Enhanced luminescence and energy transfer in Mn2+ doped CsPbCl3-xBrx perovskite nanocrystals

Photo- and Thermal-Induced Ion Migration and Phase Separation in Mn-Doped Two-Dimensional PEA2PbX4 Perovskite

Ion migration and phase separation in perovskite materials have negatively affected the solid-state lighting and display. Studying photo- and thermal-induced degradation is considered as a promising approach to understanding the luminescence mechanism and promoting practical applications. Herein, the Mn-doped two-dimensional PEA₂PbX₄ (X = Cl, Br, I) microcrystals with changing halogen composition were synthesized by an acid-assisted post-processing strategy. Then, photo- and thermal-induced degradation was studied by using steady-state and time-resolved photoluminescence (PL) spectroscopy. The band edge exciton PL peak of Mn-doped 2D PEA₂PbX₄ microcrystals was adjusted from 397 to 500 nm. The reduced Mn PL lifetime (1.37 to 0.21 ms) was monitored under ion exchange from Cl to Br to I. The degradation mechanism could be divided into two cases: (i) The halide ion migration in Mn-doped 2D perovskite under continuous illumination was revealed, suggesting that the migration of Cl ions was more accessible than that of Br and I. (ii) The PL redshift and lifetime reduction were observed after annealing at 420 K, which means that thermally induced aggregation of Mn ions resulted in the formation of Mn²⁺-Mn²⁺ dimers. In addition, the experimental results indicated that the induced B-site phase separation at high temperature annealing made the mixed perovskite phase of Pb and Mn ultimately transform into pure PEA₂PbBr₄ and PEA₂MnBr₄.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn²⁺-doped Cs₂Na_1-xAg_xBiCl₆ double perovskites that exhibit dual emission modes. Introducing Ag⁺ ions to Cs₂NaBiCl₆ samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn²⁺ dopants induce a yellow-orange emission. Importantly, Mn²⁺ doping into Cs₂Na_1-xAg_xBiCl₆ double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag⁺ ions into Cs₂NaBiCl₆ can localize wave function to the [AgCl₆]^5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn²⁺ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn²⁺ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn²⁺-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Inorganic Copper-Based Halide Perovskite for Efficient Photocatalytic CO2 Reduction

In view of the toxicity of the Pb element, exploring eco-friendly Pb-free halide perovskites with excellent photoelectric properties is of great research and practical application significance. Herein, copper-based halide perovskite CsCuCl₃ and the corresponding Br^--substituted sample (CsCuCl₂Br) are designed and explored as the catalysts for photocatalytic CO₂ reduction for the first time. A facile antisolvent recrystallization process with pre-prepared single crystals as the precursor is employed to controllably synthesize CsCuCl₃ and CsCuCl₂Br microcrystals (MCs). The electronic structure and charge transfer property analysis by theoretical and experimental investigation reveal that CsCuCl₃ possesses a satisfying bandgap (1.92 eV) and conduction band minimum (CBM) to harvest the sunlight and drive the conversion of CO₂ to CH₄ and CO. The Br^- substitution can not only narrow the bandgap but also facilitate the transportation of charge carriers. Thus, a total electron consumption rate of 44.71 μmol g^-1 h^-1 is achieved for CsCuCl₂Br MCs, which is much better than that of same-sized CsPbBr₃ microcrystals or even better than many perovskite nanocrystal photocatalysts. This study suggests that Cu-based perovskites can serve as promising candidates for artificial photosynthesis or other photocatalytic applications, which may propose a new thought to construct lead-free, low-cost photocatalysts.

In situ preparation of Mn-doped perovskite nanocrystalline films and application to white light emitting devices

Mn-doped perovskite nanocrystals have promised new optoelectronic applications due to their unique material properties. In the present study, Mn-doped perovskite nanocrystalline films were prepared in situ in a polymer matrix. The Mn-doped perovskite nanocrystals (PNCs) had good crystallinity and uniform size/spatial distributions in the polymer film. Bright dual-color emission and the long lifetime of the excited state of the dopant were observed from the host exciton and the Mn²⁺ dopant, respectively. Furthermore, magnetism was observed in the optimal Mn²⁺ concentration, implying that magnetic coupling was achieved in the Mn-doped perovskite lattice. The Mn-doped perovskite films also showed superior stability against moisture. To demonstrate the practicality of this composite film, a white light emitting device was fabricated by combining a single composite film with a blue light emitting diode; the device showed a high-quality white light emission, and the Commission Internationale De L'Eclairage (CIE) chromaticity coordinate of the white light emitting diode (WLED) (0.361, 0.326) was close to the optimal white color index. In this single-layer WLED, self-absorption among the luminous multilayers in traditional white light emitting diodes can be avoided. The study findings revealed that Mn-doped perovskite nanocrystalline films have many exciting properties, which bodes well for the fundamental study and design of high-performance optoelectronic devices.

