Lehrstuhl EP2 Uni Bayreuth

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 𝜀𝑡
and widths of the DOS distributions 𝜎DOS
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 (𝜀𝑡
 ≤ 3⁢𝜎DOS
) and deep (𝜀𝑡
 > 3⁢𝜎DOS
) 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 𝑐𝑡
, 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 𝑐𝑡
 = 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.

Advances in the development of organic field-effect transistors (OFETs), electrically gated organic semiconductors (EGOFETs), and organic electrochemical transistors (OECTs) allow for the operation of these devices at very high charge-carrier densities, where Coulomb interactions between carriers can be expected to become significant. We have studied the effects of such Coulomb interactions in an OFET-like structure using Kinetic Monte Carlo (KMC) simulations. Compared to an analogous structure where carrier-carrier interactions are neglected, we find a reduction in carrier mobility and an increase in activation energy. This effect increases with increasing gate voltage, i.e., charge density. We associate this with the emergence of a different transport regime where correlated transport prevails and where a Coulomb gap appears in a dynamic density of states (DOS), consistent with previous work. We demonstrate that at these high densities, the charges in the organic semiconductor behave like those in a Coulomb glass. In this context, the activation energy for transport is reinterpreted to relate to the structural reorganization of the carrier ensemble. Unlike in inorganic semiconductors, for the organic semiconductor system, we find the appearance of a Coulomb gap to occur even at ambient temperature, and it does not require variable-range hopping.

Quantum wells made of quasi two-dimensional organic–inorganic hybrid perovskites (2D-PKs) offer a high degree of flexibility in tailoring optoelectronic properties through carrier confinement and functional interlayers. Compared to their 3D counterparts, 2D-PKs exhibit tunable photoluminescence, excitonic binding at room temperature, and enhanced structural stability. However, the dynamics of photoinduced charge carriers and their transport properties […]