Lead-Free Hybrid Indium Perovskites with Highly Efficient and Stable Green Light Emissions

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Lead-Free Hybrid Indium Perovskites with Highly Efficient and Stable Green Light Emissions Chen Sun, Jin-Peng Zang, Yu-Qing Liu, Qian-Qian Zhong, Xin-Xin Xing, Jia-Peng Li, Cheng-Yang Yue and Xiao-Wu Lei Chen Sun School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Jin-Peng Zang School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Yu-Qing Liu School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Qian-Qian Zhong School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Xin-Xin Xing School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Jia-Peng Li School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 , Cheng-Yang Yue *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 and Xiao-Wu Lei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry, Chemical Engineer and Materials, Jining University, Qufu, Shandong, 273155 https://doi.org/10.31635/ccschem.021.202101092 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Lead halide perovskite nanocrystals (PNCs) have emerged as new-generation light-emitting materials with important potentials in optoelectronic applications. However, the toxicity of lead metal and the instabilities of PNCs overwhelmingly hinder their device engineering; thus, it is still urgent to construct new single crystalline lead-free perovskites with high-performance luminescent properties. Herein, we initially designed two novel crystalline hybrid indium chlorides, [DAPEDA]InCl6·Cl·H2O ( 1) and [DPA]3InCl6 ( 2), containing discrete octahedral [InCl6]3− units. Remarkably, bulk crystals of 1 and 2 exhibited strong broadband green light emissions, derived from self-trapped excitons (STE) with high photoluminescence quantum yields (PLQYs) of 40% and 34%, respectively, with benefits from intrinsic quantum confinement and highly localized electrons. More excitedly, the controlled Sb3+-doping strategy significantly boosted the emission efficiency with the highest PLQY of 89.2%. In addition, these indium perovskites possessed ultrahigh structural and spectral stabilities in various extreme environments exceeding typical PNCs. Combined efficient and stable luminescent performances enable these indium perovskites as excellent down-conversion green phosphors to successfully fabricate white light-emitting diodes with high color rendering index of 93. To the best of our knowledge, this work represents the first fabricated green light-emitting hybrid indium perovskites as excellent candidates for lead PNCs. Download figure Download PowerPoint Introduction Three-dimensional (3D) lead perovskite nanocrystals (PNCs) of APbX3 (A = CH3NH3, Cs+, X = Cl, Br, and I) have currently emerged as new-generation light-emitting materials with diversified advantages of adjustable bandgaps, high defect tolerance, tunable and wide light spectra across visible to near-infrared region, narrow emission bandwidth and high photoluminescence quantum yield (PLQY) stemming from quantum confinement of surrounded organic ligands.1–3 Subsequently, various synthetic modulation strategies, including morphological regulation, surface passivation, metal doping, or halogen mixing, have been massively adopted to reduce the defect concentrations, regulate the excited energy states or enhance the emission efficiencies, which optimized or diversified their luminescent properties successfully and expanded the application fields.4–6 Despite the substantial optimization evolutions toward conventional PNCs, the colloidal PNCs remain to be confronted with inevitable particle aggregation along with severe optical instabilities and PLQY degradations, which, overwhelmingly, hinder their further optoelectronic device engineering. Assuredly, 3D perovskite bulk materials were rarely reported as excitonic luminescence species because the large band dispersion of corner-shared 3D network dissociates the excitons to charge carriers readily at room temperature.7 In addition, the inflexible 3D cubic network and the simple chemical composition reduce the feasibility of structural modulation from the molecular level to regulate the luminescent properties. Inspired by the fact that the highly efficient luminescent performance of PNC originates from the quantum confinement effect of nanoscale particle size, another structural design strategy of preserving the quantum confinement has been proposed and confirmed, that is, slicing the 3D corner-shared [PbX3]− network into an zero-dimensional (0D) unit by introducing wide-bandgap organic species as spatial cations.8–10 In these bulk crystalline organic–inorganic hybrid 0D perovskites, the discrete lead halide polyhedrons are embedded periodically in the insulating organic matrix with intrinsic quantum confinement distinguishing from the external effect of PNC. Specifically, the absence of electron interaction