Multiexciton Lifetime in All-Inorganic CsPbBr3 Perovskite Nanocrystals

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Multiexciton Lifetime in All-Inorganic CsPbBr Perovskite Nanocrystals Elinore M. L. D. de Jong, Genki Yamashita, Leyre Gomez, Masaaki Ashida, Yasufumi Fujiwara, and Tom Gregorkiewicz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10551 • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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The Journal of Physical Chemistry

Multiexciton Lifetime in All-Inorganic CsPbBr3 Perovskite Nanocrystals Elinore M.L.D. de Jong*1, Genki Yamashita2, Leyre Gomez1, Masaaki Ashida2, Yasufumi Fujiwara3 and Tom Gregorkiewicz1 1

Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.

2

Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka City, Osaka 560-8531, Japan.

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Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.

ACS Paragon Plus Environment

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Abstract A lot of research has been lately conducted on perovskites, which show promising properties for many applications, ranging from optoelectronics to photovoltaics. Recently, a new class of perovskite nanocrystals (NCs) has been synthesized, namely all-inorganic cesium lead halide perovskite NCs (CsPbX3, X = Cl, Br, I), showing high photoluminescence quantum yields (5090 %), narrow emission bands and tunable emission. In this study, we investigate the ultrafast carrier dynamics in CsPbBr3 perovskite NCs using pump-probe transient induced absorption spectroscopy. We demonstrate that, depending on the excitation fluence, hot carriers with carrier temperatures up to 800 K are created upon photon excitation. We also report, for the first time, lifetimes of higher order multiexciton complexes, next to that of the biexciton. These results have implications for the application prospects of these materials for lasers, light emitting and photovoltaic devices, among others.


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Introduction Recently, perovskites attract considerable attention as possible materials for, among others, future photovoltaics due to large absorption coefficients, attractive bandgap energies, high mobilities and relatively low production costs1. Nanocrystals (NCs, nanometer-sized quantum dots) can offer some additional advantageous properties due to the quantum confinement which can modify the electrical and optical properties, making them even more suitable for some applications than the bulk form. One of the quantum confinement-induced effects is the Auger recombination rate enhancement, compared to bulk2. During the Auger recombination process excited carriers recombine nonradiatively and transfer their energy to other carriers. The Auger time constant is typically tens-to-hundreds of picoseconds, depending on the exciton multiplicity, size and shape of the NC3. It has been shown that Auger recombination strongly influences the photoluminescence (PL) at high excitation photon fluence. If the Auger recombination process is fast compared to the radiative lifetime, the Auger recombination of multiple excitons sets an upper limit to the number of emitted photons, independent on the excitation photon energy4,5. Due to efficient Auger recombination, the PL quantum yield of the material decreases drastically under strong pumping, which is detrimental for applications that rely on light emission, like light emitting diodes. The photovoltaic efficiency is also affected, since the multiexciton lifetime is typically short compared to the carrier extraction time. Thus, the investigation of multiexciton lifetimes is of essential importance for the application potential of perovskite NCs. While there are a lot of recent studies on perovskite NCs, a complete understanding of the recombination and energy relaxation processes is still lacking. Until now, most research has been conducted on hybrid (organic-inorganic) perovskites. Lately, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite NCs have been synthesized by Protesescu et al. using rather


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inexpensive commercial precursors and achieving high PL quantum yields (50-90 %), making them attractive for many applications6. In this study, we therefore investigate the carrier dynamics of cesium lead bromide (CsPbBr3) perovskite NCs by transient induced absorption (IA) measurements, which is a conventional way to investigate Auger-related dynamics.

