Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering

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1 Supporting Information Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering Dae-Yong Son 2, Seul-Gi Kim 1, Ja-Young Seo 1, Seon-Hee Lee 2, Hyunjung Shin 2, Donghwa Lee 3 and Nam-Gyu Park 1 * 1 School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Korea 2 Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea 3 Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH), Pohang 37666, Korea These authors equally contributed to this work *Corresponding author npark@skku.edu, Tel: Experimental details Tables S1 S4 Figures S1 S11 References S1

2 Experimental Synthesis of HC(NH 2 ) 2 I : Formamidinium iodide (FAI, FA = HC(NH 2 ) 2 ) was synthesized by reacting 30 ml hydroiodic acid (57 wt% in water, Sigma-Aldrich) with 15 g of formamidinium acetate (99%, Aldrich) at 0 o C. After stirring for 30 min, dark yellow precipitate was formed by evaporating the solvent at 60 o C using rotary evaporator. The solid powder was washed with ether and recrystallized from ethanol. Resulting white precipitate was dried under vacuum for 24 h and stored in glove box filled with Ar. Synthesis of CH 3 NH 3 I: Methylammonium iodide (MAI, MA = CH 3 NH 3 ) was synthesized by reacting 30 ml of hydroiodic acid (57 wt% aqueous solution, Aldrich) with 27.8 ml of methylamine (40 wt% in methanol, TCI) at 0 o C for 2 h. From the solution, a dark brown precipitate was recovered using a rotary evaporator at 60 o C for 2 h. The resulting precipitate was washed with diethyl ether several and then recrystallized from ethanol. The white precipitate was collected by filtration and dried under vacuum for 24 h before storage in a glove box filled with Ar. Synthesis of CH 3 NH 3 Br: Methylammonium bromide (MABr) was synthesized by reacting 21.6 ml of hydrobromic acid (48 wt% aqueous solution, Aldrich) with 27.8 ml of methylamine (40 wt% in methanol, TCI) at 0 o C for 2 h. From the solution, a dark brown precipitate was recovered using a rotary evaporator at 60 o C for 2 h. The resulting precipitate was washed with diethyl ether several and then recrystallized from ethanol. The white precipitate was collected by filtration and dried under vacuum for 24 h before storage in a glove box filled with Ar. Synthesis of TiO 2 nanoparticles: Anatase TiO 2 nanoparticles were synthesized by acetic acid catalyzed hydrolysis of titanium isopropoxide (97%, Aldrich), followed by autoclaving at 230 o C for 12 h. After cooling down, ~20 nm TiO 2 nanoparticles were washed with DI water several times. The 20 nm-seized TiO 2 nanoparticles were used as seed material to grow the size upto ~50 nm in the same hydrothermal procedure. Aqueous solvent in the autoclaved TiO 2 colloidal solution was replaced by ethanol for preparation of non-aqueous TiO 2 paste. Ethyl cellulose (Aldrich), lauric acid (Fluka), and terpineol (Aldrich) were added into the ethanol solution of the TiO 2 particles, which was followed by removal of ethanol using a rotary evaporator to obtain viscous pastes. For homogeneous mixing, the paste was further S2

3 treated with a three-roll mill. The nominal composition of TiO 2 /terpineol/ethylcellulose/lauric acid was 1.25/6/0.9/0.3. Solar cell fabrication. The patterned FTO glass (Pilkington, TEC-8, 8Ω/sq) was cleaned with detergent and DI water and sonicated with ethanol in an ultrasonic bath for 20 min. UV- Ozone was treated for 15 min prior to use. For the TiO 2 blocking layer (bl-tio 2 ), the cleaned FTO substrate was immersed in 20 mm aqueous TiCl 4 (Sigma Aldrich, >98%) solution at 70 o C for 20 min, washed with DI water several times and then annealed at 500 o C for 30 min. After cooling down, the mesoporous TiO 2 (ca. 50 nm) layer was deposited by spin coating at 2000 rpm for 20 s (acceleration was 500 rpm/s) using the TiO 2 paste dilluted with 1-butanol. The deposited TiO 2 film was annealed at 550 o C for 30 min, which was further treated with 20 mm aqueous TiCl 4 (Sigma-Aldrich, > 98%) solution at 70 o C for 10 min and then annealed again at 500 o C for 30 min. The all perovskite solutions were prepared by method described elsewhere [S1]. The mixed perovsktie solution ((FAPbI 3 ) (CsPbBr 3 ) ) was prepared by mixing of g of FAI, g of PbI 2 (TCI, 99.99%), g of PbBr 2 (Alfa Aesar, ultra dry, %) and g of CsBr (Alfa Aesar, ultra dry, 99.9%) in 0.50 ml of N,N -dimethylformamide (DMF, 99.8% anhydrous, Sigma Aldrich) and 0.75 μl N,Ndimethylsulfoxide (DMSO, >99.5%, Sigma Aldrich). The triple cation perovskite solution of (FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 ) was prepared by mixing of g of MABr, g of FAI, g of PbI 2, g of CsI, and g of PbBr 2 in 0.55 ml DMF and 0.75 μl DMSO. The FA/MA double cation (FA 0.85 MA 0.15 PbI 2.55 Br 0.45 ) perovskite solution was prepared by mixing of g of MABr, g of PbBr 2, g of FAI and g of PbI 2 in 0.50 ml of DMF and 0.75 μl of DMSO. MAPbI 3 (or FAPbI 3 ) were preapred by dissolving of g of MAI (0.172 g of FAI) and g PbI 2 in 0.50 ml of DMF and 0.75 μl DMSO. For KI doping, 100 mm of KI stock solution in DMF was prepared by dissolving g of KI in 10 ml of DMF. 0.1 ml of the stock solution was poured to the perovskite precursor solution for the 10 μmol KI doping ml 0.2 ml of the stock solution was mixed with the perovskite precursor solution for the case of 2 20 μmol KI doping. The total DMF volume was set 0.5 ml. Other alkali metal iodide solutions were prepared and mixed in the same way. The perovskite solution with and without KI solution were spin coated. Prior to spin-coating, all the precursor solutions were stirred at 60 o C for 1 h to dissolve all precursors completely and the perovskite solutions with alkali metal iodides were additionally stirred for S3

