Delayed Exciton Formation Involving Energetically Shallow Trap

Dec 5, 2017 - We report the occurrence of delayed exciton formation in highly emissive CsPbBr3 quantum dots, which results in anomalously slow buildup...
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Article Cite This: J. Phys. Chem. C 2017, 121, 28498−28505

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Delayed Exciton Formation Involving Energetically Shallow Trap States in Colloidal CsPbBr3 Quantum Dots Yi Wang,† Min Zhi,† and Yinthai Chan*,†,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Institute of Materials Research & Engineering, A*STAR, 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore

‡

S Supporting Information *

ABSTRACT: We report the occurrence of delayed exciton formation in highly emissive CsPbBr3 quantum dots, which results in anomalously slow buildup kinetics that are readily observed in a time-resolved photoluminescence trace. It is inferred from the dependence of the buildup kinetics on nanoparticle size, temperature, and excitation fluence that the delayed exciton formation originates from multiple carrier trapping and detrapping events between the band edge state and energetically shallow structural disorder states. A kinetic model that incorporates these carrier pathways produces fits that are in excellent agreement with the time-resolved data. Importantly, the kinetic model allows for the determination of photoluminescence quantum yield values that closely match those obtained from integrating sphere measurements, whereas merely accounting for the pathways associated with the decay kinetics produces large discrepancies. This work highlights the crucial role played by delayed exciton formation in the photoluminescence dynamics of CsPbBr3 QDs.

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INTRODUCTION

their use as the active material in low-threshold optically pumped lasers.13−17 It has been suggested that the high tolerance to defects and energetically shallow trap states are responsible for the outstanding PLQYs of CsPbX3 QDs.18−20 However, it is also well-known that the presence of trap states, notwithstanding their energy levels, is generally detrimental to PL in perovskite nanocrystals.21,22 If a significant density of shallow trap states is indeed present, it is intriguing how and why the PLQY of colloidal CsPbX3 QDs remains high. Indeed, much of the kinetics of exciton photoluminescence in these QDs is at the present time not well understood. For example, time-resolved PL (TRPL) measurements, which reflect the carrier recombination dynamics of an emissive material following optical excitation by a very short pulse of light, typically feature an extremely fast buildup of intensity followed by much slower decay. In the case of CsPbX3 QDs, however, TRPL experiments reveal an abnormally slow buildup process that was previously observed but not investigated in detail.23−25 Intuitively, the slow buildup in the TRPL profile of a semiconductor can be ascribed to the delayed formation of excitons. For TRPL experiments in which a dilute solution of colloidal CsPbX3 QDs is used, the delay in exciton formation may be due to carrier diffusion and/or trapping and detrapping events. Figure 1 provides a cartoon illustration of the TRPL

All-inorganic perovskite (namely CsPbX3, where X = Cl, Br, I) based quantum dots (QDs) are promising as chromophores because of their spectrally narrow emission profiles (∼100 meV) relative to conventional organic dyes and fluorescence wavelengths that can readily be tuned across the entire visible range (i.e., ∼400−700 nm) by varying their size and composition, thus providing the spectral bandwidth required by most applications. What is perhaps most striking is the fact that the as-synthesized QDs exhibit room temperature photoluminescence quantum yields (PLQYs) as high as 90% without the need for surface passivation by an epitaxial wide bandgap shell layer that is generally imperative for highly emissive II−VI, III−V, or IV−VI semiconductor QDs.1 In addition to the narrow emission width, these perovskite QDs possess a sharp absorption tail, which minimizes the occurrence of interparticle energy transfer and photon reabsorption within an ensemble.2 Furthermore, it has recently been reported that the CsPbX3 nanoparticles exhibit negligible spectral diffusion and minimal inhomogeneous broadening,3 facilitating high color purity in displays. Owing to these highly desirable optical properties, a rapidly growing number of efforts to develop these QDs have been made, such as optimizing their synthetic preparation via droplet microfluidics,4 tuning their optical properties via anion exchange reactions,5 and investigating the surface chemistry of the particles.6−8 To-date, the incorporation of CsPbX3 QDs in light-emitting diodes with high luminance and good color saturation has been reported,9−12 alongside © 2017 American Chemical Society

