Exciton and Coherent Phonon Dynamics in the Metal-Deficient Defect

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Exciton and Coherent Phonon Dynamics in the Metal-Deficient Defect Perovskite (CH3NH3)3Sb2I9 Mirko Scholz, Marius Morgenroth, Kawon Oum,* and Thomas Lenzer Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany S Supporting Information *

ABSTRACT: The ultrafast charge carrier dynamics of the metaldeficient lead-free perovskite derivative methylammonium antimony iodide ((CH3NH3)3Sb2I9, MASbI) is studied using femtosecond UV−vis−NIR transient absorption spectroscopy. The transient and steady-state absorption spectra show strong excitonic features. This is confirmed by an analysis of the near-band-gap absorption using Elliott’s theory and its reformulation by Tanguy with subsequent modeling of the free-carrier and exciton contributions using the Saha equation. Time scales of the different carrier scattering processes are identified. The kinetics in the UV−vis range is largely independent of pump laser fluence suggesting that the dynamics involves localized excitons, with recombination time constants of 190 ps and >5 ns. Damped oscillations in the kinetics indicate coherent phonon dynamics and strong electron-phonon coupling. Fourier transformation of the time-domain data provides the steady-state Raman spectrum of MASbI which is dominated by vibrations of the [Sb2I9]3− anion. Exciton dissociation at the interfaces with mesoporous TiO2 and a triarylamine-based hole transport material is found to be negligible. Our experiments suggest that it will be challenging to fabricate efficient photovoltaic devices from such lead-free antimony- and also bismuth-based zero-dimensional perovskite derivatives, because nonradiative exciton recombination largely governs their carrier dynamics.

1. INTRODUCTION Lead-based organic−inorganic halide perovskites show promise for challenging established thin-film solar cell technologies, however the considerable toxicity of lead and long-term stability issues with these materials might hamper their future commercialization on larger scales.1−6 This has triggered the quest for lead-free derivatives. One strategy is to maintain the ns2np0 electronic configuration of Pb(II) (n = 6) through replacement by the group 15 elements bismuth and antimony in the hope to preserve at least some of the beneficial electronic properties known from the lead-based sister compounds.7 While these lead-free systems also feature octahedral halide coordination around the trivalent metal ions, they crystallize in different structures, often denoted as “metal-deficient perovskites” or “defect perovskites”, to meet the requirement of charge neutrality.8,9 For instance, (CH3NH3)3Bi2I9 (“MABI”), the bismuth analogue of CH3NH3PbI3 (“MAPI”), forms a onethird metal-deficient 0D structure containing face-sharing [Bi2I9]3− double octahedra.10−15 The estimated exciton binding energy of MABI is high, around 300 meV, resulting in a pronounced peak in the steady-state absorption spectrum which is clearly visible even at room temperature.15−18 Our most recent ultrafast broadband transient absorption studies of MABI on mesoporous TiO2 showed that its carrier dynamics is governed by excitons, and not by free carriers such as MAPI.19−23 Exciton dissociation was achieved at the MABI © XXXX American Chemical Society

interface with mesoporous TiO2 (mp-TiO2) with an upper limit of about 12% efficiency providing some hope that this compound might be applicable to solar light harvesting if an optimized architecture is used, e.g. a bulk heterojunction arrangement.19 R e c e n t l y , K i r c h a r t z a n d co - w o r k e r s em p l o y e d (CH3NH3)3Sb2I9 (“MASbI”), the antimony analogue of MABI, in a planar heterojunction solar cell and reached a power conversion efficiency of 0.5% with a fill factor of 55% and an open-circuit voltage of 890 mV.17 Based on the much smaller exciton peak in the steady-state absorption spectrum of MASbI they suggested a lower exciton binding energy than for MABI. Such antimony-based compounds therefore might hold larger promise for future photovoltaic and optoelectronic applications than MABI. In the current paper, we investigate the carrier dynamics of MASbI deposited on mesoporous (mp) scaffolds of TiO2 using ultrafast UV−vis−NIR transient absorption spectroscopy, with particular emphasis on the importance of excitons in the carrier dynamics of this compound. For the first time, we identify characteristic coherences in the transient absorption kinetics indicating strong electron−phonon coupling. Selected results are also Received: September 27, 2017 Revised: February 10, 2018

