Emission Properties and Ultrafast Carrier Dynamics of CsPbCl3

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C: Physical Processes in Nanomaterials and Nanostructures

Emission Properties and Ultrafast Carrier Dynamics of CsPbCl Perovskite Nanocrystals 3

Ruben Ahumada-Lazo, Juan Arturo Alanis, Patrick Parkinson, David J. Binks, Samantha J. O. Hardman, James Thomas Griffiths, Florencia Wisnivesky Rocca Rivarola, Colin J. Humphreys, Caterina Ducati, and Nathaniel J.L.K. Davis J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Emission Properties and Ultrafast Carrier Dynamics of CsPbCl3 Perovskite Nanocrystals

Ruben Ahumada-Lazo1, Juan A. Alanis1, Patrick Parkinson1, David J. Binks1*, Samantha. J. O. Hardman2, James T. Griffiths3, Florencia Wisnivesky Rocca Rivarola3, Colin J. Humphrey3, ‡, Caterina Ducati3, Nathaniel. J. L. K. Davis4, †

1School

of Physics and Astronomy and the Photon Science Institute, University of

Manchester, Manchester, M13 9PL, United Kingdom 2Manchester

Institute of Biotechnology, University of Manchester, Manchester, M13

9PL, United Kingdom 3Department

of Materials Science and Metallurgy, University of Cambridge, Charles

Babbage Road, Cambridge, CB3 0FS, United Kingdom

ACS Paragon Plus Environment

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Cavendish Laboratory, University of Cambridge, J. J. Thompson Avenue, Cambridge,

CB3 0HE, United Kingdom

Author’s current affiliations: ‡School

of Engineering and Materials Science, Queen Mary University of London, London, E1 4NS,

United Kingdom. †School

of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6140, New

Zealand

ABSTRACT

Fluence-dependent photoluminescence and ultrafast transient absorption spectroscopy are used to study the dynamic behavior of carriers in CsPbCl3 perovskite nanocrystals. At low excitation fluences, the radiative recombination rate is outcompeted by significant trapping of the charge carriers which then recombine non-radiatively, resulting in weak photoluminescence. As fluence is increased, the saturation of trap states deactivates these non-radiative relaxation paths giving rise to an increase in photoluminescence at first. However, with further increases in fluence, Auger recombination of multiexcitons results in a decline in photoluminescence efficiency. Analysis of this behavior yields an


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absorption cross-section at 400 nm (3.1 eV) of 0.24 ± 0.05 x 10-14 cm2. Transient photoluminescence and absorption measurements yielded values for single exciton trapping lifetime (1.6 ± 0.7 ns), biexciton and trion lifetimes (20 ± 3 ps and 157 ± 20 ps, respectively), single exciton radiative lifetime (12.7 ± 0.2 ns), intraband cooling lifetime (290 ± 37 fs) and exciton-exciton interaction energy (10 ± 2 meV).

INTRODUCTION


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Ionic metal-halide perovskite nanocrystals (NCs), with the general formula ABX3, where A is a large cation, B a metal (Pb, Sn being the most studied), and X a halide (Cl, Br, I), have received significant attention because their optical properties make them suitable for many optoelectronic applications. 1 In particular, all-inorganic CsPbX3 NCs exhibit bright and tunable emission and have recently emerged as promising candidates to replace chalcogenide-based quantum dots (QDs), which are more susceptible to photodegradation. 2 The spectral properties of this type of NCs can be tuned by size (making use of the quantum confinement effect) or by halide composition, with emissions shifting from blue (Cl-), to green (Br-), to red (I-) in NCs of the same size. Moreover, halide exchange (of similar size anions, i.e. Cl- and Br- or Br- and I-) allows for mixed compositions with emission covering the entire visible spectrum. 3 In contrast, when NCs of CsPbI3 and CsPbCl3 are mixed together, the anion exchange is significantly reduced due to the unfavorable crystal lattice tolerance factor for iodide−chloride exchange, allowing for excitation transfer from CsPbCl3 to CsPbI3 via radiative emission. 4 CsPbX3 materials are easily synthesized by a low-cost solution method which produces monodispersed NCs that are more stable than hybrid organic-


