Free Carriers versus Excitons in CH3NH3PbI3 Perovskite Thin Films

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Free Carriers Versus Excitons in CHNHPbI Perovskite Thin Films at Low Temperatures: Charge Transfer From the Orthorhombic Phase to the Tetragonal Phase Le Quang Phuong, Yasuhiro Yamada, Masaya Nagai, Naoki Maruyama, Atsushi Wakamiya, and Yoshihiko Kanemitsu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00781 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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The Journal of Physical Chemistry Letters

Free Carriers versus Excitons in CH3NH3PbI3 Perovskite Thin Films at Low Temperatures: Charge Transfer from the Orthorhombic Phase to the Tetragonal Phase Le Quang Phuong†, Yasuhiro Yamada‡, Masaya Nagai⊥, Naoki Maruyama†, Atsushi Wakamiya†, and Yoshihiko Kanemitsu*† †

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

‡

Graduate School of Science, Chiba University, Inage, Chiba 263-8522, Japan

⊥

Graduate School of Engineering Science, Osaka University, Toyonaka , Osaka 560-8531, Japan

Corresponding Author *Email: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI3) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at low temperatures in addition to the dominant orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI3 thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures.

TOC GRAPHICS

KEYWORDS Hybrid organolead halide perovskites, free carriers, excitons, tetragonal phase, and orthorhombic phase


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Hybrid organolead halide perovskites, MAPbX3 (MA = CH3NH3, X = I, Br, Cl), have recently received worldwide attention as highly promising solar-cell materials owing to their excellent characteristics that are beneficial for solar energy-electricity conversion.[1] The photoconversion efficiency of MAPbX3-based solar cells has been rapidly improved since their initial development in 2009, and currently approaches a value as high as 22.1%,[2-10] which is comparable to those of other thin-film solar cells that are based on conventional photovoltaic materials, such as CdTe and Cu(In,Ga)(S,Se)2.[11] Superior MAPbX3-based solar cells have been developed with X = I. Many works have been conducted to thoroughly elucidate the fundamental photophysics in MAPbI3[12-20] to achieve further improvement in the photovoltaic performance of MAPbI3-based solar cells. Until now, most studies on MAPbI3 material have been performed at room temperature (RT).[12-20] However, solar cells are also extensively used in certain applications, such as the aerospace industry, where advanced photoconversion features are required to be maintained over a wide temperature range. In addition, spectroscopic studies at low temperatures will provide further understandings of the fundamental photophysics in MAPbI3 material, including the longstanding debate on excitonic effects on the photocarrier generation and recombination dynamics. Considering these facts, some recent works have focused on investigating the low-temperature fundamental optical properties of MAPbI3.[21-27] It is known that MAPbI3 single crystal experiences a phase transition, from tetragonal to orthorhombic, at 162.2 K.[28] Although the near-band-edge optical spectra of the tetragonal-phase MAPbI3 are usually composed of a single band due to two-carrier recombination,[16,20] multiplepeak structures have been observed in the diffusion reflectance[21] and photoluminescence (PL) spectra[22,23,25,26] of thin-film MAPbI3 at temperatures below the phase-transition temperatures. At


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low temperatures, the exciton binding energy of the orthorhombic-phase MAPbI3 is estimated to be within the range of 16 – 63 meV.[23,29-32] Hence, one may presume that excitons significantly contribute to the near-band-edge optical responses of the orthorhombic-phase MAPbI3, which consequently leads to the observed multiple optical components. However, the mechanisms of photocarrier generation and recombination in the orthorhombic-phase MAPbI3 have not been clarified, and thus, it is unclear whether the photocarrier dynamics in MAPbI3 are primarily determined by excitons or free carriers at low temperatures. Therefore, this is still an open and interesting issue for investigation. In complex solar cell materials, the steady-state optical responses revealed using only PL spectroscopy might not reflect fully the mechanisms of photocarrier generation and recombination.[33] Thus, a combination between PL spectroscopy and different time-resolved optical techniques provides a powerful routine to understand insightfully the photocarrier dynamics in complicated multinary compounds used for solar cell applications.[34] In this Letter, we investigated the near-band-edge dynamic properties of MAPbI3 perovskite thin films at low temperatures using various time-resolved optical techniques. The PL and optical transient absorption (TA) data consistently indicates the coexistence of tetragonal and orthorhombic phases in MAPbI3 thin films at low temperatures. The excitation-fluencedependent TA and THz TA (THz-TA) measurements point out that the optical dynamic responses of the photocarriers near the band edge in both the tetragonal and orthorhombic phases existing in MAPbI3 thin films are primarily governed by free carriers, rather than excitons, even at low temperatures. We assign the lack of excitonic features in the optical properties of the orthorhombic-phase MAPbI3 to the fast charge transfer from the orthorhombic phase to the tetragonal phase.


