coatings Engineering of Halide Cation in All-Inorganic Perovskite with Full-Color Luminescence

: All-inorganic halide perovskites are emerging as a class of superstar semiconductors with excellent optoelectronic properties and show great potential for a broad range of applications in solar cells, lighting diodes, X-ray imaging, and photodetectors. Tremendous research about their device performance has been performed since 2015. In this study, we synthesized the all-inorganic perovskite by the hot-injection method and particularly investigated their crystal structural and photoluminescence properties. By halide anion engineering, the all-inorganic perovskites showed a high-symmetry cubic phase. They also showed a tunable optical bandgap, and almost the full color luminescence was achieved (434 to 624 nm). These basic optoelectronic properties could give a guide for further development of this area.


Introduction
Recently, halide perovskites have been extensively studied due to their fascinating optoelectronic properties, such as tunable bandgaps, high absorption range and coefficients (up to 10 5 cm −1 ), long minority carrier lifetimes, low-cost fabrication process, flexible device, etc. [1,2]. A wide range of optoelectronic applications have been developed with them as the active layer, e.g., perovskite solar cells (PSCs), perovskite light-emitting diodes (PeLEDs), perovskite photodetectors, and perovskite sensors. For instance, Miyasaka et al. first discovered PSCs in 2009 [3], and the power conversion efficiency (PCE) has nowadays rapidly boosted to 25.5%, already surpassing that of commercial silicon solar cells [4]. Dan et al. reported perovskite organic light-emitting diodes (PeOLEDs) in 2014 for the first time [5], and the maximum external quantum efficiency (EQE) has today increased to over 20%, which is comparable to that of conventional OLEDs or quantum dot light-emitting diodes (QLEDs) [6]. In general, halide perovskites have a chemical formula of ABX 3 , where A represent a large monovalent cation, such as organic methylammonium (MA: CH 3 NH 3 ) and inorganic cesium (Cs). B is a bivalent metal cation, such as Pb and Sn. The X anion, as the name implies, is a halogen anion: Cl, Br, I, and mixed Cl/Br/I systems. With the inorganic Cs cation at A position, CsPbX 3 perovskite shows a significantly improve stability, which has already proven in the solid-state dye sensitized solar cells and previous PSC study [7,8].
One of the most fascinating properties of CsPbX 3 perovskite materials is their tunable bandgaps, which have allowed them to achieve astonishing breakthrough in a variety of optoelectronic applications. For instance, the ideal bandgap for PSCs is 1.34 eV [9][10][11]; the ideal bandgap for tandem photovoltaic devices (top sub-cell) is 1.70 eV [12]; and the ideal bandgaps for PeLED are 1.80, 2.30, and 2.70 eV for red, green, and blue emission, respectively. It has been proposed that in the CsPbX 3 system, the Cs cation acts to fulfill charge neutrality within the lattice and only slightly affects the electronic states [13]. The valance band of CsPbX 3 is predominately formed by mixing the halide np 6 orbitals (n is the principal quantum number, Cl: n = 3, Br: n = 4, and I: n = 5) and ns 2 orbitals from the lead (n = 5), while the conduction band mainly arises from antibonding mixing of the lead and the halide np 6 orbital; thus, the X cation plays the main role in the determination of the bandgap [14,15]. Therefore, with the mixed halogen cation, a wide broad bandgap CsPbX 3 could be achieved. Although the CsPbX 3 perovskites were reported as early as 1958 [16], investigation of their application in optoelectronic devices had not been conducted until 2015 [17,18] [18]. Motivated by this, in this study, we systematically varied the halogen anion and synthesized CsPbCl 3 , CsPbCl 2.5 Br 0.5 , CsPbCl 2 Br, CsPbCl 1.5 Br 1.5 , CsPbClBr 2 , CsPbCl 0.5 Br 2.5 , CsPbBr 3 , CsPbBr 2.5 I 0.5 , CsPbBr 2 I, CsPbBr 1.5 I 1.5 , CsPbBrI 2 , CsPbBr 0.5 I 2.5 , and CsPbI 3 by the hot-injection method, and their crystal structural and photoluminescence properties were systematically studied.

