Lead-free Cs2Ag1−xNaxIn1 − yBiyCl6 perovskite films with broad warm-yellow emission for lighting applications

Lead-free halide double perovskite Cs2AgInCl6 has been extensively studied in recent years due to the lead toxicity and poor stability of common lead halide perovskites. In this study, sodium (Na+) and bismuth (Bi3+) doped into Cs2AgInCl6 double perovskite, then Cs2Ag1−xNaxIn1 − yBiyCl6 films with broadband warm-yellow emissions were achieved by the blade coating method. Herein, Na and Bi content were changed as variables at a series of parameter optimization experiments, respectively. In the Cs2Ag1−xNaxIn1 − yBiyCl6 systems, Na+ broke the parity-forbidden transition of Cs2AgInCl6, and Bi3+ suppressed non-radiative recombination. The partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations were crucial for increasing the intensity of the PL emission. The experimental results showed that the photoluminescence quantum yield of the Cs2Ag0.4Na0.6In0.8Bi0.2Cl6 film was 66.38%, which was the highest data among all samples. It demonstrated remarkable stability under heat and ultraviolet conditions. After five thermal cycles, the PL intensity of the Cs2Ag0.4Na0.6In0.8Bi0.2Cl6 film is only reduced to approximately 5.7% of the initial value. After 720 h continuous ultraviolet irradiation, there occurred 31.9% emission decay of the film.


Na + doping
In order to find the optimal stoichiometric number in this Cs 2 Ag 1-x Na x In 1-y Bi y Cl 6 system, firstly, we fixed the Bi dopant amount (y = 0.5) and set Na content to a tunable figure (x ranging from 0 to 1, the step size was 0.2).Based on previous research 29,[43][44][45] , we found that a small amount of Bi 3+ doping is essential for the luminescence of Cs 2 AgInCl 6 .Therefore, as the intermediate value of 0 ~ 1, 0.5 was chosen as the fixed Bi doping amount to prepare Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films.Figure 1a is the XRD patterns of Cs 2 Ag 1−x Na x In 0.5 Bi 0.5 Cl 6 films (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0).It reveals that the XRD patterns mostly match that of the standard file of Cs 2 AgInCl 6 (COD number 1546186, a = b = c = 10.469Å) and Cs 2 NaInCl 6 (COD number 4003575, a = b = c = 10.5141Å).Moreover, the XRD plot indicates that the films are in cubic Fm-3 m DP structure.At 14.58°, it corresponded to the (111) peak of Cs 2 NaInCl 6 , which manifested that Na + cations were successfully incorporated into the product, Na + cations partially replaced Ag + cations at the B + sites of the LFHDPs structure.It can be seen the intensity of (111) peak almost continuously increases as x increases from 0 to 1, indicating an increase in the incorporation of Na + .The intensity of the (111) diffraction peaks is related to the Na/Ag composition through the dispersion factors of Na, Ag, and In atoms 29 .It is common to find the disorder that lies in A 2 BB'O 6 double perovskite materials, which creates antisite defects.The appearance of the (111) peak is fundamentally attributed to the ordered arrangement of B(I) and B(III) in the Cs 2 B(I)B(III)X 6 perovskites 29 .Therefore, it indicates that no additional disordering caused by Na, Ag alloying in Cs 2 Ag 1-x Na x In 0.5 Bi 0.5 Cl 6 films.From a structural perspective of Cs 2 AgInCl 6 and Cs 2 NaInCl 6 , they are both LFHDPs and have a low lattice mismatch (0.3%) 29 , which creates favorable conditions for Na alloying.We also used the above-mentioned experimental methods to make Cs 2 AgInCl 6 and Cs 2 NaInCl 6 samples in order to figure out the component of Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 miscellaneous peaks.
