We precisely investigate sodium (Na)-induced potential-induced degradation (PID) in n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules, in which open-circuit voltage (Voc) and fill factor deteriorate. Secondary ion mass spectrometry shows Na introduction into n-FE cells by a negative-bias PID stress and a reduction in Na density by positive-bias application. Scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of Na-based protrusions on the cell surface. Silicon nitride (SiNx) disappears at the position of protrusions, which is the root cause for the serious and unrecoverable PID of n-FE c-Si PV modules.
We investigate the second-stage potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules. The PID of n-FE c-Si PV modules is known to occur in three stages under negative bias stress. The second-stage PID is characterized by a reduction in fill factor (FF), due to the invasion of sodium (Na) into the depletion region of a p+–n junction and resulting increase in recombination current. The second-stage PID shows a curious independence on a negative bias voltage for the PID stress. This may indicate that the Na inducing the FF reduction comes not from the cover glass but originally exists on and/or near the cell surface. The FF reduction is recovered quite rapidly, within a few seconds, by applying a positive bias to the degraded cell. The recovered n-FE c-Si PV modules show more rapid degradation if they receive the negative bias stress again, which can be explained by Na remaining in the p+ emitter.
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