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Abstract
Polymeric light-emitting materials have been developed recently as an attractive solution-processable alternative to conventional vacuum-deposited small molecules in organic/polymeric light-emitting diodes, but they are still limited in terms of their performance, especially with low luminance and efficiency. We report on some noteworthy characteristics of a new type of single emitting layer (EML), composed of a blend of a host blue-emitting polyspirobifluorene-based copolymer and a guest yellow-emitting poly(p-phenylene vinylene) derivative copolymer. These host and guest polymers have nearly identical highest occupied molecular orbital levels of about 5.2 eV, and lowest unoccupied molecular orbital levels of about 2.4 eV and 2.9 eV, respectively, minimizing the prevailing charge-trapping properties of their blend. Even in the absence of the charge-trapping effect, it is shown that very bright green electroluminescent (EL) emission with a maximum luminance of ~142,000 cd/m2 can be realized for the blended host:guest EML at a moderate concentration (~5 wt%) of the guest polymer. Current efficiency is also observed to be up to ~14 cd/A, which is much higher than those (3.6~5.1 cd/A) of reference devices with pure host or pure guest polymeric EMLs. Moreover, there is a small change in green color emission, with CIE coordinates of (0.35, 0.60) even at high luminance, showing good color stability of the EL emission from the blended EML. These significant improvements in device performance are mainly attributed to efficient Förster resonance energy transfer between the host and guest polymers in the blended EML. Together with its simple structure and easy processability, the high brightness and efficiency of our blended polymeric EML provides a new platform for the development of solution-processable light-emitting devices and/or advanced emissive display devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Advances in electronics over the past two decades have resulted in the successful application of solution-processable polymeric semiconducting materials in a variety of devices, such as polymeric light-emitting diodes (PLEDs), polymer photovoltaic cells, polymer thin film transistors, sensors, and electronics [1–11]. The unique features of these simple and lightweight devices include their flexibility and cost-effectiveness, enabling large-scale manufacturing [1–11]. PLEDs are of particular interest; not only do they provide a simple device architecture for investigating fundamental light-emitting properties related to polymeric semiconducting materials, but they can also be used to develop various electroluminescent (EL) opto-electronic devices such as bright and efficient flat panel displays and solid-state light sources, and/or electrically driven organic lasers [1,2,12–15].
In order to improve their performance even further, several different polymeric materials and device structures have been developed. For instance, high performance PLEDs have been developed by adopting multilayered emitting layer (EML) structures [16], thick EMLs [17], and various efficient interface/electrode schemes [18]. However, imperfections in the fabricated mutilayered EMLs and/or high operating voltage are issues of some concern, and limit the use of PLEDs in several applications. To enhance the efficiency and to improve emission performance, use of polymer-blended EML in PLEDs is proposed as an alternative effective solution [19,20]. For example, blended polymeric EMLs, taking advantage of Förster resonance energy transfer (FRET) between fluorophore pairs located close together (10 nm or less) [21], have been trialed by introducing an efficient light-emitting guest polymer (acceptor) into an energy-transferring host polymer (donor) EML structure [22–25].
A polymeric blend system for EMLs was recently reported, consisting of a host poly(9,9-dioctylfluorene) copolymer (known as F8) and a guest poly (para-phenylenevinylene) copolymer (known as SuperYellow, SY) [26–28]. In this blended EML, a large number of injected holes are trapped at SY guest sites in the F8 host due to the large difference (~0.6 eV) between the highest occupied molecular orbital (HOMO) levels of F8 (~5.8 eV) and SY (~5.2 eV), in sharp contrast to their nearly identical lowest unoccupied molecular orbital (LUMO) levels (~2.7 eV). This hole-trapping effect causes the hole transport to slow down, thereby improving the balance between electron and hole transport in the EML [26,27]. This improved charge balance together with the FRET between the F8 host and the SY guest polymers significantly improves the radiative recombination of the excitons, with a maximum efficiency of up to 21~27 cd/A [26,27]. These studies show that use of a single polymeric EML, with efficient hole-trapping and FRET, could be a key element in the development of very bright and efficient next-generation PLED technologies [26–28].