Hexamethyldisilazane-Assisted Ambient Condition Mn2+ Doping Perovskite Nanocrystals

Doping Mn2+ ions into lead halide perovskite (LHP) nanocrystals (NCs) has attracted great attention in the optoelectronic fields due to the stability enhancement and unique dual-color emission characteristics arising from...

Enhance luminescence or change morphology: effect of the doping method on Cu2+-doped CsPbBr3 perovskite nanocrystals

The effect of Cu2+ on CsPbBr3 nanocrystals is compared between the hot-injection method and postsynthetic cation-exchange reaction.

Near-unity blue-orange dual-emitting Mn-doped perovskite nanocrystals with metal alloying for efficient white light-emitting diodes

The tunable dual-color emitting Mn²⁺ doped CsPbCl_3-xBr_x nanocrystals (NCs) with near-unity photoluminescence quantum yield (PL QY) were synthesized through post-treatment of metal bromide at room temperature for fabrication of efficient warm white light-emitting diodes (WLEDs). Especially, the CdBr₂ treated blue-orange emitting Mn doped NCs with various Mn/Pb molar feed ratios exhibit higher PL QY of 97% and longer Mn²⁺ PL lifetime of 0.9 ms. It is surprisingly found that the X-ray diffraction peak at 31.9° is almost not changed with increasing Br composition, meaning formation of metal alloying due to incorporation of amount of divalent cation in NCs. The strong and stable Mn²⁺ PL at temperature ranging from 80 K to 360 K are revealed and the temperature-dependent energy transfer efficiencies in Mn²⁺ doped CsPbCl_1.5Br_1.5 NCs are obtained. The enhancement mechanism of Mn²⁺ PL QY was attributed to improved energy transfer from exciton to Mn²⁺ d-d transition and suppressed defect state density after post-treatment. The efficient warm WLEDs with color rendering index of 90 and luminous efficacy of 92 lm/W at 10 mA were fabricated by combining blue-orange dual-emitting Mn²⁺ doped CsPbCl_3-xBr_x@SiO₂ and green emissive CsPbBr₃@SiO₂ NCs with violet GaN chips.

Energy vs Charge Transfer in Manganese-Doped Lead Halide Perovskites

Mn-doped lead halide perovskites exhibit long-lived dopant luminescence and enhanced host excitonic quantum yield. The contention between energy and charge transfer in sensitizing dopant luminescence in Mn-doped perovskites is investigated by state-of-the-art DFT calculations on APbX₃ perovskites (X = Cl, Br, and I). We quantitatively simulate the electronic structure of Mn-doped perovskites in various charge and spin states, providing a structural/mechanistic analysis of Mn sensitization as a function of the perovskite composition. Our analysis supports both energy- and charge-transfer mechanisms, with the latter probably preferred in Mn:CsPbCl₃ due to small energy barriers and avoidance of spin and orbital restrictions. An essential factor determining the dopant luminescence quantum yield in the case of charge transfer is the energetics of intermediate oxidized species, while bandgap resonance can well explain energy transfer. Both aspects are mediated by perovskite host band edge energetics, which is tuned in turn by the nature of the halide X.

Mn2+/Mn4+ co-doped LaM1-xAl11-yO19 (M = Mg, Zn) luminescent materials: electronic structure, energy transfer and optical thermometric properties

Dual-emitting manganese ion doped LaM_1-xAl_11-yO₁₉ (M = Mg, Zn) phosphors were prepared by substituting Zn²⁺/Mg²⁺ with Mn²⁺ and replacing Al³⁺ with Mn⁴⁺. The LaM_1-xAl_11-yO₁₉:xMn²⁺,yMn⁴⁺ phosphors show a narrow green emission band of the Mn²⁺ ions at 514 nm and a red emission band of the Mn⁴⁺ ions at 677 nm. In addition, the thermal stability of luminescence shows that the response of Mn²⁺ and Mn⁴⁺ to the temperature is obviously different in LaMAl₁₁O₁₉, implying the potential of the prepared phosphors as optical thermometers. The decay lifetime of Mn⁴⁺ was changed with temperature due to the different fluorescence intensity ratios of Mn²⁺ and Mn⁴⁺, and a dual-mode optical temperature-sensing mechanism was studied in the temperature range of -50-200 °C. The maximum relative sensitivities (S_r) are calculated as 3.22 and 3.13% K^-1, respectively. The unique optical thermometric features demonstrate the application potential of LaMAl₁₁O₁₉:Mn²⁺,Mn⁴⁺ in optical thermometry.

