Self-trapped Exciton Emission Research Articles

Boosting triplet self-trapped exciton emission in Te(IV)-doped Cs2SnCl6 perovskite variants

Perovskite variants have attracted wide interest because of the lead-free nature and strong self-trapped exciton (STE) emission. Divalent Sn(II) in CsSnX3 perovskites is easily oxidized to tetravalent Sn(IV), and the resulted Cs2SnCl6 vacancy-ordered perovskite variant exhibits poor photoluminescence property although it has a direct band gap. Controllable doping is an effective strategy to regulate the optical properties of Cs2SnX6. Herein, combining the first principles calculation and spectral analysis, we attempted to understand the luminescence mechanism of Te4+-doped Cs2SnCl6 lead-free perovskite variants. The chemical potential and defect formation energy are calculated to confirm theoretically the feasible substitutability of tetravalent Te4+ ions in Cs2SnCl6 lattices for the Sn-site. Through analysis of the absorption, emission/excitation, and time-resolved photoluminescence (PL) spectroscopy, the intense green-yellow emission in Te4+:Cs2SnCl6 was considered to originate from the triplet Te(IV) ion 3P1→1S0 STE recombination. Temperature-dependent PL spectra demonstrated the strong electron-phonon coupling that inducing an evident lattice distortion to produce STEs. We further calculated the electronic band structure and molecular orbital levels to reveal the underlying photophysical process. These results will shed light on the doping modulated luminescence properties in stable lead-free Cs2MX6 vacancy-ordered perovskite variants and be helpful to understand the optical properties and physical processes of doped perovskite variants.

Defect-Related Broadband Emission in Two-Dimensional Lead Bromide Perovskite Microsheets.

Low-dimensional hybrid lead halide perovskites (LHPs) with broadband emission (BE) have been developed as promising candidates for single-source white-light-emitting diodes. However, the underlying origin of such BE is poorly understood. Herein, dual-emissive [NH3(CH2)8NH3]PbBr4 perovskite microsheets (PMSs) with good dispersibility are successfully prepared. Besides the general narrowband emission (NE) originating from free excitons, BE (∼522 nm) is generated under a Br-poor condition, which is not observed in the single-crystal sample. Unlike self-trapped exciton emission, the BE observed in PMSs is experimentally determined to be related to bromide vacancies (VBr), thereby exhibiting quasisaturation under high excitation intensity. Femtosecond transient absorption spectroscopy first shows that the trapping time of the photogenerated electrons by acceptor-like VBr- is ∼15 ps, slower than that by surface defects (<1 ps). This study provides new insight into the underlying mechanism of BE and an effective approach to manipulating the optical properties of 2D perovskites.

Self-trapped excitons in two-dimensional perovskites.

With strong electron-phonon coupling, the self-trapped excitons are usually formed in materials, which leads to the local lattice distortion and localized excitons. The self-trapping strongly depends on the dimensionality of the materials. In the three-dimensional case, there is a potential barrier for self-trapping, whereas no such barrier is present for quasi-one-dimensional systems. Two-dimensional (2D) systems are marginal cases with a much lower potential barrier or nonexistent potential barrier for the self-trapping, leading to the easier formation of self-trapped states. Self-trapped excitons emission exhibits a broadband emission with a large Stokes shift below the bandgap. 2D perovskites are a class of layered structure material with unique optical properties and would find potential promising optoelectronic. In particular, self-trapped excitons are present in 2D perovskites and can significantly influence the optical and electrical properties of 2D perovskites due to the soft characteristic and strong electron-phonon interaction. Here, we summarized the luminescence characteristics, origins, and characterizations of self-trapped excitons in 2D perovskites and finally gave an introduction to their applications in optoelectronics.

A Highly Efficient and Stable Blue-Emitting Cs5 Cu3 Cl6 I2 with a 1D Chain Structure.

In the field of photonics, alkali copper(I) halides attract considerable attention as lead-free emitters. The intrinsic quantum confinement effects originating from low-dimensional electronic structure lead to high photoluminescence quantum yields (PLQYs). Among them, Cs3 Cu2 I5 is the most promising candidate, satisfying both high PLQY and air stability. In this study, a strategy to explore a new material meeting these requirements through the use of the mixed-anions of I- and Cl- is proposed. The expectation is maintained that the large difference in ionic radii between them likely results in the formation of a novel compound. Consequently, Cs5 Cu3 Cl6 I2 with a 1D zigzag chain structure is discovered. This material exhibits blue emission (≈462nm) with a near-unity quantum yield of 95%. An electronic structure calculation reveals that the localized nature of the valence band maximum is crucial in obtaining efficient self-trapped exciton emission. Moreover, the iodine-bridged 1D connectivity significantly enhances the chemical stability of Cs5 Cu3 Cl6 I2 , compared with the pure chloride phase. The present findings provide a new perspective for developing air-stable alkali copper(I) halides with highly efficient luminescence.

