Contact resistance and series resistance play a key role in achieving high efficiency crystalline silicon solar cells. The knowledge of the contact resistance and series resistance across the surface of a single wafer, and of the reproducibility from wafer to wafer is essential in optimizing the firing process.
Map the contact resistance of the front side metallization and optimize the metallization and firing process.
Locate shunts on solar cells and find out about their nature.
Find the locations of increased recombination (on research cells without front side metallization).
Find the regions on a solar cell with higher optical loss or lower carrier lifetime (related to local gettering and passivation).
For solar cells with more than 5 busbars or non-standard metallization prints we have the Corescan Multi Busbar PC software available. Check our support page or contact us.
- Voc scan
- LBIC scan
Contact Resistance scan
The method of contact resistance determination is based on the measurement of the potential jump at the boundary between a metal line and the silicon adjacent to it, while a current flows from the silicon into the metal line. The line contact resistance can be calculated by dividing this potential jump by the current flow into the line (per unit length of line).
In the Corescan method, light current is locally generated by a small light beam, while current flow is enabled by short-circuiting the cell externally. A potential probe centred in the beam measures the local potential and moves across the cell together with the beam, while being continuously in contact with the surface. The probe is always scanned perpendicular to the metal lines. An example of a scan line measured in this way is shown below (Rcl refers to the line contact resistance):
By making parallel scan lines the potential can be determined on the entire cell. The resulting data are presented within the Corescan software by 2D colour graphs or 3D graphs.
Corescan measurement principle
Corescan of solar cell with low and uniform contact resistance
Corescan of solar cell with high and non-uniform contact resistance
In the case of the Shuntscan, the cell is not illuminated and current flow is induced at shunt locations in the cell by applying an (adjustable) bias potential across the cell. Because the emitter has a relatively high resistance, the current flow from the adjacent silicon to a shunt will induce a potential gradient in the direction of the shunt in the silicon, which is detected by the potential probe. In the Shuntscan maps, the absolute difference between the applied bias potential and the locally measured potential is plotted, shunts being illustrated by spikes in the graph. By scanning at different bias potentials, it is also possible to investigate the (non-)linearity of individual shunts.
Shuntscan measurement principle
Shuntscan of solar cell
During a Voc scan, the local potential is measured in the centre of a scanning light beam while the cell is open-circuited. In this way, it is possible to measure a kind of local Voc, provided that the cell has no front side metallization (otherwise the potential differences across the cell are smeared out). Although the resistance of the emitter is considerable, there will certainly be current flow to the dark regions of the cell, due to the fact that the illumination is local. This causes the measured potentials to be lower than the Voc value at one sun intensity, despite the fact that the light beam is adjusted to one sun. With this method, the local current leakage through the p-n junction can be measured, shunts can be located in cells without the need for a front-side metallization, (non-visible) cracks can be detected as well as the absence of a BSF on some locations. The interpretation of the results of this method is still under investigation.
Voc scan measurement principle
Voc scan of wafer
Light Beam Induced Current scan
The LBIC scan method is not a real member of the Corescan methods family for two reasons. In the first place, it is not based on potential mapping like the other methods, and secondly the LBIC method was already widely used before the Corescan was developed. The LBIC scan method consists of the scanning of a light beam across a cell while measuring the resulting short-circuit current for each position. The usual LBIC set-ups measure LBIC with a very small beam (down to 0.1 mm) to obtain a high spatial resolution. The Corescan LBIC is a low resolution LBIC, since the beam diameter is fixed at 9 mm. It is therefore only meant as a relatively coarse method to find good and bad regions on a solar cell. An advantage of the large beam diameter is that the scan time is relatively short (5-10 minutes). A drawing of the LBIC scan method is not shown here since it is very similar to the Corescan drawing shown above, the only difference being that Isc is measured instead of V (hence the potential probe can be omitted). In principle, an LBIC scan and a Corescan could be done simultaneously, but since a Corescan damages the surface somewhat and reduces Isc by a few %, it is better to keep the scan methods seperate.