Experimental and Theoretical Investigation of the Structural and Opto‐electronic Properties of Fe‐Doped Lead‐Free Cs2AgBiCl6 Double Perovskite

Abstract Lead‐free double perovskites have emerged as stable and non‐toxic alternatives to Pb‐halide perovskites. Herein, the synthesis of Fe‐doped Cs2AgBiCl6 lead‐free double perovskites are reported that display blue emission using an antisolvent method. The crystal structure, morphology, optical properties, band structure, and stability of the Fe‐doped double perovskites were investigated systematically. Formation of the Fe‐doped Cs2AgBiCl6 double perovskite is confirmed by X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) analysis. XRD and thermo‐gravimetric analysis (TGA) shows that the Cs2AgBiCl6 double perovskite has high structural and thermal stability, respectively. Field emission scanning electron microscopy (FE‐SEM) analysis revealed the formation of dipyramidal shape Cs2AgBiCl6 crystals. Furthermore, energy‐dispersive X‐ray spectroscopy (EDS) mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements and homogenous incorporation of Fe in Cs2AgBiCl6 double perovskite. The Fe‐doped Cs2AgBiCl6 double perovskite shows a strong absorption at 380 nm. It extends up to 700 nm, suggesting that sub‐band gap states transition may originate from the surface defect of the doped perovskite material. The radiative kinetics of the crystals was studied using the time‐correlated single‐photon counting (TCSPC) technique. Lattice parameters and band gap value of the Fe‐doped Cs2AgBiCl6 double perovskites predicted by the density functional theory (DFT) calculations are confirmed by XRD and UV/Visible spectroscopy analysis. Time‐dependent photo‐response characteristics of the Fe‐doped Cs2AgBiCl6 double perovskite show fast response and recovery time of charge carriers. We believe that the successful incorporation of Fe in lead‐free, environmentally friendly Cs2AgBiCl6 double perovskite can open a new class of doped double perovskites with significant potential optoelectronics devices fabrication and photocatalytic applications.


Introduction
Lead halide A I Pb II X 3 (A = CH 3 NH 3 ,C s; X= Cl, Br,I )p erovskites have been established as efficient materials for photovoltaic (PV) and opto-electronic applicationsw ith certified powerc onversion efficiencies( PCEs) exceeding2 3%. [1][2][3][4][5][6] This hasb een attributed mainly to their outstanding intrinsic high absorption coefficient, suitableb and gap, and high chargec arrierm obility. [7][8][9][10] Despite these wonderful properties and demonstrated high PCEs, the large scale deployment of these materials is limited by the lead contents toxicity andt he intrinsic thermala nd moisture instability. [11] Significant efforts have been made to reduce the toxicity issues and improve the stability of lead halide perovskites. Various strategies have been designed and developed for the replacement of Pb. One such approach is replacing the Pb with the substitution of group IV metals with less toxicity,s uch as Ge 2 + and Sn 2 + . [12][13][14][15][16] Unfortunately,t hese elements' chemical stabilityi sp oor because they undergo facile oxidation from 2 + states to 4 + oxidation states. [17,18] Another promising approach is hetero-valent metal substitution. In this approach, two divalent Pb 2 + ions are replaced with a pair of one trivalent B 3 + and monovalent B + cation, leading to the formation of A 2 B I B III X 6 double perovskite materials, which persist the 3D crystal structure and the charge neutrality of perovskite. [19][20][21][22][23] Furthermore, incorporating another new element into the doublep erovskite structure guides the optical and structural properties depending on chemical compositions. [24] Inorganic double perovskites A 2 B I B III X 6 having variants A = Cs + ,R b + ;B I = Ag + ,N a + ,K + ,L i + ;B III = Bi 3 + ,S b 3 + ,I n 3 + ;X= Cl À ,B r À ,I À show an alternating arrangement of B I and B III ions on the octahedral sites (rock salt). [25][26][27][28][29] Therefore, double perovskitesm aterials are more attractive as they do not contain toxic Pb 2 + ions and have good thermala nd chemical stability and band gapt unability. For example, bismuth halide double perovskites such as Cs 2 AgBiCl 6 and Cs 2 AgBiBr 6 have indirect band gaps of 2.77 eV and 1.95 eV,r espectively. [30][31][32][33][34] Bandgap