In recent years, organic-inorganic, and the subsequent all-inorganic lead halide perovskites [APbX3, A=CH3NH3+, CH(NH2)2+, or Cs+, X=Cl−, Br−, or I−] have attracted the attention of many researchers because of their unique optical properties, such as narrow full width at half-maximum (FWHM), high photoluminescence quantum yield (PLQY), wide color gamut, tunable emission, and excellent optoelectronic properties [1,2]. Therefore, researchers make use of the excellent optoelectronic properties of lead halide perovskite to prepare a large-area perovskite battery [3,4]. Taking advantage of the excellent optical gain and high emission efficiency of lead halide perovskite, high-quality CsPbBr3 nanowire plasmonic lasers have been fabricated successfully by chemical vapor deposition [5]. By making use of the tunable emission of lead halide perovskite, perovskite LEDs are prepared successfully [6,7]. However, the toxicity of lead will cause irreversible damage to human intelligence, especially for the brains of young children. Meanwhile, lead halide perovskite shows poor stability to oxygen, water, or high temperature, which seriously limits its commercial application prospects.

To solve the toxicity of lead and improve the stability of lead halide perovskite, researchers actively explore efficient substituted ways to change this situation. Recently, some research groups have turned to covalent elements to replace lead elements to solve the toxicity of lead halide perovskites, such as Sn2+ [8] and Ge2+ [9]. Nevertheless, these elements are easily oxidized for the high-energy-lying 5s orbitals, which leads to the perovskite being extremely unstable in the ambient atmosphere [10], and this kind of replacement will be affected by its low PLQY or poor stability. On the other hand, in addition to replacing lead with a single element, some research groups proposed to use another effective replacement method for lead-free perovskite. The problem of lead toxicity can be solved by combining M+ (M=Ag+, Na+, or K+) and M3+ (M=Bi3+, Sb3+, or In3+) to replace double Pb2+ [11]. At the same time, this double perovskite structure also shows good stability to the natural environment, even exposed to oxygen or water vapor at high temperature. The double perovskite structure of A2M+M3+X6 completely avoids the toxicity of lead, and it also shows good environmental stability. Another effective solution is to directly replace double Pb2+ with single Sn4+ to form a lead-free defect perovskite variant. Although Cs2SnCl6 shows good thermal stability, its PLQY is very low. The main difficulty to improve application prospects is how to strengthen the PLQY. Bi-doped Cs2SnCl6 microcrystals (MCs) were successfully obtained by Tang et al. [12], and blue emission with the peak at 455 nm is obtained under the ultraviolet lamp, of which emission is attributed to the [BiSn+VCl] defect complex. Another paper reports that Sb is doped into Cs2SnCl6 to produce orange-red emission at 602 nm caused by the ionoluminescence of Sb3+ [13]. The application of tin-based perovskite to thermometry and LEDs is in continuous development. The noncontact optical thermometer with higher spatial resolution and response speed has attracted the attention of scientific researchers. It can be usually divided into two methods: the fluorescence intensity ratio (FIR) [14] and fluorescence lifetime [15] techniques. The FIR approach is based on the emission of thermally coupled energy levels. The fluorescence lifetime technique utilizes the phenomenon that the lifetime decreases with the temperature enhancement.

In this work, the steady-state PL and absorption spectra of bismuth (Bi) and antimony (Sb) codoped Cs2SnCl6 (BSCSC) MCs are measured at room temperature. Then the temperature-dependent steady-state PL and time-resolved PL (TRPL) are used to explore the luminous mechanism of BSCSC MCs. It is concluded that the peak position at 485 nm in the dual emission is attributed to the [BiSn+VCl] defect complex, and the emission of 650 nm is attributed to microsecond-timescale self-trapped excitons (STEs). Under 405 nm laser excitation, the fluorescence lifetime of BSCSC MCs is measured in the temperature range from 290 to 390 K. The maximum values of relative and absolute sensitivities are 3.82%K−1 at 345 K and 5.11ns·K−1 at 305 K. The dual emission of BSCSC MCs is applied in fabricating white-light-emitting diodes (WLEDs), avoiding the use of commercial light-emitting phosphors to reduce costs.

