Highly Efficient White-Light Emission Induced by Carboxylic Acid Dimers in a Layered Hybrid Perovskite

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE14 Jul 2022Highly Efficient White-Light Emission Induced by Carboxylic Acid Dimers in a Layered Hybrid Perovskite Lina Li, Wentao Wu, Dong Li, Chengmin Ji, Shisheng Lin, Maochun Hong and Junhua Luo Lina Li State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Wentao Wu State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Dong Li State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Chengmin Ji State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Shisheng Lin State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Maochun Hong State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 and Junhua Luo *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 School of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022 https://doi.org/10.31635/ccschem.021.202101037 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Broadband white-light emission in metal halides has been intensely explored because of their facile solution processability, structural adjustability, and high color rendering index. However, the most reported quantum yields for white-light emission remain low despite great efforts. Herein, we report a metal-halide layered perovskite, (HOOC4H9NH3)2PbBr4, showing the typical white-light emission with a highly enhanced quantum yield up to 21.2% compared to previously reported noncarboxyl layered hybrid perovskites (0.5–9%). Notably, mechanistic studies reveal that the distinctive carboxylic acid dimers largely increase the structure rigidity and in consequence reduce the nonradiative recombination induced by stretching vibration. To the best of our knowledge, this strategy is important in hybrid perovskites, which is effective and propagable to acquire prominent photoluminescence. This work will shed light on the design of highly emissive white-light materials involving intense intermolecular interaction and promote their potential in displaying application. Download figure Download PowerPoint Introduction Hybrid metal-halide perovskites have recently attracted intense attention in photovoltaic and photoelectric applications.1–6 Notably, the efficiency of CH3NH3PbI3 lab-based solar cells has achieved a new breakthrough above 25.2%.7 Meanwhile, the metal-halide perovskites feature structure flexibility, and multitudinous low-dimensional perovskites have been constructed by employing variable organic cations.8,9 Such low-dimensional perovskites possess the typical quantum confinement effects to generate tunable optical and electronic performances.10–14 Especially, white-light emission has been achieved in low-dimensional metal-halides perovskite deriving from self-trapped excitons (STEs) induced by structure distortion.15–19 White-light emission from a single-component hybrid perovskite is particularly significant, which simplifies device structure and avoids the self-absorption and color instability existing in multicomponent emitters. Furthermore, the merits of moderate thermostability, high color rendering index, and low-temperature solution processability of these low-dimensional hybrid perovskites promote their potential application. Emphatically, high photoluminescence quantum yield (PLQY) is one of the most essential figure of merits because it is indispensable to promising photoluminescent materials. However, most of the PLQYs for the reported white-light emissive hybrid perovskites are still low, such as 0.5% for (N-MEDA)PbBr4, 9% for (EDBE)PbBr4, and 1% for (C6H5C2H4NH3)2PbCl4,20–22 which hampers the practical application and drives the development of highly efficient white-light emissive hybrid perovskites. Aiming at this issue, Zhou et al.23 achieved efficient white-light emission in perovskite crystals, with a significant improvement of PLQY from 12% to 28%, by integrating the emission of Mn2+. Tang's group24 successfully realized the improvement of PLQY via co-doping Na+ and Bi3+ ions into the nonemissive double perovskite Cs2AgInCl6 to generate highly white-light emission from extrinsic STEs. Obviously, these improved PLQYs are attributed to the ion doping, which may involve potential uncertainty. In contrast, exploration of highly white-light emissive metal-halide perovskite based on intrinsic STEs remains scarce. Moreover, the study of intrinsic STEs will provide profound guidance in the future optimization of other desirable performance. Recently, Gautier et al.25 reported that the formation of deformable post-perovskite-type chains with intense confinement effects is efficient to generate intrinsic self-trapped states with an improvement of PLQY. In addition to this approach, structure rigidity induced by intermolecular interaction can readily reduce the nonradiative decay from the vibration of the organic component, which has been employed to assemble efficient and designable emitting materials.26,27 Inspired by this, we synthesized a two-dimensional (2D) hybrid perovskite, (ABA)2PbBr4 ( 