Solvent-Mediated Structural Evolution Mechanism from Cs₄PbBr₆ to CsPbBr₃ Crystals

Abstract

The study of the solvent-mediated structural evolution mechanism of the Cs₄PbBr₆ powders prepared using the solvothermal method is presented. The Cs₄PbBr₆ powders with a rhombohedral structure and an intense green emission (i.e., mainly due to the presence of complex defect states in the forbidden gap), which is stable in its solid-state form, but a distinct behavior is observed in different dispersions, easily detectable when irradiated with ultraviolet (UV) light. Depending on the polarity of the solvent, a change in the emission color from green to red is observed, easily detectable when irradiated with ultraviolet (UV) light. Our findings suggest that the solvent polarity affects the surface decomposition process, leading to a different change in composition, structure and crystal shape. This peculiar behavior plays a pivotal role in the control of the properties of Cs₄PbBr₆, and this study, therefore, offers a fundamental understanding needed for Cs₄PbBr₆ potential future applications.

1. Introduction

All-inorganic, cesium lead, bromide-related compounds are promising for optoelectronic and photovoltaic applications and have therefore received considerable attention over the last decade [1,2,3,4,5,6]. Such compounds with a perovskite structure may exist in different stoichiometries such as CsPbBr₃, Cs₄PbBr₆ and CsPb₂Br₅, which are usually characterized by a difference in the stacking of [PbX₆]^4– octahedra in the crystalline structure [1,2,3,4]. As is well-known, each crystalline structure has distinct physical and chemical properties. For instance, the bulk CsPbBr₃ and Cs₄PbBr₆ crystals have a direct bandgap (2.43 eV and 3.95 eV at room temperature, respectively) [4], with a low exciton binding energy of ~19–62 meV for CsPbBr₃ [7,8,9] and a large exciton binding energy of approximately 353 meV for Cs₄PbBr₆ [9]. Both structures are of significant interest in optoelectronic and photovoltaic applications compared to the bulk CsPb₂Br₅ phase, which exhibits an indirect bandgap of approximately 3.85 eV [10,11]. In this context, many researchers have studied various techniques for the fabrication of both CsPbBr₃ and Cs₄PbBr₆ materials, using numerous processing methods, although the crystal structure and the correlated properties are still unclear and debated in the literature [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Moreover, a significant limitation in the development of these materials is related to the large-scale fabrication, as currently, most of the available protocols to obtain Cs₄PbBr₆ crystals are based on the use of high purity reagents [19,20,21]. Consequently, it is necessary to develop new processing methods to facilitate the fabrication of large-scale, single-crystalline Cs₄PbBr₆-related compounds at a low cost, attractive to boost the emergence of new technologies, which in turn are not yet in the market due to the high cost of the final product.

Herein, we presented a solvothermal approach and a detailed study of structural, morphological and optical properties of Cs₄PbBr₆ crystals, in addition to their solvent-mediated structural evolution process. We observed that the Cs₄PbBr₆ crystals in solution exhibit a distinct change of the emitted color, depending on the type of solvent used (dimethyl sulfoxide, DMSO, N,N-dimethylformamide, DMF and methyl acetate, MeOAc). To investigate the effects of the solvent-mediated structural evolution of the Cs₄PbBr₆ materials, we used field-emission scanning electron microscopy (SEM), in addition to ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, in conjunction with electrochemical measurements. This study provides a new route for Cs₄PbBr₆ synthesis and an insight into the effects of solvent-mediated structural evolution in the photophysics responses of Cs₄PbBr₆ microcrystals as prepared.

2. Experimental

Cs₄PbBr₆ microcrystals were successfully synthesized based on the solvothermal method using approximately 5 mL of a solution of 6 M hydrobromic acid (HBr, 48%), together with 3.5 mmol of lead acetate trihydrate (Sigma-Aldrich, 99%, St. Louis, MO, USA) and 3.5 mmol of cesium carbonate (Sigma-Aldrich, 99%) in 10 mL of dimethyl sulfoxide into a Teflon autoclave at 150 °C for 120 min. Next, the resulting precipitate was then cooled to near room temperature and then immediately washed with acetone and ethanol several times in order to fully remove byproducts and dried at 80 °C overnight to obtain a green powder. A remarkable characteristic of this solvothermal approach is that it may, in principle, be scalable by an increase of the size of autoclave used [22]. The as-prepared powders were characterized using X-ray diffraction (XRD) in a 2θ range from 5° to 120° at 0.01° min⁻¹, using an X-ray diffractometer (D8 Advance, Bruker-AXS, Karlsruhe, Germany) with a Cu Kα source. Micro-Raman spectra are acquired using a Raman spectrometer model NRS-3100 (JASCO, Easton, MD, USA) using a 633 nm laser excitation source. The morphology and compositional analysis of the prepared Cs₄PbX₆ powders was analyzed using SEM (JEOL 7001F, Tokyo, Japan). In this case, the optical properties were then analyzed using UV-vis absorption spectroscopy with a Cary 300 Bio spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA) operating in the diffuse reflection mode. In this study, PL measurements, including their time-resolved PL spectra, were collected at room temperature using a Horiba-Fluorolog (Jobin Yvon, Longjumeau, France) for 350 nm excitation. The photoluminescence quantum yield (PLQY) was measured using an integrating sphere accessory with an excitation wavelength of 350 nm (Hamamatsu Photonics, Hamamatsu, Japan). For the study of the mechanism of structural evolution of these perovskites, different dispersions were prepared with concentrations of 10 mg·mL⁻¹ Cs₄PbBr₆ crystals using dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and methyl acetate (MeOAc) as the solvent. Next, for visualizing this specific process, the resulting dispersion was UV-irradiated for 120 min (UV bulb, 9 W, 365 nm). After 120 min there was no change significantly observed in our preliminary tests and, for this reason, we selected this time for our experiments. Thus, the kinetics of the solvent-mediated structural evolution of Cs₄PbBr₆ crystals were then monitored via SEM, UV-vis and PL measurements.

