Dual‐Sided Interface Optimization Enables High‐Brightness All‐Solution‐Processed ZnMgO‐Based Green Perovskite QLEDs

ABSTRACT

The degradation of the underlying perovskite quantum dot (PQD) layers by the ZnMgO ink hinders the performance of all‐solution‐processed perovskite quantum dot light‐emitting diodes (P‐QLEDs). Herein, a novel dual‐sided interface optimization is proposed for the fabrication of P‐QLEDs: (i) cross‐locking PQD surface ions with a robust zwitterionic ligand to boost their chemical and solvent tolerance; (ii) improving ink orthogonality via optimizing the solvent of ZnMgO to acetonitrile and eliminating residual hydroxyl groups from the ZnMgO surface. As a result, the ZnMgO is deposited on the PQD films without causing serious degradation, leading to improved carrier injection and radiative recombination. The optimized P‐QLEDs achieve a maximum luminance of 134 920 cd m⁻² and a peak external quantum efficiency (EQE) of 8.3%, ranking among the state‐of‐the‐art all‐solution‐processed green‐emitting devices.

A novel dual‐sided interface optimization strategy is proposed for the fabrication of P‐QLEDs without serious PQD degradation after the ZnMgO spin‐coating process. The resulting devices achieve a maximum luminance of 134 920 cd m⁻² and a peak external quantum efficiency of 8.3%, exhibiting competitive performance among state‐of‐the‐art all‐solution‐processed green P‐QLEDs.

1. Introduction

Perovskite quantum dot light‐emitting diodes (P‐QLEDs) are regarded as a highly promising candidate for the application of displays, owing to their high color purity and superior optoelectronic properties [1, 2, 3]. In the last decade, with extensive research efforts in PQD synthesis and device engineering, the external quantum efficiency (EQE) of the P‐QLEDs has rapidly improved from ≈0.1% to high values of greater than 25% [4]. It is noteworthy that champion devices among these advancements predominantly rely on organic electron transport layers (ETLs), such as TPBi and 3TPYMB. Their application faces two inherent challenges: (i) the deposition of organic ETLs is relatively expensive due to the requirement for specialized vacuum thermal evaporation equipment [5, 6, 7]; (ii) organic ETLs exhibit significantly low electron mobilities (10⁻⁵‐10⁻³ cm² V⁻¹ s⁻¹) as the electrons are confined within specific molecular pathways [8, 9, 10].

Inorganic ZnMgO nanoparticles have been successfully used as ETLs in QLEDs, especially for InP and CdSe QDs [11, 12]. In comparison to organic ETLs, the ZnMgO ETLs have the following advantages: (i) the high electron mobility up to 10⁻⁴ – 10⁻² cm² V⁻¹ s⁻¹ and a continuous and tunable conduction band, which facilitating unrestricted electron movement; (ii) the ability to form dense, smooth films via a low‐temperature solution process, owing to the high solubility and excellent ink stability [13, 14]. Indeed, considerable efforts have been devoted to optimizing the green P‐QLEDs using ZnMgO as ETLs. For instance, Chen group reported perovskite LEDs with a brightness of 21 000 cd m⁻² and an EQE of 6.36% via dispersing the ZnO nanoparticles in nonpolar n‐octane using 2‐ethyl‐1‐hexanethiol as ligands [15]. Zhong group enhanced colloidal stability of siloxane‐modified ZnMgO nanoparticles via epitaxial growth of ultrathin SiO₂ shells [16]. Seong‐Yong Cho's group adopted Al₂O₃ or TPBi interlayers to mitigate the adverse effects induced by ZnO during spin‐coating processes [17, 18]. Analogously, research has also been conducted on red CsPbI₃‐based P‐QLEDs. Zong‐Liang Tseng realized performance improvement via orthogonal solvent optimization, while the Lin group developed cross‐linked PQD films, enabling subsequent ETLs to be fabricated via solution‐processed deposition [19, 20]. Despite the impressive progress in achieving P‐QLEDs using ZnMgO ETLs, most of the state‐of‐the‐art devices still hardly exhibit systematically integrated high EQEs and high brightness, falling far behind those based on organic ETLs [21].

The performance of ZnMgO‐based P‐QLEDs is mainly limited by the degradation of the underlying PQD layer during the deposition of the ZnMgO films. The chemical and solvent tolerance of PQDs is poor due to the intrinsic instability, originating from their soft ionic properties. Meanwhile, the excess hydroxyl groups and polar ethanol (EtOH) solvents of ZnMgO nanoparticles are detrimental to the PQDs [22, 23]. We took the view that the interface engineering within P‐QLEDs, specifically between the PQDs and the ZnMgO layers, must be judiciously redesigned.

