Lead-Free Organic Manganese (II) Bromide Hybrid with Highly Efficient and Stable Green Emission for UV Photodetection

Abstract

Lead halide perovskites have been widely used in optoelectronic devices due to their excellent properties; however, the toxicity of lead and the poor stability of these perovskites hinder their further application. Herein, we report a zero-dimensional (0D) lead-free organic manganese (II) bromide hybrid compound of (TBA)₂MnBr₄ (TBA⁺ = tetrabutylammonium cation) single crystals (SCs) with great environmental stability. The (TBA)₂MnBr₄ SCs show a strong green emission peak at 518 nm with a high photoluminescence quantum yield (PLQY) of 84.98% at room temperature, which is attributed to the d-d transition of single Mn²⁺ ions, as also confirmed through density functional calculation. A green light-emitting diode was produced based on (TBA)₂MnBr₄ SCs, which exhibited CIE coordinates (0.17, 0.69) close to those of standard green. A photodetector fabricated by the (TBA)₂MnBr₄ SCs shows an obvious photo response with a rapid millisecond rise/decay response time (at 365 nm). Our findings promote the research of Mn(II)-based organic–inorganic hybrid materials and pave the way by using these materials for future high-performance optoelectronic devices.

1. Introduction

As an important semiconductor functional material, lead halide perovskite plays an important role in the light, display, and photodetector fields due to its excellent optoelectronic properties [1,2,3,4,5,6]. Although lead halide perovskites have achieved a series of remarkable results [7,8], their inherent toxicity and poor stability limit their further applications in optoelectronic devices. Therefore, the key to solving these problems is to explore new non-toxic and stable materials.

Benefiting from structure diversity, low toxicity, and excellent photoluminescence properties, such as tunable luminescence, high photoluminescence quantum yield, and high color purity, Mn(II)-based organic–inorganic hybrid metal halides (OIHMHs) are expected to replace lead halide perovskites and have broad application prospects in the field of optoelectronic devices [9,10,11,12,13]. According to previous studies, the emission of Mn(II)-based OIHMHs originates from the intrinsic d-d transition of Mn²⁺ ions [14,15,16,17,18]. In recent years, many Mn(II)-based OIHMHs have been reported and used to fabricate various optoelectronic devices [19,20,21,22,23,24,25]. For example, Chen et al. reported a green light-emitting diode (LED) based on (Ph₄P)₂MnBr₄, with a current efficiency of 32.0 cd A⁻¹, a power efficiency of 26.8 lm W⁻¹, and an external quantum efficiency of 9.6% [20]. Hyunsik et al. also reported a similar green electro-LED based on a 0D [(H₂C=CHCH₂)(C₆H₅)₃P]₂MnBr₄ compound [19], the device performance of which is close to the green device reported by Chen. Except for LED devices, Mn(II)-based compounds are also used in X-ray imaging and scintillators. Ma et al. reported an X-ray scintillator prepared from (C₃₈H₃₄P₂)MnBr₄ powder with a high light yield of about 80,000 photons MeV⁻¹ and a low detection limit of 72.8 nGy s⁻¹ [25]. Ju et al. reported two kinds of Mn-based compounds with highly efficient luminescence properties; the X-ray scintillators based on these two compounds exhibit high X-ray light yields and low detectable limits, as well as X-ray imaging demonstration with a high resolution [26]. In addition, Mn(II)-based OIHMHs have also been used to fabricate various sensors, such as the fluorescence detection of the pesticide ferbam, acetone vapor, and air humidity [21,23,27]; however, to the best of our knowledge, UV photodetectors fabricated using Mn(II)-based OIHMHs are rarely reported. Therefore, applying Mn(II)-based OIHMHs with excellent optoelectronic properties to UV photodetection is a promising route and beneficial to broaden their applications in the field of optoelectronic devices. This provides a certain reference for the use of non-toxic and stable metal halide materials in the field of photoelectric detection.