The hole-trapping effect in a polymeric EML is still limited to a few materials, however, because the difference between the HOMO levels is normally below 0.6 eV for the light-emitting polymers developed so far [29]. Moreover, it has not yet been confirmed whether or not the hole-trapping effect is essential to the achievement of such an enhanced brightness and efficiency for a blended polymeric EML. Understanding the operational mechanism of blended EMLs in high-performance PLEDs thus remains a considerable challenge.
We herein describe the use of a blended polymeric EML system in order to assess whether the performance of the blended EML system is more attributable to the charge-trapping effect or the nature of the FRET process. The blended polymeric system was obtained by mixing a host polymer of commercial blue-emitting polyspirobifluorene-based copolymer (SPB-02T) (hereafter, SPB) and a guest polymer of yellow-emitting PDY-132 (SY), and was used as a blended EML in a simple PLED structure. Although these polymers have each been studied extensively [30–33], here we find that blending them together allows us to form a single EML with no charge-trapping.
We investigated three types of polymeric EML, based on pure SPB host polymer, pure SY guest polymer, and blended SPB:SY (see Fig. 1). We contend that the strong overlap between the emission spectra of SPB and the absorption spectra of SY is key to efficient FRET [26–28]. Using these polymeric EMLs, we identified the contribution of the FRET process to the performance of the EMLs. We show that when SPB:SY = 95 wt%:5 wt% in the blended EML the device performance of the PLED was optimal, with a luminance of ~141,000 cd/m2 and a peak current efficiency of ~14 cd/A, both of which are considerably higher than those of the pure host or pure guest devices. These enhancements are attributable to the efficient FRET between the SPB host and the SY guest, even in the absence of any conventional charge-trapping effect. Although the performance of our device was not as good as that of the previous F8:SY device [26,27], the luminance and efficiency without the charge-trapping effect were more than half those of that device. It can therefore be confirmed that the FRET process plays a major role in the enhanced light-emitting performance of the polymer-blend EML. Considering the difficulty of implementing efficient charge-trapping, these results reveal the important contribution of FRET in blended polymeric EMLs, which could thus have applications in a variety of PLED devices; furthermore, their good color stability also adds to their appeal.
Fig. 1 Left: schematic illustration of the device architecture with an ITO anode, a PEDOT:PSS HIL, a blended polymeric emitting layer (EML), a CsF EIL, and an Al cathode. Right: molecular structures of the host blue-emitting copolymer of SPB-02T (SPB) and guest yellow-emitting copolymer of PDY-132 (SY) used in the blended polymeric EML.
2. Experiments
2.1 Materials and characterization of light-emitting polymers and films
All reagents were purchased from commercial sources and used without further purification. The poly(styrene sulfonic acid) doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) aqueous solution (Clevios PVP AI 4083, H.C. Starck) was purchased from Heraeus, Germany. The light-emitting polymers, blue-emitting SPB-02T (SPB for fabricating blue-PLED) and yellow-emitting PDY-132 (SY for yellow-PLED), were purchased from Merck.
Cyclic voltammetry (CV) measurements of the light-emitting polymers used were carried out in a non-aqueous electrolyte containing acetonitrile (99.8%, Aldrich) and tetrabutylammonium hexafluorophosphate (TBAPF6, 100 mM, Aldrich) at room temperature with a scanning rate of 0.05 V/s, using a potentiostat (DY2113, Digi-Ivy, Inc.) with DY2000 multi-channel potentiostat software. In the CV measurements, a glassy carbon working electrode coated with a thin polymer layer of SPB or SY was used with a platinum wire as the counter electrode. An Ag/AgCl (3.5 M KCl) electrode served as a reference. To calibrate the electrodes, the redox couple of ferrocene/ferrocenium ion (Fc/Fc+) (Sigma Aldrich) was used as a redox probe [34]. The HOMO level of the polymer film that was drop-cast on the carbon working electrode was calculated using the onset potential, determined from the intersection of two tangents drawn at the rising and background currents obtained from the CV measurements.
To investigate the features of our light-emitting polymer films, we tested three types of materials; i) pure blue-emitting SPB as a reference, ii) pure yellow-emitting SY as a comparative reference, and iii) SPB blended with SY (5 wt%), as a sample EML. UV-vis absorption spectra of the light-emitting polymer films were measured using an HP 8453 spectrophotometer. Photoluminescent spectra (PL) of the polymer films were recorded using a fluorescence spectrophotometer (Cary Eclipse Fluorescence Spectrophotometer, Agilent technologies).
