4.8 Article

Manipulating the Mixed-Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS

Journal

ADVANCED MATERIALS
Volume 31, Issue 25, Pages -

Publisher

WILEY-V C H VERLAG GMBH
DOI: 10.1002/adma.201901284

Keywords

annealing window; crystallization pathways; Cs doping; in situ GIWAXS; perovskite solar cells

Funding

  1. Research Grant Council of Hong Kong (RGC) [14314216]
  2. CUHK Direct Grant [4441636]

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Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3)) during spin-coating is revealed by in situ grazing-incidence wide-angle X-ray scattering measurements, and three phase-formation stages are identified: I) precursor solution; II) hexagonal delta-phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI-PbI2-DMSO intermediate phases, and perovskite alpha-phase. The correlated device performance and ex situ characterizations suggest the existence of an annealing window covering the duration of stage II. The spin-coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs+ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs+ will bypass the formation of secondary phases in stage III by promoting the formation of alpha-phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed-perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.

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