In situ dual-site passivation of all-inorganic perovskite quantum dots using zwitterionic taurine for enhanced defect resistance

Abstract

Metal halide perovskite quantum dots (PQDs) possess exceptional optical properties but suffer from structural instability under thermal, moisture, and light exposure due to the dynamic bonding of long-chain organic ligands and the ionic nature of their crystal structure. These instabilities increase surface defects and non-radiative recombination, limiting their potential for next-generation optoelectronic devices. In this work, we present a strategy to simultaneously control surface passivation by introducing the zwitterionic ligand taurine during the in situ synthesis of CsPbBr₃ PQDs. The SO₃⁻ group of taurine strongly coordinates with surface Pb²⁺, while the NH₃⁺ group electrostatically interacts with Br⁻ ions, enabling effective dual-site passivation at the PQD surface. As a result, taurine-based PQDs exhibit enhanced QY compared with PQDs synthesized using conventional long-chain ligands. The PQDs also maintain strong photoluminescence in aqueous environments for 140 h and demonstrate improved thermal stability at 120 °C relative to pristine CsPbBr₃ PQDs. Furthermore, dual-site passivation lowers trap density, while the short taurine ligand enhances charge mobility, yielding electronic properties favorable for device applications. This study demonstrates that zwitterionic ligand engineering is an effective strategy to simultaneously improve the stability and performance of perovskite quantum dots, offering a promising pathway for the development of high-performance optoelectronic devices.

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1. Introduction

Metal halide perovskite quantum dots (PQDs) have attracted considerable attention as promising materials for next-generation light-emitting, sensing, and optoelectronic devices due to their narrow full width at half maximum (FWHM), high photoluminescence quantum yield (QY), and composition-dependent bandgap tunability.^1–7 Their precisely adjustable emission wavelengths, facilitated by halide composition control and quantum confinement effects, combined with low-temperature, solution-processable synthesis, offer substantial advantages over conventional CdSe and InP quantum dots. Inorganic CsPbX₃ (X = Cl, Br, I) PQDs, in particular, exhibit strong potential in applications such as QLEDs, photodetectors, lasers, and solar energy harvesting.^8–15 These advantages stem from their defect tolerance, relatively high structural stability compared with CdSe and InP QDs, excellent color purity, and high luminous efficiency.

Despite these benefits, the practical implementation of CsPbX₃ PQDs is fundamentally limited by their structural instability. This instability arises from the dynamic binding property of long-chain organic surface ligands and the inherently ionic nature of the perovskite crystal structure.^16–23 Long-chain ligands are essential during nucleation and growth, but they readily desorb even at room temperature, and ligand loss accelerates under thermal stress, illumination, moisture, or polar environments. Ligand desorption increases cationic and anionic surface defect densities, promoting nonradiative recombination and causing rapid degradation of structural and photoelectrical properties. Consequently, developing more strongly binding and robust surface ligands has emerged as a critical strategy for stabilizing PQDs. Among the various approaches, zwitterionic ligand engineering, leveraging ligands that bear both positively and negatively charged functional groups, has shown notable promise for enhancing ligand binding strength, suppressing defect sites, and improving long-term luminescence stability.^24–31

Recent studies have demonstrated that ionic or multifunctional ligands containing multidentate or strongly coordinating functional groups can effectively passivate surface defects in perovskite quantum dots (PQDs). Such ligands, incorporating functionalities such as anionic moieties (e.g., –SO₃⁻, –COO⁻) and cationic groups (e.g., –NH₃⁺), have been explored to mitigate cationic and/or anionic surface defects.^32–38 For example, Yang et al. reported that strong SO₃⁻–Pb²⁺ coordination improved ligand retention and maintained PL during purification.³⁹ while Wang et al. observed reduced defect density and enhanced LED efficiency through surface reconstruction effects induced by ionic functional groups.⁴⁰ More recently, true zwitterionic ligands, which intrinsically contain both positive and negative charges within a single molecular framework (e.g., –NH₃⁺/–SO₃⁻ or –NH₃⁺/–COO⁻), have been highlighted for their potential to simultaneously interact with both cationic and anionic surface sites, thereby improving long-term stability and device performance.⁴¹ However, most reported strategies rely on long-chain alkyl zwitterionic ligand and post-synthetic ligand exchange or purification-mediated treatments, which remain limited by the inherently long ligand chains commonly used, an issue that adversely affects charge transport.

