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Abstract
The exfoliation between the electrode film and the adjacent functional layer is still a big challenge for the flexible light emitting diodes, especially for the devices dependent on the direct charge injection from the electrodes. To address this issue, we design a flexible quantum-dot light-emitting diodes (QLEDs) with a charge-generation layer (CGL) on the bottom electrode as the electron supplier. The CGL consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO can provide sufficient electron injection into the QDs, enabling a balanced charge injection. As a result, the CGL-based QLED exhibits a peak external quantum efficiency 18.6%, over 25% enhancement in comparison with the device with ZnO as the electron transport layer. Moreover, the residual electrons in the ZnO can be pulled back to the PEDOT:PSS/ZnO interface by the storage holes in the CGL, which are released and accelerates the electron injection during the next driving voltage pulse, hence improving the electroluminescence response speed of the QLEDs.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light-emitting diodes based on colloidal quantum dots (QDs), referred to as QLEDs, have attracted considerable attention owing to their unique properties benefiting from the QDs, such as high color gamut, tunable emission covering entire visible range, low-cost solution processing, etc [1–6]. To the present, the performance of QLEDs have gained much progress, approaching or even over that of organic light-emitting diodes [7]. For example, the peak brightness is over 357000, 614000, and 88900 cd/m2 for red, green, and blue QLEDs, respectively. Moreover, the external quantum efficiency (EQE) have already reached the theoretical limit of ∼20% for the three primary colored QLEDs [8,9]. Currently, most of the reported QLEDs are fabricated on the rigid glass substrates and flexible QLEDs are rarely proposed [10–14]. Actually, an efficient flexible QLED is always desirable for the foldable and wearable applications.
A main challenge in manufacturing reliable and practical flexible QLEDs is the extreme sensitivity of device performance to the charge-injection efficiency. In general, bipolar charge injection, i.e., electrons from the cathode and holes from the anode, are necessary for the QLEDs, where the injected electrons and holes formed excitons and then releasing the energy by emitting photons through radiative recombination [15–17]. The charge injection efficiency highly relies on the properties of both electrodes and the interfaces between electrode and charge-injection layer. Ohmic contact is ideal for the charge injection, which can reduce the energy loss and enhance the power efficiency of electroluminescent devices [18,19]. This issue is much more crucial for the flexible devices, for which repeated and small-radius bending processes lead to exfoliation of charge injection layers from the electrodes. Thereby, the charge injection is limited or broken off, resulting in degradation of the flexible devices.
Two strategies can be implemented to address this issue. One is to enhance the adhesion between charge injection layer and electrode, and the other refers to constructing a non-injection charge-carrier supplier, just like the cases in the alternating-current driven devices or charge-generation layer based diodes [20–23]. To date, much attention is paid to the improvement of the adhesion between conductive film and substrate [24], yet the interface of electrode/charge-injection layer is rarely mentioned. This issue urgently requires a remedy to enable ultimate mechanical flexibility and stability of the device upon bending and folding.
In this work, we fabricate a flexible QLED by constructing an efficient CGL as the electron supplier on the PET/indium tin oxide (ITO)/Ag/ITO electrode. The CGL is composed of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO bilayer, by which electron injection into the QDs is enabled through electrical field-induced electron-hole generation in the CGL rather than relying on the injection from the bottom electrode. In comparison with the general charge-injection case, this bottom cathode provides electrical contact only. As a result, the CGL decreased the dependence of the charge injection on the electrode/charge-injection interface. Moreover, the highly efficient charge-generation efficiency enables the balanced charge injection into the QD emitters, hence leading to an efficient flexible QLED with maximum EQE up to 18.6%, over 25% enhancement in comparison with the control device with conventional ZnO electron-transport layer (ETL). The bending stability of the QLED is also enhanced after replacing the ZnO ETL with a CGL, which is attributed to the introduction of CGL.
