Mitigating surface structural disorder for high-performance near-infrared quantum dot light-emitting diodes

Abstract

Perovskite quantum dots are promising for near-infrared light-emitting diodes, although they face serious challenges in limited operating device lifetime originating from the complex and disordered surface atomic states. Here, we design and use multifunctional zinc(II) 4-aminobenzenesulfonate to mitigate surface structural disorder in quantum dots, thereby achieving high-performance near-infrared light-emitting diodes. Such an additive, with four functional groups and ions (-NH₂, -S–O, -S=O, and Zn²⁺), and that shows double hydrogen–bonding effects, can strengthen surface atom termination and mitigate surface disorder, enabling an improved photoluminescence quantum yield of more than 90%. The as-fabricated light-emitting diodes demonstrated narrow electroluminescence spectra with a full width at half maximum of 42 nanometers at 789 nanometers, a maximum external quantum efficiency of 23.11% with negligible efficiency roll-off (as small as 3% at 100 milliamperes per square centimeter), a radiance of 155,851 milliwatts per steradian per square meter, and an operating lifetime of 2298 minutes at an initial radiance of 1,000 milliwatts per steradian per square meter (39,000 minutes for 190 milliwatts per steradian per square meter). This work represents a substantial improvement in radiance, efficiency roll-off, and operating stability compared with the best previously reported near-infrared perovskite quantum dot light-emitting diodes.

A multifunctional zinc(II) 4-aminobenzenesulfonate boosts performance of perovskite NIR QLEDs.

INTRODUCTION

Near-infrared (NIR, emission wavelength >750 nm) quantum dots (QDs) have shown promise for night vision, bioimaging, and clinical applications due to their narrow emission spectra and high solution compatibility (1–5). Recently, the application of NIR QDs, such as formamidinium lead triiodide (FAPbI₃) QDs, in light-emitting diode (LED) technology has achieved substantial breakthroughs, with the external quantum efficiencies (EQEs) exceeding 20% (6–8). However, the best previously reported FAPbI₃ QD-based LEDs (QLEDs) showed limited radiance (below 30,000 mW Sr⁻¹ m⁻²) and operating lifetime (100 to 120 min at an initial radiance of 1000 mW Sr⁻¹ m⁻²) with severe efficiency roll-off (dropping to below 10% under 30 mA cm⁻²), further hampering their development and industrialization (9–11).

Unlike all-inorganic perovskite QDs (e.g., CsPbI₃), organic-inorganic hybrid perovskite QDs (FAPbI₃ QDs) suffer from more complex surface issues (12–14). First, FA⁺ is prone to decompose under external environmental stimuli, such as heat, light, or humidity, due to its low C–N bond energy and strong hydrophilicity. The FA⁺ vacancy (V_FA) formation energy is only 0.54 eV and meanwhile affects Pb²⁺ vacancy (V_Pb) formation energy (decreases sharply from 1.95 to 0.86 eV) (15, 16). Second, during the purification and film-forming process of FAPbI₃ QDs, ligand loss leads to the formation of the I⁻ vacancy (V_I). Subsequently, the exposed Pb²⁺ will act as the center of nonradiative recombination. Third, the V_Pb is sensitive to moisture and easily forms V_Pb⁰-H₂O. Then, two nearby I⁻ ions approach and form an I-dimer, which in turn generates a deep trap state (15, 17). Moreover, it is reported that V_FA and V_I primarily govern the carrier recombination process, whereas V_Pb mainly influences the kinetics of the phase transition in FAPbI₃ (18). In sum, these complex and diverse surface vacancies result in QD surface structural disorders, which will accelerate QD degradation or phase transition, thereby resulting in poor QLED performance, especially under high current density or during continuous operation (19, 20).

Here, we propose a surface structural disorder-mitigating strategy by using zinc(II) 4-aminobenzenesulfonate (ZABS) to construct efficient and stable NIR perovskite QLEDs. The ZABS has four functional groups and ions, including -NH₂, -S–O, -S=O, and Zn²⁺, which could offer a precision-oriented strategy for mitigating structural disorder at the QD surface (V_FA, V_Pb, and V_I). The -NH₂ group could fill the V_FA, and the Zn²⁺ could occupy the V_Pb. The -S–O and -S=O groups from -SO₃⁻ both have strong nucleophilicity, which could interact with uncoordinated Pb²⁺ originating from V_I. Beyond that, -SO₃⁻ and -NH₂ could hydrogen bond with FA⁺ and I⁻, respectively. The target QDs exhibit reduced surface defects with an improved photoluminescence quantum yield (PLQY) of more than 90%. The as-fabricated champion QLED presents a narrow electroluminescence (EL) peak with a full width at half maximum (FWHM) of 42 nm at 789 nm, a high radiance of 155,851 mW Sr⁻¹ m⁻², and a maximum EQE of 23.11% at a current density of 0.23 mA cm⁻². The QLED exhibits negligible EQE droop, with EQEs of 20/15% at 100/200 mA cm⁻². Benefiting from the stable surface structure, the device demonstrates an operating lifetime of 2298 min at an initial radiance of 1000 mW Sr⁻¹ m⁻² (39,000 min at an initial radiance of 190 mW Sr⁻¹ m⁻²). This work discloses a facile approach to boost the performance of perovskite NIR QLEDs.

