Exciton Dynamics and Electron-Phonon Coupling Affect the

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C: Energy Conversion and Storage; Energy and Charge Transport

Exciton Dynamics and Electron-Phonon Coupling Affect the Photovoltaic Performance of the CsAgBiBr Double Perovskite 2

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Robin Kentsch, Mirko Scholz, Jonas Horn, Derck Schlettwein, Kawon Oum, and Thomas Lenzer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09911 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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The Journal of Physical Chemistry

Exciton Dynamics and Electron-Phonon Coupling Affect the Photovoltaic Performance of the Cs2AgBiBr6 Double Perovskite Robin Kentsch,† Mirko Scholz,† Jonas Horn,‡ Derck Schlettwein,*,‡ Kawon Oum,*,† Thomas Lenzer*,† †

Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany

‡

Justus-Liebig-Universität Gießen, Institut für Angewandte Physik, Heinrich-Buff-Ring 16,

35392 Gießen, Germany

AUTHOR INFORMATION Corresponding Authors * Email: [email protected], [email protected], [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT:

Lead-free double perovskites have been proposed as promising non-toxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs2AgBiBr6, in spite of considerable optimization efforts by several groups. Here we present a detailed study of Cs2AgBiBr6 thin films deposited on poly(methyl methacrylate) (PMMA) and mesoporous TiO2. Femtosecond UV-vis-NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron-phonon coupling via Fröhlich interactions is observed in terms of a pronounced coherent oscillation of a strong A1g optical phonon mode of the double perovskite at 177 cm-1. Similar behavior is also found for the related vacancy-ordered perovskite Cs3Bi2Br9 and the parent compound BiBr3. Excitonic effects and electron-phonon coupling are known to induce unwanted electron-hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.


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Introduction Lead-halide perovskites have reached high photovoltaic power conversion efficiencies1-3 with an NREL-certified top value of 23.3%.4 However, the inherent toxicity of lead and long-term stability problems regarding humidity, heat and oxygen might pose major obstacles in terms of their future commercialization.5-10 Therefore, double perovskites (also called “elpasolites”) of the general formula A2MM’X6 have been suggested as promising alternatives. Here, M and M’ are metals in the formal oxidation states +I and +III (retaining “on average” the formal +II oxidation state of lead), A is a monovalent inorganic or organic cation, and X is a halide. The prototype of this material class is Cs2AgBiBr6, which features silver and bismuth atoms with octahedral bromine coordination on alternating positions in the lattice (Figure 1(a)).11-14 It was pointed out previously that a fundamental mismatch exists between Ag d and Bi s orbitals at the band edges.15 Therefore, an indirect band gap arises, which is due to a narrower width of the conduction band.13,15 This indirect band gap transition is located at ca. 2.0 eV11,12,16,17 and is one central problem for photovoltaic applications of this system. Because of the low absorption coefficient, efficient solar light-harvesting will require thick films, which should therefore feature a large carrier diffusion length. Thermodynamically, the Ag-Bi double perovskite is stable against decomposition according to 2 Cs2AgBiBr6  Cs3Bi2Br9 + 2 AgBr + CsBr.15 Still, synthesis of phase-pure Cs2AgBiBr6 thin films using spin-coating had remained challenging, until a breakthrough was achieved by Bein and co-workers.18 Annealing of the spin-coated film above 250 °C was one of the key steps for complete conversion to the double perovskite. Yet, despite extensive attempts, the maximum PCE of optimized devices has not surpassed 2.5%,18-20 suggesting intrinsic electronic limitations.



