Enhanced piezo-response of mixed-cation copper perovskites with Cl/Br halide engineering

Abstract

Halide and cation engineering of organic–inorganic hybrid perovskites has shown a great potential for structural modulation of perovskites and enhancing their optoelectronic properties. Here, we studied the impact of Cl/Br halide engineering on the structural and piezoelectric properties of MA/Cs mixed-cation Cu-perovskite crystals. X-ray diffraction, Raman spectroscopy, and ¹³³Cs solid-state NMR were utilized to find out the nature of the perovskite crystal structure formation. Three distinct crystal structures were obtained depending on the Cl/Br content. High Cl content resulted in the formation of Br-doped (Cs/MA)CuCl₃ perovskite with the presence of paramagnetic Cu²⁺ ions. High Br content led to the formation of Cl-doped (MA/Cs)₂CuBr₄ perovskite with the presence of diamagnetic Cu⁺ ions. Equimolar Cl/Br perovskite content gave a novel crystal structure with the formation of well-dispersed diamagnetic domains. Compared to the high Cl/Br containing perovskites, the equimolar Cl/Br perovskite revealed the highest potential for piezoelectric applications with a maximum recordable piezoelectric output voltage of 5.0 V. The results provide an insight into the importance of mixed-halide and mixed-cation engineering for tailoring the perovskite structural properties towards a wide range of efficient optoelectronics.

---

Introduction

Organic–inorganic hybrid perovskite materials have recently made a dramatic and rapid impact on optoelectronic applications owing to their low processing cost, suitable bandgap, high absorption coefficient, and superior carrier transport.^1–6 Halide perovskites have the general formula ABX₃, where A is an organic or inorganic cation, B is metal, and X is halide.⁷ Lead and tin-based halide perovskites have acquired the most perovskite-related research interest. However, the environmental concerns of toxic lead (Pb) and easily oxidized tin (Sn) hinder their commercialization.^8,9 Alternatively, highly stable copper-based halide perovskites are deemed one of the next generation of lead-free perovskites to be emerging in photovoltaics,¹⁰ energy storage,¹¹ piezoelectric,¹² lighting emitting diodes,¹³ thermoelectric,¹⁴ and thermochromic smart windows applications.¹⁵

Structural modulation of halide perovskites, in terms of A-site, B-site, and X-site engineering, has revealed its potential to explore new perovskites with unique properties. One of the most common A-site engineering routes is inorganic cesium (Cs)/organic methyl ammonium (MA) mixed-cations alloying. Premkumar et al. reported highly ordered MA_1−xCs_xPbBr₃ mixed phase structures with suppressing of non-radiative recombination through incorporation of Cs⁺ cations in the MAPbBr₃ crystal structure.¹⁶ Further work showed the bimodal bandgap behavior of MA_1−xCs_xPbBr₃ single crystals where the bandgaps of mixed-cation perovskites are like those of pure CsPbBr₃ for x > 0.13 and pure MAPbBr₃ for x ≤ 0.13.¹⁷ Si et al. revealed an enhancement of the green emission properties of MAPbBr₃ thin film upon Cs-alloying to achieve a maximum external quantum efficiency of ∼2.0% for an MA_0.6Cs_0.4PbBr₃ perovskite thin film-based device.¹⁸ Cs-doping (2%) was found to enhance the photodetection property of MAPbBr₃ single crystal owing to the reduction in trap density of the mixed-cation perovskite single crystal.¹⁹ Other work discussed different morphologies such as porous sheets, nanorods and nanowires, fiber structures, and tightly bonded grains upon mixing MAPbI₃ with various amounts of Cs ions.²⁰

Most of the previous literature focused on the impact of Cs/MA mixed-cations engineering of lead-based perovskite materials. However, a few research works were reported concerning Cs/MA mixed-cations engineering of lead-free perovskite materials.²¹ Regarding our previous work, a two-dimensional phase-pure MA/Cs mixed-cation Cu-based perovskite crystal can be obtained by using an equal concentration of A-site cations [MA⁺] = [Cs⁺] and X-site halides [Cl⁻] = [Br⁻] as well through the perovskite precursor. The as-prepared perovskite exhibits a reduced bandgap (1.53 eV) with enhanced photovoltaic properties compared to single cation-based perovskites.²² Here, we study the impact of Cl/Br mixed-halides engineering on the structural properties of mixed-cation Cu-perovskite with an equal concentration of A-site cations [MA⁺] = [Cs⁺].