between spatially discrete polyhedrons resulted in a flat band structure, which firmly restrained the photoinduced excitons around each light-emitting species and significantly promoted the radiative recombination, finally leading to efficient light emission.11–13 Indeed, the versatile organic species endow the hybrid 0D perovskites with diversified structures, highly tunable emissions, and remarkable PLQYs.14,15 Moreover, the luminescence performances of 0D perovskites are almost independent of the crystal morphologies and sizes; hence, they provide higher spectral stabilities than 3D PNCs. Nevertheless, the environmental and biological toxicity of the lead metal remains an obstacle for the broad application of lead perovskites. Therefore, it is highly desirable and significant to construct new single crystalline lead-free 0D perovskites with highly efficient, versatile, and stable light-emitting performances. Under this condition, substantial lead-free metal halide perovskites based on tin, germanium, antimony, bismuth, manganese, indium, and others, were intensively explored as candidates of solid-state light-emitting materials in the large family of metal halide perovskites.16–19 Compared with instabilities of Sn2+ and Ge2+ halides, nearly fixed emission spectral ranges of Sb3+ and Mn2+-based perovskites, as well as low efficiencies of Bi3+ phases, In3+-based perovskites, are more desirable for down-converter phosphors, benefitting from the tunable light emissions, nontoxicity, and oxidation resistance abilities.20–23 In general, 0D indium perovskites display broadband light emissions with substantial potential in solid-state white light-emitting diodes (WLEDs).24–26 Previous works have unveiled that the broadband light emission originated from the intrinsic self-trapped excitons (STE) due to an excited-state structural distortion of [InX6]3− octahedrons or the strong electron-quantum coupling effect in the soft 0D crystal lattice.24–26 Regrettably, all these 0D indium perovskites exhibit strongly Stokes-shifted emission spectra with maximum peaks of over 600 nm as orange to red light emitters due to the large excited-state structural distortion level of isolated octahedrons. Up to date, none of the 0D indium-based perovskites is able to display higher-energy light emission with a maximum peak wavelength shorter than 600 nm. Besides, the emission efficiencies of 0D indium perovskites still need further improvement, considering the passable PLQYs, such as (C4H14N2)2In2Br10 (3%), Cs2InBr5·H2O (33%), and phenylmethylammine, C6H5CH2NH3 (PMA)3InBr6 (35%).24–26 In view of these shortcomings, we focus on this domain to enable the application of 0D indium perovskites for higher-energy light emitters to realize their full-color display by fine-tuning the molecular structure. Herein, we designed two new single crystalline lead-free 0D indium perovskites of [DAPEDA]InCl6·Cl·H2O (DAPEDA = 1,2-bis(3-aminopropylamino)ethane) and [DPA]3InCl6 (DPA = dipropylamine) successfully via facile wet-chemistry or a mechanical grinding solid-phase approach. Unlike the previous near red light-emitting phases, these 0D indium perovskites were highlighted with higher-energy broadband green light emissions (510–520 nm) with promising PLQYs of 34.01% and 40.40%, respectively, able to be optimized up to the highest value of 89.2% through rational Sb3+-doping strategy. The compelling luminescent performances and ultra-high stabilities enabled down-conversion green phosphors to fabricate WLEDs showcasing their potential in backlit applications. So far as we know, this work exemplifies the first green light-emitting hybrid indium perovskites with successful enrichment of the color gamut and device application field. Experimental Section Materials All the precursor materials of indium chloride tetrahydrate (InCl3·4H2O, 98%, Aladdin, Shanghai, China), DAPEDA (C8H22N4, 97%Aladdin, Shanghai, China), DPA (C6H15N, 99%, Aladdin, Shanghai, China), hydrochloric acid (HCl, 38%, SCRC, Shanghai, China), acetonitrile (99.8%, SCRC, Shanghai, China), and acetone (99.5%, SCRC, Shanghai, China) were commercially purchased and directly used without further purification. Synthesis of [DAPEDA]Cl4 Briefly, DAPEDA was added into HCl (38%) solution affording white depositions, which were filtrated and washed with ethanol several times. The final products were collected after drying in a vacuum oven. Synthesis of [DPA]Cl Briefly, DPA was added into HCl (38%) solution to form a clear solution and condensed via rotary evaporation with the formation of a white powder. The powder was washed with ethanol several times and collected after drying in a vacuum oven. Solution growth of [DAPEDA]InCl6·Cl·H2O (1) and [email protected][DAPEDA]InCl6·Cl·H2O single crystals InCl3·4H2O (0.5 mmol, 0.1466 g) and DAPEDA (0.6 mmol, 0.1046 g) were mixed and dissolved in 10 mL HCl (38%) with constant magnetic stirring and heating at about 120 °C leading to a clear solution. After slowly cooling to room temperature (25 °C), colorless layer-shaped crystals were obtained and subsequently determined as [DAPEDA]InCl6·Cl·H2O ( 1) by single-crystal X-ray diffraction (SCXRD). The crystals were then washed with ethanol, dried, and stored in a vacuum (65% yield based on In). Elemental analysis was performed and calculated: C, 17.18%; H, 5.05%; N, 10.02%. Found: C, 17.39%; H, 4.99%; N, 10.14%. Also, [email protected][DAPEDA]InCl6·Cl·H2O was prepared via a similar method to compound 1 except with the addition of SbCl3 (1 mmol, 0.2281 g). The colorless crystals of [email protected][DAPEDA]InCl6·Cl·H2O were obtained with a yield of 61%. Mechanical solid-state grinding synthesis of microscale [DPA]3InCl6 (2) A mixture of InCl3 (0.5 mmol, 0.1106 g) and [DPA]Cl (1.5 mmol, 0.2050 g) was ground together using a mortar and pestle. A white microscale powder of [DPA]3SbCl6 was obtained and used for further studies. Solution growth of [DPA]3InCl6 (2) and [email protected][DPA]3InCl6 single crystals A mixture of InCl3·4H2O (0.5 mmol, 0.1466 g) and DPA (1 mmol, 0.1012 g) was dissolved in a mixed solution of HCl (38%) (4 mL), acetonitrile (4 mL), and acetone (4 mL), and the mixture was stirred for 20 min until forming a clear solution. Large-size colorless block crystals were obtained by slow evaporation of the solution at room temperature after a week. The crystals were washed with ethanol and dried in a vacuum oven with a yield of 52% based on InCl3·4H2O. Elemental analysis performed and calculated: C, 34.09%; H, 7.63%; N, 6.63%; Found: C, 35.18%; H, 7.59%; N, 6.65%. The synthetic procedure of [email protected][DPA]3InCl6 crystal was similar to that of [DPA]3InCl6 with the addition of SbCl3 (0.8 mmol, 0.1825 g) in the mixed solution. The colorless crystals of [email protected][DPA]3InCl6 were obtained in a yield of 48%. X-ray crystallography The single-crystal data of compounds 1 and 2 were collected on the Bruker Apex II CCD diffractometer (Bruker Corporation, Billerica, Massachusetts, U.S.) with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. The crystal structures were solved by direct method and refined based on F2 using the SHELXTL-2018 program. All the non-hydrogen atoms were refined with anisotropic thermal parameters, and hydrogen atoms of organic molecules were positioned geometrically and refined isotropically. Structural refinement parameters of compounds 1-2 are summarized in Supporting Information Table S3, and important bond parameters are listed in Supporting Information Tables S4–S7. Powder X-ray diffraction The powder X-ray diffraction (PXRD) analysis was performed on Bruker D8 ADVANCE powder X-ray diffractometer (Bruker Corporation, Billerica, Massachusetts, U.S.) equipped with copper Kα radiation at a voltage of 40 kV and a current of 40 mA. The diffraction patterns were scanned over an angular range of 5–60° (2θ) with a step size of 5°/min at room temperature. A simulated powder pattern was calculated employing the Mercury 3.8 software ( https://mercury1.software.informer.com/3.8/) using the crystallographic file from a SCXRD experiment. Scanning electron microscopy measurements The morphology observation and elemental mapping were conducted by scanning electron microscopy (SEM; Zeiss Merlin Compact, Carl Zeiss AG, Oberkochen, Germany). Inductively coupled plasma measurements Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed on Agilent 5110 (Agilent Technologies Inc. California, U.S.). Thermogravimetric analysis Thermogravimetric analysis (TGA) was carried out using a Q500 SDT system (TA Instruments, Pennsylvania, U.S.). The sample was heated from room temperature to 800 °C at a rate of 5 °C·min−1 under an argon flux of 40 mL·min−1. Absorption spectrum measurements The solid-state UV–vis absorption optical spectrum for powder sample was collected using a PE Lambda 900 UV/Vis spectrophotometer (Perkin-Elmer Optoelectronics, Massachusetts, U.S.) at room temperature in the wavelength range of 200–800 nm with BaSO4 serving as a reference standard. Raman measurements The Raman measurement was performed on the powder sample in the range of 0–3500 cm−1 using Horiba Scientific LabRam HR Evolution (HORIBA Scientific, Paris, France) under 532 nm excitation wavelength. Photoluminescence property characterization The photoluminescence (PL) spectrum was performed on an Edinburgh FLS980 fluorescence spectrometer (Edinburgh Instruments, Livingston, Scotland, UK). PLQY was achieved by incorporating an integrating sphere into the FLS980 spectrofluorometer, and Xe-lamp (450 W) was utilized to generate a monochromatic light source with an excitation power of 600 μW/cm2. PLQY was calculated based on the equation: ηQE = IS/(ER − ES), where IS represents the luminescence emission spectrum of the sample, ER is the spectrum of the excitation light from the empty integrated sphere (without the sample), and ES is the excitation