The materials used are cesium chloride (CsCl, 99.9%, Aladdin), bismuth chloride (BiCl3, AR, Aladdin), antimony trichloride (SbCl3, AR, 99%, Macklin), tin chloride (SnCl2, 99%, Macklin), hydrochloric acid (HCl, AR, 37% mass fraction, Harbin Polytechnic Chem. Reag. Co., Ltd.), and isopropyl alcohol (C3H8O, AR, 99.5%, Macklin). All of these chemicals are used without any further purification.

The BSCSC MCs are synthesized with the solvothermal method [16]. 5 mmol SnCl2, 0.32–0.55 mmol BiCl3, 0.32–0.55 mmol SbCl3, and 10 mmol CsCl are dissolved in 10 mL HCl solution. A 50 mL Teflon autoclave filled with solution is heated at 180°C for 12 h. Then it is gradually cooled down to room temperature, and the cooling process is important in determining the PLQY [17]. After that, the upper solution from the precipitation in the whole sample is centrifuged and washed with isopropyl alcohol three times to separate impurities at 4000 r/min. The precipitation from the supernatant is collected via the same centrifugation method as described above. The obtained precipitate by centrifugation is dried in a vacuum oven at 60°C.

A UV-LED chip (380–390 nm) is used to fabricate the WLED and excite the phosphor-silicone mixture. The mixture contains BSCSC MCs phosphor and silicone gel A and B. First, Cs2SnCl6 phosphor is mixed with silicone gel A and B (A/B = 1/4). Then the bubbles in the silicone are removed through the vacuum chamber and overcoat the mixture on the chip. Finally, the chip is dried at 50°C for 2 h.

The 405 nm continuous-wave (CW) laser (FN-405-200mW, CNI) is used as an excitation light source for steady-state excitation. The steady-state PL spectra are collected by a spectrometer (HR4000CG-UV-NIR, Ocean Optics). The TRPL spectra are excited by frequency-adjustable picosecond laser (MDL-PS-405, CNI) pulses at 405 nm and collected by a spectrometer (SP-2500, Princeton Instruments) with an intensified CCD detector (PI-Max-1024i, Princeton Instruments). The temperature-dependent PL measurements are performed using a vacuum liquid-nitrogen cryostat (Cryo-77, Oriental Koji) with the capability to give a temperature range from 80 to 480 K.

In poor Sn4+ and rich Sn2+ conditions, the formation enthalpies (ΔH) [12] obtained from the theoretical calculations are low and even negative, which makes Bi-on-Sn and Sb-on-Sn substitution possible. For a series of BSCSC MCs, Bi and Sb are codoped into the sample, and the Bi/(Sb+Bi+Sn) and Sb/(Sb+Bi+Sn) feeding ratios in the solution are set at 6% and 10% (6%10%BSCSC MCs). The others are named as 9%10%BSCSC, 12%10%BSCSC, 9%5%BSCSC, and 9%20%BSCSC MCs, respectively. The WLEDs reported in the previous paper [13] need to mix with other luminous powders. By codoping Bi and Sb into Cs2SnCl6 at the same time, WLEDs can be obtained directly without the addition of other light-emitting powders by applying them on the light-emitting chip. Cs2SnCl6 is doped with Bi only, and the peak is found to be at 454 nm [12]. The Cs2SnCl6 is doped with Sb, and the peak is located at 630 nm [18]. The dual emission peaks of 12%10%BSCSC MCs are at 485 and 650 nm under 405 nm CW laser excitation, and the intensity at this proportion is similar. The reason for the 20–30 nm redshift on the emission might be the expansion of Bi-anion and Sb-anion bonds [19].

