Li-Salt-Free, Coevaporated Cu(TFSI)₂-Doped Hole Conductors for Efficient CH₃NH₃PbI₃ Perovskite Solar Cells

In n–i–p-type conventional perovskite solar cells (PSCs) using a doped 2,2′,7,7′-tetrakis (N,N′-di-p-methoxyphenylamine)-9,9′-spirofluorene (spiro-OMeTAD) hole transport layer (HTL), the issues of reproducibility and stability are closely associated with the redox-inactive additives lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP). Instead of these additives, copper(II) di[bis(trifluoromethylsulfonyl)imide] (Cu(TFSI)2) is demonstrated as a direct and efficient p-dopant for spiro-OMeTAD. With the adoption of the technologically relevant coevaporation technique, highly uniform, pinhole-free doped HTLs are achieved with controlled amounts of Cu(TFSI)2 and are spectroscopically and electrically characterized. Using these highly conducting doped HTLs, CH3NH3PbI3-based planar PSCs are realized, which exhibit high photoconversion efficiency (>13% with merely 4 mol % dopant) and excellent reproducibility. Also, by taking advantage of the coevaporation technique, the Cu(TFSI)2-doped HTL thickness impact on PSCs is investigated. It is observed that devices with even the thinnest (40 nm) HTL perform very similarly to the ones with a 100 nm thick HTL, which opens up cost-effective preparation strategies. Moreover, a remarkable storage stability over 218 days is observed for devices with a coevaporated Cu(TFSI)2-doped HTL, suggesting that this approach of controlled direct doping is a viable alternative to the existing arbitrarily p-doped HTL in perovskite solar cells.