Hindered Formation of Photo-inactive δ-fapbi 3. Phase and Hysteresis-free Mixed-cation Planar. Heterojunction Perovskite Solar Cells with

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1 Supporting Information Hindered Formation of Photo-inactive δ-fapbi 3 Phase and Hysteresis-free Mixed-cation Planar Heterojunction Perovskite Solar Cells with Enhanced Efficiency via Potassium Incorporation Disheng Yao, Chunmei Zhang, Ngoc Duy Pham, Yaohong Zhang, Vincent Tiing Tiong, Aijun Du, Qing Shen, Gregory J. Wilson, Hongxia Wang* School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, Australia Faculty of Informatics and Engineering, The University of Electro-Communications, Chofugaoka, Chofu, Tokyo , Japan CSIRO Energy, Solar Energy Systems, Mayfield West, NSW 2304, Australia Corresponding Author * hx.wang@qut.edu.au Experimental Section Material preparation All materials were purchased from Sigma-Aldrich and used as received unless otherwise stated. Materials for preparation of perovskite films including methyl ammonium bromide S1

2 (MABr), formamidinium iodide (FAI), methyl ammonium iodide (MAI) were purchased from Dyesol, LTD. 2, 2, 7, 7 -Tetrakis- (N, N-di-4-methoxyphenylamino)-9, 9 - spirobifluorene (Spiro-MeOTAD) was supplied by Borun New Material, China. Device fabrication Fluorine-doped tin oxide (FTO) glass (Nippon Electric Glass, 15 Ω/sq) was patterned by partially etching of FTO layer with 35.5 wt% hydrochloric acid and Zinc powder. A thorough cleaning was carried out by washing the FTO substrate in an aqueous solution of Decon-90 detergent (5%), deionised water, a mixture of ethanol, isopropanol and acetone (1:1:1, volume ratio) in sequence under sonication. The substrate was then dried by blow-n 2 gas. Prior to film deposition, the cleaned substrates were treated with UV-Ozone for 20 mins to remove any possibly residual organic solvent. SnO 2 based electron transporting layer (ETL) was deposited via spin-coating 0.1 M solution of tin (II) chloride (98%) in absolute ethanol at 3000 rpm for 30s. The film was then dried at 90 for 5 min and annealed in air at 185 for 1 h to obtain a thin and uniform SnO 2 film. Before being transferred to an N 2 -filled glove box, the film was treated again with UV-Ozone for 20 mins to clean the surface. To prepare mixed-cation K x (MA 0.17 FA 0.83 ) 1-x PbI 2.5 Br 0.5 perovskite precursor solution (1.3 M), PbI 2, PbBr 2, FAI, MABr, KI with controlled amounts were dissolved in a mixed solvent of DMSO and DMF (DMSO/DMF = 1/4, volume ratio) at room temperature for 2 h. The molar ratio of PbBr 2 /PbI 2 and MABr/FAI were both fixed at 1/5. KI/(FAI+MABr+KI) was fixed to fulfil the desired molar ratio from 0 to 10%. (PbI 2 +PbBr 2 )/(FAI+MABr+KI) =1.05/1 with slightly extra lead halides to enhance photovoltaic performance. The precursor solution for the hole transporting layer (HTL) was prepared by dissolving 72.3 mg Spiro-MeOTAD, 28.8 µl 4-tert-butylpyridine and 17.5 µl bis(trifluoromethane)sulfonimide lithium (Li-TFSI) solution (520 mg Li-TFSI in 1 ml acetonitrile) into 1 ml chlorobenzene. All these precursors were filtered by a syringe filter (pore size: 0.22 µm) prior to the film deposition. The perovskite layer was deposited onto the SnO 2 layer with two-step spin-coating procedures consisting of deposition at 2000 rpm for 10s first (ramp-up of 200 rpm/s) followed by a second deposition step at 4000 rpm for 20s (acceleration of 1000 rpm/s) ml chlorobenzene was dropped on the spinning substrate at the 10 th second during the second step. The perovskite layer was then heated at 100 for 30 min on a hot plate. After cooling down to room temperature, the hole transport layer (HTL) was deposited by spin coating the Spiro-MeTAD solution on the perovskite film at 4000 rpm for 30 s and then dry naturally in S2