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Very recently, first studies have appeared dealing with light-induced transient charge carrier processes of Cs2AgBiBr6. For instance, Bartesaghi et al. employed time-resolved microwave conductance (TRMC) with moderate time resolution to probe the formation and decay of mobile charges after pulsed photoexcitation.16 They reported the presence of short- and long-lived carriers (with maximum lifetimes reaching the microsecond timescale) and a large density of electron and hole traps, which were identified as being shallow. They claimed that exciton effects in Cs2AgBiBr6 crystals and thin films are not important.16 In contrast to Bartesaghi et al., Yang et al. reported strong excitonic features as well as substantial trapping of electrons and holes in Cs2AgBiBr6 nanoparticles. They interpreted negative signals in transient absorption spectra around 443 nm as bleach features,21 which, as we will show below, is not correct. Instead, this spectral shape is easily explained by the transient broadening of an exciton feature. Steele et al. presented steady-state Raman spectra as well as steady-state and time-resolved photoluminescence (PL) experiments on Cs2AgBiBr6 single crystals including DFT calculations.22,23 A large Fröhlich coupling constant of about 230 meV was reported, which is about 4-6 times larger than for lead perovskites, suggesting dominant scattering of electrons by longitudinal optical phonons. PL transients were fitted mathematically with up to six time constants, in a few cases even including additional stretched-exponential decay terms. These different components were then assigned to a zoo of species, including “superfast bound excitons”, “fast unbound excitons” and “slow separate free carriers”.23 It remains unclear, if such a multiexponential deconvolution is unique and if a smaller set of species with an inhomogeneous distribution of lifetimes and different decayassociated spectra might describe such data equally well. In addition, PL signals are easily swamped by contributions of strongly luminescent minority species, which might not represent the main processes governing the carrier dynamics.


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In the current work, we study the carrier dynamics of Cs2AgBiBr6 thin films by ultrafast broadband transient absorption with extended spectral coverage (spanning the UV-vis-NIR range) and high time resolution. Careful analysis using sophisticated global kinetic modeling identifies the main carrier relaxation channels as well as dynamic effects associated with strong electronphonon coupling. In particular, for the first time the deep-UV coverage provides access to the true bleach region of Cs2AgBiBr6 located below 400 nm, which was not covered in previous experiments.21 We are able to resolve several of the aforementioned conflicting interpretations regarding the charge carrier dynamics of Cs2AgBiBr6. A clear picture emerges based on a careful comparison with closely related systems such as Cs3Bi2Br9 and BiBr3. This way, general trends in bismuth-based “perovskite-inspired materials” going beyond the specific double perovskite motif are identified and backed up by electrical measurements and DFT calculations. As a result, beside the well-known problem of the indirect band gap in Ag-Bi double perovskites, our study highlights two additional important properties of these materials affecting their photovoltaic efficiency: exciton formation and strong electron-phonon coupling.

Methods

Preparation of Double Perovskite Powder and Thin Films for Optical Experiments. Cs2AgBiBr6 powder was prepared by a modification of the procedure provided by McClure et al.12 Thin films for the optical measurements were fabricated on mp-TiO2- or PMMA-covered substrates by spin-coating a solution of the pre-synthesized powder or an appropriate mixture of the respective precursor materials. Annealing at 285 °C for 30 min was imperative for obtaining phase-pure Cs2AgBiBr6 thin films.18



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Preparation and Characterization of PV Devices. A solution of the double perovskite precursor was spin-coated on pre-annealed commercial TiO2-coated transparent conductive oxide substrates. A solution of the HTM poly(bis(4phenyl)(2,4,6-trimethylphenyl)amine)

(PTAA)

and

the

dopant

lithium

bis(trifluoromethanesulfonyl)imide was spin-coated onto the double perovskite layer afterwards. Finally, a gold back-contact was evaporated onto the HTM layer. J-V characteristics were measured using an active area of 0.25 cm2.

Transient Absorption Spectroscopy. Femtosecond UV-vis and NIR broadband transient absorption spectra were obtained using two previously described setups24,25 employing the pump-supercontinuum probe (PSCP) method.26 Thin films were mounted inside a movable, nitrogen-flushed aluminum cell,27 pumped at 400 nm and probed over the wavelength ranges 260-700 and 850-1600 nm.

DFT Calculations. The program package Quantum Espresso28,29 was employed for plane-wave density functional calculations within the local density approximation using optimized norm-conserving Vanderbilt pseudopotentials. Structures of Cs2AgBiBr6 and Cs3Bi2Br9 were taken from the literature.11,30 Visualization of the most intense A1g mode was performed using XCrySDen.31 Full details of the experiments and calculations as well as XRD data confirming the phase purity of the synthesized materials are provided in the Supporting Information (Figure S1).


