Density of States of OLED Host Materials from Thermally Stimulated Luminescence

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 apply the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine
the DOS of pristine amorphous films of organic light-emitting diode (OLED) host materials.
The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl derivatives, 4,4-
bis(N-carbazolyl)-1,1-biphenyl (CBP), 3,3-di(9H-carbazol-9-yl)-1,1-biphenyl (mCBP), and 3,5-di(9Hcarbazol-
9-yl)-[1,1-biphenyl]-3-carbonitrile (mCBP-CN), and (ii) carbazole-phenyl (CP) derivatives,
1,3-bis(N-carbazolyl)benzene (mCP) and 9-[3-(9H-carbazol-9-yl)phenyl]-9H-carbazole-3-carbonitrile
(mCP-CN). TSL originates from radiative recombination of charge carriers 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 D, 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 with
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 materialsThe 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 apply the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine
the DOS of pristine amorphous films of organic light-emitting diode (OLED) host materials.
The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl derivatives, 4,4-
bis(N-carbazolyl)-1,1-biphenyl (CBP), 3,3-di(9H-carbazol-9-yl)-1,1-biphenyl (mCBP), and 3,5-di(9Hcarbazol-
9-yl)-[1,1-biphenyl]-3-carbonitrile (mCBP-CN), and (ii) carbazole-phenyl (CP) derivatives,
1,3-bis(N-carbazolyl)benzene (mCP) and 9-[3-(9H-carbazol-9-yl)phenyl]-9H-carbazole-3-carbonitrile
(mCP-CN). TSL originates from radiative recombination of charge carriers 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 D, 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 with
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