Piezoelectric materials, showing electrical change under the effect of mechanical strain, have exhibited potential application in power storage, smart sensors, energy harvesters, and artificial actuators.²³ Regarding the literature, MAPbI₃ and FAPbBr₃ lead-based halide perovskite materials have emerged in piezoelectric generators owing to their light-induced piezoelectric properties.^24–26 You et al. reported a non-centrosymmetric TMCM–MnCl₃ perovskite structure with significant enhancement of piezoelectric properties, where TMCM = trimethylchloromethylammonium cation.²⁷ Generally, embedding of piezoelectric perovskite particles through polymeric materials was found to enhance the overall piezoelectric performance.²⁸ For instance, Huang et al. reported a great enhancement of piezoelectric properties of MA₂CuCl₄/PVDF composite film with a 7 times higher piezoelectric response than PVDF film.¹² In this work, we study the piezoelectric potential of (Cs/MA) mixed-cation (Cl/Br) mixed-halide copper perovskites with embedding through the most common polymer utilized for piezoelectric devices: polydimethylsiloxane (PDMS) polymer matrix.

Experimental

Materials

Cesium bromide CsBr (≥99%), cesium chloride CsCl (≥99%), copper bromide CuBr₂ (≥99%), copper chloride CuCl₂ (≥99%), methylammonium chloride MACl (≥99%), and methylammonium bromide MABr (≥99%), as raw materials were purchased from Sigma-Aldrich. Deionized water was used as the solvent for the perovskite crystal growth. All reagents and solvents were used without further purification.

Synthesis of (CH₃NH₃)₂CuCl_4−xBr_x single crystals

As depicted in Table 1, saturated aqueous solutions of different molar ratios of cesium halide CsX (1.5 mmol), methylammonium halide MAX (1.5 mmol), and cupric halide CuX₂ (2 mmol) were prepared via dissolving the perovskite precursors in 2 mL deionized water using VORTEX for 5 min. The dissolved solutions were filtered via a 0.45 μm PVDF syringe filter. Then, the resultant solutions were left for two weeks, at room temperature, to get single crystals of (Cs/MA) mixed-cations (Cl/Br) mixed-halides copper perovskites.

Table 1 Molar ratios of raw solute materials used for preparing mixed-halide Cu-perovskites

CsCl (mol)	CsBr (mol)	MACl (mol)	MABr (mol)	CuCl2 (mol)	CuBr2 (mol)	Perovskite abbreviation
1.5	w/o	1.5	w/o	2.0	w/o	Cl7
1.5	w/o	w/o	1.5	2.0	w/o	Cl5.5Br1.5
w/o	1.5	w/o	1.5	2.0	w/o	Cl4Br3
1.5	w/o	1.5	w/o	w/o	2.0	Cl3Br4
w/o	1.5	1.5	w/o	w/o	2.0	Cl1.5Br5.5
w/o	1.5	w/o	1.5	w/o	2.0	Br7

---

Fabrication of perovskite/PDMS piezoelectric sensors

The sensor was fabricated using SYLGRAD 184 silicone elastomer kit as the source of PDMS and the curing agent. 10 wt% of perovskite powder samples were mixed with 10 : 1 (V/V) of “PDMS base material : curing agent” mixture, and stirred slowly for 20 min. This mixture was then poured into a 1 × 1 inch² glass mold and heated to 100 °C for 30 min to accelerate the curing time. After fully curing, copper tape, used as electrodes, was added to both sides of the film. Finally, the copper-taped film was surrounded with Kapton tape to shield from any external electrical effect.

Characterization

The perovskite morphological structure was confirmed via scanning electron microscopy (SEM) (JEOL) with energy dispersive spectroscopy (EDS) to obtain an elemental mapping to measure the Cl/Br content. Raman vibrational modes (JASCO, NRS4500 NMDS) were obtained using 532 nm excitation (green) lasers to characterize perovskite crystal lattice vibrations. X-ray diffraction (XRD) measurements were performed using a Rigaku Smart lab diffractometer at a wavelength of 1.5406 Å. Rietveld refinement was performed by GSASII program. Thermogravimetric analysis was conducted by TGA (TA Instruments). A Keysight Infinnivision DSOX3024T digital storage oscilloscope was used for measuring piezoelectric properties.

