Introduction

Since the first report by Tang and Van Slyke1 the organic light emitting-diode (OLED) technology has been greatly developed and become a huge mainstream in the display industry since its excellent merits for color purity, wide view angle, contrast, flexibility, and brightness takes remarkable advantages to manufacture of the electronic devices, which are closely related to our daily life2,3,4,5. In the early stage, the device architecture of OLED has basically consisted of three organic layers, known as hole transport layer, electron transport layer, and emissive layer. However, the simple device architecture cannot meet the minimal criteria of the required efficiency and durability to commercialize in the display industry. To overcome the critical issue, much effort had been attempted to improve the efficiency and durability in terms of developing the material properties and evolving the device architecture6,7,8,9,10,11,12,13,14,15,16,17. In the point of device architecture view, it is worthying noticed from the device architecture that the hole/electron injection layer (HIL/EIL) and hole/electron buffer layers (BL) were additionally adopted to mainly manipulate the balance of the hole/electron carriers and suppression of the hole/electron leakage. In addition, the pivotal materials properties such as mobility, photoluminescent quantum yield (PLQY), and internal quantum efficiency (IQE) were intensely studied to achieve the meet of the commercial standardization of efficiency and durability in the display industry. Since then, the tremendous endeavors to engineer the new organic/organometallic materials for HIL, HTL, ETL, EML, and BL have been intensely investigated and reported until now. However, designing a new material based on lithium-quinolin-8-olate (Liq)18 known as EIL, is rarely investigated and reported. In the industry and academy, Liq is typically co-deposited with ETM to manipulate the amount of electron carrier. The role of Liq is to manipulate the amount of electron carriers in terms of the injection and transport. In detail, the LUMO energy level of Liq, which is closely aligned with the work-function of cathode, aids the spontaneous injection of the electron to increase the probability of exciton recombination in EML. Moreover, the slow electron mobility of Liq plays a key role in mitigating the significant activation of the exciton-polaron quenching. Therefore, Liq is understood as a one of important components in influencing the efficiency and lifetime of OLED device. In this perspective, we assure that searching the new materials modified from Liq is also required to contribute to the enhancement of the efficiency and lifetime in OLED device. Basically, the structural feature of Liq is that the Li+ interacts with the O− and N atoms through the ionic and coordination bonding. Based on the molecular structure of Liq, the easiest design strategy is to replace the Li+ by other singly oxidized metal ions (M+). On the basis of mentioned strategy, we previously investigated the effect of different M+ on the electronic and charge transport properties of M+-quinolin-8-olate (Mq) and M2+-(quinolin-8-olate)2 (Mq2) through the density functional theory (DFT) calculation19,20. In addition, there are several attempts to modify quinolate moiety of Liq. For example, the lithium 2-methyl 8-hydroxyquiolinate (MeLiq) and lithium 2-phenyl 8-hydroxyquinoliate (PhLiq) were synthesized to understand the steric bulk effect on the aggregation behavior21. In 2008, the theoretical study for CN-substituted Liq (5-CN-Liq) was reported to gain the insight of the role of CN substituent on electronic and photophysical properties22. Despite the considering the previous reports, there is still unexplored design space in quinoline-8-olate, where the O atom can be replaced by S and Se atoms. In addition to this, the effect of the M+ ions on the quinoline-8-thiolate (Sq−) and quinoline-8-selenolate (Seq−) should be further studied to revisit the role the different M+ ion on the electronic and charge transport properties in M+-Sq− (MSq) and M+-Seq− (MSeq) complexes. Similar to the previous report20Na+, K+, Rb+, Cs+, Ag+, and Au+ ions were representatively chosen as M+.

In this study, we mainly investigated the electronic and charge transport properties of M+-QS− and M+-QSe− complexes. Moreover, the complexation behavior with ETL and absorption properties were considered to understand the disadvantageous effect on the charge transport and outcoupling efficiency for primary colors in the OLED device. To the best of our knowledge, we expect that this study will help to extend the design space as well as to gain the pivotal insight for finding new EIL materials.