Pressure-Engineered Optical and Charge Transport Properties of Mn2+/Cu2+ Codoped CsPbCl3 Perovskite Nanocrystals via Structural Progression

In this work, compared with the corresponding pure CsPbCl₃ nanocrystals (NCs) and Mn²⁺-doped CsPbCl₃ NCs, Mn²⁺/Cu²⁺-codoped CsPbCl₃ NCs exhibited improved photoluminescence (PL) and photoluminescence quantum yields (PL QYs) (57.6%), prolonged PL lifetimes (1.78 ms), and enhanced thermal endurance (523 K) as a result of efficient Mn²⁺ doping (3.66%) induced by the addition of CuCl₂. Furthermore, we applied pressure on Mn²⁺/Cu²⁺-codoped CsPbCl₃ NCs to reveal that a red shift of photoluminescence followed by a blue shift was caused by band gap evolution and related to the structural phase transition from cubic to orthorhombic. Moreover, we also found that under the preheating condition of 523 K, such phase transition exhibited obvious morphological invariance, accompanied by significantly enhanced conductivity. The pressure applied to the products treated with high temperature enlarged the electrical difference and easily intensified the interface by closer packaging. Interestingly, defect-triggered mixed ionic and electronic conducting (MIEC) was observed in annealed NCs when the applied pressure was 2.9 GPa. The pressure-dependent ionic conduction was closely related to local nanocrystal amorphization and increased deviatoric stress, as clearly described by in situ impedance spectra. Finally, retrieved products exhibited better conductivity (improved by 5-6 times) and enhanced photoelectric response than those when pressure was not applied. Our findings not only reveal the pressure-tuned optical and electrical properties via structural progression but also open up the promising exploration of more amorphous all-inorganic CsPbX₃-based photoelectric applications.

Cation-doping matters in caesium lead halide perovskite nanocrystals: from physicochemical fundamentals to optoelectronic applications

All-inorganic caesium lead halide perovskite nanocrystals (PeNCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic properties including tunable bandgaps over the entire visible spectral region, high photoluminescence quantum yields (PLQYs) close to unity and narrow emission line widths down to 10-20 nm, making them particularly suitable as promising candidates for numerous applications ranging from light-emitting diodes (LEDs), solar cells to scintillators. Despite the considerable progress made in the past six years, the real-world applications of caesium lead halide PeNCs themselves especially in the category of CsPbX₃ (X = Cl, Br and I) are still restricted by their labile crystal lattices and downgraded luminescence when exposed to ambient air conditions. Recent experimental and theoretical studies on cation doping have proven to be an effective way to significantly improve the physicochemical properties of cesium lead halide PeNCs, which would have profound implications for a range of applications. In this review, we provide a brief overview of the most recent advances in cation-doped all-inorganic caesium lead halide PeNCs, aimed at developing high-performance and long-term stable optoelectronic and photovoltaic devices, which covers areas from their fundamental considerations of cation doping, controlled synthesis methodology and novel physicochemical properties to the optoelectronic applications with an emphasis on perovskite-based LEDs and solar cells. And finally, some possible directions of future efforts toward this active research field are also proposed.

Resolving Enhanced Mn2+ Luminescence near the Surface of CsPbCl3 with Time-Resolved Cathodoluminescence Imaging

Mn²⁺ doping of lead halide perovskites has garnered recent interest because it produces stable orange luminescence in tandem with perovskite emission. Here, we observe enhanced Mn²⁺ luminescence at the edges of Mn²⁺-doped CsPbCl₃ perovskite microplates and suggest an explanation for its origin using the high spatiotemporal resolution of time-resolved cathodoluminescence (TRCL) imaging. We reveal two luminescent decay components that we attribute to two different Mn²⁺ populations. While each component appears to be present both near the surface and in the bulk, the origin of the intensity variation stems from a higher proportion of the longer lifetime component near the perovskite surface. We suggest that this higher emission is caused by an increased probability of electron-hole recombination on Mn²⁺ near the perovskite surface due to an increased trap concentration there. This observation suggests that such surface features have yet untapped potential to enhance emissive properties via control of surface-to-volume ratio.

