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Exciton photoluminescence of CsPbBr3@SiO2 quantum dots and its application as a phosphor material in light-emitting devices

Canran Zhang, Hongxiang Zhang, Ru Wang, Daotong You, Wei Wang, Chunxiang Xu, and Jun Dai

Open Access Open Access

Abstract

In this report, we mainly investigate the optical property differences between CsPbBr3@SiO2 quantum dots (QDs) and CsPbBr3 QDs. The photoluminescence demonstrates that CsPbBr3@SiO2 QDs and CsPbBr3 QDs have similar exciton binding energy. Both CsPbBr3 and CsPbBr3@SiO2 QDs present optical bandgaps and photoluminescence (PL) linewidth broadening as the temperature increases from 10 K to room temperature, which is attributed to the thermal expansion and electron-phonon coupling. The fitting results show that CsPbBr3 and CsPbBr3@SiO2 QDs have the similar bandgap thermal expansion coefficient, but the CsPbBr3@SiO2 QDs have weaker electron-phonon interaction. Temperature-dependent time-resolved photoluminescence (TRPL) demonstrates that the PL lifetime increases with the temperature and CsPbBr3@SiO2 QDs have longer PL lifetime than CsPbBr3 QDs after 110 K. In addition, the CsPbBr3@SiO2 QDs integrated on the blue light-emitting diode chip as green phosphor material show better thermal stability in ambient air.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Recently, perovskites semiconductor materials have been widely explored for practical application in the field of optoelec-tronics, such as solar cells, [1–5] light-emitting diodes (LEDs), [6–8] photo-detectors [9–11] and lasing devices [12–17]. Compared with the intrinsic thermal instability of organic and organic-inorganic hybrid perovskites, all-inorganic perovskites CsPbX3 (X = Cl, Br, and I) have higher chemical stability and exhibit a high photoluminescence quantum yield [18–21]. However, high-quantum yield Cl-based perovskite devices are difficult to obtain owing to the relatively high density of Cl vacancy defects, [22] and I-based perovskite material still cannot overcome its instability for its undesirable phase transition at room temperature and humidity sensitivity [23,24]. Among the CsPbX3 perovskites family, green CsPbBr3 shows much better quantum yield and thermal stability than CsPbCl3 and CsPbI3 [25–27]. Therefore, green light CsPbBr3 optical devices attract tremendous attention. In recent years, CsPbBr3 perovskite microstructures with different structures and morphologies have been prepared by different growth methods, such as CsPbBr3 microwire, [14,28] microsphere, [12] microplate [13] and quantum dots [8,29]. The underlying optical physics on CsPbBr3 perovskites are widely investigated to further understand this excellent semiconductor material. Zhang et al. firstly reported CsPbBr3 perovskite microwire/plates grown by the vapor deposition method, the temperature-dependence of photoluminescence reveals the exciton-phonon interaction in the CsPbBr3 microstructure [12,28]. Different from the CsPbBr3 microstructures by vapor deposition method, CsPbBr3 QDs can be easily prepared by chemical solution method and have been widely used in the field of light-emitting devices, [6,8,30,31] enhanced amplified spontaneous emission, [32,33] photocatalysis, [34] and so on. Since the first report on CsPbBr3 QDs, [29] different methods like surface passivation and element doping have been proposed to realize the CsPbBr3 QDs with high stability and high quantum yield [35–38]. Sun et al. built silica shells on the CsPbBr3 QDs surface to form the CsPbBr3@SiO2 QDs, its air stability was greatly improved [8]. Until now, much work have been done to improve the CsPbBr3@SiO2 QDs and its LED device performance [36,39,40]. However, the photoluminescence property difference of the CsPbBr3 and CsPbBr3@SiO2 QDs have not been compared. On the other hand, the PL decay process also determines the light-emitting device performance, most of the reported CsPbBr3 PL decay properties were only investigated at room temperature by single photon counting method, [6,41,42] the decay profile of the whole PL band of CsPbBr3 is rarely studied. It is also unclear whether the SiO2 can affect the PL decay process of CsPbBr3 QDs. Therefore, it is of significance to compare the effect of SiO2 coating on the optical properties of the CsPbBr3 QDs.

