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(a) Chemical structure of the monomers used to make the blue emissive polymer in the device ITO/PEDOT:PSS/polymer/Ca/Al structure. (b) Schematics of energy level alignment at the Aryl-F8/Aryl-PFB interface. Green and blue dashed elliptical lines indicate the exciplex formed at the interface and exciton formed before and after the endothermic transfer, respectively. The yellow arrow represents the endothermic exciton transfer at room temperature. The final emission from the Aryl-PFB exciton is shown with the blue wavy arrow.
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We report a blue-emitting random copolymer (termed modified Aryl-F8) consisting of three repeat units of polydioctylfluorene (F8), Aryl-polydioctylfluorene (Aryl-F8), and an aromatic amine comonomer unit, poly(bis-N,Ν′-(4-butylphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine) chemically linked to get an improved charge carrier balance without compromis...
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... et al. Appl. Phys. Lett. 112, 163301 (2018) The presence of the emission in modified Aryl-F8 can be explained by considering an efficient exciplex to exciton trans- fer, originating from an endothermic transfer process at room temperature via the energy levels of the Aryl-PFB/Aryl-F8 interface [ Fig. 1(b)]. The HOMO value for Aryl-PFB and Aryl- F8 is À5.35 eV and À5.9 eV, 9,24 while the optical bandgap is 2.75 eV and 2.9 eV, respectively. The excitation $3.05 eV excites both moieties (Aryl-PFB and Aryl-F8) but gives rise to emission from Aryl-PFB units only via intramolecular exciplex formation, as evident by the multi-component decay lifetime of modified Aryl-F8 in the TCSPC data and consequent transfer of the exciplex to the molecular exciton at the interface. The donor-acceptor nature of Aryl-PFB implies a smaller binding energy for the singlet exciton (E x ), placing the electronic bandgap close to the optical bandgap; the homopolymer nature of Aryl-F8 implies a high singlet excitonic binding energy such that its electronic bandgap is farther from the optical bandgap, creating a LUMO offset (d) within the reach of thermal excita- tion. The measured PLQE of modified Aryl-F8 ($70%) is higher than that of Aryl-PFB (40%) and F8 (55%-60%) and comparable to that of Aryl-F8. 9 Since the PL from modified Aryl-F8 is from the Aryl-PFB moiety only and enhanced as compared to pristine PFB or Aryl-PFB, we expect that energy gap between the Aryl-PFB S 1 state and the CT state of the Aryl-PFB:Aryl-F8 or F8 interface should be <0.15 eV at room ...
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... above results are discussed in the following paragraph with the aid of literature knowledge. The measured high PLQE of modified Aryl-F8 suggests that the singlet of Aryl-PFB is being repopulated via the intramolecular/intermolecular charge- transfer state as shown by the single particle model [ Fig. S1(a) in the supplementary material]. 25 Compared to F8:PFB blend systems, 20 the intramolecular/intermolecular exciplex state in modified Aryl-F8 must be numerically >2.6 eV, whereas the singlet state of Aryl-PFB is 2.75 eV compared to that of Aryl- F8 or F8 which is 2.9 eV. We expect a small (<0.15 eV) singlet-exciplex energy separation in modified Aryl-F8 [ Fig. 1(b)]. 26 In general, exciplex states have low oscillator strength and are less emissive compared to molecular excitations. 20 Intermolecular exciplex-based light emitting systems of poly- fluorene blends confirmed the presence of emissive exciplex states from the temperature induced red-shift and changes in the shape of the PL emission spectrum. 27,28 The normalized PL [ Fig. S1(b) in the supplementary material] shows no red-shifts or new features in the temperature range of 5 K-285 K, sugges- ting a dark exciplex state and/or a very weak oscillator strength. The increase in steady-state PL intensity in modified Aryl-F8 with temperature is characteristic of thermal excitation of the exciplex state to the emissive molecular exciton. 20 From the delayed PL spectra, we observe no blue-shift (Aryl-F8) at early Fig. S2(b) in the supplementary material], implying that there is ultrafast transfer of the dark exciplex state (Aryl-PFB:Aryl- F8 or F8) to the molecular Aryl-PFB excitonic state [Fig. S2(c) in the supplementary material] at room temperature. We note that this rapid transfer at room temperature allows us to achieve the final emission coming from Aryl-PFB, however, with a much higher PLQE (as compared to the Aryl-PFB film, $40%), which is equivalent to that of the homopolymer Aryl- F8 (PLQE $ 70%). Furthermore, in order to investigate the charge carrier balance, the J-V characteristics of hole-only devi- ces of Aryl-F8 and modified Aryl-F8 are studied in the device structure of ITO/PEDOT:PSS/polymer/MoO 3 /Au (Fig. S4 in the supplementary material). Carrier balance is a function of the injection rate and mobility of the two opposite carriers being injected using a common device structure in two lumines- cent polymers. We shall qualitatively quantify this by compar- ing hole-only vs. bipolar current densities for these two materials. We note that our main interest is in charge carrier balance, which dictates the overall device ...
