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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 3

Ionization and Neutralization Dynamics of CsPbBr Perovskite Nanocrystals Revealed by Double-Pump Transient Absorption Spectroscopy Satoshi Nakahara, Keiichi Ohara, Hirokazu Tahara, Go Yumoto, Tokuhisa Kawawaki, Masaki Saruyama, Ryota Sato, Toshiharu Teranishi, and Yoshihiko Kanemitsu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01554 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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The Journal of Physical Chemistry Letters

Ionization and Neutralization Dynamics of CsPbBr3 Perovskite Nanocrystals Revealed by Double-Pump Transient Absorption Spectroscopy Satoshi Nakahara, Keiichi Ohara, Hirokazu Tahara, Go Yumoto, Tokuhisa Kawawaki, Masaki Saruyama, Ryota Sato, Toshiharu Teranishi, and Yoshihiko Kanemitsu*

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

AUTHOR INFORMATION

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Ionization of nanocrystals (NCs) causes both photoluminescence intermittency and a reduction in luminescence quantum efficiency, and thus plays a critical role in the optoelectronic performance of NC-based devices. Here, we study the ionization and neutralization processes of CsPbBr3 perovskite NCs under strong photoexcitation by means of double-pump transient absorption spectroscopy. A strong initial pulse is used to generate ionized NCs, and their optical responses are investigated by varying excitation intensity and delay time of the second pump pulse. We find that charging can occur either via nonradiative Auger recombination of biexcitons or via any possible recombination of trions. The presence of the extra charge inside an ionized perovskite NCs significantly reduces its absorption cross section. The experiments reveal that ionized NCs exhibit two types of neutralization processes with time constants on the order of nanoseconds and microseconds. These results are useful for the optimal design of NC-based photonic devices.

TOC GRAPHICS


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Metal halide perovskites are a class of semiconductor materials with excellent optical properties and are anticipated for use in various photonic devices.1–5 They exhibit highly efficient luminescence due to a band structure with a direct gap, and thus they have attracted much attention as light-emitting device materials.6–10 In particular, the perovskite nanocrystals (NCs) are feasible candidates, because they exhibit high luminescence efficiencies in the whole visible spectral region even at room temperature and their emission wavelengths can be easily controlled by changing the halogen composition and/or the NC size.11–13 Furthermore, compared to the optoelectronic properties of conventional NCs such as CdSe and PbSe NCs, the optoelectronic properties of perovskite NCs are less sensitive to surface states.13 However, even for such perovskite NCs, it is found that charge trap states are formed on the NC surface of a certain fraction of the NC ensemble. These traps affect the optical response of NCs,14,15 and consequently, luminescence efficiencies of NCs can be significantly improved by chemical treatment of the surface.16–18 A deep understanding of the impact of surface trap states on exciton dynamics in perovskite NCs would be beneficial for NC design and photonic device applications.


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A trion is a stable state consisting of one electron–hole pair and one excess charge. If electrons (or holes) are trapped at the NC surface after photoexcitation, the NC’s interior state reaches a charged state and positive (or negative) trions can form inside the NC.19 Therefore, the charge trap states at the surface strongly influence the trion formation processes in NCs and NCs containing an extra charge are especially important for the generation of trions. Intentional charge injection into NCs and formation of charged NCs have been achieved by photo- and electro-chemical methods.20–22 It is well known that for a variety of NCs, the formation of trions reduces the quantum efficiency of luminescence and causes a photoluminescence intermittency due to the nonradiative Auger recombination of trions.23–25 On the other hand, it has been shown that trions reduce the threshold for optical gain,26 and the efficient amplified spontaneous emission from trions has recently been reported in perovskite NCs with long-lived trions.27 Experimental investigations have clarified that, in the ionic lead halide perovskite NCs, trions are efficiently generated and they determine the optical responses.28,29 To elucidate the NC’s fundamental optical properties and realize high-efficiency light-emitting diodes and lasers, it is indispensable to understand the generation and relaxation dynamics of trions.28–33 It has been reported


