Supporting Information

Transcription

1 Supporting Information Overcoming Short-circuit in Lead-free CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas-solid Reaction Film Fabrication Process Takamichi Yokoyama 1, 2, Duyen H. Cao 1, Constantinos C. Stoumpos 1, Tze-Bin Song 1, Yoshiharu Sato 2, Shinji Aramaki 2 and Mercouri G. Kanatzidis 1 * 1 Department of Chemistry, Northwestern University, 2145, Sheridan Road, Evanston, Illinois 60208, United States 2 Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshidacho, Aoba-ku, Yokohama , Japan Corresponding Author * m-kanatzidis@northwestern.edu S1

2 Experimental details Materials. CH3NH3I (MAI), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and 4- isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB) were purchased from Lumitec, Sigma Aldrich and TCI, respectively, and used without purification. SnI2 was prepared following a modified synthesis procedure. 1 Briefly, a 3-necked 500 ml bottom round flask was charged with 50g granulated Sn and 400 ml of 2M aqueous HCl. The mixture was then flushed with N2 for 1-2 min followed by addition of 70g of I2 in two portions. The mixture was refluxed until a clear yellow solution was obtained and unreacted, lustrous Sn chunks remain at the bottom of the flask (~2h). The yellow solution is transferred hot by decanting to another 500 ml 2-necked flask preheated in a water bath at 70 C under a flow of N2. The flask is capped under a N2 blanket and left to cool overnight. Shiny, needle-shaped, red crystals precipitate which are separated by suction filtration in vacuum under a N2 blanket. The yield is 80g of SnI2 (80%, based on total I2 content). The dry crystals were then separated in four batches transferred in fused silica tubes (500mm length, 13mm OD, 11mm ID) and flame sealed under a vacuum of 10-4 mbar. The fused silica tubes were placed in a horizontal tube furnace so that approximately 1/3 of the tubed remained outside of the furnace bore and heated to 200 o C overnight. This process effectively separates SnI4 which crystallizes exclusively on the cold end of the tube while high-purity SnI2 remains on the hot side. The tube is then broken to separate the two compounds which are ready to use for device fabrication. SnI2 used in this work was further purified by flame-sealing the remaining SnI2 under a 10-4 mbar vacuum, followed by zone-purification using the horizontal Travelling Heater Method (THM). The zone furnaces were set to 500 o C and the SnI2 tube was passed through the furnace at a rate of 5mm/h for a total of 10 passes. The tube was then transferred in a N2-filled glovebox and the deep red, almost black, ingot of SnI2 was obtained by breaking the tube. Perovskite film fabrication. FTO-coated glass (2 cm 2 cm, TEC7, 2.2 mm, Hartford Glass Co., Inc.) was patterned with diluted HCl and Zn powder. The substrates were then washed by ultrasonication with detergent-added deionized water, acetone, and isopropanol. A TiO2 compact layer was then deposited on the FTO substrate by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 2000 rpm for 30 sec, followed by annealing at 500 C for 20 minutes. Subsequently, a 350-nm-thick mesoporous TiO2 layer composed of 20 nm particles was deposited S2

