New Insights into Tunable Volatility of Ionic Materials through Counter-Ion Control

Recent development in the field of small molecular materials has led to great advances in the performance of vacuum-evaporated organic light-emitting diodes. However, as a significant class of phosphorescent emitters, ionic transition metal complexes are seldom sublimable due to the inherent ionic nature and low vapor pressure, restricting their applications in state-of-the-art devices fabricated by vacuum evaporation deposition. Here a facile, feasible and versatile strategy is shown to tune the volatility of ionic transition metal complexes through counter-ion control. By introducing counter-ions with large steric hindrance and well-dispersed charges, a series of evaporable ionic iridium complexes are developed, and efficient vapor-processed devices with a high brightness, small efficiency roll-off, and polychromic emission ranging from deep-blue to red-orange are achieved. Our findings unlock the utilization of ionic functional materials in vacuum-evaporated devices, and may open new doors for modern electronic materials technology.