In this work, we report a zero-dimensional Mn-based compound, (TBA)₂MnBr₄, which exhibits bright green emission with an emission peak at 518 nm, a full width at half maximum (FWHM) of about 50 nm, and a PLQY as high as 84.98%. The emission mechanism of (TBA)₂MnBr₄ was investigated via steady-state and transient spectroscopy in addition to first-principles calculation, which confirmed that the narrow-band green emission is derived from the d-d transition of the single Mn²⁺ ion. Stability studies show that (TBA)₂MnBr₄ has remarkable stability toward humidity, temperature, and UV light irradiation. In addition, (TBA)₂MnBr₄ powder was coated on a 365 nm UV chip to fabricate the green LED, and the Commission International de l’eclairage (CIE) chromaticity coordinates are (0.17, 0.69), which are close those of standard green. Furthermore, we fabricated a UV photodetector using bulk (TBA)₂MnBr₄ SCs, which exhibited an obvious photo response and a fast response time in the order of milliseconds under excitation at 365 nm. This work promotes the research of Mn(II)-based organic–inorganic hybrid materials and highlights the application potential of these materials in high-performance optoelectronic devices.

2. Results and Discussion

The schematic diagram of the room-temperature liquid-phase diffusion method for growing 0D (TBA)₂MnBr₄ SCs is given in Figure 1a, and a photograph of the various growth stages of (TBA)₂MnBr₄ SCs is shown in Figure S1. Typically, the C₁₆H₃₆NBr and MnBr₂·4H₂O were dissolved in a certain amount of ethanol with a molar ratio of 2:1, and a clear pale-yellow precursor solution was obtained via stirring. The toluene was then added to the solution, and the liquid levels of the ethanol precursor liquid and toluene were stratified; however, ethanol gradually diffuses into toluene over time, causing the concentration of the precursor to gradually increase and enter a supersaturated state. As ethanol continues to diffuse into toluene, tiny (TBA)₂MnBr₄ single crystals will precipitate at the junction of the liquid surfaces of ethanol and toluene and gradually grow as ethanol continues to diffuse, finally eventually sinking to the bottom of the glass bottle under the action of gravity. The entire growth cycle is 24 h to 72 h, and the length of the growth time depends on the concentration of the ethanol precursor solution. The greater the concentration of the precursor solution the shorter the growth time, but with correspondingly poorer single-crystal quality. Therefore, the concentration of the ethanol precursor determines the final quality of a single crystal. It is worth noting that we should be careful when adding toluene to the ethanol precursor solution, and to add toluene slowly along the wall of the glass bottle in small amounts and at multiple times; otherwise the toluene and ethanol precursors will mix violently, leading a large number of microcrystals to rapidly precipitate when toluene is added and the force to be too high, finally affecting the growth of single crystals. The optical photograph of (TBA)₂MnBr₄ SCs under daylight and 365 nm UV light in the environmental conditions, as shown in the inset of Figure 1a. The size of the (TBA)₂MnBr₄ SCs obtained through this method is approximately 5 mm. The experimental powder XRD (PXRD) patterns of (TBA)₂MnBr₄ correspond well with the simulation patterns from single-crystal XRD (SCXRD), indicating that the powder sample is consistent with the SC sample. The (TBA)₂MnBr₄ has a P212121 space group, and the cell parameters are a = 12.83810 Å, b = 12.89970 Å, c = 25.08200 Å, and α = β = γ = 90°. The crystal structure file of (TBA)₂MnBr₄ was obtained from the Cambridge Crystallographic Data Centre (CCDC-1902826). As shown in Figure 1b, the XRD diffraction pattern of the sample obtained through measuring is in good agreement with the simulation results and has no additional diffraction peaks, which proves the successful synthesis of pure (TBA)₂MnBr₄ free of impurities. Each unit cell of (TBA)₂MnBr₄ consists of twenty-one TBA⁺ cations and four [MnBr₄]²⁻ tetrahedra, and all cations are ordered (Figure 1c). As shown in Figure 1d, the distance of the four Mn²⁺ ions in the unit cell varies from 8.951 Å to 19.928 Å, which indicates that (TBA)₂MnBr₄ is a typical 0D structure at the molecular level. Moreover, such a large Mn-Mn distance avoids energy transfer interactions between adjacent luminescent centers and ensures that the photogenerated excitons can be effectively bound within the tetrahedral luminescent centers, which is beneficial to the production of efficient luminescence, and the details of this will be provided in the discussion later. A Hitachi SU8020 was used to collect the morphology and elemental distribution of the (TBA)₂MnBr₄ powder samples. The SEM photograph and the elemental mapping images of the powder sample are given in Figure 1e. The size of the powder sample is about 200 μm, and the elemental mapping images verified the uniform distribution of Mn and Br. In addition, the atomic ratio of Mn and Br is 8.56:34.89 (4.08:1) when analyzing the proportion of the elements (Figure S2), which is consistent with the theoretical composition of (TBA)₂MnBr₄, indicating the successful synthesis of (TBA)₂MnBr₄.