tunability has also been achieved by doping Mn 2 + ,S b 3 + ,a nd In 3 + into the Cs 2 BiAgBr 6 lattice. [35][36][37][38] For example, Locardi et al. [25] have reportedh igh photoluminescence quantum yield (PLQY) and improved the visible light emission properties for Mn-doped Cs 2 AgInCl 6. This demonstrates that doping is an effective technique to achieve superior opto-electronic properties and reduced defect state density in double perovskites. Both are essential to fabricate highly efficient opto-electronic devices.T here are, however, limited studies dedicatedt ot he comprehensive characterization of the absorption, emission, recombination dynamic processes in double perovskites. In the present study,w er eport the synthesis of ah igh-quality,l eadfree, and low-cost Fe-doped Cs 2 AgBiCl 6 double perovskite via antisolvent method. The absorption, emission, and recombination dynamic properties were compressively characterized using time-resolved photoluminescence spectroscopy (TR-PL). Photoconductivitym easurements under visible light with a standard photoelectrochemical(PEC) cell arrangement, demonstrates that the Fe-Cs 2 AgBiCl 6 samples exhibits high photocurrent. The prepared double perovskite materials not only retain their optical properties,b ut also show good thermala nd electronic stability,w hich are helpful to make devices. This work providesn ovel insightsi nto the structure-property relationships in lead-free double perovskites and offers new strategies for the development of advanced perovskite devices.

Synthesis of Cs 2 AgBiCl 6 and Fe-doped Cs 2 AgBiCl 6 double perovskite
Undoped and Fe-doped Cs 2 AgBiCl 6 double perovskites were synthesized using the antisolvent method. The schematic representation of the synthesis protocol used for the synthesis is shown in Scheme S2 (Supporting Information). For the undoped sample, salts of 0.2 mmol (101.1 mg) CsCl, 0.1 mmol (42.9 mg) AgCl and 0.1 mmol (94.5 mg) BiCl 3 were dissolved in 5mLD MSO to form a precursor solution. After that, under vigorous stirring, 100 mLoft he precursor solution was injected into 5mLi sopropanol. The mixed solution was then centrifuged at 3000 rpm for 3-6 min to remove large crystals. For the doped samples, FeCl 3 was used as the Fe precursor.F or 3% and 6% doping of Fe into Cs 2 AgBiCl 6 double perovskites, 0.006 mm (2.84 mg) and 0.012 mm (5.68 mg) FeCl 3 was taken along with the rest of the precursors, respectively.The protocol for the synthesis of doping was similar as described above.
Characterization of Cs 2 AgBiCl 6 and Fe-doped Cs 2 AgBiCl 6 double perovskite UV/Visible spectra of the undoped and Fe doped Cs 2 AgBiCl 6 double perovskites were recorded using UV-1800 spectrophotometer (SHIMADZU, Japan) in isopropanol. The surface morphology and shape of the Fe doped Cs 2 AgBiCl 6 double perovskite material were investigated under field emission scanning electron microscopy (FE-SEM) using FEI Nova NanoSEM 450, EDS:B ruker XF lash 6130 instrument. Samples were made via drop-casting on the silicon wafer and dried under reduced pressure for 12 h. Transmission electron microscopy (TEM) images were obtained from high-resolution transmission electron microscopy (HR-TEM) TALOS F-200x;a drop of the sample was placed on ac arbon-coated copper grid dried in the dark for the night. The powdered X-ray diffraction (XRD) patterns were measured on XRD, Bruker AXS D8 Advance using Cu Ka radiation within ar ange of 20-808.T hermogravimetric analysis (TGA) was recorded using aT GA-50 Shimadzu under nitrogen atmosphere. The experiment was performed from 30 to 1000 8Cw ith ah eating rate of 10 8Cmin À1 .T ime-resolved photoluminescence (TRPL) spectra were measured using at ime-correlated single-photon counting (TCSPC) system by Edinburgh EPLED-330. X-ray photoelectron spectroscopy (XPS) studies were carried out using Thermo Scientific, K-Alpha + ,U Km achine with ar esolution of 0.1 eV.T he machine can achieve vacuum > 10 À9 torr,a nd we have recorded XPS spectra for the specific element using Al Ka (1486.6 eV) radiation. The XPS signals were obtained after several scans deployed in the acquisition process. The binding energy was corrected for specimen charging through referencing C1 st o 284.6 eV.T he photo-response of the Fe doped Cs 2 AgBiCl 6 double perovskite was performed using ac onventional 3-electrode system.

