Formamidinium Tin Triiodide (FASnI3)
- FASnI3 is a lead-free tin halide perovskite featuring a 1.2–1.4 eV band gap, strong red and near-IR absorption, and structural flexibility across 3D and 2D motifs.
- It exhibits polymorphism with nearly degenerate 3D perovskite and 2D layered structures, where local FA cation disorder and Sn off-centering crucially influence band edges and electronic behavior.
- Device studies demonstrate its promise in flexible and tandem solar cells while highlighting challenges such as Sn oxidation, Sn-vacancy induced p-type self-doping, and interface engineering.
Formamidinium tin triiodide (FASnI; FA = CH(NH) or NHCHNH) is a lead-free ABX tin halide perovskite in which formamidinium occupies the A site, Sn the B site, and I the X site. In the photoactive -phase it adopts the ABX perovskite topology, and around room temperature its experimentally relevant structure is a dynamically disordered, nearly cubic perovskite with average symmetry close to 0, while local distortions remain strong. FASnI1 is intensively studied because it combines a near-optimal photovoltaic band gap with strong red and near-IR absorption and compatibility with lead-free optoelectronics; its defining limitations are Sn23Sn4 oxidation, Sn-vacancy-driven p-type self-doping, and a highly nontrivial coupling between local structure, band edges, and interfaces (Venkatanarayanan et al., 26 Nov 2025, Park et al., 7 Oct 2025, Aldamasy et al., 2023).
1. Structural motifs and crystallographic descriptions
FASnI5 supports more than one crystallographic description, and the choice of model depends strongly on the property under study. For the photoactive 6-phase, large-supercell first-principles work describes a nearly cubic average lattice with strong local distortion driven by the stereochemically active Sn(II) 7 lone pair. In that description, a 8 supercell with randomly oriented FA9 cations is the smallest model that removes macroscopic dipoles, preserves cubic symmetry, recovers local octahedral tilts, and captures the characteristic pseudo–Jahn–Teller-driven Sn off-centering; reproducing the experimental 300 K band gap requires a 0 supercell (Venkatanarayanan et al., 26 Nov 2025).
A second structural theme is polymorphism between a conventional 3D perovskite motif and a 2D layered motif. First-principles structure prediction found that HC(NH1)2SnI3 can adopt both a 3D corner-sharing SnI4 network and a 2D layered structure with alternating SnI5 octahedral layers and FA-cation layers. The two motifs are nearly degenerate: the reported 6 ranges from 7 to 8 meV/atom depending on functional, and the 3D92D barrier is 0–1 meV/atom. Phonon calculations showed both motifs to be dynamically stable, while free-energy analysis found the 3D phase thermodynamically favored over the 2D phase across the studied temperature range, but with only a small free-energy difference (Huan et al., 2015).
Static structural models used in other contexts are more specific. In one pseudo-cubic relaxation derived from experimental cubic FAPbI2, FASnI3 relaxed to 4 Å, 5 Å, and 6 Å, with 7, 8, and 9; the deviation from ideal cubic geometry was attributed largely to FA0 orientation. In surface-defect calculations, by contrast, the tetragonal phase was used with optimized bulk lattice parameters 1 Å and 2 Å. Experimentally, solution-processed films in one additive-engineering study showed a main diffraction peak around 3 assigned to the (100) plane of orthorhombic FASnI4, and no peak shift upon GeI5 addition, indicating retention of the FASnI6 crystal structure rather than bulk Sn–Ge alloying (Guo et al., 2019, Yan et al., 2022, Lai et al., 2024).
2. Electronic structure and energetic landscape
The electronic structure of FASnI7 is controlled by the Sn–I framework. In the 3D phase, the valence-band maximum is dominated by antibonding combinations of Sn 8 and I 9 orbitals, while the conduction-band minimum is largely Sn 0 in character; the 3D band structure shows dispersive, high-curvature parabolic bands around the band edges. In the 2D layered motif, broken out-of-plane connectivity produces many flat electronic bands near both valence- and conduction-band edges, lifts the cubic-like CBM degeneracy, and strongly widens the gap. At the HSE06 level, the reported gap is 1 eV for 3D FASnI2 and 3 eV for the 2D layered phase; the 3D electronic structure is described as rather similar to that of MASnI4 (Huan et al., 2015).
For the room-temperature 5-phase, the main methodological conclusion is that band edges cannot be detached from structural sampling. Reliable band edges and band gaps require a PBE0-level hybrid functional with spin-orbit coupling together with nonlocal dispersion (rVV10), because Sn relativistic effects and enhanced Sn–I covalency both matter; finite-temperature simulations further show that Sn off-centering remains local, 6-oriented, and robust against thermal fluctuations (Venkatanarayanan et al., 26 Nov 2025).
