Bifunctional Surface Engineering on SnO2 Reduces Energy Loss in Perovskite Solar Cells

　　Eui Hyuk Jung,† Bin Chen,† Koen Bertens, Maral Vafaie, Sam Teale, Andrew Proppe, Yi Hou, Tong Zhu, Chao Zheng, Edward H. Sargent*

 

　　The Edward S. Rogers Department of Electrical and Computer Engineering, University of

 

　　Toronto, Toronto, Ontario, Canada, M5S 3G4

 

　　*Correspondence to: ted.sargent@utoronto.ca

 

　　† These authors equally contributed to this work.

 

　　Materials

 

　　All chemicals used in this work are commercially available from the specified suppliers and were used without additional purification steps. Chemical list purchased from Sigma Aldrich: ammonium fluoride (NH4F), methylammonium chloride (MACl), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), diethyl ether, tin(Ⅱ) chloride dihydrate, urea, thioglycolic acid, bis(trifluoromethane)sulfoninide lithium salt (Li-TFSI), 4-tert-butylpyridine (tBP), acetonitrile, chlorobenzene, isopropanol (IPA). Chemical list purchased from Tokyo Chemical Industry Co.: formamidinium iodide (FAI), lead iodide (PbI2), methylammonium bromide (MABr), lead bromide (PbBr2). FK209 Co(Ⅲ) TFSI salt was purchased from Lumtec. Tin(Ⅳ) oxide colloidal dispersion was purchased from Alfa Aesar. 2,2’,7,7’-Tetrakis(N,N-di- p-methoxylphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) was purchased from Xi’an Polymer Light Technology Co. ITO and FTO (TEC 10) substrates were purchased from Thin Film Devices Inc. and Ossila, respectively.

 

　　Device fabrication

 

　　ITO substrates were cleaned by sequentially washing with acetone and IPA. FTO substrates were chemically etched with zinc power and HCl solution (concentrated hydrochloric acid:water= 1:3.5 v/v). The etched FTO substrates were sequentially washed with detergent, water, acetone and IPA. Before using the ITO and FTO substrates, all substrates were treated with ultraviolet-ozone for 15 min. In the case of the SnO2 layer based on nanoparticles, a diluted SnO2 nanoparticle dispersion (SnO2 (15% in water) : IPA : deionized water= 1:3:3 volume ratio) was deposited on the cleaned ITO substrates by spin-coating at 3,000 rpm for 20 s. Then, the substrates were annealed in ambient air at 150⁰C for 1 hour. In the case of the SnO2 layer

 

　　based on chemical bath deposition (CBD), the SnO2 layer was prepared as in (Elham et al. Energy Environ. Sci. 2016, 9, 3128). We used a SnCl2•2H2O solution with a concentration of

 

　　0.012 M and carried the reaction out for 4 hours in a 70⁰C water bath. For the NH4F treatment,

 

　　a NH4F solution (0.08 M in deionized water) was spin-coated on a SnO2 substrate at 5,000 rpm for 20 s after kept for 2 s on the substrate. Then, the substrate was annealed at 150⁰C for 30 min. Before making perovskite film, the substrates were UV-ozone treated for 15 min just prior to each process. To fabricate perovskite films with (FAPbI3)0.95(MAPbBr3)0.05 composition, a perovskite precursor solution was prepared by dissolving 889 mg/ml of FAPbI3, 33 mg/ml of MAPbBr3 and 33 mg/ml MACl in DMF/DMSO (8:1 v/v) mixed solvent. The solution was coated onto the SnO2 substrates by two consecutive spin-coating steps, at 1,000 and 2,500 rpm for 5 s and 20 s, respectively. During the second spin-coating (2,500 rpm), 1 ml of diethyl ether was poured onto the substrate after 15 s. Then, the substrate with intermediate phase perovskite film was put on a hot plate at 150⁰C for 10 min. For deposition of the spiro-OMeTAD layer,

 

　　1.1 ml of spiro-OMeTAD solution in chlorobenzene (90.9 mg/ml) was prepared with an addition of 23 ml of Li-TFSI solution in acetonitrile (540 mg/ml), 39 ml of tBP and 10 ml of FK209 Co Co( Ⅲ ) TFSI salt solution in acetonitrile (376 mg/ml) as dopants. The spiro- OMeTAD solution was deposited onto the perovskite surface at 1,750 rpm by using dynamic deposition method. All fabrication was performed in ambient condition. Laboratory environment are controlled by isothermal-isohumidity system at 22⁰C and 30% relative

 

　　humidity. Finally, a gold electrode was deposited by thermal evaporation.

