Free Excitons and Exciton–Phonon Coupling in CH3NH3PbI3 Single

Letter pubs.acs.org/JPCL

Free Excitons and Exciton−Phonon Coupling in CH3NH3PbI3 Single Crystals Revealed by Photocurrent and Photoluminescence Measurements at Low Temperatures Le Quang Phuong, Yumi Nakaike, Atsushi Wakamiya, and Yoshihiko Kanemitsu* Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: We investigated the exciton−phonon couplings and exciton binding energy in CH3NH3PbI3 (MAPbI3) single crystals using temperature-dependent photocurrent (PC) and photoluminescence (PL) spectroscopy. The PC and PL data provide clear evidence of the existence of excitons in orthorhombic-phase MAPbI3. The temperature-dependent PC data were found to be less influenced by the bound excitons than the PL data, and thus the PC data reflect the intrinsic scatterings of excitons. We observed that the exciton−phonon couplings were strong in MAPbI3 and determined the longitudinal optical phonon energy to be 16.1 meV. Moreover, on the basis of the temperature dependences of the PC and PL data, we evaluated the exciton binding energy to be 12.4 meV for orthorhombic-phase MAPbI3 single crystals. Our findings pave a way for using simultaneous PC and PL measurements to determine precisely fundamental properties of perovskites.

O

several emission bands,9−13 and spectral overlap impedes precise extraction of the spectral width. A combination of different spectroscopic techniques should enable proper investigation of single-crystal samples.14,15 Along with the rising debate on the electron− and exciton− phonon interactions,7,8,16 tremendous efforts have been devoted to the long-standing issue of the exciton binding energies of MAPbX3 materials.17−20 The near-band-edge optical properties of polycrystalline MAPbX3 thin films are sensitive to the films’ grain structures and compositions.21 Therefore, evaluations of the exciton binding energies of thin films from optical absorption spectroscopy might be influenced by these factors, causing the wide variation of the results acquired in previous investigations.17−20 Meanwhile, the exciton binding energies in MAPbX3 single crystals have been estimated in only a few studies with large uncertainties mainly due to the technical difficulties caused by the large thicknesses of these crystals.22 In this Letter, we describe a study of the exciton properties in orthorhombic-phase MAPbI3 single crystals using temperaturedependent photocurrent (PC) and PL spectroscopy. The obtained PC and PL data, which are presented herein, are consistent and clearly indicate the existence of free excitons in MAPbI3 single crystals at low temperatures. We further demonstrate that the temperature-dependent PC spectral width reflects the scattering processes of the free excitons more accurately than the PL spectral width. We then are able to evaluate the LO phonon energy to be 16.1 meV. In addition, by analyzing the temperature-dependent PC and PL data, we

ver the past few years, hybrid lead halide perovskite materials, in particular, methylammonium lead halide CH3NH3PbX3 (MAPbX3, X = I, Br, and Cl), have attracted global attention because of their very high potentials for uses in solar cell, light-emitting diode, photodetector, and lasing applications.1−3 The transport of charge carriers and excitons in MAPbX3 is one of the factors that most strongly influences the performances of such devices. Because the transport of charge carriers and excitons is closely correlated with the scatterings of carriers and excitons, it is of great importance to understand the scattering processes in MAPbX3 thoroughly, including the scattering by phonons and impurities. The scattering by charged impurities in MAPbX3 materials is negligible at room temperature.4−6 The strong coupling of charge carriers with phonons results in the formation of polarons,7,8 influencing the transport properties of MAPbX3. The electron−phonon interaction at high temperatures and the exciton−phonon interactions at low temperatures have mainly been evaluated through analysis of the temperature-dependent photoluminescence (PL) spectral width.9,10 However, most previous studies were performed on thin-film samples, which contain different polycrystalline grains. As a result, large spectral widths of up to several tens of millielectronvolts (meV) have been obtained even at low temperatures.9,10 Such broad spectral widths prevent correct understanding of acoustic phonon scattering at low temperatures because this scattering usually lead to broadening less than the inhomogeneous spectral width, and neglecting the acoustic phonon contribution might affect the evaluation of the interactions between excitons and longitudinal optical (LO) phonons. Therefore, studies of single crystals are needed to understand the exciton−phonon scattering at low temperatures. However, the PL spectra of MAPbX3 single crystals at low temperatures usually consist of © XXXX American Chemical Society

