1. Introduction

Alternating current (AC) field-induced electroluminescence (FIEL) devices have quite a different electroluminescence (EL) mechanisms and structures compared with those of organic light-emitting diodes (OLEDs) operated by direct current (DC) voltages [1–10]. Due to the relative simplicity of the inner structures of AC-FIEL devices, they have gained significant attention as a potential alternative to standard DC-driven OLED technologies for lighting applications [11–19]. Unlike in DC-driven OLEDs where electrons and holes are directly injected by a cathode and an anode to generate excitons in an emissive layer (EML), AC-FIEL devices utilize polarity-changing transient accumulated charges and their recombination in response to AC-field-driven charges [1,2,4,6–8]. Compared with OLEDs operated by DC fields, frequent polarity-reversal operation of OLEDs by AC-FIEL can reduce triplet–triplet or triplet–charge annihilation [20]. Moreover, dielectric layers that are usually introduced as charge accumulation interfaces for field-driven bipolar charges in AC-FIEL devices can reduce external moisture and oxygen penetration by preventing electro-chemical reactions between organic layers and electrodes, enhancing device reliability [7,21]. The AC-FIEL devices are prepared through solution-based processes with conjugated EL polymers by minimizing the inner structures [1,2,4,6–8].

Depending on the structures of AC-FIEL devices, electrons and holes for exciton formation can be thermally generated by internal charge generation layers [9,20,22–24] or externally injected by a single electrode [2,7,10] with a symmetric or asymmetric structure. Compared with symmetric AC-FIEL devices in which field-driven exciton generation is highly limited by the material characteristics of the thermal generation rate of charge generation layers, asymmetric AC-FIEL devices utilizing externally injected carriers from a single electrode can exhibit a relatively higher EL level. For higher EL properties in asymmetric AC-FIEL devices, efficient charge injection methods for both types of electron and hole carriers need to be attentively developed considering the structure of devices and the use of a single electrode [1,2,7,10] even though DC-driven OLEDs can employ two types of electrodes differently for a cathode and an anode with a lower and higher work function materials. Previous studies have demonstrated that small addition of carbon nanotubes (CNTs) into emissive or transport layers could effectively improve bipolar charge injection at a single electrode resulting in enhanced EL properties of asymmetric AC-FIEL devices with a significantly reduced threshold voltage [25–28]. In contrast to DC-driven OLEDs, ambipolar charge transport occurs within the organic layers of asymmetric AC-FIEL devices, which significantly affects transient charge balance conditions and resultant transient EL properties due to unbalanced charge injection and transport behaviors [1,2,7,10]. Thus, the time-averaged EL properties of asymmetric AC-FIEL devices are highly dependent on operation frequency conditions [4,6,20].

In this work, we propose a voltage waveform control approach for enhanced time-averaged exciton generation and improved EL properties of a CNT-doped AC-FIEL device, while preserving the simple structure of the device. Figure 1(a) shows an asymmetric AC-FIEL device structure and its operation waveform used in our experiment. A blue-emitting conjugated polymer of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) is used as an EML. Alternatively, electrons and holes are alternatively injected in response to field polarity from a single top electrode made of aluminum (Al). At the top of a bottom indium-tin-oxide (ITO) electrode, an insulator layer of polyimide (PI) was formed as a charge accumulating interface using its wide bandgap property. The energy barrier for hole injection from the Al electrode was lowered by introducing CNT dispersion into the PFO layer, as shown in Fig. 1(a). The energy band diagram of the asymmetric structure of an AC-FIEL device (ITO/PI/PFO:CNTs/Al) used in our experiment is shown in Fig. 1(b). During the positive polarity of an AC field cycle, holes are injected through CNTs rather than the PFO because CNT paths doped in the PFO provide a lower hole injection barrier condition; ϕ_CNT - ϕ_Al= 0.7 eV and E_{PFO_H} - ϕ_Al = 1.5 eV are for the hole injection into the CNT and the highest occupied molecular orbital (HOMO) level of the PFO, respectively. The injected holes, which are accumulated at the PI interface, are drifted toward the Al electrode during the polarity-reversal cycle of the AC field, generating transient exciton for the AC-FIEL device after recombining with the injected electrons. As shown in Fig. 1(b), electron injection occurs though the lowest unoccupied molecular orbital (LUMO) energy level of the PFO (the electron injection barrier: ϕ_Al - E_{PFO_L} = 1.5 eV). In this device structure, there is a large difference in charge injection and transport with unbalanced behaviors at each polarity inversion of AC field operation. Especially, when using CNT doping to enhance carrier injection, our work shows that conventional approaches to optimize CNT doping concentration and/or the operation frequency [1,2,4,5,9,10] cannot provide an efficient way to resolve the EL limitation that resulted from the carrier mobility difference between electron transport through the PFO and hole transport through the CNTs in each half cycle of AC field operation. As a viable and essential method to improve the EL properties of the AC-FIEL device, we propose an asymmetric voltage waveform operation by adjusting the duty ratio of a:b of the AC field polarity, as shown in Fig. 1(a), based on the analysis of the carrier injection and transport mechanism in the asymmetric AC-FIEL device and the transient EL behaviors of the mechanism. By employing the asymmetric voltage waveform, EL intensity could be improved by approximately 1.4 times that of conventional symmetric voltage waveform under the same operation frequency and device structure conditions.

