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Lithium bis(trifluoromethane)sulfonimide (LiTFSI) is a hydrophilic organic salt that has many uses in electric and electronic systems. Its bis(trifluoromethane)sulfonimide anion, often referred to as bistriflimide, has the useful property of coordinating weakly with cations.
LiTFSI ‘s other important property is its extremely high solubility in water: 21 molal or ≈6 kg/L of solution. Because of its solubility, it has been explored as an electrolyte for lithium-ion batteries, solar cells, and similar applications since at least the late 1980s. LiTFSI is a safer material than the formerly used salt, lithium hexafluorophosphate (LiPF6).
In a 2021 Nature article, André D. Taylor at New York University (New York City) and Yale University (New Haven, CT) and 18 colleagues there and at other institutions in the United States and the Republic of Korea reported a refinement in the use of LiTFSI in solar cells.
Photovoltaic cells made of inexpensive light-absorbing perovskites that have high power-conversion efficiencies have been developed over the past several years. To improve the cells’ ability to transport electrical charges, they are doped with a combination of LiTFSI and a semiconductor called Spiro-OMeTAD1; but this process is extremely slow.
Taylor and his fellow researchers solved the problem by bubbling carbon dioxide into a solution of spiro-OMeTAD and LiTFSI while irradiating the mixture with ultraviolet light. They then cast a film made from the solution onto the perovskite light absorber. The process can be completed in ≈1 min, compared with the older, hours-long doping procedure.
The authors state, “The CO2-treated interlayer exhibits approximately 100 times higher conductivity than a pristine [untreated] film while realizing stable, high-efficiency solar cells without any post-treatments.” They also report that their method is useful for doping π-conjugated polymers.
1. 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene, CAS Reg. No. 207739-72-8.
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