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C: Physical Processes in Nanomaterials and Nanostructures 3
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Bismuth Enhances the Stability of CHNHPbI (MAPI) Perovskite Under High Humidity
Timothy D. Siegler, Daniel W. Houck, Shin Hum Cho, Delia J Milliron, and Brian A. Korgel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11674 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018
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The Journal of Physical Chemistry
Bismuth Enhances the Stability of CH3NH3PbI3 (MAPI) Perovskite Under High Humidity
Timothy D. Siegler, Daniel W. Houck, Shin Hum Cho, Delia J. Milliron, Brian A. Korgel*
McKetta Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712-1062
*corresponding author:
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ABSTRACT Methylammonium lead iodide (CH3NH3PbI3, MAPI) is a high performance solar cell material, but lacks stability in the presence of humidity. Addition of a few percent bismuth (Bi) as a trivalent cation substitute for Pb (i.e., MAP(Bi)I) leads to enhanced stability under high (90%) relative humidity (RH). At moderate humidity (60% RH), however, MAP(Bi)I degrades more rapidly than MAPI.
Bi incorporation into MAPI either stabilizes or destabilizes the two
hydration products, MAPI.H2O and PbI2, depending on the amount of humidity in the environment.
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The Journal of Physical Chemistry
INTRODUCTION Methylammonium lead iodide (CH3NH3PbI3, MAPI) is a hybrid organic-inorganic perovskite (HOIP) and a promising semiconductor for photovoltaic devices (PVs),1,2 light-emitting diodes (LEDs),3–5 lasers,3,4 photocatalytic systems,6 and gamma ray detectors.7 However, MAPI is highly susceptible to degradation under humidity,8,9 with the reversible formation of monohydrate and dihydrate species and a final irreversible degradation into PbI2 via the following reaction pathway:8,10–14 CH3 NH3 PbI3 (s) + H2 O(g) ⇌ CH3 NH3 PbI3 ∙ H2 O(s)
(1)
4CH3 NH3 PbI3 ∙ H2 O(s) ⇌ (CH3 NH3 )4 PbI6 ∙ 2H2 O(s) + 3PbI2 (s) + 2H2 O(g)
(2)
(CH3 NH3 )4 PbI6 ∙ 2H2 O(s) ⟶ 4CH3 NH2 (g) + 4HI(g) + PbI2 (s) + 2H2 O(g)
(3)
The resulting degradation product of PbI2 is the only remaining solid material. It is certainly possible to stabilize MAPI up to several months by encapsulation in hydrophobic fluoropolymers or glass;15,16 yet, higher intrinsic stability would greatly help reach commercial viability.17 The intrinsic stability of MAPI can be improved to some extent by replacing MA+, Pb2+, or Iwith other chemical species, either by bringing the Goldschmidt tolerance factor closer to one,18,19 improving the film morphology,20–22 or strengthening hydrogen bonding between organic A site components and the halide moiety.10,23–25 For example, MA+ has been replaced with formamidinium (FA+)26 or cesium (Cs+),18,27,28 and I- has been replaced with Br-,22 Cl-,20 thiocyanate,29,30, or selenide,31 for enhanced stability. There have also been numerous reports detailing the substitution of Pb2+ with other species, but most of these do not mention the resulting stability of the materials. The addition of In3+ has been shown to stabilize CH3NH3PbI3xClx
PVs stored on a benchtop,32 and the addition of Sr2+ to CsPbI2Br significantly improved the
stability of encapsulated PVs stored at 99.0%, BHT inhibitor, Sigma), anhydrous dimethyl sulfoxide ((CH3)2SO, anhydrous 99.9%, Sigma), anhydrous dimethylformamide (HCON(CH3)2, 99.8% Sigma), ethanol (CH3CH2OH, 200 proof, Fisher Scientific), glycerol (HOCH2CH(OH)CH2OH, 98%, Fisher Scientific), and (3-aminopropyl)triethyoxysilane (H2N(CH2)3Si(OC2H5)3, >98%, Sigma) were all used as received. Preparation of MAPI and MAP(Bi)I. Glass substrates (1”x1”, Cardinal Glass) were sonicated in ethanol for 30 min and immersed in (3-aminopropyl)triethoxysilane (APTES) in ethanol (1:9 APTES:ethanol vol ratio) for 5 min. A drop of water was added to the APTES solution to promote APTES grafting. The substrates were rinsed with IPA and heated at 100⁰C in ambient air for 10 min. Smooth perovskite films were deposited on glass substrates using a modification of the procedure developed by Park, et. al.42 A MAPI precursor solution was dissolved by adding 273 mg PbI2, 95 mg MAI, and 120 mL DMSO to 480 mL DMF and stirring at room temperature for 2 hours. The resultant solution had a 1:1 molar ratio of PbI2:MAI. Likewise, 356 mg BiI3, 95 mg MAI, and 120 mL DMSO was dissolved in 480 mL DMF in a separate vial, forming a 1:1 molar ratio BiI3:MAI. After 2 hours of stirring, in order to generate the MAP(Bi)I precursor solution, the lead based solution was mixed with the bismuth based solution in a 19:1 ratio by volume (5 mol% Bi on a metals basis), forming a solution with a 0.95:0.05:1 PbI2:BiI3:MAI mole ratio.
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Spin coating was performed on glass slides coated with APTES in a nitrogen glove box with O 2