Theory and computation

Density functional theory (DFT) calculations were carried out by employing a non-local density functional of Becke’s three parameterized Lee-Yang-Parr exchange functional (B3LYP)23,24,25 with Pople’s triple zeta potential with double polarization function (6-311G**) for C, H, N, O, S, Se, Na, and K atoms and a Hay-Watt effective core potential (LANL2DZ) for Rb, Cs, Ag, and Au atoms, as implemented in the suite of Gaussian 16 program26. All molecular structures in neutral, cation, and anion state were fully optimized without symmetry constrains and their thermodynamic stabilities were verified by frequency calculations. According to the Marcus electron transfer theory27 the charge transport properties have been known to be critically influenced by reorganization energy (λh or λe) and transfer integral (th or te). In detail, hoping rates of hole and electron carriers are inversely relying on the λh or λe whereas those are proportionally depending on th or te. The λh or λe can be generally derived from Nelson’s four-point method by following equation28.

$$\:IP=\:{E}_{cation}-{E}_{neutral}\:$$
(1)
$$\:\:EA=\:{E}_{neutral}-{E}_{anion}$$
(2)
$$\:{\upeta\:}=\:\frac{(IP-EA)}{2}$$
(3)
$$\:{\upchi\:}=\:\frac{(IP+EA)}{2}$$
(4)
$$\:{\:\:\:\lambda\:}_{h}=\left({\lambda\:}_{neutral}^{cation}-{\lambda\:}_{cation}\right)+\left({\lambda\:}_{cation}^{neutral}-{\lambda\:}_{neutral}\right)$$
(5)
$$\:{\:\lambda\:}_{e}=\left({\lambda\:}_{neutal}^{anion}-{\lambda\:}_{anion}\right)+\left({\lambda\:}_{anion}^{neutral}-{\lambda\:}_{neutral}\right)$$
(6)

Transfer integral implies the transfer probability of the electron and hole carriers between two neighboring molecules. This can be computed by a certain degree of wavefunction overlap between two adjacent molecules. Therefore, the non-covalently interacted dimer structures must be considered to gain the transfer integral of hole and electron (th and te). Unfortunately, the B3LYP functional lacks the non-covalent interaction parameter, hence it is inadequate to describe the non-covalent interaction. Accordingly, Grimme’s dispersion correction with Bekes–Johnson damping (GD3BJ) was additionally utilized to describe the non-covalent interaction of pair structure29. The energy splitting by intermolecular polarization effect was further considered to obtain transfer integral. The th and te can be expressed by following equation30.

$$\:{t}^{eff}=\frac{{t}_{12-}\frac{1}{2}\left({e}_{1}+{e}_{2}\right){S}_{12}}{1-{S}_{12}^{2}},$$
(7)

where t12, e1, and e2 are matrix elements for system Hamiltonian. S12 is the spatial electron density overlap integral between two the highest occupied molecular orbitals (HOMOs) or the LUMOs. The field-independent carrier hopping rates were derived by Marcus formalism as following Eq. (5).

$$\:{k}_{ET}=\:\frac{4{\pi\:}^{2}{t}^{2}}{h}\sqrt{\frac{1}{4\pi\:\lambda\:{k}_{B}T}}exp\left(-\frac{{\left(\lambda\:+\varDelta\:G\right)}^{2}}{4\lambda\:{k}_{B}T}\right),$$
(8)

where t, λ, kB, h, T, and ΔG are defined as transfer integral, reorganization energy, Boltzmann constant, Plank constant, temperature, and Gibbs free energy, respectively.

The time dependent DFT (TDDFT) calculations were conducted to gain the absorption energies of these complexes.

Results and discussion

The role of S and Se atom in Li-based complex

To investigate the role of S and Se atoms on the electronic and charge transport property, we designed and optimized Liq, LiSq, and LiSeq. The optimized structures were depicted in Fig. 1. There are no remarkable changes in structural planarity. The structural parameters and electronic properties of Liq, LiSq, and LiSeq are listed in Table 1. Compared to Liq, the distances between S/Se and Li+ are elongated, but the distance between N and Li+ are slightly shortened. In the electronic properties, the highest occupied molecular orbital (HOMO) energy of LiSq is slightly shifted to deep level, but that of LiSeq is significantly shifted to shallow level. On the other hand, the energy level of the lowest unoccupied molecular orbital (LUMO) is gradually stabilized in the presence of the S and Se atoms. In terms of the electron injection, S and Se atoms play a beneficial role in minimizing the electron injection barrier between EIL and work-function of Al cathode (Φ = 4.2 eV). This is due to the more stabilized LUMO energy level. Moreover, chemical hardness (η) of LiSq and LiSeq is smaller than that of Liq, indicating that the LiSq and LiSeq are relatively unstable during the redox reaction. In other words, introducing S and Se atoms may play a beneficial role in activating the oxidation and reduction process. As shown in Fig. 1, the spatial distributions of HOMO and LUMOs densely spread on the organic moiety in all complexes. However, it is noticed that LiSq and LiSeq show more localized electron density in HOMO. In OLED device, the mobility, which is known as decision factor for concentration of hole and electron carriers into EML, plays an important role in determining the J-V characteristic and power efficiency. According to the Einstein relation equation31, the interrelationship between mobility and hopping rate is proportional. As we mentioned in the computational details, the hopping rate is proportional to the transfer integral and inversely proportional to the reorganization energy, according to the Marcus formalism. Comprehensively considering these relationships, it can be understood that hopping rate is one of important parameter to determine the OLED device performance. The computed parameters for charge transport property are listed in Table 2. The λh values of LiSq and LiSeq are tremendously decreased, but their λe values are significantly increased. This result indicates that the S and Se atoms play a beneficial role in suppressing the structural relaxation during the oxidation process. In contrast to this, these atoms are ineffective to reduce the structural relaxation during the reduction process. The t2h and t2h values were computed from dimer structures. The t2h of LiSq is much smaller, but that of LiSeq is slightly larger than that of Liq. On the other hand, the t2e of LiSq is much larger, but that of LiSeq is tremendously.

