Exciton Relaxation Dynamics in Perovskite Cs4PbBr6 Nanocrystals.

Lower-dimensional metal halide perovskites have been recognized as an efficient white light emitter. The broad band emission spectrum originates from the recombination of excited charge carriers through free excitons (FEs), self-trapped excitons (STEs), and defect-trapped excitons. However, the emission properties of zero-dimensional (0-D) perovskites have not been explored extensively. Here, in this work, we have performed low-temperature absorbance, photoluminescence (PL), PL excitation (PLE), PL lifetime, and Raman measurements to understand the exciton relaxation processes in Cs4PbBr6 NCs. Our experimental observations indicate that two distinct UV light spectra evolved from the photoexcited carrier recombination through FE and STE states. We emphasize that such UV light sources can be beneficial for various applications, like curing of materials, disinfection of viruses, hygiene control, etc.

Introduction

Organic–inorganic lead halide perovskites (APbX3: A = CH3NH3/HC(NH2)2/Cs/Rb; X = Cl/Br/I) have attracted immense attention due to their exceptional inherent photophysical characteristics, like high absorption coefficient, tunable band gap, visible light emission, high luminescence intensity, narrow emission linewidth, long carrier diffusion length, and balanced electron–hole mobility.[1−6] The three-dimensional (3-D) perovskite crystal structures are composed of an extended network of corner-sharing [PbX6]4– octahedra with A ions residing inside the cavities of this network. If the [PbX6]4– octahedra are not connected in all three directions, then lower-dimensional perovskite structures (i.e., two-dimensional (2-D), one-dimensional (1-D), 0-D) are developed.[6] These lower-dimensional perovskites exhibit broad band emission with excellent photoluminescence (PL) intensity and stability.[7−10] They were employed as a phosphor-lighting source in development of optically pumped white light-emitting diodes (LEDs). UV light sources are very rare in nature and potentially can be used in preserving or drying materials, disinfection of viruses, hygiene and infection control, etc.[11,12]

Nanocrystalline lead halide perovskites also display some unique features, such as high PL emission intensity, narrow band PL emission, color tunability from UV to the near-infrared wavelength region by varying either the halide constitution or size of the nanocrystals (NCs), and easy dispersibility in various liquid media.[13−16] Color-tunable perovskite NC-based LEDs have already demonstrated efficient device performance with low turn-on voltage.[14−16] These LEDs have achieved device performance comparable to the existing commercial lighting devices, revealing the tremendous progress of perovskite lighting technologies.[17]

0-D Cs4PbX6 NCs were generally synthesized via a hot injection colloidal synthesis approach under Cs-rich condition.[18−20] It has been noticed that Cs4PbI6 NCs emit white light emission at low temperatures, originated from photogenerated charge carrier recombination through intrinsic excitons, self-trapped excitons (STEs), and/or defect-trapped excitons.[21] The Cs4PbBr6 perovskite structure was first recognized more than 20 years ago;[22] however, the emission properties are still not clear and need further investigation. Various research groups have described the origin of PL emission from Cs4PbBr6 crystals in different ways.[23,24] Ray et al. proposed that the green emission appeared from the formation of impurities in the Cs4PbBr6 crystals. Jung et al. theoretically explained that the green emission originated from the native defect states. Recently, Yin et al. reported the appearance of two distinct UV emission spectra in Cs4PbBr6 NCs, which are related to the intrinsic defect Pb ions.[25] Here, in this work, we have studied the exciton relaxation processes in Cs4PbBr6 NCs in more detail. At low temperatures, we have observed two individual UV emission spectra. The first emission spectrum has a PL peak at 3.72 eV, and the other broad band emission spectra composed of two peaks centered at 3.39 and 3.12 eV, respectively. The temperature-dependent absorption, PL, PL excitation (PLE), PL lifetime, and Raman measurements revealed that the UV emission spectra originated from relaxation of charge carriers through free exciton (FE) and STE states.

