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F7GA hybrid thin films for white organic light-emitting diodes
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Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
¨rster resonance energy transfer in ternary PFO/MEH-PPV/F7GA Dual Fo hybrid thin films for white organic light-emitting diodes Bandar Ali Al-Asbahi a, b a b
Department of Physics & Astronomy, College of Sciences, King Saud University, Saudi Arabia Department of Physics, Faculty of Science, Sana’a University, Yemen
A R T I C L E I N F O
A B S T R A C T
Keywords: Ternary blend Thin films Dual f¨ orster resonance energy transfer Electroluminescence WOLED
The ternary hybrid thin films of poly [9,9′ -di-n-octylfluorenyl-2,7-diyl] (PFO)/poly [2methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/2-butyl-6- (butylamino) benzo [de] isoquinoline-1,3-dione (F7GA) were successfully prepared with various ratios of F7GA. Dual F¨ orster resonance energy transfer (FRET) from PFO to both F7GA and MEH-PPV was confirmed in the ternary thin films. The distance between PFO and F7GA molecules was smaller than the distance between PFO and MEH-PPV. Besides, white light emission was achieved with cascade energy transfer in this ternary blend system. The lifetime decay components of the acceptor emission were attributed to the presence of an exciplex between the molecules in the acceptor excited state and the donor ground state. In addition to the dual FRET, carrier trapping took place in the device, suc cessfully realizing white organic light-emitting diodes (WOLEDs). The maximum luminance of 2295 cd/m2 and a current density of 12.98 mA/cm2 at CIE coordinates of (0.31, 0.24) was achieved with the device at 1.0 wt% F7GA.
1. Introduction White organic light-emitting diodes (WOLEDs) based on conjugated polymers or small molecules have been extensively investigated due to their various potential applications for flat panel displays and large-area flexibility [1]. Although many techniques have been suggested for full-color organic displays [2], white light emission combined with the color filter method provides a relatively simple method. Since white light emission is typically achieved by mixing three major colors (blue, green, and red), at least two emissive materials are deposited in a multilayer structure [3–5], or mixed as a single layer by blending (or doping) [6–8]. Unfortunately, building multilayer devices is difficult, especially by solution processing where it is difficult to find a suitable solvent for each layer without dissolving the underlayers. Therefore, if WOLEDs can be manufactured with a simpler architecture and an easier operation by avoiding vacuum deposition of white electroluminescent components at low cost, the single-layer method is preferred. Several methods have been reported to improve the WOLED performance such as multilayer device fabrication [9], use of electron and hole transport layers [9], a perfect match between component energy level and elec ¨rster resonance trode work function [10], and the introduction of Fo energy transfer (FRET) effects in the active layer of the device [11]. The
latter technique was utilized in the current work to produce WOLED with better performance. FRET excitation in the blend requires good mixing of the two materials and good spectral overlap between acceptor absorption and donor emission [12]. Mixing both donors and acceptors for energy transfer can significantly reduce the concentration quenching of the excitons produced, improving device performance [13,14]. In this work, we used the ternary blend of poly [9,9′ -di-n-octyl fluorenyl-2,7-diyl] (synonym: PFO), poly [2methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (synonym: MEH-PPV), and 2-butyl6- (butylamino) benzo [de] isoquinoline-1,3-dione (synonym: Fluorol 7 GA or F7GA) to achieve cascaded energy transfer for tuning emission colors and improving device performance. Three different emission components of PFO/F7GA/MEH-PPV with the bandgap order of PFO (3.0 eV) > F7GA (2.3 eV) > MEH-PPV (2.2 eV) [15–17] were miscible with each other. Moreover, the donor emission spectrum (PFO) and the acceptor absorption spectrum (F7GA or MEH-PPV) overlapped signifi cantly. The main component of PFO acted as a diluent, matrix, and excitation energy donor for the ternary blend, producing light with high efficiency. Therefore, when the ternary blend was excited near the ab sorption peak wavelength of PFO, light emission from F7GA and MEH-PPV was expected, suggesting the cascade energy transfer used for WOLEDs. Since research on the photophysical mechanism of ternary
E-mail address: [email protected]. https://doi.org/10.1016/j.dyepig.2020.109011 Received 27 August 2020; Received in revised form 14 November 2020; Accepted 15 November 2020 Available online 21 November 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Bandar Ali Al-Asbahi, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109011
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hybrid systems is rare, the current research concentrated in detail on the study of dual FRET mechanism and showed how such a mechanism can develop the performance of WOLEDs.
(with schematic structure shown in Fig. S2), the deposited ITO sub strates were transferred to an electron beam chamber to create an aluminum cathode with a thickness of 150 nm and an active area of 0.076 cm2. The deposition rate and chamber pressure were 2 Å/min and 2.5 × 106 Pa, respectively. Absorption and photoluminescence (PL) spectra were collected by PerkinElmer Lambda 900 UV-VIS and PerkinElmer (LS55) Luminescent Spectrophotometers, respectively. The lifetime decays, Figs. S3 − S15, were collected using Edinburgh Instrument FLSP920 spectrophotom eter. The electroluminescence (EL) spectra and the relationship between current density (J) and bias voltage and luminance (L) were jointly ac quired by an HR2000 Ocean Optic Spectrometer and a Keithley 238 measurement system.
2. Materials and methods PFO (Mw = 58200) and MEH-PPV (Mw = 40000) were purchased from Sigma-Aldrich (St. Louis, MO, USA). F7GA (Mw = 324.41) was purchased from Exciton (Dayton, Ohio, USA). Fig. S1 shows the chem ical structures of the three materials. All materials were dissolved in a toluene solvent manufactured by Fluka (Buchs, Switzerland). Both glass and indium tin oxide (ITO) substrates were manufactured by Merck Balzers (Balzers, Liechtenstein). Glass substrates were used to deposit all samples for measuring both absorption and photoluminescence (PL), whereas the ITO substrates were used to fabricate the device. Before device fabrication, the ITO substrates were etched and patterned by exposure to hydrochloric acid (HCl) and nitric acid (HNO3) vapors at a molar ratio of 3:1. Then, the materials were sequentially exposed to acetone and isopropanol for 15 min at each step under ultra-sonication for cleaning and eliminating impurities. Each material was prepared in toluene solvent as stock. MEH-PPV was added to PFO at a fixed weight ratio of 1.0 wt% to prepare the bi nary blend while sonicating for 30 min. Different weight ratios of F7GA (0.1, 0.5, 1.0, and 5.0 wt%) were added to the binary blend for 30 min under sonication to obtain homogeneous ternary hybrid solutions. For all solutions, the concentrations of PFO and MEH-PPV were fixed at 15 mg/mL and 0.5 mg/mL, respectively. Both binary and ternary blends were deposited on the glass and etched ITO substrates (2 cm × 1.2 cm) by spin-coating technique at a rotational speed of 2000 rpm for 0.5 min. The films were baked in a vacuum oven for 10 min at 120 ◦ C to remove toluene from the films. The thickness of all films was measured by a surface profilometer (Dektak 150, Bruker)), at an average thickness of 113 nm for the binary blend and slightly increased to reach 122 nm at 5 wt % of F7GA in the ternary blend. To complete the device fabrication
3. Results and discussion 3.1. Optical properties of the ternary hybrid thin films Two emitting conjugated polymers, PFO and MEH-PPV, and a com mercial laser dye (F7GA) were utilized as a ternary hybrid system. The normalized absorbance and photoluminescence spectra for each component are shown in Fig. 1. In this figure, there is a significant overlap between the emission of PFO and the absorption of MEH-PPV and F7GA, indicating that dual FRET is likely to be present in the ternary blend from PFO to both MEH-PPV and F7GA. Fig. 2 shows the energy levels of the ternary blend components and the work functions of the electrodes. Therefore, according to Figs. 1 and 2, when the conjugated polymers are molecularly mixed within the ¨rster radius, one can expect a cascade energy transfer from PFO to Fo F7GA and then MEH-PPV [18,19]. Fig. 3 shows the absorption spectra of ternary hybrid thin films with various F7GA contents. The absorbance of the PFO/MEH-PPV binary blend was reduced from 1.8 to 1.3 with the addition of F7GA, the peak being slightly redshifted to about 5 nm. This indicated a slight increase in the conjugated length of the donor. The decrease absorbance in the
Fig. 1. Normalized absorbance (solid lines) and photoluminescence spectra (dashed lines) of PFO (blue), MEH-PPV (red), and F7GA (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
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Fig. 2. Energy levels of PFO, F7GA, and MEH-PPV in dual FRET-based WOLED devices.
