Ultrafast Charge Carrier Relaxation in Inorganic Halide Perovskite

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Ultrafast Charge Carrier Relaxation in Inorganic Halide Perovskite Single Crystals Probed by Two-Dimensional Electronic Spectroscopy Xuan Trung Nguyen, Daniel Timmer, Yevgeny Rakita, David Cahen, Alexander Steinhoff, Frank Jahnke, Christoph Lienau, and Antonietta De Sio J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01936 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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The Journal of Physical Chemistry Letters

Ultrafast Charge Carrier Relaxation in Inorganic Halide Perovskite Single Crystals Probed by Twodimensional Electronic Spectroscopy

Xuan Trung Nguyen1, Daniel Timmer1, Yevgeny Rakita2, David Cahen2, Alexander Steinhoff 3, Frank Jahnke3, Christoph Lienau1, Antonietta De Sio1 1 Institut für Physik, Carl von Ossietzky Universität, 26129 Oldenburg, Germany 2 Department of Materials & Interfaces, Weizmann Institute of Science, Israel 3 Institut für Theoretische Physik, Universität Bremen, Germany Correspondence to: [email protected]

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ABSTRACT. Halide perovskites are promising optoelectronic materials. Despite impressive device performances, especially in photovoltaics, the femtosecond dynamics of elementary optical excitations and their interactions are still debated. Here we combine ultrafast two-dimensional electronic spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe the room temperature dynamics of non-equilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic and free-carrier transitions, extracting a ~30 meV exciton binding energy, in agreement with our SBE calculations and with recent experimental studies. The 2DES dynamics indicate remarkably short, 10 ns)9, 53 and are unaffected by a variation in pump fluence by more than an order of magnitude (Figure S8). Therefore, possible excitation-energy-dependent effects, such as X-X annihilation processes or Auger recombination do not significantly influence the decay dynamics. The longer, ~700-ps decay time most likely reflects the diffusive transport of


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photoinduced charge carriers out of the probe volume, away from the sample surface, whereas the faster, ~10-ps decay may potentially orginate from the trapping of carriers into surface states. Theoretical calculations of R R (Figure 1e,f) based on the SBE (see Supporting Information for details) also exhibit a distinctly dispersive lineshape around the band-edge, in good agreement with the experiments (Figure 1b,c). The theoretical linear absorption and reflectivity (Figure S5a,b) reveal that both X and FC transitions contribute to the resonant, near-band-edge feature at low excitation densities. Efficient carrier-phonon interaction results in substantial line broadening and a polaron shift of the order of 50 meV. With increasing photoexcited carrier density n, both Pauli blocking and screening of the Coulomb interaction leads to a bleaching of the resonant peak, whereas the bandgap shifts to lower energy. The resonant peak vanishes almost completely at carrier densities of n= 1 1018 cm-3, when reaching the Mott density (Figure S5). In the nonlinear

R R spectra, this bleaching of the resonant peak results in the distinctive dispersive peak around the band-edge (Figure 1e). Its amplitude increases linearly with carrier density up to 1 1018 cm-3 and then saturates until reaching the Mott density n= 1 1018 cm-3 (Figure 1e, red and Figure S6). The maximum amplitude of the R R peak thus provides a direct measure for the transient excited carrier density in the sample. Quite strikingly and in excellent agreement with the experiment, the zero crossing of the dispersive resonant peak is virtually independent on the carrier density. Only at the highest densities it slightly blue-shifts (Figure 1f, inset), another indication of reaching the Mott transition. In agreement with these predictions, our experiments show a linear increase in amplitude and no signifcant changes in shape of the R R spectrum upon varying the pump fluence from ~8 up to ~200 µJ/cm2 (Figure S4). At ~230 µJ/cm2, the data suggest that a slight


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The Journal of Physical Chemistry Letters

saturation of the maximum amplitude to ~ 10% sets in (Figure S4d), indicating the approach of the Mott transition. All nonlinear experiments shown in Figures 1-3 are carried out at a pump fluence of 90 μJ/cm2, resulting in maximum R R amplitudes of ~2-3%. All photoinduced carrier densities are therefore well below the Mott density. By comparison with the SBE results we deduce a maximum carrier density of ~ 3 1017 cm-3. For both R R (Figures S4 and S8) and 2DES studies (Figure S11), we observe no significant effect of the pump fluence on the spectra for fluences below 165 µJ/cm2. To experimentally distinguish between X and FC contributions to the near-band-edge optical transitions, we perform 2DES under similar excitation conditions as in Figure 1b-c. The crystals are excited by a sequence of three broadband pulses in a partially collinear geometry (Figures 2a and S2a). The third order signal S ( , T , ED ) re-emitted from the sample is collected in the phase-matched direction collinear with the probe pulse in a reflection configuration (Fig S2a). In the partially collinear implementation of our 2DES setup, the measured signal arises from the real part of S with respect to the phase of probe field51 (see Section 1.2 of the Supporting Information). The time delay between the first and second pulses is termed coherence time τ, whereas the one between the second and third pulses is the waiting time T. At each waiting time T, the real part of the Fourier transform of S ( , T , ED ) along  defines an energy-energy map A2 D ( E X , T , ED ) as a function of the excitation E X and detection ED energy.44

The 2DES maps of CsPbBr3 single crystals at room temperature for selected waiting times T are shown in Fig 2b-j. Upon excitation, we observe a dominant dispersive bleaching peak at excitation

E X ~ 2.42 eV, slightly elongated along the diagonal (Figure 2b-d). The negative part of this peak


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is centered at ED ~ 2.35 eV and is highlighted in Figure 2b as a white contour line. The shape of the peak along ED is similar to that of the dispersive resonant peak in Figure 1c. This diagonal peak arises from X bleaching (Figures 2b and 3a), with the stretching along the diagonal suggesting a slight inhomogeneous broadening (Figure 3b).

Figure 2: Two-dimensional electronic spectroscopy (2DES) of CsPbBr3 single crystals at room temperature. (a) Scheme of the three-pulse-sequence interacting with the sample indicating the coherence time  and waiting time T. The measurements are performed in reflection geometry, resulting in dispersive lineshapes along the detection energy. (b-j) 2DES maps at selected waiting times T. The color code shows the real part of the 2DES signal. (b-e) At early times, a dispersive peak stretched along the diagonal (b, white contour) at an excitation energy ~2.42 eV is the signature of exciton bleaching. A cross-peak at excitation into the free carrier continuum >2.45 eV is also seen. (f-j) With increasing waiting time, the vertically-stretched cross-peak (j, white contour) redshifts and becomes dominant. Changes in cross-peak lineshape indicate carrier cooling on a

Ultrafast Charge Carrier Relaxation in Inorganic Halide Perovskite

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