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Temperature-Dependent Photoluminescence of Cesium Lead Halide Perovskite Quantum Dots : Splitting of the Photoluminescence Peaks of CsPbBr and CsPb(Br/I) Quantum Dots at Low Temperature 3

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See Mak Lee, Cheol Joo Moon, Hyunseob Lim, Younki Lee, Myong Yong Choi, and Jiwon Bang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06301 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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The Journal of Physical Chemistry

Temperature-Dependent

Photoluminescence

of

Cesium Lead Halide Perovskite Quantum Dots: Splitting of the Photoluminescence Peaks of CsPbBr3 and CsPb(Br/I)3 Quantum Dots at Low Temperature See Mak Lee†,‡,║, Cheol Joo Moon$,║, Hyunseob Lim⊥, Younki Lee‡, Myong Yong Choi *,$, and Jiwon Bang*,† †

Electronic Conversion Materials Division, Korea Institute of Ceramic Engineering and Technology, Jinju 52852, Republic of Korea

‡

School of Materials Science and Engineering, RIGET, Gyeongsang National University, Jinju 52828, Republic of Korea $

Department of Chemistry (BK21+) and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea

⊥

Department of Chemistry, Cheonnam National University (CNU), Gwangju 61186, Republic of Korea ║

Both authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT

We investigated the temperature-dependent photoluminescence (PL) properties of colloidal CsPbX3 (X = Br, I, and mixed Br/I) quantum dot (QD) samples in the 30–290 K temperature range. Temperature-dependent PL experiments reveal thermal quenching of PL, blue shifting of optical band gaps, and linewidth broadening for all CsPbX3 QD samples with increasing temperature. Interestingly, side-peak emissions that are spectrally separated from the excitonic PL peaks were observed for both CsPbBr3 and CsPb(Br/I)3 QD samples at temperatures below ~250 K. The side-peak emission for the CsPbBr3 QD sample is located at a lower energy compared to the band-edge peak, whereas that of the Br-rich CsPb(Br/I)3 alloy QD sample is located at a higher energy than that of the band-edge peak. We found that the CsPbBr3 QDs have two emissive states, a band-edge state and one involving shallow defects, which can be spectrally separated by narrowing the emission linewidths at low temperature. In the case of the Br-rich CsPb(Br/I)3 QD sample, the partial halide-segregation-induced heterogeneity of the alloy phase within the ensemble at low temperature leads to blue-shifted radiative recombination channels.

Introduction Hybrid lead-halide-based perovskites, which exhibit properties including strong absorption features in their visible spectra,1 small exciton binding energies, long chargecarrier diffusion lengths, and low trap-state densities,2 have attracted intense interest for their potential optoelectronic device applications. 1,3,4 In particular, the efficiencies of photovoltaic devices that incorporate hybrid perovskites as sensitizers have dramatically improved over the last few years, with recent device efficiencies exceeding 20%, 1,5 whereas a hybrid perovskite light-emitting diode (LED) has also shown potential with


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~10% external quantum efficiency and very high color purity. 6 However, the durabilities of hybrid perovskites are far from those required for practical use. Under operating conditions that include moisture or heat, hybrid lead halide perovskites dissociate into lead halide and volatile organic components.7 Recently, colloidal synthesis routes to allinorganic cesium lead halide perovskite quantum dots (QDs) have been reported; these QDs exhibit very bright tunable emission and narrow emission linewidths. CsPbX 3 (X = Cl, Br, I, mixed Cl/Br, and mixed Br/I) QDs exhibit tunable photoluminescence (PL) spectra over the entire visible wavelength region, with narrow linewidths of 10-40 nm, through size and composition control.8,9 It is especially notable that these materials exhibit high PL quantum yields (QYs) of up to 90%, without the requirement of a wider band gap surface passivation layer, owing to their high defect tolerance.8,10,11 Moreover, the all-inorganic structures of CsPbX3 QDs, devoid of any volatile organic components, overcome the stability issues associated with hybrid perovskites.12 These outstanding features are promising for the development of a new generation of low cost, solutionprocessable materials for optoelectronic applications.8,11-15 However, although much recent attention has been focused on the development and application of colloidal CsPbX3 QDs, their fundamental photophysical properties are currently still not fully understood. Temperature-dependent PL spectroscopy is an appropriate tool that can provide insight into the exciton behavior of semiconductors. Exciton dissociation processes, assisted by phonon scattering, can be investigated by examining thermal quenching behavior, and exciton–phonon interactions can be examined through the analysis of PL linewidths as functions of temperature.16-19 In addition, temperature-dependent PL spectroscopy also offers in-depth insight into the photophysical properties of materials, even for those that


