Ultrafast Spectral Dynamics of CsPb(BrxCl1âx)3 Mixed-Halide

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Ultrafast Spectral Dynamics of CsPb(BrxCl1-x)3 Mixed-Halide Nanocrystals Naiya Soetan, Alexander Puretzky, Kemar R. Reid, Abdelaziz Boulesbaa, Holly Zarick, Andrew Hunt, Olivia Rose, Sandra J. Rosenthal, David Geohegan, and Rizia Bardhan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00413 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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Ultrafast Spectral Dynamics of CsPb(BrxCl1-x)3 Mixed-Halide Nanocrystals Naiya Soetan,a Alexander Puretzky,b Kemar Reid,c Abdelaziz Boulesbaa,b Holly F. Zarick,a Andrew Hunt,a Olivia Rose,a Sandra Rosenthal,c, d David B. Geohegan,b and Rizia Bardhana* a

Department of Chemical and Biomolecular Engineering, cDepartment of Interdisciplinary Materials Science, & dDepartment of Chemistry, Vanderbilt University, Nashville, TN 37235, USA. bCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA E-mail: [email protected]

Abstract In this work we investigated the spectral dynamics of cesium lead mixed-halide, CsPb(BrxCl1-x)3 perovskite nanocrystals probed with complimentary spectral techniques - time-resolved photoluminescence and transient absorption spectroscopy. Mixed-halide perovskite nanocrystals were synthesized via a hot-injection method followed by anion exchange reactions. Our results show that increased Cl content in perovskite nanocrystals a) diminished the photoluminescence quantum yield and gave rise to rapid radiative recombination of carriers; b) resulted in rapid thermalization of hot carriers and low carrier temperatures which suggests weaker hot-phonon bottleneck and Burstein-Moss effects; c) decreased the bandgap renormalization energy which suggests high exciton binding energy and poor charge extraction in Cl substituted perovskite nanocrystals; and d) increased the number of carriers undergoing Auger losses, where Auger processes dominate over trap-assisted recombination. These findings provide a generalized framework to guide researchers as to when mixed-halide perovskite nanocrystals would be useful for optoelectronic technologies and when they would be detrimental to device performance. Keywords: perovskite, CsPbBr3, Transient Absorption, Cesium lead halide, Nanocrystal, CsPbCl3

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Introduction Semiconducting lead halide perovskites have recently driven a paradigm shift in a range of optoelectronic technologies due to their extraordinary photovoltaic efficiencies1-6 and bright photoluminescence.7-10 Within this family of perovskites, all inorganic cesium lead halide, CsPbX3 where X = Cl, Br, I, perovskite nanocrystals (NCs) have recently emerged as an intriguing material with large absorption coefficients, narrow emission line-widths, and highly tunable optical properties controlled by their shape, size, and composition.11-13 Unlike traditional semiconductor NCs (such as CdSe or PbS) which primarily constitute binary compounds with simple crystal structures, the three-dimensional (3D) interconnection allowed by the ABX3 (A = Cs+, B = Pb+) perovskite structure gives rise to unique properties such as high defect tolerance and screening of hot carriers.14-17 These structure-driven carrier properties in NCs were recently investigated with time-resolved spectroscopies18-23 which show the halide identity controls both inter- and intra-band relaxation of carriers. From these investigations, mixed-halide NCs, where Cl- and/or I- ions are substituted in CsPbBr3 lattice, recently emerged as a next-generation platform with the promise to substantially improve bandgap tunability, structural stability, and long-term durability.12,

24-25

A multitude of robust anion exchange reactions have been

developed to synthesize mixed halide NCs and achieve stoichiometric control of halide substitution25-30 to provide desired optical and electronic properties. Whereas recent reports on mixed halide NCs have largely focused on CsPb(Br/I)3, a complete understanding of the electronic excitations in Cl substituted CsPb(BrxCl1-x)3 NCs remains elusive thus far. In bulk methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3) perovskites, Cl substitution has been shown to improve the crystal structure, which results in better ambient stability, longer carrier diffusion lengths, and higher device efficiencies.31-32 Therefore, understanding how Cl


