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.