Methods Chemicals Cesium carbonate (Cs2CO3 99.9 %, Sigma-Aldrich), octadecene (ODE 90 %, Sigma-Aldrich), oleic acid (OA 90 %, Sigma-Aldrich), oleylamine (OLA 80-90 %, Acros), lead (II) bromide (PbBr2 98 %, Sigma-Aldrich), and toluene (ACS reagent ≥ 99.5 %, Sigma-Aldrich). All chemicals were used with no further purification, except for the drying period reported in the perovskite NCs synthesis procedure. Inorganic perovskite nanocrystals synthesis Cesium lead bromide perovskite NCs (CsPbBr3) were synthesized according with the protocol reported by Protesescu et al.6. First, Cs-oleate was prepared by mixing 0.814 g of Cs2CO3 with 40 mL of ODE and 2.5 mL of OA, all reactants dried at 120 °C for 1 h. The mixture was stirred at 150 °C in N2 atmosphere until the reaction was completed. Briefly, 5 mL of ODE and 0.188 mmol of PbBr2 were dried for 1 h at 120 °C under N2 atmosphere. After water removal, 0.5 mL of dried OLA and 0.5 mL of dried OA were added to the reaction flask and the temperature was raised up to 160 °C. After complete solvation of the lead salt, 0.4 mL of warm Cs-oleate solution was injected. A few seconds later, the NCs solution was quickly cooled down in an ice bath. The product was purified by centrifugation and redispersion in toluene.


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Optical characterization The PL spectra were obtained with a Jobin Yvon FluoroLog spectrofluorometer (Horiba) consisting of a 450 W xenon arc lamp and a double monochromator as the excitation source. The spectrum depicted in Fig. 1 was obtained under 400 nm continuous wave excitation. The optical density of the sample was measured using a Perkin Elmer Lambda 950 ultraviolet/visible/nearinfrared spectrometer. A tungsten-halogen and deuterium lamps in combination with a PMT and a Peltier-cooled PbS detector provided a detection range of 175-3300 nm. In order to obtain the optical density of a sample, the sample as well as the reference (toluene) were measured separately and their individual contributions were afterwards subtracted from each other. The transient IA spectra and dynamics have been measured using a pump-probe transient IA setup consisting of a Ti:Sapphire regenerative amplifier system (Spitfire Pro, Spectra Physics, running at 1 kHz, 800 nm center wavelength, ~35 fs pulse width, and ~3 mJ pulse energy). With this transient IA experimental setup the change in absorption of a probe pulse upon excitation of the sample with a pump pulse can be measured as function of the delay time between the two pulses. In this study, (a fraction of) the carriers are exited with a primary pump pulse of 355 nm (3.5 eV) from an optical parametric amplifier and probed with a secondary white light continuum pulse, which is generated with the help of self phase modulation in a sapphire crystal and has a relatively low intensity. The two pulses are sent through the sample with a delay time Δt with respect to each other using an optical delay stage such that the time-dependent IA characteristics can be obtained. Transmitted probe pulses were collected with a spectrometer (SpectraPro2500i, ACTON) equipped with a charge-coupled-device (CCD) camera (PIXIS256, Princeton Instruments) operating at 200 Hz. To improve the signal-to-noise ratio of the IA, we


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synchronously chopped pump pulses at 100 Hz to obtain a sequence of signals with excitation on/off (time resolution better than 1 ps). The IA differential absorbance, ΔA, is determined as: , where

(

) and

(1)

(

) are the absorbance and transmitted probe fluence with the

pump laser on (off), and where

is the incident probe light on the sample. All the measurements

have been performed at room temperature in ambient air.

Results & Discussion Sample characterization The bandgap of CsPbBr3 NCs with cubic perovskite structure can be tuned by changing the NC size, which can be modified by the synthesis temperature6. For this study, we have chosen a sample with an average NC size of about ~8.6 ± 1.5 nm (see Fig. 1(a)), exhibiting quantum confinement effects (Bohr diameter ~12 nm6). In Fig. 1(b) the optical density as a function of the wavelength as well as the PL spectrum of the studied sample are shown. The sample emits green light with its PL centered around ~522 nm (~2.4 eV), while the onset of the optical density occurs at a slightly shorter wavelength (higher energy), indicating a very small Stokes shift. The PL quantum yield is ~50-90 % and the effective PL lifetime is ~6.5 ns7.


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Figure 1. Sample characterization of CsPbBr3 perovskite nanocrystals. (a) Transmission electron microscopy (TEM) image. (b) Optical density (left y-axis, black curve) and photoluminescence spectrum (right y-axis, red curve). In the inset, a photo of the sample under ultraviolet illumination is shown.

