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Abstract
The transition dipole moment (TDM) orientation in the emission layer (EML) of organic light-emitting diodes (OLEDs) have attracted increasing attention from many researchers. But the study point at the molecular orientation in the hole transport layer (HTL) and electron transport layer (ETL) was not reported widely. In this paper, the molecular orientation of HTLs and ETLs were controlled by the deposition rate. The angle-dependent PL spectra and the variable angle spectroscopic ellipsometry (VASE) were used for evaluating the molecular orientation of B3PYMPM and TAPC, respectively. We found that fast deposition rate can boost preferentially vertical molecular orientation in both molecules and facilitate the hole and electron mobility, which was tested by the current density-voltage and capacitance-voltage curves of HODs and EODs. Moreover, the HTLs and ETLs were employed in OLED devices to verify the influence of molecular orientation on charge carrier mobility, which determined the performance of OLEDs significantly.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since C. W. Tang et al. reported organic light-emitting diodes (OLEDs), the efficiency of OLEDs has been tremendously improved. In recent years, the molecular transition dipole moment (TDM) orientation of emitters gradually become a research focus. The preferentially horizontal TDM of emitters can enhance light out-coupling efficiency (${\eta _{out}}$) and improve the performance of OLEDs [1,2]. In order to realize the preferentially horizontal TDM ratio, many researchers are trying to develop various methods, including developing new emitters with superior horizontal TDM orientation, changing substrate temperature and selecting different host for emitters [1,3–5]. However, the characteristics of the hole transport layer (HTL) and electron transport layer (ETL) are also significant for OLEDs performance, the study of molecular orientation in HTL and ETL are few reported. Although, Po-Tsung Lee et al. investigated the electrical characteristics of Alq3 thin films as ETL, they were not considered the molecular orientation in films [6]. Amir Mikaeili, et al. tried to study the current density influenced by molecular orientation of Alq3 thin film. However, the molecular orientation was not obvious changed, because of isotropy property of Alq3. Finally, they attributed the changed current density to the film roughness [7].
In this paper, a rod-shaped molecule bis-4,6-(3,5-di-3-pyridylpheny)-2-methylp-yrimidin (B3PYMPM) and a disk-shaped molecule bis-(4-(N,N-ditolylamino)phenyl)cyclohexane (TAPC) were chose as electron transport material and hole transport material, respectively. B3PYMPM and TAPC films are fabricated with various deposition rate during depositing process. We investigated the molecular orientation of these films and found that the current density, charge carrier mobility and capacitance-voltage characteristics were depended on the molecular orientation heavily. To clarify how the molecular orientation influence on the characteristics of HTL and ETL films, a systematic analysis of these phenomena was done in this work. And then, an optimized OLEDs were prepared to verify the influence of molecular orientation for OLEDs.
2. Experiment
The chemical formula of B3PYMPM and TAPC are shown in Fig. 1(a). and Fig. 1(b). Organic materials were purchased from Xi ‘an Paulette photoelectric technology co. Ltd. Prior to the deposition of the organic layers, the ITO substrates were ultrasonic treated in detergent, alcohol and acetone for 20 min, sequentially, then exposed to ultraviolet-ozone for 15 min. All the organic layers were grown under a pressure of 5×10−4 Pa by thermal evaporation. As shown in Fig. 1(c) and Fig. 1(d).
Fig. 1. (a) The chemical formula of B3PYMPM. (b) The chemical formula of TAPC. (c) The structure of EOD. (d) The structure of HOD.
The structure of electron-only device (EOD) and hole-only device (HOD) is ITO (70nm)/ B3PYMPM (45nm)/ LiF (0.7nm)/ Al (50nm) and ITO (70nm)/ HATCN (10nm)/ TAPC (35nm)/ TCTA (10nm)/ Al (50nm).