Compared with standard XRD data, Supplementary Fig. 1a shows that they fitted well at main peaks respectively, which manifests that the miscellaneous peaks did not originate from the parent Cs 2 AgInCl 6 and Cs 2 NaInCl 6 .They may be mainly produced by impurities caused by Na + and Bi 3+ cations doping.Subsequently, we analyzed the main composition of the spurious peaks in the XRD plots, as shown in Supplementary Fig. 2a,b.It can be seen that apart from Cs 2 AgInCl 6 and Cs 2 NaInCl 6 diffraction peaks, the other miscellaneous peaks are mainly the peaks of AgCl and InCl.The structural model of Na alloying and Bi-doped Cs 2 AgInCl 6 DP is shown in Fig. 1b.The cubic unit cell framework is constructed by [AgCl 6 ], [NaCl 6 ], [BiCl 6 ] and [InCl 6 ] octahedra 45 .Figure 1c is the PL spectra of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films.Compared to undoped Cs 2 AgInCl 6 , they show a broad emission spectrum.It can be obviously seen the improvement of the PL intensity from x = 0 to x = 0.6, and then began to decline from x = 0.6 to x = 1.0.The inset pictures show the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films prepared in the experiment under the 365 nm ultraviolet irradiation.Except for the sample with x = 1.0, all other samples emitted warm-yellow light.Furthermore, the optical properties of the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films were examined.In Fig. 1d we report the optical absorption spectra of these films with increasing Na content.Tauc plots are drawn based on Fig. 1d.It can be seen the six samples present a continuous increase in the bandgap from 2.78 to 3.44 eV.The causes of bandgap variations are explained later.
In order to figure out the luminescence mechanism, the internal structure of Cs 2 AgInCl 6 before and after doping is discussed.Figure 1e depicts the mechanism of parity-forbidden transitions in Cs 2 AgInCl 6 DPs.Inversion symmetry-induced parity-forbidden transitions can explain the phenomenon observed in the optical bandgap of Cs 2 AgInCl 6

36
. Electrons change the parity of their spin and orbit simultaneously during the transition process, which causes energy level splitting between Ag and In, forming even and odd forbidden bands.Transitions from VBM to CBM of carriers was parity-forbidden.Simultaneously, absorption was also forbidden.From VBM-2 to CBM, carriers were parity-allowed, which is about 1.10 eV larger than the bandgap between VBM and CBM 28,42 .After Na ions doping, on the one hand, the parity prohibition transition of Cs 2 AgInCl 6 was broken 46,47 .Radiation recombination in Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 system was allowed due to the electron wave function changing from symmetry to asymmetry 29 .During the forbidden state, the absorption coefficient was influenced but didn't affect the process of photoluminescence 31,36,43,45 .On the other hand, the electronic dimensionality of the system decreased by partially isolating [AgCl 6 ] octahedrons.Besides, trace Bi 3+ cations diminished defects and depressed the non-radiation recombination in Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 system, which further enhanced their PLQYs and PL intensity 29,44,46,48 , as discussed in the later section.Many previous studies have reported that self-trapped excitons (STEs) exsit in semiconductors with localized carriers and soft lattices can exhibit broad emission 29,46,49,50 .STEs luminescence have characteristics such as wide spectrum, low absorption, and large Stokes displacement.After Na doping, STEs were spatially confined by [NaCl 6 ] octahedra, resulting in enhanced orbital overlap of electron and hole 29,46 .In the excited state, STEs were generated through the Jahn-Teller distortion of [AgCl 6 ] octahedrons 29,43,45,46 .In our system, efficient warm-yellow broadband emission was achieved through STEs radiation recombination as the previous literature 29,30,44,51 .Besides, the higher emission intensity of PL compared to Cs 2 AgInCl 6 may be attributed to the trapped emission between states localized in the [BiCl 6 ] and [AgCl 6 ] octahedrons respectively 44 .As discussed earlier in Fig. 1d, the bandgap becomes larger.The change in the bandgap is due to the Na + doping, which affects the positions of VB in the energy band.The increase of Na + ions can increase the spatial overlap between CB and VB, thereby enhancing the oscillation intensity of electronic transitions 44 .However, the PL spectral position does not change significantly.This indicates that the PL does not originate from the emission of band-edge carriers but stems from STEs recombination.The photo holes are located in the Ag + -related internal state above the VB, while the photoelectrons are trapped in the Bi 3+ -state below the CB.The distance between these two states is almost constant, so