Solid-state nuclear magnetic resonance analysis

The ¹³³Cs and ¹H solid-state NMR spectra (ssNMR) were recorded at 11.7 T using a Bruker AVANCE III HD spectrometer and a 2.5 mm cross-polarization magic angle spinning (CP/MAS) probe operating at the Larmor frequency of ν(¹³³Cs) = 65.611 MHz and ν(¹H) = 500.180 MHz, respectively, and the following techniques were applied: (i) ¹H and ¹³³Cs VF/MAS NMR experiments; (ii) the ¹³³Cs spin–lattice relaxation times T₁(¹³³Cs) were measured using the saturation recovery and inversion recovery experiments. A temperature of 298 K was maintained for all NMR experiments and the temperature calibration was performed on Pb(NO₃)₂.²⁹ Dried samples were packed into ZrO₂ rotors and stored at laboratory temperature. All NMR spectra and relaxation build-up dependencies were processed using the Top Spin 3.5 pl2 software package. Detailed experimental parameters are provided in the ESI.†

Results and discussion

Six different mixed halides (Cl/Br) of 50% mixed-cations (Cs/MA)-based copper perovskite crystals were synthesized via slow evaporation of aqueous saturated solutions containing 1.5 : 1.5 : 2 molar ratios of CsX : MAX : CuX₂, respectively, where X = Cl or Br. Photographs of mixed halides/cations Cu perovskite crystals with different Cl/Br precursor ratios are shown in Fig. 1a. Upon increasing Br-content in the precursor solution, three major perovskite sets, whose colors are reddish-orange, brown, and brownish black, were obtained for Cl₇–Cl_5.5Br_1.5, Cl₄Br₃–Cl₃Br₄, and Cl_1.5Br_5.5–Br₇ mixed cation/halide perovskite crystal sets, respectively. This color transition was further confirmed after grinding the crystals, as depicted in Fig. 1b. Different morphological structures were observed microscopically for perovskite crystals by scanning electron microscope (SEM), as shown in Fig. S1a (ESI†). Gradual increases and decreases of Br% and Cl%, respectively, through perovskite samples were confirmed by energy dispersive spectroscopy (EDS), as presented in Fig. 1c. The atomic compositions of Cl (%) and Br (%) vary among the three previously designated main categories. The Br compositions of the perovskite sets (Cl₇–Cl_5.5Br_1.5, Cl₄Br₃–Cl₃Br₄, and Cl_1.5Br_5.5–Br₇) are in the range (0–5%, 20%, and 35–45%), respectively. EDS elemental mapping is presented in Fig. S1b and c (ESI†).

Fig. 1 Copper perovskite crystals prepared via the slow evaporation method (a) and after grinding with mortar and pestle (b) where the samples from left to right are: Cl₇, Cl_5.5Br_1.5, Cl₄Br₃, Cl₃Br₄, Cl_1.5Br_5.5, and Br₇, respectively. (c) Data from energy dispersive spectroscopy (EDS) for determining the (Cl/Br) content of perovskite crystals.

Powder X-ray diffraction (XRD) measurements performed for the perovskite powder samples reveal the appearance of three distinct crystal structures (as shown in Fig. 2a) that relate to the three distinct EDS sets. Both the (Cl₇–Cl_5.5Br_1.5) and the (Cl_1.5Br_5.5–Br₇) sets exhibit the same crystal structure as CsCuCl₃ and Cs₂CuBr₄, respectively. Meanwhile, the third (Cl₄Br₃–Cl₃Br₄) perovskite set exhibits a novel mixed cations/halides crystal structure. According to literature,^30,31 Cl₇ and Br₇ perovskites have a distorted hexagonal P6122 crystal structure and an orthorhombic Pnma(62) crystal structure, respectively (as depicted in Fig. S2 and S3, ESI†). Generally, the diffraction patterns of each perovskite set shift obviously toward smaller 2θ angles upon Br addition due to the larger effective ionic radius of Br⁻ (196 pm) compared to Cl⁻ (181 pm). As a result, Cl_5.5Br_1.5 and Cl_1.5Br_5.5 perovskites are supposed to be MA/Br-doped CsCuCl₃ and MA/Cl-doped Cs₂CuBr₄ perovskites, respectively. Rietveld refinement was performed to confirm this behavior, as depicted in Fig. S4 (ESI†), which is consistent with previous literature.

Fig. 2 Powder XRD measurements (a) and Raman measurements (b) of different Cu-perovskite powders.

