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Abstract
The interaction between the intrinsic polarity of the host material and the TADF guest material affects charge injection and transport, exciton formation, charge recombination, and emission mechanisms. Therefore, understanding and controlling the interaction between the intrinsic polarity of the host material and the TADF guest material is very important to realize efficient TADF-OLED devices. This study investigated the molecular interaction between different polar host materials and a thermally activated delayed fluorescence material (DMAc-PPM). It has been found that interaction between the host and guest (π-π stacking interaction, multiple CH/π contacts) greatly influence the molecular transition dipole moment orientation of the guest. And the OLED devices based on the strong polar host (DPEPO) exhibited the highest EQEmax and lowest luminescence intensity, while devices using the weaker polar hosts mCP and CBP achieved higher luminance and lower EQEmax. Then, the strong polar host DPEPO was mixed with the weaker polar hosts CBP and mCP, respectively. The devices prepared based on the mixed-host DPEPO: mCP showed a 2.2 times improvement in EQEmax from 6.3% to 20.1% compared to the single-host mCP. The devices prepared based on the mixed-host DPEPO: CBP showed a 3.1 times improvement in luminance intensity from 1023 cd/m2 to 4236 cd/m2 compared to the single host of DPEPO. This suggests that optimizing the polarity of host materials has the potential to enhance the performance of solution prepared OLED devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nowadays, organic light-emitting diodes (OLEDs) exhibit a multitude of advantages, including large area, light weight, small size, good flexibility, and stable performance [1–3]. Notably, OLED technology has found extensive applications in various devices such as large flat panel displays, smartphones, tablets, and other portable consumer electronics. A series of thermally activated delayed fluorescence (TADF) emitters have garnered widespread attention for their potential applications in high-performance OLEDs with almost achievable 100% internal quantum efficiency (IQE) [4]. The external quantum efficiency (EQE) of OLEDs is typically influenced by various factors, and one of the methods to enhance EQE is to improve the light outcoupling efficiency. Several light extraction techniques have demonstrated their efficacy in enhancing the out-coupling efficiency of OLEDs [5,6]. These innovative approaches aim to enhanced overall performance of OLEDs.
Similarly, the orientation of the transition dipole moment of small molecule emitters also affects the light outcoupling efficiency. In some recently published studies related to TADF, the interaction between host and TADF guest materials affects the transition dipole moment orientation of the TADF material [7–9]. Mayer Christian and Wolfgang Brütting proposed the interaction between the glass transition temperature (Tg) of the host material and the molecular weight and shape of the guest molecules, and found that with an increase in the Tg of the host material, guest molecules exhibited a horizontal orientation [10]. Kim et al. proposed that a horizontal dipole moment is induced when the linear binding geometry between the host-guest molecules is parallel to the transition dipole moment of the guest with larger binding energy [11].
On the other hand, the polar host matrices can stabilize the singlet charge transfer (CT) state of TADF materials, which leads to a smaller ΔEST and faster reverse intersystem crossing (RISC), which is beneficial for enhancing the performance of TADF-OLED devices [12–14]. Lu et al. reported a polar host DCzPO that achieved excellent charge injection, resulting in an EQE of 35.7% [15]. It has been reported that the PLQY of 4CzIPN is influenced by solvent polarity, with a value of 0.96 in nonpolar toluene and 0.54 in polar dichloromethane [16]. However, due to the host-guest dipole-dipole interaction, the presence of a strong host dipole field can lead to significant exciton quenching in blue TADF emitters [17,18]. The suitable hosts for blue emitters require good exciton confinement and charge injection capabilities. However, high-energy T1 result in high-energy S1, causing a mismatch between the frontier levels and adjacent layers, which hinders charge injection [19,20]. Hence, the appropriate host polarity is a key factor in achieving highly efficient OLED devices.