Density functional theory (DFT) details
The electronic structure calculations were performed using density functional theory (DFT) within periodic boundary conditions as implemented in the Vienna Ab initio Simulation Package (VASP). [39][40][41] The Perdew-Burke-Ernzerh of (PBE) functional [42] was used for geometry optimizations and stability,w hile for electronic structures and optical calculations, the screened hybrid functional HSE06 with 25 %H artree-Fock exchange was used, with the addition of spinorbit effects (HSE06 + SOC). [43] The valence and core electrons interactions were described with the projected augmented wave (PAW) method. [44] A3 3 3 G-centered Monkhorst-Pack [45] k-mesh and a 600 eV plane-wave cut off were used for structure optimization, while atighter k-mesh of 5 5 5w as used for the electronic structure calculations.

Photoelectrochemical (PEC) cell assembly
Scheme S1 in the Supporting Information shows the schematic of the PEC cell employed in the present study.T hree electrodes were placed inside the cell;s ynthesized Fe-doped Cs 2 AgBiCl 6 double perovskite film act as working electrode (WE), whereas platinum foil and saturated calomel electrode (SCE) were used as ac ounter electrode (CE) and ar eference electrode (RE), respectively.0 .2 m Na 2 SO 4 was used as the electrolyte. Potentiostat (Metrohm Auto-lab:P GSTAT302N) and 150 WX enon Lamp (PEC-L01) with illumination intensity of 100 mW cm À2 (AM 1.5) were used to record the I-V characteristics.

X-ray diffraction (XRD) analysis
To perform the synthesis of Fe-dopedC s 2 AgBiCl 6 double perovskite at room temperature, we implemented am odified antisolventm ethod, whicha voids the conventional heating,i njection methods, and inert gas conditions. Scheme S2 (Supporting Information) shows the actual photographs of the Fe-doped Cs 2 AgBiCl 6 double perovskite under ordinary light and blue emission at 365 nm UV light excitation. AC s 2 AgBiCl 6 double perovskite is composed of Cs + ion at the centero fc uboctahedra with alternating [BiCl 6 ] À3 and [AgCl 6 ] À5 octahedral unit, which leads to the formation of a3 Dn etwork. [31] Figure 1a shows the XRD pattern of the Fe-doped Cs 2 AgBiCl 6 double perovskite.T he presenceo fm ultiple diffraction peaks indicates that the Fe-doped double perovskites are polycrystalline. Major diffraction peaks are observed in the XRD pattern at 2q = 23.638,3 3.368,4 1.278,4 7.988,5 4.008,a nd 59.608,w hich correspond to (220), (400), (422), (440), (620) and (444) diffraction planeso fs tandard cubic doublep erovskite structure with a Fm3ms pace group andl attice parameter a1 0.91 .T hese results are consistentw ith previous literature reports, [20,21,27] confirming the formation of Fe doped and undoped Cs 2 AgBiCl 6 double perovskite via the antisolvent method. Ac arefula nalysis of the XRD pattern of the Fe-doped Cs 2 AgBiCl 6 double perovskite reveals as hift of all diffraction peaks towards ah igher diffraction angle and as ubsequent increase in the diffraction intensity with increasing Fe doping concentration. For