Surface ligand engineering of pure-red perovskite nanocrystals with enhanced stability by diphenylammonium halide molecules

Abstract

Perovskite nanocrystals (NCs) exhibit remarkable potential for light-emitting applications due to their solution processability, high photoluminescence quantum yield (PLQY), and tunable bandgaps. However, surface defects on NCs and the insulating nature of the surrounding long-chain ligands often impede the performance of the resulting perovskite light-emitting diodes (PeLEDs). Innovative strategies to address these challenges are crucial for advancing the environmental stability of perovskite NCs and high-efficiency PeLEDs. In this study, red light-emitting CsPbBr_xI_3−x NCs were synthesized via the hot-injection method, employing diphenylammonium iodide (DPAI) and diphenylammonium bromide (DPABr) as surface passivating ligands. These ligands not only compensated for surface defects of NCs through released I⁻ and Br⁻ anions but also improved charge carrier injection by π-conjugated benzene rings. Consequently, the PLQY was improved from 55% of the pristine NCs to 80% and 78% for those passivated with DPAI and DPABr ligands, respectively. The environmental stability and thermal stability of perovskite NCs were also enhanced under ambient conditions. The optimized red PeLED with the DPAI-modified perovskite NCs showed 2.8-fold higher luminance and 3.5-fold higher current efficiency than the control device. Similarly, the device based on the DPABr-modified NCs also exhibited significant improvements, showcasing the potential of surface ligand engineering with diphenylammonium halides in advancing PeLED performance.

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1. Introduction

All-inorganic cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I) possess excellent optoelectronic properties, offering advantages such as low cost, solution processability, tunable bandgaps, narrow emission bandwidth, and high photoluminescence quantum yield (PLQY).^1–3 These attributes have enabled significant advancements in applications across solar cells,^4–6 light-emitting diodes,^7–9 lasers,^10–12 and photodetectors.^13–15 CsPbBr₃ is a common perovskite material which possesses a stable crystalline structure and emits pure green light. However, red light-emitting CsPbI₃ perovskite readily induces a phase transition from the cubic phase (black phase) to the orthorhombic phase (yellow phase) when exposed to certain environmental factors, such as moisture and oxygen.¹⁶ This transformation leads to non-radiative carrier recombination and a significant reduction in device performance of perovskite light-emitting diodes (PeLEDs). To maintain a stable cubic phase, red perovskite emitters usually adopt mixed-halide perovskites with both Br and I ions. This approach not only stabilizes the perovskite structure but also enables wavelength tunability by adjusting the Br-to-I ratio to produce various emission wavelengths.^17,18 It should be noted that mixed-halide perovskites may exhibit ion migration under applied bias, resulting in phase segregation, instability in emission wavelength, and reduced device performance.¹⁹

High-quality perovskite nanocrystals (NCs) are crucial for efficient carrier recombination in PeLEDs. Ligand engineering has focused on minimizing surface defects and improving NC stability in order to achieve high device performance. Oleic acid (OA) and oleylamine (OAm) are the two widely used ligands to passivate the NC surface and prevent aggregation during synthesis. However, the long alkyl chains on OA and OAm form an insulating layer around the NCs, inhibiting charge transport and limiting the performance of PeLEDs.²⁰ Song, Zeng, and their co-workers utilized a mixed solvent system composed of hexane and ethyl acetate to purify perovskite NCs. This process can remove excess surface ligands and optimize charge transport, leading to a remarkable 50-fold increase in the external quantum efficiency (EQE).²¹ Apart from the above purification method, Park, Lee, and their co-workers employed branched ligands such as didecyldimethylammonium bromide and didodecyldimethylammonium bromide to partially replace OAm,²² improving the surface quality and optical properties of perovskite NCs. This substitution significantly enhanced the charge transport capability and stability of PeLEDs. Furthermore, organic ligands with π-conjugated benzene rings demonstrate great potential in surface passivation of perovskite NCs. Li, Tang, Jiang, and their co-workers selected phenethylammonium bromide or phenethylammonium iodide to stabilize CsPbX₃ quantum dot (QD) films via ligand-exchange.²³ Their findings revealed that the short organic ligands successfully attached to the QD surface, improving carrier injection and transport to further enhance the performance of CsPbX₃-based optoelectronic devices.

