Three novel materials (5CzICz, Cz₂ICz and Cz₃ICz), based on the indolo[3,2,1-jk]carbazole and 9H-carbazole building blocks, with high triplet energies (E_T > 2.80 eV) and good thermal stability (T_g > 101 °C) were synthesized, characterized and applied as host materials in PhOLED devices. In course of the preparation of the materials, an improved protocol for the synthesis of the indolo[3,2,1-jk]carbazole moiety has been developed. The careful molecular design of the title compounds allowed to avoid excimer formation of the indolo[3,2,1-jk]carbazole subunits in thin films. Therefore, the improved molecular design broadened the applicability of indolo[3,2,1-jk]carbazole host materials and significantly enhanced the efficiency of PhOLED devices based on these derivatives compared to previously reported indolo[3,2,1-jk]carbazole based compounds. Accordingly, employing the newly developed materials red (CE_max: 32.7 cd A⁻¹, PE_max: 31.0 lm W⁻¹, EQE_max: 20.4%), green (CE_max: 81.0 cd A⁻¹, PE_max: 87.4 lm W⁻¹, EQE_max: 21.5%) and blue (CE_max: 35.5 cd A⁻¹, PE_max: 39.9 lm W⁻¹, EQE_max: 18.0%) PhOLED devices with a remarkably low efficiency roll-off at 1000 cd m⁻² (Cz₂ICz – red: 5%; green: 0%; blue: 6%) were fabricated.