Improved Stability and Photoluminescence Yield of Mn2+-Doped CH3NH3PbCl3 Perovskite Nanocrystals

Abstract

Organic–inorganic CH₃NH₃PbCl₃ perovskite nanocrystals (PNCs) doped with Mn²⁺, CH₃NH₃Pb_xMn_1−xCl₃, have been successfully prepared using a reprecipitation method at room temperature. Structural and morphological characterizations reveal that the CH₃NH₃Pb_xMn_1−xCl₃ PNCs with cubic phase transforms from particles to cubes and increases in size from 16.2 ± 4.4 nm in average diameter to 25.3 ± 7.2 nm in cubic length after the addition of Mn²⁺ precursor. The CH₃NH₃Pb_xMn_1−xCl₃ PNCs exhibit a weak exciton emission at ~405 nm with a low absolute quantum yield (QY) of around 0.4%, but a strong Mn²⁺ dopant emission at ~610 nm with a high QY of around 15.2%, resulting from efficient energy transfer from the PNC host to the Mn²⁺ dopant via the ⁴T₁→⁶A₁ transition. In addition, the thermal and air stability of CH₃NH₃Pb_xMn_1−xCl₃ PNCs are improved due to the passivation with (3-aminopropyl) triethoxysilane (APTES), which is important for applications such as light emitting diodes (LEDs).

1. Introduction

As a promising candidate for photovoltaics and lighting application, lead halide perovskite (LHP) afford many intriguing advantages over traditional semiconductor materials such as simple fabrication [1,2,3,4], abundant precursors source, defect tolerance [5,6], high photoluminescence (PL) quantum yields (QY) [5,7,8,9], halogen dependent emission over the entire visible spectral range with narrow full width at half maximum (FWHM) [10,11], as well as long carrier lifetime and diffusion length [12]. Compared with the intensive investigations of lead bromide and iodide perovskite [13,14,15,16,17], lead chloride perovskites have received less attention due to its low PL QY and large bandgap (~2.97 eV), which is less desirable for light emitting diodes (LEDs) and solar cells applications [18].

Doping metal ions into nanocrystals is an effective approach to tuning their optical, electronic and magnetic properties, as has been widely applied to II-VI metal chalcogenide nanocrystals [19,20]. Recently, several heterovalent or isovalent metal ions such as Bi³⁺ [21], Al³⁺ [22], Sn²⁺ [23], Cd²⁺ [23], Zn²⁺ [23], and Mn²⁺ [24,25,26,27,28,29,30] have been introduced into LHP as dopants to modulate their optical properties. Among these metal ions, Mn²⁺-doped all inorganic CsPbCl₃ perovskite nanocrystals (PNCs) have attracted increasing attention owing to the appropriate band alignment of CsPbCl₃ and d-d transition of Mn²⁺ (2.15 eV, ⁶A₁→⁴T₁), resulting in efficient intraparticle energy transfer between the host exciton and dopant ions [25,31]. Moreover, the incorporation of Mn²⁺ may provide the possibility of imparting paramagnetism to the host materials [31].

In general, hot injection approach has been applied in the preparation of Mn²⁺-doped lead halide PNCs because of the homogeneous products and relatively simple purification process [32,33]. However, the highly strict conditions of this method and poor stability of the resulting PNCs greatly hinder their applications. In contrast, organic-inorganic hybrid CH₃NH₃PbCl₃ PNCs can be easily prepared via solution-based reprecipitation approach at room temperature when (3-aminopropyl) triethoxysilane (APTES) is used as a capping ligand [34,35]. Additionally, the SiO₂ shell resulting from the hydrolysis of APTES capping ligand can greatly improve the product yield and stability of CH₃NH₃PbCl₃ PNCs towards water and air [34].