In this study, we employed two diphenylammonium halide ligands, i.e., diphenylammonium iodide (DPAI) and diphenylammonium bromide (DPABr), for the surface passivation of CsPbBr_xI_3−x NCs. The chemical structures of DPAI and DPABr are provided in Fig. 1(a). The two ligands consist of a diphenylammonium ion with delocalized π electrons to enhance charge transfer on the NC surface.²⁴ Additionally, the bulky benzene ring provides protective shielding, helping to stabilize the NCs against environmental degradation.²⁵ The I⁻ and Br⁻ anions in DPAI and DPABr potentially fill halide vacancies and further passivate surface defect sites. Fig. 1(b) demonstrates the surface passivation mechanism of CsPbBr_xI_3−x NCs through incorporation of DPAI or DPABr during the hot-injection process. In general, perovskite NCs exhibit numerous surface halide vacancies which act as trap states, leading to non-radiative recombination and negative impact on the optoelectronic properties of NCs. Upon DPAI or DPABr treatment, the –NH₂⁺– group in the diphenylammonium halide ligands interacts with the NC surface. At the same time, I⁻ and Br⁻ anions help to passivate halide defects. These features of DPAI and DPABr are anticipated to enhance the luminescence, electronic properties, and stability of the modified NCs.

Fig. 1 (a) Chemical structures of DPAI and DPABr; (b) surface passivation mechanism on CsPbBr_xI_3−x NCs.

2. Results and discussion

The diphenylammonium halide (DPAI and DPABr) ligands proposed in this study offer distinct advantages over conventional ammonium and phosphonic acid-based passivation materials. Their π-conjugated benzene rings enhance charge transport while mitigating the insulating effects of long-chain ligands such as OA and OAm. Additionally, these ligands effectively passivate surface defects through halide exchange and strong interfacial interactions, leading to improved PLQY and reduced non-radiative recombination. Moreover, the bulky diphenylammonium cations offer good resistance to moisture to enhance structural stability. To verify that the diphenylammonium ion facilitates delocalized π-electrons and enhances charge transfer on the NC surface, we conducted four-point probe measurements to evaluate the electrical conductivity of perovskite NC films, as shown in Table S1 in the ESI.† It is seen that the conductivity of the DPAI- and DPABr-modified NC films significantly increased compared to the pristine NCs, with enhancements of approximately one order of magnitude. This improvement suggests that the introduction of diphenylammonium ligands facilitates better charge injection. Fig. 2(a)–(c) presents the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the pristine and DPAI (or DPABr)-modified CsPbBr_xI_3−x NCs, consistently revealing a well-defined cubic structure. Notably, the average crystal size decreased from 8.92 nm (pristine NCs) to 7.85 or 8.38 nm with the addition of DPAI or DPABr, respectively, indicating that the two ligands can effectively restrict the growth of CsPbBr_xI_3−x NCs.²⁶ The insets in Fig. 2(a)–(c) show a lattice spacing of 0.41 nm for all samples, which corresponds to the (110) plane of the cubic perovskite phase.²⁷ The HRTEM images reveal that the lattice spacing remains unchanged after surface passivation, further confirming that the perovskite crystal structure is preserved. The top-view scanning electron microscope (SEM) micrograph of the pristine NC film in Fig. 2(d) shows a porous structure with a lot of cracks on the surface, suggesting that the pristine NCs likely possess a high density of surface defects. In contrast, the DPAI-modified NC film displays a continuous and densely packed surface morphology in Fig. 2(e), resulting from the passivation ability of DPAI to compensate for defects effectively. In Fig. 2(f), a similar condensed surface for the DPABr-modified NC film is also seen due to supplies of the Br⁻ ions. The morphological difference between the DPAI- and DPABr-modified NC films can be attributed to the superior ability of the I⁻ ions in DPAI to interact with perovskite NCs and neutralize the surface defects, leading to lattice expansion and enhanced crystal growth.

Fig. 2 TEM and HRTEM images of the (a) pristine, (b) DPAI-, and (c) DPABr-modified perovskite NCs; top-view SEM micrographs of the (d) pristine, (e) DPAI-, and (f) DPABr-modified perovskite NC films.