Received: October 19, 2016 Accepted: November 16, 2016 Published: November 16, 2016 4905

DOI: 10.1021/acs.jpclett.6b02432 J. Phys. Chem. Lett. 2016, 7, 4905−4910

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The Journal of Physical Chemistry Letters obtain an exciton binding energy of 12.4 meV for MAPbI3 single crystals at low temperatures. Our results illustrate that simultaneous PC and PL measurements are versatile and powerful methods for investigating photoelectronic properties of organometal halide perovskites. Figure 1 shows representative steady-state PC and PL spectra of MAPbI3 single crystals acquired at 15 K. The inset depicts

Figure 2a−f presents the PC and PL spectra of the MAPbI3 single crystals at 40, 60, 80, 120, 160, and 200 K, respectively.

Figure 1. Typical PC (red) and PL (blue) spectra of MAPbI3 single crystals measured at 15 K. The excitation photon energy is tuned from 1.4 to 1.8 eV for PC measurements and set at 2.07 eV for PL measurements. The inset shows enlarged versions of narrow regions of the same spectra.

Figure 2. PC (red circles) and PL (blue plus signs) spectra of MAPbI3 single crystals at different temperatures. Solid red and black curves are the Gaussian fits of the PC peak and the FE PL band, respectively.

The FE band and PC peak shift to the high-energy side and become broader with increasing temperature (see Figure S2 in the SI). The sharp PC peak due to the free excitons decreases and merges with the PC contribution of the free carriers as the temperature increases beyond 100 K. When the temperature is higher than 120 K, a new emission band, which is red-shifted as the temperature increases to 160 K, arises on the low-energy side of the PL spectra, and another PC structure concurrently emerges in almost the same energy region. As the temperature increases beyond 160 K, only the low-energy emission band in the PL spectrum remains, coincident with the edge of the PC spectrum. This PL emission band and the edge of the PC spectrum are then blue-shifted as the temperature increases further. Such temperature dependence of the PL spectrum of MAPbI3 has been widely observed.9−13 The low-energy structures emerging in the PL and PC spectra as the temperature approaches the phase-transition temperature demonstrate the appearance of the high-temperature tetragonal phase. The drastic changes in the PL and PC spectra at ∼160 K represent the phase transition from the orthorhombic phase to the tetragonal phase.11,25 It is commonly accepted that the PL emission band of tetragonal-phase MAPbI3 originates from the radiative recombination of free electrons and holes in the continuum bands.13,26,27 The coincident edges of the PL and PC spectra provide additional evidence in support of the twocarrier radiative recombination model in tetragonal-phase MAPbI3. The excellent agreement between the temperature dependence of the PL spectrum, which has been well studied,9−13 and that of the PC spectrum strongly indicates that PC measurement is a reliable method for characterizing the near-band-edge photoelectrical properties of MAPbI3 single crystals. To investigate the exciton−phonon coupling in orthorhombic-phase MAPbI3 single crystals, we analyzed the temperaturedependent full widths at half-maximum (FWHM) of the FE PL band and PC peak observed at low temperatures. Considering the contribution of the spectral broadening, we used Gaussian

enlarged versions of narrow regions of the same spectra for clarity. A sharp and dominant peak is observable at 1.637 eV in the PC spectrum, followed by a region of lower PC that gradually increases with increasing photon energy. Consistently, a narrow PL emission band, denoted as FE, is also evident at 1.637 eV, as are two predominant PL bands at 1.618 and 1.61 eV, denoted as BE1 and BE2, respectively, and a broad band, denoted as D-PL, on the low-energy side. The three lowest energy PL bands were observed to decrease relative to the FE band with increasing temperature, and they disappeared completely when the temperatures exceeded 100 K (see Figure S1 in SI). Meanwhile, the FE band and PC peak were blueshifted with increasing temperature (see Figure S2 in SI); this tendency agrees with the previously reported temperaturedependent shift of the band edge of orthorhombic-phase MAPbI3 thin films.11,18 Moreover, the intensity of the FE band was found to depend linearly on the excitation fluence, and the FE, BE1, and BE2 bands decayed within a few nanoseconds after photoexcitation, unlike the D-PL band, which decayed only after several microseconds (see Figure S3 in the SI). Note that a peak structure in the PC spectrum due to exciton absorption in inorganic semiconductors has been reported.23,24 Therefore, we assign the narrow FE band in the PL spectrum and the sharp PC peak in the PC spectrum to the free excitons in the MAPbI3 single crystals. Meanwhile, the PC at the high-energy side is attributable to PC from free carriers. The slowly decaying D-PL band is related to the defects in MAPbI3, while the BE1 and BE2 bands probably originate from the bound excitons (excitons are bound to impurities or defects). Within the temporal resolution (∼20 ps) of the measurement system, no rise times were detected for the D-PL, BE1, and BE2 bands. This behavior is different from the PL dynamics in thin films consisting of two phases at low temperatures reported in ref 13. The broad, low-energy shoulder that appears in the PC spectrum in the energy region of the BE1 and BE2 bands reflects the contribution of the bound excitons to the PC signal.23,24 4906

