Red room-temperature phosphorescence (RTP), with long emission wavelength and extended lifetime, holds significant potential for a wide range of applications, including bioimaging, light-emitting diodes (LEDs), information encryption, and photodetectors (Zhao et al., 2023; Deng et al., 2023; Fateminia et al., 2017; Xu et al., 2023; Wang et al., 2019a; Wang et al., 2019b; Zhu et al., 2022; Zhao et al., 2020a). Two-dimensional (2D) hybrid halide perovskites are a novel type of organic-inorganic hybrid material, where the integration of organic and inorganic components at the molecular level provides an effective platform for energy transfer (Su et al., 2022; Li et al., 2020; Gong et al., 2018; Gao et al., 2021; Zhang et al., 2019; Peng et al., 2017; Hsu et al., 2021; Liu et al., 2022; Liu et al., 2023). The emission spectra of 2D perovskites can be systematically tuned through employing different types of organic chains to modulate the radiative recombination pathway (Deng et al., 2023; Zhao et al., 2020b; Ma et al., 2019). This unique 2D structure allows for the generation of strong and persistent RTP, making 2D hybrid halide perovskites promising candidates for the development of next-generation RTP materials. Additionally, the rigid inorganic layers in 2D hybrid halide perovskites can enhance the phosphorescence of the organic components by controlling the energy transfer from Wannier excitons to triplet excitons (Ema et al., 2008). In a previous report, Lam et al. successfully integrated organic luminophores into 2D hybrid halide perovskites, achieving red RTP through energy transfer from the inorganic layer to the organic triplet state (Hu et al., 2018). However, the resulting RTP exhibited a short lifetime of just 2.1 ms and a low photoluminescence quantum yield (PLQY) of only 3%. Therefore, although red RTP has been demonstrated in 2D hybrid halide perovskites, challenges such as low PLQY remain, highlighting the need for further development of efficient red RTP materials in 2D hybrid halide perovskites systems.

Compared with the noble metals, manganese element with an atomic number of 25 has abundant reserves, is environmentally friendly and inexpensive. Manganese ion (Mn²⁺) dopants, having a 3 d⁵ electron configuration, can introduce a new emission center in semiconductor luminescent materials, endowing them promising photoluminescence properties, such as a wide color tuning range, reversible color change, reduced self-absorption etc. Specially, Mn²⁺ ion has been widely utilized as a dopant to induce red emission in various halide perovskites (Zhang et al., 2020; Han et al., 2022; Locardi et al., 2018; Das Adhikari et al., 2017; Sun et al., 2019; Li et al., 2019; Wang et al., 2022; He et al., 2022; Chen Y. et al., 2022). For example, Wang et al. successfully synthesized zero-dimensional (0D) (ABI)₄MnBr₆ halide perovskites (ABI = 2-aminobenzimidazole), which exhibits red light emission with a peak wavelength at 629 nm and a PLQY of 80% (Yan et al., 2021). The strong red emission is attributed to the d-d orbital transition (⁴T₁→⁶A₁) of Mn²⁺ coordinated in an octahedral configuration within the 0D perovskite structure. Moreover, Zhang et al. reported the hydrothermal synthesis of a crystal by doping Mn²⁺ into another 0D double perovskite Cs₂NaInCl₆ (Chen X. et al., 2022). This transparent crystal exhibits red emission at 610 nm under ultraviolet (UV) excitation and sustains luminescence for up to 2,500 s after the excitation is turned off. In addition, Han et al. developed a Mn²⁺-doped CsCdCl₃ perovskite crystal, which demonstrates robust luminescence and RTP properties even at high temperatures (423 K) and in humid environments (He et al., 2022). The concentration of Mn²⁺ not only regulates the duration of luminescence but also enhances the PLQY from 37.1% to 91.4%. In another study, Kundu et al. synthesized 2D Mn²⁺-doped (C₄H₉NH₃)₂PbBr₄ perovskite material, and in-depth mechanism exploration confirms efficient energy transfer from host excitons to Mn²⁺, resulting in intense orange-yellow luminescence with a maximum PLQY of 37% (Biswas et al., 2017). Aforementioned findings indicate that Mn²⁺ serves as a unique luminescent center, and doping with Mn²⁺ not only induces new optical phenomena but also imparts additional properties to materials, such as enhanced stability and quantum efficiency.

