Tb³⁺ and Bi³⁺ Co-Doping of Lead-Free Cs₂NaInCl₆ Double Perovskite Nanocrystals for Tailoring Optical Properties

Abstract

Lead halide perovskites have achieved remarkable success in various photovoltaic and optoelectronic applications, especially solar cells and light-emitting diodes (LEDs). Despite the significant advances of lead halide perovskites, lead toxicity and insufficient stability limit their commercialization. Lead-free double perovskites (DPs) are potential materials to address these issues because of their non-toxicity and high stability. By doping DP nanocrystals (NCs) with lanthanide ions (Ln³⁺), it is possible to make them more stable and impart their optical properties. In this work, a variable temperature hot injection method is used to synthesize lead-free Tb³⁺-doped Cs₂NaInCl₆ DP NCs, which exhibit a major narrow green photoluminescence (PL) peak at 544 nm derived from the transition of Tb^3+ 5D₄→⁷F₅. With further Bi³⁺ co-doping, the Tb³⁺-Bi³⁺-co-doped Cs₂NaInCl₆ DP NCs are not only directly excited at 280 nm but are also excited at 310 nm and 342 nm. The latter have a higher PL intensity because partial Tb³⁺ ions are excited through more efficient energy transfer channels from the Bi³⁺ to the Tb³⁺ ions. The investigation of the underlying mechanism between the intrinsic emission of Cs₂NaInCl₆ NCs and the narrow green PL caused by lanthanide ion doping in this paper will facilitate the development of lead-free halide perovskite NCs.

1. Introduction

Lead (Pb) halide perovskites have received great research attention because of their remarkable performance in photovoltaic and optoelectronic applications, including light-emitting diodes (LEDs), solar cells, and optical pumping lasers [1,2,3,4,5,6,7]. Despite their promising properties, lead halide perovskites have not been commercialized due to their intrinsic instability and lead toxicity [8,9]. The water solubility of lead halide perovskites is associated with lead toxicity diseases involving the nervous system. To address the instability and toxicity of lead halide perovskites, researchers have been actively pursuing the development of lead-free perovskite alternatives. Sn²⁺ and Ge²⁺ have been used to replace Pb²⁺ to synthesize lead-free halide perovskites [10,11]. However, the Sn²⁺ and Ge²⁺ cations tend to oxidize to Sn⁴⁺ and Ge⁴⁺ in the ambient environment. Double perovskites (DPs) as lead-free perovskite variants containing one monovalent B⁺ cation and one trivalent B³⁺ cation to generate the [BX₆]⁵⁻ and [B’X₆]³⁻ octahedra, resulting in a three-dimensional (3D) structure of A₂BB’X₆ (A = Rb, Cs; B = Na, Ag; B’ = Bi, In, Sc, Er; X = I, Br, Cl), have received tremendous research attention due to their intense photoluminescence (PL), non-toxicity, and high stability. Nevertheless, most kinds of air-stable DP nanocrystals (NCs) exhibit forbidden optical transitions or wide band gaps [12,13,14,15], making their optoelectronic applications impractical and pushing researchers to improve their optical and optoelectronic properties.

Lanthanide ion (Ln³⁺) incorporation is a viable method to enhance the stability of DP NCs and impart optical properties via B’-site replacement. Several examples of Ln³⁺ ion doping in DP NCs have been proven experimentally, such as Ho³⁺ ion doping into Cs₂AgNaInCl₆ [16], Yb³⁺ and Mn²⁺ ions being doped into Cs₂AgBiX₆ [17], Yb³⁺ and Er³⁺ ions being doped into Cs₂AgInCl₆ [18], and Tb³⁺ and Sb³⁺ ions being doped into Cs₂NaInCl₆ [19,20]. Ln³⁺ commonly generates unique emissions with a narrow bandwidth as compared to transition metal ions, whose emissions are rather broader [21]. The energy transfer between lanthanide ions can be utilized to modulate the emissions in Ln³⁺-doped luminescent materials. In Ln³⁺-doped DPs, the energy transfer channel from the perovskite host or self-trapped excitons (STEs) to Ln³⁺ ions has been confirmed [16,22,23].