Received: September 11, 2017 Revised: December 5, 2017 Published: December 5, 2017 28498

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

Article

The Journal of Physical Chemistry C

in the supernatant was recovered by precipitating them using 1 mL of 2-propanol and centrifuging for 5 min at 5000 rpm. The recovered QDs were then dispersed in toluene for further use. The CsPbBr3 nanocrystals with the smallest size (5.7 nm edge length QDs) were obtained by following the same procedure as above with the exception that 0.39 mL of Cs-oleate was injected at 165 °C and the reaction was allowed to continue for ∼10 s before cooling down to room temperature. Five milliliters of hexane was added to the reaction flask, and the QDs were precipitated by adding 1 mL of 2-propanol. The mixture was then centrifuged for 5 min at 5000 rpm and the supernatant discarded. The processed QDs were then dispersed in toluene for further use. Transmission Electron Microscopy and X-ray Powder Diffraction. Transmission electron microscopy (TEM) images of CsPbBr3 nanocrystals were obtained using a JEOL JEM1220F microscope with a 100 kV accelerating voltage. For TEM sample preparation, a dilute drop of CsPbBr3 QDs was placed onto a 300 mesh copper grid. Excess solution was removed by an adsorbent paper, and the sample was dried under ambient conditions. X-ray diffraction (XRD) data were obtained with a diffractometer (Bruker AXS, GADDS) using Cu−K α radiation (λ = 1.540 598 Å) in the range of 10−50°. Thin film samples were prepared by drop-casting 20 μL of a concentrated CsPbBr3 QD solution onto a clean silicon wafer and drying in a glovebox under N2. This was repeated several times until a thin film of CsPbBr3 QDs was formed on the silicon substrate. Steady-State Spectroscopy. The steady-state UV−vis absorption spectra for colloidal CsPbBr3 QDs were recorded on an Agilent-8453 (Agilent Technologies) absorption spectrometer using a quartz cuvette with a path length of 1 cm. The same sample and cuvette were also used to measure the steadystate photoluminescence (PL) spectra and PL quantum yield by a Fluorolog-3 spectrofluorometer (Horiba) equipped with a Quanta-φ integrating sphere (Horiba). To minimize the effect of reabsorption on emission spectra measurements, the nanoparticle solution was diluted to ensure the absorbance at the excitation wavelength (∼405 nm) was less than 5 × 10−2. Time-Resolved Photoluminescence Measurement. The colloidal CsPbBr3 solution in a 2 mm thick quartz cuvette was excited by 100 fs pulses at 400 nm, which is the second harmonic of the 800 nm fundamental output from a 1 kHz repetition rate regenerative Ti:sapphire amplifier (Coherent, Libra-F-1K-HE-230). The excitation beam was focused by a cylindrical lens (focal length = 20 cm) into a stripe with a size of ∼0.8 × 0.04 cm2, and the transient PL signal was recorded by a time-correlated single photon counting (TCSPC) module (PicoQuant, PicoHarp 300). The same excitation source was also used to measure the fluence-dependent PL spectra, and the PL signals were recorded by a triple grating imaging spectrometer (Princeton Instruments, Acton SpectraPro SP2500). The excitation fluence was modulated by a series of neutral-density filters.

Figure 1. Schematic illustration of TRPL decay profiles with (blue line) or without (red line) a slow buildup component, which may be due to (1) radiative exciton recombination (red arrow); (2) charge diffusion followed by radiative exciton recombination (middle blue arrow); (3) carrier detrapping followed by radiative exciton recombination (right blue arrow).

profiles expected for a colloidal QD in the presence and absence of delayed exciton generation. Such markedly slow buildup kinetics (on the order of 100 ps) have previously been observed in organolead halide perovskite nanowires and nanoplates and was attributed to prolonged carrier diffusion prior to exciton formation.26 For CsPbX3 QDs, small exciton binding energies (e.g., ∼40 meV for a ∼7 nm CsPbBr3 QD1) are expected to lead to significant charge separation at room temperature. This can lead to a diffusion-limited formation of excitons, albeit at much faster time scales due to the small particle volumes of the QDs.27 Another possible reason for delayed exciton formation is the occurrence of multiple carrier trapping and detrapping events, as has been widely observed in bulk perovskites28,29 and hybrid organic−inorganic perovskite nanocrystals.30 In this work, we investigate and seek to clarify the primary mechanism for delayed exciton generation in colloidal CsPbBr3 QDs and argue that it is likely due to multiple carrier trapping and detrapping events from energetically shallow trap states. Our experimental findings suggest that the trap states arise from structural disorder rather than surface defects. Importantly, we demonstrate that taking the buildup time into account is critical for accurately determining the PLQYs of these QDs based on TRPL measurements, which is generally not necessary for prototypical semiconductor QDs such as CdSe nanocrystals.