DOI: 10.1021/acs.jpclett.6b02432 J. Phys. Chem. Lett. 2016, 7, 4905−4910

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The Journal of Physical Chemistry Letters functions to fit the FE PL band and PC peak. The FWHMs of the FE PL band and PC peak, denoted as ΓPL and ΓPC, respectively, are plotted in Figure 3 as functions of temperature.

the scatterings with acoustic and optical phonons and charged impurities.34,35 The curvature of the temperature-dependent ΓPC of MAPbI3 single crystals suggests that the contribution of charge impurity scattering, which generally causes an opposite curvature,9,35,36 is negligible; this finding is consistent with those of recent studies of carrier mobility in MAPbI3 thin films.4−6 Thus we utilized the following equation to fit the temperature-dependent ΓPC data ΓPC(T ) = Γ0 + γacT +

γLO E LO kT

( )−1

exp

(1)

where Γ0 is the temperature-independent inhomogeneous spectral width at T = 0 K; γac and γLO are the coupling strengths of the excitons with the acoustic and LO phonons, respectively; and ELO is the LO phonon energy. We obtained Γ0 = 4.1 ± 0.4 meV, γac = 93.5 ± 20.5 μeV K−1, γLO = 57 ± 22 meV, and ELO = 16.1 ± 3.4 meV. The Γ0 value for our MAPbI3 single crystals is much smaller than that reported for polycrystalline thin films.9,10 We thus are able to reveal that the coupling γac between the free excitons and the acoustic phonons in MAPbI3 is significantly greater than that in most conventional inorganic semiconductors.36−40 The acoustic phonon scattering therefore plays an important role in determining the transport properties in MAPbI3 at low temperatures.4−6 It is noteworthy that the acoustic phonon coupling at low temperatures might not be properly evaluated in previous works on polycrystalline thin films because of the large Γ0 values estimated using PL measurements. 9,10 Neglecting the acoustic phonon contribution might lead to an underestimation of the LO phonon energy in MAPbI3, deduced based on the PL spectral width. The value of ELO for orthorhombic-phase MAPbI3 single crystals, 16.1 ± 3.4 meV, achieved from our PC measurements is close to the values reported in previous studies using reflection and Raman spectroscopy.22,41 The large phonon coupling coefficients γac and γLO in MAPbI3 as compared with those in conventional inorganic semiconductors36−40 might be partly responsible for the moderate carrier mobility in MAPbI3 recently claimed.4−6,42 Because the PC peak at low temperatures is strongly associated with the existence of free excitons, we examined the temperature-dependent PC peak intensity to evaluate the binding energy of the free excitons in the orthorhombic-phase MAPbI3 single crystals. To extract the intrinsic temperature dependence of the free-exciton PC contribution ΔPCEx, we subtracted the PC intensity at excitation photon energy far above the bandgap, denoted as PCFC, from the PC peak intensity. Figure 4a,b shows the temperature dependences of the PCFC value obtained at band-to-band 2.07 eV photoexcitation and ΔPCEx, respectively. Both PCFC and ΔPCEx initially increase with temperature and then decrease in the higher-temperature region; ΔPCEx begins to decrease earlier than PCFC and does so more rapidly. We also observed an increase in the integrated intensity of the free-exciton PL band in the low-temperature region. Because most free carriers and excitons are captured by trap states at low temperatures, increasing the temperature thermally releases them, leading to the increases in PCFC and ΔPCEx. In the higher temperature region, the nonradiative loss is enhanced and causes PCFC to decrease. In addition to the nonradiative loss, the thermal dissociation of the free excitons contributes to the decrease in ΔPCEx. Assuming that the nonradiative loss at high temperatures is mainly due to the LO phonons with ELO = 16.1 meV

Figure 3. FWHMs of the PC peak ΓPC (red circles) and of the FE PL band ΓPL (blue circles) obtained for MAPbI3 single crystals as functions of temperature. The dashed black curve shows the fit for the temperature-dependent ΓPC.

