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Energy Material Advances / 2022 / Article

Research Article | Open Access

Volume 2022 |Article ID 9845942 | https://doi.org/10.34133/2022/9845942

Ming Shi, Bin Yang, Siping Liu, Ruiling Zhang, Keli Han, Can Li, Rengui Li, "Tuning Exciton Recombination Pathways in Inorganic Bismuth-Based Perovskite for Broadband Emission", Energy Material Advances, vol. 2022, Article ID 9845942, 11 pages, 2022. https://doi.org/10.34133/2022/9845942

Tuning Exciton Recombination Pathways in Inorganic Bismuth-Based Perovskite for Broadband Emission

Received19 Feb 2022
Accepted29 Mar 2022
Published15 Apr 2022

Abstract

Single-component emitters with broadband emission are attractive but challenging for illumination and display applications. The two-dimensional organic-inorganic hybrid perovskites have exhibited outstanding broad emission property due to low electronic dimensionality and strong exciton-phonon coupling. However, few layered all-inorganic lead-free perovskites with broadband emission have been explored, and the explicit mechanism of exciton recombination in them also needs in-depth understanding. Herein, the inorganic bismuth-based perovskite Cs3Bi2Br9 achieves the stable broadband emission under ambient temperature and pressure by tuning the exciton recombination pathways via antimony (Sb) doping, and the photoluminescence quantum yield (PLQY) realizes an enhancement from 2.9% to 15.9%. The photoluminescence excitation (PLE) spectra indicate that the doped Sb introduces newly extrinsic self-trapped states. The incorporation of Sb promotes the transfer of free excitons (FEs) to extrinsic self-trapped excitons (STEs) observed from Sb content-dependent steady-state PL spectra and, meanwhile, reduces the nonradiative recombination of the generated extrinsic STEs, which are primarily responsible for the remarkably enhanced broad emission. Furthermore, femtosecond transient absorption results elucidate a clear exciton dynamics, in which the transition from FEs to STEs might arise through the gradient energy levels, and finally extrinsic STEs at various energy states contribute to the broadband emission.

1. Introduction

Artificial lighting accounts for one-fifth of global electricity consumption [1], and developing efficient and stable luminescence materials is critical to avoid unnecessary waste of electric energy. The single emitters with broadband emission have recently triggered tremendous attention for artificial illumination and display applications because they can circumvent critical problems faced in the traditional mixed and multicomponent emitters such as the efficiency losses caused by self-absorption, the complex device structure, and the colors instability due to the different degradation rates of phosphors [15]. Lead halide perovskites have emerged as highly attractive next-generation optoelectronic materials for light-emitting applications due to their extraordinary photoelectric properties [69]. Notably, low-dimensional organic-inorganic hybrid lead halide perovskites dominate the research of broadband emission benefiting from their strong electron-phonon coupling interactions inducing the generation of self-trapped excitons (STEs) [1012]. However, the toxicity of lead and intrinsic instability caused by organic cations hinders their further commercial applications.

Bismuth-based halide perovskites have attracted considerable attention in optoelectronic fields due to their low toxicity, good chemical stability, and the isoelectronic configuration (6s26p0) of Bi3+ with Pb2+ [13, 14]. Among them, Cs3Bi2Br9 has emerged as an emitter for light-emitting application given a large exciton binding energy to promote the exciton recombination efficiently [15, 16]. It has been reported that Cs3Bi2Br9 exhibits blue photoluminescence with the emission at about 470 nm originating from the band edge emission [1719]. However, there are few reports on Cs3Bi2Br9 for broadband light emission at ambient temperature and pressure, although the low electronic dimensionality and strong quantum confinement brought by vacancy-ordered layered structure endow it with that potential. One of the dominant reasons is that Cs3Bi2Br9 possesses extremely strong exciton-phonon coupling because of the localized and compressed microstructure [15], which results in STEs responsible for the broad photoluminescence band being more susceptible to thermal quenching through emitting phonons nonradiatively [20, 21]; thus, Cs3Bi2Br9 only exhibits broadband emission at low temperatures [22]. Although pressure-induced photoluminescence in a broad range of spectrum for Cs3Bi2Br9 has recently been developed through isostructural transition, this broad emission property achieved by external force cannot be maintained over long periods unless applying sustained pressure [23]. The investigation of broadband emission for Cs3Bi2Br9 still lags behind, and more importantly, its underlying luminescence mechanism remains elusive.

