A Low-Cost and Lithium-Free Hole Transport Layer for Efficient and Stable Normal Perovskite Solar Cells

The most widely used material as a hole-transport layer (HTL) for effective normal perovskite solar cells (PSCs) is still 2,2 ',7,7 '-Tetrakis[N, N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (Spiro-OMeTAD), which requires heavy doping with the hydroscopic Lithium bis(trifluoromethanesulfonyl)imide (Li-tau FSI). However, the long-term stability and performance of PCSs are frequently hampered by the residual insoluble dopants in the HTL, Li+ diffusion throughout the device, dopant by-products, and the hygroscopic nature of Li-TFSI. Due to the high cost of Spiro-OMeTAD, alternative efficient low-cost HTLs, such as octakis(4-methoxyphenyl)spiro[fluorene-9,9 '-xanthene]-2,2 ',7,7 '-tetraamine) (X60), have attracted attention. However, they require doping with Li-TFSI, and the devices develop the same Li-TFSI-derived problems. Here, we propose Li-free 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) as an efficient p-type dopant of X60, resulting in a high-quality HTL with enhanced conductivity and deeper energy levels The optimized X60:EMIM-TFSI-enabled devices exhibit a higher efficiency of 21.85% and improved stability, compared to the Li-TFSI-doped X60 devices. The stability of the optimized EMIM-TFSI-doped PSCs is greatly improved, and after 1200 hr of storage under ambient conditions, the resulting PSCs maintain 85% of the initial PCE. These findings offer a fresh method for doping the cost effective X60 as the HTL with a Li-free alternative dopant for efficient, cheaper, and reliable planar PSCs.

Automated summary

The most widely used hole-transport layer (HTL) material for perovskite solar cells (PSCs) is still Spiro-OMeTAD, which requires heavy doping with Li-TFSI. However, the long-term stability and performance of PSCs are often impacted by the residual insoluble dopants and Li+ diffusion. An alternative low-cost HTL material called X60 has attracted attention, but it also requires doping with Li-TFSI and faces the same problems. In this study, we propose using a lithium-free dopamine called EMIM-TFSI as a dopant for X60, resulting in a high-quality HTL with improved conductivity and stability.