A

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION 3.1. Steady-State Structural Properties and Energy Levels. Figure 1 shows the crystal structure of MASbI and an

presented for three-layer arrangements containing the triarylamine-based hole transport material (HTM) H10124−26 in the configuration mp-TiO2/MASbI/H101 indicating hole injection from MASbI into the HTM as well as electron injection into mp-TiO2.

2. EXPERIMENTAL SECTION 2.1. Preparation of (CH3NH3)3Sb2I9 Thin Films on Mesoporous TiO2. Sintered mesoporous thin films of TiO2 with a thickness of 500−600 nm were prepared on cleaned glass slides as described in our previous publications.19,21,22,27−29 Antimony triiodide (SbI3, Alfa Aesar, 99.998%) and methylammonium iodide (Dyenamo) were dissolved either in γ-butyrolactone (GBL, Sigma-Aldrich, 99%), a GBL/DMSO mixture (molar ratio 3:2 or 2:3, DMSO from Acros Organics, 99.7%) or N,N-dimethyl formamide (DMF, Acros Organics, 99.8%, Extra Dry) for the preparation of 30 and 15 wt % “MASbI solutions”. The solutions were applied upon spinning the substrate at 500 rpm for 4 s. Afterwards the rotation speed was increased to 3000 rpm for 30 s. Antisolvent treatment was performed by spinning 70−100 μL chlorobenzene onto the thin films 14 s (GBL) or 4−6 s (GBL/DMSO or DMF) after the MASbI solution was applied. The resulting films were heated to 80 °C (GBL and DMF) or 110 °C (GBL/DMSO) for 30 min. All steps were carried out under dry nitrogen in a glove box. 2.2. Ultrafast Transient Absorption Spectroscopy. Femtosecond broadband transient absorption spectra were recorded on our three setups covering the UV−vis−NIR region (260−700, 340−760 and 850−1630 nm).30−32 They employ the pumpsupercontinuum probe technique.33 Briefly, the pump pulse (390, 400 or 505 nm, typical fluence ca. 140 μJ cm−2) was chopped at 460 Hz. Multifilament supercontinuum probe pulses covering the UV−vis region were generated in a 2 mm thick CaF2 plate using seed beams at 400 or 540 nm (50 fs pulse length, 12 μJ pulse−1). The NIR multifilament supercontinuum was produced in a 2 mm thick sapphire plate using 50 fs seed pulses at 800 nm (20 μJ pulse −1). The supercontinuum was divided into a reference and probe beam. Pump and probe beams were polarized at magic angle (54.7°) and crossed at the sample. The reference and probe beams were imaged onto the entrance slits of two separate grating spectrographs and detected by 512 element Si (UV− vis) or InGaAs (NIR) photodiode array detectors. Single-shot baseline corrections were applied. The time-resolution of the setups was 80 fs or better, with a time zero accuracy of about 10−20 fs. The thin film samples were mounted in a nitrogenflushed aluminum contact cell28 and moved within a 2 × 2 mm2 plane perpendicular to the probe beam propagation axis using an x/y piezo stage. Transient spectra at each time delay represent averages of 3000 pump−probe cycles recorded in three separate runs. Steady-state UV−vis−NIR absorption spectra were recorded using a Varian Cary 5000 spectrometer. 2.3. Characterization by XRD and SEM. The thin films were characterized by X-ray diffraction at T = 294 K using a PANalytical X’Pert MPD PRO diffractometer.19,21,22 XRD patterns were simulated with GSAS.34,35 SEM pictures were recorded employing an FEI Quanta 250 FEG. The crystal structure was visualized using VESTA 3.36

Figure 1. (A) Crystal structure of MASbI (space group P63/mmc). (B) Diagram of the relevant energy levels of mp-TiO2, MASbI and H101.