The Fourier transform infrared (FTIR) analysis provides deep insights into the chemical bonding of the pristine and DPAI (or DPABr)-modified CsPbBr_xI_3−x NCs. In Fig. 3(a), prominent absorption peaks at 2923 and 2853 cm⁻¹ are observed across all samples, attributable to the asymmetric and symmetric stretching vibrations of C–H bonds from the alkyl chains of OA and OAm ligands. A subtle peak at 3002 cm⁻¹ originates from C–H stretching of vinyl groups in the long-alkylated ligands. The characteristic peak at 1650 cm⁻¹ is associated with the bending vibration of C=C groups from OA and OAm ligands, while the two peaks at 1465 and 1390 cm⁻¹ are assigned to –CH₂ scissoring and –CH₃ symmetrical bending vibrations, respectively. The absorption peaks at 3040 and 1610 cm⁻¹ correspond to the C–H stretching and C=C stretching vibrational modes of the benzene ring, respectively. Notably, two new absorption peaks at 1580 and 1530 cm⁻¹ belong to aromatic C=C stretching vibrations, appearing in both DPAI- and DPABr-modified perovskite NCs but are absent in the pristine NCs. This provides strong evidence for successfully incorporating phenylated ammonium halide ligands onto NCs. The FTIR results confirm the effective modification of CsPbBr_xI_3−x NCs with DPAI (or DPABr) ligands. At the same time, the persistence of peaks associated with OA and OAm suggests partial replacement of original surface ligands with diphenylammonium halide ligands. In Fig. 3(b), the N 1s spectra from X-ray photoelectron spectroscopy (XPS) measurements provide further insight into the surface modifications induced by ammonium halide ligands. The pristine NCs exhibit an NH₃⁺ peak, which is attributed to residual OAm ligands from the synthesis process. After modification with DPAI (or DPABr) ligands, an additional peak corresponding to NH₂⁺ groups emerges, indicative of the successful incorporation of diphenylammonium halide ligands onto the NCs to achieve surface passivation. The co-existence of NH₂⁺ and NH₃⁺ peaks suggests that diphenylammonium halide ligands partially replace the original OAm ligands, which is consistent with the results from the FTIR experiments.

Fig. 3 (a) FTIR and (b) XPS spectra corresponding to the N 1s signal of the pristine, DPAI-, and DPABr-modified CsPbBr_xI_3−x NCs.

The electronic band alignment of the pristine, DPAI-, and DPABr-modified CsPbBr_xI_3−x NCs was investigated using an ultraviolet photoelectron spectroscopy (UPS) technique, and the corresponding UPS spectra are shown in Fig. 4(a). The work function (W_F), defined as the energy difference between the Fermi level and the vacuum level, was calculated using the equation W_F = 21.22 eV − E_cutoff,²⁸ where E_cutoff is the high binding energy cutoff around 17.9 eV. Hence, the W_F values for the pristine, DPAI-, and DPABr-modified NC films were determined to be 3.34, 3.36, and 3.27 eV, respectively. The valence band (VB) level was then calculated by subtracting the low binding energy onset (E_onset) from the W_F, which was −5.84, −5.80, and −5.75 eV for the pristine, DPAI-, and DPABr-modified NC films, respectively.²⁹ The energy band gaps (E_g) for the pristine, DPAI-, and DPABr-modified NCs were obtained from their Tauc plots in Fig. S1 in the ESI,† yielding values of 1.89, 1.83, and 1.92 eV, respectively. The conduction band (CB) levels were then calculated using the equation CB = VB + E_g, which was −3.95, −3.97, and −3.83 eV for the pristine, DPAI-, and DPABr-modified NCs, respectively. As shown in Fig. 4(b), the DPAI- and DPABr-modified NCs improve energy level alignment, effectively reducing the energy barrier between the NCs and the hole transport layer (poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine, poly-TPD). This enhanced alignment facilitates better charge carrier transport.

Fig. 4 (a) UPS spectra and (b) energy level alignment of the pristine, DPAI-, and DPABr-modified CsPbBr_xI_3−x NCs.

