Polymer light-emitting diodes (PLEDs) based on thermally activated delayed fluorescence (TADF) emitters show great potential in developing high-efficiency solution-processed light-emitting devices without the use of noble metal complexes. However, a key challenge for the development of TADF-PLEDs so far is the lack of polymer hosts with suitable triplet energy levels (E(T)s) and good carrier transport capability. Here, we report the design, synthesis, and electroluminescent properties of a novel series of bipolar poly(arylene phosphine oxide) hosts based on electron-transporting arylphosphine oxide and hole-transporting carbazole units, which show widely tunable E(T)s in the range of 2.20-3.01 eV by finely tuning the conjugation extent of the polymer backbone. The tunable E(T)s make these polymers a universal host family for all of the blue, green, and red TADF emitters. TADF-PLEDs based on these polymer hosts show promising device efficiency with external quantum efficiencies up to 15.8, 17.1, and 10.1% for blue, green, and red emissions, respectively, which are among the highest efficiencies for TADF-PLEDs. These results open an avenue for the development of TADF-PLEDs with high efficiency and full color emission in the future.