Effects of SiN_x refractive index and SiO₂ thickness on polarization‐type potential‐induced degradation in front‐emitter n‐type crystalline‐silicon photovoltaic cell modules

Abstract

This study investigated how the SiN x refractive index (RI) and SiO 2 thickness, d ox , of stacked SiN x /SiO 2 passivation layers of the front p + emitters of n-type crystalline-silicon (c-Si) photovoltaic (PV) cells affect their polarization-type potential-induced degradation (PID) behaviors. We prepared six n-type c-Si PV cells with an RI of 2.0 or 2.2 and with d ox of 9, 2, or 1 nm. Then PV modules fabricated from the cells were subjected to PID tests during which a bias of −1000 V was applied to cells with respect to the front cover glass surface.For d ox of 9 or 2 nm, rapid polarization-type PID was observed, irrespective of the RI. However, for d ox of 1 nm, the RI markedly affected the degradation behavior, and cells with an RI of 2.2 showed no degradation. These findings are attributable to carrier transport between the high RI (Si-rich) SiN x and the c-Si substrates, which can readily occur only when the SiO 2 layer is sufficiently thin for electrons to tunnel through the SiO 2 layer. These results are important for elucidating polarization-type PID mechanisms and for developing preventive measures against polarization-type PID.

Mechanistic Understanding of Polarization‐Type Potential‐Induced Degradation in Crystalline‐Silicon Photovoltaic Cell Modules

“…Effects of SiO 2 layers underneath SiN x on polarization‐type PID have been investigated. [ 30–32,34,35 ] These reports indicate that SiO 2 layers play important roles in generating polarization‐type PID. We have proposed that SiO 2 layers underneath SiN x play a role in retaining excess accumulated charges at K centers.…”

“…[ 44 ] Since the publication of our earlier work, [ 23,26 ] many results that support our hypothesis have been reported. [ 30,31,34,35 ] Jonai et al reported that Al‐BSF cells, which basically have no intentional SiO 2 layers under SiN x , show polarization‐type PID when SiO 2 is inserted between SiN x and n‐type emitters. [ 30,35 ] Suzuki et al [ 31 ] demonstrated that n‐type PERT cells without SiO 2 layers exhibit no polarization‐type PID.…”

“…In addition, for this measure, we must use sufficiently thin SiO 2 to permit electron tunneling through the SiO 2 . [ 34 ] The RI does not affect the polarization‐type PID behavior when the SiO 2 ( d ox ) thickness is greater than 2 nm. However, for d ox of approx.…”

Polarization-Type Potential-Induced Degradation in Front-Emitter p-Type and n-Type Crystalline Silicon Solar Cells

“…Recently, potential-induced degradation (PID) has been identified as a central reliability issue of photovoltaic (PV) cell modules. − Causing marked degradation in a short time, such as several months, PID is triggered by potential differences between grounded frames and the active circuit of cells in modules in the field. The PID behavior varies to a considerable degree depending on PV cell material, structure, and PID-stress intensity. , For instance, sodium-penetration-type PID (including shunting-type PID), − polarization-type PID, − and corrosion-type PID ,− are known to occur depending on these factors.…”

“…Particularly, polarization-type PID is regarded as the fastest degradation mode among all PID modes . It has been observed for crystalline silicon (c-Si) cells of several types including n-type passivated emitter and rear totally diffused (PERT) cells − and n-type interdigitated back-contact cells with a front-surface field , or a front floating emitter . Polarization-type PID is characterized by reduction of the short-circuit current density, J SC , and reduction of the open-circuit voltage, V OC .…”