 

Fig. 1. (a) Schematics of an asymmetric device structure for AC field-induced electroluminescence organic light-emitting device (AC-FIEL OLED) operated by asymmetric voltage waveform control. (b) Energy band diagram of a carbon nanotubes (CNT)-doped AC-FIEL OLED: ITO/PI/PFO:CNTs/Al.

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2. Experimental

In the device structure of the AC-FIEL OLED employed in our study, the blue-emitting conjugated polymer of PFO used as the EML was used as purchased from Sigma-Aldrich after diluting it with toluene by 1.0 wt%. After the PFO was completely dissolved, the metallic single wall CNTs (Nano-solution, SA230) were mixed with the PFO solutions by varying CNT concentration ratio up to 1.5 wt%. To enhance CNT dispersion in the solution, the CNT-mixed PFO solutions were post-treated for two hours using bath-sonication (Kodo Technical Research Co., Ltd., NXPC-2010), and then the PFO solutions with the dispersed CNTs capped by the PFOs (PFO:CNTs solution) were prepared. In this process, the CNTs were dispersed by the physical force of sonication and capped by the PFOs to improve solubility in the PFO solution [29–32].

The AC-FIEL OLEDs were fabricated using a solution-based process under normal atmospheric condition as follows. An ITO layer of 150 nm thickness used as a reference electrode was deposited on a glass substrate. Then, an ultraviolet ozone treatment was performed for 20 min to improve surface-coating uniformity with an increased surface energy of the ITO layer surface. As an insulator, PI (JSR Micro Korea, JALS-3249-R1) was spin-coated by 100 nm of thickness at 3000 rpm. The spin-coated PI layer was pre-baked at 65 °C for 10 min to evaporate solvent and post-baked at 230 °C for 30 min for PI polymerization. The CNT-dispersed PFO solution was spin-coated at 3000 rpm, which was pre-baked at 70 °C for 10 min and post-baked 180 °C for 30 min. The thickness of the CNT-dispersed PFO layer was about 90 nm at a CNT concentration of 1.0 wt%. Finally, an Al electrode of 200 nm thickness was thermally deposited as a carrier injection electrode at a high vacuum of 2 × 10⁻⁶ torr, and the fabrication of the AC-FIEL device with the ITO/PI/PFO:CNTs/Al asymmetric structure was completed based on the solution process. Furthermore, hole-only and electron-only devices were additionally prepared to assess the role of the CNT dispersion in the PFO layer during electron or hole injection and carrier transport.

The work function of CNTs was approximately 5 eV measured using a photoelectron spectrometer (Hitachi High Tech., AC-2). The LUMO and HOMO energy levels are 2.8 eV and 5.8 eV for the PFO and the PI used as an insulator layer, respectively [33]. The EL spectrum of the fabricated AC-FIEL devices was analyzed using spectroscopy equipment (Horiba Ltd., Fluoromax-4). Asymmetric AC waveforms were controlled by a waveform generator (Agilent, 33512B). Voltage-dependent EL was measured using luminance-intensity-voltage measurement equipment (Minolta, CS-100A) that was synchronized with the waveform generator. Time-resolved EL properties were characterized by a high-speed photometer (NEW FOCUS, Inc., Model 2031) synchronized with an oscilloscope (Keysight, DSO1052B).

