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Abstract
The search for heavy-metal-free quantum-dot light-emitting diodes (QD-LEDs) has greatly intensified in the past few years because device performance still falls behind that of CdSe-based QD-LEDs. Apart from the effects of nanostructures of the emitting materials, the unbalanced charge injection and transport severely affects the performance of heavy-metal-free QD-LEDs. In this work, we presented solution-processed double hole transport layers (HTLs) for improving the device performance of heavy-metal-free Cu-In-Zn-S(CIZS)/ZnS-based QD-LEDs, in which N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) as an interlayer was incorporated between the emitting layer and the HTL. Through optimizing the thickness of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl-amine (TFB) and TPD layers, a maximum external quantum efficiency (ηEQE) of 3.87% and a current efficiency of 9.20 cd A−1 were achieved in the solution-processed QD-LEDs with double-layered TFB/TPD as the HTLs, which were higher than those of the devices with pristine TFB, TPD and TFB:TPD blended layers. The performance enhancement could be attributed to the synergistic effects of the reduction of the hole injection barrier, the increase of the hole mobility and suppressed charge transfer between the HTL and the emitting layer. Furthermore, the best ηEQE of 5.61% with a mean ηEQE of 4.44 ± 0.73% was realized in the Cu-In-Zn-S-based QD-LEDs by varying the annealing temperature of TPD layer due to the more balanced charge injection and transport as well as smooth surface of TPD layer.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Quantum-dot light-emitting diodes (QD-LEDs) have attracted extensive attention due to their wide color gamut, elegant pixel design and solution processing ability as well as their inherent photo- and thermal-stability, which makes their potential applications in next-generation displays and lightening [1–7]. Since the first CdSe-based electroluminescent (EL) device was reported in 1994 [8], tremendous improvement has been achieved in the device performance of red- and green-emitting QD-LEDs through rational design of the nanostructures of quantum dots (QDs) and device architectures [9–15]. However, the emitting layers in these QD-LEDs are mostly focused on Cd- and Pb-based QDs, and their intrinsic toxicity severely limits their further commercial applications due to the release of these metal ions to the environment. Therefore, it is necessary to exploit heavy-metal-free QD-LEDs. To date, different types of heavy-metal-free semiconductor nanocrystals (NCs), such as carbon dots [16,17], ZnSe [18,19], III-V group QDs [20–24], and Cu-based NCs [25–32], have been widely used in QD-LEDs. In particular, the multinary Cu-In-S-based chalcogenide NCs are one of the most extensively studied candidates in the QD-LEDs due to their earth-abundance, excellent chemical stability and low toxicity. The strategy of compositional engineering and nanostructure tailoring achieved notable performance in the Cu-In-S-based QD-LEDs, and the maximum ηEQE could reach up to 7.3% through optimizing the shell thickness [33], which still falls behind that of CdSe-based QD-LEDs. The low device performance of the Cu-based QD-LEDs arises from not only the nanostructures of the material but also the unbalanced charge injection and transport in the QD-LEDs.
At present, the typical solution-processed QD-LEDs mostly adopt organic HTL/emitting layer/inorganic ETL hybrid architectures, in which the HTLs mainly include organic/polymer materials (such as poly-TPD, TFB, PVK) and the ETLs mainly focus on inorganic colloidal metal oxide (such as ZnO nanoparticles (NPs)) [2,4,23,25]. In such a device structure, the electron mobility of ZnO NPs is often higher than the hole mobility of the commonly used HTL, which may lead to unbalanced carrier transport. Moreover, the difference in the energy level offsets between the HTL or ETL and the emitting layer often leads to unbalanced charge injection, resulting in carrier accumulation and photoluminescence (PL) quenching of the emitting layer [34–38]. Therefore, the interfacial engineering is an effective strategy to improve the charge balance and regulate the charge mobility, and thus the device performance of Cu-In-S-based QD-LEDs can be effectively improved to some extent. To date, some groups have made some advances toward high-performance solution-processed Cu-In-S-based QD-LEDs by using doped ZnO NPs as ETL. For example, Yang et al. demonstrated that the luminance and device efficiency of the QD-LEDs based on multinary Cu-based NCs could be enhanced by using ZnO:Mg NPs as ETL as compared to those obtained device using ZnO as ETL, which could be attributed to the reduced electron injection barrier between the ETL and the emitting layer [39]. Very recently, our group developed chloride-passivated ZnO:Mg colloidal NPs to reduce the surface defects and electron mobility as well as the electron injection barrier, and thus the device efficiency of the solution-processed Cu-In-Zn-S-based QD-LEDs was enhanced by 2.6 times as compared to that of the devices using ZnO NPs as ETL [26]. These efforts on the modification of ETLs have boosted the electron injection and transport and pushed forward the device performance. To achieve more balanced charge injection and transport in the QD-LEDs, it is desirable for the HTL materials to possess both deep highest occupied molecular orbital (HOMO) and high hole mobility, which facilitates the holes injection and transport. However, the widely used HTL materials including TFB, TPD and PVK can’t satisfy the above two aspects simultaneously. Although the TFB is often used as HTL due to its high hole mobility, the large energy offset between the valence band of the emitting layer and the HOMO of TFB makes it difficult for the hole injection. Therefore, it is an effective strategy to construct double-layered HTLs or dope small molecules into TFB as HTL to take advantage of their respective merits. For example, Tan et al. incorporated tris(4-carbazoyl-9-ylphenyl)amine (TCTA) into TFB as HTL to decrease the hole injection barrier and thus the luminance and ηEQE as well as the operation lifetime of the blue QD-LEDs were dramatically increased due to more balanced charge injection [40]. Moreover, Xu et al. constructed a TPD/PVK composite HTL in the green QD-LEDs by using a solution-processed technique, and the green QD-LEDs exhibited an enhanced ηEQE of 9.22% due to the TPD modification [41]. Although the small molecule-modified HTLs possess a stepwise energy level alignment, the hole mobility of the PVK and TPD is much lower than the electron mobility of colloidal ZnO, and there is much room for enhancing the device performance. In particular, the solution-processed double-layered HTLs for CIZS-based QD-LEDs are rarely reported. In this work, solution-processed cadmium-free Cu-In-Zn-S NCs-based QD-LEDs were fabricated by using different HTLs and ZnO:Mg NPs as ETL, which exhibited a superior device performance over other doped ZnO NPs including ZnO:In or ZnO:Li NPs in the QD-LEDs [22]. Benefiting from the formation of stepwise energy level alignment and enhanced charge transport as well the reduced exciton dissociation, the double-layered TFB/TPD HTLs could boost the improvement of the device performance in the CIZS-based QD-LEDs as compared to that of pristine TPD, TFB and TPD:TFB composites. Further optimizing the thickness of TFB and TPD layers as well as the annealing temperature of TPD layer, the maximum ηEQE of the QD-LEDs with double-layered TFB/TPD as HTLs was increased dramatically from 3.40% to 5.61%, and the mean ηEQE could reach up to 4.44 ± 0.73%, which could be attributed to more balanced charge injection and transport and smooth surface of TPD layer.
2. Experimental section
2.1. QD-LEDs fabrication
All the solution-processed QD-LEDs were fabricated on glass substrates pre-patterned with ITO film and the sheet resistance is ∼20 Ω sq−1, which consisted of patterned indium tin oxide (ITO)/PEDOT:PSS/HTLs/CIZS-based NCs/ZnO:Mg NPs/Al, and the schematic device structure is depicted in Fig. 1(a). The CIZS/ZnS NCs were synthesized according to our previous work [42]. The colloidal ZnO:Mg NPs were synthesized based on our previous reports [26]. A typical device fabrication process is listed as follows: all the ITO-coated glass-substrates were sonicated and cleaned with a common routine as described in our previous report [26]. Subsequently, the as-cleaned substrates were treated by ultraviolet ozone in air for 25 min, followed by spin-coating the poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT: PSS) (AI 4083) from aqueous solution, which was then annealed at 140 °C for 15 min in air and the thickness was about 50 nm. Afterwards, the substrates were transferred to a N2-filled glove box, then the TFB (ADS 259, 7 mg mL−1 in chlorobenzene) and TPD (65181-78-4, 2 mg mL−1 in m-xylene) was sequentially spin-coated onto the PEDOT:PSS layer and the thickness was asbout 40 nm, which was annealed at different temperatures. Next, CIZS/ZnS NCs (10 mg mL−1 in toluene) and ZnO:Mg NPs (30 mg mL−1 in ethanol) were spin-coated onto the TFB/TPD bilayer, and their thickness was 45 and 80 nm, respectively, followed by baking at 60 °C for 30 min. Finally, the Al cathode layer with a thickness of 100 nm was deposited via a thermal evaporation process and the effective area is 4 mm2. To compare the device performance of the QD-LEDs with different HTLs, pristine TPD, TFB and TPD:TFB composites were spin-coated onto PEDOT:PSS layer, respectively, and other parameters were kept unchanged as the aforementioned procedures.