Tunable Dual‐Emission in Monodispersed Sb3+/Mn2+ Codoped Cs2NaInCl6 Perovskite Nanocrystals through an Energy Transfer Process

Double halide perovskites are a class of promising semiconductors applied in photocatalysis, photovoltaic devices, and emitters to replace lead halide perovskites, owing to their nontoxicity and chemical stability. However, most double perovskites always exhibit low photoluminescence quantum efficiency (PLQE) due to the indirect bandgap structure or parity-forbidden transition problem, limiting their further applications. Herein, the self-trapped excitons emission of Cs2 NaInCl6 by Sb-doping, showing a blue emission with high PLQE of 84%, is improved. Further, Sb/Mn codoped Cs2 NaInCl6 nanocrystals are successfully synthesized by the hot-injection method, showing a tunable dual-emission covering the white-light spectrum. The studies of PL properties and dynamics reveal that an energy transfer process can occur between the self-trapped excitons and dopants (Mn2+ ). The work provides a new perspective to design novel lead-free double perovskites for realizing a unique white-light emission.

Highly Efficient Self-Trapped Exciton Emission of a (MA)4Cu2Br6 Single Crystal

Recently, low-dimensional organic-inorganic lead halide perovskites have attracted much attention due to the outstanding tunable broadband emissions, while the toxicity of lead hinders their further applications in the photoelectric field. Here, we reported a novel lead-free Cu(I) based organic-inorganic perovskite-related material of (MA)4Cu2Br6 single crystal with a zero-dimensional (0D) clusters, which is a unique Cu2Br64- corner-shared tetrahedron dimer structure consisting of two connected tetrahedrons. The single crystal displays a bright broadband green emission with a high photoluminescence with quantum yield up to 93%, a large Stokes shift and very long (microsecond) PL lifetime, resulting from self-trapped excitons emission. The direct band gap characteristic of (MA)4Cu2Br6 was proved by density functional theory calculation, and its band gap was determined by experiments to be about 3.87 eV. In the temperature range of 98-258 K, the PL intensity increases gradually with the rising temperatures due to the deep-trapping out of strong electro-phonon coupling, while the PL goes down when the temperature increases over 258 K due to the phonon scattering. Deserve to mention, this new material has high chemical and light stability, in contrasting to the lead perovskite.

Broadband Photoluminescence in 2D Organic-Inorganic Hybrid Perovskites: (C7H18N2)PbBr4 and (C9H22N2)PbBr4.

Organic-inorganic hybrid perovskites have aroused intense research interest because of their excellent physical performance and potential for use in optoelectronic field. Herein, we report two new 2D hybrid lead bromides, (C7H18N2)PbBr4 [C7H18N2 is 1,7-diaminoheptane] and (C9H22N2)PbBr4 [C9H22N2 is 1,9-diaminononane], both of which possess ⟨100⟩-oriented inorganic layers consisting of corner-sharing octahedra. The optical bandgaps are experimentally determined to be 2.76 eV for (C7H18N2)PbBr4 and 2.78 eV for (C9H22N2)PbBr4. Upon 390 nm excitation, (C7H18N2)PbBr4 exhibits white-light emission centered at 600 nm, and (C9H22N2)PbBr4 exhibits red-light emission centered at 620 nm. These broad photoluminescent spectra originate from the synergistic emission of free excitons (FEs) and self-trapped excitons (STEs). This work provides a strategy for realizing single-component white-light emission and efficient red-light emission in two-dimensional perovskites, demonstrating the vast application prospects of 2D perovskites in photoelectric devices.

Manganese-Doped, Lead-Free Double Perovskite Nanocrystals for Bright Orange-Red Emission.

Lead-free halide perovskite nanocrystals (NCs) have recently attracted attention due to their nontoxicity and stability as alternatives to lead-based perovskite NCs. Here, we report undoped and manganese-doped all-inorganic, lead-free double perovskite (DP) NCs: Cs2NaInxBi1–xCl6 (0 < x < 1) and Cs2NaInxBi1–xCl6:Mn (0 ≤ x ≤ 1) NCs. Undoped NCs exhibit blue emission. Through doping Mn2+ ions into Cs2NaInxBi1–xCl6 NCs, we can avoid self-trapped exciton emission and realize bright orange red emission of Mn2+ dopants with the highest photoluminescence quantum efficiency of 44.6%. The photoluminescence (PL) is tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. Interestingly, the PL emission of Mn-doped NCs shows a red shift from the bulk size to the nanoscale. The Mn-doped NCs show good stability in air. In addition, we further prove the process of dark self-trapped state-assisted Mn2+ emission in DP NCs by ultrafast transient absorption techniques.

