Chemically Anchored PbS-2PACz CQDs Inks for Scalable HTL in Narrow-Bandgap and All-Perovskite Tandem Solar Cells

Mixed-metal narrow-bandgap (NBG) Sn-Pb perovskites are essential for achieving high-efficiency all-perovskite tandem solar cells (APTSCs). However, their single-junction performance is limited by interfacial recombination and inefficient charge extraction, due to non-uniform hole transport layers (HTLs). Here, PbS-2PACz colloidal quantum dots (CQDs) are synthesized, chemically anchoring [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) onto PbS CQDs via solution-phase ligand exchange. The resulting PbS-2PACz CQD ink demonstrates excellent colloidal stability in weakly polar solvents and yields uniform, defect-suppressing films on perovskites. To further enhance the perovskite/HTL interface, an additional 2PACz treatment deepens the valence band (-5.60 eV), reduces trap density, and optimizes energy-level alignment. Consequently, NBG perovskite solar cells utilizing PbS-2PACz with additional 2PACz achieve power conversion efficiencies (PCEs) of 22.84% ± 0.55, exceeding devices using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and 2PACz self-assembled monolayers (SAMs). Integrated into APTSCs with a 1.77 eV wide-bandgap top cell, PbS-2PACz achieves a PCE of 25.05% ± 0.41, significantly outperforming PEDOT:PSS-based tandems (21.60% ± 0.91). This work highlights PbS-2PACz as an effective HTL material that enhances hole extraction, reproducibility, and scalability for high-performance perovskite solar cells.