Herein, we have successfully incorporated a trace amount of Sb (0.13 wt.%) into the Cs3Bi2Br9 without disturbing its long-range structure. The resulted Cs3Bi2Br9:Sb exhibits a prominent broadband emission and a remarkable enhancement of PLQY, which is attributed to the regulation of the exciton recombination pathways by Sb incorporation. On the one hand, the introduction of Sb induces the transfer of FEs to extrinsic STEs evidenced by the narrow emission decrease and broad emission increase observed in Sb content-dependent PL spectra. On the other hand, the fitting carrier dynamics show that the proportion and lifetime of slow components significantly increase after Sb incorporation, indicating that the nonradiative recombination of STEs decreases, which is related to the reduction of exciton-phonon coupling due to the weakening of structural compression. Furthermore, we have drawn a clear physical picture of exciton dynamics on the broadband emission of Cs3Bi2Br9:Sb based on femtosecond transient absorption test. The Cs3Bi2Br9:Sb also exhibits excellent thermal and ambient stability for months.

2. Materials and Methods

2.1. Chemicals

Cesium bromide (CsBr) with a purity of 99.9% was purchased from Aladdin. Antimony bromide (SbBr3) with a purity of 99.995% and bismuth bromide (BiBr3) with a purity of 99% were purchased from Alfa. Hydrobromic (HBr) acid (40 wt% in water) was purchased from Sinopharm Chemical. All chemicals were used as received without further purification.

2.2. Synthesis of Samples

For the synthesis of Cs3Bi2Br9:Sb microcrystals, CsBr (3.0 mmol), BiBr3 (1.99 mmol), and SbBr3 (0.01 mmol) were added to HBr acid (15 mL, 40 wt% in water) in a flask and heated to 150°C to react for 2 hours in an oil bath and then naturally cooled to room temperature. The sample was centrifuged from the solution at 5000 rpm for one minute and then dried in the oven at 80°C for 12 hours to obtain the microcrystals. The powder samples are obtained by grinding microcrystals. For the synthesis of other Cs3SbxBi2-xBr9 samples, the synthesis methods are similar to the above, and stoichiometric CsBr (3.0 mmol), SbBr3 (x mmol), and BiBr3 ((2-x) mmol) were added to HBr acid in a flask and heated to 150-170°C to react for 2 hours and then cooled and stirred at room temperature for 1 hour. The samples were obtained by centrifugation and drying in an oven. It is worth mentioning that the femtosecond transient absorption spectroscopy performed to elucidate broadband emission mechanisms was conducted on Cs3SbxBi2-xBr9 (Sb/Bi =0.333) with relatively high Sb concentration for obtaining appreciable transient absorption signals.

2.3. Characterizations

The morphology and elemental analysis of the samples were collected by scanning electron microscopy (SEM) JSM-7900F equipped with an energy dispersive spectroscopy (EDS). The inductively coupled plasma optical emission spectrometry (ICP-OES) tests were performed using the Shimadzu ICPS-8100 emission spectrometer. The powder X-ray diffraction (PXRD) tests were conducted on Rigaku Smartlab powder diffractometer using Cu-Kα radiation. The scan rate of 20°/min was applied and in the range of 5-80° at a step size of 0.01° to record the XRD patterns. Raman spectra were characterized using a home-made Raman instrument with a 532-nm diode-pumped solid-state laser (Changchun New Industries Optoelectronics Technology Co., Ltd.). UV-visible diffuse reflectance data were recorded on the UV-vis spectrophotometer (JASCO V-650) equipped with an integrating sphere over the spectral range of 200-900 nm with a scanning rate of 400 nm/min. X-ray photoelectron spectroscopy (XPS) characterizations were carried out using a Thermo Fisher Escalab 250Xi spectrometer with monochromatized Al-Kα. Thermogravimetric analyses (TGA) were performed with a PE Diamond TG/DTA thermo-microbalance in a nitrogen atmosphere.

