Concentration- and temperature-dependent photoluminescence of CsPbBr3 perovskite quantum dots

Concentration- and temperature-dependent photoluminescence of CsPbBr3 perovskite quantum dots

Accepted Manuscript Title: Concentration- and Temperature-Dependent Photoluminescence of CsPbBr3 Perovskite Quantum Dots Authors: Yu Wang, Yue Wang, P...

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Accepted Manuscript Title: Concentration- and Temperature-Dependent Photoluminescence of CsPbBr3 Perovskite Quantum Dots Authors: Yu Wang, Yue Wang, Peng Wang, Xue Bai PII: DOI: Reference:

S0030-4026(17)30388-1 IJLEO 59040

To appear in: Received date: Accepted date:

12-9-2016 25-3-2017

Please cite this article as: Yu Wang, Yue Wang, Peng Wang, Xue Bai, Concentration- and Temperature-Dependent Photoluminescence of CsPbBr3 Perovskite Quantum Dots, Optik - International Journal for Light and Electron Optics This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Concentration- and Temperature-Dependent Photoluminescence of CsPbBr3 Perovskite Quantum Dots Yu Wang1, Yue Wang1, Peng Wang2, Xue Bai1,* 1. State Key Laboratory on Integrated Optoelectronics, and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China 2. State Key Laboratory of Superhard Materials, and College of Physics, Jilin University, Changchun 130012, China. * Corresponding authors. E-mails: [email protected] (X. Bai).

Abstract The concentration and temperature dependence of photoluminescence (PL) performance was investigated, which indicated that the main emission in CsPbBr3 perovskite quantum dots was related to the band-edge mechanism. As the concentration increased from 0.1 to 10.0 mol/L at 293 K, the center of emission band exhibited a regular redshift, as well as the peak intensity increased initially and then declined, which could be attributed to the self-absorption processes. Along with the variation of temperature (273-353 K), peak wavelength, full width of half maximum (FWHM), and peak intensity of PL spectra presented a corresponding linear changes. In addition, the temperature reversibility revealed that CsPbBr3 QDs were reliable candidates for temperature sensor and optoelectronic devices. Keywords: concentration; temperature; self-absorption; CsPbBr3; perovskite.

1. Introduction Colloidal semiconductor quantum dots (QDs) have been widely investigated because of its outstanding properties, such as high quantum yield (QY)1,2, narrow full width of half maximum (FWHM)3,4, and strong quantum confinement5,6, etc. Therefore they have been used in different field, such as solar cells7-9, light-emitting diodes10-14, and sensors15,16. Recently, perovskite QDs have attracted more and more attention due to their excellent QY of 60-90% and FWHM of 18-30 nm, which are promising in the applications of solid state lighting, display and lasing.17,18 Several works have been proposed to investigate the blinking effect, Auger recombination and photoluminescence (PL) dynamics.19,20 It is worth noting that these fundamental investigations and applications have been conducted at room temperature, and thus the obtained data need to be examined prior to their practical application at a temperature other than room temperature. Besides, this luminescence-based temperature dependence is also charming in micro-sized temperature sensing. Previous works have shown that the spectroscopic characteristics for an ensemble of CdSe, PbS, PbSe, InP, Ag2S, and ZnCuInS QDs shift with temperature.21-28 However, few works have been reported on the temperature-dependent characteristcs of spectra in perovskite QDs. In this paper, we figure out the characteristics of PL spectra in CsPbBr3 QDs at different temperatures and densities and explore their temperature sensing properties. The temperature dependences were confirmed to be linear to PL intensity, FWHM

and peak wavelength, respectively. It indicates that CsPbBr3 perovskite QDs show the similar temperature effect of PL properties, which can be a promising temperature marker. 2. Experimental section 2.1 Materials CsCO3 (99.9%), toluene, oleic acid (OA, 90%), 1-octadecene (ODE, 90%), and Oleylamine (OLA, 80%-90%) were purchased from Sigma-Aldrich. PbBr2 (98%) was purchased from Aladdin. All the chemicals above were used directly without further treatment. 2.2 Synthesis of CsPbBr3 QDs All-inorganic CsPbBr3 perovskite QDs were prepared based on the modified previous studies17. Firstly, 0.8 g CsCO3 was loaded into a 100 mL 3-neck flash which is filled with 30 mL ODE and 2.5 mL OA. Then it was dried for 2 h at 393 K and heated under N2 to 433K until CsCO3 got involved into reaction totally. Temperature of all the precursors should be over 373 K due to the low solubility of Cs-oleate in ODE at room temperature. Secondly, 0.108 g PbBr2 with 10 ml ODE was loaded into a 50 mL 3-neck flash and dried under N2 for 1h at 393 K. 1 mL dried OLA and 1 mL dried OA were injected at 393 K under N2. After the precursors were completely solved, the temperature was raised to 453 K with the rapid injection of 1 mL Cs-oleate solution. After 6 s, the reaction was quenched by using ice-water bath. The as-synthesized perovskite QDs were separated

