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Letter
Lattice Distortions Drive Electron-Hole Correlation within Micrometer-Size Lead-Iodide Perovskite Crystals Giulia Grancini, Daniele Viola, Marina Gandini, Davide Altamura, Eva Arianna Aurelia Pogna, Valerio D’Innocenzo, Ilaria Bargigia, Cinzia Giannini, Giulio Cerullo, and Annamaria Petrozza ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00607 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016
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Lattice Distortions Drive Electron-Hole Correlation
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within Micrometer-Size Lead-Iodide Perovskite
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Crystals
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Giulia Grancini†, ¦*, Daniele Viola§, Marina Gandini†, §, Davide Altamura‡, Eva Arianna
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Aurelia Pogna§, Valerio D’Innocenzo†, §, Ilaria Bargigia†, Cinzia Giannini‡, Giulio Cerullo§,
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Annamaria Petrozza†,*
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†Center for Nano Science and Technology @ Polimi, Istituto Italiano di Tecnologia, via
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Giovanni Pascoli 70/3, 20133, Milan, Italy.
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§IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano,
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Italy.
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‡Institute of Crystallography, National Research Council, via Amendola 122/O, Bari 70126,
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Italy.
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AUTHOR INFORMATION
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Corresponding Author
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*
[email protected]; *
[email protected] 16
¦ Ecole Polytechnique Fédérale de Lausanne, EPFL Valais Wal-lis, Rue de l'Industrie 17, CH-
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1951 Sion, Switzerland
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ABSTRACT. We use ultrafast transient absorption microscopy to map the effect of structural
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disorder on the photophysical properties within a micrometer-sized crystal of methylammonium
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lead iodide (CH3NH3PbI3) perovskite. We reveal a spatial inhomogeneity of the structural and
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photophysical behaviour over sub-micrometer scale induced by a local distortion of the crystal
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lattice, which affects the electron-hole correlation over the hundreds-nm length-scale.
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TOC GRAPHICS
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In recent years, perovskite materials have created much excitement in the field of solar cells,
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with certified power conversion efficiencies exceeding 22% 1, 2, challenging the established
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photovoltaic technologies. In addition, the remarkably large carrier diffusion length 3, the high
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pan-chromatic absorption with a band gap tunable from 1.1 to 3.1 eV 4 and the low non-radiative
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recombination rates 5, 6 make these materials highly attractive for a myriad of optoelectronic
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applications, ranging from lasers 7 and light emitting devices 8, 9 to photodetectors and
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phototransistors 10, 11. A great effort has been devoted to the fine-tuning of the perovskite
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composition and the optimization of the deposition protocol, in order to improve device
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properties 12-14.
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One major bottleneck in view of a rational device design is to understand how the perovskite
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local morphology affects the electronic and optical properties of the material 15-17. Bulk structural
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characterization techniques, such as X-ray diffraction (XRD), are usually employed to this
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purpose 15. Such methods, at least with tabletop laboratory equipment, interrogate macroscopic
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portions of the material, averaging out any intrinsic inhomogeneity at a local, sub-micron scale.
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Recently, to gain more insight into the spatial physicochemical inhomogeneity of perovskites, a
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few studies have correlated scanning electron microscopy (SEM), energy dispersive X-ray
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(EDX) mapping or micro-Raman spectroscopy, which provide the structural and chemical
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information, with photo-luminescence (PL) microscopy, which monitors the light-emitting
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processes 18-21. They found substantial heterogeneity in the local PL yield and lifetime, which
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were attributed to a local variation in the non-radiative decay rate at grain boundaries, where the
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density of traps was assumed to be higher 18, 19. Despite these initial studies, a clear under-
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standing of the entangled relationship between structural and photophysical properties of
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perovskites is still missing, although of utmost importance for deriving rational design guidelines
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for material processing and device optimization.