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inclusion in the CsPbBr3 NC lattice alters the exciton dynamics allows us to evaluate the utility of CsPb(BrxCl1-x)3 NCs for a range of light-harvesting and light-emitting applications. In this work, we studied the ultrafast carrier dynamics of CsPb(BrxCl1-x)3 NCs, synthesized via a hot-injection method, by combining time-resolved photoluminescence (trPL) and ultrafast transient absorption spectroscopy (TAS). Whereas trPL spectroscopy is useful in understanding the decay of emissive states, it cannot resolve sub-picosecond carrier dynamics occurring in NCs due to detection speed limitations. An ultrafast pump–probe TAS technique can reveal spectral and temporal behavior of photogenerated species and resolve carrier dynamics on sub-picosecond to nanosecond time scales. This approach has been extensively utilized to characterize perovskites.33-36 Our study shows that high Cl content in NCs resulted in low PL quantum yield (PLQY) and rapid radiative decay of carriers.

TAS measurements

showed increased Cl inclusion in the CsPbBr3 lattice results in decrease of the bandgap renormalization energy indicative of high exciton binding energies. Further, increased Cl substitution lead to rapid thermalization of carriers and low carrier temperatures weakening the hot phonon bottleneck effect37-39 and Burstein-Moss40-41 effect well-described in both bulk and nanocrystal perovskites.

Our results also indicate that increasing Cl content increases

nonradiative Auger losses, and that Auger processes dominate over trap-assisted recombination of carriers.

This work demonstrates that CsPb(BrxCl1-x)3 perovskite NCs are useful in

applications where large bandgaps and blue light emission are required, but for photovoltaic and photoemission technologies Cl substitution in NCs will likely be detrimental to devices.

Experimental Methods Preparation of Cesium Oleate (Cs-oleate):


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Cs-oleate was synthesized as reported by Protesescu et. al. 12 A 100-mL 3-neck flask was loaded with 0.814 g of Cs2CO3, 40 mL octadecene (ODE), and 2.5 mL of oleic acid (OA) and then put under vacuum for 1 h at 120 ˚C with stirring. Then, the temperature of the flask was raised to 150 ˚C and exposed to N2 until all solids had dissolved. Note: the temperature of the reaction media in the flask was slightly lower from the temperature of the oil bath; therefore, the temperature of the oil bath was set to 175 oC prior to use to achieve the desired reaction temperature within the flask. Synthesis of CsPbBr3 Nanocrystals: CsPbBr3 NCs were synthesized as detailed by Protesescu et. al.12 The following reagents were added to a 25 mL flask and stirred (~500 rpm) under vacuum in an oil bath at 120 ˚C until the lead halide (PbX2) completely dissolved (~20 min): 0.5 mL oleylamine (OLA), 0.5 mL oleic acid (OA), PbX2 (52 mg PbCl2 or 69 mg PbBr2), 5 mL octadecene (ODE). We measured the temperature of the oil bath rather than the temperature of the flask to reduce the risk of exposure of the reaction media to air and contaminants. After the PbX2 dissolved, the temperature of the oil bath containing the flask was raised to 170 ˚C using a temperature controller. Then, 0.4 mL Cs-oleate was rapidly injected. After 10 s, the flask was plunged into an ice bath to quench the reaction. CsPbBr3 NCs were a yellowish green and the CsPbCl3 NCs were milky white. The flasks were purged under vacuum before being transferred into an N2 glove box. Anion exchange reactions The anion exchange reactions were performed in an N2 glove box. To synthesize CsPb(Br0.84Cl0.16)3 NCs, 1.2 mL crude CsPbBr3 NCs and 0.3 mL crude CsPbCl3 NCs were added to 8.5 mL toluene in scintillation vials. To synthesize CsPb(Br0.78Cl0.22)3 NCs, 1.1 mL crude CsPbBr3 NCs and 0.4 mL crude CsPbCl3 NCs were added to 8.5 mL toluene in scintillation