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that the trion generation in CsPbBr3 under weak photoexcitation can be controlled by surface modification.34 Under strong photoexcitation, on the other hand, intrinsic nonradiative Auger recombination of biexcitons causes ionization of perovskite NCs and trion generation.15,34 However, the actual ionization mechanism, which is important for the trion generation, has not yet been clarified. In this Letter, we study the dynamics of ionization and neutralization of perovskite CsPbBr3 NCs using double-pump transient absorption (DPTA) spectroscopy. We extract the trion component from the transient absorption (TA) signal and analyze its intensity as a function of the time interval between the two pump pulses. The data show that ionized NCs are either generated via nonradiative Auger recombination of biexcitons or via recombination of trions (here, both radiative and nonradiative processes are contributed). Furthermore, we find that two neutralization processes take place after NC ionization. The fast neutralization of ionized NCs has a time constant of nanoseconds. The dependence of the TA signals on the laser repetition rate reveals a very slow neutralization process with a time constant of microseconds. It is considered that the former process is the fast de-trapping of charges that have been trapped in shallow traps at the NC surface and the


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latter process is due to thermal activation of carriers located at deep trap states or longrange charge transfer from the outside of the NC.

Figure 1. Single-pump transient absorption spectroscopy: (a) Two-dimensional map of the bleaching signal under strong photoexcitation (〈𝑁〉 = 6.0). (b) Excitation fluence dependence of the ratio between the extracted trion and exciton amplitudes. The black solid line is the fitting result.


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Basic sample properties. The sample used in this study is an ensemble of chemically synthesized CsPbBr3 NCs with an average edge length of 7.6 ± 1.4 nm (determined from a transmission electron microscope image). The sample preparation method is described in the Supporting Information. These NCs were dispersed in octane and placed in a 1mm thick quartz cell. The absorption spectrum and the transmission electron microscope image are shown in the Supporting Information (Figure S1). A band-gap energy of Eg = 2.44 eV (508 nm) was determined from the second-order derivative of the absorption spectrum.

Single-pump transient absorption spectroscopy. The exciton decay dynamics were investigated by employing conventional single-pump transient absorption (SPTA) spectroscopy. Experimental details are described in the Supporting Information. A whitelight pulse was used as a probe pulse and the probe timing is defined by the delay time relative to the initial pump pulse. The energy of the pump pulse was tuned to the bandgap energy of the NCs, i.e., Eex = Eg. Thus, ground-state excitons can be generated directly at the band edge. Moreover, to suppress the photocharging effect, the NCs were


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continuously stirred by a magnetic stirrer.30,34,35 The SPTA experiment revealed that the NCs exhibit a strong bleaching signal in the wavelength region corresponding to the band edge (508 nm) as shown in Figure 1a. This bleaching signal represents the occupation of the band-edge level by the excited carriers. By integrating this signal from 498 to 514 nm and analyzing the excitation intensity dependence of the obtained decay curves (see Figure S2a in the Supporting Information), the lifetimes of the different exciton complexes can be obtained.28,34 The time constants of the three exponential functions for biexcitons, trions, and excitons extracted by this analysis are 𝜏XX = 30 ps, 𝜏X ∗ = 190 ps, and 𝜏X = 4.4 ns, respectively. These values are consistent with the values previously reported.28,34 The corresponding TA amplitudes, 𝐴XX, 𝐴X ∗ , and 𝐴X, are simultaneously determined from the fitting of the decay curves to the three exponential function with these three amplitudes (see Figure S2b in the Supporting Information). Figure 1b plots the TA amplitude ratio of the trion and exciton against the excitation intensity. Here, 〈𝑁〉 is the average number of absorbed photons in a NC. It is defined by the product of the absorption cross section, 𝜎, and the excitation fluence, 𝑗. The absorption cross section 𝜎 = 1.1 × 10 ―14 cm2 was determined by fitting the fluence dependence of the TA amplitude of the exciton, 𝐴X(𝑗), to