Fig. 5 demonstrates the X-ray diffraction (XRD) patterns of the pristine, DPAI-, and DPABr-modified perovskite NCs, providing insight into the crystal structure of the NCs. The three NCs reveal distinct diffraction peaks at 2θ = 14.5°, 20.4°, 29.2°, 32.6°, and 36.1°, corresponding to the (100), (110), (200), (210), and (211) planes of the cubic α-phase, respectively.^30–32 As shown in the XRD patterns, all samples exhibit characteristic diffraction peaks corresponding to the cubic perovskite phase, indicating that the overall crystal structure remains intact after ligand modification. Moreover, no noticeable peak shifts were observed, suggesting that the incorporation of DPAI and DPABr ligands does not significantly alter the lattice parameters. As illustrated in the magnified XRD pattern on the right side, the (200) diffraction peak of the DPAI-modified NCs shifts towards a smaller 2θ angle. This shift is attributed to lattice expansion resulting from the passivation of perovskite surface defects by I⁻ ions in DPAI compared to the pristine CsPbBr_xI_3−x NCs. On the other hand, the DPABr-modified NCs exhibit an opposite behavior, with the (200) diffraction peak shifting towards a larger 2θ angle. This observation comes from the introduction of DPABr to carry out the Br⁻ exchange, leading to lattice shrinkage compared to the pristine CsPbBr_xI_3−x NCs. The phenomenon has also been reported in the previous literature when using I⁻- and Br⁻-containing ligands to modify perovskite NCs.³³

Fig. 5 XRD patterns of the pristine and modified perovskite NCs. Enlarged XRD patterns show a systematic shift in the (200) plane.

The UV-vis absorption and photoluminescence (PL) spectra of the pristine and modified perovskite NC films are depicted in Fig. 6(a), showing a red shift of 4 nm for the DPAI-modified NCs and a large blue shift of 15 nm for the DPABr-modified NCs in PL emission. This shift is attributed to halide exchange, where extra Br⁻ or I⁻ ions from the two ligands are incorporated into the perovskite lattice. The more significant blue shift observed in the DPABr-modified NCs is due to the smaller ionic radius of Br⁻, which allows it to occupy the halide sites more effectively. Additionally, the PLQY values of the pristine, DPAI-, and DPABr-modified NCs from solution state are 55%, 80%, and 78%, respectively, indicating the effectiveness of surface passivation in reducing non-radiative recombination. The PLQY values of the pristine, DPAI-, and DPABr-modified NC films were acquired to be 12%, 22%, and 18%, respectively, which were significantly lower than those in solution. This reduction is primarily due to increased defect formation and self-quenching effects in the solid-state film. On the other hand, a less pronounced PL shift for the DPAI-modified NCs can be realized due to larger I⁻ ion radius. In addition, time-resolved photoluminescence (TR-PL) decay measurements were employed to verify the radiative recombination processes in the pristine, DPAI-, and DPABr-modified NCs. The TR-PL decay curves in Fig. 6(b) were fitted using a biexponential decay model,^34,35 and detailed TR-PL decay parameters can be found in Table S2 in the ESI.† The average carrier lifetime (τ_avg) for the pristine, DPAI-, and DPABr-modified NCs is 8.76, 10.22, and 10.94 ns, respectively. The increase in τ_avg for the modified NCs is attributed to the effective passivation of perovskite surface defects by the two ligands, which simultaneously enhances radiative recombination.³⁶ This enhancement is expected to bring benefit to the performance of PeLEDs.

Fig. 6 (a) Absorption and PL emission spectra, (b) TR-PL decay curves, (c) photos of the fresh and aged films under UV light (365 nm) irradiation, and (d) temperature-dependent PL intensity of the pristine, DPAI-, and DPABr-modified CsPbBr_xI_3−x NCs.

