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Abstract
Double perovskites without lead element have attracted great attention in recent years. Further increasing the photoluminescence quantum yield of lead-free double perovskites is necessary for their potential applications. In this work, Na+ doped Cs2SnI6 nanocrystals were synthesized by hot injection method. It was displayed that all the NCs have uniform hexagonal shape with good crystallization. Energy dispersing spectroscopy and X-ray photoelectron spectroscopy proves the Na+ ions were doped in the lattice of perovskite structure. The photoluminescence intensity of doped NCs is increased by 2.7-fold than that of pure NCs. A maximum photoluminescence quantum yield of 72% is obtained. The luminous mechanism was investigated by femtosecond transient absorption spectrum and a self-trap emission was proved by the observation of ground state bleaching and photo-induced absorption signals.
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1. Introduction
Lead halide perovskites with chemical formula of ABX3 (A = Cs+, CH3NH3+, CH(NH2)2+. B = Pb2+. X = Cl-, Br-, I-) have received enormous attention due to their superior photoelectric and photovoltaic properties [1–6]. Protesescu et al. obtained monodisperse CsPbX3 colloidal nanocubes by solution-phase synthesis method [7]. The high quantum yields, narrow emission line-widths, and suitable radiative lifetimes of CsPbX3 nanocrystals (NCs) caused widely attention and continues at present [7]. Liu et al. reported a highly luminescent CsPbI3 quantum dots which achieves near 100% absolute photoluminescence quantum yield (PLQY) [8]. In 2023, our group synthesized CsPbX3 NCs in water by surface modification strategy [9]. The light absorption and photoluminescence (PL) measurements proved that the obtained CsPbX3 NCs exhibit excellent stability and luminescent properties in water [9].
The existence of lead element limits the commercial development of lead halide perovskites. Therefore, developing lead-free perovskites is necessary and urgent. Many efforts have been done to reduce the content of lead, such as doping Sn to replace Pb element. However, the introduction of Sn leads to a high defect density and low radiative recombination. In the last few years, the halide double perovskites with a formula of A2BX6 or A2B + B3 + X6 have received significant attention as candidates for lead-free halide perovskites owing to its direct band gap and long carrier lifetimes [10–16]. Yang et al. synthesized lead-free silver-bismuth halide double-perovskite NCs by substituting two divalent lead cations with one monovalent silver and one trivalent bismuth cations [17]. Locardi et al. synthesized double perovskite CsAgInCl6 NCs with and without Mn doping and observed an excellent orange emission in the NCs with Mn doping [18]. Yao et al. obtained Cs2NaBiCl6 NCs and enhanced the PLQY by doping Ag ions in the lattices of the double perovskites [19].
Cs2SnI6 is considered as an alternative because of its suitable band structure, high light absorption coefficient, high air and thermal stability [20–24]. Xu et al. synthesized Cs2SnCl6 NCs and observed a maximum PLQY value of 28% [25]. We also obtained lead-free Cs2SnX6 NCs by a hot injection method and studied the PL properties of pure Cs2SnCl6 NCs [26]. However, compared with lead-based perovskite, the PLQY of Cs2SnX6 NCs is still at a low level. How to improve the PLQY of Cs2SnX6 NCs has become a concern of researchers at present. In this work, we synthesized Na+ doped Cs2SnI6 NCs using a simple thermal injection method. The effects of different ion doping concentrations on the PLQY of Cs2SnI6 were investigated. The PLQY of Cs2SnI6 NCs was increased from 27% to 72% which is the maximum value of this kind of materials up to now.
2. Experimental section
2.1 Materials
SnI4 iodide (Macklin, 99.998%), octadecene (ODE, Aladdin, 90%), C2H3O2Na·3H2O (Macklin, 99.9%), oleic acid (OA, Aladdin, AR), Cs2CO3 (Macklin, 99.9%), oleylamine (OLA, Aladdin, 90%), hexane (Energy Chemical, 97%), were used.