example, the (400)d iffractionp eak is shiftedb y0 .638 and 0.908 relative to the undoped Cs 2 AgBiCl 6 double perovskite for3%a nd 6% Fe-doping, respectively (see Figure 1b). The shifts in diffraction peak and the increasing intensity indicate that the incorporation of smaller Fe 3 + ion (0.63 )i np lace of larger Bi 3 + ion (1.03 )i nduces contractions in the lattice. The lattice parameter contracted from 10.91 for the undoped to 10.67 and 10.60 for the 3% and 6% Fe-doped Cs 2 AgBiCl 6. We have observed an XRD peak at 46.428 in the 3% Fe-doped Cs 2 AgBiCl 6 samples, whichm ay be due to AgCl. The diffraction intensity of all the Fe-doped samples is significantly high. The full width at half-maximum is narrower,s uggesting that the Fe-doped double perovskites have excellent crystallinity.T he interplanar d-spacings (d) calculated from the diffraction peak positions( 2 q)o ft he different diffraction planes (hkl) for the Fe-doped Cs 2 AgBiCl 6 double perovskite are displayed in Ta ble S1 in the Supporting Information. It shows the clear differencesb etween the structural properties (crystallite size, strain, interplanar distance, and lattice parameters) of the pristine, 3%,a nd 6% doping of Fe into Cs 2 AgBiCl 6 double perovskites ( To investigate the structurals tability of the Fedoped Cs 2 AgBiCl 6 double perovskites,X RD measurements were carried out after six monthso nt he same samples after exposing them to ambient environmental conditions withouta ny encapsulation. Figure 1c shows the XRD pattern of the undoped and Fe-doped Cs 2 AgBiCl 6 samples taken after six months of exposure under normal temperature and pressure. Figure 1d shows the zoomed-in XRD patterns for clear visualization of the (400) diffraction peak. It has been observed that there is no significant change in diffractioni ntensity of all diffraction peaks, asw ell as no significant change or shift in the diffraction angles after six months of ambient exposure of the Fe-doped Cs 2 AgBiCl 6 perovskite. These results suggest that the Fe-doped Cs 2 AgBiCl 6 nanocrystals have excellent stabilitya fter six monthso fe xposure to ambient environmental conditions.

Thermo-gravimetric analysis (TGA)
The thermal and chemical stability of perovskite materials is an important parameter to evaluatet heir ability for device applications.T oi nvestigate the thermals tability of Fe-doped Cs 2 AgBiCl 6 perovskite, TGA was carried out. Te mperature-dependentw eightl oss of the undoped and Fe-doped Cs 2 AgBiCl 6 double perovskites is shown in Figure 2. As seen, the weight loss of the Fe-doped double perovskites is found in two steps. The first step of weightl osss tarts around5 10-715 8Cw ith 22 wt %a nd the second significant weightl oss is observed at about 78 wt %, which mainlyt akes place from 715-1000 8C. It can be seen from the thermo-gram that the Fe-doped Cs 2 AgBiCl 6 perovskite are highly stable until 510 8Ca nd the decomposition starts above 510 8C. This resulti ndicates that Fedoped Cs 2 AgBiCl 6 double perovskites have excellent thermal stability.