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Figure 1(a) shows the time-integrated PL spectra of the MAPbI3 perovskite thin film at 15 K under various fluences of 1.8-eV photoexcitation. Different with the PL spectrum at RT that is composed of a single emission band and is spectrally independent of the excitation fluence,[16] the PL spectrum at 15 K consists of a PL band peaking at 1.58 eV, denoted as T-PL, a broad band located at the low-energy region under weak excitation fluences, denoted as D-PL, and an additional PL band that emerges at approximately 1.67 eV under strong excitation fluences, denoted as O-PL. The peak energies of the T-PL and O-PL bands remain almost unchanged as the excitation fluence increases; however, that of the D-PL band shows a blueshift. Note that this multicomponent PL is not contributed by the unreacted PbI2 in the thin film samples because the excitation photon energy is below the bandgap energy of PbI2 (~2.55 eV at low temperatures). Moreover, we confirmed that the PL spectra do not change even as the samples were excited with photon energies larger than the bandgap energy of PbI2. The time-integrated intensities of these three PL bands are summarized in Figure 1(b) as a function of the excitation fluence. The intensity of the D-PL band depends linearly on the excitation fluence and tends to saturate under strong excitation fluences. Differently, those of the T-PL and O-PL bands show quadratic dependences. In addition, the D-PL band has a longer PL decay time than the T-PL and O-PL bands, and disappears as the temperature is elevated above 70 K (See Figure S2 in SI). The D-PL band locates a few hundreds of meV lower than the T-PL and O-PL bands. Therefore, the D-PL band is likely to be associated with deep defects that have formed within the MAPbI3 polycrystalline thin films. A typical transmission spectrum of the MAPbI3 thin films at 15 K is shown in Figure 1(c). A steep decrease in transmission, which reflects the near-band-edge absorptions of the orthorhombic-phase MAPbI3,[21-23,25] occurs near the peak energy of the O-PL band. Concurrently, there is no noticeable change in the time-


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integrated transmission spectrum near the peak energy of the T-PL band. Thus, the O-PL band is likely to originate from the near-band-edge recombination of the orthorhombic-phase MAPbI3.

Figure 1. (a) PL spectra of a MAPbI3 thin film at 15 K under various fluences of 1.8-eV photoexcitation. (b) Time-integrated PL intensities of the D-PL, T-PL, and O-PL bands as a function of the excitation fluence. (c) Typical transmission spectrum of the MAPbI3 thin film at 15 K. To clarify the physical origins of the T-PL and O-PL bands that were observed in the timeintegrated PL spectrum, we investigated the transient responses of the photocarriers in the MAPbI3 thin films using TA and THz-TA spectroscopy. Figure 2(a) shows the TA spectra of the MAPbI3 thin films at different delay times obtained at 15 K under 2.1-eV photoexcitation. Similar to the time-integrated PL spectra under strong excitation fluences, two photobleaching bands, denoted by T-TA and O-TA, appear in the TA spectrum. This double band structure can be detected as the temperature falls below 130 K, where a phase transition, from tetragonal to


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orthorhombic, occurs in our thin-film samples. At temperatures above 130 K, only one photobleaching band is observed in the TA spectra of the tetragonal-phase MAPbI3 (See Figures S1 and S3 in SI). The T-TA and O-TA bands at 15 K located at ~1.61 eV and ~1.68 eV, respectively, slightly higher than the peak energies of the T-PL and O-PL bands observed in the time-integrated PL spectra. By considering the transmission spectrum, and taking into account that a photobleaching TA band usually arises around the near-band-edge transitions of direct-gap semiconductors,[16,21,33,34] it is plausible that the O-TA photobleaching band relates to the nearband-edge photocarriers in the orthorhombic-phase MAPbI3.

