We developed self-assembled hybrid dielectric materials via a facile and low-temperature solution process. These dielectrics are used to facilitate ultralow operational voltage of organic thinfilm transistors. Self-assembly of bifunctional phosphonic acid and ultrathin hafnium oxide layers results in the self-assembled hybrid dielectrics. Additionally, the surface property of the top layer of hafnium oxide can be tuned by phosphonic acid-based self-assembled molecules to improve the function of the organic semiconductors. These novel hybrid dielectrics demonstrate great dielectric properties as low-level leakage current densities of <1.45×10-6 A/cm², large capacitances (up to 800 nF/cm²), thermal stability (up to 300 °C), and featureless morphology (root-mean-square roughness ˜0.3 nm). As a result, self-assembled gate dielectrics can be incorporated into thin-film transistors with p-type organic semiconductors functioning at ultralow voltages (<-2 V) to achieve enhanced performance (hole mobility: 0.88 cm²/V·s, and Ion/Ioff: > 105, threshold voltage: 0.5 V).