Results and Discussion

We have synthesized Cs4PbBr6 NCs via a hot injection method following the reported route (see the details in Section ).[20] In short, lead oxide (PbO) powder was first dissolved in 1-octadecene (ODE) in the presence of oleic acid (OAc) and oleylamine (OAm) at 150 °C in a nitrogen atmosphere. Then, the solution was cooled down to 80 °C and the tetrabutylammonium bromide (TBABr) precursor solution was injected into a PbO mixed solution. The reaction was continued for 10 min, and then, the Cs-oleate precursor was injected. The reaction was kept for an additional 5 min to grow the Cs4PbBr6 NCs, and the reaction was quenched in an ice–water bath. The NCs were precipitated through centrifugation and redispersed in toluene solvent for further characterizations and measurements.

The X-ray diffraction (XRD) pattern of Cs4PbBr6 NCs was recorded in the thin film form (see Figure S1a in the Supporting Information). The XRD spectrum matches well with the reported trigonal perovskite crystal structure (space group R3®c), and no other crystal phases were detected.[18,19] The lattice parameters of this structure are a = b = 13.7219 Å, c = 17.3153 Å, and α = β = γ = 90°. The transmission electron microscopy (TEM) image of the NCs expresses a spherical shape with an average size of 8 ± 3 nm (see Figure S1b in the Supporting Information). The absorption spectrum of the NCs was recorded in toluene solvent and is represented in Figure a. A sharp excitonic absorption peak at 3.96 eV was obtained even at room temperature, suggesting a large excitonic binding energy.[18,22] The absorption peak is independent of the size of the NCs. This is due to the fact that the [PbX6]4– octahedra are completely isolated in the crystal structure, and thus, the size of the NCs does not have any significant effect on the band gap formation. The calculated excitonic binding energy of 420 meV was obtained from the Tauc plot (see Figure b). The PL spectra of the NCs in the solution form were recorded, and the corresponding emission profiles are shown in the inset of Figure a. The PL spectrum showed a UV emission spectrum with a peak at 3.73 eV along with a small emission hump at lower energies. This green emission resulted from the existence of impurity CsPbBr3 NCs in the NC solution (see Figure S2 in the Supporting Information).[23,24]

Figure 1

(a) Absorbance spectrum and (b) Tauc plot of Cs4PbBr6 NCs. Inset of (a) represents the PL profile of the NCs.

To understand the exciton relaxation processes in the Cs4PbBr6 NCs, we have conducted temperature-dependent measurements at low temperatures in the thin film form. The optical absorption spectra of the NCs at 298 and 78 K are presented in Figure a. The variation of absorption at the full temperature range is shown in Figure S3 (in the Supporting Information). The absorption spectrum revealed a strong excitonic absorption peak at 3.96 eV along with a broad band at around 5.3 eV.[26,27] The 3.96 eV band is ascribed as the excitons formed in the isolated inorganic [PbBr6]4– octahedra. The band at 5.3 eV is attributed to the transitions from Pb(6s) + Br(4p) to Pb(6p) orbitals. At low temperatures (@ 78 K), the excitonic absorption spectrum becomes narrower and an additional peak appeared at the lower-energy range. We have fitted the absorption spectrum with the Gaussian curve, and the spectrum can be deconvoluted into two peaks, as shown in Figure b. The prominent peak at 3.96 eV indicates the lowest-energy excitons, along with a weak peak at 3.80 eV attributed to the triplet energy states.[27]

Figure 2

(a) Absorbance spectra of Cs4PbBr6 NC thin films measured at 298 and 78 K. (b) Absorbance spectrum (@ 78 K) is fitted with two Gaussian peaks. All the plots are represented in different color legends. Low-temperature PL spectra of Cs4PbBr6 NC thin films excited at (c) 4.08 eV and (d) 3.96 eV.

