Citation

Wang Song, Xie Ying, Jiang Wenchao, Liu Binghang, Shi Keying, Pan Kai. Incorporation sodium ions into monodisperse lead-free double perovskite Cs₂AgBiCl₆ nanocrystals to improve optical properties[J]. Chin. Chem. Lett., 2024, 35(3): 108521  

Song Wang^a, Ying Xie^a, Wenchao Jiang^c, Binghang Liu^a, Keying Shi^a,*, Kai Pan^a,b,*     

Lead halide perovskite nanocrystals (NCs) possess a unique three-dimensional perovskite structure and exhibit excellent carrier behavior, resulting in outstanding optical properties [1-4]. As a result, these NCs have found widespread use in optoelectronic applications such as light-emitting diodes [4-8], lasers [9-11], solar cells [12-16], non-linear optics [17], and photodetectors [18, 19]. However, their toxicity, poor moist heat, and chemical stability severely limit their potential for further applications [20-23]. Therefore, there is an urgent need to explore lead-free perovskite NCs that offer comparable electronic, structural, and optical properties to those of lead halide perovskite NCs.

Lead halide perovskite NCs can be modified by replacing two Pb²⁺ ions with one monovalent (M⁺) and one trivalent (M³⁺) metal ion to form Cs₂M⁺M³⁺X₆ double perovskite NCs (M⁺ = Na⁺, Ag⁺, etc., M³⁺ = In³⁺, Bi³⁺, Sb³⁺, etc., X = Cl, Br, I). This strategy has been proven to effectively reduce toxicity and increase the stability of the NCs [24-31]. However, these double perovskite NCs have weaker photoluminescence (PL) and lower photoluminescence quantum yield (PLQY) compared to conventional lead halide perovskite NCs. This is due to their indirect band gaps or direct band gaps characterized by the parity forbidden transitions [26, 29, 32-34], despite having a similar three-dimensional perovskite structure and electrical neutrality.

Doping or alloying double perovskite NCs with metal ions is an effective way to improve their band gap structure or parity forbidden transition, leading to enhanced PL intensity and PLQY. For instance, Xia's group prepared Cs₂AgInCl₆ NCs and Bi³⁺-doped Cs₂AgInCl₆ NCs using a simple hot-injection method [28]. Bi³⁺ doping adjusted the band gap of Cs₂AgInCl₆ NCs from 4.25 eV to 3.28 eV, and altered the emission spectrum from blue to orange. Meanwhile, Bi³⁺ doping significantly improved the PLQY of Cs₂AgInCl₆ NCs (~11.4%). Han's group prepared Cs₂NaInCl₆ NCs and Ag⁺-doped Cs₂NaInCl₆ NCs using a variable temperature hot-injection method [35]. Ag⁺ doping transformed the dark self-trapping excitons (STEs) state of Cs₂NaInCl₆ NCs into a bright STEs state, which was attributed to the interaction between [AgCl₆]⁵⁻ and [NaCl₆]⁵⁻ octahedra breaking the parity forbidden transition, resulting in bright yellow emission. The PLQY of Ag⁺-doped Cs₂NaInCl₆ NCs was significantly enhanced (~31.1%) compared to that of Cs₂NaInCl₆ NCs. Moreover, they also prepared Mn²⁺-doped Cs₂NaIn_xBi_1-xCl₆ (0 < x < 1) NCs using a colloidal synthesis strategy [36]. The emission spectrum changed from blue emission to bright orange-red emission. Meanwhile, the PLQY of Mn²⁺-doped Cs₂NaIn_xBi_1−xCl₆ (0 < x < 1) NCs can reach 44.6% compared to the undoped NCs (~38%). However, there were relatively fewer reports on alkali metal-alloyed double perovskite NCs, and the effect of such incorporation on the exciton binding energy and longitudinal optical phonons of double perovskite NCs remains unexplored.

In this work, we prepared a series of monodisperse, uniformly sized cube morphology Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts via a simple hot-injection method. By varying the amount of Na⁺ incorporation, we were able to significantly tune the band gap of the Cs₂AgBiCl₆ NCs from ~2.90 eV to ~3.50 eV. The 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs exhibited the strongest PL and the highest PLQY (12.1%) compared to other NCs, and the exciton lifetime was also extended from 17.58 ns to 36.98 ns. It was noteworthy that the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs exhibited higher exciton binding energy and longitudinal optical phonon energy than Cs₂AgBiCl₆ NCs. This indicated 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs possessed excellent thermal stability, strong exciton-phonon interactions, and lower non-radiative probability. Our findings highlight the crucial role of metal ion incorporation in regulating the band gap and optical properties of double perovskite NCs.

Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts were prepared via a simple hot-injection method (Fig. 1a). Briefly, Cs(ac), Bi(ac)₃, Ag(ac)/Na(ac) were dissolved in a mixed solution of ODE, OA, and OAm. The mixed solution was heated until the temperature reached 145 ℃, followed by the rapid injection of trimethylchlorosilane (TMS-Cl) to initiate nucleation and crystallization of the NCs (see the experimental section in Supporting information for details). Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts was obtained by adjusting the ratio of Ag/Na metal precursors (Table S1 in Supporting information). The precise Na⁺ incorporation amounts of Cs₂AgBiCl₆ NCs were obtained by inductively coupled plasma optical emission spectrometer (ICP-OES), which were 8%, 26%, 56% and 74%, respectively (Table S2 in Supporting information). In addition, the crystal structure diagrams of Cs₂AgBiCl₆ NCs and Na⁺-alloyed Cs₂AgBiCl₆ NCs were shown in Figs. 1b and c.

Figure 1

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Fig. 1. (a) Colloidal synthesis of double perovskite Cs2AgBiCl6 NCs and Na+-alloyed Cs2AgBiCl6 NCs. Crystal structure of (b) Cs2AgBiCl6 NCs and (c) Na+-alloyed Cs2AgBiCl6 NCs.

The X-ray diffraction patterns (XRD) of Cs₂AgBiCl₆ NCs with different Na⁺ amounts were shown in Fig. S1a (Supporting information). The XRD results revealed that Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts possessed a cubic double perovskite structure with Fm-3m space group [32]. The sharp diffraction peaks of the NCs indicated good crystallinity. Furthermore, as the Na⁺ incorporation amounts increased, the XRD diffraction peaks of Na⁺-alloyed Cs₂AgBiCl₆ NCs were shifted towards a smaller 2θ angle, which was attributed to the bond length of the newly formed Na-Cl bond (2.736 Å) [37] was longer than that of the Ag-Cl bond (2.708 Å) [38], resulted in lattice expansion. Fig. S1b (Supporting information) provided a magnified view of the (220) diffraction peak, further confirming the shift of the diffraction peak to smaller 2θ angles. With the increase of Na⁺ incorporation amount, the intensity of the (111) crystal plane diffraction peak gradually increases (marked with an asterisk in Fig. S1a), which was related to the difference in scattering factors of Na and Ag atoms, indicating that alloyed Cs₂Ag_1-xNa_xBiCl₆ NCs would be formed when working under the intermediate composition of Na/Ag [39].

The UV-vis absorption spectra of Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts were shown in Fig. S1c (Supporting information). The exciton absorption peak was observed to blue shift from 364 nm to 326 nm with an increase in Na⁺ incorporation amount. This corresponds to a change in exciton absorption energy from 3.40 eV to 3.80 eV (Fig. S1d in Supporting information) and an increase in PLQY from 2.4% to 12.1% (Table S3 in Supporting information). In order to display the measurement results, we added distribution histograms of the PLQY results as Fig. S2 (Supporting information). Each measurement result was either close to or equal to the average value, indicating that our results exhibited a good repeatability. Meanwhile, the band gap values of Na⁺-alloyed Cs₂AgBiCl₆ NCs were estimated according to the Tauc plots (Fig. S3 in Supporting information), revealing an increase in band gap from 2.9 eV to 3.5 eV with an increase in Na⁺ incorporation amount (Fig. S1d). This could be attributed to the participation of Na⁺ orbitals (Na-s) in the formation of the conduction band minimum (CBM) of Na⁺-alloyed Cs₂AgBiCl₆ NCs, which regulates the electronic structure of the NCs [40, 41].

PL spectra showed that Na⁺ incorporation into the Cs₂AgBiCl₆ NCs produced a broad emission peak centered at 615 nm (Fig. S1e in Supporting information), which was attributed to the STEs state [39]. With the increase of Na⁺ incorporation amounts, the intensity of the broad emission peak increases gradually, indicating more free excitons were transferred to the STEs state to form self-trapped domain excitons. There were two reasons for the difference in PLQY with different doping gradient. Firstly, Na doping broke the inversion-symmetry-induced parity-forbidden transition of the Cs₂AgBiCl₆ NCs lattice by reducing the inversion symmetry of the electron wave function at the Ag site, which changed the parity of the STEs wave function and allowed radiative recombination. Secondly, the [NaCl₆]⁵⁻ octahedra could act as barriers to restrict the spatial distribution of STEs. The incorporation amount increased from 26% to 56%, with a large doping gradient, resulting in a significant increased orbitals overlap and the transition dipole moment [42, 43]. Therefore, the PLQY increased from 7.6% to 12.1%. Interestingly, the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs possessed the strongest emission peak and the largest PLQY value (~12.1%) compared to other Na⁺-alloyed Cs₂AgBiCl₆ NCs (Table S3). However, the intensity of the emission peak decreased with the increase of Na⁺ incorporation amounts, which might be caused by the concentration quenching effect [44, 45]. In addition, Cs₂NaBiCl₆ NCs did not had a broad emission peak, which was mainly due to the existence of dark STEs state (Fig. S1f in Supporting information) [46].

