QC/QA of Cells & Modules
Solar industry will continue to experience explosive growth at least for the next decade or more. With increased affordability and adaptability of solar applications in residential and commercial, and state and local mandates, the need for a quick and reliable series of tests and measurements has intensified to ensure quality of cells and modules. Solar cells and modules are manufactured in a multitude of power levels and various conversion efficiencies. Research, Production, and QC/QA have different needs for measurement, accuracy and range of parameters that must be measured. The manufacturing of cells and modules can be classified into two distinct processes. They are Front-end and Back-end processes. Reliable inspection, monitoring and QC of both Front-end and Back-end production processes are necessary to ensure manufacturing of quality solar panels.
The use of quality control systems at the front-end prevents defective solar panels from cycling on to further process stations. This saves real money, as panels which would be rejected in the final quality check, can be sorted out before costs for the subsequent processing occur. Additionally, inspection systems can help to optimize the coating and scribing processes, in order to achieve optimal module performance at the best possible yield. Some of the parameters that are critical from QC perspective are, 1. Detection of defect in incoming glass, 2. Detection of defect of coated glass, 3. Homogeneity of layer deposition, 4. Inspection of chemical steps, 5. Inspection of laser scribing and structure.
Panels arriving at the back-end are already valuable, semi-finished products. The visual in-line control at the back-end primarily benefits the production within preset process windows. With every production step towards the line end, the costs per processed panel increase. Therefore, material defects occurring at the back-end stage usually cost a lot of money and need to be detected reliably. Line operators receive instant access to possible causes and can take immediate counteraction to prevent further damage. At the back-end, lamination and edge sealing are the production steps most crucial to guarantee a long life-time required from today‘s solar modules. They are therefore in special focus of quality and process control solutions.
Typical back-end parameters for quality measurements are, 1. Inspection of layer residue, 2. Measurement of depth of edge deletion, 3. Cell I-V Curve Measurement, 4. Defects in coating and homogeneity of AR Coating, 5. Lamination Defects, 6. Defects in contact bars, 7. Defects in drill hole, 8. Defects in back cover glass, 9. Position check for assembly parts, 10. Defects in edge sealing, 11. Cosmetic defects of finished modules.
The QC protocols are further classified as indirect and direct methods of cells and modules performance. The indirect methods usually employed are optical in nature while the direct methods are a measure of functional characteristic of the cells and modules. Typical optical measurements involve use of microscope, electroluminescence (EL) and photoluminescence (PL) of cells and modules. Electroluminescence (EL) is the use of a solar cell in a reverse format as it was originally designed. Instead of converting irradiance into electricity, at EL, electricity (supplied via the cell’s electrical contacts) is converted into IR-radiation that is emitted via the cell’s surface. The intensity of the radiation emission is an indicator for the local efficiency and quality of the conversion process. This method works well for cells and modules, but not for wafers.
GREPI is exploring the use of imaging technology and associated proprietary software as an alternative potential QC method for cells and modules. In photoluminescence the photons absorbed within cells and modules is released upon irradiation with small wave length is measured. Thermal imaging using an IR camera is another important method for the identification of temperature changes and hence efficiency of cells. This important step will help in binning cells for module manufacturing with maximum power.
Functional analysis of solar cells is done by direct method which captures the I-V curve characteristics obtained under illuminated conditions as well as capture the reverse bias characteristics of the cell under dark conditions. The illuminated I-V curve of the cell allows you to calculate parameters such as the conversion efficiency of the cell and the maximum power point of the cell. Testing the solar cell in dark conditions in reverse bias allows you to calculate such parameters as the parallel resistance of the cell and the breakdown region of the cell. To fully characterize a solar cell, a four-quadrant DC source is ideal. A four-quadrant source can sink and source current and output positive and negative current, which allows you to fully characterize the cell.