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contain complicated structures such as core/shell hetero-nanostructures.16 Nevertheless, the temperature-dependent PL properties of all-inorganic perovskite QDs have rarely been studied. Recently, Li et al.18 examined the thermal quenching behavior and carrier– phonon interactions in colloidal CsPbBr3 QDs of different sizes using temperaturedependent steady-state and time-resolved PL. In the present article, we report the temperature-dependent PL properties of colloidal CsPbX3 (X = Br, I, and mixed Br/I) QD samples in the 30–290 K temperature range. We investigated the PL intensity changes, PL spectral shifts, and linewidth broadening of the samples as functions of temperature. With increasing temperature, the PL intensities of the CsPbX3 QD samples decreased, while the peaks were blue shifted and the linewidths broadened. However, we observed distinctly asymmetric PL spectra at temperatures lower than 250 K for the CsPbBr3 and CsPb(Br/I)3 QD samples. The spectrum of the CsPbBr3 QD sample exhibited a red-shifted tail, whereas that for the Br-rich CsPb(Br/I)3 QD sample showed a blue-shifted tail at low temperature. By careful examination of the asymmetric PL spectra, the origins of the spectral asymmetry at low temperature were attributed to a shallow-defect emission that is spectrally separated from the band-edge PL for the CsPbBr3 QD sample and to partial phase separation for the CsPb(Br/I)3 QD sample.

Experimental Details Materials and Synthesis Methods Cesium carbonate (Cs2CO3, trace metals basis, 99.99%), lead(II) bromide (PbBr2, trace metals basis, 99.999%), lead(II) iodide (PbI2, trace metals basis, 99.999%), 1-octadecene (1-ODE, technical grade, 90%), oleic acid (OA, technical grade, 90%), oleylamine (OLA, technical grade,


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70%), 1-butanol (1-BuOH, anhydrous, 99.8%), and methyl acetate (MeOAc, anhydrous, 99.5%) were purchased from Sigma-Aldrich. All purchased chemicals were used without purification. (1) Preparation of cesium oleate Cs2CO3 (250 mg) was dissolved in 1-ODE (10 mL) and OA (0.8 mL) with stirring at 120 °C under vacuum for 1 h, followed by heating at 150 °C for 20 min under a flow of argon gas. The reaction mixture was maintained at 70–80 °C before injection into reaction flask. (2) Synthesis of CsPbX3 (X = Br, I, or mixed Br/I) QDs OA (0.5 mL), OLA (0.5 mL), 1-ODE (5 mL), and PbX2 (69 mg of PbBr2 for CsPbBr3; 35 mg of PbBr2 and 44 mg of PbI2 (1:1 molar ratio of Br to I) for CsPb(Br/I)3; or 80 mg of PbI2 for CsPbI3) were loaded into a 25 mL three-neck flask, dried under vacuum at 120 °C for 1 h, and then heated to 150 °C (170 °C for CsPbI3) under a flow of argon gas. The cesium oleate solution (0.4 mL for CsPbBr3 and CsPb(Br/I)3, and 0.8 mL for CsPbI3) was quickly injected at this temperature, and the mixture was stirred until CsPbX3 QDs of the desired size were obtained. To remove excess organic materials, the crude products were precipitated by adding 1-BuOH (for CsPbBr3) or MeOAc (for CsPb(Br/I)3 and CsPbI3), collected by centrifugation, and redispersed in a small amount of hexanes. The QD solutions were recentrifuged and the supernatant was collected. The CsPbX3 QDs were stored under argon gas prior to use. Low-Temperature PL Experiments CsPbX3 QD films were prepared by drop-casting the QD solution onto glass substrates. PL measurements of the QD films were performed in the 30–290 K temperature range, in 10 K increments, using a closed-cycle liquid He cryostat (APD, DE-202A). The QD films were


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attached to the sample holder. The excitation light source was the third harmonic generation of a Nd:YAG laser (Continuum, Powerlite 8010) at 10 Hz, with a 7-ns pulse width. The emitted light was collected using an ARC SpectraPro® -300i monochromator and detected by a Hamamatsu R2368 photomultiplier tube. The entrance and exit slits of the monochromator were set to 0.5 mm and the PMT voltage was 700 V. The signal was digitized and stored using a digital storage oscilloscope (LeCroy, LT322, 500 MHz, 200 MS/s), and subsequently processed with a homemade auto-data collecting program using LabVIEW® . Characterization Room-temperature absorption and PL spectra were obtained on a SCINCO UV-visible spectrophotometer (Model No. S-3100) and an Ocean Optics spectrophotometer (Model No. QE65000), respectively. The absolute PL QYs of the CsPbX3 QD samples were determined by the de Mello method20 using an integrating sphere coupled with an Ocean Optics spectrophotometer.