Surface morphology images and surface potentials of the fabricated polymer films were obtained using non-contact atomic force microscopy (AFM) and simultaneous Kelvin probe force microscopy (KPFM, FlexAFM, Nanosurf AG), respectively, by applying an AC voltage of 1 V at a frequency of 18 kHz to a Pt/Ir-coated silicon tip (resonance frequency = 87 kHz and force constant = 3.9 N/m, NanoWorld, Inc.). To calibrate the surface potential of the polymer films and to monitor the integrity of the KPFM tip, highly oriented pyrolytic graphite (HOPG, ZYB, Optigraph GmbH) was used as a reference surface [35].
2.2 Fabrication of PLEDs
Fabrication of the PLEDs followed a well established process (see Fig. 1). An indium-tin-oxide (ITO) layer (80 nm, sheet resistance 30 ohm/square) on a glass substrate (18 mm × 20 mm) was used as the transparent anode and was cleaned in an ultrasonic bath in detergent, followed by deionized water, acetone, and then isopropanol. After cleaning, the ITO glass was dried and treated with ultra-violet (UV) ozone for 5 minutes. A 40 nm- thick PEDOT:PSS layer was then spin-coated directly on the ITO anode to form the hole-injection layer (HIL). The PEDOT:PSS HIL was then annealed at 120 °C for 20 min. The thickness of the PEDOT:PSS layer was optimized at around 40 nm, measured using non-contact AFM. For the polymeric EML, a blended solution of the SPB host polymer and the SY guest polymer was prepared by dissolving the polymers in toluene, which was then spin-coated on the PEDOT:PSS HIL. The thickness of the EML was optimized at around 85 nm. Then, an electron injection layer (EIL) of CsF (2 nm) and a cathode of Al (80 nm) were thermally deposited in a vacuum at a pressure of around 2.0 × 10−4 Pa using a vacuum chamber, incorporated in a glovebox filled with an inert atmosphere. The fabricated PLEDs were then encapsulated within a UV-cured epoxy resin for stable air operation. The active area of the fabricated device was 2 mm × 3 mm.
Current density-voltage-luminance (J-V-L) and CIE-coordinate measurements of the fabricated PLEDs were performed at ambient temperature using a computer-controlled chromameter (CS-200, Minolta) with a source meter (Keithley 2400). EL spectra of the devices were also obtained using a spectrometer (HR4000, Ocean Optics). In order to investigate the exciton lifetime, the time-resolved PL spectra of the EMLs were recorded using a time-correlated single photon counting system (TCSPC, Fluotime 300, Picoquant) with an excitation wavelength of 405.5 nm.
3. Results and discussion
Our first challenge relates to the energy bands, i.e., the HOMO and LUMO levels of SPB and SY as measured by UV-vis absorption spectra and CV. We initially recorded the UV-vis absorption spectra of the SPB and SY polymer films, observing absorption peaks at 390 nm and 440 nm, with absorption edges at 436 nm and 528 nm, respectively, as shown in Fig. 2(a). The optical band-gap values of SPB and SY were thus estimated to be 2.84 eV and 2.35 eV, respectively.
Fig. 2 (a) The UV-Vis absorption spectra of the spin-coated films of the pure SPB polymer (blue curve), the pure SY polymer (dark yellow curve), and the blended polymers of SPB:SY (95:5 wt%, green curve). (b) Cyclic voltammograms of SPB (upper) and SY (lower) for a scan rate of 0.05 V/s with a sweeping voltage range of −1.0 to + 1.0 V/VAg/AgCl. For comparison, the cyclic voltammogram of 1.0 mM ferrocene (Fc) is also shown in the upper figure (gray curve) (E1/2(Fc/Fc+) = + 0.45 V, glassy carbon working electrode, scan rate = 0.05 V/s). The insets in (b) show the respective energy level diagrams of SPB and SY.