RESULTS

Surface structural disorder mitigating strategy

We optimized the traditional hot-injection method and synthesized FAPbI₃ QDs without inert gas protection (Fig. 1A); see details in the Materials and Methods. The FA-oleate precursor was injected into a PbI₂ solution for a 5-s reaction to synthesize a crude FAPbI₃ QD solution, followed by two rounds of purification. However, such unprocessed QDs suffer from metastable surface structural disorder (V_FA, V_Pb, and V_I), which intensifies the degradation pathways of materials and devices (21). Thus, we designed and introduced ZABS with four functional groups and ions (-NH₂, -S–O, -S=O, and Zn²⁺), suit the remedy to the case, thus mitigating the structural disorder at the QD surface. We performed density functional theory (DFT) calculations to calculate the molecular electrostatic potential (ESP) distribution maps of ZABS, as shown in fig. S1. The ZABS exhibited distinct site specificity: The -NH₂ and Zn²⁺ corresponded to the electropositive region, and the -S–O and -S=O corresponded to the electronegative region. Theoretically, the -NH₂ could fill the V_FA, and the Zn²⁺ could occupy the V_Pb. The -S–O and -S=O groups both have strong nucleophilicity, which could interact with uncoordinated Pb²⁺ originating from V_I. In addition, -SO₃⁻ and -NH₂ could hydrogen bond with FA⁺ and I⁻, respectively. Thus, we expected the ZABS to inhibit QD decomposition, enabling QDs with stable lattice structures and fewer defects, as shown in Fig. 1B. On the basis of our speculations above, ZABS can inhibit FAPbI₃ QD decomposition, thereby preserving the QD lattice’s integrity. As a result, the stronger, narrower, and stable PL should be observed in the target QDs. Meanwhile, the target QD particles should be larger and have a narrower size distribution (Fig. 1C).

Fig. 1. Surface structural disorder–mitigating strategy.

(A) Schematic illustration of synthesized pristine and target FAPbI₃ QDs. The pristine QDs exhibit metastable surface structural disorder, leading to their degradation. (B) The ZABS with four functional groups and ions (-NH₂, -S–O, -S=O, and Zn²⁺), together with the double hydrogen bond effects, could mitigate surface structural disorder and maintain the surface stability. (C) Schematic illustration of the effects of the proposed ZABS treatment on QDs’ optical and microstructural properties. The pristine QDs suffer from surface structural disorder, which causes their particle size to decrease and defocus over time or during the film-forming process, further leading to a decline in the intensity of the PL, broadening of the PL emission, and a blue shift in the peak position.

Optical properties and microstructure

We began by examining the QDs’ optical properties. The corresponding PL and absorption spectra of the pristine and target QD solution are shown in Fig. 2 (A and B, respectively). The pristine QD solution exhibited NIR emission at 785 nm with an FWHM of 48 nm, whereas the target QD solution displayed a 5-nm redshift and narrower emission at 790 nm with an FWHM of 44 nm. The redshift of PL and absorption, and a narrower FWHM, indicated a more uniform size distribution in the target QDs originating from the inhibited crystal decomposition. The target QD solution with optimal addition ratio (fig. S2) had a higher PLQY of 92% than the pristine QD solution (72%), suggesting enhanced radiative recombination in the target QDs (Fig. 2C) (22). In addition, the target QDs exhibited improved stability. The target QDs maintained 86% of their initial PLQY after 2 months of air storage, whereas the pristine QDs had degraded (Fig. 2D). The optical properties of Zn(AC)₂- and Zn(OTs)₂-treated QDs confirmed the synergistic effects of four functional groups and ions boost QD’s performance to the optimal level (fig. S3).

Fig. 2. Optical and microstructural properties.

