Zn-TFSI ₂ is introduced as a powerful p-dopant for spiro-MeOTAD in perovskite solar cells which not only outperforms Li-TFSI but also achieves outstanding long term stability.

One of the most prominent hole‐transporting material (HTM) for hybrid perovskite solar cells has been 2,2″,7,7″‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), which is commonly doped with metal bis(trifluoromethylsulfonyl)imide (M(TFSI)_n) salts that contribute to generating the active radical cation HTM species. The underlying role of the metal cation, however, remains elusive. Here, the effect of metal cations (M = Li, Zn, Ca, Cu, and Sc) on doping spiro‐OMeTAD is analyzed by a combination of techniques, including electron paramagnetic resonance spectroscopy and cyclic voltammetry, which is complemented by photovoltaic device and hole mobility analysis. As a result, the authors reveal the superiority of Zn(TFSI)₂ salts in device performances as compared to the others, including redox‐active Cu(TFSI)₂. This analysis thereby unravels new design principles for dopant engineering in HTMs for hybrid perovskite photovoltaics.