SUPPORTING INFORMATION. Unraveling Charge Carriers Generation, Diffusion and Recombination in

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1 SUPPORTING INFORMATION Unraveling Charge Carriers Generation, Diffusion and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film Piotr Piatkowski 1, Boiko Cohen 1, Carlito S. Ponseca Jr 2, Manuel Salado 3, Samrana Kazim 3, Shahzada Ahmad 3, Villy Sundström 2 and Abderrazzak Douhal*,1 1 Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., Toledo, Spain. 2 Division of Chemical Physics, Lund University, Box 124, Lund, Sweden. 3 Abengoa Research, Abengoa, Campus Palmas Altas, C/Energia Solar, Sevilla , Spain. 1

2 Methods: Formamidinium lead triiodide synthesis Materials. Unless otherwise stated, all materials were purchased from Sigma- Aldrich and used as received. Perovskite precursor synthesis: Formamidinium iodide (FAI) was synthesized following a previously reported procedure. 1 Formamidinium acetate was dissolved in a double molar excess of aqueous hydriodic acid (HI) (57%, w/w), and stirred at 50 C for 2 h. The excess solvent remaining HI was then removed with the help of rotary evaporator at 70 o C to obtain a slightly yellow powder. The yellow precipitate was then washed repeatedly with dry diethyl ether and recrystallized from absolute ethanol to yield a white crystalline powder, which was kept overnight at 60 o C for vacuum drying. To synthesize Formamidinium lead triiodide (FAPbI 3 ) precursor solutions, FAI and PbI 2 were dissolved in anhydrous dimethyl sulfoxide (DMSO) in a 1:1 molar ratio, at 0.8 M of each reagent, to result in a 0.8 M perovskite precursor solution. Substrate cleaning: Quartz substrates were used for all optical spectroscopy measurements. Prior to deposit any perovskite film, quartz substrates were cleaned by acetone and dry nitrogen. Film Preparation: FAPbI 3 was deposited by one step process using toluene solvent dripping method. The solution was prepared inside an argon glove box with moisture and oxygen controlled conditions (H 2 O level: <1ppm and O 2 level: <10 ppm). Perovskite precursor solution composed of PbI 2 (0.8 M), and a FAI (0.8 M) were dissolved in dimethyl sulfoxide (DMSO) and kept stirring at 80ºC overnight. 2

3 Afterwards, the solution was spin-coated on a quartz substrate at 1000 rpm for 10 s and then 6000 rpm for 30 s. During the second step, toluene was dripped at the center of the substrate during the last seconds. After solvent treatment, the films were transferred onto a hot plate and annealed at 150ºC for 60 minutes for the perovskite film formation. Then, the samples were left to cool down gradually until room temperature (~25 C) and was probed for characterization. Characterization of FAPbI 3 Scanning Electron Microscope (SEM): A Hitachi S-4800 field emission scanning electron microscope at power 2 kv was used to acquire surface and cross sectional SEM images. Cross sectional SEM images were obtained to determine the thickness of the perovskite film. The structural analysis of the samples was performed by X-ray powder diffraction (XRD) using a Rigaku powder diffractometer (Cu source). UV-VIS spectrometer: UV-visible absorption spectrum of the perovskite film was recorded using JASCO V-670 spectrophotometer. To eliminate the contribution of the scattered light to the absorption spectra we carried out the diffuse reflectance measurements using 60 mm integrating sphere ISN Femtosecond Transient Absorption Spectrophotometer The femtosecond (fs) transient UV-visible setup used here has been described elsewhere. 3 Briefly, it consists on a Ti:sapphire oscillator (TISSA 50, CDP Systems) pumped by a 5 W diode-laser (Verdi 5, Coherent). The seed pulse (30 fs, 450 mw at 86 MHz) centred at 800 nm is directed to an amplifier (Legend-USP, Coherent). The 3

4 amplified fundamental beam (50 fs, 1 W at 1 khz) is then directed through an optical parametric amplifier for wavelength conversion (CDP Systems). The pump intensities used ranged from ~ 40 to 250 µw (depending on the nature of the experiment) and the excitation wavelength was 600 nm. All the spectra analysed in the uv-visible region were corrected for the chirp of the white light continuum. The transient absorption measurements were performed in the spectral ranges of nm (UV-visible region) and nm (near infrared (NIR) region). To avoid sample degradation, the sample was moved during the measurement using XY translational stage. All the experiments were performed at room temperature (293 K). Sub-Picosecond Time-resolved Terahertz Spectrophotometer The terahertz (THz) experiments were done using a chirped pulse amplification (CPA) setup. The output (30 fs, 470 mw at 86 MHz) centered at 800 nm of a Ti:sapphire oscillator (TISSA 50, CDP Systems) pumped by a 5 W diode laser (Verdi 5, Coherent) is directed to a regenerative amplifier (Legend-USP, Coherent). The amplified fundamental beam centered at 800 nm (50 fs, 1W, 1 khz) is split into three parts. The first one ( 700 mw) is directed through an optical parametric amplifier (OPA) for wavelength conversion and a selected output from OPA is frequency-doubled in a 1-mm BBO crystal to give the desired wavelength. The resulting beam is sent through a long (up to 10 ns) delay line (H2W Technologies) to excite the sample in the time-resolved terahertz (THz) experiment. The second part of the fundamental beam (~ 200 mw) generates the THz probe in a ZnTe crystal by optical rectification. The third part ( 1 mw) is used for electro-optic detection of THz in another ZnTe crystal. During the measurements the setup is continuously purged with dry nitrogen. The studies were 4

