Additive-free, Low-temperature Crystallization of Stable α-FAPbI3 Perovskite
Tian Du a
a Friedrich-Alexander-Universität Erlangen-Nürnberg, Institute Materials for Electronics and Energy Technology, Department of Materials Science and Engineering, Energy Campus Nürnberg, Fürther Straße, 250, Nürnberg, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Tian Du, presentation 074
Publication date: 20th April 2022

Formamidinium lead triiodide (FAPbI3) has emerged as a promising candidate for efficient and stable perovskite solar cells (PSC) among the family of metal halide perovskites (MHPs), due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium based perovskites. Crystallization of phase-pure α-FAPbI3 conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI3 is metastable at room temperature. Here we show an aerosol assisted crystallization (AAC), reliant on the interaction of vaporized Lewis base solvents with the FAPbI3 films, to convert yellow δ-FAPbI3 into black α-FAPbI3 at only 100 °C using precursor solutions containing only lead iodide (PbI2) and formamidinium iodide (FAI) with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvement in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering and density functional theory simulation, we identify 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 α-FAPbI3. This overcomes strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3. Accordingly, we show the pure FAPbI3 p-i-n solar cells, facilitated by the low-temperature (≤ 100 °C) AAC processing. They demonstrate increases in both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

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