The luminance, current density and electroluminescence (EL) spectra were measured using a Keithley 2400 source meter and a Spectra Scan PR655. The surface morphologies of these films were measured by using a Dimension Icon atomic force microscope (AFM) from BRUKER. The angle-dependent PL spectra were measured to analyze the molecular orientation in B3PYMPM films by the Molecular orientation characteristic measurement system from HAMAMATSU. The variable angle spectroscopic ellipsometry (VASE; M-2000, J. A. Woollam) was used for evaluating molecular orientation in TAPC by measuring birefringence. The impedance spectroscopy was measured with a Solartron 1260 impedance analyzer.
The detailed structure of the OLED devices is ITO (70nm)/ HAT-CN (10nm)/ TAPC (35nm)/ TCTA (10nm)/ TCTA: B3PYMPM: green dopant Ir(ppy)2acac (30nm, 10 mol%)/ B3PYMPM (45nm)/ LiF (0.7nm)/ Al (50nm). 1,1-bis-(4-bis(4-methylphenyl)-amino-phenyl)-cyclohexan (TAPC) was used as the hole transport layer (HTL), bis-4,6-(3,5-di-3-pyridylpheny)-2-methylp-yrimidin (B3PYMPM) as the electron transport layer (ETL). Bis(2-phe-nylpyridine) iridium (III)-acetylacetonate [Ir(ppy)2acac] was doped in the exciplex forming co-host system of TCTA and B3PYMPM used as the EML. The EMLs of the TCTA: B3PYMPM co-host films were doped with the dyes at doping concentration of 10 mol%.
3. Molecular orientation, surface morphology and single charge devices
3.1 Molecular orientation of electron transport layers and hole transport layers
Both of the angle-dependent PL spectra and the VASE can be used for providing the molecular orientation of pure films, which can measure different optical properties of materials. For the rod-shaped molecule B3PYMPM, the angle-dependent PL spectra were measured and provided the ratio of molecules with vertical orientation through fitting with the theoretical simulation. A model of organic microcavity light emitting diode structure proposed by WASEY and BARNES was utilized to simulate the angle-dependent PL spectra. The simulation is based on a dipole embedded in a multilayer film and uses a transfer-matrix formalism to calculate the Fresnel refraction and transmission coefficients at each interface [8–10]. The coordinate system was established and the substrate plane was defined as x-y plane and the normal vector of substrate plane defined as z direction. The molecular orientation of rod-shaped molecule B3PYMPM can be characterized by the anisotropy factor $\alpha $, which is defined as the fraction of vertical molecular orientation (${P_z}$) divide by the total of molecular orientation (${P_x},{P_y},{P_z}$). If the molecular orientation was completely random or vertical, the $\alpha $ is well equal to 0.33 or 1, since there is a third or complete contribution from vertical molecular orientation. [11–14].
The VASE measured TAPC films deposited on Si substrates to analyze anisotropy. The incident angle of VASE measurement was varied from 45° to 65° with 5° increments. The VASE data were fitted with a uniaxial anisotropy model using Gaussian-Cauchy type model and all the mean square errors (MSE) are less than 7. For disk-shaped molecule TAPC, the VASE was used and provided the order parameter S, which defined as (1):
As shown in Fig. 2(a), where $\theta $ is the angle between the normal vector of the molecule plane (vertical to the molecular dipole moment) and the direction vertical to the substrate surface, $\langle \sim \rangle $ indicates the ensemble average, where ${k_o}$ and ${k_e}$ are the ordinary and extraordinary extinction coefficients in the directions parallel and vertical to the substrate at the 371nm absorption wavelength attributed to the molecular orientation, respectively [15–17]. Figure 2(b)-(d), S = 1 represents the molecular orientation is complete parallel to substrate plane, S = −0.5 represents the molecular orientation is complete vertical to substrate plane, S = 0 represents the molecules are randomly oriented.
Fig. 2. (a) The schematic diagram of disk-shaped molecular orientation. (b) S = 1, molecular orientation is completely horizontal. (c) S = −0.5, molecular orientation is completely vertical. (d) S = 0, molecular orientation is random.