Stable Yellow Light-Emitting Devices Based on Ternary Copper Halides with Broadband Emissive Self-Trapped Excitons.

Great successes have been achieved in developing perovskite light-emitting devices (LEDs) with blue, green, red, and near-infrared emissions. However, as key optoelectronic devices, yellow-colored perovskite LEDs remain challenging, mainly due to the inevitable halide separation in mixed halide perovskites under high bias, causing undesired color change of devices. In addition to this color-missing problem, the intrinsic toxicity and poor stability of conventional lead-halide perovskites also restrict their practical applications. We herein report the fabrication of stable yellow LEDs based on a ternary copper halide CsCu2I3, addressing the color instability and toxicity issues facing current perovskite yellow LED's compromise. Joint experiment-theory characterizations indicate that the yellow electroluminescence originates from the broadband emission of self-trapped excitons centered at 550 nm as well as the comparable and reasonably low carrier effective masses favorable for charge transport. With a maximum luminance of 47.5 cd/m2 and an external quantum efficiency of 0.17%, the fabricated yellow LEDs exhibit a long half-lifetime of 5.2 h at 25 °C and still function properly at 60 °C with a half-lifetime of 2.2 h, which benefits from the superior resistance of CsCu2I3 to heat, moisture, and oxidation in ambient environmental conditions. The results obtained promise the copper halides with broadband light emission as an environment-friendly and stable yellow emitter for the LEDs compatible with practical applications.

Influence of the ratio of rare earth oxyorthosilicate R2SiO5 (R = La, Y) hosts on the structure and optical properties of co-doped Pr3+ /Dy3+ phosphors

Praseodymium (Pr3+) and dysprosium (Dy3+) co-doped mixed lanthanum/yttrium oxyorthosilicates (La2-xYxSiO5, x = 0.5, 1 and 2) phosphors were synthesized using urea-assisted combustion method. The X-ray diffraction (XRD) patterns showed the pure monoclinic phases of La2SiO5 (x = 0) and Y2SiO5 (x = 2) and mixed phases of La2-xYxSiO5. The diffuse reflectance spectra of the phosphors showed a broad absorption peak with a maximum around 376 nm assigned to O2−→ Pr4+ charge transfer band. The band gaps of the Pr3+/Dy3+ co-doped La2-xYxSiO5 increased from 2.7 to 5.1 eV as the value of x increased from 0 to 2. The luminescent properties were measured when the samples were excited using three different sources, namely 450 W Xenon lamp, 325 He-Cd laser and electron beam. The photoluminescence (PL) emission spectra recorded using monochromatized Xenon lamp showed tunable emission for different excitation wavelengths. For example, when the PL spectra were recorded under 294 nm excitation, using Xenon lamp, emissions from self-trapped excitons (STE) in SiO2, f→f transitions of Dy3+ and Pr3+ were observed. The emission monitored under 349 nm excitation showed the STE peak and the four emission lines of Dy3+. However, when the phosphors were excited using a 325 nm He-Cd laser, the STE emission and four Dy3+ lines were observed in all the samples, but the four manifolds emission from the 1D2→3H4 transition of Pr3+ were only observed for x = 1.5 and 2. Furthermore, the cathodoluminescence (CL) spectra showed the STE band, four emission lines of Dy3+ and four manifolds emission from the 1D2→3H4 transition of Pr3+. The tunable colours of the phosphors were shown in International Commission on Illumination (CIE) coordinate diagram.

Highly Efficient Blue Emission from Self-Trapped Excitons in Stable Sb3+-Doped Cs2NaInCl6 Double Perovskites.

Highly efficient blue-emitting three-dimensional (3D) lead-free halide perovskites with excellent stability have attracted worldwide attention. Herein, a doping route was adopted to incorporate Sb3+ ions into the Cs2NaInCl6 for decorating the electronic band structure. Due to the moderate electron-phonon coupling, the Sb3+-doped Cs2NaInCl6 double perovskites showed a narrow and relatively unusual blue emission of self-trapped excitons (STEs). Density functional theory (DFT) calculation indicated that the doped Sb3+ ions could break the parity-forbidden transition rule and modulate the density of state (DOS) population effectively to boost the PLQY of STEs drastically. The optimized Sb3+:Cs2NaInCl6 exhibited a PLQY of up to 75.89% and excellent stability under the consecutive illumination of 365 nm UV light for 1000 h. This kind of highly efficient lead-free Sb3+-doped Cs2NaInCl6 double perovskites may overcome the bottlenecks of severe toxicity and insufficient stability and therefore have an extensive application in the scarce blue photonic and optoelectronic fields.

