Lead Sulfide Quantum Dots as a Bifunctional Layer for Efficient and Stable All-Inorganic Cesium Lead Iodide Perovskite Solar Cells

All-inorganic cesium lead iodide (CsPbI₃) perovskite has aroused great attention while their spontaneous phase transition from the active black α-phase to the undesired yellow δ-phase at room temperature has restrained their further application. In addition, the widely-used organic hole-transporting materials (HTMs), such as 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD), with hygroscopic additives not only accelerate the phase transition but also hamper the commercialization process because of its high price and complicated synthesis procedure. Herein, we employ colloidal lead sulfide quantum dots (PbS QDs) synthesized by a hot-injection method as both hole-transporting material (HTM) and water-repelling barrier for all-inorganic α-CsPbI₃ perovskite solar cells (PSCs). The CsPbI₃ films capped with PbS QDs as HTM demonstrate much improved phase-stability and can remain the black α-phase for at least 30 d without any encapsulation in air. The PSCs with PbS QDs as HTM showed high performance with a power conversion efficiency (PCE) of 7.07%. Furthermore, 89% of the original PCE is remained after storage for 504 h without any encapsulation due to strong moisture resistance resulting from hydrophobic ligands of PbS QDs.