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Results and Discussion

Steady-State Absorption and PL Spectra of Cs2AgBiBr6. The absorption spectrum of the Cs2AgBiBr6 thin film shows a slow rise below 540 nm. The indirect band gap of this material is located at 614 nm (see Figures S2 and S3(a), Supporting Information). At lower wavelengths, we observe a characteristic sharp peak at 437 nm (2.84 eV, Figure 1(b), black line). We assign this to considerable excitonic character. This is consistent with the interpretation of similar features in so-called “vacancy-ordered perovskites” of the general formula A3Bi2X9,32-34 such as, e.g., the layered compound Cs3Bi2Br9, see also Figure S4 in the Supporting Information.

Figure 1. (a) Crystal structure of the double perovskite Cs2AgBiBr6 based on ref. 11, drawn using VESTA 3.14 (b) Steady-state absorption (black) and photoluminescence (red, exc = 360 nm) of a



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Cs2AgBiBr6 thin film on PMMA. The PL signal above 700 nm is not shown because of substantial second-order stray-light contributions of the excitation beam. (c) Band energy diagram for n-i-p devices based on Cs2AgBiBr6. (d) J-V curves including characteristic cell parameters and a picture of a representative solar cell device. Red: 50 mV s-1 forward scan, VOC = 1.02 V, JSC = 1.42 mA cm-2, FF = 0.57, PCE = 0.82%. Blue: 50 mV s-1 reverse scan, VOC = 1.01 V, JSC = 1.43 mA cm-2, FF = 0.53, PCE = 0.77%. Black: dark scan. The absorption edge of a strong direct transition is located at 387 nm (3.20 eV), as determined from a Tauc plot analysis (Figure S3(b), Supporting Information). An estimate of the exciton binding energy was obtained by fitting the region of the direct transition using the model of Elliott.35,36 Details of the procedure are provided in the Supporting Information (Figure S5) and in our previous paper dealing with excitonic absorption features in (CH3NH3)3Sb2I9.37 The fit provides a substantial exciton binding energy of 268 meV, which is much larger than the average thermal energy at 296 K (26 meV). Substantial values were also found previously for other bismuth compounds, such as (CH3NH3)3Bi2I9 (> 300 meV)32,37 and BiI3 (160-180 meV).38 We note that we did not analyze excitonic contributions in the spectral region of the indirect band gap because of the very weak absorption of the thin films and the superimposed interference structure in the absorption spectrum. The peak of the weak steady-state photoluminescence (PL) spectrum of the Cs2AgBiBr6 thin film in Figure 1(b) (red line) shows a large red shift with respect to the position of the direct transition and the exciton absorption. This strong stabilization and the broadness of the PL band are consistent with the emission of self-trapped excitons and strong exciton-phonon interaction, as suggested for A3M2I9 compounds (A = Cs, Rb; M = Bi, Sb)39 and systems such as silver chloride.40


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Construction of Solar Cells Based on Cs2AgBiBr6. We

constructed

n-i-p

devices

using

the

configuration

FTO/cp-TiO2/mp-

TiO2/Cs2AgBiBr6/PTTA/Au. The energy diagram in Figure 1(c), based on literature values,11,16,41,42 shows satisfactory band alignment with the double perovskite. The J-V curve in Figure 1(d) clearly confirms well-defined diode characteristics of the chosen contact architecture and features a substantial open-circuit voltage of 1.0 V, in very good agreement with the difference of 1.1 eV between the TiO2 CB and the PTAA HOMO, but a low short-circuit current. The PCE of 0.82% confirms the weak performance of this double perovskite found in previous studies18,19 suggesting inherent limitations regarding the charge carrier dynamics and carrier transport in this material. The lower value compared with literature18,19 is likely related to the reduced film thickness and the not fully optimized hole transport layer. Wavelength-dependent contributions to the photocurrent density are discussed in detail in the Supporting Information (Figure S6).

Broadband Transient Absorption Experiments for Cs2AgBiBr6. Figure 2 shows a representative example of a transient absorption measurement for a Cs2AgBiBr6 thin film deposited on top of an inert PMMA layer on a glass substrate.



