Low-breakdown-voltage TOPCon IBC solar cells to improve shading tolerance

Researchers from the Delft University of Technology in the Netherlands have investigated how fine-tuning the reverse characteristics of interdigitated back-contacted (IBC) solar cells based on tunnel oxide passivated contacts (TOPCon) technology could help to improve the performance of PV modules, mainly by increasing the shading tolerance and limiting the operating temperature of shaded solar cells.

“Our work had a double objective: on the one hand, to show how it is possible to decrease the breakdown voltage (BDV) of TOPCon IBC solar cells, and on the other hand to demonstrate how modules using low-breakdown-voltage cells are more shade-tolerant,” researcher Patrizio Manganiello told pv magazine. “We used this approach in all applications where partial shading conditions are common,  such as for example in urban-integrated PV.”

The researchers described their findings in “Low-breakdown-voltage solar cells for shading-tolerant photovoltaic modules,” which was recently published in Cell Reports Physical Science. They simulated the breakdown characteristics of IBC solar cells based on polysilicon and silicon monoxide (poly-Si/SiO_x) passivating contacts.

“Typically, in IBC solar cells based on TOPCon technology, the emitter and the back surface field (BSF) regions are physically isolated to prevent shunting because of diffusion of dopant atoms into the c-Si base during the thermal processing steps,” they explained. “However, it has been experimentally demonstrated that it is possible to design highly efficient TOPCon IBC cells with contiguous BSF and emitter regions, where the p+ and n+ fingers are separated by a compensated poly-Si region, forming a p-i-n junction.”

They said the breakdown voltage can be tuned without significantly degrading the efficiency of the solar cell. Through a device-level simulation, they claim to have shown that it is possible to reduce the BDV in TOPCon IBC solar cells by simply reducing the distance between the emitter and the BSF, without creating a shunt.

“Simulations of partially shaded PV modules indicate that, if the BDV can be reduced to 0.3 V, then the energy yield could be boosted up to 20% for a PV module with three bypass diodes that is shaded approximately 20% of the time,” the scientists said. “The simulations results are supported by outdoor experiments showing that, under partial shading conditions, a PV module made with IBC cells with a BDV of 3 V produced an average of 4.2% more energy than a PV module with FBC solar cells with BDVs larger than 10 V and six bypass diodes.”

Despite these encouraging results, the research group said there is still much left to be done to bring this cell technology close to commercial production.

“The feasibility of manufacturing cost-effective solar cells with breakdown voltage as low as 0.3 V has yet to be demonstrated,” said Manganiello.