Effect of polar molecules on the transport and localization of charge carriers in molecular materials

Abstract

The in uence of polar species on the transport of charge carriers in molecular solids is reviewed. Guest molecules with permanent dipole moments introduced into molecular crystals can act as traps localizing carriers provided certain energetic conditions are ful lled. In this case their polar character may manifest itself in a eld-dependent decrease of the trap depth. Moreover, irrespective of the positions of their energy levels, polar impurities may locally modify the polarization energy thus creating traps on neighbouring molecules. The e ect of polar dopants in disordered molecular solids can be described within the model due to B assler: the presence of polar species broadens the density-of-states function thus decreasing the e ective mobility o f c harge carriers.

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

“…A polar impurity molecule leads to an electric field dependent trap depth though. [45] Even if a polar impurity does not lead to a positive trap depth, its dipole moment modifies the local value of the polarization energy since we have highly polarizable π-orbitals in organic semiconductors. This results in traps in the vicinity of the water molecules.…”

“…[45,46] The net effect is a significant broadening of the trap DOS at the insulator-semiconductor interface. [45] 3. Dielectric constant of the gate dielectric It has been suggested that the polarity of the gate dielectric surface impedes the charge transport as described in the following.…”

The impact of 1 wt% of Ir(ppy) 3 on trapping sites and radiative recombination centres in PVK and PVK/PBD blend seen by thermoluminescence

“…TL spectra, registered in temperature range 15-320 K, reveal that trapping sites 31 localised on the matrices and on Ir(ppy) 3 molecules exist in both investigated systems. The traps localised 32 on Ir(ppy) 3 have depth about 0.4 eV and they dominate in the doped PVK. At the same concentration of 33 the dopant molecules in the PVK/PBD matrix, the traps located on the matrix dominated.…”

Blue iridium complexes as electron trapping sites and efficient recombination centres in poly(N-vinylcarbazole) seen by spectrally resolved thermoluminescence

“…Thus, the energy of some sites of PVK matrix localised in neighbourhood of the dopant can be lowered and these sites might work as slightly deeper traps as compared with those in undoped PVK. It should be underlined that the presence of polar additives has an influence on the transport process in disordered molecular solids and such effects were reported and analysed by many research groups [25][26][27][28]. The effect of dipolar disorder was also studied by using the TL method [29,30].…”