In this work, we introduce a strategy for incorporating the zwitterionic ligand taurine directly during the in situ synthesis of CsPbBr₃ PQDs. The SO₃⁻ group of taurine strongly coordinates with surface Pb²⁺ ions, while the NH₃⁺ group electrostatically interacts with bromide ions, enabling simultaneous suppression of cationic and anionic defects. As a result, taurine–CsPbBr₃ PQDs exhibit improved colloidal dispersion, enhanced PL efficiency, and markedly increased stability against air, water, and thermal stress compared with PQDs synthesized using conventional long-chain organic ligands. Additionally, the intrinsically short chain length of taurine reduces trap density and improves charge mobility, offering significant performance advantages for PQDs-based optoelectronic devices.^39,42–48 Overall, this work presents an effective surface-engineering strategy for CsPbBr₃ PQDs using zwitterionic ligands and demonstrates the strong potential of zwitterionic functional group incorporation for enabling stable, high-performance perovskite-based optoelectronic devices.

2. Experimental

2.1. Chemical materials

Cesium bromide (CsBr, 99.99%, TCI), lead bromide (PbBr₂, 99.99%, TCI), taurine (99%, Sigma-Aldrich), n-octylamine (99%, Sigma-Aldrich), oleic acid (90%, Sigma-Aldrich), N,N-dimethylformamide (DMF, ≥99.9%, Sigma-Aldrich), toluene (99.8%, Sigma-Aldrich), n-hexane (95%, Sigma-Aldrich), [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM, >99.5%, 1-material), SnO₂ (15% in H₂O colloidal dispersion solution, Alfa Aesar) and rhodamine B (analytical standard, Sigma-Aldrich) were used for QY measurement. Deionized water (DIW) was purified using a Water Purification System (Human, POWER), producing water with a resistivity of 18.2 MΩ cm.

2.2. Synthesis of pristine CsPbBr₃ PQD solution

CsBr (0.0213 g, 0.01 mmol) and PbBr₂ (0.036 g, 0.01 mmol) were added to a flask along with 1 mL of DMF and 15 μL of octylamine. The mixture was sonicated for 2 min to obtain a clear precursor solution. In a separate 10 mL flask, 5 mL of toluene and 1 mL of oleic acid were combined and stirred at 600 rpm. The precursor solution was rapidly injected into this toluene/oleic acid mixture, immediately yielding a yellow–green dispersion. After 5 s, the solution was centrifuged at 8000 rpm for 5 min. The supernatant was discarded, and the precipitate was dried for 2 min. The dried solid was redispersed in 2 mL of hexane and centrifuged again, and the supernatant was collected to yield a hexane dispersion of CsPbBr₃ PQDs.

2.3. Synthesis of taurine–CsPbBr₃ PQDs solution

Taurine solutions of various concentrations (0, 6, 12, and 24 wt%) were prepared by dissolving taurine in 1 mL of DMF in a three-neck flask. The mixture was heated to 60 °C under a N₂ atmosphere and stirred for 30 min to ensure the zwitterionic property of taurine in DMF solution. The perovskite precursors CsBr (0.0213 g, 0.01 mmol) and PbBr₂ (0.036 g, 0.01 mmol) were placed in a separate 10 mL flask, and 1 mL of DMF along with the prepared taurine solution were added. This mixture was sonicated for 3 min to promote coordination between the sulfonate group and Pb²⁺. Subsequently, 15 μL of octylamine was added and the solution was sonicated for an additional 2 min. In a separate 10 mL flask, 5 mL of toluene and 1.7 mL of oleic acid were stirred at 600 rpm. The taurine-containing precursor solution was then rapidly injected into this mixture, immediately forming a yellow–green solution. After 5 s, the solution was centrifuged at 8000 rpm for 5 min. The supernatant was discarded and the precipitate was dried and the solid was redispersed in 2 mL of hexane. After re-dispersion, the supernatant was collected to obtain taurine–CsPbBr₃ PQDs dispersed in hexane.

2.4. Device fabrication

Glass/ITO substrates were cleaned by sequential ultrasonication in DIW, acetone, and isopropanol for 20 min each, followed by N₂ drying and UV–ozone treatment for 20 min. SnO₂ was then spin-coated onto the ITO substrate at 3000 rpm for 30 s and annealing for 30 min at 150 °C. CsPbBr₃ and taurine–CsPbBr₃ PQDs were spin-coated at 1000 rpm for 10 min inside a glovebox (Ar atmosphere). A 1 : 1 (v/v) mixture of chloroform and chlorobenzene (0.5 mL total) containing PC₆₀BM at 200 g L⁻¹ was then spin-coated at 2000 rpm for 40 s. Finally, a 100 nm Ag electrode was deposited by thermal evaporation.