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the expression 𝐴(1 ― e ―𝜎 ∙ 𝑗), which is a widely used expression since the number of photons absorbed by a NC follows a Poisson distribution.36–38 To analyze the relative strength of trion formation, we employed the fitting curve 𝐴X ∗ /𝐴X(𝑗) ∝ (1 ― e ―𝜎 ∙ 𝑗 ― 𝜎 ∙ 𝑗 ∙ e ―𝜎 ∙ 𝑗)/(1 ― e ―𝜎 ∙ 𝑗), 34 whose result is shown by the solid curve in Figure 1b. We find that trions are hardly generated under weak photoexcitation. Here, we consider that the trion formation efficiency is independent of the excitation fluence, because the degeneracy of the band-edge states is two in perovskite NCs30 and the laser pulses in this experiment resonantly excite the band-edge state. Then, the trion formation and NC ionization behaviors in this experiment are different from those observed in NCs under high-energy photon excitation.39


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Figure 2. Double-pump transient absorption (DPTA) spectroscopy. (a) Two-dimensional map of the TA signal under strong photoexcitation (〈𝑁〉 = 5.1) for a pump-pulse interval 𝛥𝑡 = 811 ps. (b) Pump-pulse interval dependence of the integrated TA signal. (c) Illustration of carrier dynamics under double-pump photoexcitation.

Double-pump transient absorption spectroscopy. We conducted the DPTA spectroscopy to investigate the carrier dynamics of ionized NCs. In the DPTA experiments, we employed two pump pulses (hereafter referred to as the 1st pump and 2nd pump) with


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same photon energy Eex = 2.44 eV. This method allows us to investigate the influence of the band-edge ground-state excitons prepared by the 1st pump on the dynamics of excitons resonantly generated by the 2nd pump. To probe the decay dynamics of different exciton states in strongly excited NCs where ionization is likely to occur, we fixed the excitation fluence of the 1st pump to 𝑗1st = 4.7 × 1014 cm ―2 (〈𝑁〉 = 5.1). The obtained bleaching spectrum for 𝛥𝑡 = 811 ps and the spectrally integrated TA signals from 498 to 514 nm for selected 𝛥𝑡 are shown in Figures 2a and 2b, respectively. By increasing the pump-pulse interval 𝛥𝑡, a new bleaching signal clearly appears. According to the Poisson distribution analysis, this strong photoexcitation by the 1st pump leads to absorption of at least one photon by more than 99% of the NCs, and absorption of at least two photons by more than 95% of the NCs. In the following discussion we assume that multiple-exciton states containing three or more electron–hole pairs cannot be generated since the degeneracy of the band-edge level is two.30 Since biexcitons (and trions in NCs with surface traps) are generated by the strong 1st pump pulse and they recombine relatively fast, the 2nd pump pulse can only excite NCs containing single excitons or ionized NCs. Hence, the formation mechanism of ionized NCs can be investigated.


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By changing the time interval 𝛥𝑡 between two pump pulses, it is possible to control the population of the ionized NCs after 𝛥𝑡. In the present experiment, 𝛥𝑡 was varied between 11 and 811 ps. The excitation fluence of the 2nd pump pulse, 𝑗2nd, was changed from 0.20 × 1014 to 3.5 × 1014 cm ―2 (〈𝑁〉 = 0.21 to 3.8).

Figure 3. TA signals in the DPTA experiment for Δ𝑡 = 811 ps. (a) Excitation fluence dependence of the TA dynamics. (b) TA dynamics obtained by subtracting the SPTA signals from the DPTA signals. These TA difference signals correspond to the effective bleaching signals induced by carriers generated by the 2nd pump. (c) Corresponding TA difference amplitudes of excitons, trions, and biexcitons generated by the 2nd pump as a function of the excitation fluence of the 2nd pump pulse.