3. Field-polarity-dependent EL mechanisms on CNT-doped AC-FIEL OLED

Figure 2 shows the operating principle of the AC-FIEL device according to the polarity inversion of applied fields. In this structure, two charge carriers of electrons and holes are injected from the single Al electrode into the PFO layer. Considering the energy band diagram shown in Fig. 1(b), during the positive field cycles of the AC fields, the holes are injected through the CNTs dispersed in the PFO EML, drifted by the positive polarity field, and then accumulated at the PFO/PI interface. During the negative field cycles of the AC fields, the accumulated holes are drifted toward the Al electrode, recombining with the electrons that are injected from the Al electrode through the PFO layer and moving toward the PFO/PI interface. These alternative electron and hole injections and their electron-hole recombination generate excitons for EL in the EML. To explain the transient EL dynamics in AC-FIEL devices, unbalanced charge injection and transport behaviors need to be considered in this asymmetric device structure. In terms of injection barriers, the hole injection barrier is lowered by approximately half a level of that of the electron injection by the CNTs dispersed in the PFO layer. In addition, the hole mobility of CNTs (∼ 20,000 cm²/Vs) is much faster than the electron mobility of PFO (∼ 10⁻⁵ cm²/Vs) [34,35]. Therefore, under symmetric waveform operation, transient EL due to exciton generation occurs dominantly at the PFO layer near the Al electrode during the negative field cycles of the AC waveform, i.e., during the relatively slow carrier (electron) injection with fast carriers (holes) consumptions. Light emission rarely occurs during a faster carrier injection cycle because electrons injected during negative polarity cycles are sufficiently exhausted by recombination with a relatively large concentration of accumulated holes. For both polarity cycles of AC operation, electron-hole pairs are weakly generated in the CNTs and can contribute to weak lighting during positive polarity cycles. However, most light emission occurs transiently during negative polarity cycles for electron injection in our asymmetric structure of the AC-FIEL device. Therefore, more accumulated holes are needed to increase the number of excitons for time-averaged effective light emission. Furthermore, considering that the faster carriers of the holes can rapidly escape through an electrode without recombining with slowly injected electrons, controlling operation frequency while preserving symmetric AC waveform cannot be an efficient method to enhance exciton generation. On the other hand, asymmetric AC waveform capable of longer hole injection duration and shorter electron injection duration (or hole escaping) can effectively improve exciton generation and higher light emission in AC-FIEL OLEDs.

 

Fig. 2. Schematics of the FIEL phenomena of the AC-FIEL OLED according to polarity inversion of applied fields.

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4. Results and discussion

4.1 Improving hole injection by CNT doping in AC-FIEL

Before investigating the effects of the asymmetric waveform control on AC-FIEL properties, luminance-voltage (L-V) properties according to the operation frequency with symmetric voltage waveform were characterized by varying CNT concentrations in the EML of the asymmetric device structure shown in Fig. 1. Under the AC field operation, the L-V properties of the maximum EL and turn-on (threshold) voltages levels are highly dependent on the impedance properties of the EML that are different from CNT concentrations [1,2,7,10,25–28], which needs to be clarified, in advance, under our experimental condition.

Figure 3 shows the L-V curves of AC-FIEL OLEDs according to the CNT concentrations, where the symmetric waveform of an AC field was applied at 600 kHz operation frequency. Compared with the AC-FIEL OLED prepared without CNT doping in the EML layer, a small amount of CNT doping could effectively improve L-V properties to achieve maximum luminance and turn-on voltages. At an operating AC voltage of 40 V, the luminance of 0.12 cd/m² obtained without CNT doping was improved to 5.25 cd/m² by 0.3 wt% of CNT doping due to the lowered injection energy barrier of hole injection paths by the doped CNTs, as discussed with Fig. 2 [25–28]. As CNT concentration increased, luminance was gradually increased up to a CNT concentration of 1.0 wt% where the saturated luminance was 17.85 cd/m² at 40 V and 600 kHz. Under the same operation frequency condition, an increase in CNT concentration in the EML can provide more hole accumulation at the PFO/PI interface due to the fast hole mobility of the CNTs through the increased hole injection paths. Considering the high conductivity of CNTs, the reduced resistance level of the EML and a strengthened field at the CNT ends can be also attributed to the improved properties of the L-V curves and the increase in CNT concentrations [1,2,4,5,9,10]. However, the excess CNT doping concentration degraded the L-V properties of the AC-FIEL OLED, and luminance obtained at CNT doping 1.5 wt% deteriorated to 2.89 cd/m² at the same AC operating conditions. Hole injection would be further increased during the positive field cycle by increasing CNT concentration, but excitons with degrading luminance due to energy transfer from recombination cites to CNTs may be increased at a high CNT concentration [36,37]. Despite the PFO-capping of CNTs introduced in our experiment, CNT aggregation at a high CNT concentration may result in internal field non-uniformity with degrading luminance [7,36]. Turn-on voltage (applied AC voltage for 1 cd/m²) properties according to CNT concentrations show the same tendency based on luminance levels: 22.5 V, 19 V, and 24.5 V for CNT concentrations of 0.3 wt%, 1.0 wt%, and 1.5 wt%, respectively.