Traditional research has primarily focused on vacuum-based films, but solution-processed devices are gaining significance in flexible organic light-emitting diodes. Therefore, in this study, a solution-treated host-guest system was used as the emission layer, and three different polarities of the host materials were selected to analyze the effect of the polarity of the host on the photophysical properties of the guest TADF material and the overall performance of the device. And by controlling the polarity of the host, optimal optimization between efficiency and luminance was achieved.
2. Experiment
In the experiment, different polar materials including DPEPO, mCP and CBP were chosen as hosts, and thermally activated delayed fluorescence material DMAc-PPM was chosen as the guest material, all of which were purchased from Xi’an Polaroid Optoelectronics Technology Co.Ltd. Their molecular structures, dipole moments calculated using Gaussian, and electrostatic potentials (ESPs) generated using Multifwn and VDM are shown in Fig. 1 [21]. DPEPO, with the strongest polarity, has a dipole moment of 7.84 D, while mCP and CBP, with weaker polarities, have dipole moments of 1.38 D and 0 D, respectively. Chloroform was chosen as the solvent and a mixed solution of the host and guest was prepared at a concentration of 10 mg/ml. The resulting mixture was spin-coated onto a well-cleaned substrate. The samples were annealed at 40°C for 20 min to remove the residual solvent.
The device structure of the OLED is as follows: ITO/ PEDOT: PSS (40 nm)/ EML (40 nm)/ TPBi (40 nm)/ LiF (1 nm)/ Al (70 nm). ITO is used as the anode for the device, the aqueous solution of PEDOT: PSS serves as the hole transport layer. TPBi is used as the electron transport layer, while LiF and Al form the composite cathode. Only the emission layer and hole transport layer were spin coated, while the other layers are deposited through thermal evaporation under a pressure below 5 × 10−4 Pa.
The ultraviolet-visible (UV-Vis) absorption spectra was measured using a UV-Vis spectrophotometer (UV-3310, Hitachi). The photoluminescence (PL) spectra and PLQY were measured using a fluorescence spectrometer (FLS-920). The Keithley 2400 and spectral scan PR655 were used to test the current density of single-carrier devices and the electrical performance of OLED devices, respectively. The surface roughness of the EML films were estimated using atomic force microscopy (AFM) based on the Bruker method. The molecular orientation measurement system (C13472-01 optical system from Hamamatsu Photonics) was used to determine the molecular transition dipole moment orientation. Experimental data were simulated using U6039-09 standard software.
3. Results and discussions
3.1 Optical physical properties
As illustrated in Fig. 2(a), the absorption of the TADF material DMAc-PPM and the PL spectra of the host materials DPEPO, mCP and CBP were measured. Notably, the absorption peaks attributed to the TADF material DMAc-PPM showed significantly overlap with the photoluminescence peaks exhibited by the host materials [22]. This implies the existence of energy transfer from the host to the guest materials. The förster energy transfer radius ($\textrm{R}_0^6$) from the host materials DPEPO, CBP and mCP to the guest material DMAc-PPM can be expressed using the following formula:
Fig. 2. The absorption spectra of DMAc-PPM and the PL spectra of DPEPO, CBP and mCP, (a) the film, (b) the 10−5 mol/L solution of chloroform, (c) the transient PL spectra of blended film.