Materials and Methods
Preparation of Cs-oleate: Oleic acid (OA, 90%), oleylamine (OAm, 90%), octadecene (ODE, 99%), Cs 2 CO 3 (98%), lead iodide (99%), cesium bromide (>98%), cesium chloride (98%), and cesium iodide (99%) were purchased from Sigma-Aldrich (Shanghai, China). Cs 2 CO 3 (0.4 g, 1.23 mmol), 15 mL ODE, and 1.25 mL OA were loaded into a 50 mL threenecked flask and dried for 0.5 h at 100 • C with stirring and then heated under N 2 to 150 • C until all Cs 2 CO 3 reacted with OA (transparent solution). Because Cs-oleate precipitated out of ODE at below 100 • C, it had to be annealed to 150 • C before CsPbX 3 perovskite synthesis. Synthesis of CsPbX 3 : All-inorganic perovskites were synthesized by a hot-injection method according to Protesescu's work [17]. In this study, 0.36 mmol PbX 2 (X = Cl, Br, and I), OAm (1.0 mL), oleylamine (1.0 mL), and octadecene (10 mL) were added to a three-necked round-bottom flask (25 mL). The resulting mixture was heated to 100 • C with stirring and maintained for 0.5 h. At this time, the water residue was removed by nitrogen purging and vacuum aspiration, and the mixture was heated to 160 • C until the PbX 2 precursor was completely dissolved. Then, the hot cesium oleate precursor solution (1 mL) was quickly injected into the above reaction mixture. After 5 s reaction, the flask was quickly transferred to the ice bath, and the obtained CsPbX 3 was kept by centrifugation at 10,000 rpm for 10 min and stored in cyclohexane (4 mL) before further use.
Synthesis of CsPbX 3 (X = Cl, Br, or I) nanocrystals containing mixed halogens: a mmol of PbX 2 (X = Cl, Br, or I) and (0.36 − a) mmol of another PbX 2 (X = Cl, Br, or I), OA (1.0 mL), OAm (1.0 mL), and ODE (10 mL) were added to a three-necked roundbottom flask (50 mL). Then, it was annealed to 100 • C with stirring and maintained for 30 min. Then, it was heated to 160 • C until the PbX 2 precursor was completely dissolved. The hot cesium OA solution (1 mL) was quickly injected into the above solution, and after 5 s reaction, the flask was quickly transferred to the ice bath. The obtained CsPbX 3 perovskite was kept by centrifugation at 10,000 rpm for 5 min and stored in cyclohexane prior to further use. The added amount were as follows: CsPbCl 3 (PbCl 2 : 100.12 mg), CsPbCl 2.5 Br 0. 5  Billerica, MA, USA). First, 100 µL of the sample was taken onto the slide and dried under nitrogen atmosphere to make cyclohexane volatilize, which was then tested and analyzed.
Photoluminescence characterization: The experiment was carried out using a fluorescence spectrometer model Fluorolog-3. First, 100 µL of the sample was taken and added to a quartz cuvette. Then, 4 mL of cyclohexane was added, and the luminescence wavelengths under 365 nm excitation were tested and analyzed.