Transmission electron microscopy (TEM) images (Figs. 2a and c, Fig. S4 in Supporting information) and the histograms of the edge length distribution (Figs. 2e and f, Fig. S4) demonstrated the microscopic morphology and edge length distribution of Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts, respectively. The Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts exhibited uniform, monodisperse cubic morphology. The edge length of the Na⁺-alloyed Cs₂AgBiCl₆ NCs gradually increased from 9.15 ± 0.89 nm to 9.63 ± 0.86 nm with increased Na⁺ incorporation amounts (Fig. S5 in Supporting information). This was attributed to the larger lattice constant of Cs₂NaBiCl₆ NCs (10.84 Å) compared to that of Cs₂AgBiCl₆ NCs (10.78 Å). The 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were further investigated due to their highest PLQY (12.1%). High-resolution TEM (HRTEM) images of Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ showed that both NCs had clear, highly crystalline lattice fringes and exhibited a cubic crystal structure corresponding to the (220) crystal plane (Figs. 2b and d). The lattice constant of the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs was found to have increased slightly from 0.380 nm (Cs₂AgBiCl₆ NCs) to 0.384 nm. This could be attributed to the formation of a longer Na-Cl bond, compared to the Ag-Cl bond, which resulted in lattice expansion, which was consistent with the XRD results discussed earlier. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping of 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were shown in Figs. 2g-l. The presence and homogeneous distribution of Cs, Na, Ag, Bi and Cl elements in the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs confirmed the successful incorporation of Na⁺ into the Cs₂AgBiCl₆ NCs.

Figure 2

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Fig. 2. Transmission electron microscopy (TEM) images of (a) Cs2AgBiCl6 NCs and (d) 56% Na+-alloyed Cs2AgBiCl6 NCs. High-resolution TEM (HRTEM) images of (b) Cs2AgBiCl6 NCs and (e) 56% Na+-alloyed Cs2AgBiCl6 NCs. Edge length distribution histogram of (c) Cs2AgBiCl6 NCs and (f) 56% Na+-alloyed Cs2AgBiCl6 NCs. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of (g) 56% Na+-alloyed Cs2AgBiCl6 NCs and corresponding elemental mapping images of (h) Cs, (i) Na, (j) Ag, (k) Bi, and (l) Cl, respectively. Scale bars are 20 nm.

In order to further confirm the existence of Na⁺, the Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were tested by X-ray photoelectron spectroscopy (XPS) [47]. Compared with Cs₂AgBiCl₆ NCs, an additional clear Na 1s peak appeared on the full XPS spectrum of 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs (Fig. S6a in Supporting information), meanwhile, in the high-resolution XPS spectrum of Na 1s (Fig. S6b in Supporting information), a Na 1s peak was displayed at 1070.7 eV, which confirmed the successful incorporation of Na⁺ into the Cs₂AgBiCl₆ NCs. The high-resolution XPS spectra of Cs 3d, Ag 3d, Bi 4f, and Cl 2p were shown in Figs. S6c-f (Supporting information). When compared to Cs₂AgBiCl₆ NCs, the binding energy peaks of Ag 3d and Cl 2p of 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were shifted towards lower binding energy (Figs. S6d and f), suggesting that the incorporation of Na⁺ weakens the Ag-Cl bond. This was attributed to the lower electronegativity of Na relative to Ag, which resulted in a stronger Na-Cl bond [48, 49]. Moreover, the peaks of binding energy for Cs 3d and Bi 4f were slightly shifted towards the lower binding energy (Figs. S6c and e). This was attributed to the increased electron density of Cl around Cs and the weakened interaction between Bi and Cl [50].

Fig. 3a showed the time-resolved PL (TRPL) decay curves of Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs. The TRPL decay curve was fitted with a double exponential decay [51], and the luminescence lifetime decay time (τ_i) and relative amplitude (A_i) were obtained, which was summarized in Table S4 (Supporting information). According to formula , the average decay times of Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were calculated to be 17.58 ns and 36.98 ns, respectively. This indicated that the incorporation of Na⁺ can effectively enhance the exciton lifetime of Cs₂AgBiCl₆ NCs and prolonged the exciton lifetime by about 2.1 times, which could be attributed to the formation of STEs state resulting from the incorporation of Na⁺ [39]. Based on the results discussed above, a possible PL mechanism of Na⁺-alloyed Cs₂AgBiCl₆ NCs was proposed (Fig. 3b). Under the light excitation of 365 nm, the carriers were excited from the ground state to the excited state, which led to the formation free excitons locally. These free excitons undergo non-radiative relaxation and transferred to the STEs state, resulting in the formation of STEs. Finally, the recombination of STEs generated a broad emission peak.