there is no variation of the PL peak energy 44 .The schematic depiction of the energy levels involved in the photophysics of CsAg 1 − x Na x In 0.5 Bi 0.5 Cl 6 films is shown in Supplementary Fig. 3. Then we measured the PLQYs of the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films.The testing method of PLQY is detailed in the Supplementary information.Supplementary Fig. 4 shows a test plot of one of the Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 films.The inset represents a magnified view of the emission spectrum.From Fig. 1f, when x = 0.6, Cs 2 Ag 0.4 Na 0.6 In 0.5 Bi 0.5 Cl 6 film shows the highest value of PLQY (21.6%).The testing method and diagram for PLQY are included in the Supplementary information.As the Na content continues to increase, conversely, PLQY decreases.This is consistent with the PL results.An important factor may be attributed to the increase of the non-radiation recombination caused by electron-phonon coupling in the system 29,44 .When x = 0, x = 1.0, non-radiation recombination occupied a dominant stage, with fluorescence quenching.To understand the recombination dynamics of the exciton, time-resolved PL measurements are performed as shown in Fig. 1f.All decay curves are fitted well by a biexponential decay function, and the fitting parameters are listed in Supplementary Table 1.The average lifetime (τ aver ) is calculated according to the Eqs (equations) [52][53][54] , where τ 1 refers to the fitted short lifetime, τ 2 refers to the fitted long lifetime.τ 1 and τ 2 may originate from parity-allowable transitions and parity-forbidden transitions respectively 8,9,55 .A 1 and A 2 are the weights of two exponential functions, Φ refers to PLQY, K R, and K NR refers to the radiation transition rate and the non-radiative transition rate, respectively.It reveals that the sample of x = 0 has the biggest value of τ ave , the sample of x = 0.4 has the biggest value of K R , and the sample of x = 0.6 has the biggest value of PLQY.when x = 0.6, the maximum value of radiative recombination proportion (K R /K NR ) is 0.274.Combined Eq. ( 3) in the Supplementary information, the larger the K R /K NR value, the larger the Φ value.Φ is proportional to K R /K NR .The K R /K NR value can demonstrate the relationship between radiative recombination and non-radiative recombination.For example, the sample of x = 0.4, and x = 0.6, the obtained K R /K NR value are 0.272, and 0.274 respectively.Their PLQY are 21.26%, and 21.6% respectively, which is the relatively high PLQY value among the six samples.The enhanced PLQY, nearly twice as high as other samples, demonstrates the increase of Na + incorporation content can raise the radiative recombination proportion.Overall, K R /K NR value increases from 0.116 to 0.274, and then reduces to 0.122.The change values of Φ present a trend of first rising and then falling.It increases from 10.39 to 21.6%, which suggests that Na + doping plays a significant role in enhancing the radiative recombination probability, thus the PLQY is improved, and it then reduces to 10.67%.From this, it can be seen that the Φ value is influenced by the K R /K NR value.The sample of x = 0.6 possesses the highest K R /K NR value.Combined Fig. 1c, TRPL result and PL spectrum of Cs 2 Ag 0.4 Na 0.6 In 0.5 Bi 0.5 Cl 6 both prove that x = 0.6 is the suitable Na + doping ratio.Supplementary Fig. 5is the SEM image of Cs 2 AgInCl 6 .Cs 2 AgInCl 6 tends to grow into homogeneous films with a large number of holes and cracks.In Fig. 2, SEM was used to analyze the morphology evolution of the films at different Na content.Interestingly, it can be seen that the morphology of the films changes from irregular shapes, such as flaky, and strip-shaped to octahedral shapes.The irregular shapes, such as the bulk-shaped or particle-shaped products may achieve through the aggregation of the nanocrystals.This is due to the effect of soft agglomeration of electrostatic attraction or van der Waals forces.Compared with Supplementary Fig. 5, the SEM images of Fig. 2a-f doped with Na and Bi show that it is composed of separated particles with uncertain appearance and size.Starting with the sample with x = 0.4, some obvious octahedral shapes began to appear as circled in the picture.As the value of x increases, the SEM image shows more and more octahedral.Octahedral shaped crystals are usually considered to be more complete and regular crystal forms, and their crystal quality may be higher.They may have lower surface energy and fewer defects compared to other shapes, which may have a positive impact on their optoelectronic performance 30 .However, even though the x = 1.0 film has the most octahedra in the SEM, its luminescence efficiency didn't improve because it does not contain Ag and cannot be alloyed with Na for luminescence.Combined the