Stoichiometry‐Controlled Cobalt Sulfide‐Based Hole Transport Layers for Perovskite Solar Cells

Abstract Most hole transport layers (HTLs) used in perovskite solar cells (PSCs) require extrinsic doping to enhance conductivity and modify energy levels. However, such doping often induces structural disorder, compositional inhomogeneity, and band edge distortion owing to dopant segregation, reducing hole mobility and interfacial charge trapping. Herein, Co _x S _y with controlled stoichiometry is reported as a dopant‐free inorganic HTL. Three phase‐pure compositions, namely, CoS, Co ₄ S ₃ , and Co ₉ S ₈ , are synthesized using a hot‐injection method by controlling the injection temperature of a sulfur‐oleylamine precursor. Each stoichiometrically defined Co _x S _y HTL possessed distinct valence band positions, enabling systematic control of band alignment with that of the perovskite layer. PSCs containing these HTLs exhibited power conversion efficiencies (PCEs) of up to 18.65% and open‐circuit voltages of up to 1.09 V. To further enhance hole transport and charge collection efficiency, a bilayer HTL composed of Co _x S _y and 2,2 ′ ,7,7 ′ ‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9 ′ ‐spirobifluorene (spiro‐OMeTAD) is introduced. The PSC containing a Co ₄ S ₃ /spiro‐OMeTAD bilayer HTL exhibited a PCE of 24.41%. Moreover, the thermal and operational stabilities of the PSCs containing the Co _x S _y HTLs are better than those of the PSCs employing conventional spiro‐OMeTAD‐only HTLs. This strategy can expand the utility of previously underutilized nonstoichiometric materials as functional HTLs in high‐photovoltaic‐performance PSCs.