Comprehensive study of potential‐induced degradation in silicon heterojunction photovoltaic cell modules

Abstract

Abstract Accelerated tests were used to study potential‐induced degradation (PID) in photovoltaic (PV) modules fabricated from silicon heterojunction (SHJ) solar cells containing tungsten‐doped indium oxide (IWO) transparent conductive films on both sides of the cells and a rear‐side emitter. A negative bias of −1000 V was applied to a module with respect to the cover glass surface in a chamber maintained at 85°C, which significantly reduced the cell's short‐circuit current density ( J sc ) within several days. Based on dark current density‐voltage and external quantum efficiency measurements, the reduction in the J sc was attributed to optical losses rather than carrier recombination. X‐ray absorption fine structure spectroscopy showed the formation of metallic indium (In) in the IWO layers of a degraded cell, which suggests that the root cause of the optical loss was a darkening of the front IWO layers caused by the precipitation of metallic In. In extremely severe PID tests, the SHJ PV modules exhibited not only a further reduction in the J sc but also a moderate reduction in the open‐circuit voltage ( V oc ). These J sc and V oc reductions were probably caused by sodium being introduced into the base region of the cells. A comparison of the PID test results of the SHJ PV modules with those of other types of PV modules indicates that SHJ PV modules have a relatively high resistance to PID. As a module with an ionomer encapsulant exhibited little degradation, their high resistances to PID may be further improved by using encapsulants with high electrical resistances.