Understanding the Influence of Anion Exchange on the Hole Transport Layer for Efficient and Humidity-Stable Perovskite Solar Cells

High-performance perovskite solar cells (PSCs) are readily degradable by moisture, leading to high demand for a water-repelling efficient hole transport layer (HTL). In this study, we proposed an anion-exchange approach to replace the conventional hygroscopic dopant anion-bis(trifluoromethanesulfonyl)imide (TFSI-)-with a hydrophobic dopant anion capable of effectively doping into a 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) matrix. By varying the size of dopant anions, we successfully controlled electrostatic interactions between spiro-OMeTAD and the dopant anion. Hexafluorophosphate (PF6-) demonstrated the highest p-doping anion-exchange capability because the optimal-sized PF6- enabled a strong electrostatic interaction between spiro-OMeTAD•+ and PF6- while resulting in poor affinity between Li+ and PF6-. The resulting PF6-doped spiro-OMeTAD HTL not only produced favorable energy band alignment with perovskite but also improved film conductivity. Correspondingly, the PSCs based on the PF6-doped HTL exhibited a higher power conversion efficiency (PCE) of 20.78% than the reference TFSI-based PSCs of 19.04%. Besides device performance, the superior hydrophobic nature of PF6- enabled the HTL to prevent water penetration into the perovskite layer, improving long-term stability against moisture. The PF6-based PSCs exhibited enhanced humidity stability while maintaining 92% of the initial PCE for 1180 h at a relative humidity of 25% under ambient conditions.