From the left of Fig. 1(a), the crystal structure of BSCSC MCs shows a vacancy-ordered double perovskite structure with isolated [AX6]2− octahedra [20]. The powders are white in the daylight, which with different elements or contents show different colors under the UV lamp (365 nm) as shown in the right of Fig. 1(a). Through controlling the Bi and Sb feeding contents, the emission of BSCSC MCs can be tuned, even to white-light emission. As shown in Fig. 1(b), whether Bi- (or Sb-)doped Cs2SnCl6 and 12%10%BSCSC MCs, each diffraction peak of all samples has a perfect match for the Cs2SnCl6 crystal structure (JCPDF#70-2413), which has an Fm3¯m space group, and almost no impurity phase can be detected. It is noted that the diffraction peak (111) shows a fairly small peak shift as shown in the right side of Fig. 2(b). Compared with Cs2SnCl6 MCs, the X-ray diffraction (XRD) peaks of doped Cs2SnCl6 MCs have a slight peak shift, which can be proved that Bi3+ and Sb3+ are successfully doped into Cs2SnCl6 MCs. As shown in Fig. 1(c), the sharp optical absorption edge of Cs2SnCl6 at 320 nm is very close to the previously reported paper that showed excitonic absorption at 317 nm [21]. There is another absorption edge found at ∼390nm for Bi, Sb, and Bi-and-Sb-doped Cs2SnCl6 MCs. The absorption spectra show that only the doped samples have additional absorption peaks between 350 and 400 nm. The corresponding bandgap is calculated through the Tauc plot and labeled in Fig. 1(c). It can be seen that the bandgap of Cs2SnCl6 samples doped with Bi (or Sb) is smaller than that of undoped samples. The bandgap of 12%10%BSCSC MCs is the smallest in all samples due to the formation of the defect bands [12]. As shown in Fig. 1(d), the characteristic peak positions of every element can be observed from the X-ray photoelectron spectroscopy (XPS) survey spectrum. The characteristic peaks located at 539.8 and 532.1 eV correspond to Sb3+3d3/2 and 3d5/2, respectively. The Bi3+4f5/2 and 4f7/2 are located at 165.0 and 159.9 eV. Because of the instability of Sn2+, it is easy to oxidize to Sn4+ in the process of solvothermal reaction. Those peaks located at 496.0 and 487.6 eV are attributed to Sn4+3d3/2 and 3d5/2; meanwhile, it proves that oxidation results in the solvothermal reaction. It can be observed that the size of these MCs is from hundreds of nanometers to several micrometers by using a scanning electron microscope (SEM) as shown in Fig. 1(e), and it depicts the energy dispersive X-ray energy spectrum (EDS) mapping of the 12%10%BSCSC MCs. Through the observation of the mapping diagram containing five elements of Cs, Cl, Sn, Bi, and Sb, all the elements are uniformly distributed and perfectly overlapped, which demonstrates the successful incorporation of Sb and Bi elements. To study the composition of elements in the samples and their valences, XPS of 12%10%BSCSC MCs is measured. As shown in Fig. 1(f), the elemental analysis of 12%10%BSCSC MCs is obtained from the EDS and corresponding Cs∶Sn∶Cl≈2∶1∶6. It is difficult to measure the content of Sb by EDS because the content is small. Then the chemical compositions of Sb and Bi are measured by inductively coupled plasma optical emission spectrometry (ICP-OES). The Bi/(Bi+Sb+Sn) and Sb/(Bi+Sb+Sn) measured by ICP-OES are 1.14% and 0.66%. This indicates that Sb and Bi can only be partially incorporated into Cs2SnCl6 MCs.

