Potential-free sodium-induced degradation of silicon heterojunction solar cells

Silicon-based photovoltaic (PV) modules suffer from potential-induced degradation (PID) caused by sodium (Na) permeation, which is present in large quantities in soda-lime glass. Here, we report that Na atoms can decrease the performance of amorphous/crystalline silicon heterojunction (SHJ) solar cells without the help of a voltage bias. The three degradation stages are investigated in this work. First, H2O molecules open channels for Na transport in the transparent conductive oxide (TCO), while the device performance remains almost unchanged. Next, when Na atoms reach the boron-doped hydrogenated amorphous silicon (p-a-Si:H), the field passivation is poisoned, leading to a great decline in the fill factor (FF), whereas the open-circuit voltage (V-oc) only slightly declines. Finally, Na atoms further diffuse into the intrinsic a-Si:H layer and c-Si surface, resulting in a substantial decrease in V-oc. These findings have important implications for the installation of SHJ solar modules in Na-abundant environments. As a feasible solution, we demonstrate that a compact SiO2 thin film can efficiently prevent H2O molecules from penetrating into the TCO layer and therefore guarantee a long-term stable operation of SHJ solar cells.