Results
We began by studying how the X cation influenced the optoelectronic properties based on CsPbX 3 with monohalogen (Cl, Br, and I). Figure 1a exhibits their XRD patterns monitored in the 2θ range of 10 • to 45 • , which matched well with the literature. With hotinjection preparation methods, the halogen cation engineering did not change the crystal structure. All CsPbX 3 showed a high-symmetry cubic phase with the space group pm3m due to the high reaction temperature in the preparation process and also the contribution from the surface energy of the nanocrystal [17,19,20]. The optical properties of these CsPbX 3 were studied by stable photoluminescence, as shown in Figure 1b. We found that an increase in the cation size resulted in a reduction in the optical bandgap, which was 2.90 eV for CsPbCl 3 , 2.40 eV for CsPbBr 3 , and 2.00 eV for CsPbI 3 . The difference in optical properties originated from the difference in the ionic radius of halide ions with six-fold coordination, which was 1.81, 1.96, and 2.20 Å for Cl − , Br − , and I − , respectively. The CsPbCl 3 perovskite exhibited the PL peak at 434 nm with full width at half maximum (FWHM) = 16 nm (blue color emission); the CsPbBr 3 perovskite showed PL peak at 507 nm and FWHM = 23 nm (green color emission); and the CsPbI 3 perovskite showed PL peak at 625 nm with FWHM = 37 nm (red color emission).
Coatings 2021, 11, x FOR PEER REVIEW 3 of 6 X-ray diffraction (XRD) pattern characterization: The experiment was performed by XRD diffraction device (Bruker D8 Advanced diffractometer with Cu Kα radiation, Bruker, Billerica, MA, USA). First, 100 μL of the sample was taken onto the slide and dried under nitrogen atmosphere to make cyclohexane volatilize, which was then tested and analyzed.
Photoluminescence characterization: The experiment was carried out using a fluorescence spectrometer model Fluorolog-3. First, 100 μL of the sample was taken and added to a quartz cuvette. Then, 4 mL of cyclohexane was added, and the luminescence wavelengths under 365 nm excitation were tested and analyzed.

Results
We began by studying how the X cation influenced the optoelectronic properties based on CsPbX3 with monohalogen (Cl, Br, and I). Figure 1a exhibits their XRD patterns monitored in the 2θ range of 10° to 45°, which matched well with the literature. With hotinjection preparation methods, the halogen cation engineering did not change the crystal structure. All CsPbX3 showed a high-symmetry cubic phase with the space group pm3 ̅ m due to the high reaction temperature in the preparation process and also the contribution from the surface energy of the nanocrystal [17,19,20]. CsPbCl3 showed three dominant diffraction peaks: properties of these CsPbX3 were studied by stable photoluminescence, as shown in Figure  1b. We found that an increase in the cation size resulted in a reduction in the optical bandgap, which was 2.90 eV for CsPbCl3, 2.40 eV for CsPbBr3, and 2.00 eV for CsPbI3. The difference in optical properties originated from the difference in the ionic radius of halide ions with six-fold coordination, which was 1.81, 1.96, and 2.20 Å for Cl − , Br − , and I − , respectively. The CsPbCl3 perovskite exhibited the PL peak at 434 nm with full width at half maximum (FWHM) = 16 nm (blue color emission); the CsPbBr3 perovskite showed PL peak at 507 nm and FWHM = 23 nm (green color emission); and the CsPbI3 perovskite showed PL peak at 625 nm with FWHM = 37 nm (red color emission). We then studied the influence of mixed halogen cation based on CsPbClyBr3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). From the XRD patterns (Figure 2a), the diffraction peaks exhibited  We then studied the influence of mixed halogen cation based on CsPbCl y Br 3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). From the XRD patterns (Figure 2a), the diffraction peaks exhibited successive shift, which was linearly dependent on the halide composition. The composition modulation of halide anions does not affect the cationic sublattice; meanwhile, the cubic structure is still maintained [21]. Moreover, it means the precursor solutions should be uniform during the film fabrication procedure. From the PL spectra shown in Figure 2b, we also found the successively red shift emission peaks, which were 434 nm for CsPbCl 3 , 446 nm for CsPbCl 2.5 Br 0.5 , 453 nm for CsPbCl 2 Br, 471 nm for CsPbCl 1.5 Br 1.5 , 490 nm for CsPbClBr 2 , 499 nm for CsPbCl 0.5 Br 2.5 , and 507 nm for CsPbBr 3 perovskites, respectively. The redshift of the PL peaks indicated that the Cl anion was gradually replaced by Br to form CsPbCl y Br 3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3).
