Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP27)
Publication date: 22nd October 2026
Formamidinium lead iodide (FAPbI3) with a cubic perovskite structure is the key light absorber in high-efficiency perovskite solar cells. In contrast, FA-rich lead iodides such as FA2PbI4 have been reported only as thin films or impurity phases, and their structures were assigned to the Ruddlesden–Popper (RP) type without reliable characterization due to the limitations of polycrystalline films.
We have previously shown that ordered columnar defects can be introduced into FAPbI3 using thiocyanate (SCN−), leading to the homologous series FAn+1Pbn−1I3n−1.5(SCN)0.5 [1–3]. In this talk, I will focus on isolating two new FA-rich perovskite-derived phases, FA2PbI4 and FA3PbI5, in bulk form [4].
Using solid-state synthesis and synchrotron X-ray diffraction, we found that FA2PbI4 does not adopt the RP-type structure with (100)p-oriented layers. Instead, it forms a (110)p-oriented layered perovskite with an uncommon eclipsed stacking of corner-sharing PbI6 layers and minimal octahedral distortion. FA3PbI5 forms one-dimensional corner-sharing octahedral chains that preserve the cubic perovskite geometry, unlike the face-sharing network of δ-FAPbI3. Both structures can be described as defect-ordered perovskites, in which one-third (FA2PbI4) or one-half (FA3PbI5) of the [PbI]+ units are removed in an ordered columnar manner and the vacant sites are occupied by FA+ cations. Together with FAPbI3, these compounds form the homologous series FAn+1Pbn−1I3n−1 (n = ∞, 3, 2), representing a dimensional reduction from 3D to 2D to 1D.
This dimensional reduction is directly reflected in the optical properties: the band gap increases from ~1.5 eV (FAPbI3) to ~2.1 eV (FA2PbI4) and ~2.4 eV (FA3PbI5), and FA2PbI4 and FA3PbI5 show red (~650 nm) and green (~561 nm) photoluminescence, respectively. Moreover, both phases remain stable in air for several days, whereas FAPbI3 powder rapidly transforms into the δ-phase. These results provide a new approach to controlling structural dimensionality using FA as both the A-site and interlayer cation.
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