Thin Films and Solar Cells Based On. Semiconducting 2D Ruddlesden-Popper

Transcription

1 Supporting Information for Thin Films and Solar Cells Based On Semiconducting 2D Ruddlesden-Popper (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n 1 Sn n I 3n+1 Perovskites Duyen H. Cao, 1 Constantinos C. Stoumpos, 1 Takamichi Yokoyama, 1, 2 Jenna L. Logsdon, 1 Tze- Bin Song, 1 Omar K. Farha, 1, 3 Michael R. Wasielewski, 1 Joseph T. Hupp, 1 Mercouri G. Kanatzidis 1* 1 Department of Chemistry, and Argonne-Northwestern Solar Energy Research (ANSER) Center Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA 2 Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshidacho, Aoba-ku, Yokohama , Japan 3 Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Corresponding Author * m-kanatzidis@northwestern.edu 1

2 Experimental details Materials. FTO-coated glass (TEC7, 2.2 mm) was purchased from Hartford Glass Co. Inc. Transparent titania (TiO 2 ) paste (Dyesol 18NR-T) was purchased from DyeSol. Methylammonium iodide (MAI) and butylammonium iodide (BAI) were synthesized by neutralizing equimolar amounts of aqueous hydroiodic acid (HI) and the corresponding aqueous alkylamine (ph ~7). The white alkylammonium iodide precipitates were collected by evaporation of the solvent using rotary evaporation at 60 C. Tetrakis(pentafluorophenyl)borate (TPFB) was purchased from Tokyo Chemical Industry Co, Ltd. Low temperature thermoplastic sealant (Dyesol LTD.) was used for device encapsulation. All other chemicals were purchased from Sigma-Aldrich and used as received unless otherwise noted. Synthesis (n-ch 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Sn 4 I 13. SnCl 2.2H 2 O powder (2.257 g, 10 mmol) was dissolved in a mixture of 57% w/w aqueous HI solution (20.0 ml, 152 mmol) and 50% aqueous H 3 PO 2 (6.8 ml, 62.0 mmol) by heating on a hot plate at 200 C under constant magnetic stirring for about 5 min until a bright yellow solution was obtained. Subsequent addition of solid CH 3 NH 3 I (1.192 g, 7.5 mmol) to the hot yellow solution initially caused black powder precipitation. The stirring was kept going until a clear bright yellow solution was achieved. n- CH 3 (CH 2 ) 3 NH 3 I (1.005 g, 5 mmol), was added to the hot yellow solution. The stirring was then discontinued, and the solution was left to cool to room temperature during which time black rectangular-shaped plates gradually crystallized out. The precipitation was completed after ~ 2 hours. The crystals were then isolated by suction filtration and dried in vacuum. The reaction yield was g (68.8 %). The powder was further purified and homogenized by annealing in 2

3 vacuum. Specifically, the powder was loaded in a 9 mm pyrex tube in a N 2 glove box, evacuated at 10-4 bar, and flame sealed. The sealed tube was then placed in a sand bath at 200 C vertically in such a way that the part of the tube that has no perovskite material stays out of the bath. (n-ch 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 2 Sn 3 I 10. The synthesis procedure of Sn 3 I 10 is identical to that of Sn 4 I 13 except for the amount of starting materials and the reaction yield. In particular, SnCl 2.2H 2 O (2.257 g, 10 mmol), CH 3 NH 3 I (1.060 g, 6.67 mmol), and n-ch 3 (CH 2 ) 3 NH 3 I (1.340 g, 6.67 mmol) were used. The reaction yield was g (82.7 %). Material characterization. XRD patterns of bulk materials and thin films were collected in air using a Rigaku MiniFlex600 X-ray diffractometer (Cu Kα, Å). Optical diffusereflectance spectra were collected in air using a Shimadzu UV-3600 PC double-beam, doublemonochromator spectrophotometer operating from 300 to 1240 nm. BaSO 4 was used as a nonabsorbing reflectance reference. Diffuse reflectance spectra were converted to absorption spectra using the Kubelka-Munk equation, α/s = (1-R) 2 /2R, where R is the percentage of reflected light, and α and S are the absorption and scattering coefficients, respectively. 1 Absorption coefficient (α) is estimated according to the following equation, = ln( + ) where T is the transmission, R is the reflection of the thin film, and L is the film thickness value obtained from cross-sectional scanning electron microscope (SEM) images. SEM was acquired at an accelerating voltage of 10 to 15 kv using a Hitachi SU8030 instrument equipped with an Oxford X-max 80 SDD EDS detector. Steady-state and time-resolved fluorescence data were collected at room temperature using a streak camera system (Hamamatsu C4334 Streakscope). 3