Temperature-dependent PL spectra of Cs4PbBr6 NC thin films at different excitation energies are represented in Figure c and d. The inspection of PL spectra at room temperature is very important. As the thin film was excited at 4.08 eV (larger than the excitonic band gap energy of 3.96 eV) at room temperature, a narrow emission band centered at 3.72 eV (A band) appeared in PL spectra, as shown in Figure c. However, no significant PL emission was detected in the lower-energy range. While lowering the temperature, the PL intensity of A band increased continuously (five times @78 K) compared to the PL intensity at room temperature (see Figure S5 in the Supporting Information). No obvious PL shift of the A band was observed upon lowering the temperature, while the full width at half-maxima (FWHM) remained almost constant. Interestingly when the temperature reached below 223 K, an additional faint emission band appeared in the lower-energy region. We have fitted the emission spectrum recorded at 78 K with the Gaussian curve in the energy range of 3.42–2.80 eV. The PL spectrum deconvoluted into two emission spectra with peak positions at 3.39 eV (B band) and 3.12 eV (C band), as shown in Figure S7 (see the Supporting Information).

The NC thin film was also excited at 3.96 eV (similar to the excitonic band gap energy), and a nearly similar PL spectrum was observed at room temperature, as shown in Figure d. However, during the cooling process, the emission intensities of B and C bands increased must faster than the PL intensity of the A band (see Figure S4 in the Supporting Information). A shifting in PL peak positions of B and C bands was observed (see Figure S7 in the Supporting Information), and the emission FWHM also varied with the temperature (see Figure S6 in the Supporting Information). At low temperatures, the PL spectra became narrower due to the reduction of phonon vibrational energy. The simultaneous existence of these three emission bands strongly suggests the presence of various types of optical transitions from the excited state to the ground state.

We have also performed the PLE measurements at 78 K under different excitation energies, and the resultant spectra are represented in Figure a and b. It turns out that the PL spectra strongly depend on the excitation energies, and distinct PL profiles are visible under different excitation energies. The PL spectrum is composed of three emission bands having peak positions at 3.72 eV (A band), 3.39 eV (B band), and 3.12 eV (C band). The pseudocolored plot between the excitation wavelength and emission wavelength at 78 K is shown in Figure c. PLE spectra revealed that the PL emission intensity of the A band attained the maximum value under excitation at 4.08 eV energy, while B and C bands attained maximum emission intensity values when the excitation energy was 3.96 eV. Specifically, if the NC sample was excited larger than excitonic band gap energy (4.08 eV), the emission intensity of the A band was much stronger than the intensity of B and C bands. If the sample was excited with a similar value of the excitonic band gap energy (3.96 eV), the corresponding PL intensities from B and C bands became more intense compared to the emission intensity of the A band.

Figure 3

(a, b) PL and PLE spectra of Cs4PbBr6 NC thin films at 78 K. (c) Pseudocolor plot between the excitation wavelength vs emission wavelength at 78 K. (d) Temperature-dependent Raman spectra of the Cs4PbBr6 NC film excited with a 532 nm laser.

It is well known that if the electron–phonon coupling in the perovskite structure is weak, then the PL emission originates from the relaxation of charge carriers through FE states and results in a narrow emission spectrum.[8,28−31] On the other hand, if the electron–phonon coupling is strong, a broad band emission appears from the recombination through different energy states. From our experimental observations, we understand that the A band was originated from the exciton recombination from FE states because of very small Stoke’s shift between the excitonic absorption peak and emission peak. The other two emission bands (B and C bands) exhibited large Stoke’s shift and could be originated from charge carrier relaxation from STEs and/or defect-trapped excitons.

To understand whether any perovskite crystal phase transition causes such PL behavior at low temperatures, we have performed temperature-dependent Raman measurements (excited with a 532 nm laser) for Cs4PbBr6 NC thin films from 298 to 78 K. We observed four distinct Raman peaks (∼46, ∼74, ∼87, and ∼128 cm–1) at 78 K, as shown in Figure d. The appearance of these four Raman peaks at lower wavenumbers signifies different crystal lattice vibration modes, such as the opposite movement of Pb ions toward Cs ions, bending mode of Pb–Br bonds, opposite movement of Cs and Br ions, and opposite movement of Cs and Pb ions.[32] A little shift of Raman peaks to higher wavenumbers with a decrease in temperature is due to the contraction of crystal volume, while the crystal structure remains unchanged. These observations conclude that the enhancement of the emission intensity at lower temperatures is not associated with any phase transition in the perovskite crystal structure.