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vials. To obtain CsPb(Br0.55Cl0.45)3 NCs, 1.5 mL crude CsPbBr3 NCs and 7 mg ZnCl227, 30 were added to 8.5 mL toluene. As controls, in 2 separate scintillation vials, 1.5 mL CsPbBr3 were combined with 8.5 mL toluene and 1.5 mL CsPbCl3 were combined with 8.5 mL toluene. Solutions were stirred for 1 hr to ensure that the anion exchange reactions went to completion.2526

Purification of CsPbX3 NCs Upon completion of the anion exchange reactions, the scintillation vials were removed from the glove box and left on a counter for 10 min to allow large particles to settle to the bottom of the vials, as evidenced by a layer of fine powder at the bottom of the vial. Note that the solution retained its color after the removal of the aggregates. For each vial, the supernatant was transferred to a 15-mL centrifuge tube and chilled in a freezer for 5 min. Then, the centrifuge tubes were placed in a centrifuge for 10 min at 8805 rcf. The supernatant was discarded, and the pellets were dispersed in a mixture comprised of 7 parts hexanes and 3 parts ethyl acetate. The tubes were chilled for 5 min in the freezer and centrifuged for 10 min at 8805 rcf. The supernatant was removed, and the pellets were dispersed in hexane. CsPbX3 solid-state substrate preparation For transient absorption (TA), time-resolved photoluminescence (trPL), steady state photoluminescence (ssPL), absorbance, and x-ray diffraction (XRD) measurements, glass microscope slides were cut and sonicated in a 1:1 v:v ratio of 2-propanol and acetone. The slides were then dried using compressed air. The colloidal solutions were drop cast onto glass substrates. The samples were transferred to the N2 glove box and coated in a 10 mg/mL solution of PMMA dissolved in chlorobenzene. For TEM measurements, colloidal solutions of the nanocrystals in hexane were drop cast onto copper grids and dried under ambient conditions.


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Characterization The images of the NCs were taken using an Osiris Transmission Electron Microscope (TEM) at 200 keV or a Zeiss Merlin Scanning Electron Microscope between 3 and 10 kV. Samples prepared for TEM/SEM were not coated with PMMA. A Scintag XGEN-4000 X-ray diffractometer with CuKa providing the radiation at a wavelength of λ = 0.154 nm obtained Xray Diffraction (XRD) spectra for the NCs. Absorbance spectra of the samples were taken using a Varian Cary 5000 UV-vis-NIR spectrophotometer with dual-beam capabilities. Transient Absorption Spectroscopy Transient absorption measurements were taken using a homemade pump-probe system, a Ti:sapphire amplifier (Legend USP-HE, Coherent) seeded by pulses generated by a titanium sapphire oscillator (Micra, Coherent). It produces 50 fs, 800 nm pulses with a 1000 Hz repetition rate. The output energy was 2.5 mJ per pulse. An Nd:YLF laser (Evolution-30, Coherent) was used for pumping the Legend amplifier. Part (~4 µJ per pulse) of the output of the Legend amplifier was focused on a 2 nm-thick sapphire window to produce a white light continuum (WLC) probe ranging from 450 – 900 nm. A set of parabolic mirrors collimated and focused the WLC on the sample to minimize temporal chirp in the probe. A spectrometer/CCD (USB2000ES, Ocean Optics) was used to detect the transmitted probe via a 100 µm core fiber. Approximately 50 µJ per pulse of the 800 nm laser was doubled using a BBO crystal to produce the 400 nm-pump-pulse. The pump beam passed through a delay line to vary the time-delay between the pump and the probe. The changes in absorbance were measured between every two successive laser shots by chopping the pump beam at 500 Hz. The spot size of the pump was 100 µm, and the spot size of the probe was 70 µm. The sample was excited by pulses with energies from 1.5 – 6 nJ.