In this work, a series of 2D hybrid halide perovskite luminescent material were synthesized by doping Mn²⁺ into 2D hybrid halide perovskites (C₈H₉NO₂)₂CdCl₄, abbreviated as P-MACC. The electron-donating alkoxy groups on the benzene ring were designed to fine-tune the electron density and shift their triplet levels. As expected, the resulted materials P-MACC:x%Mn, where x represents the theoretical molar doping percentage of Mn²⁺, exhibit strong red phosphorescence emission at 625 nm, attributed to the ⁴T₁→⁶A₁ transition of Mn²⁺. At room temperature, P-MACC:20% Mn demonstrates a maximum PLQY of 66.89%, which is eight times higher than that of P-MACC (8.69%), with Mn²⁺ ions lifetime of 12 ms. Time-resolved photoluminescence spectra reveal an efficient energy transfer from the triplet states of P-MACC to the dopant Mn²⁺, facilitating the high-efficiency red RTP. Subsequently, based on the superior luminescent properties of P-MACC:20% Mn, electroluminescent LEDs were assembled to highlight their potential as highly efficient luminescent materials for illumination applications.

Powder X-ray diffraction analysis was conducted on as-prepared halide perovskites P-MACC and its Mn²⁺-doped counterparts to determine whether the doping process affects the materials’ phase structure. As shown in Figure 1A, the PXRD patterns indicate that P-MACC:x%Mn exhibits PXRD characteristics similar to those of P-MACC, with strong periodic diffraction peaks along the (00L) planes. No significant diffraction peaks from other crystal planes were observed, further confirming that P-MACC:x%Mn shares the same space group as P-MACC, and the original material’s structure remains intact. Moreover, P-MACC:x%Mn retains the preferred orientation along the c-axis, which favors the formation of a layered structure with alternating inorganic and organic layers (Smith et al., 2019). Besides, the sharp diffraction peaks and increased diffraction intensity of the (00L) planes suggest that Mn²⁺ doping enhances the material’s crystallinity.

2D hybrid halide perovskite materials are always characterized by a layered structure, typically described by the interlayer spacing parameter (Mao et al., 2019). The interlayer spacing d refers to the distance between two adjacent inorganic layers in the material’s structure. This spacing can be calculated using the data from the first diffraction peak in the PXRD pattern and Bragg’s equation (2dsinθ = nλ, where θ denotes the diffraction angle, λ stands for the X-ray wavelength, and n refers to the diffraction order). As shown in Figure 1B, within the doping range of 0.1%–20% Mn²⁺, the interlayer spacing shows a slight decreasing trend. This decrease is attributed to the smaller ionic radius of Mn²⁺ compared to Cd²⁺, leading to lattice contraction and thus a shift of the diffraction peak at 2θ ≈ 5° towards higher angles (Liu et al., 2018a; Liu et al., 2018b; Liu et al., 2018c; Liu Z. et al., 2017). However, when the Mn²⁺ doping concentration reaches 30%, the first diffraction peak exhibits an anomalous shift toward lower angles. This phenomenon may be due to that excess Mn²⁺ not only substitutes Cd²⁺ in the inorganic layers but also occupies interlayer spaces, leading to an increase in the interlayer spacing.

The positions of absorption peaks in Fourier-transform infrared (FTIR) spectroscopy can be used to identify specific functional groups and chemical bonds within a material, allowing for further investigation into the internal structure and vibrational modes of organic components before and after Mn²⁺ doping. As illustrated in Supplementary Figure S4, both P-MACC and P-MACC:20% Mn exhibit N-H vibrational peaks corresponding to the amino group (-NH₂), consistent with the characteristic vibrational peaks of the reactants' chemical structures. The N-H absorption peak of–NH₂ in P-MACC appears around 3,066 cm^-1, and in P-MACC:20% Mn, it shifts slightly to around 3,064 cm^-1, indicating no significant peak position displacement. Furthermore, a comparison of the FTIR spectra reveals that the shapes, intensities, and positions of other absorption peaks remain remarkably similar, suggesting that the incorporation of Mn²⁺ at a certain concentration does not significantly affect the infrared vibrations of organic amines in the compounds. Notably, the FTIR spectrum of the P-MACC perovskite shows a prominent characteristic vibrational peak around 3,500 cm^-1, which is likely attributed to the hydroxyl (-OH) peak from water in the air.