2.5. Characterization

The CsPbBr₃ and taurine–CsPbBr₃ PQDs were characterized using transmission electron microscopy (TEM) with a JEOL JEM-2100 instrument and scanning transmission electron microscopy (STEM) with a JEOL JEM-ARM200F. The X-ray diffraction (XRD) patterns of the perovskite powders were obtained using a Bruker AXS New D8-Advance diffractometer equipped with Cu–Kα radiation. Film thickness and cross-sectional morphology were examined using a SIGMA field-emission scanning electron microscope (FE-SEM, Carl Zeiss) operated at an accelerating voltage of 5 kV. Functional groups of the PQDs were analyzed using a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer in attenuated total reflection (ATR) mode. Absorbance measurements of the CsPbBr₃ and taurine–CsPbBr₃ PQDs dispersed in n-hexane were recorded using a Perkin–Elmer Lambda 365 spectrophotometer. Photoluminescence (PL) spectra were collected using a Hitachi F-7000 fluorescence spectrophotometer with a monochromatic Xe lamp as the excitation source. Relative QY values for CsPbBr₃ and taurine–CsPbBr₃ PQDs were determined using the Hitachi F-7000 and the Perkin–Elmer Lambda 365 spectrophotometers, with rhodamine B as the reference dye. The absorbance of the reference solution was adjusted to approximately 0.1 prior to measurement. The stability of the CsPbBr₃ and taurine–CsPbBr₃ PQDs was assessed under various conditions through storage tests conducted under an ambient atmosphere. X-ray photoelectron spectroscopy (XPS) analysis of PQDs thin films was performed using a K-alpha instrument (Thermo Fisher Scientific).

3. Results and discussion

As schematically illustrated in Fig. 1, CsPbBr₃ PQDs and taurine–CsPbBr₃ PQDs were synthesized using a slightly modified split-ligand-mediated reprecipitation method.⁴⁹ In this system, the sulfonate group of taurine forms a direct strong coordination bond with the surface Pb²⁺ ions of the PQDs. The wt% of taurine was defined relative to the PbBr₂ content. Taurine was introduced in situ into the perovskite precursor solution to promote efficient anchoring of the sulfonate groups to the PQDs surface. In the conventional synthesis route, taurine does not fully dissolve when mixed with the DMF/octylamine-based perovskite precursor under ultrasonication. To address this solubility limitation, DMF and taurine were first added to a three-necked flask and stirred at 60 °C for 30 min under a nitrogen atmosphere to ensure complete dissolution. The resulting taurine–DMF solution was then added to the perovskite precursor, followed by the addition of octylamine, and the mixture was ultrasonicated to allow full incorporation of taurine. Upon injecting the precursor solution into a nonpolar solvent, CsPbBr₃ PQDs were formed due to the solubility contrast, while sulfonate groups simultaneously coordinated to surface Pb²⁺ sites. The proposed surface passivation mechanism of taurine on CsPbBr₃ PQDs is schematically shown in Fig. 1. Typically, CsPbBr₃ PQDs synthesized at room temperature are capped with long-chain organic ligands such as oleic acid and oleylamine, which bind dynamically to the PQD surface. Although this dynamic binding initially stabilizes the PQDs surface, ligand desorption readily occurs over time, during purification, or under polar, moist, or thermal environments. As a result, the formation of Br vacancies and undercoordinated Pb²⁺ sites increases non-radiative recombination pathways, thereby degrading optical efficiency and structural stability.^16,39,41 Taurine, a zwitterionic ligand, provides two complementary binding modes that are not accessible to conventional single-functional ligands. First, the sulfonate (–SO₃⁻) group forms a strong Lewis acid–base interaction with exposed Pb²⁺ ions. This binding mode is known to exhibit greater coordination strength, higher stability and bond dissociation energy than carboxylate-based ligands, allowing sulfonates to more effectively suppress Pb²⁺ exposure.^39,41,48 Second, the positively charged ammonium (–NH₃⁺) group forms electrostatic interactions with nearby bromide ions or halide vacancies, inhibiting Br loss and preventing the formation of additional halide vacancies.

Fig. 1 Schematics of pristine CsPbBr₃ and zwitterionic taurine–CsPbBr₃ PQD synthesis method, and the surface passivation mechanism of perovskite crystals.

This dual-site passivation reduces the density of vacancy formation and leads to a reduction in surface trap states.^45,46 Because taurine simultaneously passivates both Pb- and Br-related surface defects within a single ligand, unlike conventional long-chain organic ligands, its incorporation during CsPbBr₃ PQD synthesis yields higher QY, improved long-term storage stability, and enhanced resistance to water- and thermally induced degradation.