Lead-free Cs₂NaInCl₆ DP NCs were synthesized using a variable temperature hot injection method in this study. The undoped Cs₂NaInCl₆ NCs had little PL, while the Tb³⁺-doped Cs₂NaInCl₆ DP NCs exhibited a characteristic emission of Tb³⁺ from the transitions of ⁵D₄→⁷F₅, ⁵D₄→⁷F₆, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃. With further Bi³⁺ co-doping, the Tb³⁺-Bi³⁺-co-doped Cs₂NaInCl₆ NCs were not only directly excited at 280 nm but were also excited at 310 nm and 342 nm, with the latter obtaining a higher PL intensity because partial Tb³⁺ ions in Tb³⁺-Bi³⁺-co-doped NCs are excited by more efficient energy transfer from the Bi³⁺ to the Tb³⁺ ions. This work investigates the underlying mechanism between the intrinsic emission of Cs₂NaInCl₆ DP NCs and the narrow green PL resulting from Ln³⁺ ion doping. It will facilitate the development of lead-free halide perovskite NCs and expand their application in optoelectronics.

Due to its high stability and nontoxicity, lead-free Cs₂NaInCl₆ DP NC has attracted excellent research attention, especially on its optical properties and optoelectronic applications. However, more efforts are needed to achieve tunable band gaps and light emission in specific applications. Other lanthanide ion doping, such as Ho³⁺, Er³⁺, and Yb³⁺ [16,17,18], can be used to modulate the band gap and light emission. The mixing of monovalent or trivalent metals in DP NCs, that is, isovalent doping, such as Na⁺/Ag⁺, Ag⁺/Cu⁺, and In³⁺/Sb³⁺ [23,24,25,26,27], is an effective strategy. On the other hand, heterovalent doping has also made great progress in stages, among which divalent manganese ion is one of the representative dopants [28,29]. Although these doping strategies provide a variety of options for band gap and light emission modulation, the strategies are prone to defect formation, so the optimization of nanomaterial growth and device fabrication is critical for optoelectronic applications. In addition, the dimensional regulation of DP NCs could help to adjust their electronic structures to extend the absorption spectra from the ultraviolet-visible region to the near-infrared region, which would widen their applications in photovoltaic devices [30].

2. Materials and Methods

Materials: Indium acetate (In(OAc)₃, 99.99%), silver acetate (Ag(OAc), 99.99%), oleic acid (OA, 90%), and 1-octadecene (90%) were purchased from Alfa Aesar Chemical Co., Ltd. (Shanghai, China). Terbium acetate hydrate (Tb(OAc)₃∙nH₂O, 99.99%), sodium acetate (Na(OAc), 99.99%), cesium acetate (Cs(OAc), 99.99%), oleylamine (OLA, 80%), and n-hexane (97%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Chlorotrimethylsilane (TMSCl, 99%) and bismuth acetate (Bi(OAc)₃, 99.99%) were purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China). All chemicals were utilized directly as they were received without further purification.

Sample Preparation: An optimized hot injection method was used to synthesize the Cs₂NaInCl₆ NCs. In detail, 131.4 mg In(OAc)₃, 36.9 mg Na(OAc), and 125.0 mg Cs(OAc) were mixed with octadecene (10 mL), oleylamine (0.65 mL), and oleic acid (2.9 mL), placed in a 50 mL two-necked flask, and heated at 105 °C for 80 min under vacuum. Using nitrogen protection, the reaction solution was heated at a rate of 7 °C/min to 190 °C, with 0.5 mL of TMSCl being quickly injected at 180 °C, after 20 s, and quickly cooled down to room temperature in an ice-water bath. After that, the mixture was centrifuged at 10,000 rpm for 20 min. The supernatant was separated off. To obtain colloidal Cs₂NaInCl₆ NCs, the precipitate was washed with 5 mL of toluene, centrifuged for 5 min at 10,000 rpm, redispersed with sonication in 5 mL of hexane, and centrifuged for 5 min at 5000 rpm. The Tb³⁺- and Bi³⁺-ion-doped Cs₂NaInCl₆ NCs were synthesized using the same method, except for adding varied feed ratios of Tb(OAc)₃∙nH₂O or Bi(OAc)₃ at the first step.