Fig. 3. Absorption spectra of binary PFO/MEH-PPV hybrid thin films with various amounts of F7GA. The inset is the absorption spectra in the range of 400–450 nm used to determine the cut-off wavelength.
ternary blend relative to the binary blend can be attributed to the fact that small molecules of F7GA can fill the spaces between the polymer chains, resulting in reduced light scattering and thus reduced the absorbance of the ternary blend. Moreover, since no new absorption peaks were detected in the increments of F7GA content, the ternary blends formed no dimers and had no ground-state interactions. The absorbance edge in the wavelength range of 400–450 nm, shown in the inset of Fig. 3, gradually redshifted with the increment of F7GA content. The absorption edge of each content of F7GA in the binary blend thin film was used to determine the cut-off wavelength (λcut-off) by the “Ab sorption Spectrum Fitting method” and calculate the width of the localized tails [20]. The energy that indicates the extension of tail depth levels into the forbidden energy gap below the absorption edge is called the energy tail (Etail). All λcut-off and Etail values are listed in Table 1. A gradual decrease in the Etail values was detected, indicating a decrease in topological, compositional, or structural disorders [21]. This suggests that the ternary blend has less compositional and structural disorders
Table 1 Optical parameters of ternary PFO/MEH-PPV/F7GA blend thin films. F7GA (wt. %)
λcut-off (nm)
Etail (eV)
σ × 10-3
0 0.1 0.5 1.0 5.0
424 429 441 443 444
2.924 2.890 2.811 2.799 2.792
8.890 8.995 9.247 9.289 9.309
compared to the PFO/MEH-PPV binary blend. The broadening/shrinkage of the optical absorption edge due to the electron-phonon or exciton-phonon interactions can be estimated from the steepness parameter (σ) values. As shown in Table 1, an increase in σ or a decrease in Etail with increasing F7GA can be ascribed to a decrease in the localized density of electronic states within the forbidden band gap of the ternary hybrids [22,23]. 3
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Fig. 4 shows the photoluminescence spectra of ternary PFO/MEHPPV/F7GA hybrid thin films with various contents of F7GA at the excitation wavelength of 355 nm. In the binary PFO/MEH-PPV blend thin films, one peak and one shoulder were observed for PFO at 435 nm and 450 nm, respectively, and for MEH-PPV at 568 nm and 607 nm, respectively. By adding F7GA, a new peak, referring to F7GA, appeared at about 490 nm)with higher intensity than its pure emission (and redshifted with increasing F7GA content. This peak was attributed to the efficient energy transfer from the PFO to the F7GA molecules, while redshifting was attributed to radiative migration due to self-absorption [24,25]. On the other hand, the intensity of the main peak corre sponding to PFO decreased dramatically, its shoulder gradually redshifted, and then disappeared in the blend thin film with 5.0 wt% F7GA content. Furthermore, the systematic decrease in PFO peak in tensity increased F7GA peak intensity, and the presence of MEH-PPV peaks due to the addition of F7GA confirmed the possibility of dual FRET occurrence in ternary blend thin films from PFO to F7GA and MEH-PPV. At 5 wt% F7GA content, the disappearance of the PFO peak intensity was probably due to complete energy transfer in the system, while the intensities of both F7GA and MEH-PPV peaks decreased instead of increasing. This indicates that some of their molecules were not emitted as fluorescence, but acted as dark quenchers without fluorescence due to the possibility of exciplex formation [24,26]. On the other hand, by incorporating the F7GA content into the binary blend of PFO/MEH-PPV, at ratios in the range of 0.1–1.0 wt%, three peaks cor responding to blue, green, and red emissions appeared, resulting in white light emission. Moreover, at high F7GA content (5 wt %), the PFO emission peak almost disappeared and the F7GA emission peak was significantly reduced, while most of the emissions mainly generated from MEH-PPV. This observation confirmed the cascade energy transfer in this ternary blend system [19]. Moreover, it is important to note that controlling energy transfer in a ternary blend system plays an important role in tuning the spectrum of white light emission.
