Potential-induced Degradation at Interdigitated Back Contact Solar Cells

Abstract

Potential-induced degradation (PID) is characterized by the power loss of solar modules under high voltage stress across the module layer stack between framing/glass surface and solar cells. Standard silicon solar cells with a front side emitter may suffer from PID through massive shunting (PID-s) under high voltage stress conditions. PID was also reported in the past for cell concepts with a local emitter at the back side. For this case the underlying physical mechanism is not fully understood.In this contribution the PID effect is investigated for interdigitated back contact solar cells (IBC cells). Parts of the front side of the cells are exposed to high-voltage stress using a recently developed cell test setup at variable temperature, voltage and polarity. Cells are investigated by means of electroluminescence (EL), IR-thermography, illuminated and dark I-V measurements before as well as after PID tests. Cell fragments are investigated after PID stressing using the electron beam induced current (EBIC) method in combination with scanning electron microscopy (SEM).PID tests with a positive voltage respect to the grounded cell cause a locally degraded EL signal in the region where the PID stress was applied. In contrast, PID tests with the opposite polarity do not affect the EL behavior at all. I-V curves and thermography images indicate that PID stress does not significantly increase Rshunt. The local decrease of the EL intensity indicates increased non-radiative recombination. SEM/EBIC reveals neither local shunts nor distinct local degradation of the EBIC signal that could be attributed to PID tests with positive voltage.The results indicate a degradation process related to a degradation of the front side passivation layer (PID-p), in contrast to the well-known PID-s effect. Based on the results a model concept for PID-p of IBC solar cells is proposed. Accordingly, the potential impact on the module power output under the influence of high voltage stress is assessed.