In this paper, we investigated the effects of SiO2 coating on the optical properties of the CsPbBr3 QDs. The temperature-dependent photoluminescence was measured from 10 K to room temperature, the PL band presents obvious blue shift and continuous widening as the temperature increases, the effect of SiO2 coating on the phonon-related optical properties of CsPbBr3 QDs were discussed. The temperature-dependent time-resolved PL was also measured by the streak camera, the effect of SiO2 coating on the PL lifetime was revealed. Finally, CsPbBr3@SiO2 QDs shows better thermal stability in air ambient than CsPbBr3 QDs when they are served as phosphor material on the light-emitting diode chips.

2. Experimental

2.1 Preparation of CsPbBr3 and CsPbBr3@SiO2 perovskite quantum dots

Firstly, we prepare the pure CsPbBr3 quantum dots. Cs2CO3 (1.25 mmol, 99.9%), oleic acid (OA) (1.25 mL, 99%) and 1-octadecene (ODE) (15 mL, 90%) were mixed into 100 mL three-neck flask and the mixture was dried for 1h at 120 °C under the protection of N2. Then, the reaction temperature was increased to 160 °C with magnetic stirring until the reaction mixture became a transparent solution. After about 30 min reaction, the Cs2CO3 powder can be completely dissolved. The cesium oleate precursor is prepared. In a 100 mL three-neck flask, 0.2 mmol PbBr2 was added, followed by the addition of 5 mL of ODE, the mixture was heated at 120 °C for 1h under the protection of N2. Then, the reaction temperature was increased to 160 °C, OA (0.5 mL) and OAm (0.5 mL) were injected into the reaction. When the solution became clear, the Cs-oleate precursor (0.6 mL) was quickly injected. After 5 s, the reaction mixture was cooled down to room temperature by an ice−water bath for the next purification process. Finally, the obtained quantum dots raw product was centrifuged at 9000 rpm for 15 min. After centrifugation, the supernatant was poured out, and the CsPbBr3 quantum dots deposited at the bottom of the centrifuge tube were redispersed in toluene. Then, the CsPbBr3 quantum dots dispersion solution was centrifuged at 12000 rpm for 10 min. After the purification, the CsPbBr3 quantum dots were stored as dispersion in hexane.

The similar method reported by Sun et al. was employed to prepare the CsPbBr3@SiO2 perovskite quantum dots [7]. Different from the production process of pure CsPbBr3 quantum dots, after the PbBr2 mixture was heated at 120 °C for 1 h under the protection of N2, OA (0.5 mL), OAm (0.5 mL) together with APTES (1 mL) were slowly added. Also, when the solution became clear, the Cs-oleate precursor (0.6 mL) was quickly injected. Then, the flask was opened to the air with continuous stirring for 1.5 h at 50% humidity, then the CsPbBr3@SiO2 QDs solution can be obtained.

2.2 Experimental methods of temperature-dependent PL and TRPL

The prepared CsPbBr3 and CsPbBr3@SiO2 quantum dots solutions were spin-coated onto 1 cm×1 cm quartz glass substrate to obtain for temperature-dependent optical test. The temperature-dependent PL was measured by low temperature system (Janis 150c) and the spectrometer (SP 2500i, Acton) equipped with CCD. The samples were placed in a quartz container where the temperature was cooled down to 10 K by liquid Helium system, and the PL was collected as the temperature was changed from 10 K to room temperature, meanwhile, the temperature-dependent TRPL was measured by the streak camera (Optronis GmbH SC-10). In the PL experiment, the samples were excited by 325 nm femtosecond laser (pulse duration of 150 fs, repetition rate of 1 kHz). In order to observe the carrier dynamics behavior of the pure CsPbBr3 QDs in more detail, we used a larger grating when measuring its TRPL spectra.