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Figure 3a shows the excitation fluence dependence of the TA decay dynamics obtained in the DPTA experiment for 𝛥𝑡 = 811 ps. It can be confirmed that the bleaching signal induced by the 2nd pump increases with its fluence. To extract the influence of the 2nd pump pulse excitation on the TA signals, we subtracted the SPTA signals from the DPST signals in Figure 3a and summarized the data in Figure 3b. We find that these TA difference signals have very fast time constants on the order of several 100 ps. We assumed that these decay dynamics can be explained by excitons, trions, and biexcitons, and performed the fitting of these decay curves using the three exponential decay function with the lifetimes determined in the SPTA measurements. The excitation fluence dependences of the corresponding TA difference amplitudes are shown in Figure 3c. We (2) note that biexcitons (𝐴(2) XX ) and trions (𝐴X ∗ ) are generated by the 2nd-pump pulse, while

almost no excitons (𝐴(2) X ) were generated. This result shows that the 1st-pump pulse is sufficiently strong to prepare single excitons or single charges in almost all NCs. Therefore, the trion TA amplitude induced by the second pump pulse, 𝐴(2) X ∗ , is related to the number of NCs that have been ionized as a result of the strong excitation by the 1st-


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pump pulse. Consequently, the dependence of 𝐴(2) X ∗ on Δ𝑡 reflects the dynamics of the ionization of NCs induced by the 1st-pump pulse. In order to discuss the formation of ionized NCs, we first evaluate the absorption cross section and the temporal evolution of the number of ionized NCs. In a neutral NC, two electron–hole pairs can be generated by photoexcitation in NCs with a doubly degenerate lowest energy level. In an ionized NC, however, the number of photoexcitable electron– hole pairs decreases due to the presence of one charge carrier at the lowest band-edge state. Similarly, a general decrease of the number of photoexcitable producible electron– hole pairs has been observed for resonantly generated multiple excitons in other NCs.37 Since the degeneracy of the band-edge level of perovskite NCs is two,30 it is expected that the absorption cross section of ionized NCs, 𝜎C, is about half of that of neutral NCs, 𝜎N. It is known that the exciton energy levels in perovskite NCs split due to the electron– hole exchange interaction, and thus the singlet and triplet states are formed.32 However, since the reported splitting exciton energy is several meV, we can ignore this splitting at room temperature.40 Since the pump pulse energy is equal to the band-gap energy, the trions are generated from ionized NCs by absorbing at least one photon. The TA


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difference amplitude of trions generated by the 2nd pump pulse at a delay time Δ𝑡, 𝐴(2) X ∗ (Δ 𝑡), can be written as the product of the number of ionized NCs, 𝐴C(Δ𝑡), and the probability ― 𝜎C ∙ 𝑗 of absorbing one or more photons, 1 ― 𝑒 ― 𝜎C ∙ 𝑗, i.e., 𝐴(2) ). X ∗ (Δ𝑡) = 𝐴C(Δ𝑡)(1 ― 𝑒

Therefore, by analyzing the dependences of 𝐴(2) X ∗ (Δ𝑡) on the 2nd pump fluence and the delay time Δ𝑡, we are able to obtain the absorption cross section of an ionized NC and the temporal profile of 𝐴C(Δ𝑡). We performed a global fitting of the 2nd-pump fluence dependence of 𝐴(2) X ∗ for all measured Δ𝑡 that reproduces the experimental data very well (see Figure S3 in the Supporting Information). The obtained value 𝜎C = 4.0 × 10 ―15 cm2 = 𝜎N/2.7 is in good agreement with the expected result. This value suggests that the trions generated by the 2nd pump are mainly generated from NCs containing only a single charge. The difference between the obtained factor 2.7 and the ideal factor 2 is considered to be a result from the existence of higher-order transitions such as those from doubly ionized NCs. Further theoretical studies are needed to clarify the whole spectrum of absorption processes in ionized NCs. To clarify the ionization and neutralization mechanisms, the amplitude 𝐴C(Δ𝑡) determined from the fitting is shown in Figure 4a. This graph describes the temporal evolution of the number of ionized NCs


