Recent researches show that nickel oxide (NiOx) thin films can be utilized as an efficient and stable hole transport layer (HTL) in inverted planar perovskite solar cells (PSCs) replacing costly and chemically unstable organic materials. Nevertheless, thermal annealing process is necessary for NiOx made by conventional methods with metallic dopants, which is not applicable to flexible substrates with low melting points or temperature-sensitive materials. In this work, annealing-free Al-doped NiOx nano-particles (NPs) were synthesized and employed as a HTL in PSCs. Without using any Dimethyl sulfoxide (DMSO) additive in devices, the electrical conductivity, morphology, external quantum efficiency, and power conversion efficiency were all significantly enhanced after 2% Al doping in NiOx. The Ultraviolet Photoemission Spectroscopy (UPS) was conducted to explore the mechanisms behind the device improvements. An interfacial dipole of 0.3 eV was found between the ITO electrode and the NiOx layer directing away from the active layer, boosting the hole extraction efficiency at ITO/MAPbI3 (MA = methylamine) interface. This work not only provides a deep insight of the ITO/NiOx NPs/perovskite interface but also proposes a new strategy for further development of highly efficient and reliable PSCs.