2.4. Photoluminescence (PL) and Transient Absorption Measurements

The steady-state PL emission spectra and photoluminescence quantum yield (PLQY) were recorded using the FLS1000 Edinburgh Instruments spectrofluorometer equipped with the integrating sphere under an excitation wavelength of 350 nm unless otherwise specified. Time-resolved photoluminescence (TRPL) measurements were conducted on a PL-scanned imaging microscope equipped with a time-correlated single photon counting (TCSPC) module. Transient absorption spectra (TAS) were performed on a home-made femtosecond pump-probe setup. The laser pulses (800 nm, 50 fs pulse length, and 1 kHz repetition rate) were generated from a Ti:sapphire femtosecond laser source (Hurricane, Spectra-Physics). The laser wavelength was changed through an optical parametric amplifier. The super-continuum generated from a thin CaF2 plate was used as the probe light. Through placing a Berek compensator in the pump beam, the mutual polarization between the pump and probe beams was set to the magic angle (54.7°). There was no sign of degradation during a long time of scanning. The spot size and excitation power were applied to determine the excitation fluence (pump wavelength: 340 nm).

3. Results

3.1. Chemical Composition and Structure

The Sb doped Cs3Bi2Br9 (Cs3Bi2Br9:Sb) was synthesized through the slow cooling method in hydrobromic acid solution, in which about 0.01 mmol SbBr3 (~0.2 wt.%) was added to the synthesis of 1.0 mmol Cs3Bi2Br9. The scanning electron microscopy (SEM) images display the layered morphologies of Cs3Bi2Br9:Sb, and the corresponding elemental mapping results clearly show that Cs, Bi, Br, and Sb elements were uniformly distributed throughout the samples (Figure 1(a)). The energy dispersive spectroscopy (EDS) further confirms that Cs3Bi2Br9:Sb are composed of Cs, Bi, Br, and Sb, and the content of the introduced Sb element is very low (Figure 1(b) and Figure S1). Then the inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out to determine the precise Sb content of Cs3Bi2Br9:Sb, which shows that the exact incorporated content of Sb is 0.13 wt% (Table S1). Noted that most of the feeding Sb elements were incorporated into the host structure of Cs3Bi2Br9, which is closely related to the high lattice match degree between Cs3Bi2Br9 and Cs3Sb2Br9 (Table S2). Based on this, Sb3+ could be incorporated uniformly into Cs3Bi2Br9 to replace Bi3+ and form SbBr6 octahedra, without introducing new phases of impurities [24]. The X-ray photoelectron spectroscopy (XPS) of Cs3Bi2Br9:Sb displays the characteristic peaks for Cs, Bi, Br, and Sb (Figure S2), and the peaks located at 530.4 and 540.4 eV correspond to 3d5/2 and 3d3/2 of Sb3+, respectively, suggesting that the valence state of Sb does not change after incorporation into the host (Figure S3). According to the above EDS, ICP-OES, and XPS analysis, we concluded that the Sb was incorporated into the Cs3Bi2Br9 successfully.

To obtain insights into the structure evolution from Cs3Bi2Br9 to Cs3Bi2Br9:Sb, powder XRD and Raman measurements were performed. As shown in Figure 1(c), all the diffraction peaks of Cs3Bi2Br9:Sb agree well with the Cs3Bi2Br9 crystal structure with a space group of P-3 m1, and no impurity phases and no peak shift appeared after the Sb incorporation. This indicates that the trace amount of Sb does not affect the long-range crystal structure of Cs3Bi2Br9. The Raman peaks at 162 cm-1 and 187 cm-1 can be assigned to the two different modes of stretching vibration of the shorter Bi-Br bonds in BiBr6 octahedra (Figure 1(d)) [17]. Notably, although the Raman peaks have no shift, the peak intensity of Cs3Bi2Br9:Sb decreases compared with that of Cs3Bi2Br9, demonstrating that the vibration coupling strength of BiBr6 octahedra has substantially reduced [20]. This phenomenon may be due to the weakening of octahedral compression, which could affect the exciton-phonon interactions and thus influence the photogenerated excitons recombination process.