The optical properties of BSCSC MCs are characterized at room and low temperature. Figure 2(a) shows the steady-state PL spectra of 9% Bi and y% (y=10, 30, and 40) Sb and 5.5%47% BSCSC under excitation wavelength of 360 nm. For a polarizable halide perovskite lattice structure, light-induced transient lattice deformation results in excitons being localized, which represents a type of bound state for efficient radiative recombination called STEs [12], Keeping Bi doping content unchanged, increasing the Sb doping content can get a gradual enhancement of orange PL emission. The enhancement in PL intensity may be attributed to the increase in Sb content leading to more STE production, which means more energy should belong to the [BiSn+VCl] transfer to STEs through intersystem crossing (ISC) [22]. There is no quenching phenomenon with high-concentration doping; it can be ruled out that the emission at 650 nm is attributed to ionoluminescence [23]. The broad spectra and the large Stokes shift of the peak position of 650 nm are attributed to STEs [24,25]. Figure 2(b) shows the PL spectra of x% (x=6, 9, and 12) Bi and 10% Sb codoped Cs2SnCl6 MCs. There are fewer energy transfers to STEs with the increase of Bi feeding contents, and that makes the PL intensity of blue emission peak at 485 nm be gradually enhance. The blue emission is attributed to the [BiSn+VCl] defect complex at the luminescent centers, and almost no peak shift is consistent with previously reported [12]. PL spectra of 12%10%BSCSC MCs under different laser pump fluence are measured under the 405 nm CW laser at room temperature as shown in Fig. 2(c). The PL intensity increases gradually, and the FWHM has a slight increase with the pump laser fluence increasing from 35 to 5729mW/cm2. With the material excited by laser for a long time, the surface temperature of the MCs is rising. The thermal effect has occurred, which causes a slight increase in the FWHM. The influence of pump fluence on PL intensity is shown in Fig. 2(d). The green and red solid lines are linear fitting curves with the spectrum integral intensity at the peak positions of 485 and 650 nm, the slopes of which are 0.80 and 0.75. The correlation coefficient (R2) is 0.99, and the linear fitting curves reveal the existence of single-photon excitation.

The temperature-dependent PL spectra are efficient for studying the optical properties of materials, especially for understanding the luminous mechanism of substances [26]. As shown in Fig. 3, temperature-dependent PL measurements are performed for 12%10%BSCSC MCs within the temperature range of 80–380 K, which shows the same change as in the reported paper [27]. The small quantity of STEs is caused by weak electron–phonon interactions in a low-temperature environment, and the PL intensity is weakened. In the temperature range of 80–260 K, with the increase of temperature, the electron–phonon interactions are enhanced gradually, and more STEs are generated simultaneously [16]. In the range of 260–380 K, as the temperature continues to rise, the lattice vibration is enhanced. The lattice relaxation of the luminescent center increases, more phonons are coupled to excitons, and the nonradiative transition probability increases [25]. This leads to a reduction in luminous efficiency decrease, and the thermal quenching phenomenon begins to increase. The corresponding pseudocolor map is shown in Fig. 3(b), and it shows the PL intensity with the temperature rising, too. The temperature dependence of the integrated emission intensity I(T) can be described [28] as I(T)=I01+A0e−Eb/KBT,(1)where I0 is the integrated emission intensity at 0 K [29]. A0 is a fitting parameter, Eb is the exciton binding energy, and KB represents the Boltzmann constant. As can be seen from Fig. 3(c), the Eb obtained by fitting the experimental data is 594.0 meV, and it is much larger than that of lead halide perovskite [30], such as CsPbBr3 with 50.0–70.0 meV [31–33]. The correlation coefficient (R2) is 0.99, which shows that the fitting line is very consistent with the actual measured data from 260 to 380 K. When KBT≥Eb, the thermal quenching occurs. In the range of 80–260 K, the intensity of PL increases with the increase of temperature, which is different from that of lead halide perovskites [32]. It extracts that the emission peaks with the temperature change, which shows an abnormal blueshift monotonically from 658 to 628 nm and 500 to 486 nm with increasing temperature from 80 to 380 K as shown in Fig. 3(d). The blueshifts may be ascribed to the interaction between electron–phonon coupling, thermal expansion [34], and lattice deformation [35]. Photon energy of the emission peak is nearly linear with temperature, and the shift of emission peak with temperature change can be expressed by a temperature coefficient α=∂E/∂T, which is 0.14 and 0.20 meV/K for PL emission peaks at 650 and 485 nm, respectively.

From 80 to 320 K, the photon energy or wavelength of the emission peak is almost a constant; this temperature-independent chromaticity makes them as a class of materials for potential application to cold-light LED.

