Tailoring red-NIR luminescence in rare earth doped lead-free layered double perovskites by bivalent manganese energy bridge

doi:10.21203/rs.3.rs-1767466/v1

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Tailoring red-NIR luminescence in rare earth doped lead-free layered double perovskites by bivalent manganese energy bridge

https://doi.org/10.21203/rs.3.rs-1767466/v1

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Combining rare earth ions (RE³⁺) in halide perovskites is a hopeful strategy to produce interesting optical properties in visible and near-infrared (NIR) regions. Here, RE³⁺ (RE = Ho, Er, Tm, Nd) are successfully introduced into lead-free Cs₄Mn_1-wCd_wBi₂Cl₁₂ (0 ≤ w ≤ 1) layered double perovskites through a facile hydrothermal method. Besides the strong orange-red Mn²⁺ emission, the as-prepared materials exhibit effective characteristic emission of RE³⁺ in the red and NIR region via constructing Mn²⁺ energy bridge. With Cd²⁺ gradually replacing Mn²⁺ in Cs₄Mn_1-wCd_wBi₂Cl₁₂:RE³⁺, the photoluminescence (PL) of RE³⁺ in red and NIR range shows a decreased trend until it disappeared (w = 1). The energy transfer processes of [BiCl₆]^3-→Mn²⁺→RE³⁺ are confirmed and corresponding mechanism is explored. According to the manipulation of Mn²⁺/Cd²⁺ component, an evolution process from indirect to direct band gap is verified via a combined experimental–theoretical approach. White LEDs are fabricated by employing RE³⁺-doped Cs₄MnBi₂Cl₁₂ on near-ultraviolet (n-UV) LED chips, achieving an improved color rendering index (CRI) from 83.3 to 93.0 compared to parent compound. Co-doping RE³⁺ (RE = Ho, Er, Tm, Nd) into Cs₄MnBi₂Cl₁₂ shows a wide range of full-spectral emission covering the NIR I and NIR II regions, showing great potential for NIR detection applications.

Metal halide

red-NIR luminescence

lead-free layered double perovskites

rare earth

Mn2+ energy bridge

Phosphor-converted light-emitting diodes (pc-LEDs) have drawn extensive attention from academic to industrial fields due to their high energy efficiency, low power consumption, environmental friendliness, and highly tunable optical properties.^1,2 Most white pc-LEDs pair blue LED chips with yellow Y₃Al₅O₁₂:Ce³⁺ phosphors. One limitation of this approach is the lack of a red component in the emission light, which is considered as a “cool white”. For applications requiring the warmer white light, a combination of tricolor (blue, green, red) phosphors is used with near-ultraviolet (n-UV) LED chips.³ Regardless of combination method, red luminescent material plays a key role in improving the lighting and backlit display quality of white pc-LEDs.⁴ In recent decades, near-infrared (NIR) luminescent materials are attracting much attention for applications in food composition analysis, night-vision technologies, bio-sensing.^5–7 As an emerging field, researchers have made considerable efforts to explore new NIR emitters, such as Eu²⁺/Cr³⁺/Bi³⁺-doped NIR bulk luminescent materials, CH₃NH₃PbI₃ halide perovskite, Sb³⁺-doped Cs₂ZnCl₄ metal halide materials and Ag₂S quantum dots based LEDs.^8–12 In these both regard, the development of red and NIR luminescent materials provides new opportunities for both white and NIR pc-LED applications.

Low dimensional lead halide perovskite has emerged as an important class of tricolor luminescent materials.¹³ In these perovskites, the Pb sites have a coordination number (CN) of 6. These octahedral sites in perovskites are suitable for incorporating rare earth (RE) dopants, since RE³⁺ ions prefer to occupy CN ≥ 6.^14,15 Although these lead halide perovskites have some advantages, the major issues are their poor stability and toxicity.¹⁶ Various attempts have been made to discover lead-free metal halide perovskites with efficient optical and optoelectronic properties, such as double A₂B(I)B(III)X₆, vacancy ordered A₂B(IV)X₆, and Bi(III)-, Sb(III)-, In(III)-, Ag(I)-, Cu(I)-, Sn(II)-, Mn(II)-based lead-free perovskites.^17–25 In this category, lead-free layered double perovskites, such as Cs₄Mn_1 − wCd_wBi₂Cl₁₂, comprising one layer of [Cd/MnCl₆]⁴⁻ octahedra inserted in between two layers of [BiCl₆]³⁻ octahedra, are emerging.^26–28 Combining RE³⁺ ions in halide perovskites is considered as a promising approach to developing more efficient and stable luminescent materials.²⁹ Meanwhile, tunable and stable photoluminescence (PL) covering the full visible–NIR (VIS-NIR) gamut is available with delicate composition engineering. To the best of our knowledge, many successful cases of RE³⁺-doped and RE³⁺-based lead-free perovskites have been reported. For example, Tang’s and Du’s group doped Ho³⁺ or Yb³⁺/Er³⁺ ions into Cs₂Ag_0.6Na_0.4InCl₆ to achieve an efficient multimodal luminescent material;^30,31 Yb³⁺ or Er³⁺ doping into Cs₂AgInCl₆ was explored by Nag’s group to achieve optical functionality in the NIR window.³² Er³⁺/Yb³⁺-based halide double perovskites were explored for realizing efficient NIR emission.^33,34 Several works also demonstrated the Mn²⁺→RE³⁺ (RE = Er, Nd, Ho) energy transfers in the perovskite systems for enhancing NIR emission intensity.^35–37 Inspired by these studies, it is valuable to reveal the new RE³⁺ doping effects on lead-free perovskite and impart the optical properties in the VIS and NIR ranges.