As illustrated in Figure 1, a dual‐sided interface optimization is proposed for the fabrication of green P‐QLEDs via all‐solution process: (i) the ODPE‐FAPbBr₃ PQDs are obtained from OA‐FAPbBr₃ PQDs via performing ligand exchange of thermally dynamic OA by zwitterionic 1‐octyl‐2‐dodecanol‐3‐phosphoethanolamine (ODPE), leading to improved chemical stability and solvent resistance via forming tightly cross‐locked geometric fitness of the anionic and cationic moieties into the lattice sites of PQDs; (ii) the excess unbound hydroxyl (OH⁻) groups on the ZnMgO surface are removed via two‐stage purification, along with the solvent's optimization from EtOH to acetonitrile (ACN). Consequently, a high‐performance all‐solution‐processed P‐QLED is fabricated with the emission peak at 531 nm, demonstrating one of the highest brightness up to 134 920 cd m⁻² and EQE of 8.3%.

FIGURE 1.

Schematic comparison of ZnMgO/PQD heterojunction fabrication with and without using the dual‐sided interface optimization strategy.

2. Structure and Morphology of FAPbBr₃ PQDs

As shown in the HAADF‐STEM images of the two PQD samples (OA‐FAPbBr₃ and ODPE‐FAPbBr₃) in Figure 2a,b, both samples exhibit a cubic morphology. ODPE‐FAPbBr₃ PQDs possess a size distribution of 8.29 ± 0.66 nm, narrower than the 8.53 ± 1.15 nm of OA‐FAPbBr₃ (Figure S1). For the dynamic bonding between OA and PQDs, these ligands can be easily stripped off during the isolation/purification process, resulting in undercoordinated Pb²⁺ clusters (<2 nm) at the surface of the OA‐FAPbBr₃ PQDs [24, 25]. The bright Pb clusters are fully eliminated for ODPE‐FAPbBr₃, which indicates a notable enhancement in the phase purity. The powdered X‐ray diffraction (XRD) was recorded in Figure 2c _. The identical diffraction peaks at 15.1, 30.2, and 33.9 degrees correspond to the (100), (200), and (210) planes, matching well the α‐FAPbBr₃ structure [26, 27, 28]. The successful replacement of OA by ODPE ligands on the PQD surface was confirmed by Fourier‐transform infrared (FT‐IR) spectroscopy (Figure 2d). The successful exchange is evidenced by the appearance of an absorption peak at 1210 cm⁻¹ (asymmetric PO₂ ⁻ double bond stretches in ODPE) and the concurrent disappearance of the peak at 1533 cm^{− 1} (carboxyl antisymmetric stretch of OA), respectively [29, 30].

FIGURE 2.

Structural and morphological characterization of FAPbBr₃ PQDs. HAADF‐STEM images of OA‐FAPbBr₃ (a) and ODPE‐FAPbBr₃ (b). (c) XRD patterns and (d) FT‐IR results of OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs.

To further demonstrate the binding configuration, we calculated the interaction of ligands (e.g., OA and ODPE) with the FAPbBr₃ surface using density functional theory (DFT). The differential charge density of surface structures is compared in Figure S2, where yellow and blue indicate regions of electron accumulation and depletion. The carboxylate groups of OA coordinate with undercoordinated Pb²⁺, as evidenced by interfacial charge transfer. Meanwhile, the phosphate groups of ODPE interact with surface Pb atoms, whereas its ammonium groups occupy the FA sites on PQDs. This dual‐site binding mode is also experimentally confirmed by the simultaneous red‐shifts of PO₄ ³⁻ and NH₃ ⁺ groups of ODPE resulting from the weakened bonds upon binding to the FAPbBr₃ surface, as observed in the FT‐IR spectra in Figure S3. The binding energy is defined as E _b = E _{ligand+surface}–E _ligand–E _surface, where the latter three terms are the energy of the surface and bound ligand, the energy of the isolated ligand, and the energy of the bare surface, respectively. Our calculated binding energy for ODPE, at −5.58 eV, is large compared to the calculated values for OA (−4.03 eV). The large binding energy originates from the synergistic contributions of anionic and cationic binding moieties, which enables cross‐locking of the PQD surface.