In this work, we demonstrate that Mn²⁺ can be doped into CH₃NH₃PbCl₃ PNCs via a reprecipitation method. By adjusting the doping concentration of Mn²⁺, CH₃NH₃Pb_xMn_1−xCl₃ PNCs with a strong dopant emission located at 610 nm were obtained. Besides the greatly improved overall QY of CH₃NH₃Pb_xMn_1−xCl₃ PNCs (from ~0.4% to ~15.6%) upon Mn²⁺ doping, the thermal and air stability have also been enhanced by the SiO₂ shell formed from hydrolysis of APTES. The combined stability and high PL QY make these PNCs potentially useful for various photonics applications including LEDs.

2. Experimental Section

2.1. Materials

All the chemicals were used as received without further purification, including PbCl₂ (99.99%, Aladdin Chemical Co., Ltd., Shanghai, China), CH₃NH₂ (40%, TCI Chemical Co., Ltd., Tokyo, Japan), MnCl₂ (99%, Aladdin Chemical Co., Ltd., Shanghai, China), APTES (99%, Aladdin Chemical Co., Ltd., Shanghai, China), HCl (37.5%, Xilong Scientific Co., Ltd., Shantou, China), toluene (AR, Xilong Scientific Co., Ltd., Shantou, China), N,N-dimethylformamide (DMF, spectroscopic grade, 99.9%, Aladdin Chemical Co., Ltd., Shanghai, China), oleic acid (85%, TCI Chemical Co., Ltd., Tokyo, Japan), octylamine (99%, Fisher Scientific Co., Ltd., Pittsburgh, PA, US), ethyl acetate (99%, Aladdin Chemical Co., Ltd., Shanghai, China).

Mn²⁺-doped CH₃NH₃PbCl₃ PNCs: CH₃NH₃Cl was prepared according to a reported procedure [18]. The Mn²⁺-doped CH₃NH₃PbCl₃ PNCs were synthesized following a method reported previously with some small changes [1]. 1.5 mL precursor solution of PNCs was prepared by dissolving 0.05 mmol CH₃NH₃Cl, 0.05 mmol (PbCl₂ + MnCl₂), 0.05 mmol APTES or octylamine and 40 μL oleic acid in 1.5 mL DMF solvent and ultrasonicated until the solution became transparent. Then, 10 μL, 25 μL, 50 μL and 100 μL of the precursor solution were injected slowly into 5 mL toluene to form CH₃NH₃PbCl₃ PNCs solution. Mn²⁺-doped CH₃NH₃PbCl₃ PNCs with different doping concentration of Mn²⁺ were prepared by varying the ratio between PbCl₂ and MnCl₂. The solid samples were obtained by adding ethyl acetate in toluene to precipitate CH₃NH₃PbCl₃ PNCs and centrifuged at 4000 rpm subsequently, then re-dispersed in toluene. These steps were repeated three times. At last, the dry samples were collected by placing the samples at 50 °C oven overnight.

Mn²⁺-doped CH₃NH₃PbCl₃ PNCs films: The films were prepared by dropping purified solution on microslides and dried at room temperature.

2.2. Characterization

Absorption and photoluminescence properties of the samples were collected with circular dichroism spectra (MOS450, Biologic Science Instruments, Seyssinet-Pariset, France) and fluorescence spectra (PTI QM-TM, Photon Technology International, Ontario, Canada), respectively. Absolute QY was recorded on HAMAMATSU C11347 spectrometer (Hamamatsu Photonics Instruments, Hamamatsu City, Japan) by testing samples in toluene. Luminescence lifetime measurements were collected on EDINBURGH INSTRUMENTS (FLS920, Edinburgh Instruments Ltd., Livingston, UK). X-Ray diffraction (XRD, MiniFlex 600, Rigaku Corporation, Tokyo, Japan) analysis was used to obtain the crystalline phase. The scanning angle range was 10–50° (2θ) with a rate of 3°/min. Temperature-dependent Powder XRD (Ultima IV, Rigaku Corporation, Tokyo, Japan) were studied to indicate the thermal stability of samples. Transmission electron microscopy (TEM, Tecnai G2 F20, FEI Technologies, Hillsboro, OR, USA) were carried out to obtain the morphology and interlayer spacing of samples at an accelerating voltage of 200 kV. The elemental analysis was conducted on inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 720, Agilent Technologies, Santa Clara, CA, USA).

