Perovskite solar cells (PeSCs) prepared with single crystals (SCs) would, in theory, exhibit higher efficiencies than those of polycrystalline devices because SCs exhibit lower trap densities and superior charge transport. Most of the crystallization methods proposed so far for the synthesis of asymmetric PeSCs require additional interfacial layers to facilitate the diffusion of the precursor ions. Nevertheless, the electrical resistance of the interfacial layer increases the series resistance (Rs) of the solar cells, thereby limiting the performance. In this study, we prepare single-crystal PeSCs featuring modified hole-transport layers (HTLs) that improved the power conversion efficiency (PCE). After blending p-type 4-isopropyl-4´-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate into the interfacial layer, the values of Rs decrease and the PCEs of the single-crystal PeSCs are improved notably. At the optimal concentration, the PCE can be increased to 14.99%; the champion PCE achieves 15.67%. Further, stability tests reveal that the doping process has little effect on the device reliability. Thus, the quality of the HTL plays an important role in determining the performance of single-crystal PeSCs.