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Figure 2. Femtosecond UV-vis-NIR transient absorption spectra of the double perovskite Cs2AgBiBr6 on PMMA after excitation at 400 nm for different time ranges of interest. In (a), the second derivative of the steady-state absorption spectrum (blue) as well as the stimulated emission spectrum of Cs2AgBiBr6 (orange) are included for comparison. The latter was determined from the steady-state photoluminescence spectrum in Figure 1(b). Species-associated spectra (SAS) from a global kinetic analysis of the data using the consecutive kinetic scheme DP1  DP2  DP3  DP4  DP0, where DP0 is the steady-state absorption of the double perovskite (DP), are provided in Figure 3(a) including the species lifetimes 1 to 4. Representative kinetic fits are presented in the Supporting Information (Figures S7-S10).


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Figure 3. (a) Species-associated spectra (SAS) of the transient double perovskite “species” DP1 to DP4 (indicating different stages of carrier relaxation) with associated lifetimes 1 to 4 for Cs2AgBiBr6/PMMA. DP0 = steady-state absorption. (b) and (c) Examples for the contributions of the individual species to the transient absorption spectra at 0.15 and 100 ps, respectively, including the total fit (cyan) to the experimental data (open circles). Figure 2(a) shows the dynamics around zero time delay. The spectra consist of a bleach below 380 nm and an oscillatory feature above 380 nm. The latter resembles the second derivative of the steady-state absorption spectrum (dotted blue line) with additional absorption toward shorter and longer wavelengths: This spectral feature clearly indicates broadening of the exciton resonance peak due to exciton-exciton interactions, exciton screening and carrier heating (red SAS in Figure 3(a)).37,43-45 We do not observe any transient stimulated emission (SE) features above 540 nm. This is deduced from a comparison with the separately measured steady-state SE (dotted orange line).



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The finding is consistent with the indirect band gap of this material. Therefore, SE requires phonon assistance and the transition is too weak to be observed. Spectral features above 800 nm are not very prominent and assigned to weak intraband absorption of the double perovskite. Up to 2 ps, spectral dynamics only occur in the exciton band above 380 nm (Figure 2(b)). The decay of the oscillatory features is consistent with a recovery and narrowing of the exciton peak (orange SAS in Figure 3(a)). The time constants of 1 = 0.15 and 2 = 3.6 ps are assigned to relaxation by carrier-carrier (cc) and predominantly carrier-optical phonon (cop) scattering. No spectral decay is observed below 380 nm, so electron-hole recombination does not take place in this time regime. On longer time scales (up to 1000 ps, Figure 2(c)), the spectra show a more uniform decay, also involving the band below 380 nm. The dynamics must therefore include electron-hole recombination. In addition, the narrowing of the SAS (green and blue lines in Figure 3(a)) suggests superimposed carrier cooling processes such as optical-acoustic phonon (oap) scattering as well as acoustic phonon relaxation. We obtain two time constants 3 = 290 ps and 4 > 5 ns. A discussion of the recombination dynamics on longer time scales (based on separate transient emission experiments) is provided in the Supporting Information (Figures S11 and S12). Contributions of the individual “species” to the characteristic exciton-related second-derivative shape of the transient spectra at 0.15 and 100 ps are shown in Figure 3(b) and (c). We note that very similar exciton dynamics with essentially the same time constants is found for the double perovskite on mesoporous TiO2 (Figures S13-S18, Supporting Information), so the carrier relaxation processes are largely independent of the respective underlayer.

Strong Electron-Phonon Coupling in Cs2AgBiBr6.


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In addition to the excitonic contributions, which will make the extraction of electron and holes in a PV device more difficult, we also observe clear indications for strong electron-phonon coupling in the double perovskite. Such coupling is known to accelerate recombination and to decrease mobility of charge carriers in PV thin films.46,47 The coupling is clearly captured in the kinetics of Figure 4(a): The transients of Cs2AgBiBr6 on PMMA and mesoporous TiO2 show the same characteristic pronounced oscillations (green and blue lines). The origin of the oscillations is ascribed to impulsive stimulated Raman scattering (ISRS).48 It is important to note that the coherent phonon oscillations of the Cs2AgBiBr6 double perovskite are much more pronounced than in prototypical lead perovskites where they are strongly damped and barely observable.49,50 The reason for this is likely the much larger Fröhlich coupling in the case of the double perovskite.22 Using a simple picture based on chemical intuition, this can be ascribed to the larger polarity (larger difference in electronegativity) of the Ag-Br and Bi-Br “bonds” in this double perovskite semiconductor compared with Pb-I. A frequency analysis of the oscillations was performed by Fourier transformation of the complete data set for Cs2AgBiBr6/PMMA. The resulting frequency map is shown in Figure 4(b), and spectral cuts can be found in Figure 4(c) (for the corresponding data set of Cs2AgBiBr6/mpTiO2 see Figure S19, Supporting Information). The analysis reveals one dominant frequency component at 177 cm-1, which agrees well with the value of 175 cm-1 reported in experimental steady-state Raman spectra (red stick spectrum in Figure 4(c)).11,22 Interestingly, we find pronounced resonance effects for the Raman features, as indicated by the intensity fluctuations in the frequency map and the respective spectral cuts of Figure 4(c). The intensity of this longitudinal optical A1g phonon mode (corresponding to the amplitude of the beating pattern in the transient absorption kinetics) becomes particularly enhanced when exciting near the absorption peak at 437