3. Results and Discussion

3.1. Structural and Morphological Characterizations

The structure and morphology of the prepared samples were determined by XRD and TEM measurements, respectively. For the nominal doping concentration of Mn²⁺ in 0~75 at.% range, the XRD pattern (Figure 1) shows a series of diffraction peaks that can all be attributed to cubic phase CH₃NH₃PbCl₃ perovskite structure (space group: Pm$\overline{3}$m), demonstrating that the incorporation of Mn²⁺ has little effect on the crystal structure of CH₃NH₃PbCl₃ [18]. With the addition of 90 at. % MnCl₂, some diffraction peaks attributed to CH₃NH₃Cl and MnCl₂ precursors could be clearly observed due to the excess amount of precursors. Note that the broad band ranging from 15° to 38° is characteristic of amorphous silica resulting from the hydrolysis of APTES, which has been clearly elucidated in our previous work [34]. To further determine the concentration of Mn²⁺ dopant, ICP-AES was used, with results summarized in Table S1. In the synthesis of Mn²⁺-doped CH₃NH₃PbCl₃ PNCs, it was found that APTES would react with MnCl₂ to form a puce complex at a high concentration of Mn²⁺ precursor, as shown in Figure S1. The color of CH₃NH₃Pb_xMn_1−xCl₃ PNCs capped with APTES under room light is deeper than that of CH₃NH₃Pb_xMn_1−xCl₃ PNCs passivated with octylamine owing to the adsorption of Mn²⁺-APTES complex, indicating that the detected concentration in Table S1 are higher than the actual concentration of Mn²⁺ in the crystal lattice.

To determine the morphology evolution and size distribution, TEM images were recorded, as shown in Figure 2. For undoped CH₃NH₃PbCl₃ PNCs (Figure 2a), the PNCs are mostly spherical with an average diameter of 16.2 ± 4.4 nm. The PNCs are embedded in amorphous silica matrix (Figure S2), which results from the hydrolysis of APTES. When MnCl₂ precursor was added, the resulting Mn²⁺-doped PNCs are mostly cubic with an average dimension increasing from 10.4 ± 5.1 nm (Figure 2b) to 25.3 ± 7.2 nm (Figure 2e) as the Mn²⁺ concentration was increased from 25 to 90 at. %. Due to the complex reaction between APTES and the Mn²⁺ precursor, the concentration of APTES ligands decreased after adding MnCl₂ precursor. Higher concentration of APTES can facilitate the formation of small spherical PNCs due to the large steric hindrance of branched APTES [34]. Likewise, with the increase of Mn²⁺ precursor concentration, the lower concentration of APTES will result in large cubic PNCs, which is consistent with the TEM results of this work. High-resolution TEM (HRTEM, Figure 2f) reveals that the lattice space of undoped PNCs is around 0.28 nm, corresponding to the (200) crystal plane of cubic CH₃NH₃PbCl₃.

3.2. Optical Properties

Similar to Mn²⁺-doped all inorganic CsPbCl₃ PNCs, new emission band may be introduced in Mn²⁺-doped organic-inorganic CH₃NH₃PbCl₃ PNCs, as shown in the PL spectra in Figure 3, where the electronic absorption spectra are also shown. For undoped CH₃NH₃PbCl₃ PNCs, the absorption spectrum exhibits a sharp rise starting around 400 nm and an excitonic peak at ~373 nm. In comparison, the excitonic peak positions of all the Mn²⁺-doped CH₃NH₃PbCl₃ PNCs shift 4–10 nm towards long wavelength owing to a significant increase in average particle size. For the PL spectra, Mn²⁺-doped CH₃NH₃PbCl₃ PNCs feature a broad emission band peaked at 610 nm, attributed to the spin-forbidden ⁴T₁→⁶A₁ transition of Mn²⁺ [31], along with a narrow luminescence band located at ~400 nm, which is assigned to intrinsic excitonic emission of the CH₃NH₃PbCl₃ host. Compared to the Mn²⁺ dopant emission (~590 nm) in CsPbCl₃ PNCs [28,31], Mn²⁺ dopant emission in the CH₃NH₃PbCl₃ system significantly red shifts by ~20 nm, possibly caused by the crystal lattice difference between cubic CH₃NH₃PbCl₃ and cubic/tetragonal CsPbCl₃. The lattice parameters of cubic CH₃NH₃PbCl₃ (space group: Pm$\overline{3}$m, a = b = c = 5.685 Å) are a little larger than that of cubic CsPbCl₃ (a = b = c = 5.605 Å) and tetragonal CsPbCl₃ (a = b = 5.590 Å, c = 5.630 Å) [18,36], resulting in the smaller energy difference (Δ_O) of Mn²⁺ d-orbitals.