To estimate the origin of A, B, and C bands, we have performed the excitation-power-dependent PL measurements for Cs4PbBr6 NC thin films at 78 K. The nature of the exciton relaxation process can be estimated from the relationship between the integral PL intensity from the NCs and the excitation power density of the laser source. We have excited the NC thin film with laser energy of 4.00 eV and detected integral PL intensities at 3.72 and 3.31 eV distinctly. It will help us to understand the PL behaviors of A, B, and C bands with variations in the excitation laser power density. We observed that the PL spectra of these bands remained identical even after the excitation power density increased by two orders of magnitude. We have plotted the integral PL intensity versus excitation power density, and the corresponding plots are represented in Figure a and b. Both integral PL intensities at 3.72 and 3.31 eV increased linearly with the external excitation power density that was varied from 0.02 to 1 mW/cm2 at 78 K. No sign of PL saturation characteristics was observed. If the PL emission originated from the permanent defect/trap states in the perovskite structure, a PL saturation behavior should be expected as these trap states became filled at higher exciton power densities. By fitting the integral PL intensity with a power law of the form I α L (where I is the integral PL intensity, L is the power density of the excitation laser, and k is the fitting constant), we got the k values of 1.034 and 1.006 for 3.72 and 3.31 eV energies, respectively. Since the value of k is very close to 1, we attribute that these emission bands originate from recombination of FEs or STEs, not from any defect/trap states.[8,33,34]

Figure 4

Power-dependent integral PL intensities of Cs4PbBr6 NCs at (a) 3.72 eV and (b) 3.31 eV, under different excitation power densities of 4.00 eV laser. Data points are plotted with logarithmic coordinates. The red lines are the linear fitting results of data points. (c) Normalized PL decay spectra and (d) average PL lifetime vs energy plot of Cs4PbBr6 NC thin films at 78 K excited with a 4.00 eV energy. Schematic energy diagrams of carrier relaxation processes in the Cs4PbBr6 structure at (e) room temperature and (f) low temperatures below 200 K (FCs = free carriers; FEs = free excitons; STEs = self-trapped excitons; GS = ground state). PL emission is shown with dotted arrow lines.

To investigate the PL decay dynamics of the FEs and STEs, we have performed the low-temperature time-resolved PL (TRPL) measurements (@78 K) for Cs4PbBr6 NC thin films at different characteristic emission energy values while excited with a 4.00 eV energy source, as shown in Figure c. The PL decay profiles are well fitted with a two-exponential decay function, I(t) = I0 [a1 exp(−t/τ1) + a2 exp(−t/τ2)], where I(t) is the PL intensity at time t, I0 is the initial PL intensity at t = 0, τ1 and τ2 are the PL lifetimes of two components, and a1 and a2 are the corresponding amplitudes. The fitting results are listed in Table T1 in the Supporting Information.

The average PL lifetimes according to the equation τave = ∑aτ2/∑aτ are calculated for each decay and plotted in Figure d. It shows that the average PL lifetime increases with a decrease in the emission energy of the exciting photons.

The PL lifetime results revealed that the FEs relax very fast in the nanosecond time scale. The lifetime values became slightly longer for lower-energy regions, signifying the formation of STE states. These STE states were developed through interaction of excitons with the surrounding lattice having distorted structures, which is more commonly seen in many lower-dimensional layered lead bromide perovskites.[28−31,35] The inhomogeneous distribution of photon energies originates from recombination through STE states and resulted in broadening the PL emission spectra. The greater lattice distortions in the crystal structure resulted in formation of deeper STE states. These deeper STEs emit lower-energy photons and take longer time to recombine.