In this work, we successfully introduce the RE³⁺ (RE = Ho, Er, Tm, Nd) ions into the lattice of lead-free Cs₄Mn_1 − wCd_wBi₂Cl₁₂ layered double perovskite crystals through a facile hydrothermal method. For the as-prepared Cd parent crystal, there is no luminescence of RE³⁺ due to mismatched energy level between the conduction band of host matrix and the excited state of RE³⁺ ions. Interestingly, tailoring efficient red-NIR emission arise by combination of Mn²⁺ and RE³⁺ emission after importing Mn²⁺ bridge. First-principle calculations are used to investigate the electronic and optical properties of Cs₄Mn_1 − wCd_wBi₂Cl₁₂, and the RE³⁺-doping effect on the intense red-NIR luminescence in this solid solution. The Mn²⁺ modifies the projected density of states (PDOS) at the band, matching with RE³⁺ and giving rise to an optical absorption bridge at a lower energy. This absorbed energy is then efficiently transferred to RE³⁺ f-electrons, promoting the red emission of Mn²⁺ and RE³⁺ ions as well as typical NIR I and NIR II region emission of RE³⁺ ions. The successful adjustment of band gap and Mn²⁺-RE³⁺ combination will expand the utility of lead-free halide layered double perovskites in some new photoelectric areas, such as LEDs, biosensing, NIR detection, optical communication in the future.

Synthesis and characterization

Rare earth RE³⁺ (RE = Ho, Er, Tm, Nd) ions doped Cs₄Mn_1 − wCd_wBi₂Cl₁₂ crystals were synthesized by the hydrothermal method as well as by changing the concentration of RE³⁺ and stoichiometry of Mn/Cd in the precursor. Details can be found in the Supporting Information (SI). Considering the same valence (+ 3) and similar ionic radii of Bi³⁺ and Ho³⁺, Tm³⁺, Er³⁺, Nd³⁺, these RE³⁺ ions tend to occupy the lattice of Bi³⁺ ions in the rhombohedral structure of Cs₄MnBi₂Cl₁₂, forming the [Ho/Tm/Er/NdCl₆] octahedra (Fig. 1a). In this structure, each [BiCl₆]³⁻ octahedron is surrounded by three Mn²⁺-centered octahedra. The crystal structures of Cs₄MnBi₂Cl₁₂ and Ho³⁺-doped Cs₄MnBi₂Cl₁₂ were examined using high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD). The HRTEM images of Cs₄MnBi₂Cl₁₂ and Ho³⁺-doped Cs₄MnBi₂Cl₁₂ display clear lattice fringes with lattice spacing of 3.82 Å and 3.90 Å, respectively, which could be indexed as crystal plane (2, -1, 0) of rhombohedra of Cs₄MnBi₂Cl₁₂ phase (Fig. 1b and 1c). The increase in the interplanar spacing value of the (2, -1, 0) plane from 3.82 Å to 3.90 Å further confirms the lattice expansion after Ho³⁺ introducing into host. The SAED patterns of Cs₄MnBi₂Cl₁₂ and Ho³⁺-doped Cs₄MnBi₂Cl₁₂ show very sharp diffraction spots (Fig. 1d and 1e), proving the single crystal characteristics for the studied samples. XRD pattern of Ho³⁺-doped Cs₄MnBi₂Cl₁₂ proves the phase purity (Figure S1). The actual RE³⁺ doping concentration in Cs₄MnBi₂Cl₁₂:xRE³⁺ (RE = Ho, Er, Tm, Nd) can be estimated by using inductively coupled plasma optical emission spectrometer (ICP-OES). The doping concentrations were found to deviate considerably from the nominal content of the Cs₄MnBi₂Cl₁₂ precursor (Table S1), as was previously reported for RE³⁺-doped compounds.^31,38,39 Scanning electron microscopy mapping was applied to analyze the composition of each compound (Figure S2). All the elements are evenly overspread in the images, with no element enrichment, indicating that there is no phase separation in the as-synthesized perovskite materials. This result implies the successful doping of RE³⁺ ions into the Cs₄MnBi₂Cl₁₂ host lattice. The transparent Cs₄MnBi₂Cl₁₂ and Ho³⁺-doped Cs₄MnBi₂Cl₁₂ single crystals exhibit intense orange and orange-red emission upon UV light illumination (Fig. 1f and 1g). Figure 1h depicts the normalized Ho L₃-edge extended X-ray absorption fine structure (EXAFS) spectra of Ho³⁺-doped Cs₄MnBi₂Cl₁₂. The EXAFS oscillations are clearly observed by the function spectrum k³χ(k) (Fig. 1i). Figure 1j shows the Fourier transformed Ho L₃-edge EXAFS spectra k³χ(k). By fitting within the allowable margin of error, the bond length and coordination information of Ho³⁺ in Cs₄MnBi₂Cl₁₂ were obtained. EXAFS fitting parameters at the Ho L₃-edge is provided in the Table S4. As expected, the coordination number of Ho is ~ 6, which coordinated with six chlorine atoms. The bond length of Ho-Cl is ~ 2.58 Å, which is closer to the Bi-Cl bond length, confirming that Ho³⁺ is replacing Bi³⁺ rather than Mn²⁺ in Cs₄MnBi₂Cl₁₂. Similarly, Er³⁺, Tm³⁺, Nd³⁺ ions can also be doped into this host, and Raman spectroscopy shows that the doping does not induce phase transitions and additional vibrational modes (Figure S3). From the electron paramagnetic resonance (EPR) spectra of Cs₄Mn_0.23Cd_0.77Bi₂Cl₁₂, Cs₄MnBi₂Cl₁₂ and Ho³⁺-doped Cs₄MnBi₂Cl₁₂, a single broad line is observed (centered around 3480 G, effective g value g ~ 2.0, Figure S4) due to the high Mn concentration. This can be attributed to the fact that Mn ions are coupled by dipolar or exchange magnetic interactions. In this case, the hyperfine sextet disappears due to fluctuations of the dipolar or exchange interactions.