2. Experimental details
The QLEDs were fabricated by employing an inverted device structure consisting of PET/ITO/Ag/ITO (IAI)/CGL/QDs (∼20 nm)/4,4'-bis(9-carbazolyl)−2,2'-biphenyl (CBP, 60 nm)/MoO3 (8 nm)/Al (∼100 nm). Before use, the patterned PET/IAI substrates are carefully cleaned according to the routine described in our previous report [10]. The CGL was built through spin-coating PEDOT:PSS (AI4083, 3000 rpm for 60 s in air) and ZnO (35 mg/mL in ethanol, 2500 rpm for 60 s in N2-filled glovebox) in sequence on the substrates and thermal annealing at 80 °C for 30 min each. Then QD solution (12 mg/mL in octane) were spin-coated on the ZnO films at 2500 rpm for 60 s followed by a thermal annealing at 80 °C for 15 min. At last, the CBP, MoO3 and Al were deposited by thermal evaporation in a chamber jointed with the glovebox. The pressure in the evaporation chamber is below 4×10−4 Pa and the deposition rates were around 0.2, 0.05, and 0.4 nm/s for CBP, MoO3 and Al, respectively. The device with ZnO as the ETL was also fabricated with the same procedures as described above. The commercialized PET/IAI substrates were purchased from KANGDEXIN (Beijing). The QDs with a core/shell structure of CdSe/ZnCdSe/ZnS were obtained from Mesolight Ltd. Co., the PL and absorption spectra of QDs in octane and solid form are shown in Fig. 1(a). And PEDOT:PSS (AI4083), CBP, and MoO3 were purchased from Xi′an Polymer Light Technology Co. All the materials were used as received without any further purifying processes.
Fig. 1. (a) PL and absorption spectra of QDs in octane and solid form. (b) Transmission spectra of PET/IAI and PET/ITO substrates.
The photoelectrical characteristics of QLEDs were measured by means of a programmable power supply (Keithley 2400) combined with a luminance meter (Minolta LS-110). The electroluminescence (EL) and photoluminescence (PL) spectra of the QD films were probed by a spectrometer (Ocean Optics Maya 2000pro). For the PL measurement, the excitation wavelength of 400 nm from a semiconductor laser was used. The room temperature absorption spectrum was measured through an ultraviolet/visible spectrometer (UV 1700, Shimadzu). The capacitance‒voltage properties were recorded by a precision LCR meter (Tonghui TH2829C). The transient EL (TrEL) response was probed through a home-built measured system, including a signal generator (RIGOL DG5102), a photomultiplier tube (Zolix PMTH-S1-CR131), and a digital oscilloscope (RIGOL DS4054). All the measurements were implemented in air and without any encapsulation for the devices.
3. Results and discussion
The photoelectrical properties of the PET/IAI substrate were described in our previous report [10] and the transmission spectra of PET/IAI and PET/ITO substrate as shown in Fig. 1(b). The IAI transparent conductive film exhibits outstanding transmittance in the range of visible light and ultra-stable sheet resistance after bending over 2000 times, which make it suitable to fabricate flexible QLEDs. Here we first fabricate a PET/IAI/PEDOT:PSS (∼40 nm)/ZnO (∼45 nm)/Al (100 nm) device to examine the electrical properties of the CGL. Figure 2(a) shows the current density‒voltage (J‒V) curves of this device under forward (IAI is used as anode, charge-injection current) and reverse (IAI is used as cathode, charge-generation current) driving modes, and the insets schematically shown the working mechanism of the CGL under different electric field. The similar current densities under forward and inverse directions imply that the PEDOT:PSS/ZnO possess excellent charge generation capability without extra power loss as reported previously [23,25]. Therefore, the CGL can also provide enough electron injection and hence efficient exciton formation on the QD emitters. This is a prerequisite for the fabrication of efficient QLEDs. The capacitance characteristics of the CGL device are shown in Fig. 2(b) as the function of frequency under different voltages. The capacitance of the CGL device approaches the same value under high frequency larger than 300 kHz, which equals to the geometry capacitance of the IAI/CGL/Al parallel capacitor. It is notable that the capacitance of the CGL device under 0 V bias is remarkably high under low frequency. Considering that the capacitance was measured by applied a small sinusoidal alternating-current signal with peak intensity of 0.1 V. Therefore, the large capacitance is attributed to the generated charge-induced capacitance increase as reported by Terai et al. [26]. This indicates that the charge can be generated under such small sinusoidal alternating-current voltage, further demonstrating that the CGL possesses highly efficient charge-generation capability and negligible voltage loss as observed in Fig.2a. In comparison, the capacitance at forward driving voltage of 2 V is similar to that in the high frequency region, which might be induced by the generated holes and electrons accumulated at IAI cathode and Al anode, respectively.