Transmission electron microscopy (TEM) images were collected with a JEOL JEM-2000EX microscope (operated at 200 kV) and a JEOL JEM-4010 microscope (operated at 400 kV). An Oxford Instruments 7244 energy dispersive X-ray spectrometer was used to collect energy dispersive X-ray spectra. X-ray diffraction (XRD) measurements were performed using a Bruker-AXS D8 Advance X-ray diffractometer equipped with a Cu-K X-ray source; samples for analysis were prepared on glass substrates. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a PHI 5000 VersaProbe spectrometer equipped with an Al Kα X-ray source (1486.6 eV). All binding energies in the XPS data were calibrated with reference to the C–C bond in the C 1s spectrum (284.6 eV).

Results and Discussion


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The colloidal CsPbBr3 (CPB), CsPb(Br/I)3 (CP(Br/I)), and CsPbI3 (CPI) QD samples were prepared using the hot injection approach.8 Energy dispersive X-ray spectroscopy (EDS) revealed that the CP(Br/I) alloy perovskite in this study is Br-rich; the stoichiometric ratio of Br to I for our CP(Br/I) sample is 0.77:0.23. The absorption and PL spectra of the as-prepared CsPbX3 (CPX) QDs in hexanes at room temperature are shown in Figures 1a and 1b. The absorption profiles and PL peaks are tuned via compositional modulation. PL peaks are observed at 2.47 eV for CPB QDs, 2.29 eV for CP(Br/I) QDs, and 1.82 eV for CPI QDs. The all-inorganic perovskite QD samples in dilute solution exhibit bright PL. The absolute PL QYs of the purified CPB, CP(Br/I), and CPI QD samples in hexanes at room temperature are 57%, 48%, and 39%, respectively. The crystal structures of the CPX QD samples were elucidated by powder XRD, as represented in Figure 1d. The CPX QD samples are cubic in shape, with a monodisperse size distribution, as determined by TEM (Figure 1e). The average edge lengths of the CPB, CP(Br/I), and CPI QD samples are 11.0 (  1.8), 12.6 (  1.7), and 16.9 (  2.6) nm, respectively (Figure S1), which are slightly larger than their exciton Bohr diameters; the exciton Bohr diameters of CPB and CPI are about 7 and 12 nm, respectively.8 Therefore, all of these CPX QD samples are considered to belong to the weak exciton confinement regime, and the dielectric constants of the CPX QD samples cannot be significantly different from those of the bulk materials.21 The absorption spectra of the CPX QD samples with weak confinement lack distinct band-edge transition peaks, mainly owing to the reduced oscillator strengths of their first exciton transitions relative to those of QDs in the strong quantum confinement regime.8 To quantify the band-edge transitions in the absorption profiles, we calculated the first minimum from the second derivatives of the absorption cross-sections.22,23 Figure 1c displays the second derivative of each absorption spectrum and the corresponding PL spectrum of each investigated sample. The Stokes


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shifts for the CPB, CP(Br/I), and CPI QD samples were determined to be 29, 72, and 41 meV, respectively. The large Stokes-shift feature of the CP(Br/I) QD sample compared with those of the pure CPX samples is possibly due to a nonlinear relationship within the alloy composition. 24 The Stoke shifts of the samples are comparable to previously reported data,8,23 and show that the room-temperature PL emissions of the CPX samples mainly originate from the band edge, or near the band-edge excitonic transition, and not from deep-trap related transitions. To conduct temperature-dependent PL measurements, the CPX QD films were excited with a 3.50 eV pulsed laser (7 ns pulse width) from 30 to 290 K, with temperature increments of 10 K. The pulse fluence was determined to be 8 µJ/cm2, which corresponds to an average number of photons absorbed of about 0.1 per pulse, as estimated from the absorption cross-sections of the CPX QDs.23 The average exciton number of these QDs should be smaller than the average number of absorbed photons per pulse because the exciton lifetimes of the CPX QDs8,18,23 are similar to the pulse duration under our experimental conditions. Consequently, most of the CPX QD excitations in this study refer to single exciton states. Figure 2 displays the PL spectra of CPX QD film samples as functions of temperature. In the case of the CP(Br/I) films, a background PL signal, originating mainly from the glass substrate, is also observed at the blue side of the PL peak. The CPX QD film samples exhibit broad PL spectral features compared with those observed in their dilute solution phases. Size defocusing might be expected to occur during and/or after film fabrication.25-27 As the temperature is increased from 30 to 290 K, the band-edge PL intensity decreases and linewidth broadening is observed for each CPX QD sample. Thermally activated exciton dissociation suppresses radiative recombination and enhances carrier–phonon temperatures,16-18,28 as discussed in detail below. On the other hand, the PL peak of each CPX QD sample is continuously blue-shifted with increasing temperature. This


