Density of States of OLED Host Materials from Thermally Stimulated Luminescence
| Autor: | Jan Genoe, Heinz Bässler, Andrey Kadashchuk, A. Vakhnin, Andrei Stankevych, Ivan I. Fishchuk, Denis Andrienko, Anna Köhler, Leanne Paterson |
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| Rok vydání: | 2021 |
| Předmět: | |
| Zdroj: | Physical Review Applied |
| ISSN: | 2331-7019 0277-786X |
| DOI: | 10.1103/physrevapplied.15.044050 |
| Popis: | 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. ispartof: Physical Review Applied vol:15 issue:4 status: Published online |
| Databáze: | OpenAIRE |
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