To evaluate the stability of NC films, the pristine and modified NC films were stored in an atmosphere of 25 °C and 70% relative humidity. The photos of the fresh and aged films under UV light (365 nm) irradiation are displayed in Fig. 6(c). After 5 days storage, the pristine NC film partially emitted green light, likely due to halide migration to produce phase separation. In contrast, DPAI- and DPABr-modified NC films maintained their red luminescence. After 15 days storage, the emission of the pristine NC film completely transferred to green; meanwhile, the modified NC films still showed red emission, meaning prohibition of halide exchange. The color change in the pristine NCs after 15 days of storage is attributed to phase transition and halide segregation, which occur more readily due to the lack of effective surface passivation. Without protective ligands, environmental factors such as moisture and oxygen further accelerate these instabilities. Our result highlights the superior stability of DPAI- and DPABr-modified NC films under ambient conditions, which is attributed to effective surface passivation and enhanced resistance provided by the two ligands. The thermal stability of perovskite NC solutions in hexane was evaluated by monitoring their PL intensity as a function of temperature, as shown in Fig. 6(d). At 70 °C, the pristine NC solution significantly reduced its PL intensity below 50% of the initial value. In contrast, the DPAI- and DPABr-modified NC solutions retained 93% and 74% of their initial PL intensities, respectively. At a temperature of 110 °C, the PL intensity of the pristine NC solution further decreased to 11% of its initial value, whereas the DPAI- and DPABr-modified NC solutions maintained approximately 70% of their initial intensities. The enhanced thermal stability can be realized with effective surface passivation by incorporating DPAI and DPABr ligands. The π-conjugated benzene rings in these ligands provide good resistance to moisture, which mitigates thermal degradation and enhances the overall stability of the NCs under elevated temperatures.

Fig. 7(a) presents the structure of PeLEDs with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/(poly-TPD)/perovskite NCs/1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi)/LiF/Al configuration, where the pristine, DPAI- and DPABr-modified NCs serve as the active layer. The cross-sectional SEM image of the device is exhibited in Fig. 7(b), indicating layer thicknesses of 150, 25, 20, 40, and 100 nm for ITO, PEDOT:PSS/poly-TPD, perovskite, TPBi, and LiF/Al, respectively. The energy level alignment of the PeLEDs, as shown in Fig. 4(b) in the previous part, demonstrates that the modified NCs result in better-matched energy level alignment within the device, facilitating more efficient charge injection and transport. It is noted that for the energy levels of PEDOT:PSS, poly-TPD, TPBi, and LiF/Al the reader is referred to the previous literature.³⁷ The influence of different molar concentrations of DPAI on device performance was investigated, as shown in Fig. S2 (ESI†). The results indicate that an optimal concentration exists for achieving the highest current efficiency and luminance, with the peak performance observed at 0.3 mmol. The molar concentration of DPABr was determined to be the same (0.3 mmol). To evaluate the optoelectronic performance of PeLEDs, the current density–voltage–luminance (J–V–L) and current efficiency–current density (CE–J) characteristics are presented in Fig. 7(c) and (d), respectively. The corresponding electroluminescence (EL) wavelength, turn-on voltage (V_on), maximum luminance (L_max), and peak CE (η_max) are listed in Table 1. As shown in Fig. 7(c), the DPAI-modified PeLED exhibited the lowest V_on of 3.28 V and the highest L_max of 913 cd m⁻², which is approximately 2.8- and 1.6-fold higher than that of the control device (325 cd m⁻²) and the DPABr-modified device (562 cd m⁻²). This highlights the enhanced charge injection and recombination efficiency in the DPAI-modified device. Similarly, the CE–J curves in Fig. 7(d) reveal that the DPAI-modified PeLED achieved a η_max of 0.97 cd A⁻¹, which is nearly 3.5-fold higher than that of the control device (0.28 cd A⁻¹) and approximately twice as high as the DPABr-modified device (0.49 cd A⁻¹). The improvement in luminance and efficiency is attributed to the DPAI and DPABr ligands that passivate surface defects and align energy levels of perovskite NCs, facilitating better charge transport and reducing non-radiative recombination pathways. The EQE–J curves and EQE statistical distribution of 20 devices are provided in Fig. S3(a) and (b) (ESI†). The peak EQE values of the pristine, DPAI-, and DPABr-modified devices are 2.0%, 6.9%, and 3.5%, respectively, and our devices possess good reproducibility in EQE. As shown in Fig. 7(e), the introduction of diphenylammonium halide ligands can lead to partial halide exchange, shifting the bandgap and consequently altering the EL emission peak. DPABr-modified NCs are found to have a slight blue shift due to Br⁻ incorporation. Besides, the EL spectra of the PeLEDs exhibit a slight red shift compared to the PL spectra of CsPbBr_xI_3−x NCs. This shift can be ascribed to field-driven ion migration and Förster resonance energy transfer under operational conditions.³⁸ These mechanisms cause red-shifted emission wavelength, particularly in mixed-halide perovskite materials. To further investigate the operational stability of PeLEDs, the half-lifetime (T₅₀) was recorded and compared. Fig. 7(f) presents the luminance evolution over time under a constant voltage of 5 V, and the results indicate that the T₅₀ values for the pristine, DPAI-, and DPABr-modified devices are 188, 800, and 488 s, respectively. It can be realized that DPAI and DPABr effectively passivated surface defects of the perovskite NCs, enhancing operational stability and extending the working lifetime of PeLEDs. An overview of the performance for red PeLEDs from previous literature and our work has been added as Table S3 in the ESI.† Currently, red PeLEDs can achieve record peak EQEs of 29.04–32.14%, along with prolonged T₅₀ of 43.7–60.0 h.³⁹ Frankly, the luminance of our devices is moderate, but their EQE values are lower compared to other works. Limited by our processing facilities, the performance of the fabricated devices was not impressive. Nevertheless, our study demonstrates a facile and inexpensive method to passivate red perovskite NCs with dual halide ions, and hydrophobicity brought by benzene rings mitigates moisture-induced degradation to enhance environmental stability.