Figure 2. (a) TA spectra of a MAPbI3 thin film at various delay times under 2.1-eV photoexcitation at 15 K. (b) THz-TA decay dynamics of the MAPbI3 thin film at 18 K under 1.67- (red circles) and 1.6-eV (blue circles) excitation. The black solid lines represent double (for red circles) and single exponential fits.


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Regarding the T-TA band, experiments under a resonant excitation of the T-TA band, which has a photon energy lower than the O-TA band energy, may be helpful to gain more insights. However, it is difficult to conduct optical TA measurements under resonant excitation because of strong laser scattering. This challenge could be overcome by using a probe light in the THz frequency region, which has a much longer wavelength than that of an excitation laser. Figure 2(b) shows the normalized THz-TA decay dynamics obtained under excitation with photon energies of 1.6 and 1.67 eV, which are close to the resonant excitation of the T-TA and O-TA bands, respectively. The THz-TA kinetic traces are very similar to the optical TA decay dynamics probed at the T-TA and O-TA bands. The THz-TA kinetic trace achieved in our THzTA measurements directly reflects the transient responses of the free carriers (See Figure S4 in SI). Thus, the experimental observation of the THz-TA decay dynamics strongly indicates that free carriers are generated near the band edge, even under 1.6-eV photoexcitation. In addition, the kinetic trace of the free carriers under 1.6-eV excitation can be described using a single exponential decay, meanwhile, that obtained under 1.67-eV excitation is nonexponential. This dissimilarity implies different physical characteristics of free carriers created under 1.6- and 1.67-eV photoexcitation. Taking into account entirely the transmission, PL, optical TA and THz-TA results, we consider that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at temperatures below the phase-transition temperature of 130 K. We confirmed that this two-phase coexistence is always observed in the polycrystalline thin-film samples regardless of cooling rate and excitation conditions, but never observed in the single-crystal MAPbI3 (data are not shown here).[24] A recent work has shown that the crystalline structure of grains in MAPbI3 thin film is quite sensitive to the surrounding external force.[35] Thus, we consider that a change in the


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structural phase might occur due to the strain imposed on the boundaries of the grains in the polycrystalline thin films.[36] Therefore, the T-PL and T-TA bands observed in the PL and TA spectra are likely to be originated from the tetragonal phase. On the other hand, the O-PL and OTA bands are owing to the orthorhombic phase. Here, it is important to mention that PL measurement is widely utilized to detect extremely-low-density localized states because PL occurs efficiently at low-energy localized states after energy relaxation. Thus, our optical measurements can probe the low-density low-energy tetragonal phase.

Figure 3. TA decay dynamics monitored at (a) the O-TA and (b) T-TA bands of a MAPbI3 thin film under various fluences of 2.1-eV photoexcitation at 15 K. The inverse of normalized TA decay dynamics monitored at (c) the O-TA and (d) T-TA bands as a function of the delay time. The solid lines are for guidance.