PL spectra (solid line) and absorption (dashed line) of the (A) pristine and (B) target FAPbI₃ QDs. (C) PLQYs of QDs. (D) PLQY evolution of QDs with changing storage time. (E) STEM images of the pristine and target FAPbI₃ QDs. Scale bars, 50 nm. (F) The particle size distributions with Gauss fitting of the pristine and target FAPbI₃ QDs. The target QDs showed larger average size and more concentrated size distributions. (G) XRD results of the QDs. (H) FTIR spectra of QDs and ZABS. XPS spectra of (I) N 1s, (J) Pb 4f, and (K) I 3d of the pristine and target QD films. (L) Quantitative XPS analysis.

We then used aberration-corrected scanning transmission electron microscopy (STEM) and x-ray diffraction (XRD) to characterize the microstructures of FAPbI₃ QDs. At low magnification, the target QDs exhibited homogeneous spatial distributions across the observed region, whereas the pristine QDs exhibited obvious aggregation (fig. S4). Further high-magnification STEM observations revealed that the pristine FAPbI₃ QDs showed incomplete lattice structures and uneven size distributions, whereas the target QDs displayed decent crystal structures with even size distributions (Fig. 2E). The target QDs had a larger statistical average size of 23.0 ± 2.1 nm than the pristine QDs (17.7 ± 4.4 nm), which confirms that such a strategy could mitigate surface structural disorder and inhibit QD degradation (Fig. 2F). Element distribution mapping of target QDs confirmed Zn²⁺ has been successfully incorporated into the target FAPbI₃ QDs (fig. S5). The XRD results are shown in Fig. 2G. The diffraction peaks at 14°, 19°, 24°, 28°, and 31° in the XRD patterns correspond to the (100), (110), (111), (200), and (210) crystal planes of α-phase FAPbI₃, respectively (23). The target QDs exhibited preferred orientation with (100) and (200) planes, indicating improved crystallinity and a more concentrated size distribution compared with pristine QDs. The main diffraction peaks of the target QDs shifted toward higher angles (fig. S6A), indicating lattice contraction induced by Zn²⁺ incorporation (fig. S6B). The narrower typical diffraction peak confirmed the larger crystal size of the target QDs, consistent with PL and STEM results. In addition, we observed peaks corresponding to the δ-phase FAPbI₃ in the pristine QDs, which might originate from surface lattice distortion or from water oxygen–induced phase transition. The optical and microstructural measurements revealed that ZABS could mitigate QD surface structural disorder, thereby improving optical properties and structural stability.

Surface states and chemical environment

We used Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) to gain more detailed information concerning the surface state of QDs. As shown in Fig. 2H and figs. S7 and S8, compared with the pristine FAPbI₃ QDs, the target QDs exhibited pronounced changes in peaks of the N–H vibrations (~3200 to 3500 cm⁻¹). Specifically, the N–H stretching vibration in the 3200 to 3500 cm⁻¹ region broadened and shifted slightly toward lower wave numbers after ZABS incorporation, indicating a modified protonation environment and strengthened hydrogen-bonding interactions involving the amine groups, surface I⁻, and FA⁺ cations (24). In addition, the characteristic S=O stretching vibrations associated with sulfonate groups (~1000 to 1250 cm⁻¹) exhibited peak broadening in the target QDs, suggesting coordination and electrostatic interactions between the sulfonate groups and undercoordinated Pb²⁺ on the QD surface (25, 26). Changes in the intensity of the C=O stretching vibrations (~1710 cm⁻¹) of the OA ligand for target QDs (including attached and detached OA) can be attributed to the fact that sulfonate groups partially replace and reorganize carboxylate coordination on the QD surface (27). Zn²⁺ could stabilize the sulfonate ligand configuration and facilitate robust interfacial anchoring, as reflected indirectly in the N–H and S=O stretching vibrational features. The interaction between ligands [oleic acid/oleyl amine (OA/OAm)] and the QD surface is highly dynamic and labile, allowing ligands to easily fall off during the QDs’ storage and film-forming processes. ZABS was added in the target QDs and preferentially bound to the QD surface because of its stronger coordination, making the density of OA/OAm in target QDs lower than that of pristine QDs (fig. S8).

