A spiro-configured bipolar molecule (CSC) that possesses high triplet energy, suitable energy levels, and balanced ambipolar carrier mobilities was successfully applied as an efficient host material, compatible with various iridium-based green phosphors, to realize highly efficient single-layer PhOLEDs of a maximum external quantum efficiency up to 8.3% (31.4 cd A(-1)) at a practical brightness of 1000 cd m(-2) (8 V).