In order to deeply understand the inside photophysical properties of (TBA)₂MnBr₄, we carried out steady-state photoluminescence (PL) spectroscopy (Figure 2a) and UV-Vis absorption spectroscopy (Figure S3a) studies on it. Under the 365 nm excitation, the (TBA)₂MnBr₄ shows an emission band centered at 518 nm with an FWHM of 50 nm, which is attributed to the d-d transition (⁴T₁(G) → ⁶A₁(S)) of single Mn²⁺ ions. Benefitting from the large Mn-Mn distance, the photogenerated excitons are bound within the [MnBr₄]²⁻ tetrahedra, resulting in a high PLQY (84.98%, Figure S4), which would be discussed in the theoretical calculation. The photoluminescence excitation (PLE) spectra of the emission band centered at 518 nm were also collected, which show seven excitation bands located at ~273 nm, ~288 nm, ~365 nm, ~374 nm, ~433 nm, ~450 nm, and ~468 nm, corresponding to the ⁶A₁(S) → ⁴A₂(F), ⁶A₁(S) → ⁴T₁(F), ⁶A₁(S) → ⁴E(D), ⁶A₁(S) → ⁴T₂(D), ⁶A₁(S) → ⁴A₁(G), ⁴E_g(G), ⁶A₁(S) → ⁴T₂(G), and ⁶A₁(S) → ⁴T₁(G) transitions, respectively [16,26,28]. The absorption spectrum of (TBA)₂MnBr₄ also shows seven absorption bands (Figure S3a), which are consistent with the PLE spectrum. As shown in Figure S3b, the bandgap value of (TBA)₂MnBr₄ is calculated as 2.50 eV according to Tauc’s rule: (αhν)^1/n = A(hν − E_g) (n = 2 for a direct bandgap and n = 1/2 for an indirect bandgap). Figure 2b provides details about the energy absorption and transfer, non-radiative transition, and emission process of a [MnBr₄]²⁻ tetrahedral. Under the excitation of high-energy UV light, the electron absorbs energy and transitions from the ground-state ⁶A₁ to the high-energy excited state of Mn²⁺ ions and transfers to the ⁴T₁ energy level through non-radiative transition, finally transitioning back to the ground-state ⁶A₁ through radiative recombination. Due to the four-coordinate condition, the intensity of the crystal field is weak [29,30,31,32]. Therefore, the energy level difference between ⁴T₁ and the ground-state ⁶A₁ is large, which ultimately produces a green light emission of 518 nm. Figure 2c presents the PL decay curve of the (TBA)₂MnBr₄ under the excitation of 365 nm, monitored at 518 nm at room temperature. The decay curve can be fitted by the following single exponential decay equation [33]:

$$I(t)=I_0+A{\mathrm{e}}^{\frac{-t}{\tau }}$$

where I(t) and I₀ are the PL intensity at time t and t ≫ τ, A is a constant, and τ is the PL decay time. The PL decay time of (TBA)₂MnBr₄ is calculated to be 0.267 ms, which is consistent with that of other reports [19,22,34], further indicating that the green emission of (TBA)₂MnBr₄ originates from the d-d transition (⁴T₁(G) → ⁶A₁(S)) of a single [MnBr₄]²⁻ tetrahedron. Figure 2d shows the Raman spectra of (TBA)₂MnBr₄ excited by a 633 nm laser at room temperature. The Raman modes at a low wavenumber (<400 cm⁻¹) come from the vibrations of a [MnBr₄]²⁻ tetrahedron [16,30,35], and the vibrations of tetrabutylammonium cations are located at high wavenumbers [36]; the detailed Raman peak positions and corresponding Raman mode assignments are listed in Table S1. The strongest Raman mode is located at 80.43 cm⁻¹. This is usually associated with acoustics with polaron properties [37]. Furthermore, it is worth noting that the Raman mode at 152.89 cm⁻¹ can be viewed as the overtone of the 80.43 cm⁻¹, and the Raman mode at 253.59 cm⁻¹ can be approximately regarded as the triple frequency of the 80.43 cm⁻¹ mode, which indicates the presence of multi-phonon modes in (TBA)₂MnBr₄, indicating strong electron–phonon coupling [37].

Key information such as crystal field strength, electron–phonon interactions, etc., can be obtained through temperature-dependent PL spectroscopy [38,39]. Herein, we used 365 nm ultraviolet light as the excitation source and collected the PL spectrum of (TBA)₂MnBr4 single crystals in the temperature range of 80 K–360 K. As shown in Figure 3a, all of the PL peaks shift towards a lower energy direction with increasing temperature (from 509 nm to 529 nm), and are accompanied by spectral broadening. This phenomenon is contrary to the blue shift of the PL peaks with an increase in the temperature of Mn-based organic–inorganic hybrid metal halides reported in other works in the literature [17,40,41]. Since the ⁴T₁(G) → ⁶A₁(S) transition energy is inversely proportional to the crystal field strength [42], the crystal field strength of (TBA)₂MnBr₄ at room temperature is higher than 80 K. At 360 K, the PL intensity of (TBA)₂MnBr₄ is about 60% of the PL intensity at 80 K (Figure 3b), indicating that (TBA)₂MnBr₄ has high thermal stability and good thermal quenching resistance, which makes it expected to be applied in the field of solid-state lighting. To further determine the thermal stability of (TBA)₂MnBr₄, we fit the binding energy (E_b) using the Arrhenius formula [43]:

$$I(t)=I_0/A{\mathrm{e}}^{{-E}_b/k_{B}T}$$

where $I_0$ is the PL intensity at 0 K, I(t) is the PL intensity at temperature T, A is a constant, E_b is the binding energy, and k_b is the Boltzmann constant. The binding energy obtained as a function of PL intensity and temperature is 39.43 meV (Figure 3c), which is larger than the room temperature thermal energy (about 26 meV). The high activation energy (E_b) of (TBA)₂MnBr₄ makes it difficult for the excited electrons to overcome the nonradiative transition energy barrier, thus ensuring the excellent thermal stability of (TBA)₂MnBr₄. To study the interaction between electrons and lattice vibrations in (TBA)₂MnBr₄, we calculated the Huang–Rhys factor, S, of (TBA)₂MnBr₄. The Huang–Rhys factor, S, can be solved by using the following formula [44]:

$$\mathrm{FWHM}\left(T\right)=2.36\mathit{\hslash }\mathsf{\omega }\sqrt{S\coth \left(\frac{\mathit{\hslash }\mathsf{\omega }}{2k_{B}T}\right)}$$

(1)

where ℏω is the phonon energy and k_b is the Boltzmann constant. The hyperbolic function, coth(x), can be expressed as follows:

$$\coth \left(x\right)=\frac{{\mathrm{e}}^x+{\mathrm{e}}^{-x}}{{\mathrm{e}}^x-{\mathrm{e}}^{-x}}=1+\frac{1}{{\mathrm{e}}^{2x}-1}$$

(2)