by a completely centrifugation29,30. After the centrifugation, the products were redissolved in toluene for the further research. 2.3 Characterizations A Shimadzu 3600 UV-Vis spectrophotometer was used to measure the UV-Vis absorption spectra. The emission spectra of liquid-type QD-LED were measured by a Zolix Omni 位300 monochromator/spectrograph. Transmission electron microscopy (TEM) TECNAIF20 was utilized to obtain the images of the morphological structure of perovskite QDs. 3. Results and discussion Fig. 1 shows the absorption and PL spectra of CsPbBr3 QDs with the concentration of 0.5 mol/L at 293K. It is obvious that the peak positions of first excitonic absorption and PL band locate on 500 nm and 520 nm, respectively, while its FWHM is 21 nm. The Stokes shift, which is the difference between the peak wavelengths of PL spectra and the first excitonic absorption, is just 20 nm. Because of the tiny shift, the absorbed photon and the emitted photon have almost the same energy. According to the PL spectra, the symmetry of the curve is quite high and the tailing is not obvious in the range of long wavelength, which reveals that the band-gap emission is the main recombination in CsPbBr3 QDs.

The concentration dependence of PL spectra has been shown by changing the concentration of CsPbBr3 QD solution with the same particle size. Fig. 2 (a) shows PL spectra of CsPbBr3 QD solution with the different concentrations at 293K. The peak wavelength increases continuously with the increase of concentration, which is also displayed in Fig. 2 (b). While the concentration increases from 0.1 to 10 mol/L, the peak wavelength rises from 517.5 nm to 527.0 nm, which means the peak wavelength of PL spectra exhibits a redshift. It can be observed that the concentration at the maximum PL intensity is in the range of 1-2 mol/L.The peak intensity does not increase as expected with the increasing concentration, which should be mainly due to the self-absorption. The Stokes shift is so small that the large amount of emission in the short wavelength is absorbed strongly, which induces that the PL peak shifts to the red side. Therefore, the peak wavelength exhibits a continuous redshift as the concentration goes up. With the increase of QD concentration, more and more QDs were pumped, then the QD emission become stronger. However, when the QD concentration is high enough, the self-absorption is strong enough to weaken the emission, thus, the QD emission dropped dramatically. Therefore, a maximum value exists, which is confirmed to be in the range of 1-2 mol/L experimentally. The temperature is another significant factor that can influence the features of PL spectra. Fig. 3 exhibits the temperature-dependent PL spectra of QDs with of three specific concentrations. The temperature-dependent parameters are summarized in Fig. 4 including the peak wavelength, the peak intensity and FWHM. As shown in Fig. 4, FWHM is broadened, the intensity decreases and peak wavelength shifts to red with

the increasing temperature. The good linearity can be achieved. Therefore, the relationship of FWHM and peak wavelength versus temperature can be described as, 位(饾憞) = a 饾憞 + 饾惗1 , 螖位(饾憞) = b 饾憞 + 饾惗2 , where 位 and 螖位 mean peak wavelength and FWHM at certain temperature, respectively; T is the temperature. 位 and 螖位 have the temperature coefficients (a,b) and the undetermined constant (C1, C2) which is obviously dependent on the concentrations of CsPbBr3 QD solution. The corresponding fitting results are listed in Table 1.

