Inspect solar cells without a microscope
Use a short-wave infrared camera to capture the image of an entire wafer.
Chun-Fu Lin and Tai-Shan Liao, National Applied Research Laboratories, Hsinchu, Taiwan; Edited by Martin Rowe and Fran Granville -- EDN, November 26, 2009
Solar cells convert light energy into electricity, making them a renewable energy source. Solar-cell manufacturers often use SEMs (scanning electron microscopes) to detect defects in solar cells while they're still in wafer form. Although SEMs can see down to a solar cell's grain structure, they can be slow because their scan area is small. A SEM must scan a wafer many times to cover it.
Instead of using a SEM, you can use an SWIR (shortwave-infrared) camera system to detect defective cells. You can take advantage of a solar cell's electroluminescence signature to find defects on a solar cell. A cell's light has a wavelength of about 1.1 micron, which results when you apply a forward bias voltage and forward operating current of at least 7A to the cell. An SWIR sensor can provide an image of an entire wafer, eliminating the need to scan the wafer. The sensor identifies defects by detecting a wafer's electroluminescence.
Figure 1 shows the system, which uses an SWIR sensor that converts an image into an analog voltage. A preamplifer boosts the signal to a level sufficient for an ADC in a digital-processing module to digitize the analog signal at 10M samples/sec.
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The ADC's digital output travels through an LVDS (low-voltage-differential-signaling) data interface to a Dalsa frame-grabber card in a computer. Custom image-processing software, written in C++, processes the data, producing an image of the entire wafer on the computer's screen.
The board containing the sensor, preamplifier, and ADC also has a microcontroller, which generates a clock signal for the timing of the sensor and the ADC. An RS-232 communications port on the Atmel microcontroller allows it to communicate with a PC to get commands from the user who set parameters such as the SWIR sensor's operating mode. A timing-driver circuit sends the clock signal to the SWIR sensor.
Figure 2 shows the image from the SWIR camera circuit. This image shows the intensity distribution of the cell's light output. A homogenous intensity-distribution image is essential for a high-quality solar cell, but solar cells always show some inconsistencies. All defects resulting in a local reduction of the carrier concentration are visible on the electroluminescence image as dark bars.