3.2. Dual energy transfer parameters The mechanism of dual energy transfer from PFO (as a donor, D) to both F7GA and MEH-PPV (as acceptors, A) can be described by several parameters such as donor quantum yield in the presence of acceptor (ØDA), donor lifetime in the presence of acceptor (τDA), energy transfer probability (PDA), and energy transfer efficiency (η). All these parame ters were calculated and listed in Table 2. The proposed homogeneous quenching method showed that the addition of F7GA dramatically reduced the ∅DA and τDA values of PFO in ternary hybrid thin films, suggesting the possibility of radiative energy transfer. However, their short values were compared to those of the pristine PFO thin films (∅D = 0.72 and τD = 346 ps) [24], confirming efficient dual energy transfer from PFO to both acceptors. Both PDA and η values can be estimated based on the emission in tensity of PFO in the presence of the acceptors. Both were enhanced from 25.5 to 522.2 ns− 1 and from 88.7% to 99.4%, respectively, with the addition of F7GA to the hybrid thin films. This finding was another evidence of efficient dual energy transfer in ternary hybrid thin films. 3.3. Fo¨rster resonance energy transfer (FRET) parameters This section focuses on FRET from PFO to MEH-PPV in the presence of F7GA and from PFO to F7GA in the presence of MEH-PPV. The type of Table 2 Dual energy transfer (ET) parameters from PFO to both MEH-PPV and F7GA thin films. F7GA content (wt. %)
ØDA
τDA (ps)
PDA (ns)−
0 0.1 0.5 1.0 5.0
0.08 0.06 0.05 0.02 0.004
39.1 30.9 24.4 11.5 1.91
25.5 32.4 40.9 86.4 522.2
Fig. 4. Photoluminescence spectra of binary PFO/MEH-PPV hybrid thin film at various contents of F7GA. 4
1
η (%) 88.7 91.1 92.9 96.6 99.4
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energy transfer can be determined from the Ro values. The following equation can be used to calculate the Ro values in isotropic media [24, 27]: ∫ 5.89 × 10− 5 φD 5.89 × 10− 5 φD FD (λ)εA (λ) λ4 dλ = R6o = J(λ), 4 n n4
achieved in the ternary hybrid thin films. The distance between the donor and acceptor molecules (RDA) can be ¨rster radius and emission spectra of the donor in calculated using the Fo the presence or absence of the acceptors. Since the energy level of F7GA is closer to that of PFO than that of MEH-PPV (Fig. 2), the distance be tween the PFO and F7GA molecules is expected to be smaller than that of PFO and MEH-PPV. This expectation was confirmed by calculating RDA. As shown in Table 3, the distance between PFO and F7GA molecules ranged from 52.2 to 15.6 Å and the distance between PFO and MEH-PPV molecules ranged from 60.0 to 31.3 Å. On the other hand, the RDA values were more than 10 Å and less than 1.5 Ro, confirming that charge transfer from PFO to F7GA or MEH-PPV can be ignored [31]. This interpretation is consistent with previous results where there was a large overlap between PFO emission and F7GA and MEH-PPV absorbance, in addition to decreased PFO emission and increased acceptor emission [32,33]. However, as expected from the energy level diagram (Fig. 2), there may be a charge trapping effect between the F7GA and MEH-PPV acceptors and thus exciplex states can be formed at high content of the acceptor [34]. The formation of exciplexes, which generally formed by blending an acceptor with a donor, can be expected in the current blend from the very small distance (RDA) between electron-acceptor and electron-donor molecules, on the order of a few nanometers at most as confirmed in the other reports [35–37]. Moreover, as mentioned in previous reports [38,39], the formation of the exciplexes can be confirmed from the long decay components of acceptors in the blends, on the order of a nanosecond. This will be proved in the section below.
where n, FD (λ), and εA (λ) are the refractive index of the solvent (here for toluene n = 1.496), the normalized emission of the donor (extracted from Fig. 4), and the decadic molar extinction coefficient of the acceptor, respectively. The εA (λ) values of the acceptor as a function of λ, which are typically in units of M− 1 cm− 1, have been estimated using the Beer–Lambert law [27,28]:
εA (λ) =
A (λ) A (λ)Mw = Ct dt
where A (λ) is the absorbance as a function of λ (extracted from Fig. 3), d is the density of the acceptor, and t is the film thickness. Figs. 5 and 6 were used to calculate the integral values J(λ) for PFO/MEH-PPV in presence of F7GA and for PFO/F7GA in presence of MEH-PPV, respec tively, where λ varies from 365 to 700 nm. These values of J(λ) were employed to estimate the Ro values between PFO and MEH-PPV in the presence of F7GA and between PFO and F7GA in the presence of MEHPPV, as listed in Table 3. The Ro values for energy transfer from PFO to MEH-PPV in the presence of F7GA ranged from 84.6 to 74.4 Å, while for energy transfer from PFO to F7GA in the presence of MEH-PPV ranged from 76.9 to 37.0 Å. It can be seen that all these values were in the range of 10–100 Å, confirming the domination of non-radiative energy transfer ¨rster type) in the ternary hybrid thin films [29,30]. (Fo In the dual FRET mechanism, the PFO molecules are illuminated with light energy (355 nm, ~3.50 eV). Then, their oscillating dipoles can be generated and resonated with the dipoles of F7GA and MEH-PPV molecules. Subsequently, the excited state energy travels in space from the PFO molecules to the F7GA molecules without electron ex change (dipole-dipole interaction). Non-radiative energy transfer ¨rster type) occurs when the PFO molecules return to the ground state (Fo and the F7GA molecules enter the excited state. Since similar processes occur for the MEH-PPV molecules, dual FRET mechanism can be
3.4. Lifetime decay Unlike the calculated fluorescence lifetime, the mean fluorescence lifetime includes radiative emission and obtained from analysis lifetime decay [40]. Fig. 7 shows the lifetime decay obtained at 435 nm, 495 nm, and 570 nm, corresponding to the emission regions of PFO, F7GA, and MEH-PPV, respectively. The values of mean fluorescence lifetime are presented in Table 4. The existence of one or two lifetimes can be explained by the existence of one or two conformers [41]. This makes it possible to compare various decays and understand the impact of F7GA
Fig. 5. FD(λ) εA(λ) λ4 vis wavelength of ET from PFO to MEH-PPV in the presence of various amounts of F7GA. 5
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Fig. 6. FD(λ) εA(λ) λ4 vis wavelength of ET from PFO to various amounts of F7GA in the presence of 1.0 wt% of MEH-PPV.
to the binary blend, a new peak began to appear gradually with refer ence to the green emission from F7GA. The existence of three mixed colors (blue, green, and red) in all devices based on the ternary hybrids provided a great possibility to realize WOLED. Compared to the PL spectra with 5 wt% F7GA, the blue emission disappeared in the EL spectrum. The PFO shoulder peak at 450 nm in the PL spectrum dis appeared in the EL spectrum. This suggests that the carriers trapped by acceptors simultaneously occurred in addition to the normal dual FRET. In the case of PL, photoexcitation did not generate free carriers. Fig. 9 shows the influence of F7GA content on the current density (J) of a binary PFO/MEH-PPV hybrid under the forward bias (V) condition. A systematic decrease in the current density was observed with increasing F7GA content, improving the resistivity of the light-emitting layer [45]. This decrease in the current density suggested an increase in the exciton confinement and recombination efficiency, which are important for improving device performance [46,47]. Furthermore, this J-V behavior can be confirmed by the occurrence of charge trapping processes and dual FRET in the device based on ternary blends, which is consistent with a previous report [32]. On the other hand, the turn-on voltage decreased by increasing F7GA content, leading to the improve ment of device performance. Fig. 10 shows the luminance vs. current density characteristics of the ITO/PFO: MEH-PPV: diff. wt.% F7GA/Al devices. The maximum lumi nance of 2295 cd/m2 was achieved for the device with 1.0 wt% F7GA at the current density of 12.98 mA/cm2 and CIE coordinates of (0.31, 0.24), confirming white light emission. Table 5 shows the maximum luminance of each device, its corresponding current density, and CIE coordinates. For the device with 5 wt% F7GA, the luminance was significantly reduced and the device deviated from the white light emission. This is consistent with the results of the PL and EL spectra where the energy transfer is complete and the blue color disappears.