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EXPERIMENTAL SECTION Synthesis of CsPbBr3 QDs. The synthesis of CsPbBr3 quantum dots in this work is based on a previously reported procedure1 with slight modifications. Typically, in a three-neck round-bottom flask, 136 mg of lead bromide, 2 mL of oleic acid, 2 mL of oleylamine, and 10 mL of 1-octadecene (ODE) were degassed under vacuum at 80 °C for 1 h. The temperature was then increased to 185 °C under N2, whereupon 0.78 mL of Cs-oleate (0.13 M in ODE) was swiftly injected into the mixture. This initiated the nucleation and growth of the perovskite nanocrystals. After 5 min, the reaction mixture was cooled down to room temperature, and 5 mL of hexane was added to the reaction flask. This yields a bimodal distribution of particle sizes. To recover the larger of the two sizes, 2 mL of reaction solution was centrifuged for 5 min at 5000 rpm, and the precipitate (13.0 nm edge length QDs) was dispersed in toluene. The smaller of the two sizes (9.2 nm edge length QDs)

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RESULTS AND DISCUSSION To investigate the origins of the slow buildup time in the TRPL profile, we first synthesized colloidal CsPbBr3 QDs based on a previously reported method1 (see details in the Experimental Section). The CsPbBr3 QDs were then dispersed in toluene for further characterization. Analysis by TEM, as presented in Figure 2a, revealed cubelike nanoparticles with an average edge length of about 13.0 ± 1.1 nm (Figure 2b). This length is 28499

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

Article

The Journal of Physical Chemistry C

distribution of shallow intragap states near the band edge.32,33 The room temperature TRPL profile of the CsPbBr3 QDs is given in Figure 2g, where the PL decay kinetics are fitted by a multiexponential function yielding an average decay lifetime (⟨τ⟩) of ∼3.3 ns. The multiexponential decay kinetics in CsPbBr3 QDs has commonly been observed1,34,35 and has been suggested to arise from imperfect surface passivation.36 Zooming into the first 500 ps of the TRPL trace (inset of Figure 2g), a steep rise within the first 100 ps followed by a gentler one that extends to 500 ps may be observed. The fwhm of the instrument response function (IRF) is ∼50 ps; therefore, the more gradual rise component, which we will refer to as the slow buildup, is within the temporal resolution of the detector and cannot be dismissed as a measurement artifact. Given that the slow buildup time is premised on the facile thermal dissociation of excitons under ambient conditions, low temperature should disfavor exciton dissociation, thereby resulting in a faster buildup time. We investigated the temperature dependence of the TRPL buildup profiles at two distinct temperatures as depicted in Figure 3a. To avoid

Figure 2. (a) TEM image of typical CsPbBr3 QDs. The inset is a photograph of a toluene solution of QDs under UV lamp excitation (λ = 365 nm). (b) The size histogram counted from the TEM image. (c) XRD spectrum of CsPbBr3 QDs (orange) and the reference standard for cubic CsPbBr3 (green). (d) The UV−vis absorption (solid black line) and PL emission (dashed red line) spectra of CsPbBr3 QDs in toluene. (e) Semilog plot of band edge absorption spectrum. (f) Tauc plot of CsPbBr3 QDs. (g) Representative TRPL profile in CsPbBr3 QDs. Inset: buildup profile at short delay times. The orange, blue, and green lines are the experimental data, the fit to the decay component, and the instrument response function (IRF) respectively.