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In order to determine which type of near-band-edge excitation, i.e., excitons or free carriers, predominantly controls the optical dynamic responses in each phase of the MAPbI3 polycrystalline thin films at low temperatures, we performed excitation-fluence-dependent TA measurements. Figures 3(a) and (b) show the TA decay dynamics monitored at the O-TA and TTA bands, respectively, under various fluences of 2.1-eV photoexcitation at 15 K. The inverses of the normalized TA kinetic traces are plotted in Figures 3(c) and (d). Obviously, over the entire range of excitation fluences, the inverses of the TA decay dynamics monitored at the T-TA and O-TA bands depend linearly on the elapsed time after photoexcitation. Such a linear dependence on the delay time is also observed for the inverse of the TA kinetic traces recorded at RT (See Figure S5 in SI). This particular delay-time dependence of the inverse of the TA decay dynamics can be attributed to two-carrier recombination[16,17] or bimolecular exciton-exciton interactions[17,37] predominantly take place. Note that the TA spectrum remains unchanged with the delay time up to the nanosecond scale. The typical lifetime of biexcitons in semiconductors varies from a few tens to hundreds of picoseconds.[38,39] Assuming that the observed photobleaching bands are owed to biexcitons, additional photobleaching bands should emerge at the high-energy region because fresh excitons are generated following the decay of the biexcitons.[39] Additionally, the T-PL band can be observed in the time-integrated PL spectra even under weak excitation fluences where biexcitons are unlikely to be formed. Moreover, the photoconductivity spectra in the THz frequency region at low temperatures are similar to that measured at RT, and exhibits a unique frequency-dependent response of free carriers (See Figure S4 in SI). Therefore, the dynamic behavior of the photocarriers generated in both the tetragonal and orthorhombic phases in the MAPbI3 thin films at 15 K is primarily determined by two-carrier electron-hole


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recombination. This conclusion enables us to reasonably interpret the quadratic dependences of the T-PL and O-PL bands on the excitation fluence, which is shown in Figure 1(b).

Figure 4. (a) Two-dimensional contour image of time-resolved PL spectrum of a MAPbI3 thin film obtained under a fluence of 274 nJ/cm2 of 1.8-eV photoexcitation at 15 K. (b) PL spectra at different times elapsed after photoexcitation. (c) PL decay dynamics probed at the O-PL (dashed lines) and T-PL (solid lines) bands of the MAPbI3 thin films under various excitation fluences. The black dashed line represents the response function of the measurement system. Our previous study revealed an exciton binding energy of 6 meV at high temperatures for the MAPbI3 thin films,[23] which were prepared with the same procedure of those used in this work. Similar exciton binding energies of the tetragonal-phase MAPbI3 were observed recently by other groups.[31,32] Thus, it is understandable that the photocarrier dynamics in the tetragonal-


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phase MAPbI3 at RT[16,17] and even at 15 K are fundamentally controlled by free carriers. Meanwhile, the exciton binding energy of the orthorhombic-phase MAPbI3 was evaluated to be in the range of 16 – 63 meV.[23,29-32] Therefore, it is surprising that our experimental results indicate that free carriers, rather than excitons, contribute to the near-band-edge optical responses of the orthorhombic-phase MAPbI3. To reveal the physical reasons for the lack of excitonic features in the PL spectra, we focused on the transient decay of the O-PL and T-PL emission bands in the early delay time. Figure 4(a) shows a two-dimensional contour image of a representative time-resolved PL spectrum of the MAPbI3 thin films at 15 K measured under an excitation fluence of 274 nJ/cm2 with a time scale of 5 ns. The PL spectra at different times elapsed after photoexcitation are plotted in Figure 4(b). The O-PL band immediately emerges upon photoexcitation, and then quickly decays within 1 ns. As the O-PL band rapidly decays, the T-PL band simultaneously increases to its maximum along with a slight redshift of the peak energy. The redshift of the TPL peak energy at early elapsed times probably reflects the relaxation of the hot photocarriers. The PL decay dynamics monitored at the peak energies of the T-PL and O-PL bands under various excitation fluences are shown in Figure 4(c). The rise time of the T-PL band is almost equivalent to the fast decay of the O-PL band. Under weak excitation condition, the rise time of the T-PL band could not be resolved due to the limitation of temporal resolution of measurement system, which is approximately 35 ps. The consistency between the rise and decay times of the T- PL and O-PL bands, respectively, indicates very fast charge transfer from the orthorhombic phase to the tetragonal phase. We consider that the fast charge transfer potentially suppresses the formation of excitons in the orthorhombic phase.


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In conclusion, we studied the photocarrier dynamics in MAPbI3 polycrystalline thin films using a combination of various time-resolved optical spectroscopy including time-resolved PL, TA and THz-TA. We found that a minor volume fraction of the high-temperature tetragonal phase still maintains in the MAPbI3 thin films at low temperatures in addition to the dominant low-temperature orthorhombic phase. The near-band-edge optical responses of the photocarriers in both the tetragonal phase and the orthorhombic phase in the MAPbI3 thin films at low temperatures are primarily determined by free carriers rather than excitons. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase in the MAPbI3 thin films at low temperatures.