 

　　Characterization

 

　　The samples were mounted on a stainless-steel mounting plate. XPS measurements were

 

　　performed with the Thermo Scientific K-Alpha system. An Al Kα source was used, and the takeoff angle was 90°. To account for sample charging, the XPS spectra were calibrated to the C 1s peak at 284.8 eV. DFT calculations in this work were performed using Vienna Ab initio Simulation Package (VASP) with projector augmented wave (PAW) method.1-2 The cut-off energy was set to 400 eV. Generalized gradient approximation (GGA) with the Perdew, Burke and Ernzerhof (PBE) functional was used for exchange-correlation.3 Bulk SnO2 supercell with

 

　　(110) plane normal as z-axis was first relaxed until all forces on atoms are below 0.02 eV/Å. Then a symmetric five-layer (110) SnO2 slab was constructed from the relaxed 2x2x3 SnO2 bulk supercell with 18 Å vacuum space between periodic slab. The middle SnO2 layer was fixed to simulate the bulk region.4 Monkhorst−Pack k-point mesh for the Brillouin zone sampling of was used:5 (8x4x4) for bulk, (8x4x1) for slab and (15x7x1) for slab DOS calculation. Steady-state PL and time-resolved PL were measured using a Horiba Fluorolog time correlated single-photon-counting system with photomultiplier tube detectors. The light was illuminated from the top surface of perovskite film. For steady-state PL measurements, the excitation source is a monochromated Xe lamp (peak wavelength at 540 nm with a line width of 2 nm). For time-resolved PL, we used a green laser diode (λ = 504 nm) as the excitation source with an excitation power density of 5 mW/cm2. The PL decay curves were fitted with biexponential components to obtain a fast and a slow decay lifetime. UPS measurements were carried out on the samples. A He-Ⅰa lamp (hn= 21.22 eV) was used as the radiation source and the samples were kept under -5 V bias during the measurement. The current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under AM 1.5G illumination (100 mW/cm2, calibrated with reference solar cell from Newport) from a solar simulator (Newport, Class A). During the measurement, an aperture shade mask with 0.049 cm2 area was used for all devices.

 

　　1. Kresse, G.; Furthmüller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54 (16), 11169-11186.

 

　　2. Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B 1994, 50 (24), 17953-17979.

 

　　3. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple.

 

　　Phys. Rev. Lett. 1996, 77 (18), 3865-3868.

 

　　4. Duan, Y., Electronic properties and stabilities of bulk and low-index surfaces of SnO in comparison with SnO2: A first-principles density functional approach with an empirical correction of van der Waals interactions. Phys. Rev. B 2008, 77 (4), 045332.

 

　　5. Monkhorst, H. J.; Pack, J. D., Special points for Brillouin-zone integrations. Phys. Rev. B

 

　　1976, 13 (12), 5188-5192.

 

 

　　Figure S1. (a) Schematic of water dissociation into two hydroxyl groups at (110) SnO2 surface. Bandstructure and partial density of states (PDOS) of (b) terminal fluorine surface (FT) and (c) substitutional fluorine surface (FB).

 

 

　　Figure S2. Current-voltage (I-V) linear plots of (a) control and (b) NH4F-treated device (ITO/SnO2/perovskite/Au configuration). Average resistances were calculated from 8 devices.

 

 

　　Figure S3. Statistical PCE and VOC values for 12 perovskite solar cells based on pristine (red) and NH4F-treated (blue) SnO2 electron transport layer. Unfilled and filled circles indicate the values of perovskite solar cells based on SnO2 electron transport layer from nanoparticle and chemical bath deposition, respectively.

 

 

　　Figure S4. Current density-voltage (J-V) curves of (a) control and (c) NH4F-treated device based on SnO2 nanoparticle (NP) measured along reverse (blue) and forward (red) bias scanning direction. Stabilized PCE of the corresponding devices (b,d) measured at each maximum power point.

 

 

　　Figure S5. Current density-voltage (J-V) curves of (a) control and (c) NH4F-treated device based on chemical bath deposition (CBD) substrate measured along reverse (blue) and forward (red) bias scanning direction. Stabilized PCE of the corresponding devices (b,d) measured at each maximum power point.