3.2. Optical Properties

To assess the effects of Sb incorporation on the photophysical properties, UV-vis absorption and steady-state photoluminescence (PL) spectra were performed on Cs3Bi2Br9 and Cs3Bi2Br9:Sb. As shown in Figure 2(a), the pristine and Sb-doped Cs3Bi2Br9 present an obvious excitonic absorption peak, indicating that free carriers are easily bound in the lattice, which is related to the strongly distorted structure of BiBr6 octahedra [15]. Upon Sb incorporation, the absorption spectra show a nonnegligible absorption widening, illustrating the Sb dopants toward a critical role in modulating the optical properties of Cs3Bi2Br9. The absorption broadening can also be seen from the apparent color change of the samples, and the original bright yellow is deepened after Sb incorporation (insets in Figure 2(a)). The steady-state PL spectra of Cs3Bi2Br9 show two characteristic emission peaks, a narrow peak around 490 nm and a broad peak around 780 nm (inset in Figure 2(b)). The narrow emission at 490 nm is close to the band gap energy, and therefore, it is attributed to the band edge emission of FEs [17]. For the broad emission, it has been reported that it is originated from radiative decay of STEs given its broadband and large stokes shift characteristics [23], which is widely observed in low-dimensional halide perovskites due to strong exciton-phonon interaction [1012]. Moreover, the time-resolved photoluminescence (TRPL) measurements of high-quality single crystals and powders ruled out that the broad emission was associated with defect-assisted processes since they have identical lifetimes (Figure S4) [20]. Furthermore, the TRPL for broadband emission of Cs3Bi2Br9:Sb shows an average lifetime of 60.3 ns that is two orders of magnitude longer than that of FEs (Figure S5), and combined with the near-infrared emission characteristics with a large Stokes shift, we speculate that the broadband emission comes from the triplet STEs [25]. The Cs3Bi2Br9:Sb exhibits only the enhanced ultra-broadband emission ranging from about 550 to more than 850 nm, while the narrow emission peak at the band edge disappeared (Figure 2(b)). This suggests that the intrinsic radiative recombination from FEs is suppressed while the broadband emission assigned to STEs is enhanced after the incorporation of Sb, indicating more energy transfer from FEs to STEs. This may be due to the lower energy barrier between FEs and the new STEs induced by Sb incorporation [20]. Photoluminescence excitation (PLE) spectra test further confirms that (Figure 2(c)). It is clear that Cs3Bi2Br9 and Cs3Bi2Br9:Sb have completely different PLE spectra under 780-nm emission wavelength, which means that the introduction of Sb brings new extrinsic self-trapped states. The excitation cut-off edge of Cs3Bi2Br9 is about 400 nm, which is significantly smaller than the absorption band edge (Figure 2(a)), indicating that the transfer of FEs to intrinsic STEs must overcome a very high energy barrier. After Sb doping, the excitation wavelength for broadband emission is extended to almost 500 nm, which closely matches the absorption spectrum of Cs3Bi2Br9:Sb. This indicates that the transfer barrier between FEs and extrinsic STEs is obviously reduced with Sb incorporation, and the FEs are easily trapped by the relaxed extrinsic STEs. The PLE spectra with 490-nm emission wavelength show that the emission intensity from FEs decreases after Sb doping (Figure S6), which further illustrates the transfer of FEs to STEs with the incorporation of Sb into Cs3Bi2Br9. The PL spectra under different excitation wavelengths also affirmed the energy transfer between FEs and STEs (Figure S7). For Cs3Bi2Br9, as the excitation energy decreases from 350 nm to 400 nm, the generated hot FEs are not enough to surmount the potential barrier, so the narrow emission peak from FEs radiation recombination increases, while the broadband emission from STEs decreases. However, for Cs3Bi2Br9:Sb, the excitation wavelength increases from 350 nm to 400 nm, and the broad emission is enhanced, which may be that the FEs originally transferred to the intrinsic STEs are trapped by the extrinsic STEs.