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inorganic perovskites. 5 They are also highly ionic and stoichiometric which reduces the number of structural defects that can act as carrier traps, giving high photoluminescence (PL) quantum yields (PLQYs) without the need of core/shell structures.2 However, the high surface-to-volume ratio of NCs can inherently induce significant trapping of carriers at the dangling bonds of undercoordinated surface atoms if these are not properly passivated. 6 Moreover, because of the ionic nature of these all inorganic perovskites, their interactions with capping ligands are also very ionic and unstable, making their colloidal solubility, PLQY and even their structural integrity highly susceptible to reaction conditions and purification methods. 7 In addition, photochemical degradation has been observed to decrease the PL emission intensity in perovskite NCs exposed to laser light by creating surface defects which act as non-radiative recombination centers. 8

These newly developed materials are very promising for applications as varied as quantum emitters, 8 color-converting phosphors, 9 lasers, 10 photovoltaics 11 and lightemitting diodes. 12,13 This has motivated the study of the carrier dynamics and quantum


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confinement effects for different sizes and compositions of this family of perovskites. 14 A few studies have reported the dynamics of single and multi-excitons in CsPbI3, CsPbBr3 and mixed CsPbI1.5Br1.5 compositions, finding very interesting similarities and differences with more common colloidal QDs 14–16. For example, in perovskite NCs the decay of multiexcitons and charged excitons is also dominated by very fast Auger recombination. Biexciton lifetimes have been found to scale linearly with NC volume, V, regardless of the differences in electronic structure for many of the different types of quantum dots studied in the literature. Perovskite NCs also follow this trend but only when under the strong confinement regime, i.e. NCs with edge lengths close to or less than the exciton Bohr radius. Larger perovskite NCs (in the weak confinement regime) deviate from this “universal volume scaling” showing lifetimes with sublinear dependencies on V. 14,16 In addition, the absorption cross-sections of these NCs have been found to also scale linearly with volume as in other QDs, but are almost an order of magnitude smaller than those of CdSe QDs of the same size. 14 The sub-picosecond intraband relaxation of hot carriers in perovskite quantum dots is also of interest because its exact mechanism is still not fully understood. This is believed to happen


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through either Auger energy transfer, coupling to surface ligands or multiphonon emission in quantum dots, in contrast to the phonon emission cascade observed for bulk materials. 14

Understanding these properties is fundamental for the further development of devices based on perovskite NCs. However, studies on carrier dynamics have not yet been extended to CsPbCl3, which is of interest because of its bright and spectrally-narrow blue luminescence. 2 In the present work we use fluence-dependent photoluminescence (PL) and ultrafast transient absorption (TA) techniques to uncover the dynamic behavior of carriers under different excitation regimes in CsPbCl3 NCs.

EXPERIMENTAL METHODS

Nanocrystal synthesis

NCs CsPbCl3 were prepared using a solution based method previously reported by Protesescu et al.

2

and deposited on transparent support films for PL and CL

measurements. Briefly: Cs2CO3 (0.814 g, 99.9%) was loaded into 100 mL three-neck


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flask along with octadecene (ODE, 30 mL, 90%) and oleic acid (2.5 mL, OA, 90%), the mixture was dried for 2 h at 120 °C under N2. The solution temperature was then lowered to 100 °C. ODE (75 mL), oleylamine (7.5 mL, OLA, 90%), and dried OA (7.5 mL, PbCl2 (0.675g, 99.99%) and 5 mL of trioctylphosphine (TOP, 97%) (to solubilize PbCl2) were loaded into a 250 mL three-neck flask and dried under vacuum for 2 h at 120 °C. After complete solubilization of the PbCl2 salt, the temperature was raised to 170°C and the Cs-oleate solution (6.0 mL, 0.125 M in ODE, prepared as described above) was quickly injected. After 10 s, the reaction mixture was cooled in an ice-water bath. The NCs were transferred to an argon purged glove box (H2O and O2 < 1 ppm) precipitated from solution by the addition of equal volume anhydrous butanol (BuOH, 99%) (ODE:BuOH = 1:1 by volume). After centrifugation, the supernatant was discarded and the NCs were redispersed in anhydrous hexane (99%) and precipitated again with the addition of BuOH (hexane:BuOH = 1:1 by volume). These were redispersed in hexane. The nanocrystal dispersion was filtered through a 0.2 μm PTFE filter and diluted in hexane before use. Sample concentrations were determined by allowing the


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solvent from a known volume to evaporate and then weighing the residue. Details of the sample preparation for TA are given in the supporting information.

Scanning transmission electron microscopy and Nano-cathodoluminescence Samples for STEM were prepared by drop-casting 40 mg/mL perovskite nanocrystal solution in hexane on a carbon coated 200 mesh copper grid in an argon filled glove box. Atomic resolution ADF-STEM was performed on a probe corrected FEI Titan, at 300 keV with 60 pA. Nano-CL was performed in a STEM operated at 80 kV. Miniature elliptical mirrors positioned around the specimen were used to collect the light emitted from each position in the sample as the sub-nanometre electron probe was rastered across the specimen. The collected light was coupled to optical fibers and detected on a photomultiplier tube.

