Dipole-tunable interfacial engineering strategy for high-performance all-inorganic red quantum-dot light-emitting diodes

, the vast energy offset and the high trap density at the NiO X /QDs interface limit hole injection leading to fluorescence quenching and hampering the performance. Here, we present self-assembled monolayers (SAMs) with phosphonic acid (PA) anchoring groups modifying NiO X hole transport layer (HTL) to tune energy level and passivate trap states. This strategy facilitates hole injection owning to the well-aligned energy level by interface dipole, downshifting the vacuum level, reducing the hole injection barrier from 0.94 eV to 0.28 eV. Meanwhile, it mitigates the interfacial recombination by passivating surface hydroxyl group (-OH) and oxygen vacancy (V O ) traps in NiO X . The electron leakage from QDs toward NiO X HTL is significantly suppressed. The all-inorganic R-QLEDs exhibit one of the highest maximum luminance, external quantum efficiency and operational lifetime of 88980 cd m (cid:0) 2 , 10.3 % and 335045 h (T 50 @100 cd m (cid:0) 2 ), respectively. The as-proposed interface engineering provides an effective design principle for high-performance AI-QLEDs for future outdoor and optical projection-type display applications.

The AI-QLEDs with NiOx HTL perform much less than conventional QLEDs with hybrid CTLs [16][17][18].The main reason is the significant energy barrier between NiO X and QDs, resulting in insufficient hole injection and increased Auger recombination.In addition, high traps at the NiO X /QDs interface also serve as non-radiative recombination centers [19].To overcome such a challenge, ionic doping [20,21], surface treatment [22,23], and insulating insertion layers have been performed to regulate energy-level alignment and inhibit emission quenching [24,25].Although these strategies can relatively improve the device performance, it still lags behind hybrid CTLs-based QLEDs.Therefore, techniques that simultaneously improve device stability and efficiency must be developed.
Self-assembled monolayers (SAMs) have emerged as the ideal selection to reconcile inorganic surfaces due to their versatility, good surface loading, tunable dipole moment, and high order in photoelectric devices [26][27][28].Anchoring groups of SAMs are responsible for binding to the metal or metal oxide surface, which can regulate the work function (WF), tune the shape of the energy barrier at the interface, and passivate trap states [27,29,30].Terminal groups are responsible for adjusting the surface and interface properties, such as wettability and electronic coupling with the overlayer [28,31,32].Encouraged by these benefits, the excellent properties of SAMs materials make them promising candidates for NiOx/QDs interface modifications to enhance the AI-QLEDs performance.