Fig. 1
Fig. 1
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The optimized molecular structure of MQ, MSQ, and MSeQ complexes.

Table 1 The structural, electronic, and charge transport parameters of liq, lisq, and liseq.
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smaller than that of Liq. To exactly compare the charge transport property, the hopping rates of hole and electron (kh and ke) were calculated. As a result, the computed kh of LiSq is slightly decreased while that of LiSeq is 10-times faster than Liq. However, the calculated ke values of LiSq and LiSeq are smaller than that of Liq. From these results, it is worthying noticed that introducing the S and Se atoms plays a beneficial role in lowering the electron injection barrier, whereas these atoms seem to be ineffective to improve the electron transport property. Nonetheless, the further investigation is necessary to clearly judge the role of S and Se atoms on charge transport property. This is because the strong perturbation effect of the different M+ on the MO energy and charge transport property was previously reported.

M+ (M = Na, K, Rb, Cs, Ag, and Au) effect

The optimized structures of MSq and MSeq complexes (M = Na, K, Rb, Cs, Ag, and Au) are denoted in Fig. 2. Similar to LiSq, and LiSeq, the structural planarity of a series of M+Sq− and M+Seq− complexes are sustained. Compared to Li-based complexes, dS/Se−M and dN−M values are significantly increased due to the large van der Waals (vdW) radius.

Compared to Liq, the HOMO and LUMO energies of MSq and MSeq complexes (M = Na, K, Rb, and Cs) are shifted to deeper level, but these are more destabilized than those of LiSq and LiSeq. On the other hand, the HOMO and LUMO energies of MSq and MSeq complexes (M = Ag and Au) shifted to the deeper level, compared to Li-based complexes. This result implies the advantageous role of Ag+ and Au+ on the reduction of energy barrier between EIL and cathode.

Table 2 The structural and electronic parameters of a series of MSq and MSeq (M = Na, K, Rb, Cs, Ag, and Au).
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The spatial distributions of HOMOs and LUMOs for all M+Sq− and M+Seq− (M = Na, K, Rb, Cs, Ag, and Au) are depicted in Fig. 2. For NaSq, NaSeq, and KSeq, HOMO and LUMO shapes are quite similar to LiSq and LiSeq. On the other hand, it is interested to notice that the spatial distribution of HOMO predominantly lies on the Sq/Seq moiety while that of LUMO mainly spreads on the metal ion in a series of MSq (M = K, Rb, Cs, Ag, and Au) and MSeq (M = Rb, Cs, Ag, and Au). Moreover, the contribution of S and Se atomic orbitals to both HOMO and LUMO is meaningfully increased in AgSq, AuSq, AgSeq, and AuSeq. From MO shapes, the transition characteristic can be assigned as ligand to metal charge transfer (LMCT). Similar to previous report20 the metal ion plays a key determinant to change the MO shapes. As the change in M+ ions, the calculated LUMO energies of MSq and MSeq (M = Na, Rb, Cs, Ag, and Au) are more stable than Liq. This result indicates that the most complexes are expected to reveal the superior electron injection property than Liq due to the decrease in the energy difference between work-function of Al-cathode and LUMO energy.

Fig. 2
Fig. 2
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The optimized molecular structure of MQ, MSQ, and MSeQ complexes.