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trend is opposite to that observed for conventional semiconductors, which show emission red shifts with increasing temperature owing to lattice dilation.16,17,29,30 Note that the valence band maxima (VBM) and conduction band minima (CBM) in common semiconductors originate from bonding and antibonding orbital pairs; hence, the energy gap between the VBM and CBM decreases with lattice dilation.31,32 On the contrary, in the case of the band-edge states of lead halide perovskites, the VBM is formed by strong a s-p antibonding interaction between Pb and Br, whereas Pb 6p bonding orbital states mainly contribute to the CBM. Therefore, lattice dilation decreases the VBM potential energy while slightly increasing the CBM potential energy;31,32 consequently, the band gap of a lead halide perovskite is blue-shifted with increasing temperature, as observed in this study and in the literature.18,19,33 It is noteworthy that the PL spectra of both the CPB and CP(Br/I) QD samples show distinct asymmetry at temperatures lower than 250 K (Figures 2a and b). This asymmetry arises from a side-peak emission that is spectrally separated from the band-edge peak and is revealed at low temperature. The side peak generated at low temperature in the CPB QD sample is located at a lower energy than the band-edge peak, whereas that of the Br-rich CP(Br/I) QD sample has a higher energy than the band-edge peak. The anomalous emission-peak splitting behavior exhibited by organometallic halide perovskite samples at low temperature has been reported by several groups, and this phenomenon is related to crystal structure transitions in these perovskite samples.19,33,34 Organometallic halide perovskites generally have tetragonal structures at room temperature and undergo transitions to orthorhombic structures below the structure transition temperature (~160 K).19,33-35 The crystal structure transition of lead halide perovskites is associated with large energetic shifts; hence, two PL emissions exist simultaneously owing to the presence of two crystal structures near the structure transition temperature.19,33,34 The all-


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inorganic lead halides crystallize in orthorhombic, tetragonal, and cubic polymorphs. The cubic structures exist at high temperatures and thermodynamically convert to the orthorhombic structures, through the tetragonal structures, with decreasing temperature.8,12,35 For example, CPB undergoes a phase transition from a cubic to a tetragonal structure at ~400 K, and a further phase transition to an orthorhombic phase occurs at ~360 K.36,37 In the case of the cubic CPI crystal, which is known as quite metastable state,8,12 the cubic to orthorhombic phase transition occurs at ~590 K.36,38 Cubic CPI (black phase) has quite different optical properties than orthorhombic CPI (yellow phase). Cubic CPI has a narrow band gap (~1.7 eV), whereas orthorhombic CPI has a wide band gap (~3 eV) that is PL inactive.8,12,39 We examined the CPX QDs by XRD at varying temperatures, from ambient to 123 K (Figure S2). The XRD patterns are similar to those of previous reported cubic CPX QD samples (Figure 1d),8,40,41 and cooling did not lead to remarkable changes in the crystal structures of our CPB and CP(Br/I) QD samples. However, it is difficult to distinguish between cubic and orthorhombic structures using broadened laboratory XRD data.9 For the CPI QD sample, which is quite metastable in the cubic phase, the XRD patterns also appeared to correspond to the orthorhombic phase. However, the absence of a sharp absorption peak at ~3 eV, which is characteristic of orthorhombic phase formation,12,39 even after incubating in liquid nitrogen, indicated that the CPI QD sample is likely to have a cubic structure rather than an orthorhombic structure (Figure S3). Indeed, the bandedge PL intensity of the cubic CPI QD sample increases as the temperature decreases to 30 K (Figure 2c), revealing that the cubic structure is maintained at cryogenic temperature. Therefore, we thought that our CPX QD samples could be more stable in their cubic phases, even at cryogenic temperature; this phenomenon is mainly due to quantum size effects that include large surface energy contributions.8,12