spectrum for exciting the sample. The time-resolved decay data were carried out using the PL spectrofluorometer with a picosecond pulsed diode laser. The average lifetime was obtained by exponential fitting. Theoretical band calculation The single-crystal data were directly used to calculate the electronic band structure using the CASTEP software ( http://www.castep.org/CASTEP/GettingCASTEP). The total energies were calculated using the density functional theory (DFT) using Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation. The interactions between the ionic cores and the electrons were described by the norm-conserving pseudopotential. Hence, the C-2s22p2, N-2s22p3, H-1s1, In-5s25p2, and Cl-3s2sp5 orbitals were adopted as valence electrons. The number of plane wave included in the basis sets were determined by cut-off energy of 340 eV. Other calculating parameters and convergence criteria were set by the default values of the CASTEP code. Fabrication of WLED device The WLED was fabricated by integrating the Sb-doped compounds 1 or 2, the commercial red phosphor of K2SiF6∶Mn4+, and the blue phosphor BaMgAl10O17∶Eu2+ on an UV LED chip (365 nm). The phosphors were thoroughly mixed with epoxy resin, and the mixture obtained was coated on the LED chip. The optoelectronic properties, including electroluminescence (EL) spectrum, correlated color temperature (CCT), color rendering index (CRI), and Commission Internationale de l'Eclairage (CIE) color coordinates of the WLED were collected using an integrating sphere spectroradiometer system (PCE-2000B; EVERFINE Corporation, Hangzhou, China). Results and Discussion Colorless bulk single crystals of [DAPEDA]InCl6·Cl·H2O ( 1) and [DPA]3InCl6 ( 2) were prepared using facile wet-chemistry methods (Figure 1a). Specifically, we performed gradual evaporation of saturated HCl solution containing precursor InCl3·4H2O and corresponding organic salts. After the successful growth of single crystals and determination of crystal structure, we attempted the solid-state reaction route considering that solvent-free mechanochemical reaction is more economically applicable with multiple advantages, including low pollution, high production yield, simplified assembly technology, short reaction time, and reduced energy consumption, compared with the conventional wet-chemistry synthetic approach. As is known, conventional wet-chemistry synthetic techniques inescapably involve the massive use of hazardous organic solvents in assembly, separation, and purification processes, which are harmful to the environment and cause pollution to water bodies and soil. Based on these considerations, prepared the title compounds using a mechanochemical grinding solid-state reaction route. As indicated in Figure 1d, compound 2 was obtained through the mechanochemical grinding solid-state reaction from precursor materials within a fast time of about 60 s and 100% yield; the PXRD pattern was consistent with the simulated result from the crystal structure ( Supporting Information Figure S1). Compared with the demand precise controlling technique, tedious reaction process, and low-yields of wet-chemistry preparation avenues, this solvent-free mechanochemical grinding reaction method was highly cost-effective, simple, and time-saving. The facile and versatile preparation methods used provided sufficient assembly feasibility for optoelectronic device engineering. Figure 1 | Normal characterizations of compounds 1 and 2: (a) The solution evaporation synthetic route, (b) structure of [InCl6]3− unit, and (c) packing crystal structure of 1 along the b-axis. (d) The mechanochemical grinding solid-state synthetic route, (e) structure of [InCl6]3− unit, and (f) the crystal structure of compound 2 along the b-axis. Download figure Download PowerPoint The structures of compounds 1 and 2 were determined by using SCXRD. Compound 1 crystallizes in the monoclinic space group (P21/c), and the structure contains isolated [InCl6]3− octahedrons surrounded by [DAPEDA]4+ cations with dissociative Cl− ions as charge-balanced units (Figure 1b). The long-chain-like [DAPEDA]4+ cations feature closely paralleled stacking via weak molecular forces to form insulating organic walls to separate the isolated [InCl6]3− octahedrons with abundant N–H···Cl bonding interactions (H···Cl: 2.7–3.6 Å) (Figure 1c). Compound 2 belongs to the trigonal system (R3c), and the structure is composed of [InCl6]3− octahedrons and [DPA]+ cations (Figure 1e). The [DPA]+ cations feature a chiral column-like arrangement along the c-axis, resulting in cabined quasi-one-dimensional tunnels among them to enclose the parallel [InCl6]3− units (Figure 1f). In both compounds 1 and 2, all the [InCl6]3− octahedrons displayed parallel arrangement with closest In···In distance of 7.336 and 9.163 Å, respectively, circumventing the electron interactions between neighboring octahedrons.27 Therefore, these compounds belong to