September 8, 2009--Goodrich ISR Systems (Princeton, NJ; formerly Sensors Unlimited) manufactures sensors and cameras based on indium-gallium-arsenide (InGaAs) imaging technology that can be used in photovoltaic (PV) inspection to detect solar-cell and solar-panel flaws. These InGaAs imagers can easily capture the glow emitted by the PV junction of the solar cell under forward-bias conditions in an electroluminescent imaging system (see also "Short-wave infrared cameras characterize thin-film solar cells").
Operating at video rates, short-wave infrared (SWIR) InGaAs cameras image the glow from within the cell, 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.
In the first part of the video, solar cell electroluminescence (EL) inspection takes place at 60 frames per second (fps). The clip shows a small solar-cell section being driven to luminance by a square-wave pulse from a function generator running at 1 Hz. Note the structure evident in the luminescent image, including the obvious dead areas. The imagery was captured under dim lighting conditions with the Goodrich InGaAs KT camera, using high gain with 16 ms exposures while recording at 60 fps. All images in the video were captured with 320 x 256 pixel resolution. For higher resolution or more field of view, Goodrich also manufactures 640 x 256 pixel resolution cameras.
In the second part of the video, SWIR electroluminescence imagery from Goodrich's KT camera detects the lack of efficiency uniformity in this 36-cell, commercial solar panel. The 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.
Finally, the latter section of the video shows simultaneous comparison of SWIR electroluminescence from one pair of mono-crystalline modules encapsulated in clear epoxy (top of video) and from another pair of thin-film copper indium diselenide solar modules deposited on glass (bottom of video). The voltage across all modules was ramped from 4.5 V to 6.5 V several times during the segment. The video demonstrates that cell luminescence, which is an indicator of photovoltaic cell efficiency and manufacturing uniformity, is quite different, both within and between cells of the same type. These cells were purchased over the internet. The video demonstrates the wide variation of quality in solar-cell products currently available, and also shows that the electroluminescent inspection method is useful for different types of photovoltaic designs.
For more information, go to www.goodrich.com/isr.
--Posted by Gail Overton, [email protected]; www.laserfocusworld.com.