Fig. 1a, it can be seen that the (220) peak intensity of the sample with x = 0.6 is the highest.It may indicate that the crystal structure of the x = 0.6 sample has a better crystallinity with a more ordered arrangement of atoms in the (220) crystal plane.Next, the energy dispersive X-ray spectroscopy (EDS) was carried out to detect the element distribution of the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films by verifying Cs, Ag, Na, In, Bi, and Cl elements.The corresponding EDS spectra are displayed in Supplementary Fig. 6.According to the element ratios measured by EDS (Supplementary Table 2) and Supplementary Fig. 8 www.nature.com/scientificreports/some potential patterns can be discovered.Intriguingly, before adding Na + , the content of Ag is very low, making it difficult to enter the system.After the addition of Na, it greatly promotes the integration of Ag into the system.With Na incorporation increases, the incorporation amount of Ag tends to promote, as well as Bi, which is favorable for Bi 3+ successfully substituting part of In 3+ .It should be noted that this phenomenon is based on appropriate Na incorporation.For example, when x increases from 0 to 0.2, the percentage of Ag increases from 0.71 to 1.94%, Bi 3+ increases from 3.02 to 6.1%.According to the observations of atomic content and element ratio in Supplementary Fig. 7a and b, we made a conjecture that a certain amount of Na doping into the DP structure can promote Ag and Bi content in the system.Figure 3a-f show the XPS results of the composition of the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films.Cs, Ag, In, Bi, Cl all had two split peaks locating at around 737.88 eV and 723.88 eV, 373.68 eV and 367.78 eV, 452.88 eV and 445.18 eV, 164.68 eV and 159.28 eV, 199.63 eV and 198.12 eV, which were assigned to Cs 3d 3/2 and Cs 3d 5/2 , Ag 3d 3/2 and Ag 3d 5/2 , In 3d 3/2 and In 3d 5/2 , Bi 4f 5/2 and Bi 4f 7/2 , Cl 2p 1/2 , Cl 2p 3/2 electronic levels, respectively.Regarding to Cs, the change in peak position is minimal as shown in Fig. 3a.This indicates that Cs are relatively stable in the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 system regardless of Na + doping.Figure 3c depicts a peak locates at 1071.2 eV corresponding to Na 1 s.Starting from x = 0.4, the peak of Na 1 s is more pronounced, which suggests some Ag + ions were successfully substituted by Na + ions as expected.Similarly, Fig. 3b also confirms this conclusion.When x = 1.0, there is no Ag element in the Cs 2 NaIn 0.5 Bi 0.5 Cl 6 film.So, it's obviously there aren't Ag 3d 3/2 and Ag 3d 5/2 peaks.Interestingly, it was found that In 3d 3/2 and In 3d 5/2 peaks of Cs 2 NaIn 0.5 Bi 0.5 Cl 6 shift towards high binding energy direction compared to other films (shown in Fig. 3d).This may be because more [InCl 6 ] octahedra were formed in the pure Na system.Additionally, Fig. 3f shows Cl 2p 1/2 and Cl 2p 3/2 move towards the direction of high binding energy as Na + doped.The reason for this may be due to excessive Na doping, resulting in an increase in [NaCl 6 ] octahedra.

Bi 3+ doping
Based on PL and PLQY of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films, we set x = 0.6 and change the value of y in the system of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 (y = 0, 0.2, 0.4, 0.6, 0.8, 1.0).The composition of the Cs 2 Ag 0.4 Na 0.6 In 1-y Bi y Cl 6 films was modulated by systematically changing the value of y from 0 to 1.The XRD patterns of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films are shown in Fig. 4a.It demonstrates that these films are still in cubic Fm-3 m DP structure after adding Bi 3+ dopant.We can notice that the (220) diffraction peak gradually offset towards the direction where the value of 2θ decreases, from 23.87 to 23.27°.According to the Bragg equation, when some Bi 3+ ions (1.17 Å) replace the position of In 3+ ions (0.94 Å), the crystal cell parameters will increase, and the d value will also increase.Therefore, the value of θ decreases.The XRD diffraction peak will shift to the left.Figure 4b shows the PL spectra of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films.The PL intensity of the y = 0.2 sample is much greater than other samples.The inset pictures show the luminescence of Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 films under excitation of a 365 nm ultraviolet lamp.Figure 4c is the absorption spectra of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films.Tauc plots demonstrate that the bandgap value continues to decrease as the y value increases.It is shown in the inset image of Fig. 4c.This is in stark contrast with the bandgap phenomenon of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 samples.These bandgap results maintain a range from 2.79 to 3.13 eV, which is in good agreement with previous studies 25,37,43 .