approximately 2 times of the Bohr exciton diameter (∼7 nm), which suggests that quantum confinement effects should be relatively weak. Structural characterization by XRD, as illustrated in Figure 2c, shows that the synthesized nanocubes are cubic which is consistent with previous reports on CsPbBr3 QDs.1 Figure 2d shows the UV−vis absorption and PL emission spectra of the 13.0 nm edge length CsPbBr3 QDs dispersed in toluene. The sample features a small Stokes shift (∼20 meV, see Figure S1 of the Supporting Information), as is characteristic of most direct band gap semiconductors; the excitonic absorption peak is nearly featureless, which is indicative of weak quantum confinement. Concomitantly, the PL peak is symmetric with a relatively narrow full width at half-maximum (fwhm) of ∼80 meV, which is typical of large (>10 nm edge length) CsPbBr3 particles. A semilog plot of the absorption spectra, as illustrated in Figure 2e, shows an exponential relation between absorbance and photon energy below the band gap, which can be assigned to an Urbach tail.31 Analysis of the semilog plots yielded Urbach energy (EUr) and band gap energy (Eg) values of 17.4 meV and 2.40 eV, respectively. The value of Eg could be further verified by the Tauc plot shown in Figure 2f. The small value of EUr, which is lower than the thermal energy kBTR of ∼25.7 meV at room temperature, suggests a small degree of structural disorder and a narrow

Figure 3. (a) TRPL measurements of a sample of 13.0 nm edge length CsPbBr3 QDs at 180 and 280 K. Inset shows the TRPL data for a longer time window. (b) The TRPL trace recorded at different detection wavelengths in 13.0 nm edge length CsPbBr3 QDs. (c) The TRPL kinetics of CsPbBr3 QDs with average edge lengths of 13.0, 9.2 and 5.7 nm. (d) The TRPL kinetics for a 13.0 nm edge length CsPbBr3 QD sample at an average exciton occupancy ⟨N⟩ of 0.02, 0.51, and 0.72. Note that for all the figures, the open circles/triangles are experimental data while the dashed curves are a guide to the eye.

freezing of the solvent at low temperature, we employed a thin random-packed film of particles for the TRPL measurements. We found only slight differences in the TRPL kinetics between the thin film and a solution of particles at room temperature (see Figure S2), which gave confidence to our thin film measurements. As seen from Figure 3a, the buildup time at lower temperature is substantially reduced, consistent with the notion of a suppressed thermal dissociation of excitons. It is also observed that the decay component is faster (inset of Figure 3a), which cannot be attributed to the opening of a fast nonradiative channel since the quantum yield of the film did 28500

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

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The Journal of Physical Chemistry C

Figure 4. (a) Proposed schematic of dominant PL kinetic pathways following photoexcitation in semiconductor nanocrystals. Photogenerated excitons generally recombine radiatively or can be captured by surface states and undergo nonradiative relaxation to the ground state (red arrows). Where the exciton binding energy is small, thermal dissociation into charge carriers can occur. The carriers can either be trapped by surface states or structural disorder states or reform into excitons (blue arrows). (b) Fitting of TRPL data based on PL kinetics that include (blue curve) and exclude (red curve) pathways associated with carriers in the band edge state. The common fitting parameters in the two cases are tIRF = 50 ps and τPL = 3.3 ns, where tIRF is the fwhm of the IRF and τPL is the decay lifetime. The value of τrise, i.e., the time constant of the buildup kinetics, was set to be the IRF value for the red curve and 150 ps for the blue curve.

