The proper bandgap and exceptional photostability enable CsPbI3 as a potential candidate for indoor photovoltaics (IPVs), but indoor power conversion efficiency (PCE) is impeded by serious nonradiative recombination stemming from challenges in incomplete DMAPbI3 conversion and lattice structure distortion. Here, the coplanar symmetric structure of hexyl sulfide (HS) is employed to functionalize the CsPbI3 layer for fabricating highly efficient IPVs. The hydrogen bond between HS and DMAI promotes the conversion of DMAPbI3 to CsPbI3, while the coplanar symmetric structure enhances crystalline order. Simultaneously, surface sulfidation during HS-induced growth results in the in situ formation of PbS, spontaneously creating a CsPbI3 N-P homojunction to enhance band alignment and carrier mobility. As a result, the CsPbI3&HS devices achieve an impressive indoor PCE of 39.90% (Pin: 334.6 μW cm−2, Pout: 133.5 μW cm−2) under LED@2968 K, 1062 lux, and maintain over 90% initial PCE for 800 h at ∼30% air ambient humidity.