3. Results and Discussion

The pure Cs₄PbBr₆ has a typical structure with an ionic character higher than the one in the CsPbBr₃ structure, due to an increase of the Pb-Br bond length in Cs₄PbBr₆ [7,23]. In turn, this structure is considered much more tolerant of the incorporation of defects, compatible with many desired optoelectronic applications, especially in light-emitting devices or as battery components [23]. Consequently, this implies a stronger PL emission for Cs₄PbBr₆ crystals. However, the origin of this strong PL emission for the Cs₄PbBr₆ crystals is still debated [3,4,7,8]. These properties are supported by their crystalline structure, in which the clusters of (PbBr₆) are isolated and separated by ions of Cs [24]. As briefly mentioned above, the Cs₄PbBr₆ powder is synthesized for the first time from the solvothermal method and first characterized by the XRD measurements. Figure 1a illustrates the XRD patterns of Cs₄PbBr₆ powders that were processed under solvothermal conditions, where all the peaks can be correctly indexed to the rhombohedral phase with space group (ICSD # 162158). A high intensity of the (113) face for the as-prepared Cs₄PbBr₆ crystals (compared to the ICSD # 162158) indicates a predominance of this exposed face (113) related to the solvothermal conditions used in this study. Figure 1b shows the crystallographic unit cell of the rhombohedral Cs₄PbBr₆ structure (belonging to space group R-3c (n° 167)) that is typically formed by octahedral (PbX₆) clusters separated by Cs ions [7,23,24]. The reaction at this temperature produces an assembly of cubic-like morphologies, which in turn leads to distinct shapes, as shown in the SEM images of the Cs₄PbBr₆ powders (Figure 1c). In addition, the EDS data (Figure 1d–g) confirms the presence of Cs, Pb and Br in agreement with the expected stoichiometry for the as-prepared Cs₄PbBr₆ crystals.

The room-temperature micro-Raman spectra of single-crystalline Cs₄PbBr₆ (Figure 2A), display two Raman-active bands at approximately 80.64 cm⁻¹ and 121.30 cm⁻¹, which are attributed to the vibration modes of the octahedral (PbX₆) clusters in the rhombohedral Cs₄PbBr₆ structure [7,25,26]. Additionally, the Raman intensity (I_80.64/I_121.30) ratio found for these crystals may be a possible indication that the halide vacancies are predominant in this structure. To investigate the optical response of the Cs₄PbBr₆ powders, the UV-vis and PL measurements are carried out. The Cs₄PbBr₆ materials exhibit an optical absorption spectrum that is governed by well-studied direct electronic transitions [7,27] and Figure 2B shows the UV-vis spectra of the Cs₄PbBr₆ powders with a peak absorption at approximately 520 nm. The calculated bandgap of the Cs₄PbBr₆ is ~3.59 eV (Figure 2B), according to the Kubelka–Munk method [28], a significantly lower value if compared with the literature (3.95 eV) [8]. The PL band in Figure 2D has a maximum emission at about 540 nm, in accordance with previous reports [3,7,9,29].

Notably, the PL emission spectra are then deconvoluted into three sub-peaks using the Gaussian function, as shown in Figure 2D, in which each of these components represents a different defect state within the forbidden bandgap [27]. The observed PL profile can be attributed to specific structural distortions in the octahedral (PbX₆) clusters [3,7,9,29,30]. Moreover, the PLQY measured for the Cs₄PbBr₆ powders is about 15%, indicating that the solvothermal method is an alternative way to synthesize a good-quality material on a large scale. As the lifetime constant (τ₁ = 19.63 ns) is usually associated with excitonic recombination, while τ₂ = 19.64 ms can be attributed to the different defect states that are generated during the synthesis of such samples [30]. However, the PLQY and decay time results for as-prepared pure Cs₄PbBr₆ microcrystals are in general lower and slower than reported in the literature [3,31]. However, it must be emphasized that different synthetic routes prepared the same materials used in this comparison. It is known that materials processing is based on defect engineering. Thus, the final product will have more or fewer defects depending on the experimental conditions used in their manufacturing [32,33]. From this perspective, a similar material in terms of its physical characteristics (such as morphology, size, composition and structure) but prepared from the use of different synthetic routes exhibits completely different properties [32,33,34].