To further estimate the effects of the incorporation of Sb on carrier dynamics, transient absorption measurements were carried out. An obvious difference in ground-state bleach (GSB) signals for Cs3Bi2Br9 and Cs3Bi2Br9:Sb was observed (Figure S8). The decay of GSB can be fitted with three prominent processes: a fast component (~10 ps) from FEs trapping, a moderate component (~100 ps) assigned to radiative recombination of FEs or nonradiative decay of STEs, and a slow process (>1 ns) identified as the radiative decay of STEs (Figure S9 and Table S3) [26, 27]. Compared with Cs3Bi2Br9, the obvious difference is that the proportion of moderate components in Cs3Bi2Br9:Sb decreased significantly (from 20% to 0.3%), while the proportion of slow components increased obviously (from 74.7% to 99.6%). That is, after Sb incorporation, more FEs were trapped to form STEs and then underwent radiation recombination to generate broadband emission, which is in good agreement with the above PL spectra analysis. In addition, the lifetime of the slow component of Cs3Bi2Br9:Sb (15.6 ns) is markedly longer than that of Cs3Bi2Br9 (1.2 ns). Considering that the long lifetime component originates from the radiative recombination of STEs, the incorporation of Sb introduces new STEs with a longer lifetime, which indicates a reduction in nonradiative decay of STEs and thus enhances the intensity of broadband photoluminescence as seen in PL spectra. We further study the PLQY values of Cs3Bi2Br9 and Cs3Bi2Br9:Sb, and that for Cs3Bi2Br9:Sb reaches 15.9%, which is more than 5-fold compared to the pure Cs3Bi2Br9 (Figure 2(b)). The images of the two samples under UV lamp (350 nm) also evidence the increase in photoluminescence intensity, in which the dark yellow photoluminescence of Cs3Bi2Br9 transformed into the bright orange of Cs3Bi2Br9:Sb (insets in Figure 2(b)). Based on the above analysis, the introduction of Sb in Cs3Bi2Br9 promotes the transfer of FEs to the STEs and inhibits STEs nonradiative decay with a short lifetime, thus observably improving the PLQY from broadband emission.

3.3. Sb Content-Dependent Photophysical Properties

In order to investigate the incorporated Sb content-dependent photophysical properties, we further tune the atomic ratio of Sb/Bi in Cs3SbxBi2-xBr9 (0 ≤ x ≤2). The XRD results show that the samples with different Sb/Bi ratios still maintain the P-3 m1 crystal structure, but the diffraction peaks shift to higher angles when an excess amount of Sb was incorporated due to the lattice contraction caused by the smaller Sb3+ radius than Bi3+ (Figure S10). This indicates that the introduction of Sb occupying Bi sites does not change the long-range structure of Cs3Bi2Br9, which is consistent with our previous study [24]. As can be seen from the UV-vis absorption spectra (Figure S11), the absorption band edge presents a gradual red shift with increasing Sb content, which is because the outermost orbitals of Sb ions participate in the contribution of energy band structure [28]. We further performed the steady-state PL spectra of Cs3SbxBi2-xBr9 to understand the trend of the PL properties as a function of Sb concentration (Figure 3(a)). At the low Sb content region (Sb/Bi <0.01), the narrow emission peak at 490 nm from FEs radiative decay gradually decreases and finally disappears with the increase of Sb content, while the broadband emission around 780 nm assigned to radiative recombination of STEs rapidly increases and reaches the maximum value in the same region (Figure S12), which further explains the transfer of FEs to STEs with the incorporation of Sb into Cs3Bi2Br9. The slight blue shift of the narrow emission peak might be due to the octahedral tilting microscopically caused by the incorporation of Sb, and similar phenomenon has been observed in other halide perovskites [29]. The PL intensity then gradually decreases at the higher Sb concentration (Sb/Bi >0.01), and when Bi was completely replaced by Sb, the PLQY of Cs3Sb2Br9 was as low as 1.1% (Figure S13). This manifests that the concentration quenching initiates to play a role at the higher Sb content [30], and the low PLQY of Cs3Sb2Br9 was caused by the thermal quenching owing to the small thermal activation energy [31].

The TRPL of FEs and STEs recombination were carried out on samples with different Sb content. The lifetime of FEs barely changes before and after Sb doping (Figure S14a and Table S4), which indicates that the introduction of Sb does not affect the radiative recombination dynamics of FEs. For the STEs recombination, the lifetime increases with the incorporation of trace Sb, indicating that the nonradiative recombination is suppressed, but it turned to decrease with increasing Sb content (Figure S14b and Table S4), suggesting the increased nonradiative recombination process and causing the fast lifetime [32]. The above processes are consistent with the changes of Sb content-dependent PL spectra. In addition, the temperature-dependent PL spectra of Cs3Bi2Br9 and Cs3Bi2Br9:Sb were also performed. The PL intensity of them increases monotonously with the decrease of temperature due to the inhibition of nonradiative recombination at low temperature (Figure S15a and S15b) [20]. Based on the temperature-dependent PL, the activation energy (Ea) of Cs3Bi2Br9 and Cs3Bi2Br9:Sb can be estimated (Figure S15c and S15d), which signifies the energy barrier for excitons binding to nonradiative centers. It is worth noting that the activation energy is elevated after Sb incorporation, which prevents excitons energy from being dissipated by phonons and facilitates the radiative recombination [20, 33], further verifying the enhanced PL intensity of Cs3Bi2Br9:Sb.