Figure 2. Experimental XRD pattern of a MASbI/mp-TiO2 thin film including a fivefold magnification (A) compared with simulations for mp-TiO2 (B) and MASbI (C), including assignments of the Bragg reflexes. Details of the simulation procedure are provided in the text.

energy level diagram of mp-TiO2, MASbI and H101. Figure 2 provides the XRD pattern of the MASbI/mp-TiO2 thin film including a five times magnification (black lines). Simulations for mp-TiO2 (space group I41/amd)37 and MASbI including peak assignments are shown as magenta and blue lines, respectively. Excellent agreement of the simulated and experimentally determined XRD peak positions of MASbI were obtained using the published structure of Yamada et al. for Cs3Sb2I9 (space group P63/mmc) using a zero point correction of 0.0945°, a = 8.535 Å and c = 21.479 Å.38 Voids were assumed for the methylammonium cations in the simulation. We note that slight deviations in the relative intensities of individual peaks between experiment and powder-based simulations are due to texture effects of the thin films, e.g. due to a preferred orientation and/or growth direction of the crystallites on the substrate. For instance, in the experimental XRD pattern the (00n) reflexes are more intense indicating a somewhat preferred growth of the MASbI crystals with the c-axis B

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

spectrum indicated as dotted lines. Corresponding broadband transient absorption spectra at 10 ps are presented in panel (B). The steady-state spectra show a rise below 600 nm and a clear shoulder around 470 nm. The latter is assigned to excitonic absorption.8,17 This feature is less pronounced than in the structurally related bismuth-analogue MABI, as demonstrated in Figure S1A (Supporting Information). We will show below that the exciton binding energy of MASbI is larger than 200 meV and thus substantial. The analysis based on the second derivative of the steady-state spectra in Figure S1B shows that the exciton feature of MASbI is slightly blue-shifted by about 1500 cm−1 (190 meV). The transient broadband absorption spectra at 10 ps in Figure 3B closely resemble the second derivative of the steadystate absorption spectrum down to 360 nm. As in our previous study of MABI, we take this spectral signature as one strong indication for the presence of excitons. Further support for this conclusion will be provided below. Antisolvent treatment yields smaller crystallites, see the SEM images in panels (C,D), yet the impact on the carrier dynamics in this size regime is minor. The same holds for samples prepared using DMF (with and without antisolvent) where we find an even smaller crystallite size (Figure S2, Supporting Information). Figure 4 shows the complete temporal evolution up to 1 ns delay time for the UV−vis broadband transient absorption

perpendicular to the substrate, yet the effect is less pronounced than for MABI.19 The crystal structure of MASbI in Figure 1A features isolated face-sharing [Sb2I9]3− double-octahedra (“dimers”) and therefore represents a 0D molecular salt.38 It resembles the structure of its bismuth sister compound10 which was previously investigated by us using UV−vis−NIR broadband transient absorption spectroscopy.19 We note that a second polymorph of MASbI exists consisting of 2D corrugated layers of polyanions (space group P3m ̅ 1), however it is not formed under the synthesis conditions applied in the current work.38,39 The energy levels of the systems under investigation are highlighted in Figure 1B. Estimates for the location of the valence band (VB) and the optical band gap of the 0D form of MASbI were reported by Boopathi et al. as −5.50 and 2.2 eV, respectively.40 As will be discussed in more detail below, this “band gap” value of 2.2 eV should be better interpreted as the low-energy edge of an exciton feature, whereas the actual direct band gap (continuum absorption) starts at ca. 2.9 eV. A comparison with the energy levels of mp-TiO241 shows that upon photoexcitation it should be energetically feasible to inject conduction band (CB) electrons from MASbI into the CB of mp-TiO2. Likewise, the HOMO level of H101 is favorably located,24 and therefore hole injection from MASbI into the HTM should be possible. 3.2. Steady-State and Transient Absorption Spectra of MASbI. Figure 3A shows the steady-state absorption spectra of MASbI on mp-TiO2 obtained by deposition without (red solid line) and with antisolvent treatment (blue solid line), including the respective second derivative of each absorption

