G. Gordillo

Electrical and morphological properties of low resistivity Mo thin films prepared by magnetron sputtering

Mo thin films have been deposited using a DC magnetron sputtering system with an S-gun configuration electrode and characterized electrically and morphologically. The influence of the sputtering gas pressure and glow discharge (GD) power, on the electrical resistivity of Mo thin films and on the contact resistivity of Mo to Cu(In,Ga)Se2 (CIGS) films was determined through an exhaustive parameter study. This study also allowed us to find the conditions to deposit Mo films with suitable properties for its use as back contact of solar cells based on CIGS. Resistivities smaller than 1x10-4 W.cm and contact resistivities smaller than 0.3 Wcm2 were found. Mo films with these characteristics are suitable for back contacts in solar cells based on CIGS. It was also found that the Mo thin films, deposited by DC magnetron sputtering on CIGS thin films, act effectively as ohmic contacts. The main contribution of this work was to obtain Mo thin films with adequate properties to be used as back contact for CIGS based solar cells using a DC sputtering system with S-gun configuration electrode, which allows growing the film with better surface quality and at a higher deposition rate than those deposited using the conventional planar RF sputtering system.

Optical and structural studies on SnS films grown by co-evaporation

Polycrystalline SnS thin films were grown on glass substrates using a novel procedure involving a chemical reaction between the precursor species evaporated simultaneously. This is a relatively new material, which exhibits excellent properties to be used as absorbent layer in solar cells. X-ray diffraction (XRD) measurements indicate that the synthesized samples grow in several phases (SnS, SnS2 and Sn2S3) depending upon the deposition conditions. However, through an exhaustive parameter study, conditions were found to grow thin films predominantly in the SnS phase with orthorhombic structure. It was found that this type of compound presents p-type conductivity, a high absorption coefficient (greater than 10 4 cm i1 ) and an energy band gap Eg of about 1.3 eV, indicating that this compound has good properties to perform as absorbent layer in thin film solar cells.

Synthesis and characterization of highly transparent and conductive SnO2:F and In2O3:Sn thin films deposited by spray pyrolysis

Highly transparent and conductive thin films of SnO2:F and In2O3:Sn (ITO) have been prepared on glass substrates using the simple pyrolitic (spray) method. Through an exhaustive parameter study and using as diagnostic method for the film quality a figure of merit defined as a function of both, the transmittance and the electric resistivity, the conditions to prepare the SnO2:F and ITO films, with adequate properties to be used as transparent front contact for solar cells, were achieved. A relevant contribution of this work is related with the deposition of SnO2:F and ITO films with the mentioned characteristics, using a solution synthesized in our laboratory by dissolving the precursor metals in HCl. Transparent conducting oxide (TCO) thin films were obtained, with transmittances greater than 80% and resistivities smaller than 7x10 - 4 W·cm, results which are comparable with those obtained using commercial reactants. Results concerning the influence of the synthesis parameters on the optical, electrical and structural properties of the TCO films are reported.

Statistical modeling for global solar radiation forecasting in Bogotá

Based on measurements of global solar radiation carried out in the National University of Colombia (located in Bogotá, at 74° 4′ West, 4° 35′ North, 2580 m altitude) for the period 2003 – 2009, a statistical modeling based on time series was made to forecast the accumulated mean daily global solar radiation in Bogotá. This analysis was done using the programs EXCEL 2007 and SPSS 16; the lost data were determined with the technique of lineal trend. Using the test of Dickey - Fuller it was established that the time series are not stationary, but seasonal. Using the methodology of Box and Jenkins to convert the original data to a stationary series, it was found that the series which better fitted to the one built with experimentally measured data, can be obtained using the ARIMA (1, 0, 0) model, which can be mathematically represented by the equation X<inf>t</inf> = 3562.78 + 0.518 X<inf>t−1</inf> + ε<inf>t</inf>, where the error ε<inf>t</inf> is distributed like a normal (0,1) white noise

CGS based solar cells with In2S3 buffer layer deposited by CBD and coevaporation

In this paper we investigated In2S3 as substitute for CdS, which is conventionally used as buffer layer in chalcopyrite based solar cells. In2S3 thin films were deposited by CBD and co-evaporation methods and these were employed as buffer layer in CuGaSe2 based solar cells. Previous to the device fabrication, comparative study was carried out on In2S3 thin films properties deposited from chemical bath containing thioacetamide, Indium Chloride, and sodium citrate, and In2S3 thin films prepared by co-evaporation from its constituents elements. The influence of synthesis conditions on the growth rate, optical, structural and morphological properties of the as-grown In2S3 thin films have been carried out with Spectrophotometry, X-ray diffraction and AFM microscopy techniques. Suitable conditions were found for reproducible and good quality In2S3 thin films synthesis. By depositing In2S3 thin films as buffer layers in CuGaSe2 configuration, a maximum solar cell efficiency of 6% was achieved, whilst the reference solar cell with CdS/CuGaSe2 on similar absorber exhibited 7% efficiency.

