Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal–halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl–. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.
|Original language||English (US)|
|Journal||Journal of the American Chemical Society|
|State||Published - Apr 28 2023|
Bibliographical noteKAUST Repository Item: Exported on 2023-05-01
Acknowledged KAUST grant number(s): OSR-2018-CARF/CCF-3079, OSR-2019-CRG8-4095
Acknowledgements: This work was partly funded by the Engineering and Physical Sciences Research Council (EPSRC) U.K. through grants (EP/V010840/1, EP/V027131/1, EP/T028513/1, EP/T025077/1, EP/S004947/1, EP/L01551X/1, EP/R029431, EP/T015063/1, EP/R029946/1, EP/P033229/1) and has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 764787 (PH) and no. 861985 (PEROCUBE). Financial support was also gratefully received from the DFG (CH 1672/3-1), King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR-2018-CARF/CCF-3079, OSR-2019-CRG8-4095), the Basque Government (PIBA_2022_1_0031 and EC_2022_1_0011) and the Spanish Government (PID2021-129084OB-I00, RTI2018-101782-B-I00, and RED2022-134344). B.M.G. and S.Z. thank the Rank Prize Fund for their support. D.J.K. acknowledges the support of the University of Warwick. The UK High-Field Solid-State NMR Facility used in this research was funded by EPSRC and BBSRC (EP/T015063/1), as well as, for the 1 GHz instrument, EP/R029946/1. B.K.S. acknowledges University College, Oxford, for the Oxford-Radcliffe scholarship. M.J.G. is grateful for access to the X-ray facilities at the Materials Characterization Laboratory at the ISIS Facility. S.S. and M.R.F. accessed computational resources via membership of the UK’s HEC Materials Chemistry Consortium. S.C. acknowledges the Polymat Foundation for a postdoctoral research contract. J.L.D. acknowledges the Polymat Foundation and Ikerbasque, Basque Foundation for Science, for an “Ikerbasque Research Associate” contract. E.Y.-H.H. thanks Xaar for PhD scholarship sponsorship. The authors thank Seth Marder and Steve Barlow for useful discussions related to this work.
ASJC Scopus subject areas
- Colloid and Surface Chemistry