Incorporation of High-Mobility and Room-Temperature-Deposited Cu_xS as a Hole Transport Layer for Efficient and Stable Organo-Lead Halide Perovskite Solar Cells

The most widely used hole transport layer (HTL) for n-i-p perovskite solar cells (PVSCs), 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), suffers a severe degradation from the permeation of moisture and direct contact with the metal electrode, mainly due to hygroscopic additives and the presence of pinholes in the spiro-OMeTAD film. To overcome this problem, we developed a stable inorganic copper-based chalcogenide (Cu_xS, x = 1.75) that can cooperate with spiro to serve as the HTL for planar n-i-p PVSCs. The Cu_xS HTL has two main functions: 1) enhancing hole transport due to its high intrinsic mobility and proper energy level alignment, resulting in a better charge transfer and reduced charge recombination; 2) protecting the spiro layer from damage from both moisture and the top Au anode, through the formation of a physical hydrophobic buffer layer. PVSCs with enhanced power conversion efficiencies (PCEs) are realized through this simple approach, yielding the highest PCE of 18.58% and a steady-state PCE of 17.91%. Furthermore, benefiting from the hydrophobic nature of Cu_xS, PVSCs retained over 90% of their initial efficiency, even after storage in air with approximately 40% humidity for 1000 h without encapsulation. This study demonstrates that Cu_xS is a potential hole transport material for fabricating low-cost and efficient PVSCs with long-term stability.