Unraveling the Ultrafast Exciton Relaxation and Hidden Energy State

Feb 12, 2018 - Recently organic–inorganic lead-halide perovskite nanoparticles (NPs) have been attractive as low-cost and high-conversion-efficient ...
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Cite This: J. Phys. Chem. C 2018, 122, 5209−5214

Unraveling the Ultrafast Exciton Relaxation and Hidden Energy State in CH3NH3PbBr3 Nanoparticles Tetsuro Katayama,* Harunobu Suenaga, Tomoki Okuhata, Sadahiro Masuo, and Naoto Tamai* Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan

J. Phys. Chem. C 2018.122:5209-5214. Downloaded from pubs.acs.org by UNIV OF ROCHESTER on 08/13/18. For personal use only.

S Supporting Information *

ABSTRACT: Recently organic−inorganic lead-halide perovskite nanoparticles (NPs) have been attractive as low-cost and high-conversionefficient solar cells and light-emitting diode. The generation of the exciton and dissociation into free carriers are quite important for primary photoelectric conversion processes. In this study, we have examined the initial exciton dynamics of CH3NH3PbBr3 (MAPbBr3) NPs by femtosecond transient absorption spectroscopy and picosecond time-resolved luminescence spectroscopy. The ultrafast exciton quenching with a time constant of 200 fs was observed, which may be related with longitudinal optical phonon and/or lurching motion of MA cation in the excited state. In addition, higher electronic state with a short lifetime was clearly detected by the excitation intensity dependence of time-resolved luminescence and transient absorption spectra. These findings of MAPbBr3 NPs are very important not only for understanding the generation of charge carrier but also for constructing the high-efficiency charge separation and electric luminescence systems.

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11%.21 The internal conversion efficiency was over 80%. These high conversion efficiencies could not be simply explained by an exciton binding energy of NPs higher than the thermal energy at room temperature. Single- and multiple-exciton dynamics of perovskite NPs have been reported by means of transient absorption and timeresolved luminescence measurements.22−25 Scholes group first demonstrated the transient absorption dynamics of MAPbI3 NPs.22 Klimov group revealed the size dependence of Auger recombination of excitons in Cs-based quantum dots (QDs).23 However, an initial stage of the relaxation of exciton has not been discussed because of the limitation of the monitoring wavelength and signal-to-noise ratio. In this study, we have examined the initial exciton dynamics by femtosecond transient absorption spectroscopy with a temporal resolution of 30 fs and picosecond time-resolved luminescence spectroscopy. The spectral change in the initial stage and the excitation intensity dependence of the luminescence spectra revealed the hidden electronic state and Coulombic screening between electron and hole in MAPbBr3 NPs within a time constant of about 200 fs. This time constant is similar to a rotational “wobbling-in-acone” motion of MA cation molecules measured by femtosecond IR transient absorption spectroscopy and molecular dynamics (MD) calculation.26,27 Comprehensive understanding of the exciton dynamics of NPs is very important not only for

INTRODUCTION Recently organic−inorganic lead-halide perovskite materials have been attractive as low cost and high conversion efficient solar cells since the first report of 2009.1−5 The generation of the exciton and the dissociation into free carriers are quite important for primary photoelectric conversion processes. Previously, the correlation between the generation of carriers and the oscillator strength of exciton on the ground state was discussed in the device of a perovskite solar cell by means of transient absorption microscopy.6 The size effects of the perovskite crystal and nanoparticles (NPs) were also discussed with other time-resolved spectroscopy.7−10 These phenomena were interpreted in terms of a large dielectric constant induced from the inhomogeneous surface charge following the photoexcitation. 8 Large dielectric constants can reduce the Coulombic attractive fields of electrons and holes. The drastic change of the Coulombic attractive fields of the electron and hole, following the photoexcitation, was also reported in vinyl11−13 and π-conjugated polymers.14−16 Thus, the process reducing the Coulombic attractive fields between electrons and holes is very important for photoelectric conversion systems. Even though the photoelectric conversion efficiency of CH3NH3PbBr3 (MAPbBr3) film was reported as over 10%,17 the exciton binding energy of 60 meV reported for MAPbBr3 of bulk crystal is higher than the thermal energy at room temperature (25 meV).18 In addition, although the exciton binding energy of MAPbBr3 NPs with an average size of 3.3 nm is 375 meV,19,20 the photoelectric conversion efficiency of the solar cell device prepared with MAPbBr3 NPs was reported as © 2018 American Chemical Society

Received: January 30, 2018 Revised: February 8, 2018 Published: February 12, 2018 5209

DOI: 10.1021/acs.jpcc.8b01051 J. Phys. Chem. C 2018, 122, 5209−5214

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The Journal of Physical Chemistry C

