COVER I STORY
power can be measured (module on the right side).
Long term promise Manufacturers have to offer a long-term guarantee of their products. Therefore the reliable detection of micro cracks becomes essential for them and they will be forced to proof the quality to their clients with electroluminescence images.
Test module before thermo cycling
Electroluminescence inspection technology can be used for crystalline and thin film modules to provide valuable information about the production process. In the case of thin-film modules the focus is on the detection of short circuits and shunts resulting from the coating and scribing processes. Defects on crystalline solar modules can be segmented into three defect sources:
Test module after thermo cycling
supply to apply current and voltage to the solar module under test. Serial connection of the solar cells within a module resembles serial connection of diodes. Thus if the voltage of roughly 0,7 V – for each cell, or 42 V for a 60 cell module – is exceeded by the power supply, a current will flow through the series connection. The current is limited by the control of the power supply unit. Each solar cell will emit light near the band gap of silicon (1100 nm). Special cameras with a high sensitivity in the NIR spectrum are needed to capture an EL image using this very little light. This effect makes invisible defects visible, inactive areas for example will appear dark, active areas bright. Most of the critical defects are occurring during production become visible through this technology. That is why electroluminescence inspection is so important for this industry.
Influence of the module quality over the lifetime
Many defects are not visible to the human eye, but can influence the power output of a module over its lifetime. Most critical of all are the so-called micro cracks. Those defects have initially no influence on the measured power coming from the sun- simulator. That’s why micro cracks will not be detected, but it is proven that micro cracks have an influence on power output over the time through mechanical and thermal stress. The images below are showing this effect. On the left side the small test module with micro cracks provides 100% of the calculated power, but after 150 thermo cycles a huge drop in the output
14
www.solar-international.net I Issue III 2012
a) cell production process, b) module production process, c) transport and installation process.
The solar module production process covers numerous possible causes of bad solar module quality. First of all, the soldering process is one of the critical process steps for the mechanical stability of the solar cells. Running the stringing machine with slightly erroneous parameters can cause serious defects like cracks or finger interruptions. Using an electroluminescence string tester – even off-line – will help to get this process under control. But also the cross connection process and the lamination process can cause further mechanical damage of the solar cells. Inspecting the module before it enters the lamination process will enable the manufacturer to replace defective cells.
Image comparison between low resolution and high resolution EL systems
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40