perfect 0D hybrid perovskites with [InCl6]3− units periodically embedding in the wide bandgap organic matrix to exhibit the intrinsic photophysical properties of individual species. The high purities of bulk crystals for compounds 1 and 2 are confirmed by the coincidence of measured PXRD patterns with simulated results from single-crystal data without any visible impurities ( Supporting Information Figure S2). The chemical compositions of the single crystals were checked by semi-quantitative energy-dispersive X-ray spectroscopy (EDX), and the average In:Cl mole ratios of 1:7.4 and 1:6.3 for compounds 1 and 2 are closing to the refined results obtained from the SCXRD, respectively ( Supporting Information Figures S3 and S4). At the same time, elemental mappings by SEM confirmed that all the C, N, and were on the surface of bulk crystals 1 and characterizations indicated high with crystal structures and chemical compositions for the bulk crystals 1 and Supporting Information Figure the solid-state UV–vis absorption the results that both compounds 1 and 2 display absorption with almost cut-off at nm and a strong excitonic absorption peak around nm. In addition, the broad absorption in the range of and nm were to the electron within the organic ( Supporting Information Figure Under nm UV both the bulk crystals of compounds 1 and 2 displayed green light by the and further the PL properties of compounds 1 and 2 using PL excitation and emission as well as time-resolved PL decay the photophysical parameters are summarized in Table The PL excitation spectra of both compounds broad band characterizations with two peaks at nm for compound and nm by two weak excitation peaks of and 340 nm for compound 2 ( Supporting Information Figure The excitation spectra were almost consistent with the absorption spectra that the PL emissions originated from the intrinsic bulk crystals and from surface the nm UV light compound 1 exhibited a broadband green light emission at nm with a at of nm and a large of nm (Figure compound 2 a single broadband green emission nm) at nm with of nm and of nm (Figure The perfect broadband emission spectra the single optical in compounds 1 and The PL emission and were to of lead such as and hybrid ( Supporting Information Table The corresponding coordinates were calculated to be and for compounds 1 and 2, respectively (Figure The PL emission of compounds 1 and 2 be by the distortion of octahedrons. In the unit of compound the bond and bond in ranges of and with a distortion of = and = respectively, the unit of compound 2 to a with of without any distortion and 1e). The octahedral distortion level resulted in PL emission and of compound such has been confirmed in reported hybrid metal The large indicated the which is of for and luminescent Figure 2 | The PL characterizations of compounds 1 and 2 at and of bulk crystals of 1 and 2 under light and UV light and excitation and emission spectra of 1 and 2 (e) and g) PL decay of 1 and 2 Download figure Download PowerPoint Table 1 | of for All the in at 1 340 2 is the PLQY and is the average PL and are the radiation and To the green light emission efficiencies of compounds 1 and 2, we performed the excitation PLQYs at The results the PLQY values along with the of excitation in which the highest values are corresponding to the highest excitation wavelength ( Supporting Information Figure the highest PLQYs of and 34.01% for compounds 1 and 2 far of hybrid lead halides, such as and on ( Supporting Information Figure and Table is that these PLQYs at room temperature are in the of compared with high-performance hybrid such as and To the best of our knowledge, compounds 1 and 2 the first green light-emitting hybrid indium perovskites with promising PLQYs, a new structural to efficient green the fact that almost excitation and emission spectra from the isolated [InCl6]3− optical the PLQYs of compounds 1 and 2 from the of environments around [InCl6]3− species. has been reported that the of optical species an in the modulation of between radiative and the of optical species the PLQY by the for the hybrid metal of single-crystal structures more abundant weak hydrogen bonding interactions between [InCl6]3− and organic species in compound which the crystal and the molecular ( Supporting Information Figure and Tables and we performed a analysis of the weak interactions between and species by surface calculation to the interactions within the crystal structure by using As in Supporting Information Figure the a more color the interactions in compound 1 than As a the crystal and in compound 1 reduced the thermal of [InCl6]3− species and the further leading to higher To the photophysical of broadband green light emissions, we performed time-resolved PL the maximum emission at compound the PL decay was by a with a and and a and which an average lifetime of (Figure The PL decay of compound 2 a single exponential with an average lifetime