Steady-state absorption and PL properties

The similarity of absorption spectra (250–400 nm) of Cs₄CdBi₂Cl₁₂, Cs₄MnBi₂Cl₁₂, Ho³⁺-doped Cs₄MnBi₂Cl₁₂ confirms that three compounds can be considered as insulating hosts. Their optical properties are controlled mainly by local electronic transitions associated with isolated [BiCl₆]³⁻ centers (Fig. 2a). Interestingly, Ho³⁺-doped and undoped Cs₄MnBi₂Cl₁₂ give rise to intense new absorption with peak at ~ 430 (⁶A_1g→⁴T_2g) and ~ 520 nm (⁶A_1g→⁴T_2g) compared with Cs₄CdBi₂Cl₁₂. The new absorption should arise from the contribution of Mn²⁺ orbitals at the band edges. The obtained light-orange crystals of Ho³⁺-doped Cs₄MnBi₂Cl₁₂ show a bright orange-red emission upon 365 nm UV light excitation, similar to pure Cs₄MnBi₂Cl₁₂ (Fig. 2b). Figure 2b and 2c show PL spectra of Cs₄CdBi₂Cl₁₂, Ho³⁺-doped and undoped Cs₄MnBi₂Cl₁₂ in VIS and NIR region, respectively. Besides the broad-band emission derived from Mn²⁺ (⁴T_2g→⁶A_1g), there is a sharp emission peak located at 645 nm, corresponding to the ⁵F₈→⁵I₈ transition of Ho³⁺. In addition to the characteristic emission in VIS region, RE³⁺ ions usually present rich PL spectra in NIR region. Obvious emission around 985, 1194 and 1488 nm correspond to the transitions of Ho^{3+ 5}F₅→⁵I₇, ⁵I₆→⁵I₈, and ⁵F₅→⁵I₆, respectively (Fig. 2c). From the PL decay curves of Ho³⁺-doped Cs₄MnBi₂Cl₁₂ in Fig. 2d, the average PL lifetime are 0.058, 1.246, 1.192 ms monitored at 596, 645, 985 nm, respectively.

After monitoring the PL peak at 596, 645, 985 nm, the Ho³⁺-doped Cs₄MnBi₂Cl₁₂ presents similar PLE spectrum profile (Fig. 2e). This is composed of a strong broad excitation from 250 to 400 nm and two distinct excitation peaks at ∼430 and ∼520 nm associated with spin-forbidden d − d transition of octahedrally coordinated high-spin Mn²⁺. In fact, the above PLE spectra resemble the PLE spectrum monitored at 596 nm without Ho³⁺ emission, indicating the existence of an energy transfer channel from Cs₄MnBi₂Cl₁₂ to Ho³⁺. Beyond that the PL intensity of RE ions with f–f transition is usually sensitive to the excitation wavelength. However, with the excitation wavelength varying from 310 to 430 nm, the relative PL intensity between Ho³⁺ and Mn²⁺ emission is almost constant (Figure S5). Accordingly, the indirect excitation may be the dominant excitation resource for RE³⁺ ions, with an optimal excitation wavelength of ~ 370 nm. Ho³⁺ ions are not directly excited but excited via an energy transfer channel from the Cs₄MnBi₂Cl₁₂ host. Because f–f transition of RE³⁺ ions is forbidden and thus incapable to compete with the excitation of perovskite host. The fact that the radiation light only excites the host but the characteristic emission of Ho³⁺ ions directly confirm the existence of energy transfer. On the contrary, no characteristic emissions are observed in Ho³⁺-doped Cs₄CdBi₂Cl₁₂, indicating that Ho³⁺ will emit red-NIR light when Mn²⁺ acts as a bridge to transfer energy to Ho³⁺, as shown in Fig. 2f.