TEM imaging and particle-size distribution analysis were conducted to evaluate the effect of taurine concentration on the morphology and size of CsPbBr₃ PQDs. As shown in Fig. 2a–d, both pristine CsPbBr₃ PQDs and taurine–CsPbBr₃ PQDs exhibit a cubic crystal structure. The particle-size distributions reveal mean diameters of 10.75 nm for the pristine sample, 10.24 nm for 6 wt% taurine, 10.49 nm for 12 wt% taurine, and 10.78 nm for 24 wt% taurine. Taurine treatment did not have a significant effect on particle size, and taurine treatment inhibited particle aggregation. This behavior is attributed to the dynamic binding nature of long-chain ligands such as oleic acid and octylamine, which readily desorb during purification or air exposure, leading to surface instability and promoting interparticle aggregation.¹⁶ In contrast, the taurine-incorporated samples show a tendency toward suppressed aggregation due to effective surface immobilization, where the sulfonate (–SO₃⁻) groups strongly coordinate to surface Pb²⁺ sites and the ammonium (–NH₃⁺) groups electrostatically interact with surface bromide ions. This effect is most pronounced for the 6 wt% taurine sample, in which a tendency toward a narrower particle-size distribution and reduced aggregation is observed. At higher taurine loadings (12 wt% and 24 wt%), however, excess ligands partially hinder precursor diffusion and surface rearrangement, inducing ligand clustering and resulting in reduced morphological uniformity compared with the optimized 6 wt% sample.

Fig. 2 TEM and HR-TEM images of CsPbBr₃ PQDs synthesized with different taurine loadings: (a) 0, (b) 6, (c) 12, and (d) 24 wt% and corresponding particle size distributions.

The crystal structures of the synthesized CsPbBr₃ and taurine–CsPbBr₃ PQDs were examined using X-ray diffraction (XRD). As shown in Fig. 3a, all samples show diffraction patterns characteristic of cubic-phase CsPbBr₃ PQDs, indicating high crystallinity. The diffraction peaks of taurine–CsPbBr₃ PQDs exhibit a shift to higher 2θ values relative to the pristine sample (see Table S1). According to Bragg's law (nλ = 2d sin θ), the diffraction angle θ is inversely related to the interplanar spacing d. Therefore, a decrease in d leads to an increase in θ, corresponding to a shift toward higher 2θ values. The peak shifts to higher 2θ observed in the taurine-containing samples thus indicate a slight reduction in lattice spacing. This trend is qualitatively consistent with the lattice spacing variations estimated from HR-TEM analysis of the (110) plane, indicating a lattice-level modulation induced by taurine incorporation. The Fourier transform infrared (FT-IR) spectra in Fig. 3b provide further insight into the bonding and vibrational characteristics of the PQDs. All samples show vibrations associated with the hydrocarbon chains of native ligands (v(C–H_x) and v(C=O)) from oleic acid and octylamine. Taurine-containing samples additionally exhibit peaks assigned to ammonium (v(N–H₃⁺)) and sulfonate (v_as(S=O)) vibrations, confirming incorporation of taurine's zwitterionic functional groups on the CsPbBr₃ PQDs surface (detailed assignments are presented in Fig. S1). Fig. 3c shows the PL intensity and absorbance of CsPbBr₃ PQDs with different taurine concentrations. The PL peak positions for CsPbBr₃ are 527.5, 517.6, 516.3, and 521.3 nm for 0, 6, 12, and 24 wt%, respectively in Table S2. The overall blue shift observed upon taurine addition results in the observed particles being located in the weak quantum confinement region, as shown in Fig. S2. As shown in Fig. S3, the bandgap values extracted from Tauc plots are provided to facilitate the interpretation of the electronic structure. As shown in Fig. S4, the PL intensity of taurine–CsPbBr₃ PQDs followed the trend of 6 > 12 > 24 > 0 wt%, with the 6 wt% taurine sample exhibiting the most pronounced increase compared with pristine CsPbBr₃ PQDs. Relative QY values were determined using rhodamine B dissolved in ethanol (QY = 0.65) as the standard.^50–52 The relationship between the standard and the sample for the comparative QY calculation is expressed by eqn (1):