3 Results and discussion

Transmission electron microscope (TEM) observations on the CsPbBr3 and CsPbBr3@SiO2 QDs are shown in Fig. 1(a-b), respectively. The morphology of the CsPbBr3 and CsPbBr3@SiO2 QDs do not have any obvious differences, and the white arrows in Fig. 1(b) indicate the SiO2 layer. As shown in the TEM image of the CsPbBr3 and CsPbBr3@SiO2 QDs, the pure CsPbBr3 QDs aggregate together with an average size of ∼ 9 nm (Fig. 1(c)), however, the CsPbBr3@SiO2 QDs randomly scattered with better dispersion and larger particle size of ∼ 20 nm (Fig. 1(d)). After coating the SiO2, the size of the QDs has almost doubled. In addition, the High-resolution TEM (HRTEM) image in the inset of Fig. 1(a-b) further demonstrates that the CsPbBr3@SiO2 QDs show a similar crystal structure with the pure CsPbBr3 QDs, indicating that the coating of the SiO2 has no influence on the basic structure of the QDs. The pure CsPbBr3 QDs and CsPbBr3@SiO2 QDs both show a lattice spacing of ∼ 4.1 Å, which corresponds to the <100> face of the cubic CsPbBr3 perovskite phase.

 figure: Fig. 1.

Fig. 1. (a) TEM of the CsPbBr3 QDs. Scale bar is 20 nm. (inset: HRTEM of the CsPbBr3 QDs. Scale bar is 5 nm.). (b) TEM of the CsPbBr3@SiO2 QDs. Scale bar is 20 nm. (inset: HRTEM of the CsPbBr3@SiO2 QDs. Scale bar is 5 nm.) The white arrows indicate SiO2 layer, and the black arrow shows the QDs in SiO2 matrix. (c-d) Size distribution of CsPbBr3 and CsPbBr3@SiO2 QDs. (e) Standard XRD pattern of cubic phase PDF#54-0752.

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Figure 1(e) shows the X-ray diffraction (XRD) pattern of the CsPbBr3 (blue line) and CsPbBr3@SiO2 QDs (green line). The CsPbBr3@SiO2 QDs show the similar additional diffraction peaks to the pure CsPbBr3 QDs (PDF#54-0752). In addition, the FWHM of the XRD peaks of the CsPbBr3@SiO2 QDs are about a half of the pure CsPbBr3 QDs which shows that the particle size of CsPbBr3@SiO2 QDs are twice as large as CsPbBr3 QDs according to the Scherrer equation.

Figure 2(a-b) show the contour mapping of the temperature-dependent PL for CsPbBr3 and CsPbBr3@SiO2 QDs from 10 K to 300 K, and their corresponding normalized temperature-dependent PL spectrum are shown in Fig. 2(f-g).

Two typical features can be easily identified: (1) The PL intensity decreases continuously and (2) PL peaks shifts to the shorter wavelength as the sample temperature increases from 10 K to 300 K. Figure 2(c) shows the relationship between the integrated PL intensity and the temperature (1000/T). The integrated PL intensity can be well fitted by the Arrhenius equation for semiconductor luminescent materials [43,44]:

$$I(T) = \frac{{I{}_0}}{{1 + A{e^{ - {\textstyle{{{E_B}} \over {{K_B}T}}}}}}}$$
Where I(T) and I(0) are the integrated PL intensities at the temperature T K and 0 K, respectively. A is a fitted constant, KB is Boltzmann constant, and EB is exciton binding energy. Here the exciton binding energy EB of CsPbBr3 QDs and CsPbBr3@SiO2 QDs can be fitted as 18.16 meV and 19.08 meV, respectively. This indicates that the SiO2 coating can hardly affect the exciton binding energy.

 figure: Fig. 2.

Fig. 2. Temperature-dependent PL mapping of (a) CsPbBr3 QDs and (b) CsPbBr3@SiO2 QDs for the temperature region [10-300 K]. (c) The relationship of integrated PL intensity and 1000/temperature of CsPbBr3 QDs and CsPbBr3@SiO2 QDs, respectively. The temperature-dependent (d) band gap and (e) PL FWHM of CsPbBr3 QDs and CsPbBr3@SiO2 QDs, respectively. Normalized Temperature-dependent PL of (f) CsPbBr3 QDs and (g) CsPbBr3@SiO2 QDs for the temperature region [20-300 K].