Figure 4. Broadband transient absorption spectra of MASbI on mpTiO2 upon photoexcitation at 400 nm. (A) without antisolvent; (B) with antisolvent chlorobenzene. In both cases MASbI was prepared in a GBL/DMSO solvent mixture (molar ratio 3:2) with subsequent heating at 110 °C for 30 min.

spectra of MASbI on mp-TiO2 after photoexcitation at 400 nm. Corresponding contour plots including the NIR region are provided in Figure S3. The samples without (A) and with (B) antisolvent treatment exhibit very similar dynamics, despite the considerable difference in crystallite size: the characteristic shape of the transient spectra already emerges at very early times (top panels). The spectra may be broadly divided into two regions: a negative signal below 360 nm (bleach band) and an oscillating feature above 360 nm (exciton band) which largely resembles the shape of the second derivative of the steady-state absorption spectrum (see also Figure 3A). The latter suggests the presence of bound excitons and may be

Figure 3. (A) Steady-state absorption spectra of MASbI on mp-TiO2 without (red solid line) and with (blue solid line) antisolvent treatment, including the respective second derivatives (dotted lines). (B) Corresponding broadband transient absorption spectra at 10 ps upon photoexcitation at 400 nm (same color-coding). (C,D) SEM images for both thin films (length of vertical image border corresponds to 100 μm) including sixfold (C) and sixtyfold (D) magnifications. C

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C explained either by a trapped-carrier-induced Stark effect42,43 or alternatively by a biexciton effect.44−46 In the time range 0.1− 10 ps (middle and bottom panels) the bleach band below 360 nm does not change. In contrast, the spectral development in the exciton band above 360 nm involves a decay. From 10 ps onward (bottom panels) the complete spectrum uniformly decays. The specific carrier relaxation processes responsible for this spectral dynamics will be discussed in section 3.4. 3.3. Estimates for the Exciton Binding Energy of MASbI and the Contribution of Excitons to Its Carrier Dynamics. Our steady-state and transient absorption spectra suggest a pronounced impact of excitons on the carrier dynamics of MASbI. Unfortunately no experimental measurements of the exciton binding energy of MASbI exist. For MABI a value of ≥0.3 eV has been estimated by Kawai et al. based on temperature-dependent optical absorption spectra of single crystals.16,47 Because the exciton feature of MASbI is smaller, Hebig et al. came to the qualitative conclusion that its exciton binding energy should be smaller than in MABI.17 We therefore obtained estimates for the exciton binding energy as follows: the near band-gap steady-state absorption α(E) was simulated in the framework of Elliott’s model for Wannier excitons48−51 and its reformulation by Tanguy.52 The latter also takes the change of the refractive index into account and should therefore provide a more realistic description. Details of the simulation procedure can be found in the Supporting Information. Figure 5 shows the resulting fits of the

value of (2.83−2.48) eV = 0.35 eV which is in reasonable agreement with the estimate of ≥0.3 eV based on the lowtemperature MABI absorption spectra of Kawai and coworkers.16 An analogous procedure for MASbI yields (3.01− 2.67) eV = 0.34 eV. Again this value is consistent with the two estimates 0.22 and 0.41 eV obtained from the models of Elliott and Tanguy. In any case, our estimates suggest that the exciton binding energy of MASbI is substantial (Eb ≈ 0.2−0.4 eV) and much larger than the average thermal energy at 296 K (0.026 eV). The impact of this fairly large exciton binding energy on the carrier dynamics is underlined by a numerical simulation of the contribution of free charges and excitons at equilibrium. For that we follow the approach described by D’Innocenzo et al. which is based on the Saha equation51,53−55 x2 1 ⎛ 2πμkBT ⎞ ⎟ = ⎜ 1−x n ⎝ h2 ⎠

