S-1 Supporting Information
Supporting Information Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement Yuanyuan Zhou,†,* Mengjin Yang,‡ Shuping Pang,¶ Kai Zhu,‡,* Nitin P. Padture†,* †
School of Engineering, Brown University, Providence, RI 02912, USA Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401, USA ¶ Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China ‡
*
Corresponding authors: [email protected]; [email protected]
Raw Materials. All reagent grade chemicals were obtained commercially from Sigma-Aldrich, St. Louis, MO, unless noted otherwise. Lead iodine acid (HPbI3) powders were prepared using an antisolvent vapor-assisted crystallization approach as described in our previous report [ref. S1]. Briefly, 0.461 g of PbI2 and 0.224 g of hydroiodic acid (57 wt% in water, unstabilized, Sigma-Aldrich, USA) were mixed and dissolved in 0.493 g of N,N-dimethylformamide (DMF; 99.8%, Sigma-Aldrich, USA) solvent to form a 50 wt% HPbI3 solution. The as-prepared HPbI3 solution was then heated at 80 ˚C in the chlorobenzene (CBE) vapor environment overnight using an experimental setup similar to that described in the literature (ref. S2). During the heat-treatment, the CBE molecules diffused into the HPbI3 DMF solution, which reduce the solubility of HPbI3. As a result, light yellow needle-like HPbI3 crystals are formed. The as-crystallized HPbI3 solid is then collected and washed, and then dried at 60 ˚C for 10 h under vacuum. Methylamine (CH3NH2) gas was synthesized as follows: 10 g CH3NH4Cl (98%) and 10 g KOH (85%) powders were sequentially dissolved in 100 mL H2O and then heated to 60 ˚C. The resulting gas passes through a CaO dryer to remove any moisture. CH3NH2 (anhydrous, ≥98%) can be also available commercially from Sigma-Aldrich. Raw Film Fabrication. Thin films of MAPbI3 perovskite are fabricated using as-reported methods (one-step spincoating method (ref. S3), two-step dipping method (ref. S4), antisolvent treatment method (ref. S5) and methylamine-gas treatment method (ref. S1)). For the one-step spincoating method, a 40 wt% precursor solution of PbI2 and MAI (molar ratio 1:1) in N,N-dimethylformamide (DMF; 99.8%) was first made and then spin-coated on the substrate at 6000 rpm for 20s. This was followed by thermal annealing at 150 ˚C for 2 min, resulting in MAPbI3 perovskite thin films consisting of one-dimensional structures. For the two-step method, a 40 wt% PbI2 solution in DMF was spin-coated (6000 rpm, 40 s) and the dried. The PbI2 film is sequentially dipped in the 10 mg ml-1 MAI solution in isopropanol for 1 hr and then washed and dried at 150 ˚C for 2 min. The obtained MAPbI3 perovskite thin film consists of three dimensional cuboids. For the antisolvent treatment method, first, a solution of PbI2 and MAI (molar ratio 1:1) in the polar solvent (N-Methyl-2-pyrrolidone/γ-Butyrolactone, 7:3 (v/v)) was spin-coated onto the substrate, and the solution coated substrate was vertically dipped in an ~50 ml anhydrous DEE bath. The film was kept immersed until a brown film formed on the substrate after 2 min. The film was then taken out and
S-1
dried at 100 ˚C for 2 min. The as-fabricated film exhibit the typical polycrystalline nature with compact grains. For methylamine gas treatment methods, a thin film of MAPbI3 (or HPbI3) was first fabricated via the one-step spincoating method using its 40 wt% DMF solution. The as-formed film was then placed in the CH3NH2 gas environment for 2-3 s at room temperature and was then removed to the ambient quickly. The resultant MAPbI3 thin film shows ultra-smooth morphology with textured characteristics. Morphology-Preserved Perovskite Conversion. To convert to the FAPbI3 perovskite, the as-prepared MAPbI3 perovskite thin films via the above typical methods were heated at 150 ˚C for 1 to 4 min in an atmosphere of H2N-CH=NH (formylimidamide or FA) gas. The FA gas was generated by reacting HC(NH2)2CH3COOH salt (formamidine acetate or FA(Ac)) with