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In our previous work we demonstrated that the perovskite film crystallization process and its
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microstructure affect the molecular interactions within the hybrid crystal 15 and consequently
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impact on the observed photophysical properties of the thin film 16, 17. In particular, we have
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suggested that the presence of a long-range order affects the vibrational degree of freedom of the
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organic dipole with respect to the inorganic cage, as well as the lattice dielectric response, which
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determines the charge screening within the semiconductor 16. It is thus well understood that for
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polycrystalline samples, where the crystal coherence can be easily disrupted (e.g. by grain
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boundaries and interaction with environmental agents) no excitonic transition at the edge of the
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absorption spectrum is expected and their photo-induced dynamics are solely governed by
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uncorrelated charge carriers at any excitation density and temperature 16. On the other hand,
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when the crystalline coherence length is increased, carrier correlation effects start to appear, with
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an exciton binding energy which increases from virtually zero to a few tens of meV 22, 23. Despite
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the demonstration of the two regimes, it is not clear yet over which spatial length scale the
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crystal coherence length affects the photophysical properties of the semiconductor. To address
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this question, large micrometer-size crystals would be the ideal sample. However, they often
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show an intense absorption which, combined with severe artifacts 24, hampers accurate optical
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investigations by the transient absorption (TA) technique 25. To overcome this issue here we
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have synthesized large aspect-ratio platelets of methylammonium lead iodide (CH3NH3PbI3)
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which, thanks to their moderate absorbance, allow us to directly map the photo-carriers dynamics
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with sub-micrometer spatial resolution by a combination of space- and time-resolved optical
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spectroscopy 26, 27. We observe at the boundaries of the single platelet a higher degree of
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structural disorder, which induces a local modulation of the semiconductor optoelectronic
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properties, representing a possible source of defect states. CH3NH3PbI3 micrometer-large crystals
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are grown by a two-step deposition protocol 16 on a glass substrate and are covered by a thick
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layer of polymethyl methacrylate in order to preserve them from interaction with water and
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oxygen. The crystals have a lateral size of a few micrometers (around 5x4 µm2 large) and a
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thickness of ≈1 µm (see cross section of the SEM image re-ported in the Supporting Information
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as Fig. S1). Figure 1a shows the SEM image (scale bar 1 µm) along with the optical absorption
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image of the crystal investigated (Fig. 1b). The linear absorption image at 680 nm shows a
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homogeneous signal across the crystal, indicative of a very smooth film with uniform thickness.
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This is of key importance for probing the photophysics on different spots within the crystal,
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because it ensures homogeneous excitation density across it. Figure S2 shows the micro-
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absorption spectrum within one crystal, which is in good agreement with the measurement
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reported in 24. To assess the crystal phase and degree of crystallinity (coherence length) of the
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sample, we measure the XRD spectrum of a distribution of similar platelets, as reported in Fig.
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1c. The sharp diffraction peaks, indexed as the tetragonal CH3NH3PbI3 structure (COD #
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4335638), denote a high level of crystallinity. The fitting of the XRD profile (see SI for details),
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retrieves a coherence length larger than 330 nm (instrument resolution limited estimate). This
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indicates that the micrometer-large perovskite crystal shown in the SEM image is made of
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monocrystalline regions extending at least over 330 nm.
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Figure 1. a, SEM image of the crystal under investigation, scale bar: 1 µm; b, Linear optical
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absorption image at 680 nm for the same sample, scale bar 2 µm. 1 and 2 are two characteristic
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regions that will be further investigated; c, XRD spectrum (experimental: dots; fit: red line) of
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the sample. The refined unit cell parameters are: a=8.699 Å, b= 8.699 Å and c=12.421 Å).
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Theoretical investigations of molecular dynamics suggest that, despite the sharp XRD peaks,
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perovskites present structural fluctuations at room temperature 28, reflecting the intrinsic
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extraordinary “jelly” nature of these semiconductors. Thus, to gain a comprehensive picture of
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the structural properties of the investigated sample it is important to characterize the lattice
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vibrations as well. Figure 2 shows the resonant micro-Raman spectra measured in two
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characteristic regions of the crystal under investigation, which are indicated in Fig. 1b: region 1
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(Fig. 2a), closer to the center of the sample, and region 2 (Fig. 2b), closer to the border, about 2
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microns far from the center.
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Figure 2 Micro-Raman spectra at regions 1 (a) and 2 (b) of the platelet shown in Fig. 1b,
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Excitation is at 532 nm.
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The main features of the Raman spectra match with those reported in the literature 15, 16, 21.