In addition, all excitonic emissions of Mn²⁺-doped CH₃NH₃PbCl₃ PNCs show a slight red-shift (~4 nm) with respect to that of undoped CH₃NH₃PbCl₃ PNCs, consistent with red-shifted absorption spectra, attributed to the increase in size of PNCs. Intriguingly, with the increased concentration of Mn²⁺ dopant, the PL intensity of host increases first and then decreases, which is highly unusual. On one hand, the Mn²⁺ dopant ions may remove the preexisting structural defects and enhance the excitonic emission of hosts [25,37]. At the low doping concentration, both the intensity of excitionic emission and dopant emission are increased after 25 at. % Mn²⁺ doping, attributed to the dominant defect passivation of Mn²⁺ and exciton-to-Mn²⁺ energy transfer. On the other hand, the excitonic emission will be quenched upon the incorporation of Mn²⁺. With the addition of a higher Mn²⁺ concentration, the transfer of excitonic energy from the host to Mn²⁺ dopants becomes more dominant, giving rise to the decrease of excitonic emission intensity and enhancement of dopant emission intensity. Importantly, the overall PL QY was highly improved from 0.4% (undoped) to 15.6% (75 at. % Mn²⁺-doped), indicating efficient excitons-to-Mn²⁺ energy transfer, which is desired for light emitting applications.

To demonstrate that the Mn²⁺ dopant emission does not originate from the Mn²⁺-APTES complex, contrast test was conducted as shown in Figure S3. Without the formation of CH₃NH₃PbCl₃ perovskite, no absorption and emission band can be observed. In the range of 380–700 nm, no emission can be seen for MnCl₂ precursor, indicating that the Mn²⁺ dopant emission results from the host absorption. In addition, PLE spectra are collected to further illustrate the emission mechanism of Mn²⁺-doped CH₃NH₃PbCl₃ PNCs. As shown in Figure S4, a sharp onset located at ~400 nm can be clearly observed for all Mn²⁺-doped CH₃NH₃PbCl₃ PNCs, consistent with the absorption of undoped CH₃NH₃PbCl₃ PNCs. These results display that the emission band centered at 610 nm originates from the absorption of CH₃NH₃PbCl₃ PNCs, manifesting the energy transfer from host to dopants.

The volume ratio between precursor solution and anti-solvent (toluene) is an important factor in the synthesis of CH₃NH₃PbCl₃ PNCs when adopting the reprecipitation strategy. Figure S5 displays the PL spectrum of Mn²⁺-doped CH₃NH₃PbCl₃ PNCs solution prepared with different volume ratios. The Mn²⁺ dopant emission greatly increases in the range of 50–200 volume ratio after normalizing to excitonic emission, implying that more Mn²⁺ ions are incorporated into the crystal lattice of CH₃NH₃PbCl₃. However, the intensity of dopant emission is diminished when reaching 500 toluene/precursor volume ratio, owing to the poor crystallinity of the host at high Mn²⁺ dopant concentration, thereby lower the transfer efficiency of photo-induced excitons between the host and dopants due to the competition of charge transfer to trap states [28].