Therefore, Equation (1) can be written as follows:

$$\mathrm{FWHM}\left(T\right)=2.36\mathit{\hslash }\mathsf{\omega }\sqrt{S\left(1+\frac{1}{{\mathrm{e}}^{\frac{\mathit{\hslash }\mathsf{\omega }}{k_{b}T}}-1}\right)}$$

(3)

$${\mathrm{FWHM}}^2\left(T\right)=5.57\times S{\times \left(\mathit{\hslash }\mathsf{\omega }\right)}^2\left(1+\frac{1}{{\mathrm{e}}^{\frac{\mathit{\hslash }\mathsf{\omega }}{k_{b}T}}-1}\right)$$

(4)

When $\frac{\mathit{\hslash }\mathsf{\omega }}{k_{b}T}$ is small enough, ${\mathrm{e}}^{\frac{\mathit{\hslash }\mathsf{\omega }}{k_{b}T}}-1$ can be approximately expressed as $\frac{\mathit{\hslash }\mathsf{\omega }}{{2k}_{b}T}$; therefore, Equation (4) can be written as follows:

$${\mathrm{FWHM}}^2\left(T\right)=5.57\times S{\times \left(\mathit{\hslash }\mathsf{\omega }\right)}^2\left(1+\frac{1}{\frac{\mathit{\hslash }\mathsf{\omega }}{{2k}_{b}T}}\right)$$

(5)

This can be further expressed as follows:

$${\mathrm{FWHM}}^2\left(T\right)=a+\frac{b}{\frac{1}{{2k}_{b}T}}$$

(6)

where a = 5.57 × S × (ℏω)² and b = 5.57 × S × (ℏω). The Huang–Rhys factor, S, defines the degree of electron–phonon coupling [16,17]. The value of $\frac{1}{{2k}_{b}T}$ and FWHM² can be obtained according to the PL spectrum from 80 K to 360 K. As shown in Figure 3d, taking $\frac{1}{{2k}_{b}T}$ as the horizontal axis and FWHM² as the vertical axis, the Huang–Rhys factor, S, and phonon energy, ℏω, are obtained by fitting, corresponding to S = 63.42 and $\hslash \omega$ = 2.93 meV. Such a large S may be related to the organic cations in (TBA)₂MnBr₄. According to the research of Sargent et al., electron–phonon interaction is related to the stiffness of organic cations; greater stiffness of the organic cations leads to stronger lattice vibration, and the corresponding electron–phonon interaction will be stronger [44]. Due to the tetrabutylammonium cation having four C-C-C-C chains, belonging to soft organic cations with rigidity, which can generate strong lattice vibration, the result is a large S. The large Huang–Rhys factor, S, indicates that (TBA)₂MnBr₄ has strong electron–phonon coupling, which is consistent with the results obtained through Raman spectroscopy.

Density functional theory (DFT) calculations were used to deeply understand the PL mechanism and electronic structure of (TBA)₂MnBr₄. The energy band structure of (TBA)₂MnBr₄ is shown in Figure 4a; (TBA)₂MnBr₄ exhibits flat band edges, indicating that (TBA)₂MnBr₄ has highly localized electronic states, consistent with the electronic structure of 0D organic–inorganic hybrid metal halides [40,45]. Furthermore, the flat bands of both the valence band maximum (VBM) and the conduction band minimum (CBM) indicate that no obvious intermolecular coupling within a [MnBr₄]²⁻ tetrahedron can be found, which proves that each [MnBr₄]²⁻ tetrahedron can be regarded as an independent emission center [21], which is the key to producing highly efficient luminescence [46]. At the same time, the large Mn²⁺ ions’ distance ensures that there is no obvious energy transfer between adjacent [MnBr₄]²⁻ tetrahedral luminescent centers, ultimately producing efficient green light emission. The calculated bandgap of (TBA)₂MnBr₄ at the Г point in the Brillouin zone is about 2.52 eV, which is consistent with the experimental bandgap value (2.50 eV). Figure 4b shows the calculated total of each element’s orbital-resolved partial densities of states (DOSs); the valence band maximum (VBM) is composed of Mn-3d and Br-4p orbitals and the conduction band minimum (CBM) is composed of Mn-3d orbitals, while the organic molecules do not contribute to the frontier orbitals, which indicates that the bandgap of (TBA)₂MnBr₄ is determined by the inorganic [MnBr₄]²⁻ tetrahedron. The large-sized tetrabutylammonium bromide organic molecules play a role in supporting the skeleton and effectively isolating the [MnBr₄]²⁻ tetrahedral luminescent center in the structure. In addition, the sharp peaks near the VBM and CBM indicate that the valence and conduction bands of (TBA)₂MnBr₄ are almost dispersion-free, indicating negligible electronic coupling within [MnBr₄]²⁻ [47].