Er³⁺/Tm³⁺/Nd³⁺ - singly doped Cs₄MnBi₂Cl₁₂ samples also exhibit their f-f characteristic emissions in the VIS and NIR region. Figure S6 illustrates the PLE/PL spectra of RE³⁺-doped Cs₄MnBi₂Cl₁₂ (RE = Er, Tm, Nd) and AM 1.5 solar spectrum over wavelengths of 280–1650 nm. The PLE spectrum of Er³⁺/Tm³⁺/Nd³⁺-doped Cs₄MnBi₂Cl₁₂ by monitoring the PL peak at VIS and NIR range are similar to that of monitored at 596 nm without Er³⁺/Tm³⁺/Nd³⁺ emission, implying the existence of the energy transfer channel from Cs₄MnBi₂Cl₁₂ to Er³⁺/Tm³⁺/Nd³⁺. From the PL spectrum of Er³⁺-doped Cs₄MnBi₂Cl₁₂, besides the broad emission from Mn²⁺, the sharp emission peaks located at 672, 983, 1158, 1540 nm, correspond to the transitions ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, ⁴I_13/2→⁴I_15/2 of Er³⁺, respectively. The sharp emission peaks at 694, 802, 1211, 1432 nm from Tm³⁺-doped Cs₄MnBi₂Cl₁₂ are ascribed to the transitions ³F₃, ³H₄, ³H₅→³H₆, ³H₄→³F₄ of Tm³⁺, respectively. The sharp emission peaks located at 886, 1061, 1344 nm from PL spectrum of Nd³⁺-doped Cs₄MnBi₂Cl₁₂, correspond to the transitions ⁴F_3/2→⁴I_9/2, ⁴I_11/2, ⁴I_13/2 of Nd³⁺, respectively. Moreover, the PLE spectrum consists of several bands in the UV–VIS range, which is consistent with the maximum photon flux region of the solar spectrum. In addition, RE³⁺-doped Cs₄MnBi₂Cl₁₂ (RE = Ho, Er, Tm, Nd) give an intense orange-red emission in 500–780 nm and NIR emission from 800 to 1650 nm, matching well with the optimal spectral response of solar cells. Therefore, RE³⁺-doped Cs₄MnBi₂Cl₁₂ materials are promising solar spectral converters by achieving efficient NIR emission for solar cells. RE³⁺-doped Cs₄MnBi₂Cl₁₂ (RE = Ho, Er, Tm) with different concentrations were also prepared for further investigated. XRD patterns of RE³⁺-doped Cs₄MnBi₂Cl₁₂ demonstrate pure phases for all syntheses (Figure S7a-S7c). As RE³⁺ ions concentration increases, the characteristic emission bands of RE³⁺ ions gradually become obvious, which not only supplement the red-light component, but also achieve full-spectrum NIR emission (Figure S7d-S7e). The photoluminescence quantum yield (PLQY) for Cs₄MnBi₂Cl₁₂ is ~ 26% under 370 nm UV light excitation, and RE³⁺ doping can increase the PLQY to 35–59% owing to the superposition of the additional RE³⁺ emissions (Figure S7f-S7i). A schematic variation of emission color in the Commission Internationale de l´Eclairage (CIE) diagram by a change in the Ho³⁺ concentration depicts a color tuning in orange-red region (Figure S8a). The decreased PL lifetime decay curves of Mn²⁺ emission in Ho³⁺-doped Cs₄MnBi₂Cl₁₂ with different Ho³⁺ concentrations substantially prove the existence of energy transfer (Figure S8b). Energy transfer from halide perovskite hosts to RE ions may provide new avenues for achieving some exciting optical properties and goals.

Woodward et al. reported that replacing Cd²⁺ with Mn²⁺ to form Cs₄Mn_1 − wCd_wBi₂Cl₁₂ inhibits energy migration to defect sites where nonradiative decay can occur.⁴⁰ The PL intensity in the Cs₄Mn_1 − wCd_wBi₂Cl₁₂ prepared via precipitation reaction reaches a maximum near w = 0.73, where each [BiCl₆]³⁻ sensitizer is approximately on average adjacent to one Mn²⁺ activator, and the PLQY reaches 57%. Here, Cs₄Mn_1 − wCd_wBi₂Cl₁₂ (w = 0–1) prepared by hydrothermal method exhibits a maximum PLQY of 93% when w = 0.77. XRD patterns indicate pure phases for all syntheses (Figure S9a). The broadening of 6s²→6s¹p¹ transition is attributed to the increase of electronic dimensionality with enhancing Mn incorporation (Figure S9b). The bands at 430 and 520 nm are associated with the spin-forbidden ⁶A₁(S)→⁴T₂(G) and ⁶A₁(S)→⁴T₁(G) transitions, respectively. These transitions are forbidden in octahedral coordination, but magnetic coupling between Mn²⁺ pairs can relax the parity selection rules. The PLE intensity of the both bands steadily increase as increases Mn content due to the larger amount of Mn²⁺-coupled pairs. The overlap of absorption spectra indicates that Mn²⁺ emission originates from the absorption of host (Figure S9d). The PL spectra of the studied Cs₄Mn_1 − wCd_wBi₂Cl₁₂ (w = 0–1) samples reach a maximum near 596 nm (Figure S9c). The emission position hardly changes with increasing w, and the maximum PL intensity is observed when 77% Mn²⁺ ions were replaced by Cd²⁺. In Cs₄MnBi₂Cl₁₂, each Bi³⁺ ion has three Mn²⁺ adjacencies. The relatively low PLQY of ∼26% measured for the w = 0 sample suggests that energy transfer to the killer site is too easy in Cs₄MnBi₂Cl₁₂ end member (Figure S9e-S9f). Incorporation of Cd²⁺ acts as electron-isolation of Mn − Bi − Mn network, which increases radiative recombination at Mn²⁺ site. It is worth noting that w = 0.67 is quite close to the replacement level where the PL is strongest. On average, this means that each Mn²⁺-centered octahedron neighbors one Bi³⁺-centered octahedron. These results could help us to conclude that the optimal structure puts a single Mn²⁺ ion next to each [BiCl₆]³⁻ octahedron.

