Supporting Information. Compact Layer Free Mixed-Cation Lead Mixed-

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1 Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2018 Supporting Information Compact Layer Free Mixed-Cation Lead Mixed- Halide Perovskite Solar Cells Zhelu Hu, Hengyang Xiang, Mathilde Schoenauer Sebag, Laurent Billot, Lionel Aigouy, and Zhuoying Chen,* LPEM, ESPCI Paris, PSL Research University, Sorbonne Universités, UPMC, CNRS, 10 Rue Vauquelin, F Paris, France Corresponding Author * zhuoying.chen@espci.fr I. Experimental section All chemicals were purchased from Sigma-Aldrich and Alfa Aesar unless indicated otherwise and used as received. Synthesis of formamidinium iodide (FAI). FAI was prepared according to the method reported by Wozny et al. 1 with minor modifications. In a typical preparation, g of formamidinium acetate was added to 125 ml of methanol in a 250-mL-volumn Erlenmeyer flask that was immersed in an ice bath. 24 ml of hydriodic acid (57 wt %) was then added dropwise. The reaction was left stirring for approximately 2 hours at room temperature. The precipitate was collected by evaporating the entire reaction product at 55 C during 2 hours. The obtained 1

2 solids after evaporation was dissolved in isopropanol and precipitated by adding diethyl ether and subsequent centrifugation. This precipitation and centrifuge procedure was further repeated three times. The final white powders obtained was dried in a vacuum oven at 50 C overnight before being transferred to an Ar-filled glovebox for storage and further use. Perovskite thin film & device fabrication. The FTO-coated glass substrates were cleaned by four sonication baths in 2% hallmanex detergent solution, deionized water, acetone and isopropanol. Blow-dried substrates were then further cleaned by oxygen plasma for 10 minutes. To prepared the perovskite precursor solution, four chemicals including FAI, CsI, PbBr 2 and PbI 2 were dissolved in anhydrous N,N-dimethylformamide (DMF) to obtain a stoichiometric solution toward the composition of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 with five different molar concentrations of 0.95M, 1.10M, 1.25M, 1.40M, and 1.50M respectively. According to the method reported by McMeekin et al, 2 we added 54.7 µl of 57 wt% hydroiodic acid (HI) and 27.3 µl of 48 wt% hydrobromic acid (HBr) into 1 ml of 0.95M perovskite precursor solution. Based on these values the amounts of HI and HBr were modified linearly as the precursor concentration increases. The precursor solutions were then stirred at room temperature for at least 72 hours inside an Ar-filled glovebox. The perovskite precursor solution was spin-coated inside the glovebox on bare FTO or cp-tio 2 /FTO substrates at 2000 rpm for 45 s. After drying on a hot plate at 70 C for 1 minute, samples were annealed in an oven at 185 C for 90 minutes to induce the complete perovskite crystallization of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3. To complete the solar cell structure, on FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskites a layer of hole transport layer (HTL), 2-7,7 -tetrakis(n,n-di-p-methoxyphenylamine)-9,9 -spirobifluorene (Spiro- OMeTAD), was deposited by spin-coating (at 2000 rpm for 45 s) a chlorobenzene solution containing 96 mg/ml Spiro-OMeTAD. Additives of 32 μl of lithium bis(trifluoromethanesulfonyl)imide (170 mg/ml in 1-butanol) and 10 μl of 4-tertbutylpyridine were added into 1 ml of Spiro-OMeTAD chlorobenzene solution. The samples were then 2

3 placed into a desiccator for about 12 hours in order to oxidizer the hole-transport layer. Au electrodes were then thermally evaporated through a shadow mask on top of the Spiro- OMeTAD layer under vacuum of Torr. The shadow mask used in this work defined the area of the solar cells to be mm 2. For devices with an ETL of cp-tio 2, on cleaned FTO substrates cp-tio 2 was deposited by spin-coating (at 4000 rpm) a sol-gel precursor solution based on titanium(iv) isopropoxide according to the method reported by Edri et al. 3 After spin-coating substrates are annealed in an oven in air first at 120 C for 15 minutes and then at 450 C for 1 hour. Structural and optical characterizations. SEM characterizations were performed with a FEI Magellan 400 system with a standard field emission gun source. XRD spectrum were obtained by a PANalytical X Pert X-ray diffractometer using Cu-K radiation. UV-Visible absorption spectra were recorded in transmission mode by an Ocean Optics HL-2000 fiber-coupled tungsten halogen lamp and an Ocean Optics HR4000 spectrometer ( nm). Steadystate photoluminescence (PL) was recorded in a reflection mode by the same Ocean Optics spectrometer with an excitation source at 365 nm (Ocean Optics LLS-365). For PL decay measurements, samples are excited by a pulsed laser at = nm (pulsed period = 2 µs). Time-resolved PL signal centered at 735 nm were recorded by a time-correlated single photon counting (TCSPC) setup (SPC-130-EM, Becker & Hickl GmbH). All film thicknesses were measured using a profilometer (Veeco Dektak). Photovoltaic Characterization. Solar cell current-voltage characteristics were measured in an Ar-filled glovebox by a computer-controlled Keithley 2612B source measurement unit (SMU). Devices were illuminated through the transparent substrate (FTO/glass) side by a class ABB (ASTM E927-10) Newport LCS-100 solar simulator with an AM 1.5G filter operated at 1 SUN. The light intensity was first calibrated by a calibrated Si reference solar cell (ReRa Solutions B.V.). For the external quantum efficiency (EQE) measurements, a monochromatic light beam 3