 

Fig. 3. Luminance-voltage (L-V) characteristics of AC-FIEL OLED according to the CNT concentration doped in the PFO layer at the 600 kHz operation frequency with symmetric AC waveform.

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The effect of CNT doping in the EML of the AC-FIEL OLED and the resulting dominant carrier injection route in response to the polarity inversion of the AC field are investigated by preparing two types of CNT-doped PFO devices by replacing the PI layer of Fig. 1 with alumina and PEDOT:PSS layer for an electron-only device and a hole-only device, respectively. As shown in Figs. 4(a) and 4(b), the alumina and PEDOT:PSS (Sigma-Aldrich Co.) layer works as the hole-blocking and electron-blocking layer, respectively. Both layers were made through a printing process for both single-carrier devices, and the structures of the devices were identical to the AC-FIEL OLED shown in Fig. 1 except for the layer used for hole-blocking or electron-blocking. To prepare an alumina precursor solution, alumina acetate was obtained by mixing 2 g aluminum chloride anhydrous (Sigma-Aldrich Co., Ltd, ≥99.99%) with 10 ml acetic acid (Sigma-Aldrich Co., Ltd, ≥99.7%), and then the alumina precursor solution was prepared by dissolving 10 mg of the aluminum acetate colloid in 1 ml 2-methoxyethanol under stirring for 1 h [38,39]. The alumina thin film layer was prepared on the ITO substrate by spin-coating at 3000 rpm for 30 s, followed by a two-step baking process at 150 ^°C for 30 min and 180 ^°C for 30 min in an ambient condition.

 

Fig. 4. Energy band diagrams for (a) the electron-only and (b) hole-only devices. (c) and (d) are the current-voltage characteristics of the electron-only and hole-only devices, respectively.

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When we measured the current-voltage (I-V) properties of the electron-only device according to CNT concentrations, we found that the addition of CNTs has a negligible effect on electron injection and transport properties, as shown in Fig. 4(c), with identical I-V curves at different CNT concentrations. This means that electron injection from the Al electrode to the EML dominantly occurs through the LUMO level of the PFO irrespective of CNT doping. On the other hand, the I-V curves of Fig. 4(d) obtained with the hole-only device showed that hole current levels were much increased with an increase in CNT concentrations, indicating that the holes were injected through CNTs instead of the HOMO level of the PFO due to the lowered injection barrier. Before CNT doping, the work function level of the Al electrode is positioned close to the middle of the PFO bandgap, as shown in Fig. 1. After CNT doping into the PFO, the hole injection barrier could be much lowered than the electron injection barrier, and the threshold voltage for the carrier injection can be much lowered for holes than for electrons after CNT doping. Current levels evaluated for single-carrier devices were also much higher in the hole-only device than the electron-only device. The fast hole mobility level in the CNTs compared with the electron mobility in the PFO can be also attributed to the increased hole current levels after CNT doping.

