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1 Supporting Online Material for Lead-Free Solid State Organic-Inorganic Halide Perovskite Solar Cells Feng Hao, 1 Constantinos C. Stoumpos, 1 Hanh Cao, 1 Robert P. H. Chang, 2 Mercouri G. Kanatzidis 1* *Corresponding author. m-kanatzidis@northwestern.edu NATURE PHOTONICS 1

2 1. SnI 2 synthesis and purification. It is found the quality of SnI 2 material is critical to determine the final purity of as synthesized CH 3 NH 3 SnI 3. The synthesis and purification method can be found in our previous publication. 1 In brief, granulated Sn was first reacted with 2M aqueous HCl under N 2 flow for 1-2 min. Sufficient I 2 was added subsequently into the solution. As the reaction proceeds the color of the solutions changes progressively to red and finally to yellow. Excess Sn was added to remove any traces of I 2 or, less likely, O 2 present. Large red needles of SnI 2 crystals can be formed from cooling the solution and then isolated by filtration, followed by copious washing with 0.01M degassed HCl and drying in a vacuum oven at 50 o C. Note that combined action of oxygen and water on the material results in rapid oxidation to orange SnI 4, while distilled water causes hydrolysis to white Sn(OH) 2. As evidently indicated in the TG curves below, the SnI 2 easily gets oxidation to SnI 4 when exposed in air, corresponding to a sharp weight loss happened around 150 o C. The latter weight loss can be index to the melting of SnI 2 itself. After purification, the Sn 4+ impurity can be efficiently removed. Figure S1. Thermal gravimetric analysis (TGA) of commercial, as-synthesized and purified SnI 2. 2 NATURE PHOTONICS

3 SUPPLEMENTARY INFORMATION 2. Valence band energy (E VB ) of the CH 3 NH 3 SnI 3 perovskite. Figure S2. Ultraviolet photoelectron spectrum of the synthesized CH 3 NH 3 SnI 3. The binding energy is calibrated with respect to He I photon energy (21.21 ev). The valence band energy can be estimated to be ~ ev below vacuum level. NATURE PHOTONICS 3

4 3. Device photovoltaic performance with a thinner active layer (~ 150 nm) Figure S3. Photovoltaic characteristics for the devices with CH 3 NH 3 SnI 3 perovskite with an active layer thickness of 150 nm. 4 NATURE PHOTONICS

5 SUPPLEMENTARY INFORMATION 4. Surface morphology of the CH 3 NH 3 SnI 3 perovskite films Figure S4. Representative SEM images of CH 3 NH 3 SnI 3 film on top of the mesoporous TiO 2 layers with different magnifications. The CH 3 NH 3 SnI 3 perovskite can penetrate well into the TiO 2 films, thus forming an interconnected film. However, as evidently demonstrated in (b), there are still some pinhole on the perovskite film, which might cause short-circuiting, charge leaking, and large series resistance of the final device. NATURE PHOTONICS 5

6 5. Preliminary stability test of the CH 3 NH 3 SnI 3 perovskite solar cell It is known that this particular tin perovskite material has poor atmospheric stability compared to the Pb analog. 2, 3, 4 Therefore, a preliminary stability investigation of the CH 3 NH 3 SnI 3 perovskite solar cell was performed by storing the devices under nitrogen glove box after careful sealing. The I-V performance was monitored in a time range of 24 hour. The devices retained almost 80% of the initial performance in the first 12 hours. Performance loss was mainly from the photocurrent density and the fill factor. This happens primarily due to the inevitable oxidation induced during the fabrication process. Even better stability can be expected if more advanced sealing technique are adopted. Table S1. Preliminary stability test of the CH 3 NH 3 SnI 3 perovskite solar cell. Devices were carefully sealed using the commercial Surlyn film (DuPont 1702) under heat and storage under nitrogen atmosphere. Storage time (h) J sc (ma cm -2 ) V oc (V) FF PCE (%) Retained percents (%) References 1. Stoumpos CC, Malliakas CD, Kanatzidis MG. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near- Infrared Photoluminescent Properties. Inorg Chem 2013, 52(15): Mitzi DB. Synthesis, structure, and properties of organic- inorganic perovskites and related materials. Progress in Inorganic Chemistry, Vol , 48: Mitzi DB, Dimitrakopoulos CD, Kosbar LL. Structurally tailored organic- inorganic perovskites: Optical properties and solution- processed channel materials for thin- film transistors. Chem Mater 2001, 13(10): Hodes G. Perovskite- Based Solar Cells. Science 2013, 342(6156): NATURE PHOTONICS