3/2

⎛ E ⎞ exp⎜ − b ⎟ ⎝ kBT ⎠

(1)

Here x is the ratio of the density of free carriers to the total excitation density (i.e. nfc/n). The latter is given by the sum of nfc and the exciton density nexc, i.e. n = nfc + nexc. The reduced mass is taken from the calculations of Brandt et al. for the closely related compound Cs3Sb2I9 (μ = 0.44 me, with me being the electron mass).56 Eb represents the exciton binding energy determined above. In Figure 6 we plot x as a function of n for a

Figure 6. Fraction x of the free carrier density as a function of the total photoexcitation density n of MASbI. Curves from right to left are for increasing exciton binding energy in the range 200−450 meV with 50 meV increments. Reduced mass μ = 0.44me and temperature T = 296 K.

Figure 5. Fits of the MASbI absorption spectrum near the band gap. (A) Elliott’s model using the parameters Eg = 2.88 eV, Eb = 0.22 eV, Γ = 0.13 eV and E2g = 2.86 eV. (B) Elliott’s model within the reformulation of Tanguy using the parameters Eg = 3.00 eV, Eb = 0.41 eV, Γ = 0.16 eV and E2g = 2.88 eV. Blue: discrete part, magenta: continuum part, red: total fit, black: experimental spectrum.

representative set of exciton binding energies (200−450 meV) covering the estimates for Eb discussed above. We find that for typical photoexcitation densities in our experiments (>1016 cm−3) excitons are clearly the dominant species. 3.4. Analysis of Transient Absorption Spectra. We analyzed the time-resolved absorption spectra of MASbI using a global kinetic modelling procedure.30,57,58 It was found that the simplest mechanism capturing the essentials of the dynamics involved four steps which are assigned to different carrier relaxation processes on well-separated time scales. Therefore we employed a consecutive kinetic model involving five “species” Mi, described by M1 → M2 → M3 → M4 → M0 with the respective time constants τ 1 , τ 2 , τ 3 and τ 4 (systematically increasing). M1 is the species originally prepared

band edge region (red) to the experimental data (black), including the corresponding discrete (blue) and continuum contributions (magenta). We obtain an exciton binding energy of 0.22 eV for the simple Elliott model (panel (A)) and 0.41 eV for the more accurate Tanguy model (panel (B)). Another estimate of Eb is possible by determining the energy difference between the two minima in the second-derivative plot, see Figure S1B (Supporting Information). The lowerenergy minimum is located at the peak of the exciton band, and the second minimum approximately describes the location of the rise of the absorption continuum. For MABI one obtains a D

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

photoexcited carriers57,59,60 in combination with band-gap renormalization.57,59,61,62 It decays with a time constant τ1 = 0.14 ps yielding the magenta-colored spectrum. The time scale is typical for carrier−carrier and the fastest part of carrier− optical phonon scattering. Further relaxation involves the slower portion of carrier−optical phonon scattering as well as the onset of acoustic photon relaxation21,22,57,63 with a time constant τ2 = 2.2 ps. This leads to the green spectrum which is slightly narrower. We note that there is no decay in the bleach region of the transient spectra up to 10 ps, see the middle and bottom panels of Figure 4A,B. Therefore no electron−hole recombination happens on this early time scale. Only the decay at later times involves recovery of the bleach band. The green spectrum in Figure 7 decays with a time constant τ3 = 190 ps yielding the blue spectrum. The blue spectrum finally decays very slowly to the violet spectrum for T = 296 K (steady-state M0). We may estimate a time constant τ4 > 5 ns, because our experiment covers only a time window up to 1.5 ns. We associate the latter two decay processes predominantly with exciton recombination which is, especially in the 10−100 ps regime, also overlaid by some contributions of slow acoustic phonon relaxation.22,63 3.5. Fluence-Dependent Kinetics. Kinetic traces at the peak of the exciton band (500 nm) were compared for different fluences of the pump beam. They were subsequently analyzed to learn more about the decay and recombination mechanisms. Representative kinetic data are provided in Figure 9A,B for