To investigate the PL decay of CH₃NH₃Pb_xMn_1−xCl₃ PNCs, time-resolved PL spectra are obtained. All the PL decay kinetics were fitted with a single exponential curve, as shown in Figure 4. An expected decrease of excitonic lifetime from 2.40 ns to 2.04 ns can be observed with increasing the Mn²⁺ concentration from 0% to 75% (Figure 4a), due to the energy transfer from the host to Mn²⁺ dopants. Interestingly, the small change of exciton emission lifetime indicates the insignificant effect of dopant states on the overall exciton decay in the CH₃NH₃PbCl₃ PNCs. Since the introduction of Mn²⁺ dopants provides a new and efficient decay pathway of the excitons, which leads to the radiative decay of Mn²⁺ via the ⁴T₁→⁶A₁ transition, the overall PL QY of the doped PNCs was improved [28].

PL decay curve of Mn²⁺ dopant emission located at 610 nm was also studied and presented in Figure 4b. A very long lifetime of 1.2 ms was observed resulting from the spin-forbidden nature of Mn^{2+ 4}T₁→⁶A₁ transition, suggesting the successful incorporation of Mn²⁺ [28,38].

3.3. Stability Investigation

Although moisture stability of CH₃NH₃PbCl₃ has been greatly improved, their practical applications are still restricted by their low decomposition temperature, which is determined by their low formation energies [39]. A recent study found that the formation energy of Mn²⁺-doped CsPbBr₃ is calculated to be ~5% larger than that of pure CsPbBr₃ using DFT simulation, suggesting the better thermal stability of CsPbBr₃ after Mn²⁺ doping with a specific concentration [39]. In this work, the thermal stability of pure CH₃NH₃PbCl₃ and CH₃NH₃Pb_xMn_1−xCl₃ PNCs has also been studied using temperature-dependent XRD, as presented in Figure 5. Without the addition of MnCl₂ precursor, CH₃NH₃PbCl₃ PNCs capped with octylamine (Figure 5a) show poor thermal stability. Some diffraction peaks due to impurities could be observed when the temperature was increased up to 170 °C. CH₃NH₃Pb_xMn_1−xCl₃ PNCs passivated with octylamine were relatively stable under 170 °C, but still to be decomposed at 180 °C (Figure 5b). In contrast, both CH₃NH₃PbCl₃ PNCs and CH₃NH₃Pb_xMn_1−xCl₃ PNCs passivated with APTES show better thermal stability, as shown in Figure 5c,d. The decomposition temperature was significantly improved by 20–30 °C compared to PNCs passivated with octylamine. The improved thermal stability of CH₃NH₃PbCl₃ PNCs passivated with APTES may be due to the SiO₂ coating layer, which maintains the morphology and crystal structure of CH₃NH₃PbCl₃ as a result of the higher thermal stability of SiO₂.

The air stability of CH₃NH₃Pb_xMn_1−xCl₃ PNCs films was also studied, as shown in Figure S6. The PL intensity CH₃NH₃Pb_xMn_1−xCl₃ PNCs passivated with APTES and octylamine drop ~9% and ~73% after one week, indicating the better air stability of CH₃NH₃Pb_xMn_1−xCl₃ PNCs capped with APTES.

4. Conclusions

Doping of Mn²⁺ into organic-inorganic CH₃NH₃PbCl₃ PNCs has been demonstrated using a versatile reprecipitation approach. The PL QY of the obtained samples are greatly improved after Mn²⁺ doping as a result of efficient energy transfer from the host to dopants. Furthermore, both thermal and air stability are enhanced due to the presence of SiO₂ on the PNC surface. Importantly, compared to the Mn²⁺ dopant emission (~590 nm) in all inorganic CsPbCl₃ system, the larger crystal lattice of CH₃NH₃PbCl₃ results in a lower energy splitting of Mn²⁺ d-orbitals, thereby emitting at a longer wavelength (~610 nm). In this manner, Mn²⁺ dopant emission may be tuned by modulating the crystal lattice through substitution of different components in perovskites.