Figure 1. (a) Time-resolved luminescence spectra (exc. at 480 nm, ⟨N⟩ = 7.2), (b) luminescence decay dynamics at 505 and 525 nm with different excitation intensities, (c) excitation intensity dependence of luminescence spectra of MAPbBr3 QDs in toluene solution observed at 0 ps after excitation, and (d) luminescence intensity ratio of 505 nm against 525 nm (I505nm/I525nm) of QDs as a function of the excitation intensity.

sample for determining the quantum yield of the luminescence. The average particle size of NPs was estimated to be 5.5 ± 0.5 nm from STEM analyses, as shown Figure S1. Figure S1 shows the steady-state absorption and luminescence spectra of MAPbBr3 NPs in toluene solution. The absorption and luminescence spectra of the centrifuged MAPbBr3 NPs are apparently different from those of the precipitate in toluene solution. The lifetimes (amplitudes) of the centrifuged MAPbBr3 NPs were estimated to be 8 ns (0.85) and 23 ns (0.15) from double exponential fits to the luminescence decay, as shown in Figure S2. The luminescence decay of redispersed MAPbBr3 has much longer lifetime components over 200 ns as compared with MAPbBr3 NPs because of the contribution of large-sized NPs.

understanding the generation of charge carrier but also for constructing the high-efficiency charge-separation systems.

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EXPERIMENTAL SETUP UV−vis absorption and luminescence spectra were recorded using a Hitachi U-4100 and Fluorolog-3 (Jobin Yvon Spex), respectively. The structures of MAPbBr3 NPs were characterized by scanning transmission electron microscopy (STEM; Tecnai 20, 200 keV, FEI). Transient absorption spectra were measured by means of femtosecond pump-probe experiments. Light source was an amplified mode-locked Ti:sapphire laser (Solstice, Spectra-Physics), and the state-selective excitation experiments were performed by a noncollinear optical parametric amplifier (TOPAS-white, Light Conversion Ltd.). Excitation wavelengths were converted into 515 nm for transient absorption and 490 nm for time-resolved luminescence measurements. Transient absorption spectra were probed by delayed pulses of a femtosecond white-light continuum, generated by focusing a fundamental laser pulse into a CaF2 plate. Probe light was detected by a C-MOS detector (Hamamatsu, PMA-20). The temporal resolution was ca. 30 fs. Time-resolved luminescence spectra were measured by using a streak camera with 1 kHz reputation rate (Hamamatsu, synchronous blanking unit M5678 and synchroscan sweep unit M5675). The temporal resolution was ca. 6 ps. Longer lifetime components were measured by a time-correlated single-photon counting method. The sample solution was agitated with a stirring bar during measurements. Lead (II) bromide (PbBr2, Aldrich, 99.999%) methylamine hydrobromide (CH3NH2 HBr, TCI, 99%), and n-otylamine (Wako, 98%) were purchased and used without further purification. NPs were synthesized by a reprecipitation method.19 Before the measurements, the solution was centrifuged with a speed of 12 000 rpm. The supernatant was used for the transient absorption measurements. The solvent of redispersed NPs was toluene. Luminescence quantum yield was ca. 70%. Coumarin 343 in ethanol was used as the reference

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RESULTS AND DISCUSSION Time-Resolved Emission Spectra. Figure 1a shows timeresolved luminescence for MAPbBr3 NPs excited at 490 nm and detected at various delay times after excitation. Two luminescence peaks around 505 and 530 nm were observed just after excitation. The peak around at 505 nm disappeared very quickly within an instrument response function. The emission at 530 nm remained over 20 ps after excitation and then shifted to 525 nm within 300 ps. Figure 1b shows the luminescence decays observed at 505 and 525 nm with different excitation intensities. At lower excitation intensity of 4.8 × 1012 photon/ cm2 (average excitation number per NPs ⟨N⟩ = 0.1, cross section at 480 nm was estimated as (2.2 ± 0.7) × 1014 cm2 by using a general calculation method),23 a single exponential decay was observed with a time constant longer than 2 ns. On the other hand, the luminescence decay at ⟨N⟩ = 7.2 was described by a sum of two exponential functions with a time constant of 55 ps (0.5) and 1.7 ns (0.5). The amplitude ratio between 55 ps and 1.7 ns increased with increasing the excitation intensity. The faster decay component of 55 ps is also observed in femtosecond transient absorption dynamics at higher excitation intensities, as illustrated in Figure S3. Thus, 5210

DOI: 10.1021/acs.jpcc.8b01051 J. Phys. Chem. C 2018, 122, 5209−5214

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The Journal of Physical Chemistry C

Figure 2. (a) Transient absorption spectra for MAPbBr3 QDs in toluene solution (exc. at 515 nm, ⟨N⟩ = 0.7), (b) transient absorption spectra within 1 ps after excitation (exc. at 515 nm, ⟨N⟩ = 0.3), and (c) peak energy shift of the positive signal around 500 nm.