Photoluminescence (PL) measurements The steady state PL was measured using a FluoroLog 3 spectrometer. Ensembles of the NCs were studied by fluence-dependent photoluminescence (PL) spectroscopy. This was performed using a pulsed Ti:sapphire (RegA 9000) laser source at a repetition rate of 250 kHz and 170 fs pulse duration. A frequency doubler (PHOTOP


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TP-2000B) was used to generate the excitation wavelength of 400 nm. The laser fluence was varied with a neutral density filter wheel and measured by a power meter. The spot size at the sample position was measured to be 1.3 x 10-3 mm2 after defocusing with an objective lens. The emitted light was collected by an optic fiber and directed into a spectrometer (Ocean Optics) after passing through a long pass filter to eliminate scattered light from the source. More details of this experimental setup are given in reference 17.17 Photoluminescence decay transients were recorded using a time correlated single photon counting (TCSPC) system previously reported.18 Here, a mode-locked Ti:sapphire laser (Mai-Tai HP, Spectra Physics) is used to produce 100 fs pulses at a repetition rate of 80 MHz and 720 nm wavelength. The repetition rate is then reduced to 4 MHz by an acousto-optic pulse picker (APE Select) and the initial wavelength halved (to 360 or 400 nm) via second harmonic generation (APE harmonic generator). These pulses were passed through a lens with 5 cm focal length to reduce the spot size and a series of neutral density filters were used to excite the solution of NCs with an range of photon fluences from ~1.7 x 10-13 to ~4 x 10-14 photons·cm-2 per pulse. The PL emission


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of the samples was collected and focused after a 405 nm pass-band filter into a monochromator (Spex 1870c) and detected at the PL peak by a multi-channel plate (Hamamatsu R3809U-50). The time correlation of the detected photons was performed using a PC electronics card (TCC900) from Edinburgh Instruments.

Absorption and Transient absorption (TA) measurements Steady-state absorption spectra were taken using a Perkin Elmer lamda-1050 spectrometer. A previously reported

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Ti:sapphire amplifier system (Spectra Physics Solstice Ace)

was used to generate 800 nm pulses at 1 kHz for the TA experiments. A portion of this beam was used to pump an optical parametric amplifier (Topas Prime) with an associated NIR-UV-Vis unit to achieve the 100 fs pump pulses at 350 nm with a beam diameter of 360 μm. The pulse energy could be reduced using a series of reflective neutral density filters to give pump fluences from ~3 x 1012 to ~3 x 1015 photons·cm2 per pulse. The rest of the laser output was passed through a CaF2 crystal to generate a white light continuum which was used as the probe to record changes in absorption


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between 360 and 750 nm. Ultrafast broadband transient absorption measurements were carried out at randomly ordered time points in a Helios (Ultrafast Systems LLC) spectrometer (-5 ps to 3 ns, ∼0.2 ps resolution). The sample was magnetically stirred to avoid photocharging effects during the measurements.

RESULTS AND DISCUSSION

The sample analyzed in this study consisted of irregular CsPbCl3 NCs with a range of sizes and aspect ratios as can be seen in the scanning transmission electron microscope (STEM) image inset in figure 1. The average length of the nanocrystals in the sample is 8 nm and standard deviation of the length distribution was ± 2 nm. Since the exciton Bohr radius for CsPbCl3 is 2.5 nm, these nanocrystal sizes correspond to the weak confinement regime and so little or no size-dependence to their properties is expected. 2 The size dispersion is approaching that typically demonstrated for similarlysized CsPbBr3 nanocrystals, with values of 8.1 ± 1.1 nm and 7.7 ± 1.1 nm reported.14,15 Further details about size distribution of the sample and the size and shape


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dependence of emission are given in the Supporting Information. The lattice of the atomic structure (figure 1 inset) corresponds to the thermally-stable cubic perovskite crystal structure, with no extended defects; we therefore attribute the trapping processes described below to surface states. Also shown in figure 1 are the steadystate absorbance and PL spectra. The absorbance spectrum (continuous line) shows a steep onset and a very clear feature at 3.08 eV (403 nm) which is identified with the first excitonic state. Absorption peaks corresponding to a second and third excited state can also be seen at 3.18 eV (390 nm) and 3.30 eV (375 nm), respectively. The photoluminescence spectrum (dashed line) shows an emission peak centered at 3.05 eV (406 nm) with a 80 meV linewidth at the FWHM, which is comparable to that of previously reported perovskite NCs. 2 The inset also shows a photograph of the bright blue emission observed from a solution of CsPbCl3 NCs with a concentration of 1 mg/mL under UV excitation (365 nm).