A study of trap and recombination centers in MAPbI3 perovskites

Trapping and recombination processes in thin films of CH3NH3PbI3 (MAPbI3) were studied by means of transient photoconductivity measurements and theoretical simulations of the relaxation curves resulting from the photocurrent measurements; in particular, the influence of temperature as well as of the sample temperature and intensity of illumination and pressure inside the measurement system on the photoconductivity response, were studied. The experimental curves of photocurrent were analyzed using the real part of the Fourier transform. The study revealed that the photocurrent of the MAPbI3 films, measured at atmospheric pressure, is mainly governed by surface related processes induced by chemisorption and desorption of oxygen, whereas the photocurrent resulting from measurements performed in a vacuum is mainly governed by bulk related processes. It was found that, in general, the photocurrent response is affected by both trap assisted fast recombination processes and traps whose activation process is delayed, with the contribution in the intensity of the photocurrent of the first process being greater that of the second one. Evidence that the MAPbI3 film exhibits a deep trap state at around 459 meV attributed to trap assisted recombination was found; furthermore, the MAPbI3 films present shallow trap states at 129 and 24 meV that correspond to trap states whose activation process is delayed.

One-step diffusion membrane assisted CBD synthesis and characterization of Cu2SnS3 thin films

This paper present a novel method for growing thin films of Cu2SnS3 (CTS) using a solution-based chemical route consisting of simultaneous precipitation of Cu2-xS and SnS2 performed by diffusion membranes assisted CBD technique. Diffusion membranes are used to optimize the kinetic growth through a moderate control of the releasing metal into the solution. The conditions in terms of concentration of metal species, sulfide anion and temperature required for the precipitation of the Cu2SnS3 compound were determined through a study of chemical equilibrium of the system SnCl2, Na3C6H5O72H2O, CuCl2 and Na2S2O35H2O. These conditions were obtained solving the equilibrium equations with the help of the Visual MINTEQ 3.0 package, supported on the program MINTEQA2. X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the structural properties of the CTS films. Optical, morphological and electrical properties were also studied by spectral transmittance, atomic force microscopy (AFM) and resistivity vs temperature measurements. XRD and Raman measurements confirmed the formation of the Cu2SnS3 phase.

XPS analysis and structural characterization of CZTS thin films prepared using solution and vacuum based deposition techniques

This work describes a procedures to grow single phase Cu2ZnSnS4 (CZTS) thin films with tetragonal-kesterite type structure, using solution and vacuum based deposition techniques. The solution based deposition approach involves sequential deposition of Cu2SnS3 (CTS) and ZnS films, where the CTS compound is synthesized in one step process by simultaneous precipitation of Cu2S and SnS2 performed by diffusion membranes assisted CBD technique and the vacuum based approach includes sequential evaporation of the elemental metallic precursors under a flux of sulphur supplied from an effusion cell. X-ray diffraction analysis (XRD) which is mostly used for the phase identification can not clearly distinguish the formation of secondary phases such as Cu2SnS3, since both compounds have the same diffraction pattern; therefore the use of a complementary technique is needed. Raman scattering analysis was used to distinguish these phases. Further, the chemical elemental composition and oxidation states of both type of CZTS films were studied by X-ray photoelectron spectroscopy (XPS) analysis.

Phase identification and AES depth profile analysis of Cu(In,Ga)Se2 thin films

This work presents results related with phase identification and study of the homogeneity in the chemical composition of Cu(In,Ga)Se2 (CIGS) thin films grown by a chemical reaction of the precursor species evaporated sequentially on a soda-lime glass substrate, in a two or three-stage process. For that, the CIGS samples were characterized through X-ray diffraction (XRD) and Auger Electron Spectroscopy (AES) depth profile measurements, respectively. The presence of secondary phases growing superficially was identified through small angle XRD measurements. Theoretical simulation of the XRD spectra, carried out with the help of the PowderCell package, confirmed this result. The results showed that the deposition conditions affect the homogeneity of the chemical composition of the CIGS films, as well as the phase in which these films grow. In general, the samples grown in a two stage process present a mixture of the Cu(In,Ga)Se2 phase with the secondary In2Se3 and Cu2Se phases, whereas, the films grown in a three stage process do it, mainly, in the Cu(In,Ga)Se2 phase with a tetragonal (chalcopyrite type) structure and better homogeneity. CIGS films with characteristics like those found for the three stage samples, have demonstrated good properties for its use as absorber layers in thin film solar cells.

Electrical and optical properties of thin films with a SnS2 ? Bi2S3 alloy grown by sulphurization

In this work, thin films of SnS2 with increased Bi content were grown by sulphurization of a thin film of Sn:Bi alloy, at temperatures around 300°C. The effect of the Bi concentration on the optical, electrical and structural properties was determined through measurements of spectral transmittance, conductivity and x-ray diffraction XRD respectively. It was found that the optical constants (refractive index n, absorption coefficient α and energy gap Eg) and the electrical conductivity are significantly affected by the Bi concentration. In particular, a variation of the energy gap between 1.44 and 1.63 eV and a change of the conductivity greater than three orders of magnitude were observed when the content of Bi in the Sn:Bi alloy varied between 0 and 100 %. The analysis of the XRD measurements allowed us to find that the SnS: Bi films grow with a mixture of the SnS2 and Bi2S3 phases, independently of the Bi content.