Considering its potential application in several emerging optoelectronic technologies, studies on its structural transformation and structural evolution process may reveal important chemical information regarding the stability of the material. As well-known, the as-prepared Cs₄PbBr₆ powders exhibit high intrinsic stability (e.g., both moisture and oxygen) [8]. Interestingly, we observed an unusual behavior of the as-prepared Cs₄PbBr₆ powders in solution (Figure 3A). Based on the polarity of the solvent used in this study (DMSO, DMF, MeOAc), a change in the color emitted (in solution) from green to red is observed. A detailed understanding of this unusual behavior is essential for the solution-processed devices, since this feature may ultimately affect the device performance. For a better understanding of the main steps of this structural evolution mechanism, we initially evaluated the photophysics properties of the three different dispersions, in DMSO, DMF and MeOAc, respectively, by means of UV-vis and PL measurements, which are compared in Figure 3B–E. Due to the low dispersibility of Cs₄PbBr₆ in MeOAc, it is hard to obtain reliable measurements, so only the comparison between the powder in DMSO and DMF is reported.

As expected, we observed a blue shift in the UV-vis absorbance spectra as a function of solvent polarity used in this experiment [35]. Therefore, we can observe that the change in solvent polarity alters their structural evolution kinetics behavior, suggesting that a lower polarity is responsible for a higher structural evolution rate for the Cs₄PbBr₆ crystals. In fact, the UV-vis absorption spectrum for all dispersions shows a small variation in time, being more meaningful for minor polarity solvent. These findings are in good agreement with PL measures, which show a smaller reduction of intensity PL for the Cs₄PbBr₆ in DMSO over time. In all cases, Cs₄PbBr₆ crystals in DMSO were the most optically stable dispersions. These photophysical results reveal a partial structural transformation of these samples. Our data thus suggest a possible stabilization of defect states by the solvent that likely also acts as a passivating agent.

From a structural point of view, these octahedral (PbX₆) clusters are isolated in the rhombohedral Cs₄PbBr₆ structure, which may indicate a weak interaction between them [7,23,24,27], and that the surface of these structures is highly flexible. Hence, we propose that the solvents can induce a partial structural evolution of the surface of the Cs₄PbBr₆ crystals, due to the interaction with the octahedral (PbX₆) clusters, in turn exchanging the CsBr and creating new surface states that modulate the emission of the solution. The extraction of CsBr is in fact already studied and leads to a phase transformation of Cs₄PbBr₆ into CsPbBr₃ [36,37,38], as we observed from the BSE-SEM images (Figure 3D) and EDS measurements in different regions after 120 min of solution aging. The chemical and morphological alteration of the crystals confirms that the solvent contributes to the decomposition of Cs₄PbBr₆ as we detect two different compositions (Cs₄PbBr₆ and CsPbBr₃) and also the CsBr extracted residual outside the crystal (see Figure 4). The structural modifications in the solution are therefore the reason why the emission in the solution changes with the polarity of the solvent, which induces a partial chemical and structural transformation of the Cs₄PbBr₆ to CsPbBr₃, as well as a partial amorphization of the surface of these crystals.

In addition, the different suspensions of these crystals play a critical role in controlling the CsPbBr₃ final size. According to our SEM images (Figure 1 and Figure 4), it seems that the structural evolution process is a sort of Kirkendall effect [39,40]. Furthermore, these results suggest that this process is not only limited to the surface of the material studied, but might in principle occur in the entire crystal, as well. Therefore, these kinetic data provide evidence of the critical role of the solvent in determining the properties of Cs₄PbBr₆. These results are consistent with the shift observed in the UV-vis and PL data. Also, it is worth highlighting that Cs₄PbBr₆ has a high dispersion in DMSO, which hence is the most suitable solvent for solution-based optoelectronic devices.

4. Conclusions

In summary, we reported the synthesis of the pure Cs₄PbBr₆ crystals by solvothermal method, with a rhombohedral structure, a green emission at normal room temperature (mainly due to the presence of complex defect states in the forbidden gap) and exhibiting a high PLQY. Moreover, we have demonstrated a solvent-mediated structural evolution process of the Cs₄PbBr₆ crystals to shine a light on the intrinsic stability of the as-prepared material. According to our findings, the chemical transformation of the surface of Cs₄PbBr₆ is promoted by the solvent, which can induce a color change for Cs₄PbBr₆ dispersions. This is related to the different extraction rates of the CsBr species (i.e., originating at the Cs₄PbBr₆ surface), which in turn depends on the polarity of the solvent used. With the increase of the solvent polarity, the structural evolution of the Cs₄PbBr₆ crystals is faster, resulting in a blue shift of the solution emission and a rearrangement of the structure and morphology following a Kirkendall-like mechanism. The modulation of phase composition Cs₄PbBr₆/CsPbBr₃ may therefore be beneficial for the performance of these devices, since the relative band position of these materials would, in theory, facilitate the recombination of holes and electrons. Furthermore, such effects could pave the way for the emergence of novel optoelectronic applications based on Cs₄PbBr₆/CsPbBr₃ materials.