Raman vibrational modes of the mixed cations/halides perovskites were examined in the range (50–500 cm⁻¹) to further prove their trimodal behavior, as depicted in Fig. 2b. Three different Raman modes were obtained from the three perovskite sets which are consistent with their three different crystal structures, as ascribed in XRD measurements. In the (Cl₇–Cl_5.5Br_1.5) perovskite category, three Raman modes, around 115, 180, and 285 cm⁻¹, are assigned to ω(E₂), the X–Cu–X wagging, τ(E₂), the X–Cu–X twisting/torsion, and ν_s(E₂), the X–Cu–X symmetric stretching, where X is Cl and/or Br, respectively. Meanwhile, the (Cl_1.5Br_5.5–Br₇) perovskite category Raman mode around 184 cm⁻¹ is ascribed to the symmetric Cu–Br stretching mode. It is worth noting that the (Cl₄Br₃–Cl₃Br₄) perovskite set has a similar Raman modes behaviour as the (Cl₇–Cl_5.5Br_1.5) perovskite set with blue shifting of the second Raman peak toward higher wavenumber around 200 cm⁻¹.

¹³³Cs solid-state NMR (ssNMR) analyses were performed on the copper-based perovskites with mixed halides (Cl/Br) and mixed counterions (MA/Cs). To gain deeper insight into the structure of the prepared systems, ¹³³Cs VF/MAS NMR spin-echo³² and T₁-relaxation experiments^33,34 were conducted. This approach is based on the fact that the presence of unpaired electron(s) induces detectable changes in the chemical shift of observed nuclei and a reduction in ¹³³Cs NMR T₁-relaxation times. The reason for this observation is the fact that the presence of paramagnetic metal centers (e.g. Cu²⁺) causes extremely rapid longitudinal and transverse relaxation of the nearby nuclei due to strong electron–spin couplings.^35,36 The relaxation build-up curves of the detected ¹³³Cs NMR signal(s) were analysed by single-exponential functions and values of T₁(¹³³Cs) relaxation times of individual components are summarized in Table 2. The inversion-recovery technique was used for spins with fast relaxation behavior, while the saturation-recovery experiment was used for spins with relatively slow relaxation times. The investigated samples can be divided into three groups based on the recorded ¹³³Cs VF/MAS NMR spectra, as depicted in Fig. 3 and Fig. S5 (ESI†).

Table 2 The ¹³³Cs spin–lattice relaxation times T₁(¹³³Cs) with relative amount determined for each spectral component of the perovskite samples

^133Cs spin–lattice relaxation times T1(^133Cs) [s, ms]
Sample/δiso	1200 ppm	950 ppm	294 ppm	215 ppm	165 ppm
Cl7	13.5 ms	—	—	—	—
Cl5.5Br1.5	15 ms	24.5 ms	—	—	—
Cl4Br3	—	—	25 ms	750 ms	10 s
Cl3Br4	—	—	—	750 ms	14 s

---

Sample/δiso	2900 ppm	2750 ppm	600 ppm	155 ppm	145 ppm
Cl1.5Br5.5	2 ms	13.3 ms	6 ms	—	—
Br7	2 ms	—	6 ms	15 s	31 s

---

Fig. 3 Experimental ¹³³Cs VF/MAS NMR spectra of (a) Cl₇, Cl_5.5Br_1.5, (b) Cl₄Br₃, Cl₃Br₄, and (c) Cl_1.5Br_5.5, Br₇, respectively. The asterisks (*ssb) denote spinning sidebands.

The first group consists of samples with low bromine content (Cl₇–Cl_5.5Br_1.5), where the dominant NMR signal is located at approximately 1200 ppm (Fig. 3a). The Cl₇ perovskite sample exhibited one symmetric and very narrow signal corresponding to a single crystallographic position of Cs ions. Based on literature data, this signal was attributed to (Cs/MA)CuCl₃ which is consistent with XRD and Raman results.³⁷

In the case of Cl_5.5Br_1.5 perovskite, which has a slightly increased bromine anion content, two broad, not fully resolved signals at around 1200 ppm and 950 ppm were detected in the ¹³³Cs VF/MAS NMR spectrum. The presence of two signals indicates two preferred positions of Cs⁺ ions in the perovskite matrix, where one or more Cl⁻ anions are replaced by Br atoms, respectively. The incorporation of Br⁻ into the matrix is random, which is evident from the visible signal broadening in the spectrum of Cl_5.5Br_1.5. However, the similar chemical shift of the detected signals in both systems suggests a similar crystallographic group, as discussed in the XRD section, with slight distortion in the symmetry group for Cl_5.5Br_1.5 perovskite. Moreover, the detected signals in both systems show a significant upfield shift of the ¹³³Cs NMR signals, indicating the close proximity of unpaired electrons (Cu²⁺) to Cs counterions. Clear evidence of the presence of paramagnetic Cu²⁺ ions is provided by the ¹³³Cs spin–lattice relaxation experiments, which yield very fast T₁(¹³³Cs) times of around 20 ms, as shown in Table 2. The ¹³³Cs spin–lattice relaxation processes were monitored using the inversion-recovery technique and easily described using a single-exponential function for all three signals, which indicates uniform distribution/dispersion of the ions within the domains.

