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Density of States of OLED Host Materials from Thermally Stimulated Luminescence

Title data

Stankevych, Andrei ; Vakhnin, Alexander ; Andrienko, Denis ; Paterson, Leanne ; Genoe, Jan ; Fishchuk, Ivan ; Bässler, Heinz ; Köhler, Anna ; Kadashchuk, Andrey:
Density of States of OLED Host Materials from Thermally Stimulated Luminescence.
In: Physical Review Applied. Vol. 15 (April 2021) Issue 4 . - No. 044050.
ISSN 2331-7019
DOI: https://doi.org/10.1103/PhysRevApplied.15.044050

Abstract in another language

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(9H-carbazol-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.

Further data

Item Type: Article in a journal
Refereed: Yes
Institutions of the University: Faculties > Faculty of Mathematics, Physics und Computer Science > Department of Physics
Faculties > Faculty of Mathematics, Physics und Computer Science > Department of Physics > Chair Experimental Physics II - Optoelectronics of Soft Matter
Faculties > Faculty of Mathematics, Physics und Computer Science > Department of Physics > Chair Experimental Physics II - Optoelectronics of Soft Matter > Chair Experimental Physics II - Optoelectronics of Soft Matter - Univ.-Prof. Dr. Anna Köhler
Result of work at the UBT: Yes
DDC Subjects: 500 Science > 530 Physics
Date Deposited: 06 May 2021 07:10
Last Modified: 06 May 2021 07:10
URI: https://eref.uni-bayreuth.de/id/eprint/65103