Fig. 1. (a) Schematic device structure and (b) energy level diagram of all-solution-processed QD-LEDs with a multilayered structure of ITO/PEDOT:PSS/TFB/TPD/CIZS/ZnS NCs/ZnO:Mg NPs/Al. (c) Absorption (black line) and PL spectra (red line) of CIZS/ZnS NCs dispersed in toluene as well as EL spectrum of the QD-LEDs under driving voltage of 5 V. Inset: digital photo of emission from the device under driving voltage of 5 V.
2.2. Characterizations
Absorption spectra were measured on an Ocean Optics USB 2000 spectrophotometer and the photoluminescence (PL) spectra were performed on a FLUORAT-02-PANORAMA spectrophotometer. The time-resolved PL spectra were measured via a time-correlated single-photon counting method using an Edinburgh F900 steady/transient state fluorescence spectrometer. The current density-luminance-voltage (J-L-V) characteristics of QD-LEDs were recorded on a Keithley 2400 source meter and a Keithley 2000 multimeter coupled with a calibrated Si photodiode, and the electroluminescence (EL) spectra were taken using an OceanOptics USB 2000 spectrometer with the devices driven at a constant current using a Keithley 2400 source meter. The surface morphology and topographic images including the root-mean-square (RMS) values with and without TPD were analyzed by an atomic force microscope (AFM, Bruker Multimode 8). All the measurements were performed at room temperature.
3. Results and discussion
Figure 1(b) depicts the energy level diagram of the QD-LEDs with double-layered TFB/TPD as HTLs. In this device, the yellow-emitting CIZS/ZnS NCs were selected as the emitting layer, whose mean size was less than 5 nm and their relative photoluminescence quantum yield (PLQY) was around 80%. The NCs exhibited a broad PL emission peak of 558 nm and a significant red-shift as compared to the absorption peak (Fig. 1(c)). Our previous work demonstrated that a maximum peak ηEQE of 4.05% was obtained in the solution-processed and yellow-colored CIZS-based QD-LEDs through engineering the ETLs [26]. Herein, ZnO:Mg NPs were chosen as the ETLs due to their more suitable energy level alignment and reduced electron transport. In our previous report, TFB was often used as HTL in the QD-LEDs due to its high holes mobility (∼10−2 cm2 V−1 S−1). However, a large energy offset between the valence band level of NCs and the HOMO level of TFB is not beneficial for the hole injection (Fig. 1(b)). Since small-molecule TPD possesses relatively high hole mobility (∼10−3 cm2 V−1 S−1) and deeper HOMO energy level, TPD as an interlayer is incorporated between TFB and the emitting layer. In the fabrication process of double-layered TFB/TPD films via a solution-processed technique, the spin-coating of TPD layer on the underlying TFB layer often involves the dissolution of underlying layer. To avoid such a case, the m-xylene is selected to dissolve TPD, which causes the least damage to the underlying layer TFB from chlorobenzene solvent [36]. Figure 1(c) depicts the EL spectrum of the device, and it can be seen that the emission profile is similar to the PL spectrum, and a little of red-shift (6 nm) is observed in the EL maximum as compared to that of the PL maximum, indicating that the EL emission mainly comes from the CIZS/ZnS NCs. The inset of Fig. 1(c) presents the digital photo of the QD-LEDs under the driving voltage of 5 V, and a yellow bright emission is observed from the QD-LEDs.
