SUPPLEMENTARY INFORMATION. Direct Monitoring of Ultrafast Electron and Hole Dynamics in Perovskite. Solar Cells

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1 Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics. This journal is the Owner Societies SUPPLEMENTARY INFORMATION Direct Monitoring of Ultrafast Electron and Hole Dynamics in Perovskite Solar Cells Authors: Piotr Piatkowski 1, Boiko Cohen 1, Francisco Javier Ramos 2, Maria Di Nunzio 1, Mohammad Khaja Nazeeruddin 3, Michael Grätzel 3, Shahzada Ahmad 2 and Abderrazzak Douhal 1 * Affiliations: 1 Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S/N, Toledo, Spain. 2 Abengoa Research, C/ Energía Solar nº 1, Campus Palmas Altas-41014, Sevilla, Spain. 3 Laboratory of Photonics and Interfaces (LPI), Institute of Chemical Science and Engineering Faculty of Basic Science, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne Switzerland. * Correspondence to: abderrazzak.douhal@uclm.es (AD), Supplementary Materials: Materials and Methods Figures 1S to 7S

2 2 Methods Device Fabrication: FTO-coated glass, NSG-10 (Nippon Sheet Glass) 1mm thick was employed and was etched prior to use. The substrates were cleaned with Hellmanex and further rinsed with deionized water and ethanol; subsequently ultra-sonicated in 2-propanol for 15 minutes and dried with compressed air. Prior to compact layer deposition, the samples were cleaned with an ultraviolet/o 3 treatment for 30 minutes. A TiO 2 compact layer was deposited by spray pyrolysis at 450 ºC using 0.5 ml of titanium di-isopropoxidebis(acetyl acetonate) precursor solution (75% in 2-propanol, Sigma Aldrich) in 19.5 ml of pure ethanol using O 2 as carrier gas. After deposition, the samples were further heated for 30 minutes at 450 ºC to enhance the anatase formation. Once the samples are at room temperature, they were, immersed in a 0.02 M TiCl 4 solution in deionized water at 70 ºC for 30 minutes. Finally, the samples were washed with deionized water, heated at 500 ºC for 15 minutes and cool down slowly. Note that the TiCl 4 treatment is not on top of mesoporous TiO 2 film, rather used to prepare a thin compact layer of nm on FTO. The methodology we are adopting is aimed at making a compact layer to avoid any possibilities of direct contact of HTM with the FTO. Mesoporous layers were then deposited on top of it, made of titania (mp-tio 2 ). The paste was diluted in pure ethanol (1g of paste in 3.5 g of pure ethanol) and kept under stirring until used. 35 µl of diluted solution was spun-coated (at 5000 r.p.m, 30 s) over the compact titania layer. The samples were then heated in sequence at 125 ºC (5 min), 325 ºC (5 min), 375 ºC (5 min), 450 ºC (15 min) and finally 500 ºC (15 min). The perovskite layer, CH 3 NH 3 PbI 3 was deposited using the novel sequential deposition method. 1 Lead iodide (PbI 2 ) was first deposited by spin coating (6500 r.p.m, 30 s) using 50 µl per cell of 1M PbI 2 solution in N,N-dimethylformamide (DMF) stock solution

3 3 which was kept at 70ºC under vigorous stirring. After spin coating of PbI 2 the cells were placed onto a hot plate at 70 ºC and annealed for 30 minutes. After cooling down to room temperature, the cells were dipped in the methyl ammonium iodide solution in 2-propanol (8mg/mL) for 20 s (a color change from yellow to dark brown-black was observed), and were rinsed in pure 2-propanol and dried using the spin coater (4000 r.p.m., for 30 s). The samples were annealed at 70 ºC for 30 minutes. Spiro-OMeTAD (2,2,7,7 -tetrakis(n,n-di-p-methoxyphenyamine)-9,9- spirobifluorene) was selected as hole transporting material (HTM) for the cells. 35 µl of solution was deposited by spin coating (4000 r.p.m., 30s). The solution was prepared by dissolving 72.3 mg of Spiro-OMeTAD in 1mL of chlorobenzene µl of tris(2-(1hpyrazol-1-yl)-4-tert-butylpyrydine)cobalt(iii) bis(trifluoromethylsulphonyl)imide (FK209) stock solution (400mg of FK209 in 1 ml of acetonitrile), 17.5 µl of lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) stock solution (520 mg of LiTFSI in 1mL of acetonitrile) and 28.8 µl of 4-tert-butylpyridine were also added to the solution as dopants. For the thickness of the layers: TiO 2 layer: nm, mesoporous TiO 2 and perovskite layer: nm, the perovksite layer: ~ 200 nm and the Spiro-OMeTAD layer: nm. Femtosecond Transient Absorption Spectrophotometer. A multichannel visible detector head was used in the ExciPro pump-probe transient absorption spectrometer (CDP Systems). The multichannel detector head contains two new detectors for probe and reference beams, respectively, and allows for pump probe transient absorption measurements with weak (single filament) femtosecond white light continuum, as well as working with accumulation of one pulse of femtosecond continuum before readout and giving saturating signal through the visible region. The detectors are Si based and have detection in the spectral range of 200 nm 1100 nm. They are back thinned linear image sensors with an internal electronic shutter for spectrometers. These image sensors use a