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Figure 1. Absorption and PL spectra of a solution of CsPbCl3 NCs. Inset shows high resolution STEM images and a photograph of the blue emission under UV radiation.

In figure 2, the relative photoluminescence quantum efficiency, QEPL(defined as the ratio between the time-averaged, spectrally-integrated PL signal and the excitation fluence, normalized to the maximum value of this ratio) is plotted as a function of the photon fluence of the excitation laser pulse,𝐽𝑝. The sample consisted of a film of


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CsPbCl3 NCs (0.1 ml of a 20 mg·ml-1) deposited on a quartz substrate (0.8 cm2). As can be seen, the QEPL first increases with photon fluence then remains approximately constant for moderate fluences before finally reducing as the photon fluence is further increased. This behavior can be explained by considering how the competition between trapping, radiative recombination and Auger recombination changes with photon fluence. (Shallow surface traps have been reported to arise from lead-rich surfaces, i.e. uncoordinated Pb atoms due to surface Pb - Cl ion pair vacancies in this type of NCs, affecting the PLQYs and giving multiexponential PL decays.20,21) For low fluences, there is negligible probability that one NC will absorb more than one photon per excitation pulse, so only single excitons are produced. The photoluminescence efficiency thus depends on both the fraction of NCs that are trap-free, and hence completely emissive, and the competition between radiative recombination and non-radiative trap-mediated recombination pathways in those NCs with traps. The increasing saturation of traps produced as the excitation fluence is increased results in the growth of QEPL observed initially. As only single excitons are created in a NC at low fluences, this saturation corresponds to the deactivation of a non-radiative recombination channel for longer than


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the interval between excitation pulses (4 μs in this study). Long-lived changes in the emission properties of perovskite NCs of up to 1 s in duration have been observed in PL intermittency studies. 8 With further increase in fluence, the probability of more than one photon being absorbed per nanocrystal per pulse is now no longer negligible. This results in the increase in QEPL due to trap saturation being offset by a decrease in QEPL due to Auger recombination of multiple excitons. Initially, these two effects broadly balance each other giving rise to a plateau region over a small fluence range (~4 - 9 x 1013 photons.cm-2). Since Auger recombination has a characteristic lifetime that is much shorter than that of radiative recombination 14 and thus dominates when multi-excitons form, the QEPL continues to reduce with further increase in fluence as the formation of multiexcitons becomes more and more prevalent. To understand this trend with relation to the number of excitons in each NC, the absorption cross-section, 𝜎, was found by fitting the data in figure 2 to the following equation:


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〈𝑁〉 𝑒 ―〈𝑁〉

𝑄𝐸𝑃𝐿 = 𝑆(〈𝑁〉) + 1 ―

𝑒 ―〈𝑁〉

(1).

where 〈𝑁〉 = 𝜎𝐽𝑝 is the average number of photons absorbed per nanocrystal per excitation pulse. Equation (1) was derived by combining a sigmoidal growth function, 22 𝑆(〈𝑁〉), with an expression based on the assumption that photon absorption events follow a Poisson distribution and single excitons are significantly more likely to recombine radiatively and contribute to the PL than multiexcitons. 14 The derivation of this equation is detailed in the supporting information. This cross-section was found to be 𝜎 = (0.24 ± 0.05) x 10-14 cm2 for an excitation photon energy of 3.1 eV (400 nm), which is in good agreement with values reported for CsPbI3 and CsPbBr3 when scaled to match the NC sizes using the linear dependency with volume observed previously. 14 This 𝜎 value was used to calculate the values of 〈𝑁〉 plotted along the top horizontal axis in figure 2. Similar data obtained from a solution of the NCs is shown in the Supporting Information.


