R. Mendoza-Pérez

Influence of the growth conditions and postdeposition treatments upon the grain boundary barrier height of CdTe thin films deposited by close space vapor transport

CdTethin filmsdeposited by close space vapor transport (CSVT) under variable growth conditions and postthermal and chemical treatments were studied by means of photoconductivitymeasurements in the temperature range of 90–300 K. The influences of the depositiontemperature gradient, the oxygen content in the growth chamber, and the CdCl 2 treatment and chemical etching upon the intergrain barrier height of CSVT polycrystallineCdTethin films were determined. The grain boundary barrier height (E bφ ) as a function of the intensity of the incident light was analyzed, and values of E bφ under illumination of 100 mW/cm2 (AM1) were obtained in each case. These results are important for improving CdS/CdTe solar cell performance in the near future.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Photovoltaic modules processing of CdS/CdTe by CSVT in 40 cm 2

We present in this work the fabrication of photovoltaic modules of the CdS/CdTe type, the first ones processed in Mexico in large area (40 cm2 and contact area of 3 cm2); these photovoltaic modules are constituted of the n-type cadmium sulfide (CdS) semiconductor, as the window material, and the p-type cadmium telluride (CdTe) semiconductor as the absorbing material. The back contacts were deposited by vacuum thermal evaporation. The electrical characteristics were determined by the measurement of current on voltage I vs. V. Two independent series of photovoltaic modules have been processed: in the first series the CdS was processed by the technique of Chemical Bath Deposition (CBD), the CdTe was deposited by the conventional technique of Closed Spaced Vapor Transport and the device consists of a photovoltaic module constituted of 9 independent solar cells with approximate 3 cm2 contact area. Of the total of these 9 solar cells, 6 have almost the same l-V characteristics in dark as well as in illumination AM1.5 (100 mW/cm2), with an average efficiency of 5.9 ± 0.3%, and the three remaining cells have approximately 3% of photovoltaic efficiency. The maximum power of each solar cell is of 18.5 mW. The power obtained by the prototype of photovoltaic module is approximately of 105 mW or 0.105 W in 18 cm2, which could scale up to 58 W/m2. The second series consists of photovoltaic modules, totally processed in a new CSVT system (completely designed, machined and assembled in our workshop and laboratory) including 9 interconnected solar cells to each another, with 3 cm2 of contact area, approximately.

Raman measurements on GaN thin films for PV - purposes

Raman scattering (RS) is a very important experimental tool to characterize the optical modes and another elementary excitations of materials. Among other issues it can determine for example the degree of crystalline quality and point defects like local modes. Therefore GaN - thin films and related compounds for photovoltaic purposes and as processed by several systems have been measured by this technique. The films were grown by Molecular Beam Epitaxy (MBE), Close Spaced Vapor Transport (CSVT) and Laser Ablation (LA) with the use of optimal growth parameters and substrates. Gallium nitride crystallizes in the wurtzite structure with 4 atoms in the unit cell and presents 7 allowed Raman modes of A1, E1, and E2 symmetries. In this work we present and discussed our Raman experiments where particularly the detection of the E2 and A1 modes are illustrated in these nitride semiconductor compounds.

Gallium nitride thin films as processed by several techniques: Their possible applications for PV-devices

We present in this work the characterization studies carried on GaN — thin films as processed by the Close Spaced Vapor Technique (CSVT), Laser Ablation (LA), and Molecular Beam Epitaxy (MBE), under particular growth parameters for each of the three techniques. The films characterization was performed by x-ray diffraction (X-RD), Photoluminescence (PL), Raman spectroscopy, optical transmission, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). With these results an analysis of the samples was done, with an aim for a possible application of these thin films for PV-devices.

Optical properties of CdS thin films obtained by CSVT under different growth conditions and their influence in prototype PV devices

We present in this work studies of photoluminescence at room temperature, AFM and optical transmission measurements of Cadmium Sulphide (CdS) films grown by the Closed Space Vapor Transport (CSVT) technique. Normally as-grown CdS-CSVT thin film samples do not show luminescence at room temperature, due mainly to the high density of deep non-radiative recombination centers at the band gap, thus in principle it is logic to think that a change of the critical growth parameters allows us to promote a reduction of the density of defects giving rise to a better quality of CdS-CSVT films, which can show luminescence at room temperature and are also suitable for PV-applications. Morphological changes in the films are also showed by AFM measurements, when we use different kinds of atmospheric molecular species (Ar, N2 and O2/Ar) in the growth chamber. Our PV-devices in the superstrate configuration, using these CdS thin films as windows layer, present also a relative increment in the conversion efficiency, when the pressure in the growth chamber increases. The comparative J-V measurements of the solar cells processed in this work are also illustrated and discussed.