Near-Unity Red Mn2+ Photoluminescence Quantum Yield of Doped CsPbCl3 Nanocrystals with Cd Incorporation

Although Mn²⁺ doping in semiconductor nanocrystals (NCs) has been studied for nearly three decades, the near 100% photoluminescence (PL) quantum yield (QY) of Mn²⁺ emission has never been realized so far. Herein, greatly improved PL QYs of Mn²⁺ emissions are reported in Mn²⁺-doped CsPbCl₃ NCs with various Mn²⁺ doping concentrations after CdCl₂ post-treatment at room temperature. Specifically, the near-unity QY and near single-exponential decay of red Mn²⁺ emission peaking at 627 nm in doped CsPbCl₃ NCs are obtained for the first time. The temperature dependence of steady-state and time-resolved PL spectra reveals that the CdCl₂ post-treatment significantly reduces the nonradiative defect states and enhances the energy transfer from host to Mn²⁺ ions. Moreover, the Mn²⁺:CsPbCl₃ NCs after CdCl₂ post-treatment exhibit robust stability and high PL QYs after multipurification. The results will provide an effective route to obtain doped perovskite NCs with high performance for white lighting emitting diodes.

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 Cs₂Na_1-xAg_xBiCl₆ double perovskites for engineering the band gap and tailoring the energy transfer. The as-prepared Cs₂Na_1-xAg_xBiCl₆ (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 Cs₂Na_1-xAg_xBiCl₆, 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.

Investigation of the Mn dopant-enhanced photoluminescence performance of lead-free Cs2AgInCl6 double perovskite crystals

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

Rapid synthesis of hybrid methylammonium lead iodide perovskite quantum dots and rich MnI2 substitution favouring Pb-free warm white LED applications

We present a facile room temperature synthesis of CH₃NH₃Pb_1-x Mn _x I₃ perovskite quantum dots (PQDs) substituting manganese (Mn²⁺) at the lead (Pb²⁺) sites to minimize environmental pollution and make it commercially feasible. By varying the concentration of Mn²⁺ from 0 to 60%, the PQDs exhibit strong color tunability from red to orange color suggesting successful energy transfer due to Mn²⁺ inclusion. We observed a high external photoluminescence quantum yield (PLQY) of 98% for unsubstituted CH₃NH₃PbI₃ and >50% for up to 15% Mn²⁺ substituted PQDs. The average lifetime of PQDs was found to shorten with increasing Mn²⁺ replacement. We demonstrate a white LED prototype by employing the CH₃NH₃Pb_1-x Mn _x I₃ PQDs with green QDs on a blue LED chip. The CRI and CCT value varying from 92 to 80 and 5100 K to 2900 K, respectively, indicate the usability of the Mn²⁺ substituted PQDs as efficient warm white LEDs with a promising CRI and good stability.

Improved Doping and Emission Efficiencies of Mn-Doped CsPbCl3 Perovskite Nanocrystals via Nickel Chloride

It is challenging to improve the emission efficiency of Mn-doped CsPbCl₃ (Mn:CsPbCl₃) nanocrystals (NCs) because the excellent optical performances are dependent on high doping efficiency and few defects and traps. Steady-state and time-resolved photoluminescence (PL) spectroscopies were used to investigate the luminescence properties of Mn:CsPbCl₃ NCs with different Mn doping levels synthesized in the presence of nickel chloride. The doping efficiency of Mn ions in Mn:CsPbCl₃ NCs was greatly enhanced in the presence of NiCl₂, and the PL wavelength of Mn²⁺ ions was tuned from 594 to 638 nm by varying the concentration of dopant Mn from 0.11% to 15.25%. The high emission quantum yields of Mn:CsPbCl₃ NCs with orange and red emissions peaked at 600 and 620 nm in hexane were 70% and 39%, respectively. The improvement in doping and emission efficiencies of Mn²⁺ was attributed to the enhanced formation energies of the Mn doping under the Mn and Ni codoped configuration and the resulting reduction of defects and traps in Mn:CsPbCl₃ NCs with incorporation of Ni²⁺ ions.