3 the glove box. Finally, the device fabrication was finished by depositing a 100 nm gold layer on the prepared sample as the back contact electrode through an e-beam evaporation process at 10-6 Torr pressure. Computational Detail The calculations were performed using density functional theory (DFT) within generalized gradient approximation of the Perdew-Burke-Ernzerh of functional, as implemented in the Vienna ab initio simulation package (VASP). S1, S2 The structures in this work were fully relaxed until energy and force converged to 10-6 ev and 0.001eV/Å, respectively. An energy cut-off of 500 ev was used for the plane-wave representation of the wave-functions. For interstitial doping and substitutional doping, the unit cell containing one MA, five FA (MAFA 5 Pb 6 I 15 Br 3 ) and two MA, ten FA (MA 2 FA 10 Pb 12 I 30 Br 6 ) are adopted, respectively. The energy required to doping K to MA 2 FA 10 Pb 12 I 30 Br 6 is. S3 Thus, the formation energy ( ) for interstitial doping and substitutional doping are calculated based on = and = +, respectively.,,, and are the total energies of KMAFA 5 Pb 6 I 15 Br 3, MAFA 5 Pb 6 I 15 Br 3, K, KMA 2 FA 9 Pb 12 I 30 Br 6, and FA, respectively. Characterization The top-view and cross-sectional scanning electron microscopy (SEM) images of the samples were obtained using a field emission scanning electron microscope (FESEM JOEL 7001F) at an acceleration voltage of 5 kv and 20 kv respectively. X-ray diffraction pattern (XRD, Rigaku Smartlab) of the perovskite films deposited on FTO glass substrates was obtained with an XRD facility with monochromatic CuKα (λ=0.154 nm) excitation source at a scan rate of 1.5 /min and step size of X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectra (UPS, HeI as resonance line, photo energy is ev) were measured through Kratos AXIS Supra photoelectron spectrometer. UV-visible transmittance and absorbance spectrum was detected by Cary 50 UV-visible spectrometer. Scanning Kelvin Probe Force Microscopy (KPFM) was operated on the perovskite film using NSG-03 Pt coated cantilever under a uniformed light illumination at room temperature. Ultrafast transient absorption (TA) spectroscopy was used to study the photo-excited carrier dynamics of the perovskite film. The laser source was generated by a titanium/sapphire laser (CPA- 2010, Clark-MXR Inc.) with a wavelength of 760 nm, a pulse width of 150 fs and a repetition S3

4 rate of 1 khz. The output of the laser was then split into two parts. One part was incident on a sapphire plate to generate white light for the probe beam. The other part was employed to pump an optical parametric amplifier (OPA, a TOAPS from Quantronix) to generate light pulses which was used as the pump light to excite the sample. In this study, all the samples were measured using the pump light wavelength of 470 nm with pump fluence of 3 µj/cm 2 at room temperature. The time-resolved TA spectra at 760 nm were obtained with a temporal resolution of 100 fs. The performance of perovskite solar cells (I-V measurement) was tested under irradiation of 100 mw/cm 2 (AM1.5, 1 sun) through a solar simulator (Oriel Sol3A, Newport) equipped with 450W Xenon lamp. The active area of the device was cm 2 using a black mask to avoid scattered light. The scan rate in the I-V measurement was 10 mv/s and no preconditioning such as light soaking or voltage bias were applied with the devices. Quantum efficiency system (IQE 200B, Newport) under AC mode was used to measure the external quantum efficiency (EQE) measurement. Impedance spectroscopy (IS) was performed at open-circuit voltage in a frequency range from 1 MHz to 0.1 Hz by an electrical workstation (VSP BioLogic Science Instruments). The light intensity dependence of open-circuit voltage (V oc ) was carried out by the I-V measurement under different illumination intensities which was manipulated by settling neutral density optical filters with transmittance of 1%, 11%, 19%, 30% and 55% between the cells and the solar simulator. The open-circuit voltage decay was measured by an electrochemical workstation (BioLogic) to monitor the open-circuit voltage (V oc ) relaxation behaviour of cells in dark as a function of time. A white LED lamp equivalent to 0.15 sun illumination intensity was used to illuminate the cell for ~3s to build voltage before it was switched off to detect the open-circuit voltage decay (OCVD). The long-term stability of the PSCs performance was conducted by recording the efficiency of PSCs without encapsulation which were stored in an ambient environment (temperature ~15-25 C, relative humidity 30-45%). The performance was measured on the 1 st, 3 rd, 7 th, 14 th and 21 st respectively. Supporting Result S4

