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

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_xS_y with controlled stoichiometry is reported as a dopant-free inorganic HTL. Three phase-pure compositions, namely, CoS, Co₄S₃, and Co₉S₈, are synthesized using a hot-injection method by controlling the injection temperature of a sulfur-oleylamine precursor. Each stoichiometrically defined Co_xS_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_xS_y and 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) is introduced. The PSC containing a Co₄S₃/spiro-OMeTAD bilayer HTL exhibited a PCE of 24.41%. Moreover, the thermal and operational stabilities of the PSCs containing the Co_xS_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.