Under an AC field operation, the L-V properties of AC-FIEL OLEDs are highly dependent on operation frequency, charge carrier dynamics, and impedance behaviors, as shown in Fig. 5. Figure 5(a) shows the L-V characteristics of AC-FIEL OLEDs prepared by CNT concentration of 1.0 wt% and measured by varying the operation frequency conditions with symmetric voltage waveform. In Fig. 5(b), luminance levels obtained at an operating voltage of 40 V were presented by operation frequency, where luminance levels of AC-FIEL prepared without CNT doping in the EML were co-plotted for comparison. As increasing operation frequency, luminance levels initially increased and then decreased, identically for both types of the AC-FIEL devices (with or without CNT doping). Under a relatively lower operation frequency condition, accumulated charge carriers at the PFO/PI interface during each half cycle of the AC field can be quenched by traps and result in non-radiative losses before the polarity reversal of the AC field [7]. Thus, radiative electron-hole recombination can be increased with increasing operation frequency. In the other aspect, as operation frequency increases, the capacitive reactance of the whole device is inversely proportional to the operation frequency and the root-mean-square AC current level by both of injected electrons and holes in response to AC field can be increased [10]. This also can be attributed to the increasing luminance behavior by the increased excitons with increasing the operation frequency at a relatively lower operation frequency regime. However, at a high operation frequency greater than the maximum luminance operation frequency, the carrier dynamics of charge injection and transport cannot follow the high-speed switching of the polarity reversal of the AC field, and light emission through electron-hole recombination decreases [20]. Particularly, in our asymmetric device structure, the maximum luminance operation frequency level is mainly limited by the charge dynamics of electrons undergoing carrier injection and transport through the PFO layer without using the CNT paths. In the CNT-doped AC-FIEL device, the maximum luminance operation frequency was measured at about 600 kHz. The luminance level at 600 kHz was about 14.4 times larger than the device luminance at 300 kHz, as shown in Fig. 5(b). The maximum luminance operation frequency of the AC-FIEL device without CNT doping was relatively lower than that with CNT doping due to relatively poor carrier injection and transport with higher impedance and without CNT doping in the EML, which was obtained at about 300 kHz. As shown in Fig. 5, the luminance-frequency characteristics of the AC-FIEL OLEDs are highly dependent on the charge carrier dynamics resulting from the AC field. More importantly, the significant mismatch behaviors of the charge dynamics between electrons and holes, which is certainly present in asymmetric structures, would severely affect the L-V properties of the CNT-doped AC-FIEL OLEDs, but investigation on these effects has not yet been explored. The limitation of luminance level due to charge dynamics mismatch cannot be resolved by only controlling the operation frequency while maintaining equal carrier injection interval for electrons and holes; thus, the introduction of a driving scheme utilizing asymmetric voltage waveform is essentially needed.

 

Fig. 5. (a) Frequency-dependent L-V characteristics of AC-FIEL OLEDs prepared under the same doping concentration (1.0 wt%) of CNTs, where symmetric AC waveforms were used. (b) Luminance-frequency (L-f) characteristics of AC-FIEL OLEDs, measured at the operating voltage of 40 V_pp.

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4.2 Enhanced EL by using asymmetric voltage waveform in AC-FIEL

Investigations on the effects of asymmetric voltage waveform on AC-FIEL properties were performed by measuring spectral EL properties to check whether luminance variations are due to a change of the waveform-dependent charge recombination effect or to the fluorenone defect effects caused by field-induced oxidation in the PFO layer [40–45]. Figure 6 is the spectral EL intensities of the AC-FIEL OLEDs according to the voltage waveform control, operated at 600 kHz and 40 V, where Figs. 6(a) and 6(b) correspond to the condition of the EML prepared without and with CNT doping (1.0 wt%), respectively. The relative duty ratio of a:b within one cycle of the AC field and its AC field polarity directions follows the schematics shown in Fig. 1(a). For both types of AC-FIEL OLEDs, Fig. 6 shows that spectral intensity levels are highly dependent on voltage waveform variations, but spectral intensity profiles are quite similar in each device type. Compared with the spectral intensity profiles of Fig. 6(a), the spectral intensity profiles of Fig. 6(b) show additional light emission in the green spectral band, and this spectral light emission corresponds to exciton relaxation due to fluorenone defects caused by locally concentrated electric fields at CNT ends [40]. However, the relative properties of the spectral intensity profiles of Fig. 6(b) obtained by the voltage waveform variation show that during our evaluation of the CNT-doped AC-FIEL OLED, there is no additional fluorenone defect that could cause variation in luminance levels [40–45].

 

Fig. 6. Spectral EL intensities of AC-FIEL OLEDs according to the voltage waveform control (the operation frequency and voltage: 600 kHz and 40 V, respectively); (a) PFO without CNTs and (b) PFO with CNTs (the doping concentration: 1.0 wt%).