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As shown in Fig. 2(a, b), the PL center wavelength (λcenter) of the CsPbBr3 QDs and CsPbBr3@SiO2 QDs present blue shift from 527 to 513 nm and from 537 to 523 nm, when the temperature increases from 10 K to 300 K. The CsPbBr3 and CsPbBr3@SiO2 QDs present the same broadening of optical band gap Eg (Fig. 2(d)) from 10 K to 300 K, which is common for the inorganic CsPbX3 (X= Br and I) perovskite as the temperature increases [41,42]. The temperature-dependent band gap can be written as [45]:

$${E_g}(T) = {E_0} + {A_{TE}}T + {A_{EP}}[{\textstyle{2 \over {\exp ( \hbar \omega /{K_B}T) - 1}}} + 1]$$
where E0 is the band gap at 0 K, ATE is the thermal effect coefficient for band gap, AEP is the electron-phonon coupling coefficient, ћω is the average optical phonon energy. Four parameters can be fitted as E0 = 2.34 eV, ATE = 0.389 meV/K, AEP = −421 meV, ћω = 75 meV for CsPbBr3 QDs, and E0 = 2.30 eV, ATE = 0.374 meV/K, AEP = −167 meV, ћω = 52 meV for CsPbBr3@SiO2 QDs. The CsPbBr3 QDs always has wide band gap than that of CsPbBr3@SiO2 QDs, because the size of the CsPbBr3 QDs are almost half of CsPbBr3@SiO2 QDs (see Fig. 1), the quantum size effect makes the smaller QDs have larger bandgap. In Fig. 2(d), the bandgap approximately linearly changes for CsPbBr3 QDs when T is lower than 150 K, and for CsPbBr3@SiO2 QDs when T is lower than 120 K. The slopes of the linear band gap are similar for CsPbBr3 and CsPbBr3@SiO2 QDs, which indicates that they have the similar thermal expansion coefficient ATE. When the temperature is above 150 K, the bandgap of CsPbBr3 QDs and CsPbBr3@SiO2 QDs increase with a decreasing slope and approaches to the maximum as the temperature increases to room temperature. The sublinear behavior of the bandgap at high temperature (T > 150 K) originates from the synergistic effect of thermal expansion and electron-phonon interaction [28]. The result indicates that the electron-phonon interaction leading to the reduction of the band gap at high temperature. The lower electron-phonon coupling coefficient of CsPbBr3@SiO2 QDs indicates that the SiO2 coating on the CsPbBr3 QDs can decrease the electron-phonon coupling. The dielectric constant of matrix has strong effect on the strength of electron-phonon interaction [46]. We speculate that SiO2 is an insulator that protects the QDs, the effect of the substrate and solution on QDs could be blocked, which leads to the decrease in electron-phonon coupling of CsPbBr3@SiO2 QDs. In addition, the magnititude of ћω determines the temperature point at which the electron-phonon interaction starts to play the dominant role. The lower ћω of the CsPbBr3@SiO2 QDs makes electron-phonon coupling effect occure at lower temperature of 120 K.

It is well known that the PL FWHM can be widened as the temperature increase. Here, the wider PL FWHM of CsPbBr3 QDs without SiO2 coating is due to the inhomogeneous quantum dots and homogeneous broadening originated from the stronger carrier-phonon interaction. It is considered that two kinds of phonon (acoustic phonon, longitudinal optical phonon) affect the PL band width [28,45]. Figure 2(e) shows the temperature-dependent FWHM on 1/Temperature of CsPbBr3 and CsPbBr3@SiO2 QDs sample. The relationship between the FWHM and the temperature can be fitted by the following equation [28,45,47]:

$$\Gamma (T) = {\Gamma _{inh}} + \sigma T + {\textstyle{{{\Gamma _{Lo}}} \over {\exp ({E_{LO}}/{k_B}T) - 1}}}$$
Where Γinh is the inhomogeneous band width caused by the inhomogenity of the sample, such as the size and shape of the quantum dots, σ is the exciton-acoustic phonon coupling coefficient. ΓLO is the exciton-longitudinal (LO) mode optical phonon coupling coefficient. ELO is the longitudinal mode optical phonon energy. The fitted parameters are Γinh = 31.75 meV, σ = 0.1703 meV/K, ΓLO = 160.609 meV, ELO = 65.05 meV for CsPbBr3 QDs and Γinh = 17.1398 meV, σ = 0.11384 meV/K, ΓLO = 100 meV, ELO = 44.34 meV for CsPbBr3@SiO2 QDs. We can clearly see that the FWHM of the CsPbBr3@SiO2 QDs is about half smaller than CsPbBr3 QDs and the Γinh of the CsPbBr3@SiO2 QDs is also half smaller than the CsPbBr3 QDs as shown by Fig. 2(a-b) and Fig. 2(e), which indicates that the CsPbBr3@SiO2 QDs have better homogeneity in shape and surface roughness than the CsPbBr3 QDs. In addition, the ΓLO and ELO of the CsPbBr3@SiO2 QDs are smaller than the CsPbBr3 QDs, which consistent with the AEP and ћω in the bandgap fitting results. This result futher indicates that the SiO2 coating on the CsPbBr3 QDs can decrease the electron-phonon (especially LO phonon) coupling. The σ of CsPbBr3 and CsPbBr3@SiO2 QDs are very small, suggesting that the exciton-acoustic phonon coupling contribution to the broadening of PL is weak [28,45]. The LO phonon energies are close to the average optical phonon energy ћω fitted by Eq (2), suggesting that the LO phonon dominates the exciton-phonon interaction process [28].