Considering the dependence of the STE states on the deformability of the lattice and exciton-phonon interactions, the broadband emission originating from the STEs should be correlated to the transformation of crystal structure. We further analyze the variation of PL properties with the incorporation of Sb from the perspective of microstructure modulation. The exact crystal structure information of Cs3Bi2Br9 has been resolved by single-crystal X-ray diffraction [17]. The Bi-Br bond length (2.71 Å for Bi-Br2) is obviously shorter than the theoretical value (2.99 Å), implying strongly compressed octahedra and distorted structure (Figure 3(b) and Table S5), which could bring about extremely strong exciton-phonon coupling [15]. The strong exciton-phonon interactions not only cause the excited state energy to be dissipated by phonons, leading to nonradiative recombination, but also prompt the excitons being trapped by nonradiative centers, resulting in PL quenching [20]. After Sb incorporation, the Cs3SbxBi2-xBr9 structure has been analyzed by powder X-ray diffraction refinement [24]. The Sb-Br bonds (2.76 Å for Sb-Br2) present an equal or even longer length than the theoretical value (2.72 Å), representing a locally relaxed lattice structure (Figure 3(b) and Table S5), which is also confirmed by the lower Raman peak intensity of Cs3Bi2Br9:Sb (Figure 1(d)). This indicates the weakened coupling strength for local octahedra vibration, further reducing the vibrational overlap between the ground and excited-state potential energy surfaces and thus suppressing the nonradiative recombination processes [20]. When further increasing the Sb concentration, the isolate SbBr6 octahedra would not be surrounded by BiBr6 octahedra, and the coterminous SbBr6 clusters would appear (Figure S16), which not only decreases the localization of excitons, but also leads to severe self-absorption [34], which is also one reason why the PL intensity degradation appears at higher Sb content.

3.4. Photoluminescence Mechanism

To further reveal the photo-induced exciton dynamics and understand the radiative and nonradiative mechanism, femtosecond transient absorption spectra were conducted on Cs3SbxBi2-xBr9. Figure 4(a) shows the pseudocolor transient absorption plot. A broad photo-induced absorption (PIA) signal was observed across the region, providing additional evidence of STE states that are photo-induced. A negative ground-state bleach (GSB) signal truncates the PIA signal at around 440 nm, which arises from the filling effect of states [35]. Over a wide range of time scales (ps-ns), the variation of positive PIA signal is almost synchronous with negative GSB signal, indicating that STEs are derived from the trapping of FEs and such a process occurs at a very fast timescale (Figure 4(b)). The rise time of the PIA signal exhibits the dependence on probe wavelength, and the shorter wavelength (high excitation energy), the longer the PIA rise time (Figure 4(c)). This attests the presence of STEs at different energy levels, and the STEs at lower energy levels take a longer time to arise. These spectral results show that the transition from FEs to STEs proceeds through the gradient energy levels, first to high energy states, then to low energy states, and finally STEs at various energy states emit a broadband emission. The formation process of PIA is still ultrafast (less than 0.5 ps) despite the different rise time, indicating the low energy barrier separating the FEs and STEs [36], which is consistent with the above PLE spectra analysis. In addition, a shorter wavelength evolving stronger PIA signal intensity in Figure 4(c) also demonstrates that most FEs transition to the STE states with smaller energy. Extracting the decay curves of PIA signal at different wavelengths, it was found that the rapidly decaying components also show wavelength dependence in the short-wavelength region, which again proves the existence of gradient energy levels, and the initial decay of STEs in the high energy states is slower (Figure S17a). However, the PIA decay signals in the long-wavelength region are almost identical, indicating that the PIA in this region was originated from the same STEs states (Figure S17b).