From Fig. 4d, when y = 0.  PL spectra are shown in Fig. 4d.In Supplementary Table 3, K R /K NR increase from y = 0 to y = 0.2.It can be seen that K R /K NR up to19.8 at y = 0.2, which is the highest data among all of the six Cs 2 Ag 1-x Na x In 0.5 Bi 0.5 Cl 6 samples.Its PLQY, 66.38%, is also the highest value in all samples.The extremely high K R /K NR and PLQY manifest that radiation recombination occupies a more important position than non-radiation recombination in Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample.When y = 0.4, the value of K R /K NR suddenly drops to 0.313, then it decreases from y = 0.6 to y = 1.0, from 0.313 to 0.258.Combined Fig. 4b,d, PL and PLQY are associated with non-radiative loss process.These curves also have a trend of first rising and then falling.There lies an interesting phenomenon that the effect of Bi 3+ cations doping on PL and PLQY is more significant than Na + ion doping.Compared Fig. 1f with Fig. 4d, for Na + doping, the PLQY value varies within the range of 10.39-21.6%.While for Bi 3+ doping, PLQY value changes more dramatically.It rapidly increased from 14.37 to 66.38%, and then began to steadily decline, maintaining a value above 20%.Firstly, this is attribute to the appropriate Na + and Bi 3+ doping ratios found in the Cs 2 Ag 1−x Na x In 1 − y Bi y Cl 6 system.Secondly, it is due to the further promotion of the radiative recombination process upon Bi 3+ doping.y = 0.2, y = 0.4, y = 0.6, y = 0.8, and y = 1.0 samples all exhibit higher K R and K R /K NR than the sample of y = 0. Considering their same Na + doping ratio, an obvious difference between y = 0 and other samples is that it does not contain Bi 3+ content.For this reason, it is speculated that radiative recombination ratio can be promoted with Bi 3+ doping.Thridly, the Bi 3+ incorporation is considered to improve crystal quality and promote exciton localization 29 .The K NR value of the sample with y = 0 is more than twice that of the sample with y = 0.2.With Bi 3+ doping, the radiative localization is promoted, defects of the films are passivated and within a certain range, non-radiative recombination loss is suppressed, which further enhances the PLQY 29 .
The PLQY value of the sample with y = 0.2 is more than three times that of other samples.PLQY has a huge decrease from y = 0.4.Two factors may account for the decreased PLQY upon further increasing the Bi content.K R value began to sharply decline from the sample y = 0.4.According to Eq. (1), Fermi's golden rule, K R is proportional to the square of the transition dipole moment.The decrease in transition dipole moment caused by the orbital spatial overlap between electrons and holes for STEs may be the first reason, which affects electron-hole recombination and reduces the probability of electronic transitions 29 .The second reason is the increased nonradiative loss, which can be seen from the gradually increasing K NR value.It may be attributed to the phonon emission from the recombination between some photoexcited electrons and holes.In summary, all data prove that selecting x = 0.6, and y = 0.2 as the Na optimal alloying and Bi doping parameters is suitable for the purpose of high-quality and high-performance thin films.
Figure 5a-f show the SEM images of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films.Figure 5a shows that the Cs 2 Ag 0.4 Na 0.6 InCl 6 film consists of irregular nanoparticles.It could be noticed that the sample of y = 0.2 began to have octahedral shapes, which indicates the doping of Bi 3+ can change the morphology of the films, appearing as octahedral 30 .