Figure 3. Excitation intensity dependence of transient absorption spectra for MAPbBr3 QDs in toluene solution observed at 200 fs after excitation. (a) Excitation wavelength was at 515 nm and (b) at 400 nm.

observed at 490 nm. Negative signals indicate the loss of the ground state, and a positive signal can be attributed to the photoinduced absorption (PA) of MAPbBr3.8 Figure 2b illustrates PA spectra in the initial time region up to 10 ps. As clearly shown in the figure, the peak wavelength shifts to blue from 505 to 490 nm accompanied with the spectral broadening. The time constant of the spectral shift was estimated to be 200 fs with the energy shift of ca. 90 meV, as shown in Figure 2c. Previously, photoinduced giant dielectric constant was reported in the organic−inorganic halide perovskite system.28 Increase of the dielectric constant should reduce the Coulombic attractive field between hole and electron pair, which was called “exciton quenching” in inorganic−organic perovskite crystal with MD simulation study.26 In the calculation, the steady-state absorption spectrum changes from “QDs”-like to “bulk”-like spectrum without exciton absorption. The photoinduced exciton quenching might induce a negative signal as a result of disappearance of exciton absorption, and thus the blue-shift of PA signal in Figure 2b could be interpreted in terms of increase of the negative signal. The exciton quenching was also observed with carrier− exciton interaction.8 Thus, multiple excitons generated at higher excitation intensity might induce the exciton quenching originated from the electron/hole−exciton interaction. Figure 3a illustrates the excitation intensity dependence of the transient absorption spectra at 200 fs after 515 nm excitation. The PA spectrum shifted to blue with an increase in the excitation intensity. On the other hand, the maximum wavelength of the bleach signal around 515 nm does not change. This behavior suggests that the blue-shift of the

the lifetime of 55 ps is probably because of biexiton Auger recombination. The binding energy of biexciton was estimated as ΔXX = 21 meV from the difference of luminescence maximum between spectral components of 55 ps and 1.7 ns by using global analysis, as shown in Figure S4. This value was close to the thermal energy of room temperature (25 meV). The excitation intensity dependence of luminescence spectra for MAPbBr3 NPs just after the excitation (0 ps) is illustrated in Figure 1c. The peak at 505 nm was clearly observed at higher excitation intensity and the peak wavelength almost constant, irrespective of the excitation intensity. The ratio of the luminescence intensity of 505 nm against 525 nm is given in Figure 1d. As clearly shown in the figure, the ratio increases nonlinearly over ⟨N⟩ = 2. Recently, Klimov group reported a similar luminescence behavior from multiexcitons in the Csbased perovskite QDs (diameter = 6.3 to 11.2 nm), in which a new luminescence spectrum with a very short lifetime was superimposed at shorter wavelength region.23 In addition, the luminescence spectrum from higher energy states was reported in iodine-based bulk perovskite film using a femtosecond optical Kerr gating method.24 By considering the previous results, the spectrum at 505 nm is probably because of the luminescence from energy higher than the band-edge state of MAPbBr3 NPs. Femtosecond TA Measurements. For deciphering the ultrafast excitation dynamics in MAPbBr3 NPs, femtosecond transient absorption spectroscopy was utilized. Figure 2a shows the time evolution of transient absorption spectra at low excitation intensity. Negative signal was observed immediately after excitation at 515 nm, and a positive absorption was 5211

DOI: 10.1021/acs.jpcc.8b01051 J. Phys. Chem. C 2018, 122, 5209−5214

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The Journal of Physical Chemistry C

Figure 4. (a) Transient absorption dynamics of MAPbBr3 QDs excited at 515 nm (⟨N⟩ = 2.0) and observed at 490 nm. (b) Vibrational components of the vibrational components were calculated by maximum entropy method (MEM) from the residual component of the time profiles of transient absorbance at 490 nm. Residual component of the time profiles at 490 nm was obtained by means of fitting with an exponential function. Residual components and fast Fourier transform (FFT) spectra are shown in Figure S6.

