Supporting Information

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1 Supporting Information Amorphous Metal Oxide Blocking Layers for Highly Efficient Low Temperature Brookite TiO 2 -based Perovskite Solar Cells Atsushi Kogo,*, Yoshitaka Sanehira, Youhei Numata, Masashi Ikegami, and Tsutomu Miyasaka*, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba, Ibaraki , Japan. Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa, , Japan. *Corresponding authors. kogo.atsushi@aist.go.jp (A. K.), miyasaka@toin.ac.jp (T. M.) S-1

2 Experimental details Indium tin oxide (ITO) glass substrates (12 /sq, GEOMATEC) were cleaned by sonication in 2% Hellmanex aqueous solution for 30 min. After rinsed with deionised water, the substrates were cleaned with sonication in acetone, followed by rinsing with 2-propanol. Then the substrates were treated with UV ozone for 10 min. TiCl 4 solution was prepared by diluting an aqueous solution of TiCl 4 ( wt%, Wako Chemicals) with 1-butanol. SnCl 2 solution were prepared by dissolving SnCl 2 2H 2 O in 1-butanol. Blocking layers (BLs) of amorphous TiO x and SnO x were formed by spin-coating the TiCl 4 and the SnCl 2 solutions on ITO-coated glasses at 3000 rpm for 45 s followed by drying at 150 o C for 1 h. The thickness of BLs was controlled by the concentration of TiCl 4 and SnCl 2 (Fig. S1). A brookite TiO 2 slurry (PECC-B01, Peccell Technologies, Inc., particle size nm) was diluted to 5 vol% with ethanol and was then spin-coated in a two steps program at 500 and 3000 rpm for 5 and 30 s, respectively and dried at 150 o C for 1 h. A precursor solution of CH 3 NH 3 PbI 3 was prepared by dissolving CH 3 NH 3 I (1.3 M) and PbI 2 (1.3 M) in a mixed solvent of DMF and DMSO (4:1 (v:v)). After UV ozone treatment of the brookite TiO 2 -coated substrates, the precursor solution was spin-coated at 5000 rpm for 30 s in dry atmosphere. 300 L of chlorobenzene was dropped 5 s after the beginning of spin-coating. The substrates were annealed at 105 o C for 10 min. 41 mg of 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-ometad) was dissolved in 480 L of chlorobenzene with additive of 4-tert-butylpyridine (16.27 L), 0.2 M solution of lithium bis(trifluoro methanesulfonyl)imide in acetonitrile (28.37 L) and 0.2 M solution of tris(2-(1h-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(iii) tris(bis(trifluoromethylsulfonyl)imide in acetonitrile (5.06 L). The spiro-ometad solution was spin-coated on CH 3 NH 3 PbI 3 layer at 4000 rpm for 20 s and aged overnight in dry air. Finally, Au contacts were formed by thermal vapour deposition. Photovoltaic characteristics of all the devices were measured with Keithley 2400 source meter under 1 sun illumination through a black mask (aperture area of 0.09 cm 2, thickness of 400 m). Peccell Technologies PEC-L01 solar simulator (AM 1.5 G, 100 mw cm -2 ) was employed as a light source with a reference crystalline Si cell (BS-520, calibrated and certified by Bunkou Keiki) for correction of intensity. Photocurrent density-voltage (J-V) curves were measured without any preconditioning of the solar cells with scan speed and dwell time of V s -1 and 0.05 s, respectively. External quantum efficiency (EQE) action spectra of the devices were measured with Peccell Technologies, PEC-S20 action spectrum measurement setup. For characterization, an S-2

3 UV spectrophotometer (UV-1800, SHIMADZU), an X-ray diffractometer (D8 Discover, Bruker) with CuK radiation source and a scanning electron microscope (SU8000, HITACHI) were employed. Photoelectron yield spectroscopy system (BIP-KV201, Bunkou Keiki) was employed to measure work function of materials in vacuum condition. We employed a quartz substrate for measuring SnO x absorption spectrum, since ITO substrate has narrower bandgap (~3.7 ev) than SnO x (4.25 ev). Figure S1. Dependence of (a) TiO x and (b) SnO x thickness on concentration of precursor solution. S-3

4 Figure S2. Dependence of (a) short-circuit photocurrent density, (b) open-circuit voltage, (c) fill factor, and (d) series resistance on TiO x thickness in CH 3 NH 3 PbI 3 solar cells with TiO x /brookite TiO 2 bilayer electron collector measured under 1 sun illumination in backward voltage scan (1.2 V 0.1 V). Figure S3. A histogram of PCE showing performance distributions of CH 3 NH 3 PbI 3 perovskite solar cells with TiO x (thickness ~8 nm)/brookite TiO 2 electron collector. S-4

5 Figure S4. Dependence of (a) power conversion efficiency, (b) short-circuit photocurrent density, (c) open-circuit voltage, (d) fill factor, and (e) series resistance on TiO x thickness in CH 3 NH 3 PbI 3 solar cells with TiO x /brookite TiO 2 bilayer electron collector measured under 1 sun illumination in forward voltage scan ( 0.1 V 1.2 V). S-5

6 Figure S5. J-V curves of CH 3 NH 3 PbI 3 solar cells with TiO x (thickness = 8 nm)/brookite TiO 2 electron collector stored in dry air for 0 days (red), 2 days (green), 9 days (blue), and 16 days (black). Forward ( 0.1 V 1.2 V) and backward (1.2 V 0.1 V) scans are indicated as solid and dashed lines, respectively. Figure S6. (a) A SEM image of ITO glass coated with 8-nm thick SnO x. (b) An XRD profile of the SnO x layer (thickness ~120 nm). Figure S7. (a) A schematic illustration of resistance measurement of BLs. (b) I-V slope data of TiO x /ITO (red), SnO x /ITO (blue), and ITO (black) substrates. S-6

7 Figure S8. (a) Photoelectron yield spectrum of brookite TiO 2 (black), TiO x (red), and SnO x (blue). (b) Absorption spectra of ITO substrates before and after coating of TiO x. (c) Tauc plot of TiO x layer. (d) Absorption spectra of quartz substrates before and after coating of SnO x. (e) Tauc plot of SnO x layer. (f) Absorption spectra of glass substrates before and after coating of brookite TiO 2. (g) Tauc plot of brookite TiO 2 layer. S-7

8 Table S1. Device performance of CH 3 NH 3 PbI 3 solar cells with TiO x /brookite TiO 2 bilayer electron collector stored in dry air for 2 days. Scan direction: 1.2 V 0.1 V. J SC V OC / ma cm 2 / V FF PCE / % Cell Cell Cell Cell Cell Cell Cell Cell S-8