3 on the compact layer by spin-coating a TiO2 paste (Dyesol DSL 18NR-T) diluted in anhydrous ethanol (1:3.5 weight ratio) at 4000 rpm for 30 sec. The mesoporous TiO2 film was then annealed at 500 C for 15 min. Finally, the substrate was treated with 0.02 M aqueous TiCl4 solution at 70 C for 30 min, followed by annealing at 500 C for 15 min. All substrates were heated at 200 C for 10 min in a N2-filled glovebox prior to device fabrication. VASP-film method. SnI2 solution was prepared from dissolving 400 mg SnI2 in 1 ml of N,Ndimethylformamide (DMF). The solution was stirred at room temperature for 1 h and filtered through a syringe filter with 0.45 m GHP membrane before use. 60 l of the solution was dropped on to the mesoporous TiO2 substrate and the substrate was spun at 2000 rpm for 30 sec to form the SnI2 film. The MAI chamber was set up by placing a petri-dish covered with a lid on a 150 C preheated hotplate, followed by placing MAI powder in the center of the dish. The MAI powder was kept at 150 C for 1 min. SnI2 film was then transferred to the petri dish to react with the MAI vapor for a set time. In the case of LT-VASP, the SnI2 film was maintained at lower temperature by having a glass spacer between the petri-dish and the device substrate. All these procedures were performed in a N2-filled glovebox. Prior to the film formation, the temperature of the glass/fto substrate on the spacer was measured using a thermocouple. For the LT-VASP, four glass substrates (length, width and thickness were about 3 cm, 2 cm and 2 mm, respectively) were stacked and used as the spacer to set the temperature of the device substrate around 60 C. One-step film method. MASnI3 solution was prepared from dissolving mg of SnI2 and mg of MAI in 1 ml of DMF. The solution was stirred at room temperature for 1 h and filtered before use. 60 l of MASnI3 solution was dropped on to the mesoporous-tio2 substrate and the substrate was spun at 2000 rpm for 60 sec to form the MASnI3 perovskite film. The film was then annealed at 70 C for 10 min. All these procedures were performed in the N2-flowed glovebox. Perovskite film characterization. Transmittance measurements were performed from 200 nm to 1200 nm at room temperature using a Shimadzu UV-3600 PC double-beam, doublemonochromator spectrophotometer. UV-VIS absorption spectra was calculated from transmittance by using an equation of = -log(t), where and T are absorbance and transmittance, respectively. XRD was collected using a CPS 120 INEL X-ray diffractometer (Cu Kα, Å) operating at S3

4 40 kv and 20 ma and equipped with a position-sensitive detector. Hitachi 4800 SEM was used for surface morphology study. Energy level study was performed with Kelvin prove (KP technology); the film was exposed to air for approximately 90 s before the measurement started. For the time evolution studies, the KP measurements were performed for a few min, and on two different spots on the film. Solar cell fabrication and characterization. A hole transporting material solution was prepared from dissolving 32 mg of PTAA and 3.6 mg of TPFB in 1.6 ml of chlorobenzene as HTM, and kept under stirring for 1 h at 70 C in the N2-filled glovebox. The HTM was then deposited on the MASnI3 perovskite film to form HTL by spin-coating at 1500rpm for 30 sec, followed by annealing at 70 C for 5 min. To complete the device fabrication, a 120 nm gold layer was thermally evaporated on top of the HTL in a vacuum chamber. During the process of transferring the devices to the thermal evaporator chamber they were exposed to air for about 5 min. After the evaporation, the devices were exposed to air for about 5 min again and then brought back into the glovebox for encapsulation using a 30- m-thick hot-melting polymer and a microscope coverslip. J-V measurements were carried out in air under 1-sun illumination (AM 1.5G, 100mW/cm 2 ) using a certified solar simulator (Abet Technologies) and Keithley 2400 source meter. The device active area was between 0.15 and 0.18 cm 2, defined by the overlapping area between the un-etched FTO glass and Au electrode. IPCE was characterized using an Oriel model QE-PV-SI instrument equipped with a NIST-certified Si diode. Monochromatic light was generated from an Oriel 300 W lamp. The IPCE measurements were performed without light bias. S4

5 Figure S1. Absorption spectra and photos of MASnI3 films on FTO/TiO2 substrates. VASP were performed for 0 min (neat SnI2 film), 1 min and 10 min. Figure S2. Cross-sectional SEM image of LT-VASP film on FTO/TiO2 substrate, indicating the dimension of each layer. S5

6 Figure S3. ( h ) 2 of one-step, VASP, and LT-VASP MASnI3 films as a function of Energy (i.e. Tauc plot for direct bandgap., h and indicate absorbance, the plank constant and wavenumber, respectively). The bandgaps of one-step, VASP and LT-VASP films are estimated to be 1.35eV, 1.27eV and 1.26V, respectively. Figure S4. Hysteresis behavior in J-V characteristics of a LT-VASP MASnI 3 device. Reference (1) Moser, W.; Trevena, I. C. Crystal structure of Tin(II) Iodide. J. Chem. Soc. D: Chem. Comm., 1969, 1, S6