In order to understand the carrier recombination dynamics at different temperature, the temperature dependent countering mapping TRPL of the CsPbBr3 and CsPbBr3@SiO2 QDs were measured from 10 K to 200 K as shown in Fig. 3, and the vertical dimension of time range is 1400 ps for each slice. Figure 3(a-b) clearly shows that the carrier decay process is prolonged as the temperature increases from 10 to 200 K. The exact lifetime of the PL center increase from 160 ps to 340 ps for the CsPbBr3 QDs, and from 50 ps to 410 ps for the CsPbBr3@SiO2 QDs, as shown in Fig. 4(a-b). For some inorganic semiconductor and CsPbBr3 microstructures, the prolonged PL lifetime at higher temperature is also observed, which is attributed to the exciton thermal dissociation [48]. When the temperature is higher than 110 K, the PL lifetime of CsPbBr3@SiO2 QDs is longer than that of CsPbBr3 QDs, this indicates that CsPbBr3@SiO2 QDs are more suitable for light-emitting materials. In addition, the blueshift of the PL center (black arrow in Fig. 3(a)) can also be demonstrated on the TRPL spectra. Figure 4(c-d) show the representative TRPL of the PL center wavelength at 10, 50, 100, 150 and 200 K for CsPbBr3 and CsPbBr3@SiO2 QDs, all of the TRPL present monoexponential decay process which corresponds to mono-exciton recombination process. Different from the reported CsPbBr3 nanostructure with two exciton PL lifetime components (exciton, trapped exciton) [49] and Auger recombination, [50] the single PL lifetime of our sample indicates the high quality of the crystalline with few luminescent trap states [14,49].

 figure: Fig. 3.

Fig. 3. TRPL of (a) CsPbBr3 QDs and (b) CsPbBr3@SiO2 QDs at different temperature.

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 figure: Fig. 4.

Fig. 4. The lifetime of the (a) CsPbBr3 and (b) CsPbBr3@SiO2 QDs at different temperature.The decay process for the center PL wavelength at different temperatures of (c) CsPbBr3 QDs and (d) CsPbBr3@SiO2 QDs.

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To investigate the improvement of stability of the CsPbBr3@SiO2 QDs, we measured PL of the CsPbBr3 QDs and CsPbBr3@SiO2 QDs in air ambient (Fig. 5(a-b)). When these samples were excited by 325 nm laser, the PL intensity of CsPbBr3 QDs decreased to 40% of its original intensity after 16 days, while the PL intensity CsPbBr3@SiO2 QDs decreased much less than CsPbBr3 QDs. Further CsPbBr3 and CsPbBr3@SiO2 QDs were dried, we mixed them with silicone resin solution, respectively. Then the same mount CsPbBr3/silicone resin and CsPbBr3@SiO2 QDs/silicone resin were dipped on LED chips and solidified after 1hour heating at 50 °C. As the LED is working and dissipating heat (surface temperature ≈ 70 °C), we then tested the thermal stability of CsPbBr3 QDs and CsPbBr3@SiO2 QDs. Figure 5(c) and (d) show the PL spectra of the CsPbBr3 and CsPbBr3@SiO2 QDs excited by the blue LED chip (λcenter = 450 nm). As can be seen from the inset of Fig. 5(c) and (d), at the same working current, the CsPbBr3@SiO2 QDs on the LED surface emit brighter light than the CsPbBr3 QDs. After 12 hours of continuous working, the PL intensity of the CsPbBr3@SiO2 QDs on the LED chip is hardly reduced. However, the PL intensity of CsPbBr3 QDs on LED chip is reduced by 40% only after 60 minutes. This indicates that the CsPbBr3@SiO2 QDs show much better thermal stability when it is integrated on the LED chips.

 figure: Fig. 5.