3. Photophysical Properties and Stability of FAPbBr₃ PQDs

The photophysics of OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs are elucidated via the femtosecond time‐resolved transient absorption (TA) spectroscopy in Figure 3a,b and Figure S4. Both PQD samples exhibit two pronounced photo‐bleaching peaks at about 518 nm and about 497 nm, which are attributed to the 1s and 2s excitons [31]. Notably, the enhanced transient‐absorption signal at the two exciton resonances for the target sample compared with the control (around 2.7 times at 2s exciton and 1.3 times at 1s exciton) may be attributed to the enhanced Coulombic interaction and excitonic effect because of ODPE coordination. The bleach recovery kinetics at 518 nm were analyzed by fitting the decay curves with a bi‐exponential function, as illustrated in Figure 3c. The bleach recovery kinetic traces of OA‐FAPbBr₃ PQDs show one fast component (τ₁) of 116 ps and a long component (τ₂) of 4.27 ns. The proportion of τ₁ is up to 19% in the total decay, indicating a large amount of carrier trapping by defects. In contrast, the ODPE‐FAPbBr₃ PQDs exhibit a markedly prolonged τ₁ of 266 ps and a drastically reduced proportion of 2.5%. The decay profile is dominated by the long τ₂ of 10.2 ns, thus exhibiting near‐single‐exponential behavior. Meanwhile, the time‐resolved PL spectra (TRPL) of ODPE‐FAPbBr₃ PQDs are measured in Figure 4f. The ODPE‐FAPbBr₃ PQDs also show single exponential properties with a lifetime of 9.69 ns, consistent with the TA results. This is consistent with the higher PL intensity of ODPE‐FAPbBr₃ PQDs relative to OA‐FAPbBr₃ counterparts (Figure 3d). Moreover, all‐inorganic CsPbBr₃, CsPb(I/Br)₃ and CsPb(Br/Cl)₃ PQDs were also implemented with OA‐to‐ODPE ligand exchange, and all samples achieved enhanced PL intensity (Figure S5). This demonstrates the generality of ODPE in eliminating the defects via stabilizing the PQD surface [32, 33].

FIGURE 3.

Photophysics and stability of PQDs. TA spectra of (a) OA‐FAPbBr₃ and (b) ODPE‐FAPbBr₃ PQDs. The insets are the photophysical diagrams describing the carrier recombination. (c) Bleach recovery kinetics and bi‐exponential fitting results measured at 518 nm. (d) PL intensity and PLQY of OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs as prepared and after 7 d. Optical stability of the OA‐FAPbBr₃ (e) and ODPE‐FAPbBr₃ (f) PQDs against different purification cycles (1–5). The insets are the corresponding digital photographs.

FIGURE 4.

Surface and solvent optimization of ZnMgO ink. (a) TEM image of ZnMgO nanoparticulates. (b) Dynamic light scattering (DLS) and c) zeta potential of control ZnMgO (C‐ZnMgO) and surface‐polished ZnMgO (P‐ZnMgO). (d) XPS spectra of the O1s signal measured from C‐ZnMgO and M‐ZnMgO. (e) PL spectra and (f) TRPL of ODPE‐FAPbBr₃ films after spin‐coating different ZnMgO.

The optical stability for OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs was explored after 7 d of storage under ambient conditions. A significant degradation is observed in OA‐FAPbBr₃ PQDs: the PLQY falls sharply from 86% to 45.0%, along with a 3.4 nm emission broadening in the full width at half maximum (FWHM). In contrast, the ODPE‐FAPbBr₃ PQDs exhibit a negligible decrease in PLQY (from 93% to 92%) and a minimal broadening in FWHM (0.3 nm). As shown in the inset of Figure 3d, the ODPE‐FAPbBr₃ solution showed no discernible change, while the OA‐FAPbBr₃ solution turned noticeably yellow, indicating improved stability of the ODPE‐capped PQDs. Furthermore, the anti‐solvent stability of PQDs across successive purification cycles is evaluated in Figure 3e,f and Figure S6. A progressive reduction in PLQY is observed for OA‐FAPbBr₃ PQDs with increasing purification cycles, falling from 85.8% initially to merely 8.9% after the fifth cycle. Meanwhile, a distinct 14 nm red‐shift (from 534 to 548 nm) and a significant broadening of the FWHM (from 22.2 to 34.7 nm) are observed. The observed changes are attributed to the instability of OA‐FAPbBr₃ PQDs, primarily caused by the loss of OA ligands from the PQD surface during purification, leading to subsequent phase segregation or agglomeration. Unlike the monodentate binding of OA, ODPE ligands adopt a cross‐locked bidentate binding mode on PQD surfaces. As illustrated in Figure 3f, the ODPE‐FAPbBr₃ PQDs exhibit markedly enhanced anti‐solvent stability, maintaining stable PL profiles without red‐shift or broadening and nearly constant PLQY values throughout 1–5 purification cycles. Therefore, the ODPE‐FAPbBr₃ PQDs lay an important groundwork for all‐solution‐processed ZnMgO‐based perovskite QLEDs.