Judd-Ofelt and quantum cutting analysis of Eu3+ or Pr3+ doped β-NaGdF4 nanorods obtained by rapid coprecipitation method

Pure hexagonal β-NaGdF4 and Eu3+ or Pr3+-doped β-NaGdF4 nanocrystals in shape of nanorods of ∼140 nm of length have been synthesized using a simple and rapid coprecipitation method. XRD analysis evidenced pure single phases exhibiting characteristic luminescence of Eu3+ or Pr3+ ions when excited under UV or blue radiations. Samples gave rise to intense orange-red emission and quite white emission in the case of respectively Eu3+ or Pr3+ doped samples. Optical properties are discussed in the frame of Judd-Ofelt theory and considering that an energy transfer occurs between active ions. Notably it was found that the quantum-cutting by a two-step energy transfer from Gd3+ to Eu3+ can improve the red emission of Eu3+ ions under near-VUV excitation. Such quantum-cutting mechanism has been also considered in the case of Pr3+ doped β-NaGdF4. However, the efficiency of both processes in comparison with literature suggest that only one part of the energy in the excited states within Gd3+ can be transferred to Eu3+ for its red emission. In the case of Pr3+ ions, the energy is more probably released through the self-trapped exciton emission certainly due to the lack of resonant VUV excitation.

Anisotropy of Excitons in Two-Dimensional Perovskite Crystals.

Two-dimensional (2D) perovskites show great potential for optoelectronic applications due to their bandgap tunability, extremely large excition binding energy, and large crystal anisotropy compared with their three-dimensional counterparts. To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in 2D perovskites. Here, we investigate exciton anisotropy within the crystallographic plane and cross plane of (C4H9NH3)2PbI4 2D perovskite crystals by polarization-resolved photoluminescence, reflection, and photoconductivity studies. We observe a polarization-dependent emission evolution and an enhanced self-trapped exciton emission with an oblique incident excitation from the cross plane. Furthermore, the anisotropy of excitons in (C4H9NH3)2PbI4 2D perovskite crystals is identified by polarization-resolved photoluminescence and photoconductivity measurement, and a completely opposite polarization-dependent behavior was observed for free excitons and self-trapped excitons. We attribute this different anisotropy to the existence of out-of-plane excitons and different optical selection rule for free excitons and self-trapped excitons. Our findings will shed light on designing and improving the performance of exciton-based optoelectronic devices in 2D perovskites.

Mechanism for tunable broadband white photoluminescence of one-dimensional (C4N2H14)2Pb1-xMnxBr4 perovskite microcrystals

This work demonstrated that tunable photoluminescence of the low-dimensional hybrid halide perovskites via the doping strategy is feasible, and hence paves the way towards the investigation and application of high-performance single photoluminescence perovskite material with broadband white light emission. This work reported the first synthesis of one-dimensional (1D) (C4N2H14)2Pb1-xMnxBr4 perovskite microcrystals (MCs) with tunable broadband white-light emission and long-lived STE (self-trapped exciton) emission. The overall photoluminescence mechanism under the regulation of defect engineering of 1D (C4N2H14)2Pb1-xMnxBr4 MCs was formulated for the first time. Photoluminescence mechanism of these MCs involves competition of FE (free exciton) and STE and 4T1 states. This competitive mechanism further promotes photoluminescence tunability ranging from blue-white to white to orange-red for Mn-doped MCs. Moreover, time-resolved emission spectra show the changes of lifetime of these MCs at different emission wavelength. These perovskite MCs have a long-lived STE emission, which indicates that the more effective exciton combinations are occurring in them. For comparation, the Mn-doping strategy can regulate structural defect of these MCs, which promotes or suppresses transitions of FE→STE, FE→4T1, STE→FE and further tunes photoluminescence. This work paves the way towards the investigation and development of optoelectronic application of perovskite MCs material.

Pressure-Induced Remarkable Enhancement of Self-Trapped Exciton Emission in One-Dimensional CsCu2I3 with Tetrahedral Units.