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As shown in Figure 2a, the CPB QD sample exhibits asymmetric PL spectra with a red-shifted side peak that is clearly observed below ~250 K. When the temperature is returned to 290 K, the side peak become featureless and the original PL features of the CPB QD sample are restored (Figure S4). The asymmetric PL spectrum is characterized by two distinct peaks that can be fitted by two Gaussian peaks. The fitted spectrum of the CPB QD sample at 250 K is shown by dotted lines in Figure 3a. We followed the changes in peak energies (Figure 3b) and linewidths (Figure 3c) of the band-edge peak (P1) and side peak (P2) in the CPB QD sample as the temperature was varied from 30 to 250 K. Both the band-edge peak and side peak become gradually blue shifted with increasing temperature; however, the band-edge peak seems to be slightly more sensitive to thermal band gap changes than the side peak. The difference in energy between the band-edge peak and the red-shifted side peak is 130 meV at 30 K, rising to 138 meV at 250 K. To investigate the effect of particle size on the temperature-dependent PL peak splitting, we prepared a 12.8 nm CPB (L-CPB) QD sample (Figure S5), which is larger than the CPB QD sample. The L-CPB QD sample, which has an optical band gap of 2.39 eV at room temperature, also exhibited similar temperature-dependent PL properties, with a new side peak clearly appearing at temperatures below ~250 K. The energy separation between the band-edge peak and side peak of this sample is 75 meV at 20 K, rising to 85 meV at 250 K (Figure S5). The energy difference between these two CPB QD emissions becomes narrower with increasing particle size. Phonon side bands may also be responsible for the red-shifted side peaks observed for our CPB samples. However, the energy spacing between the side peak and the band-edge peak is larger than the longitudinal optical (LO) phonon energy of ~20 meV for bulk42 and nanosized18 CPB samples. We also ruled out the possibility that these red-shifted side peaks result from


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amplified spontaneous emissions (ASEs) from the CPB QD samples. Yakunin et al. 15 reported that the room-temperature ASE threshold of a CPB QD sample is 5.3 µJ/cm2 with a 100-fs pulse; however, this value increases to 450 µJ/cm2 when excited by a 10-ns pulse, which is a significantly higher fluence than that used in our experiments (excitation density: 8 µJ/cm2 with a 7-ns laser pulse). The red-shifted side peak is also observed by continuous-wave excitation at a sufficiently low excitation power (2 mW/cm2) (Figure S4). Instead, this red-shifted side peak of the CPB QD sample could be a defect-related emission band, which can be verified by conducting excitation-power-dependent PL measurements. Figure 3e shows a logarithmic plot of the intensity (IP) of the band-edge peak and red-shifted side peak as a function of the excitation intensity (I0). The data can be fitted by a power law of the form IP ∝ I0α, with the α exponents of the band-edge peak and side peak evaluated as 1.07  0.03 and 1.54  0.16, respectively. The nonlinear trend observed for the red-shifted side peak suggests that the recombination is influenced by free carriers in defect states.43 We also observed that the redshifted side emission band shows slower decay kinetics than the band-edge excitonic PL band in the CPB QD sample (Figure S6a). However, for the CPI QD sample, no distinguishable difference was observed between PL decay profiles at the blue-side and red-side of the PL peak under our experimental condition. (Figure S6b), which rules out the possibility that the slow decay rate observed at the lower energy side of the CPB QDs PL results from large QDs in the particle size distribution obtained under our experimental conditions. The slow decay kinetics of the red-shifted side peak in the CPB QD sample also reveals defect-related transition characteristics owing to the poor overlap integral of the carrier wave functions.43,44 Special classes of VBM s-p antibonding character and CBM bonding character in CPB crystals result in point defects that are generally shallow in character.10,11,32,45,46 Kang et al.32 calculated defect


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densities and defect transition levels of intrinsic point defects, vacancies, interstitials, and antisites in CPB crystals. They argued that most of the intrinsic point defects in CPB crystals induce shallow transition levels. In lead halide perovskites, vacancy defects have low formation energies, whereas interstitial or antisite defects have much higher formation energies; thus, vacancy defects could be the dominant defect.32,45,46 From the XPS spectra of the CPB QD sample, we estimate a stoichiometric ratio for Cs, Pb, and Br of 0.7:1:2.7 on the surface and 0.7:1:2.3 within the interior (Figure S7). These ratios indicate that a pair of Schottky vacancies (Cs and Br vacancies) could exist in the interior of the CPB QDs, as well as Cs vacancies on the surface. Both Cs and Br vacancy defects can create shallow transition levels,32 and thus, such vacancies in our CPB QD samples may be the origin of the defect states. The integrated emission intensity ratio of the band edge to defect state in the CPB QD sample obtained from the deconvoluted Gaussian fit is plotted as a function of temperature in Figure 3d. The band-edge peak is clearly dominant over the temperature range examined in this study. The results show that the emission intensity (i.e., the carrier population of each state) ratio is almost insensitive to temperature up to 250 K. This observation suggests that the activation energy for carrier transfer between the band-edge and defect states is not overcome, even at 250 K, and both emissive states coexist at room temperature. Meanwhile, the linewidths of both emissive states in the CPB QD sample are broadened through scattering between charge carriers and phonons (Figure 3c and S5).16-19 The linewidth of the band-edge peak ranges from 190 to 233 meV with increasing temperature, whereas that of the shallow defect peak ranges from 83 to 112 meV (Figure 3c). At low temperature, the CPB shallow-defect-related emission peak is spectrally distinguishable from the band-edge PL peak because of linewidth narrowing of the two emissive states. However, the shallow-defect-state emission, which is close to the band edge in energy, is


