Potential-induced degradation in bifacial silicon heterojunction solar modules: Insights and mitigation strategies

Abstract

Potential-induced degradation (PID) may be a serious concern in photovoltaic (PV) modules and plants, particularly when approaching high system voltages (1500+ V). Here, we investigate PID occurring in bifacial rear-emitter silicon heterojunction (SHJ) solar cells encapsulated in a glass/glass (G/G) module configuration with ethylene vinyl acetate (EVA) as an encapsulant. PID testing was performed at 85°C in 85% relative humidity (RH), and the solar cells were subjected to −1 kV and +1 kV for up to 800 h. SHJ cells were found to degrade when subjected to −1 kV, and to a lesser extent when left unbiased in damp heat (DH) conditions, while the application of +1 kV prevented degradation. Although prone to PID after extended test durations, the SHJ mini-modules investigated in this study noticeably passed the industry standard (IEC 61215:2021) PID test of 96 h. The degradation was primarily characterized by losses in short-circuit current (I_SC) at the front side, followed by fill factor (FF) and open-circuit voltage (V_OC). A cross-sectional transmission electronic microscopy analysis of the laminates subjected to −1 kV highlighted a transport of sodium (Na) through the transparent conductive oxide (TCO), reaching the amorphous Si/TCO interface. The samples tested in DH conditions and with positive PID test conditions did not exhibit such a migration of Na. To account for these observations, we updated a previously proposed model describing the sensitivity of SHJ cells to water. In our degradation model, moisture in the module corrodes the glass, creating sodium hydroxide (NaOH) that then percolate through the EVA before reaching the SHJ cell. The application of a high negative bias amplifies the previous mechanism by increasing the availability of Na⁺ and also enhances the drift of Na⁺ through the EVA to the cell. Finally, we demonstrate that PID can be mitigated or suppressed at the module level by using a high-volume resistivity encapsulant with a low water vapor transmission rate (WVTR) or by encapsulating SHJ solar cells in a configuration impermeable to water (e.g., using an edge sealant).