The overlap integral ${\rm J} = \smallint \overline {{\rm I}_{\rm D}} {\rm \varepsilon }_{\rm A}{\rm \lambda }^4{\rm d}{\rm \lambda }$, k2 is typically assumed to be 2/3, ${\mathrm{\Phi }_\textrm{D}}$ represents the PLQY of the donor, n denotes the refractive index of the medium [23,24]. Based on Fig. 2(b), the energy transfer radii are calculated to be 3.6 nm, 3.0 nm and 2.8 nm, respectively. As a result, DPEPO is capable of transferring a significantly greater amount of energy to the guest. The transient PL spectra of the blended films with different materials as hosts were tested as shown in Fig. 2(c) (The guest component is doped at a ratio of 20 wt% in the blended films, which is the optimized result from previous work [23]). As the polarity of the host increases, the fast fluorescence lifetime of the blended film becomes longer, and the radiative decay rate (kr) of guest decreases, with values of 6.87 × 107 s-1, 6.28 × 107 s-1 and 5.84 × 107 s-1, respectively. At the same time, the delayed lifetime of the blend films increases with the increase in polarity, indicating an increase in ΔEST [25]. This can be attributed to the presence of potential π-π interactions with CBP and mCP as hosts, leading to a small ΔEST [26,27]. The photoluminescence quantum yields (PLQY) was also measured and the corresponding become larger, 54%, 61% and 81%, respectively. It can be seen that a higher förster energy transfer radii corresponds to a higher energy transfer rate. There is an optimal compromise among energy transfer efficiency, radiative decay rate, and ΔEST, leading to a higher PLQY when DPEPO is used as the host.
Further investigation was carried out on the PL spectra and absorption of the blended films. As shown in Fig. 3, the blended films exhibited strongest absorption peaks at wavelengths of 274 nm, 297 nm and 296 nm, respectively, which can be attributed to the π-π* transition. Due to the intramolecular charge transfer (ICT) transition, a weak absorption peak was detected near 380 nm when DPEPO was used as the host material. In this case, electrons are transferred within the molecule from the peripheral donor part to the central pyrimidine acceptor part, induced by the presence of DMAc [28]. Furthermore, the emission peak of the blended film based on DPEPO as the host exhibited a redshift, which is commonly attributed to the polar nature of the host material. Therefore, compared to CBP and mCP, DPEPO is a strongly polar host, resulting in a corresponding redshift in the PL spectra [29,30].
Fig. 3. (a) Normalized absorption spectra of blended films, (b) normalized pl spectrum of blended films, (c) angle-dependent PL spectrum of blended films.
Additionally, the molecular transition dipole orientations were also tested for the guest pure film and blended films with different hosts. The emission dipole consists of Px, Py, and Pz dipoles. The horizontal orientation is defined as Θ=Θh = (Px + Py)/(Px + Py + Pz). Figure 4 presents a schematic of the experimental setup for angle-dependent PL spectrum measurements. The measurements range from 0° to 90°, in 2° increments, under UV excitation (365 nm continuous laser diode, with an incident angle of 45°). The sample is affixed onto a hemispherical cylindrical lens using optical gel and placed on a rotating sample stage. Subsequently, a fiber optic spectrometer detects the spectra of sample at different angles between the detector and the sample surface. The orientation of the molecular transition dipole moments is determined by comparing these measurement results with optical simulation [10,23,31]
As shown in Fig. 3(c), the Θ of the guest pure film and blended films based on DPEPO, mCP and CBP as the hosts are 0.65, 0.67, 0.71 and 0.75 respectively. The blended films have achieved a better horizontal transition dipole moment orientation compared to the pure film of the guest. It implies that the interaction between the host and guest can indeed optimize the proportion of the horizontal molecular transition dipole moment orientation of the guest. Furthermore the lower host polarity leads to a higher Θ. Based on CBP as the host, the best horizontal molecular transition dipole ratio was achieved. The DPEPO exhibits a relatively large dipole moment of 7.84 D, whereas the guest material DMAc-PPM has a significantly smaller dipole moment of 0.48 D. Due to the smaller dipole moment of the guest molecules, the dipole-dipole interaction between the host and guest has a relatively weak influence on the orientation of the guest molecule transition dipole moment [8]. It is conceivable that there might be additional latent interaction forces at play between the host and guest molecules, which could have an influence on the orientation of the guest molecule transition dipole moment.
