Role of a trap in disordered OLED host-guest systems

Extrinsic traps created by dopants or impurities are ubiquitous in organic semiconductors and can critically influence charge transport. Here we report a comprehensive study, using low-temperature thermally stimulated luminescence measurements complemented by quantum mechanics and molecular dynamics (QM-MD) calculations, as well as kinetic Monte Carlo (KMC) simulations, to investigate trapping and energetic relaxation of charge carriers in amorphous host-guest systems containing the widely investigated thermally activated delayed fluorescence (TADF) emitter 10-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9,9-dimethyl-9,10-dihydro-acridine (DMAC-TRZ) in two different hosts. DMAC-TRZ guest acts as a shallow trap in 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) and a deep trap in 1,3-bis (triphenylsilyl) benzene (UGH3), each forming an additional offset Gaussian density of states (DOS) distribution, as supported by the QM-MD calculations. These calculations accurately reproduce the trap depths ε t and widths of the DOS distributions σ DOS in the considered compositions, showing that the nominal trap depth is independent of the host-guest ratio. By experiment and KMC simulations we found that the trapping behavior differs significantly for shallow ( ε t ≤ 3 σ DOS ) and deep ( ε t > 3 σ DOS ) cases. Shallow traps do not behave as distinct traps but rather broaden the host DOS and increase the density of low-energy states, causing the average activation energy to depend on guest concentration c t , in line with the “effective disorder” concept. By contrast, when a trap is deep, the energy needed to release the trapped charges to the host is independent of the guest concentration. Furthermore, for deep traps, we observe that charge detrapping occurs simultaneously via both guest-to-host and guest-to-guest charge-transfer pathways, with the latter regime becoming dominant for concentrations exceeding c t = 5%. Remarkably, this guest-guest transfer already sets in at only 1% trap concentration. We attribute this to local guest clustering and superexchange-mediated intercluster transfer.