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Organic optoelectronic devices that can be fabricated at low cost have attracted considerable attention because they can absorb light over a wide frequency range and have high conversion efficiency, as well as being lightweight and flexible. Moreover, their performance can be significantly affected by the choice of the charge-selective interlayer material. Nonstoichiometric nickel oxide (NiO x ) is an excellent material for the hole-transporting layer (HTL) of organic optoelectronic devices because of the good alignment of its valence band position with the highest occupied molecular orbital level of many p-type polymers. Herein, we report a simple low-temperature process for the synthesis of NiO x nanoparticles (NPs) that can be well dispersed in solution for long-term storage and easily used to form thin NiO x NP layers. NiO x NP-based organic photodiode (OPD) devices demonstrated high specific detectivity ( D *) values of 10 12 -10 13 jones under various light intensities and negative biases. The D * value of the NiO x NP-based OPD device was 4 times higher than that of a conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based device, an enhancement that originated mainly from the 16 times decreased leakage current. The NiO x NP-based OPD device demonstrated better reliability over a wide range of light intensities and operational biases in comparison to a device with a conventional sol-gel-processed NiO x film. More importantly, the NiO x NP-based OPD showed long-term device stability superior to those of the PEDOT:PSS and sol-gel-processed NiO x -based devices. We highlight that our low-temperature solution-processable NiO x NP-based HTL could become a crucial component in the fabrication of stable high-performance OPDs.