Optical and electronic bandgap tailoring

To search for a suitable metal halide perovskite for achieving excellent optoelectronic properties in the VIS and NIR region, the optical properties of RE³⁺ doping in the Cs₄Mn_1 − wCd_wBi₂Cl₁₂ have been investigated in detail. XRD patterns reveal the pure phase products for all syntheses (Figure S10). Variations in the PL spectra of Cs₄Mn_1 − wCd_wBi₂Cl₁₂:RE³⁺ (w = 0.1, 0.3, 0.5, 0.7, 0.9) samples depending on the w values (different Cd/Mn ratios) are shown in Fig. 3a-3c for RE = Ho, Er, Tm, respectively. The PL intensity of RE³⁺ characteristic emission bands for all the samples gradually decrease with increasing w values. While the PL intensity of Mn²⁺ reaches a maximum when w = 0.7, which is consistent with the observation in RE³⁺-undoped Cs₄Mn_1 − wCd_wBi₂Cl₁₂. The gradual weakening of PL intensity of RE³⁺ characteristic emission bands ascribed to the indirect-to-direct bandgap transition is observed. The optical bandgap properties of Cs₄Mn_1 − wCd_wBi₂Cl₁₂ were measured by UV-VIS diffuse reflectance (DR) spectroscopy. The experimentally obtained indirect and direct bandgaps are shown in Fig. 3d. Cs₄MnBi₂Cl₁₂ has the characteristics of an indirect band gap semiconductor, with a shallow absorption at 2.72 eV (indirect band gap) and a large absorption at 3.00 eV (direct band gap). Moreover, the linear region of the T_auc plot of (F(R)hv]^1/2 versus hv reveals the expected phonon-assisted processes, with phonon absorption and emission transitions occurring at 2.72 and 3.00 eV, respectively. The indirect band gap of this material is estimated as 2.72 eV. With introducing Cd²⁺ into Cs₄MnBi₂Cl₁₂, the absorption edge becomes larger. Cs₄CdBi₂Cl₁₂ possesses a characteristic of direct band gap with absorption edge at 3.40 eV, according to the T_auc plot. Schematic illustration of possible electronic dual-bandgap structure for Cs₄CdBi₂Cl₁₂ and Cs₄MnBi₂Cl₁₂ is displayed in Figure S11. To investigate the electrical and optical contributions of the two end members in Cs₄Mn_1 − wCd_wBi₂Cl₁₂, we performed the first-principle calculations for Cs₄MnBi₂Cl₁₂ and Cs₄CdBi₂Cl₁₂. Figure 3e and 3f show the calculated band structures of original Cs₄CdBi₂Cl₁₂ and Cs₄MnBi₂Cl₁₂. For Cs₄CdBi₂Cl₁₂, the valence band maximum (VBM) mainly consists of the Cl 3p orbitals, the conduction band minimum (CBM) is mainly composed of Bi 6p orbitals. They are both located at B point, leading to a direct band gap of 3.0 eV, which is smaller than experimental measurements of 3.40 eV. The energy difference between experimental observations and theoretical calculations is understandable because density functional theory-Perdew-Burke-Ernzerhof (DFT-PBE) calculations underestimate the band gap. Nevertheless, such difference does not affect our qualitative analysis on the electronic and optical properties of Cs₄Mn_1 − wCd_wBi₂Cl₁₂. For Cs₄MnBi₂Cl₁₂, the VBM and CBM mainly originate from the Mn 3d and Bi 6p orbitals, respectively (Fig. 3f). The p–d repulsive force between the Mn 3d and Bi 6p orbitals pushes the CBM state at G point to a lower energy, leading to an indirect band gap of 1.9 eV.

The projected density of states (PDOS) for Cs₄CdBi₂Cl₁₂ shows overlap of Cl 3p in the deeper valence and conduction band states (Figure S12a). A large overlap of Bi 6p orbitals in the conduction band range can be observed. For Cs₄MnBi₂Cl₁₂, the states near both the VBM and CBM are both contributed by the Mn states (Figure S12b). Such change in the band edge states is expected to change the properties of optical transitions close to the band gap energy. The CBM states in Ho³⁺-doped Cs₄MnBi₂Cl₁₂ (Fig. 3g) are mainly composed of Bi 6p, Mn 3d and Ho 3d states, while the CBM states in Ho³⁺-doped Cs₄CdBi₂Cl₁₂ (Fig. 3h) are mainly contributed by the Bi 6p orbitals with neglectable amount of Ho 3d states. Actually, the Bi 6p and Mn 3d states are mixed into the unoccupied Ho 3d state, while the Cl 3p is mixed into the occupied Ho 3d state. Thus, the Bi 6p and Mn 3d states can contribute to the PL process by the optically allowed transitions to the Cl 3p state. On the contrary, for Ho³⁺-doped Cs₄CdBi₂Cl₁₂, the optical transition from the Bi 6p state mixing with unoccupied Ho 3d to the Cl 3p state mixed into the occupied Ho 3d state is neglectable because of its parity forbidden nature (Fig. 3h). Moreover, the amount of Ho 3d state mixed into the Bi 5p state is very small, resulting in the loss of PL intensity (Fig. 3a). For Cd/Mn solid solution, the CBM is mainly contributed by Mn 3d rather than Cd 5s, meaning that the optical properties of solid solution are mainly dominated by Mn constituent.