Motivated by the FTIR results, XPS measurements were performed to elucidate the corresponding evolution of surface chemical states (Fig. 2, I to L; figs. S9 and S10; and table S1). The core level spectrum of N 1s can be resolved into the -NH₂ peak (from FA⁺ and ZABS) and the protonated -NH₂ (-NH₃⁺), as shown in Fig. 2I (28). The -NH₃⁺ peak of target QDs shifted toward higher binding energy compared with pristine and Zn(OTs)₂-treated QDs, while -NH₂ peaks showed no obvious shifts for all QDs, indicating that protonated -NH₂ (-NH₃⁺) in ZABS could fill the V_FA in QDs. The core level spectrum of Pb 4f for Zn(OTs)₂-treated and target QDs both shifted toward lower binding energy compared with pristine QDs (Fig. 2J), indicating the enhanced electronegativity of Pb²⁺ due to stronger coordination with -SO₃⁻ in Zn(OTs)₂ and ZABS (29). The small peaks at ~141 and 136 eV in pristine QDs can be attributed to the V_I, which induced the reduction of Pb²⁺. The core level spectrum of I 3d for target QDs shifted slightly toward higher binding energy compared with pristine and Zn(OTs)₂-treated QDs, which confirmed the hydrogen bond between -NH₂ in ZABS with I⁻ in target QDs (Fig. 2K and fig. S9). In the O 1s core level spectrum, the observation of an additional signal at 529.5 eV in the pristine QDs can be attributed to the oxidation of surface-exposed Pb²⁺ in FAPbI₃ QDs by ambient oxygen and residual moisture, resulting in the formation of Pb-based oxides (fig. S10A). The -SO₃⁻ in Zn(OTs)₂ and ZABS offered strong interactions with surface-exposed Pb²⁺ and enhanced the stability of QDs in the atmosphere, which is consistent with the appearance of Pb⁰ signal in the core level spectrum of Pb 4f for pristine QDs (Fig. 2J). The shifts of the core level spectrum of O 1s at 532.5 eV in Zn(OTs)₂-treated and target QDs suggest Zn(OTs)₂ and ZABS could replace some of the weakly bound OA/OAm ligands, which is consistent with FTIR results (fig. S8). In the Zn 2p core level spectrum, an additional peak appeared in target QDs, suggesting the introduction of Zn²⁺ (fig. S10B). As shown in the quantitative XPS analysis in Fig. 3L, the I/Pb and N/Pb content ratios increased, while the oxide/Pb content ratio sharply decreased after introducing ZABS, suggesting decreased surface vacancies (V_FA and V_I) and improved surface stability. In sum, the FTIR and XPS results confirmed that all the functional groups and ions in ZABS play a substantial role in improving the surface state of QDs: The -NH₂ formed hydrogen bonds with I⁻ to prevent its detachment (V_I), and protonated -NH₂ can fill V_FA; the -SO₃⁻ strongly interacted with the surface-exposed Pb²⁺ to inhibit its reduction and the formation of Pb-based oxides; Zn²⁺ filled V_Pb to maintain the overall stability of the QD lattice.

Fig. 3. Trap states and exciton dynamics.

(A) TRPL spectra and fitted curves of the QDs. (B) The temperature-dependent TRPL curves of the QD films ranging from 77 to 297 K with a step of 10 K. (C) The corresponding PL lifetime of the QD films at different temperatures. (D) TA spectra of QDs. (E) TA spectra of the pristine QDs recorded at different delay times. (F) TA spectra of the target QDs recorded at different delay times. (G) Energy diagram and schematic structures of the hole-only devices. (H) Current-voltage characteristics of the control hole-only device. (I) Current-voltage characteristics of the target hole-only device.

Exciton behavior and trap states

We analyzed the exciton behavior of QDs using time-resolved PL (TRPL) decay, temperature-dependent TRPL spectra, and ultrafast transient absorption (TA) spectroscopy. For TRPL, we collected and fitted the decay curves by Eq. 1 and calculated the average PL lifetime (τ_avg) by Eq. 2 as follows

$$I=A_1\mathit{\text{exp}}(-\frac{t}{{\mathrm{\tau }}_1})+A_2\mathit{\text{exp}}(-\frac{t}{{\mathrm{\tau }}_2})$$ (1)

$${\mathrm{\tau }}_{\mathit{avg}}=\frac{A_1{{\mathrm{\tau }}_1}^2+A_2{{\mathrm{\tau }}_2}^2}{A_1{\mathrm{\tau }}_1+A_2{\mathrm{\tau }}_2}$$ (2)

The calculated τ_avg of the pristine and target QDs were 93.54 and 127.73 ns, respectively, as shown in Fig. 3A and table S2 (13). We then collected temperature-dependent TRPL spectra ranging from 77 to 297 K with a step of 10 K (Fig. 3B). The weak electron-phonon interaction at low temperatures (<200 K) strongly restricted nonradiative recombination, with carrier decay dominated by the radiative transition of single excitons (30). With rising temperature, the activated thermal excitation inhibited exciton recombination and thereby extended the PL lifetime. Meanwhile, gaps in the lifetimes of pristine and target QDs at high temperatures (>200 K) began to emerge (Fig. 3C). This indicates that target QDs with mitigated surface structural disorder have fewer trap states.