X-ray photoelectron spectroscopy (XPS) analysis
Quantitative analysis of the electronic structuresa nd chemical properties of Fe-doped Cs 2 AgBiCl 6 double perovskite has been performed by XPS analysis.F igure 3a illustrates the survey and high-resolution XPS spectra of Fe-doped Cs 2 AgBiCl 6 double perovskite. As seen, the peaks corresponding to cesium  Figure 3b-f are the narrow scan XPS spectra for 3d-Cs, 3d-Ag,4 f-Bi, 2p-Cl and 2p-Fe elements, respectively.I nF igure 3b,t wo strong peaks were observed for the narrow XPS spectra of 3d-Cs. The peak at 728.1 eV is due to 3d 5/2 -Cs and peak at 742.1 eV due to 3d 3/2 -Cs consistent with the standard Cs element. The two peaks were disjoint with an energy value of 14.0 eV.T he 3d-Ag spectrac onsist of two peaks at 371.7 and 377.6 eV associated with Ag 3d 5/2 and Ag 3d 3/2 ,r espectively.T he core-level peaks for Bi are found at the binding energy 163.0 eV due to Bi 4f 7/2 and 168.5 eV due to 4f 5/2 .T he energy separation between these two peaks was measured 5.5 eV, which is ac haracteristic signalf rom the Bi 3 + species (Figure 3d). The 2p-Cl spectra composed of two peaks at 202.0 and 203.7 eV originating from Cl 2p 3/2 and Cl 2p 1/2 ,r espectively ( Figure 3e). Figure 3f shows the 2p-Fe XPS spectra of the Fedoped Cs 2 AgBiCl 6 double perovskite material. The 2p-Fe has two peaks with binding energies 710.78 and 724.10 eV which are related to Fe 2p 3/2 and 2p 1/2 ,r espectively.T he binding energy of all the elements is slightly shifteda th igherv alues than the reported one, demonstrating that the stronger M-Cl interaction in the [BiCl 6 ] 3À and [AgCl 6 ] 5À octahedra and leads to differing the chemical environmenta round Bi, Ag, andC so n Fe-incorporation. The resultsa re analogous to that of earlier reported systems. [46][47][48] These results confirmt he effective doping of Fe in Cs 2 AgBiCl 6 double perovskite material. The XPS spectra of the pristine Cs 2 AgBiCl 6 and 3% Fe-doped double perovskite are provided in Figures S1 and S2 in the Supporting Information.

Electron microscopy analysis
Field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) were used to study surfacem orphology and topography of the Fe-doped Cs 2 AgBiCl 6 double perovskite. The FE-SEM images of the Fe-dopedC s 2 AgBiCl 6 double perovskite at different resolutions are showni nF igure 4a1-a3. It showst he formation of micrometer-sized multifaceted crystals, including several uniform dipyramidalu nits having as ize of 1-2 mm. Ac lose-up picture in the magnified images for one microcrystal (Figure 4c)s hows perfect dipyramidal units with ac ubic crystal system. The fine fac-     Energy-dispersive X-ray spectroscopy (EDS) analysis The compositional analysis of Fe-doped Cs 2 AgBiCl 6 double perovskite was carried out using EnergyD ispersive X-ray Analysis (EDS) technique. At ypical EDS spectrum recordedi nt he binding energy region of 0-10 KeV for the Fe-doped Cs 2 AgBiCl 6 double perovskite is shown in Figure 5a.T he EDS mapping (Figure 5b,5 c, 5d,5 e, and 5f)s hows the overlapping of Cs, Ag, Bi, Cl, andF ee lements, respectively.I ti ndicates that Fe is homogeneously incorporated in the entire Cs 2 AgBiCl 6 double perovskite lattice. EDS pattern of the elements displays peaks at 0.7 and at 6.4 keV,w hich confirms the presence of Fe in the crystal lattice.