elucidate the effect of particle size on the buildup time, CsPbBr3 QDs with different average edge lengths of 13.0 ± 1.1, 9.2 ± 0.6, and 5.7 ± 0.7 nm were synthesized (see Figure S3). Figure 3c shows the TRPL kinetics for the as-synthesized QDs, where the average exciton number per particle (⟨N⟩) is kept the same across the different samples by tuning the pump fluence. Details on the determination of ⟨N⟩ are given in the Supporting Information (Figure S4). It is interesting to see from Figure 3c that the buildup time remains comparable across all three samples. While the PLQYs of all three samples may be different (its effects on buildup time are discussed later), the data strongly indicate that the rate-determining process in the slow buildup cannot be attributed solely to the delayed exciton formation from free carriers. We also investigated the interplay between the slow buildup of PL and multiexcitonic Auger recombination, which is an extremely fast nonradiative recombination pathway that can dominate the early TRPL profile at sufficiently high pump fluence. We used 13.0 nm edge length QD samples and optically excited them at 400 nm by 100 fs pulses from a regeneratively amplified Ti:Sapph with a repetition rate of 1 kHz. As seen from Figure 3d, the slow buildup becomes less pronounced with the increase of pump fluence, with ⟨N⟩ varying from 0.02 to 0.72. This is surprising, given that the 13.0 nm edge length CsPbBr3 QD is expected to exhibit weak quantum confinement and its TRPL profile should not be heavily influenced by Auger recombination effects. We find, however, that this is not the case. At a pump fluence corresponding to ⟨N⟩ = 0.72, the proportion of biexcitons in the 13 nm edge length QDs is estimated to be ∼13% based on Poisson distribution. Given a fast Auger dominated biexciton lifetime of ∼340 ps as determined from a known subtraction procedure,34 it is conceivable that this relatively low proportion of biexcitons (as well as higher order multiexcitons) is sufficient to mask the slow buildup profile. It is therefore important to employ pump fluences that correspond to ⟨N⟩ ≪ 0.72 to reliably observe and characterize the slow buildup profile in such QD systems. It has been widely suggested that due to energetically shallow intragap trap states (EUr ∼ 15 meV32,33,40,41), the dynamics of

not appear to decrease at lower temperature. A more plausible explanation is the reduced dissociation of excitons and a concurrent increase in the radiative rate at low temperature.37 However, while the reduced buildup time at low temperature can be explained by a lowered rate of thermally induced exciton dissociation, why the buildup time is anomalously slow at room temperature remains unanswered. Although the buildup time in semiconductor nanocrystals is often attributed to hot-exciton relaxation and at times interparticle energy transfer, neither of them can be used to explicate the slow buildup observed in Figure 2g. The time constant of hot-exciton relaxation in CsPbBr3 QD (typically less than 5 ps38,39) is much shorter than that of the slow buildup (>100 ps); the effect of interparticle energy transfer (e.g., from smaller to larger particles) should be negligible in a dilute solution of particles, which can be verified by the fact that their PL lifetimes are independent of the detection wavelength. This is seen in the TRPL spectrum in Figure 3b, where the decay lifetimes detected at 500, 517, and 530 nm are 3.5 ± 0.2, 3.6 ± 0.1, and 3.8 ± 0.1 ns, respectively. In view of their large exciton dissociation rates and long carrier diffusion lengths,27 one possible explanation for the slow buildup time in CsPbBr3 QDs is delayed exciton formation by prolonged carrier diffusion. This supposes that the exciton is formed after carrier photogeneration and diffusion within the nanoparticle. However, although a slow buildup TRPL profile induced by carrier diffusion has been observed in micron-sized organolead halide perovskite nanowires and nanoplates,26 the time scale associated with carrier diffusion in our CsPbBr3 QDs is estimated to be within the IRF of our TRPL setup given their small size (∼13.0 nm edge length) and high carrier mobility (∼4500 cm2 V−1 s−1).27 Subsequently, carrier diffusion cannot be the basis for the observed slow buildup. It is also possible that delayed exciton formation results from the transition between free carriers and excitons. In such a scenario, different particle sizes spanning various levels of quantum confinement should yield dissimilar buildup profiles since the energy difference between the band edge state (where free carriers reside) and the first excitonic state, i.e., the exciton binding energy, will be distinct for each particle size. To 28501

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

Article

The Journal of Physical Chemistry C

Figure 5. (a) UV−vis absorption (a) and PL emission (b) spectra of CsPbBr3 QDs in toluene with different post-treatments. The inset of panel (a) is the semilog plot of band edge absorption spectrum to show the Urbach tail of the tested samples.