The TRPL of the two emission peaks needs to be measured. As shown in Fig. 4(a), the PL decay curve of 9%y%BSCSC (y=0, 5, 10, and 20) MCs is measured at 485 nm excited by a 405 nm picosecond laser. Under the same synthesized condition that the content of Bi remains unchanged with the increase of Sb, the TRPL decay curve is fitted well by double exponential function Eq. (2): I(t)=A1+B1e−t/τ1+C1e−t/τ2,(2)where I(t) is the PL intensity at time t. A1 represents the PL baseline, and B1 and C1 represent the amplitudes of PL. τ1 represents the lifetime of radiative transition only at the peak of 485 nm, and the value of C1 is zero. When Bi is only doped into Cs2SnCl6, the lifetime we measured is 305.1 ns. When keeping Bi contents unchanged and increasing Sb contents, the lifetime of the [BiSn+VCl] emission center gradually becomes shorter. It demonstrates that [BiSn+VCl] is more competitive than STEs in vying for the excited electrons [36]. Figure 4(b) presents the PL decay curves of x%10%BSCSC (x=0, 6, 9, and 12) MCs. Due to the nonradiative transition part, C1 is not zero at this time. τ1 is the lifetime of nonradiative transition of free exciton, and τ2 corresponds to the lifetime of the radiative transition at 650 nm, which is consistent with the microsecond-scale lifetime of other double perovskites [25]. The lifetimes of τ1 and τ2 in Sb-doped Cs2SnCl6 MCs are 0.74 and 5.67 μs, respectively. Compared with [BiSn+VCl], STEs occupy more energy proportion [36] when keeping Sb feeding contents unchanged and increasing Bi feeding contents. It can be observed that the lifetimes of both the radiative transition and the nonradiative transition begin to shorten. The PL decay process of 12%10%BSCSC MCs from 150 to 900 ns at 485 nm emission is shown in Fig. 4(c). It can be clearly distinguished that the PL intensity declines with the increase of the number of frames. Figure 4(d) shows the PL decay process of 12%10%BSCSC MCs from 4.0 to 15.0 μs at 650 nm emission.

Figures 5(a) and 5(b) show the temperature-dependent TRPL decay curves at the wavelengths of 485 and 650 nm. More and more local electronic states promote the formation of STEs, which is caused by the fact that the Cs2SnCl6 substrate is 0D perovskite derivatives [37]. It shows the lifetime with the temperature rising in Fig. 5(c). Due to the weaker electron–phonon interaction at low-temperature conditions, the nonradiative recombination caused by free excitons is obvious in the temperature range from 80 to 260 K. With the increase of temperature, the electron–phonon interaction becomes stronger, and the PL decay is mainly dominated by STEs. When the temperature is higher than 300 K, thermal quenching makes the radiative recombination weaken and the nonradiative recombination strengthen, which results in the loss of lifetime. Figure 5(d) depicts a schematic energy diagram for BSCSC MCs. For undoped Cs2SnCl6, the valence band maximum (VBM) is derived from Cl 3p states, while the conduction band minimum (CBM) consists of the antibonding states of Sn 5s and Cl 3p orbitals. When Cs2SnCl6 is codoped with Bi and Sb, it leads to the upward shift of VBM [12]. Furthermore, the lattice deformation makes the energy of the ground state rise, which increases the lattice deformation energy Ed [38,39]. In the reported papers, there are a lot of ions with ns2 outermost electronic configuration acting as emission centers. Usually, there is a ground state with ns2 outer electrons S10. Triplet states P3n (n=0, 1, 2) and single state P11 are formed by the splitting of the excited state. According to Hund’s rules, it can be arranged according to the excited state energy as P30<P31<P32<P11. Due to electric and magnetic selection rules, the transitions of S10−P30 and S10−P32 are forbidden. The transitions of S10−P31 and S10−P11 are parity allowed due to spin-orbit coupling [24,40]. Due to the energy transition [22], part of energy belongs to [BiSn+VCl] transitions to STEs by ISC with the increase of Sb-doping content. In the process of energy transfer, part of the energy undergoes a radiative transition from P11 to S10, exhibiting an emission at 485 nm. Due to ISC, part of the energy is transferred from P11 to P31 in a nonradiative way, and then it continues to be transferred to S10 in the form of a radiative transition accompanied by a PL emission at 650 nm.

