Investigating Reliability Attributes of Silicon Photovoltaic Cells: An Overview
Reliability attributes are being developed on a wide variety of advanced single-crystal silicon solar cells. Two separate investigations: cell-contact integrity (metal-to-silicon adherence), and cracked cells identified with fracture-strength-reducing flaws are discussed. In the cell-contact-integrity investigation, analysis of contact pull-strength data shows that cell types made with different metalization technologies, i.e., vacuum, plated, screen-printed and soldered, have appreciably different reliability attributes. In the second investigation, fracture strength was measured using Czochralskli wafers and cells taken at various stages of processing and differences were noted. Fracture strength, which is believed to be governed by flaws introduced during wafer sawing, was observed to improve (increase) after chemical polishing and other process steps that tend to remove surface and edge flaws.
Accelerated Stress Testing of Terrestrial Solar Cells
The results of a program to investigate the reliability characteristics of unencapsulated low-cost terrestrial solar cells using accelerated stress testing are presented. Four types of cells were investigated. Reliability (or parametric degradation) factors appropriate to the cell technologies and use conditions were studied and a schedule of accelerated stress tests was synthesized. An electrical measurement procedure capable of distinguishing small chanes in cell electrical parameters was established. A data analysis and management system was derived, and stress test fixturing and material flow procedures were set up after consideration was given to the number of cells to be stress tested and measured and the nature of the information to be obtained from the process. Based on both electrical parameters and metalization adherence strength, significant degradation was shown by some cell types in some stress tests. Other combinations of cell types and stress tests resulted in no detectable cell degradation. Analysis of the origins of the differences in degradation is continuing.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.