The cyclic voltammograms of SPB and SY are shown in Fig. 2(b), and indicate that both polymers reveal a quasi-reversible p-doping/dedoping (oxidation/reduction) process at positive potentials. The HOMO levels of these polymers, relative to the vacuum level, were estimated with respect to the redox potential of a probe of ferrocene according to the following equation [34]: EHOMO = [−(Eox/onset − E1/2(Fc/Fc+)) − 4.8] eV, in which 4.8 eV is the absolute energy level of the ferrocene/ferrocenium (Fc/Fc+) redox couple below the vacuum level [34]. Using the observed value of 0.45 V for E1/2(FC/FC+) of ferrocene, the HOMO level of SPB was −5.27 eV, estimated from the onset oxidation potentials (Eox/onset) of the polymer (Eox/onset = 0.92 V). This HOMO level of SPB is almost identical to that (−5.24 eV) of SY, estimated from its Eox/onset of 0.89 V. We note that the HOMO level (−5.27 eV) of SPB is much lower than that (~5.8 eV) of its fluorine-based polyer analog F8, which was used as the host polymer in previous research [26–28]. The low HOMO level of SPB may be due to the presence of a hole-transporting amine unit in the polymer backbone of SPB, which causes the SPB polymer to be more electron-donating [32].
Based on the optical band-gap values and the HOMO levels of SPB and SY, the LUMO levels of the polymers can be estimated at −2.43 eV and −2.89 eV, respectively. The HOMO and LUMO values obtained here are in relatively good agreement with those reported previously [26–28,32,33] (see Table 1). We therefore contend that the SY guest polymer may not trap the holes injected into the blended SPB:SY EML, because of their nearly identical HOMO levels in the EML, in contrast to those of the F8:SY EML [26–28].
Table 1. Summary of the energy levels of the SPB and SY polymers
The basic physical and electronic properties of the three polymeric films were investigated as follows. We obtained the surface morphologies and surface potentials of the fabricated polymeric layers using non-contact AFM and simultaneous KPFM measurements (Fig. 3). The left-hand panel in Fig. 3(a) shows a topographic AFM image of the SPB EML. Its surface appears fairly smooth (RMS roughness ~0.34 nm). This AFM result was confirmed by the corresponding local surface potential map of the contact potential differences (VCPDs) for the SPB EML (Right-hand panel in Fig. 3(a)), which reveals relatively low surface potentials on the EML surface with respect to the HOPG reference surface (0.86 V). From the KPFM map, the estimated average VCPD for the SPB EML was approximately 0.17 V. For comparison, we also investigated the SY layer in the same way, as shown in Fig. 3(b). The SY layer presents an AFM morphology with an RMS roughness of ~0.47 nm and an average VCPD of ~0.13 V, which is similar to that (0.17 V) of the SPB EML. Similarly, the blended SPB:SY layer (5 wt%) exhibited a smooth AFM image with an RMS roughness of 0.36 nm and an average VCPD of ~0.23 V (Fig. 3(c)). These results for surface roughness and surface potential of the SPB:SY layer are almost identical to those of the SPB reference EML, indicating that the SY guest was homogeneously dispersed into the SPB host polymer layer, without any noticeable aggregation and/or phase separation of the SY guest in the blended EML. We also note the almost identical surface potentials (VCPD) of the three EMLs studied, implying hardly any differences in their electrical properties, mainly due to their similar HOMO levels.
Fig. 3 AFM topographic images (left) and their corresponding KPFM surface potential maps (right) observed for the spin-coated SPB (a), SY (b), and SPB:SY (c) layers.
To investigate the EL performances of the three polymeric EMLs described above, we fabricated PLEDs, including a transparent indium tin oxide (ITO) anode, a PEDOT:PSS hole injecting layer (HIL, 40 nm), a polymeric emitting layer (EML, 85 nm), a CsF electron injecting layer (EIL, 2 nm), and an Al metal cathode (80 nm), in sequence (Fig. 1(a), see also the Experimental Section for details of the structure and fabrication). We now describe the device characteristics in terms of the current (J) and luminance (L) as a function of applied voltage (V) for the sample PLED with the blended SPB:SY EMLs, in comparison with two reference PLEDs fabricated using the pure SPB EML (for the blue-PLED) and the pure SY EML (for the yellow-PLED) (Fig. 4). As shown in Fig. 4(a), the current density-voltage (J-V) characteristics of the sample PLEDs show that the current densities flowing through the blended EML were slightly higher than those flowing through the pure SPB EML, which may be due to the addition of SY into the SPB EML. However, the current densities are still far lower than those flowing through the pure SY EML. This result indicates that blending a small amount of the SY guest into the SPB host causes virtually little change to the electrical properties of the blended EML compared with those of the pure SPB EML.