NaOH. The resulting gas passed through a CaO dryer and reacted with MAPbI3, with forming FAPbI3 perovskite thin films that mimic the MAPbI3 morphologies. The simple experimental setup is shown schematically in Figure S2. Materials Characterization. X-ray diffraction (XRD) patterns were obtained using an X-ray diffractormeter (D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.5406 Å); 0.02˚ step . UV-vis absorption spectra of the thin films were recorded using spectrometer (U-4100, Hitachi, Japan). Surface and cross-sections (fractured) morphology of the perovskite solar cells (PSCs) were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Germany). Device Fabrication and Characterization. For the fabrication of the PSCs, a compact TiO2 electron-transporting layer (ETL) was first deposited on pre-patterned FTO-coated glass (TEC15, Hartford Glass Co., Hartford City, IN) by spray pyrolysis at 450 ˚C. Mesoporous TiO2 layer was spin-coated at 2000 rpm for 35 s from TiO2 paste, which consists of 5.4 % TiO2 nanoparticles and 1.6 % ethyl cellulose in terpineol/ethanol (3/7 weight ratio) solution. The mesoporous layer was sintered at 450 ˚C for 30 min. 60 wt% HPbI3 DMF solution is then spin-coated at 6000 rpm for 20 s to form an HPbI3 thin film, which is followed by heat-treatment at 150 ˚C for 30 s. Once cooling down to the room temperature, the HPbI3 thin film is exposed to CH3NH2 gas for 2 s, and rapidly degassed by removing the gas atmosphere, resulting in a black MAPbI3 film. The film is finally heated at 150 ˚C for 2 min. Then the morphology-preserved perovskite conversion process was conducted which was followed by spin-coating a hole-transporting material (HTM) solution, which consisted of 80 mg 2,2’,7,7’-tetrakis(N,N-dip-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD), 30 µl bis(trifluoromethane) sulfonimide lithium salt stock solution (500 mg Li-TFSI in 1 ml acetonitrile), and 30 µl 4-tert-butylpyridine (TBP), and 1 ml chlorobenzene solvent. The HTM spin-coating process was performed in a dry-air atmosphere with humidity
School of Engineering, Brown University, Providence, RI 02912, USA Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401, USA ¶ Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China ‡
*
Corresponding authors: [email protected]; [email protected]
Raw Materials. All reagent grade chemicals were obtained commercially from Sigma-Aldrich, St. Louis, MO, unless noted otherwise. Lead iodine acid (HPbI3) powders were prepared using an antisolvent vapor-assisted crystallization approach as described in our previous report [ref. S1]. Briefly, 0.461 g of PbI2 and 0.224 g of hydroiodic acid (57 wt% in water, unstabilized, Sigma-Aldrich, USA) were mixed and dissolved in 0.493 g of N,N-dimethylformamide (DMF; 99.8%, Sigma-Aldrich, USA) solvent to form a 50 wt% HPbI3 solution. The as-prepared HPbI3 solution was then heated at 80 ˚C in the chlorobenzene (CBE) vapor environment overnight using an experimental setup similar to that described in the literature (ref. S2). During the heat-treatment, the CBE molecules diffused into the HPbI3 DMF solution, which reduce the solubility of HPbI3. As a result, light yellow needle-like HPbI3 crystals are formed. The as-crystallized HPbI3 solid is then collected and washed, and then dried at 60 ˚C for 10 h under vacuum. Methylamine (CH3NH2) gas was synthesized as follows: 10 g CH3NH4Cl (98%) and 10 g KOH (85%) powders were sequentially dissolved in 100 mL H2O and then heated to 60 ˚C. The resulting gas passes through a CaO dryer to remove any moisture. CH3NH2 (anhydrous, ≥98%) can be also available commercially from Sigma-Aldrich. Raw Film Fabrication. Thin films of MAPbI3 perovskite are fabricated using as-reported methods (one-step spincoating method (ref. S3), two-step dipping method (ref. S4), antisolvent treatment method (ref. S5) and methylamine-gas treatment method (ref. S1)). For the one-step spincoating method, a 40 