Because of the high exciton binding energy of Cs4PbBr6 NCs, very few free carriers (FCs) will generate upon excitation considering the equilibrium between excitons and FCs. In lower-dimensional perovskites, the photogenerated FCs typically form FEs first and then relax through FE and STE states.[35] The FEs interact with the phonons and as a result emit photons with a very narrow energy range. During the relaxation process, excitons tend to form STEs due to lattice distortions (Ep > Eat, where Ep is the phonon energy and Eat is the trapping energy of charge carriers), as illustrated in the schematic diagram in Figure e and f. The STEs may decay through radiative or nonradiative recombination processes, resulting in broad emission spectra. At room temperature, we observed only a narrow emission that appeared from recombination of FEs (see Figure e) because the lattice has sufficient thermal energy to detrap carriers from STE states back to the FE states (Ep > Ead, where Ead is the detrapping energy of charge carriers).[8,28,31] However, upon cooling below 200 K, the thermal energy was insufficient for carriers in the STE states to detrap back to the FE states (Ep < Ead). As the temperature decreases, the PL intensity of both emission bands enhanced due to reduction of nonradiative recombination pathways (see Figure f). At lower temperatures, the PL intensity from STE states increased faster than the emission from FE states.

Conclusions

In conclusion, we have investigated the exciton relaxation processes in Cs4PbBr6 perovskite NCs by absorption, PL, PLE, Raman, and PL lifetime measurements. We observed an excitonic absorption band peak at 3.96 eV and a UV emission spectrum with a peak position at 3.72 eV at room temperature. Stoke’s shift of 240 meV resulted from the charge carrier’s recombination through FE states. At low temperatures, a new UV emission band was obtained that was composed of two emission spectra with peaks centered at 3.39 and 3.12 eV. From the PLE, TRPL, and power-dependent PL measurements, we concluded that the emission band originated from exciton recombination through STE states. TRPL experiments revealed that the PL lifetimes depend on the emission energy positions of STE states, which can be attributed to the existence of multiple emissive STE states. When the excitation energy was larger than the excitonic absorption energy, the emission from FE states was dominant. When the excitation energy was equal to the excitonic absorption energy, STE emission bands became more intense compared to FE emission bands. Our comprehensive studies revealed the properties and mechanism of light emission processes in Cs4PbBr6 NCs and offer a great platform for UV-light-emitting applications.

Experimental Section

Materials

Lead oxide (PbO; 99.999%, trace metals basis), cesium carbonate (Cs2CO3; 99.995%, trace metals basis), tetrabutylammonium bromide (TBABr; 98%), oleic acid (OAc; 90%), oleylamine (OAm; 70%), 1-octadecene (ODE; 90%), and toluene (anhydrous, 99.7%) were purchased from Sigma-Aldrich. All chemicals were used without further purification.

Preparation of the Cs-oleate Precursor

Cs2CO3 (2.5 mmol, 814 mg) powder was mixed in OAc (2.5 mL) and ODE (40 mL) in a 100 mL three-necked round-bottom glass flask.[20] At first, the solution was dried under vacuum for 1 h at 120 °C. Then, the reaction was heated under a nitrogen atmosphere for another 1 h at 150 °C until all Cs2CO3 reacted with OAc and a transparent solution was obtained. Before injection during synthesis of NCs, the Cs-oleate precursor was preheated to 100 °C.

Preparation of the TBABr Precursor

TBABr (2 mmol) powder was mixed in OAm (7 mL) and ODE (3 mL) in a 100 mL round-bottom flask.[20] The mixture was kept under vacuum conditions for 1.5 h at 150 °C and then subsequently heated at 200 °C for 1 h under a nitrogen atmosphere. A pale-yellow colored solution was obtained. Before injection during synthesis of NCs, the TBABr precursor was warmed to 100 °C.

Synthesis of Cs4PbBr6 NCs

PbO (0.2 mmol, 45 mg), OAm (0.2 mL), OAc (1 mL), and ODE (3 mL) were mixed in a round-bottom glass flask.[20] The mixture was first dried under vacuum conditions for 1.5 h at 100 °C, after which the temperature was dropped to 80 °C under a flow of nitrogen gas and kept for an additional 1 h. Then, the TBABr precursor (2 mL) was swiftly injected and the reaction was continued for 10 min. Later, the Cs-oleate precursor (0.4 mL) was injected into the reaction mixture. The reaction was extended for 5 min to grow the NCs and quenched in a cold ice–water bath. The As-synthesized NCs were precipitated via centrifugation and redispersed in toluene solvent.