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

Abstract

Various types of potential‐induced degradation (PID) with fundamentally different physical root causes have been observed recently ranging from shunting (PID‐s), corrosion (PID‐c) to degradation of surface passivation (PID‐p). Herein, PID‐p is investigated for the first time at the rear side of bifacial silicon solar cells with enhanced time‐resolution during degradation and recovery under simultaneous illumination. The tests reveal fast transient changes of rear‐side short‐circuit current within minutes and thus allow to understand the degradation process related to PID‐p. The short‐circuit current with rear‐side illumination first decreases significantly by more than 80 % resulting in an intermediate degraded state. Subsequently, it increases to a regenerated state. Under reversed bias, a total recovery of the cell parameters is observed and confirmed by I–V measurements under standard testing conditions (STCs). The underlying transient PID‐p degradation and recovery mechanism is well described by a PC‐1D simulation of the field‐induced band bending at the rear surface. Finally, a qualitative model involving the migration of mobile charged species and their impact on the charge equilibrium in the field effect passivation layer is presented. Thus, time‐resolved analysis of transient PID effects can serve as a clear characteristic for PID of polarization type.

Electrochemical degradation modes in bifacial silicon photovoltaic modules

“…In particular, recent work describes a PID-p mechanism without Na+ caused by a change in charge state of K-centers in the silicon nitride layer. 24,25 Although we observe PID-p in our G/EVA/G samples, the roles of Na+ or other charged species require further investigation.…”

“…Recent studies suggest PID can manifest in varying forms, for example, shunting (PID-s), [18][19][20] polarization (PID-p), 16,[21][22][23][24][25] and metallization or cell-stack corrosion (PID-c). 26,27 All PID types generally arise from movement of charged species caused by an electric field between the operating solar cells and the module's grounded frame.…”

“…16,22,23 It has alternatively been proposed that the charge accumulation causing PID-p results from a field-induced change in charge state of the passivation stack (i.e., oxidation of silicon nitride or oxide layers). [23][24][25] It is unclear how this second possible PID-p mechanism would vary between G/G and G/TB structures, as this mechanism would not depend on ionic species introduced from the packaging. However, differences in the surface conductivity of glass and TB surfaces may impact the electric field distribution and thus the extent of PID.…”

“…Here, we address the different degradation mechanisms in bifacial silicon PV packaged in G/G and G/TB structures with either EVA or POE encapsulants, where we test these packaging schemes against both moisture ingress and high system voltages that can cause PID. Recent studies suggest PID can manifest in varying forms, for example, shunting (PID‐s), 18–20 polarization (PID‐p), 16,21–25 and metallization or cell‐stack corrosion (PID‐c) 26,27 . All PID types generally arise from movement of charged species caused by an electric field between the operating solar cells and the module's grounded frame 28 .…”

“…On the back side of G/G packaged cells, some studies have suggested Na+ accumulation at the antireflection coating could cause PID‐p, where the positive ions counteract field‐effect passivation and enhance rear‐side surface recombination 16,22,23 . It has alternatively been proposed that the charge accumulation causing PID‐p results from a field‐induced change in charge state of the passivation stack (i.e., oxidation of silicon nitride or oxide layers) 23–25 . It is unclear how this second possible PID‐p mechanism would vary between G/G and G/TB structures, as this mechanism would not depend on ionic species introduced from the packaging.…”