EXPERIMENTAL METHODS Thin Film Preparation: Bare MAPbI3 thin films were fabricated using two-step method on quartz substrates for measurements in THz frequency region. In a glove box filled with an inert gas, a 1.0 M solution of PbI2 (L0279 for perovskite precursor, Tokyo Chemical Industry Co., Ltd., Japan) in dehydrated dimethylformamide at 70 °C was deposited on a quartz substrate by spin-coating (slope 5 s, 6500 rpm, 5 s, slope 5 s). The resulting yellow film was annealed on a hot plate at 70 °C for 1 h. The annealed PbI2 film was dipped in a 0.06 M 2-propanol solution of CH3NH3I (Tokyo Chemical Industry Co., Ltd., Japan) for 40 s. The formed perovskite film was then annealed on a hot plate at 70 °C for 1 h. After fabrication, the samples were kept in vacuum at room temperature for several days before the optical measurements; this process helps to reduce significantly the unreacted PbI2, and subsequently, improves the quality of thin-film


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samples.[40] A typical SEM image of a MAPbI3 thin film is shown in Figure S1. For low temperature measurements, the samples were attached to a cold finger of a He-flow-type cryostat. Transmission spectroscopy: A white-light picosecond laser was utilized as excitation source in the transmission measurement. Monochromatic lights with photon energies ranging from 1.46 to 1.8 eV were extracted using a monochromator, and chopped by an optical chopper with a frequency of 400 Hz, and then focused loosely on the sample surface. The transmitted light was detected using a Si photodiode and recorded by a lock-in amplifier. PL and TA spectroscopy: PL and TA measurements were performed based on a Yb:KGW regenerative amplified laser (pulse duration: ~300 fs, repetition rate: 50 – 100 kHz). The wavelength-tunable excitation lights emitted from an optical parametric amplifier were focused on the sample with spot sizes of ~200 and ~400 µm in the PL and TA measurements, respectively. The PL signal was collected in a backward configuration, and then directed to a 50cm monochromator equipped with a liquid-nitrogen-cooled Si charge-coupled device. The spectral response of all steady-state PL measurements was calibrated using a standard lamp. Time-resolved PL measurements were conducted using a visible streak camera and a monochromator. In TA measurements, the excitation light was chopped with a frequency of 130 Hz and directed to overlap with the white-light probe on the sample surfaces. The white-light probe pulses were generated by focusing the seed 1030-nm light on a sapphire crystal, and irradiated upon the sample with a spot size of 200 µm. The transmitted probe light was collected by a Si detector and a monochromator. THz-TA spectroscopy: THz-TA measurements were carried out using a Ti:sapphire regenerative amplified laser (1-kHz repetition rate and 35-fs pulse duration). Wavelength-tunable excitation pump pulses emitted from an optical parametric amplifier were chopped with a


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frequency of 250 Hz and the collimated excitation beam then was guided to the samples, which were attached to a cold finger with a 2-mm-diameter hole of a He-flow-type cryostat. A THzprobe pulse generated from two-color pumped air plasma was directed to the excitation spot with an incident angle of 30o. The electric-field profile of the transmitted THz pulse was detected using the electro-optic sampling method by utilizing a 1-mm-thick ZnTe crystal. The detectable frequency ranged from 0.5 to 2.5 THz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Sample morphology, transmission spectra at different temperatures, time-resolved PL spectra and PL decay dynamics at 15 K, steady-state PL spectra at different temperatures, timeintegrated optical TA spectra at different temperatures, typical THz photoconductivity spectra at 18 K and 297 K. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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Part of this work was supported by JST-CREST and the MEXT Project of Integrated Research on Chemical Synthesis. We thank G. Yamashita (Osaka University) for supports in THz transient absorption measurement. REFERENCES (1) Stranks, S. D.; Snaith, H. J. Metal-halide Perovskites for Photovoltaic and Light-emitting Devices. Nat. Nanotech. 2015, 10, 391-402. (2) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051 (3) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J.E.; et al. Lead Iodide Perovskite Sensitized All-solid-state Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (4) 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. (5) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.;Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as A Route to High-performance Perovskite-sensitized Solar Cells. Nature 2013, 499, 316-319. (6) Liu, M; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (7) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546.