In addition to the FIR, the PL decay lifetime with temperature dependence can also be used to apply in temperature measurement. To show the variation of the lifetime with temperature, the lifetime of 485 nm emission attributed to S10−P11 in the temperature range of 290–390 K is calculated as shown in Fig. 5(e). The relationship of lifetime and temperature can be fitted well by the Arrhenius equation [41]: τ′(T)=τ′01+De−ΔE/KBT,(3)1τ′=1τ′0(1+De−ΔE/KBT),(4)where τ′(T) and τ′0 are the lifetime of T and 0 K, and D is a fitting parameter. ΔE is the thermal-quenching activation energy, which is 448.7 meV after fitting. The sensitivity of optical thermometers is of great significance for temperature sensing. The SR and SA are significant parameters to evaluate thermometric material. The SA represents the change of lifetime over a unit temperature, and SR is defined as the relative change. The SR and SA can be calculated respectively by the following equations [14]: SR=|1τ′dτ′dT|×100%,(5)SA=|dτ′dT|.(6)

Figure 5(f) exhibits the change of SR and SA when the temperature rises. The maximum value of SR is determined to be 3.82%K−1 at 345 K, which indicates higher sensitivity. The SA shows the same variation tendency, the maximum value of which is 5.11ns·K−1 at 305 K. Table 1 summarizes some thermometric parameters about different sensing materials. Compared with Gd3Al5O12:Mn and BaLaMgNbO6:Dy, Mn, the SR value of our sample based on the fluorescence lifetime technique is the highest, which means the sensitivity is pretty good. These parameters of high SR and SA show that this kind of material is suitable for noncontact temperature measurement. In short, these experiment results demonstrate that BSCSC MCs are promising for thermometry because of their outstanding sensitivity as well as excellent stability.Table 1.

Several Thermometric Materials and Their Relative Sensitivity

Most WLED devices [44,45] are made by mixing perovskite powder with other commercial phosphors [30] in a proportion that contains rare-earth elements. The spectra of BSCSC MCs contain two emission peaks at 485 and 650 nm, of which the intensity can be controlled by different content, achieving the effect of controlling the color of light emission. The BSCSC MCs after grinding are mixed with a curable resin coat on a commercial UV-380 LED chip directly, and then the chip is dried at 50°C in a vacuum environment for 2 h to solidify the curable resin. Figure 6(a) shows the working WLED and the corresponding PL spectrum. Then the stability test of the WLED is carried out within 0–200 min. As shown in the inset of Fig. 6(a), it is concluded that the PL intensity can be maintained about at 68%. As seen in Fig. 6(b), the CIE coordinates of our WLED are located at (0.30, 0.37) with a color temperature of 6800 K after tuning the Bi and Sb feeding ratio (5.5%47%). The responding PL intensity increases with the increase of current, showing good stability. These properties make codoped Bi and Sb a promising candidate luminescent phosphor material.

In summary, we successfully synthesized a series of BSCSC MCs with different Bi and Sb contents by using the solvothermal method. Compared with undoped Cs2SnCl6 MCs, dual broad emissions can be observed, of which emission peaks are located at 485 and 650 nm. Phonon–electron interaction and thermal quenching make the PL intensity exhibit the phenomenon of first strengthening and then weakening with the temperature rise. Temperature-dependent PL and TRPL reveal that 650 nm emission primarily originates from STEs due to the incorporation of Sb and the interconversion between radiative and nonradiative transition. The [BiSn+VCl] as the luminescent center is responsible for the 485 nm emission. The lifetime of this perovskite luminescence is used for noncontact temperature measurement for the first time. The maximum SR and SA values of 3.82%K−1 and 5.11ns·K−1 show better sensitivity than other materials, which indicates that there are application prospects in noncontact temperature measurement. By changing different feeding content of Bi and Sb to make phosphors and coating those on the 380 nm LED chip to make WLEDs, and adding no other commercial phosphors, this codoping provides a new idea for reducing costs in the future.