Fig. 4 Current density-voltage (J-V) (a), luminance-voltage (L-V) (b), luminance efficiency-voltage (ηC-V) (c), and luminance efficiency-luminance (ηC-L) (d) characteristics of the PLEDs using the SPB, SY, and blended SPB:SY EMLs.
It is nevertheless interesting to note that differently from the J-V characteristics, the sample PLED with the SPB:SY EML exhibits a significantly improved luminance-voltage (L-V) performance, superior to those of the blue-PLED and yellow-PLED (Fig. 4(b)). For example, the maximum luminance of the sample PLED is L ~142,000 cd/m2 at V = 16.5 V, with a low threshold voltage (Vonset) of ~2.7 V. This maximum luminance L is ~5.9 times that (L ~24,000 cd/m2 at V = 16.5 V, Vonset of ~3.2 V) of the blue-PLED, and is also much higher than that (L ~51,000 cd/m2 at V = 13.0 V, Vonset of ~2.2 V) of the yellow-PLED. This improved outcome clearly indicates the enhanced radiative recombination process in the blended SPB:SY EML. The current efficiency ηC of the sample device thus also shows remarkable improvement, to well in excess of those of the blue-PLED and yellow-PLED (Fig. 4(c)). For instance, the highest observed ηC value was approximately 14.3 cd/A, at V = 4.5 V, which is much higher than those of the reference PLEDs (5.1 cd/A for the yellow-PLED and 3.6 cd/A for the blue-PLED). Even at a luminance of 500 cd/m2, the sample PLED emitted EL light with an ηC of 13.7 cd/A, which is approximately 4.6 times that (ηC = 3.0 cd/A) of the blue-PLED and 2.8 times that (ηC = 4.9 cd/A) of the yellow-PLED. Moreover, even at high luminance levels of 5,000 cd/m2, the sample PLED maintains an enhanced ηC of ~13.9 cd/A (Fig. 4(d)), which offers another important advantage: given this value of ηC, the same brightness can be obtained for a lower J, providing a prolonged device lifetime [36]. Thus, it is clear that the SPB:SY EML in the sample PLED provided improved device performance both in terms of the luminance L and the current efficiency ηC, even though the cathode was not fully optimized. Notably, for the blended SPB:SY EMLs, we also observed that when a higher concentration of SY of more than 5 wt% was used, its EL emission and efficiency was reduced because higher doping leads to aggregation and concentration quenching of the guest emission [30,31]. Details of the device performances are summarized in Table 2, and compared with those of other reported devices; the best device performance is that offered by the F8:SY EML. As shown in the table, the maximum brightness and efficiency of our sample device (5 wt%) was more than half those of the previous F8:SY PLED, even without the charge-trapping effect.
Table 2. Summary of the Device Performance Outcomes of PLEDs with the SPB, SY, and SPB:SY EMLs
In order to understand the charge-transport dynamics in the blended EML, single charge carrier devices, hole-only and electron-only devices, were fabricated and investigated. To form hole-only devices (HODs), the CsF/Al cathode in the PLEDs was replaced by a MoO3/Ag electrode (ITO/PEDOT:PSS/EML/MoO3/Ag) [37]. Figure 5(a) shows the hole-current density vs. applied voltage (Jh-V) characteristics for the HODs, whose energy level diagram is shown in the inset of the figure. In the Jh-V observations, the ITO electrode was biased positively to allow hole transport from the ITO anode to be studied in the HODs. The characteristic Jh-V curves in the figure show that the hole current density of the SPB:SY HOD is slightly lower than those of the other reference HODs; after mixing a small amount (5.0 wt%) of SY in SPB, the current density Jh (~0.09 mA/cm2 at 1.5 V) of the SPB:SY HOD becomes approximately one-third of that (~0.29 mA/cm2 at 1.5 V) of the SPB HOD. This outcome shows that the hole transport through the SPB:SY active layer of the SPB:SY HODs was not significantly different from that through the SPB active layer of the reference SPB HOD. This is at odds with the previous finding that the previous F8:SY HOD exhibited a severe reduction in Jh of ~3 orders of magnitude due to the hole-trapping effect [27]. The small decrease in Jh of our SPB:SY HOD may be a result of the small difference between the HOMO levels of SPB and SY, resulting in a sharp reduction in the hole-trapping effect.
