A Generic Route of Hydrophobic Doping in Hole Transporting Material to Increase Longevity of Perovskite Solar Cells

The use of hydrophobic dopant is paramount for the success of organic semiconductors. Here we demonstrate the use of an N-heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI), to induce dual functionality: as p-type dopant and as additive for state-of-the-art Spiro-OMeTAD hole-transporting material (HTM). This ionic liquid doped HTM was then implemented for perovskite solar cell fabrication and delivered competitive results. The dopant has the ability to substitute state-of-the-art ultra-hygroscopic lithium salt (LiTFSI) and problematic 4-tertbutylpyridine (t-BP) additive. BMPyTFSI was found to increase the conductivity of Spiro-OMeTAD and its use as dopant in HTM reduces charge recombination, improves the film formation by reducing the pinholes on the HTM surface, and allows fabrication of efficient devices. Competitive air stability of solar cells in comparison with their state-of-the-art dopant was found, and these findings open up a broad range of organic semiconductors for hydrophobic ionic liquid-based doping.