5 performed under ambient conditions and no additional constant illumination has been used. Flash photolysis Spectrophotometer The ns-s flash photolysis setup is described elsewhere. 4 In brief, it consists of a LKS.60 laser flash photolysis spectrometer (Applied Photophysics) and a Vibrant (HE) 355 II laser (Opotek) as a pump pulse source (5 ns time duration). The signal from the OPO (pumped by Q-switched Nd:YAG laser, Brilliant, Quantel) at 440 nm was used for sample excitation. The probing light source was a 150 W xenon arc lamp. The light transmitted through the sample was dispersed by a monochromator and detected by either visible or near-infrared photomultipliers (Applied Photophysics R928 and Hamamatsu H10330 A, respectively), both coupled to a digital oscilloscope (Agilent Infiniium DS08064A, 600 MHz, 4 GSa/s). Calculations of electron-hole recombination rate constants and total diffusion length of charge carriers The THz dynamics presented in Figures 2b in the main text were fitted by wellknown equation. 5,6 Accordingly, the recombination dynamic of generated charge density (n) in semiconductor can be described as follows: = k n k n k n S1 The first coefficient (k T ) is related to the nonradiative deactivation by trap states and its values was taken from flash photolysis experiments (k T1 = 12 s -1 and k T2 = 143 s -1 ), k BM corresponds to the radiative and the nonradiative bimolecular deactivation of generated charges and k A describes the processes of Auger recombination. Furthermore, the equation S1 gives the total recombination rate (R): 5

6 R= = k +k k n k n S2 The differential equation (Equation S1) has been fitted to the experimental THz data by χ 2 minimization to obtain the values of the recombination rate constants. In the calculation, we assumed that the exciton yield generation (ϕ) is 1. 6 Otherwise, the real calculated rate constants are equal to: ϕk BM and ϕ 2 k A. The calculations take into account the charge density profile variation in space by dividing the perovskite layer (210 nm) into 50 equally thick sheets. 5 Assuming that the initial charge density (after pulse excitation) can be described by an exponential function (Equation S4) we compute the decay for each of the sheet individually. 6 = S3 δ is the charge density, δ 0 is the charge density at l = 0, α(λ) describes the absorption coefficient at the wavelength of excitation and l corresponds to the distance from the first sheet (taken at l = 0) of the FAPbI 3 film. The values of both the mobility and the total recombination rate constants enabled to calculate the charge carriers diffusion length L: = S4 Where D is the diffusion constant given by: = S5 In this equation, µ is the mobility, k B represents the Boltzmann constant, T is the temperature and e is the elementary charge. 6

7 Figure S1: Cross-section SEM (a) of the studied FAPbI 3 film (thickness, about 210 nm) and (b) top view image of the microstructure. (c) X-ray diffraction spectrum of the FAPbI 3 sample. 7

8 500 nm 600 nm Absorbance 760 nm nm Wavelength (nm) Figure S2: UV-visible absorption spectrum of the FAPbI 3 film measured by the spectrometer with the integration sphere. The arrows indicate the used excitation wavelengths in the THz experiments. σ/(n exc e 0 ) (cm 2 V -1 s -1 ) ps 2 ps Real Imaginary Frequency (THz) Figure S3: Real and imaginary parts of the THz spectra for FAPbI 3 film upon excitation at 760 nm at 2 (open squares) and 20 ps (solid circles) pump-probe delay times using a pump fluence of ph/cm 2. 8

9 E/E (Normalized to 1) FAPbI 3 GaAs Time (ps) 4 5 Figure S4: Time-dependent THz signals (in a 5-ps window) of FAPbI 3 film and GaAs crystal upon excitation at 800 nm and pump fluence of ph/cm OD nm 650 nm Time / µs Figure S5: Flash photolysis transient absorption signals from excited FAPbI 3 film. The signals were recorded upon excitation at 440 nm and observation at 580 nm (black line) and 650 nm (blue line). The red, dashed lines correspond to the best exponential fits of the experimental data. 9

10 Initial charge distribution (a.u.) λ ex = 500 nm λ ex = 600 nm λ ex = 760 nm λ ex = 800 nm Distance (nm) Figure S6: Variation of the initial charge distribution in FAPbI 3 film (~210 nm) upon excitation at 500, 600, 760 and 800 nm at the same pump fluence. The distributions were calculated using equation S3. 10

11 References (1). G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz and H. J. Snaith, Energy Environ. Sci. 2014, 7, (2). Y. Tian, I. G. Scheblykin, J. Phys. Chem. Lett. 2015, 6, (3). M. Ziółek, I. Tacchini, M. T. Martínez, X. Yang, L. Sun and A. Douhal, Phys. Chem. Chem. Phys. 2011, 13, (4). M. Ziółek, C. Martín, L. Sun and A. Douhal, J. Phys. Chem. C. 2012, 116, (5). P. Piatkowski, B. Cohen, F. J. Ramos, M. Di Nunzio, M. K. Nazeeruddin, M. Graetzel, S. Ahmad, A. Douhal, Phys. Chem. Chem. Phys. 2015, 17, (6). Ch. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. 2014, 26,