where F, L, and n represent the integrated PL area, optical path length, and solvent refractive index, respectively. ε, C, and Φ denote the molar absorptivity, concentration, and relative quantum yield. Because the three parameters ε, C, and Φ are linearly related to absorbance (A) through the Beer–Lambert law, the QY can be expressed in terms of the gradient obtained from the slope of PL intensity versus absorbance. Subscripts R and P refer to the reference (rhodamine B) and the PQD sample, respectively. The PL and absorbance spectra of rhodamine B used for relative QY calculations are provided in Fig. S5 and S6. The variables applied in the relative QY calculations are summarized in Table 1 and Table S3. As shown in Fig. 3d, the relative QY values of the taurine–CsPbBr₃ PQDs were 0.723, 0.867, 0.811, and 0.750 for 0, 6, 12, and 24 wt% taurine, respectively. We further evaluated the optimal conditions by adding the wt% taurine content condition. As shown in Fig. S7 and Table S4, among the key performance indicators of PQDs, the quantum yield (QY) was compared, and the results showed that the 6 wt% condition yielded the best QY. The QY values for each wt% condition show a decreasing trend with increasing concentration, starting from 6 wt%. Therefore, the maximum QY observed at 6 wt% is considered a reliable indicator of the most effective surface defect passivation achieved through taurine addition. The observed performance degradation at higher concentrations (12 wt% and 24 wt%) is due to ligand clustering due to excess ligand. In this case, ligands attached to the particle surface or free ligands tend to aggregate, resulting in a decrease in both luminescence performance and stability. Because the relative QY values were determined under identical absorption conditions, the observed PL intensity trends are consistent with the corresponding QY variations. This enhancement is attributed to the zwitterionic bifunctional groups of taurine, which effectively reduce perovskite surface defects and bromide vacancies, thereby significantly suppressing non-radiative recombination and greatly improving the optical performance of the PQDs.

Fig. 3 (a) XRD patterns, (b) FT-IR spectra, (c) normalized UV–vis absorption and PL spectra, and (d) relative quantum yield for synthesized CsPbBr₃ PQDs calculated with reference to a rhodamine B standard value of 0.65.

Table 1 Parameters used to determine the relative QY of CsPbBr₃ PQDs

Taurine	Refractive index	Gradient (F A^−1)	Quantum yield
0 wt%	1.375	1.13 × 10^5	0.723
6 wt%	1.375	1.33 × 10^5	0.867
12 wt%	1.375	1.25 × 10^5	0.811
24 wt%	1.375	1.15 × 10^5	0.750

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To investigate whether the observed enhancement in PLQY originates from the suppression of non-radiative recombination processes, time-resolved photoluminescence (TRPL) measurements were conducted. The TRPL decay curves were analysed by selecting the effective decay region, excluding the initial rise immediately after photoexcitation as well as the noise-dominated tail at long delay times. t₀ avoids reliance on assumptions associated with a specific exponential decay model, and the average lifetime was defined using the first-moment mean lifetime, 〈t〉. The 〈t〉 calculation is expressed by eqn (2):⁵³

I(t) is the baseline-corrected PL intensity and t is the delay time. The peak position of the decay curve after baseline correction was defined as t₀. To systematically exclude the rising edge, the integration start time was set to t₁ = t₀ and the integration end time t₂ was fixed at 250 ns for all samples to exclude the noise-dominated tail region. The same criteria were applied to all samples to ensure reproducibility and comparability. As a result, t₀ values of 11.23, 19.53, 16.30 and 11.23 ns and average lifetimes of 38.20, 44.60, 41.92, and 38.74 ns were obtained for the samples with taurine concentrations of 0, 6, 12, and 24 wt%, respectively, with the 6 wt% taurine–CsPbBr₃ PQDs exhibiting the longest 〈t〉. This prolonged lifetime indicates that non-radiative recombination pathways associated with surface defects are effectively suppressed upon taurine incorporation. The dual-site binding mechanism of taurine plays a critical role in this process: the sulfonate group coordinates with undercoordinated Pb²⁺ sites, while the ammonium group compensates halide-related vacancies. Such simultaneous passivation of cationic and anionic defect sites effectively suppresses defect-induced trap states, thereby reducing trap-assisted non-radiative recombination. In contrast, the 〈t〉 values for the 12 wt% and 24 wt% samples show a decreasing trend, which can be attributed to surface saturation effects or the emergence of localized non-radiative pathways at excessive ligand concentrations. These TRPL results qualitatively agree well with the PLQY trends, both identifying 6 wt% as the optimal taurine concentration and providing evidence that taurine-based surface passivation effectively enhances the optical properties of CsPbBr₃ PQDs. Detailed TRPL decay curves and the corresponding analysis procedures are provided in Fig. S8.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical states and bonding environments of CsPbBr₃ PQDs with and without taurine incorporation. To elucidate the influence of taurine on surface interactions, the core-level spectra of Cs, Pb, Br, S, and N were individually analyzed, as shown in Fig. 4a–e. Fig. 4a shows the Cs 3d spectra.

Fig. 4 XPS spectra of w/o and w/ taurine (6 wt%) CsPbBr₃ PQDs: (a) Cs 3d, (b) Pb 4f, (c) Br 3d, (d) S 2p.