Thin-film experiments sharpen this picture. Additive-free, solvent-free co-evaporation of SnI7 and FAI yielded highly crystalline FASnI8 with an optical band gap 9 eV, a sharp absorption onset, a steep Urbach tail, and photoluminescence near 0 nm with a narrow FWHM of 1 nm. The same study reported that typical spin-coated “state-of-the-art” FASnI2 films often show 3 eV together with broader, blue-shifted PL and a more pronounced Moss–Burstein effect, linking the wider apparent gap to strain, disorder, and heavy p-type doping (Park et al., 7 Oct 2025).
Band-edge metrology by Kelvin probe and photoelectron yield spectroscopy established an energetic landscape that differs qualitatively from the one usually assumed for Pb perovskites. For FASnI4 containing 10% SnF5, the Fermi level lies only 6–7 eV above the valence-band maximum, and Hall, PL/TRPL, and KP–PYS converge to a hole concentration of 8 cm9. In the same framework, the PEDOT/FASnI0 valence-band offset is about 1 eV, while the FASnI2/C3 conduction-band offset is negative, quantified as about 4 eV from KP–PYS and as 5 eV in drift-diffusion simulations. This is the basis for describing the energetic system of tin perovskites as “deformed” and in need of CTLs designed specifically for FASnI6 rather than inherited from Pb-perovskite stacks (Aldamasy et al., 2023).
3. Elasticity, ductility, and anisotropy
FASnI7 is mechanically soft even within the already compliant class of hybrid halide perovskites. In the pseudo-cubic cell, the independent elastic constants are 8, 9, 0, 1, 2, 3, 4, 5, and 6 GPa. These values satisfy the elastic stability conditions, so the structure is mechanically stable at 0 K within the DFT treatment. The tensor is strongly anisotropic: 7, 8 is significantly larger than 9 and 0, and 1, which correlates with the FA-induced distortions along the 2 and 3 directions (Guo et al., 2019).
Polycrystalline Voigt–Reuss–Hill averages confirm the softness of the material. For FASnI4, the Hill bulk modulus is 5 GPa, the shear modulus is 6 GPa, and the Young’s modulus is 7 GPa. The Pugh ratio is 8, the Poisson ratio is 9, and the universal anisotropy index is 00. Within the criteria used in that study, both 01 and 02 indicate ductility, so FASnI03 is ductile in the Pugh sense. The authors further note that FASnI04 is softer than FASnBr05 and slightly softer than FAPbI06, while also being the least anisotropic of the four FABX07 systems considered.
The microscopic origin of this mechanical response is twofold. First, the planar FA08 cation distorts the inorganic framework and weakens the elastic response; the paper states that “the Young's moduli of FA-based perovskites are lower than that of MA-based perovskites, which is due to larger FA09 weakening the inorganic framework.” Second, projected crystal orbital Hamilton population analysis gives an averaged 10 eV, weaker than Pb–I 11 eV) and weaker than Sn–Br 12 eV). The combination of weaker Sn–I bonding and FA-induced geometric distortion accounts for the low stiffness, easy shear, and directional dependence of FASnI13 (Guo et al., 2019).
4. Defects, oxidation, and passivation chemistry
The central defect-chemistry problem in FASnI14 is the coupling between Sn vacancies and Sn oxidation. In a surface-defect analysis, 15 is identified as the dominant defect in both Sn-rich and I-rich conditions, and the degradation pathway is summarized by
16
On the Sn-deficient FASnI17 (301) surface, removing one surface Sn atom leaves five dangling I atoms, strongly distorts the local octahedral network, shortens nearby Sn–I bonds from 18 Å to 19–20 Å, and introduces deep defective states in the band gap. These states are localized mainly on a neighboring subsurface Sn atom rather than on the dangling I atoms, making them efficient nonradiative recombination centers and a microscopic origin of p-type self-doping (Yan et al., 2022).
Lewis-base passivation was proposed as a chemically guided route to heal such defective surfaces. In that framework, the defective FASnI21 surface is treated as having a “somehow hard Lewis acid nature,” and passivation performance is correlated with molecular hardness 22. The hard bases edamine 23 and Isatin-Cl 24 donate electrons to FASnI25, with 26 for edamine and 27 for Isatin-Cl; by contrast, p-TA 28 and EDT 29 show inverse charge transfer, 30 and 31, respectively. The dual effect emphasized in that work is charge redistribution of the 32-related defect state together with saturation of dangling states and reduction of deep gap states (Yan et al., 2022).
Processing studies show that the same defect chemistry can be altered at film level. In co-evaporated FASnI33, XPS gives a dominant Sn 34 peak at 35 eV assigned to Sn36, only a minor higher-binding-energy component at 37 eV, and an I 38 single peak at 39 eV. UPS places the Fermi level near mid-gap, and the resistivity is dramatically larger than in solution-processed films: 40 k41m for P(FAI0.5), 42m for P(FAI0.75), 43m for P(FAI1.0), and 44m for P(FAI1.5), versus 45–46m for solution films without additives and 47m with 10% SnF48. The same films show negligible Moss–Burstein shift and are described as nearly intrinsic (Park et al., 7 Oct 2025).
