Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.

years, because of growing concerns over the energy security. Among different types of photovoltaics, silicon solar cells give power conversion efficiencies (PCEs) of over 26% with excellent durability. This technology has already reached the market, and the products are readily available. However as recent technologies, such as flexible smartphones and IoT (internet of things), evolve into more versatile and portable electronics, there is a shift of demand from performance-centric technologies to versatility-oriented technologies. In other words, the paradigm is shifting to flexible, wearable, and light-weight electronic devices. Energy-generating devices are also following the same path. This means that the thin-film solar cell technologies, such as organic solar cells (OSCs) [1][2][3] and perovskite solar cells (PSCs), [4,5] are considered to be the wave of the future with their critical attributes of being ultrathin (<1 mm), light-weight, and solution-processable. [6] These emerging thin-film solar cells have the potential to equal or surpass the PCE of silicon solar cells while having major advantages in terms of production cost, enhanced design and a variety of new functionalities. Furthermore, tandem photovoltaics, [7] which are combined solar cells of PSCs, silicon solar cells, [8] or OSCs, [9,10] are another new and promising category for the future photovoltaic technology. Despite different names of solar cell types, the device structure is largely the same. They commonly share the typical configuration of one photoactive layer in the center, two charge selective layers above and below the active layer, and two electrodes at each end-at least one of which has to be transparent so that sunlight can pass through it. While there has been an intense efficiency race within the emerging thin-film solar cell community, less attention has been paid to other features, such as flexibility and stability. These traits can be enhanced by using new electrodes and charge-selective layers. Not only the functionalities but also photovoltaic performance is heavily dependent on the electrode and the charge-selective layers. Many scientists around the world, thus far, have focused on these layers by developing novel materials and modifying conventional materials to push the boundaries of current thin-film photovoltaic technology.Abundant and mechanically resilient carbon allotropes are composed of carbon atoms only, yet manifest different electronic properties depending on their configurations and arrangements. Of the carbon species, carbon nanotubes (CNTs) are promising materials to be incorporated into the thin-film solar cells owing to a wide range of properties ranging from conductors to semiconductors with different bandgaps based Emerging solar cells, namely, organic solar cells and perovskite solar cells, are the thin-film photovoltaics that have light to electricity conversion efficiencies close to that of silicon solar cells while possessing advantages in having additional functionalities, facile-processability, an...

Triflic acid dispersed in an apolar solvent exhibited a superior doping effect and stability on carbon nanotube electrodes. The carbon nanotube electrode-based perovskite solar cells exceeded the metal electrode-based counterpart in terms of efficiency.