1D/3D Heterojunction for High‐Efficiency Hole‐Transport Layer‐Free Carbon‐Based Wide‐Bandgap Perovskite Solar Cells

ABSTRACT Hole‐transport layer (HTL)‐free carbon‐based perovskite solar cells (C‐PSCs) exhibit significant advantages in terms of low cost and superior stability. However, high defect density and significant energy level mismatch at the perovskite/carbon interface severely limit power conversion efficiency (PCE). Constructing low‐dimensional perovskite on the surface of 3D perovskite to form heterojunctions has been demonstrated to effectively passivate defects and modulate energy levels. Nevertheless, the strong Cs ⁺ ‐[PbI ₆ ] ⁴⁻ binding in high‐Cs‐content perovskites hinders such heterojunction formation. Herein, by systematically investigating quaternary ammonium salts with varying alkyl‐chain lengths, we demonstrate that tetrapropylammonium bromide with a proper size induces a stable 1D TPAPb ₂ I ₅ phase on high‐Cs‐content wide‐bandgap perovskite. The formed 1D/3D heterojunction effectively passivates surface defects and optimizes interfacial energy level alignment, significantly reducing interfacial nonradiative recombination. Consequently, the PCE of HTL‐free wide‐bandgap C‐PSCs is improved to a record value of 20.14% with enhanced device stability.