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overwhelmed by the band-edge-state emission near room temperature. Consequently, the side peak could not be spectrally distinguished from the band-edge peak at those temperatures because the linewidths of the two states are too broad to allow separation of the defect-related side peak from the dominant band-edge PL peak. A CPB QD sample with a narrow size distribution shows asymmetric PL spectra, even at room temperature (Figure S4), presumably because of reduced spectral overlap between the band-edge state and the defect state owing to linewidth narrowing of the PL emission from both states. The CP(Br/I) QD PL spectra also exhibit asymmetric behavior, with a blue-shifted tail at low temperature (Figure 2b). The PL spectrum of the CP(Br/I) QD sample was fitted by summing two Gaussian peaks, enabling us to isolate the blue-tailed emission peak (P2) from the main peak (P1) in the 30–240 K temperature range (Figure 4). The side peak of the CP(Br/I) QD sample has different characteristics to that of the CPB QD samples, as mentioned above. Firstly, the emission energy of the side peak is more sensitive to temperature than the main peak, and the energy difference between these peaks widens with increasing temperature (Figure 4b). Secondly, the integrated emission intensity ratio of the side peak to the main peak increases with decreasing temperature (Figure 4d). Thirdly, the broad linewidth of the side peak is insensitive to temperature up to 180 K, but it becomes narrower as the temperature is further elevated (Figure 4c). In contrast to the defect-related side peak exhibited by the CPB QD sample, which is still present at room temperature but obscured by the band-edge PL, the radiative transition state above the band edge of the CP(Br/I) sample does not exist at room temperature, but is generated at lower temperatures; the population of carriers in this state increases with decreasing temperature. In addition, the insensitivity of the side-peak linewidth to temperature over the 30– 180 K range, followed by gradual line narrowing above 180 K, suggests that the energy


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distribution of this state originates from disorder and imperfections rather than carrier–phonon scattering.16,19 We speculate that the asymmetric PL emission of the CP(Br/I) QD sample partially originates from temperature-dependent changes in the alloy. Recently, reversible photoinduced phase separation of mixed Br/I perovskite films has been demonstrated, which leads to the formation of I-rich low-band-gap domains.47-49 The formation energies of mixed halide perovskites have positive values because internal strain exists between differently sized halides. Consequently, the alloys are not thermodynamically stable if the formation entropy cannot compensate for the unfavorable formation energy of mixing.50,51 For example, Yin et al.50 reported that the miscibility gap temperature, which is the temperature below which the alloy is insoluble and phase separation of the alloying elements occurs, is 223 K for CP(Br/I). Our CP(Br/I) QD sample is fully mixed at room temperature, which is well above the miscibility gap temperature determined by Yin et al.50 However, below the miscibility gap temperature, the mixed Br/I phase undergoes separation owing to spinodal decomposition.51 In the CP(Br/I) QD sample, the large surface strain of the nanocrystals can cause the mixed halide phase to become more unstable. The halide ions that are energetically unstable in the mixed alloy phase migrate through the halide vacancies52 or surface toward local energy minimum phases in these nanosized particles. Moreover, the nanosized particles reduce the ion diffusion activation energy, which may allow ion migration to occur even at low temperature.53 Halide segregation is thermodynamically promoted at lower temperatures, creating further disorder and phase separation in the CP(Br/I) QD sample. The CP(Br/I) QD sample in this study is Br-rich as mentioned above. We speculate that the blue-shifted side peak of the Br-rich CP(Br/I) QD at low temperature originates from the partial spatial separation of majority CPB and minority CPI domains. In our Br-rich CP(Br/I)