Fig. 7 (a) Device structure, (b) cross-sectional SEM image, (c) J–V–L, (d) CE–J, and (e) EL spectra of PeLEDs with a snapshot of the emitted DPAI-modified PeLED captured at 9 V; (f) luminance evolution curves of the pristine, DPAI-, and DPABr-modified PeLEDs at a fixed voltage of 5 V.

Table 1 Device performance of PeLEDs based on the pristine, DPAI-, and DPABr-modified CsPbBr_xI_3−x NCs

Perovskite	EL (nm)	V on (V)	L max (cd m^−2@V)	η max (cd A^−1@V)
Pristine	666	3.84	325@11.2	0.28@4.31
DPAI-modified	665	3.28	913@9.09	0.97@4.5
DPABr-modified	654	3.56	562@8.25	0.49@3.56

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To investigate the trap density in perovskite NCs and evaluate the effect of surface passivation by DPAI and DPABr ligands, space-charge-limited current (SCLC) measurements were conducted. Electron-only devices were fabricated with the structure of ITO/PCBM/pristine, DPAI-, or DPABr-modified NCs/TPBi/LiF/Al, and their dark J–V characteristics are depicted in Fig. 8(a)–(c). The trap-filled limit voltage (V_TFL) corresponds to the transition point from the ohmic region (where current increases linearly with voltage) to the trap-filled region (the current rises sharply due to the filling of trap states at this stage). The V_TFL values for the pristine, DPAI-, and DPABr-modified NC films were determined to be 0.76, 0.42, and 0.53 V, respectively. The corresponding trap densities (N_trap) were then calculated using the following eqn (1):

where ε₀, ε_r, q, and L represent the vacuum permittivity (ε₀ = 8.854 × 10⁻¹² F m⁻¹), relative dielectric constant of the perovskite (ε_r = 33),⁴⁰ elementary charge (q = 1.602 × 10⁻¹⁹C), and thickness of the CsPbBr_xI_3−x NC film (L = 20 nm). The calculated N_trap for the pristine, DPAI-, and DPABr-modified NCs are 6.92 × 10¹⁸, 3.82 × 10¹⁸, and 4.82 × 10¹⁸ cm⁻³, respectively, revealing that introducing DPAI and DPABr ligands can reduce the trap density in perovskite NCs and improve the device performance. Hole-only devices were fabricated to quantify the density of hole defect states and evaluate the passivation effect on halogen vacancy defects. The J–V characteristics of hole-only devices are provided as Fig. S4 in the ESI.† The calculated N_trap from hole-only devices for the pristine, DPAI-, and DPABr-modified NCs are 1.51 × 10¹⁹, 1.29 × 10¹⁹, and 1.41 × 10¹⁹ cm⁻³, respectively, revealing that introducing DPAI and DPABr ligands can reduce the hole defect density in perovskite NCs. The J–V characteristics of the pristine, DPAI- and DPABr-modified electron-only and hole-only devices are provided as Fig. S5 (ESI†), and the extracted carrier mobility and the electron-to-hole mobility (μ_e/μ_h) ratio are summarized in Table S4 (ESI†). The μ_e/μ_h ratio decreased after surface passivation with DPAI and DPABr ligands, indicating more balanced charge transport and reduced recombination losses. These results further confirm that diphenylammonium halide ligands effectively modulate carrier transport properties, enhancing overall device performance.