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Figure 2. Normalized photoluminescence quantum efficiency, QEPL, as function of photon fluence, 𝐽𝑝, and average number of photons absorbed per pulse per nanocrystal, 〈𝑁〉. The fit shown is to eqn. S1.1

Figure 3a-c show contour plots of the pump-induced absorption change, ∆𝐴, spectra as a function of delay time for NC samples in solution at three representative pump fluences corresponding to 〈𝑁〉 = 0.007, 0.11 and 6.73 (as calculated from the excitation fluence using the absorption cross-section obtained from the fitting to the 𝑄𝐸𝑃𝐿 data as described above). A strong bleach feature (negative ∆A shown in green and blue)


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centered at 403 nm is observed in all the plots. This feature corresponds to the lowestenergy absorption feature identified in the steady-state absorption spectrum and is attributed to state-filling of the 2-fold degenerate band-edge states, as previously reported. 14 Photoinduced absorption features (positive ∆A shown in red) are also observed at both sides of the bleach peak. All these features progressively become more intense and broad as the pump fluence and hence 〈𝑁〉 are increased. Similar plots for the other pump fluences used in this study are shown in the Supporting Information. Figure 3d shows fractional absorption change, ∆A/A, transients recorded at the 403 nm bleach peak for different 〈𝑁〉 values. For low pump fluences where 〈𝑁〉 1 since that reduces the time between photon absorption events for a particular NC to its minimum value i.e. the interval between excitation pulses. At significantly lower 〈𝑁〉 values, an individual nanocrystal will not, on average, absorb a photon during every excitation pulse. Trions have been observed in perovskite NCs to form even under sample stirring conditions under high excitation fluences. 15 As in the case of neutral multiexcitons, the decay of trions in QDs is dominated by Auger recombination. For a negative trion, this can be described in terms of Coulomb scattering between the two conduction-band electrons, where one of them recombines with the hole in the valence band while its energy is transferred to the other electron which is excited higher in the conduction band. The similar process for a positive trion involves the Coulomb scattering of the two valence band holes. Theoretically, the ratio of Auger recombination lifetimes between trions and biexcitons has been calculated to be 4 for QDs with mirror-symmetric conduction and valence bands. 23 However, from experimental measurements it has been found to be around 5 for CsPbBr1.5I1.5 and CsPbBr3 NCs. 14,15 In our case, the ratio obtained using the lifetimes extracted from the CsPbCl3 transients is ~8. This deviation from the theoretical prediction has been attributed to the possible asymmetries in the positive and negative trions pathways in this type of NCs. 14 PL intermittency has been associated with the fluctuation between neutral single excitons (ON state) and trions (OFF state) arising from pump-induced photoionization in perovskite NCs, where the fraction of time the NC stays in the OFF state increases with increasing pump intensity. 8 This reduces emission efficiencies and is unfavorable for many applications. 14


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Figure 4a shows the time-evolution of the near band edge transient spectra of the CsPbCl3 sample at 〈𝑁〉 ≪ 1. Here, the spectral dynamics at early delay times are product of the interaction between the hot photogenerated exciton (created by the pump pulse) as it cools to the band edges and the exciton created by the probe pulse. This biexciton effect is responsible for the observed derivative-like shape of the spectra. In this figure, the photoinduced absorption peak at 3.01 eV (412 nm) is labelled as A while the absorption bleach peak at 3.07 eV (403 nm) is denoted as B. The fact that a photoinduced feature appears at the low-energy side of the absorption bleach indicates an attractive exciton-exciton interaction. The magnitude of this interaction energy can be calculated form the ratio between the amplitudes of these two peaks, i.e. A/B, which in this case yielded a value A/B = 0.24, corresponding to an exciton-exciton interaction energy ∆𝑋𝑋 of 10 ± 2 meV. 14,24 Very similar values of 12 meV and 11 meV have been reported previously for CsPbI3 and CsPbBr1.5I1.5 NCs, respectively, using the same technique. 14 Exciton-exciton interaction energies calculated by this method had shown no dependence with nanocrystal size. However, when calculated from fluence-dependent timeresolved PL spectra, the ∆𝑋𝑋 values obtained for CsPbI3 and CsPbBr3 NCs are larger, up to 100 meV, and show size dependency. 16,24 Over time (figure 4b), the bleach and photoabsorption features show complementary growth and decay behaviors, respectively as the band edge state is filled by the intraband relaxation of the hot carriers. Mono-exponential fitting to both of these transients gave an intraband cooling lifetime 𝜏𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 290 ± 37 fs. This also agrees with the reported values for CsPbI3 and CsPbBr3 by Makarov et al. 14 These fast cooling rates are in agreement with the efficient channeling of hot carriers to the emitting energy states observed from PLE measurements on CsPbI3, CsPbBr3 and CsPbCl3 NCs. 25


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Figure 4. a) Spectral dynamics at early delay times for 〈N〉

Emission Properties and Ultrafast Carrier Dynamics of CsPbCl3

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