Backsheet Durability Sequence

Backsheet cracks that allow moisture to enter a PV module can severely impact field performance. Other signs of backsheet aging, such as yellowing and chalking, are often signs of accelerated backsheet degradation. Kiwa PVEL’s BDS has been designed to evaluate the module’s backsheet for these field-reported issues.

BDS combines conditions of high temperature/high humidity that stress the backsheet and break down any hydrolytically weak bonds; high energy ultraviolet (UV) photons that can break polymer chains in backsheets, resulting in degraded mechanical properties; and cyclic thermal/mechanical stress that allow for the full level of material degradation to be realized.

As per Section 8.1, BDS testing will occur on mini-modules, rather than full size modules. Unlike other PQP tests, BDS results on mini-modules (with the cells and interconnect wires geometry mimicking full size modules) are representative for full-sized modules.

The test begins with 200 hours of DH as per Section 5.2. The module is then placed in a UV chamber and receives a rear-side UV exposure at 80°C BPT. The exposure time is tuned to cause equivalent damage to 65 kWh/m² of UV (280-400 nm) with an AM1.5G spectrum. The damage model is based on a state-of-the-art understanding of the dependence of backsheet aging on test conditions.  Following this UV exposure, the module undergoes thermal cycling for 50 cycles (TC50) plus 10 cycles of humidity freeze (HF10) as described in Section 5.3. The process of UV65, followed by TC50 and HF10 then repeats two more times for a total of three cycles. The final testing step is a UV exposure of equivalent 6.5 kWh/m² of AM1.5G for photobleaching, which removes any minor discolorations that can be a result of the climate chamber testing and aren’t observed in the field.

Characterizations occur pre-stress and after each UV dose, and include VI and color measurement. IV and EL measurements will not be taken by Kiwa PVEL during these characterization steps, as BDS is not focused on electrical performance or cell behavior. Additionally, WL is not taken during BDS testing as field-cracked modules have been shown to pass the WL testing, while showing obvious cracking. Visual inspection of micro-cracks in the backsheet is more effective in detecting backsheet failures. As per IEC TS 63209-1:2021, VI will include a magnification of 10X or greater, focused on the 10 locations on the rear-side where colorimeter measurements are taken.

The BDS results will be included in a separate Interim and Final BDS Report.

As per Appendix B – Kiwa PVEL PQP: BOM Test Requirements, BDS is not required on glass//glass modules.