Fig. 2. (a) J‒V and (b) capacitance‒frequency properties of PET/IAI/PEDOT:PSS/ZnO/Al device. IAI is used as anode at the forward bias and as cathode at the inverse case, corresponding charge injection and generation, respectively. Inset in Fig.2a depicts the working mechanism of CGL.
To confirm the feasibility of this CGL as an electron supplier, a flexible QLED were fabricated with an inverted device structure as schematically depicted in Fig.3a. The flexible QLED with ZnO as the ETL is also built as the control device. The device performance is summarized in Table 1. The PL and EL spectrum of the QD film are shown in Fig.3b. A pure EL emission identical with the PL spectrum of QD film is observed without any parasitic contribution from the charge-transport layers. These results manifest that the excitons are primarily formed on the QDs in these two QLEDs and the CGL has hardly effect on the exciton formation zone.
Table 1. Summary of device performance for the flexible QLEDsa
Figure 3(c) shows the current density–voltage–luminance (J–V–L) properties of these two flexible QLEDs. The current density of the CGL-based QLED is much lower than that of ZnO ETL QLED at the same driving voltages. In order to interpret this phenomenon, we calculated the relative change of the current density (ΔJR) as a function of driving voltages as shown in Fig.3d, where ΔJR= (JCGL−JETL) ∕ JETL, JCGL and JETL are the current densities of the CGL and ZnO ETL based QLED, respectively. The positive values of ΔJR indicate an increased current density of CGL-based QLED compared with the ZnO ETL device. In contrast, the negative ΔJR represents a reduced charge injection into the CGL QLED. The higher current density of CGL-based QLED under low operating voltage range (less than 2 V) is attributed to the charging current of the large capacitance contributed by the CGL as discussed in Fig.2b. Considering that the same hole-transport layers are used in these two QLEDs, which means similar hole transport/injection, we therefore deduce that the decreased current density of CGL QLED originates from the reduced electron injection from the CGL into the QD emitters. The reduced current density of CGL-based QLED in comparison with the ZnO ETL device after the QLED turning-on (higher than 2 V) should be resulted from the change of the electrical-field distribution across the device with driving voltage increased, which should be induced by the change of the charge distribution in the device and therefore affects the charge generation processes of the CGL. Accordingly, the luminance of CGL based QLED is also lower than that of ZnO ETL device at the same driving voltages owing to the reduced electron injection. As shown in Fig.3e, the capacitance‒voltage characteristics of these two QLEDs reveal the charge storage properties in the CGL. In comparison with ZnO ETL-based device, the CGL-based QLED shows larger peak capacitance, which indicates that the charges (here should be holes) are stored in the CGL (primarily in the PEDOT:PSS as reported previously [27]). This will be discussed in detail in the following text. In addition, the capacitance of CGL based device drops more slowly from the peak than that of ETL device. This should originate from the reduced electron injection in the CGL-based device, which leads to a more balanced charge injection, and hence a slow consumption of hole and electron carriers and slow decline of capacitance from the peak value.
