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

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 -1000V was applied to a module with respect to the cover glass surface in a chamber maintained at 85 degrees 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.