The stability of materials is crucial for the fabrication of high-performance optoelectronic devices. Before fabricating optoelectronic devices using (TBA)₂MnBr₄ SCs, we first evaluated the stability of (TBA)₂MnBr₄ SCs. As shown in Figure S5a, the PXRD pattern of (TBA)₂MnBr₄ showed that no additional diffraction peaks were generated and that there was no shift in diffraction peaks compared with as-synthesized samples after they are stored for 60 days in ambient conditions (T ≈ 30 °C and RH ≈ 70%). The TGA measurement found that the initial decomposition temperature of (TBA)₂MnBr₄ is about 220 °C, indicating that (TBA)₂MnBr₄ has a high thermal stability (Figure S5b). The (TBA)₂MnBr₄ can still maintain a high PLQY after being stored for 60 days in ambient conditions, about 90% of that of the as-synthesized samples (Figure S5c). In addition, the PL intensity of the sample attenuated by about 5% after 4 h of UV irradiation, indicating that (TBA)₂MnBr₄ has high UV stability (Figure S5d). The above results show that (TBA)₂MnBr₄ has excellent stability and is very suitable for the preparation of high-performance optoelectronic devices.

The efficient luminescence properties and excellent stability of (TBA)₂MnBr₄ make it a promising green phosphor for solid-state lighting applications [48]. Therefore, (TBA)₂MnBr₄ powder was coated on the 365 nm GaN UV chip, and a bright green light was obtained after switching on the power, as shown in the insert of Figure S6a. To compare the green emission of the (TBA)₂MnBr₄ with standard green light, their CIE coordinates, through the use of a CIE chromaticity diagram based on the PL spectrum, were collected. The coordinates of the green emission of (TBA)₂MnBr₄ are (0.17, 0.69), which are close to those of standard green (Figure S6a). Meanwhile, with an increase in voltage or current the spectral shape remains almost unchanged (Figure S6b,c), indicating that (TBA)₂MnBr₄ has excellent stability and great application potential as a green phosphor.

Bulk single crystals with a narrow bandgap, high carrier mobility, and steady-state photocurrent are excellent candidates for the fabrication of high-performance photosensitive devices [49,50,51,52]. According to our literature survey on the optoelectronic applications of manganese-based metal halides in recent years, Mn(II)-based metal halides are currently mainly used in the fields of X-ray scintillators, LEDs, and sensors, while reports in the field of photodetectors are still rare. Table S2 summarizes the main application directions of typical Mn(II)-based metal halides in recent years. Based on this, we used the as-grown large-sized (TBA)₂MnBr₄ single crystals to prepare ultraviolet photodetectors to explore the application potential of this type of material in the field of photodetectors. A UV photodetector with a planar metal–crystal–metal structure of Ag/(TBA)₂MnBr₄/Ag is shown in the inset of Figure 5b. The quality of (TBA)₂MnBr₄ SCs was first evaluated before characterizing the performance of the device. The density of trap states (n_trap) is an important indicator for evaluating the quality of a single crystal [53]. The silver paste was brushed on both sides of the (TBA)₂MnBr₄ SCs with a thickness of 3.5 mm as electrodes (inset of Figure 5a), and the I–V curve was tested under dark conditions. As shown in Figure 5a, the dark I–V curve can be divided into three parts on the log–log scale: (1) the Ohmic region (I ∝ V^{n = 1}, V < 56.72 V)—in this region the current increases linearly with voltage; (2) the Child’s region (I ∝ V^{n = 2}, V > 66.83 V)—in this region the current exhibits quadratic voltage; and (3) the trap fill limited (TFL) region (I ∝ V^{n = 1}, 56.72 V < V < 66.83V). The voltage value at the inflection points between the Ohmic region and the TFL region is called the trap fill limited voltage (V_TFL), and the n_trap can be defined by the following formula [47]:

