Potential Induced Degradation Evaluation of Damp Heat Stressed PV Modules

Potential induced degradation (PID) mode is one of the most critical degradation modes in the photovoltaic (PV) modules. The PID issue is highly dependent on the conductivity of the materials (especially encapsulant and glass surface) and the adhesion strengths of interfaces (glass/encapsulant, encapsulant/cell and encapsulant/backsheet). In the field-aged modules, the material conductivity typically increases due to aging of materials, weakened interface and cemented soiling of glass surface. Currently, the PID tests are performed on the fresh modules as per IEC standards' requirements. In the fresh modules, the encapsulant conductivity is very low (compared to the field-aged modules) and the adhesion strengths of interfaces are high. Therefore, the PID loss in the current tests represent only the loss which would happen in the fresh modules and young modules, not in the long-term field-aged modules. In the current work, we have used two pre-stressed modules to represent the long-term field-aged PV modules. The pre-stressing was done on two glass/polymer modules for 2000 hours at 85°C/85%RH according to IEC 61215 standard. After the damp-heat prestressing, these two modules were subjected to PID (+ve PID on one module; -ve PID on the other module) in an environmental chamber at 1000 V, 60°C, and 85% RH, according to IEC 62804-1. These modules experienced 6.3-7.5% power degradation which is beyond the allowed limit of 5% by the standard. Based on these results, we believe that the PID tests may need to be performed on the unstressed/fresh and pre-stressed modules to appropriately represent the PID issue corresponding to the short-term and long-term field-aged modules, respectively.