Opticalproperties UV/Visible absorption spectroscopy was used to investigate the optical properties and light absorption capability of the Fedoped Cs 2 AgBiCl 6 double perovskite. Figure 6a showst he room-temperature absorption spectrao ft he undoped and Fedoped Cs 2 AgBiCl 6 double perovskite. Fe-doped Cs 2 AgBiCl 6 double perovskite samples reveal as harpa bsorption peak with the first electronic transition peak at 373 nm (3.31 eV) and 380 nm (3.27 eV). The absorption peak also shows al ong tail extendedt o7 00 nm, whichs uggestst hat the transition from sub-bandg ap states may originate from the surface defecto f the Fe-doped Cs 2 AgBiCl 6 double perovskite material. The absorptions pectra of the Fe-doped Cs 2 AgBiCl 6 double perovskite reveal asmall changeinpeak positions and shapes. It is consistent with the point that the 6s!6p transition of the Bi 3 + ions largely directed the first electronic transition of Fe-doped Cs 2 AgBiCl 6 microcrystal. [20,37] The absorption peak suggests negligible perturbations in the electronic structure of Cs 2 AgBiCl 6 when Fe 3 + is incorporated into the lattice.T he UV/ Visible spectra are used to estimate the optical band gap of Fe-doped Cs 2 AgBiCl 6 perovskite. Absorbance and incident photon energy can be directly relatedu sing equation, [49] ahv in which h is Planck's constant, a is the absorption coefficient, n is the frequencyo fl ight, Ci st he proportionality constant, E g is the band gap, and n is integer 1/2 or 2d epending on the materiala nd whether it is ad irect or indirect band gap. The inset of Figure 6a shows the typical Ta uc's plot for Fe-doped Cs 2 AgBiCl 6 double perovskite used for the estimation of the optical band gap. The indirectb and gap of the Fe-doped Cs 2 AgBiCl 6 double perovskite is estimated at 2.45 eV,w hereas the pristine Cs 2 AgBiCl 6 has an indirectb and gap of 2.77 eV. The band gap reduction after Fe-doping may be due to the overlap of the Ag-d/Cl-p orbitala nd the Fe-3d orbital during the construction of the valence band and new VBM shiftingt owards ah igher energy level as revealed by DFT calculations ( Figure 8). As imilar result has been observed in Cu-doped Cs 2 AgInCl 6 ,S b-doped Cs 2 AgBiBr 6 and Ti-doped Cs 2 AgBiBr 6 . [48,50] The decreasei nb and gap suggestst hat the Fe-doped Cs 2 AgBiBr 6 double perovskite materiali samore suitable absorberf or tandem solar devicea pplications. We examined the photophysical properties and determined the Urbach energy (E u )o fF e-doped double perovskitem aterials. It shows structural disorders,e ffects of impurities,a nd electron-phonon interactions in the absorption phenomenon. [51] The Urbach energy is estimated by using, [52] a ¼ a o exp hv E u ð2Þ in which a is the absorption coefficient, and hv is the photon energy.T he Urbach energy is then estimated by plottingl n(a) as af unctiono f( hv)s hown in Figure S3 (Supporting Information). The reciprocal of the slopeo fal inear fit gives the value of Urbach energy.T he Urbach energy for the undoped and Fedoped Cs 2 AgBiCl 6 is found to be 1.2 eV and 1.0 eV,r espectively. The room temperature photoluminescence (PL) spectrumo f the undoped and Fe-doped Cs 2 AgBiCl 6 double perovskites excited at aw avelength of 360 nm is shown in Figure 6b.A sc an be seen, the Fe-doped Cs 2 AgBiCl 6 double perovskite exhibit a major PL peak at 412 nm and at iny shoulder( 419 nm), and these appear as ar esult of band edge emission and sub-band gap trap states emission, respectively.T he major PL peak is red shifted by 9nmf rom the undoped Cs 2 AgBiCl 6 perovskite. It is interesting to notet hat the band gap values obtained using the PL spectrum are slightly highert han those obtained from the UV/Visible spectrum. It can be attributedt ot he band-edge excitonic irradiative luminescence.
To get furtheri nsights into the optical properties of doped Cs 2 AgBiCl 6 double perovskite material, time-resolved photoluminescence (TRPL) analysish as been carried out. Figure 6c showst he time-resolved photoluminescence emission spectra for Fe-doped Cs 2 AgBiCl 6 double perovskite materials in isopropanol using al aser excitation wavelength of 412 nm. The TRPL traces at 412 nm wavelength show an initially fast decay,f ollowed by as low decay with at ail. The PL decay time of the Fe-doped Cs 2 AgBiCl 6 double perovskite is reduced as compared to the undoped Cs 2 AgBiCl 6 doublep erovskite materials. [37] It may be the enhancement in the non-radiative recombination rate. [48] The emission decay traces can be well fitted to bi-exponential function, [53][54][55] At in which t 1 and t 2 are the decay constants for the fast and slow components of the traces, respectively.T he short-lived component (t 1 )a nd al ong-lived component (t 2 )a re estimated at 1.71 ns and 9.41 ns at 412 nm excitation wavelength. The calculated kinetic parameters are displayed in Ta ble 2. The average carrier lifetime for the Fe-doped Cs 2 AgBiCl 6 double perovskite at excitation wavelength 412 nm is in the order of 8.41 ns. It is reported that the biexponential decay corresponds to two different phenomena. First, the short life-time PL component is due to the recombination of initially generated excitons, while the second is the long-life time PL attributed to recombination of excitond uring the contribution of surface states, which act as stable excitons at room temperature. [56] However, more investigation is necessary to get additional insights into the phenomenonofe xciton dynamic.