Fig. 5. (a-b) PL spectra of the CsPbBr3 and CsPbBr3@SiO2 QDs measured at different working time. (c-d) PL spectra of the CsPbBr3 and CsPbBr3@SiO2 QDs measured at different working time on blue LED chips.

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4. Conclusion

In summary, the temperature-dependent PL and TRPL of CsPbBr3 and CsPbBr3@SiO2 QDs were investigated. CsPbBr3@SiO2 QDs show the similar exciton binding energy as CsPbBr3 QDs after SiO2 coating. Both CsPbBr3 and CsPbBr3@SiO2 QDs present continuous broadening of optical bandgap from 10 K to 300 K, the exact thermal expansion induced bandgap broadening and electron-phonon interaction induced bandgap narrowing were discussed, the result indicates that the electron-phonon coupling can be decreased in the CsPbBr3@SiO2 QDs. The temperature-dependent TRPL demonstrates that the PL lifetime increases with the temperature, and CsPbBr3@SiO2 QDs have longer PL lifetime than CsPbBr3 QDs at high temperature. In addition, CsPbBr3@SiO2 QDs show better thermal stability in air ambient than CsPbBr3 QDs when they were integrated on blue light-emitting diode chips. The above conclusions indicate that CsPbBr3@SiO2 QDs are more suitable for light-emitting materials.

Funding

National Natural Science Foundation of China (11874185, 11981240363); Qinglan Project of Jiangsu Province of China.

Disclosures

The authors declare no conflicts of interest.

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Figures (5)
Fig. 1.
Fig. 1. (a) TEM of the CsPbBr3 QDs. Scale bar is 20 nm. (inset: HRTEM of the CsPbBr3 QDs. Scale bar is 5 nm.). (b) TEM of the CsPbBr3@SiO2 QDs. Scale bar is 20 nm. (inset: HRTEM of the CsPbBr3@SiO2 QDs. Scale bar is 5 nm.) The white arrows indicate SiO2 layer, and the black arrow shows the QDs in SiO2 matrix. (c-d) Size distribution of CsPbBr3 and CsPbBr3@SiO2 QDs. (e) Standard XRD pattern of cubic phase PDF#54-0752.

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Fig. 2.
Fig. 2. Temperature-dependent PL mapping of (a) CsPbBr3 QDs and (b) CsPbBr3@SiO2 QDs for the temperature region [10-300 K]. (c) The relationship of integrated PL intensity and 1000/temperature of CsPbBr3 QDs and CsPbBr3@SiO2 QDs, respectively. The temperature-dependent (d) band gap and (e) PL FWHM of CsPbBr3 QDs and CsPbBr3@SiO2 QDs, respectively. Normalized Temperature-dependent PL of (f) CsPbBr3 QDs and (g) CsPbBr3@SiO2 QDs for the temperature region [20-300 K].

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Fig. 3.
Fig. 3. TRPL of (a) CsPbBr3 QDs and (b) CsPbBr3@SiO2 QDs at different temperature.

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Fig. 4.
Fig. 4. The lifetime of the (a) CsPbBr3 and (b) CsPbBr3@SiO2 QDs at different temperature.The decay process for the center PL wavelength at different temperatures of (c) CsPbBr3 QDs and (d) CsPbBr3@SiO2 QDs.

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Fig. 5.
Fig. 5. (a-b) PL spectra of the CsPbBr3 and CsPbBr3@SiO2 QDs measured at different working time. (c-d) PL spectra of the CsPbBr3 and CsPbBr3@SiO2 QDs measured at different working time on blue LED chips.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

I ( T ) = I 0 1 + A e − E B K B T
E g ( T ) = E 0 + A T E T + A E P [ 2 exp ⁡ ( ℏ ω / K B T ) − 1 + 1 ]
Γ ( T ) = Γ i n h + σ T + Γ L o exp ⁡ ( E L O / k B T ) − 1
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