There exists π-π interaction between the carbazole-based hosts mCP and CBP, where the extended π-conjugation of CBP leads to stronger π-π stacking. However, DPEPO has a small conjugation length, and there is almost no π-π interaction [27]. The molecular orientation at the interface is closely related to the π-π interaction between the host and guest molecules, where stronger π-π interaction is more favorable for the molecular horizontal orientation [8,33]. Moreover, the pyridine end of DMAc-PPM has four bulky methyl groups, and multiple CH/π contacts between the methyl groups and the π-plane facilitate molecular transition dipole moment horizontal orientation [33,34]. Therefore, the film based on the host CBP exhibits superior horizontal orientation. Further investigations were conducted on the electrical characteristics of OLED devices based on different hosts.
3.2 Electrical physical properties
Based on single hosts DPEPO, mCP and CBP, the corresponding OLED devices and hole-only devices were prepared with the following structures: ITO/ PEDOT: PSS (40 nm)/ EML (40 nm)/ TPBi (40 nm)/ LiF (1 nm)/ Al (70 nm), ITO/ PEDOT: PSS (40 nm)/ EML (40 nm)/ MoO3 (10 nm)/ Al (70 nm). The optoelectronic properties of OLED devices were tested and the results are shown in Fig. 5. The performance parameters of the OLED devices are listed in Table 1. The devices based on single hosts mCP and CBP achieved the maximum luminance of 6643 cd/m2 and 4868 cd/m2, respectively. It is worth mentioning that devices based on single host DPEPO exhibited the lowest luminance of 1023 cd/m2. However, the OLED devices based on DPEPO achieved the highest EQE (EQEmax = 20.9%), surpassing the relatively modest values of 6.3% for mCP and 5.5% for CBP. The EQE of a device can be expressed by the following formula [23,32,35]:
Fig. 5. The devices based on the single hosts (a) the luminance-voltage (L-V) characteristics, (b) current density-voltage characteristics (J-V) (inset: the normalized EL spectra), (c) the power efficiency-current density (PE-J) and current efficiency-current density (C-J) characteristics, (d) the EQE-current density (EQE-J) characteristics.
Table 1. Performance parameters of OLED devices
EQE is a metric that shows the efficiency of a device in converting injected electrons and holes into light emission. IQE is the internal quantum efficiency and ${\eta _{out}}$ is the light output coupling efficiency. The $\mathrm{\gamma }$ represents the carrier balance factor, ${\eta _{S/T}}$ is the fraction of excitons allowed to decay radiatively, ${q_{eff}}$ is effective quantum yield. The OLEDs based on single hosts DPEPO, mCP and CBP exhibited a decreased EQEmax from 20.9% for DPEPO to 6.3% for mCP and 5.5% for CBP. For the device with DPEPO host, there is a strong efficiency roll-off. When the luminance was 100 cd/m2, the EQE of the device with DPEPO host reduced to 14.7%, which is still higher than that of mCP (6.2%) and CBP (4.8%). The variations of EQEmax on polarity of host is in agreement with that of PLQY of blend films. However, the higher host polarity leads to a lower Θ, which means a lower light out-coupling efficiency. Then the EQE of OLEDs is a synergy effect of PLQY and Θ.
From the current density-voltage curves of the hole only devices (HODs) shown in Fig. 6(a), the HODs with single host mCP and CBP exhibit significantly higher current density compared to that of single host DPEPO at the same voltage. It can be attributed to the carbazole groups, which act as hole-transporting units. As shown in Fig. 6(b), when DPEPO, mCP and CBP was used as host, the hole injection barrier is 0.9 eV, 0.6 eV and 0.7 eV, respectively. The higher hole injection barrier hinders the hole injection and lows the current density of HODs. Furthermore, the proportion of horizontally oriented molecular transition dipoles in the blend emission layer is significantly higher for the single hosts of mCP (Θ=0.71) and CBP (Θ=0.75), compared to that of single host DPEPO (Θ=0.65). The increase in horizontal molecular orientation is more favorable for the injection and transport of charge carriers [36,37].
Fig. 6. (a) Current density- voltage curves of hole-only devices based on single hosts, (b) the structure of device.
