Synthesis and Characterization of Spinel Cobaltite (Co₃O₄) Thin Films for Function as Hole Transport Materials in Organometallic Halide Perovskite Solar Cells

Conventional inorganic p-type conductive oxides, for example, NiO, CuOX, and CuCrOX, can serve as low-cost and efficient hole transport materials for wide-bandgap organolead halide perovskites [for example, MAPbI3] but fail for low-bandgap Sn-rich organometallic perovskites, for example, (FASnI3)0.6(MAPbI3)0.4, where MA = (CH3NH3) and FA = (HC(NH2)2). In this work, we explore spinel Co3O4-based p-type conductive oxides as hole transport materials in organometallic halide MAPbI3 and (FASnI3)0.6(MAPbI3)0.4 perovskite solar cells. We examine the structural, crystalline, optical, electrical, photo-electrochemical, and surface chemistry properties of spin-coated Co3O4 films without and with lithium doping. We find that lithium doping improves hole mobilities and film optical transparency and causes a lithium-enriched overlayer (e.g., LiCoO2) forming at the Co3O4 film surface. As a result, lithium doping can maximize the hole transport properties of Co3O4 in our inverted planar perovskite solar cells, achieving about 14 and 7% light-to-electricity power conversion efficiencies (PCEs) for perovskite halides MAPbI3 and (FASnI3)0.6(MAPbI3)0.4, respectively. This work underscores that cobaltite spinels hold promise for application as working HTLs for all kinds of organometallic halide perovskites.