Publication date: 28th August 2024
Hybrid perovskite materials, such as formamidinium lead iodide (FAPbI3) are one of the most promising photovoltaic materials due to their near-ideal bandgap (1.48 eV). However, the spontaneous phase transition from the photoactive perovskite phase (α-FAPbI3) to an inactive non-perovskite phase (δ-FAPbI3) complicates the application of FAPbI3 in perovskite solar cells. To solve this issue, cations such as Cs+, Rb+ and K+ are often used in the perovskite synthesis. It has been shown by solid-state NMR spectroscopy that Rb+ cannot dope into the perovskite lattice in double halide systems, but instead forms an additional non-perovskite phase. To date, little is known about this non-perovskite phase. Herein, we used solid-state NMR spectroscopy to show that FA+ substitutes into the Rb-based non-perovskite phases to form FAyRb1-yPb2Br5 and FAyRb1-yPbI3 for bromide and iodide perovskites, respectively. This is shown by changes in the 1H and 87Rb chemical shifts, the 1H–87Rb heteronuclear correlation (HETCOR) spectra, and complete dephasing in the 87Rb{1H} rotational echo double resonance (REDOR) spectra. Moreover, we simulate the REDOR dephasing curves to estimate the amount of FA+ substituted into the non-perovskite phase, finding up to ~60% FA+ incorporation for the bromide and ~15% for the iodide systems. Finally, photoluminescence (PL) measurements showed an increased signal intensity upon substitution of FA+ into non-perovskite phase, which implies improved crystallinity and lower defect density. We hypothesize that the substitution of FA+ into the Rb-based non-perovskite phase can modulate the interaction with the perovskite phase, which may be the reason for the observed enhancement in power conversion efficiency and the stability of the PSCs with Rb+.
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