V. Manivannan

Advances in continuous, in-line processing of stable CdS/CdTe devices

A continuous, in-line process suitable for high throughput manufacturing of CdS/CdTe photovoltaic devices has been demonstrated. Utilizing this process, devices with efficiencies of 13% has been fabricated with a low iron soda lime glass (3″×3″) with ant-reflection coatings. The process has been extended to large area devices (16″ ×16″ substrate size). After CdCl<inf>2</inf> treatment, devices showed V<inf>oc</inf> ≫ 700 mV and J<inf>sc</inf> ≫ 20 mA/cm². This performance is similar to the performance of small area devices which showed good stability. Also we have employed Spectroscopic Ellipsometry (SE) as a nondestructive tool to characterize CdS/CdTe heterojunction specifically studying the effects of chemical treatment on the optical properties of the thin-film layers.

Synthesis, and measurement of structural and magnetic properties, of La1-xNaxFeO3(0.0≤x≤0.3) perovskite oxides

Abstract Polycrystalline La1-xNaxFeO3 (0.0≤x≤0.3) solid solutions were synthesized by a solution combustion method using glycine as a fuel. The combustion synthesized compounds were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), diffused reflectance (DR) in the UV-vis, and magnetic measurements. The crystal structure examined by X-ray powder diffraction indicated that the samples were single phase and crystallized in an orthorhombic (space group, Pbnm no.62) structure. The parent and sodium-substituted compounds showed canted anti-ferromagnetic behavior associated with an increase in magnetic moment as Na content increased. The changes in magnetic properties of the materials were correlated to the changes in structural features, as shown by Rietveld structural refinement of the materials.

Stable Cu-Based Back Contacts for CdTe Thin Film Photovoltaic Devices

Cadmium telluride (CdTe) thin film photovoltaic devices fabricated in a-line process developed at Colorado State University (CSU) have shown stability during long-term (over a 5 year period) accelerated stress testing. These devices have a copper (Cu) containing back contact. The Cu profile as measured by secondary ion mass spectrometry characterization shows, for the maximum stressed device (23,399 h), that there is a significant (two times) change in the concentration of secondary Cu ions in the bulk of the material; however, the Cu concentration gradient at the back of the device has no significant change, and the CdS layer has no significant Cu concentration increase at open-circuit bias and 65°C temperature conditions. This indicates that with a proper CdCl 2 treatment, Cu can be used to form the back contact for CdTe devices with acceptable stability. These devices have a projected field lifetime of greater than 60 years.

Synthesis of MSn(OH)6 (where M = Mg, Ca, Zn, Mn, or Cu) materials at room temperature

Applying a metathesis approach (MCl2·xH2O+Na2SnO3·xH2O→MSn(OH)6+NaCl+xH2O), Schoenfliesite-type materials with general formula MSn(OH)6 (where M=Ca, Cu, Mg, Mn or Zn) were synthesized at room-temperature. The high lattice-energy of the by-product alkali halide NaCl drives the reaction in the forward direction leading to the formation of the desired materials. The materials synthesized were characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) to elucidate structural and micro structural features.

Effect of Chemical Treatment on the Optical Properties of a Cadmium Telluride Photovoltaic Device Investigated by Spectroscopic Ellipsometry

Spectroscopic ellipsometry has been successfully used to characterize the CdS/CdTe heterojunction solar cell deposited on TEC15 glass. The effects of copper treatment on the optical properties of a cadmium chloride treated photovoltaic device were investigated using ellipsometry. No changes in either the band gaps or critical points of CdTe layer were noticed as a result of copper treatment. The copper treated CdTe layer exhibited a higher refractive index in the visible and longer wavelengths (≤3 eV), as compared with the untreated layer. This was attributed to the increased disorder in the case of copper treated layers.

Continuous in-line processing of CdS/CdTe devices: Process control using XRF and efficient heating

A simple, quick, and non-destructive way to measure deposited Cu, used for back contact formation, and thickness of CdS and CdTe layers in the device is desirable for manufacturing process control. A simple X-ray Fluorescence (XRF) system was investigated for these measurements. Experiments show that 5 min. polychromatic XRF spectra can be used to measure CdTe and CdS thickness, and accurate 30 sec. measurements are possible. Statistical integrity of Cu XRF measurements was investigated. XRF measurements are able to differentiate Cu concentrations within 10% of nominal, making it useful for process control. A description of a new, improved in-line R&D deposition system is also presented. Thermal modeling results show the new NiCr wire based heating system will reach desired operating temperatures while providing opportunities for improved temperature uniformity and energy usage.

Measurement of cooling rates of a superstrate cooling apparatus for an integrated in-line manufacturing process for thin-film photovoltaic devices

A pilot-scale semiconductor deposition system for the continuous production of CdTe based solar cells has been developed. During initial process optimization it was found that glass superstrates needed to be cooled before the back contact application. In order to shorten the cycle time for cooling glass superstrates, a close contact cooling apparatus with conduction through the gas phase for use in a vacuum chamber is being developed. Initial measurements of the cooling rates for a test cooling apparatus indicate that 76×76×2mm3 superstrates can be cooled from 170to30°C in a period of 4min.

Performance of In-Line Manufactured CdTe Thin Film Photovoltaic Devices

In-line manufactured CdTe thin film photovoltaic devices were evaluated for their performance with respect to power conversion efficiency and reliability (stability). A National Renewable Energy Laboratory (NREL) certified power conversion efficiency (η) of 12.44% was measured for one CdTe device. A mean efficiency of 11.4% for 53 devices is shown. Results of reliability studies give a conservative estimate of -1% relative degradation per year in efficiency over a 20-year lifetime for these CdTe devices.

Effect of chemical treatment on the optical properties of a cdte photovoltaic device investigated by spectroscopic ellipsometry

Variable angle Spectroscopic Ellipsometry (SE) was used to study CdTe PV devices at multiple points in the production process, with specific attention to the effect of applying of CdCl2 and Cu treatment. Results suggest the potential value of this characterization technique.

Effect of microstructure on the performance of stressed cadmium telluride photovoltaic (PV) devices

Our group has demonstrated a commercially viable, continuous, in-line process to fabricate PV devices with efficiencies of 13% on a low iron soda-lime glass (3„×3„) with anti-reflection coatings. The process has been extended to large area devices (16„×16„ substrate size). In this paper we present our results on the microstructural features of PV devices subjected to stress conditions. The results help to gain insight into the stability and lifetime of CdTe PV modules.