Researchers from Ghana’s Kwame Nkrumah University of Science and Technology Kumasi and the U.K.’s Teesside University have identified parameters and techniques they say could be used to design and manufacture robust, moisture-resistant PV modules for the tropics.
The research, presented in the paper Robust crystalline silicon photovoltaic module (c-Si PVM) for the tropical climate: future facing the technology – published in African Science and on the ScienceDirect website – reviews factors which accelerate degradation of solar panels in tropical climates and analyzes cell interconnection technology for potential application in the production of modules with increased thermo-mechanical reliability in tropical regions.
The researchers identified the main causes of module degradation in packaging material; solder joints; adhesion; delamination; moisture accumulation; semiconductor device thermal challenges; and interconnection technology. The latter was highlighted as one of the main causes of panel performance degradation in hot, humid climates where module temperature can reach an average high of around 80 degrees Celsius at midday and low of 15 degrees Celsius at midnight.
The scientists added, the miniaturization trend for manufacturing solar cells with thinner wafers could be partly responsible for a larger incidence of microcracks and, as a result, of reduction in module output. The Anglo-Ghanaian group said infrared soldering of copper ribbons to form interconnection strings between cells was the main cause of cracking. “With reduced wafer thickness, wafer breakage during stringing and tabbing [processes] is also expected to rise,” the paper noted.
Conventional front-to-back cell interconnection methods are set up in a manner which causes a kink in the copper ribbons as they connect the back to the front side, according to the researchers. “This geometric distortion induces stresses in the copper ribbons during manufacture, which is aggravated further by high thermal loading during operation in tropical ambient temperatures,” stated the paper.
The research group said the heavy hail storms and high winds which characterize the tropics worsen micro-cracks in the cells and cause module failure. Elevated ambient temperatures, too, raise the rate of inter-metallic compound growth in module interconnections, according to the paper, in turn increasing interconnection failure in tropics-based panels.
The authors of the paper said several new interconnection arrangements are emerging. These include smartwire and multi-busbar approaches, shingled modules and the manufacturing of efficient mini-modules with little silver consumption and with or without copper-based metallization. However, the researchers said, none of those innovations is considered ideal to address degradation in the tropics.
The Kwame Nkrumah-Teesside group said the interconnection design seen in back-contact cells could help eliminate recombination losses caused by humidity. The researchers proposed a back-junction, back-contact (BJBC) interconnection solution with selective laser soldering for the manufacture of robust crystalline silicon panels for tropical sites.
That type of module architecture is based on an inter-digitated finger structure and busbar which harvests current from individual fingers, with p and n junctions on the rear side. “Metallization pattern on the front surface is virtually absent and therefore there is no feature present on the surface to shadow the incident photon flux,” the scientists said. “In addition … low series resistance of the metal pattern is achieved, as the metallization can cover about one half of the back surface.”
Unlike front-contacted cells, current conduction in BJBC devices does not go through the emitter, ensuring there is no trade-off between grid shading and series resistance losses, the group added. The Anglo-Ghanaian group said rear function could be optimized in terms of the lowest saturation current only. “Research simulation and measurements reported, show that the busbar regions of BJBC solar cells reduce the fill factor and short-circuit current density,” stated the paper.
The researchers also analyzed alternative back-contact technologies, including emitter wrap-through and metallization wrap-through approaches. The group also demonstrated how the use of electrical conductive adhesives for cell interconnection was found to produce modules with lower residual stresses.