The effects of the incorporation of TPD into the HTLs on the device performance of the QD-LEDs are exemplified based on different QD-LEDs by using pristine TFB, double-layered TFB/TPD (TFB: 6 mg mL−1, TPD: 2 mg mL−1), TFB:TPD (6:2, and the total concentration of 8 mg mL−1) composites and pristine TPD as HTLs. The corresponding J-V-L characteristics and device efficiency as a function of current density are given in Fig. 2. As shown in Fig. 2(a), the device with only TPD as HTL exhibits a poor device performance, such as a high current density and a very low maximum luminance of less than 500 cd m−2, which may arise from the unconnected TPD film, hence leading to a large leakage current. In contrast, the other three devices exhibit a low turn-on voltage of about 2.6 V and a rapid increase in the current density and luminance after turn-on voltage. Furthermore, the devices with TFB:TPD composites and TFB/TPD bilayer as HTLs show higher maximum luminance than the device with only TFB as HTL, and the device with double-layered TFB/TPD as HTLs exhibits the highest luminance of 3545 cd m−2 at 4.4 V, indicating that the incorporation of TPD into the TFB as HTLs plays a positive role in the enhancement of the luminance. The comparison of ηEQE and current efficiency as a function of current density is shown in Fig. 2(b), and the device with TPD as the HTL displays a very low peak ηEQE of 0.08% and a peak current efficiency of 0.22 cd A−1, but the device with TFB as the HTL shows a relatively high peak ηEQE of 2.58%, corresponding to a peak current efficiency of 6.28 cd A−1, which is comparable with the data reported previously [26]. After incorporation of TPD into the TFB, a peak ηEQE of 2.90% and a peak current efficiency of 7.07 cd A−1 can be achieved in the device with TFB:TPD composites as HTL. It is exciting that the double-layered TFB/TPD HTLs enabled the device to possess a high peak ηEQE of 3.40% and a peak current efficiency of 8.29 cd A−1, which is enhanced about 1.32 times as compared to that of the device with only TFB as HTL. It is obviously seen that the device with double-layered TFB/TPD HTLs exhibits an improved performance as compared to the device with TFB:TPD composites as the HTL, which may be associated with the difficulties in finding the optimum content of TPD in the TFB under the premise of high-quality of films [41]. As mentioned above that the incorporation of TPD could compensate for the large energy offset between TFB and the NCs, but it should be noted that the hole mobility of TPD is lower than that of TFB. To maximum the function of the small molecule and polymer HTLs, it is important to control the thickness of the HTLs to optimize the device performance. On the other hand, the optimization of the HTL thickness could effectively tune the exciton recombination zone. As a result, a series of QD-LEDs were fabricated, in which the thickness of HTLs was controlled by varying the concentration. Figures 3(a) and 3(b) show the J-V-L characteristics and device efficiencies of the device with different thicknesses of TPD, in which the TPD concentration is changed from 1.5 to 2.5 mg mL−1 while the TFB concentration is kept at 6 mg mL−1. It is clearly observed that the highest luminance of 3512 cd m−2 and the best ηEQE of 3.40% can be achieved in the device with TPD concentration of 2 mg mL−1. Further optimization was performed by varying the TFB concentration from 6 to 8 mg mL−1 while the TPD concentration was kept 2 mg mL−1. The corresponding J-V-L characteristics and device efficiencies indicate that the peak ηEQE can be enhanced to be 3.87% when the TFB concentration is 7 mg mL−1 (Figs. 3(c) and 3(d)). Therefore, it can be concluded that the optimization of each HTL is important to the improvement of the device performance for the CIZS-based QD-LEDs with double-layered HTLs.
Fig. 2. (a) The J-L-V characteristics and (b) current efficiency and ηEQE as a function of current density for the QD-LEDs using pristine TFB, double-layered TFB/TPD (TFB C = 6 mg mL−1, TPD C = 2 mg mL−1), TFB:TPD composites and pristine TPD as HTLs.
Fig. 3. (a) The J-L-V characteristics and (b) current efficiency and ηEQE as a function of current density for the QD-LEDs with different TPD concentrations while the TFB concentration is kept 6 mg mL−1. (c) The J-L-V characteristics and (d) current efficiency and ηEQE versus current density for the QD-LEDs with different TFB concentrations when the TPD concentration is 2 mg mL−1.
