Reducing Carrier Density in Formamidinium Tin Perovskites. and Its Beneficial Effects on Stability and Efficiency of Perovskite

Transcription

1 Supporting Information Reducing Carrier Density in Formamidinium Tin Perovskites and Its Beneficial Effects on Stability and Efficiency of Perovskite Solar Cells Seon Joo Lee, Seong Sik Shin, Jino Im, Tae Kyu Ahn, Jun Hong Noh,, Nam Joong Jeon, Sang Il Seok,,*, and Jangwon Seo,* Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong-gu, Daejeon 34114, Republic of Korea Department of Energy Science, Sungkyunkwan University, 2066 Seobu-ro, Jangsan-gu, Suwon , Republic of Korea School of Civil, Environmental and Architectural Engineering, Korea University, Seoul , Republic of Korea School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea *Corresponding authors. 1

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3 Experimental Methods Chemicals: Hydriodic acid (99.95 %, 57 wt.% in H 2 O), hydrobromic acid (48 wt.% in H 2 O), formamidine acetate salt (99 %), titanium diisopropoxide bis(acetylacetonate) (75 wt.% in isopropanol), N,N-dimethylformamide (anhydrous, 99.8 %), dimethyl sulfoxide (anhydrous, 99.9 %), tin(ii) fluoride (99 %), toluene (anhydrous, 99.8 %), chlorobenzene (anhydrous, 99.8 %), bis(trifluoromethane) sulfonimide ( 95.0 %), acetonitrile (anhydrous, 99.8 %), 2,6-lutidine ( 99 %) were purchased from Aldrich. Ethanol and diethyl ether were purchased from Burdick & Jackson. 2,2,7,7 -tetrakis(n,n-bis(pmethoxyphenyl)amino)-9,9 -spirobifluorene (Spiro-OMeTAD) was purchased from Luminescence Technology Corp (Lumtec). All chemicals mentioned above were used without further purification. Synthesis of formamidinium iodide (CH(NH 2 ) 2 I, FAI): CH(NH 2 ) 2 I (FAI) was synthesized according to the literature ml of hydriodic acid was added to 250 ml round-bottom flask which is containing 15 g of formamidine acetate and the reaction was proceeded at 0 o C for 2 h with vigorous stirring. The precipitate was recovered by evaporating the reaction mixture at 50 o C for 1h. The product was dissolved in ethanol and recrystallized by diethyl ether. The resulting FAI was collected by filtration and dried in a vacuum oven at 60 o C overnight. Synthesis of formamidinium bromide (CH(NH 2 ) 2 Br, FABr): CH(NH 2 ) 2 Br (FABr) was synthesized in the same manner as FAI except using hydrobromic acid instead of hydriodic acid. Device fabrication: A dense blocking layer of TiO 2 with 60 nm thickness was prepared by spray pyrolysis. 20 mm titanium diisopropoxide bis(acetylacetonate) diluted in ethanol was spread onto a F-doped SnO 2 (FTO, Pilkington, TEC8) substrate at 450 o C. This 3

4 procedure is for preventing a direct contact between FTO and the hole-conducting layer. A mesoporous TiO 2 with 400 nm thickness was deposited onto the blocking TiO 2 /FTO substrate by spin coating at 2,000 rpm for 50 s using a diluted TiO 2 paste and sintered at 500 o C for 1 h. The TiO 2 paste was prepared by the same procedure as described in our previous report. 1 The CH(NH 2 ) 2 SnI 3 (FASnI 3 )-based perovskite absorbing layers were prepared in glove box. FAI (1 mmol) and SnI 2 (1 mmol) were dissolved in 1 ml of DMF and DMSO mixed solvent (4:1 volume ratio) and then SnF 2 was added. FABr instead of FAI was used to introduce Br into the FASnI 3 perovskite lattice. The resulting solution was coated onto the mesoporous TiO 2 /blocking TiO 2 /FTO substrate via a twostep spin-coating process, at 1,000 rpm and 5,000 rpm for 40 s and 50 s, respectively. During the second spin-coating step, 1 ml of toluene was dripped onto the substrates. The substrate was annealed at 60 o C for 1 h. For the comparison of light stability, Pbbased perovskite layer was fabricated using a solution of formamidinium lead iodide (1.3 mmol) and methylammonium lead bromide (0.1 mmol) in 0.9 ml of DMF and DMSO mixed solvent (8:1 volume ratio). The resulting solution was deposited onto the prepared mesoporous TiO 2 layer via a two-step spin-coating process, at 1,000 rpm and 5,000 rpm for 5 s and 50 s, respectively. During the second spin-coating step, 1 ml of ethyl ether was dripped on to the substrates and the substrate was annealed at 100 o C for 1 h. A solution of spiro-ometad/chlorobenzene (72.3 mg/ml) containing 17.5 µl of bis(trifluoromethane) sulfonimide/acetonitrile (509 mg/ml) and 30 µl of 2,6-lutidine was spin-coated on the perovskite layer at 3,000 rpm for 30 s. Finally, 60 nm thick gold electrode was deposited onto the devices using a thermal evaporator while maintaining N 2 condition. The effective active area was fixed at 0.16 cm 2. The devices were thermally encapsulated with cover glass (2.5 cm 1.5 cm) using a polymer and an epoxy resin 4