$${\mathrm{V}}_{\mathrm{TFL}}=\frac{\mathrm{e}{d}^2{n}_{\mathrm{trap}}}{2\epsilon {\epsilon }_0}$$

where e is the elemental charge, d = 3.5 mm is the thickness of (TBA)₂MnBr₄ SCs, ε is the relative dielectric constant of (TBA)₂MnBr₄ SCs, and ε₀ is the vacuum permittivity. A Tonghui TH2828 precision LCR meter was used to measure the capacitance of the Ag/(TBA)₂MnBr₄/Ag device, and the ε = 28.5 is calculated by the formula $\epsilon =\frac{\mathit{Ct}}{{\epsilon }_{0}A}$, where A = 1.44 × 10⁻⁶ m² is the area of the Ag electrodes. Therefore, the trap density, n_trap, is estimated to be about 1.46 × 10¹⁰ cm⁻². The lower trap density indicates that (TBA)₂MnBr₄ SCs have the quality of high crystallinity, making them an ideal material for fabricating photodetectors. Unfortunately, due to the weak excitation intensity of the xenon lamp, we could not observe the obvious optoelectronic signal of the device with excitation by the xenon lamp. Therefore, a fixed-wavelength laser (λ = 365 nm) was utilized as an excitation source with which to study the optoelectronic properties of the (TBA)₂MnBr₄ device. The I–V curves of the device under dark and light conditions are shown in Figure 5b, and the device exhibits a distinct photo response after being excited by UV light compared to dark conditions. The I–V curve of the device shows relatively symmetrical characteristics. Here, we defined the rectification ratio as the ratio between the forward bias current and reverse bias current [54]. Therefore, the rectification ratio of the device is approximately equal to 1 here. The time-dependent photocurrent response of the (TBA)₂MnBr₄ device is shown in Figure 5c. After multiple on–off cycles, the device can still work stably, with high reproducibility. Benefiting from the high crystalline quality, the grain boundary recombination of carriers is reduced [55], and the on–off ratio of the device is 8.73 under an electrode distance of about 5 mm. Therefore, the on–off ratio can be further improved if the distance between electrodes is further narrowed or a more suitable electrode material is selected [40,56]. The rise time (t_rise) and decay time (t_fall) were calculated to be 103.91 ms and 98.02 ms, respectively, close to the values of previously reported Mn-based metal halide photodetectors [56]. The fast photo response of the device was attributed to the high quality of the (TBA)₂MnBr₄ SCs. Our results promote the research of Mn(II)-based organic–inorganic hybrid materials and highlight the application potential of these materials in high-performance optoelectronic devices.

3. Conclusions

In conclusion, we synthesized a new lead-free 0D Mn(II)-based organic–inorganic hybrid metal bromide (TBA)₂MnBr₄. This compound exhibits strong green emission with peaking at 518 nm, which is attributed to the d-d transition of the single Mn²⁺ ion. In addition, the as-synthesized (TBA)₂MnBr₄ shows a high PLQY of 84.98% and remarkable stability toward humidity, temperature, and UV irradiation. Meanwhile, a green LED was fabricated via coated (TBA)₂MnBr₄ on the 365 nm UV chip, and the coordinates of the green emission of (TBA)₂MnBr₄ are (0.17, 0.69), which are close those of standard green. Finally, a UV photodetector based on bulk (TBA)₂MnBr₄ SCs was fabricated, which exhibits a distinct positive photo response and a fast response time in the order of milliseconds. Our results promote the research of Mn(II)-based organic–inorganic hybrid materials and pave the way by using these materials for future high-performance optoelectronic devices.