Table 3 Parameters of dual FRET from PFO to both MEH-PPV and F7GA. For ET from PFO to MEH-PPV in presence of F7GA
For ET from PFO to F7GA in presence of MEH-PPV
F7GA in the blend (wt. %)
J(λ) × 1016
J(λ) × 1015
0 0.1 0.5 1.0 5.0
4.33 3.07 2.78 3.22 2.01
R0 (Å)
RDA (Å)
τET
(ps)
− 1
R0 (Å)
RDA (Å)
τET
76.9 60.6 52.8 37.0
52.2 39.4 30.1 15.6
102 20.4 10.1 1.94
(ps)
− 1
(M . cm− 1. nm4)
(M . cm− 1. nm4) 84.6 79.9 78.6 80.5 74.4
60.0 54.2 51.1 45.9 31.3
44.1 33.9 26.3 11.9 1.92
24.5 5.83 2.56 0.31
content on hybrids. As shown in Table 4, the addition of the acceptor contents significantly reduced the mean fluorescence lifetime at 435 nm, confirming the efficient energy transfer in ternary hybrid thin films. On the other hand, the offset energy levels between PFO and F7GA on one side and between PFO and MEH-PPV on the other side indicated the formation of exciplexes between the donor and the two acceptors. Consequently, the long decay components of F7GA and MEH-PPV emissions were decayed at 495 nm and 570 nm, respectively, due to the existence of exciplexes between the acceptor excited state molecules and the donor ground-state molecules. This agrees with previous reports on PFO/MEH-PPV [26,39] and PFO/F7GA hybrid thin films [42]. As evidenced in recent reports of ternary blend [43,44], the dual FRET from PFO to both F7GA and MEH-PPV can be also confirmed from the shortened lifetime of PFO and enhanced that of MEH-PPV and F7GA. 3.5. White organic light-emitting diode (WOLED) characterization
4. Conclusion
Fig. 8 shows the EL spectra of all devices. The spectra show a broad visible emission from 400 to 700 nm. Devices based on the binary PFO/ MEH-PPV blend displayed two emission peaks. The first peak was associated with blue emission from PFO and the second peak was associated with red emission from MEH-PPV. With the addition of F7GA
The ternary PFO/MEH-PPV/F7GA hybrid thin films with various weight ratios of F7GA were used to investigate the dual FRET mecha nism and WOLED performance. Efficient dual FRET from PFO to F7GA 6
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Fig. 7. Lifetime decay in ternary hybrid thin films at (a) λem = 435 nm, (b) λem = 495 nm, and (c) λem = 570 nm. Table 4 Main fitting parameters and mean fluorescence lifetime (τ) of PFO/MEH-PPV thin films at 0, 0.1, 0.5, 1.0, and 5.0 wt% F7GA. At λem. 435 nm F7GA (wt. %) 0 0.1 0.5 1.0 5.0
At λem. 495 nm Relative amplitude
Donor lifetime
B
τ (ps)
0.495 0.542 0.271 0.427 0.474
211 195 134 110 87
χ2
Relative amplitude
Acceptor lifetime (ps)
τ (ps)
0.911 0.821 1.199 0.796 0.787
– 0.141 0.097 0.086 0.075
– 628 1417 1916 876
– 3879 4432 4540 3070
– 0.026 0.039 0.04 0.056
– 5792 6158 6266 3756
χ2
–
0.907 1.200 0.786 0.723
At λem. 570 nm F7GA (wt. %)
Relative amplitude
0 0.1 0.5 1.0 5.0
0.032 0.169 0.194 0.14 0.132
Acceptor lifetime (ps) 0.002 0.008 0.016 0.019 0.014
827 700 804 1095 1112
and MEH-PPV was confirmed by the large overlap between donor emission and acceptor absorptions, in addition to the values of energy transfer parameters and fluorescence lifetime. The dual energy transfer efficiency was enhanced from 88.7 to 99.4% with the addition of F7GA (0–5.0 wt%) to the hybrid thin films. For energy transfer from PFO to MEH-PPV and from PFO to F7GA, the Ro values were in the range 84.6–74.4 Å and 76.9–37.0 Å, respectively, confirming the dominant
2678 4696 4967 5233 4539
τ (ps)
χ2
1138 1663 2209 2723 2147
0.964 0.844 0.900 0.825 0.867
¨rster type energy transfer in the blend. The distance between PFO and Fo F7GA molecules was in the range of 52.2–15.6 Å and the distance be tween PFO and MEH-PPV molecules was in the range 60.0–31.3 Å, confirming that the charge transfer from donor to the two acceptors can be ignored. The mean fluorescence lifetime at 435 nm decreased significantly with increasing acceptor contents, confirming efficient energy transfer in the ternary hybrid thin films. But at a longer lifetime, 7
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Fig. 8. EL spectra of PFO/MEH-PPV with different contents of the F7GA device at voltages corresponding to maximum luminance.
Fig. 9. J-V characteristics of ternary hybrids based on OLED devices.
the components decayed at 495 nm and 570 nm. This suggests that an exciplex exists between the donor and the two acceptors. The presence of peaks corresponding to the blue, green, and red colors in the PL and EL spectra confirms white light emission and realizes WOLEDs. How ever, the charge trapping effect between F7GA and MEH-PPV can occur
in WOLED devices combined with dual FRET, as proven by the device characteristics. The optimal WOLED belonged to an emissive layer with 1.0 wt% F7GA in a ternary blend with a maximum luminance of 2295 cd/m2 at a current density of 12.98 mA/cm2 and CIE coordinate of (0.31, 0.24). 8
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Fig. 10. L-J characteristics of ternary hybrids based on OLED devices.
Acknowledgment
Table 5 Performance of the ITO/PFO: MEH-PPV: diff. wt.% F7GA/Al devices. F7GA (wt. %)
J (mA/cm2)a
Lmax (cd/m2)
CIE coordinatesa (x, y)
0 0.1 0.5 1.0 5.0
19.16 12.03 23.76 12.98 7.81
1726 2069 1218 2295 414
(0.38, (0.31, (0.31, (0.30, (0.29,
a
The author extends his appreciation to the Deputyship for Research & Innovation, “Ministry of Education” in Saudi Arabia for funding this research work through the project number IFKSUHI-1440-037.
0.51) 0.32) 0.26) 0.24) 0.22)
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.109011.
At maximum luminance.
Funding
Supplementary materials
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Supplementary material associated with this article can be found, in the online version, at doi:
Associated content
References
Supplemental information includes: lifetime decay with theoretical curves for all ternary samples at various emission wavelengths; the chemical structures of PFO, MEH-PPV and F7GA; the schematic struc ture of the ternary hybrid-based OLED. This material is available free of charge via the internet.
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CRediT authorship contribution statement Bandar Ali Al-Asbahi: Conceptualization, Methodology, Investiga tion, Visualization, Formal analysis, Writing. Declaration of competing interest 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.
