New insight into the grafted transition metal ions in trilacunary Keggin polyoxometalates dopants for efficient and stable perovskite solar cells

Currently, the quality of hole transport layers (HTLs) plays a vital role in achieving the high-efficiency perovskite solar cells (PSCs). The 2,2′,7,7′-tetrakis-(N, N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD) based HTLs usually suffer from pinholes and defects which would cause the performance degradation. Here, a series of polyoxometalate (POM) crystals with mixing different transition metal ions, named Na₁₀[M₂(solvent)₆(WO₂)₂(BiW₉O₃₃)₂] (BiW₁₀M, M = Mn, Co), are synthesized and used as dopants to improve the properties of Spiro-OMeTAD based HTLs. We observe that the effects of oxidation abilities for Spiro-OMeTAD could be tuned by mixing transition metal ions. Besides, the morphologies of HTLs are significantly improved and the power conversion efficiencies (PCEs) of BiW₁₀M doped PSCs are enhanced from 18.48% (control device) to 20.31% (BiW₁₀Mn) and 20.12% (BiW₁₀Co), respectively. Moreover, doping transition metal ions can also prevent the water penetration, thus the device stability of BiW₁₀M doped PSCs is also obviously enhanced. In this work, metal ions doped POMs are proved to be low-cost and facile chemical additives for oxidizing Spiro-OMeTAD, which provide an efficient strategy to fabricate high-performance and stable PSC devices.