4 was obtained from a white light source and an Oriel Cornerstone monochromator (and appropriate order sorting filters to eliminate higher order grating reflections). The monochromatic illumination was chopped at 37 Hz and calibrated by a NIST-calibrated Si photodiode. The solar cell short-circuit current I sc under each monochromatic wavelength was measured in air by a Stanford Research systems SR570 low-noise current preamplifier and a SR810 DSP lock-in amplifier. The EQE spectrum was determined by EQE(%) = I sc (A) P(W) 1240 λ(nm) 100 I sc (A) is the measured short-circuit current. where P(W) is the power of the monochromatic illumination and II. Supporting figures II.1 Supporting SEM characterizations 4

5 Figure S1. (a) Low-magnification (3 500x) scanning electron microscope (SEM) images of the FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskite thin films deposited from a precursor solution of 0.95M, 1.10M, 1.25M, 1.40M, and 1.55M concentration. (b) High magnification (15 000x) SEM image of a FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskite thin film deposited from a 1.25M precursor solution exhibiting large grains with a lateral dimension up to 12 microns. II.2 Supporting XRD spectra Figure S2. The XRD spectrum of a spin-coated sample by a 1.25M FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskite precursor solution before 185 C annealing (black curve), presented together with the XRD spectra of the same sample after 185 C annealing (green curve), FAI (red curve), and a bare FTO/glass substrate (grey curve). P indicates diffractions possibly from the early formation of perovskite phase. S indicates diffractions from the FTO substrate. * indicates diffractions from PbI 2 and/or PbBr 2. 5

6 II.3 Tauc plot Figure S3. Tauc plot of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 assuming direct band gap allowing the determination of its optical band gap from intercept to be 1.72 ev. II.4 SEM characterization of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 deposited on a cp-tio 2 /FTO substrate: In particular, by the same precursor concentration optimized on bare FTO substrates (1.25M), on cp-tio 2 /FTO substrates we obtained only perovskite films with poor substrate coverage together with a reduced film thickness ( 400 nm). By increasing the precursor concentration to 1.40M, on cp-tio 2 /FTO we improved significantly the perovskite surface coverage with an increased film thickness of 500 nm despite the fact that some pinholes are still visible from SEM characterizations. 6

7 Figure S4. SEM image of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 thin films deposited on a cp-tio 2 /FTO substrate from a precursor solution of 1.25M (left image) and 1.40M (right image) concentration. II.5 Experiments applying chlorobenzene as the antisolvent during the deposition of perovskite films: As shown in the device characteristics below, we did not find any obvious improvement on our devices with the use of antisolvent during the deposition of perovskite layer compared to the case without. Figure S5. J-V characteristics under AM1.5G illumination (100 mw cm-2) of an ETL-free FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 perovskite solar cells fabricated without (green dots) and with (blue triangles) the use of antisolvent (chlorobenzene) during the deposition of the perovskite layer. 7

8 II.6 Experiments formulating FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 precursor solution to compare the following four situations: (1) with both HI and HBr additives (this is the default configuration used in our manuscript, described in the experimental section of the supporting information); (2) with only HI additive; (3) with only HBr additive; (4) without HI nor HBr additive. As shown in the optical images in the figure below, we observed very different film color right after spin-coating (before annealing): While films with the two additives (the default configuration used in our manuscript) showed only yellowish color after spin-coating, films with only one additive or films without any additive showed already brownish dark color, likely a result of perovskite crystallization. As stated in the main text of our manuscript, the crystallization of our perovskite film (with both additives) mainly happened during the annealing stage. The combination of the two additives thus seems to slow down the crystallization process compared to the situation (2), (3), and (4). The fast-crystallization process happened during the spin-coating process is, however, not favorable for the formation of a smooth perovskite film with high surface coverage. This is confirmed by the SEM characterizations of annealed FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 films deposited in the (2), (3) and (4) situation. Compared to the default configuration (SEM images shown in Figure 1 of the main text), perovskite films deposited in the (2), (3) and (4) situation showed poor surface coverage without the formation of a continuous film. They are thus not suitable for the fabrication of planar perovskite solar cells. 8

9 Figure S6. (a) Optical images of the precursor solution, and the films before annealing (i.e. right after spin-coating of the respective solution) and the films after thermal annealing. Solution (1) was formulated with both HI and HBr additives (this is the default configuration used in our manuscript, described in the experimental section); solution (2) was formulated with only HI additive; solution (3) was formulated with only HBr additive; solution (4) was formed without any HI nor HBr additive. (b), (c), and (d) show the SEM image of the annealed perovskite film from the (2), (3), and (4) situation, respectively. REFERENCES (1) Wozny, S.; Yang, M.; Nardes, A. M.; Mercado, C. C.; Ferrere, S.; Reese, M. O.; Zhou, W.; Zhu, K. Controlled Humidity Study on the Formation of Higher Efficiency Formamidinium Lead Triiodide-Based Solar Cells. Chem. Mater. 2015, 27 (13), DOI: /acs.chemmater.5b (2) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Horantner, M. 9

10 T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B.; et al. A mixed-cation lead mixedhalide perovskite absorber for tandem solar cells. Science (80-. ). 2016, 351 (6269), DOI: /science.aad5845. (3) Edri, E.; Kirmayer, S.; Cahen, D.; Hodes, G. High open-circuit voltage solar cells based on organic-inorganic lead bromide perovskite. J. Phys. Chem. Lett. 2013, 4 (6), DOI: /jz400348q. 10