The second (Cl₄Br₃–Cl₃Br₄) perovskite group can be defined as the systems with equimolar chlorine and bromine contents. In these samples, the dominant NMR signal was detected at 215 ppm, as depicted in Fig. 3b, indicating copper-based perovskites with an equal ratio of chlorine and bromine. In the case of the Cl₄Br₃ sample, two additional signals were detected with chemical shifts of 294 ppm and 165 ppm, respectively. The signal at 294 ppm was attributed to chlorine-rich domains with T₁(¹³³Cs) time 25 ms, while the signal at 165 ppm was attributed to bromine-rich domains. Furthermore, the observed T₁(¹³³Cs) time of the bromine-rich domains is around 10 s. The Cl₃Br₄ sample, with a higher bromine content, exhibited only two relatively sharp signals at 215 ppm and 161 ppm, corresponding to the copper-based perovskite with an equal ratio of halides phase and bromine-rich domains, respectively. Additionally, this sample was probed by a ¹³³Cs VF/MAS NMR experiment with a long repetition delay of 100 s, as shown in the inset of Fig. 3b, where the signal intensity of the bromine-rich domains is significantly increased. Similar behavior also confirms the ¹³³Cs spin–lattice relaxation experiments with similar T₁(¹³³Cs) times, as shown in Table 2. These results suggest that the bromine-rich domains form a homogeneously dispersed phase where Cu²⁺ is reduced to Cu¹⁺ (in other words, no effect of paramagnetic centers was observed in this phase) with domain size >20 Å. The spin–lattice ¹³³Cs relaxation process was monitored using the saturation-recovery technique for the spins resonating at 165 ppm, while an inversion-recovery experiment was applied for the spins resonating at 215 ppm and 294 ppm (Table 2).

The last group can be defined as high bromine content (Cl_1.5Br_5.5–Br₇), and corresponding ¹³³Cs VF/MAS NMR spectra are depicted in Fig. 3c. The ¹³³Cs VF/MAS NMR spectrum of the Br₇ sample (fully bromine perovskite) exhibited two sharp and symmetric signals at ca. 2900 ppm and 600 ppm, respectively. Based on literature data, these two signals correspond to two different Cs sites in (MA/Cs)₂CuBr₄, which is consistent with XRD and Raman data.³⁸ The determined T₁(¹³³Cs) times of 2 ms and 6 ms, respectively, clearly confirm the presence of paramagnetic Cu²⁺ ions in close proximity. Furthermore, two additional diamagnetic Cs sites at 145 and 155 ppm were observed in the case of the ¹³³Cs VF/MAS NMR experiment with a 100 s recycle delay (Fig. 3c, inset). The diamagnetic nature of these detected signals was confirmed using the T₁(¹³³Cs) relaxation experiment, where T₁(¹³³Cs) times of 31 s and 15 s, respectively, were determined. This also indicates that the (MA/Cs)₂CuBr₄ system forms well-dispersed diamagnetic domains within the investigated matrix.

For the Cl_1.5Br_5.5 sample, two broad dominant signals with similar chemical shifts and T₁(¹³³Cs) times as for Br₇ sample were detected. The broadening reflects the random distribution of Cl⁻/Br⁻ anions in the perovskite matrix and suggests the formation of a fully amorphous system. Moreover, a broad extra signal appeared at around 2750 ppm, which was assigned to a minor unknown but paramagnetic phase (see Table 2). In this case, the relaxation process is very fast, and the corresponding build-up curve is noisy. Consequently, the determined relaxation time, T₁(¹³³Cs) = 13.30 ms, should be considered relatively uncertain.

Moreover, the wide dispersion of the ¹³³Cs NMR shifts and the fast T₁(¹³³Cs) times of all detected signals (2900 ppm, 2750 ppm, 600 ppm) suggest the presence of diamagnetic and paramagnetic domains with random distribution in the perovskite matrix. Generally, it can be concluded that the increase in Br⁻ anions induces the formation of well-dispersed diamagnetic domains, where copper is reduced from Cu²⁺ to Cu¹⁺ with roughly estimated domain sizes up to 40 Å.

