Solar inspection: Cheaper and easier with Photonfocus and CMOS technology

Solving the solar cell inspection cost issue with the Photonfocus EL1-D1312

The rise in domestic and commercial demand for solar cell technology has led to the need to produce more efficient and cheaper solar cell panels.

One of the issues in providing solar-cell energy generation products has been finding a low-cost solution for the identification of defective solar cells and panels during manufacture in order to improve the efficiencies and low the costs.

Addressing this challenge is Photonfocus' EL1-D1312 CMOS camera, a detection tool that can be applied to specific parts of the inspection process to test for quality in solar cell manufacture.

What technologies exist for solar cell quality control?
Soalr Cells are made from Silicon and are manufactured from Silicon ingots which are blocks of silicon. The blocks are sliced into thin wafers which are then etched and doped to produce a solar cell. Solar cells are then integrated into solar modules and a number of solar modules are put together to create a solar panel - which is the end result.

There are defects in the original block of silicon such as cracks that ultimately find their way into the solar cell and so reduce its efficiency to produce energy. These cracks are sometimes on the surface of the cell and sometimes below the surface. Those defects on the surface can be detected using visible light with standard area scan or line scan cameras. The defects below the surface cannot be seen with visible light and so must be detected with an alternative means.

Silicon is to some extent transparent to near-infrared light and so this can be used to detect defect features and anamolies below the surface of silicon. This requires a camera that is sensitive to near infrared light. That;s where the Photnofocus EL1-D1312 comes into the picture.

There are two techniques withwhich infra-red light can be used to inspect solar cells for defects:
*** Photoluminescence; and
*** Electroluminescence

How are they different?
If using the photoluminescence process, a laser excites the solar cell which then emits infra-red light. The emitted light is then captured by the camera and the resultant image shows what is under the surface of the cell. Being partially transparent to infrared light this allows cracks, breaks and other defects below the surface can be seen.

In the electroluminescence process, a voltage source is connected to the finished cell and a current fed through it resulting in the cell emitting infra red light. A camera captures the infrared light and the resultant image displays defects in the cell that are beow the surface.
What are the advantages and disadvantages of using each type of technology?
	The advantage of using photoluminescence in solar cell inspection is that quality control can be conducted at any stage in the production process. On the other hand, inspection via electroluminescence needs the cell to be finished before it is tested. The major disadvantage with photoluminescence; however, is that he quantity of light emitted is very low and so the camera used must be very sensitive. Typically the cameras used are cooled back-thinned CCD cameras or InGaAs cameras. These cameras are very expensive. However with the recent release of the new Photonfocus' EL1-D1312,, solar cell inspection can be done when the solar cell is finished and before a module is built, using low-cost, non-cooled CMOS camera technology.
EL1-D1312 CMOS sensor

What can be seen using the electroluminescence process?
OPTICAL IMAGE: The above two images shows a solar cell as viewed with an optical microscope		compared with	ELECTROLUMINESCENCE IMAGE The same cell showing intrinsic defects (e.g. inhomogeneity) and external faults (e.g. cracks, broken finger) made clearly visible by EL imaging.

How can the electroluminescence process be applied to solar cell quality control? Fault detection via the electroluminescence process can occur at two points: (1) when the individual cell is finished and (2) when a series of cells are aggregated to form a module. The following flow diagrams indicate where EL can be deployed.
	The illustration on the left demonstrates the point at which EL enters into the process, post-cell completion; the camera's function being to screen/test for finder interruptions, homogeneity, micro-cracks and local shunts. The diagram below describes the further role of the EL camera again, post-cell finishing, but at the module-preparation stage. Using a lower-cost EL CMOS camera in part of the quality-control process results in money-saving efficiencies while still protecting the integrity of the final product.
Quality control points during cell production (above)
Quality control points during module production

Why use electroluminescence and the Photonfocus EL1-D1312 CMOS camera? What are its benefits?
	The diagram on the left compares the quantum efficiency of the Photonfocus EL1-D1312 CMOS sensor as compared to the ICX285 which is regarded as one of the most sensitive CCD sensors to NIR. The EL1 has more than two times the sensitivity at 1000nm The Photonfocus EL1-D1312 camera: ** is an especially designed solution for electroluminescence applications ** has an uncooled CMOS sensor ** has good NIR sensitivity ** needs short exposure times (400ms) ** is a fraction of the cost of cooled back-thinned Si-CCD cameras

What components are needed to capture images?
** Photonfocus EL1-D1312-160-CL-12 camera
** Photonfocus EL Demo Software
** Dalsa Framegrabber
** Dalsa Sapera T software driver
** Optimised NIR lens, CameraLink cables, etc (Note: The use of a suitable NIR lens makes a significant difference)

About the A1312 CMOS sensor
The A1312 image sensor used in the EL1 is designed and fabricated in a 0.35µm CMOS technology optimized for image sensors to achieve an outstanding sensitivity and quantum efficiency. The resolution of the new sensors is 1312 x 1082 pixels, with 8µm x 8µm square pixels and over 60% fill factor with an excellent image quality. The new A1312 CMOS sensor has an extended spectral range covering 350nm to over 1100nm.

Quick specifications
• 1312 x 1082 pixel resolution
• 8µm x 8µm square pixels
• Up to 108 fps @ full resolution
• High sensitivity over wide spectral range from 320 to 1100 nm.
• High Quantum efficiency (> 50%)
• Sensor without cover glass
• Up to 12 bit grayscale resolution
• Global shutter avoiding image distortions, ideal for high speed applications
• CameraLink® interface
• Dimensions 60 x 60 x 45 mm

Summary of Adept Electronic Solutions are "The Machine Vision and Imaging Specialists" and distributor of Photonfocus products in Australia and New Zealand. To find out more about any Photonfocus machine vision product please email us at: adept@adept.net.au or call us at Perth (08) 92425411 / Sydney (02) 99792599 / Melbourne (03) 95555621 or use our online contact us page.