Optical bleach Before we will discuss the IA dynamics and determine the multiexciton lifetimes, we will first examine the IA spectra. Figure 2 shows a typical IA spectrum under 355 nm (3.5 eV) excitation after the carriers have relaxed to the lowest excited states, i.e. Δt = 15 ps. Noteworthy, the IA intensity is negative for the depicted probe wavelength range (ΔA < 0); a smaller fraction of the probe light is absorbed in combination with the excitation pump beam (excited sample) than without the pump beam (unexcited sample). This transient bleaching signal, which is characteristic for direct bandgap materials8, occurs since the electrons and holes in the conduction and valence bands, which are created upon excitation by the pump pulse, reduce the absorption of probe photons for the corresponding transitions (specific probe wavelengths) because of Pauli exclusion principle. It is sometimes also referred to as Pauli blocking or statefilling. Excited state absorption, also called free carrier absorption, which arises from intraband


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absorption (also referred to as intraexcitonic absorption for semiconductor quantum dots9) can also add to the measured IA signal of semiconductors with a positive amplitude (ΔA > 0). The absorption cross-section of excited state absorption decreases significant with increasing probe energy (Drude model with a (Eprobe)-2 dependence)10,11. The probability of this intraband absorption is therefore much lower than that of the interband absorption process (also called linear absorption) in the visible region where we are probing. The maximum of the bleach appears around ~515 nm (~2.41 eV), at a slightly shorter wavelength (higher energy) than the maximum of the PL spectrum (~522 nm, ~2.38 eV).

Figure 2. Induced absorption spectrum. Typical induced absorption spectrum under 355 nm (3.5 eV) excitation for a time delay of Δt = 15 ps.

State-filling With increasing excitation pump fluence, multiple excitations per NC will occur and biexcitons and multiexcitons will be generated. When we compare the normalized IA spectra for different excitation pump fluencies after initial relaxation to the band-edge states (Δt = 3 ps9) with each other (see Fig. 3(a)), we observe that the IA spectrum broadens and becomes asymmetric with


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increasing excitation pump fluence, while the spectra at long delay time (Δt = 500 ps) appear similar after normalization (see Fig. SI1). The relative larger contribution of the blue high energy (short wavelength) side of the IA spectrum indicates the presence of carriers in higher excited states and is commonly assigned to the state-filling effect, whereby higher excited states become populated due to complete filling of the lower excited states close to the band-edge, with limited degeneracy12. In NCs, the discrete energy levels result from quantum confinement restricting the number of electronic states that can be populated. The electrons rapidly relax to the bottom of the conduction band, which results in a bleach of its corresponding optical transition. With growing excitation pump fluence, the number of absorbed photons per NC rises and therefore the occupancy of the lowest energy levels at the band-edge increases, slowing down the relaxation of carriers from higher excited states to the lowest energy level close to the band-edge. This will lead to longer occupancy of the higher excited states and, consequently, to a bleach of their optical transitions. Thus, with increasing excitation pump fluence, more optical transitions are progressively bleached due to the increasing population of the lowest states, resulting in an enhanced carrier population temperature, which represents an average of both electron-and holecarrier distribution temperature. With increasing delay time, carriers have more time to relax to lower states and/or recombine, leading to a decreasing population of carriers in the higher excited states, while the lowest excited states are still occupied. Therefore, the additional contribution to the blue high energy (short wavelength) side of the IA spectrum becomes less prominent with increasing time delay between the pump and the probe pulse. A similar behavior has been observed before by Wu et al. for CsPbBr3 perovskite NCs13, but also for organicinorganic lead halide perovskites14 and silicon (Si) NCs15.


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Figure 3. Induced absorption spectra for several excitation pump fluencies showing photoexcited carrier distributions and their “cooling”. (a) Normalized induced absorption spectra for different excitation pump fluencies at a time delay of Δt = 3 ps under 355 nm (3.5 eV) excitation. Note that the actual amplitudes of the peaks are negative (see Fig. 2), corresponding to bleaching. (b) Estimated photoexcited carrier temperature as a function of the delay time between pump and probe for several excitation pump power fluencies obtained by fitting the short wavelength (high energy) side of the IA spectra between 495 and 507 nm (2.45-2.51 eV) to a Boltzmann distribution and

, where

is the Fermi energy,

is Boltzmann constant

is the carrier temperature.

