Density of States of OLED Host Materials from Thermally Stimulated Luminescence

Abstract

The electronic density-of-states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors, while its accurate determination is still a challenging task. We have applied the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of OLED host materials. The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl (CBP) derivatives: CBP, mCBP, and mCBP-CN, and (ii) carbazolephenyl (CP) derivatives: mCP and mCP-CN. TSL originates from radiative recombination of chargecarriers thermally released from the lower energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution, with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments, varying from 0 to 6 Debye, in a similar manner within each series, in line with the dipolar disorder model. The same molecular dipole moment, however, leads to a broader DOS of CP compared to CBP derivatives. Using computer simulations, we attribute the difference between the series to a smaller polarizability of cations in CP-derivatives, leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.

New organic dye-sensitized solar cells based on the D–A–π–A structure for efficient DSSCs: DFT/TD-DFT investigations

“…Numerous studies have shown that examination of the density of states can explain the charge-transfer characteristics of organic semiconductors. 108 Indeed, it also provides the contribution of the MOs of each molecule's different components to the overall system. The GaussSum can be used to determine the density of states (DOS) function.…”

On Analytical Modeling of Hopping Transport of Charge Carriers and Excitations in Materials with Correlated Disorder

“…The agreement is good, especially at moderate field strengths. Equation 21 gives the correct value and field dependence of hole mobility in nonpolar organic material 4,4′-bis( N -carbazolyl)-1,1′-biphenyl (CBP), as measured in previous work, 30 providing the independently determined 31 values σ = 0.125 eV and μ 0 = 0.8 cm 2 V –1 s –1 . We obtain the inverse localization radius γ = 2.5/ a 0 from fitting; hence, ν 0 = 10 15 s –1 from eq 21 .…”

“…The proposed approach has been tested by comparison with the known results of MC modeling of charge carrier mobility in organic quadrupole glasses (where a similar type of correlation exists). Using the same approach, an analytic model of the diffusion coefficient and mobility for quadrupole glasses has been developed and justified by comparison with the experimental data. , …”

Charge transport in the spatially correlated exponential random energy landscape: effect of the nonpositive correlation function

“…47 Recently the shape of the DOS in several organic glasses serving as the popular charge transporting materials has been studied. 48 In two materials with molecules having low dipole moments (i.e., the very type of the materials where we can expect the dominant quadrupolar contribution to the DOS) the exponential tails of the DOS have been observed, while for materials with molecules having high dipole moments the DOS has the Gaussian shape. Some degree of the local order is natural for the arrangement of spacious and highly asymmetric organic molecules, thus the development of the exponential tails seems to be a common phenomenon.…”