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Steady State PL Spectroscopy The PL spectra were measured in a custom-built micro-PL setup, that included a spectrophotometer (Acton SP2300) equipped with a CCD camera (Princeton Instruments, Pixis 256B) coupled to a microscope (Olympus). The samples were excited with a continuous wavediode-pumped solid-state laser (Excelsior, Spectra Physics, 532 nm, 100 mW) through an upright microscope using a 100x-long working distance objective with NA (numeric aperture) = 0.8 (beam spot ~ 1 µm). The typical incident laser power on a sample was maintained at few µW to reduce possible laser heating and damaging of the samples during PL spectra acquisition. The PL light was analyzed by a spectrometer (Spectra Pro 2300i, Acton, f = 0.3 m) that was coupled to the microscope and equipped with 150, 600, and 1800 groves/mm gratings and a CCD camera (Pixis 256BR, Princeton Instruments). The typical acquisition times were varied from 0.5 to 5 s depending on the PL intensity. Time-resolved PL Spectroscopy Time-resolved photoluminescence measurements were performed using a custom-built epifluorescence microscope. Samples were excited under wide-field illumination using a 400 nm pulsed source (200 fs pulse duration) at a 250 KHz repetition rate utilizing the frequency doubled output of a Coherent RegA Ti:Sapphire Oscillator. PL from the samples was filtered with an appropriate long-pass filter and directed onto a single-photon avalanche photo-diode (SPAD, Micro Photon Devices, PD-050-0TC). A time-correlated single photon-counting unit (TCSPC, PicoHarp 300, ∼ 35 ps) was used to generate a histogram of photon arrival times. PL quantum yield (PLQY) measurements


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Samples were washed as they were in the characterization section, suspended in hexane, dried under vacuum, and suspended in anhydrous hexane in an N2 environment. This was to ensure that the samples in solution were similar to the samples in solid-state used discussed previously. It was assumed that the PLQYs calculated in solution would not vary significantly from the solid-state samples, and that any variations between the solid-state PLQY and the solution-state PLQY would be consistent across all samples. PL spectra were collected on a PTI QuantaMaster fluorescence spectrophotometer using a 75 W Xe arc lamp as the excitation source. PL was measured with a 1 s integration time and a 1 nm slit width. Quantum Yield (QY) measurements were determined by comparing the PL of the NCs to that a reference dye (Coumarin 521 in EtOH, QY ~75% or Coumarin 47 in EtOH, QY ~ 50%), using the equation,

= ×

×

×

2

2

,

where QYStandard is the quantum yield of standard dye, A is the area of the peak of the sample or the dye, and n is the refractive index of the solvent of the sample or the solvent of the dye.

42-44

Samples and dyes used in these measurements were diluted with hexane and EtOH, respectively, so that the optical density at the wavelength of the absorption peak of the dye was less than 0.2.

Results and Discussions In this work we synthesized CsPbX3 (X = Br, Cl) NCs via a hot-injection process as previously reported.12 Mixed halide CsPb(BrxCl1-x)3 NCs were obtained via anion exchange reactions starting with CsPbBr3 NCs and using ZnCl2 or CsPbCl3 as the chloride source. We found anion exchange using ZnCl2 was rapid and effective at shifting the band edge, as observed previously27 and resulted in 45% Cl-substituted CsPb(Br0.55Cl0.45)3 NCs. In comparison, when


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CsPbCl3 and CsPbBr3 NCs of similar morphologies were combined together, slow ion migration resulted in NCs with 16% and 22% Cl content, CsPb(Br0.84Cl0.16)3 and CsPb(Br0.78Cl0.22)3 NCs.25, 45

The Cl substitution in our sample was confirmed with STEM-EDS, which showed 16, 22, and