Figure 3

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Fig. 3. (a) Time-resolved PL (TRPL) decay profiles of Cs2AgBiCl6 NCs (wine) and 56% Na+-alloyed Cs2AgBiCl6 NCs (orange). (b) The PL mechanism of Na+-alloyed Cs2AgBiCl6 (DS: defect state; STEs: self-trapped excitons).

To investigate the impact of Na⁺ incorporation on the PL properties of Cs₂AgBiCl₆ NCs, we obtained temperature-dependent PL spectra of both the Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs within the range of 80-300 K. These spectra were showed in Figs. 4a-d. The temperature-dependent PL spectra provided insight into the exciton binding energy (E_b) and the exciton-phonon interaction of the NCs. As shown in Figs. 4a and b, the PL intensities of both NCs decrease with increasing temperature, which was attributed to the creation of more non-radiative decay channels with increasing temperature [52]. The variation of PL intensity with temperature (Fig. 4e) was fitted according to Eq. 1 [7, 53].

(1)

Figure 4

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Fig. 4. (a) Waterfall maps of temperature-dependent PL spectra of (a) Cs2AgBiCl6 NCs and (b) 56% Na+-alloyed Cs2AgBiCl6 NCs. The 2D color maps of temperature-dependent PL spectra of (c) Cs2AgBiCl6 NCs and (d) 56% Na+-alloyed Cs2AgBiCl6 NCs. (e) Integrated PL intensity and (f) FWHM as a function of temperature.

where I(T) and I₀ are the integrated PL intensity at temperature T and 0 K, respectively, E_b is the exciton binding energy, and k_b is the Boltzmann constant. The fitting results in Fig. 4e revealed that the E_b for Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were 170 meV and 223 meV, respectively. The significant increase in E_b for 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs suggested a higher probability of radiative recombination compared to Cs₂AgBiCl₆ NCs [54]. Moreover, this result indicates that 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs exhibited relatively high thermal stability [55]. However, the PLQY of 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs was relatively low compared to other reported double perovskites [35, 42]. This was attributed to the fact that 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were an indirect band gap material. Meanwhile, the colloidal synthesis led to the production of surface defects, which further decreased the PLQY.

Meanwhile, the interaction between excitons and phonons during carrier recombination was studied by analyzing the broadening behavior of the full-width half-maximum (FWHM) of the PL peak. Fig. 4f showed the variation of FWHM with temperature for Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs. The variation of FWHM with temperature was fitted according to Eq. 2 [56, 57].

(2)

where Γ(T) is the FWHM of the sample at temperature T, Γ₀ is the inhomogeneous broadening contribution at 0 K, Γ_op is the exciton-optical phonon coefficient, ℏω_op is the longitudinal optical phonon energy, and k_b is the Boltzmann constant. Based on the fitting results in Fig. 4f, we determined that the ℏω values for Cs₂AgBiCl₆ NCs and 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs were 64.3 meV and 83.7 meV, respectively. The Cs₂AgBiCl₆ NCs exhibited a smaller ℏω_op value, indicating that more phonons were generated and become non-radiative recombination centers. In contrast, 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs had a larger ℏω_op value, which suggested strong exciton-phonon interactions and lower non-radiative probability during exciton recombination [55].

In this work, we successfully prepared monodisperse, uniformly sized, cubic morphology of Cs₂AgBiCl₆ NCs with different Na⁺ incorporation amounts via a simple hot-injection method. The incorporation of Na⁺ directly affects the electronic structure and optical properties of Cs₂AgBiCl₆ NCs. By varying the amount of Na⁺ incorporation, the band gap of the NCs was significantly tuned from ~2.90 eV to ~3.50 eV. Meanwhile, the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs had the stronger PL and the higher PLQY (~12.1%) compared to the Cs₂AgBiCl₆ NCs (~2.4%), and the exciton lifetime was also extended ~2.1 times than Cs₂AgBiCl₆ NCs. Furthermore, temperature-dependent PL spectroscopy studies revealed that the 56% Na⁺-alloyed Cs₂AgBiCl₆ NCs exhibited higher longitudinal optical phonon energy and exciton binding energy compared to the Cs₂AgBiCl₆ NCs, indicating their strong exciton-phonon interactions during exciton recombination, lower non-radiative probability, and good thermal stability. Overall, this study offers an effective approach for enhancing the optical properties of lead-free double perovskite NCs, which further advanced the potential application of their in the field of optoelectronics.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

We gratefully acknowledge the support of the National Natural Science Foundation of China (No. 21473051), the Natural Science Foundation of Heilongjiang Province (No. LH2019B014) and Youth Science and Technology Innovation Team Project of Heilongjiang Province (No. 2018-KYYWF-1593).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108521.