by photoexcitation of M0, where the latter corresponds to MASbI in thermal equilibrium (T = 296 K). A sum of Gaussian functions was employed to describe the species-associated spectra (SAS) of the species Mi. The resulting spectra and time constants are provided in Figure 7. An example for the species

Figure 7. Absorption spectra of MASbI at different stages of carrier relaxation. The transient species M1−M4 in the kinetic model are associated with the corresponding time constants τ1−τ4, with M0 being the steady-state absorption at 296 K. The inset shows the same spectra over a wider wavelength range. Photoexcitation was performed at 400 nm.

Figure 9. Fluence-dependent kinetics of MASbI at the probe wavelength 500 nm. (A) Early-time kinetics up to 2.3 ps. (B) Longer-time kinetics up to 1500 ps. (C) Transient absorption amplitude at 0.13 ps vs. fluence of the pump laser beam. (D) Time constants τ1−τ3 extracted from multiexponential fits as a function of fluence. Fluences: 86, 173, 260, 346 and 432 μJ cm−2.

Figure 8. Contributions of the species M0−M4 to the total fit of the transient experimental spectrum at 0.15 ps. The inset shows a magnification.

contributions to the spectrum at 0.15 ps is shown in Figure 8. Species contributions to the spectrum at 100 ps and kinetics on short and long time scales at four representative wavelengths are shown in Figures S4−S8 (Supporting Information). The main feature of the early-time spectrum (red line) in Figure 7 is the strongly broadened and depleted exciton band which extends toward longer wavelengths. This band shape leads to the second-derivative-like features in the transient absorption spectra, as shown in Figure 4. This spectral feature is a hallmark of Coulomb screening of the excitons by

short and long times, respectively. Plots of the absorption amplitude as a function of pump fluence provide straight lines. One example is shown for t = 0.13 ps (peak of the kinetics) in Figure 9C. Upon normalization of the transients all curves fall on top of each other. This kinetic behavior largely differs from 3D lead-based halide perovskites, such as MAPI. In that case the transients are dominated by bimolecular recombination kinetics of free carriers for carrier densities in the range 1017 to 10 19 E