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... above results are discussed in the following paragraph with the aid of literature knowledge. The measured high PLQE of modified Aryl-F8 suggests that the singlet of Aryl-PFB is being repopulated via the intramolecular/intermolecular charge- transfer state as shown by the single particle model [ Fig. S1(a) in the supplementary material]. 25 Compared to F8:PFB blend systems, 20 the intramolecular/intermolecular exciplex state in modified Aryl-F8 must be numerically >2.6 eV, whereas the singlet state of Aryl-PFB is 2.75 eV compared to that of Aryl- F8 or F8 which is 2.9 eV. We expect a small (<0.15 eV) singlet-exciplex energy separation in modified Aryl-F8 [ Fig. 1(b)]. 26 In general, exciplex states have low oscillator strength and are less emissive compared to molecular excitations. 20 Intermolecular exciplex-based light emitting systems of poly- fluorene blends confirmed the presence of emissive exciplex states from the temperature induced red-shift and changes in the shape of the PL emission spectrum. 27,28 The normalized PL [ Fig. S1(b) in the supplementary material] shows no red-shifts or new features in the temperature range of 5 K-285 K, sugges- ting a dark exciplex state and/or a very weak oscillator strength. The increase in steady-state PL intensity in modified Aryl-F8 with temperature is characteristic of thermal excitation of the exciplex state to the emissive molecular exciton. 20 From the delayed PL spectra, we observe no blue-shift (Aryl-F8) at early Fig. S2(b) in the supplementary material], implying that there is ultrafast transfer of the dark exciplex state (Aryl-PFB:Aryl- F8 or F8) to the molecular Aryl-PFB excitonic state [Fig. S2(c) in the supplementary material] at room temperature. We note that this rapid transfer at room temperature allows us to achieve the final emission coming from Aryl-PFB, however, with a much higher PLQE (as compared to the Aryl-PFB film, $40%), which is equivalent to that of the homopolymer Aryl- F8 (PLQE $ 70%). Furthermore, in order to investigate the charge carrier balance, the J-V characteristics of hole-only devi- ces of Aryl-F8 and modified Aryl-F8 are studied in the device structure of ITO/PEDOT:PSS/polymer/MoO 3 /Au (Fig. S4 in the supplementary material). Carrier balance is a function of the injection rate and mobility of the two opposite carriers being injected using a common device structure in two lumines- cent polymers. We shall qualitatively quantify this by compar- ing hole-only vs. bipolar current densities for these two materials. We note that our main interest is in charge carrier balance, which dictates the overall device ...
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... above results are discussed in the following paragraph with the aid of literature knowledge. The measured high PLQE of modified Aryl-F8 suggests that the singlet of Aryl-PFB is being repopulated via the intramolecular/intermolecular charge- transfer state as shown by the single particle model [ Fig. S1(a) in the supplementary material]. 25 Compared to F8:PFB blend systems, 20 the intramolecular/intermolecular exciplex state in modified Aryl-F8 must be numerically >2.6 eV, whereas the singlet state of Aryl-PFB is 2.75 eV compared to that of Aryl- F8 or F8 which is 2.9 eV. We expect a small (<0.15 eV) singlet-exciplex energy separation in modified Aryl-F8 [ Fig. 1(b)]. 26 In general, exciplex states have low oscillator strength and are less emissive compared to molecular excitations. 20 Intermolecular exciplex-based light emitting systems of poly- fluorene blends confirmed the presence of emissive exciplex states from the temperature induced red-shift and changes in the shape of the PL emission spectrum. 27,28 The normalized PL [ Fig. S1(b) in the supplementary material] shows no red-shifts or new features in the temperature range of 5 K-285 K, sugges- ting a dark exciplex state and/or a very weak oscillator strength. The increase in steady-state PL intensity in modified Aryl-F8 with temperature is characteristic of thermal excitation of the exciplex state to the emissive molecular exciton. 20 From the delayed PL spectra, we observe no blue-shift (Aryl-F8) at early Fig. S2(b) in the supplementary material], implying that there is ultrafast transfer of the dark exciplex state (Aryl-PFB:Aryl- F8 or F8) to the molecular Aryl-PFB excitonic state [Fig. S2(c) in the supplementary material] at room temperature. We note that this rapid transfer at room temperature allows us to achieve the final emission coming from Aryl-PFB, however, with a much higher PLQE (as compared to the Aryl-PFB film, $40%), which is equivalent to that of the homopolymer Aryl- F8 (PLQE $ 70%). Furthermore, in order to investigate the charge carrier balance, the J-V characteristics of hole-only devi- ces of Aryl-F8 and modified Aryl-F8 are studied in the device structure of ITO/PEDOT:PSS/polymer/MoO 3 /Au (Fig. S4 in the supplementary material). Carrier balance is a function of the injection rate and mobility of the two opposite carriers being injected using a common device structure in two lumines- cent polymers. We shall qualitatively quantify this by compar- ing hole-only vs. bipolar current densities for these two materials. We note that our main interest is in charge carrier balance, which dictates the overall device ...
