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.