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When the AC-FIEL OLED was prepared without CNT doping, the spectral intensities of the asymmetric waveforms (a:b = 4:6 or 6:4) were lower than that of the symmetric waveform (a:b = 5:5) following the conventional operation scheme. As shown in Fig. 1(b), the injection barrier heights for electrons and holes produced by the Al electrode are quite similar to each other without the introduction of CNT doping in the PFO layer. In addition, the charge transports of the PFO layer also show relatively similar properties for electrons (∼10⁻⁵ cm²/Vs) and holes (∼2.5×10⁻⁵ cm²/Vs) [35]. Thus, a higher spectral intensity level is achieved by using symmetric waveforms than using asymmetric waveforms. However, in the CNT-doped AC-FIEL OLED, when we used the asymmetric waveform of a:b = 6:4, the spectral intensity level was significantly higher than when we used the symmetric waveform, as shown in Fig. 6(b). In the reversal case of a:b = 4:6, the spectral intensity level when using the asymmetric waveform was lower than when using the symmetric waveform. For symmetric waveform operation, the spectral intensity levels showed a different pattern based on polarity types that are chosen for relatively longer or shorter injection times within one AC field cycle.

Figure 7(a) shows the L-V characteristics of the CNT-doped AC-FIEL OLED (CNT doping concentration of 1.0 wt%) based on the relative duty ratio of the applied polarity in the voltage waveform (the operation frequency: 600 kHz). At 40 V, the luminance level obtained by using the asymmetric waveform of a:b = 6:4 was 25.05 cd/m², which was approximately 1.4 times higher than the luminance level (17.97 cd/m²) obtained by using the conventional symmetric waveform of a:b = 5:5. Similar to the spectral intensity behavior shown in Fig. 6(b), the luminance level (11.84 cd/m²) was lowered at the asymmetric waveform of a:b = 4:6 compared to the luminance level at the symmetric waveform. As shown in Fig. 6, the PFO has two emission peaks at 430 nm and 455 nm wavelengths in the blue spectral regime. When we evaluated the spectral EL intensities for two PFO emission peaks with the results of Fig. 6(b), the relative intensity ratio of 1.81:1:0.66 was obtained at 430 nm emission peak for the voltage waveform conditions of a:b = 6:4, 5:5, and 4:6, respectively, and that of 1.65:1:0.68 was obtained for 455 nm. The enhancement effects by the voltage waveform control were higher in the PFO peak intensities than in the luminance levels. The waveform-dependent luminance behaviors and charge dynamics mechanism in the CNT-doped AC-FIEL OLEDs can be explained by polarity-dependent transient EL properties.

 

Fig. 7. (a) L-V characteristics of the CNT-doped AC-FIEL OLEDs according the AC voltage waveforms (operation frequency of 600 kHz, and CNT doping concentration of 1.0 wt%). (b) and (c) are the time-resolved EL properties of Fig. 6(a); (b) a:b = 5:5 and (c) a:b = 6:4.

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Figures 7(b) and 7(c) show the time-resolved EL (TREL) results of the CNT-doped AC-FIEL OLEDs using the symmetric (a:b = 5:5) and asymmetric (a:b = 6:4) voltage waveforms, respectively. As discussed with Figs. 3 and 4, due to a significant reduction in hole injection barrier conditions after CNT doping in the PFO layer, holes can be easily injected through dispersed CNT paths than through electrons produced by the Al electrode. However, the energy barrier for the electron injection was unchanged by the CNT doping as shown in Fig. 4. In addition, the charge transport properties of the holes became much faster by the CNT paths dispersed within the PFOs. Thus, the hole carriers can be sufficiently accumulated at the PFO/PI interface during the positive polarity cycles of the AC field. But, considering the hole carrier paths though the CNTs dispersed within the PFOs as shown in Fig. 1, the much faster carrier of the accumulated holes can be also drifted to the Al electrode during the negative polarity cycles of the AC field without forming exitons for EL. When AC field polarity is changed from positive to negative, relatively slow electron carriers are injected and recombined with holes drifted by the negative field. During the negative polarity cycles of the AC field, the injected slow electron carriers are quickly consumed with the generation of excitons predictably near the Al injection electrode as depicted in Fig. 2, and, thus, the accumulation of electrons at the PFO/PI interface is negligible. For this reason, as shown in Figs. 7(b) and 7(c), transient light emission occurs mainly during the negative polarity periods of the AC field.

