Modified NiOx nanoparticles (NPs) developed via surface engineering are applied to a hole transport layer (HTL) in n-i-p-structured perovskite solar cells (PSCs). Hexanoic acid (HA) as a surfactant improves the dispersibility of NiOx NPs in chlorobenzene (CB). The conductivity of the NiOx-HA film is 1.20x10-5S cm-1, which is superior to that of 2,2 ',7,7 '-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9 '-spirobifluorene (spiro-OMeTAD) with dopants. The NiOx-HA film shows better hole extraction properties compared with the pristine NiOx film. The NiOx-HA NPs form closely packed and pinhole-free films, leading to improved device performance with a power conversion efficiency from 5.5% to 13.1%.