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generated by the 1st pump pulse. In the region Δ𝑡 < 100 ps, 𝐴C(Δ𝑡) exhibits a steep increase and reaches a maximum at Δ𝑡 ≈ 100 ps. Then, for 𝛥𝑡 > 100 ps, it slowly decreases. The initial increase and later decrease of the ionized-NC amplitude in Figure 4a reflects the ionization and neutralization dynamics of the NCs. To explain this time dependence of the ionized NCs quantitatively, we use the following rate equations: d𝐴XX(𝑡)

= ― 𝑘XX𝐴XX(𝑡),

(1a)

= ― 𝑘X ∗ 𝐴X ∗ (𝑡),

(1b)

= + 𝑘1𝐴XX(𝑡) + 𝑘2𝐴X ∗ (𝑡) ― 𝑘C𝐴C(𝑡) .

(1c)

d𝑡 d𝐴X ∗ (𝑡) d𝑡 d𝐴C(𝑡) d𝑡

Here, 𝐴XX(𝑡), 𝐴X ∗ (𝑡), and 𝐴C(𝑡) represent the numbers of NCs containing one biexciton, one trion, and a single charge at time 𝑡, respectively. 𝑘XX and 𝑘X ∗ are the recombination rates of biexcitons and trions, respectively. 𝑘1 (𝑘2) in equation (1c) describes the generation rate of ionized NCs by recombination of biexcitons (trions). When these equations are solved under the initial condition: 𝐴XX(0) = 𝐴XX,0, 𝐴X ∗ (0) = 𝐴X ∗ ,0, and 𝐴C(0) = 0 at time t = 0, i.e., immediately after the 1st-pump excitation, we obtain the following solutions:


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𝐴XX(𝑡) = 𝐴XX,0e ― 𝑘XX𝑡,

(2a)

― 𝑘𝑋 ∗ 𝑡

(2b)

𝐴X ∗ (𝑡) = 𝐴X ∗ ,0e

𝐴C(𝑡) =

𝑘1 𝑘XX ― 𝑘C

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𝐴XX,0(e ― 𝑘C𝑡 ― e ― 𝑘XX𝑡) +

,

𝑘2 𝑘X ∗ ― 𝑘C

𝐴X ∗ ,0(e ― 𝑘C𝑡 ― e

― 𝑘𝑋 ∗ 𝑡

).

(2c)

Using equation (2c), we can perform the numerical analysis of the data of 𝐴C(Δ𝑡) shown in Figure 4a. The parameters, 𝐴XX,0, 𝐴X ∗ ,0, 𝑘XX ( = 1/𝜏XX), and 𝑘X ∗ ( = 1/𝜏X ∗ ) were already determined in the SPTA experiments. The three variables 𝑘1, 𝑘2, and 𝑘C are fitting coefficients. The fitting result using equation (2c) is the solid line in Figure 4a, which shows that the experimental results are very well reproduced. From this analysis, we obtain the generation rate of ionized NCs from biexcitons, 𝑘1 = 21 ± 1 ns ―1, that from trions, 𝑘2 = 4.3 ± 0.3 ns ―1, and the neutralization rate of ionized NCs is 𝑘C = 1.0 ± 0.1 ns ―1. The ratio 𝑘1/𝑘𝑋𝑋 = 0.62 describes the generation efficiency of an ionized NC in case of biexciton recombination. Furthermore, the ratio 𝑘2/𝑘𝑋 ∗ = 0.82 represents the generation efficiency of an ionized NCs in case of trion recombination. Trions are more likely to generate ionized NCs because both radiative and nonradiative Auger recombination processes of trions contribute to the generation of ionized NCs. In addition,


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the time constant 𝜏C = 1/𝑘𝐶 = 990 ps is equivalent to the lifetime of an ionized NC, and it suggests that the fast neutralization of ionized NCs occurs within nanoseconds.

Figure 4. (a) Pump-pulse interval dependence of the number of ionized NCs generated by the 1st pump. (b) Repetition rate dependence of the TA signals.