Characterization: Powder X-ray diffraction (PXRD) was carried out at room temperature with a PANalytical Empyrean diffractometer (Malvern Panalytical Ltd., Malvern, UK) under Cu K radiation (λ = 1.54056). The transmission electron microscopy (TEM) measurements and energy dispersive spectroscopy (EDS) mapping were conducted using the JEM-2100 (Japan Electronics Co., Ltd., Tokyco, Japan) for microstructure observation and elemental distribution analysis. PerkinElmer 8300 (Perkin Elmer, Waltham, MA, USA)was used for inductively coupled plasma optical emission spectrometer (ICP-OES) measurements to determine the concentration of specified elements in the samples. Optical diffuse reflectance was measured using a Shimadzu UV 2550 spectrometer (Shimadzu, Kyoto, Japan) equipped with an integrating sphere over the spectral range from 200 nm to 900 nm, with BaSO₄ as the complete reflectance reference. The absorption spectra were obtained by transforming the reflectance data using the Kubelka–Munk equation, $\mathrm{F}\left(R\right)={\left(1-R\right)}^2{\left(2R\right)}^2=\alpha /S$, where R is the reflectance, and α and S are the absorption and scattering coefficients, respectively. Photoluminescence excitation (PLE) and PL spectra were obtained using the Horiba PTI QuantaMaster 400 (Horiba, Shanghai, China). The PL lifetime measurement was carried out using a home-built time-correlated single photon counting system. The excitation beam was a nanosecond pulse diode laser, and the optical detector was a single photon counting module.

3. Results and Discussion

As shown in Figure 1a, the lattice structure of the Cs₂NaInCl₆ DP NCs crystallizes in a highly symmetric cubic structure (F$\stackrel{-}{\mathrm{m}}$3m space group). Corner-connected [NaCl₆]⁵⁻ and [InCl₆]³⁻ octahedrons construct a 3D framework with Cs⁺ inserted in the octahedron’s cavities [31,32]. Figure 1b shows the PXRD patterns of the undoped and Tb³⁺-doped Cs₂NaInCl₆ DP NCs. The XRD peaks of the undoped Cs₂NaInCl₆ NCs at 2θ values of 14.4°, 23.8°, 28.1°, 29.3°, 34.0°, 41.9°, 48.9°, and 55.0° correspond to (111), (220), (311), (222), (400), (422), (440), and (620) lattice planes, respectively [33,34]. There is no detectable impurity phase in the doped NCs, implying that no phase separation occurred and the lattice structure remains unchanged. The actual doping concentrations in these samples are far lower than the feeding ratios revealed by the ICP-OES measurements (

Table 1. The ICP-OES results of different feed ratios of Tb³⁺-doped Cs₂NaInCl₆ DP NCs.

