Assessment of hydrogen-induced contact resistance in TOPCon solar cells

Scientists at the University of Oxford and the University of New South Wales have looked at the impact of inducing hydrogen-induced contact resistance (HIRC) in tunnel oxide passivated contact (TOPCon) solar cells and passivated emitter and rear contact (PERC) solar cells.

Their study looks at the capabilities of HIRC to reduce the changes in series resistance (Rs) in both kinds of solar cells. The changes are mainly due to the post-firing thermal processes used to manufacture the cells themselves, and are responsible for affecting their fill factor and overall efficiency.

“We show, for the first time, that HICR can be induced in TOPCon solar cells in a similar way as it has been reported for PERC,” the scientists explained. “The application of cycling forward current and reverse bias conditions controls the speed and extent of the contact resistance degradation.”

They conducted their experiment on standard commercial full-size p-type mono PERC cells and n-type TOPCon cells from two undisclosed producers. They found that the degradation purely occurs at the n-type silicon to silver (Ag) contact on both cell architectures.

For PERC cells, the results improved on previous studies by pinpointing the origin of contact resistance in the front contact. The researchers designed and created a new geometry of PERC samples, allowing them to isolate three distinct current pathways.

“These include pathway one: from the front Ag contact to the rear aluminum (Al) contact, pathway two: from the front Ag contact to a modified front Al contact, and pathway three: from a modified front Al contact to the rear Al contact,” they said.

They found that only the first two pathways contributed to change in Rs. They concluded “that the contact resistance at the front Ag/Si interface with the n + emitter is the primary cause of HICR in PERC solar cells.”

For TOPCon cells, increased series resistance is reportedly related to an increase in contact resistance between the Ag contact and the n-type silicon region. However, transmission line method (TLM) measurements also showed that this occurs on the rear surface between the Ag contact and the heavily doped n-type polysilicon layer, rather than at the front surface near the junction, as in PERC.

“In both cases the application of a reverse bias was able to reverse the effect to a certain extent,” the scientists said.

The TOPCon structure differs from PERC in a few main ways. First, it has a higher value of series resistance at the end of the reserve bias cycle, which indicates “a more significant irreversible component,” according to the scientists. Second, its temperature response is different, as it exhibits a substantial increase in series resistance at lower temperatures.

“TOPcon structures show a greater dependence on forward bias current with very small current having almost no impact on the observed series resistance,” added the scientists. “The results presented herein thus have significant implications for the development of processing strategies that mitigate degradation and enhance stability in TOPCon solar cells.”

The group introduced its findings in “Observations of contact resistance in TOPCon and PERC solar cells,” which was recently published in Solar Energy Materials and Solar Cells.