5 under nitrogen atmosphere. Characterization: The morphology of FASnI 3 films and full device were investigated using field emission scanning electron microscopy (Mira 3 LMU, Tescan) operated at 20 kv. X-ray diffraction (XRD) patterns of the prepared films were measured on a Rigaku SmartLab X-ray diffractometer. The absorption spectra of perovskite films were collected using a Shimadzu UV 2550 spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) studies were carried out using a Thermo VG Scientific Sigma Probe. The external quantum efficiency (EQE) was measured using a power source (Newport 300 W Xenon lamp, 66920) with a monochromator (Newport Cornerstone 260) and a multimeter (Keithley 2001). Timeresolved photoluminescence (TRPL) curves were recorded using a commercial Time- Correlated Single Photon Counting (TCSPC) system (FluoTim 200, PicoQuant). Samples were photoexcited by picosecond diode laser of 470 nm (LDH-P-C-670, PicoQuant) with a variable repetition rate (1MHz) (since the reference was measured at 1MHz). The emitted PL was spectrally dispersed with monochromater (ScienceTech 9030) for each PL signal, and was collected by a fast photon multiplier tube (PMT) detector (PMA182, PicoQuant GmbH) with a magic angle (54.7 ) arrangement. The incident angle of excitation pulse was set to be about 30 with respect to the sample. The resulting instrumental response function was about 160 ps in full-width-half-maximum. In addition, a cutoff filter (FF01-692nm, Semrock) was applied to block the remaining scattering. Transient photovoltage decay measurements were performed using a nanosecond laser (10 Hz, NT342A-10, EKSPLA) as a small perturbation light source and a Xe lamp (150 W, Zolix) as a bias light source. The device was directly connected to a digital oscilloscope (500 MHz, DSO-X 3054A, Agilent) and the input impedance of the 5

6 oscilloscope was set to 1 MΩ for an open circuit condition. The bias light intensity was controlled by neutral density filters for various open circuit voltages (V oc ) and a strongly attenuated laser pulse of 550 nm, which generated a voltage transient ( V) that did not exceed 20 mv. The J-V curves were measured using a solar simulator (Newport, Oriel Class A, 91195A) under standard air mass 1.5 global (AM 1.5G) illumination with an irradiation intensity of 100 mw cm -2 (Keithley 2420) and a calibrated Si-reference cell certified by the National Renewable Energy Laboratory, USA. The J-V curves were measured by reverse scan (forward bias (1.2 V) short circuit (0 V)) or forward scan (short circuit (0 V) forward bias (1.2 V)). The step voltage and the delay time, which is a delay set at each voltage step before measuring each current, were set to 10 mv and 10 ms, respectively. The J-V curves for all devices were measured by masking the active area using a metal mask with an area of cm 2. Time-dependent current, dark current, and capacitance voltage measurements were conducted using a potentiostat (PGSTAT302N, Autolab). Computational Methods First-principle electronic structure calculations were carried out for crystal structure optimization and total energy calculations within the density functional theory (DFT) formalism. Planewave basis set (cutoff energy = 550 ev), projector augmented wave method 2 and PBE-sol exchange-correlation functional 3 were employed, which are implemented in Vienna Ab-initio Simulation Package. 34 For structural optimization, both internal coordinates and lattice parameters were relaxed. To describe alloying Br contents into the perovskite lattice, we used a 2x2x2 super cell which accommodates 24 halogen sites. In this work, we considered 4 different configurations with x=0.0, 0.125, 6