Cs2NaInCl6:Tb3+ Feeding Ratios	In (mg/L)	Tb (mg/L)	In (%)	Tb (%)	Tb/In Actual Ratios
Tb/In = 0.3	17.68	0.035	15.37	0.022	0.001432
Tb/In = 0.6	13.22	0.049	11.50	0.031	0.002681
Tb/In = 1.0	17.33	0.106	15.07	0.067	0.004424
Tb/In = 1.3	13.63	0.097	11.85	0.061	0.005147
Tb/In = 1.6	16.67	0.121	14.50	0.076	0.005250
Tb/In = 2.0	34.63	0.285	30.11	0.179	0.005952

). Tb³⁺ ions are considered to replace In³⁺ ions in the crystalline lattice of Cs₂NaInCl₆ DP NCs [34,35,36]. The EDS result indicates that the molar ratio of major elements in Cs₂NaInCl₆ NCs is close to the ratio of 2:1:1:6 (

Table 2. The EDS results of Tb³⁺-doped Cs₂NaInCl₆ DP NCs.

Element	Weight%	Atomic%
Na	3.83	10.19
Cl	34.93	60.18
In	20.99	11.16
Cs	39.95	18.36
Tb	0.29	0.11

). As shown in Figure 1c, the TEM image shows that the cubic-shaped Tb³⁺-doped Cs₂NaInCl₆ DP NCs are evenly distributed with an average size of about 11 nm. The high-resolution TEM (HRTEM) picture of Tb³⁺-doped NCs demonstrates excellent crystallinity with 0.272 nm and 0.379 nm lattice spacing values matched to the (400) and (220) crystal planes (Figure 1d).

The optical properties of the Tb³⁺-doped Cs₂NaInCl₆ DP NCs were investigated using steady-state PL and absorption spectra. The PL spectra under 280 nm excitation for Cs₂NaInCl₆ NCs with different Tb³⁺ doping ratios are shown in Figure 2a. The Tb³⁺-doped Cs₂NaInCl₆ NCs exhibit a major narrow green PL peak at 544 nm derived from the transition of Tb^{3+ 5}D₄→⁷F₅, with three other small emission peaks at 490 nm, 583 nm, and 622 nm derived from the transitions of Tb^{3+ 5}D₄→⁷F₆, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃ [21,37]. The PL intensity increases dramatically when the feeding ratio of doping agents is increased, while the peak location stays constant. The optimum Tb/In atomic feeding ratio is 1.6. After further increasing the Tb³⁺ doping amount, the PL intensity drops due to the concentration quenching effect. For clarity, the following discussion will focus on the optimal doping ratio samples. In the diffuse reflection absorption spectra (Figure 2b), a major absorption peak at 217 nm is observed for the Tb³⁺-doped NCs. The corresponding Tauc plot exhibits a wide band gap of 5.42 eV. The PLE and PL spectra of the Tb³⁺-doped NCs are shown in Figure 2c. A major green emission peak at 550 nm with a large Stokes shift of 270 nm is observed. The narrow green emission should be attributed to the characteristic emission of Tb³⁺ [21,37], while the undoped NCs are non-luminous, which indicates that Tb³⁺ ions are excited via energy transfer channels from the Cs₂NaInCl₆ host to Tb³⁺ ions. The photophysical properties of the Tb³⁺-doped NCs were investigated using transient PL spectra. As shown in Figure 2d, the PL lifetime of the Tb³⁺-doped NCs is fitted with exponential function with an extremely long lifetime (τ = 62 μs), which is attributed to the recombination process of excited Tb³⁺ ions involving an energy transfer from the Cs₂NaInCl₆ host to excite Tb³⁺ ions.

The use of a co-doping strategy to modify the PL properties of metal halide DPs has attracted a great deal of attention [23,24,25,26]. The trivalent Bi³⁺ cations are chosen for co-doping with Tb³⁺ ions for Cs₂NaInCl₆ DP NCs. The PXRD patterns of the Tb³⁺-Bi³⁺-co-doped Cs₂NaInCl₆ NCs are shown in Figure 3a. The XRD patterns of Tb³⁺-Bi³⁺-co-doped NCs are similar to those of Tb³⁺-singly-doped NCs, indicating that the lattice structure is not significantly altered with an additional 5% Bi³⁺ dopant. The TEM image of the Tb³⁺-Bi³⁺-co-doped NCs is shown in Figure 4a. The HRTEM image of the Tb³⁺-Bi³⁺-co-doped NCs shows excellent crystallinity with lattice spacing values of 0.271 nm and 0.380 nm matching to the (400) and (220) crystal planes (Figure 4b), indicating that the trace Bi³⁺ dopant does not significantly change the lattice distance. The mixing of Bi³⁺ and In³⁺ trivalent ions is considered to be random in the B’-site in the crystalline lattice [31]. The EDS element mappings show that the Cs, Na, In, Cl, Tb, and Bi elements are homogeneously distributed in the NCs, indicating the character of a single-phase compound (Figure 4d–i).

