3.4. Charge transport rate and stability
The reorganization energy is closely related to the charge transport
rate. As can be seen from equation 1, the lower reorganization energy
values will promote a higher hopping rate. Of DTS and
derivatives 1-11 , the electron reorganization energies of the
hole are smaller than those of the hole. Actually, it is commonly found
that materials based on DTS compounds should be suitable for
the electron transport layer [1]. In addition, the values
λe of all studied compounds have emerged from Table 4,
which are lower than that of CBP , but this one is in contrast
with the case of λh. In further detail, the results make
a clear order when the compound 10 comprises the lowest value
λe of 0.278 eV, and better than compound 11(0.471 eV) and monomeric group DTS and 1-9(0.338-0.474 eV). In comparison with compound DTS and in
contrast to molecules 4 and 7-8 , λeenergies are scaled down in the case of 1-3 , 5-6 , and9 . Nevertheless, except for compound 5 (0.318 eV), all
of the remaining compounds fail to decrease their λhrate in comparison with that of DTS (0.399 eV). Agreement with
the mention above, the lager of λh of compounds3 , 6 , 8 , 9 , 10 can be
explained by the significant change of the position of SMe in the
neutral and cation forms. The calculation improves the influence of
phenyl units and also advocates for using pyridyl groups to stimulate a
better rate in the charge transport.
Ionization potential (IP) and electron affinity (EA) are likely two main
factors to estimate the hole and electron injection energy barriers of
OLED materials [2,21]. It appears that IP shows oxidation potent
and, in agreement with HOMO energy level, the EA corresponds to the LUMO
energy level which presents a reduction capacity of a molecule [2].
Materials with smaller IP behave as good hole transporters while
materials contain larger EA is compatible with electron transport
devices [18-20]. The data from Table 5 also reveals that in each
studied compound, the value of the vertical IPv is
always found to be larger than that of adiabatic IPa,
whereas the reverse trend is a fact in the case of EAvand EAa. The second evidence is to obtain, when
spontaneously compared with parent molecule DTS , derivatives1-11 possess smaller IPv/IPaand larger EAv/EAa values.
Significantly, the theoretically computed
IPv/IPa values of compounds 5and 10-11 are the lowest numbers, at 6.419/6.250, 6.122/5.795
and 6.162/5.716 eV, respectively. This is even lower than CBP’svalues at 6.577/6.540 eV. Similarly, the largest
EAv/EAa values belong to molecules5 (1.162/1.363 eV), 9 (1.000/1.198 eV), 10(1.388/1.546 eV) and 11 (1.344/1.623 eV), which successfully
demonstrated that the larger values than CBP (0.443/0.687 eV).
Consequently, dimeric DTS compounds (10 , 11 ) are
responsible for enhancing the hole and electron injection capacity in
OLED materials. Meanwhile, π-expansion which is performed by
substituting pyridyl units is a priority for charge injection enhance in
DTS monomer. The chemical hardness η is naturally used to estimate the
stability of the charge transport and luminescent materials [10].
This quantitative parameter has emerged as a measure of resistance to
charge transfer. Inspection Table 5 indicates that the chemical hardness
η values range from 1.399 to 2.022 eV. Due to having the lowest η,
compound 10 may be become a good candidate to liberate
electrons.