The pristine CsPbBr₃ PQDs exhibit Cs 3d_3/2 and Cs 3d_5/2 peaks at 738.01 and 724.07 eV, respectively. After incorporating taurine at 6 wt%, both peaks shift slightly toward higher binding energies, appearing at 738.38 and 724.50 eV. This shift is not due to direct chemical binding of Cs⁺ ions to taurine, but rather to changes in the local electrostatic environment induced by surface reorganization following the introduction of taurine. More pronounced changes are observed in the Pb 4f spectra (Fig. 4b). In the pristine sample, the Pb 4f_5/2 and Pb 4f_7/2 peaks can be deconvoluted into two components corresponding to Pb–Br interactions (143.18 and 138.29 eV) and Pb–oleate surface species (141.93 and 137.08 eV). Upon taurine incorporation, the Pb–Br-related peaks shift to higher binding energies (143.65 and 138.78 eV), while the Pb–oleate-associated component evolves into a new feature located at 142.57 and 137.66 eV. This newly emerged component is attributed to S-related surface interactions (denoted as Pb–S), indicating a modification of the Pb-centered surface bonding environment. The Br 3d spectra (Fig. 4c) also exhibit systematic binding-energy shifts. The pristine PQDs show Br 3d_3/2 and Br 3d_5/2 peaks at 69.55 and 68.37 eV, respectively, whereas the taurine-treated sample shows corresponding peaks at 69.84 and 68.65 eV. These shifts suggest a modification of the Br-related surface bonding state and local electrostatic potential following taurine incorporation. Fig. 4d shows the S 2p spectra. No discernible S 2p signal is detected in the pristine sample, while a distinct S 2p peak centered at 168.70 eV emerges after taurine incorporation, which is characteristic of sulfonate (S=O) species. This observation provides direct evidence for the incorporation of taurine-derived functional groups on the PQD surface. Finally, the N 1s spectra (Fig. S9) shows a peak at 401.45 eV for the pristine sample, which shifts slightly to 401.65 eV after taurine incorporation. This binding-energy range corresponds to ammonium-like (NH₃⁺) nitrogen species, indicating the presence of positively charged taurine moieties at the PQD surface. So, taurine–CsPbBr₃ PQDs strengthened electrostatic interaction at the PQD surface. Overall, the emergence of the S 2p signal, the evolution of Pb 4f surface components, and the consistent binding-energy shifts observed across Cs, Pb, Br, and N core levels collectively demonstrate that taurine incorporation effectively modulates the surface chemical environment and local electrostatic potential of CsPbBr₃ PQDs. These results support the role of taurine as an effective in situ surface passivation strategy rather than a conventional post-synthetic ligand-exchange process.

The inherently low stability of PQDs in air, aqueous, and thermal environments is a well-known limitation for their practical application.⁵⁴ To assess the effect of taurine on stability, we conducted contact angle, surface energy, and environmental stability measurements in aqueous, thermal, and ambient air environments. The stability measurements were conducted under ambient laboratory conditions at approximately 25 ± 2 °C and 40 ± 5% relative humidity. To evaluate the resistance to water of PQDs, we fabricated thin films of CsPbBr₃ and taurine–CsPbBr₃ PQDs, and DIW droplets were dropped to determine contact angles and surface energies. The surface energy was calculated using DIW and diiodomethane (DIM) droplets via the Owens–Wendt method,⁵⁵ with the relevant variables summarized in Table S5. The relationship between the contact angle and the surface energy for the samples is expressed by eqn (3) and (4):

where θ is the contact angle between the droplet and the thin film coated with taurine–CsPbBr₃ PQDs, and γ_lv, γ^d_lv, and γ^p_lv are the surface free energy and dispersive and polar components of the surface energy of the liquid droplet (DIW or DIM), respectively (eqn (3) and (4)). As shown in Fig. 5a, the contact angle increased in the order 6 wt% > 12 wt% > 24 wt% > pristine CsPbBr₃ PQDs, with the highest value of 65.35° observed for the 6 wt% taurine sample and the lowest value of 58.33° for the pristine CsPbBr₃ PQDs. Correspondingly, surface energies were lowest for the 6 wt% sample (50.79 mJ m⁻²) and highest for pristine CsPbBr₃ PQDs (55.07 mJ m⁻²). These trends can be attributed to the weak dynamic binding of long-chain organic ligands in pristine PQDs, which leads to ligand desorption and exposure of uncoordinated surface ions. The resulting surface defects reduce contact angles and increase surface energies when exposed to DIW. In contrast, the incorporation of taurine at the optimal concentration (6 wt%) enhances surface stabilization through strong dual-site passivation, improving surface immobilization and dispersion stability. This effect is reflected in the higher contact angle and lower surface energy for the 6 wt% sample. Collectively, these results indicate that taurine incorporation not only enhances water resistance but also contributes to improved long-term storage stability and thermal durability of CsPbBr₃ PQDs.