The B3PYMPM films of 45 nm thickness were fabricated with various deposition rates of 0.10 Å/s, 0.25 Å/s, 1.20 Å/s and 5.50 Å/s. The fitted anisotropy factor $\alpha $ of B3PYMPM films are shown in Fig. 3. The scattered points and solid black lines represent the experimental data and the theoretical simulation, respectively. With the slow deposition rate of 0.10 Å/s, B3PYMPM film obtain a lower vertical molecular orientation ratio of $\alpha $ = 0.22. When the deposition rate gradually increased, the ratio of vertical molecular orientation grew. The anisotropy factor $\alpha $ = 0.28, 0.36, and 0.46 are corresponding to the deposition rates of 0.25 Å/s, 1.20 Å/s and 5.50 Å/s, respectively.
Fig. 3. Angle-dependent PL intensity and theoretical simulation of B3PYMPM films based on different deposition rate.
In additional, the TAPC films of 35nm thickness were fabricated at deposition rates of 0.25 Å/s, 1.20 Å/s, 3.00 Å/s and 5.50 Å/s. From the results of VASE, the order parameters S was calculated (see section S1 of Supplement 1). The order parameters S is estimated by birefringence, as shown in Fig. 4. The value of S decreased along with increasing deposition rate, which indicated the increasing vertical molecular orientation ratio. Therefore, fast deposition rate enables both B3PYMPM and TAPC films to obtain a higher vertical molecular orientation ratio.
3.2 Surface morphology of B3PYMPM and TAPC films
To make the influence of deposition rate on films morphology and the relationship between films surface roughness and molecular orientation clear, we measured the surface morphology of films. Four points were measured on each sample. The root mean squared roughness ${R_q}$ is a critical indicator for roughness of films. The AFM images, ${R_q}$ results (${R_{q1}}$, ${R_{q2}}$, ${R_{q3}}$ and ${R_{q4}}$) and average ${R_q}$ of B3PYMPM and TAPC films deposited on silicon substrates with different deposition rates were shown in Fig. 5 and listed in Table 1, respectively. For the B3PYMPM films, the highest average ${R_q}$ of 0.393 existed in 0.10 Å/s film, which can be attribute to the impurities mixed into film during deposition process, because the ultralow rate needed a lot of time to finish the complete deposition process [17]. Both of B3PYMPM and TAPC films, from 0.25 Å/s to 5.50 Å/s, the fast deposition rate caused the increased average ${R_q}$, which originate from the growing number of molecules with vertical orientation.
Table 1. Four root mean squared roughness (${R_q}$) and the average ${R_q}$ of B3PYMPM and TAPC films with different deposition rates.
The side view of B3PYMPM film with horizontally and vertically oriented molecules are shown in Fig. 6(a). Horizontally aligned long axis of B3PYMPM facilitates the formation of smooth film surface, which leads to lower ${R_q}$ at low deposition rate. When the long axis of B3PYMPM is vertical to the substrate, the film surface is uneven and has a jagged shape, which cause the rough film surface. Figure 6(b) shows the top view of horizontally aligned TAPC molecules and the side view of vertically aligned TAPC molecules, respectively. Horizontally aligned molecular plane of TAPC also forms smooth film surface to reduce the roughness. From the side view of TAPC film with vertically aligned molecules, TAPC molecular plane vertical to substrate leads to the increased ${R_q}$.”
Fig. 6. The relationship between films surface roughness and molecular orientation. Horizontally and vertically aligned (a) B3PYMPM and (b) TAPC molecules.