We then used ultrafast TA spectroscopy to gain further insights into the electronic evolution in QDs. We photoexcited the sample at 467 nm—an energy high enough to generate carriers across the wider bandgap barrier (Fig. 3D). Notably, the target QDs showed reduced trapping compared to the pristine QDs, consistent with their higher PLQY. The representative time-dependent decay kinetics of ground-state bleach (GSB) signals in both QD samples are shown in Fig. 3 (E and F, respectively). We observed narrowing and redshift of the GSB signals after mitigating surface structural disorder in QDs, which is consistent with optical spectral analysis. An 8.8-nm redshift of the GSB peak was highlighted by an arrow in target QDs (Fig. 3F), which is less pronounced than pristine QDs (12.1 nm; Fig. 3E). This reduced redshift indicates a flatter band distribution and reduced band-tail defect states in target QDs, thus decreasing the charge transport barrier and boosting radiative transition probability (31). All these results above indicate that the mitigated surface structural disorder effectively eliminated defects and suppressed the trap-related nonradiative recombination (32, 33).

To study the film-forming characteristics and prepare for subsequent device research, we applied scanning electron microscopy (SEM) and atomic force microscopy (AFM) to characterize the microstructures of FAPbI₃ QD films. For the pristine QDs, the partial particles aggregated together, and the boundaries between the particles are not distinct (fig. S11A). The target QDs exhibited clear, distinct shapes and larger crystal size, consistent with STEM and PL results. In addition, the target QD solution exhibited good film-forming properties, and the substrate was completely covered (fig. S11B). As AFM results showed in fig. S12, the target QD film showed a smoother surface with root mean square roughness (Rq) values of 3.58 nm, while the Rq of the pristine QD film was 4.43 nm. The target QDs maintained good film-forming properties after storage for 1 month, and the as-fabricated QD film showed good morphology (fig. S13), indicating that the target QDs exhibit improved structural stability.

The smooth and complete QD film was beneficial for carrier transport in electrical devices. We then applied space charge–limited current measurements to characterize the electrical properties of FAPbI₃ QD films. We constructed the hole-only devices according to the structure of indium tin oxide (ITO)/poly(ethylene dioxythiophene):polystyrene sulfonate mixed with nafion perfluorinated ionomer (PEDOT:PSS:PFI)/poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)/QDs/molybdenum trioxide (MoO_x)/Al (Fig. 3G) (29). The trap-filled limit voltages (V_TFL) were 0.73 V (Fig. 3H) and 0.48 V (Fig. 3I) for the control and target hole-only devices, respectively. The hole trap density was calculated by Eq. 3 as follows

$$N=2{\mathrm{\epsilon }}_0{\mathrm{\epsilon }}_r{V}_{\text{TFL}}/e{L}^2$$ (3)

where ε₀ is the vacuum permittivity, ε_r is the relative dielectric constant of FAPbI₃ QDs (46.9), e is the elementary charge, and L is the QD film thickness (6). The calculated hole trap densities decreased from 2.37 × 10¹⁸ cm⁻³ to 1.56 × 10¹⁸ cm⁻³, suggesting lower defect density in the target QD film.

EL performance of NIR QLEDs

We spin-coated the FAPbI₃ QD as the emitting layer to fabricate QLEDs based on a device configuration of ITO/PEDOT:PSS:PFI/PTAA/QD film/TPBi/LiF/Al (Fig. 4A), along with the corresponding cross-sectional SEM image and energy diagram in Fig. 4B. The corresponding current density-voltage (J-V) and radiance-voltage (R-V) curves of the control and target QLEDs are shown in Fig. 4 (C and D). Both the control and target QLEDs displayed a turn-on voltage of approximately 2.4 V (Fig. 4D). In the subthreshold regime below 2.4 V, the target QLED exhibited lower leakage current density than the control QLED, which signifies enhanced QD film quality and optimized interfacial properties (Fig. 4C). The target QLED showed higher current densities than the control device, indicating that fewer long-chain OA/OAm ligands contribute to better conductivity of target QDs. The target QLED achieved the highest radiance of 155,851 mW Sr⁻¹ m⁻², which was threefold higher than that of the control device (51,439 mW Sr m⁻²). Thanks to the reduction of surface defects caused by mitigating surface structural disorder, the target QLED showed a maximal EQE of 23.11% at 0.23 mA cm⁻², much higher than that of the control device (Fig. 4E). The statistics of 20 devices, presented in Fig. 4F, suggested that such a strategy has a high level of reproducibility. The corresponding EL peak of the target QLED was centered at 789 nm with an FWHM of 42 nm, showing a redshift and a narrower EL peak than the control device (Fig. 4G), consistent with the PL results. Notably, as the voltage increased from 4 to 7 V, the target QLED maintained stable EL (Δ_Peak < 1 nm) and EL FWHM (from 42 to 43 nm), whereas the control device continued to blue-shift and broaden (Δ_Peak = 7 nm for EL, from 43 to 45 nm for EL FWHM), which can be attributed to the stable surface and phase structure of the target QDs under high applied voltages or current densities (Fig. 4H).