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
¨rster resonance energy transfer in ternary PFO/MEH-PPV/F7GA Dual Fo hybrid thin films for white organic light-emitting diodes Bandar Ali Al-Asbahi a, b a b
Department of Physics & Astronomy, College of Sciences, King Saud University, Saudi Arabia Department of Physics, Faculty of Science, Sana’a University, Yemen
A R T I C L E I N F O
A B S T R A C T
Keywords: Ternary blend Thin films Dual f¨ orster resonance energy transfer Electroluminescence WOLED
The ternary hybrid thin films of poly [9,9′ -di-n-octylfluorenyl-2,7-diyl] (PFO)/poly [2methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/2-butyl-6- (butylamino) benzo [de] isoquinoline-1,3-dione (F7GA) were successfully prepared with various ratios of F7GA. Dual F¨ orster resonance energy transfer (FRET) from PFO to both F7GA and MEH-PPV was confirmed in the ternary thin films. The distance between PFO and F7GA molecules was smaller than the distance between PFO and MEH-PPV. Besides, white light emission was achieved with cascade energy transfer in this ternary blend system. The lifetime decay components of the acceptor emission were attributed to the presence of an exciplex between the molecules in the acceptor excited state and the donor ground state. In addition to the dual FRET, carrier trapping took place in the device, suc cessfully realizing white organic light-emitting diodes (WOLEDs). The maximum luminance of 2295 cd/m2 and a current density of 12.98 mA/cm2 at CIE coordinates of (0.31, 0.24) was achieved with the device at 1.0 wt% F7GA.
1. Introduction White organic light-emitting diodes (WOLEDs) based on conjugated polymers or small molecules have been extensively investigated due to their various potential applications for flat panel displays and large-area flexibility [1]. Although many techniques have been suggested for full-color organic displays [2], white light emission combined with the color filter method provides a relatively simple method. Since white light emission is typically achieved by mixing three major colors (blue, green, and red), at least two emissive materials are deposited in a multilayer structure [3–5], or mixed as a single layer by blending (or doping) [6–8]. Unfortunately, building multilayer devices is difficult, especially by solution processing where it is difficult to find a suitable solvent for each layer without dissolving the underlayers. Therefore, if WOLEDs can be manufactured with a simpler architecture and an easier operation by avoiding vacuum deposition of white electroluminescent components at low cost, the single-layer method is preferred. Several methods have been reported to improve the WOLED performance such as multilayer device fabrication [9], use of electron and hole transport layers [9], a perfect match between component energy level and elec ¨rster resonance trode work function [10], and the introduction of Fo energy transfer (FRET) effects in the active layer of the device [11]. The
latter technique was utilized in the current work to produce WOLED with better performance. FRET excitation in the blend requires good mixing of the two materials and good spectral overlap between acceptor absorption and donor emission [12]. Mixing both donors and acceptors for energy transfer can significantly reduce the concentration quenching of the excitons produced, improving device performance [13,14]. In this work, we used the ternary blend of poly [9,9′ -di-n-octyl fluorenyl-2,7-diyl] (synonym: PFO), poly [2methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (synonym: MEH-PPV), and 2-butyl6- (butylamino) benzo [de] isoquinoline-1,3-dione (synonym: Fluorol 7 GA or F7GA) to achieve cascaded energy transfer for tuning emission colors and improving device performance. Three different emission components of PFO/F7GA/MEH-PPV with the bandgap order of PFO (3.0 eV) > F7GA (2.3 eV) > MEH-PPV (2.2 eV) [15–17] were miscible with each other. Moreover, the donor emission spectrum (PFO) and the acceptor absorption spectrum (F7GA or MEH-PPV) overlapped signifi cantly. The main component of PFO acted as a diluent, matrix, and excitation energy donor for the ternary blend, producing light with high efficiency. Therefore, when the ternary blend was excited near the ab sorption peak wavelength of PFO, light emission from F7GA and MEH-PPV was expected, suggesting the cascade energy transfer used for WOLEDs. Since research on the photophysical mechanism of ternary
E-mail address: [email protected]. https://doi.org/10.1016/j.dyepig.2020.109011 Received 27 August 2020; Received in revised form 14 November 2020; Accepted 15 November 2020 Available online 21 November 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Bandar Ali Al-Asbahi, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109011
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hybrid systems is rare, the current research concentrated in detail on the study of dual FRET mechanism and showed how such a mechanism can develop the performance of WOLEDs.
(with schematic structure shown in Fig. S2), the deposited ITO sub strates were transferred to an electron beam chamber to create an aluminum cathode with a thickness of 150 nm and an active area of 0.076 cm2. The deposition rate and chamber pressure were 2 Å/min and 2.5 × 106 Pa, respectively. Absorption and photoluminescence (PL) spectra were collected by PerkinElmer Lambda 900 UV-VIS and PerkinElmer (LS55) Luminescent Spectrophotometers, respectively. The lifetime decays, Figs. S3 − S15, were collected using Edinburgh Instrument FLSP920 spectrophotom eter. The electroluminescence (EL) spectra and the relationship between current density (J) and bias voltage and luminance (L) were jointly ac quired by an HR2000 Ocean Optic Spectrometer and a Keithley 238 measurement system.
2. Materials and methods PFO (Mw = 58200) and MEH-PPV (Mw = 40000) were purchased from Sigma-Aldrich (St. Louis, MO, USA). F7GA (Mw = 324.41) was purchased from Exciton (Dayton, Ohio, USA). Fig. S1 shows the chem ical structures of the three materials. All materials were dissolved in a toluene solvent manufactured by Fluka (Buchs, Switzerland). Both glass and indium tin oxide (ITO) substrates were manufactured by Merck Balzers (Balzers, Liechtenstein). Glass substrates were used to deposit all samples for measuring both absorption and photoluminescence (PL), whereas the ITO substrates were used to fabricate the device. Before device fabrication, the ITO substrates were etched and patterned by exposure to hydrochloric acid (HCl) and nitric acid (HNO3) vapors at a molar ratio of 3:1. Then, the materials were sequentially exposed to acetone and isopropanol for 15 min at each step under ultra-sonication for cleaning and eliminating impurities. Each material was prepared in toluene solvent as stock. MEH-PPV was added to PFO at a fixed weight ratio of 1.0 wt% to prepare the bi nary blend while sonicating for 30 min. Different weight ratios of F7GA (0.1, 0.5, 1.0, and 5.0 wt%) were added to the binary blend for 30 min under sonication to obtain homogeneous ternary hybrid solutions. For all solutions, the concentrations of PFO and MEH-PPV were fixed at 15 mg/mL and 0.5 mg/mL, respectively. Both binary and ternary blends were deposited on the glass and etched ITO substrates (2 cm × 1.2 cm) by spin-coating technique at a rotational speed of 2000 rpm for 0.5 min. The films were baked in a vacuum oven for 10 min at 120 ◦ C to remove toluene from the films. The thickness of all films was measured by a surface profilometer (Dektak 150, Bruker)), at an average thickness of 113 nm for the binary blend and slightly increased to reach 122 nm at 5 wt % of F7GA in the ternary blend. To complete the device fabrication
3. Results and discussion 3.1. Optical properties of the ternary hybrid thin films Two emitting conjugated polymers, PFO and MEH-PPV, and a com mercial laser dye (F7GA) were utilized as a ternary hybrid system. The normalized absorbance and photoluminescence spectra for each component are shown in Fig. 1. In this figure, there is a significant overlap between the emission of PFO and the absorption of MEH-PPV and F7GA, indicating that dual FRET is likely to be present in the ternary blend from PFO to both MEH-PPV and F7GA. Fig. 2 shows the energy levels of the ternary blend components and the work functions of the electrodes. Therefore, according to Figs. 1 and 2, when the conjugated polymers are molecularly mixed within the ¨rster radius, one can expect a cascade energy transfer from PFO to Fo F7GA and then MEH-PPV [18,19]. Fig. 3 shows the absorption spectra of ternary hybrid thin films with various F7GA contents. The absorbance of the PFO/MEH-PPV binary blend was reduced from 1.8 to 1.3 with the addition of F7GA, the peak being slightly redshifted to about 5 nm. This indicated a slight increase in the conjugated length of the donor. The decrease absorbance in the
Fig. 1. Normalized absorbance (solid lines) and photoluminescence spectra (dashed lines) of PFO (blue), MEH-PPV (red), and F7GA (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
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Fig. 2. Energy levels of PFO, F7GA, and MEH-PPV in dual FRET-based WOLED devices.