3.3 Current density and carrier mobility of single carrier devices
The charge balance factor $\gamma $ is an extremely critical element for performance of OLED devices [18–20]. In order to investigate the influence of molecular orientation to electrical property of EODs and HODs with B3PYMPM and TAPC films as electron transport layer and hole transport layer, the EODs and HODs were fabricated with the structures as shown in Fig. 1(c) and (d). The current density-voltage characteristics of EODs are shown in Fig. 7(a). With the increasing deposition rate and the vertical molecular orientation ratio, current density has a continuous improvement under the same voltage. For example, at 5V, the current density is 24.02 mA/cm2 for 0.10 Å/s, 52.07 mA/cm2 for 0.25 Å/s, 79.36 mA/cm2 for 1.20 Å/s and 106.41 mA/cm2 for 5.50 Å/s. The space charge limited current (SCLC) regime was analyzed according to the Mott−Gurney theory which is expressed as (2):
where J is the current density, ${\varepsilon _0}$ is the vacuum permittivity, ${\varepsilon _r}$ is the dielectric constant, $\mu $ is the carrier mobility, E is the electric field and L is the thickness of film. The relationship of carrier mobility $\mu $ and electric field E can be expressed by Poole-Frenkel formula (3): where ${\mu _0}$ is the zero field carrier mobility, $\beta $ is the Poole-Frenkel coefficient [21–23]. The electron mobility of different EODs was shown in Fig. 7(b). Fast deposition rate led to high electron mobility. The impedance spectroscopy analysis of EODs also gives the same results (see section S2 of Supplement 1). There is almost one order of magnitude difference between 5.50 Å/s and 0.10 Å/s of EODs. The result indicates that the improvement of vertical molecular orientation ratio can facilitate current density and electron mobility of EODs.The current density and hole mobility of HODs are shown in Fig. 8. As shown in Fig. 8(a), like EODs, the current density of EODs also increased with increasing deposition rate and ratio of vertical molecular orientation under the same voltage. As seen in Fig. 8(b), the hole mobility of EOD of 5.50 Å/s is also one order greater than that of EOD of 0.25 Å/s. Comparing to EODs, the hole mobility is greater than electron mobility. Although, the worst hole mobility of HODs is the one deposited at 0.25 Å/s, which is still faster than 5.50 Å/s electron mobility, the best one of EODs.
3.4 Capacitance measurement for single carrier devices
To further understand the transport and accumulation of charge carrier, the capacitance-voltage characteristics for single carrier devices were measured [18–20]. The capacitance-voltage curve of EODs and HODs are shown in Fig. 9(a) and Fig. 9(b), respectively. For both EODs and HODs, the capacitance increased with increasing voltage. At the same voltage, the EODs and HODs obtained higher capacitance when the deposition rate gradually increased. This phenomenon indicates that preferentially vertical molecular orientation enhanced the charge carrier transport, which facilitates the accumulation of charge carrier in single carrier devices. In the measured voltage range, the capacitance of HODs are more than two orders greater than that of EODs, because hole mobility is two orders greater than electron mobility.
3.5 Molecular orientation dependent on the deposition process
According to the results above, the molecular orientation can be controlled by the molecular deposition process based on various deposition rates. The surface equilibration mechanism was used for explaining the anisotropic molecular orientation [16,24–30]. In the organic film, the diffusion coefficient of surface is 108 times faster than that of bulk when the substrate temperature is at the glass transition temperature (${T_g}$). When the substrate temperature is lower than ${T_g}$, the surface molecule mobility is also faster than bulk molecule mobility seriously [24]. Therefore, the bulk molecule mobility is essentially negligible when the substrate temperature is below ${T_g}$ during deposition.
The unique symmetry axis of rod-shaped B3PYMPM is parallel to the molecular long axis and the unique symmetry axis of disk-shaped TAPC is vertical to molecular plane [16]. High surface molecule mobility during the deposition facilitated the rod-shaped B3PYMPM molecules in the free surface toward to horizontal arrangement. For disk-shaped TAPC molecules, high surface mobility also facilitated the unique symmetry axis of TAPC toward vertical orientation, which means TAPC molecules toward horizontal orientation [16,24,25]. Figure 10 illustrates the molecular deposition process of rod-shaped B3PYMPM and disk-shaped TAPC molecules with slow and fast deposition rate, respectively. The transverse is the formation of molecular orientation along with deposition time and the longitudinal is the molecular deposition direction. Figure 10(a) and Fig. 10(c), at the slow deposition rate, when the molecule first contacts the substrate, molecules in the free surface are toward to horizontal arrangement because higher surface mobility cause molecular migration on the substrate plane. Molecules have enough time to adjust their position to horizontal to the substrate because of slow deposition rate. As the deposition continues, surface molecules are buried by subsequent deposited molecules, which forms the film with low ratio of vertical orientation. Figure 10(b) and Fig. 10(d), at the fast deposition rate, the number of deposited molecules increase through the same space compare with that of low deposition rate condition. The fast deposition rate and the increased molecular number cause that the molecules reached the substrate early have insufficient time to adjust their position and fail to adjust the molecular orientation toward to horizontal arrangement. As further deposition continues, the subsequent molecules buried them quickly. And then, the surface molecules become the bulk molecules which lose the ability toward to horizontal orientation. This is the reason for the formation of high ratio of vertical orientation in the films. High ratio of vertical orientation facilitates the dipoles contact compactly and accelerated the hopping charge carrier transport.
