Efforts to stabilize photoactive formamidinium (FA)–based halide perovskites for perovskite photovoltaics have focused on the growth of cubic formamidinium lead iodide (a-FAPbI3) phases by empirically alloying with cesium, methylammonium (MA) cations, or both. We show that such stabilized FA-rich perovskites are noncubic and exhibit ~2° octahedral tilting at room temperature. This tilting, resolvable only with the use of local nanostructure characterization techniques, imparts phase stability by frustrating transitions from photoactive to hexagonal phases. Although the bulk phase appears stable when examined macroscopically, heterogeneous cation distributions allow microscopically unstable regions to form; we found that these transitioned to hexagonal polytypes, leading to local trap-assisted performance losses and photoinstabilities. Using surface-bound ethylenediaminetetraacetic acid, we engineered an octahedral tilt into pure a-FAPbI3 thin films without any cation alloying. The templated photoactive FAPbI3 film was extremely stable against thermal, environmental, and light stressors.
Bibliographical noteFunding Information:
We acknowledge J. Parker and P. Quinn for support during experiments on the I14 beamline at Diamond Light Source. We thank Diamond Light Source for access and support in use of beamline I14 (proposal number sp20420) and the electron Physical Science Imaging Centre (ePSIC; Instrument E02 and proposal number MG24111) that contributed to the results presented here. Via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by the Engineering and Physical Sciences Research Council (EPSRC; EP/L000202), this work used the ARCHER UK National Supercomputing Service (www.archer.ac.uk). Supported by a National University of Ireland Travelling Studentship (T.A.S.D.); a Newton International Fellowship funded by the Royal Society and SERB (S.N.); the Royal Society and Tata Group (UF150033) (S.D.S.); EPSRC grant EP/ R008779/1 (P.A.M.); the European Research Council under the European Union’s Horizon 2020 research and innovation program (HYPERION grant agreement 756962); EPSRC grant EP/R023980/ 1; a George and Lilian Schiff Studentship, Winton Studentship, EPSRC studentship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship (K.F.); Marie Skłodowska-Curie actions (grant agreements 841136, 841265, and 841386, respectively) under the European Union’s Horizon 2020 research and innovation program (D.J.K., E.M.T., and M.A.); scholarships from the British Spanish Society and the Sir Richard Stapley Educational Trust (A.N.I.); an EPSRC studentship (S.M.); the Royal Society (A.A.); a University of Leeds academic fellowship (S.M.C.); an EPSRC studentship (P.C.); EU ERC Advanced Fellowship DLV-835073 (C.P.G.); National Research Foundation of Korea grant 2018R1C1B6008728; and the European Union's Horizon 2020 INFRAIA program (ESTEEM3–grant agreement 823717).
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