45% Cl content (Table S1) in the NCs. The substitution of Br ions with Cl resulted in a visual color change that was observable under UV light illumination (Fig. 1a) and was spectrally characterized as a blue shift in the absorption band edge (Fig. 1b). This blue shift is attributable to shifts in the valence band maximum to more negative potentials and the conduction band minimum to less negative potentials, leading to an increase in the bandgap.46 The observed blue shift is also related to changes in the PbX64- octahedra, where higher Cl content increases the PbX bond length. The increase in the Pb-X bond length subsequently decreases the dielectric constant and increases the exciton binding energy of the NCs, resulting in an increase in the absorption cross section with Cl substitution.24

The CsPb(BrxCl1-x)3 mixed halide NCs

maintained their cubic or orthorhombic morphology for all Cl substituted samples as observed in TEM micrographs (Fig. 1c, Fig. S1) and XRD spectra (Fig. 1d).24, 26-27, 45 The mean sizes of the NCs were obtained from over 90 NCs and range from 13.6 – 19.3 nm, shown in Figure S2. XRD spectra showed that Cl substitution increased 2θ values due to the decrease in interplanar spacing associated with the smaller ionic radius of Cl relative to Br.26-27 The 45% Cl-substituted mixedhalide NCs also showed the presence of CsPb2Br5, which is an alternate perovskite phase.47 CsPbBr3 nanocrystals exist in both orthorhombic and cubic phases. Whereas the two phases are almost indistinguishable in their XRD spectra, a clear peak splitting is observable at 2θ ~ 30° for the orthorhombic phase.9, 48 We did not observe peak splitting in our CsPbBr3 nanocrystals, and therefore we concluded that the nanocrystals used in this study are cubic phase. The cubic phase has been reported to occur when high temperatures are used during colloidal synthesis as well as


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has been attributed to the high surface energy of the nanocrystals.48 Note that we also synthesized CsPbCl3 pure halide NCs (Fig. S3) but could not study the spectral dynamics since their absorption and emission spectra were outside the spectral range of our experimental apparatus.

Figure 1: Characterization of CsPb(BrxCl1-x)3 perovskite nanocrystals (NCs). (a) Photo of colloidal solutions of NCs with and without UV illumination. The % Cl in each sample is indicated in the photo. (b) Absorbance spectra of NCs showing blueshift in band edge with increased Cl content. (c) Representative TEM micrograph of CsPb(Br0.55Cl0.45)3 NCs showing single crystal lattice. (d) XRD spectra showing the cubic crystal structure of CsPb(BrxCl1-x)3 NCs where # indicates the presence of CsPb2Br5.


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Figure 2: Photoluminescence spectral dynamics of CsPb(BrxCl1-x)3 perovskite nanocrystals. (a) Steady state PL emission spectra showing a blue-shift with increasing Cl concentration, (b) timeresolved PL showing faster decay kinetics with increasing Cl concentration, and (c) rapid radiative decay rates with increasing Cl. Spectra are color coordinated in (a) and (b).

To understand the impact of Cl substitution on the spectral dynamics of CsPb(BrxCl1-x)3 NCs, we examined the steady-state photoluminescence (ssPL) spectra of the NCs in solid-state, where NCs were deposited on glass substrates. The blue-shift observed in the PL spectra (Fig. 2a) with increasing Cl substitution in CsPb(BrxCl1-x)3 NCs resembles the trends observed in the absorbance spectra, which is expected since CsPbX3 is a direct bandgap material. Since the dimensions of the NCs were well above the Bohr diameters of both CsPbCl3 and CsPbBr3, the shift in the band edge can be attributed to the changes in composition rather than any minor morphological differences in the NCs.27 Further, the full width at half maximum (FWHM) of the ssPL peaks remained consistent and narrow among the different NC samples which suggests homogeneity of the composition of mixed-halide NCs. The FWHMs of the NCs with 0, 16, 22, and 45% Cl substitution were 15.5, 16.1, 16.0 and 18.2 nm, respectively. STEM-EDS analysis of the samples (Table S1, Figures S4, S5) further reflected their compositional homogeneity. We also studied the PL decay kinetics of the mixed-halide NCs with time-resolved PL (trPL) to understand the impact of Cl content on the radiative lifetimes. The PL decay was fit with a triexponential decay function yielding three time constants (Table S2) which were then used to


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calculate the amplitude weighted average PL lifetime given by =

! ! . !