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QDs, the CPI cluster size could be smaller than the CPB cluster size within the ensemble, which would increase the band-gap energy by strongly confined excitons in the small CPI or CPB domains. However, an I-rich CP(Br1-x/Ix) QD sample (x = 0.67) with a size of 11.5 nm shows an asymmetric PL band with a red-shifted shoulder when the temperature is decreased. Moreover, the red tail disappears and the original symmetric PL features are restored when the temperature is returned to room temperature (Figure S8). We thought that the high iodide concentration could form larger CPI clusters to create a lower energy state than in the fully mixed CP(Br/I) QD alloy after phase separation at low temperature. However, we did not observe splitting or noticeable broadening of the XRD patterns owing to phase separation below the miscibility gap temperature (Figure S9). Presumably, broadening of the XRD patterns of the QDs and CPI/CPB clusters, as well as peak shifts caused by lattice contraction in the XRD patterns, could mask the presence of CPI and CPB clusters, with only a small fraction of ensemble QDs. Further investigation, such as EDS mapping analysis, is required to characterize the phase separation in these samples. The PL intensities, peak energies, and linewidths of the excitonic band-edge PL of the CPX samples, as functions of temperature, are displayed in Figure 5. In the case of the CPB and CP(Br/I) QD samples, we plotted the band-edge PL data obtained from the deconvoluted Gaussian fit to exclude the side peaks in the original PL spectra. The integrated PL intensity for each CPX sample decreases with increasing temperature. Thermal energy activates exciton dissociation, which competes with radiative recombination.16-18 In particular, compared with the other samples, the CPB sample exhibits noticeable thermal quenching at lower temperatures, presumably because the hot carriers are not only thermalized to the band-edge state, but are also trapped in shallow defect states, even at low temperature, as discussed above. The temperaturedependent PL intensity (I(T)) can be fitted to the Arrhenius equation, as shown in Equation 1:


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I(T) 

I0 1  Ae

 E a /kBT

(1)

where I0 is intensity at 0 K, Ea is the activation energy for exciton dissociation, A is the preexponential coefficient, and kB is the Boltzmann constant. The open circles in Figure 5a represent raw data points for the PL intensities at given temperatures, and the solid lines represent the best least-squares Arrhenius fits. From these plots, activation energies of 15.6, 28.2, and 45.1 meV were determined for the CPB, CP(Br/I), and CPI QDs, respectively. Figure 5b shows the PL spectral shifts of the CPX QDs, in which the PL energy at 30 K was subtracted from the measured PL energies to reveal the changes in the optical band gaps with temperature. As discussed above, the PL peak is blue-shifted with increasing temperature. The CP(Br/I) QD sample is definitely less sensitive to thermal band gap changes than the CPB and CPI samples, which may be related to changes in the alloy phase with temperature. Br and I are more miscible in CP(Br/I) when the temperature is high. The mixed alloy environments in the CP(Br/I) QD sample lead to a red-shifted band gap through bowing in the band gap energy.54 In contrast, dilatation of the lattice results in a blue-shifted band gap as the temperature increases. Therefore, the CP(Br/I) QD sample may have reduced sensitivity to band gap changes with temperature. The PL linewidth changes as functions of temperature for the CPX samples are displayed in Figure 5c. As the temperature is increased, the linewidth of the CPI QD sample gradually increases. However, the CPB QD and CP(Br/I) QD samples show nonmonotonic temperature dependences. Below a critical temperature, the linewidths become narrower as the temperature increases, after which the linewidths increase rapidly as the temperature exceeds the critical temperature. The linewidth reduction observed for our Br-containing perovskite samples may


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result from the thermal accumulation of excitons in localized states, where they decay to the ground state with similar energies.18 The PL linewidths of nanocrystals are convolutions of inhomogeneous

and

homogeneous

broadenings.16-19

The

temperature-independent

inhomogeneous broadening is related to the polydispersity of the nanocrystal ensemble, including size, shape, and composition,16,17 whereas the temperature-dependent homogeneous broadening results from carrier–phonon scattering.16-19 The temperature dependence of the excitonic-peak linewidths (Г(T)) were fitted to the following simple equation (Equation 2):18,19