At 15 K, ΓPL and ΓPC are similar and approximately equal to 5.6 meV. As the temperature increases, both ΓPL and ΓPC increase. Interestingly, ΓPL surpasses ΓPC in the high-temperature region. The values of ΓPL and ΓPC obtained from our MAPbI3 single crystals are much smaller than those previously reported for polycrystalline thin films.9,10 The PC signal is determined primarily by the absorption coefficient, carrier extraction efficiency, and carrier loss rate. For direct-gap semiconductors, photocarriers are generated closer to the sample surface if a higher photon energy is used for excitation, which is a result of optical penetration depth shortening due to increased absorption. Because numerous nonradiative sites exist near the sample surface, photogenerated carriers are quickly captured by trapping sites. Consequently, PC spectra usually decrease significantly in the above-band-gap energy regions.14,28 However, considerable reduction at high energies does not appear in our PC spectra. This result indicates that the surface state density of the MAPbI3 single crystals is low, consistent with recent reports.29,30 Thus the PC spectra measured in our experiments were less influenced by nonradiative surface loss and reflect the intrinsic transport properties of photocarriers in MAPbI3 single crystals. Because the free carrier extraction efficiency is likely to be independent of the photoexcitation energy, the shapes of the PC spectra mainly reflect that of the absorption spectrum, and the increase in ΓPC with increasing temperature is due to the spectral broadening of the freeexciton absorption peak. Because bound excitons usually possess giant oscillator strengths at low temperatures, the intrinsic free-exciton PL properties are obscured by the dominant contribution of the bound excitons.31 The broader spectral width ΓPL and lower peak energy of the FE band in comparison with those of the PC at the higher-temperature region (see Figure S2 in SI) imply that the bound excitons affect the PL spectra. In addition, the PL spectra of MAPbI3 might be influenced by the photonrecycling effect.27,32,33 Therefore, the temperature-dependent ΓPC reflects more accurately the scatterings of the free excitons in MAPbI3 single crystals at low temperatures than the temperature-dependent ΓPL. In semiconductors, the temperature-dependent spectral broadening of the exciton absorption is generally caused by 4907

DOI: 10.1021/acs.jpclett.6b02432 J. Phys. Chem. Lett. 2016, 7, 4905−4910

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The Journal of Physical Chemistry Letters

resulting solution was filtered and poured into a screw-cap vial, which was placed on a hot plate at 110 °C for 6 days to produce a single crystal with typical dimensions of 5 × 5 × 3 mm3. For PC measurement, Ti/Au electrodes were evaporated on MAPbI3 single crystals by a resistive heating in vacuum. Two rectangular-shaped electrodes were separated by 50 μm. The samples were mounted onto a coldfinger of a He-flow-type cryostat for temperature-dependent PC and PL measurements. PC and PL Spectroscopy. In the PL and PC measurements, tunable-wavelength monochromatic light extracted from a broadband white-light picosecond laser (40 MHz) using a monochromator was chopped by an optical chopper with a frequency of 400 Hz and then utilized as the photoexcitation source. The excitation photon energy was tuned from 1.4 to 2.2 eV for PC measurements, and was set at 2.07 eV for PL measurements. The PC signal under a bias voltage was amplified and converted to voltage using an I−V converter and then was collected by a lock-in amplifier. The PL signal was recorded by a liquid-nitrogen-cooled Si charge-coupled device through a 30 cm monochromator. The spectral resolution of the PL measurement system is ∼1.3 meV. In the time-resolved PL measurements, the 2.07 eV pulse emitted from an optical parametric amplifier pumped by a Yb:KGW (potassium gadolinium tungstate) regenerative amplified laser with a pulse duration of 300 fs and a repetition rated of 20 kHz was used to excite the MAPbI3 single crystals. The PL decays were recorded using a visible streak camera. The temporal resolution of the system is ∼20 ps.