4. Solvent and Surface Optimization of ZnMgO Ink

The colloidal ZnMgO nanoparticles were synthesized with a low‐temperature sol‐gel method (C‐ZnMgO) [34]. The morphologies of C‐ZnMgO were characterized by TEM in Figure 4a, where ZnMgO shows an average size of ≈4 nm. Because ZnMgO is formed via the hydrolysis reaction of Zn²⁺ in an alkaline environment, its surface contains residual unbound hydroxyl (OH‐) groups, which are detrimental to the PQDs. We then polished the ZnMgO surface to remove the OH⁻ via two‐stage purification (P‐ZnMgO). As illustrated by the dynamic light scattering (DLS) and zeta potential in Figure 4b,c, the P‐ZnMgO samples exhibit a smaller average particle size (18 nm) and a narrower size distribution compared to C‐ZnMgO (21 nm), indicating the decreased thickness of the electrical double layer. Meanwhile, the zeta potential shifted from −9.87 to −5.94 mV. This demonstrates that the effective removal of unbound OH⁻ groups from the C‐ZnMgO. The presence of trace unbound OH⁻ groups in the C‐ZnMgO ink is further verified, as evidenced by the formation of blue Cu(OH)₂ precipitate when excess ZnMgO ink is added dropwise into the Cu²⁺ solution (Figure S7). To further verify the above, the XPS spectra of O 1s signal were measured in Figure 4d. The spectra of O 1s are deconvoluted into two peaks centered at 529.8 and 531.5 eV, in which the peak with lower binding energy is associated with the oxygen atoms in a ZnMgO lattice, and the one with higher binding energy is mainly attributed to the hydroxyl groups [35]. The OH⁻ component ratio in P‐ZnMgO is approximately 61%, which is comparatively lower than the 67% observed in C‐ZnMgO, matched well with the above results.

The second detrimental species in ZnMgO ink are the polar solvents. The ZnMgO dissolved in EtOH (or ACN) is abbreviated as ZnMgO‐EtOH (or ZnMgO‐ACN). The PL intensity was recorded before/after spin‐coating ZnMgO ink on the prepared PQD films. As depicted in Figure 4e and Figure S8, both FAPbBr₃ PQDs degrade drastically upon exposure to C‐ZnMgO ink, with their emission almost completely diminished. By contrast, FAPbBr₃ PQDs treated with P‐ZnMgO maintain high PL intensity. This confirms that unbound OH⁻ in C‐ZnMgO ink is the primary cause for PQD degradation. Additionally, we also explored the solvent effects of ACN and EtOH. The PQDs treated with ZnMgO‐ACN ink maintain a PL intensity retention of 42%, over twice that of the ZnMgO‐EtOH counterpart at only 17%. This indicates that solvents exert a secondary effect on the degradation of PQDs. The corresponding TRPL was measured in Figure 4f. The average PL lifetime (τ_ave) of the ODPE‐FAPbBr₃ films treated with P‐ZnMgO‐ACN is 1.09 ns, which is longer than the 0.96 ns of P‐ZnMgO‐EtOH. Thus, the P‐ZnMgO‐ACN ink demonstrates high orthogonality to the ODPE‐PQD films, thereby fulfilling the other essential requirement for realizing solution‐processed ZnMgO‐based P‐QLEDs in our study.

5. Device Performance of ZnMgO‐Based Perovskite QLEDs

P‐QLEDs were fabricated using OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs as the emissive layers for the control and target devices, respectively. As seen from the energy level diagram in Figure 5a, the holes are injected and transported to the valence band (VB) of OA‐PQD via the HOMO of TFB, while electrons are from the conduction band (CB) of ZnMgO to that of PQDs. As revealed by ultraviolet photoelectron spectroscopy (UPS) and absorption spectra (Figure S9), the VB of the ODPE‐capped FAPbBr₃ PQDs shifts downward by 0.06 eV relative to that of the OA‐capped counterparts. This shift establishes low energy barrier for electron injection. The transient electroluminescence (TREL) was recorded in Figure S10. The target devices based on ODPE‐FAPbBr₃ PQDs exhibit a carrier delay time (t_d) of 3.41 µs, which is faster than the 3.57 µs measured for the control devices. This shorter delay indicates smoother electron transfer/injection and improved carrier balance in our devices.