The absorption spectra of the Tb³⁺-Bi³⁺-co-doped NCs are shown in Figure 3b. Except for the major absorption peak at 223 nm, another absorption peak is detected at 325 nm, which should be attributed to the correlated 6s²-6s¹6p¹ transitions of Bi³⁺ ions. The insert Tauc plot shows a band gap of 3.70 eV for the Tb³⁺-Bi³⁺-co-doped NCs, indicating that Bi³⁺ co-doping can lower the band gap of the DP NCs because the energy can transfer directly from the Bi³⁺ ions to the Tb³⁺ ions. As shown in Figure 3c, the PLE spectrum of Tb³⁺-Bi³⁺-co-doped NCs shows three PLE peaks at 280 nm, 310 nm, and 342 nm, which is different from the Tb³⁺-singly-doped NCs. Similar PL emission peaks at 549 nm with large Stokes shifts of 269 nm, 238 nm, and 206 nm are observed. With Bi³⁺ co-doping, which decreased the energy of absorbed photons, the samples are not only directly excited at 280 nm but are also excited at 310 nm and 342 nm. The latter obtain a higher PL intensity because partial Tb³⁺ ions in co-doped NCs are excited through more efficient energy transfer channels from the Bi³⁺ to the Tb³⁺ ions, consistent with the previous reports [16,22]. The PL spectra of the Tb+-singly-doped and Tb³⁺-Bi³⁺-co-doped Cs₂NaInCl₆ NCs are shown in Figure 3d. The higher PL intensity of Tb³⁺-Bi³⁺-co-doped NCs than that of Tb³⁺-singly-doped NCs demonstrates that the energy transfer channel built by introducing Bi³⁺ ions is more favorable for the highly efficient luminescence of Tb³⁺ ions than the intrinsic excitation band.

4. Conclusions

In conclusion, we report the Tb³⁺ and Bi³⁺ doping of Cs₂NaInCl₆ DP NCs for narrow green PL. The Tb³⁺-doped Cs₂NaInCl₆ DP NCs exhibit a major narrow green PL peak at 544 nm, derived from the transition of Tb^{3+ 5}D₄→⁷F₅, with three other small emission peaks at 490 nm, 583 nm, and 622 nm derived from the transitions of Tb^{3+ 5}D₄→⁷F₆, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃. The ultra-long PL lifetime of about 62 μs corresponds to the recombination process of excited Tb³⁺ ions, involving an energy transfer from the Cs₂NaInCl₆ host to excite Tb³⁺ ions. The additional two stronger PLE peaks at 310 nm and 342 nm caused by further Bi³⁺ co-doping indicate that partial Tb³⁺ ions in Tb³⁺-Bi³⁺-co-doped NCs are not only directly excited at 280 nm but are also more efficiently excited through energy transfer channels from the Bi³⁺ to the Tb³⁺ ions. The emission intensity of Tb³⁺-Bi³⁺-co-doped NCs is much higher than that of Tb³⁺-singly-doped NCs, indicating that introducing Bi³⁺ ions is more favorable for the highly efficient luminescence of Tb³⁺ ions by providing more efficient energy transfer channels. This work provides an effective method for producing lead-free halide DPs with excellent optical properties, and this mechanism has great potential for tailoring the optical properties of DPs.