Density of States of OLED Host Materials from Thermally Stimulated Luminescence

Andrei Stankevych, Alexander Vakhnin, Denis Andrienko, Leanne Paterson, Jan Genoe, Ivan Fishchuk, Heinz Bässler, Anna Köhler, Andrey Kadashchuk

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

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 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(9 H-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.
Original languageEnglish (US)
JournalPhysical Review Applied
Volume15
Issue number4
DOIs
StatePublished - Apr 29 2021
Externally publishedYes

Bibliographical note

KAUST Repository Item: Exported on 2021-05-03
Acknowledged KAUST grant number(s): CRG
Acknowledgements: The authors acknowledge funding through the EU Marie Skłodowska-Curie ITN TADFlife grant (Grant No. 812872). This research is also supported by the European Research Council under ERC Grant No. 835133 (ULTRALUX), VW Foundation, and by the National Academy of
Science of Ukraine (Project No. VC/205) and NRFU Grant No. 2020.01/0144. The authors thank Rama Dhali from the University of Parma for calculation of the dipole moments of the materials studied in this paper. D.A. thanks the BMBF for an InterPhase grant (Grant No. FKZ 13N13661) and the European Union Horizon 2020 research and innovation program “Widening materials models” under Grant
No. 646259 (MOSTOPHOS). This research is supported by the King Abdullah University of Science and Technology (KAUST), via the Competitive Research Grants (CRG) Program. D.A. acknowledges KAUST for hosting his sabbatical.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.

ASJC Scopus subject areas

  • General Physics and Astronomy

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