Fig. 8 SCLC characteristics for the (a) pristine, (b) DPAI-, and (c) DPABr-modified devices. The insets show the structure of the electron-only devices based on different perovskite NCs.

3. Conclusions

In this study, red light-emitting CsPbBr_xI_3−x NCs were synthesized via the hot-injection method and passivated with DPAI or DPABr ligands successfully. The FTIR and XPS results proved the successful incorporation of diphenylammonium halide ligands onto the NCs. These ligands compensated for surface defects of NCs through released I⁻ and Br⁻ anions and improved charge carrier injection by π-conjugated benzene rings. The average crystal size of perovskite NCs decreased by introducing DPAI or DPABr ligands. The UPS experiment suggested better charge carrier transport from poly-TPD to NCs after modification with diphenylammonium halide ligands. The PLQY increased from 55% of the original NCs to 80% and 78% for the DPAI- and DPABr-modified NCs, respectively. The environmental and thermal stability of perovskite NCs were also enhanced in ambient conditions. PeLEDs based on the DPAI-modified NCs demonstrated the best performance, with a 3.5-fold increase in η_max and a 2.8-fold enhancement in L_max compared to the control device. The PeLEDs based on the DPAI- and DPABr-modified NCs also had longer operational lifetime. These findings provide valuable insights into surface ligand engineering for achieving efficient and stable perovskite optoelectronic devices.

4. Experimental section

4.1 Materials

Diphenylamine (DPA, 98%), hydriodic acid (HI, 57 wt% in water), hydrobromic acid (HBr, 48 wt% in water), and 1-octadecene (ODE, purity 90%) were procured from Thermo Fisher Scientific. Lead(II) iodide (PbI₂, purity 99.9985%) and lead(II) bromide (PbBr₂, purity 99%) were obtained from Alfa Aesar. Cesium carbonate (Cs₂CO₃, purity 99.99%) and OA (purity 90%) were bought from Sigma-Aldrich. OAm (purity 99%) was obtained from Acros. ITO-coated glass substrates (15 Ω per square) were procured from Aimcore Technology. PEDOT:PSS aqueous solution (UR-AI4083) was purchased from Heraeus Precious Metals GmbH & Co. KG. Poly-TPD was purchased from 1-Material Inc. TPBi was acquired from Shine Material Technology. Other reagents and solvents were sourced from suppliers such as Alfa Aesar, Acros, Macron, or ECHO Chemical Co., Ltd. and used without further purification.

4.2 Synthesis of DPAI and DPABr

To synthesize the ionic ligand DPAI (or DPABr), 1.0 g (5.9 mmol) of DPA was dissolved in 15 mL of ethanol under stirring at 0 °C. Subsequently, 0.86 mL (11.3 mmol) of HI (or 0.74 mL of HBr, 13.5 mmol) was added dropwise to the DPA solution and stirred for 2 h. The mixture was then subjected to rotary evaporation to remove ethanol. The crude product was re-dissolved in 10 mL of ethanol and precipitated in 200 mL of diethyl ether. Finally, the filtered product was dried under vacuum for 24 h to give DPAI (or DPABr) solids with an average yield of 47–48%.

4.3 Synthesis of CsPbBr_xI_3−x NCs

To prepare Cs-oleate, Cs₂CO₃ (407 mg, 1.25 mmol), 1.25 mL of OA (3.9 mmol), and 20 mL of ODE were added to a 50 mL two-neck flask, evacuated for 30 min, and heated to 120 °C for degassing. Nitrogen was loaded into the reaction for 10 min and the flask was re-evacuated for 20 min. Then, nitrogen was loaded into the flask again and the reaction temperature was increased to 160 °C. The mixture was heated until all solids were dissolved to give the Cs-oleate which was stored in an ambient environment. The solution was heated to 110 °C before use.