To further elucidate the reason for the performance improvement of the CIZS-based QD-LEDs enabled by solution-processed double-layered TFB/TPD HTLs, the energy level comparison of two types of HTLs, NCs and ETL is illustrated in Fig. 4(a), and the energy offset (0.2 eV) between the HOMO levels of TPD and CIZS/ZnS NCs is smaller than that between TFB and NCs (0.4 eV), and thus the stepwise energy level alignment is formed at the HTLs/NCs interface, which is beneficial for the hole injection. Moreover, the large energy offset between the LUMO level of TPD and the conduction band of NCs makes it difficult for the leakage of electrons into the HTLs, and the exciton recombination is confined in the emitting layer, leading to the enhancement of the device performance. Based on the energy level comparison, hole-only devices were fabricated with a structure of ITO/PEDOT:PSS/HTLs/NCs/TFB/Al, and the electron-only device with a structure of ITO/ZnO:Mg/NCs/ZnO:Mg/Al was also fabricated as a reference. The corresponding J-V characteristics are shown in Fig. 4(b), and the current density of the device with double-layered HTLs is increased dramatically as compared with that of the device with only TPD or TFB HTL, suggesting once again that the incorporation of TPD between TFB and NCs can promote the hole injection. Although the hole mobility of the double-layered HTLs is lower than the electron mobility of ZnO:Mg ETL, the difference of the charge mobility is reduced greatly, indicating that the transport of the electrons and holes in the QD-LEDs is more balanced, boosting the improvement of the luminance and efficiencies of the QD-LEDs. Apart from the charge injection, the energy transfer process is important for the EL in the QD-LEDs. It is generally accepted that the excitons are often formed at the interface between the NCs and HTLs in the hybrid QD-LEDs [43]. In our devices, the electrons transport is faster than the holes in the emitting layer, and thus some electrons could accumulate at the HTLs/NCs interfaces, leading to the leakage of electrons to recombine with holes in the HTLs. To illustrate the contribution of energy transfer to the enhancement of device performance, the comparison between the absorption of NCs and the PL spectra of different HTLs is depicted in Fig. 4(c). It is clearly seen that the spectral overlap between the PL spectrum of the TPD and the absorption spectrum of the NCs is larger than that between the PL spectrum of TFB and the absorption spectrum of NCs, indicating that the exciton energy transfer from TPD to NCs is more likely to take place. To verify the viewpoint, the PL spectra of the NCs films deposited on different HTLs on glass substrates were measured by using 370 nm monochromatic light as the excitation source, which is given in Fig. 4(d). As compared to pure NCs films, the PL intensity of the NCs films deposited on different HTLs is enhanced, and the NCs film on double-layered TFB/TPD HTLs exhibits the strongest PL emission, which suggests the energy transfer from the HTLs to the NCs contributes greatly to the PL intensity enhancement of the NCs.
Fig. 4. (a) Schematic energy level diagram of TFB, TPD and NC layer. (b) J-V characteristics of hole-only devices with the architecture of ITO/PEDOT:PSS/HTLs/CIZS/ZnS NCs/TFB/Al and electron-only device with the architecture of ITO/ZnO:Mg NPs/CIZS/ZnS NCs/ZnO:Mg NPs/Al. (c) The absorption spectrum of NCs and PL spectra of TFB and TPD. (d) The PL spectra of the NC films and NC films contacting with different HTLs including pristine TFB, double-layered TFB/TPD and pristine TPD, and the wavelength is 370 nm.
It was demonstrated in previous reports that the low glass transition temperature (Tg=65 °C) and low packaging density of TPD are harmful to the device performance of QD-LEDs, but high annealing temperature often brought about the degradation of the film [44]. As a result, it is important to find the optimum annealing temperature for improving the device performance of QD-LEDs. Figures 5(a) and 5(b) depict the J-V-L characteristics and device efficiencies versus current density of the QD-LEDs under different annealing temperatures of TPD films (60 °C, 90 °C, 120 °C and 150 °C). As shown in Fig. 5(a), the devices with TPD annealed at different temperature exhibit a low turn-on voltage of 2.7 V, and the current density and luminance increase rapidly after the driving voltage is above the turn-on voltage (about 2.7 V). However, the current density with an annealing temperature of 120 °C is lower than other devices at the tested voltage range. As a result, the best peak ηEQE of 5.61% and current efficiency of 13.1 cd A−1 at a luminance of 100 cd m−2 is achieved in the device under the annealing temperature of 120 °C, which is improved by 1.8 and 1.9 times as compared to the devices under the TPD annealing temperature of 60 and 150 °C, respectively (Fig. 5(b)). All the data mentioned above support that the optimized device performance can be achieved in the CIZS/ZnS-based QD-LEDs under the TPD annealing temperature of 120 °C. It should be noted that the best peak ηEQE of 5.61% stands for the best value of the cadmium-free CIZS/ZnS-based QD-LEDs by just through the interfacial engineering. The comparison of the device performance of this work with other CIZS/ZnS-based devices reported previously is summarized in Table 1. Figure 5(c) depicts the statistical histograms of peak ηEQE from 24 devices from different batches, and the average peak ηEQE is 4.44 ± 0.73%, indicating good reproducibility of the device performance. Figure 5(d) shows the corresponding EL spectra at different driving voltages of the device with TPD annealing at 120 °C, and the EL intensity becomes intensified with the driving voltage increasing from 3 to 7 V. The EL peak of 565 nm remains unchanged under different driving voltages, suggesting that the EL emission is independent of the electric fields [22].
