Luminescence Imaging For Solar QC

Are you looking for a reliable and almost universally applicable quality control (QC) method in the field of PV – then it is likely that Luminescence imaging is the technology of choice, as shown in the first TaiyangNews Market Survey on Luminescence Imaging Systems, which my colleague Shravan Chunduri and me recently published.

Luminescence imaging has evolved into a crucial and reliable characterization principle in PV – used almost from upstream production start in silicon crystallization processing down to the very end of the value chain in system integration and operations. This is true for EL and PL, even though their applications are somewhat different.

Why use luminescence imaging for quality control?

The luminescence imaging technique is simple and detects a wide range of defects for quality control – from microcracks to finger disruptions and shunts at the cell level and soldering induced damages at the module level, and much more. In addition,
the information can also be used to develop or fine tune manufacturing processes. While the module manufacturing segment has completely adapted the technique, not everyone is reaping all of the benefits. As of now, the EL image of a panel is mainly used
at the end of the module assembly process for documentation in case of insurance and warranty claims. But the technology can do much better than this. When EL is used before the lamination step,as many of the top manufacturers now do, it avoids feeding defect strings into the final module.

The information obtained by EL/PL images can
also be used to correct any anomalies in production tools. But so far, the majority of module makers is not making use of such automatic defect detection algorithms. However, with cycle times reduced to less than 20 seconds, many luminescence imaging tools suppliers are confident that module companies will eventually opt for fully automatic solutions.

The rise of photoluminescence

While quality control in module production is the mainstay for luminescence imaging, the technology is quickly spreading its applications along the entire solar value chain. PL based ingot characterization
is very interesting to help in sorting and process optimization, but it's even more impressive in wafering, where manufacturers can not only use PL to identify efficiency limiting defects in as-cut wafers, but software of some tools even allows to forecast the efficiency potential for every wafer processed into a solar cell.

PL of semi processed wafers is mainly used
for process development and research related applications. Applying EL/PL for finished cells is also very useful, especially when integrated into
IV testing. In addition to quality control, production inspection also enables process optimization. Inspection of installed modules is also an interesting and spreading application.

Within the luminescence imaging field there is one interesting observation to make: While in the past PL was only utilized in those parts of the solar
value chain where EL could not be applied – that
is ingots/wafers as they have no electrical contacts needed for EL –, their reach is increasingly getting wider. EL images of cells show both material and series resistance defects confounded in the same image, but it is not so easy to separate these effects, whereas PL primarily shows material defects, claims BT Imaging, a pioneer in PL technology. But you see PL now even used for PV system inspection: Solarzentrum, for example, uses sunlight as the source to trigger the luminous response from the PV module for its PL measurements. However, for characterization of modules in factories, EL remains the first choice because it is rather easy to excite large substrates with electrical biasing compared to using a uniform light source. On top, testing a finished module with EL can be combined with IV characterization.

What's next?

There are several hot topics in luminescence imaging. While the focus is on further improving crack detection, both at the cell and module level, another important theme is speed. Apparently, manufacturers are also interested in getting more comprehensive tools, such as combined IV & EL inspection or wafer sorting combined with PL.

However, as production costs continue to go down, putting further pressure on manufacturers to improve yields, we will see more luminescence imagining systems used to optimize the production process from ingots to modules as well for solar system checks – and if needed in combination with other quality control technologies.

Download your free copy of the TaiyangNews Market Survey on Luminescence Imaging Systems here.