Homo‐Electronic‐Type Inner‐ and Outer‐Wall Double‐Wall Carbon Nanotubes for Record‐Efficiency Carbon/Silicon Heterojunction Solar Cells

ABSTRACT A fundamental and long‐standing question in carbon nanotube‐silicon (CNT/Si) photovoltaics is whether semiconducting or metallic nanotubes form better heterojunctions. For double‐wall carbon nanotubes (DWCNTs)—where both inner and outer walls can be either semiconducting (S) or metallic (M)—this question becomes more complex, as the electronic synergy between the two walls governs junction behavior. The lack of electronically pure DWCNT materials has, however, precluded a definitive answer. Here, we resolve this via side‐by‐side comparison of high‐purity DWCNTs with homo‐electronic‐type inner and outer walls (S@S and M@M) in heterojunction solar cells. Using optimized Nafion‐based passivation and doping, both S@S and M@M devices achieve record efficiencies >22% on large active areas (≈10 cm 2 ), outperforming mixed‐type cells. Through systematic optoelectronic and junction‐physics analyses, we demonstrate that the device performance is dictated by the DWCNT electronic type. M@M devices benefit from higher built‐in potential and lower series resistance, yielding marginally higher efficiencies, while S@S devices exhibit the lowest film‐internal recombination. Beyond establishing a design pathway for high‐performance CNT/Si 1D/3D heterojunction, this work provides a materials principle—electronic uniformity in low‐dimensional passivating contacts—applicable to other emerging heterojunction systems, such as perovskite and tandem solar cells, where tailored interfacial energetics and suppressed recombination are equally crucial.