Inline quality checks

Inline quality checks

The current-voltage (I-V) characteristics for every solar cell are routinely measured as part of quality control, but also to match the solar cells for the subsequent PV module manufacturing. Obviously, there is a lot of information you can get from an I-V measurement. However, this measurement is done at the end of the process and, consequently, it can take significant time before problems in early manufacturing steps are detected. Consequently, there is much scope for in-line metrology to ensure that all the processes are performing according to their specifications. The most common in-line quality checks used in solar cell manufacturing are focussing on the following process steps.

Texturing

The texturing process for both mono and multicrystalline silicon is quite a delicate process and is strongly dependent on the incoming wafer properties, the chemical batch composition, and the process parameters. Manufacturers often use optical inspection units that detect visible impurities and get some insight into the surface reflection. The focus is typically not on obtaining the absolute reflection value, however, it is more relying on relative changes to ensure the stability of the process. Using carefully designed algorithms, out-of-spec wafers can automatically be taken out of the production line. 100% detection is relatively straightforward using tools from e.g. GP Solar (http://www.gpsolar.de/en/inspect/cellinspection/tex-q/).

Diffusion

The phosphorus diffusion process is one of the most critical process steps in industrial silicon solar cell manufacturing. Small deviations in the process can have a severe impact on the final solar cell performance e.g. due to contacting issues or increased recombination losses. Consequently, there is a lot of interest to quantify the diffused wafers. Two common approaches used for the measurement of diffused layers in manufacturing are based on junction photovoltage and infrared transmission and reflection.

In the junction photovoltage technique, a modulated light beam is impinging on a small area, and the resulting junction photovoltage is measured close to the excitation, as well as a distance d from the excitation. The voltage difference between the two measurement points depends strongly on the sheet resistance of the underlying layer and can consequently by a proper calibration be used for to quantify the sheet resistance in a contactless way. More detail can be found on https://www.semilab.hu/category/products/sheet-resistance-measurement-0

Another method relies on the difference in infrared reflection and transmission due to free carrier absorption due to the high doping density in the diffused layer. Free carrier absorption sharply increases in the infrared part of the solar cell and thus typically light in the 3-10 ┬Ám is used for this purpose. By combining reflection and transmission it even allows the measurement of a front and rear diffusion as is typical in bifacial n-type solar cells. More detail can be found on https://aurorasolartech.com/

Antireflection coating

The silicon nitride antireflection coating give the solar cell its characteristic deep blue colour. In addition, this layer also provides bulk and surface passivation which is key to achieve a high solar cell efficiency. The main property solar cell manufacturers want to ensure is the aesthetics of the solar cell, and consequently, the in-line metrology focusses on the colour uniformity. The colour of the silicon nitride antireflection coating predominantly relies on the optical thickness of the film (i.e. the physical thickness times its refractive index) and the most common root causes for the colour to change are a change in physical thickness e.g. due to obstructed gas flow (e.g. a blocked gas shower in the deposition tool) or a change in silicon nitride composition (e.g. due to parasitic deposition on the quartz tubes used in microwave-based PECVD reactors). More information can e.g. be found on http://www.gpsolar.de/en/inspect/cellinspection/col-q/

Screen printing

Screen printing and the subsequent firing step form the front and rear contact of the solar cell. There is obviously a lot that can go wrong in these process steps and, consequently, it is very common to have in-line inspection of the printing process to ensure that the process is running properly. These detection systems typically rely on optical inspection with sophisticated illumination and multi-image stitching to allow for the detection of the screen print quality (e.g. finger and busbar width, metal coverage). More information can e.g. be found on http://www.gpsolar.de/en/inspect/cellinspection/cell-q/ The detection of finger breakage is typically done using electroluminescence imaging as this is very sensitive to this defect.