Carrier temperature and cooling Since the IA spectra at Δt = 500 ps look similar for different excitation pump fluencies, the carrier temperature is expected to be close to room temperature, while at shorter time delays the carrier temperature is most likely enhanced (especially for the highest investigated excitation pump fluencies). By fitting the short wavelength (high energy) tail of the IA spectrum for each time delay between pump and probe to a Boltzmann distribution, we can estimate the


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photoexcited carrier temperature as a function of time. Figure 3(b) shows the estimated carrier temperature, illustrating that it can reach values up to ~800 K, depending on the excitation pump fluence. The carriers cool within a couple picoseconds to room temperature. These carrier cooling times are of the same order of magnitude as found by Price et al. for films of organicinorganic lead halide perovskites (CH3NH3PbI3), where time constants ranging from 230 fs to 770 fs, depending on the excitation pump fluence, have been determined14. This slow down of the carrier cooling is commonly assigned to the “hot-phonon bottleneck”14,16,17. As expected, the higher the excitation photon fluence, the longer it takes for the carriers to arrive to room temperature (see Fig. 3(b)). It is interesting to note that we do not observe any clear signature for carriers in higher excited states under low fluence excitation (the normalized IA spectra do not change as function of time), despite an excitation photon energy which is much larger than the bandgap and which will definitely create carriers in high excited states far above the band-edge. This strongly suggests that the effective cooling of the carriers (in high excited states) in this low excitation photon fluence regime is so fast that it escapes detection. For higher excitation fluencies, the number of absorbed photons per NC increases, slowing down the effective cooling of the carriers in high excited states towards the bottom of the conduction band, making them detectable in the IA measurement. This slow down of the relaxation of carriers in states above the band-edge is also visible in the IA dynamics, by means of a rise of the IA intensity in case of strong pumping (see Fig. 4). Specifically, the IA bleaching amplitude is proportional to the carrier population of a certain energy level. Right after excitation (Δt = 0 ps), mainly higher excited states will be occupied, while the band-edge states are initially not (fully) occupied. Therefore, the bleaching amplitude of corresponding band-edge transition will not have reached its maximum yet. Afterwards, the carriers will cool down to the band-edge states leading to the


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increased occupation of these levels at the time delay Δt after initial photon excitation. Therefore, we expect the IA intensity to reach its maximum at a later delay time, when the probe wavelength is increased (so when a lower energy level is probed) and/or when the excitation photon fluence is increased (due to the slowdown of the carrier cooling). As shown in Fig. 4, this is in agreement with our observations.

Figure 4. Rise time of the induced absorption dynamics. (a) Normalized induced absorption transient for three probe wavelengths zoomed-in to the first 10 ps. (b) Normalized induced absorption transient under low (blue) and high (magenta) excitation photon fluence zoomed-in to the first 10 ps for a probe wavelength of ~514 nm (~2.41 eV).

Induced absorption dynamics Multiexciton lifetime In order to determine the biexciton and possible higher order multiexciton recombination times of the sample, we have measured the IA transients under different excitation pump fluencies. A typical IA transient, where the IA intensity at a certain probe wavelength is plotted as function of delay time, is shown in Fig. 5. In general, the initial IA amplitude (Δt = 0 ps) is directly proportional to the number of generated free carriers upon photon excitation and increases


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therefore linearly with the incoming excitation photon fluence. Subsequently, the free carriers can undergo different relaxation and recombination processes, such as relaxation to the bottom of the conduction band and non-radiative Auger recombination. By studying the IA transients, the characteristic times of these processes can be determined. At a long time delay (e.g. Δt = 500 ps), all multicarrier interaction processes have been completed (Auger lifetime is typically 10-200 ps, depending on the NC size and the excitation photon fluence18) and the IA dynamics from this time delay onwards is therefore related to relaxation and recombination of singly-excited NCs. Consequently, the IA amplitude at long delay times shows signs of saturation with the excitation photon fluencies, indicating the excitation regime where all NCs have already absorbed at least a single photon. When normalizing the IA transients for different excitation powers at a long delay time value of Δt = 500 ps, we expect them to be identical in the low fluence regime where multiparticle processes (like Auger recombination) do not take place. This is exactly what we observe for the lowest excitation photon fluencies (not shown). In this fluence regime, the IA transients follow a relatively slow decay, with a time constant of a few nanoseconds, which largely exceeds our experimentally-probed 0.5 ns time window. The IA dynamics are similar, implying that Auger recombination does not take place, with each NC containing at most a single exciton. Thus, we know that this is the “less-than-one-exciton-per-NC” regime which will be important to extract the multiexciton lifetimes, as we will discuss later. Upon increasing excitation photon fluence, a rapidly decaying component becomes apparent (see Fig. 6(a)) which is a signature of a rapid Auger decay of multiple excitons localized in a single NC. These multiexcitons could be created by the excitation pump beam or by carrier multiplication 19. In this case, we exclude the latter (which is basically the inverse of an Auger process whereby two electron-hole pairs recombine to produce a single highly energetic electron-hole pair) since the


