Doping organic hole-transport materials for high-performance perovskite solar cells

Single-junction and tandem perovskite solar cells(PSCs)have achieved impressive power conversion efficiencies(PCEs)of 25.7％and 31.3％,respectively,which makes it to be one of next-generation photovoltaic technologies[1-9].Inter-face engineering[3,5,10-12],composition engineering[13]and ad-ditive engineering[7,14,15]have made remarkable contribu-tions to efficiency enhancement.Compared with efficiency,the long-term operational stability of PSCs jogs along,which is far from the requirements of commercialization.Currently,almost all regular n-i-p PSCs were accomplished with classic-al organic hole-transport materials(HTMs),i.e.,PTAA[16]and spiro-OMeTAD[2,4,6].However,the highly efficient PSCs with the above organic hole-transport layers(HTL)usually suffer from instability.To facilitate hole transport and extraction,LiTF-SI and tBP are frequently employed to dope organic HTLs but this would sacrifice device stability.The use of these hygro-scopic p-dopants endows the devices with poor moisture sta-bility.It is worth noting that small-sized lithium ion(Li+)can easily diffuse into perovskite layer and metal electrode,which deteriorates device performance[17].Consequently,a critical challenge limiting commercial applications of PSCs is the trade-off between high efficiency and high stability.The tradi-tional doping strategy with LiTFSI and tBP requires a long time(usually several days)to reach optimal doping and device performance,which is not good for mass production.In addition,owing to the intrinsic soft nature of perovskites,iodide ions can easily migrate and diffuse into the HTL[18,19],and then interact with positive radicals in HTLs,diminishing hole transport[16].