4 515 nm laser pulses were utilized as the excitation source, which were generated by the same high repetition rate (100 khz) ultrafast laser system. Work function was probed using ultraviolet photoelectron spectroscopy (UPS) under high vacuum with He I gas as an ultraviolet light source (21.20 ev) on thin film materials. Fermi level was probed using Kelvin Probe in which the perovskite films were exposed to air for approximately 90 seconds before the measurement began. Device fabrication Mesoporous TiO 2 layer. Conductive FTO-coated glass (2 x 2 cm) was patterned by etching a 0.5 cm strip away with zinc powder and 4M HCl acid, followed by ultrasonicating in a detergent solution (Alcanox water mixture), acetone, and isopropanol. The films were then dried by an air flow followed by oven drying at 85 C for 30 min. An ~ 40-nm-thick TiO 2 compact layer was deposited onto the FTO substrates by spin coating a mildly acidic solution of 0.3 ml of titanium isopropoxide and 0.1 ml of 37% HCl in 5 ml anhydrous ethanol, at 2000 rpm for 30 sec. The compact layer was annealed in air at 500 C for 15 min. Subsequently, a mesoporous TiO 2 layer composed of 20-nm-sized particles was deposited on the compact TiO 2 layer substrates by spincoating at 1000 rpm for 30 sec using a commercial TiO 2 paste (Dyesol 18NR-T) diluted in anhydrous ethanol (2:7, weight ratio). The mesoporous TiO 2 films were gradually heated (8 o C/min) to 500 C for 15 min and slowly cooled down to room temperature (8 o C/min). The room temperature TiO 2 films were then soaked in a 0.05 M aqueous solution of TiCl 4 for 30 min at 70 C, rinsed with deionized water, and annealed at 500 o C for 15 min. Note that all processes after this point were carried out in a N 2 glove box. 4

5 2D Perovskite layer. The perovskite light absorbing material precursor solution was prepared by dissolving 0.2 M (BA) 2 (MA) 2 Sn 3 I 10 (0.367 g, 0.2 mmol) or 0.2 M (BA) 2 (MA) 2 Sn 4 I 13 (0.474 g, 0.2 mmol), and 0.2 M SnF 2 (0.031 g, 0.2 mmol) in 1 ml DMF under constant magnetic stirring at 70 C for 1 hour. For triethylphosphine (TEP), M concentration was employed by adding it directly into the perovskite solution (5 ul in 1 ml DMF). The 2D films were then fabricated by spin coating the perovskite solution on 120 C preheated mesoporous TiO 2 substrates at 3000 rpm for 30 seconds. The films were subsequently annealed at 75 C for 5 min and cooled down to room temperature. Remaining layers. The hole transporting material (HTM) solution was prepared by dissolving 20 mg of PTAA and 2.3 mg of TPFB in 1.0 ml of chlorobenzene solvent under stirring at 70 C for 1 hour. The HTM solution was then deposited on the perovskite layer by spin-coating at 1500 rpm for 30 sec, followed by annealing at 75 C for 5 min. To complete the device fabrication process, 100 nm of gold was thermal evaporated on top of the HTM layer. Encapsulation was done by using a 30-µm thick hot melting polymer and a microscope glass. The device active area was in the range of 0.14 to 0.18 cm 2. Solar cell device characterization. J-V measurements were carried out in air under 1-sun illumination (AM 1.5G, 100mW/cm2) using a certified solar simulator (Abet Technologies) and Keithley 2400 source meter. External quantum efficiency (EQE) 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 EQE measurements were performed without light bias. 5

6 Figure S1. Left: photograph of 2D tin iodide perovskite polycrystalline bulk materials in HI/H 3 PO 2 solution, Right: SEM image of layered Sn 3 I 10 crystals. Figure S2. Powder XRD patterns of (BA) 2 (MA) 2 Sn 3 I 10 and (BA) 2 (MA) 2 Sn 4 I 13 compounds. The calculated XRD pattern is derived from single crystal structure data. 6

7 Figure S3. Absorption coefficient (α) of (BA) 2 (MA) 2 Sn 3 I 10 compound vs. energy (T is transmittance, R is reflectance, and L is thickness of Sn 3 I 10 thin film). Figure S4. XRD patterns of mesoporous TiO 2 and mesoporous TiO 2 -Sn 3 I 10 thin films. 7

8 Figure S5. XRD patterns of planar Sn 3 I 10 and Sn 4 I 13 thin films fabricated on different temperature (25 C and 120 C) substrates. The (202) peak region of both films is zoomed in and highlighted by the green rectangles. 8

9 Figure S6. Photovoltaic performance of n = 3 Sn 3 I 10 device without using SnF 2 additive. Figure S7. Electrical resistance of a pristine Sn 4 I 13 film. 9

10 Figure S8. XRD patterns of planar Sn 3 I 10 and Sn 4 I 13 thin films before and after introducing SnF 2 and triethylphosphine (TEP) additives; the thin film orientation is not affected by the additives. Figure S9. Cross-sectional SEM image of a representative Sn 3 I 10 device. 10

11 Figure S10. Optical band gap spectra of planar thin films of a) MASnI 3, b) (BA) 2 (MA) 2 Sn 4 I 13, and c) (BA) 2 (MA) 2 Sn 3 I 10 before and after adding SnF 2 and TEP additives; d) Comparison of the three perovskite films. 11

12 Figure S11. Ultraviolet photoemission spectroscopy (UPS) data of an FTO-Sn 3 I 10 film (left: full spectra, top right: magnification of second cut-off, and bottom right: magnification of Fermi level). Figure S12. Kelvin Probe spectra of MASnI 3, (BA) 2 (MA) 2 Sn 4 I 13, and (BA) 2 (MA) 2 Sn 3 I 10 films vs. air-exposure time. 12

13 Figure S13. Photograph of an encapsulated (BA) 2 (MA) 2 Sn 3 I 10 solar cell device after intentionally exposing to air for several days. The dark brown color of the perovskite in the region protected by the 1 cm x 2 cm glass slide remained but faded in the unprotected region. It is anticipated that better device encapsulation engineering will be beneficial for the long-term stability of 2D perovskite solar cells. Figure S14. Photovoltaic performance of a MASnI 3 -SnF 2 device without TEP showing an almost short-circuited output. References (1) Kubelka, P.; Munk, F. Ein Beitrag zur Optik der Farbanstriche. Z. Tech. Phys. (Leipzig). 1931, 12,