Result and discussion
We fabricated AI-QLEDs consisting of multiple layers with ITO/ NiO X / or NiO X /SAMs QDs/ZnMgO/Al architecture (Fig. 1a).Lowtemperature (100 ℃) solution-processed NiO X nanocrystal films were used as HTL.The SAMs in ethanol were deposited at the surface of NiO X using spin-coating method, and afterwards, the layers were washed with pure ethanol and annealed to remove all the weakly bonded molecules.CdZnSe/ZnS core/shell QDs with PL emission of 623 nm were used as emitting layer (Fig. S1a).Fig. S1b is the transmission electron microscopy (TEM) image of CdZnSe/ ZnS QDs.The average size of QDs is 15.1 ± 0.5 nm.We used SAMs: MeO-2PACz, Me-4PACz and 2PACz with different molecular dipole moments as interfacial monolayers between NiO X and QDs to improve the electroluminescence properties.The molecular structures of SAMs are shown in Fig. 1a.SAMs molecules are attached to the NiO X surface by reacting with the hydroxyl groups (-OH).Fig. 1b shows the normalized electroluminescence (EL) spectra of AI-QLEDs based on NiO X and SAM modify NiO X .The typical EL peak of 2PACz modified-NiO X -based QLEDs is located at 630 nm with a narrow full width at half-maximum (FWHM) of 24.2 nm, corresponding to the Commission Internationalede l'Eclairage (CIE) color coordinates of (0.68, 0.32), demonstrating high color-saturated red emission.Remarkably, the SAMs-modified devices display FWHM narrower peak than the control device (28.6 nm) and are accompanied by a blue-shift of the peak EL from 642 nm to 630 nm.This could be attributed to the interface engineering alleviated charge imbalance, reducing the generation of Joule heating by non-radiative Auger recombination, thus blue shift EL [33].The narrower emission for SAMs-modified devices was attributed to the reduction of interfacial defect states, which decreased the luminescence induced by defect states under an applied bias, suggesting more exciton confined in QD and narrowing the FWHM.where the driving voltage attains a 1 cd m − 2 luminance, decreased from 3.2 V to 1.9 V after modifying NiO X with 2PACz.The AI-QLEDs based on NiO X /2PACz showed peak luminance, current efficiency, and EQE of 89880 cd m − 2 , 14.06 cd A − 1 , and 10.30 %, respectively.Detailed device parameters are summarized in Table S1.In addition, the EQE distribution features of 10 devices are presented in Fig. 1e.The 2PACz-NiO Xbased devices yielded an average EQE of 8.2 % with a relative standard deviation of 0.95 %, outperforming the control devices.We believe that the improvement of device performance results from reducing the energy barrier and facilitating the injection of holes into QDs.Remarkably, the leakage current was considerably reduced by 1-2 orders of magnitude compared to NiO X -based devices, which is critical to improve performance.We consider that one reason is more holes are injected into QDs recombinating with electrons, alleviating the accumulation of electrons.Another reason is that reducing the electron capture by passivating NiO X defect states.
To ascertain the reason for performance improvement of AI-QLEDs based on SAMs interfacial modification NiOx HTL, synchrotron-based two-dimensional (2D) grazing incidence X-ray diffraction (GIXRD) was carried out to investigate the QD film's characteristics.Fig. 2a and b shows the integrated GIXRD patterns of QDs deposited on NiO X and NiO X /2PACz films.The QDs on NiO X /2PACz exhibited considerably sharper diffraction rings than the control sample, which is attributed to SAM molecules anchored to the NiO X surface to form strong bonds, reducing interfacial defects and increasing the QD coverage per unit area, thus enhancing the diffraction intensity [34,35].This was reflected in the out-of-plane line profiles in Fig. 2c.The enhanced peak intensities confirmed the better performance of QDs film.Improved performance is known to assist efficient recombination of electrons and holes in QD films, facilitating optimum EL performance of AI-QLEDs [36].
To further verify the interaction between the SAMs and NiOx, we performed X-ray photoelectron spectroscopy (XPS) to evaluate the surface chemistries of NiOx with and without SAMs.The presence of P 2p and N 1 s peaks for the NiO X /SAM sample at binding energies of 133.2 eV and 400.2 eV from XPS measurements confirmed the binding of SAM on the NiO X surface (Fig. 2d, e and Fig. S2).For clarity, the fine XPS spectra of Ni 2p, O 1 s, and C 1 s have been fitted with multiple Gaussians (Fig. 2f, S3, and S4).The O 1 s spectrum of the modified surfaces were fitted with additional C-O-C components (Fig. S4b), which belong to the methoxy groups of MeO-2PACz [37].Also, the additional C-N, and C-P components were discovered with SAM-modified samples by fitting the C 1 s spectrum (Fig. S3b and S4c).The Ni 2p peak for Ni 2+ is shifted by 0.36 eV towards higher binding energy after interfacial modification.The homogeneous shift in the binding energy suggested a chemical interaction of SAMs modifying NiO X .This is ascribed to the PA anchoring group has two hydroxyls and one phosphoryl group, which could anchor on the surface-OH groups of NiO X with mono-, bi-, and tridentate binding modes (Fig. S5) [23,38].Due to the presence of molecular dipoles introduced by SAMs, the chemical interaction of SAMs with NiO X will affect the energy-level alignment at the HTL/QDs interface [38].