Figure 4. Temperature dependences of (a) the PC intensity PCFC obtained from band-to-band excitation and (b) the free-exciton PC contribution ΔPCEx. The dashed curves are the fitting curves.

and considering the thermal activation from trap states at low temperatures, we were able to explain the temperature dependence of PCFC, as shown by the black dashed curve in Figure 4a (see the fitting procedures in the SI). We then used the results to fit the temperature-dependent ΔPCEx and obtained an exciton binding energy Eb of 12.4 ± 1.2 meV for the MAPbI3 single crystals at low temperatures. The exciton binding energy we determined is very similar to that observed using magneto-absorption measurement in a recent study.20 Note that our value is obtained for the single crystals, and thus it is free of the influences of grain structures and compositions that might affect evaluation of the exciton binding energy for polycrystalline thin films. The analysis using only PL measurement might overestimate the exciton binding energy because of the existence of nonradiative recombination processes. In conclusion, we studied the exciton−phonon coupling and the exciton binding energy in MAPbI3 single crystals using temperature-dependent PC and PL spectroscopy. The spectroscopic data clearly indicated the existence of free excitons in MAPbI3 single crystals at low temperatures. We found that PC spectroscopy is more appropriate than PL spectroscopy for evaluating the scattering processes of free excitons in MAPbI3 at low temperatures. We determined that the exciton−phonon coupling strengths in MAPbI3 are stronger than those in conventional semiconductors and estimate the LO phonon energy to be 16.1 meV. In addition, we proposed a simple model to explain the temperature-dependent PC data and obtained an exciton binding energy of 12.4 meV for MAPbI3 single crystals at low temperatures. Our findings provide a deep understanding of the scattering processes of excitons and the near-band-edge photoelectrical properties of perovskites.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02432. Fitting procedures; temperature-dependent PL spectra and PL and PC peak energies; excitation-fluencedependent intensity of the FE band at 15 K; and PL decay dynamics of different emission bands at 15 K. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoshihiko Kanemitsu: 0000-0002-0788-131X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was supported by JST-CREST and JSPS KAKENHI (16F16017). We thank Prof. Y. Yamada (Chiba University) and T. Yamada (Kyoto University) for support in preparing the electrodes and Dr. D. M. Tex and Dr. H. Tahara (Kyoto University) for fruitful discussion.

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EXPERIMENTAL METHODS Sample Preparation. CH3NH3PbI3 (MAPbI3) single crystals were prepared according to previously established protocol.27 In brief, MAPbI3 single crystals were fabricated from 1.1 M γbutyrolactone solution at 110 °C. Purified lead iodide (L0279 for perovskite precursor, Tokyo Chemical Industry, Japan) (1383 mg, 3.0 mmol) and methylammonium iodide (Tokyo Chemical Industry, Japan) (477 mg, 3.0 mmol) were dissolved in dry γ-butyrolactone (2.7 mL) at 70 °C by stirring. The

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DOI: 10.1021/acs.jpclett.6b02432 J. Phys. Chem. Lett. 2016, 7, 4905−4910

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The Journal of Physical Chemistry Letters (38) Malikova, L.; Krystek, W.; Pollak, F.; Dai, N.; Cavus, A.; Tamargo, M. C. Temperature Dependence of the Direct Gaps of ZnSe and Zn0.56Cd0.44Se. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 1819. (39) Gopal, A. V.; Kumar, R.; Vengurlekar, A. S.; Bosacchi, A.; Franchi, S.; Pfeiffer, L. N. Photoluminescence Study of Exciton− optical Phonon Scattering in Bulk GaAs and GaAs Quantum Wells. J. Appl. Phys. 2000, 87, 1858. (40) Kanemitsu, Y.; Nagai, T.; Yamada, Y.; Taguchi, T. Temperature Dependence of Free-exciton Luminescence in Cubic CdS Films. Appl. Phys. Lett. 2003, 82, 388. (41) Quarti, C.; Grancini, G.; Mosconi, E.; Bruno, P.; Ball, J. M.; Lee, M. M.; Snaith, H. J.; Petrozza, A.; Angelis, F. D. The Raman Spectrum of the CH3NH3PbI3 Hybrid Perovskite: Interplay of Theory and Experiment. J. Phys. Chem. Lett. 2014, 5, 279−284. (42) Brenner, T. M.; Egger, D. A.; Rappe, A. M.; Kronik, L.; Hodes, G.; Cahen, D. Are Mobilities in Hybrid Organic−Inorganic Halide Perovskites Actually “High”? J. Phys. Chem. Lett. 2015, 6, 4754−4757.

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DOI: 10.1021/acs.jpclett.6b02432 J. Phys. Chem. Lett. 2016, 7, 4905−4910