FIGURE 5.

Device performance of ZnMgO‐based P‐QLEDs. (a) energy level diagram of P‐QLEDs using OA‐FAPbBr₃ and ODPE‐FAPbBr₃ PQDs as the emissive layer. (b) EL spectra of the target P‐QLEDs under various voltage bias. (c) J–V–L and (d) EQE–V–CE curves of both P‐QLEDs. (e) Operational stability of P‐QLEDs evaluated at the initial luminance of 5000 cd m⁻². (f) Comparison of EQE and brightness of our P‐QLEDs with reported similar device architectures.

Both the P‐QLEDs show pure‐green electroluminescence (EL) centered at 531 nm, corresponding to the CIE color coordinates located at (0.1981, 0.7567) (Figure 5b and Figure S11). Moreover, the EL spectra exhibit spectral stability, showing negligible shift as the driving voltage increases from 3 to 6 V. We compared the current density–EQE (J–EQE) and current density—voltage–luminance (J–V–L) curves of our control and target devices in Figure 5c,d, respectively. Compared with the P‐QLED processed from OA‐FAPbBr₃ PQDs, the target devices based on ODPE‐FAPbBr₃ PQDs exhibit significantly higher current densities and luminance across the whole device operation range, verifying the improved charge injection/transport. The target devices reach a peak EQE of 8.3% and current efficiency of 36.42 cd A⁻¹ at a brightness of around 4952 cd m⁻². In addition, the device demonstrates a notably high maximum brightness of 134 920 cd m⁻², which is ≈4.4 times higher than that of the device based on OA‐FAPbBr₃ PQDs (30 601 cd m⁻²). The luminance of devices is responsible for the non‐radiative heat generation during operation. Surface temperature evolution for both devices with OA‐PQDs and ODPE‐PQDs is shown in Figure S12. The surface temperature of our target device increases by only 2.5°C (from 21.3°C to 23.8°C) when the bias voltage rises from 2 to 4 V, compared to a 3.2°C increase (from 24.4°C to 27.6°C) for the control devices. The low heat generation ensures exceptionally high operational stability for our target devices. The operational stability was then evaluated in Figure 5e and Figure S13. The target devices achieve a lifetime (T₉₀) of up to 2.8 min (the time required for the luminance to decay to 90% of its initial value) at an initial luminance of 5000 cd m⁻², representing a 28‐fold improvement over the control device. Meanwhile, at a lower initial luminance of 400 cd m⁻², the T₉₀ lifetime of the target devices can be extended to over 1000 min. Our target devices represent one of the best‐performing solution‐processed ZnMgO‐based P‐QLEDs as illustrated in Figure 5f and Table S1 [15, 36, 37, 38, 39, 40, 41, 42, 43].

In conclusion, a dual‐sided interface optimization was proposed for the fabrication of all‐solution‐processed ZnMgO‐based green P‐QLEDs. Specifically, the surface ions of the PQDs were strengthened by exchanging the dynamic OA ligands with zwitterionic ODPE, which greatly enhanced their chemical and solvent tolerance via forming cross‐locked configurations. Moreover, the surface defects in the PQDs were minimized by ODPE, resulting in a high PLQY of 93% and a nearly single‐exponential decay. Meanwhile, detrimental species in the ZnMgO ink, including unbound hydroxyl groups and polar EtOH solvents, were removed via a two‐stage purification process. Consequently, the degradation of underlying emissive PQD films by the ZnMgO ink was effectively addressed. The following achievements were obtained for all‐solution‐processed ZnMgO‐based P‐QLEDs: (i) our target device achieved a peak EQE of 8.3% and a current efficiency of 36.42 cd A^{− 1}, representing a 1.5‐fold enhancement over the control devices; (ii) a maximum brightness of up to 134 920 cd m^{− 2} was also achieved, representing one of the highest values reported for solution‐processed ZnMgO‐based P‐QLEDs; (iii) the target devices showed improved operational stability with a T₉₀ lifetime of 2.8 min, which is 28 times longer than that of control device.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smtd70815‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.