(T)  Γinh  Γ LO (e

ELO/kBT

 1) 1

(2)

where the first term (Γinh) is the temperature-independent inhomogeneous broadening, and the second term represents the carrier–LO phonon interaction; ΓLO is the carrier–LO phonon coupling coefficient, ELO is the longitudinal phonon energy, and kB is the Boltzmann constant.18,19 In the case of the nonmonotonic temperature dependences, we have only fitted data for values above the critical temperature, namely 120 K for CPB and 150 K for CP(Br/I). From the best least-squares fits, the values of ELO (and ГLO) were found to be 36.2 meV (and 99.3 meV) for CP(Br/I) and 19.6 meV (and 31.7 meV) for CPI QD samples. Unfortunately, we were unable to extract meaningful parameters for the temperature dependence of the linewidth data for the CPB QD sample, which exhibited large deviations from the fitted curve (R2 = 0.430). Previously, LO phonon energies were reported to be 20.4 meV for bulk CPB42 and 16.1 meV for bulk CPI,54 which are close to our value for the CPI QD sample. However, our CP(Br/I) alloy QDs have an LO phonon energy that is more than twice that of bulk CPB and CPI, 42,55 which was unexpected. We believe that lattice strain induced by the different sizes of Br and I within


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the CP(Br/I) alloy50,51 gives rise to shifts in the phonon vibration energies relative to those of pure lead halide perovskite samples.

Conclusion We have studied the temperature-dependent PL properties, including thermal PL quenching, band-gap blue shifting, and linewidth broadening, of colloidal CPB, CP(Br/I), and CPI QD samples over the 30 to 290 K temperature range. The CPB QD sample has two emissive states: a band-edge state and a shallow defect state. The shallow-defect emission peak was successfully separated from the band-edge PL peak and characterized by reducing the temperature of the CPB QD samples. The alloyed CP(Br/I) QD sample exhibits unique temperature-dependent photophysical properties, including smaller band-gap changes and a higher LO phonon energy than those of the pure CPX materials. In addition, the Br-rich CP(Br/I) alloy QD sample generates a new blue-shifted emissive state that is activated at lower temperatures. Understanding the photophysical properties of CPX QDs is useful for the potential applications of these QDs in optoelectronics, including displays and photonic devices.


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FIGURES

Figure 1. Room-temperature (a) absorption spectra, (b) PL spectra, and (c) second derivative absorption spectra (open circles), plotted with the corresponding PL spectra (filled circles), of CsPbBr3 (blue), CsPb(Br/I)3 (green), and CsPbI3 (red) QD samples in hexanes. (d) Powder XRD patterns, in which the bars on the bottom and top represent the bulk structures of CsPbBr3 and CsPb(Br/I)3, respectively, and (e) TEM images of the CsPbBr3, CsPb(Br/I)3, and CsPbI3 QD samples. Scale bars correspond to 50 nm.


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Figure 2. PL spectra of (a) CsPbBr3, (b) CsPb(Br/I)3, and (c) CsPbI3 QD films as functions of temperature.


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Figure 3. (a–d) Temperature-dependent PL properties of the 11.0 nm CsPbBr3 QD sample. (a) PL spectrum at 250 K (green) fitted by the sum of two Gaussian peaks (wine), with separate P1 (black) and P2 (red) components. (b) Emission peak energies and (c) linewidths of P1 (black) and P2 (red) as functions of temperature. (d) Integrated emission of P1 relative to P2 plotted against temperature. (e) Excitation intensity (I0) dependence of the emission intensity of P1 (black) and P2 (red) in the 11.2 nm CsPbBr3 QD sample at 20 K.


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Figure 4. (a) PL spectrum of the CsPb(Br/I)3 QD sample at 200 K (green) fitted to the sum of two Gaussian peaks (wine), with separate P1 (black) and P2 (blue) components. (b) Emission peak energies and (c) linewidths of P1 (black) and P2 (blue) in the CsPb(Br/I)3 QD sample as functions of temperature. (d) Integrated emission of P2 relative to that of P1 for the CsPb(Br/I)3 QD sample as a function of temperature.

Figure 5. (a) Band-edge PL intensities, (b) band-edge PL peak center differences obtained by subtracting the PL peak energy at 30 K, and (c) band-edge PL linewidth differences obtained by subtracting the PL linewidth at 30 K as functions of temperature for the CsPbBr3 (blue), CsPb(Br/I)3 (green), and CsPbI3 (red) QD samples.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Size distribution histograms, XRD spectra, absorption spectra of CsPbI3 QDs, temperaturedependent PL spectra of CsPbBr3 and CsPb(Br/I)3, time-resolve PL spectra, and XPS spectra (Figures S1–S9).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected]. Author Contributions S. M. Lee and C. J. Moon contributed equally to this work. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Basic Science Research Program though the National Research Foundation (NRF) of Korea funded by the Korea government (MSIT) (1007099 and 2017M2B2A9A02049940).


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