Scanning electron microscopy (SEM) can be used to observe the microstructure of materials, providing insight into their morphological characteristics. SEM analysis was conducted on the 2Dhybrid halide perovskites P-MACC and P-MACC:20% Mn. As shown in Figures 2A, B, both P-MACC and P-MACC:20% Mn exhibit a layered structure formed by the stacking of sheet-like growths. This is attributed to the influence of larger organic ligands in 2D perovskites, which results in a more pronounced layered morphology compared to three-dimensional halide perovskites (Castelli et al., 2019). Additionally, this indicates that the material retains the layered characteristics of 2D perovskites after Mn²⁺ doping. Elemental mapping can further reveal the distribution of elements within the materials. As shown in Figures 2C, D, both P-MACC and P-MACC:20% Mn contain Cd and Cl elements, while P-MACC:20% Mn also includes Mn, with all elements uniformly distributed throughout both materials. Figures 2E, F display the energy-dispersive X-ray spectroscopy (EDS) spectra for P-MACC and P-MACC:20% Mn, respectively. The comparison confirms the presence of Mn²⁺ in the P-MACC:20% Mn, although at a relatively low doping level (1.39%), which is consistent with Inductively coupled plasma mass spectrometry (ICP-MS) result (1.51%). This finding further demonstrates the successful incorporation of Mn²⁺ into P-MACC host. Additionally, the actual molar ratios of Cd to Cl in both materials are approximately 1:4.11 and 1:3.70, respectively, which are close to the theoretical molar ratio of 1:4 for 2D perovskites, confirming that both materials exhibit the characteristics of 2D structure and the chemical formula should be (P-MA)₂(Cd_1-xMn_x)Cl₄.

To further confirm the successful incorporation of Mn²⁺ into P-MACC host, we conducted electron paramagnetic resonance (EPR) spectroscopy analysis. Supplementary Figure S5 displays the EPR spectrum of P-MACC:20% Mn, revealing a sextet hyperfine coupling pattern characteristic of the unpaired 3 d⁵ electron spin (S = 5/2) interacting with the nuclear spin (I = 5/2) (Mantel et al., 2004). The average hyperfine coupling constant for P-MACC:20% Mn is 9.38 mT (93.8 G), which is comparable to the hyperfine coupling constant observed in Mn-doped L₂PbX₄ systems. As a result, the sextet hyperfine splitting confirms the presence of Mn²⁺ within P-MACC crystal lattice.

Thermal stability is a crucial property of halide perovskites, with higher thermal stability generally indicating superior performance. To investigate the changes in thermal stability before and after Mn²⁺ doping, thermogravimetric analysis (TGA) was conducted on P-MACC and P-MACC:20% Mn over a temperature range of 25°C–800°C. As shown in Supplementary Figure S6, the initial thermal decomposition temperature of P-MACC:20% Mn is 284°C, which is 17°C higher than that of P-MACC, indicating that the introduction of Mn²⁺ enhances the thermal stability of P-MACC. In addition, both P-MACC and P-MACC:20%Mn exhibit two distinct decomposition stages. For P-MACC, the first stage begins at 267°C with a mass loss of approximately 32%, attributed to the volatilization of organic components. The second stage starts at 432°C, with a mass loss of around 42%, primarily due to the volatilization of inorganic CdCl₂. In contrast, the first decomposition stage for P-MACC:20% Mn begins at 284°C with a mass loss of about 39%, also due to the volatilization of organic molecules. Notably, P-MACC:20% Mn shows a minor mass loss of approximately 1.5% before reaching 284°C, likely due to the desorption of a small amount of solvent or moisture. The second decomposition stage begins at 470°C, with a mass loss close to 32%, similarly attributed to the volatilization of inorganic CdCl₂.

To investigate the impact of Mn²⁺ doping on the luminescent properties of the materials, we characterized their optical properties using fluorescence and phosphorescence spectroscopy. Figure 3A shows the fluorescence spectra of the pristine and Mn²⁺-doped 2Dhybrid halide perovskites. For a fair comparison of intensity, all samples were prepared with equal amounts of powder and excited at a wavelength of 270 nm. The emission spectrum of pure P-MACC exhibits a sharp emission peak at 320 nm, accompanied by a weak broad emission band at longer wavelengths. In contrast, P-MACC:x%Mn materials display a strong broad emission centered at 625 nm. Although there is also an emission peak at 320 nm, its intensity decreases with increasing Mn²⁺ doping, while the emission intensity at 625 nm gradually intensifies. Figure 3B presents the phosphorescence spectra of the materials. The pristine P-MACC shows dual emission peaks at 525 nm and 615 nm. However, after Mn²⁺ doping, the peak at 525 nm gradually disappears, as shown in the inset of Figure 3B, and the spectrum shifts to a broad emission with a dominant peak centered at 625 nm. We speculate that the energy at 525 nm is being transferred to the 625 nm peak, thereby achieving higher emission efficiency.

