Module Fabrication Using Flexible Solar Cells

Achievement date: 
2016
Outcome/accomplishment: 

Researchers at the NSF-funded Quantum Energy and Sustainable Solar Technologies (QESST) Engineering Research Center (ERC) demonstrated highly flexible solar cell modules using thin 153 cm2 silicon crystalline cells and transparent fluoropolymer foil. The highly flexible solar cells show efficiencies consistent with the original cells even after bending over 200 times around a cylinder with a radius of 4 cm.

Impact/benefits: 

Balance-of-system (BOS) is the leading cost in photovoltaics (PV). Lightweight modules have the potential to reduce BOS cost and increase PV deployment. The availability of flexible and high performance solar cells based on a cost competitive and high manufacturing-throughput technology provides opportunities for new applications in portable PV and building integration (BIPV) devices. The QESST modules using wire interconnections are both lightweight and highly flexible, with efficiencies greatly exceeding existing commercial flexible modules using thin films.

Explanation/Background: 

Lighter and more flexible solar cell modules enable PV installations on weight-constrained roofs used in industrial buildings. Their compact size reduces transportation costs across the supply chain and enables more aesthetically pleasing solutions. The fluoropolymer commonly used as front sheet in the QESST cells has over 95 percent optical transparency with lower glare, allowing module use where glare is a critical safety issue (in places like airports). PV roof installations using lightweight and highly flexible solar cell modules become less costly, safer, and simpler as minimal adhesive mounting set-ups are required.

To succeed, the flexible solar cell modules must improve not only the power-to-weight ratio, but also the power-to-area performance. Traditionally lightweight and flexible modules are associated with organic and thin films deposited over flexible substrates with low efficiency. By replacing the traditional thick cells at 120 µm with 60 µm cells, the modules become more flexible. The silicon crystalline heterojunction solar cells developed at QESST are 65 µm-thick with efficiencies up to 18.4 percent.

In order to understand the likely failure mechanism in long term and extreme use, testing examined both the flexibility of the solar cells and that of their interconnections beyond the standard requirements for semi-rigid lightweight modules. QESST examined module durability using electroluminescence; cracks in the solar cells and interconnections were induced by mechanical stress during module bending. Two interconnection solutions were explored: ribbons soldered to busbars and indium-wires directly bonded to the cell fingers. Wire interconnections proved highly flexible with potential applications in lightweight modules for building integrated and portable PV power. The silicon crystalline heterojunction solar cells can be flexed 200 times around a bend radius of 4 cm without changes in efficiency.