The rise and decay of GSB signal represent the whole process of FEs generation, transition to form STEs, and subsequent decay and radiative recombination of STEs. The fitted rise time of GSB is 126 fs (inset of Figure 4(d)), representing the formation of FEs, which is slightly shorter than that of PIA, further suggesting the STEs from the trapping of FEs. The decay of GSB could be fitted by four prominent components:  ps (ultrafast),  ps (fast),  ps (moderate), and  ns (slow) (Figure 4(d) and Table S6). The ultrafast component (τ1) and the fast component (τ2) can be ascribed to the trapping process of FEs, and the fast component (τ2) is comparable to the lifetime in Cs3Bi2Br9 [19]. In addition, the cooling process of hot STEs should also be within a few picoseconds, affirmed by the blue shift of the excited state absorption over the short decay time range (Figure S18) [36]. Considering that the narrow emission from FEs observed in Cs3Bi2Br9 disappeared after the incorporation of high concentration Sb, the moderate-lived component (τ3) is associated with the energy transfer from the intrinsic STEs to the extrinsic STEs induced by Sb [36]. Noted that the above result cannot exclude that the radiative recombination process of FEs is also within the time scale of the moderate component. The final slow component (τ4) can be identified as the radiative recombination of STEs (>1 ns). Based on the above analysis, the mechanism of photo-induced exciton dynamics for Cs3SbxBi2-xBr9 can be depicted in Figure 4(e). After photoexcitation, ground state carriers are excited to produce excited state FEs. Most of the FEs are directly trapped by the newly emerging STEs states induced by the incorporation of Sb, which subsequently undergo cooling and decay to the ground state. Some FEs are firstly trapped by the intrinsic STEs states and then proceed with the energy transfer to the extrinsic STEs that are mainly responsible for the ultra-broadband emission.

3.5. Structural and Optical Stability

Furthermore, we explored the stability of Cs3Bi2Br9:Sb with superior broadband emission. The thermal stability was studied using thermogravimetric analysis, and the results are shown in Figure 5(a). The extrapolated onset temperature of Cs3Bi2Br9:Sb is about 495°C, slightly lower than 507°C of Cs3Bi2Br9 due to the incorporation of more easily decomposed Sb species, but still much higher than the decomposition temperature of hybrid halide perovskites (about 240°C for MAPbI3) [37] and comparable with CsPbBr3 (about 580°C) [38], indicating the excellent thermal stability of Cs3Bi2Br9:Sb emitter. The ambient stability was also investigated by exposing the samples to ambient air over months. The XRD pattern, absorption, and PL spectra remain unchanged even after more than six months (Figures 5(b)–5(d)), demonstrating that Cs3Bi2Br9:Sb has the high structural and optical stability under air conditions.

4. Discussion

In summary, we have demonstrated that the stable broadband emission can be realized on the bismuth-based perovskite Cs3Bi2Br9 by tuning the exciton recombination pathways via Sb doping. The incorporation of Sb endows Cs3Bi2Br9:Sb with the remarkable broadband emission from the generated extrinsic STEs and obviously enhanced PLQY from 2.9% to 15.9% under ambient temperature and pressure. The Sb content-dependent steady-state PL spectra clearly evidenced that more FEs transfer to extrinsic STE states with the introduction of Sb, and the doped Sb reduces the nonradiative recombination of the generated extrinsic STEs, which are primarily responsible for the remarkably enhanced broad emission. Femtosecond transient absorption spectra reveal the presence of different energy levels of STEs, and after photoexcitation, the excited FEs transfer to STEs undergoing through the gradient energy levels, and the extrinsic STEs at various energy states contribute to the broadband emission. Moreover, the Cs3Bi2Br9:Sb exhibits excellent thermal and ambient stability for months, which paves a way for the potential luminescence applications for the lead-free halide perovskites.

Data Availability

All data presented in the paper and the supporting information are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Authors’ Contributions

R.L. and C.L. conceived the idea and revised the manuscript. M.S. designed and performed most of the experiments and data analysis. B.Y., S.L., R.Z., and K.H. contributed to the steady-state PL and transient absorption spectroscopy tests and data analysis. All authors discussed the results and contributed to the manuscript.

Acknowledgments

The work was supported by the National Key Research and Development Program of China (2021YFA1502300), conducted by the Fundamental Research Center of Artificial Photosynthesis (FReCAP), and financially supported by the National Natural Science Foundation of China (22088102). R.L. thanks the support from the National Natural Science Foundation of China (22090033) and Youth Innovation Promotion Association of Chinese Academy of Sciences.

Supplementary Materials

Figures S1-S18 and Tables S1-S6 show the SEM-EDS spectra, XPS, TRPL, steady-state PL spectra, TAS spectra, XRD, UV-vis spectra, structural refinement data, and calculated PL lifetimes. (Supplementary Materials). (Supplementary Materials)

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