(1) K R = Generally, most of these films are mixed in flakes, rods, and octahedra.As the y value increases, the grains gradually grow from flakes to octahedra.The increase in Bi 3+ cations doping is beneficial for the growth of more octahedra in the Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 system.In Fig. 4b, the PL intensity of the y = 0.2 sample is the highest, and there is not much difference in PL intensity between the y = 0.4 sample and y = 0.6 sample, y = 0.8 sample and y = 1.0 sample.In Fig. 5e,f, It can be observed that the crystals has grown.The decrease in PL strength may be due to the introduction of lattice defects or deformation caused by grain growth.In order to understand the effect of Na + and Bi 3+ doping on lattice parameters, the crystal data of (220) crystal plane were analyzed based on the XRD patterns in Figs.1a and 4a, which is shown in Supplementary Tables 4 and 5. Futhermore, the Williamson-Hall analysis based on XRD patterns (as shown in Supplementary Tables 6 and 7) suggested the average grain size was about 64.57nm for y = 0.2 sample.The data in Supplementary Tables 6 and 7 are obtained based on the fitting results in Supplementary Figs. 8 and 9, respectively.EDS spectra and the element proportions of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films are shown in Supplementary Fig. 10 and Supplementary Table 8.According to Supplementary Fig. 11 and Supplementary Table 8, with y increase from 0 to 1, the doping percentage of Bi 3+ continues to increase, from 0 to 10.33%.In general, the ratio of Ag also increases as the incorporation amount of Bi 3+ .This indicates that to a certain extent, an increase in Bi 3+ content will also promote the content of Ag in the system.For Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film, according to Supplementary Fig. 11a, the atomic percentage of Cs, Ag,  www.nature.com/scientificreports/moving towards the low binding energy.Na 1 s peak locate at around 1070.78 eV.For In 3+ , there is an evident change at y = 0.8 and y = 1.0.This indicates that more Bi 3+ cations have replaced In 3+ cations, thus reducing the binding energy of In 3d 3/2 and In3d 5/2 .Instead, Bi 4f 5/2 , Bi 4f 7/2 move toward the direction of high binding energy from y = 0 to y = 1.0 as shown in Fig. 6e.For Cl 2p 1/2 and Cl 2p 3/2 , the Bi 3+ doping samples had a lower binding energy than that of non-doping sample as observed in Fig. 6f, which may derive from the environment change after Bi 3+ doping.

Stability and the light emitting devices
Then we conducted a cold and hot cycling experiment to test the thermal stability of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample.One cycle refers to the temperature heated from 20 to 100 °C and then cooled from 100 to 20 °C.The Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample was measured at five cycles and the remaining PL intensity of the sample at each cycle point was recorded as shown in Fig. 7a.It can be easily seen that the remaining PL emission intensity of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample decreased as the temperature increased during the heating process and increased during the cooling process.During five cycles, the remaining PL intensity decreased slightly, but the overall change was not significant.When five thermal cycles are finished, the sample still preserve their relative emission intensity of 94.3%. Figure 7b displays a Commission International de I'Eclairage color coordinates (CIE) chart of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample during the thermal stability test.From the picture, we can see that the color coordinates are concentrated in the warm-yellow light region, implying that the thermal cycling process didn't have a significant impact on the luminescent color of the sample.From the correlated color temperature (CCT) chart (Supplementary Fig. 12), it also declares that the sample emitted relatively stable light.Moreover, we conducted thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) analysis of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 slurry to further measure its thermal stability (Supplementary Fig. 13).
The DSC curve indicates our sample began to crystallize around 170 °C and began to decompose around 560 °C, which is consistent with the pattern in previous literature 29,31 .Such high thermal stability gets benefit from the resistance of thermal stress for all-inorganic perovskites compared to inorganic-organic hybrid perovskites.
The TGA curve displays a big mass drop from 22 to 178 °C, which may drive from the evaporation of DMSO in the slurry.Figure 7c displays the normalized PL spectra of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample measured at the temperature range from 153 to 393 K.The contour plot is also shown in Fig. 7d.Interestingly, PL spectra occurs a blueshift phonomenon, which should attribute to the electron-phonon interraction and the crystal lattice thermal expansion 56,57 .Due to the thermally activated nonradiatie recombination, the PL intensity gradually decreased with the increasement of the temperature.
To clarify the internal mechanism of the effect of the temperature on the luminescence intensity of the film, the Arrhenius Eq. ( 2) was used to fit the curve that the normolized PL intensity as a function of temperature for Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample, which is shown in Fig. 7e.
where I(T) and I 0 refer to the PL intensities at temperatures TK and 0 K, respectively.A is the fitting constant, E b is the exciton binding energy, and K b is the Boltzmann constant.The fitting results reveals the E b value is 299.5 meV.With a certain range, the large the value, the more favorable it is for excition survival at RT and even a high temeprtaure [58][59][60] .It is noticed that the PL spectral bandwidth widens with the rising of temperature, which is related to electron-phonon coupling.More specifically, acoustic phonons and longitudinal optical (LO) phonons can affect PL bandwidth.In the high-temperature region, PL broadening is mainly controlled by LO phonons.As the temperature further decreases, there is a crossover between acoustic phonons and LO phonons, thus the interaction with acoustic phonons controls the broadening 61 .Figure 7f depicts the PL spectra, PLQY of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample under the continuously 365 nm UV irradiation in 30 days to evaluate its photostability.After a continuous irradiation of 720 h, 31.9% of the initial PL intensity, 41.2% of the initial PLQY were diminished.
