Enhancing UV light stability in commercial silicon HJT solar cells and modules

Abstract

Silicon heterojunction thin film solar cells are sensitive to ultraviolet (UV) light. SIMS analysis shows that UV light ≈ 365 nm disassociates Si-H bonds, resulting in hydrogen migration away from the a-Si:H/c-Si interface and the formation of metastable defects. These defects contribute to a degradation in V_oc and FF, ultimately reducing overall solar cell efficiency (ƞ). To mitigate this effect, the lower UV-damaged continuous PECVD process is developed by optimizing the hydrogen content to 33 % for $i_1$ (a-SiO_x:H) and 25 % for $i_2$(a-Si:H). As a result, the effective carrier lifetime inclines to ≈ 3.6  ms and reduces the UV-induced degradation (UVID) from 1.59 % to 0.71 %. Furthermore, a comprehensive correlation between UV power, exposure time, and temperature has been established. The longer UV duration and higher UV intensity exacerbate UVID, whereas higher temperatures can mitigate UVID in Si HJT solar cells. Even when the UV power is set to 100 % no UVID occurs at temperatures ≈ 110 °C and beyond. This finding suggests that Si HJT solar cells and modules exhibit long-term stability making them particularly well-suited for high-temperature environments. The high-intensity light soaking recovery treatment (60suns) is applied to restore the efficiency of HJT solar cells after short and prolonged UV exposure. The ƞ can be fully recovered after short-term UV exposure UV 6 kWh/m², 12 kWh/m², and UV 20 kWh/m². In contrast, cells cannot be fully recovered after prolonged UV exposure to 60 kWh/m² and 80 kWh/m², even after multiple LS treatments. To further enhance the UV resistance the HJT cells are encapsulated with UV-cut EVA films to form modules. The UV-cut encapsulated modules exhibit superior resistance under UV 20 kWh/m², 40 kWh/m², 60 kWh/m², and 80 kWh/m². This study advances the understanding of the UVID mechanism in HJT cells and proposes a viable mitigation strategy.