The surface roughness of the emission layer is an issue for hole injection, then the surface roughness of the blend emission layer with different hosts was tested and the AFM images are depicted in Fig. 7. The surface roughness values are 0.280 nm, 0.235 nm and 0.256 nm for DPEPO, mCP and CBP, respectively. The relatively smoother interface was obtained when mCP and CBP was used as host, which is more conducive to carrier injection and transportion. The excellent horizontal orientation of the molecular transition dipole moments, lower hole injection barriers, and smoother interfaces work synergistically to facilitate that the devices utilizing single hosts mCP and CBP display significantly higher luminance.
From the aforementioned study, it was discovered that the OLED devices based on the EML with high polarity single host DPEPO achieved the highest EQEmax but the lowest luminance. Conversely, the OLED devices based on the EML with low polarity single hosts CBP and mCP exhibited higher luminance but lower EQEmax. In order to enhance both the EQE and luminance, the high polarity host DPEPO was mixed with the lower polarity of mCP and CBP in a 1:1 ratio to construct the mixed host with a compromise polarity. As illustrated in Fig. 8, the absorption spectra of the guest materials and PL spectra of the mixed hosts were studied. The overlap between the PL spectrum of the mixed host and the absorption spectrum of the guest ensures the presence of energy transfer. Compared to the single host DPEPO, the device based on the mixed host DPEPO: mCP significantly increases luminance by 95%, while still reaching an EQEmax of 20.1%. Compared to the devices based on the single host CBP, the mixed host DPEPO: CBP showed a 1.4 times increase in EQEmax, while the mixed host DPEPO: mCP showed a 2.2 times increase in EQEmax compared to the devices based on the single host mCP.
Fig. 8. (a) Normalized PL spectrum of the mixed host and absorption spectrum of the guest, along with the PL spectrum of the emissive layer based on the mixed host, the devices of mixed host (b) the luminance-voltage (L-V) characteristics and current density-voltage characteristics (J-V), (c) the power efficiency-current density (PE - J) and current efficiency-current density (CE - J) characteristics, (d) EQE-current density characteristics (inset: the normalized EL spectra).
The EQEmax show much difference of the OLED based on the DPEPO: mCP and DPEPO: CBP. First, according to Fig. 9 combined with the energy transfer formula [24]
Fig. 9. The transient PL decay curve of the different mixed host in the presence and absence of the guest
4. Conclusion
In this research, the host with different polarities were utilized to enhance the luminescence properties of TADF material of DMAc-PPM. The interactions between the host and guest were found to greatly influence the photophysical properties of the guest. And the host with weak polarity demonstrate significantly better horizontal orientation of DMAc-PPM molecule (75% for CBP and 71% for mCP) compared to the host with strong polarity (65% for DPEPO). For OLEDs, the EQEmax based on single host material DPEPO reached 20.9%, albeit with lower luminance. Conversely, the EQEmax based on single host mCP and CBP hosts exhibit higher luminance but lower EQEmax values (mCP: 6.3%, CBP: 5.5%). The mixed hosts were used to enhance both the EQE and luminescence intensity of the OLEDs. In particular, the higher EQEmax (20.1%) of the mixed host DPEPO: mCP was improved by 2.2 times compared to the single host mCP, and also showed superior luminance compared to the single host DPEPO. It is evident that the polarity of the host material plays a significant role in determining the efficiency of the corresponding devices.
Funding
National Natural Science Foundation of China (61775089); Natural Science Foundation of Shandong Province (ZR2020KB18, ZR2022MF240); the Project of Liaocheng University (318011904, 318051650); the Special Construction Project Fund for Shandong Province Taishan Scholars.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61775089), and the Natural Science Foundation of Shandong Province (ZR2020KB18, ZR2022MF240), the Project of Liaocheng University (318011904 and 318051650), and the Special Construction Project Fund for Shandong Province Taishan Scholars.
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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