Fensha
Cai
a,
Yufei
Tu
b,
Dadi
Tian
a,
Yan
Fang
a,
Bo
Hou
c,
Muhammad
Ishaq
d,
Xiaohong
Jiang
a,
Meng
Li
a,
Shujie
Wang
*a and
Zuliang
Du
*a
aKey Lab for Special Functional Materials of Ministry of Education, National & Local Joint Engineering Research Center for High-efficiency Display and Lighting Technology, School of Materials Science and Engineering, and Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, China. E-mail: wsj@henu.edu.cn; zld@henu.edu.cn
bSchool of Electronics Information and, Intelligent Manufacturing, Sias University, Xinzheng, China
cSchool of Physics and Astronomy, Cardiff University, Cardiff, Wales CF24 3AA, UK
dInstitute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China
First published on 22nd May 2023
Zinc oxide nanoparticles (ZnO NPs) have been actively pursued as the most effective electron transport layer for quantum-dot light-emitting diodes (QLEDs) in light of their unique optical and electronic properties and low-temperature processing. However, the high electron mobility and smooth energy level alignment at QDs/ZnO/cathode interfaces cause electron over-injection, which aggravates non-radiative Auger recombination. Meanwhile, the abundant defects hydroxyl group (–OH) and oxygen vacancies (OV) in ZnO NPs act as trap states inducing exciton quenching, which synergistically reduces the effective radiation recombination for degrading the device performance. Here, we develop a bifunctional surface engineering strategy to synthesize ZnO NPs with low defect density and high environmental stability by using ethylenediaminetetraacetic acid dipotassium salt (EDTAK) as an additive. The additive effectively passivates surface defects in ZnO NPs and induces chemical doping simultaneously. Bifunctional engineering alleviates electron excess injection by elevating the conduction band level of ZnO to promote charge balance. As a result, state-of-the-art blue QLEDs with an EQE of 16.31% and a T50@100 cd m−2 of 1685 h are achieved, providing a novel and effective strategy to fabricate blue QLEDs with high efficiency and a long operating lifetime.
For the state-of-the-art QLED device, ZnO NPs are the most commonly employed as the electron transport layer (ETL) because of their high electron mobility and optimal energy band alignment.9–11 However, QLEDs with the ZnO ETL generally lead to charge imbalance in the QDs due to a non-negligible energetic barrier for hole injection. Consequently, the over-accumulation of electrons at the hole transport layer (HTL) and QD interface leads to increasing non-radiative Auger recombination.12 Additionally, low-temperature processed ZnO NPs exhibit a relatively open hexagonal close-packed lattice structure with the Zn atoms occupying only half of the tetrahedral sites, which therefore lead to trap states of Zn interstitials (Zni) and OV.13,14 Previous reports revealed that –OH defects exist on the surface of ZnO NPs during the solution process.15 All these defects could act as charge-trapping centers, resulting in exciton quenching at the QD/ETL interface.16,17
In view of the aforementioned issues, various strategies have been proposed to balance charge injection and suppress interfacial exciton quenching. For instance: inserting insulating layers such as poly(methyl methacrylate) (PMMA),18 poly-[(9,9-bis(30-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN),19 Al2O320 and polyethylenimine (PEI)21 between QDs and the ETL; doping ZnO NPs with metal elements including Mg,22 Li,23 Al,24 Sn,25 Be,26 and Ga;27 altering the surface of ZnO NPs with organic materials.28–31 Although these strategies can improve the device performance, it is challenging to precisely adjust the thickness of the ultrathin interlayer. A thin interlayer is ineffective in blocking electrons and can even have tunneling effects that promote electron injection. A thick interlayer increases resistance and isolates charge transport. In addition, doped ZnO NP solutions appear to be particularly chemically reactive and have a propensity to agglomerate in the environment.32 Therefore, in order to reduce electron over-injection and improve stability, techniques that can simultaneously control energy levels and passivate defects in ZnO NPs must be developed.
Here, we present a bifunctional strategy that uses EDTAK to tailor ZnO (EK-ZnO) as the ETL in blue QLEDs with improved efficiency and operational stability. The detailed analysis indicates that EDTAK can significantly passivate –OH and OV defects to relieve exciton quenching. The chemical interaction contributes to upshifting the conduction band minimum (CBM) of ZnO to impede the injection of excess electrons. Meanwhile, the stability of ZnO NPs is increased due to the chelation function of EDTAK. Utilizing such bifunctional EK-ZnO as the ETL, the fabricated QLED displays the peak luminance value of 20060 cd m−2 and the EQE value of 16.31%. Peak values and EQE both improved by 42% and 53% over the control device, respectively. The device operating stability (T50@100 cd m−2) is enhanced by 10-fold from 170 h to 1685 h. The proposed strategy in this manuscript highlights enormous potential for developing highly reliable and efficient QLEDs and other optoelectronic devices.
