COF-anchored carbon nanotubes enable ultrathin, lightweight substrates for high-specific-power flexible GaAs solar cells

The growing demand for lightweight power systems in satellites, aerospace vehicles, and other advanced technologies has placed increasing emphasis on the specific power of solar cells. GaAs-based III–V solar cells are particularly attractive for space applications because of their high conversion efficiency and favorable photovoltaic performance. However, further improvements in specific power require not only efficient photovoltaic materials but also substantial reductions in device weight. Flexible solar cells commonly use polymer films as supporting substrates, making substrate density and mechanical reliability critical factors in overall device performance.

Polyimide (PI) is widely used in flexible electronics because of its excellent mechanical properties and thermal stability. Nevertheless, conventional PI films are relatively dense, and introducing pores to reduce their weight often weakens their mechanical strength. Carbon nanotubes have been investigated as reinforcing fillers, but their tendency to aggregate and their weak interfacial interactions with polymer matrices can limit stress transfer and reinforcement efficiency. Achieving lightweight PI films without sacrificing mechanical robustness therefore remains a significant challenge.

To overcome this limitation, the researchers developed a composite reinforcement strategy combining multi-walled carbon nanotubes with covalent organic frameworks. Porous COF shells were directly grown on carboxylated MWCNTs through a solvothermal process, producing coaxial MWCNT-COF hybrids. These hybrid nanomaterials were subsequently incorporated into a polyamide acid precursor through in situ polymerization, followed by film casting and thermal imidization to obtain ultrathin MWCNT-COF/PI composite films.

Unlike conventional approaches based on the physical blending of nanofillers, this strategy establishes strong chemical connections between the COF shells and carbon nanotubes while improving their integration into the PI matrix. The resulting three-dimensional interconnected network combines the mechanical reinforcement provided by MWCNTs with the lightweight porous architecture of COFs.

The mechanical reinforcement arises from the complementary functions of MWCNTs and COFs. Carbon nanotubes provide rigid structural support and efficient pathways for transferring mechanical stress, while the covalently anchored COF layers improve interfacial adhesion and help distribute the nanotubes throughout the polymer matrix. The porous interconnected structure simultaneously reduces material density and promotes more uniform stress distribution. Computational analyses further supported the beneficial interactions between COF-grafted nanotubes and the polymer precursor, providing additional insight into the reinforcement mechanism.

The optimized composite film containing 2 wt% MWCNT-COF exhibited a density of only 0.73 g cm⁻³, compared with 1.33 g cm⁻³ for pristine PI, corresponding to a 45.1% reduction. Remarkably, this reduction in weight was accompanied by improved mechanical properties. The tensile strength increased from 72.01 to 97.53 MPa, while the Young’s modulus reached 1.53 GPa, representing improvements of 35.4% and approximately 109%, respectively. The composite films also retained good thermal stability, with no significant mass loss observed below 500 °C. These results demonstrate that rational interfacial and structural engineering can alleviate the conventional trade-off between low density and high mechanical strength.

To demonstrate the practical benefits of the composite films, the researchers fabricated flexible GaAs solar cells using 6 μm-thick MWCNT-COF/PI substrates. Through epitaxial inverted growth, low-temperature metal bonding, and transfer processes, the ultrathin polymer films were integrated with high-efficiency III–V photovoltaic structures. The mechanically robust substrates provided effective support for the fragile semiconductor layers while minimizing the overall device weight.

Under AM0 illumination conditions, the flexible dual-junction GaAs solar cells achieved a power conversion efficiency of 26.6% and a specific power of 8998 W kg⁻¹. The triple-junction devices delivered an efficiency of 31.5% and a specific power of 7749 W kg⁻¹. These results demonstrate how the combination of high-efficiency photovoltaic structures and ultralight substrates can significantly improve power output relative to device mass.

Mechanical reliability was also examined under repeated deformation. The flexible solar cells exhibited nearly unchanged current–voltage characteristics at different bending radii. Even after 2000 bending cycles at a radius of 10 mm, the devices maintained excellent efficiency retention, confirming that the composite substrates can effectively support photovoltaic operation under repeated mechanical stress.

This work demonstrates an effective materials engineering strategy for combining low density, high mechanical strength, and flexibility in ultrathin polymer substrates. By integrating the porous architecture of COFs with the mechanical reinforcement of carbon nanotubes, the researchers developed PI composite films that support high-specific-power flexible GaAs solar cells without compromising mechanical reliability. Beyond flexible photovoltaics, this approach offers a promising direction for lightweight aerospace power systems, flexible electronics, and other energy devices requiring both structural durability and reduced weight.

Title: Covalent Organic Framework-Anchored Carbon Nanotubes Enabling Ultra-Thin Robust Polyimide Films for High-Specific-Power Flexible GaAs Solar Cells

Journal Name: Nano-Micro Letters

DOI: https://doi.org/10.1007/s40820-026-02326-1

URL: https://scholarcast.cast.org.cn/top-news/2114309945202880512

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