1, ABA+ = HOOC4H9NH3+). Hybrid perovskite 1 features layered perovskite structure with distinctive carboxylic acid dimers between the bilayered organic cations.28 Significantly, 1 displays an efficient broadband white-light emission with a highly enhanced PLQY up to 21.2%, which is more than twice the highest reported value, 9%, in 2D hybrid perovskites. Such significant improvement is attributed to the formation of carboxylic acid dimers that could largely increase structure rigidity and, therefore, reduce the nonradiative recombination induced by stretching vibration, which will be an effective strategy to enhance the intrinsic STEs-based photoluminescent properties. Experimental Methods Material synthesis Pb(CH3COO)2·3H2O (1.9 g, 5 mmol) was dissolved in 40% w/w aqueous HBr solution (30.0 mL) by heating to boiling under constant magnetic stirring to give a clear solution. Subsequent addition of HOOC4H9NH2 (1.03 g, 10 mmol) to the hot solution formed a white precipitate, which dissolved under stirring to afford a clear solution. Finally, lamelliform crystals were obtained after cooling the solution to room temperature ( Supporting Information Figure S1). Materials characterization Powder X-ray diffraction (PXRD) was performed on a Rigaku MiniFlex diffractometer (Rigaku Corporation, Tokyo, Japan) at room temperature. The diffraction patterns were collected in the 2θ range of 5–40°. The experimental PXRD patterns match well with the simulated data based on the single-crystal structure, which confirm the pure phase of 1. Thermogravimetric analysis (TGA) was performed on STA 449C (Netsch, Selb, Germany) Jupiter thermal analyzer ranging from room temperature to 600 °C. Infrared (IR) spectroscopy was carried out on a VERTEX 70 (Bruker, Karlsruhe, Germany) IR spectrometer in the range of 4000–500 cm−1. The UV absorptions in the solid state were measured at room temperature on a PE Lambda 900 UV–vis spectrophotometer (PerkinElmer, Waltham, Massachusetts, USA). The fluorescence measurements were performed on an FLS920 spectrometer (Edinburgh Photonics; Edinburgh Instruments, Livingston, United Kingdom): Emission lifetime in the solid state were determined using picosecond pulsed diode lasers of the EPL at 500 nm excitation. The luminescence decay was fitted by a bi-exponential curve in the OriginPro.9.1. The photoluminescent quantum yield was measured by using an integrating sphere on the Edinburgh FLS920 fluorescence spectrometer (Edinburgh Instruments, Livingston, United Kingdom) with the error at ±5%. The image of luminescence was taken under the excitation of the UV lamp at 365 nm. Single-crystal XRD measurement of 1 and (BA)2PbBr4 was performed on a Bruker D8 diffractometer (Bruker, Karlsruhe, Germany) with the Mo Kα radiation at 300 K for comparison. The data were processed by the Crystalclear software package (Bruker, Karlsruhe, Germany). The structures were solved by direct methods and refined by the full-matrix least-squares refinements on F2 using SHELX-97. Results and Discussion Hybrid perovskite 1 was synthesized in a solution of concentrated hydrobromic acid containing γ-aminobutyric acid and lead acetate trihydrate. The consistency of measured PXRD patterns and those simulated based on single-crystal structure confirms the pure phase ( Supporting Information Figure S2). This compound is environmentally stable at ambient conditions and is thermally stable up to 479 K ( Supporting Information Figure S3). Single-crystal XRD reveals that 1 adopts the layered Ruddlesden–Popper type structure.29,30 As shown in Figure 1a, ABA+ are the bilayered cations that separate the inorganic layers just like BA+ in the well-known layered perovskite (BA)2PbBr4 (BA = n-butylammonium).31 Uniquely, different than the Van der Waals' interactions of the bilayered cations in traditional Ruddlesden–Popper perovskite, carboxylic acid dimers are constructed in the bilayered organic cations of 1. As exhibited in Figure 1b, dimers are formed via a pair of O–H⋯O intermolecular bonds. The carboxyl groups, acting as donor and acceptor in this context, create dimers in the R22(8) moieties,32 which are typical for carboxylic acids. Selected bond lengths and angles are provided in Supporting Information Table S1. The formation of such dimers was further confirmed via the IR spectrum of 1 by the characteristic stretching vibration absorption band of C=O (1702 cm−1) in typical dimerized carboxylic acids ( Supporting Information Figure S4). Finally, a three-dimensional (3D) network is constructed via N–H⋯Br hydrogen bonds between the organic cations and inorganic layers ( Supporting Information Figure S5 and Table S1). The distinctively alternating distribution of organic and inorganic elements in layered perovskite can be considered a quantum-well structure. Considering the unique quantum and dielectric confinements in a quantum-well structure, the photoinduced carriers can be intensely confined in the inorganic layer to readily form the exciton.10,33 In addition, benefitting from the strong crystal rigidity induced by carboxylic