relations, an expression for nex(t), which is proportional to the PL intensity in the TRPL spectrum, may be obtained (see Supporting Information for details). Figure 4b shows a fit to the TRPL data at early times based on the model described in Figure 4a (see Table S1 for details). It is readily seen that the fit is in excellent agreement with the data, which lends confidence to the MT model. For comparison, a fit based on the same model but without the participation of the band edge state is also shown in the same plot. A large discrepancy with the experimental data that is prominently reflected in the residual plot in the lower panel of Figure 4b highlights the need to consider the dynamics of free carriers in understanding the TRPL profile of CsPbBr3 QDs. It should be mentioned that in the case of semiconductor QDs such as CdSe with strong exciton binding energies on the order of hundreds of meV,43 there is little need to consider thermal dissociation of the exciton into free carriers at room temperature. Case in point: the TRPL curves for 3.5 nm CdSe QDs (binding energy ∼300 meV43) can be fitted well without the need to consider the kinetics of free carriers in the band edge state (see Figure S7). To further establish the validity of the model described in Figure 4a, we use it to determine the PLQYs of CsPbBr3 QDs. The PLQY is a manifestation of the various radiative and nonradiative processes that occur in the QDs following excitation and gives a good indication of the exactitude of the dominant PL kinetics suggested by our proposed model. Crucially, its accuracy can be independently verified by integrating sphere measurements that directly give the absolute quantum yield of a sample. To carry out the PLQY measurements, we employed CsPbBr3 QDs with an absolute QY of ∼58% and separately subjected them to different chemical treatments. One of the samples was precipitated and washed in acetone before redispersion in toluene, causing its PLQY to be reduced to 21% due to the partial loss of surface ligands. Another sample was treated with ammonium thiocyanate (NH4SCN), which has recently been reported to significantly increase the PLQYs of CsPbBr3 QDs by removing excess Pb2+.36 Subsequently, the NH4SCN-treated QDs exhibited an absolute QY of 92%. The UV−vis absorption and PL emission spectra of these three different CsPbBr3 QD samples are given in Figures 5a and 5b, respectively. The invariance of the absorption profile and emission position on the two QD samples is indicative that the size and composition of the nanoparticles were unchanged by the postsynthetic treatments, which suggests that the intrinsic radiative lifetime of the different samples should be the same. It is noteworthy that

carrier recombination in bulk perovskites often follow the socalled multiple-trapping (MT) model.28,29 In the MT model, charge carriers presumably shuttle rapidly between energetically shallow traps and the band edge state via frequent trapping and detrapping events, resulting in delayed exciton generation.42 In the case of CsPbBr3 QDs, the fairly small EUr (∼16−18 meV for all 3 particle sizes, see Supporting Information Figure S5) suggests that the carriers in these shallow intragap states corresponding to the Urbach tail can be thermally detrapped at room temperature (kBTR ∼ 25.7 meV). After considering the different plausible PL kinetic pathways that can give rise to slow buildup in the TRPL profile (see Figure S6 for details), we put forward a phenomenological kinetic model that is consistent with all the prior experimental observations made, as illustrated in Figure 4a. In this model, in addition to the lowest energy excitonic and surface states, the band edge state is taken into account given the small exciton binding energy of perovskite nanoparticles and exciton dissociation into carriers at room temperature.1,27 Further, we propose the existence of energetically shallow trap states that are associated with the Urbach tail in the absorption spectra as mentioned earlier. The Urbach tail is often associated with structural disorder,32,33 and we therefore refer to these shallow trap states as “structural disorder states”. Photogenerated charge carriers in the band edge state may either populate the structural disorder states or form excitons. Unlike surface states, the structural disorder states are even more energetically shallow, and carriers trapped in these states can be thermally detrapped to the band edge state. The trapping and detrapping of carriers to and from the structural disorder states result in delayed exciton formation and account for the slow buildup TRPL profile. As with most excitonic chromophores and with no loss in generality, the ensuing PL decay kinetics may be attributed to excitons that either radiatively recombine or relax to the ground state nonradiatively via surface states. By defining the carrier and exciton density as nc and nex, respectively, their temporal evolution may be expressed as dnc(t ) d n (t ) = −(kex + k t)nc(t ), ex dt dt = kexnc(t ) − (k r + k nr)nex (t )

where kex is the rate constant of the transition from the band edge state to the first excitonic state, kt is the rate constant of the transition from the band edge state to the surface trap states, and kr and knr are the rate constants of radiative exciton recombination and nonradiative relaxation from surface trap states to the ground state, respectively. From the above 28502