Fig. 5 J-V curves of hole-only devices (a) and electron-only devices (b) using the SPB, SY, and blended SPB:SY layers. The fitting results are shown using dotted curves for the Jh-V curves in (a) (see Table 3). The insets show the energy band diagrams of the hole-only and electron-only devices.
The hole transport curve was analyzed to assess the hole mobility and the hole conduction in the SPB:SY EML, by fitting a simplified space-charge-limited current (SCLC) model; [38–40]. Curve fitting for the Jh-V data in Fig. 5(a) gives a value (~1.26 × 10−8 cm2/(V s)) for the zero field mobility of holes μ0 for the SPB:SY layer, which is slightly lower than that (μ0 ~4.75 × 10−8 cm2/(V s)) for the SPB layer (See Table 3). This contrasts with the significant reduction in μ0 by 3 orders of magnitude for the previous F8:SY layer [27]. Similar to the behavior of μ0, the field-effect mobility coefficient βF also shows a similar value of ~1.41 × 10−3 cm1/2 V−1/2 for the SPB:SY layer, compared with that (~1.10 × 10−3 cm1/2 V−1/2) for the pure SPB layer (Table 3). Thus, it is clear that introduction of a small amount of the SY guest into the SPB host does not suppress severely the hole-current in the SPB:SY EML.
Table 3. Summary of the hole transport parameters extracted by fitting hole-only currents injected from the ITO/PEDOT:PSS electrode
Next, in order to fabricate the electron-only devices (EODs), the ITO/PEDOT:PSS anode in the PLEDs was replaced by an ITO/ZnO/Cs2CO3 electrode (ITO/ ZnO/Cs2CO3/EML/CsF/Al) [41]. Figure 5(b) shows the electron-current density vs. applied voltage (Je-V) characteristics for the EODs fabricated. In these observations, the Al electrode was biased negatively to study the electron transport from the Al cathode in our EODs. The Je-V curves of the EODs shown in Fig. 5(b) did not fit well with the SCLC and Pool-Frankel model [27,39]. However, the Je-V curve of the SPB:SY EOD shows slightly higher electron-current densities than those of the SPB EOD; pure SPB exhibits an electron current density Je ~4.63 × 10−2 mA/cm2 at V = 6.0 V, increasing slightly to Je ~7.35 × 10−2 mA/cm2 for the blended SPB:SY. Through this small increase in Je, it can be shown that the addition of the SY guest into the SPB host does not suppress the electron-current in the SPB:SY EML, even though there is a large difference between the LUMO levels of SPB and SY. This slight increase in electron current may be caused by the reduction of the electron injection barrier for the SPB:SY layer due to the introduction of the SY guest, whose LUMO level is between the LUMO level of the SPB host and the work function of the CsF/Al electrode. Thus we confirm that the addition of the SY guest into the SPB host introduces no charge-trapping effect in the SPB:SY layer.
Next, our investigation focused on the light-emission mechanism of the blended SPB:SY EML. Figure 6(a) shows the normalized EL and PL emission spectra of the three EMLs in the PLEDs. All the plots exhibited EL spectra similar to the PL spectra, indicating that the mechanisms are analogous. The EL (PL) spectra for the pure SPB EML showed the well known blue emission with a strong electronic 0-0 peak at ~457 (456) nm, a strong first shoulder peak located at about 487 (484) nm, and a weak second shoulder peak at about 522 (521) nm, which is attributable to the characteristic emission of the spirobifluorene unit of the SPB polymer [33]. For the SY EML, the EL (PL) spectra showed yellow emission, having a strong electronic 0-0 peak at 552 (552) nm and a first vibronic 0-1 peak at 590 (591) nm [30,31]. In contrast with the EL (PL) spectra of the reference EMLs, the EL (PL) spectra of the blended EML showed a strong green emission peak located at around 526 (534) nm, between the emission peaks of the pure SPB and SY polymers, as shown in the figure. Thus, the green emission from the SPB:SY EML can be understood in terms of intermolecular interaction between SPB and SY.
