Damp Heat Performance of Silicon Heterojunction Solar Cells with Reactive Silver Ink Metallization

The use of reactive silver ink (RSI) metallization for silicon heterojunction (SHJ) solar cells has been shown to outperform standard low-temperature silver paste (LT-SP) with a dramatically different form factor due to its markedly improved electrical properties, which results in 88% lower finger silver usage. As is necessary with any new technology, it must pass accelerated testing in order to ensure viability compared to the current standard. Damp heat testing is vital as it evaluates various modes of degradation: such as, moisture ingress, temperature driven diffusion processes, encapsulant decomposition, and particularly corrosion of interconnects and electrodes. Degradation of contacts are especially crucial for SHJ solar cells due to the required increase in silver content of the silver paste arising from processing temperatures below 200 degrees C. In order to gain an understanding of these mechanisms and their role, SHJ mini-modules fabricated with two different metallization types: LT-SP and RSI, as well as two common encapsulants: ethylene vinyl acetate (EVA) and polyolefin elastomer (POE) were damp heat tested. These modules fabricated with a glass-backsheet configuration were subsequently exposed to 85 degrees C/ 85% relative humidity for 1000 hours total. The EVA samples generally demonstrated higher rates of degradation versus POE as has been previously found, and the RSI is observed to have similar degradation rates when compared with these LT-SP samples. Out of the four sample sets, the RSI/POE cells demonstrated the lowest degradation of 1.5% absolute efficiency over the course of the test, which emphasizes the impact of interconnect corrosion due to acetic acid in EVA. The results suggest that degradation is dominated by the cell itself rather than any issues related to the novel silver ink technology, which showcases for the first time high performance of SHJ with 88% less finger silver usage.