The presence of the second, organic counterion, methylammonium (MA), in all investigated systems, was confirmed by ¹H VF/MAS NMR experiments, as shown in Fig. 4. Furthermore, ¹H VF/MAS NMR provides not only information about the presence of the organic counterion but also suggests ordering of MA molecules in Cu-based perovskites. The ¹H VF/MAS NMR spectra clearly indicate that MA counterions in fully chlorine-containing Cl₇ and fully bromine-containing Br₇ perovskites are well ordered. Additionally, a trend is observed where an increased concentration of Br⁻ anions in the matrix leads to a more disordered arrangement of MA counterions. This trend is exemplified by the Cl_1.5Br_5.5 sample, where only a broad and unresolved signal was detected. This observation is consistent with the ¹³³Cs VF/MAS NMR spectrum of the Cl_1.5Br_5.5 sample (Fig. 3c), which also suggests the presence of disordered Cs⁺ counterions.

Fig. 4 Experimental ¹H VF/MAS NMR (2-loops) spectra of the perovskite samples. The asterisks (*ssb) denote spinning sidebands.

The thermal stability of the mixed cations Cu-perovskites was studied through thermogravimetric analysis (TGA). The decomposition profiles for Cl₇, Cl₄Br₃, and Br₇ Cu perovskites are composed of two steps, as shown in Fig. S6 (ESI†). The first step is accompanied by weight loss of both organic ammonium halides (MAX) and cesium halides (CsX). The second step is possibly accompanied by the loss of copper halides (CuX₂). The first weight loss is at 320 °C, 190 °C, and 220 °C for Cl₇, Cl₄Br₃, and Br₇ perovskites, respectively, which reveals an increase in the perovskite thermal stability in the order Cl₄Br₃ < Br₇ < Cl₇. Different thermal decomposition profiles of perovskite samples corroborate their different crystal structures which are consistent with the XRD, Raman, and NMR results.

To explore the piezoelectric potential application of the mixed cations/halides copper-based perovskite samples, we fabricated a “Cu/perovskite–PDMS composite/Cu” cell configuration, as shown in Fig. S7 (ESI†), to measure the piezoelectric response. Fig. 5 exhibits the piezoelectric sensing mechanism under a pressing/releasing process. Upon pressing, voltage is generated owing to the polarization of perovskite particles embedded in the PDMS matrix whereas the releasing process leads to demolishing both the perovskite polarization and the pressure-dependent voltage as well. Regarding the literature,³⁹ the 10 wt% of PDMS composite materials was supposed to be the optimized concentration for getting the highest piezoelectric response. As a result, all perovskite/PDMS composites were prepared with a 10 wt% perovskite concentration. Fig. 6 exhibits the piezo voltage of perovskite/PDMS composites under the periodic pressure/release process with a knocking pen. Under vertical compression, the Cl₇, Cl₄Br₃, and Br₇ perovskite–PDMS composite sensors show 2 times, 5 times, and 3 times higher piezoelectric response than that of the pure PDMS sensor, respectively. Notably, combining of mixed cations/halides copper perovskites with PDMS polymer creates a synergistic effect, leading to enhanced piezoelectric properties owing to their strong interaction between copper perovskites and PDMS polymer. Furthermore, the equimolar Cl/Br content perovskite matrix, Cl₄Br₃–PDMS composite sensor, shows the highest piezoelectric response rather than that of both high Cl content and high Br content samples, Cl₇–PDMS and Br₇–PDMS composite sensors, respectively. This might be attributed to both the two-dimensional crystallinity and the diamagnetic domains of Cl₄Br₃ perovskite, as discussed in XRD and NMR results. Comparison of output voltage of halide perovskites-based piezoelectric nanogenerators are shown in Table S1 (ESI†). These results reveal the potential of cation/halide engineering of halide perovskites towards fabrication of efficient piezoelectric generators.

Fig. 5 Piezoelectric sensor configuration and its working mechanism under pressing/releasing process.

Fig. 6 Piezoelectric properties of the perovskite/PDMS composite films: output voltage of PDMS (black), Cl₇/PDMS (red), Cl₄Br₃/PDMS (yellow), and Br₇/PDMS (dark red) composite films under a periodic pressuring and releasing process with a knocking pen.

Conclusions

In summary, we studied the (Cl/Br) mixed halides engineering of (50%Cs/50%MA) mixed cations copper perovskite. Three distinct crystal structures were obtained depending on the Cl/Br content. The piezoelectric sensors based on the equimolar Cl/Br-mixed cation perovskite showed the highest potential for piezoelectric generation. The key figures of merit revealed in this work highlights the potential of emerging lead-free copper halide perovskites for future commercialization of piezoelectric devices.

