In the early stage, Marks and co-workers [ 13 ] reported a striking performance improvement of OSCs by replacing PEDOT:PSS with NiO x fi lm using a pulsed-laser deposition technology. From then on, NiO x HTLs have been reported for organic optoelectronics by various preparation methods, such as thermal evaporation, [ 14 ] sputtering, [ 9a ] and solution process. [ 9d , 15 ] Among them, solution process method is desirable for low-cost, large-scale and roll-to-roll production. Olson and co-workers [ 16 ] proposed a solution-processed NiO x fi lm as highly effi cient HTL in OSCs. The functional NiO x HTL was fabricated through annealing the precursor fi lm at a temperature of 275 °C. So and co-workers [ 17 ] also presented a NiO x fi lm by using monoethanolamine (MEA) to react with Ni in ethanol solution followed by thermally converting (275 °C) coordination complexes ions [Ni(MEA) 2 (OAc)]+ into high-quality NiO x . Meanwhile, solution-processed NiO x at 150 °C has also been realized. Ma and co-workers [ 18 ] reported a solution-processed NiO x fi lm for OSCs using oxygen-plasma treatment and annealing treatment simultaneously. Zhang et. al. reported that the colloidal NiO nanoparticles are used as the anode buffer layer in OSCs without high temperature post-annealing to induce decomposition and crystallization. [ 9f ] For a long period, the studies of NiO x HTLs were focused on utilizing sol-gel methods with thermally converting the precursor solution to NiO x thin fi lms. In the process of device fabrications, thermal annealing process and oxygen-plasma treatment may be simultaneously required, which hinders the applications of NiO x in fl exible optoelectronic devices. Instead of precursor method, an approach to signifi cantly reduce the processing temperature of TMO HTLs is to directly use high-quality colloidal nanoparticles (NPs). Jin and co-workers demonstrated a facile and general strategy based on ligand protection for the synthesis of unstable colloidal NiO nanocrystals. [ 19 ] Fattakhova-Rohlfi ng and co-workers described the preparation of ultrasmall, crystalline, and dispersible NiO nanoparticles, which are promising candidates as catalysts for electrochemical oxygen generation. [ 9e ] Herein, we will demonstrate a facile chemical precipitation method which is robust and simple for direct preparation of high-quality non-stoichiometric NiO x NPs. Remarkably, by using this method, NiO x HTL fi lm can be formed through a room-temperature solution process without any post-treatments during device fabrication. Interestingly, our results show that the NiO x NPs fi lm can function as effective HTLs over a wide range of annealing temperatures from room temperature to 150 °C. Very good optoelectronic performances utilizing the NiO x NPs fi lm as HTLs have been demonstrated in both OSCs and OLEDs. High power conversion effi ciency (PCE) of 9.16% (best 9.28%) was achieved in OSCs using NiO x