More detailed explanations can be addressed with the TREL result of Fig. 7(b), obtained by the symmetric waveform operation at the maximum luminance operation frequency, where the transient light emission occurring during electron injection periods stops before the polarity inversion of the AC field. This means that the number of accumulated holes are quickly reduced under the negative field by recombining with injected electrons and also by the field-induced hole current through the Al electrode via the CNT paths without hole blocking function unlike the DC-operated multi-layered OLED structure. Considering the fast hole mobility in CNTs and the hole current without encountering injected electrons, a longer hole injection period is needed and that can be achieved by using asymmetric voltage waveform at a maximum luminance operation frequency. Thus, compared with the AC-FIEL OLED operated by the symmetric (a:b = 5:5) voltage waveform, the TREL result of the AC-FIEL OLED operated by the asymmetric (a:b = 6:4) voltage waveform showed a higher EL intensity by the increased hole injection and accumulation time under the same operation frequency condition, as shown in Figs. 7(b) and 7(c). On the contrary, the increase in the relative time for the negative field duration rather decreased the luminance, as shown in Figs. 7(a) and 7(b), even though this period corresponds to transient light emission. Certainly, by introducing an injection electrode with a lower work function level than the Al electrode, electron injection can be improved to further improve L-V properties [46]. However, in the asymmetric AC-FIEL structure where CNTs were doped into the EML to improve injection and transport properties, our results show that an operation scheme using the asymmetric waveform adjusted to allow a relatively longer injection time for a faster carrier is essential to achieve improved L-V characteristics.

5. Conclusion

Compared with OLEDs operated by the DC field, the asymmetric structure of AC-FIEL OLEDs has large different mechanisms in the charge injections and their transports with inevitable unbalanced behaviors between electrons and holes. Due to the frequent polarity-reversal operation of the AC-FIEL OLEDs, the FIEL properties due to exciton generation are highly dependent on the operation frequency of AC fields. As both electron and hole carriers need to be injected by a single electrode, carrier injections are highly limited, resulting in an insufficient luminance level. By providing additional carrier injection paths through the use of dopants, such as the CNTs in our experiment, in the EML, exciton density can be increased, luminance level can be improved, and turn-on voltage can be reduced while at the same time preserving the simplicity of devices’ process and structure. Our results show that the conventional methods for optimizing CNT doping concentration and/or operation frequency cannot sufficiently address the AC-FIEL limitation regarding severely unbalanced charge dynamics in CNT-doped AC-FIEL OLEDs. As a viable method to enhance FIEL properties, as well as to improve exciton generation, under AC field operation, we proposed the device operation scheme in which the asymmetric voltage waveform is used by adjusting the duty ratio of a:b of the AC field polarity. Due to the highly unbalanced charge transport conditions caused by doped CNTs in the EML, the TREL results showed that the transiently alternating lighting properties in the CNT-doped AC-FIEL OLEDs and transient light emission occurred at the maximum luminance operation frequency during slow carrier injection. Under conventional AC operation with the symmetric waveform (a:b = 5:5) where a and b are the relative duty time for the hole and electron injection in each AC field cycle, respectively, faster hole carriers, in our experiment, can be frequently injected and accumulated at the PFO/PI interface during the positive field of cycles rather than electron carriers. However, due to the effects of fast charge transport toward the Al electrode through the CNT paths during the negatively reversed field of cycles, the accumulated holes at the PFO/PI interfaces need to be further increased to improve luminance by increasing the relative duty time of the positive field before polarity change. Thus, transient light intensity and luminance properties can be further improved by using the asymmetric waveform designed to allow longer injection time for faster carrier, i.e. holes in our experiment, despite the reduced relative duty time of light emission. When we changed the operation waveform from the conventional symmetric to asymmetric (a:b = 6:4), the luminance level was improved by 1.4 times in the same device structure. The proposed operation scheme using an asymmetric voltage waveform is a simple and efficient method to enhance EL properties of AC-FIEL OLEDs operated by a single electrode for both holes and electrons. Although the temporal asymmetric waveform was applied to the AC-FIEL OLEDs in our work, it is expected that a waveform with asymmetry in magnitude of positive and negative voltages can be also explored for enhanced luminance levels.