Fig. 1 (a) XPS spectra of N 1s, (b) UV-vis absorption spectra, (c) FTIR spectra and (d) XRD patterns of ZnO and EK-ZnO films. |
Furthermore, atomic force microscopy (AFM) was carried out to study the surface topography of ZnO and EK-ZnO film surfaces deposited on bare glass. As shown in Fig. S5,† the root-mean-square (RMS) values of both samples showed negligible changes, suggesting a uniform coverage of the surface of ZnO films with no agglomeration and it was pinhole-free.
To investigate the passivation effect of additive molecules on typical defects such as –OH and OV of ZnO NPs, we characterized the steady state PL of ZnO and EK-ZnO solutions. The PL emission peaks around 375 nm were ascribed to near-band-edge emission, which originated from intrinsic luminescence of ZnO exciton recombination.22,29 The peaks around 524 nm were ascribed to defect state emission such as OV, –OH and interstitial oxygen generated during the synthesis.36 The defect state emission might have originated from the photo-excited electrons in the CBM of ZnO relaxing to the mid-gap defect states and recombining radiatively to the holes in the valence band.37 The defect state emission intensity could reflect the density of defect states to a certain degree.38 According to Fig. 2a, the additive molecules could successfully passivate the defects and lower the defect density in ZnO NPs by reducing the PL intensity of defect state emission and increasing near-band-edge emission (375 nm).
To further elucidate the defect's passivation effect, we performed X-ray photoemission spectroscopy (XPS) to analyze the binding energy of Zn 2p and O 1s core levels before and after EDTAK modification. As shown in Fig. 2b, the O ls core level spectra were deconvoluted with Gaussian peaks for identifying the change after modification with EDTAK. Both pristine ZnO and EK-ZnO surfaces exhibited a peak at ∼530.0 eV corresponding to the metal oxide (OM). The shoulders at ∼531.4 and ∼532.0 eV were assigned to OV and OOH, respectively.30,34,39 The percentages for these species are summarized in Fig. 2c, which clearly shows that both OV and OOH attenuate distinctly, accompanied by an increase in OM. The XPS results and FTIR analyses demonstrated the deprotonation of carboxyl in EDTAK reacted with the surface-OH defects and K-doped passivation of the OV defects. Notably, the Zn 2p peaks of EK-ZnO presented a slight shift to a lower binding energy compared with the unmodified samples, further suggesting a chemical interaction between the ZnO NPs and additive molecules.31
To confirm the passivation effects of EDTAK, we performed steady-state PL and time-resolved PL decay (TRPL) measurements for QD films on quartz, ZnO and EK-ZnO substrates, respectively. QD films on ZnO and EK-ZnO exhibited weaker PL emission compared with that on quartz, consistent with the exciton quenching at the QD/ZnO interface by defect capturing. The quartz/QDs/EK-ZnO sample showed stronger PL emission than the quartz/QDs/ZnO sample. This implies reduced quenching of QDs at the interface resulting increased radiative recombination. The average exciton lifetime (τavg) was extracted by a bi-exponential fitting function.40τavg was rapidly reduced to 4.35 ns for quartz/QDs/ZnO relative to 5.88 ns for quartz/QDs, and then was gradually increased to 5.13 ns in the EK-ZnO sample (Table S1†), indicating that the present additives were effective in suppressing the interfacial non-radiative recombination at the QDs/ZnO interface.
Based on these observations, the passivation effect was attributed to the chemical interaction between the ZnO NPs and additive molecules. Reduction of surface –OH defects via an acid–base neutralization reaction and OV defects was induced by K+ doping into ZnO simultaneously, which will tune the energy-level alignment at this interface.33,34 These changes were revealed by the UV-vis absorption spectra and ultraviolet photoemission spectroscopy (UPS) analysis.
The UV-vis absorption spectra demonstrated an optical band gap (Eg) of 3.52 eV for ZnO and 3.49 eV for EK-ZnO, consistent with previous reports (Fig. 3a).41,42Fig. 3b shows the secondary photoelectron cutoff (Ecut-off) and valence band region (Eonset) for ZnO and EK-ZnO. The work functions (WF) of pristine ZnO and EK-ZnO were determined to be 3.80 and 3.53 eV, respectively, using the equation: WF = 21.22 − Ecutoff.43 Combined with band gap, the CBM of pristine ZnO and EK-ZnO were found to be −3.97 and −3.69 eV, respectively, which is demonstrated in the corresponding energy level diagram (Fig. 3c). An increased energy barrier for electron transport at the EK-ZnO/Al interface impedes injection of excess electrons and facilitates charge balance in the device. Meanwhile, the CBM of EK-ZnO was slightly higher than that of QDs, which resulted in reduction of space-electron accumulation at the QD/EK-ZnO interface, thereby suppressing the degradation at the QD-ETL junction to enhance the stability.12 Fig. S6† exhibits the UPS spectra of the QD film on ITO, for which the CBM and valence band maximum were 3.71 and 6.47 eV, respectively. A single-carrier device of electron-only devices (EODs) with a structure of ITO/ETL/QDs/ETL/Al and hole-only devices (HODs) with a structure of ITO/PEDOT:PSS/PF8Cz/QDs/MoO3/Al were prepared to verify this hybrid configuration (Fig. 3d). The electron current decreased by a unit order of magnitude in EK-ZnO-based QLEDs, much closer to the hole current compared with ZnO-based devices. These results indicate that the upshift CBM of ZnO by additive treatment promotes the balance of charge injection in the QDs.
