and Δ Ev are determined from C– V fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor–semiconductor and semimetal–semiconductor heterojunctions.

Structural and optical properties of Cu(In,Ga)Se 2 grown by close-spaced vapor transport technique

Polycrystalline thin films CuGa 0.3In 0.7Se 2 (CIGS) were deposited onto coated soda-lime glass and transparent conducting oxide (SnO 2) substrates at 480 °C substrate temperature with a low cost and potentially large area deposition method by using close-spaced vapor transport technique (CSVT) designed in our laboratory (LAMPS). Morphological, structural and compositional properties were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. Optical transmission spectra were used to determine absorption coefficient and gap energy of the sample. Using photoacoustic spectroscopy, we report the activation energies of shallow and deep levels. Finally, we discuss the results obtained.

Physical properties of Bi2Te3 and Sb2Te3 films deposited by close space vapor transport

Near-stoichiometric bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) films were grown by the close space vapor transport (CSVT) method in a relatively simple fashion. The dependence of the film properties on the substrate temperature was explored over a wide range by keeping the source-to-substrate thermal gradient fixed at 300 °C and 350 °C for the Bi2Te3 and Sb2Te3 film growth, respectively. A Seebeck coefficient (α) of 255 µV K−1 and a power factor of 20.5 × 10−4 W K−2 m−1 were obtained for Bi2Te3 at a substrate temperature of 350 °C; meanwhile, the maxima of the above parameters for the antimony telluride films (108 µV K−1 and 3.8 × 10−4 W K−2 m−1, respectively) were reached at a substrate temperature of 450 °C.

Close space vapor transport method for Bi 2Te 3 thin films deposition: Influence of the type of substrate

Bismuth telluride thin films have been grown by close space vapor transport (CSVT) technique as a function of substrate temperature ( T sub). Both N- and P-type samples can be obtained by this method which is a relatively simple procedure, which makes the method interesting for technological applications. The samples were deposited onto amorphous glass and polycrystalline CdTe film substrates in the substrate temperature range 300–425 °C, with a fixed gradient between source and substrate of 300 °C. The influence of the type of substrate and substrate temperature in the CSVT chamber on the physical properties of the films is presented and discussed.

CdTe solar cell degradation studies with the use of CdS as the window material

We present in this work the degradation effects with time in thin film CdTe/CdS solar cells, where the CdS and CdTe layers are deposited by chemical bath deposition (CBD) and close space vapor transport (CSVT), respectively. The CdS thin films were grown from different baths by varying the S/Cd ratio. The variation of the S/Cd ratio allowed us to control the morphology and the density of defects, thus giving rise to better quality CBD CdS films. Depending on the S/Cd ratio an improvement of the morphology and capacitance signal was observed, these factors have also an influence on the open-circuit voltage, short-circuit current density, fill factor and conversion efficiency of the solar cell. The variation with time of these parameters in our devices was tracked during a period of 3 years measured directly on the exposed back contact regions (CdTe/Cu/Au). A discussion on the deterioration of the photovoltaic (PV) performance of the solar cells is presented in correlation with the local environmental conditions. This particular environment has contamination, and represents another type of stress for standard PV operations. These conditions reduce the mean life time of solar cells beyond short periods; this can be of interest for PV community.

Chemically-deposited Te layers improving the parameters of back contacts for CdTe solar cells

A chemical bath process was carried out for the deposition of Te layers on CdTe films grown by the close-space vapor transport technique (CSVT) on conducting SnO 2:F substrates. The Te layers were chemically-deposited on as-grown CdTe films and on previously CdCl 2-treated CdTe ones. After Te deposition, the CdTe films were annealed at temperatures from 200 to 400 °C. The Te layer on top of the CdTe films was studied by Raman Spectroscopy and by Scanning Electron Microscopy. The electrical resistivity of the annealed CdTe films was determined from current versus voltage measurements in a sandwich configuration, employing gold contacts on top of the CdTe modified surface. The results show that the combined effect of the Te layer on CdTe together with previous CdCl 2 treatment improves the electrical properties of CSVT-CdTe films. These results are quite promising for increasing performance of CdS/CdTe solar cells.

HFCVD and CSVT techniques working together to produce nanostructured tungsten oxide

Hot Filament Chemical Vapour Deposition (HFCVD) was used to deposit tungsten oxide clusters on a graphite surface using a tungsten filament as source. These clusters were subsequently transported onto quartz and Si substrates where they transformed into WO 3 − X nanowires. The transport of tungsten species from source to substrate was made through the Close Spaced Vapour Transport (CSVT) technique. The synthesized nanostructures were characterized by means of SEM, EDS, XRD techniques as well as by Raman spectroscopy. The results show that the nanowires have a monoclinic structure, a diameter about 100 nm and they were extended many tenths of micrometers.