Table 3 The charge transport parameters of MSq and MSeq (M = Na, K, Rb, Cs, Ag, and Au).
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The computed λ, t, and hopping rate (k) of hole and electron carriers were summarized in Table 3. The calculated λh values of MSq and MSeq (M = Na, Ag, and Au) are smaller than that of LiSq and LiSeq, while those of MSq and MSeq (M = K, Rb, and Cs) are slightly larger than that of LiSq and LiSeq. On the other hand, the λe values of MSq and MSeq (M = Na, K, Rb, and Cs) are remarkably decremented, but those of MSq and MSeq (M = Ag and Au) are significantly increased, compared to those of LiSq and LiSeq. From these results, it is clearly understood that Ag and Au plays an advantageous role in suppressing the structural relaxation under the oxidation process. However, alkali metals (Na, K, Cs, and Se) efficiently suppress the structural relaxation under the reduction process. Nonetheless, λe values of all complexes are much larger than Liq. Let us turn to see the t2h and t2e. Compared to Li-based complexes, the computed t2h values are significantly incremented except NaSq NaSeq, RbSq, CsSq, and AgSq. In the case of t2e, NaSq, KSeq, RbSq, and CsSq are computed to be less unfavorable than LiSq. Considering the role of EIL materials, it is interested to mention that Ag+ and Au+ surprisingly activate the probability of the intermolecular electron transfer. To exactly compare the charge transport characteristic of these complexes, the derived kh and ke values are compared. The calculated kh of Liq, LiSq and LiSeq are 3.41 × 1013 s− 1, 1.48 × 1013 s− 1 and 2.19 × 1014 s− 1, which are greatly increased in KSq, KSeq, CsSeq, AgSq, AgSeq, Auq, and AuSeq with the magnitude range of 1014–1017 s− 1. For ke, the derived rates of MSq and MSeq (M = Na, K, Rb, Cs, Ag, and Au) are computed to be in the range of 1012–1017 s− 1. Among them, MSq (M = K, Ag, and Au) and MSeq (M = Na, Rb, Cs, Ag, and Au) complexes are expected to be 10–100 times faster than Liq. Although the structural changes are inefficiently suppressed during the redox reaction, the intermolecular overlap of molecular orbitals helps to achieve the excellent hopping rates.

In other words, these results indicates that the most contributable parameter of charge transport is the transfer integral in these molecular systems. As already mentioned in the introduction, A key role of EIL material is to greatly increase the amount of injected electron from cathode and efficiently transfer the electron carrier into adjacent organic layer. Accordingly, the small injection barrier and good electron transport properties are essentially required to deliver a large amount of electron carriers into the EML, leading to the enhancement of exciton recombination. From the comprehensive consideration of LUMO energy and ke results, it is expected that Ag and Au ions binding with Sq and Seq moiety play an advantageous role in reducing the energy barrier and enhancing the ke.

Absorption property

In the top emitting device, the light is extracted through the cathode. To maximize the efficiency of the top-emitting device, three primary color lights must be entirely extracted without any loss of photon by absorption of ET-side layers. With this perspective, the absorption property of EIL should be carefully considered as one of parameter to effect on the device efficiency. Prior to compute the absorption energy, the adequate DFT functional should be chosen to accurately predict the absorption energy. Accordingly, B3LYP23,24,25 CAM-B3LYP32ωB97xD33 and LC-ωHPBE34,35,36 functionals with 6-311G** basis set were examined to compare the absorption energy of Liq in experiment. As a result, the computed absorption energies of Liq by different DFT functionals are 2.799 eV (B3LYP), 3.256 eV (CAM-B3LYP), 3.290 eV (ωB97xD), and 3.642 eV (LC-ωHPBE), respectively, which correspond to the absorption wavelengths of 443.02, 380.84, 376.90, and 340.47 nm, respectively. In the experiment, the absorption wavelength of Liq was measured at 365 nm, which is in good agreement with the result of the ωB97xD calculation.

Table 4 The calculated absorption energy and oscillator strength. The absorption wavelengths and oscillator strengths for 2nd excited state are listed in parentheses.
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Therefore, the absorption energies of all designed complexes were computed by TD-ωB97xD/6-311G** level of theory and the corresponding values are listed in Table 4. The absorption energies of all complexes are shifted to lower energy than Liq. Given the emission wavelengths of primary colors in commercial OLED devices (blue = 455 nm, green = 530 nm, red = 630 nm), the most of complexes are expected not to remarkably influence on the efficiency drop of top-emitting OLED devices except CsSeq, AgSq, AgSeq, AuSq, and AuSeq. It is important to note that CsSeq, AgSq, AgSeq, AuSq, and AuSeq may very minorly impact on the efficiency drop of the blue emission due to the slight overlap of the absorption and emission wavelengths. Specifically, the blue absorption of CsSq, AgSq, and AgSeq is originated from the 1st excited state, while that of AuSq and AuSeq occurs in the 2nd excited state. (See Fig. S2) Nonetheless, the computed oscillator strengths of all designed molecules are too small to abundantly absorb the photons. This result indicates that a series of MSq and MSeq complexes may hardly impact on the efficiency drop in OLED devices.