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(8) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-performance Solar Cells. Nature 2015, 517, 476-480. (9) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Highperformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234-1237. (10) http://www.nrel.gov/ncpv/images/efficiency_chart.jpg for record cell efficiencies. (11) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables. Prog. Photovoltaics 2015, 23, 805-812. (12) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole Diffusion Lengths Exceeding 1 Micrometer in An Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341344. (13) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-range Balanced Electron- and Hole-transport Lengths in Organicinorganic CH3NH3PbI3. Science 2013, 342, 344-347. (14) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J., Herz, L. M. High Charge Carrier Mobilities and Lifetimes in Organometal Trihalide Perovskites. Adv. Mater. 2014, 26, 1584-1589. (15) Ponseca, C. S.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; et al. Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-long Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189-5192. (16) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Photocarrier Recombination Dynamics in Perovskite CH3NH3PbI3 for Solar Cell Applications. J. Am. Chem. Soc. 2014, 136, 11610-11613.


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(17) Manser, J. S.; Kamat, P. V. Band Filling with Free Charge Carriers in Organometal Halide Perovskites. Nat. Photon. 2014, 8, 737-743 and references therein. (18) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qui, J.; Cao, L.; Huang, J. Electron-hole Diffusion Lengths > 175 µm in Solution-grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. (19) Shi, D; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-state Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519522. (20) Yamada, Y.; Yamada, T.; Phuong, L. Q.; Maruyama, N.; Nishimura, H.; Wakamiya, A.; Murata, Y.; Kanemitsu, Y. Dynamic Optical Properties of CH3NH3PbI3 Single Crystals as Revealed by One- and Two-photon Excited Photoluminescence Measurements. J. Am. Chem. Soc. 2015, 137, 10456-10459. (21) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Near-band-edge Optical Responses of Solution-processed Organic−inorganic Hybrid Perovskite CH3NH3PbI3 on Mesoporous TiO2 Electrodes. Appl. Phys. Express 2014, 7, 032302. (22) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge Carrier Recombination Channels in Low-temperature Phase of Organic-inorganic Lead Halide Perovskite Thin Films. APL Mater. 2014, 2, 081513. (23) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Photoelectronic Responses in Solution-processed Perovskite CH3NH3PbI3 Solar Cells Studied by Photoluminescence and Photoabsorption Spectroscopy. IEEE J. Photovoltaics 2015, 5, 401-405. (24) Fang, H.-H.; Raissa, R.; Abdu-Aguye, M.; Adjokatse, S.; Blake, G. R.; Even, J.; Loi, M. A. Photophysics of Organic-inorganic Hybrid Lead Iodide Perovskite Single Crystals. Adv. Funct. Mater. 2015, 25, 2378-2385.