On the doping problem of CdTe films: The bismuth case

The controlled increase of hole concentration is an important issue and still an unsolved problem for polycrystalline CdTe-based solar cells. The typical hole concentration of as-grown CdTe thin-films goes up to 10 13 cm − 3 , depending on the specific growth technique. The highest electron concentration obtained for CdS, the suitable window partner material of CdTe, is around 10 15 cm − 3 . Thus, the PV-performance of a CdS/CdTe device can be optimized if the hole concentration in CdTe is increased. We have faced up this problem by studying the electrical properties of two types of CdTe films: CdTe films grown by Close Space Vapor Transport using a CdTe:Bi powder as the starting material and CdTe sputtered films doped by implantation with different Bi-doses. Temperature-dependent resistivity and Hall effect measurements and a discussion on the efficiency of both doping processes are presented.

Study of the physical properties of Bi doped CdTe thin films deposited by close space vapour transport

Bi doped cadmium telluride (CdTe:Bi) thin films were grown on glass-substrates by the close space vapour transport method. CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi concentration in the range between 1 × 10 17 and 8 × 10 18 cm − 3 , were used in powder form for CdTe:Bi thin film deposition. Dark conductivity and photoconductivity measurements in the 90–300 K temperature range and determination by photoacoustic spectroscopy of the optical-absorption coefficient of the films in the 1.0 to 2.4 eV spectral region were carried out. The influence of Bi doping levels upon the intergrain barrier height and other associated grain boundary parameters of the polycrystalline CdTe:Bi thin films were determined from electrical, optical and morphological characterization.

A study of the optical absorption in CdTe by photoacoustic spectroscopy

We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determination of the optical-absorption coefficient of CdTe thin films, in the spectral region near to the fundamental absorption edge, ranging from 1.0 eV to 2.4 eV, using an open cell in the transmission configuration. We applied this method to the optical characterization of CdTe layers for several values of their thickness. These CdTe samples were deposited by closed-space vapor transport (CSVT) technique under different growth conditions.

Preparation and Study of CuInSe2 Thin Films Obtained by the Technique of Close-Spaced Vapour Transport with Open Reactor

Preparation and Study of CuInSe2 Thin Films Obtained by the Technique of Close-Spaced Vapour Transport with Open Reactor

Morphology of nanostructured GaP on GaAs: Synthesis by the close-spaced vapor transport technique

Nanostructures of gallium phosphide (GaP) were fabricated through the close-spaced vapor transport technique (CSVT). Such nanostructures were grown on crystalline gallium arsenide (GaAs) by using a GaP powder source in the absence of any catalyst. Nanostructured GaP products were structurally and morphologically characterized by means of physicochemical and spectroscopic techniques, such as SEM, EDS, XRD, and NMR. Results showed that GaP structures present a nanoflower-like morphology. Indeed, these nanoflowers are constituted by numerous nanowires. The diameters of the GaP nanowires were in the interval of 80–300 nm, with lengths varying from several to tens of micrometers.

ZnSe Nanobelts and Nanowires Synthesized by a Closed Space Vapor Transport Technique

ZnSe nanobelts and nanowires have been simultaneously synthesized by a closed-space vapor transport technique without using any carrier gas. The products are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. It is shown that the nanobelts and nanowires are 50−90 nm in thickness or diameter and hundreds of micrometers in length. Structure analysis indicates that the nanowires take the zinc blende structure, while the nanobelts are single crystalline or bicrystalline wurtzite phase. The as-synthesized products exhibit rich Raman modes and a strong near-band-edge luminescence peak centered at 476 nm. The nanobelts and nanowires grow via a vapor−solid mechanism mediated by a polycrystalline ZnSe film.

Surface Morphologies and Optical Properties of Homoepitaxial ZnO Grown by Close-Spaced Chemical Vapor Transport

AbstractChemical vapor transport (CVT) using carbon as a transporting agent is studied for homoepitaxial growth on O-polar ZnO substrates. To increase growth rate at high temperatures, we keep a substrate close to ZnO source powder. Surface smoothness and crystal quality of epilayers are remarkably improved by increasing a substrate temperature. Smooth surfaces are observed on the epilayer grown at substrate temperatures above 920°C.