Fig. 4. Performance of NIR QLEDs.

(A) Schematic illustration of the device structure and the corresponding cross-sectional SEM image. Scale bar, 200 nm. (B) Energy level alignment of the NIR QLED. (C) Current density–voltage (J-V) curves. (D) Radiance-voltage (R-V) curves. (E) EQE curves as a function of current density. (F) Histogram of the peak EQEs obtained from 20 devices for the control and target QLEDs. (G) EL spectra of the NIR QLEDs. (H) The EL peak position and EL FWHM evolution of QLEDs with changing voltage.

We compared the maximum EQE and the EQE at 30 mA cm⁻² in this work with those in other representative reports (fig. S15). Other reported NIR QLEDs all exhibited a severe roll-off, with EQEs dropping sharply below 10% at a current density of 30 mA cm⁻², whereas this work maintained an EQE of more than 20%. The target QLED also exhibited exceptionally low-efficiency roll-off at higher current densities, maintaining an EQE of 20% at 100 mA cm⁻² (15% at 200 mA cm⁻²). As shown in fig. S16, these four groups and ions in ZABS played distinct roles and jointly contributed to the comprehensive improvement of the device’s performance.

Operating stability of NIR QLEDs

The excellent electro-optical conversion performance and the extremely low efficiency roll-off meet our expectations for the device’s operating stability. We then evaluated the operating stability of QLEDs by testing their radiance decay under different initial radiance (R₀), as shown in Fig. 5 (A to F). We estimated the T₅₀ using an empirical scaling law for simulating LED degradation

$${R_0}^{\mathrm{n}} T_{50}=\text{constant}$$ (4)

Fig. 5. Stability of NIR QLEDs.

(A) The operating time of the control QLEDs as a function of initial radiance. The dashed line fits the T₅₀ data to the equation R₀ⁿ T₅₀ = Constant, where n is the acceleration factor. (B) Radiance changes of the control QLEDs over time under different initial radiance. (C) Voltage changes of the control QLED over time under a constant current of 0.1 mA (current density of 3.33 mA cm⁻²). (D) The operating time of the target QLEDs as a function of initial radiance. The dashed line fits the T₅₀ data to the equation R₀ⁿ T₅₀ = Constant, where n is the acceleration factor. (E) Radiance changes of the target QLEDs over time under different initial radiance. (F) Voltage changes of the target QLED over time under a constant current of 0.1 mA (current density of 3.33 mA cm⁻²). (G) Summary of the reported NIR perovskite QLED characteristics based on maximum radiance, EQE, and operating time. (H) Comparison of five characteristics (EL FWHM, EQE, Radiance, Roll-off, and T₅₀) of representative high-performance NIR perovskite QLEDs and this work. Roll-off was determined by the device’s current density when its EQE dropped to 10%. (I) Comparison of this work and other reported NIR QLEDs based on other types of QDs.

where n is the acceleration factor, and the value was obtained by fitting T₅₀ under different R₀ (13). The fitted values of n for the control and target QLEDs are 1.72 and 1.71, respectively. The corresponding T₅₀ at an R₀ of 1000 mW Sr⁻¹ m⁻² were 437 and 2298 min, respectively, as shown in Fig. 5 (A and D). The target QLED demonstrated a 15-fold higher operational stability than the best previously reported NIR perovskite QLEDs (110 to 160 min). The estimated T₅₀ at an R₀ of 190 mW Sr⁻¹ m⁻² (equivalent to 100 cd m⁻² for visible green emission) was 37,000 min for the target QLED. The voltage of the target QLED remained almost constant during operation at 0.1 mA (current density of 3.33 mA cm⁻²), rising from 3.35 to 3.37 V (ΔV = 0.02 V) at the initial 126 min (3.42 to 3.62 V for the control device, ΔV = 0.2 V), as shown in Fig. 5 (C and F).

