Rapid progression and subsequent saturation of polarization-type potential-induced degradation of n-type front-emitter crystalline-silicon photovoltaic modules

Abstract

In this study, we investigated progression of potential-induced degradation (PID) in photovoltaic modules fabricated from n-type-based crystalline-silicon cells with front p+ emitters. In PID tests in which a bias of −1000 V was applied to the modules, they started to degrade within 5 s and their degradation saturated within 60 s. This behavior suggested that the PID was caused by positive charge accumulation in the front passivation films. Performing PID tests with a bias of −1500 V revealed that the degradation rate strongly depended on the applied bias whereas the saturation value was independent of the applied bias. Regeneration tests on degraded modules previously subjected to PID tests for durations of 5 and 10 min were performed by applying a positive bias of +1000 V. All the degraded modules completely recovered their performance losses within 60 s regardless of the degradation test duration. On the basis of these results, we proposed that these positive charges originate from positively charged K centers formed by extracting electrons from neutral and negatively charged K centers. This model readily explains the observed degradation and regeneration behavior. To test our model, we determined the fixed positive charge densities (Qf) of a silicon nitride passivation film before and after PID, for which it was found that Qf showed similar saturation behavior. Additionally, the saturated Qf value was of the same order as K center density. These results support our model involving a charging process of K centers.

Reduced power degradation in bifacial PERC modules by a rear silicon oxide additive layer

“…These K centers can be neutral K 0 , positively charged K + and negatively charged K − depending on the number of electrons on the dangling bond 30 . When the cell is negatively biased, electrons are extracted from K 0 and K − centers, leaving only K + centers 29 . One study actually showed that solar modules based on cells without SiN x did not experience PID‐p, 20 possibly due to the absence of K + centers.…”

“…In fact, studies have shown that solar modules with Na-free substrate still suffered from PID-p. 26,27 One possible source of positive charges proposed by Yamaguchi et al is K centers in SiN x film itself, which are dangling bonds of Si atoms backbonded to three N atoms. 28,29 These K centers can be neutral K 0 , positively charged K + and negatively charged K − depending on the number of electrons on the dangling bond. 30 When the cell is negatively biased, electrons are extracted from K 0 and K − centers, leaving only K + centers.…”

“…30 When the cell is negatively biased, electrons are extracted from K 0 and K − centers, leaving only K + centers. 29 One study actually showed that solar modules based on cells without SiN x did not experience PID-p, 20 possibly due to the absence of K + centers. Besides that, the higher breakdown voltage of the AlO x + SiO x + SiN x stack shown in Table 1 We propose that the SiO x layer hinders the transfer of positive charges from the SiN x layer to the AlO x layer when subjected to negative bias, thereby maintaining high fixed negative charge at the rear surface for effective surface passivation.…”

Time‐Resolved Investigation of Transient Field Effect Passivation States during Potential‐Induced Degradation and Recovery of Bifacial Silicon Solar Cells

“…For n‐type PERT solar cells, Yamaguchi et al have proposed that PID‐p is caused by a change of K center charge state in the silicon nitride layer. [ 14 ] These results are supported by capacitance–voltage ( C – V ) and electron‐spin‐resonance (ESR) that revealed density of K‐centers and their charge state. In case of p‐type PERC+, Luo et al have discussed that positively charged sodium ions accumulate in the silicon nitride layer and compensate the negatives charges of the aluminum oxide layer, leading to failure of the field effect passivation.…”

Potential‐induced degradation in photovoltaic modules composed of interdigitated back contact solar cells in photovoltaic systems under actual operating conditions

“…14,15,[24][25][26][27][28][29] The PID-p in n-type IBC solar cells occurs under positive potential conditions, 9,[30][31][32] whereas the PID-p in n-type solar cells with a front p + emitter is observed under negative potential conditions. 14,15,[24][25][26][27][28][29] Both can be explained by the surface recombination of minority carriers arising from the charge fixed on the silicon dangling bonds back-bonded with nitrogen, known as "K centers," in the SiN x ARC layer. 9,24,25 If the ARC of p-type c-Si solar cells is constructed by the SiN x stacked on SiO 2 layers, PID-p can occur even in the p-type c-Si solar cells by applying a high positive voltage.…”