Self-trapped exciton (STE) emissions derived from inorganic octahedral units make metal halide perovskites promising photoluminescence materials for light-emitting applications. However, there is still little understanding of the intrinsic STE emissions in metal halide perovskites or derivatives with nonoctahedral units. In this work, via high pressure compression, remarkable STE emission enhancement is, for the first time, realized in one-dimensional CsCu2I3 crystals with {CuCl4} tetrahedral units. The intertetrahedral distortionis believed to induce the slight emission enhancement of the ambient phase under initial compression. Notably, the obvious structural distortions of both inter- and intratetrahedra are responsible for the significant emission enhancement of the high pressure phase. This work not only sheds light on the structure-optical property relationships of tetrahedron-based halide complexes, but also may provide guidance for the design and fabrication of highly luminescent metal halides.

Emission Trend of Multiple Self-Trapped Excitons in Luminescent 1D Copper Halides

Low-dimensional metal halides are promising luminescent materials with efficient self-trapped exciton (STE) emission at room temperature; however, the understanding of optical behaviors and trends ...

Facile solution synthesis of Bi3+/Yb3+ ions co-doped Cs2Na0.6Ag0.4InCl6 double perovskites with near-infrared emission.

Double perovskites have attracted wide attention due to their low toxicity and good thermal stability. Doping with active elements is a promising method to endow perovskites with excellent optical, electrical or magnetic properties. Herein, a facile aqueous solution method is developed to synthesize Bi3+-Yb3+ co-doped Cs2Na0.6Ag0.4InCl6 double perovskite. Besides the intrinsic warm white self-trapped exciton (STE) emission that originated from the Cs2Na0.6Ag0.4InCl6 host, the samples also exhibit a strong near-infrared emission assigned to the 2F5/2 → 2F7/2 transition of octahedral coordinated Yb3+ ions upon excitation with ultraviolet light. The optimized sample shows a markedly high photoluminescence quantum yield of 84.7%. The photophysical studies such as photoluminescence (PL) and photoluminescence excitation spectra, temperature-dependent PL spectra, and PL decay curves reveal that the strong Yb3+ emission is attributed to the energy transfer from STEs to the Yb3+ ions. Finally, the lead-free double perovskite phosphor-converted LED containing visible and NIR regions was fabricated, with 8.92 mW output power at 100 mA drive current. The results show the great potential of the as-prepared crystals in optoelectronics applications.

Mn-doped 2D Sn-based perovskites with energy transfer from self-trapped excitons to dopants for warm white light-emitting diodes

This work shows the competitive relationship between self-trapped exciton (STE) emission from the host crystal and dopant Mn d–d transition emission.

Optical anisotropy of one-dimensional perovskite C4N2H14PbI4 crystals

Optical anisotropy is essential for the polarization-sensitive optoelectronic devices. Recently, in-plane anisotropy is demonstrated in various 2D layered materials. It has been proved that organic-inorganic perovskites possess excellent optical properties; however, the anisotropy of organic-inorganic perovskites is rarely reported because of the in-plane isotropic structure. Here, we report a large optical anisotropy of one-dimensional organic lead iodine perovskites C4N2H14PbI4 crystals, including emission, excitation and reflection anisotropy. An emission linear dichroic ratio of 5.5 and an excitation linear dichroic ration of 7 are achieved respectively, which are much larger than the in-plane anisotropy of 2D layered materials. The large optical anisotropy can be ascribed to the anisotropic dipole moment in the unique 1D chain structure. In addition, the PL of the 1D C4N2H14PbI4 crystal is dominated by the broadband self-trapped exciton emission due to the quantum confinement effect and strong electron-phonon interaction. Our results advocate that 1D perovskites are promising in the application of broadband polarization-sensitive optoelectronic devices.

Homo- and Heterovalent Doping-Mediated Self-Trapped Exciton Emission and Energy Transfer in Mn-Doped Cs2Na1-xAgxBiCl6 Double Perovskites.

Double perovskites exhibit low toxicity, intrinsic thermodynamic stability, and small carrier effective mass. Herein, a novel doping route was adopted to incorporate Mn ions into Cs2Na1-xAgxBiCl6 double perovskites for engineering the band gap and tailoring the energy transfer. The as-prepared Cs2Na1-xAgxBiCl6 (0 < x < 1) exhibited excellent photoluminescence and a broad self-trapped exciton (STE) band from 500 to 900 nm, which exhibited an abnormal emission peak blue shift with increasing temperature. For Mn-doped Cs2Na1-xAgxBiCl6, the two photoluminescence (PL) bands from d-d transition emission of Mn ions and STEs were always observed simultaneously in the PL window. The distinct energy-transfer channel from the Mn2+ ion guest to the double-perovskite host resulted in the dominant Mn2+ emission. Our results will be helpful for further understanding the nature of the photophysics of double perovskites.