We performed UPS to investigate the energy levels of NiO X with and without SAMs.Fig. 3a, and b, Fig. S6a, and b display the samples' secondary photoelectron cutoff (E cut-off ) and valence band region (E on- set ). SAM-treated NiO X could effectively regulate the energy level.Specifically, the NiO X with 2PACz interface layer downshifted the valence band maximum (VBM) of pristine NiO X film from − 5.16 eV to − 5.82 eV based on the equation: VBM = 21.22 -(E cutoff -E onset ).This would significantly lower the hole-injection barrier and establish a better band alignment at the HTL/QDs interface, alleviating the charge imbalance in QDs.The difference in VBM of NiO X /SAMs is attributed to the different molecular dipole moments of MeO-2PACz, Me-4PACz, and 2PACz.For more details, theoretical DFT calculations were carried out to probe the electronic properties of the three SAMs.SAM's electrostatic potential (ESP) was calculated to determine its electron density distribution (Fig. S7), which strongly depends on the electron-donating ability of the terminal groups [39].The calculated molecular dipole moment value is ~1.01 D for MeO-2PACz, ~1.6 D for Me-4PACz and ~1.9 D for 2PACz (Fig. S8).The positive dipole shifts the WF of the NiO X toward higher absolute numbers.Fig. 3f illustrates the energy level alignment at the NiOx/QDs interface and 2PACz-modified-NiO X /QDs interface.2PACz downshifts the VBM of the NiO X HTL by 0.66 eV, larger than MeO-2PACz (0.22 eV) and Me-4PACz (0.29 eV) (Fig. S9).The UV-vis absorption spectrum combined with the Tauc plots (Fig. S10) were checked to determine the band gap (E g ) of NiO X .Data of NiO X with and without SAM samples extracted from the UV-vis and UPS measurements   S2.The schematic illustration of device energy band is shown in Fig. S11.In summary, the NiO X /2PACz HTL has the lowest energy barrier with QDs, reducing the energy barrier from 0.94 eV to 0.28 eV, thus achieves the most improved charge balance, consistent with the above device performance results.
The carrier dynamics of QD films on NiO X with and without 2PACz treatment were studied by TA spectroscopy.Fig. 4a, b shows the TA 2D maps of QD films on NiO X and NiO X /2PACz substrates.Both samples show negative values for ground-state bleaching maximum at around 620 nm, which coincides with the exciton absorption position in the steady-state UV-vis absorption spectrum.Fig. 4c compares the transition dynamics of QDs on NiO X and NiO X /2PACz at 620 nm.For the CdSe-based QDs, the conduction-band electrons overwhelmingly dominate the exciton bleach with negligible contributions from the   valence-band holes [40,41].The 2PACz treated sample exhibits a slower delay, indicating a lower electrons capture, which resulting the suppressed exciton quenching in 2PACz treated-NiO X /QDs interface.
The steady-state PL and time-resolved PL (TRPL) spectroscopy analysis agree with the results of TA kinetics.The QDs on 2PACz-treated NiO X film show a higher PL intensity than the unmodified sample under the same measurement conditions (Fig. S12).Such a dramatic improvement is attributed to the passivated non-radiative recombination centers at the NiO X /QDs interface and the enhanced radiative recombination.TRPL spectroscopy curves (Fig. 4d) were fitted by a biexponential decay model, and the results are shown in Table S3.The average recombination lifetime (τ avg ) of NiO X /QDs films is 12.43 ns, which is lower than QD film (17.57ns), owing to the exciton quenching by trap states in NiO X .A prolonged τ avg of 14.93 ns was achieved for 2PACz-treated NiO X /QDs films.This confirms that 2PACz-treated NiO X is an effective approach to reduce quenching at the interface by passivating surface-OH.We further carried out DFT method to discover the passivation effect of 2PACz on NiO X .Our simulation indicates that the V O , which created more localized in-gap states, is passivated by the phosphoryl group of 2PACz (Fig. 4e).The O atom from the phosphoryl group occupied the V O .It relocated the V O defect away from the in-gap region, thus reducing trap states density in the interfacial of NiO X /QDs and enhancing radiative recombination [30,42].
Fig. 5a and b illustrates injection and recombination mechanisms for NiO X -based AI-QLEDs to emphasize the origin of the improved device performance.Overall, SAMs' improved EL performance in SAM-based NiO X AI-QLEDs is attributed to a bifunctional surface treatment.The interface engineering strategy effectively reduced non-radiative recombination by defect-capturing charge carriers, improved energylevel alignment, and significantly promoted hole injection into QDs.The reduced non-radiative recombination is attributed to the defect passivation of surface-OH and V O in NiO X .The improved level alignment results from interfacial dipoles of SAM downshifted the VBM of 0.66 eV, reduced the energy barrier of NiO X /QD from 0.94 eV to 0.28 eV.To better prove the charge-injection balance, we fabricated single-carrier device of the hole-only devices (HOD) with the structure of ITO/ HTLs/QDs/MoO 3 /Al and the electron-only devices (EODs) with a structure of ITO/ETL/QDs/ETL/Al (Fig. 5c).The current density of HOD based on SAMs-treated NiO X was 2-3 orders of magnitude higher than that of pristine NiO X -based HOD, much closer to the electron current.These results reveal that massive hole injection into QDs that could indeed balance the carrier transport, which contributes to achieving highly efficient EL performance.
The device's operational stability was also explored in ambient air (Fig. 5d).We observed that the 2PACz treated-NiO X -based device expands the lifetime.The T 50 lifetime at an initial luminance of 18430 cd m − 2 (T 50 @18430 cd m − 2 ) of AI-QLEDs is determined to be 28.1 h, corresponding to a T 50 @100 cd m − 2 of 335045 h or a T 50 @1000 cd m − 2 of 5319 h from the empirical equation of L n 0 T 50 = constant with an acceleration factor (n) of 1.80. 16The enhanced operational stability is due to decreased non-radiative Auger recombination.The peak EQE and the operational stability of our champion device is one of the highest value among red AI-QLEDs (Fig. S13 and Table S4) [16,18,25,43].