Fig. 3. Photoelectrical characteristic of QLEDs based on ZnO ETL and CGL. (a) Device structure; (b) EL and PL spectra, (c) J‒V‒L, (d) relative change of the current density ΔJR, (e) capacitance‒voltage, and (f) PE‒V‒EQE curves.
Figure 3(f) shows the power efficiency (PE)–V–EQE curves of these two flexible QLEDs. A substantial enhancement for both PE (from 17.6 to 21.0 lm/W) and EQE (from 14.9% to 18.6%) is obtained for the CGL-based QLED relative to the ZnO ETL device. Combining with the above discussion, we can deduce that the efficiency enhancement should originate from the reduced electron injection into the QD emitters, which improves the charge balance as reported in previous literatures [4,28]. Additionally, the peak PEs of these two QLEDs are achieved at similar luminance of 500–600 cd/m2 that is required for the display applications. This result indicates that the suppression of electron injection can largely improve the power conversion efficiency of QLED without sacrificing the device luminance although the peak luminance is reduced.
Additionally, the influence of the storage charges in the CGL on the QLEDs is further examined. The TrEL measurements are carried out to these two flexible QLEDs and the results are shown in Fig. 4 in a semilogrithm scale. The EL intensities of the TrEL spectra are normalized according to the intensity values at 50 µs. It can be observed that the CGL-based QLED presents faster EL turn-on than the ZnO ETL device, which means the CGL enhances the response speed of QLED to the electrical stimulation. The fast-response capability of the electroluminescent devices is critical to the visible light communication applications based on light-emitting diodes [29]. Recently, Li et al. reported a fast-response QLED by employing a charge storage layer [30]. Here, the fast response of the CGL-based QLED is also ascribed to the storage holes in the PEDOT:PSS. When a voltage pulse is applied to the QLED, the generated electrons in the CGL are transported through ZnO layer and then injected into the QDs. And the generated holes are drafted towards the IAI/PEDOT:PSS interface and are in part stored in the PEDOT:PSS layer [31]. When the driving voltage pulse is turned off, the storage holes will pull the residual electrons in the ZnO layer back to the PEDOT:PSS/ZnO interface. These electrons will be released during the next electrical signal stimulation and be injected into the QD emitters, resulting in a fast EL onset and overshoot. Higher driving voltage will lead to more residual electrons, and consequently stronger EL overshoot. That is indeed the case as shown in Fig. 5(a). In comparison, the ZnO ETL based device exhibits a rather weak EL overshoot even at high driving voltage of 6 V (Fig. 5(b)).
Fig. 4. TrEL characteristic of QLEDs based on ZnO ETL and CGL. (a-c) Rising and (d-f) falling edges at different driving voltages of 4, 5, and 6 V.
Fig. 5. (a,b) Rising and (c,d) falling edges of TrEL spectra of QLEDs based on (a,c) CGL and (b,d) ZnO ETL driving under voltages of 4, 5, and 6 V.
The decays of the falling edges of the CGL- and ZnO ETL-based QLEDs also show voltage-dependent characteristics, higher driving voltages leading to faster EL decays as shown in Fig. 5(c), and 5(d). However, the CGL suppresses the dependence of EL decay on the driving voltage. This might be owing to the large amount of storage holes in the PEDOT:PSS, which leads to high electrical field intensity across the device. Consequently, the CGL-based QLED presents faster EL decay than that of ZnO ETL device as shown in Fig. 4(d) and 4(e). Finally, the EL decays of these two QLED are almost the same when the devices are driven by high voltage of 6 V (Fig. 4(f)). Given the large amount of storage holes that will shield the external applied electrical field, then the EL decay must be insensitive to the offset voltage for the CGL based QLED. Therefore, we measured the EL overshoot and decay under different offset voltages to verify the hole storage effect and the results were shown in Fig. 6. The driving voltage mode for the TrEL measurements is schematically depicted in Fig.6a. Offset voltages were applied to evaluate the effect of the residual charges on the EL response. As our above hypothesis, the offset voltage dependent overshoot of the CGL-based device as shown in Fig.6b implies that the residual charge is indeed sensitive to the offset voltage. And the difference between EL decays of CGL-based QLED under different offset voltages are indiscernible as shown in Fig.6c. In contrast, the EL decay processes are highly dependent on the offset voltages for the ZnO ETL device (Fig. 6(d)). An inverse offset voltage accelerates the EL decay because the residual charges are extracted from the exciton information zone [32]. On the contrary, the EL decay processes are prolonged as a positive offset voltage is applied to the device, which is ascribed to the increased resident duration of the residual charges in the device [33].