Fig. 5 (a) Surface energy and contact angle of CsPbBr₃ PQDs film with different amounts of taurine (0, 6, 12, and 24 wt%) measured using DIW. (b) Photograph of deionized water (DIW) and different amounts of taurine (0, 6, 12 and 24 wt%) after 0, 70 and 140 h. Relative PL intensity of CsPbBr₃ PQDs with different taurine contents under (c) aqueous, (d) thermal, and (e) ambient air conditions.

As shown in Fig. 5b, DIW and hexane-dispersed PQDs were mixed in a 1 : 1 ratio, forming a DIW bottom layer and a hexane-dispersed PQD top layer. Because the PQDs in hexane are in direct contact with DIW at the interface, this configuration is well suited for evaluating the structural stability of the perovskite quantum dots. As shown in Fig. 5b and c, the PL intensities of all samples decreased significantly over time, except for the 6 wt% taurine sample. After 140 h, PL retention was 75.2% for the 6 wt% taurine sample, compared with 29.4% for pristine PQDs, 18.6% for 12 wt% taurine, and 16.1% for 24 wt% taurine. While the 12 wt% and 24 wt% taurine samples exhibited weak photoluminescence, and pristine CsPbBr₃ PQDs showed diminished emission, the 6 wt% taurine sample retained strong luminescence even after 140 h in an aqueous environment. These results indicate that strong surface passivation via ligand binding enhances luminescence in aqueous environments, whereas excessive ligand loading or weak ligand binding accelerates the degradation of PQD particles. Thermal stability was assessed by heating PQDs for 10 min at temperatures from 20 °C to 140 °C in 20 °C increments. As shown in Fig. 5d, PL retention at 120 °C was 62.1% for 6 wt% taurine, 48.4% for 12 wt% taurine, 33.8% for pristine PQDs, and 26.7% for 24 wt% taurine. These results demonstrate that dual-site passivation by taurine ligands effectively inhibits decomposition of the PQD particles due to the desoprtion of the surface ligands under thermal stress. PL stability under ambient air conditions is shown in Fig. 5e. After 450 h, the PL retention was 71.0% for 6 wt% taurine, 63.5% for 12 wt% taurine, 59.0% for pristine PQDs, and 53.6% for 24 wt% taurine. This behavior can be attributed to taurine's zwitterionic functional groups (–SO₃⁻ and –NH₃⁺), which enable dual-site passivation: the sulfonate group coordinates strongly with Pb²⁺, while the ammonium group electrostatically interacts with halides, suppressing halide loss and reducing defect formation. Fig. S1 and Fig. 3b show that the strongest sulfonate and ammonium vibrational peaks were observed at 6 wt% taurine, followed by 12 wt% and 24 wt% taurine, indicating that only the optimal concentration enables efficient surface binding. At higher concentrations, excess taurine ligands remain unbound as free ligands, leading to ligand clustering or particle agglomeration, which significantly reduces stability. In DMF, taurine's functional groups are exposed when the PQD surface is not fully passivated, accelerating degradation, particularly in aqueous environments. This effect is more pronounced in an aqueous environment than under thermal or air conditions. Consequently, relative QY follows the order 6 wt% taurine > 12 wt% taurine > 24 wt% taurine > pristine PQDs, with excess ligand leading to lower stability over time. These results indicate that an optimal taurine concentration of 6 wt% provides the most effective defect passivation, suppressing perovskite structural degradation and enabling excellent stability under water, heat, and air exposure. Overall, the introduction of zwitterionic ligands at optimal concentrations significantly enhances the environmental stability of CsPbBr₃ PQDs. Therefore, the consistent stability trends observed across water, thermal, and air conditions directly correlate with the extent of taurine-induced defect passivation. The prolonged carrier lifetime observed in the TRPL measurements at 6 wt% taurine, together with the highest QY retention, confirms that balanced dual-site passivation effectively suppresses non-radiative recombination and environmental degradation.

Conventional long-chain organic ligands possess extended alkyl chains that act as electrical insulators, limiting charge transport in devices. In contrast, incorporating taurine ligands with short alkyl chains enhances charge transport relative to long-chain ligands. To quantify this effect, the space-charge-limited current (SCLC) method was employed to investigate charge transport and determine trap-state density. Electron-only devices with the configuration ITO/SnO₂/taurine–CsPbBr₃ PQDs/PCBM/Ag were fabricated, as illustrated in Fig. 6f.