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excitation energy of 3.5 eV (355 nm) is not sufficient for carrier multiplication to take place (Eexc 274 μJ/cm2), gives us an estimation of the effective multiexciton lifetime, which spans from ~70 to ~35 ps for the highest investigated photon fluence (see Fig. 6(c) and Fig. SI2(b)). The observed enhancement of the multiexciton decay rate with increasing number of excitons per NCs is in agreement with the literature3,21. The biexciton lifetime determined here for ~8.6 nm sized CsPbBr3 perovskite NCs is faintly shorter than that found by Wang et al. for slightly larger sized CsPbBr3 NCs, namely ~105 ps for ~9 nm24. Faster biexciton lifetimes of ~44 ps are reported for ~7 nm CsPbBr3 NCs25. Very recently, Makarov et al. determined biexciton lifetimes of ~38 ps and ~47 ps for ~8.1 nm and ~9.3 nm CsPbBr3 NCs, respectively20. They have also investigated cesium-based perovskite NCs of CsPbI3 and determined the biexciton lifetime of ~92 ps for NCs of 11.2 nm, similar (~100 ps) as Park et al. for ~11 nm sized CsPbI3 NCs26. Nanocrystals made of the indirect bandgap material Si of ~5-6 nm have biexciton lifetimes of ~18-33 ps23; this is significantly shorter but it is important to note that the lifetime decreases rapidly for smaller NC3. In summary, most of the reported lifetime values for similarly sized all-inorganic cesium lead halide perovskite NCs are shorter than obtained by us, but also larger values can be found in literature for other materials, e.g. ~350 ps for ~8 nm sized CdSe quantum dots21. Usually NCs of different compositions show a similar biexciton lifetime dependence on the NC volume3, but Makarov et al. recently showed that all-inorganic cesium lead halide perovskite NCs deviate from that general trend; they observed a sublinear scaling with the NC volume and significantly shorter (by a factor ~5-10) biexciton lifetimes than in previous reports20. The biexciton lifetime of ~85 ps found in this study is about two times longer than that reported by Makarov et al. (~38-47 ps) for the same NC size


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and will therefore lay slightly closer to the values generally observed for other materials, like CdSe and PbSe. Differences in size determination, fabrication procedure and/or NC surface could possibly be responsible for these variations.

Figure 5. Induced absorption transient. Typical induced absorption transient at the maximum of the bleach under 355 nm (3.5 eV) excitation.

Figure 6. Fluence dependence of the normalized induced absorption transients under 355 nm (3.5 eV) excitation at the maximum of the bleach. (a) Excitation pump fluence dependence of the induced absorption dynamics normalized to their photobleach at long time delay (Δt = 500 ps). Note that the actual amplitudes are negative (see Fig. 5), corresponding to bleaching. (b) Biexciton lifetimes found by fitting the normalized IA dynamics obtained for the lowest


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excitation pump fluencies, after subtraction of the contribution of a singly-excited nanocrystal (see also Fig. SI2(a)), to a single exponential function. (c) Multiexciton lifetimes determined by fitting the IA dynamics obtained for the highest excitation pump fluencies, after subtraction of the contribution of a doubly-excited nanocrystal (see also Fig. SI2(b)), to a single exponential function.

Degeneracy From the fluence dependence of the IA, information about the degeneracy of the band-edge states can be extracted. Makarov et al. observed that no other lifetimes than that of a biexciton were visible in their fluence dependent PL and IA transients, indicating a degeneracy of the band-edge states of two20. In our study, we clearly observe also contributions from higher order multiexciton complexes: the IA transients obtained at high fluence pumping cannot be fitted to a single exponential function after subtraction of the contribution of single exciton recombination. Also, the determined multiexciton lifetimes decrease significantly with the excitation fluence (see Fig. 6(c)) as one would expect with increasing number of generated excitons per NC3. Remarkably, the initial IA amplitude is ~4 for the highest investigated excitation fluencies (see Fig. 6(a)). In other words, the carrier concentration is ~4 times higher directly after photon excitation than at 500 ps after excitation by the pump pulse. This strongly suggests that the degeneracy of the lowest band-edge states are higher than two.