Fig. 6. (a) The driving mode for the TrEL measurements. (b) Rising and (b,c) falling edges of TrEL for QLEDs based on (b,c) CGL and (d) ZnO ETL at different offset voltages from −2 V to 1 V.
To further explore the influence of the CGL on the device, we also evaluate the steady-state EL intensity of the TrEL response. As can be seen from Fig. 7, the steady-state (or saturated) EL intensities also rely on the offset voltages. The negative offset voltage enhances the EL emission and the positive one reduces the EL intensities. The steady-state EL intensities at 300 µs are extracted and shown in Fig. 7(c) as a function of offset voltage. The data are normalized according to the intensities without offset voltage. We can see that the dependence of the steady-state EL emission on the offset voltage is much weaker for CGL QLED than that of ZnO ETL device. These results further verify that the storage holes in CGL does shields the external electrical field, thereby improving the EL emission reliability of QLEDs under complexed electrical field conditions. This is essential to the optical communication applications.
Fig. 7. TrEL response as a function of offset voltage, (a) ETL and (b) CGL devices. (c) Saturated EL intensity properties of ETL and CGL devices.
Finally, the bending fatigue test of the flexible QLEDs was implemented to evaluate the influence of CGL on the bending stability of devices. The bending radius is 5 mm. Figure 8 shows the photographs of these two kinds of flexible QLEDs with bending times from 0 to 1000. We can see that some dark spots emerge within the pixel area after bending 200 times for the ZnO ETL device. In comparison, the CGL-based QLED deliver a strong resistance to the bending. The dark spots appear after a 500-times bending as shown in the bottom panel in Fig. 8. These results demonstrate that the bending stability of the flexible QLED can be improved by using CGL as the electron supplier instead of ZnO ETL. The detailed mechanism on the bending stability is so far unclear and needs to be investigated in detail. However, our present work provides a possible approach to enhance the flexibility of the QLEDs.
Fig. 8. Photographs of QLEDs for ZnO ETL (top panel) and CGL (bottom panel) under a driving voltage of 4 V. The numbers shown in the photographs is the bending times with a bending radius of 5 mm.
4. Conclusion
An efficient, fast-response and bending-stable flexible red QLED with inverted architecture is obtained by employing a CGL as the electron supplier instead of the commonly used ZnO ETL. Due to the excellent charge-generation efficiency, the CGL based QLED exhibits high EQE up to 18.6%, approaching the theoretical limit. Moreover, balanced charge injection reduces the electrical power consumption in the CGL-based QLED, leading to a ∼20% enhancement for the power efficiency (from 17.6 to 21.0 lm/W) in compared with the ZnO ETL-based QLED. Additionally, the bending fatigue test for these two kinds of flexible QLEDs manifests that the CGL-based QLED possesses better stability than the ZnO ETL-based device. This might be to the different roles of IAI electrodes in the CGL and ZnO ETL-based QLEDs. The introduction of CGL between electrode and QDs decouples the bottom cathode from the charge injection. We believe our results can provide another pathway to construct bending-stable flexible QLEDs.
Funding
the Key Science Fund of Educational Department of Henan Province of China (22A140008); National Natural Science Foundation of China (11974141, 12074148).
Acknowledgments
This work was supported by the program of the National Natural Science Foundation of China (Nos. 11974141 and 12074148), the Key Science Fund of Educational Department of Henan Province of China (22A140008).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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