2.2 Synthesis of undoped and Na+ doped Cs2SnI6 NCs
15 mL of ODE, together with OA (0.5 mL) and OLA (0.5 mL), were added into a 100 mL three-necked flask. Then, 0.1625 g of Cs2CO3 and C2H3O2Na · 3H2O (for sodium doping) were added to the reaction vessel, the reactant was dried at 120 °C for an hour under vacuum. The flask was heated to 150 °C in N2 atmosphere until Cs2CO3 was completely dissolved. The resulting Cs-Oleate solution was stored under natural conditions and dissolved at 150 °C before use.
0.1468 g SnI4 with ODE (5 mL), OA (0.2 mL), and OLA (0.2 mL) were loaded into a 100 mL three-necked flask, keeping in a vacuum with the temperature of 80 °C for 15 min. Then, the mixture is heated to 220 °C under N2 protection, and the prepared cesium oleate solution or sodium-containing cesium oleate solution (for sodium doping) is rapidly injected. One minute after injection, cool the mixture in an ice bath. After cooling to room temperature, transfer the solution to a centrifuge tube and centrifuge for 5 minutes at a speed of 7000 revolutions per minute. The precipitates are Cs2SnI6 NCs, and then are dispersed in hexane for cleaning with centrifugation. Finally, the cleaned Cs2SnI6 NCs are eventually dispersed in hexane.
2.3 Characterization
The crystalline phase and structure were checked by X-ray diffraction (XRD, Rigaku SmartLab) with Cu Kα in 2θ-ω scan mode. The morphologies and energy dispersing spectroscopy (EDS) element mapping of Cs2SnI6 NCs were tested on transmission electron microscopy (TEM, JEM-2100PLUS). The UV-vis absorption spectra were collected by a UV-VIS-NIR spectrophotometer (PerkinElmer, Lambda 1050). The PL and time-resolved PL (TRPL) spectra were measured by a fluorescent spectrometer (Edinburgh, FLS1000). The PLQY was measured using FLS1000 instrument equipped with the integrating sphere. Femtosecond transient absorption spectrum (TAS) was performed using Ultrafast Helios System. The 800-nm monochromatic light from a femtosecond laser with 35 fs and 1 kHz frequency was split into two beams: a 400 nm pump laser pulse and a quasi-continuous white probe laser pulse with wavelength of 450-750 nm.
3. Results and discussion
3.1 Characterization of Cs2SnI6 NCs
The crystalline phase of the obtained NCs was checked by wide angle XRD 2θ-ω scan as shown in Fig. 1(a). Compared with the standard PDF card of #73-0330, all the reflection peaks can be identified as the anti-fluorite structure Cs2SnI6. It indicates that the obtained NCs have a single phase of double-perovskite without any impurity phase. As mentioned in other works, the doping of elements may change the structure of perovskite crystals [27,28]. However, the Na-doped Cs2SnI6 are not stable in air under X-ray illumination. Therefore, the change in the structure with doping concentration cannot be studied by XRD at present. Surface passivation or coating maybe an efficient way to further improve the stability of the doped NCs. Absorption spectrum of the Cs2SnI6 NCs was shown in Fig. 1(b). The clearly absorption starting at around 690 nm with a corresponding optical band gap of 1.8 eV. No significant change of the absorption edge was observed indicates the doping of Na ions have little effect on the band gap of the Cs2SnI6 NCs. Figure 1(c) shows the atomic scheme of the crystals to indicate the position of Na+ dopant. In this work, the optical measurements are performed in hexane solution and the Cs2SnI6 NCs are stable in hexane under UV-light illumination as shown in Fig. 1(d-h). As shown in Fig. 1(i-j), the pure Cs2SnI6 can exist in air during XRD measurement. However, the Na-doped Cs2SnI6 goes bad very quickly in air under X-ray illumination as shown in Fig. 1(k-l).
Fig. 1. (a) XRD pattern of Cs2SnI6 NCs and the standard PDF card. (b) Optical absorption of the pure and Na+ doped Cs2SnI6 NCs. (c) The atomic scheme of the crystal which indicates the position of Na+ dopant. (d-h) Optical pictures of pure and doped Cs2SnI6 NCs under 375 nm excitation in hexane. (i-j) The obtained pure Cs2SnI6 NCs and the powder after XRD measurement. (k-l) The obtained Na-doped Cs2SnI6 NCs and the powder which are not stable in air.