In order to explore the possibility of applying our materials to LED field, we put Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 samples on ultraviolet light chips (365 nm) to fabricate their corresponding LEDs.Supplementary Fig. 14a, b record the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 samples and Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 samples CIE chart respectively.Supplementary Fig. 15a,b record their CCT respectively.Supplementary Table 9 presents the CCT and CRI of the Cs 2 Ag 0.4 Na 0.6 In 1-y Bi y Cl 6 LEDs.It is noticed that the CCT of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 samples locate in a range from 2969 to 5913 K, the CCT of Cs 2 Ag 0.4 Na 0.6 In 1-y Bi y Cl 6 samples locate in a range from 2727 to 3517 K, which manifests the samples prepared through our fabrication approach could adjust the color of LED light within a wide range.Importantly, it will be highly potential for the application in making LEDs.Figure 8a,b show the CCT, color purity, and PL spectra of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED under different driving currents, respectively.For example, under 100 mA, the CIE color coordinate is (0.48, 0.44), CCT is 2732 K, the high color rendering index (CRI) is 85.5.With the increase of the driving current, the CCT and the color purity nearly had no change.However, the increase of the PL intensity was clearly observed.Additionally, CIE color coordinates of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED is shown in Supplementary Fig. 16.

Conclusions
In conclusion, we have demonstrated a facile blade coating avenue, with alkali metal Na + and Bi 3+ doping into Cs 2 AgInCl 6 , via parament design optimization to prepare Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 films.By Na + alloying (60% mol) and Bi 3+ alloying (20% mol), Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film exhibited a broad spectrum warm-yellow PL with a quantum yield of 66.38%.It is found that doping with Na + and Bi 3+ can break parity-forbidden transition, decrease the defect density and promote the radiative recombination, and greatly improve the luminescence performance of the samples.To verify its photostability and thermal stability, the corresponding experiments were conducted.After 720 h continuous ultraviolet irradiation, only 31.9% emission decay.And after five thermal cycles, the PL intensity is only reduced to 5.7% of the initial value.Furthermore, Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film was used to fabricate a LED, which shows the CIE color coordinates of (0.48, 0.44), the CCT of 2732 K, and the CRI of 85.5 under 100 mA, falling in the warm-yellow light region.More work and research should focus on approaches to improve the pinhole and defect of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film.In addition, due to the broadband warm-yellow emission, it will be involved to the full-spectrum lighting devices in our future work.

Synthesis of Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 films
As illustrated in Fig. 9, firstly, CsCl, AgCl and NaCl, InCl 3 and BiCl 3 molar ratio 2:1:1 was put into a mortar.Then fully ground the mixture, adding DMSO solvent to form a slightly sticky paste.Through fully ground the mixture, a facile and low-cost blade coating method was used to transfer the as-prepared slurry to glass substrates and subsequently heated at 150 °C for 10 min on a hot plate for the evaporation of DMSO and crystallization for Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0), (y = 0, y = 0.2, y = 0.4, y = 0.6, y = 0.8, y = 1.0) films.For example, when x = 0.2, y = 0.5, typically, 4 mmol CsCl (0.673 g), 1.6 mmol AgCl (0.229 g), 0.4 mmol NaCl (0.023 g), 1 mmol InCl 3 (0.221 g) and 1 mmol BiCl 3 (0.315 g) were dissolved in 1 mL dimethyl sulfoxide (DMSO) to form the Cs 2 Ag 0.8 Na 0.2 In 0.5 Bi 0.5 Cl 6 slurry.After these steps, the paste was applied to form Cs 2 Ag 0.8 Na 0.2 In 0.5 Bi 0.5 Cl 6 film.Wherein, the speed of the blade coater was set as 5 mm/s.The reason for using DMSO here was that a solvent with strong polarity and high boiling point was critical for the crystallization of LFHDPs, which was further beneficial for the synthesis of high-quality films 30 .Additionally, the chemical mixture that needs to be ground can be better dissolved in polar solvents.Especially for AgCl powder with poor solubility.