Fig. 3. Absorption spectra of binary PFO/MEH-PPV hybrid thin films with various amounts of F7GA. The inset is the absorption spectra in the range of 400–450 nm used to determine the cut-off wavelength.
ternary blend relative to the binary blend can be attributed to the fact that small molecules of F7GA can fill the spaces between the polymer chains, resulting in reduced light scattering and thus reduced the absorbance of the ternary blend. Moreover, since no new absorption peaks were detected in the increments of F7GA content, the ternary blends formed no dimers and had no ground-state interactions. The absorbance edge in the wavelength range of 400–450 nm, shown in the inset of Fig. 3, gradually redshifted with the increment of F7GA content. The absorption edge of each content of F7GA in the binary blend thin film was used to determine the cut-off wavelength (λcut-off) by the “Ab sorption Spectrum Fitting method” and calculate the width of the localized tails [20]. The energy that indicates the extension of tail depth levels into the forbidden energy gap below the absorption edge is called the energy tail (Etail). All λcut-off and Etail values are listed in Table 1. A gradual decrease in the Etail values was detected, indicating a decrease in topological, compositional, or structural disorders [21]. This suggests that the ternary blend has less compositional and structural disorders
Table 1 Optical parameters of ternary PFO/MEH-PPV/F7GA blend thin films. F7GA (wt. %)
λcut-off (nm)
Etail (eV)
σ × 10-3
0 0.1 0.5 1.0 5.0
424 429 441 443 444
2.924 2.890 2.811 2.799 2.792
8.890 8.995 9.247 9.289 9.309
compared to the PFO/MEH-PPV binary blend. The broadening/shrinkage of the optical absorption edge due to the electron-phonon or exciton-phonon interactions can be estimated from the steepness parameter (σ) values. As shown in Table 1, an increase in σ or a decrease in Etail with increasing F7GA can be ascribed to a decrease in the localized density of electronic states within the forbidden band gap of the ternary hybrids [22,23]. 3
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Fig. 4 shows the photoluminescence spectra of ternary PFO/MEHPPV/F7GA hybrid thin films with various contents of F7GA at the excitation wavelength of 355 nm. In the binary PFO/MEH-PPV blend thin films, one peak and one shoulder were observed for PFO at 435 nm and 450 nm, respectively, and for MEH-PPV at 568 nm and 607 nm, respectively. By adding F7GA, a new peak, referring to F7GA, appeared at about 490 nm)with higher intensity than its pure emission (and redshifted with increasing F7GA content. This peak was attributed to the efficient energy transfer from the PFO to the F7GA molecules, while redshifting was attributed to radiative migration due to self-absorption [24,25]. On the other hand, the intensity of the main peak corre sponding to PFO decreased dramatically, its shoulder gradually redshifted, and then disappeared in the blend thin film with 5.0 wt% F7GA content. Furthermore, the systematic decrease in PFO peak in tensity increased F7GA peak intensity, and the presence of MEH-PPV peaks due to the addition of F7GA confirmed the possibility of dual FRET occurrence in ternary blend thin films from PFO to F7GA and MEH-PPV. At 5 wt% F7GA content, the disappearance of the PFO peak intensity was probably due to complete energy transfer in the system, while the intensities of both F7GA and MEH-PPV peaks decreased instead of increasing. This indicates that some of their molecules were not emitted as fluorescence, but acted as dark quenchers without fluorescence due to the possibility of exciplex formation [24,26]. On the other hand, by incorporating the F7GA content into the binary blend of PFO/MEH-PPV, at ratios in the range of 0.1–1.0 wt%, three peaks cor responding to blue, green, and red emissions appeared, resulting in white light emission. Moreover, at high F7GA content (5 wt %), the PFO emission peak almost disappeared and the F7GA emission peak was significantly reduced, while most of the emissions mainly generated from MEH-PPV. This observation confirmed the cascade energy transfer in this ternary blend system [19]. Moreover, it is important to note that controlling energy transfer in a ternary blend system plays an important role in tuning the spectrum of white light emission.