Conclusion
In summary, we have successfully integrated the SAM as an interface modification interlayer in all-inorganic NiO X -based red AI-QLEDs.The bifunctional interface engineering strategy favorably regulates the WF of NiO X by interfacial dipoles and heals the surface defects.The defect states at the interface of NiO X /QDs can be well passivated by carbazolebased SAMs with PA anchoring groups, which well suppress nonradiative recombination.Benefiting from the improved energy-level alignment, reduced hole injection barrier, and enhanced charge balance, 2PACz modified NiO X enables the AI-QLEDs with a record-high EQE and luminance of 10.3 % and 89880 cd m − 2 .More importantly, the AI-QLEDs show a remarkable improvement in operational stability, the superior stability T 50 @ 100 cd m − 2 reaches 335045 h.This strategy demonstrates that interface modification provides a simple and efficient way to develop high-performance AI-QLEDs.

Fig. 1c ,Fig. 1 .
Fig. 1.(a) Schematic illustration of SAMs' device and molecule structures.The zoom-in visualizes SAMs binding to the NiO X surface.(b) Normalized EL spectra.Inset, CIE coordinate of red AI-QLEDs based on 2PACz modified NiOx HTL.(c) J-V-L characteristics, (d) EQE-CE-L, and (e) EQE statistics for AI-QLEDs based on NiO X and NiO X /SAMs HTLs are shown in boxplots with normal distribution curves.

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Fig. 3 .
Fig. 3. UPS spectra of NiO X and NiO X /2PACz on ITO.(a) the secondary electron cutoff and (b) valence-band edge region.(c) Schematic illustration of energy level alignment at the NiOx/QDs interface (left) and 2PACz-modified-NiO X /QDs interface (right).

Fig. 4 .
Fig. 4. TA response of (a) NiO X /QDs and (b) NiO X /2PACz/QDs films.(c) TA delay at a specific wavelength.(d) TRPL decay for the pristine QDs, NiO X /QDs, and NiO X /2PACz/QDs films deposited on quartz substrates.(e) First-principles simulation of passivation effect of 2PACz on NiO X (V O ) surfaces.

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Fig. 5 .
Fig. 5. Schematic illustration of proposed carrier injection and recombination mechanism for (a) NiO X -based and (b) NiO X /2PACz-based devices.(c) Current densityvoltage characteristics of hole-only devices based on NiO X and NiO X /SAMs HTLs.(d) Operational lifetimes of devices based on NiO X and NiO X /2PACz HTLs.

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