Charge trapping for controllable persistent luminescence in organics

Persistent luminescence, long-lived emission from inorganic or organic materials after the cessation of excitation, receives considerable attention in the field of optoelectronics. Despite great achievements in the past decades, the performance of organic materials still lags behind their inorganic counterparts, which have thousands of years of history. This is largely caused by the limited understanding of the mechanisms involved in organic materials. Here we report trap-induced persistent luminescence (TIP) in organic host-guest materials, with controllable trap depths from 0.11 to 0.56 eV and tunable afterglow emission at wavelengths from 507 to 669 nm via energy level engineering. The TIP phenomenon in a typical TN@TPBi film lasts for more than 24 h, with additional energy stored at room temperature for over 1 week. It is found that the trap depth in TIP is probably determined by the energy gap between the lowest unoccupied molecular orbitals of the radical anions of the host and guest molecules, matching well with density functional theory calculations. TIP was also observed after electrical excitation, demonstrating the potential of exploiting the semiconductor features of the organic hosts. These results provide a fundamental principle to design metal-free organic emitters of persistent luminescence, thereby expanding their applications in fields such as medical delivery identification, semiconductor devices, and imaging techniques.