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C cm−3,21,64 and consequently pronounced changes in the shape of the transients occur upon altering the pump fluence, compare e.g. with Figure 5 of ref 21. In contrast, the kinetic traces of MASbI suggest that the dynamics is largely governed by the intrinsic behavior of localized excitons. It is therefore independent of the absolute number density of excitons in the semiconductor material. A reasonable fit of all transients is achieved using four exponentials employing the time constants from the global analysis described in section 3.4, as plotted in Figure 9D, i.e. τ1 = 0.14 ± 0.02 ps (A1 = 66 ± 3%), τ2 = 2.20 ± 0.13 ps (A2 = 11 ± 1%), τ3 = 190 ± 11 ps (A3 = 9 ± 3%) and τ4 > 5 ns (A4 = 14 ± 1%). Polaron formation and self-trapping of excitons have been suggested by McCall et al. based on their steady-state Raman scattering and photoluminescence (PL) studies of several A3M2I9 compounds (A = Cs, Rb; M = Bi, Sb).8 Such a picture is consistent with the kinetics observed here, resulting from excitons which are locally trapped in the lattice and nonradiatively recombine on longer time scales.8 In addition, strong evidence for the presence of localized excitons has been found in the PL studies of Ni et al. and our previous studies for MABI.19,65 3.6. Direct Observation and Analysis of Coherent Phonon Dynamics in MASbI. The trapping of excitons is consistent with a strong interaction between the charge carriers and the lattice, resulting in pronounced electron−phonon coupling.8,66 A closer inspection of the kinetics in Figure 9A identifies weak oscillations appearing at early times which are superimposed on the transients. For an accurate analysis of these features, separate measurements were obtained with higher time resolution (60 fs). A contour plot of this transient absorption data and eight kinetic traces including fit lines are shown in Figure 10. Such oscillations indicate strong coherent motion of optical phonons modulating the dielectric constant and thereby the transmission and reflectance of the material. In MAPI, oscillatory features in transient absorption spectra have been observed only at low temperatures (77 K)67 and only recently identified by 2D electronic spectroscopy at room temperature.68 The strong oscillations observed here suggest the presence of strong electron−phonon coupling in 0D vacancyordered perovskite materials, such as MASbI. In order to fit the kinetic traces, four adjacent wavelength slots of a measured transient absorption spectrum were averaged and cut off at 4.5 ps. A global fit function consisting of one step-function, two exponential decays and one damped cosine-function (with wavelength-independent phase-shift), convolved with a Gaussian function to describe the finite experimental time-resolution, was employed to obtain a reasonable fit of the experimental signals. The two decay time constants were fixed at the global τ1 and τ2 values obtained from the aforementioned measurements (showing weaker oscillations due to the slightly worse time resolution of ca. 80 fs) and were not further optimized as they already provided an excellent description of the decays. The optimized parameters for the wavenumber ν̃ and damping time constant τd of the coherent mode were employed to fit the complete transient spectrum up to 4.5 ps. We obtain ν̃ = 100 cm−1 for the prominent oscillation which coincides with the intense broad peak in the steady-state Raman spectrum of MASbI arising from three modes centered at 95, 110 and 125 cm−1, as reported by Jagodzinski and Laane,69 and τd ≈ 0.6 ps for the damping time constant of the coherent motion. While the

Figure 10. Contour plot of transient absorption data of MASbI on mp-TiO2 including eight kinetic transients covering the UV−vis region. For each kinetics, the optimized fit (red line) consists of two exponential decays (green and magenta lines), a step-function (orange line) and a damped cosine function (black line), all convolved with a Gaussian function. λPump = 400 nm.

description of the transients is good (Figure 10), the fit of the oscillatory pattern is still not perfect. The experimentally observed oscillations were therefore analyzed further in the following way: first, the pure oscillatory contribution to the experimental spectrum was obtained by subtracting the global fit without the damped cosine part from the experimental data. Each time trace was then zero-padded to three times its original length and multiplied with an exponential-decay apodization function. The frequency magnitude was calculated from a Fourier transformation of the modified experimental time trace. The resulting phonon frequency spectra were not particularly sensitive to the chosen amount of zero-padding and the type of apodization function. Frequency spectra were averaged over selected wavelength intervals. The results of this analysis are provided in Figure 11. The vibrational spectra extracted from our transient absorption data completely recover the steady-state Raman spectrum of MASbI reported by Jagodzinski and Laane which is shown as a stick spectrum in the bottom panel:69,70 According to their analysis the peak at 170 cm−1 is assigned to stretching modes of the external Sb−I bonds of the [Sb2I9]3− anion. The prominent peaks at 110 and 125 cm−1 are due to the bridging Sb−I bonds which are weaker and feature larger bond lengths.70 The peaks at 95 and 64 cm−1 are assigned to bending modes of the anion, and the broader contributions at even lower frequencies arise from lattice modes of MASbI.69 We note that this type of Raman spectrum is characteristic for vacancy-ordered 0D A3M2I9 perovskite derivatives, whereas layered 2D polymorphs lack the intense mode at 110 cm−1.8 F