Specific state-to-state transitions In contrast to the extended investigations of CdSe and PbSe semiconductor NCs, the field of allinorganic perovskite NCs is relatively young; theoretical as well as experimental knowledge is


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not as well developed. Also, the size distribution of the all-inorganic perovskite NC ensembles is by far not as monodisperse as for PbSe and CdSe NCs, being an obstacle for the investigations on specific state-to-state transition rates (of electrons as well as holes)27. Due to the broad size distribution, monitoring the dynamics with specificity in the initial and final excitonic states remains a key difficulty; as a result, a mixture of 1S, 1P or 2S is adressed9,28. Once the production of highly monodisperse samples and theoretical knowledge about this quite new class of all-inorganic perovskite NCs has developed, state-specific “resonant” IA spectroscopy will become available, possibly yielding much more detailed information.

Conclusions In conclusion, using ultrafast IA we have observed the formation of multiple exciton complexes, with multiplicity higher than two in CsPbBr3 perovskite NCs of ~8.6 nm. The Auger time constants of τ2~85 ps and τx>2~35-70 ps have been determined for biexciton and higher order exciton complexes, respectively. From that, a higher than two degeneracy of the ground state in these materials has been concluded. The relatively short bi- and multiexciton lifetimes are unfavorable for applications in light emitting devices and photovoltaic cells, indicating the need for ways to suppress Auger recombination. On the other hand, the higher degeneracy of the ground state is good news for possible laser applications.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge: Normalized IA spectra for several excitation pump fluencies for a delay time of Δt = 500 ps (Fig. SI1); Induced absorption dynamics under 355 nm excitation for different excitation pump fluencies (Fig. SI2).

AUTHOR INFORMATION Corresponding Author: *

Tel (+31) 205258443, e-mail [email protected]

Author Contributions EdJ and TG conceived the project and designed the experiments; LG synthesized the samples; GY and EdJ performed the IA measurements; EdJ analyzed the data; EdJ and TG interpreted the data and co-wrote the manuscript; YF and MA facilitated the IA measurements. All the authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs. The authors also thank Osaka University for the International Joint Research Promotion Program.

REFERENCES (1)

Kazim, S.; Nazeeruddin, M. K.; Grätzel, M.; Ahmad, S. Perovskite as Light Harvester: A Game Changer in Photovoltaics. Angew. Chemie - Int. Ed. 2014, 53, 2812–2824.

(2)

Kharchenko, V. A.; Rosen, M. Auger Relaxation Processes in Semiconductor Nanocrystals and Quantum Wells. J. Lumin. 1996, 70, 158–169.

(3)

Klimov, V. I. Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2007, 58, 635–673.

(4)

Timmerman, D.; Izeddin, I.; Gregorkiewicz, T. Saturation of Luminescence from Si Nanocrystals Embedded in SiO2. Phys. Status Solidi Appl. Mater. Sci. 2010, 207, 183–187.

(5)

de Jong, E. M. L. D.; Rutjes, H.; Valenta, J.; Trinh, M. T.; Poddubny, A. N.; Yassievich, I. N.; Capretti, A.; Gregorkiewicz, T. Thermally-Stimulated Exciton Emission in Si Nanocrystals. Rev.

(6)

Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang,


20

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692–3696. (7)

de Weerd, C.; Gomez Navascues, L.; Zhang, H.; Buma, W. J.; Nedelcu, G.; Kovalenko, M. V.; Gregorkiewicz, T. Energy Transfer Between Inorganic Perovskite Nanocrystals. J. Phys. Chem. C 2016, 120, 13310–13315.

(8)

Guyot-Sionnest, P.; Shim, M.; Matranga, C.; Hines, M. Intraband Relaxation in CdSe Quantum Dots. Phys. Rev. B 1999, 60, 2181–2184.

