Inhibition of Amine–Water Proton Exchange Stabilizes Perovskite Ink for Scalable Solar Cell Fabrication

Abstract

Ambient air processing is desirable for the industrial fabrication of perovskite solar cells. Here, we show that perovskite ink containing methylammonium and formamidinium in Nmethyl-2-pyrrolidone and N,N-dimethylformamide, a cosolvent composition that satisfies prerequisites for upscaling solar cell fabrication, degrades within a day in ambient air. From 1 H NMR spectroscopic analysis, we find that water proton exchange with methylammonium and formamidinium facilitates the aminolysis of formamidinium by methylamine. The addition of elemental sulfur inhibits this proton exchange process via sulfur−amine reactions, resulting in a stable perovskite ink with an extrapolated half-life of 6300 h. The control ink aged for 1 day does not form perovskite films for solar cell fabrication, while the sulfur-stabilized ink is reproducibly used to make devices with efficiencies >15% when aged for over 1 month. The stabilized ink is suitable for upscaling perovskite solar cell fabrication, with efficiencies up to 17% for blade-coated devices.

PbI2 Reagent Impurities Catalyze Mixed-Cation Halide Perovskite Ink Degradation

“…The presence of PbO (which does not dissolve well) caused nearly complete degradation (96%), which is also consistent with the fact that PbO can react with 3 equivalents of MAI to make MAPbI 3 and H 2 O, releasing 2 equivalents of methyl­amine. It is most notable that water has such a small effect on the MFA reaction, given a recent report, yet logical since DMSO more readily accepts a proton than H 2 O in these aprotic solvent environments. − …”

“…5 The presence of PbO (which does not dissolve well) caused nearly complete degradation (96%), which is also consistent with the fact that PbO can react with 3 equivalents of MAI to make MAPbI 3 and H 2 O, releasing 2 equivalents of methylamine. It is most notable that water has such a small effect on the MFA reaction, given a recent report, 6 yet logical since DMSO more readily accepts a proton than H 2 O in these aprotic solvent environments. 7−9 Overall, these results unambiguously indicate the sensitivity of mixed MA:FA perovskite inks' stability to specif ic impurities in PbI 2 reagents, namely OAc − , PbO, and likely PbIOH, while other impurities, like H 2 O and NO 3 − , are relatively benign to this particular degradation pathway (H 2 O may affect other pathways, though, such as hydrolysis reactions, that then cause a cascade of side reactions).…”

Solution Aging Promotes the Formation of Hexagonal Polytypes in Mixed-Cation/-Halide Perovskites

“…The physical properties of perovskite thin films may differ considerably depending on the deposition method and precursor preparation conditions, therefore significantly altering the experimental data. ,, Perovskite precursor ink concentration, temperature, stoichiometry, and aging time are all factors that may impact the chemical–physical characteristics of perovskite films . It was recently observed, for example, that the aging time of the precursor solution before its deposition on a substrate is a critical parameter that has a considerable impact on overall thin-film formation, morphology, and crystallinity, as well as solar cell performance. − In general, device reproducibility issues stem from nonuniform film coverage, , defect densities, ion or vacancy migration, , and, in mixed cation/halide, the coexistence of different polymorphs . The complex precursor solution chemistry, characterized by a labile equilibrium of ionic species and aggregation/complexation phenomena, has a huge implication for perovskite’s nucleation and crystallization process, determining the resulting film characteristics. , …”

Dissolution Modulation of Formamidinium‐Based Perovskite for Regulated Crystallinity in Printable Mesoscopic Solar Cells

“…[5][6][7] HPs are generally prepared via facile solutionprocessable methods, thus the precursor solution is the central focus. [8][9][10][11] Taking full advantage of HPs' excellent electronic and optoelectronic properties for high device performance depends on wellcontrolled crystal quality. [12][13][14] Deep insight into the precursor solution of HPs by evaluating their dissolution behaviors benefits the precise crystallization modulation for HPs.HPs are compounds with the formula of ABX 3 , where A is the monovalent cation such as formamidinium/methylammonium/cesium (FA þ /MA þ /Cs þ ), B is the divalent cation such as lead (II)/tin (II) (Pb 2þ /Sn 2þ ), and X is the halide ion such as iodide/bromide/chloride (I À /Br À /Cl À ).…”

“…[5][6][7] HPs are generally prepared via facile solutionprocessable methods, thus the precursor solution is the central focus. [8][9][10][11] Taking full advantage of HPs' excellent electronic and optoelectronic properties for high device performance depends on wellcontrolled crystal quality. [12][13][14] Deep insight into the precursor solution of HPs by evaluating their dissolution behaviors benefits the precise crystallization modulation for HPs.…”