According to the Varshni equation31, Eg (饾惗, 饾憞) = Eg (饾惗, 0) 鈭

伪(饾惗)饾憞 2 , 饾憞 + 尾(饾惗)

where Eg(C,0) is the band gap of QDs at 0K; 伪(C) is the temperature coefficient of the QDs; and 尾(C) is the Debye temperature (胃D) of the semiconductor at 0 K. when the temperature goes up, the band energy would decrease. FWHM is determined by three factors including an inhomogeneous broadening factor and two homogeneous broadening factors, which are induced by optical and acoustic phonon鈭抏xciton interactions, respectively. In Fig. 4 (b), it is shown that with the variation of temperature in the range of 273 K to 353 K, FWHM changes linearly. According to the following equation32,

螕(饾憞) = 螕inh + 蟽饾憞 +

螕LO , 饾惛 exp ( LO ) 鈭 1 饾憳B 饾憞

where 螕inh is the inhomogeneous FWHM of QDs and temperature-independent, which is caused by the fluctuations of different factors such ascompound, pattern, volume, etc.; 蟽 is the coefficient of exciton鈭抋coustic phonon coupling; 螕LO is the intensity of exciton鈭扡O鈭抪honon coupling and ELO is the LO-phonon energy. FWHM should rise with the increase of temperature at the same time according to the above theoretical formula, which agrees with the experimental phenomenon.

Considering the complication of temperature-dependent integrated PL intensity, the intensity should vary with temperature at the same concentration. Therefore, integrated PL intensity follows the equation below with temperature33, IPL (饾憞) =


1 + Aexp (鈭

鈭抦 饾惛a 饾惛 ) + B [exp ( LO ) 鈭 1] 饾憳B 饾憞 饾憳B 饾憞

where IPL(T) is the integrated PL intensity at temperature T; I0 is the integrated PL intensity at 0 K; A and B are the ratios of the radiative lifetime in QDs and the capture time from emitting centers to nonradiative recombination centers; m is the amount of LO phonons involved in thermal escape of carriers; ELO is their energy; Ea is the thermal activation energy. When the temperature becomes higher, the intensity goes down. In current case, there is a good agreement between the phenomenon and the equation.

Fig. 5 gives the peak intensities of CsPbBr3 QD solution with the concentration of 0.5 mol/L at different given temperatures. The sample suffers from two cycles of heating processes. Only slight change is observed, which demonstrates that CsPbBr3 QDs could be a potential temperature marker.

4. Conclusions In summary, the concentration and temperature dependence of CsPbBr3 QDs were investigated. According to the absorption and PL spectra, it exhibits a small Stokes shift, implying that the band-gap luminescence is quite important in the whole luminescence. The temperature-dependent PL spectra of CsPbBr3 QDs indicate the good linearity. These linear relationships between temperature (273K 鈥 353K) and peak intensity, peak wavelength, FWHM, respectively, can be explained by the Varshni equation and other related fomula. Besides, the concentration of CsPbBr3 QDs is confirmed to influence PL spectra because of the self-absorption. Peak wavelength shifts to red with the increase of concentration while the peak intensity rises first and then falls. Finally, the good thermal stability was demonstrated, which indicated that CsPbBr3 QDs were promising in the application of temperature sensor.

Conflict of Interests All authors declare that this paper does not have any content with conflict of interests.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (61106039, 51272084, 61306078, 61225018, 61475062), the Jilin Province Key Fund (20140204079GX).

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Fig. 1 UV-Vis absorption (a, black) and PL spectra (a, blue) of CsPbBr3 QDs with the concentration of 0.5 mol/L at 293 K and TEM image of CsPbBr3 perovskite QDs (b).

Fig. 2 Concentration dependence of PL spectra (a) and peak wavelength (b, red) and peak intensity (b, blue) of CsPbBr3 QDs at 293K

Fig. 3 PL spectra of CsPbBr3 QDs with three concentrations at temperature of 273 鈥 353 K: (a) 0.5 mol/L, (b) 5.0 mol/L, and (c) 10.0 mol/L.

Fig. 4 Temperature dependence of FWHM (a), peak intensity (b) and peak wavelength (c) and the corresponding fitting results for QD solution with three concentrations: 0.5 mol/L (red), 5.0 mol/L (green), and 10.0 mol/L (blue).

Fig. 5 Integrated peak intensities of CsPbBr3 QDs in different processes. Black, red, green scatters are the intensities of the heating process, cooling process, and reheating process, respectively. Solid lines are the fitting curves.

Table 1 Fitting intercept of temperature-induced FWHM and peak wavelength based on equations Concentration