High voltage bias testing of thin film pv modules, adhesional strength and surface analysis for pv module durability and study of back contact molybdenum for thin film cigs2 solar cells

Keywords

Molybdenum; Photovoltaic cells; Thin films

Abstract

This thesis primarily focuses on three different aspects of photovoltaic (PV) modules. The first aspect covered the study of the reliability of thin film PV modules. This work describes high voltage bias testing of thin-film PV modules in hot and humid conditions of Florida. This high voltage setup is the second of its kind in USA (first one at NREL) and the one and only in the entire world in hot and humid conditions. High leakage currents generated in thin film PV modules can lead to electromigration and degradation, and thus become important issue for reliability and safety. Sudden rise in relative humidity (RH) caused by sudden drop in temperature during the day resulted in sharp peaks in leakage current. Leakage currents for a typical clear sunny day in summer were approximately double compared to corresponding values in winter. Low-resistance paths created with the passage of time increased the leakage current continuously indicating probable exponential growth in leakage currents. Leakage currents increased proportionately to applied voltage bias, at least up to 600 V, showing potential of the technique as an excellent acceleration test. Corrosion and complete delamination of cell of a large region in less than 18 months also show that all thin modules must be tested under high voltage bias. The second aspect addressed the issues related to the durability and reliability of crystalline silicon PV modules. Past 25 years research on durability and reliability of crystalline silicon modules aimed at achieving a service lifetime of 25 years and more. However there are still few critical issues to be rectified for achieving this goal. One semi-crystalline silicon, two of monocrystalline silicon and two of multi crystalline silicon PV modules were tested for adhesional strength analysis. The modules were subjected to 11 visual inspection and were cored till glass. Cored samples were further characterized by optical microscopy, scannmg electron microscopy (SEM), energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). These results indicate the presence of impurities like sodium, phosphorus, titanium, fluorine and iodine. Sodium might have diffused from the glass onto the cell, phosphorus from n-type dopant, titanium from antireflection coating, iodine from solder flux and Fluorine was due to contamination appearing as white patches on the cell. Fluorine might have been introduced during cleaning or storage or from the tedlar at the back of the module, which is fluoropolymer. Another aspect dealt with the study of the effect of deposition parameters on the morphological and electrical properties of sputtered deposited molybdenum (Mo), the most commonly used back contact material for thin film solar cells. Thin Mo films were obtained on 7" x 4"clean glass substrate as well as y~"x 6" one mil thick titanium foil using magnetron sputtering with varying power and pressure. The thickness and surface roughness of these films was measured using optical profilometer. Four-point probe technique was used for measuring the resistivity of these films. Surface morphology was studied using scanning electron microscopy (SEM). Depending on the bending of thin foil deposited, the stress on the films was measured. Varying the deposition parameters and altering the cycles of deposition, films with improved surface morphology and reduced stress were obtained. These parameters were further used for deposition of back contact molybdenum for successful fabrication of 4" x 4" highly efficient CIGS2 thin film solar cells on glass, stainless steel (5 mil thick) and titanium foils (1 mil thick) for novel space as well as terrestrial applications.

Notes

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Graduation Date

2003

Advisor

Dhere, Neelkanth G.

Degree

Master of Science (M.S.)

College

College of Engineering

Department

Mechanical, Materials, and Aerospace Engineering

Format

PDF

Pages

96 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0029084

Subjects

Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic

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