(9)

Kambhampati, P. Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale. J. Phys. Chem. C 2011, 115, 22089–22109.

(10)

Smith, R. A. Semiconductors; Cambridge University Press: New York, 1959.

(11)

De Boer, W. D. A. M.; De Jong, E. M. L. D.; Timmerman, D.; Gregorkiewicz, T.; Zhang, H.; Buma, W. J.; Poddubny, A. N.; Prokofiev, A. A.; Yassievich, I. N. Carrier Dynamics in Si Nanocrystals in an SiO2 Matrix Investigated by Transient Light Absorption. Phys. Rev. B Condens. Matter Mater. Phys. 2013, 88, 155304.

(12)

Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers. Nano Lett. 2006, 6, 424–429.

(13)

Wu, K.; Liang, G.; Shang, Q.; Ren, Y.; Kong, D.; Lian, T. Ultrafast Interfacial Electron and Hole Transfer from CsPbBr3 Perovskite Quantum Dots. J. Am. Chem. Soc. 2015, 137,


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

12792–12795. (14)

Price, M. B.; Butkus, J.; Jellicoe, T. C.; Sadhanala, A.; Briane, A.; Halpert, J. E.; Broch, K.; Hodgkiss, J. M.; Friend, R. H.; Deschler, F. Hot-Carrier Cooling and Photoinduced Refractive Index Changes in Organic–inorganic Lead Halide Perovskites. Nat. Commun. 2015, 6, 8420.

(15)

Zhang, P.; Feng, Y.; Wen, X.; Cao, W.; Anthony, R.; Kortshagen, U.; Conibeer, G.; Huang, S. Generation of Hot Carrier Population in Colloidal Silicon Quantum Dots for HighEfficiency Photovoltaics. Sol. Energy Mater. Sol. Cells 2016, 145, 391–396.

(16)

Rosenwaks, Y.; Hanna, M. C.; Levi, D. H.; Szmyd, D. M.; Ahrenkiel, R. K.; Nozik, A. J. HotCarrier Cooling in GaAs: Quantum Wells versus Bulk. Phys. Rev. B 1993, 48, 14675–14678.

(17)

Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Colloidal Silicon Nanocrystals. Nano Lett. 2007, 7, 2506–2512.

(18)

Robel, I.; Gresback, R.; Kortshagen, U.; Schaller, R. D.; Klimov, V. I. Universal SizeDependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett. 2009, 102, 177404.

(19)

Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601.

(20)

Makarov, N. S.; Guo, S.; Isaienko, O.; Liu, W.; Robel, I.; Klimov, V. I. Spectral and Dynamical Properties of Single Excitons, Biexcitons, and Trions in Cesium-Lead-Halide


22

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Perovskite Quantum Dots. Nano Lett. 2016, 16, 2349–2362. (21)

Klimov, V. I.; Mikhailovsky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots. Science 2000, 287, 1011–1013.

(22)

Barzykin, A. V; Tachiya, M. Stochastic Models of Charge Carrier Dynamics in Semiconducting Nanosystems. J. Phys. Condens. Matter 2007, 19, 65105.

(23)

Trinh, M. T.; Limpens, R.; Gregorkiewicz, T. Experimental Investigations and Modeling of Auger Recombination in Silicon Nanocrystals. J. Phys. Chem. C 2013, 117, 5963–5968.

(24)

Wang, Y.; Li, X.; Song, J.; Xiao, L.; Zeng, H.; Sun, H. All-Inorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics. Adv. Mater. 2015, 27, 7101–7108.

(25)

Wu, K.; Liang, G.; Shang, Q.; Ren, Y.; Kong, D.; Lian, T. Ultrafast Interfacial Electron and Hole Transfer from CsPbBr3 Perovskite Quantum Dots. J. Am. Chem. Soc. 2015, 137, 12792-12795.

(26)

Park, Y.-S.; Guo, S.; Makarov, N. S.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386–10393.

(27)

Klimov, V. I. Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals. J. Phys. Chem. B 2000, 104, 6112–6123.

(28)

Kambhampati, P. Unraveling the Structure and Dynamics of Excitons in Semiconductor


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Quantum Dots. Acc. Chem. Res. 2011, 44, 1–13.


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Multiexciton Lifetime in All-Inorganic CsPbBr3 Perovskite Nanocrystals

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