Fig. 3 (a) Tauc plots and (b) UPS spectra of ZnO and EK-ZnO films. (c) Schematic energy level diagram. (d) Current density–voltage characteristics of the electron-only and hole-only devices. |
For further assessment of the effect of EDTAK-modified ZnO NPs on the device performance, blue QLEDs with the structure of ITO/PEDOT:PSS/PF8Cz/QDs/ETL/Al were also fabricated as demonstrated in Fig. 4a. CdSe/ZnSe/ZnS core/shell QDs with a central emission wavelength of 459 nm with full width at half maximum (FWHM) of 25 nm was used as an EML (Fig. S7a†). The photoluminescence quantum yield (PLQY) was about 65%. Fig. S7b† shows the TEM image of CdSe/ZnSe/ZnS QDs with an average size of 10 nm. Normalized EL spectra of QLEDs driven under 3.2 V exhibited pure blue emission at 468 nm without any parasitic contribution from the other neighboring layers (Fig. 4b). The inset of Fig. 4b displays photos of QLEDs operating at 3.2 V, corresponding to the Commission Internationale de I'Eclairage color coordinates of (0.129, 0.068). This illustrates that the charge transport layers have no effect on exciton radiative recombination. Notably, Fig. S8 and S9† exhibit the EL spectra and CIE coordinates of our QLEDs with no obvious change from 2.6 to 5.0 V, indicating substantial stability.
The current–voltage–luminance (J–V–L), EQE–luminance (EQE–L) and current efficiency–luminance (CE–L) characteristics of QLEDs based on ZnO and EK-ZnO ETL exhibit lower current density compared with the ZnO-based devices (Fig. 4c and d). The results validated that the electrons injection in EK-ZnO-based devices were less efficient compared with that of the ZnO-based devices, which was ascribed to the increased barrier for electron injection. A considerable increase in luminance was due to restraining Auger recombination by excess electrons.44 As a result, devices based on EK-ZnO ETL achieved a maximum luminance and peak EQE of 20060 cd m−2 and 16.31%, respectively, which were remarkably higher than those (14140 cd m−2 and 10.67%) of the ZnO ETL devices (Fig. 1d). The relevant device parameters are summarized in Table 1. Furthermore, a histogram of the maximum EQE of the optimized devices showed that an average EQE of the 18 devices was 15.68% with a low standard deviation of 0.64, suggesting high device reproducibility (Fig. S10†). The operational T50 lifetime (when the luminance drops to 50% of its initial value) of EK-ZnO-based QLEDs was found to be 5.05 h at an initial luminance of 3457 cd m−2, equivalent to 1686 h at 100 cd m−2 by applying an acceleration factor of 1.64 (Fig. 4d).5,45 The enhanced T50 lifetime for the optimized devices is attributed to reduced Auger recombination and the improved stability of ZnO NPs by EDTAK chelation.46 Additionally, the excellent stability of EK-ZnO was revealed by keeping ZnO and EK-ZnO solution in an ambient atmosphere with a relative humidity of 35–40% for 3 days, it was clear for EK-ZnO solution, while the ZnO solution became turbid (Fig. S11†).
Device | EL (nm) | V on (V) | L MAX (cd m−2) | CE (cd A−1) | PE (lm W−1) | EQE (%) |
---|---|---|---|---|---|---|
ZnO | 468 | 2.4 | 14140 | 6.76 | 7.08 | 10.67 |
EK-ZnO | 468 | 2.4 | 20060 | 9.89 | 9.71 | 16.31 |
These systematic analyses and results concluded the improved efficiency of the QLEDs based on the EK-ZnO ETLs, which was attributed to the bifunctional engineering strategy implemented on ZnO using EDTAK. Fig. S12† illustrates the schematic of performance enhancement for devices with primordial ZnO and additive-treatment ZnO films. In the ZnO-based device, the electron transfer barrier was smooth, where excess electrons accumulated at the PF8Cz/QD interface, inducing non-radiative Auger recombination. In addition, the abundant defects (–OH and OV) on the ZnO surface acted as trap states, inducing exciton quenching, which synergistically reduced the radiation recombination ratio for degrading device performance. Similarly, for the QLED devices based on the EK-ZnO ETL, the carboxyl in EDTAK significantly passivated –OH defects, meanwhile doping K+ ions passivated OV in ZnO and the interfacial QDs/ETL, thus suppressing exciton quenching. The chemical interaction contributed to an elevated conduction band energy level of ZnO, increased the electron injection barrier at the ETL/Al cathode and alleviated the electron over-injection, facilitating charge balance. Therefore, the bifunctional surface engineering strategy synergistically enhanced the proportion of radiative recombination, resulting in improved device performance.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr01194a |
This journal is © The Royal Society of Chemistry 2023 |