Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3

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Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr Single Crystal 3

Olga A. Lozhkina, Vsevolod I. Yudin, Anna A. Murashkina, Vladimir V. Shilovskikh, Valentin G. Davydov, Ruslan Kevorkyants, Alexei V Emeline, Yury V. Kapitonov, and Detlef W. Bahnemann J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02979 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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The Journal of Physical Chemistry Letters

Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3 Single Crystal Olga A. Lozhkina1, Vyacheslav I. Yudin1, Anna A. Murashkina1, Vladimir V. Shilovskikh1, Valentin G. Davydov1, Ruslan Kevorkyants1, Alexei V. Emeline1*, Yury V. Kapitonov1, Detlef W. Bahnemann1,2 1

Saint-Petersburg State University, ul. Ulyanovskaya 1, Saint-Petersburg, 198504, Russia.

2

Leibniz University of Hannover, Callinstrasse 3, Hannover, 30167, Germany.

*Corresponding Author [email protected]

We present optical study of MAPbBr3 single crystal grown from solution. The crystal Pm3m symmetry was confirmed by electron backscatter diffraction. Our major attention was focused on optical effects related to the excitonic states in MAPbBr3. Photoluminescence temperature dependence of narrow exciton resonance showed encouragingly low inhomogeneous broadening Γ ≈ 0.5 meV that allows to distinguish the signals from free excitons and those arising from recombination of excitons localized on defects. Excitonic origin of the resonance was proved by its superlinear pump intensity dependence in contrast to the linear behavior of the defect-assisted recombination bands. For the first time the phonon replicas originating from free exciton recombination accompanied by partial energy transfer to the phonons, were observed in highresolution PL spectra and confirmed by independent low-temperature Raman scattering experiments. In turn, low temperature low frequency Raman scattering studies let us to resolve the structure of low frequency phonon spectrum.

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KEYWORDS Lead-halide perovskites, low temperature photoluminescence, inhomogeneous broadening, phonon replicas, low temperature Raman scattering. In the recent years lead-halide perovskites have drawn considerable attention due to their superior optoelectronic properties such as extremely high luminescence efficiencies1 and substantial charge carrier diffusion lengths2. These properties pave the way for a number of technologically important photo-physical applications, e.g.solar energy conversion. At present, solar cells based on these perovskites achieve conversion efficiencies of ~20%3. Electronic properties of the photoactive lead-halide perovskites are due to electronic energy levels of the lead and halogen atoms. Main contributions into their valence bands come from the halogen’s p-electrons and the lead’s 5 s-electron lone pair antibonding orbitals. The perovskite’s conduction bands consist mostly of the halogen’s p-electrons and the lead 5 p-electron bonding orbitals4. This leads to a defect tolerance of the perovskite’s electronic structure. That is, the formation energy of deep defects is high5, which, in turn, enables facile synthesis of the leadhalide perovskites. Besides the use in solar cells, the lead-halide perovskites can be utilized in X-ray6 and γdetectors7, optical imaging8, and other devices. This requires thorough understanding of fundamental properties of the perovskites and their relation to the device’s performance. In


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general, optical and vibrational characteristics of materials are best understood in the studies on their single crystals due to elimination of phenomena related to grain boundaries. Several recent works were devoted to the solution-processed perovskite single crystals of large size (up to a few mm) and high optical quality9-12. In this work we present optical study of high-quality solutionprocessed methylammonium lead tribromide single crystal. The recorded low temperature PL spectrum of the MAPbBr3 single crystal is shown in the Figure 1a. We assign the most pronounced peak of the figure centered at 2.250 eV to recombination of the free exciton (FE). The two orders of magnitude weaker blue-shifted peak at 2.265 eV we attribute to the band-to-band transition (radiative recombination, RR) based on the perovskite's band gap and the exciton binding energy determined in the paper 12. The redshifted, broad, low-intensity bands correspond to recombination of the excitons localized on defects (e.g. Shockley-Read-Hall recombination centers, SRH). Sharp satellites from low-energy side of the FE peak are attributed to phonon replicas and will be discussed below.

Figure 1. PL spectrum of MAPbBr3 single crystal (log scale) (a), Normalized pump intensity dependence of PL spectra (b), inset – peaks' intensity dependencies. Assigned peaks: FE – free exciton, RR – radiative recombination, SRH – Shockley-Read-Hall recombination.”


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Pump intensity dependence of the PL spectra of MAPbBr3 recorded at T = 8 K is shown in the Figure 1b. The FE peak demonstrates superlinear dependence (see inset of the Figure 1b), which is indicative of its excitonic origin. Intensity of the SRH peak shows linear pump dependence and follows quasi single-particle approximation, as long as the concentration of charge carriers is considerably lower than that of the charge traps. Temperature dependence of the PL spectra of MAPbBr3 is shown in the Figure 2a. As the temperature increases the PL peaks broaden and become blue-shifted due to the electron-phonon coupling12 and crystal lattice thermal expansion13. Narrowness of the exciton resonance makes it possible to follow homogeneous component down to helium temperatures where this broadening increases linearly with the temperature that could be explained by scattering of the excitons on acoustic phonons in the Lee-Koteles model14. Based on this model we estimate acoustic phonon scattering coefficient of the single crystal of MAPbBr3 at 0.07 meV/K. Extrapolation of the FE line width to 0 K gives value of inhomogeneous broadening Г ~0.5 meV. Such small broadening indicates good optical quality of the MAPbBr3 single crystal. The obtained value is just half an order of magnitude larger than that of the best GaAs epitaxial heterostructures15. The Figure 2a demonstrates dependence of the PL intensity on a temperature. The measured intensities were fit using Arrhenius-like equation16 resulting in exciton binding energy Eb = 13±1 meV (Fig. 2b). This value is in a good agreement with binding energy determined by different approaches in MAPbBr3 single crystal12 in contrast to the more diverse data for polycrystalline materials17, 18. Temperature dependence of the FE, dominant SRH peak positions, and their linear approximation are depicted in the inset of the Figure 2b. The observed rise of energy is unusual for semiconductors whose band gap energy generally decreases with temperature-caused interatomic spacing rise19. However, the linear dependencies in the inset and their tangents are