Energy transfer mechanisms

The luminescence mechanisms of Cs₄Mn_1 − wCd_wBi₂Cl₁₂ have been discussed in previous reports.^27,41,42 Du et al. considered that the weak PL of Cs₄CdBi₂Cl₁₂ is far from the corresponding band-edge emission suggesting the defect emission.⁴³ Kuang et al. recently reported the crystal structure of Cs₄MnBi₂Cl₁₂, which presents a typical red emission of Mn²⁺ at 610 nm.⁴⁴ Each BiCl₆³⁻ octahedron is surrounded by three Mn²⁺-centered octahedra. Diluting the concentration of Mn²⁺ by alloying with Cd²⁺ in Cs₄Mn_1 − wCd_wBi₂Cl₁₂ results in a significant enhancement of PL intensity. Cd: Mn = 2:1, namely, one Mn²⁺-centered octahedron neighbors each Bi³⁺-centered octahedron, is quite close to the substitution level with the strongest PL intensity. Schematic diagram of local structure and luminescence mechanism in RE³⁺-doped Cs₄Mn_1 − wCd_wBi₂Cl₁₂ is proposed in Fig. 4a. Although the strongest PL intensity of Mn²⁺ is the sample at Cd:Mn = 2:1, the transferred energy to the RE³⁺ is very small due to the less Mn²⁺ energy bridge, resulting in a weak RE³⁺ luminescence. In RE³⁺-doped Cs₄MnBi₂Cl₁₂, [BiCl₆]³⁻ will efficiently transfer energy to RE³⁺ through the more bridge function of [MnCl₆]⁴⁻. Figure 4b describes the process of optical absorption and emission in RE³⁺-doped Cs₄MnBi₂Cl₁₂. [BiCl₆]³⁻ octahedron plays an important role as a UV light photosensitizer in exciton production. The Mn²⁺ in [MnCl₆]⁴⁻ octahedron is both the exciton acceptor and emission center to afford the intense PL via ⁴T_1g→⁶A_1g transition. The exciton transfer process from [BiCl₆]³⁻ octahedron to [MnCl₆]⁴⁻ octahedron effectively increases the exciton density in ⁴T_1g, and increasing PL quantum yield. The excitation energy is then transferred to the excited state energy level of RE³⁺. RE³⁺ f-electrons then de-excite via multiple transitions and give transition emission. Generally, defects in the Cs₄CdBi₂Cl₁₂ capture energy from [BiCl₆]³⁻, and most of the energy is released through non-radiative transitions, which makes it impossible to further transfer the energy to RE³⁺ ions. In addition, the energy levels of [BiCl₆]³⁻ and [CdCl₆]⁴⁻ do not match the energy levels of RE³⁺, resulting in RE³⁺ ions with forbidden f-f transitions cannot emit light in the Cs₄CdBi₂Cl₁₂ host. On the contrary, as Mn²⁺ replaces Cd²⁺, the energy level of Mn²⁺ in [MnCl₆]⁴⁻ octahedron can efficiently match the energy level of RE³⁺, which can better absorb the energy from Mn²⁺ to produce light. Under n-UV light excitation, the electrons in the ground state levels of [BiCl₆]³⁻ are excited to the excited state levels, then the excited electrons release energy through two energy transfer processes. Firstly, the energy in excited energy levels could be transferred to ⁴T_1g energy levels of Mn²⁺ and return back to the ground ⁶A_1g energy level with 596 nm orangish-red light. Secondly, the energy in excited ⁴T_1g energy levels of Mn²⁺ could be transferred to ⁵F₅, ⁴F_9/2, ³F₃ energy levels of RE³⁺ (RE = Ho, Er, Tm). Compared with the energy level of [CdCl₆]⁴⁻, the lowest energy level of Mn²⁺ easily transfers energy to ⁵F₅, ⁴F_9/2, ³F₃ energy levels of RE³⁺ (RE = Ho, Er, Tm), as a result, the [BiCl₆]³⁻→Mn²⁺→RE³⁺ energy transfer occurs by Mn²⁺ energy bridge. Moreover, as the Mn²⁺ content increases, the energy transfer efficiency is becoming higher and reaches the highest when it is completely replaced (w = 0), and the strongest RE³⁺ luminescence is achieved.