TEM morphologies of NCs with different Na+ doping ratios are displayed in Fig. 2(a)-(e). It can be seen that all the NCs have uniform hexagonal shape with good crystallization. The size of the NCs is around 35 nm. The histograms of the sizes from the NCs with different doping concentrations were shown in the inset of Fig. 2. It can be observed that with the larger ratio, the sizes are larger. The zoom-in image of Fig. 2(c) is shown in the Fig. 2(f) and the corresponding EDS mapping of Cs, Sn, I, and Na are shown in the Fig. 2(g), (h), (i), and (j), respectively. The exhibit of Na element in the NCs was proved by EDS map in the Fig. 2(j). Furthermore, it also can be observed that all the elements are uniformly distributed in the NC.
Fig. 2. TEM patterns of Cs2SnI6 NCs with different Na+ doping ratios: (a) 0, (b) 0.2, (c) 0.4, (d) 0.6, and (e) 0.8, respectively. The insets are histograms of the sizes from the NCs. (f) The zoom-in image of (c). (g-j) The EDS elemental mappings (Cs, Sn, I, Na) of Cs2SnI6: Na NCs.
3.2 PL and TRPL analysis
Figure 3(a) exhibits the steady-state PL spectra of Cs2SnI6:Na NCs with different doping ratios. All the spectra show an emission peak at around 690 nm which position was not influenced by the Na+ ions doping. The black line is the PL of Cs2SnI6 NCs without Na+ ions doping, which was used for comparison. It can be observed that all the doped NCs have a higher PL emission intensity which proves the efficiency of the Na+ doping. With the doping concentration of Na+ ions increasing, the PL intensity increases to a maximum value and then decreases when doping too much Na+ ions. The best doping ratio of Cs and Na is 1: 0.4. Further study of the PL properties was analyzed based on the PLQY results as shown in Fig. 3(b). PLQY is defined as the number of photons absorbed divided by the number of photons emitted. It is an important parameter for the quantitative comparison of luminescence properties. Figure 3(b) demonstrates that the doping of Na+ ions can greatly increase the PLQY of the NCs. Compared with the pure NCs, the PLQY intensity increased by 2.7-fold when the doping ration of Na+ ions is 0.4. Further increasing of doping ratio reduced the PLQY intensity. The maximum PLQY of the Na+ doped Cs2SnI6 is 72.86% in this work which value is higher than other reports of this kind of material [25,29,30]. The higher PLQY efficiency is considered to be the main reason that results a better PL emission property.
Fig. 3. PL (a) and PLQY (b) of Cs2SnI6 NCs with different Na+ doping ratios. (c) TRPL of Cs2SnI6 NCs with different Na+ doping ratios. (d) Comparison diagram of average lifetime and PLQY versus doped Na+ ratios.
To better understand the influence of Na+ ions doping on the PL properties of Cs2SnI6 NCs, The TRPL of NCs with different Na+ doping ratios was characterized in Fig. 3(c). The obtained TRPL curves can be fitted by the formula of [31]:
Table 1. The TRPL fitting parameters for the NCs with different Na+ doping concentration.