7 0.250, and in CH(NH 2 ) 2 SnI 3(1-x) Br 3x. Heat of formation were evaluated with the following formula: =, where E is total energy from DFT calculation and is total energy of each chemical element in its reference form. In the calculation of heat of formation, the formamidinium molecule (CH(NH 2 ) 2 ) was considered as a single element. Defect formation energy of Sn vacancy (V Sn ) was calculated for x=0.0, 0.125, 0.250, and as shown in the following formula: = + + +, where is chemical potential for element i and is Fermi level of defect system. 5 term is a correction term resulted from the finite cell size in the calculation. 6 A 2x2x2 super cell was considered to model a defect configuration. During structural relaxation, only internal coordinates were relaxed and lattice parameters were fixed at the relaxed one from the bulk calculations. In this study, only Sn vacancy in neutral charge status were considered, with respect to reference phase of each chemical element. 7

8 Table S1. Elemental analysis for FASn(Br x I 1-x ) 3 perovskite films. (FA = CH(NH 2 ) 2 ) molar ratio of theoretical Br atomic% I atomic% FAI:FABr in precursor chemical formula (from EDS) (from EDS) solution 1:0 FASnI :1 FASn(Br 0.08 I 0.92 ) :2 FASn(Br 0.17 I 0.83 ) :3 FASn(Br 0.25 I 0.75 ) :1 FASn(Br 0.33 I 0.67 ) Figure S1. a)-e) Top view scanning electron microscopy (SEM) images of FASnI 3 perovskite films with various Br content. a) is for Br 0 mol%, b) Br 8 mol%, c) Br 17 mol%, d) Br 25 mol%, and e) Br 33 mol%. f) Cross-sectional SEM image of FASnI 3 perovskite film containing Br deposited on mesoporous TiO 2 layer. 8

9 Table S2. Lattice parameters of FASn(Br x I 1-x ) 3 perovskite calculated from XRD reflections where x=0, 0.08, 0.17, 0.25, and a (A ) b (A ) c (A ) FASnI FASn(Br 0.08 I 0.92 ) FASn(Br 0.17 I 0.83 ) FASn(Br 0.25 I 0.75 ) FASn(Br 0.33 I 0.67 ) Figure S2. Tauc plots of FASnI 3 perovskite films with various Br content. 9

10 Table S3. Photovoltaic parameters of FASnI 3 -based perovskite solar cells (PSCs) prepared without and with Br in the absence of SnF 2. J sc (ma/cm 2 ) V oc (V) FF (%) PCE FASnI Br-doped FASnI (%) Figure S3. Heat of formation ( ) of CH(NH 2 ) 2 SnI 3(1-x) Br 3x in neutral charge status are depicted as a function of x. 10

11 Figure S4. Top view SEM images of FASnI 3 perovskite films a) without Br and b) with Br prepared in the absence of SnF 2. Figure S5. Dark current density of the bare FASnI 3 and Br-doped FASnI 3 PSCs. 11

12 Figure S6. Defect formation energy ( ) of Sn vacancy in neutral charge status CH(NH 2 ) 2 SnI 3(1-x) Br 3x are depicted as a function of x. 12

13 Figure S7. Ultraviolet photoelectron spectroscopy (UPS) spectra of the bare FASnI 3 and Br-doped FASnI 3 perovskite films. 13

14 Figure S8. Histograms of a) short circuit current density (J sc ), b) open circuit voltage (V oc ), c) fill factor (FF), and d) power conversion efficiency (PCE) of the optimized bare FASnI 3 and Br-doped FASnI 3 PSCs for 40 devices. 14

15 Figure S9. Photocurrent density voltage (J V) curves of Br-doped FASnI 3 PSC measured by forward (J sc V oc ) and reverse (V oc J sc ) scans. 15

16 Figure S10. Normalized a) J sc, b) V oc, and c) FF of the encapsulated Sn-based PSC under continuous light exposure for 1000 h. 16

17 Figure S11. Atomic% of the Sn and Ti of the Sn-based perovskite film as a function of etching time. 17

18 Figure S12. XPS spectra on the Sn-based perovskite surface with different etching time (from 15 sec to 75 sec). 18

19 Figure S13. XPS spectra of the Ti (2p) bands on the Sn-based perovskite surface with different etching time. Figure S14. Normalized PCE of the encapsulated Sn-based PSCs without Br with the same structure of FTO/bl-TiO 2 /mp-tio 2 /perovskite/spiro-ometad/au under continuous light exposure. 19

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