Lightweight yet strong: a new foundation for high-power flexible solar cells

As aerospace systems, satellites, and emerging flexible electronics demand increasingly lightweight power sources, solar-cell substrates must deliver exceptional mechanical strength without adding unnecessary mass. Although polyimide (PI) films offer excellent thermal stability and flexibility, their relatively high density limits the specific power of thin-film solar cells, while introducing porosity can weaken their mechanical integrity. Researchers from Zhengzhou University of Light Industry, the Shanghai Institute of Space Power-Sources, Shanghai Ocean University, Shanghai Jiao Tong University, and the Shanghai Institute of Technology have developed an ultralight, mechanically robust composite film by integrating covalent organic frameworks (COFs) with multi-walled carbon nanotubes (MWCNTs). The resulting substrate enables flexible gallium arsenide (GaAs) solar cells to achieve exceptional specific power while maintaining performance during repeated bending. 

Why This Lightweight Solar-Cell Platform Matters

For space-based and other weight-sensitive applications, increasing power output relative to device mass is critical. Conventional PI substrates combine good thermal stability with mechanical strength, but their density can exceed 1.3 g cm^-3. Simply introducing pores to reduce weight often compromises strength, while conventional CNT reinforcement may suffer from aggregation and weak interfacial bonding with the polymer matrix. Overcoming this strength–density trade-off is therefore essential for developing flexible solar cells that are both lightweight and mechanically reliable.

Innovative Design and Mechanism

The researchers constructed a coaxial MWCNT–COF hybrid by growing a porous COF layer directly onto carboxylated carbon nanotubes through a solvothermal process. These hybrids were then incorporated into a PI matrix through in situ polymerization to form an interconnected, layered porous network. The CNTs provide mechanical reinforcement, while the covalently bonded COF shells improve interfacial compatibility and facilitate stress transfer between the nanotubes and polymer. This architecture helps suppress CNT aggregation, strengthens the composite interface, and introduces porosity that reduces density. Combined with PI’s thermally stable molecular structure, the design achieves lightweight construction without sacrificing mechanical integrity.

Outstanding Performance

The optimized MWCNT–COF/PI film achieves a tensile strength of 97.53 MPa and a Young’s modulus of 1.53 GPa, while its density falls to just 0.73 g cm^-3—a 45.1% reduction compared with the unmodified PI film. The composite also exhibits improved hydrophobicity and maintains thermal stability, with no significant mass loss below 500 °C. Using an ultrathin, 6 μm-thick composite film as the substrate, the researchers fabricated flexible dual-junction and triple-junction GaAs solar cells with power-conversion efficiencies of 26.6% and 31.5% under AM0 illumination, respectively. Their corresponding specific powers reached 8,998 and 7,749 W kg^-1. The devices also retained excellent photovoltaic performance after 2,000 bending cycles at a 10 mm radius, demonstrating their mechanical durability.

Applications and Future Outlook

This COF-anchored nanotube strategy offers a promising route toward lightweight, durable substrates for flexible photovoltaics. By combining high mechanical strength, low density, thermal stability, and reliable photovoltaic performance, the composite films could support the development of power systems for satellites, aerospace vehicles, and other mass-constrained applications. More broadly, the work demonstrates how rational interfacial engineering and three-dimensional network design can address competing requirements in advanced polymer composites, providing a useful materials-design strategy for next-generation flexible electronic and energy devices.