Measurement and characterization
PL and PLQY measurements were performed by using the OmniFluo900 fluorescence spectrophotometer (Beijing Zhuolihanguang Instrument Co., Ltd, China).The PLQYs were measured at room temperature (RT) using a xenon lamp as excitation source coupled with an integrating sphere.Time-resolved PL experiments were  conducted using the Edinburgh FLS1000 Instrument (Edinburgh Instruments Limited, UK).The XRD analysis was performed on a D/max 2200PC X-ray diffractometer (Nikkei Electric Co., Ltd, Japan), equipped with a 3 kW CuK α ceramic X-ray tube.The experiment measurement 2 theta range is from 10 to 80°, with a scanning step of 0.04°.XPS measurements were carried out with an ESCALAB 250Xi instrument from the manufacturer (Temo Fisher Scientific, America).Samples were taken using a ZEISS GeminiSEM 300 scanning electron microscope (SEM) equipped with a Schottky Field Electron Emission Gun (Zeiss, Germany).Energy-dispersive spectroscopy (EDS) was used Oxford Xplore to evaluate the elemental ratios.The UV-visible absorption spectra were recorded using a Shimadzu UV-3600i Plus UV-vis absorption spectrophotometer (Shimadzu, Japan).TGA and DSC were measured by a STA 449/DSC 200 Simultaneous Thermal Analyzer (NETZSCH, Germany).

Figure 1 .
Figure 1.Structural characterizations and optical property of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films.(a) XRD patterns of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 at different value of x.(b) Structural model of Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 .(c) PL spectra of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films at increasing x from x = 0 to x = 1 with 0.2 steps.The samples were all excited under 350 nm ultraviolet.The inset picture is the Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films prepared by our experiment procedures under the 365 nm ultraviolet irradiation.(d) Optical absorption spectra of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 films at increasing x from x = 0 to x = 1 with 0.2 steps, and Tauc plots (the inset picture).(e) Fluorescent mechanism of (I) Cs 2 AgInCl 6 films, (II) and after Bi doping, and Na-doping films (f), Time-resolved PL decay curves of Cs 2 Ag 1 − x Na x In 0.5 Bi 0.5 Cl 6 measured at 590 nm, λ exc = 350 nm.PLQY results are shown in the upper right corner.

Figure 4 .Figure 5 .
Figure 4. Structural characterizations and optical property of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films.(a) XRD patterns of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 at different value of y.(b) PL spectra of films at increasing y from y = 0 to y = 1 with 0.2 steps.The inset picture are the samples of y = 0.2, 0.4, 0.6, 0.8, 1.0.They were excited under 350 nm ultraviolet.(c) Optical absorption spectra of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films at increasing y from y = 0 to y = 1 with 0.2 steps, and Tauc plots (the inset picture).(d) Time-resolved PL decay curves of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 measured at 590 nm, λ exc = 350 nm, except for y = 0 sample (λ exc = 310 nm, measured at 540 nm), and PLQY results (the inset picture).

+Figure 7 .
Figure 7. Stability of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film.(a) Heating/cooling PL tests of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample at a temperature of 20-100 °C.(b) CIE chart of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 sample during the thermal stability test.(c) Photoluminescence spectra of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film measured at different temperatures from 153 to 393 K.(d) Contour plot.e, Integrated PL emission intensity of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film as a function of temperature from 153 to 393 K. (f) Photoluminescence spectra of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film measured at fixed interval (5 days) in 30 days, the inset picture is the PLQY of the sample.

Figure 8 .
Figure 8. Stability of Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED.(a) CCT, color purity of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED with different driving currents (from 50 to 400 mA) under 365 nm UV irradiation.The insets show the structure diagram of the LED and a photo of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED.(b) Normalized PL spectra of the Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 LED.

Figure 9 .
Figure 9. Experiment synthesis procedures of Cs 2 Ag 1 − x Na x In 1 − y Bi y Cl 6 films.
2, Cs 2 Ag 0.4 Na 0.6 In 0.8 Bi 0.2 Cl 6 film shows the highest value of PLQY (66.38%).Similarly, time-resolved PL measurements of Cs 2 Ag 0.4 Na 0.6 In 1 − y Bi y Cl 6 films were also carried out.The time-resolved