3.2. Dual energy transfer parameters The mechanism of dual energy transfer from PFO (as a donor, D) to both F7GA and MEH-PPV (as acceptors, A) can be described by several parameters such as donor quantum yield in the presence of acceptor (ØDA), donor lifetime in the presence of acceptor (τDA), energy transfer probability (PDA), and energy transfer efficiency (η). All these parame ters were calculated and listed in Table 2. The proposed homogeneous quenching method showed that the addition of F7GA dramatically reduced the ∅DA and τDA values of PFO in ternary hybrid thin films, suggesting the possibility of radiative energy transfer. However, their short values were compared to those of the pristine PFO thin films (∅D = 0.72 and τD = 346 ps) [24], confirming efficient dual energy transfer from PFO to both acceptors. Both PDA and η values can be estimated based on the emission in tensity of PFO in the presence of the acceptors. Both were enhanced from 25.5 to 522.2 ns− 1 and from 88.7% to 99.4%, respectively, with the addition of F7GA to the hybrid thin films. This finding was another evidence of efficient dual energy transfer in ternary hybrid thin films. 3.3. Fo¨rster resonance energy transfer (FRET) parameters This section focuses on FRET from PFO to MEH-PPV in the presence of F7GA and from PFO to F7GA in the presence of MEH-PPV. The type of Table 2 Dual energy transfer (ET) parameters from PFO to both MEH-PPV and F7GA thin films. F7GA content (wt. %)
ØDA
τDA (ps)
PDA (ns)−
0 0.1 0.5 1.0 5.0
0.08 0.06 0.05 0.02 0.004
39.1 30.9 24.4 11.5 1.91
25.5 32.4 40.9 86.4 522.2
Fig. 4. Photoluminescence spectra of binary PFO/MEH-PPV hybrid thin film at various contents of F7GA. 4
1
η (%) 88.7 91.1 92.9 96.6 99.4
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energy transfer can be determined from the Ro values. The following equation can be used to calculate the Ro values in isotropic media [24, 27]: ∫ 5.89 × 10− 5 φD 5.89 × 10− 5 φD FD (λ)εA (λ) λ4 dλ = R6o = J(λ), 4 n n4
achieved in the ternary hybrid thin films. The distance between the donor and acceptor molecules (RDA) can be ¨rster radius and emission spectra of the donor in calculated using the Fo the presence or absence of the acceptors. Since the energy level of F7GA is closer to that of PFO than that of MEH-PPV (Fig. 2), the distance be tween the PFO and F7GA molecules is expected to be smaller than that of PFO and MEH-PPV. This expectation was confirmed by calculating RDA. As shown in Table 3, the distance between PFO and F7GA molecules ranged from 52.2 to 15.6 Å and the distance between PFO and MEH-PPV molecules ranged from 60.0 to 31.3 Å. On the other hand, the RDA values were more than 10 Å and less than 1.5 Ro, confirming that charge transfer from PFO to F7GA or MEH-PPV can be ignored [31]. This interpretation is consistent with previous results where there was a large overlap between PFO emission and F7GA and MEH-PPV absorbance, in addition to decreased PFO emission and increased acceptor emission [32,33]. However, as expected from the energy level diagram (Fig. 2), there may be a charge trapping effect between the F7GA and MEH-PPV acceptors and thus exciplex states can be formed at high content of the acceptor [34]. The formation of exciplexes, which generally formed by blending an acceptor with a donor, can be expected in the current blend from the very small distance (RDA) between electron-acceptor and electron-donor molecules, on the order of a few nanometers at most as confirmed in the other reports [35–37]. Moreover, as mentioned in previous reports [38,39], the formation of the exciplexes can be confirmed from the long decay components of acceptors in the blends, on the order of a nanosecond. This will be proved in the section below.
where n, FD (λ), and εA (λ) are the refractive index of the solvent (here for toluene n = 1.496), the normalized emission of the donor (extracted from Fig. 4), and the decadic molar extinction coefficient of the acceptor, respectively. The εA (λ) values of the acceptor as a function of λ, which are typically in units of M− 1 cm− 1, have been estimated using the Beer–Lambert law [27,28]:
εA (λ) =
A (λ) A (λ)Mw = Ct dt
where A (λ) is the absorbance as a function of λ (extracted from Fig. 3), d is the density of the acceptor, and t is the film thickness. Figs. 5 and 6 were used to calculate the integral values J(λ) for PFO/MEH-PPV in presence of F7GA and for PFO/F7GA in presence of MEH-PPV, respec tively, where λ varies from 365 to 700 nm. These values of J(λ) were employed to estimate the Ro values between PFO and MEH-PPV in the presence of F7GA and between PFO and F7GA in the presence of MEHPPV, as listed in Table 3. The Ro values for energy transfer from PFO to MEH-PPV in the presence of F7GA ranged from 84.6 to 74.4 Å, while for energy transfer from PFO to F7GA in the presence of MEH-PPV ranged from 76.9 to 37.0 Å. It can be seen that all these values were in the range of 10–100 Å, confirming the domination of non-radiative energy transfer ¨rster type) in the ternary hybrid thin films [29,30]. (Fo In the dual FRET mechanism, the PFO molecules are illuminated with light energy (355 nm, ~3.50 eV). Then, their oscillating dipoles can be generated and resonated with the dipoles of F7GA and MEH-PPV molecules. Subsequently, the excited state energy travels in space from the PFO molecules to the F7GA molecules without electron ex change (dipole-dipole interaction). Non-radiative energy transfer ¨rster type) occurs when the PFO molecules return to the ground state (Fo and the F7GA molecules enter the excited state. Since similar processes occur for the MEH-PPV molecules, dual FRET mechanism can be
3.4. Lifetime decay Unlike the calculated fluorescence lifetime, the mean fluorescence lifetime includes radiative emission and obtained from analysis lifetime decay [40]. Fig. 7 shows the lifetime decay obtained at 435 nm, 495 nm, and 570 nm, corresponding to the emission regions of PFO, F7GA, and MEH-PPV, respectively. The values of mean fluorescence lifetime are presented in Table 4. The existence of one or two lifetimes can be explained by the existence of one or two conformers [41]. This makes it possible to compare various decays and understand the impact of F7GA
Fig. 5. FD(λ) εA(λ) λ4 vis wavelength of ET from PFO to MEH-PPV in the presence of various amounts of F7GA. 5
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Fig. 6. FD(λ) εA(λ) λ4 vis wavelength of ET from PFO to various amounts of F7GA in the presence of 1.0 wt% of MEH-PPV.
to the binary blend, a new peak began to appear gradually with refer ence to the green emission from F7GA. The existence of three mixed colors (blue, green, and red) in all devices based on the ternary hybrids provided a great possibility to realize WOLED. Compared to the PL spectra with 5 wt% F7GA, the blue emission disappeared in the EL spectrum. The PFO shoulder peak at 450 nm in the PL spectrum dis appeared in the EL spectrum. This suggests that the carriers trapped by acceptors simultaneously occurred in addition to the normal dual FRET. In the case of PL, photoexcitation did not generate free carriers. Fig. 9 shows the influence of F7GA content on the current density (J) of a binary PFO/MEH-PPV hybrid under the forward bias (V) condition. A systematic decrease in the current density was observed with increasing F7GA content, improving the resistivity of the light-emitting layer [45]. This decrease in the current density suggested an increase in the exciton confinement and recombination efficiency, which are important for improving device performance [46,47]. Furthermore, this J-V behavior can be confirmed by the occurrence of charge trapping processes and dual FRET in the device based on ternary blends, which is consistent with a previous report [32]. On the other hand, the turn-on voltage decreased by increasing F7GA content, leading to the improve ment of device performance. Fig. 10 shows the luminance vs. current density characteristics of the ITO/PFO: MEH-PPV: diff. wt.% F7GA/Al devices. The maximum lumi nance of 2295 cd/m2 was achieved for the device with 1.0 wt% F7GA at the current density of 12.98 mA/cm2 and CIE coordinates of (0.31, 0.24), confirming white light emission. Table 5 shows the maximum luminance of each device, its corresponding current density, and CIE coordinates. For the device with 5 wt% F7GA, the luminance was significantly reduced and the device deviated from the white light emission. This is consistent with the results of the PL and EL spectra where the energy transfer is complete and the blue color disappears.
