AbstractThe performance of five hole‐transporting layers (HTLs) is investigated in both single‐junction perovskite and Cu(In, Ga)Se₂ (CIGSe)‐perovskite tandem solar cells: nickel oxide (NiO_x,), copper‐doped nickel oxide (NiO_x:Cu), NiO_x+SAM, NiO_x:Cu+SAM, and SAM, where SAM is the [2‐(3,‐6Dimethoxy‐9H‐carbazol‐9yl)ethyl]phosphonic acid (MeO‐2PACz) self‐assembled monolayer. The performance of the devices is correlated to the charge‐carrier dynamics at the HTL/perovskite interface and the limiting factors of these HTLs are analyzed by performing time‐resolved and absolute photoluminescence ((Tr)PL), transient surface photovoltage (tr‐SPV), and X‐ray/UV photoemission spectroscopy (XPS/UPS) measurements on indium tin oxide (ITO)/HTL/perovskite and CIGSe/HTL/perovskite stacks. A high quasi‐Fermi level splitting to open‐circuit (QFLS‐V_oc) deficit is detected for the NiO_x‐based devices, attributed to electron trapping and poor hole extraction at the NiO_x‐perovskite interface and a low carrier effective lifetime in the bulk of the perovskite. Simultaneously, doping the NiO_x with 2% Cu and passivating its surface with MeO‐2PACz suppresses the electron trapping, enhances the holes extraction, reduces the non‐radiative interfacial recombination, and improves the band alignment. Due to this superior interfacial charge‐carrier dynamics, NiO_x:Cu+SAM is found to be the most suitable HTL for the monolithic CIGSe‐perovskite tandem devices, enabling a power‐conversion efficiency (PCE) of 23.4%, V_oc of 1.72V, and a fill factor (FF) of 71%, while the remaining four HTLs suffer from prominent V_oc and FF losses.