Applied Science and Convergence Technology 2018; 27(6): 161-165
Published online November 30, 2018
Copyright © The Korean Vacuum Society.
Department of Physics, Kangwon National University, Gangwon-do 24341, Republic of Korea
Correspondence to:*Corresponding author: E-mail: email@example.com
Organic light-emitting diodes (OLEDs) have received much attention for application the in next-generation display due to their many advantages. To increase the device performance of OLEDs, a host-dopant system in the emission layer has been used. In blue OLEDs, anthracene-based materials have been used as a host material. To understand the device behavior of OLEDs, a fundamental study on the electronic properties of organic semiconductors is necessary. In this study, theoretical calculations using density functional theory were performed to investigate the electronic structure and charge transport ability of 9,10-diphenyl-2-((3-trifluoromethyl)phenyl)anthracene (ATFP-Ph), 9,10-di([1,1′-biphenyl]-4-(trifluoromethyl)phenyl)anthracene (ATFP-BiPh), and 9,10-di(naphthalene-2-yl)-2-(3-(trifluoromethyl)phenyl)anthracene (ATFP-Naph). All molecules have similar lowest unoccupied molecular orbital and highest occupied molecular orbital energy levels, and thus their charge injection abilities from the adjacent layer to the host material are similar. However, ATFP-Ph has significantly lower hole and electron reorganization energy (
Keywords: Organic light-emitting diode, Reorganization energy, Density functional theory, Host material
Organic light-emitting diodes (OLEDs) have received great attention due to their wide angle viewing, thin thickness, high contrast, and mechanical flexibility . Since the first report of a multilayer structure of OLEDs by Tang , device performance of OLEDs has been significantly improved, and now they utilized in the display for televisions and smartphones. To increase the quantum efficiency, a host-dopant system is used for light emission. The holes and electrons are transported to the host material, then generated excitons transfer to the dopant material to emit light with the desired color.
Well-aligned energy levels with adjacent layers and high and ambipolar charge mobility are required for an efficient host material . Anthracene-based materials are widely used [e.g. 9,10-bis(2-naphthyl)anthracene (ADN)] as a host material . Recently, 9,10-diphenyl-2-((3-trifluoromethyl) phenyl)anthracene (ATFP-Ph), 9,10-di([1,1′-biphenyl]-4-(trifluoromethyl)phenyl)anthracene (ATFP-BiPh), and 9,10-di(naphthalene-2-yl)-2-(3(trifluoromethyl)phenyl) anthracene (ATFP-Naph) were synthesized, and OLEDs were characterized using these host materials and a 4,4′-bis[4-(di-p-tolylamino)styryl] biphenyl (DPAVBi) dopant material . Among these, OLEDs with ATFP-Ph showed much higher device performance than OLEDs with ATFP-BiPh and ATFP-Naph. This could be attributed to the superior charge transport characteristics of ATFP-Ph over ATFP-BiPh and ATFP-Naph. However, the origin of different device performance is still not well understood.
In this study, we investigated the electronic structure and charge transport ability of ATFP-Ph, ATFP-BiPh, and ATFP-Naph host materials using density functional theory (DFT) calculations. Frontier energy levels, molecular orbital, reorganization energy (
DFT calculations were performed at a single molecule of ATFP-Ph, ATFP-BiPh, and ATFP-Naph. Because organic semiconductors interact with each other with weak van der Waals type forces, the intermolecular coupling is small and thus the electronic structure of a single molecule well approximates that of a film . In this study, a hybrid functional of Becke three parameters exchange and Lee-Yang-Parr correlation (B3LYP) and a 6–31G(d,p) basis set implemented in a GAUSSIAN 09 package were used to calculate the total energy, geometry optimization, and vibrational frequency calculations [7–10].
Hopping is the major mechanism of charge transport in organic semiconductors during device operation. In this case, the semi-classical Marcus theory can be applied to explain the charge transport process . The charge hopping rate
Figure 1 shows the optimized geometry of ATFP-Ph, ATFP-BiPh, and ATFP-Naph derived from DFT calculations. Anthracene and trifluoromethyl-phenyl moieties are common in the three molecules, but the side groups vary as phenyl, biphenyl, and naphthyl moieties. The electron-withdrawing property of trifluoromethyl-methyl moiety lowers the energy levels to match the charge transport levels of the adjacent layers in OLEDs. The side groups are indicated with the red arrow and the chemical formula is shown in the inset. These side groups are flexibly connected with the anthracene moiety, and thus the geometry of side groups can be easily changed during charge transfer.
Figure 2(a) shows the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of ATFP-Ph, ATFP-BiPh, and ATFP-Naph molecules determined by DFT calculations. The calculated LUMO and HOMO levels of the three molecules are almost the same. The LUMO and HOMO levels of ATFP-Ph are −1.85 and −5.22 eV, the values of ATFP-BiPh are −1.87 and −5.23 eV, and the values of ATFP-Naph are −1.86 and −5.23 eV, respectively. This result is in good agreement with the values measured by cyclic voltammetry and UV-vis absorption reported in the literature . Figure 2(b) shows the LUMO and HOMO distribution of ATFP-Ph, ATFP-BiPh, and ATFP-Naph. Both LUMO and HOMO mainly originate from the anthracene and trifluoromethyl-phenyl moieties. The side phenyl, biphenyl, and naphthyl moieties contribute little to the LUMO and HOMO. Considering that, all molecules have similar LUMO and HOMO levels. In OLEDs, electrons are transported from the electron transport layer (ETL) and holes are transported from the hole transport layer (HTL) to the host layer. For efficient charge transport, the energetic offset between the HOMO levels of HTL/host and the LUMO levels of ETL/host should be minimized. In these host materials, the LUMO and HOMO levels are almost the same, and the charge injection abilities are similar. Therefore, the difference in device performance cannot be attributed to energy level differences.
To examine the
To understand the origin of different
To analyze the charge distribution in detail, Mulliken charge analysis was performed. Since the anthracene and trifluoromethyl-phenyl moieties are common in the molecules, only charge distributions in phenyl, biphenyl, and naphtyl moieties were compared.
In this study, the electronic structure and charge transport properties of anthracene-based host materials, ATFP-Ph, ATFP-BiPh, and ATFP-Naph, were investigated using DFT calculations. All molecules have similar LUMO and HOMO levels, and thus charge injection to these host materials would be similar in OLEDs. However, both hole and electron
This study was supported by National Research Foundation of Korea (NRF-2018R1D1A1B07051050).
Mulliken charge analysis of a phenyl moiety in ATFP-Ph, biphenyl moiety in ATFP-BiPh, and naphthyl moiety in ATFP-Naph.