4 2 h in ambient condition. The solutions were filtered with 0.45 μm pore sized filter (Whatman). The filtered solutions were spin-coated on the TiO 2 film at 4000 rpm for 20 s, where 0.3 ml of diethyl ether was dripped after spinning for 12 s. The transparent brownish adduct film was formed right after deposition, which was heated at 145 o C for 40 min. The 20 μl of spiro-meotad solution, containing of 86.2 mg spiro-meotad (Merck), 35.2 μl of 4- tert-butyl pyridine and 16.7 μl of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solution (600 mg LiTSFI in 1 ml acetonitrile (99.8%, Sigma Aldrich) in 1 ml of chlorobenzen), was spin-coated on the perovskite layer at 3000 rpm for 30 s. Finally, the Au electrode was evaporated by using the thermal evaporator at a constant evaporation rate of 0.05 nm/s. Characterizations. Current density-voltage (J-V) curves were measured under AM 1.5G one sun (100 mw/cm 2 ) illumination using a solar simulator (Oriel Sol 3A, class AAA) equipped with 450 W Xenon lamp (Newport 6280NS) and a Kiethley 2400 source meter. The light intensity was adjusted by NREL-calibrated Si solar cell having KG-5 filter. The device was covered with a metal mask with aperture area of cm 2. Impedance spectroscopy measurements were performed under one sun illumination (100mW/cm 2 ) and in the dark using PGSTAT 128N (Autolab, Eco-Chemie). DC voltage was applied from 0.1 V to 0.9 V at 0.1 V intervals with small perturbation of AC 20 mv sinusoidal signal. Frequency ranged from 100 mhz to 10 khz. Trap density was estimated using SCLC (Space-Charge-Limited Current) method. Devices with the FTO/perovskite/Au layout were measured in the dark from 0 V to 2 V with the scan rate of 1000 ms. The observed response was analyzed according to SCLC theory. Trap density (n t ) was estimated using the relation V TFL = en t d 2 /2 0 [S2], where V TFL is the onset voltage of the trap-filled-limit, e electric charge ( C), dielectric constant, 0 the vacuum permittivity ( F/cm) and d film thickness (Figure S9). The dielectric constant was calculated using the equation of = Cd/A 0, where C is capacitance at high frequency (~10-4 Hz), d thickness of perovskite, A area, and 0 the vacuum permittivity [S3, S4]. X-ray diffraction (XRD) patterns of (FAPbI 3 ) (CsPbBr 3 ) perovskites doped with 0, 2, 4, S4