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However, going into the detail, first of all one can notice a greater sharpness of the bands in
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region 1, which is generally a signature of higher structural order. This comes along with the
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presence of a defined peak at ∼160 cm−1, assigned to libration of the methylammonium cation 29.
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This feature becomes sharper when the organic cation has a reduced degree of freedom within
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the crystal unit, i.e. by reducing temperature 29. This feature is expected to become sharper in a
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more ordered crystal-line arrangement, i. e. when the organic cations fix their position in a
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preferential “head to tail” geometry. 29 The presence of this marker, according to Ref. 29 and
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Ref. 15, proves that the degree of freedom of the organic cation is the dynamical switcher able to
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induce, through hydrogen bonding interaction, the local distortion of the lattice, and to tune the
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optoelectronic properties of the material modifying the local dielectric properties, as we will
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show in the following. In region 1 the cations have a reduced degree of freedom within the
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crystal unit, resulting in a lower lattice strain. Thus, despite the high quality of the crystalline
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sample, micro-Raman reveals that the lattice vibrations still present in-homogeneity and
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indicates a different degree of order/disorder at the micrometer length scale.
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To correlate structural and optoelectronic properties of the perovskite, we have measured the
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spatially resolved photoinduced dynamics by means of ultrafast TA microscopy 26, 27, 30. With
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respect to standard ultrafast TA spectroscopy, which interrogates macroscopic volumes of tens to
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hundreds of micrometers, TA microscopy, with its sub-micron spatial resolution, is able to
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resolve spatial inhomogeneities in the photophysical response of the sample. Our ultrafast TA
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microscope has been described in detail before 26, 27. Briefly, it starts from an amplified
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Ti:Sapphire laser, whose output is divided into pump and probe lines. An optical parametric
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amplifier generates ~70-fs pump pulses tunable in the 500-750 nm range, while the probe is
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obtained by white light continuum generation in a 2-mm-thick sapphire plate. The collinear
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pump and probe pulses are focused by a reflective microscope objective with 15x magnification
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to avoid chromatic aberrations. At the sample position, the pump and probe beams have
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diameters of 1500 nm and 800 nm, respectively. The pump beam is modulated by a mechanical
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chopper and the differential transmission (∆T/T) spectrum of the probe is measured by a
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spectrometer, with an overall temporal resolution of ≈100 fs. Figure S3 shows a schematic of the
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setup of the TA microscope. Thanks to the sub-micrometer spatial resolution, we are able to
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measure the TA signal exactly in the regions 1 and 2, previously characterized by micro-Raman.
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Figure 3 reports the two-dimensional ∆T/T (λ, τ) maps, as a function of probe wavelength λ
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and pump-probe delay τ, recorded at region 1 (Fig. 3a) and at region 2 (Fig. 3b), respectively,
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with the pump wavelength tuned to 650 nm.
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Figure 3. a, Two dimensional ∆T/T (λ, τ) map, upon excitation at 650 nm, measured in region 1
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of the CH3NH3PbI3 crystal; b, ∆T/T (λ, τ) map in region 2; c, ∆T/T spectra at 1 ps pump-probe
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delay in regions t 1 and 2 (red circles and blue squares, respectively). d, Scheme of the energy
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levels of the perovskite including conduction and valence band (CB, VB) and the excitonic
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levels and a cartoon of the ∆T/T signals showing the free carriers and excitonic features. e, ∆T/T
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time traces at selected probe wavelengths as indicated in the legend for regions 1 and 2.
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Excitation density is ≈1018photons/cm3.
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These maps highlight an inhomogeneity in the photophysical response of the sample on the
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micrometer length scale, which is confirmed by the space dependent TA map of the crystal for λ
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= 780 nm and τ =200 fs, reported in Figure S4. Figure 3c shows the corresponding ∆T/T spectra
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at 1 ps delay. Spectral differences are remarkable: region 1 (red circles) shows a modulation,
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with a positive band (red in the ∆T/T map of Fig. 3a) close to 810 nm and a blue-shifted negative
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band (blue in the map) around 780 nm. Region 2, on the other hand, shows a dominant broad
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positive band in the 760-790 nm wavelength range and a negative signal at shorter and longer
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wavelengths. We start the analysis with the region 2, which is close to the center of the platelet.
