Ligand‐Induced In Situ Epitaxial Growth of PbI2 Nanosheets/MAPbI3 Heterojunction Realizes High‐Performance HTM‐Free Carbon‐Based MAPbI3 Solar Cells

Abstract Hole‐transporting layer‐free carbon‐based perovskite solar cells (HTL‐free C‐PSCs) hold great promise for photovoltaic applications due to their low cost and outstanding stability. However, the low power conversion efficiency (PCE) of HTL‐free C‐PSCs mainly results from grain boundaries (GBs). Here, epitaxial growth is proposed to rationally design a hybrid nanostructure of PbI 2 nanosheets/perovskite with the desired photovoltaic properties. A post‐treatment technique using tri(2,2,2‐trifluoromethyl) phosphate (TFEP) to induce in situ epitaxial growth of PbI 2 nanosheets at the GBs of perovskite films realizes high‐performance HTL‐free C‐PSCs. The structure model and high‐resolution transmission electron microscope unravel the epitaxial growth mechanism. The epitaxial growth of oriented PbI 2 nanosheets generates the PbI 2 /perovskite heterojunction, which not only passivates defects but forms type‐I band alignment, avoiding carrier loss. Additionally, Fourier‐transform infrared spectroscopy, 31 P NMR, and 1 H NMR spectra reveal the passivation effect and hydrogen bonding interaction between TFEP and perovskite. As a result, the V OC is remarkably boosted from 1.04 to 1.10 V, leading to a substantial gain in PCE from 14.97% to 17.78%. In addition, the unencapsulated PSC maintains the initial PCE of 80.1% for 1440 h under air ambient of 40% RH. The work offers a fresh perspective on the rational design of high‐performance HTL‐free C‐PSCs.