Fig. 6 (a) Normalized EL (solid curves) and PL (excitation wavelength: 350 nm) (dotted curves) spectra of the SPB, SY, and blended SPB:SY EMLs. (b) Normalized PL spectra of the blended SPB:SY (5 wt%) EML for various excitation wavelengths, together with the UV-Vis absorption of SY (dotted red curve) and emission spectra of SPB (blue dotted curve). (c) Left: time-resolved PL spectra of the PLEDs with the SPB, SY, and SPB:SY EMLs. Right: relevant energy-level diagrams of the blended SPB:SY EML.
In order to clarify the emission mechanism of the SPB:SY EML, we measured other PL characteristics, as shown in Figs. 6(b) and (c). Figure 6(b) shows the PL spectra of the SPB:SY (5 wt%) EML at various excitation wavelengths. As shown in the figure, the PL spectra of the SPB:SY EML show almost identical spectral shapes, even for a long excitation wavelength of 500 nm, which can excite only the SY guest polymer; the PL emission shows clear blueshifted spectra with an electronic weak 0-0 peak at 520 nm, a strong vibronic peak at 528 nm, and a shoulder peak at 542 nm. This result indicates that the PL emission spectra of the SPB:SY mixture can be explained by the emission of SY: the blue-shifted spectra observed is caused by diluted SY polymers, with reduced intermolecular interaction between the SY polymers without aggregation in the host matrix [30,31]. We thus note that the strong blue-shifted emission of the blended SPB:SY EML at about 520 nm is attributable to the enhanced emission of the diluted SY guest via efficient energy transfer (FRET) from the SPB host. The efficient FRET is mainly due to the strong overlap between the emission spectra of SPB and the absorption spectra of SY (dotted curves in Fig. 6(b)) [26–28]. Note that this emission due to the energy transfer cannot ideally compensate for the blue emission from the SPB polymer in the region of 457 nm, as shown in Fig. 6(a). More sophisticated photophysical characterizations are required to quantify the exact contribution of the emission from the SPB:SY EML (further details will be reported elsewhere).
Next, time-resolved PL spectra were also measured to investigate the fluorescent lifetime of the SPB, SY, and SPB:SY EMLs, and the curves are shown in Fig. 6(c). It was observed that the fluorescent decay curves of both the reference polymers fitted well with the double-exponential decay function (Table 4). The average fluorescent lifetimes, < τ >s, of SPB and SY are < τSPB > ~1.78 ns and < τSY > ~0.90 ns. Interestingly, the blended SPB:SY EML exhibited double exponential decay, with an average PL decay lifetime (< τSPB:SY >) of 0.98 ns, which is obviously reduced and close to that (< τSY > ~0.90 ns) of the pure SY EML, as shown in the figure. The reduced < τSPB:SY > of the SPB:SY EML compared with the SPB EML confirms a FRET to SY guests. We also calculated the FRET efficiency, ηFRET, for the SPB:SY EML using the relationship ηFRET = (1 - τSPB:SY / τSPB) [42]; ηFRET = 22.6% when calculated from τ1 and ηFRET = 44.9% when calculated from < τ >. This high FRET efficiency indicates significant intermixing of these two polymers in the SPB:SY EML. Hence, we verify the efficient FRET process between the SPB host and the SY guest in the blended SPB:SY EML. Detailed results of the fluorescent lifetimes are summarized in Table 4.
Table 4. PL decay times (τis) and relative amplitudes (Ais) for the SPB, SY, and SPB:SY EMLs
Hence, it is clear that although the performance of our device with SPB:SY EML is not as high as that of the previous F8:SY EML, the luminance and efficiency of our device without the charge-trapping effect are greatly improved, being more than half those of the previous F8:SY device. These results show that a major element of the enhanced device performance for the blended EML is attributable to the FRET process, and the FRET of blended polymer EMLs can be a very effective means of achieving high performance PLEDs.