Steady-state temperature-dependent PL properties

Photophysical mechanisms of RE³⁺-doped Cs₄MnBi₂Cl₁₂ were further experimentally investigated via steady-state temperature-dependent PL spectra. To better illustrate the temperature-dependence of emission, the contour plots are shown in Fig. 5a-5c and Figure S13. Generally, the PL intensity of Mn²⁺ (⁴T_1g→⁶A_1g) decreases with increasing temperatures for three RE³⁺-doped Cs₄MnBi₂Cl₁₂ samples because of an enhancement of non-radiative transition probability. This is ascribed to the enhanced thermal vibration of the matrix lattice at high temperature, the increase of thermally activated phonons and the strengthened interaction between electrons and phonons.^45,46 Based on previous studies, the relationship between PL intensity (I_i) and temperature (T) is expressed as follows:^47,48

here N(T) is the excited states’ population at a certain temperature T, τ denotes the lifetime of excited states, P represents the phonon number, ℏω is the phonon energy, R and W_NR are the radiative and non-radiative transition rate, respectively. After Eq. 3 is merged into Eq. 1, the equation is expressed as:

where ΔE is the energy gap between the excited and ground states. It is clear from Eq. 4 that the PL intensity is mainly affected by two processes: non-radioactive decay and thermal agitation. In the low temperature range, the thermal distribution rate is high while the non-radiation rate is low, resulting in an increase of PL intensity with increasing temperatures. Nevertheless, the non-radiation rate rapidly increases in the high temperature range and the radiation probability significantly decreases. Thus, the PL intensity decreases at high temperatures. In addition, there is a clearly observable difference in temperature-dependence between these RE³⁺ and Mn²⁺ emission peaks. The RE³⁺ emission perform a weaker thermal quenching compared with Mn²⁺ emission. This suggests that the 4f electron of RE³⁺ ions is well screened from the surrounding defect sites, through the closed 5s²5p⁶ outer shell electrons of RE³⁺.³² The specific thermal quenching trend of these emission peaks at VIS and NIR regions is described in Fig. 5d-5f. For Ho³⁺-doped Cs₄MnBi₂Cl₁₂, the integrated PL intensity of Ho³⁺ emission (⁵F₆→⁵I₈, ⁵F₆→⁵I₇) increases with maximum PL intensity at 250 K and then decreases throughout the temperature range of 100 to 400 K due to the associative effects of nonradioactive decay and thermal agitation. For Er³⁺-doped Cs₄MnBi₂Cl₁₂, there are three energy transitions of NIR emission for Er³⁺: ⁴I_9/2→⁴I_15/2, ⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2. Their integrated PL intensity does not change much in the temperature range from 100 to 250 K, and then declines rapidly with increasing temperatures. There is an obvious temperature-dependent difference between NIR and VIS emission peaks, and VIS emission exhibits a stronger thermal quenching. This is largely attributed to the photon energy at VIS region is much higher than that at NIR region. According to Eqs. 1 − 4, the non-radiative decay and quenching activation energy are more significant at VIS region. As a result, the PL intensity at VIS region reduces more strongly with increasing temperatures. For Tm³⁺-doped Cs₄MnBi₂Cl₁₂, a temperature-dependent difference among these NIR emission peaks is clearly observed. The PL thermal quenching from ³H₅→³H₆ is particularly strong. This is attributed to the phenomenon of phonon-assisted population inversion for Tm³⁺ in Cs₄MnBi₂Cl₁₂, increasing the population of the ³H₅ state with the increase of temperature (100–450 K). In addition, the energy transfer should be strongly dependent on the temperature due to the energy difference between excited states of interacting ions. Mn²⁺-Tm³⁺ energy transfer makes the weaker thermal quenching from ³H₄→³H₆. The integrated PL intensity of Tm³⁺ emission (³H₄→³H₆) increases with maximum intensity at 300 K and then decreases due to the cooperative effects of nonradioactive decay and thermal agitation. Moreover, RE³⁺-doped Cs₄MnBi₂Cl₁₂ single crystals exhibit outstanding stability toward ambient conditions. Their excellent structural stability is proved by no distinct decompositions in XRD patterns (Figure S14a) under storage at ambient conditions for 200 days. The stability of these single crystals in various organic solvents was also examined. As shown in Figure S14b, they remained highly stable in ethanol (EtOH), isopropanol (IPA), toluene (PhMe), Acetone (ACE) and hexyl hydride (HDI), and even for 100 days, which is confirmed by the XRD patterns (Figure S14c). Due to the ionic nature of Cs₄MnBi₂Cl₁₂, the emission faded when exposed in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) for 10 h.

Fabrication and performance of pc-LEDs

In order to investigate the practical performances in LED applications of RE³⁺-doped Cs₄MnBi₂Cl₁₂, pc-LEDs were fabricated by assembling the optimized samples with commercially available 400 nm LED chips. Firstly, RE³⁺-doped and undoped Cs₄MnBi₂Cl₁₂ (RE = Ho, Er, Tm) samples with and without the commercial blue BAM:Eu²⁺ and green (BaSr)₂SiO₄:Eu²⁺ phosphors were constructed. The single-color pc-LEDs of RE³⁺-doped and undoped Cs₄MnBi₂Cl₁₂ emit intense orange-red/red light (Fig. 6a) with the CIE xy coordinates of (0.576, 0.399), (0.589, 0.394), (0.579, 0.391) and (0.574, 0.399) for without and with RE = Ho, Er, Tm doping, respectively (yellow symbol in Figure S15). Four fabricated pc-WLEDs of RE³⁺-doped and undoped Cs₄MnBi₂Cl₁₂ produce warm white light with full-spectrum covering the whole VIS region (Fig. 6b). The CIE xy coordinates are (0.349, 0.394), (0.384, 0.395), (0.358, 0.392) and (0.352, 0.376), respectively (white symbol in Figure S15).