acid dimers, prominent photoluminescent properties were expected for 1. Figure 1 | Layered perovskite structure of (BA)2PbBr4 (a) and (ABA)2PbBr4 (b). Download figure Download PowerPoint Different from the layered perovskite (BA)2PbBr4, which displays a narrow blue emission originating from radiative transition of free excitons (FEs), 1 presents a bright white-light emission upon 365 nm excitation (Figure 2a). The photoluminescence (PL) spectrum discloses that 1 exhibits a broadband emission that ranges from 390 to 700 nm with a maximum emission peak at 415 nm (Figure 2b). As shown in Figure 2c, the International Commission on Illumination (CIE) 1931 chromaticity coordinates are (0.22, 0.22) for 1 at 295 K, corresponding to the bluish white-light region. Simultaneously, 1 has a high color rendering index reaching 85, satisfying the demand for color-critical lighting devices.34 Notably, the quantum yield of 1 was measured to be 21.2% ( Supporting Information Figure S6), which is significantly higher than those of previously reported 2D hybrid perovskites featuring white-light emission, such as 0.5% for (N-MEDA)PbBr4, 9% for (EDBE)PbBr4, and 1% for (C6H5C2H4NH3)2PbCl4.20–22 What is more, such value is even higher than those of ultrastable, cationic lead-halide layered materials, [Pb2X2]2+[O2C(CH)2CO2] (X = F, Cl, Br; 1.8–11.8%).35 In addition, benefitting from its high environmental stability, the emission of a sample stored for 2 months showed negligible decrease compared with the original emission ( Supporting Information Figure S7). Figure 2 | (a) Photograph of white-light emission for 1 under 365 nm excitation. (b) Optical absorbance and steady-state PL spectra of 1. (c) Chromaticity coordination of 1 under 295, 130, and 77 K respectively. (d) Emissions of excited STEs in 1 upon 360 nm at 295 and 77 K. Download figure Download PowerPoint The mechanism of the white-light emission in 1 is discussed further. As shown in Supporting Information Figure S8, the consistency of emission between different particle sizes discloses that the surface defect is not responsible for this white-light emission. In addition, variable-temperature measurement exhibits that the absolute intensities of both the narrow and broad emission band increase as the temperature decreases, likely due to the reduction of nonradiative recombination ( Supporting Information Figure S9). And at the temperature of 77 K, the emission of 1 also locates in the white-light region with CIE chromaticity coordinates of (0.24, 0.26) as shown in Figure 2c. In 1, the high energy narrowband emission can be reasonably ascribed to the radiative transition of FEs because of the approximate level to the absorption edge (Figure 2b), which is a feature of 2D Pb–Br perovskites. The double peak of high energy emission is probably caused by the self-absorption of the exciton emission, which tends to be present in large crystals.36 The obviously different PL spectra in the short wavelength region between microcrystal and bulk crystal further confirms the existence of self-absorption ( Supporting Information Figure S8). As the temperature decreases, the low energy broadband emission becomes narrower with a red shift (Figure 2d), which can be attributed to STEs resulting from electron–phonon interactions in the distorted structure. The almost identical excitation spectra of emission peaks at 395, 415, 495 nm further confirm the above deductions ( Supporting Information Figure S10). Further structure analyses indicate that instead of the local structural distortions in the metal coordination, distortions arising from inter-octahedral tilting result in the broadband white-light emission. As shown in Figure 3a, the distances of Pb–Br range from 2.967 to 3.033 Å, indicating the PbBr6 octahedra are minimally distorted. In contrast, the Pb–Br–Pb bond angle (142.1°) deviates from the ideal value of 180° by approximately 37.9° (Figure 3b). The Pb–Br–Pb angles can be further separated into the in-plane distortion angle (θin) and the out-of-plane distortion angle (θout), where the plane is defined by three adjacent Pb atoms within the inorganic layer (Figures 3c and 3d). In 1, the θin and θout angles are calculated to be 29.9 and 22.6, respectively. Such values are distinctly larger than those of recently reported broadband white-light emissive 2D hybrid perovskite (CH3CH2NH3)4Pb3Cl10.16 Therein, the out-of-plane component is the key structural parameter for the broad white-light emission testified by the previous work.28 Figure 3 | Structure distortion in 1. (a) PbBr6 unit showing "close-to-ideal" octahedral coordination geometry. (b) Scheme of inter-octahedral tilting in a single Pb–Br layer, and its decomposition into out-of-plane (c) and in-plane (d). Download figure Download PowerPoint In 1, the role of the organic cation on the white-light emission was explored, and the Raman spectroscopy of organic cation and 1 were performed, respectively. As shown in Figure 4a, new modes and changes are observed for ABA+ in 1 compared with those of ABABr, indicating