5 (a) (b) (c) (d) Figure S1. (a) XPS full spectrum of different mixed-cation perovskite films, high resolution sweeping for (b) K 2p in the perovskite films with 0%, 3%, 5% K +, (c) Pb 4f, (d) I 3d. S5

6 Figure S2. (a)-(d) Side views of four different types of substitutional K doping positions for KMA 2 FA 9 Pb 12 I 20 Br 6 crystal structure. Different species are shown as light orange (Br), dark orange (I), blue (N), white (H), purple (K), light grey (C), and dark grey (Pb) which are also marked in the Figure (a). Four different K positions are considered. S6

7 Table S1 the corresponding K-formation energy (ev) and the lattice parameters (Å) for four different K substitutional doping positions for KMA 2 FA 9 Pb 12 I 10 Br 6. 1 st -4 th presents the corresponding doping positions in Figure 2 (a)-(f), and a, b, c means the lattice parameters along x, y, z directions, respectively. For comparison, the lattice parameters for MA 2 FA 10 Pb 12 I 10 Br 6 are also given. 1 st 2 nd 3 rd 4 th MA 2 FA 10 Pb 12 I 10 Br 6 K + formation energy (ev) Lattice parameters a= a= a= a= a= (Å) b=6.259 b=6.344 b=6.355 b=6.273 b=6.348 c= c= c= c= c= S7

8 (a) (b) Roughness=33.3 nm Average CPD=289.4mV (c) (d) Roughness=31.8 nm Average CPD=442.3mV Figure S3. KPFM topography of the (a) MA 0.17 FA 0.83 PbI 2.5 Br 0.5 /SnO 2 /FTO film and (c) K 0.03 (MA 0.17 FA 0.83 ) 0.97 PbI 2.5 Br 0.5 /SnO 2 /FTO film; (b and d) their corresponding contact potential difference (CPD) images. S8

9 (a) (b) (c) (d) Figure S4. Statistical parameters of (a) J sc, (b) V oc, (c) FF and (d) PCEs measured under both forward and reverse scanning for 20 cells respect to the mixed-cation perovskite with different K + contents from 0% to 10%. S9

10 (a) 0% K (b) 3% K (c) 10% K (d) 20% K Figure S5. Top-view SEM images of perovskite films. (a) MA0.17FA0.83PbI2.5Br0.5, (b) K0.03(MA0.17FA0.83)0.97PbI2.5Br0.5, (c) K0.1(MA0.17FA0.83)0.9PbI2.5Br0.5, (d) K0.2(MA0.17FA0.83)0.8PbI2.5Br0.5. Figure S6. Nyquist plots of MA-FA solar cell and 3% K-doped device at open-circuit voltage with a frequency range from 1 MHz to 0.1 Hz under AM 1.5G 1 sun light illumination. S10

11 Table S2 The extracted EIS parameters of perovskite solar cells with different K + contents. Sample R s (Ω) R 1 (Ω) R 2 (Ω) R 1 +R 2 (Ω) C s (F) C g (F) 0% K e e -8 3% K e e -8 (a) (b) (c) Figure S7. (a)-(b) J-V curves of the MAPbI 3 solar cell and mixed-cation perovskite devices with different K + contents under different light soaking times. (c) Stabilized photocurrent measured at a maximum power point under 1 sun light illumination. S11

12 Table S3 The calculated photoexcited carrier lifetime (τ) from the KTS fitting curves. Sample τ (ps) 0% K 1044±31 1% K 1670±68 3% K 1461±37 5% K 1666±61 10% K 1573±72 Figure S8. Open-circuit Voltage (V oc ) as a function of light illumination intensity. Figure S9. Long-term stability of reference MA-FA and 3% K-doped solar cell in the air without encapsulation under 30-45% relative humidity. S12

13 Reference (S1) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, (S2) Kresse, G.; Furthmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comp. Mater. Sci. 1996, 6, (S3) Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K. Organic-Inorganic Hybrid Lead Iodide Perovskite Featuring Zero Dipole Moment Guanidinium Cations: A Theoretical Analysis. J. Phy. Chem. C 2015, 119, S13