Photo-response properties
The photoelectrochemical( PCE) experimentation of Fe-doped Cs 2 AgBiCl 6 doublep erovskite was carriedo ut using linear sweep current density-voltage (J-V)t echniqueu nder dark and light visible light irradiation. Figure 7a shows the current density-voltage (J-V)c urves for the Fe-doped Cs 2 AgBiCl 6 double perovskite under dark and visible light illumination. As ignificant increasei nt he negative current under visible light illumination for the Fe-doped Cs 2 AgBiCl 6 double perovskite is observed. Figure 7b shows the time-dependent current response of the Fe-doped Cs 2 AgBiCl 6 doublep erovskite for repeated

Density functional theory (DFT) analysis
The experimental results were corroborated by first-principles density functional theory (DFT) calculations.T he optimized structures of the undoped and Fe-doped Cs 2 AgBiCI 6 materials in the cubic crystal structure (Fm " 3m space group) are shown in Figure 8. The lattice parameters are 10.885 for pristine Cs 2 AgBiCI 6 and1 0.782 for the Fe-doped Cs 2 AgBiCI 6 ,w hich indicate that Fe substitutional doping at Bi site results in contraction of the lattice, owing to the smalleri onic radius of   of Bi by Fe resulted in the introduction of donor states close to the top of the valenceb and. The partial density of states (PDOS) plots (Figure 8e and Figure 8f)s hows that the valence band to be composed mainly of CI-p orbitals, whereas the conductionband is composed moreo fB i-p orbitals.

Conclusions
We have successfully synthesized Fe-doped Cs 2 AgBiCl 6 double perovskite that displays blue emission via an antisolvent method. The formation of Fe-doped Cs 2 AgBiCl 6 double perovskite is confirmed by XRD and XPS analyses. XRD pattern of the Cs 2 AgBiCl 6 perovskite recorded after six months of exposure to ambient environmental conditions shows that the material has high structurals tability. Also, TGA results indicate that both Fe-doped Cs 2 AgBiCl 6 perovskites have high thermal stability( 510 8C). FE-SEM analysisr evealed the formation of dipyramidal in shape Fe-doped Cs 2 AgBiCl 6 double perovskite and EDS mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements, with the Fe homogeneouslyi ncorporated in the entire Cs 2 AgBiCl 6 perovskite lattice. The Fe-doped Cs 2 AgBiCl 6 double perovskite shows as harp absorptionp eak at 380 nm and extendsu pt o7 00 nm suggesting the transition from subband gap states may originate from the surface defect of Fedoped Cs 2 AgBiCl 6 perovskite material. Lattice parameters and band gap values of the Fe-doped Cs 2 AgBiCl 6 double perovskites predicted by the DFT calculations are confirmedb yX RD and UV/Visible spectroscopy analysis. Finally,t ime-dependent photoresponse characteristics for Fe-doped Cs 2 AgBiCl 6 double perovskite show fast response and recovery time of chargecarriers. The displayed hight hermal stabilitya nd excellent response and recovery time of charge carriers in the Fe-doped Cs 2 AgBiCl 6 double perovskite make it as uitablel ead-free solar absorber for various opto-electronics applications, including photocatalysis and photovoltaics.