2, NREL, Golden, Colorado, United States
Improving the reliability and durability of photovoltaic (PV) modules is necessary to reduce the levelized cost of electricity of PV installations. However, due to the extended deployment times (> 25 years) and multicomponent construction of these devices there are many potential degradation and failure modes. Two components which are most notably linked to module end-of-life failure include the solar cell contact metallization and polymeric encapsulation. In this framework here is presented a degradation analysis of PV ribbon wire solder joints and polymeric encapsulation in field-exposed PV modules. The modules have been deployed for 4 to 28 years in many different regions, including in desert, temperate, continental, and tropical climates. Surface and cross-sectional morphological (SEM, optical microscopy) and chemical (FTIR, Raman) characterization of the module layers has revealed several common patterns. Poor soldering practices have been identified in many of the modules, including the newer modules. These include incomplete soldering due to poor wetting and ribbon wire alignment. In addition to initially poor soldering, several types of solder joint degradation are observed, including phase separation, grain coarsening, corrosion, and cracking.
The metallization layer of a solar cell is covered by an ethylene vinyl acetate (EVA) encapsulation layer on the sun-exposed side of the modules. The most apparent degradation of this layer is yellowing, which reduces the amount of low wavelength light arriving at the solar cell, thereby decreasing extractable current. Degradation of the metallization and encapsulation do not occur in isolation, and in fact they have a direct impact on one another. The photochemical degradation of the EVA leads to formation of corrosive acetic acid which remains mostly trapped within the cells. There are also thermomechanical stresses generated between the soft, rubbery encapsulation layer, and comparatively hard metal layers, promoting and accelerating the cracking observed by by cross-sectional analysis. The metallization also prompts changes in the EVA layer, most notably by formation of carboxylate salts in the polymer, which are only found in regions where there is direct contact between EVA and PV metallization.
The degradation of the metallization and encapsulation layers has various implications for the lifetime of PV modules. Poor electrical contact with the ribbon wiring increases series resistance of the solar cells, reducing overall power. Degradation of the encapsulation means it is less capable of fulfilling its intended purpose, e.g. optical coupler, oxygen/moisture barrier. Prior work has examined each of these systems in isolation and on a very lmited number of specimens. This work correlates degradation of these two components on a comparatively large sample set, with modules from a very broad range of ages, climates, and PV technologies.