3.3 XPS and TAS analysis
The XPS overview spectra of Na+ doped Cs2SnI6 NCs was showed in Fig. 4(a). Except for the carbon peak resulting from the environment, other peaks are attributed to the Cs, Sn, I and Na elements of Cs2SnI6:Na NCs. This indicates no impurity elements exists in the Cs2SnI6:Na NCs during the synthesized process. The Na element exhibit a weak peak in Fig. 4(b) which arise from the core level spectra of Na 1s. The existence of Na elements in the NCs have been proved by EDS elemental mappings in Fig. 2. However, the Na+ binding energy is low and the noisy signal affect the accurate analysis. Hence, the effect of doped Na ions on the Cs2SnI6 NCs was analyzed by comparing the changing of Cs 3d and Sn 3d peaks. Due to the spin-orbit splitting [25], Cs 3d state split into Cs 3d5/2 and Cs 3d3/2 peaks were displayed in Fig. 4(c). The doped of Na+ does not shift the binding energy of Cs 3d peaks in Cs2SnI6 NCs. The peaks with binding energy of 724.1 eV and 738.2 eV can be ascribed to the Cs 3d5/2 and Cs 3d3/2 respectively, which is consistent with the previous studies [34]. As shown in Fig. 4(d), the core level spectra of Sn 3d exhibits Sn 3d5/2 and Sn 3d3/2 two peaks due to the spin-orbit splitting. The binding energy of Sn 3d decreases with the doping of Na+, which may be attribute to the influence of Na on the bonding state of Sn. In the Cs2SnI6 NCs without Na doping, the binding energy of Sn 3d5/2 peaks is at 486.7 eV and the binding energy of Sn 3d3/2 peaks is at 495.2 eV. In the Cs2SnI6:Na NCs, the binding energy of Sn 3d5/2 peaks and Sn 3d3/2 peaks is at 486.2 eV and 494.7 eV respectively. According to reports in Ref. [34,35], the binding energy of Sn2+ is about 485.8 eV in Sn 3d5/2 peaks. Hence, the Sn valence in our Cs2SnI6 NCs and Cs2SnI6:Na NCs are both Sn4+. The slight shift of Sn4+ 3d peaks in the doped Cs2SnI6 NCs may be related to the introduction of Na+ ions. The exist of Sn4+ ions also means that the doped Na+ are mainly instead of Cs ions which did not reduce the valence state of Sn ions.
Fig. 4. (a) Overview XPS spectrum of the Na+ doped Cs2SnI6 NCs. (b) XPS core-level spectrum of the Na 1s peak for the doped Cs2SnI6 NCs. Cs 3d (c) and Sn 3d (d) spectra of NCs with and without Na+ doping.
Figure 5(a) shows transient absorption curve of the pure and Na+ doped NCs. The ground state bleaching (GSB) signal is present in both pure and Na+ doped NCs. The two main differences of pure and doped NCs are GSB red-shift and the photo-induced absorption (PIA) signal. For the doped NCs, a GSB signal located near 475 nm and a PIA signal centered at 525 nm can be clearly observed. The GSB amplitude was achieved by carries population inversion under the pump condition. The photo-generated carrier concentration is increased and fill the conduction band. Due to bandgap renormalization, GSB signal of Na+ doped NCs shifts compared with pure NCs. Some carriers absorb energy and jump to higher energy levels, which is the origin of PIA. In contrast, the pure sample has a lower carrier concentration and radiative irradiation soon after leaping into the conduction band. An energy level diagram of Cs2SnI6 is presented from the experimental results in Fig. 5(b). Both GSB and PIA signals indicate that high energy is required to excite the carriers, which is one of the typical features of self-trap emission which results a PL emission at around 690 nm as shown in the Fig. 3.
Fig. 5. (a) Transient absorption spectrum of the pure and Na+ doped Cs2SnI6 NCs. (b) The photo-emission diagram of Cs2SnI6 materials excited by the ultra-fast laser.
4. Conclusion
In summary, Na+ doped Cs2SnI6 NCs with red emission were fabricated by the hot injection method. XRD and TEM images demonstrate that all the Cs2SnI6 NCs have uniform hexagonal shape with good crystallization. XPS spectra reveals the successful doping of Na+ ions in the double perovskite lattices. The PL intensity of Cs2SnI6 NCs can be improved by a suitable doping ratio. Additionally, the maximum PLQY of the NCs above 72% which is the highest value in this kind of materials up to now. TRPL results indicate that the NCs have a long average lifetime which is benefit to the PL emission. The ground state bleaching and photo-induced absorption signals were observed in the transient absorption spectrum which indicates typical self-trap emission in the NCs. This work may be useful for the understanding of PL mechanism in the lead-free double-perovskites for future applications in photoelectric devices.
Funding
National Natural Science Foundation of China (11904198, 51872161, 51902179).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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