wt% precursor solution of PbI2 and MAI (molar ratio 1:1) in N,N-dimethylformamide (DMF; 99.8%) was first made and then spin-coated on the substrate at 6000 rpm for 20s. This was followed by thermal annealing at 150 ˚C for 2 min, resulting in MAPbI3 perovskite thin films consisting of one-dimensional structures. For the two-step method, a 40 wt% PbI2 solution in DMF was spin-coated (6000 rpm, 40 s) and the dried. The PbI2 film is sequentially dipped in the 10 mg ml-1 MAI solution in isopropanol for 1 hr and then washed and dried at 150 ˚C for 2 min. The obtained MAPbI3 perovskite thin film consists of three dimensional cuboids. For the antisolvent treatment method, first, a solution of PbI2 and MAI (molar ratio 1:1) in the polar solvent (N-Methyl-2-pyrrolidone/γ-Butyrolactone, 7:3 (v/v)) was spin-coated onto the substrate, and the solution coated substrate was vertically dipped in an ~50 ml anhydrous DEE bath. The film was kept immersed until a brown film formed on the substrate after 2 min. The film was then taken out and
S-1
dried at 100 ˚C for 2 min. The as-fabricated film exhibit the typical polycrystalline nature with compact grains. For methylamine gas treatment methods, a thin film of MAPbI3 (or HPbI3) was first fabricated via the one-step spincoating method using its 40 wt% DMF solution. The as-formed film was then placed in the CH3NH2 gas environment for 2-3 s at room temperature and was then removed to the ambient quickly. The resultant MAPbI3 thin film shows ultra-smooth morphology with textured characteristics. Morphology-Preserved Perovskite Conversion. To convert to the FAPbI3 perovskite, the as-prepared MAPbI3 perovskite thin films via the above typical methods were heated at 150 ˚C for 1 to 4 min in an atmosphere of H2N-CH=NH (formylimidamide or FA) gas. The FA gas was generated by reacting HC(NH2)2CH3COOH salt (formamidine acetate or FA(Ac)) with NaOH. The resulting gas passed through a CaO dryer and reacted with MAPbI3, with forming FAPbI3 perovskite thin films that mimic the MAPbI3 morphologies. The simple experimental setup is shown schematically in Figure S2. Materials Characterization. X-ray diffraction (XRD) patterns were obtained using an X-ray diffractormeter (D8 Advance, Bruker, Germany) with Cu Kα radiation (λ=1.5406 Å); 0.02˚ step . UV-vis absorption spectra of the thin films were recorded using spectrometer (U-4100, Hitachi, Japan). Surface and cross-sections (fractured) morphology of the perovskite solar cells (PSCs) were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Germany). Device Fabrication and Characterization. For the fabrication of the PSCs, a compact TiO2 electron-transporting layer (ETL) was first deposited on pre-patterned FTO-coated glass (TEC15, Hartford Glass Co., Hartford City, IN) by spray pyrolysis at 450 ˚C. Mesoporous TiO2 layer was spin-coated at 2000 rpm for 35 s from TiO2 paste, which consists of 5.4 % TiO2 nanoparticles and 1.6 % ethyl cellulose in terpineol/ethanol (3/7 weight ratio) solution. The mesoporous layer was sintered at 450 ˚C for 30 min. 60 wt% HPbI3 DMF solution is then spin-coated at 6000 rpm for 20 s to form an HPbI3 thin film, which is followed by heat-treatment at 150 ˚C for 30 s. Once cooling down to the room temperature, the HPbI3 thin film is exposed to CH3NH2 gas for 2 s, and rapidly degassed by removing the gas atmosphere, resulting in a black MAPbI3 film. The film is finally heated at 150 ˚C for 2 min. Then the morphology-preserved perovskite conversion process was conducted which was followed by spin-coating a hole-transporting material (HTM) solution, which consisted of 80 mg 2,2’,7,7’-tetrakis(N,N-dip-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD), 30 µl bis(trifluoromethane) sulfonimide lithium salt stock solution (500 mg Li-TFSI in 1 ml acetonitrile), and 30 µl 4-tert-butylpyridine (TBP), and 1 ml chlorobenzene solvent. The HTM spin-coating process was performed in a dry-air atmosphere with humidity