The τavg

is useful as it accounts for change in both the time constants and the amplitude, and it is used to calculate the radiative decay rates discussed below. As illustrated in Table 1, τavg was in the range of 0.7 - 4 ns which correlates well to that reported in the literature.8, 10, 12, 24 We observed that Cl content in mixed-halide NCs results in a faster lifetimes which can be understood from the trends observed in photoluminescence quantum yield (PLQY) measurements (Table 1). The PLQYs for CsPbBr3, CsPb(Br0.84Cl0.16)3, CsPb(Br0.78Cl0.22)3, and CsPb(Br0.57Cl0.45)3 were 0.49, 0.30, 0.09, and 0.04, respectively. The short τavg of the Cl substituted NCs implies more rapid recombination of electron/hole pairs which would reduce the PLQY. This observation parallels to literature reports for NCs with high Cl content.25, 27 Note that our PLQYs are lower than those reported in the literature due to differences in sample preparation. To enable well-resolved TAS and trPL measurements our samples were thoroughly purified, which removed most ligands, and then deposited onto solid substrates as described in the methods section. The measured PLQY also provides an understanding of the radiative and non-radiative ()*+

decay rate of the NCs, which is given by "#$%&%' =

,-.

where τavg accounts for both

radiative and nonradiative processes in recombination dynamics. The nonradiative decay rate can be calculated as follows49: =

/0,12,32-4 /0,12,32-4 /5650,12,32-4

. The calculated kradiative and knonradiative

are provided in Table 1. The calculated kradiative values as a function of Cl content (Fig. 2c) show a decrease in radiative decay rate with the presence of Cl, and increase in non-radiative decay rate (Table 1). Anion-dependent variations in kradiative have been correlated to the Kane energy in perovskite NCs.21 In II−VI, III−V, and group-IV semiconductors, the material composition has minimal impact on the Kane energy because the wave functions of the band-edge states in the 12 ACS Paragon Plus Environment

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conduction and valence band states are nearly identical. However, in perovskites the lowest energy valence band state has a mixed character influenced by the halide 3p/4p and Pb 6s orbitals suggesting the anion identity should impact the degree of mixing of these orbitals. This would result in variations in the anion-dependent Kane energy that likely explains the observed decrease in the radiative decay rate.21 Experimentally, it is also likely that during the anion exchange of Br- with Cl-, high concentrations of Cl- led to an increase in surface defects that could have increased the prevalence of nonradiative decay pathways, leading to low kradiative, and an increase in knon-radiative with increasing Cl. Table 1: The amplitude-weighted lifetime, τavg, ± standard error obtained from n = 4 samples per NC, photoluminescence quantum yield, (PLQY), radiative recombination rates (krad), and nonradiative recombination rates (knr) obtained from tri-exponential fits of time-resolved photoluminescence decays. Time-resolved Photoluminescence Spectroscopy τavg (ns)

PLQY

krad (ns-1)

knr (ns-1)

CsPbBr3

3.66 ± 0.07

0.49

0.134

0.14

CsPb(Br0.84Cl0.16)3

3.67 ± 0.07

0.30

0.082

0.19

CsPb(Br0.78Cl0.22)3

1.19 ± 0.04

0.09

0.075

0.77

CsPb(Br0.55Cl0.45)3

0.70 ± 0.05

0.04

0.057

1.36


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Figure 3: Evolutionary transient absorption spectra of (a) all samples at 0.5 ps time delay and of (b) CsPb(Br0.84Cl0.16)3 at 4 different time delays. All samples were excited at 6.5 µW. (c) Bandgap renormalization energy probed as a function of Cl substitution in nanocrystals. Error bars are from n= 3 samples for each NC composition. *Due to the limitations of our setup, we could not calculate the renormalization energy of CsPb(Br0.55Cl0.45)3 (see text for more details).