To synthesize perovskite NCs, a mixture containing PbI₂ (63.6 mg, 0.137 mmol), PbBr₂ (18.4 mg, 0.05 mmol), OA (0.5 mL, 1.58 mmol), OAm (0.5 mL, 1.51 mmol), and ODE (5 mL) was loaded in a 50 mL two-neck flask, evacuated for 30 min, and heated to 120 °C for degassing. Nitrogen was loaded into the reaction for 10 min and the flask was re-evacuated for 20 min. Afterward, nitrogen was loaded into the flask again, and the reaction mixture was heated to 160 °C with vigorous stirring until the solids were completely dissolved to form Pb-oleate. Then, 0.4 mL of Cs-oleate was injected into the Pb-oleate solution using a syringe. After reacting for 5 s, the reaction mixture was quickly immersed in an ice-water bath and centrifuged at 8500 rpm for 10 min to preserve the precipitate. Next, 0.5 mL of hexane and 2 mL of ethyl acetate were added to the centrifuge tube and the solution was centrifuged at 8500 rpm for 10 min. The precipitate was collected and dispersed in 1 mL of hexane for further utilization. To obtain DPAI (or DPABr)-modified CsPbBr_xI_3−x NCs, 0.3 mmol% of DPAI (or DPABr) relative to the sum of PbI₂ and PbBr₂ moles were added into the PbI₂/PbBr₂ solution. The synthetic procedure was the same as that of the pristine CsPbBr_xI_3−x NCs.

4.4 Device fabrication

Light-emitting devices were constructed with the architecture of ITO/PEDOT:PSS/poly-TPD/pristine or modified perovskite NCs/TPBi/LiF/Al. The patterned ITO substrates underwent a sequential cleaning process using detergent, deionized water, acetone, and isopropanol under ultrasonication for 20 min each, followed by nitrogen purging and oxygen plasma treatment. The filtrated PEDOT:PSS solution was spin-coated onto the ITO substrate at 4000 rpm for 40 s, followed by annealing at 150 °C for 15 min in air. The substrate was then transferred to a nitrogen-filled glove box for the deposition of subsequent layers. Poly-TPD (8 mg mL⁻¹ in chlorobenzene) was spin-cast from its solution onto the PEDOT:PSS layer at 3000 rpm for 40 s and annealed at 150 °C for 20 mi. The CsPbBr_xI_3−x NCs solution was then spin-coated onto the poly-TPD layer at 3000 rpm for 40 s. Finally, 40 nm of TPBi, 0.5 nm of LiF, and 100 nm of aluminum electrodes were sequentially deposited by thermal evaporation under a base pressure of 8 × 10⁻⁶ Torr. The active area of each device for performance evaluation was 1 mm².

4.5 Characterization

XPS and UPS measurements were conducted using a ULVAC-PHI PHI 5000 Versaprobe II to analyze the elemental composition and energy levels of the pristine and DPAI (or DPABr)-modified CsPbBr_xI_3−x NCs. The Al anode (hν = 1486.6 eV) and a He(I) (hν = 21.22 eV) discharge lamp were utilized as the excitation source for XPS and UPS measurements, respectively. The XRD patterns of perovskite films were measured using a Bruker D8 Discover diffractometer. The FTIR spectra were acquired using a Thermo Scientific Nicolet iS-10 spectrometer. The dimensions of CsPbBr_xI_3−x NCs were assessed through a JEOL JEM-3010 TEM. The HRTEM images were obtained using a Talos F200X G2 field emission TEM to analyze the detailed crystal structure and lattice fringes. The top-view and cross-sectional SEM micrographs of samples were obtained using an ultrahigh-resolution ZEISS AURIGA Crossbeam SEM. The PL and absorption spectra of CsPbBr_xI_3−x NCs were obtained using a Princeton Instruments Acton 2150 spectrophotometer, utilizing a xenon lamp (ABET Technologies LS 150) as the light source. TR-PL decay signals were measured using a PicoQuant MultiHarp 150 4N module, in conjunction with a photomultiplier tube through an Andor Kymera 328i spectrometer with a time resolution of 160 ps. EL spectra and device performance of PeLEDs were obtained using an Agilent 4155C semiconductor parameter analyzer in conjunction with an Ocean Optics USB2000+ spectrometer.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to acknowledge financial support from the National Science and Technology Council of Taiwan (grant no. NSTC 113-2221-E-A49-161-MY3).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00390c

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