Journal
ADVANCED MATERIALS
Volume 34, Issue 9, Pages -Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adma.202107850
Keywords
additive-free; aerosol-assisted crystallization; formamidinium lead triiodide; stability; strain
Categories
Funding
- EPSRC Plastic Electronics CDT [EP/L016702/1]
- QMUL-EPSRC Impact Accelerator Account
- Stephen and Anna Hui Scholarship (Imperial College London)
- Royal Commission for the Exhibition of 1851
- US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division [DE-AC02-05-CH11231]
- National Research Foundation of Korea (NRF) - Ministry of Science and ICT [NRF-2017K1A1A2013153]
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Aerosol-assisted crystallization (AAC) method is used to convert yellow delta-FAPbI(3) into black alpha-FAPbI(3) at a lower temperature, resulting in significantly improved stability, crystallinity, and photoluminescence yield. Relaxation of residual tensile strains during AAC is identified as the key factor for the formation of phase-stable alpha-FAPbI(3). Pure FAPbI(3) p-i-n solar cells fabricated using low-temperature AAC processing demonstrate higher power conversion efficiency and operational stability compared to those fabricated using high-temperature annealed films.
Formamidinium lead triiodide (FAPbI(3)) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure alpha-FAPbI(3) conventionally requires high-temperature thermal annealing at 150 degrees C whilst the obtained alpha-FAPbI(3) is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow delta-FAPbI(3) into black alpha-FAPbI(3) at only 100 degrees C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained alpha-FAPbI(3) exhibits remarkably enhanced stability compared to the 150 degrees C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable alpha-FAPbI(3). This overcomes the strain-induced lattice expansion that is known to cause the metastability of alpha-FAPbI(3). Accordingly, pure FAPbI(3) p-i-n solar cells are reported, facilitated by the low-temperature (<= 100 degrees C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 degrees C annealed films.
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