Finally, we investigated the green-color emitting capability of the sample PLED, together with that of the blue- and yellow-emitting reference PLEDs (Fig. 7(a)). Here, the PLEDs were produced using active areas of 3 × 2 mm2. Snapshots were obtained during operation of the PLEDs at two applied voltages, of 5.0 V and 12.0 V. In comparison with the reference PLEDs, the EL light output from the sample PLED was clearly bright green. The CIE (Commission Internationale de l'Eclairage) 1931 chromaticity diagram relative to the EL emission of the sample PLED shows a green color, with CIE coordinates of (0.35, 0.60) lying in the yellowish-green region, compared with the blue (0.18, 0.31) and yellow (0.47, 0.52) color coordinates for the pure host SPB and the pure guest SY reference devices, respectively (the CIE color coordinates are shown in Fig. 7(b)). We note the small changes in the green color at small rates of (dx/dL ~2.9 × 10−7 (cd/m2)−1, dy/dL ~0.7 × 10−7 (cd/m2)−1) even at high luminance, exhibiting an excellent color stability of the blended SPB:SY EML, despite the fact that the FRET emission cannot compensate that well for the blue emission (~458 nm) from the SPB polymer host, as shown in Fig. 6(a). This high color stability of the sample PLED can be attributed to the molecularly homogeneous dispersion of SY guest polymers without aggregation in the host SPB polymers, enabling efficient energy transfer between FRET pairs of SPB:SY in the blended EML.
Fig. 7 (a) Photographs of three PLEDs (18 × 20 mm2) with SPB, SY, and blended SPB:SY EMLs in operation at 5.0 V (upper) and 12.0 V (lower), showing the bright green EL emission from the blended SPB:SY EMLs (SPB:SY = 95:5 wt%). The active area of each PLED is 2 × 3 mm2. (b) The CIE chromaticity diagram of the EL emissions of the PLEDs studied for various applied voltages. The arrow shows the direction assigned to the high voltage applied.
The foregoing results clearly demonstrate the remarkable light-emitting performance of the FRET donor SPB and acceptor SY pair in the blended EML. To the best of our knowledge, this is the first demonstration of high-performance green PLEDs without any charge-trapping effect, exhibiting greatly enhanced brightness and current efficiency, exceeding 142,000 cd/m2 and 14 cd/A, respectively. This thus demonstrated that the concept of the FRET process in blended polymer EMLs can be a very effective and general way of fabricating high-performance solution-processed PLEDs with high brightness and improved efficiency. Furthermore, considering the difficulty of realizing an efficient charge-trapping effect, the combination of these FRET-based EMLs together with a charge-balancing architecture, if necessary, for solution-processed PLEDs clearly presages the development of inexpensive, fast, large-area, color-stable, and high-performance light-emitting devices.
4. Summary
In summary, we have herein explored a new type of single polymeric layer of EML, consisting of a blue-emitting polyspirobifluorene-based host SPB copolymer blended with a yellow-emitting poly(p-phenylene vinylene) derivative guest SY polymer, both of which possess nearly identical HOMO levels of about 5.2 eV. Bright and efficient green emission can be realized in the physical blend of polymeric EML with the SPB host and the SY guest, even in the absence of the charge-trapping effect. It is confirmed that low-voltage operation and an enhanced luminance of up to ~142,000 cd/m2 can be achieved with CIE coordinates of (0.35, 0.60), even for a simple PLED structure. Moreover, the current efficiency of the sample PLED was at least ~14 cd/A, which was much higher than those (3.6 and 5.1 cd/A) of reference PLEDs with a pure SPB EML and a pure SY EML, respectively. These significant improvements in device performance are attributable to the efficient FRET of the SPB:SY blended EML. Together with their simple structure and easy processability, highly bright, efficient, and color-stable FRET-based polymeric EMLs provide a new platform for the development of advanced light-emitting devices and/or solution-processable emissive display devices.
Funding
National Research Foundation of Korea (NRF) (2017M3C1A9069592, 2017R1A2A1A17069729); Kwangwoon University (2018).
Acknowledgments
The authors also thank Jaewoo Park for his support and help with the CV measurements.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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