After we looked at alternative metallization approaches in the previous story of our HJT cell production series, this feature is about IV characterization, a process step, where HJT cells also require optimization. This is due to the fact that cell architectures such as HJT that have high open circuit voltages requires longer voltage sweep times for precise and accurate measurement of their I-V characteristics. The details of HJT cell testing were covered in the presentation on "High Throughput IV-Classification Of High Efficiency Silicon Solar Cells" of Dr. Sascha Esefelder, Director Product Development of Wavelabs, at the recent TaiyangNews Virtual Conference on Solar Cell Production Equipment & Processing Materials 2023 (see recording here). While PERC cells show no significant difference when cutting down the measurement time from 1 second to 20 milliseconds, HJT cells show a higher efficiency when the measuring time is reduced downwards from 500 milliseconds.
"250 milliseconds is already way too fast to get a reasonable measurement for HJT cells," said Esefelder. Understanding the discrepancy is made easier by the PC1D simulation with a 30-millisecond flash. The simulation demonstrated a bump at Pmpp in the reserve scan, measuring from Voc to Isc. This bump is manifested as higher cell efficiency that rises with shorter measurement times. This well-known phenomenon is often referred as high cell internal capacitance. According to a recent scientific study cited by Esefelder, a HJT module's measurement uncertainty ranges from one to 10% for measurement times under 100 milliseconds.
The industry standard for IV characterization, on the other hand, calls for a cycle time of 0.72 seconds, during which several tasks must be completed. Transporting the cell to the measurement position, closing the contacts, taking the measurement, opening the contacts, and then moving the cell away from the measurement position are the main steps.
Taking out the time required for other steps leaves 120 milliseconds for the total measurement time. Additionally, a number of measurements must typically be completed during this period. These include the basic front side IV characterization, halfsun measurement from the front to determine the series resistance, dark-IV measurement to determine shunts and finally electroluminescence imaging for mapping the local defects, which now also became standard. There are also a few additional options such as Suns-Voc, quasi-EQE and IR imaging.
In order to facilitate all these, the measurement time has to be below 30 milliseconds, according to Esefelder, while HJT due to its high internal capacitance cannot support measurement times below 250 milliseconds. To bride this gap, WAVELABS developed the RapidWAVE algorithm, which drastically cuts the measurement times of HJT cells to just 20 milliseconds. The algorithm generates IV curves that closely resemble those from steady state measurements.
The SINUS-300 testing tool platform of the company not only features the function, but also enables measuring times spanning to a few seconds to cross check the accuracy of the fast measurement on the floor. The IV testing platform from WAVELAB also supports cut cell measurement, whether it be a 1/2 or 1/3rd, which is useful for HJT fabs that have begun implementing half wafer processing. Within a single flash, the cell slices are contacted and measured separately. According to Esefelder, the solution is also RapidWave compatible and simple to retrofit on-site.
For more details on HJT, download the TaiyangNews report on Heterojunction Solar Technology 2023 for free here.