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nm and on its red edge. Such a resonance enhancement of Raman lines, well known from seminal steady-state Raman studies e.g. by Shen and co-workers,51 has been identified previously by us in transient absorption experiments of other “halide-perovskite-inspired” materials based on group 15 elements, e.g. BiI3 and (CH3NH3)3Sb2I9.37,45 It was also more recently proposed based on the results of steady-state Raman experiments for Cs2AgBiBr6 crystals.22 One expects such strong enhancements of Raman bands in cases, where the incident wavelength (in our case the respective portion of the broadband supercontinuum probe light) and the wavelength of the scattered photon are in resonance with excitonic levels.51 This observation provides further support for the presence of considerable excitonic contributions in the region of the strong absorption peak of Cs2AgBiBr6 at 437 nm (Figure 1(b)). A full assignment of the Cs2AgBiBr6 Raman spectrum is obtained from our DFT calculations. For space group Fm 3 m (No. 225) we expect four Raman-active modes (stick spectrum in Figure 4(c)): The dominant mode at 169 cm-1 is of A1g symmetry, in good agreement with our measured 177 cm-1, and mainly involves concerted motion of the Br atoms (symmetric stretching of the BiBr and Ag-Br bonds of the octahedral cages in Figure 4(d)). In addition, a doubly degenerate Eg mode (129 cm-1) as well as two triply degenerate T2g modes (59 and 22 cm-1) are found, where the latter one has very weak intensity. The values are in satisfactory agreement with the line positions in the nonresonant Raman spectrum of Steele et al., who reported phonon frequencies of 175, 135, 75 and 40 cm-1 for the respective phonon modes in a bulk crystal.22 In general, our phonon calculations appear to underestimate the experimental values in a systematic fashion.


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Figure 4. (a) Coherent oscillations for Cs2AgBiBr6/PMMA, Cs2AgBiBr6/mp-TiO2, Cs3Bi2Br9/mpTiO2 and BiBr3/mp-TiO2. (b) Fourier amplitude map over the complete spectral range for Cs2AgBiBr6/PMMA. (c) Line positions and intensities from the nonresonant Raman experiments (exc = 808 nm) of ref. 22 (red) and our DFT calculations (blue) shown as stick spectra (normalized to the highest-intensity peak) including assignments as well as averaged spectral slices extracted from the Fourier amplitude map in (b) for Cs2AgBiBr6/PMMA. (d) Visualization of the calculated intense Raman-active phonon mode of Cs2AgBiBr6 at 169 cm-1 having A1g symmetry (blue: bismuth, grey: silver, red: bromine, magenta: cesium). Displacement vectors are indicated as small light-green arrows.

Broadband Transient Absorption Spectroscopy of the Vacancy-Ordered Perovskite Cs3Bi2Br9. Figure 5 shows the results of transient absorption experiments for Cs3Bi2Br9 on mesoporous TiO2 after excitation at 400 nm. The dynamics shows striking similarities to Cs2AgBiBr6 (Figure 2 and Figure S13, Supporting Information), most prominently the second-derivative-type



appearance of the spectra. This also suggests considerable excitonic contributions in this compound, which can be formally denoted as a one-third bismuth-deficient perovskite (“CsBi2/3Br3”). Spectral differences are mainly observed below 390 nm, where Cs3Bi2Br9 shows absorption instead of the bleach seen in Cs2AgBiBr6. The very weak signal in the NIR region is due to intraband absorption of Cs3Bi2Br9. Electron injection into mesoporous TiO2 within the observed time regime is therefore negligible, as in the case of the Cs2AgBiBr6 double perovskite.