Complexation with ETL

Previously, we reported the complexation effect of Liq with electron transport material (ETM) on the charge transport property37 which critically reduces kh and ke due to increase in λ and decrease in t2.With this perspective, the complexation effect of newly designed EIL complexes with ETM on the charge transport property was investigated. The most popular core skeleton for ETM is triazine moiety since the strong electron acceptability and structural planarity are beneficial to efficiently transport the electron carrier. Therefore, 2,4,6-tris(biphenyl-3yl)-1,3,5-triazine (T2T) was selected as the representative ETL material in this study. For triazine-based ETM, the root cause of the complexation effect on the small t2 can be understood that planar structure of triazine core can be broken since the intramolecular H-bonds defined as C-H…N H-bond are dismantled by coordination bond between N and M+. Accordingly, the bulkiness of ETL material is increased, thereby the inefficient intermolecular interactions enable the reduction of the molecular orbital overlap. In addition, the coordinated EIL material induces the shielding effect to interrupt the effective overlap of the molecular orbitals. Therefore, we solely computed λh and λe of coordinated structures between individual designed EIL material and T2T. These values are listed in Table 5 and their Δλ (λT2T−MXq (X= S and Se) - λT2T−Liq) values are depicted in Fig. 3. In addition, the optimized structures were denoted in Fig S1.

Table 5 The computed λh and λe of T2T-MSq and T2T-MSeq complexes.
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Fig. 3
Fig. 3
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The chart graph for difference between λT2T−MXq (X= S and Se) and λT2T−Liq.

Compared to both λ of T2T, the complexation between T2T and MSq/MSeq clearly impacts to the increment of λh and λe. As shown in Fig. 3, the λh values of T2T-MSq and T2T-MSeq (M = Li, Na, K, Rb, Cs, Ag, and Au) complexes are decremented, compared to T2T-Liq complex. In addition, the λe values of T2T-NaSeq, T2T-KSeq, T2T-AgSeq, T2T-AuSq, and T2T-AuSeq are slightly decreased while those of T2T-NaSq, T2T-KSq, T2T-RbSq, T2T-RbSeq, T2T-CsSq, T2T-CsSeq, and T2T-AgSq are slightly increased. As aforementioned, Liq is co-deposited with ETM to form ETL, resulting in the presence of the undesired complex between Liq and ETM. Therefore, forming a complex between ETM and EIL interrupts the spontaneous electron transfer between adjacent molecules, reducing the amount of electron carriers into EML. With this perspective, NaSeq, KSeq, AgSeq, AuSq, and AuSeq are expected to slightly mitigate the weakening the electron transport property under the complexation with ETM.

For commercial OLED device, the demands for highly efficient OLED device with excellent power efficiency have been gradually increased. With this perspective, it is expected that AuSq and AuSeq play a pivotal role in improving the efficiency and the power efficiency since the fast electron transport property and small injection barrier of these materials decrease the turn-on voltage and increase the electron carrier concentration. Moreover, these materials are expected to slightly weaken the electron transport property under the complexation with ETL material. Accordingly, we propose AuSq and AuSeq as candidate EIL material to replace the current Liq for achieving the highly efficient OLED device.

Conclusion

In this study, the candidate EIL materials, which can be used as alternative materials of Liq, were designed and investigated with DFT computation. Based on the quinolate skeleton, the S and Se atoms were utilized to replace the O in the quinolate moiety. In addition, the effect of introducing the different singly oxidized metal ions was also considered. As increase the demand for high power efficiency of OLED device, the electron transport and electron injection properties play an important role in enhancing these performances since the J-V characteristic of OLED device strongly depends on the electron transport. Therefore, the development of charge transport property for EIL is essentially required to further improve the power efficiency. From this perspective, with Li+ ion, the effect of introducing S and Se atoms on the electron transport property is diadvantageous. However, the electron transport property is significantly enhanced with different M ions except NaSq, KSeq, RbSq, and CsSq. In the point of view for the side effect of EIL material on the performance of OLED device, the absorption energies of designed EIL materials and λh/e of ETM-MSq/MSeq complexes were additionally considered. Except CsSeq, AgSq, and AgSeq, the effect of the absorption energies on the device performance can be negligible in all designed materials. In addition to this, NaSeq, KSeq, AgSeq, AuSq, and AuSeq are beneficial to weaken the increase of λe via forming a complex with ETM. From comprehensive analysis of our theoretical results, we suggest that the utilization of AuSq or AuSeq as EIL is expected to reveal the superior device performance, compared to conventional Liq.