Electron Spin Resonance and Ultra Violet (UV) Photoluminescence of Ge Implanted CuGaSe2 Thin Films Prepared by the CCSVT (Chemical Close-spaced Vapor Transport) Technique

AbstractNon-equilibrium ion implantation of Ge in p-type polycrystalline thin film CuGaSe2 (CGSe) prepared by Chemical Close-spaced Vapor Transport (CCSVT) has been performed with the goal to achieve n-type doping of this chalcopyrite semiconductor. Using Electron Spin Resonance (ESR) it is shown that Ge implantation induces a paramagnetic specie at g = 2.003. A model is proposed that assigns the ESR signal to electrons trapped by donor states that are electrically inactive. Moreover, UV photoluminescence of Ge implanted films has evidenced a new peak emission at 1.47 eV, which is resolved as a radiative recombination of a hole bound to the native copper vacancy and an electron bound to a deep donor with an ionization energy of ED =360±10 meV.

Effect of substrate temperature and source grain size on the structural and electrical properties of CSVT grown Cu(In1−xGax)Se2 thin films

Polycrystalline CIGS thin film layers, used as a solar cell absorber, have been prepared on glass substrates by a low cost Close Spaced Vapor Transport Technique (CSVT) at different growth conditions for different substrate temperatures (450, 500 and 550 °C) and source grain sizes. The structural quality and evolution of defects in the CIGS thin films have been studied in terms of the substrate temperature and the source grain size. Scanning electron microscopy and atomic force microscopy analyses show that for a substrate temperature of 500 °C and the large source grain size, the layers present larger columnar crystallites and better morphology. For a same substrate temperature of 500 °C and different source grain sizes, it was found that the crystallites structure disappears for source grain sizes less than 20 μm. These results are discussed in terms of supersaturation rate.Admittance spectroscopy measurements carried out on CIGS thin films obtained from large source grains size reveal the presence of two defects with an activation energy of 116 meV and 45 meV for layers grown at a substrate temperature of 550 °C while the defects detected in samples grown at 500 °C had much smaller activation energies around 35 meV.The observed deep trap levels are tentatively assigned to the well known N1 centres. The effect of growth conditions on the defect energy levels and the defect densities are discussed.

Physical properties of Bi doped CdTe thin films grown by the CSVT method

A study of the physical properties of CdTe thin films doped with Bi is presented. CdTe:Bi thin films were deposited by the close space vapor transport (CSVT) technique using powdered CdTe:Bi crystals grown by the vertical Bridgman method. CdTe:Bi crystals were obtained with nominal Bi doping concentrations varying in the 1 1017–8 1018 cm 3 range. The physical properties of CdTe:Bi thin films were studied performing photoluminescence, X-ray, SEM, photoacoustic spectroscopy and resistivity measurements. We observed a decrease of the resistivity values of CdTe:Bi films with the Bi content as low as 6 105O-cm for Bi concentrations of 8 1018 cm 3. These are meaningful results for CdTe-based solar cells. r 2006 Published by Elsevier B.V

CuGaSe 2–CuGa 3Se 5 phase transition in CCSVT-grown thin films

The system CuGaSe 2–CuGa 3Se 5 in thin films has been investigated. Layer synthesis was carried out by chemical close-spaced vapour transport (CCSVT) using Cu precursors on Mo/soda-lime glass substrates. The extension of deposition times in a two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 1 to 3, allowing the study of the phase transition mentioned above. Films showing chalcopyrite (1:1:2), OVC (1:3:5) and both phases were grown. X-ray emission spectroscopy and X-ray diffraction (XRD) techniques have been combined for a compositional and structural study of this material system probing both bulk and near surface properties of the films. This analysis was also extended to the rear-surface investigation of selected two-phase thin films and complemented with surface sensitive photoelectron spectroscopy (PES). From these results a growth model is presented for CuGa 3Se 5 formation in gallium rich, CCSVT-grown CuGa x Se y -films.

Influence of the CdCl 2 thermal annealing on the luminescent properties of CdS‐CSVT thin films

In this work we present results about the influence of a postdeposition heat treatment with CdCl2 on CdS films grown by the Close Spaced Vapour Transport (CSVT) technique. Such heat treatment has been used as a standard procedure to improve the overall performance of CdTe/CdS based solar cells. The films were studied by analyzing the luminescent properties in the range 10–300 K. The room temperature PL-spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low energy side at 1.80 eV approximately. After the CdCl2 thermal annealing, the spectrum, also at 300 K, showed better PL characteristics: a strong band in the low energy side at 1.67 eV and a band in the high energy side at 2.47 eV. The analysis at lower temperatures allows us to observe the evolution of these bands. The high energy band becomes more intense and shifts to higher energies reaching an energy close to the energy band-gap at 10 K. The analysis of the PL intensity as a function of temperature allows us also to apply a theoretical model for the quenching of the PL-intensity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)