Table 3 Parameters of dual FRET from PFO to both MEH-PPV and F7GA. For ET from PFO to MEH-PPV in presence of F7GA
For ET from PFO to F7GA in presence of MEH-PPV
F7GA in the blend (wt. %)
J(λ) × 1016
J(λ) × 1015
0 0.1 0.5 1.0 5.0
4.33 3.07 2.78 3.22 2.01
R0 (Å)
RDA (Å)
τET
(ps)
− 1
R0 (Å)
RDA (Å)
τET
76.9 60.6 52.8 37.0
52.2 39.4 30.1 15.6
102 20.4 10.1 1.94
(ps)
− 1
(M . cm− 1. nm4)
(M . cm− 1. nm4) 84.6 79.9 78.6 80.5 74.4
60.0 54.2 51.1 45.9 31.3
44.1 33.9 26.3 11.9 1.92
24.5 5.83 2.56 0.31
content on hybrids. As shown in Table 4, the addition of the acceptor contents significantly reduced the mean fluorescence lifetime at 435 nm, confirming the efficient energy transfer in ternary hybrid thin films. On the other hand, the offset energy levels between PFO and F7GA on one side and between PFO and MEH-PPV on the other side indicated the formation of exciplexes between the donor and the two acceptors. Consequently, the long decay components of F7GA and MEH-PPV emissions were decayed at 495 nm and 570 nm, respectively, due to the existence of exciplexes between the acceptor excited state molecules and the donor ground-state molecules. This agrees with previous reports on PFO/MEH-PPV [26,39] and PFO/F7GA hybrid thin films [42]. As evidenced in recent reports of ternary blend [43,44], the dual FRET from PFO to both F7GA and MEH-PPV can be also confirmed from the shortened lifetime of PFO and enhanced that of MEH-PPV and F7GA. 3.5. White organic light-emitting diode (WOLED) characterization
4. Conclusion
Fig. 8 shows the EL spectra of all devices. The spectra show a broad visible emission from 400 to 700 nm. Devices based on the binary PFO/ MEH-PPV blend displayed two emission peaks. The first peak was associated with blue emission from PFO and the second peak was associated with red emission from MEH-PPV. With the addition of F7GA
The ternary PFO/MEH-PPV/F7GA hybrid thin films with various weight ratios of F7GA were used to investigate the dual FRET mecha nism and WOLED performance. Efficient dual FRET from PFO to F7GA 6
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Fig. 7. Lifetime decay in ternary hybrid thin films at (a) λem = 435 nm, (b) λem = 495 nm, and (c) λem = 570 nm. Table 4 Main fitting parameters and mean fluorescence lifetime (τ) of PFO/MEH-PPV thin films at 0, 0.1, 0.5, 1.0, and 5.0 wt% F7GA. At λem. 435 nm F7GA (wt. %) 0 0.1 0.5 1.0 5.0
At λem. 495 nm Relative amplitude
Donor lifetime
B
τ (ps)
0.495 0.542 0.271 0.427 0.474
211 195 134 110 87
χ2
Relative amplitude
Acceptor lifetime (ps)
τ (ps)
0.911 0.821 1.199 0.796 0.787
– 0.141 0.097 0.086 0.075
– 628 1417 1916 876
– 3879 4432 4540 3070
– 0.026 0.039 0.04 0.056
– 5792 6158 6266 3756
χ2
–
0.907 1.200 0.786 0.723
At λem. 570 nm F7GA (wt. %)
Relative amplitude
0 0.1 0.5 1.0 5.0
0.032 0.169 0.194 0.14 0.132
Acceptor lifetime (ps) 0.002 0.008 0.016 0.019 0.014
827 700 804 1095 1112
and MEH-PPV was confirmed by the large overlap between donor emission and acceptor absorptions, in addition to the values of energy transfer parameters and fluorescence lifetime. The dual energy transfer efficiency was enhanced from 88.7 to 99.4% with the addition of F7GA (0–5.0 wt%) to the hybrid thin films. For energy transfer from PFO to MEH-PPV and from PFO to F7GA, the Ro values were in the range 84.6–74.4 Å and 76.9–37.0 Å, respectively, confirming the dominant
2678 4696 4967 5233 4539
τ (ps)
χ2
1138 1663 2209 2723 2147
0.964 0.844 0.900 0.825 0.867
¨rster type energy transfer in the blend. The distance between PFO and Fo F7GA molecules was in the range of 52.2–15.6 Å and the distance be tween PFO and MEH-PPV molecules was in the range 60.0–31.3 Å, confirming that the charge transfer from donor to the two acceptors can be ignored. The mean fluorescence lifetime at 435 nm decreased significantly with increasing acceptor contents, confirming efficient energy transfer in the ternary hybrid thin films. But at a longer lifetime, 7
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Fig. 8. EL spectra of PFO/MEH-PPV with different contents of the F7GA device at voltages corresponding to maximum luminance.
Fig. 9. J-V characteristics of ternary hybrids based on OLED devices.
the components decayed at 495 nm and 570 nm. This suggests that an exciplex exists between the donor and the two acceptors. The presence of peaks corresponding to the blue, green, and red colors in the PL and EL spectra confirms white light emission and realizes WOLEDs. How ever, the charge trapping effect between F7GA and MEH-PPV can occur
in WOLED devices combined with dual FRET, as proven by the device characteristics. The optimal WOLED belonged to an emissive layer with 1.0 wt% F7GA in a ternary blend with a maximum luminance of 2295 cd/m2 at a current density of 12.98 mA/cm2 and CIE coordinate of (0.31, 0.24). 8
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Fig. 10. L-J characteristics of ternary hybrids based on OLED devices.
Acknowledgment
Table 5 Performance of the ITO/PFO: MEH-PPV: diff. wt.% F7GA/Al devices. F7GA (wt. %)
J (mA/cm2)a
Lmax (cd/m2)
CIE coordinatesa (x, y)
0 0.1 0.5 1.0 5.0
19.16 12.03 23.76 12.98 7.81
1726 2069 1218 2295 414
(0.38, (0.31, (0.31, (0.30, (0.29,
a
The author extends his appreciation to the Deputyship for Research & Innovation, “Ministry of Education” in Saudi Arabia for funding this research work through the project number IFKSUHI-1440-037.
0.51) 0.32) 0.26) 0.24) 0.22)
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.109011.
At maximum luminance.
Funding
Supplementary materials
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Supplementary material associated with this article can be found, in the online version, at doi:
Associated content
References
Supplemental information includes: lifetime decay with theoretical curves for all ternary samples at various emission wavelengths; the chemical structures of PFO, MEH-PPV and F7GA; the schematic struc ture of the ternary hybrid-based OLED. This material is available free of charge via the internet.
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CRediT authorship contribution statement Bandar Ali Al-Asbahi: Conceptualization, Methodology, Investiga tion, Visualization, Formal analysis, Writing. Declaration of competing interest 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.
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