Tunable photoluminescence and an enhanced photoelectric response of Mn2+-doped CsPbCl3 perovskite nanocrystals via pressure-induced structure evolution

Mn²⁺:CsPbCl₃ nanocrystals (NCs) were synthesized using a modified one-pot injection method, which exhibits significantly improved thermal stability. For the first time, the pressure-treated optical and structural properties of synthetic Mn²⁺:CsPbCl₃ NCs were further investigated, and their associated intriguing electrical and photoelectric properties were revealed from impedance spectra and photocurrent measurements under compression. The pressure-dependent photoluminescence experienced an initial redshift before 1.7 GPa followed by a continuous blueshift, as evidenced by the bandgap shifts. High-pressure XRD spectra uncovered a cubic-to-orthorhombic structural transition at about 1.1 GPa and subsequent amorphization upon further compression, which was fully reversible. Furthermore, the sample annealing from 340 K drove grain growth and decreased grain boundary resistance at ambient pressure. The compression further decreased the grain boundary barrier and improved the electrical conductivity (up to ∼10^-2Ω^-1 cm^-1) of the thermally annealed Mn²⁺:CsPbCl₃ NC surface. Simultaneous photocurrent enhancement of thermally annealed NCs was also achieved as expected, and reached optimal performance at 0.7 GPa. Strikingly, after the pressure cycling (loading-releasing), the results show that thermally annealed Mn²⁺:CsPbCl₃ NCs gained preservable higher electrical conductivity (∼10 times increase) and an improved photoelectric response compared to the ambient state before compression. This work proves that high pressure is useful for opening the versatility in the structure and properties of metal-halide perovskite nanocrystals leading to a promising way for superior optoelectronic materials-by-design.

Doping Mn(II) in All-Inorganic Ruddlesden-Popper Phase of Tetragonal Cs2PbCl2I2 Perovskite Nanoplatelets

Doping Mn(II) in inorganic Ruddlesden-Popper phase Cs₂PbCl₂I₂ perovskite nanoplatelets is reported. The host nanostructures were prepared with a calculative protocol taking the exact required composition of Cs(I) and Pb(II) and injecting the preformed mixed oleylammonium chlorides and iodides at optimized reaction temperature. Reactions were optimized with various halides and their mixtures, but the stable phase of the Cs₂PbX₄ system was obtained only for the chloride-iodide mixed-halide system. Introduction of Mn(II) along with Pb(II), resulted in successful light-emitting doped nanocrystals. Measuring the photoluminescence and the decay lifetimes at room and liquid nitrogen temperatures, the variations in the excitonic, self-trapped, and Mn dopant emission properties were compared with those of the chalcogenide and perovskite nanocrystals.

Room temperature synthesis of Mn-doped Cs3Pb6.48Cl16 perovskite nanocrystals with pure dopant emission and temperature-dependent photoluminescence

We report Mn-doped Cs₃Pb_6.48Cl₁₆ nanocrystals with pure dopant emission synthesized at room temperature.

Fe2+ doped in CsPbCl3 perovskite nanocrystals: impact on the luminescence and magnetic properties

All inorganic halide perovskite nanocrystals (NCs) have wider practical applications owing to their good properties, whereas the photoluminescence quantum yield (PLQY) of the purple emissive CsPbCl₃ NCs is too low to apply in multi-color displays. In this study, earth-abundant Fe²⁺ metal ions were successfully incorporated into the lattice of CsPbCl₃ NCs with the partial replacement of the sites of Pb²⁺ ions. The impacts of Fe²⁺ ions on the luminescence and magnetic properties of CsPbCl₃ NCs were studied using photoluminescence spectroscopy (PL), X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and a vibrating sample magnetometer (VSM). CsPb_1−xFe_xCl₃ NCs, with x = 0, 0.1, 0.2, and 0.3, were synthesized at 170 °C. It was found that an appropriate amount of Fe²⁺ doping not only improved the homogeneity of the size of NCs, but also enhanced the PLQY and average PL lifetimes. An obvious hysteresis behavior was observed for the NCs, and there was a significant change in the saturation magnetization value with the increase in the Fe²⁺ concentration.

The CsPb_1−xFe_xCl₃ NCs were synthesized and an appropriate amount of Fe²⁺ doping can enhance PLQY and average PL lifetimes. Meanwhile, an obvious hysteresis behavior has been observed for the NCs.

Improved ultraviolet radiation stability of Mn2+-doped CsPbCl3 nanocrystals via B-site Sn doping

A facile and effective B-site Sn doping strategy is used to grow Mn²⁺-doped CsPbCl₃ (Mn:CsPbCl₃) nanocrystals with enhanced ultraviolet irradiation stability and luminescence properties.