5 6, 8 and 10 μmol KI were obtained using a Rigaku SmartLab (45 kv, 200 ma) under graphite-monochromated Cu-K radiation at two theta interval of 0.01 o and the scan rate of 3 o /min. To avoid thermal expansion, the temperature was maintained 23 o C. XRD data were analyzed using EXPO2014 program [S5]. To determine space group and crystal structure, we analyzed the XRD data according to the user manual provided by The indexing process was performed by the N-TREOR setting in EXPO2014. Reflections were indexed by cubic unit cell based on Pm-3m space group. The structural parameters of heavy atoms such as Cs, Pb and I were determined by using the direct method (DM) in the EXPO2014 software. Plan-view and cross-sectional morphologies were investigated by means of a scanning electron microscope (SEM, JSM-7600F, JEOL). Absorption and reflectance spectra were measured using an ultraviolet/visible spectrometer (Lambda 45, Perkin Elmer). Conductive atomic force microscopy (c-afm) was measured by a commercial atomic force microscope (SPA-400, SII, Japan) using Rh coated Si tips (SI-DF20-R, SII) with a typical resonant frequency of 130 khz and a spring constant of 15 N/m. All images were acquired with a contact force of -2.2 nn, and bias voltages of -1.0 V at a scan rates of 0.5~1.0 Hz. Negative biases were applied to the substrate, while the tip was grounded. The current and topographic images were obtained in ambient, simultaneously. All the CAFM measurements were performed under a white LED (W , Seoul Semiconductor, Korea) illumination 2 cm away from the samples at an angle (α) of ~30. The maximum Intensity (at α = 0) of white LED is 10 mw/cm 2. The LED intensity at α = 30 can be estimated from the supplier technical data sheet, and the value is ~8 mw/cm 2. Computational details. First-principle density functional theory (DFT) calculations were performed with projector augmented wave (PAW) method [S6-S8] for the exchangecorrelation potential as implemented in Vienna Ab-initio Simulation Package (VASP) code [S9]. The optimized lattice parameter for the cubic MAPbI 3 and FAPbI 3 system was obtained as and A for unit-cells. For FA MA PbI 2.55 Br 0.45 mixed cation system, one FA of 8 formula unit was switched to MA. Periodic boundary condition and Monkhorst-Pack k-point sampling [S10] with a Г-centred k-point grid of was used for the Brillouin zone integration. An energy cutoff of 500 ev was used for the plane-wave S5

6 representation of the wavefunctions and the 5d electrons of Pb were considered as valence electrons. Atomic structures were relaxed until all Hellman-Feynman forces were below 0.01 ev/å. Climbing-image nudged elastic band method [S11] with 25 images was used for locating minimum energy pathways. Defect formation energy is calculated by subtracting the energy of non-defective system and defect interstitial from total energy of the system including defect. BCC crystal structure is used to calculate the energy of K interstitial. S6

7 Table S1. Photovoltaic parameters of short-circuit photocurrent density (J sc ), open-circuit voltage (V oc ), fill factor (FF) and power conversion efficiency (PCE) of pristine and 10 μmol KI doped FA 0.85 MA 0.15 PbI 2.55 Br 0.45, FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3, MAPbI 3 and FAPbI 3, measured at reverse (rev.) and forward (for.) scans at 130 mv/s (= voltage settling time of 200 ms) under AM 1.5G one sun illumination. FA 0.85 MA 0.15 PbI 2.55 Br 0.45 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for Average (rev.) Average (for.) S7

8 FA 0.85 MA 0.15 PbI 2.55 Br 0.45 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for Average (rev.) Average (for.) S8

9 FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for Average (rev.) Average (for.) S9

10 FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for ms-rev ms-for Average (rev.) Average (for.) S10

11 MAPbI 3 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev Average (rev.) Average (for.) MAPbI 3 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev Average (rev.) Average (for.) S11

12 FAPbI 3 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev Average (rev.) Average (for.) FAPbI 3 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms - for ms - rev ms - for ms - rev ms - for ms - rev ms - for ms - rev Average (rev.) Average (for.) S12

13 Table S2. Scan rate dependent photovoltaic parameters of short-circuit photocurrent density (J sc ), open-circuit voltage (V oc ), fill factor (FF) and power conversion efficiency (PCE) for dual cation and triple cation perovskites without (w/o) and with 10 μmol KI. Scan rate was represented by voltage settling time, measured under one sun illumination. FA 0.85 MA 0.15 PbI 2.55 Br 0.45 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for Average FA 0.85 MA 0.15 PbI 2.55 Br 0.45 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for Average S13

14 FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 (w/o KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for Average FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 (10 μmol KI) J sc (ma/cm 2 ) V oc (V) FF PCE (%) 200 ms-rev ms-for ms-rev ms-for ms-rev ms-for Average S14

15 Table S3. Photovoltaic parameters of short-circuit photocurrent density (J sc ), open-circuit voltage (V oc ), fill factor (FF) and power conversion efficiency (PCE) of pristine (w/o KI) and 2, 4, 6, 8 and 10 μmol KI doped (FAPbBr 3 ) (CsPbBr 3 ) 0.125, measured at reverse and forward scans at 130 mv/s (= voltage settling time 200 ms) under AM 1.5G one sun illumination. Concentration of KI J sc (ma/cm 2 ) V oc (V) FF PCE (%) w/o KI rev w/o KI for μmol KI rev μmol KI for μmol KI rev μmol KI for μmol KI rev μmol KI for μmol KI rev μmol KI for μmol KI rev μmol KI for μmol KI for μmol KI rev μmol KI for μmol KI rev S15