Whereas steady-state PL and trPL measure the recombination dynamics occurring on nanosecond timescales, ultrafast transient absorption (TA) spectroscopy provides an understanding of the dynamics occurring in picosecond timescales. The TA evolutionary spectra of CsPbBr3 and Cl substituted NC samples (Fig. 3a) showed a photobleach (PB) feature near the band edge which were at ~514, 497, 490 and 462 nm for CsPbBr3, CsPb(Br0.84Cl0.16)3, CsPb(Br0.78Cl0.22)3, and CsPb(Br0.57Cl0.45)3, respectively. The PB is attributed to excitation of valence band electrons to the conduction band by the pump pulse, which consequently saturates the conduction band edge and prevents further light absorption by the samples. The TA spectral characteristics mirror the trends observed in the absorbance spectra (Fig. 1b) as the PB feature blue shifts with increasing Cl content. A positive photoinduced absorption (PIA) peak was also observed (Fig. 3a, b) due to conduction band electrons moving to higher energy states, resulting in vacant electronic states near the band edge. This allows more electrons to transverse the bandgap, resulting in a positive amplitude of the transient signal.19, 50-51 The PB and PIA peak positions allowed us to calculate the bandgap renormalization energy (∆Ern), obtained from the difference in energy between PB and PIA (Fig. 3c, Table S3).37 When semiconductors are 14 ACS Paragon Plus Environment

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photoexcited, the density of electrons in the conduction band increases, resulting in Coulombic interactions between the electrons of the conduction band and increased electron impurity scattering, leading to the renormalization of the band gap.52-55 This increases the energy of the valence-band maximum and decreases the energy of the conduction-band minimum, resulting in a narrower band gap, reduced coulombic interactions, and larger exciton binding energies.37 We observed a decrease in ∆Ern from 0.18 to 0.11 eV, calculated at 2 ps time delay after photoexcitation, with increasing Cl substitution (Fig. 3a), which suggests that 16 – 45% Clcontent results in high exciton binding energies. This leads to less efficient exciton dissociation and carrier extraction, which is not favorable in most optoelectronic applications where exciton dissociation is indispensable. This is supported by literature findings where CsPbCl3 has an exciton binding energy of 75 meV, and CsPbBr3 has an exciton binding energy of 40 meV.12 The rapid decline in ∆Ern is also indicative of bandgap narrowing in high Cl doped NCs; this suggests Cl substituted CsPbBr3 NCs would not be suitable for photovoltaic applications, as bandgap reduction would manifest as lower open circuit voltage obtained from devices. We note that we could not calculate the renormalization energy of CsPb(Br0.55Cl0.45)3 since our white light probe ranges from 450 – 900 nm and part of the PIA band was cut off. This limitation arose due to the new calibration of the position of the neutral density filter which determines the pump fluence hitting the sample. In addition to the peak positions of PB band, their full width at half maximum (FWHM) is also related to the thermal relaxation of carriers. We observed larger FWHM of the PB bands at early timescales and larger FWHM of the PB bands of all samples at higher pump powers (Table S4, Figure S6). This observation is not surprising as increasing pump intensities result in higher carrier densities, which would consequently slow down thermal relaxation.

PB


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broadening is also related to the Burstein-Moss effect,39, 56 a model for electronic band filling that correlates broadening of the PB band to charge carrier accumulation and assumes the change in optical band gap scales with the charge density raised to the power of 2/3. We observed that the Burstein-Moss effect was most prominent in the pure halide NCs and decreased with increasing Cl content. This manifested as an increased rate of carrier cooling with high Cl substitution in CsPbBr3 NCs. These results are in agreement with our previous report,41 and those in the literature on bulk perovskites, which demonstrated higher Burstein-Moss shift in pure halide MAPbI3 perovskites and substitution of I- with Br- resulted in rapid carrier cooling.57 This is further explained below in the discussion of bleach formation lifetimes and carrier temperatures of the CsPbBr3 and Cl-substituted NCs. Table 2: Lifetimes obtained by fitting the photobleach band in transient absorption spectra at 6.5 µW excitation power for the different Cl substituted perovskite NCs. Here τr represents rise time, τ1 is decay by Auger processes, and τ2 is trap-assisted recombination. A1 and A2 are amplitudes correlating to τ1 and τ2. The standard error is achieved from n = 3 samples for each Cl content. %Cl 0