Author contributions

Amr Elattar conceived the idea for the manuscript and designed the experiments. Amr Elattar synthesized the materials and conducted SEM, Raman, and XRD characterizations. Libor Kobera and Andrii Mahun conducted and wrote the solid-state NMR analysis. Christopher Munoz conducted the piezoelectric application. Amr Elattar wrote the first manuscript. All authors discussed the results and commented on the manuscript at all stages. Jiri Brus led the assessment of the NMR part. Mohammed Jasim Uddin led the assessment of the piezoelectric application part. Yasuhiko Hayashi, Okenwa Okoli, and Tarik Dickens coordinated the entire project.

Data availability

The data supporting this article has been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Amr Elattar, Tarik Dickens, and Okenwa Okoli acknowledge the NNSA MSIPP I-AM EMPOWER'D (Grant No. DE-NA0004004) at the FAMU-FSU College of Engineering. Libor Kobera, Andrii Mahun and Jiri Brus acknowledge the financial support from the Grant Agency of the Czech Republic (Grant GA24-10199S), Dr Siegrist’s lab for XRD usage, and the DOW SURE program for donation of desktop SEM program.

References

1. E. H. Jung, N. J. Jeon, E. Y. Park, C. S. Moon, T. J. Shin, T.-Y. Yang, J. H. Noh and J. Seo, Nature, 2019, 567, 511–515.
2. Z. Liu, J. Hu, H. Jiao, L. Li, G. Zheng, Y. Chen, Y. Huang, Q. Zhang, C. Shen, Q. Chen and H. Zhou, Adv. Mater., 2017, 29, 1606774.
3. R. Yang, R. Li, Y. Cao, Y. Wei, Y. Miao, W. L. Tan, X. Jiao, H. Chen, L. Zhang, Q. Chen, H. Zhang, W. Zou, Y. Wang, M. Yang, C. Yi, N. Wang, F. Gao, C. R. McNeill, T. Qin, J. Wang and W. Huang, Adv. Mater., 2018, 30, 1804771.
4. Q. Chen, J. Wu, X. Ou, B. Huang, J. Almutlaq, A. A. Zhumekenov, X. Guan, S. Han, L. Liang, Z. Yi, J. Li, X. Xie, Y. Wang, Y. Li, D. Fan, D. B. L. Teh, A. H. All, O. F. Mohammed, O. M. Bakr, T. Wu, M. Bettinelli, H. Yang, W. Huang and X. Liu, Nature, 2018, 561, 88–93.
5. H. Cho, S.-H. Jeong, M.-H. Park, Y.-H. Kim, C. Wolf, C.-L. Lee, J. H. Heo, A. Sadhanala, N. Myoung, S. Yoo, S. H. Im, R. H. Friend and T.-W. Lee, Science, 2015, 350, 1222–1225.
6. J. Zhao, L. Zhao, Y. Deng, X. Xiao, Z. Ni, S. Xu and J. Huang, Nat. Photonics, 2020, 14, 612–617.
7. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051.
8. J. Li, H.-L. Cao, W.-B. Jiao, Q. Wang, M. Wei, I. Cantone, J. Lü and A. Abate, Nat. Commun., 2020, 11, 310.
9. L. Chen, S. Fu, Y. Li, N. Sun, Y. Yan and Z. Song, Adv. Sci., 2024, 11, 2304811.
10. I. Hamdi, Y. Khan, F. Aouaini, J. H. Seo, H.-J. Koo, M. M. Turnbull, B. Walker and H. Naïli, J. Mater. Chem. C, 2022, 10, 3738–3745.
11. P. Pandey, N. Sharma, R. A. Panchal, S. W. Gosavi and S. Ogale, ChemSusChem, 2019, 12, 3742–3746.
12. S. Huang, G. Tang, H. Huang, X. Wu, P. Zhou, L. Zou, L. Xie, J. Deng, X. Wang, H. Zhong and J. Hong, Sci. Bull., 2018, 63, 1254–1259.
13. S. Lin, Z. Ma, X. Ji, Q. Zhou, W. Chu, J. Zhang, Y. Liu, Y. Han, L. Lian, M. Jia, X. Chen, D. Wu, X. Li, Y. Zhang, C. Shan and Z. Shi, Adv. Mater., 2024, 36, 2313570.
