The quest for more efficient, stable, and cost-effective solar energy has led researchers to explore the remarkable properties of carbon nanotubes (CNTs), and recent advances suggest these microscopic cylindrical structures could play a pivotal role in the next generation of photovoltaic technologies. While traditional solar cells face limitations in efficiency and flexibility, CNTs are being integrated into various solar cell architectures to address these challenges, serving as transparent electrodes, charge transport enhancers, and light absorbers.
Single-walled carbon nanotubes (SWCNTs), in particular, have demonstrated an exceptional ability to function as both front and back electrodes in bifacial perovskite solar cells, achieving a bifaciality factor of over 98% and a power generation density exceeding 36%. This means these cells can efficiently capture light from both sides, significantly boosting energy yield without a proportional increase in cost. The potential of CNTs extends to creating lightweight, flexible, and even foldable solar panels. SWCNT-based devices have been fabricated with a high power-per-weight value of 73.75 W g⁻¹, showcasing their promise for applications where weight is a critical factor, such as in aerospace or portable electronics.
One of the most transformative impacts of CNTs is their ability to enhance the performance of perovskite solar cells (PSCs), a promising and rapidly advancing photovoltaic technology. A key challenge with PSCs is maintaining high efficiency under varied lighting conditions and ensuring long-term stability. Here, carbon nanotubes are being incorporated as an additive during the fabrication process to improve charge transfer and suppress charge recombination. For instance, integrating multi-walled carbon nanotubes (MWCNTs) into the perovskite layer via an antisolvent engineering strategy has yielded an outstanding indoor power conversion efficiency (PCE) of 32.63% under 1000 lx LED illumination.
This breakthrough is crucial for powering Internet of Things (IoT) devices and self-powered electronics using ambient light. The CNTs facilitate more efficient charge extraction and faster interfacial charge transfer by increasing surface conductivity and providing direct conductive pathways, which significantly reduces energy losses that typically occur when electrons and holes recombine before being collected. Furthermore, the intrinsic hydrophobicity of carbon nanotubes helps protect the perovskite layer from moisture-induced degradation, a major obstacle to device longevity, leading to cells that retain over 50% of their initial efficiency after 150 days of ambient storage without encapsulation.
The versatility of carbon nanotubes is evident in their ability to replace more expensive and less flexible materials in conventional solar cell designs. A primary function is serving as a high-performance transparent electrode, offering a compelling alternative to the commonly used but costly and brittle indium tin oxide (ITO). SWCNT films have been successfully employed as transparent window electrodes in polymer/silicon hybrid solar cells, achieving a power conversion efficiency of up to 7.0%. Their high optical transparency, good electrical conductivity, and mechanical flexibility allow for simpler, low-temperature, and solution-based fabrication processes, bypassing the need for high-vacuum metal deposition.
This approach not only reduces manufacturing costs but also enables the creation of flexible modules that can be integrated into building facades and other curved surfaces. Acid treatment of SWCNT networks has been shown to further enhance their conductivity, enabling ITO-free perovskite solar cells to reach over 24% PCE, with flexible versions maintaining roughly 23% efficiency and over 95% of their original performance after one month under high temperatures, high humidity, and continuous solar irradiation. This remarkable stability, combined with the ability to produce meter-scale SWCNT films using a roll-to-roll process, positions CNT-based electrodes as a practical, scalable solution for the future of sustainable energy.
Beyond their role in electrodes, CNTs are proving to be invaluable as charge-selective materials and as components in advanced cell architectures. Functionalized CNTs are being used as additives to boost the performance of hole-transporting materials (HTMs), which are essential layers that collect positive charges in a solar cell. For example, adding porphyrin-functionalized SWCNTs to the commonly used HTM spiro-OMeTAD optimized energy level alignment and improved the contact interface with the perovskite, resulting in an enhanced PCE of 19.8% compared to 18.0% for reference devices. The hydrophobic nature of these CNTs also contributed to improved device stability under ambient conditions.
Furthermore, CNTs are enabling the development of novel tandem solar cells, which stack different light-absorbing materials to achieve higher efficiencies than single cells. Researchers have demonstrated a four-terminal tandem device integrating a semitransparent SWCNT-based perovskite solar cell with an SWCNT-silicon heterojunction solar cell, achieving a combined efficiency of over 22%. This innovative approach leverages the unique properties of SWCNTs in both sub-cells, showcasing a pathway to economical and highly efficient solar power. In a completely different application of their light-absorbing prowess, vertically aligned carbon nanotube arrays (VACNTs) have been engineered to possess near-perfect light absorption, with an ultrahigh average absorptance of 99.94% in the near-infrared range.
This “blackbody” like property allows them to efficiently convert sunlight into heat, powering a solar-thermal electric generator that achieved a record output power density of 458.4 μW cm⁻², demonstrating the potential of CNTs in thermal energy harvesting applications as well. From enhancing charge transfer in indoor photovoltaics to creating durable, flexible, and ultra-efficient tandem cells, carbon nanotubes are undeniably boosting the potential of solar energy across multiple fronts.
