Degradation in Glass/Glass Bifacial PV With Emerging Encapsulants and Half-Cut Cells

DuraMAT's sequential stress testing has identified degradation modes related to unstable rear-side passivation layers on bifacial photovoltaic (PV) cells.

Sequential stress testing can identify durability issues for bifacial crystalline silicon PV modules beyond those caught by existing qualification tests. Here, we study bifacial cells with variations in the rear-side color ranging from blue to yellow, indicating inconsistencies in rear-side cell passivation. We apply a modified IEC 63209-2 sequential stress procedure, consisting of damp heat (DH200), full-spectrum light exposure (A3-2000), thermal cycling (TC50), and humidity/freeze (HF10).

We observe a 1%–2% rear-side power increase after the initial DH200 stress step, followed by up to 17% degradation for subsequent UV-visible light exposure incident on the front side of the short-circuited cells for 2,000 hours at 20% relative humidity and 65°C air temperature (90°C black panel). This result, combined with additional control test sequences, shows that the stressor sequence of damp heat followed by light exposure is required to produce this degradation mode, and that degradation may not occur under light exposure alone.

Interestingly, we observe less degradation for half-cells compared to full-cell mini-modules. Additionally, we observe that cells with greater yellowness on the rear side begin with higher voltages but exhibit more severe degradation. Nondestructive methods, including the PV external quantum efficiency and imaging with photoluminescence and electroluminescence, reveal degradation signatures of rear-side surface recombination.