The above analysis revealed a fast neutralization process of ionized NCs. In the experiments, we used a low laser repetition rate to reduce the effects of processes with time constants on the order of microseconds on the TA signals in the pico- and nanosecond time region. On the other hand, it has been reported that ionized perovskite NCs require more than several μs for their neutralization.41 To monitor such slow neutralization


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processes, we also changed the repetition rate of the pump laser. Here, the sample was stirred during the experiment as well. We found that, as the repetition rate increases, the bleaching signals for negative delay times (that is, the region where TA signals from the previous pump pulse are visible) increase (Figure S4). Figure 4b shows the averaged signal amplitude in this time region as a function of the inverse laser repetition rate. By performing the fitting of these data with a single-exponential function, we obtained a time constant of 12 μs. Since the exciton lifetime is 4.4 ns, it is reasonable to consider that this very long decay component represents the slow neutralization of ionized NCs. From the above, we understand that there exist two types of neutralization processes for ionized NCs. The fast neutralization suggests that the remaining extra charges are trapped at relatively shallow surface states and the de-trapping of the charges leads to neutralization. This charge trapping and de-trapping at the NC surface determines the rapid change between ionized and neutral conditions of NCs, and thus is closely related to the photoluminescence intermittency. In fact, it has been reported that surface treatments are able to modify the carrier tapping conditions, leading to an observation of two

kinds

of

photoluminescence

intermittency

in

single-dot

spectroscopy


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measurements.29 The slow neutralization is another process where one carrier has left the NC (for example, the ligand gets charges) or has been trapped at deep traps at the surface. It is believed that the slow neutralization of NCs is caused by the required carrier transfer over a long distance (from the ligand to the inside of the NCs) and/or the slow escape rate of charges trapped at deep surface states. In conclusion, we studied the trion dynamics involving ionization and neutralization processes of CsPbBr3 perovskite NCs. By employing DPTA spectroscopy, we investigated the effect of photoexcited carriers that have been prepared by a strong 1st pump pulse, on the TA signals induced by the 2nd pump pulse. In contrary to the longlived exciton signal obtained by the SPTA spectroscopy, the DPTA difference signals were governed by components with lifetimes less than several 100 ps. The analysis of these difference decay curves revealed that only biexcitons and trions are generated by the 2nd pump in addition to those that have been prepared by the 1st pump. By studying the 2nd-pump fluence dependence of the trion TA difference signal, we clarified that the averaged absorption cross section of ionized NCs is smaller than that of neutral NCs, because of the existence of the extra charge inside the NC. From the analysis of the trion


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TA difference signal as a function of the time interval between the two pump pulses under a weak 2nd pump, we revealed that the ionized NCs can be generated via both nonradiative Auger recombination of biexcitons and any possible recombination of trions. Furthermore, the data revealed that fast and slow neutralization processes occur in ionized NCs.

ASSOCIATED CONTENT

Supporting Information. Sample synthesis, Transient absorption spectroscopy, Singlepulse transient absorption spectroscopy, Repetition-rate dependent transient absorption signals. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID


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Yoshihiko Kanemitsu: 0000-0002-0788-131X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

Part of this work was supported by JST-CREST (JPMJCR16N3).

REFERENCES

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(2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites.

Science 2012, 338, 643–647.


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(5) Kanemitsu, Y.; Handa, T. Photophysics of Metal Halide Perovskites: From Materials to Devices. Jpn. J. Appl. Phys. 2018, 57, 090101.

(6) Kanemitsu, Y. Luminescence Spectroscopy of Lead-Halide Perovskites: Materials Properties and Application as Photovoltaic Devices. J. Mater. Chem. C 2017, 5, 3427–3437.

(7) Yamada, Y.; Yamada, T.; Kanemitsu, Y. Free Carrier Radiative Recombination and Photon Recycling in Lead Halide Perovskite Solar Cell Materials. Bull. Chem. Soc.

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