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similar to those of other perovskite materials, e.g. polycrystalline MAPbX313, 20, 21, FAPbBr321, and CsSnI322. The temperature-independent splitting between the FE and the SRH main peak of 26±5 meV indicates the recombination through shallow traps. Such defects could result from excessive MABr precursor in the solution used for preparation of the perovskite23.

Figure 2. Dependence of PL spectra (a) and PL intensities (b) of the single crystal of MAPbBr3 on a temperature. Inset in (b) shows the spectral positions of FE and the main SRH peaks as a function of temperature. In the PL spectrum of the MAPbBr3 single crystal at 1.4 K we have observed several red shifted weak and narrow bands. These bands can be attributed to phonon replicas, which originate from exciton recombination accompanied by partial energy transfer to the MAPbBr3 crystal lattice vibrations (phonons). Since phonon modes in the crystal are quantized, the replicas are shifted with respect to the FE resonance by characteristic energy portions. For the purpose of their identification we have conducted Raman scattering study of MAPbBr3 single crystal at temperatures down to 7 K for the first time. The Figure 3a shows the obtained temperature dependence of the Raman scattering spectrum. Temperature decrease from liquid nitrogen24-26 to liquid helium results in the narrowing of the band widths by several times making


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it possible to observe the structure of the 100 and 140 cm-1 bands. These peaks were attributed to vibrational modes according to the modeling presented in paper 24 (Fig. 3a, full list of phonon frequencies and vibrational modes could be found in the Table S1 of Supplementary Information). All observed modes correspond to cage vibrations – simultaneous movement of Pb, Br, and MA as a whole, whereas MA molecular modes should be typically at the positions higher than 300 cm-1. Comparison of the PL at 1.4 K and the Raman scattering spectra at 7 K reveals that the shifts of the weak red-shifted bands off the FE peak (Fig. 3b) strongly correlate with the energies of lattice phonons with the most intense observed peaks: based on the theoretical analysis the 7 K spectrum peaks at 38 cm-1 and 48 cm-1 should correspond to [PbBr6] octahedra twist (TO), at 67 cm-1 – to MA pumping around N and at 71 cm-1 – to lurching of MA (LO-like) modes. Therefore, we conclude that these red-shifted bands are phonon replicas of the free exciton. Observation of these phonon replicas is possible because of extremely small inhomogeneous broadening of the excitonic resonance, which, in turn, manifests high optical quality of the studied single crystal.


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Figure 3. Temperature dependence of Raman scattering spectra of the MAPbBr3 single crystal (a) and comparison between low-temperature Raman and PL spectra revealing phonon replicas (FE peak is set as zero shift) (b). Phonon modes: OT – [PbBr6] octahedra twist, OD – [PbBr6] octahedra distortion, MA pumping – methylammonium pumping around N. In conclusion, we have presented optical study of single crystal of lead-halide perovskite MAPbBr3 grown from supersaturated solution. Superior optical quality of the crystal enabled the study of various optical effects related to its excitonic states. Extremely low value of inhomogeneous broadening of excitonic resonance in the crystal (~0.5 meV) allowed for distinction of signals from free excitons and those arising from recombination of excitons weakly localized on defects. We have established the dependence of homogenous broadening of the free exciton band on a temperature down to a few Kelvins and have determined the value of acoustic phonon scattering coefficient (0.07 meV/K). For the very first time we were able to observe the phonon replicas i.e. bands originating from free exciton recombination, which is accompanied by partial energy transfer to the MAPbBr3 crystal lattice vibrations (phonons), in the high-resolution PL spectra. This phenomenon was confirmed by independent low-temperature Raman scattering experiments. Low temperature high resolution low frequency Raman scattering studies let us resolve the structure of phonon spectrum at 100 cm-1 and 140 cm-1 bands observed elsewhere25– 27

. Synthesis procedure and structure characterization of MAPbBr3, and experimental details

of PL measurements are given in Supplementary Information. ACKNOWLEDGMENT The present study was performed within the Project “Establishment of the Laboratory “Photoactive Nanocomposite Materials” No. 14.Z50.31.0016 supported by a Mega-grant of the


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Government of the Russian Federation. Yu.V. Kapitonov and O. A. Lozhkina acknowledge Russian Science Foundation (grant 17-72-10070). The work was carried out using equipment of the resource centers “Nanophotonics”, “Geomodel”, and “Centre for Optical and Laser Materials Research” of Saint-Petersburg State University. REFERENCES (1)

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Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3

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