Fig. 6 J–V curves of electron-only SCLC devices fabricated with CsPbBr₃ PQDs containing different taurine concentrations: (a) 0 wt%, (b) 6 wt%, (c) 12 wt%, and (d) 24 wt%. (e) Electron mobility and trap density of taurine–CsPbBr₃ PQDs films extracted from SCLC measurements. (f) Schematic illustration of the electron-only device structure: ITO/SnO₂/taurine–CsPbBr₃ PQDs/PCBM/Ag.

As shown in Table 2 and Fig. 6a–d, the current–voltage characteristics of the devices were analyzed to determine the trap-filled limit voltages (V_TFL), which were 0.748 V, 0.339 V, 0.499 V, and 0.588 V for the 0, 6, 12, and 24 wt% taurine concentrations, respectively. The trap-state density (N_trap) was calculated from V_TFL using eqn (5):⁵⁶

Table 2 Electron-only SCLC device parameters of CsPbBr₃ PQDs with varying taurine contents

Taurine	VTFL [V]	Electron mobility [cm^2 V^−1 S^−1]	Trap density [# cm^−3]
0 wt%	0.748	2.82 × 10^−6	1.26 × 10^17
6 wt%	0.339	7.50 × 10^−6	5.21 × 10^16
12 wt%	0.499	3.49 × 10^−6	9.20 × 10^16
24 wt%	0.588	3.09 × 10^−6	1.04 × 10^17

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In eqn (5), ε represents the permittivity of CsPbBr₃ PQDs, ε₀ is the permittivity of vacuum, and d is the thickness of the CsPbBr₃ PQDs layer, which varies with taurine wt%. The dielectric constant (ε) of CsPbBr₃ PQDs is 16.4. Layer thicknesses of the CsPbBr₃ PQDs with varying taurine concentrations were measured and found to be 44.46, 46.21, 42.43, and 42.25 nm for 0, 6, 12, and 24 wt% taurine, respectively (Fig. S10).^23,57 Charge mobility was calculated using the Mott–Gurney equation:⁵⁸

In eqn (6), μ is the electron mobility and J_SCLC is space-charge-limited current density. Using eqn (5) and (6), the device based on 6 wt% taurine–CsPbBr₃ PQDs exhibited a trap density of 5.21 × 10¹⁶ cm⁻³ and an electron mobility of 7.50 × 10⁻⁶ cm² V⁻¹ s⁻¹, corresponding to approximately a two-fold reduction in trap density and a 2.5-fold increase in mobility compared with pristine CsPbBr₃ PQDs (Fig. 6e). The short-chain, zwitterionic taurine ligands effectively passivate surface defects arising from ligand desorption while mitigating the charge transport limitations associated with conventional long-chain ligands. Consequently, taurine–CsPbBr₃ PQDs exhibit significantly enhanced electrical and optical performance, highlighting their potential for high-performance optoelectronic applications. In addition, key performance metrics, including trap density, QY, and stability, are summarized and compared with those of previously reported CsPbBr₃ PQDs in Table S6, confirming the effectiveness of the present surface passivation strategy.

4. Conclusion

In this work, taurine-functionalized CsPbBr₃ PQDs were successfully synthesized via an in situ ligand incorporation strategy using a zwitterionic ligand. This approach effectively mitigated ligand desorption and halide loss, which are major limitations of conventional long-chain organic ligands. As a result, surface defect passivation was significantly enhanced, leading to superior optical performance, including a 1.5-fold increase in photoluminescence quantum yield compared with pristine CsPbBr₃ PQDs. The water and thermal stability of the PQD solutions were also markedly improved, highlighting the crucial role of dual-site coordination in maintaining structural and optical integrity under harsh conditions. Furthermore, electron-only devices fabricated with CsPbBr₃ PQDs at the optimal taurine concentration exhibited a 2.5-fold increase in charge mobility and a two-fold reduction in trap density relative to devices based on pristine CsPbBr₃ PQDs, demonstrating substantial improvements in electrical properties. Collectively, these results show that the dual-site coordination and short-chain property of taurine enable effective surface defect passivation while simultaneously enhancing device performance. This strategy provides a promising and broadly applicable approach for the development of high-stability, high-performance perovskite-based optoelectronic devices.

Author contributions

J. H. Choi and J. Y. Kim: conceptualization, methodology, formal analysis, investigation, writing – original draft, and visualization; Y. M. Lee: investigation and formal analysis; H. Lee, J. W. Park: investigation; D. H. Wang: conceptualization, methodology, writing – review & editing, supervision, project administration, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6qi00251j.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT) (Grant No. 2023R1A2C2008021). This research was also supported by the Chung-Ang University Graduate Research Scholarship in 2025.

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Footnote

† These authors contributed equally to this work.

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