16 Table S4. Capacitance (C), trap-filled-limit voltage (V TFL ), trap density (n t ), dielectric constant ( ), and area of (FAPbBr 3 ) (CsPbBr 3 ) with and without (w/o) KI. Concentration of KI C ( 10-9 F) V TFL (V) ε n t ( cm -3 ) Area (cm 2 ) w/o KI μmol μmol μmol μmol μmol μmol μmol S16

17 Figure S1. Scan rate dependent current density-voltage curves of perovskite solar cells employing (A,B) FA 0.85 MA 0.15 PbI 2.55 Br 0.45 and (C, D) FA 0.85 MA 0.1 Cs 0.05 PbI 2.7 Br 0.3 without and with 10 μmol KI, measured at reverse (solid lines) and forward (dash-dotted lines) at different scan rate from 200 ms (130 mv/s) to 2000 ms (13 mv/s) under AM 1.5 G one sun illumination condition. S17

18 Figure S2. (A) Absorbance spectra, (B) Tauc plot, (C) tolerance factor, and (D) absorption coefficient and Urbach tail energy (E u, inset) of (FAPbI 3 ) 1-x (CsPbBr 3 ) x. S18

19 Figure S3. (A)-(F) Plane-view SEM images of (FAPbI 3 ) 1-x (CsPbBr 3 ) x where the x ranging from 0.05 to 0.2. S19

20 Figure S4. (A)-(F) Plan-view SEM images of (FAPbI 3 ) (CsPbBr 3 ) perovskite films doped with 2, 4, 6, 8 and 10 μmol KI together with pristine perovskite (w/o KI). S20

21 Figure S5. (a) X-ray diffraction (XRD) patterns of (FAPbI 3 ) (CsPbBr 3 ) perovskite films doped with 2, 4, 6, 8 and 10 μmol KI together with pristine perovskite (w/o KI). All the peaks could be indexed to be cubic phase using EXPO2014 program. (b) Lattice parameter as function of KI concentration showing gradual increase in lattice parameter with KI concentration upto 8 μmol. S21

22 Figure S6. (A)-(H) J-V curves of the perovskite solar cells employing (FAPbBr 3 ) (CsPbBr 3 ) doped with 2, 4, 6, 8, 10, 15 and 20 μmol KI and without KI (pristine, w/o KI), measured at reverse (filled black circles) and forward (open red circles) at the scan rate 130 mv/s (= voltage settling time of 200 ms) under AM 1.5 G one sun illumination condition. S22

23 Figure S7. (A) Stead-state photocurrent measured at voltage at maximum power under one sun illumination for the perovskite solar cells employing (FAPbBr 3 ) (CsPbBr 3 ) doped with 2 and 10 μmol KI and without KI (w/o KI). (B) First derivative data of time-dependent photocurrent, showing faster kinetics for the 10 μmol KI doped perovskite than for the pristine perovskite without KI doping. S23

24 Figure S8. Dark current-voltage curves of (FAPbBr 3 ) (CsPbBr 3 ) perovskite films doped with KI (2 20 μmol) together with pristine (w/o KI), showing ohmic region with n =1 and trap-filled-limit region with n>3. Device structure is schematically shown in the left on top. S24

25 Figure S9. Cross-sectional SEM images of (FAPbBr 3 ) (CsPbBr 3 ) perovskite layer doped with KI (2 20 μmol) and pristine without KI doping. Perovskite layers were deposited on FTO glass substrate. Top layer is Au electrode. The scale bar was 1 μm. S25

26 Figure S10. Capacitance-Voltage curves of (FAPbBr 3 ) (CsPbBr 3 ) perovskite layer doped with 10 μmol KI and without KI doping. The measured frequency ranged from 10 KHz to 100 KHz. The first derivative data (inset) showing a typical p-type shape. S26

27 Figure S11. AFM topography images (left) and corresponding C-AFM images (right) of (FAPbBr 3 ) (CsPbBr 3 ) perovskite layer (A,C) without KI and (B,D) with 10 μmol KI doping. Perovskite films were deposited on FTO substrate. All images were obtained under illumination condition. The external biases were applied to FTO substrate with (A,B) -1.0 V and (C,D) +1.0 V, where the darker contrast corresponds to a higher current when negative biases were applied, while the brighter contrast corresponds to a higher current when positive biases were applied. (E) Mean current observed from four different batches. A big difference in current depending on bias polarity is indicative of rectifying effect of perovskite films. S27

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