4 4 resistive gate structure that allows high-speed transfer and readout at 1 khz. Each detector gives 1024 active pixels. Pixel size is 28 (horizontal, H) x1000 (vertical, V) μm and the sensitive area is (V) x (H) mm. Dynamic Burstein-Moss shift To give an explanation of the fluence-dependent blue shift and broadening of the 760 nm bleach, we applied a Burstein-Moss model. The density of the accumulated charge carrier n induced by the pulse of light changes upon increasing of the pump fluence and both the band shift and broadening indicate a charge carrier accumulation. The band gap shift is given by the equation 2-3 ΔE BM g = ħ2 2m eh (3π 2 n) 2/3 S1 ΔE BM g m where is the optical bandgap change, eh is the reduced effective mass and ħ is the reduced Planck constant. Furthermore, the total optical transition energy E g is defined as 3 E g = E 0 g + ΔEBM g S2 E 0 g = 1.55eV is a value of the perovskite band gap. In similarity with previous reports, we used a full-width at half-maximum as a measure of the Burstein-Moss shift ΔE BM g. 3-4 The effective mass of the exciton calculated form the value of the slope (4 ps delay) is 0.1m 0 and 0.13m 0 (m 0 is the free electron mass) for FTO/perovskite and FTO/TPS, respectively, which is in agreement with the value reported by Tanaka et al. (0.15 m 0 ). Calculation of electron-hole recombination rate constants

5 5 To fit the data shown in Figures 3 and 4 in the main text, we have used a well known equation. 6-8 Accordingly, the recombination dynamic of generated charge density (n) in semiconductor can be described as follows: dn dt = (k SHR + k Surf )n k BM n2 k A n 3 S3 The first coefficient (k SHR ) is the Shockley-Read-Hall (SHR) term, the second one (k Surf ) is the coefficient connected with the nonradiative deactivation by surface trap states, k BM corresponds to the radiative and the nonradiative bimolecular deactivation of generated charge and k A describes the processes of Auger recombination. The above equation serves to define the total recombination rate (R): R = dn ndt = (k SHR + k Surf ) k BM n k A n2 S4 In order to calculate coefficients k the differential equation (Equation 1S) has been fitted to the experimental data by minimization and the fitting function contained only the nonlinear terms. The resulted curves well reproduced the experimental data. The k SHR and k Surf coefficients were calculated from the results of the bi-exponential fit to the data obtained from flash photolysis measurements. Our calculations take into account the spatially varying charge density profile by dividing the perovskite layer into 50 equally thick sheets. The thickness of the perovskite layer for FTO/perovskite and FTO/TPS was 200 nm and 400 nm, respectively. Assuming that the charge density can be described by exponential function we compute the decay for each of the sheet individually. The initially generated charge density n 0 is a function of absorbed photon density and is given by equation:

6 6 n 0 = Eλα(λ,E) hca eff S5 where E is the pulse energy of wavelength is the energy and the wavelength dependent absorption coefficient, A eff is the effective area of the overlap between the pump and the probe pulse, h and c are Planck constant and speed of light respectively. The above equation was taken from. 8 However, it has been modified because of nonlinear dependence of the probe signal on the pump fluence (Figure 3c-d in the main text). To test the power dependent absorption, we have measured the absorbance of light at 580 nm at different pump fluences (Figure 4d in the main text). It turned out that the absorption of the perovskite decreases and finally saturates when the energy of the pulse increases. This phenomenon is called saturable absorption and most materials show this behaviour at high optical intensities. 9,10 The obtained results were fitted by simply two-level saturable absorber model with approximate form 10 α(e) = α S + α NS = α E E S + α NS S6 is the energy dependent absorption coefficient while S and NS are saturable and nonsaturable absorption components respectively. E is the energy of absorbed light, and E S the saturation energy. To fit the experimental data visible in Figure 3c-d in the main text, we have used Equation 5S, which is a modified version of Equation 4S: I = I 0 I E E s S7

7 7 where I 0 is a saturated optical density. Figure S1. Normalized (to the absorbance value at 750 nm) visible absorption spectrum of FTO/titania-perovskite-spiro-OMeTAD (TPS) film.

8 8 Figure S2. Transient absorption spectra. (a, b) Change of the optical density ( OD) of FTO/perovskite and FTO/titania-perovskite-spiro-OMeTAD (TPS) excited at 460 nm and gated at three representative pump-probe delay times. (c) Transient absorption spectra of FTO/perovskite upon excitation at 750 nm at 0.1 and 5 ps delay. Inset: Differential absorption spectra of the FTO/perovskite film in the 500 nm region, excited at 750 nm at selected pump probe delays.

9 9 Figure S3. Decay of the femtosecond transient signals (in terms of change in absorbance, OD) of FTO/TPS sample probed at 860 nm (dark solid circles), 900 nm (blue solid triangles) and 920 nm (red open circles). Solid lines are from the best multi-exponential fits of the experimental data. The excitation wavelength was 460 nm, the IRF was 100 fs and the pump fluence was 2 J/cm 2.

10 10 Figure S4. Decay of the femtosecond transient signal of the FTO/TPS film excited at 430 nm and probed at 460 nm ( ) and at 505 nm ( ). Inset: Differential absorption spectra of the FTO/TPS film excited at 430 nm at selected pump probe delays.

11 11 Figure S5. Effect of fs-pump fluence on the inverse optical density for FTO/TPS ( ex = 580 nm, obs = 760 nm).

12 12 Figure S6. Flash photolysis decays f FTO/perovskite (black squares) and FTO/TPS (red circles) along with the bi-exponential fits (solid lines). Excitation was at 460 nm and observation at 580 nm. The pump fluence was 80 J/cm 2.

13 13 Figure S7. J V curve for a typical cell measured at a simulated AM1.5G solar irradiation at 1 Sun.

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