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

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across all three samples, it may be seen that the amplitude of the more gradual buildup time increases with sample PLQY. Within our model, this trend may be understood as follows: given that carrier trapping due to surface defects and structural disorder are competing nonradiative pathways, better surface passivation suppresses carrier trapping to surface states which in turn increases the yield of delayed exciton formation (Figure 4a). By taking the kinetic pathways of delayed exciton generation into account, we recalculate the PLQYs from the TRPL data (see Supporting Information for details, Figure S10 and Table S2) and obtain PLQY values of 26% and 91% in the acetone-washed and the NH4SCN-treated sample, respectively. These recalculated values are in close agreement with the absolute PLQY values obtained by integrating sphere, providing further support for our proposed picture of the PL dynamics in colloidal CsPbBr3 QDs.

the Urbach energy from the absorption spectra of the treated samples did not change (EUr = 17 ± 1 meV, inset of Figure 5a), which is in line with our proposition that the structural disorder states are distinct from surface states that arise from imperfect surface passivation. The TRPL profiles in the as-synthesized and differently treated CsPbBr3 QD samples are shown in Figure 6a. From the

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CONCLUSION In summary, the abnormally slow buildup of TRPL in CsPbBr3 QDs is ascribed to the delayed formation of excitons that results from carrier trapping and detrapping events between the band edge state and energetically shallow structural disorder states. This conclusion was reached by considering the dependence of the buildup time on nanoparticle size, temperature, and excitation fluence. A phenomenological model that takes into account the kinetic pathways of carriers in these states is proposed and produces fits that are in excellent agreement with the experimental TRPL spectra. The validity of the model is further supported by its determination of the PLQY from TRPL data that is close to the values obtained by integrating sphere measurements. This study indicates that the PL kinetics in all-inorganic perovskite QDs can differ significantly from the well-studied cadmium chalcogenide QDs and should be carefully considered when utilizing them for light-emitting applications.

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Figure 6. (a) TRPL kinetics of CsPbBr3 QDs with different chemical post-treatments, where the open circles are experimental data and the solid curves are fitting results based on the MT model. The PLQYs and the average decay lifetimes are included in the different panels. (b) TRPL profiles at short delay times corresponding to the samples in (a), where the IRF-limited, fast buildup component has been normalized.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09040. Stokes-shift determination; TRPL comparison between CsPbBr3-based film and solution; TEM characterization and steady-state spectra, determination of the absorption cross section, semilog absorption and Tauc plots for CsPbBr3 with different nanoparticle sizes; modeling the PL kinetics pathways in CsPbBr3 QDs; rate expression of the TRPL kinetics; comparison of the buildup kinetics between CsPbBr3 and CdSe QDs; steady-state spectra, TRPL, and PLQY calculation of CdSe QDs; calculation of PLQY from TRPL in CsPbBr3 QDs (PDF)

fits to the TRPL spectra, the average PL lifetime ⟨τ⟩ for the untreated, acetone-washed, and NH4SCN-treated QDs are 3.3 ± 0.1, 2.1 ± 0.1, and 4.3 ± 0.1 ns, respectively. Given the often used relation that PLQY = ⟨τ⟩/τr, where τr is the radiative lifetime,34 one may infer that relative to the untreated sample, the PLQY for the acetone-washed and NH4SCN-treated samples are 36% and 75%, respectively. These two values are derived from considering the competition between radiative and nonradiative (i.e., relaxation via surface states) recombination processes involving the exciton, which is essentially captured by the PL decay kinetics. While the determination of PLQY from the expression ⟨τ⟩/τr works well for CdSe QDs (see Figures S8 and S9), the large disagreement between TRPL-derived and integrating sphere obtained values of PLQYs in CsPbBr3 QDs implies that the buildup kinetics need to be accounted for as well. This is evident in Figure 6b, which shows very significant changes in the buildup times across the three samples. After normalizing the initial IRF limited rise-time

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yinthai Chan: 0000-0002-8471-9009 Notes

The authors declare no competing financial interest. 28503

DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505

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ACKNOWLEDGMENTS The authors acknowledge funding support from a A*STAR Science & Engineering Research Council Public Sector Funding (Project no. 142100076).

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DOI: 10.1021/acs.jpcc.7b09040 J. Phys. Chem. C 2017, 121, 28498−28505