Fig. 5. (a) The J-L-V characteristics and (b) current efficiency and ηEQE as a function of current density for the QD-LEDs with TPD annealing at different temperature (60 °C, 90 °C, 120 °C and 150 °C). (c) Statistical histograms of ηEQE measured from 24 devices. (d) EL spectra of QD-LEDs with TPD annealed at 120 °C under different driving voltages.
Table 1. Comparison of the device performance of this work with other CIS-based device reported previously.
To further exploit the reason for the enhancement of the device performance of QD-LEDs under different TPD annealing temperatures, the film morphology of the double-layered HTLs under different annealing temperatures was firstly evaluated by AFM measurement, which is given in Figs. 6(a)–6(c). A continuous film is observed except some small solids in the AFM images, but there is little difference in the film morphology. However, the Root-mean-squared (RMS) roughness of the film annealed at 120 °C is smaller than that of the film annealed at 90 and 150 °C, suggesting that the smooth film could be obtained at 120 °C. Besides, the holes mobility was then studied by measuring the J-V characteristics of the hole-only device with a structure of ITO/PEDOT:PSS/HTLs/CIZS/ZnS NCs/TFB/Al under different annealing temperatures of TPD films, and the corresponding results are shown in Fig. 6(d). The current density of the device with TPD treated at 120 °C is increased significantly as compared to that of the device treated at 90 and 150 °C, suggesting that the appropriate annealing temperature of TPD film is beneficial for the hole injection and transport, boosting the enhancement of the device performance. Additionally, we measured the PL spectra of the NC films deposited on the double-layered HTLs with different TPD annealing temperatures (Fig. 6(e)), and the PL intensity of the film annealed at 120 °C is stronger than that of the film annealed at 90 and 150 °C, indicating that the PL intensity of the NCs is enhanced rather than quenched through annealing at 120 °C. This phenomenon may be associated with the increased exciton energy transfer from the HTLs to the NCs enabled by annealing the TPD film at 120 °C. Therefore, the double-layered HTLs combine the merits of TFB and TPD, such as high holes mobility, matched energy level alignment as well as the efficient energy transfer from HTLs to NCs, which thus promotes the charge balance and boosts the improvement of the device performance of CIZS/ZnS-based QD-LEDs.
Fig. 6. (a-c) AFM images of the films consisted of ITO/PEDOT:PSS/TFB/TPD under different annealing temperatures: (a) 90 °C, (b) 120 °C and (c) 150 °C. (d) J-V characteristics of hole-only devices with a structure of ITO/PEDOT:PSS/HTLs/NCs/TFB/Al. (d) PL spectra of NC films deposited on different double-layered TFB/TPD films under different annealing temperatures, and the exciton wavelength is 370 nm.
4. Conclusion
In summary, we demonstrate solution-processed double-layered TFB/TPD HTLs play a vital role in the improvement of device performance in cadmium-free CIZS/ZnS-based QD-LEDs. The small molecule TPD is incorporated to combine with polymeric TFB to construct double-layered HTLs, which provide not only a stepwise energy level alignment between TFB and the NCs but also high hole mobility and smoother HTL surface, leading to more balanced charge injection and transport. The best-performing QD-LEDs exhibit a mean peak ηEQE of 4.44 ± 0.73% with a champion peak ηEQE of 5.61% by finely tuning the thickness of each HTL and the annealing temperature of the TPD layer, which is comparable to the highest value ever reported for the all-solution-processed CIZS/ZnS-based QD-LEDs. The usage of solution-processed double-layered HTLs offers a simple and effective strategy to improve the device performance of QD-LEDs, which sheds light on the combination of small molecular and polymeric HTLs for applications in solution-processed QD-LEDs.
Funding
Fundamental Research Funds for the Central Universities (2019RC020); National Natural Science Foundation of China (61674011, 61735004, 61974009); Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commission (NERE201903).
Disclosures
The authors declare no conflicts of interest.
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