DOI: 10.1021/acs.jpcc.7b09609 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

induced via the displacive excitation of coherent phonons (DECP) mechanism.71,75 As such it would be very similar to the observation of Gühr and Schwentner where a lowfrequency coherent zone boundary phonon was detected in a solid argon matrix upon photoexcitation of an embedded Br2 chromophore.74 In the case of MASbI, the photoexcited [Sb2I9]3− chromophore provides a kick to the surrounding lattice initiating the low-frequency Raman response. The current findings triggered us to re-inspect our transient absorption experiments for the bismuth-analogue MABI.19 Indeed, an in-depth analysis of that data also reveals strongly damped and fairly noisy oscillatory features at early times with a period of ca. 250 fs, see e.g. the red transient at the probe wavelength 350 nm in Figure 2B of our previous paper.19 The resulting frequency of about 130 cm−1 is in good agreement with strong peaks found in the steady-state Raman spectra for the 0D polymorphs of the related systems ((CH3)2NH2)3Bi2I9 and Cs3Bi2I9.8,76 Therefore we conclude that pronounced coherent phonon dynamics is a general feature of antimonyand bismuth-based 0D A3M2I9 systems. As such it will be also interesting to compare the current findings with those for 2D layered A3M2X9 polymorphs in future experiments. 3.7. Electron and Hole Injection Processes at the Interfaces. The energy diagram of Figure 1B shows that electron injection from the CB of MASbI to the CB of mesoporous TiO2 and hole injection from the VB of MASbI to the HTM H101 is energetically feasible. We have recently demonstrated for mp-TiO2/MAPI/H10125,26 and mp-TiO2/ MABI/H10119 thin films that the electron and hole injection processes may be monitored conveniently by ultrafast broadband transient absorption spectroscopy in the NIR range. Corresponding results for mp-TiO2/MASbI/H101 are presented in Figure 12. Photoexcitation was performed at a wavelength of 505 nm so that only MASbI, but not the HTM is photoexcited.25,26

Figure 11. Contour plot of the vibrational contributions (in cm−1) to the coherent oscillations of MASbI on mp-TiO2 including six spectral cuts providing vibrational spectra averaged over the wavelength intervals indicated. The bottom panel shows a stick spectrum of the experimental Raman data for MASbI including intensities from ref 69 (vertical axis break at 20). Note the reciprocal representation for the wavelength λ used in the contour plot which is equivalent to an energy scale. Several spurious peaks at 400 nm arise from stray-light contamination by the pump pulse.

The relatively short decoherence time (τd ≈ 0.6 ps) leads to an increase of the already present spectral line broadening in this system, so that in our spectra obtained from Fourier transformation e.g. the peaks at 95 and 125 cm−1 appear only as shoulders of the main peak at 110 cm−1. The contributions of the individual vibrations to the oscillatory pattern vary considerably with wavelength. For instance, the traces in the wavelength ranges 370−387, 406− 442 and 476−524 nm contain all frequency components. In contrast, the spectra in the regions 442−476, 524−556 and 557−696 nm are weak and quite featureless. The slices for the two latter wavelength intervals feature mainly low-frequency modes. These results suggest the following picture of the coherent dynamics in this 0D molecular salt: photoexcitation induces pronounced vibrational coherences localized on the [Sb2I9]3− unit69 which most likely arise from impulsive stimulated Raman scattering (ISRS) processes upon photoexcitation of the anionic chromophore.71 The oscillations are particularly pronounced in the region of the excitonic absorption features and above the band gap. This suggests an enhancement66 of the coherent features in those probe wavelength ranges which are in or close to resonance with the exciton absorption feature. The dynamics observed here for this 0D vacancy-ordered perovskite derivative resembles the coherent response of localized photoexcited host chromophores in matrices. A similar behavior was e.g. detected previously for halogen molecules in solid rare gases.72−74 Vibrational decoherence in the (sub)picosecond range is a result of chromophore interactions with the surrounding matrix. We suggest that the low-frequency (