Shortwave Infrared (SWIR) Imaging for Photovoltaic Inspection

Machine vision is one of the key tools that photovoltaic manufacturers are using to improve solar cell uniformity and efficiency, thereby improving yield, and driving down the cost of each cell. While researchers are experimenting to boost conversion efficiencies of several competing PV cell types, manufacturers are working hard to reduce their production costs of cell types that can most readily ramp up into volume production. In the above video, the SWIR electroluminescence imagery clearly shows that the cells have unwelcome and widely varying regions of efficiency. Goodrich's KT shortwave infrared-InGaAs camera detects the lack of efficiency uniformity in this 36-cell, commercial solar panel. The SWIR camera sweeps across a panel composed of 3 rows of 12 cells, with the panel nominally rated as 18 V output. Applying that same voltage to the solar panel causes the cells to glow at different intensity levels (this was recorded during the day in a room with drawn mini-blinds). Careful viewing of the cells shows that several contain defects; zooming in to the cells to the left of center reveals that one of the cells is cracked, with some of the portions dead. Note in the bottom row of cells, near the end of the video, an unwanted defect shows up consistently in the top section of many of these cells. Sensors Unlimited - Goodrich ISR Systems, Princeton, NJ (www.sensorsinc.com) manufactures sensors and cameras based on indium gallium arsenide (InGaAs) imaging technology that can be used in photovoltaic inspection to detect solar cell and solar panel flaws. These InGaAs imagers can easily capture the glow emitted by the photovoltaic (PV) junction of the solar cell under forward bias conditions in an electroluminescent imaging system. Operating at video rates, SWIR-InGaAs cameras image the glow from within the cell, thus revealing any non-uniformity in photodiode junction or in the associated optical layers above. This helps cell manufacturers, panel manufacturers, and solar system installers to screen out weak cells. The cameras can also detect developing problems in older cells that might be a result of poor or decaying seals and they can assist in matching cells of similar efficiencies. The latter application is important in order to obtain the highest power output from solar modules and panels and to improve the reliability of the whole solar energy generation system.