p‑Doping of a Hole Transport Material via a Poly(ionic liquid) for over 20% Efficiency and Hysteresis-Free Perovskite Solar Cells

An efficient metal-free formulation of a hole transport material (HTM) based on an ionic liquid polymer is developed for n–i–p perovskite solar cells (PSCs), to address reproducibility issues related to the use of complex dopant mixtures based on lithium salts and cobalt coordination complexes. The conductivity of the HTM is thus significantly improved by 4 orders of magnitude, up to 1.9 × 10–3 S·cm–1, using poly­(1-butyl-3-vinylimidazolium bis­(trifluoromethylsulfonyl)­imide) (PVBI-TFSI) as dopant. Introduced in the FTO/c-TiO2/mp-TiO2/K0.05 (MA0.15FA0.85)0.95PbI2.55Br0.45/HTM/Au PSC configuration, PVBI-TFSI-HTM formulation shows power conversion efficiency as high as 20.3%, versus 18.4% for the standard lithium salt-HTM formulation, with considerably reduced hysteresis and excellent reproducibility. Mechanistic investigations suggest that PVBI-TFSI acts as a source of protons promoting the HTM oxidation.