that the ABA+ in the perovskite is in a different conformation because of its strong interaction with the inorganic layer, readily affecting the distortion of the [PbBr4]2− framework.37 Regarding the interaction between the organic element and distorted inorganic layer, the whole recombination process can be deduced as shown in Figure 4b. The photoinduced electron–hole pairs form FEs first, which then transform into STEs due to strong electron–phonon interactions in the deformed lattice. Simultaneously, such generated STEs can thermally escape into FEs. In addition, due to the distinct interactions of organic molecules with inorganic lead-halide networks, nonradiative decay originating from the organic molecules is involved in the whole process, and such nonradiative transfer readily induces the PL quenching in metal-halide perovskites.38 Strikingly, unlike the relatively weak white-light emission in the previously reported metal-halide layered perovskites, prominent white-light emission with high quantum yield of 21.2% is obtained for 1. This performance can be ascribed to the featured carboxylic acid dimers, which are known to form strong intermolecular interaction to generate intense structure rigidity. Such structure rigidity of 1 can be verified by atomic displacements quantified via equivalent isotropic displacement parameter from single-crystal XRD. As shown in Supporting Information Table S2, atoms in 1 exhibit smaller atomic displacements compared with corresponding atoms in (BA)2PbBr4, indicating less activated phonon modes in 1. As a result, the quenching of the molecular vibrations can be largely reduced to give rise to such high quantum yield for 1, which is the highest among the reported layered hybrid perovskites ( Supporting Information Table S3). Similar molecular packing of phenyl rings giving rise to higher structure rigidity to achieve intensely bright PL has been also realized in (PhCH2CH2NH3)2PbBr4.38 This work is important to achieve enhanced quantum yields for white PL in hybrid perovskite by introducing the distinct carboxylic acid dimers. Figure 4 | (a) Raman spectra of the organic cation salts (black) and the formed perovskite (blue), indicating the active mode changes of the organic cation upon perovskite formation. (b) Photoluminescence mechanism in 1. The nonradiative deactivation through the organic molecule is involved. Download figure Download PowerPoint To evaluate its practical applications in optoelectronics, a light-emitting diode (LED) device was fabricated by coating a commercial LED chip (365 nm) with 1. As shown in Figure 5a, the LED device exhibits bright bluish white-light emission with the corresponding CIE chromaticity coordinates of (0.22, 0.21) and a high color rendering index of 87. The emission spectra of a white-light LED device under different electric currents from 50 to 300 mA were performed. As exhibited in Figure 5b, the emission shape basically remains unchanged while the emission intensity enhances along with increases in the electric currents. These results indicate the possibility of using compound 1 for applications in the white-light emissive optoelectronic fields. Figure 5 | (a) PLspectrum of LED device (365 nm) with 1 (inset: the white-light emission of the LED operated at 100 mA). (b) PL spectra of LED device with 1 under different power currents. Download figure Download PowerPoint Conclusion A 2D hybrid perovskite, (ABA)2PbBr4, involving distinctive carboxylic acid dimers was acquired. It shows the typical broadband white-light emission originating from the synergistic effect of FEs and STEs. Significantly, benefitting from the strong structure rigidity induced by carboxylic acid dimers, this compound exhibits a highly enhanced quantum yield up to 21.2%, which reaches the highest value among the 2D layered hybrid perovskites. Such results indicate that intermolecular interaction may be an effective strategy to achieve highly enhanced photoluminescent performance and further promote their potentials in display applications. Supporting Information Supporting Information is available and includes single-crystal XRD data, PXRD patterns, TGA, IR, PL of Figures S1–S10, and Tables S1–S3. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (grant nos. 21971238, 21833010, 21875251, 21975258, and 21921001), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (grant no. ZDBS-LY-SLH024), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB20010200), and Youth Innovation Promotion of CAS. References 1. Lee M. M.; Teuscher J.; Miyasaka T.; Murakami T. N.; Snaith H. J.Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites.Science2012, 338, 643–647. Google Scholar 2. Yang W. S.; Noh J. H.; Kim Y. C.; Ryu S.; Seo J.; Seok S. I.High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange.Science2015, 348, 1234–12347. Google Scholar 3. Sessolo M.; Bolink H. J.Perovskite Solar Cells Join the Major League.Science2015, 350, 917. Google Scholar 4. Tsai H.; Nie W.; Blancon J.-C.; Stoumpos C. 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