Applied Science and Convergence Technology 2020; 29(6): 149-153
Published online November 30, 2020
Copyright © The Korean Vacuum Society.
Department of Physics and Institute for Accelerator Science, Kangwon National University, Chuncheon 24341, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
Atomic substitution in an organic semiconductor alters the intermolecular and intramolecular interactions in the solid state. Thus, an understanding of such electronic interactions is crucial in designing an efficient molecule. In this study, the effect of the substitution of two hydrogen atoms in the tetracene core to sulfur in benzopyrazine-fused tetracene was investigated via theoretical methods. The reorganization energy and electronic coupling of 5,6-substituted benzopyrazine-fused tetracene and its 5,6,11,12-substituted disulfide were calculated using density functional theory. By sulfur substitution, the hole reorganization energy increases but the electron reorganization energy decreases. Meanwhile, the highest values of both hole and electron electronic couplings decrease. As a result, the calculated hole mobility decreased significantly. However, the calculated electron mobility increased slightly, indicating that the charge transport type changes from p-type to ambipolar. These results indicate that atomic substitution significantly affects the charge transport ability of organic semiconductors.
Keywords: Density functional theory, Reorganization energy, Electronic coupling, Organic semiconductor
Organic semiconductors are promising materials for a wide range of device applications, including organic light-emitting diodes, organic photovoltaics, and organic field-effect transistors (OFETs) . One of the major advantages of organic semiconductors is their mechanical flexibility . The carrier mobility (
In general, organic semiconductors comprise an expanded structure of aromatic rings such as polyacenes . One of the representative polyacenes is tetracene, which has four aromatic rings. The electronic properties of tetracene can be adjusted by attaching additional functional groups or via the atomic substitution of hydrogen atoms . Such modified materials are beneficial for electronic devices. For example, rubrene, which is a tetracene-derivative with four phenyl groups, exhibits excellent hole mobility (
Recently, benzopyrazine-fused tetracene derivatives have been suggested as efficient materials for OFETs . The chemical structures of 5,6-substituted benzopyrazine-fused tetracene (TBPy) and its 5,6, 11,12-substituted disulfide (TBPyS) are shown in Fig. 1. By the substitution of two hydrogen atoms in the tetracene core to sulfur, the crystal structure changed significantly. Consequently, the
In this study, the charge transport properties of TBPy and TBPyS were investigated via theoretical calculations. The reorganization energy (
The theoretical estimation of μ in organic solids based on the Marcus theory has been widely investigated in recent decades. The detailed theory and method are described in our previous reports and in the literature [14–18]. Briefly, the μ of organic solids can be estimated using the Einstein relation as follows:
λ can be evaluated using four single-point energies as follows:
DFT calculations of single-point energy and geometry optimization were performed with a Becke-style three-parameter exchange and the Lee–Yang–Parr correlation (B3LYP) hybrid functional [19– 21], whereas that of
Figure 2 shows (a) the calculated Kohn–Sham energy levels of the LUMO and HOMO and (b) the electrostatic potential surfaces of TBPy and TBPyS. In Fig. 2(a), the molecular orbitals of TBPy and TBPyS are shown. The LUMO and HOMO shapes of TBPy and TBPyS were similar, but some contributions from the substituted sulfur atoms were observed. In TBPy, the LUMO and HOMO levels were evaluated to be ˗2.55 and ˗5.02 eV, respectively. However, in TBPyS, the LUMO and HOMO levels were evaluated to be ˗2.63 and ˗4.82 eV, respectively. Hence, the fundamental gap decreased from 2.47 to 2.19 eV by the substitution of the two hydrogen atoms to sulfur. This can be explained by the broader distribution of wave functions on the molecule. To evaluate the contributions of the sulfur atoms to the LUMO and HOMO, molecular orbital coefficients were calculated. In TBPy, no contribution (0%) of the two hydrogen atoms in the tetracene core was discovered. However, in TBPyS, the two sulfur atoms contributed to the LUMO and HOMO by 16.4% and 23.3%, respectively. Hence, the sulfur substitution significantly affected the frontier orbitals that contributed primarily to charge transport. From the perspective of hole injection in OFETs, the energy barrier between the Fermi level of metal electrodes and the HOMO level of an organic semiconductor significant affects the device performance . Considering other p-type organic semiconductors, their HOMO levels calculated at the same level [B3LYP/6-31G(d)] were ˗4.86 eV for tetracene, ˗4.67 eV for rubrene, and ˗4.60 eV for pentacene. Hence, the HOMO level of TBPyS was comparably shallow, but that of TBPy was deeper. This can imply that the formation of an Ohmic contact of TBPy with electrodes may be more difficult than that of TBPyS. Hence, an interlayer with a high work function (e.g., molybdenum trioxide ) would be necessary.
As shown in Fig. 2(b), the two hydrogen atoms in TBPy were more positively charged than the two sulfur atoms in TBPyS. To quantitatively evaluate the charge distribution, Mulliken charge analysis was performed for the neutral and charged states. The results indicate that the charge on the hydrogen atom in the neutral, cation, and anion states of TBPy were 0.134684e, 0.181382e, and 0.08363e, respectively. Hence, the change in charge between the cation and neutral states was 0.046698e, whereas that between the anion and neutral states was ˗0.051320e. Meanwhile, the charges on the sulfur atom in the neutral, cation, and anion states of TBPyS were 0.124066e, 0.235474e, and 0.027316e, respectively. Therefore, the change in charge between the cation and neutral states was 0.111408e, whereas that between the anion and neutral states was ˗0.096750e. Accordingly, the changes in charge between the neutral and charged states of TBPyS were larger than those of TBPy. This is attributable to the higher contributions of the sulfur atoms to the LUMO and HOMO.
Table I shows the calculated hole reorganization energy (λh) and electron reorganization energy (
The possible hopping PWs of TBPy and TBPyS in the crystal structure are shown in Fig. 3 and related parameters are presented in Tables II and III. The reference molecule is colored in red, whereas PWs with nearest neighbor molecules are numbered. The crystal structures were obtained from the crystallographic information file in the literature .
Meanwhile, in TBPyS, the PWs with the shortest
Using the calculated charge transport parameters, the theoretical
In this study, the effect of the substitution of hydrogen atoms in the t tracene core to sulfur was investigated via theoretical calculations. By sulfur substitution,
This study was supported by the National Research Foundation of Korea (NRF-2018R1D1A1B07051050 and 2018R1A6A1A03025582).