Coatings 2021, 11, x FOR PEER REVIEW 4 of 6 successive shift, which was linearly dependent on the halide composition. The composition modulation of halide anions does not affect the cationic sublattice; meanwhile, the cubic structure is still maintained [21]. Moreover, it means the precursor solutions should be uniform during the film fabrication procedure. From the PL spectra shown in Figure  2b, we also found the successively red shift emission peaks, which were 434 nm for CsP-bCl3, 446 nm for CsPbCl2.5Br0.5, 453 nm for CsPbCl2Br, 471 nm for CsPbCl1.5Br1.5, 490 nm for CsPbClBr2, 499 nm for CsPbCl0.5Br2.5, and 507 nm for CsPbBr3 perovskites, respectively. The redshift of the PL peaks indicated that the Cl anion was gradually replaced by Br to form CsPbClyBr3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). We further studied the influence of mixed halogen cation based on CsPbBryI3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). They exhibited similar trends as observed in CsPb(Cl/Br)3 perovskites (as shown in Figure 3). The XRD peaks showed a successive shift, which was linearly dependent on the composition. The successive redshift of the photoluminescence peak was also observed, indicating that the Br anion was gradually mixed with I anion and formed CsPbBryI3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3) perovskite.   We further studied the influence of mixed halogen cation based on CsPbBr y I 3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). They exhibited similar trends as observed in CsPb(Cl/Br) 3 perovskites (as shown in Figure 3). The XRD peaks showed a successive shift, which was linearly dependent on the composition. The successive redshift of the photoluminescence peak was also observed, indicating that the Br anion was gradually mixed with I anion and formed CsPbBr y I 3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3) perovskite.
Coatings 2021, 11, x FOR PEER REVIEW 4 of 6 successive shift, which was linearly dependent on the halide composition. The composition modulation of halide anions does not affect the cationic sublattice; meanwhile, the cubic structure is still maintained [21]. Moreover, it means the precursor solutions should be uniform during the film fabrication procedure. From the PL spectra shown in Figure  2b, we also found the successively red shift emission peaks, which were 434 nm for CsP-bCl3, 446 nm for CsPbCl2.5Br0.5, 453 nm for CsPbCl2Br, 471 nm for CsPbCl1.5Br1.5, 490 nm for CsPbClBr2, 499 nm for CsPbCl0.5Br2.5, and 507 nm for CsPbBr3 perovskites, respectively. The redshift of the PL peaks indicated that the Cl anion was gradually replaced by Br to form CsPbClyBr3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). We further studied the influence of mixed halogen cation based on CsPbBryI3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3). They exhibited similar trends as observed in CsPb(Cl/Br)3 perovskites (as shown in Figure 3). The XRD peaks showed a successive shift, which was linearly dependent on the composition. The successive redshift of the photoluminescence peak was also observed, indicating that the Br anion was gradually mixed with I anion and formed CsPbBryI3−y (y = 0, 0.5, 1, 1.5, 2, 2.5, and 3) perovskite.

Conclusions
In summary, we synthesized and characterized CsPbX 3 by adjusting the halogen anions (Cl, Br, I, mixed Cl/Br, and mixed Br/I). With controllable hot-injection methods, these all-inorganic perovskites exhibited a high-symmetry cubic phase. Meanwhile, by halogen anion engineering, they showed a successive tunable optical bandgap with luminescence covering the entire visible range (434-625 nm). Moreover, the successive bandgap Coatings 2021, 11, 330 5 of 6 energy of CsPbX 3 were found to have a linear relationship with the halogen content. We are convinced that the present findings will be helpful for the development of promising related application, such as all-inorganic perovskite solar cells, perovskite light-emitting diodes, and perovskite photodetectors.