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(25) Milot, R. L.; Eperon, G. E.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Temperaturedependent Charge-carrier Dynamics in CH3NH3PbI3 Perovskite Thin Films. Adv. Funct. Mater. 2015, 25, 6218-6227. (26) Kong, W.; Ye, Z.; Qi, Z.; Zhang, B.; Wang, M.; Rahimi-Iman, A.; Wu, H. Characterization of An Abnormal Photoluminescence Behavior upon Crystal-phase Transition of Perovskite CH3NH3PbI3. Phys. Chem. Chem. Phys. 2015, 17, 1640516411. (27) Tahara, H.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Experimental Evidence of Localized Shallow States in Orthorhombic Phase of CH3NH3PbI3 Perovskite Thin Films Revealed by Photocurrent Beat Spectroscopy. J. Phys. Chem. C 2016, 120, 5347-5352. (28) Poglitsch, A.; Weber, D. Dynamic Disorder in Methylammoniumtrihalogenoplumbates (II) Observed by Millimeter-wave Spectroscopy. J. Chem. Phys. 1987, 87, 6373-6378. (29) Hirasawa, M.; Ishihara, T.; Goto, T.; Uchida, K.; Miura, N. Magnetoabsorption of The Lowest Exciton in Perovskite-type Compound (CH3NH3)PbI3. Physica B 1994, 201, 427-430. (30) Tanaka, K.; Takahashi, T.; Ban, T.; Kondo, T.; Uchida, K.; Miura, N. Comparative Study on The Excitons in Lead-halide-based Perovskite-type Crystals CH3NH3PbBr3 CH3NH3PbI3. Solid State Commun. 2003, 127, 619-623. (31) Lin, Q.; Armin, A.; Nagiri, R. C. R; Burn, P. L.; Meredith, P. Electro-optics of Perovskite Solar Cells. Nat. Photon. 2015, 9, 106-112. (32) Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugal, O.; Wang, J. T.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct Measurement of The Exciton Binding Energy and Effective Masses for Charge Carriers in Organic-inorganic Tri-halide Perovskites. Nat. Phys. 2015, 11, 582-587. (33) Phuong, L. Q.; Okano, M.; Yamashita, G.; Nagai, M.; Ashida, M.; Nagaoka, A.; Yoshino, K.; Kanemitsu, Y. Free-carrier Dynamics and Band Tails in Cu2ZnSn(SxSe1-


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x)4:

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Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

6 The Journal of Physical Chemistry Letters

10

6

10

5

10

10

T-PL

(a)

15 K

548 nJ/cm

4

10

3

10

2

D-PL

10

1

10

5

10

O-PL

2

PL Intensity (arb. units)


Page 217 of 24

2

0.98 nJ/cm (c)

(b) 2

4

I

10

3

10

2

1

10

I

1

2

10

I

0

D-PL T-PL O-PL

10

-1

10 1.3

1.4

1.5

1.6

-2 10 0 ACS Paragon Plus Environment 1.7 1.8

Photon Energy (eV)

10

1

2

10 10 2 Excitation Density (nJ/cm )

3

10

-3

∆OD (x 10 ) -∆OD (Normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Page 22 of 24 8 The Journal of Physical Chemistry Letters O-TA 15 K 6 10 ps 300 ps 4 750 ps T-TA 2 (a) 1500 ps

0 1.4

1.5

1.5

1.6 1.7 1.8 Photon Energy (eV) THz-TA

1.0

1.9

2.0

Excitation 1.67 eV 1.60 eV

0.5 (b) 0.0


0

200

400 600 Time (ps)

800

1000

Page 23 of 24

Eprobe = 1.66 - 1.69 eV 15 K

0.3

Eprobe = 1.66 - 1.69 eV -1

6 ∆OD

∆OD

4

(a)

(c) 2 15

0.51 µJ/cm

2

1.02 µJ/cm

2

2.04 µJ/cm

2

4.07 µJ/cm 8.15µJ/cm

2

2

21.4 µJ/cm

2

-1

∆OD

Eprobe = 1.59 - 1.61 eV (b)

10

5

0

500

1000 Time (ps)

Eprobe = 1.59 - 1.61 eV

∆OD

1 2 3 4 0.2 5 6 7 8 0.1 9 10 11 0.0 120.06 13 14 15 160.04 17 18 19 200.02 21 22 23 240.00 25 26

The Journal of Physical Chemistry Letters 8

(d)


1500

0

500

1000 Time (ps)

1500


0

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(c)

T-PL O-PL

1 2 3 4 600

15 K 0 -14 ps 6 - 67 ps 107 - 168 ps 1430 - 1490 ps

400

T-PL O-PL

548 nJ/cm

PL Intensity (normalized)


Time (ns)

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

(a)

274 nJ/cm 137 nJ/cm 70 nJ/cm

35 nJ/cm

200

9 nJ/cm

(b) 1.50


1.55 1.60 1.65 Photon Energy (eV)

0.0

0.5 1.0 Time (ns)

2

2

2

2

17 nJ/cm

0

2

2

2

2

4

Free Carriers versus Excitons in CH3NH3PbI3 Perovskite Thin Films

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