We summarized, to the best of our knowledge, all reported high-performance NIR perovskite QLEDs in Table 1 and compared them with this work (Fig. 5G). This work demonstrated a high radiance of more than 150,000 mW Sr⁻¹ m⁻² with the narrowest EL (42 nm) and longest wavelength (789 nm), whereas the highest radiance reported in other studies is less than 30,000 mW Sr⁻¹ m⁻². This work also demonstrated an exceptionally low roll-off, maintaining an EQE of 20/15% at 100/200 mA cm⁻², whereas EQEs reported in other studies drop below 10% at ~30 mA cm⁻². The T₅₀ at an R₀ of 1000 mW Sr⁻¹ m⁻² of this work (2298 min) far exceeds the highest performance in other reports (160 min). In summary, this work demonstrated a 5-fold increase in radiance, a 7-fold improvement in efficiency roll-off, and a 15-fold increase in operational stability compared to the best previously reported NIR QLEDs (Fig. 5H) (6–8). Not only that, but it also represented one of the highest efficiencies among all types of NIR QLEDs to our best knowledge (Fig. 5I) (5, 34).

Table 1. Device performance of typical reported NIR perovskite QLEDs and this work.

*The time it took to drop to half of the initial radiance of 1000 mW Sr⁻¹ m⁻².

EL peak (nm)	EL FWHM (nm)	Radiance (mW Sr−1 m−2)	EQEmax (%)	Roll-off (EQE @30/100/200 mA cm−2)			T50* (min)	Reference
772	46	12,810	15.4	10	4	2	15	(4)
780	44	29,500	20.79	10	/	/	105	(6)
776	45	24,000	22.86	10	/	/	110	(7)
770	43	~15,000	21.82	8	/	/	117	(8)
776	46	~30,000	25.48	12	/	/	160	(36)
789	42	155,851	23.11	21	20	15	2298	This work

DISCUSSION

In summary, we have proposed a surface structural disorder-mitigating strategy for high-performance FAPbI₃ QDs, thereby further enabling efficient, bright, and stable NIR QLEDs. The ZABS could offer four functional groups and ions, mitigating various types of structural disorder at the QD surface and maintaining the structural stability of the QDs. Optical properties and microstructure analysis confirmed that ZABS could inhibit QD degradation and maintain the integrity of the QD lattice. The surface and trap states of QDs suggested the suppressed nonradiative recombination upon ZABS treatment. The as-fabricated QLED exhibited a narrow EL peak with an FWHM of 42 nm at 789 nm, a high radiance of 155,851 mW Sr⁻¹ m⁻², and a maximum EQE of 23.11% with small efficiency roll-off. The device demonstrated a T₅₀ of 2298 min at an R₀ of 1000 mW Sr⁻¹ m⁻² (39,000 min at an R₀ of 190 mW Sr⁻¹ m⁻²). This work highlights the crucial role of mitigating QD surface structural disorder in enhancing NIR perovskite QLEDs, thereby paving the way for high-performance QLEDs in night vision, bioimaging, and clinical applications.

MATERIALS AND METHODS

Materials

OAm (80 to 90%), OA (80 to 90%), 1-octadecene (ODE, >80%), isopropanol (99.7%), ethyl acetate (99%), and toluene (≥99.5%) were purchased from Aladdin. Nafion perfluorinated ionomer (tetrafluoroethylene-perfluoro-3, 6-dioxa-4-methyl-7-octenesulfonic acid copolymer) and FA-acetate (99%) were purchased from Sigma-Aldrich. Lead iodide (PbI₂, 99.99%) and PTAA were purchased from Advanced Eletion Technology Co. Ltd. PEDOT: PSS (Baytron P VPAl 4083) and TPBi were purchased from Xi’an Yuri Solar Co. Ltd. ITO glass was designed and purchased from Advanced Eletion Technology Co. Ltd.

Synthesis of FAPbI₃ QDs

In a typical FAPbI₃ QD synthesis, we first prepared the FA precursor by mixing FAAC (0.521 g) with OA (10 ml) and stirring until the mixture became clear and transparent. Then, PbI₂ (0.25 g, 0.6 mmol), OAm (1.5 ml), OA (2 ml), and ODE (10 ml) were added to a beaker or sample bottle in the atmosphere. The mixture was stirred at 120°C for 15 min. After that, the temperature was lowered to 80°C and maintained for an additional 10 min. Using a syringe, 3 ml of the FA precursor solution was injected into the mixture. Then, the mixture was allowed to react for 5 to 10 s to obtain the crude FAPbI₃ QD solution. The beaker was immediately transferred to an ice-water bath until the crude FAPbI₃ QD solution reached room temperature.