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The spectral shape of the ∆T/T at 1 ps closely resembles the one usually observed in
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polycrystalline thin films of CH3NH3PbI3 29-33. The positive band peaking around 780 nm is
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ascribed to a photobleaching (PB) signal due to phase space filling of free charge carriers at the
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band edge. The PB signal forms within 350 fs (see dynamics in Fig. 3e) and slowly decays in the
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investigated time window. This fast rise is associated to the carrier cooling due to carrier-carrier
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and carrier–phonon scattering processes 16, 32-36. At longer wavelengths, the sub-band-gap
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photoinduced absorption (PA) results from band gap renormalization, as previously observed 16,
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32, 34
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10 ps), differently from what reported in literature. An accurate assignment of this long-lived PA
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deserves further investigations, out of the scope of the present work. In agreement with previous
. It forms in a few hundreds of fs, but it does not decay in our probing time window (about
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studies, we assign 32 the broad PA band for wavelengths shorter than 720 nm to a change in the
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refractive index due to the photo-generation of free carriers within the semiconductor. This band
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should be flat, while a peak at around 730nm can be observed, suggesting the presence of a
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“derivative-like” feature convoluted with the PB, though of little entity. Region 1, corresponding
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to the center of the platelet, shows a dominant “derivative-like” feature of the ∆T/T spectrum,
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with a large red-shift of ≈ 30 nm (corresponding to nearly 60 meV) with comparison to point 2
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and an intense positive band peaking at 810 nm. ∆T/T dynamics at selected probe wavelengths
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(close to the peak of the positive bands) are reported in Fig. 3e and in Fig. S5 on longer time
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scale. From the dynamics in Fig. 3e the positive band in region 1 shows a resolution-limited rise
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time and an initial fast decay within the first ps, followed by a slower decay outside our temporal
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observation window (see also Fig. S5). A sharp negative band peaking at 770 nm forms together
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with the red positive one. This spectral behavior can be assigned to self-normalization of the
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exciton energy (that is a blue-shift of the exciton absorption) due to exciton–exciton and exciton–
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carrier interactions 16, 35, 37-38, leading to a derivative shape of the ∆T/T spectrum. This spectral
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feature can be considered as a fingerprint of an enhanced electron-hole correlation. A scheme of
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the electronic energy levels of the perovskites in terms of conduction and valence band, as well
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as the excitonic level with the associated blue-shift, are drawn in the scheme in Fig. 3d along
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with a cartoon of the resulting ∆T/T features expected for the two extreme scenarios where “free-
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carriers” or “excitonic” features dominate. Thus, the TA signal in point 1 is the result of a
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concomitant reduction of the semiconductor band-gap, which has been associated to a change in
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the lattice strain, along with a reduction of the local dielectric constant driven by an enhancement
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in the local intermolecular order, which affects the degree of self-normalization of the exciton
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energy and stabilizes it. The TA microscopy experiments thus indicate that the electron-hole
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correlation is higher in region 1, with reduced local dielectric constant, with respect to region 2.
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In conclusion, the combination of Raman microscopy with femtosecond TA microscopy has
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allowed us to locally probe molecular interactions in a micrometer-large perovskite crystal and to
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correlate local structural disorder with a modification of the optoelectronic properties of the
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semiconductor. In particular, by locally probing the photophysical properties of a micrometer-
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size perovskite crystal with sub-micron spatial resolution, we provide evidence that an enhanced
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structural disorder, when moving from the center to the border of the crystal, is responsible for
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an increase of the band-gap. In addition, the local disorder affects the degree of electron-hole
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screening, thus inducing a spatial dependence of carrier correlation at the sub-micrometer scale,
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according to the local lattice strain. Our work provides new fundamental insights for the
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manipulation of perovskites as active materials in solar cells and light emitting devices.
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Modulating the density of grain boundaries and defect states can be a strategy to change the
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photophysical response of the material and thus specifically tune its functionality.
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ASSOCIATED CONTENT
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Supporting Information includes experimental methods and details along with additional data.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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G.C. acknowledges support by the European Research Council Advanced Grant STRATUS
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(ERC-2011-AdG No. 291198). This project has received funding from the European Union's
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Horizon 2020 research and innovation programme under grant agreement no 654148 Laserlab-
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Europe (project CUSBO002194).
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