Tailoring the interfacial chemical interaction for. high efficiency perovskite solar cells

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1 Supporting Information Tailoring the interfacial chemical interaction for high efficiency perovskite solar cells Lijian Zuo, Qi Chen, Nicholas De Marco, Yao-Tsung Hsieh, Huajun Chen, Pengyu Sun, Sheng-Yung Chang, Hongxiang Zhao, Shiqi Dong, Yang Yang * Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USA. California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA. KEYWORDS: Perovskite solar cells, Interfacial chemical interactions, Trap states passivation, interfacial charge transfer, self-assembly monolayer. Methods Materials. The methylammonium iodide was home-synthesized, and the lead iodide was purchased from Alfa Aesar and used as received. The Spiro-OMeTAD was purchased from the Lumitech Corp. and used as received. Other materials not specified were from the Sigma Aldrich. 1

2 Device fabrication. The patterned ITO glass substrates were cleaned by an ultra-sonicator in an acetone, water, and iso-propanol bath. The deposition of followed previous work. The aged SnCl 2 was dissolved in ethanol with a concentration of 0.1 M. The films were obtained by heating the spin-coated SnCl 2 films on a 180 o C hot plate for 1 h. After cooling to room temperature, the glass/ito/ substrates were transferred into a UV-Ozone cleaner for 15 mins. The deposition of SAMs follow previous procedure. Most he SAMs can be dissolved in methanol at a concentration of ~ 0.5 mg/ml, except 4-amino-benzoic acid. The 4-amino-benzoic acid was dissolved in a methanol:water=3:1 mixture (0.5 mg/ml). All these SAMs were spun onto the at a speed of 3000 rpm, and heated on a 120 o C hot plate for 15 min. Afterward, excessive SAMs on the were washed away by methanol or methanol:water mixture. The perovskite films were deposited by a sequential two steps solution process. Hot PbI 2 solution (600 mg/ml, in dimethylformamide (DMF)) was spun onto the glass/ito/ substrates at 3,000 rpm for 40 s. After annealing on a 75 o C hot plate for 15 mins, the MAI:MACl solution (70:7 mg/ml, in isopropanol (IPA)) was dripped onto the PbI 2 films to form the PbI 2 -MAI complex films. For the traditional two-steps solution process, the complex films were put on a 135 o C hot plate for 15 min. The 2,2,7,7 -Tetrakis(N,N-di-p-methoxyphenylamine)-9,9 - spirobifluorene (Spiro-OMeTAD) solution (80 mg/ml in chlorobezene with 3% 4-tertbutylpyridine and 9 mg lithium bis(trifluoromethanesulfonyl)imide (TFSI)) was spun onto the perovskite film as hole conductor. The devices were completed by evaporating 80 nm gold in a vacuum chamber (base pressure 5*10-4 pa). Device Characterization. The device efficiencies were measured by a keithley source meter under AM 1.5G 1 sun irradiance as generated by a thermo oriel solar simulator. The light 2

3 intensity were adjusted by a KG-5 Si diode, that can be traced back to the national renewable energy lab (NREL). The devices were measured in ambient air at temperature of 22 degree centigrade. Different scan directions (reverse scan: from 1.2 V to 0 V, and forward scan: from 0 V to 1.2 V) were recorded at a scan rate of 0.5 V/s for the hysteresis study. The device pixel has the dimension of 0.2 cm by 0.7 cm, and accurate device area is defined by a aperture of cm 2, and the device photocurrent show little variation with the aperture area. And for the steady state output measurement, the solar cells were put under the simulated AM 1.5 G, 1 sun illumination to record the photocurrent under a bias voltage. The EQE is measured on a Enli tech measurement system. Transient Photovoltage Decay Measurements. A white light bias was generated from an array of diodes (Molex ) to simulate a 1 sun working condition. A pulsed red dye laser (Rhodamine 6G, 590nm) pumped by a nitrogen laser (LSI VSL-337ND-S) was used as the perturbation source, with a pulse width of 4 ns and a repetition frequency of 10 Hz. The perturbation light intensity was attenuated to keep the amplitude of transient Voc ( Voc) below 5 mv so that Voc << Voc. Voltage dynamics were recorded on a digital oscilloscope (Tektronix DPO 4104B), and voltages at open circuit were measured over a MΩ and a 50Ω resistor, respectively. X-Ray Diffraction Analysis. 0-2θ scans were obtained using a Bruker D8 Discover Powder X- Ray Diffractometer equipped with a push plug gobel mirror and a monochromatic CuKα ( λ = Å) radiation source. Photoluminescence (PL) and Time Resolved Photoluminescence (TRPL) Measurements: A Horiba Jobin Yvon system with a red light source with an excitation at 640 nm was used to 3

4 conduct PL measurements. TRPL spectra was obtained using a Picoharp 300 with timecorrelated single-photon counting capabilities. A picosecond diode laser provided excitation at a wavelength of 640 nm with a repetition frequency of 1 MHz (PDL 800B). Scanning Electron Microscopy (SEM). An emission Nova 230 NanoSEM was used to obtain SEM images with an electron beam acceleration in the range of 500V to 30 kv. X-ray photonelectron spectroscopy and ultra-violet photonelectron spectroscopy. X-ray or ultra-violet photoelectron spectroscopy were carried out in Kratos DLD XPS system. Optical simulation: Optical simulations based on the transfer matrix formalism (TMF) were used to calculate the interference of reflected and transmitted light at each interface within the stratified devices. All the simulations are based on the assumptions of planar interfaces and total isotropy for all layers. However, the interference within the glass substrates is ignored because their thicknesses (2 mm) are much higher than the wavelengths of the simulated incident beams. In addition, 100% internal quantum efficiency and the AM1.5 intensity spectrum (ASTM G173-03) are assumed to calculate the theoretically maximum photocurrent density. The parameters for the optical simulation is from the literature report. [1,2] 4

5 Ref Intensity (a.u.) BA-SAM C3-SAM PA-SAM CBA-SAM ABA-SAM Binding Energy (ev) Figure S1. XPS spectra of N 1s of SnO2 surface with different SAMs modification. Figure S2. SEM image of the perovskite films on different SAMs modified SnO2 substrates. a) None, b) Benzoic acid, c) 4-pyridinecarboxylic acid, d) 4-cyanobenzoic acid, e) 3Aminopropanoic acid, f) 4-aminobenzoic acid. 5

6 Intensity (a.u.) 100k /ABA-SAM /C3-SAM /CBA-SAM /PA-SAM /BA-SAM θ (degree) Figure S3. XRD patterns of perovskite film on substrates with or without SAMs. 6

7 Figure S4. Optical intensity distribution in the perovskite solar cells. Reference [1] Y. Jiang, I. Almansouri, S. Huang, T. Young, Y. Li, Y. Peng, Q. Hou, L. Spiccia, U. Bach, Y.-B. Cheng, M. A. Green, A. Ho-Baillie, J. Mater. Chem. C 2016, 4, [2] C.-W. Chen, S.-Y. Hsiao, C.-Y. Chen, H.-W. Kang, Z.-Y. Huang, H.-W. Lin, J. Mater. Chem. A 2015, 3,