To gain a clearer understanding of the materials’ color characteristics, we processed the fluorescence and phosphorescence spectra data using the Commission Internationale de l’Eclairage (CIE) standard color measurement system. As shown in Figures 3C, D, the fluorescence color coordinate of P-MACC is (0.41, 0.39), placing the color in the orange-yellow region. With the introduction of Mn²⁺ doping, the color coordinates shift to around (0.61, 0.37), moving the color towards the red region. For phosphorescence, the color coordinate of P-MACC is (0.54, 0.42), and Mn²⁺ doping shifts the coordinate to approximately (0.61, 0.36). Overall, both the fluorescence and phosphorescence color regions of MACC:x%Mn are predominantly orange-yellow, but with increasing Mn²⁺ doping, the color shifts progressively towards red. Aforementioned results can be visually confirmed by the photographs of P-MACC:x% Mn powders with varying Mn²⁺ doping concentrations under ambient and UV light (Supplementary Figure S7).

To confirm that the 625 nm emission peak is attributable to the internal energy level transitions of Mn²⁺, we conducted relevant characterizations on both pristine P-MACC and Mn²⁺-doped counterparts. Figure 4A presents the photoluminescence excitation (PLE) spectra of P-MACC with varying Mn²⁺ doping concentrations. The sharp exciton peaks in the 200 nm–350 nm range are attributed to the absorption of the host excitons. Interestingly, MACC:x%Mn samples show additional characteristic peaks in the 400 nm–450 nm range, which correlate positively with the Mn²⁺ concentration. This result suggests that in the doped system, not all absorption peaks arise from the host excitons, but intermediate bandgap states also emerge with Mn²⁺ doping. The relatively low intensity of these intermediate bandgap states indicates that the emission energy of Mn²⁺ primarily originates from the absorption of the host excitons. To further understand the emission behavior of Mn²⁺ within P-MACC host, we examined the temperature-dependent emission spectra of P-MACC:5% Mn over a range of 78 K–298 K. As shown in Figure 4B, as the temperature increases from 78 K to 298 K, the emission band of MACC:5% Mn shows a blue shift from 655 nm to 625 nm. This shift is due to the thermal expansion of the host lattice, which reduces the d-d orbital splitting of Mn²⁺, consequently widening the ⁴T₁→⁶A₁ bandgap (Zhang et al., 2020; Yuan et al., 2017). This behavior is similar to the characteristics observed in Mn²⁺-doped ZnS and ZnSe nanocrystals (Yuan et al., 2014).

To verify the energy transfer between P-MACC host Mn²⁺ dopant, we conducted time-resolved spectroscopy measurements on the phosphorescence peaks of the materials before and after Mn²⁺ doping. As shown in Figure 5, with initially increasing doping concentration of Mn²⁺, the emission lifetime of P-MACC host at 525 nm gradually quenches, while the lifetime at 625 nm shows a slight increase. However, as the doping concentration further increases, the lifetime at 625 nm remains nearly unchanged. The slight increase in lifetime at 625 nm may be due to some degree of energy transfer from the excitons at the nearby energy level. Since the energy at 615 nm is relatively low, the increase in Mn²⁺ concentration does not significantly extend the lifetime at 625 nm. Combined the energy quenching of P-MACC host and increased luminescence intensity of Mn²⁺, it is evident that the energy at 525 nm is utilized to enhance the overall luminescence of the materials.

To gain a clearer understanding of the spectral peak lifetimes before and after Mn²⁺ doping, we conducted time-resolved emission spectroscopy (TRES) characterizations for P-MACC and P-MACC:20%Mn, as shown in Supplementary Figure S8. Results reveal that, as time progresses, the emission intensity gradually decreases for both samples. Under the same delay conditions, P-MACC:20% Mn exhibits the strongest emission intensity in the range of 560 nm–660 nm, while P-MACC shows the strongest emission intensity in the 450 nm–600 nm range. This result indicates that Mn²⁺ doping alters the peak position of the strongest emission intensity. Additionally, the emission intensity of P-MACC and P-MACC:20%Mn reach their approximately three orders of magnitude at 0.75 s and 1.5 s, respectively. We also collected the time-resolved decay curves of P-MACC:x%Mn monitored at 320nm, 525 nm and 625 nm, respectively, as shown in Figure 5 and Supplementary Figure S9. We found that the fluorescence lifetimes are nearly consistent (∼1.99 ns). We hypothesize that there is a large energy gap between the fluorescence emission peak of the P-MACC host and the emission of Mn²⁺ ions, which may make energy transfer difficult (Supplementary Figure S9). Clearly, P-MACC has a longer lifetime across the entire spectral range than P-MACC:20% Mn. Specifically, the lifetime at the 525 nm peak is significantly longer in P-MACC compared to P-MACC:20%Mn, further confirming that the energy at 525 nm decreases after Mn²⁺ doping. Therefore, we can calculate the energy transfer efficiency (Φ) using the following equation Φ = 1- τ/τ₀ where τ, τ₀ are the lifetime monitored at 525 nm in the presence and absence of Mn²⁺ ions, respectively. The Φ reaches 76% for P-MACC:20%Mn suggesting that a large proportion of triplet excitons has been transferred to the excited states of Mn²⁺ ions.

