Pure Hydrocarbon‐Based Polycyclic Aromatic Hydrocarbon Hosts Enable Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

Pure hydrocarbon (PHC) molecules, composed exclusively of carbon and hydrogen, exhibit excellent stability and hold promising potential for applications in optoelectronic devices. Herein, a series of PHC host materials are proposed, namely ( R )‐4′“”‐(9,9′‐spirobi[fluoren]‐2‐yl)‐2,4′“‐bi(9,9′‐spirobi[fluorene]) (SP‐2), ( R )‐4′”“‐(9,9′‐spirobi[fluoren]‐3‐yl)‐3,4′”‐bi(9,9′‐spirobi[fluorene]) (SP‐3), and ( R )‐4′‐(9,9′‐spirobi[fluoren]‐4‐yl)‐4,4′'‐bi(9,9′‐spirobi[fluorene]) (SP‐4), based on the parent molecule spirobifluorene (SBF), designed through different connection sites to achieve varying degrees of molecular skeleton distortion. The resulting isomers exhibit high singlet and triplet energy levels, a wide HOMO/LUMO energy gap, and favorable thermal stability. These outstanding properties strongly support their potential as host materials for phosphorescent organic light‐emitting diodes (PhOLEDs). As a result, green PhOLEDs based on SP‐2 achieved a high external quantum efficiency (EQE) of 24.7% with minimal efficiency roll‐off, which ranks among the best EL performances reported for PhOLEDs based on pure hydrocarbon host materials. This demonstrates that tailoring the degree of conjugation and three‐dimensionality through different linkage sites is effective in creating host materials that successfully balance charge transport and exciton confinement.