Coherent Longitudinal Optical Phonon Motions. To reveal the Coulombic screening mechanism coupled with the phonon vibration of the perovskite, vibrational coherence of the perovskite NPs was precisely analyzed. Figure 4a shows the time profiles of the transient absorbance monitored at 490 nm. Vibrational coherence was observed in the transient signal, where several vibrational components were superimposed in the signal. Vibrational spectra were analyzed by an MEM, as shown in Figure 4b. Four peaks were observed and their energies were 1000, 780, 510, and 120 cm−1 for MAPbBr3 NPs. 1000, 780, and 510 cm−1 were also observed in pure toluene. The low-frequency mode of 120 cm−1 was not observed in the toluene solvent. Hence, the low-frequency mode is probably because of the phonon vibrational mode in perovskite. Previously, longitudinal optical (LO) phonon mode and lurching mode of MA cation was reported in the wavenumber region of 100−150 cm−1 by resonance Raman spectroscopy of the perovskite NPs and a single crystal.33,34 This frequency of LO phonon mode is similar to our experimental data, with a period of 120 cm−1 also close to the time constant of exciton quenching. Thus, the phonon mode might assist the ion displacement of MA cation, inducing the Coulombic screening between electron and hole.

absorption band around 490 nm is probably because of the Coulombic screening between electron and hole originated from the carrier−exciton interaction.21,22 Thus, the blue-shift of the PA signal with a time constant of 200 fs in Figure 2b at low excitation intensity can be ascribed to exciton quenching. In addition, the existence of ultrafast rotational “wobbling-in-acone” motion of MA cation with a time constant of ∼300 fs in the excited state of bulk MAPbI3 has been recently reported.26 The Coulombic screening between electron and hole can be expected by the increase of dielectric constant induced by the oriental motion of MA cation.26,28 Thus, the blue-shift of PA with a time constant of 200 fs in Figure 2b probably originated from change of dielectric constant with the ultrafast rotational motion of MA cation, although the time constant is a little faster than that in the bulk. Figure 3b shows the excitation intensity dependence of transient absorption spectra at 400 nm excitation. A positive signal around 525 nm was observed at 400 nm excitation. The new band might be resulted by the interference of 1P and 1S29 with the red-shift of absorption edge. In addition, with the increase of the excitation intensity, the spectral width of the bleach signal greatly increased, and an additional peak appeared around 490 nm. This result indicates that the electronic state higher than 1S(e) is occupied with the increasing excitation intensity. It has been recently reported that the bleach shoulder shorter than 490 nm appeared at high excitation intensity, which was originated from the new trapping site with a lifetime of >10 ns.30 However, as shown in Figure S5, the shoulder at 490 nm disappeared with a time constant of ca. 200 fs. Hence, this behavior might not be caused from the trapping sites at higher excitation intensity. This behavior is explicable as the relaxation from higher electronic state, for example, 1P(e) reported as II−VI semiconductor QD systems.29,31 The hidden electronic state might appear with luminescence at 505 nm and bleach signal at 490 nm with higher excitation intensity condition, just after the excitation. The relaxation time constant of hidden electronic state was almost the same as the initial spectral change of PA. Thus, the disappearance of hidden electronic state suggests the reduction of quantum confinements with increasing the dielectric constant in NPs. Slow hot-carrier relaxation and efficient hotelectron transfer dynamics with several hundred femtoseconds and a yield more than 80% in MAPbBr3 perovskite nanocrystal have been reported.32 Such an efficient hot-electron transfer probably progresses via hidden electronic state with a lifetime of 200 fs. The QD-like state might have an advantage for slowcarrier relaxation and electron-transfer reaction because of phonon bottleneck process.

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CONCLUSIONS In conclusion, ultrafast exciton dynamics in MAPbBr3 NPs was observed by time-resolved luminescence and femtosecond transient absorption spectroscopy. The exciton quenching and hidden states were revealed by precise analyses of time-resolved spectroscopic data. These phenomena suggest the electronic state change from “QD”-like to “bulk”-like state with decreasing the quantum confinement effect. Vibrational coherence data with a period of 120 cm−1 also suggest that the MA cation motion with LO phonon mode in NPs assists the exciton quenching. Such a dynamical Coulombic screening reduces the binding energy of exciton, leading to high electric charge separation and photoelectric conversion efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01051. Scanning TEM image and steady-state absorption and luminescence spectra, luminescence decay, excitation intensity dependence of transient absorption decay, decay-associated spectra of the time-resolved luminescence, excitation intensity dependence of transient absorption, and FFT spectra (PDF) 5212

DOI: 10.1021/acs.jpcc.8b01051 J. Phys. Chem. C 2018, 122, 5209−5214

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +81-(0)79-565-8364 (T.K.). *E-mail: [email protected](N.T.). ORCID

Tetsuro Katayama: 0000-0002-3214-2228 Sadahiro Masuo: 0000-0003-4828-5968 Naoto Tamai: 0000-0002-7343-6564 Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the JSPS KAKENHI (grant number; JP26107005 in Scientific Research on Innovative Areas “Photosynergetics” and 17K14084) and the CASIO foundation (grant number; 9347).

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DOI: 10.1021/acs.jpcc.8b01051 J. Phys. Chem. C 2018, 122, 5209−5214