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Figure 5. Femtosecond UV-vis-NIR transient absorption spectra of Cs3Bi2Br9 on mesoporous TiO2 after excitation at 400 nm for different time ranges of interest. The smoothed second derivative of the steady-state absorption spectrum of Cs3Bi2Br9 is included in panel (a) as a blue dotted line. Figure 6(a) contains SAS of Cs3Bi2Br9, which were obtained by fitting the kinetics by the consecutive scheme C1  C2  C3  C4  C0, with C0 being the steady-state absorption of


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Cs3Bi2Br9. Two selected examples for fits of the spectra at 0.15 and 100 ps are shown in Figure 6(b) and (c). Representative kinetic fits can be found in the Supporting Information (Figures S20S23).

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Figure 6. (a) Species-associated spectra (SAS) of the transient “species” C1 to C4 (indicating different stages of carrier relaxation of the vacancy-ordered perovskite) with associated lifetimes

1 to 4 for Cs3Bi2Br9/mp-TiO2. C0 = steady-state absorption. (b) and (c) Examples for the contributions of the individual species to the transient absorption spectra at 0.15 and 100 ps, respectively, including the total fit (cyan) to the experimental data (open circles). As in the case of the double perovskite, the initial broadening and subsequent narrowing of the exciton band at 434 nm34 is clearly visible, here with an initial small shift toward larger wavelengths in the first short-lived SAS (red). The four time constants 1 to 4 (0.14, 4.8, 200 and



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> 5000 ps) are required for fitting the specific carrier relaxation processes. They are very similar to those obtained for the double perovskite and are assigned to the same carrier processes already described for Cs2AgBiBr6. We note that the “parent compound” BiBr3 also displays similar spectral and kinetic behavior (Figures S24-S29, Supporting Information), so excitonic features appear to be common features of the bismuth-containing “perovskite-inspired” materials studied here as well as antimony-based compounds.37

Strong Electron-Phonon Coupling in the Case of Cs3Bi2Br9. Pronounced oscillations are also found in the short-time kinetics of Cs3Bi2Br9 on mp-TiO2 (Figure 4(a), red line). The oscillatory beating pattern resembles that of the double perovskite, yet with a larger phonon wavenumber (slightly faster oscillation). The transient spectra were analyzed further by Fourier transformation of the complete data set. The resulting frequency map, selected spectral cuts and assignments of the Raman-active phonon modes from our DFT calculations, including a comparison with experimental steady-state Raman data, are provided in Figure 7. Cs3Bi2Br9 belongs to the space group P 3 m1 (No. 164). Therefore one expects nine Ramanactive phonon modes (four A1g and five Eg), as shown by the blue stick spectrum in Figure 7(b). The spectrum is dominated by the A1g mode at 191 cm-1 (DFT calculation: 189 cm-1) and involves Bi-Br stretching motion, see Figure 8. In nice agreement with experiment, this phonon mode has a higher frequency than the strongest A1g mode of the double perovskite (Figure 4(c) and S19, Supporting Information). A weaker doubly degenerate Eg mode (DFT calculation: 170 cm-1) is likely responsible for the weak shoulder of the dominant 191 cm-1 peak, see also the green spectral cut in Figure 7(b). In general, our calculated Raman spectrum shows good agreement with the


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experimental steady-state Raman spectrum of Baran and co-workers.52 As in the case of the double perovskite, we observe enhancement of the Raman features close to the excitonic peak.

(a)

480

Fourier amp. / 10-2

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3.0 2.4

460

1.8 1.2

440

0.6 0.0 420

0

100

200

300

Wavenumber / cm-1

(b)

A1g

Calc. (DFT) Exp.