These four fabricated pc-WLEDs exhibit low correlated color temperature (CCT) of 4999, 4033, 4732, 4870 K and high CRI (Ra = 83.3, 93.0, 87.9, 88.3) for undoping, doping with RE = Ho, Er, Tm, respectively. It is worth noting that the RE³⁺ doping results in the decreased CCT value and increased CRI value, because the contribution of red component is greater than that of orange-red component in pc-WLEDs. As shown in the images, the working devices based on the mixed luminescent materials emit bright warm white light, which demonstrates that RE³⁺-doped Cs₄MnBi₂Cl₁₂ samples are promising for lighting applications. Secondly, the full-spectrum from VIS to short wavelength infrared (VIS-SWIR) LED was designed by RE³⁺-codoped Cs₄MnBi₂Cl₁₂ (RE = Ho, Er, Tm) samples and the PL spectrum is given in Fig. 6c. This PL emission spectrum is composed of a broad emission band at 596 nm due to the ⁴T_2g→⁶A_1g transition of Mn²⁺ and other narrow emission band at 645, 802, 985, 1158, 1432, 1540 nm due to ⁵F₅→⁵I₈ (Ho³⁺), ³H₄→³H₆ (Tm³⁺), ⁵F₅→⁵I₇ (Ho³⁺), ⁴I_11/2→⁴I_15/2 (Er³⁺), ³H₄→³F₄ (Tm³⁺), ⁴I_13/2→⁴I_15/2 (Er³⁺) transitions. Due to the unique spectral characteristics of SWIR light, such as invisible to the naked eye and special penetration ability, the fabricated SWIR LED can be used for night vision surveillance and penetration applications. Under a fluorescent light, the part of the emission hand covered by the 750 nm filter is invisible on viewing by a visible camera (Fig. 6d-i). When the NIR pc-LED is added and turned on, the obscured hand is clear in the NIR camera. The NIR emission from the pc-LED can pass through the filter. Furthermore, the NIR camera can also detect the tissue damage in fruits, which is difficult to detect by the naked eye (Fig. 6d-ii). NIR spectroscopy is considered as an effective method for qualitative and quantitative analysis in the food and agricultural industries due to the different NIR optical absorption responses.⁴⁹ To demonstrate the application prospect of NIR spectral analysis, the NIR PL spectra penetrated by NIR light emitted by RE³⁺-codoped Cs₄MnBi₂Cl₁₂ (RE = Ho, Er, Tm) were measured with water and ethanol as representatives (Fig. 6e). Clearly, this NIR-emitting phosphor shows a characteristic NIR absorption response. Three absorption band ranges located at around 950–1000, 1150–1250 and 1400–1550 nm can be detected (Fig. 6f). They correspond to the characteristic overtones of the O–H stretch and C-H stretch according to the absorption spectrum of water and ethanol, respectively.

In summary, a series of RE³⁺ (RE = Ho, Er, Tm, Nd) doped Cs₄Mn_1 − wCd_wBi₂Cl₁₂ single crystals are prepared by a facile hydrothermal method. The RE³⁺-doped lead-free layered double perovskite keeps the rhombohedra structure and combines efficient Mn²⁺ and RE³⁺ emission covering the red light to NIR region (520–1650 nm) and improving PLQY via building Mn²⁺ energy bridge. By gradually substitution of Cd²⁺ with Mn²⁺, the band gap changes from indirect band gap to direct band gap, which is confirmed by experimental observation and theoretical calculation. Furthermore, we explored the associated emission mechanism through NIR spectra and PL attenuation, confirming the existence of the energy transfer channel from Mn²⁺ to RE³⁺. The [BiCl₆]³⁻→Mn²⁺→RE³⁺ energy transfer occurs by Mn²⁺ energy bridge because of the matched energy level of Mn²⁺ in [MnCl₆]⁴⁻ octahedron and RE³⁺. Thanks to the effective RE³⁺ emission, the CRI of pc-WLEDs using RE³⁺-doped Cs₄MnBi₂Cl₁₂ increases from 83.3 to 93.0 compared to the pristine Cs₄MnBi₂Cl₁₂. Finally, we demonstrate a promising NIR pc-LED based on RE³⁺-doped Cs₄MnBi₂Cl₁₂ for night-vision and penetration applications. Such strategies imparting optical functionality into metal halide perovskites may lead to some future applications such as optical fiber communication, NIR LEDs, and NIR sensors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC No. 51932009, 51720105015, 51929201, 52072349, 52172166), Science and Technology Cooperation Project between Chinese and Australian Governments (2017YFE0132300), the Natural Science Foundation of Zhejiang Province (LR22E020004).

Conflict of interest

The authors declare no competing financial interests.

Supplementary information accompanies the manuscript on the Light: Science & Applications website (http://www.nature.com/lsa/)

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Tailoring red-NIR luminescence in rare earth doped lead-free layered double perovskites by bivalent manganese energy bridge

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