To further validate the hypothesis that the energy at 525 nm is utilized to enhance the material’s luminescence, we conducted PLQY characterizations on P-MACC and P-MACC:20% Mn samples. Figure 6A illustrates the PLQY of the materials as a function of Mn²⁺ doping concentration. As the doping percentage rises from 0.1% to 20%, PLQY increases from 8.69% to 66.38%, an improvement of nearly 8 times. This enhancement is attributed to the introduction of new energy levels by Mn²⁺ and effective energy transfer, thereby enabling efficient Mn²⁺ emission (Liu H. et al., 2017). However, when the doping percentage exceeds 20%, excessive Mn²⁺ leads to the degraded quality of host crystal lattice and enhanced Mn²⁺-Mn²⁺ interactions, causing luminescence quenching (Yuan et al., 2017). Figure 6B shows the relationship between the relative luminescence intensity at 525 nm and 625 nm and the Mn²⁺ content after spectral normalization. As the Mn²⁺ content increases, the relative intensity declines, indicating that the energy at 525 nm is transferred to Mn²⁺, which in turn induces efficient phosphorescence emission. Based on the above discussion, emission mechanism for the high-efficiency luminescence of P-MACC:x%Mn is proposed, as depicted in Figure 6C. Under UV excitation, the system is excited to the S₁ state, which can undergo intersystem crossing to the T₁ state, followed by a transition to the self-trapping exciton (STE) state, resulting in corresponding energy level emission. Mn²⁺ doping introduces a new energy level (internal transition ⁴T₁→⁶A₁), enabling energy transfer from STE state to ⁴T₁ state, or from the T₁ state to ⁴T₁ state, thereby facilitating the ⁴T₁→⁶A₁ radiative recombination. The high-efficiency luminescence of the Mn²⁺-doped materials is attributed to two factors: 1) Mn²⁺ doping enables efficient utilization of energy; 2) Mn²⁺ provides an effective pathway for the recombination of ⁴T₁ excited state to ⁶A₁ ground state. As a result, the PLQY of P-MACC:x%Mn is remarkably improved compared to pristine P-MACC.

Based on the excellent luminescent properties and thermal stability of P-MACC:x%Mn, we demonstrate their lighting applications. P-MACC:20% Mn samples are selected to directly coat onto LEDs chips, and the devices are driven with varying voltages to achieve emission, followed by spectral testing. Figure 7A shows the fluorescence spectra under different voltage drives, with the inset displaying photos of the fabricated LED devices. As the voltage increases, the emission intensity increases accordingly. Figure 7B presents the phosphorescence spectra under different driving voltage, that higher working voltage leads to stronger phosphorescence. Figures 7C, D indicate that the color coordinates of both fluorescence and phosphorescence remain relatively stable with varying voltages, demonstrating the materials’ excellent voltage stability.

In this work, a series of 2D hybrid halide perovskite P-MACC:x%Mn were synthesized by partially substituting Cd²⁺ with Mn²⁺, achieving efficient red RTP emission. Structural characterizations revealed that the successful incorporation of Mn²⁺ did not alter the original 2D perovskite layer structure. The P-MACC:x%Mn exhibited strong red phosphorescence emission at 625 nm, attributed to the ⁴T₁→⁶A₁ energy transition of Mn²⁺. In addition, the introduction of Mn²⁺ significantly enhanced the luminescence efficiency, with the PLQY increasing from 8.69% to 66.38%, representing an 8.0-fold improvement. As the Mn²⁺ doping concentration increased, the lifetime at 525 nm gradually decreased, which is resulted from that Mn²⁺ doping introduced new luminescent energy levels and enabled efficient energy transfer from host to Mn²⁺. Based on the superior optical properties of P-MACC:20% Mn, the material was coated onto LEDs to create red-emitting devices. This study demonstrates that Mn²⁺ plays a crucial role in improving the luminescence efficiency of 2D hybrid halide perovskites, providing a pathway for achieving efficient RTP in 2D hybrid halide perovskite systems.