Eg

Intensity / arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Eg E A1g g

Eg A1g

Eg A1g

465-485 nm 455-465 nm

440-455 nm 433-440 nm 421-433 nm 410-421 nm

0

100

200

300

Wavenumber / cm-1

Figure 7. (a) Fourier amplitude map over the complete spectral range for Cs3Bi2Br9/mp-TiO2 upon excitation at 400 nm. (b) Line positions and intensities from the nonresonant Raman experiments (exc = 1063 nm) in x(zz)y geometry at 300 K of ref. 52 (red) and our DFT calculations (blue) shown as stick spectra (normalized to the highest-intensity peak) including assignments as well as averaged spectral slices extracted from the Fourier amplitude map in (a).



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Figure 8. Visualization of the calculated intense Raman-active phonon mode of Cs3Bi2Br9 at 189 cm-1 having A1g symmetry (blue: bismuth, red: bromine, magenta: cesium). Displacement vectors are indicated as light-green arrows. The results therefore suggest that strong electron-phonon coupling also exists in the silver-free system Cs3Bi2Br9, which consists of corner-sharing BiBr6 octahedra forming corrugated layers. We note that the parent compound BiBr3 also shows oscillations (Fig. 4(a), black line, dominant mode of about 92 cm-1), which are however strongly damped. Taking all this evidence together and combining it with our previous results for BiI3,45 it appears to be reasonable to assume that strong electron-phonon coupling via Fröhlich interactions22 are a hallmark of such bismuth-based and also antimony-based materials37 with octahedral halide coordination, regardless of their dimensionality.


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Conclusions In summary, the current study has highlighted two crucial properties affecting the less than ideal charge separation in the prototypical lead-free double perovskite Cs2AgBiBr6, beside the wellknown problem of its indirect band gap: (a) excitonic contributions (identified in the steady-state and ultrafast transient absorption experiments) and (b) strong electron-phonon coupling via Fröhlich interactions (which is known to induce faster recombination). Both effects also appear in the two other bismuth-based thin films studied here. Such inherent electronic material properties need to be addressed properly in future PV designs. The efficiency of exciton separation could be e.g. improved by appropriate engineering of the interfaces between the charge-extracting layers and the double perovskite. In addition, double perovskite materials need to be discovered, which possess a direct band gap and are at the same time thermodynamically stable.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.xxxxxxx. Experimental and computational details, x-ray diffraction data, calculated band diagram of Cs2AgBiBr6, Tauc plot analysis of the Cs2AgBiBr6 absorption spectrum, comparison of steadystate absorption spectra, estimating the exciton binding energy of Cs2AgBiBr6, IPCE measurements for Cs2AgBiBr6, kinetic fits for PSCP spectra of Cs2AgBiBr6/PMMA, results of TCSPC experiments for Cs2AgBiBr6/PMMA, transient absorption of Cs2AgBiBr6/mp-TiO2 and kinetic modeling, FT analysis of coherent oscillations for Cs2AgBiBr6 on mp-TiO2, kinetic fits



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for PSCP spectra of Cs3Bi2Br9/mp-TiO2, transient absorption of BiBr3/mp-TiO2 and kinetic modeling (PDF)

AUTHOR INFORMATION Corresponding Authors *Emails: [email protected], [email protected], [email protected].

ORCID Robin Kentsch: 0000-0002-0023-8532. Mirko Scholz: 0000-0002-5648-7925. Derck Schlettwein: 0000-0002-3446-196X. Kawon Oum: 0000-0001-6137-2236. Thomas Lenzer: 0000-0002-0766-709X. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS DS, KO and TL acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) through grants SCHL 340/21-3, OU 58/10-3 and LE 926/11-3 and the RTG 2204. We are thankful to M. Morgenroth for TCSPC measurements and extensive lab assistance, J. Weber and J. Schmedt


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auf der Günne (University of Siegen) for using their oven for thin film sintering and their diffractometer for recording powder XRD patterns, and to T. Kowald, S. Afflerbach, C. Pritzel and H.F. Trettin (University of Siegen) for providing thin film XRD patterns. We are also grateful to N.P. Ernsting (Humboldt University Berlin, Germany), J. Troe, K. Luther, J. Schroeder, D. Schwarzer and A.M. Wodtke (Georg August University Göttingen, Germany) as well as R. Rueß and the Center of Materials Research LaMa (Justus Liebig University Giessen) for their continuous support and advice. RK thanks the Deutsche Bundesstiftung Umwelt (DBU) for a stipend.

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