14. Z. Kang, H. Xiong, B. Wu, L. Jiang, B. Fan, A. Yang, B. Sa, J. Li, L. Lin and Y. Qiu, EcoMat, 2022, 4, e12163.
15. A. Elattar, H. Suzuki, R. Mishima, K. Nakao, H. Ota, T. Nishikawa, H. Inoue, A. K. K. Kyaw and Y. Hayashi, J. Mater. Chem. C, 2021, 9, 3264–3270.
16. S. Premkumar, K. Kundu and S. Umapathy, Nanoscale, 2019, 11, 10292–10305.
17. F. Liu, F. Wang, K. R. Hansen and X.-Y. Zhu, J. Phys. Chem. C, 2019, 123, 14865–14870.
18. J. Si, Y. Liu, N. Wang, M. Xu, J. Li, H. He, J. Wang and Y. Jin, Nano Res., 2017, 10, 1329–1335.
19. L. Li, M. Su, X. Qiu, X. Zhao, X. Zhu, W. Wei, L. Huang, G. Li, Y. Wang and W. H. Sun, Ceram. Int., 2022, 48, 436–445.
20. G. Murugadoss, R. Thangamuthu, S. Vijayaraghavan, H. Kanda and S. Ito, Electrochim. Acta, 2017, 257, 267–280.
21. M. S. Adli Azizman, A. W. Azhari, N. Ibrahim, D. S. Che Halin, S. Sepeai, N. A. Ludin, M. N. Md Nor and L. N. Ho, Heliyon, 2024, 10, e29676.
22. A. Elattar, W. Li, H. Suzuki, T. Kambe, T. Nishikawa, A. K. K. Kyaw and Y. Hayashi, Chem. – Eur. J., 2022, 28, e202104316.
23. C. R. Bowen, H. A. Kim, P. M. Weaver and S. Dunn, Energy Environ. Sci., 2014, 7, 25–44.
24. M. Coll, A. Gomez, E. Mas-Marza, O. Almora, G. Garcia-Belmonte, M. Campoy-Quiles and J. Bisquert, J. Phys. Chem. Lett., 2015, 6, 1408–1413.
25. T. Paul, S. Maiti, U. Mukherjee, S. Mondal, A. Sahoo and K. K. Chattopadhyay, Mater. Lett., 2021, 301, 130264.
26. Y.-J. Kim, T.-V. Dang, H.-J. Choi, B.-J. Park, J.-H. Eom, H.-A. Song, D. Seol, Y. Kim, S.-H. Shin, J. Nah and S.-G. Yoon, J. Mater. Chem. A, 2016, 4, 756–763.
27. Y.-M. You, W.-Q. Liao, D. Zhao, H.-Y. Ye, Y. Zhang, Q. Zhou, X. Niu, J. Wang, P.-F. Li, D.-W. Fu, Z. Wang, S. Gao, K. Yang, J.-M. Liu, J. Li, Y. Yan and R.-G. Xiong, Science, 2017, 357, 306–309.
28. H. Park, C. Ha and J.-H. Lee, J. Mater. Chem. A, 2020, 8, 24353–24367.
29. J. Brus, Solid State Nucl. Magn. Reson., 2000, 16, 151–160.
30. A. W. Schlueter, R. A. Jacobson and R. E. Rundle, Inorg. Chem., 1966, 5, 277–280.
31. N. Krüger, S. Belz, F. Schossau, A. A. Haghighirad, P. T. Cong, B. Wolf, S. Gottlieb-Schoenmeyer, F. Ritter and W. Assmus, Cryst. Growth Des., 2010, 10, 4456–4462.
32. E. L. Hahn, Phys. Rev., 1950, 80, 580–594.
33. D. J. Kubicki, D. Prochowicz, A. Pinon, G. Stevanato, A. Hofstetter, S. M. Zakeeruddin, M. Grätzel and L. Emsley, J. Mater. Chem. A, 2019, 7, 2326–2333.
34. F. Ji, F. Wang, L. Kobera, S. Abbrent, J. Brus, W. Ning and F. Gao, Chem. Sci., 2021, 12, 1730–1735.
35. A. R. Haase, M. A. Kerber, D. Kessler, J. Kronenbitter, H. Krüger, O. Lutz, M. Müller and A. Nolle, Z. Naturforsch., A: Phys., Phys. Chem., Kosmophys., 1977, 32, 952–956.
36. S. Hayashi and K. Hayamizu, Bull. Chem. Soc. Jpn., 1991, 64, 685–687.
37. A. R. Lim, J. K. Jung and S. Y. Jeong, J. Phys. Soc. Jpn., 2002, 71, 332–335.
38. A. R. Lim, Appl. Magn. Reson., 2017, 48, 889–899.
39. K. Jeronimo, V. Koutsos, R. Cheung and E. Mastropaolo, Sensors, 2021, 21, 5873.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00970c

---