τr 0.60 ± 0.02

16

A1

A2

0.42 ± 0.01

τ1 5.94 ± 0.31

0.58 ± 0.01

τ2 830.84 ± 14.10

0.41 ± 0.01

0.44 ± 0.02

33.62 ± 2.66

0.56 ± 0.02

333.38 ± 10.85

22

0.32 ± 0.02

0.50 ± 0.03

33.89 ± 3.00

0.50 ± 0.03

397.22 ± 28.01

45

0.33 ± 0.02

0.66 ± 0.08

13.88 ± 0.10

0.34 ± 0.08

149.75 ± 31.82


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Figure 4: TA dynamics at early times. (a) Photobleach formation dynamics as a function of time for various Cl substitution. (b) Lifetimes for the formation of the photobleach. Error bars are from n= 3 samples for each NC composition. (c) Carrier temperature as a function of time for varying amounts of chlorine. Note that the limitations of the TA setup prevented the estimation of the carrier temperature for CsPb(Br0.55Cl0.45)3. (d) Carrier temperature as a function of pump fluence in CsPbBr3 perovskite nanocrystals. All samples were excited at 6.5 µW. Thermalization of hot carriers can be tracked by the PB formation in TA spectra, which occurred within the first 2 ps after photoexcitation (Fig. 4a). The rise times for bleach formation were obtained from an exponential fit of the PB formation dynamics; the amplitudes and lifetimes extracted from the fits are listed in Table 2. Upon ultrafast excitation of NCs, carriers with higher energies than the bandgap undergo intra-band relaxation and carrier thermalization, resulting in the accumulation of carriers at the edge of the conduction band.39, 51 We observed that the rise time decreases with increasing Cl content in CsPbBr3 NCs, implying rapid thermalization of carriers with increasing Cl.

However, minimal change in rise time was 17

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observed at 45% Cl content suggesting that the hot carrier population decreased with Cl content but is insensitive beyond 16%. These results corroborate the diminished Burstein-Moss effect with Cl substitution discussed above. We also examined the impact of the pump power on the thermalization lifetimes of the mixed-halide NCs between 2.5, 4.4, and 6.5 µW. The thermalization lifetime increased with rising pump fluence for the NCs as expected (Table S5, Fig. S7). Following excitation by a femtosecond pump pulse, hot carriers are generated in NCs that cool on sub picosecond time scales by dissipating the absorbed optical energy as lattice heat via longitudinal optical phonon decay. As the carriers thermalize, a nonequilibrium carrierphonon distribution results.22, 58-60 With rising pump fluence, the number of absorbed photons per NC increases, resulting in an increase in the occupancy of the lowest energy levels at the band edge. This process, known as the “hot phonon bottleneck,” slows down the relaxation of hot carriers, which results in long-lived hot carrier population.38,

57

The trends observed in

carrier thermalization both as a function of Cl content (Fig. 4b) and pump power (Fig. S7) indicated that hot phonon bottleneck is strongest in CsPbBr3 NCs and diminishes with increasing Cl content in mixed halide NCs. We also estimated the temperature of the hot carrier population in the different samples by fitting the tail of the high-energy photoinduced absorption peaks (see Figure S8) to a Boltzman distribution at different delay times given by: ∆ ∝ :(

Ultrafast Spectral Dynamics of CsPb(BrxCl1âx)3 Mixed-Halide

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