Solvent engineering with hexafluorobenzene for reproducible 2D/3D perovskite heterojunction solar cells

Inserting two-dimensional (2D) perovskite layers at the contact interfaces of three-dimensional (3D) perovskite solar cells (PSCs) reduces surface defects and suppresses ion migration. However, these 2D layers are often inhomogeneous with excess organic ligands, jeopardizing charge extraction and device reproducibility. Here, we introduce a binary solvent post-treatment using isopropanol (IPA) and hexafluorobenzene (HFB) that selectively removes unbound ligands while promoting a homogeneous 2D perovskite layer. Guided by solubility screening and thin-film characterization, we identify an IPA:HFB composition that minimizes 2D phase dissolution while efficiently removing residual phenethylammonium iodide. Treated 2D/3D films exhibit improved optoelectronic properties. Consequently, inverted PSCs achieve a champion power-conversion efficiency of 26.10%. Accelerated thermal (ISOS-D-2) and photothermal (ISOS-L-2) stress tests show ∼67% efficiency retention after >7,200 h and ∼80% after >1,000 h, respectively. This ligand-agnostic solvent-washing strategy complements ligand-reactivity approaches and extends to other spacer cations, offering a broadly applicable route to dimensional engineering of perovskite thin films.