Enhanced moisture stability of mixed cation perovskite solar cells enabled by a room-temperature solution-processed organic-inorganic bilayer hole transport layer

Despite excellent power conversion efficiency (PCE), reliability of photovoltaic performance against humidity remains a fatal problem of perovskite solar cells (PSCs). We demonstrate here a room-temperature solution-processed bilayer hole transport layer (HTL) composed of copper thiocyanate (CuSCN) and 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) as a robust HTL for highly stable PSCs. The bilayer HTL affords efficient hole transport properties with suppressed trap density at the interface of perovskite absorber and anode, delivering a comparable PCE to that of single HTL of Spiro-OMeTAD. Remarkably, the CuSCN capping confers hydrophobicity to Spiro-OMeTAD/perovskite, which effectively protects the perovskite absorbing layer from moisture invasion in highly humid condition. As a result, the bilayer HTL substantially improves the ambient stability of the mixed cation-based PSCs, which retains >88% of their initial efficiency under ambient atmosphere for >700 h without encapsulation. The bilayer HTL provides a practical approach for developing novel HTLs in PSCs with enhanced ambient stability and photovoltaic performance.