We purified the crude QD solution, referencing our previous report (29). The crude QD solution was first transferred to centrifuge tubes, and isopropanol was added in three times the volume. The mixture was then centrifuged at 8000g for 1 min. The supernatant was discarded, and toluene was added to disperse the precipitate. Next, OAmI/toluene (0.05 mol liter⁻¹) and ethyl acetate were added in turn. The mixture was then again centrifuged at 8000g for 1 min. The precipitate obtained after centrifugation was dispersed in n-octane to achieve pristine FAPbI₃ QDs, and the concentration was adjusted by changing the volume of n-octane. For the target QDs, ZABS was added together with PbI₂ at the very beginning, and the remaining procedures remain unchanged.

Device fabrication

We fabricated the QLED devices as described in our previous report (13). The PEDOT:PSS was mixed with the PFI at a mass ratio of 1:1 and then stirred until thoroughly mixed. For fabricating the hole-transporting layer, the mixture was filtered and spin-coated onto the ITO glass at 4000g for 45 s in air. The PEDOT:PSS:FPI film was annealed at 150°C for 20 to 30 min. PTAA/chlorobenzene (5 mg ml⁻¹) was spin-coated at 4000g for 45 s in a nitrogen-filled glovebox. The PTAA layer was annealed at 120°C for 20 to 30 min. For the QD layer, 60 to 70 μl QD solution was spin-coated at 2000g for 45 s. Notably, the QD layer does not require annealing. Next, the substrate was transferred to a vacuum-evaporation system. The electron-transporting layer (TPBi, 40 nm) and the electrode (LiF/Al, 1/100 nm) were evaporated under a vacuum of ~2 × 10⁻⁴ Pa. The overlapping part of the ITO and Al electrodes determines the shape and area of the emitting region.

Theoretical calculation

DFT calculations were performed using the Gaussian 16 package. Geometry optimization was carried out with the B3LYP functional, using the D3BJ dispersion correction and the 6-311 g(d,p) basis set for all atoms. ESP analysis was performed using the optimized coordinates with the Multiwfn 3.8 package, and the results were visualized with the VESTA package (35).

Characterization and device measurements

PL spectra of QDs were collected by a Cary Eclipse Fluorescence Spectrophotometer from Agilent. Ultraviolet-visible absorption spectra of QDs were collected by UV-3600 from Shimadzu SSL. For STEM measurements, the QD solution was diluted 1:30. The diluted QD solution was then dropped onto a carbon-coated copper grid, and the STEM test was conducted after the copper grid dried. STEM images were obtained using the aberration-corrected Thermo Fisher Scientific Themis Z 3.2 microscope. The STEM images were obtained at 300 kV, with a beam current of 3 pA, a convergence semiangle of 25 mrad, and a collection semiangle of 26 to 157 mrad. TRPL measurements were conducted using a time-correlated single-photon counting spectrofluorometer (FLS920, Edinburgh Instruments, UK). XPS measurements were performed using an ESCALAB Xi+ spectrometer (Thermo Fisher Scientific, USA). A monochromatized Al Kα x-ray source was used under the magnetic lens mode. The x-ray spot size was 500 μm, with an acceleration voltage of 15 kV and an emission current of 10 mA. The vacuum level in the analysis chamber was better than 1.0 × 10⁻⁹ mbar to minimize surface contamination during measurements. Survey scans were acquired with a pass energy of 160 eV and a step size of 1.0 eV to identify the elemental composition of the sample surface. High-resolution narrow scans were recorded with a pass energy of 20 eV and a step size of 0.05 eV for detailed analysis of the chemical states of the target elements. FTIR measurements were obtained using a Horiba VERTEX 70 with ATR mode. TA measurements were conducted using a femtosecond TA spectrometer consisting of a femtosecond laser (PH2-10 W, Light Conversion), optical parametric amplifier (ORPHEUS, Light Conversion), and femtosecond TA spectroscopy system (HARPIA-TA, Light Conversion). The QD solution was diluted 10 times with n-octane solvent. SEM measurements were conducted using a HITACHI SU-8010 cold-field emission scanning electron microscope. AFM images were obtained using a Veeco D3100 AFM tool. XRD measurements were conducted using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The XRD samples were prepared by directly dropping the QD solution onto the glass substrate in air and allowing it to dry. The EL characteristics of QLEDs were measured using an integrated LED testing system comprising a Keithley 2400 source, a fiber integration sphere, and a PMA-12 spectrometer (designed by Ocean Optics Co. Ltd.) in a nitrogen-filled glovebox at room temperature. For the operating stability test of QLEDs, the device was driven in constant-current mode at a specified initial radiance, and the device’s radiance over time was recorded.

Supplementary Materials

This PDF file includes:

Figs. S1 to S16

Tables S1 and S2