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... Aryl-F8 and F8 [ Fig. 1(a)] are known blue emitting active materials for PLEDs, 9,18 whereas the aromatic amine comonomer covalently bonded to F8 is PFB and used as a hole-transporting layer in PLEDs. 19 The three moieties are chemically linked to synthesize a random copolymer (modified Aryl-F8) where the ratio of the Aryl-F8:F8:amine comonomer ...
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... energy level of modified Aryl-F8 and Aryl-F8 is determined separately using cyclic-voltammetry measurements. Figure 1(b) provides an indicative diagram of the type II heterojunction and the contributions of the Aryl-PFB and Aryl-F8 units towards the overall polymer energy level landscape. ...
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... et al. Appl. Phys. Lett. 112, 163301 (2018) The presence of the emission in modified Aryl-F8 can be explained by considering an efficient exciplex to exciton transfer, originating from an endothermic transfer process at room temperature via the energy levels of the Aryl-PFB/Aryl-F8 interface [ Fig. 1(b)]. The HOMO value for Aryl-PFB and Aryl-F8 is À5.35 eV and À5.9 eV, 9,24 while the optical bandgap is 2.75 eV and 2.9 eV, respectively. The excitation $3.05 eV excites both moieties (Aryl-PFB and Aryl-F8) but gives rise to emission from Aryl-PFB units only via intramolecular exciplex formation, as evident by the multi-component decay ...
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... above results are discussed in the following paragraph with the aid of literature knowledge. The measured high PLQE of modified Aryl-F8 suggests that the singlet of Aryl-PFB is being repopulated via the intramolecular/intermolecular chargetransfer state as shown by the single particle model [ Fig. S1(a) in the supplementary material]. 25 Compared to F8:PFB blend systems, 20 the intramolecular/intermolecular exciplex state in modified Aryl-F8 must be numerically >2.6 eV, whereas the singlet state of Aryl-PFB is 2.75 eV compared to that of Aryl-F8 or F8 which is 2.9 eV. We expect a small (<0.15 eV) singlet-exciplex energy separation in ...
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... supplementary material]. 25 Compared to F8:PFB blend systems, 20 the intramolecular/intermolecular exciplex state in modified Aryl-F8 must be numerically >2.6 eV, whereas the singlet state of Aryl-PFB is 2.75 eV compared to that of Aryl-F8 or F8 which is 2.9 eV. We expect a small (<0.15 eV) singlet-exciplex energy separation in modified Aryl-F8 [ Fig. 1(b)]. 26 In general, exciplex states have low oscillator strength and are less emissive compared to molecular excitations. 20 Intermolecular exciplex-based light emitting systems of polyfluorene blends confirmed the presence of emissive exciplex states from the temperature induced red-shift and changes in the shape of the PL emission ...
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... states have low oscillator strength and are less emissive compared to molecular excitations. 20 Intermolecular exciplex-based light emitting systems of polyfluorene blends confirmed the presence of emissive exciplex states from the temperature induced red-shift and changes in the shape of the PL emission spectrum. 27,28 The normalized PL [ Fig. S1(b) in the supplementary material] shows no red-shifts or new features in the temperature range of 5 K-285 K, suggesting a dark exciplex state and/or a very weak oscillator strength. The increase in steady-state PL intensity in modified Aryl-F8 with temperature is characteristic of thermal excitation of the exciplex state to the emissive ...
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