Fig. 10. The formation of molecular orientation during deposition process for (a) B3PYMPM with low deposition rate and (b) B3PYMPM with fast deposition rate and (c) TAPC with low deposition rate and (d) TAPC with fast deposition rate. Black arrows represent the tendency that molecules toward to horizontal orientation, red crosses represent the molecules lose the ability toward to horizontal orientation.
4. Performance of OLED devices
A certain TAPC film deposited at 0.25 Å/s and four B3PYMPM films deposited at 0.10 Å/s, 0.25 Å/s, 1.20 Å/s and 5.50 Å/s were employed in OLED devices. As shown in Fig. 11(a), under the same voltage, the luminance increased with increasing B3PYMPM deposition rate.
Fig. 11. (a). The luminance and (b). The current density of OLED devices based on various deposition of B3PYMPM.The inserted picture provides EL spectra of OLED devices.
For example, when the driving voltage is 19V, the luminance is 11340 cd/m2 for 0.10 Å/s, 22250 cd/m2 for 0.25 Å/s, 37710 cd/m2 for 1.20 Å/s and 46670 cd/m2 for 5.50 Å/s. At the same time, as shown in Fig. 11(b), the OLED device of fast B3PYMPM deposition rate obtains high current density at the same voltage. The EL spectra of devices are shown in the inserted picture. The peaks of all EL spectra are located at 520nm, which indicates that the light emission position in EML was not changed though the increasing number of electrons. Figure 12 illustrate the transport process of hole and electron in the OLED devices based on different deposition rate of B3PYMPM films. When the deposition rate of B3PYMPM rising from 0.10 Å/s to 5.50Å/s, the ratio of the vertically oriented molecules increased, the number of electrons which combined with holes in the EML increased too, which led to charge carrier balance. Therefore, the improvement of luminance and current density of OLED devices can attribute to the increased charge carrier balance due to the increased ratio of vertically oriented molecules and electron mobility in B3PYMPM films.
Fig. 12. The transport process of hole and electron in the OLED devices based on (a) 0.10 Å/s, (b) 0.25 Å/s, (c) 1.20 Å/s and (d) 5.50 Å/s B3PYMPM films.
5. Conclusion
The molecular orientation in ETL and HTL were controlled based on various deposition rate. We found that the ratio of vertically oriented molecules in ETLs and HTLs increased at fast deposition rate, though the surface morphology of films were not changed seriously. The preferentially vertical molecular orientation led to great improvement for current density, mobility and capacitance of EODs and HODs. We have systematically analyzed the molecular deposition process to explain the origin of molecular orientation and how the vertically oriented molecules facilitate the charge carrier transport. Moreover, the OLED devices were fabricate based on ETL with different molecular orientations. Evidently, the OLED devices including ETL with preferentially vertical molecular orientation obtained higher luminance and current density, which verify our experimental results that vertical molecular orientation can facilitate charge carrier mobility.
Funding
Liaocheng University (318011904, 318051650); Natural Science Foundation of Shandong Province (ZR2017BF009); National Natural Science Foundation of China (61775089).
Acknowledgments
We acknowledge support from the Special Construction Project Fund for Shandong Province Taishan Scholars. The authors would like to thank Yangyang Guo, Tingting Liu, Hui Du, Xue Qin, Shuhui Lv, Junyi Liu and Yanfang Ren for their valuable inputs during the course of this work.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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