Additive engineering reaches related conclusions by a different route. In GeI49-assisted solution processing, Ge does not substitute Sn in the bulk lattice; instead, Ge-rich complexes accumulate first at the bottom interface and regulate nucleation and oriented growth. With 5% GeI50, the carrier lifetime from single-exponential TRPL rises from 51 ns to 52 ns, the Sn53 XPS signal disappears, the recombination resistance increases from 54 k55 to 56 k57, and the transient photovoltage decay time increases from 58s to 59s. In that study, GeI60 thus acts simultaneously as crystallization regulator, interfacial chemical modifier, and oxidation suppressor (Lai et al., 2024).
5. Film formation, morphology, and interfacial charge extraction
Vacuum co-evaporation and solution deposition reveal how strongly FASnI61 properties depend on crystallization pathway. In the co-evaporation route, separate SnI62 and FAI sources are controlled by quartz crystal microbalances, with a base pressure of 63 mTorr and four nominal FAI/SnI64 rate ratios, P(FAI0.5), P(FAI0.75), P(FAI1.0), and P(FAI1.5). The optimum region is P(FAI1.0): films are pinhole-free, flat, compact, and 65 nm thick, XRD peaks align closely with ideal 66/Amm2 simulations, and both lower FWHM and small peak shifts toward higher 67 indicate reduced microstrain and better crystallinity. By contrast, P(FAI0.5) and P(FAI1.5) give amorphous phases without long-range order (Park et al., 7 Oct 2025).
In solution-processed films, microstructure can be redirected by interfacial seeding. Adding GeI68 strengthens the FASnI69 (100) peak at 70, reduces its FWHM by about 71 at 5% GeI72, and increases the average domain size while improving coverage and reducing pinholes. At 10% GeI73, extra peaks at 74 and 75 match SnI76, and morphology degrades again. The proposed model is bottom-up crystallization: low-solubility Ge–F / Ge–I complexes precipitate first near the substrate, form a Ge-rich seed layer, and promote oriented growth of subsequent FASnI77 crystals (Lai et al., 2024).
Interface-resolved spectroscopy shows that FASnI78 charge extraction is highly asymmetric. Transient surface photovoltage measurements found that PEDOT extracts holes better than NiO79 and current carbazole-based SAMs, but the PEDOT/FASnI80 buried interface still suffers from substantial recombination and weak selectivity. On the electron side, C81, PCBM, and ICBA all extract electrons, with C82 giving the largest tr-SPV amplitude, up to 83 mV, despite its unfavorable conduction-band cliff. A separate layer-by-layer KP–PYS study showed that evaporated Ag diffuses into BCP to form a BCP:Ag blend with 84 eV, effectively a degenerated n-type layer that sharpens electron extraction and contributes strongly to the built-in field of ITO/PEDOT/FASnI85/C86/BCP/Ag devices (Aldamasy et al., 2023).
6. Device implementations, tandem roles, and methodological constraints
FASnI87 is a practical absorber in flexible and rigid p–i–n tin-perovskite solar cells. In a flexible architecture on PEN/ITO/PEDOT:PSS, using FASnI88 with 10 mol% SnF89 and 5 mol% GeI90, the champion device after light soaking reached a PCE of 91, 92 V, 93 mA cm94, and 95; the 0% GeI96 reference gave 97, 98 V, 99 mA cm00, and 01. Encapsulated rigid devices with 5% GeI02 retained 03 of initial PCE after 04 h maximum-power-point tracking at 05C, while multiple rigid devices stored in N06 retained 07 of their initial PCE after 08 h (Lai et al., 2024).
In multijunction modeling, FASnI09 is the narrow- to intermediate-gap workhorse of lead-free tandems. One finite-difference time-domain plus finite-element study used 10 eV, 11, 12 cm13 V14 s15, 16 ns, 17 cm18 s19, and 20 cm21 s22 for the FASnI23 absorber. In that framework, an optimized single-junction MgF24/ITO/ZnO/TiO25/FASnI26/Spiro-OMeTAD/Au cell reached 27, 28 mA cm29, 30 V, and 31. An all-perovskite KSnI32/FASnI33 tandem reached 34, 35 mA cm36, 37 V, and 38, while a KSnI39/FASnI40/ACZTSe triple-junction reached 41, 42 mA cm43, 44 V, and 45. In the triple-junction case, FASnI46 is the current-limiting middle junction (Rahman et al., 8 Nov 2025).
These implementations coexist with substantial methodological and physical constraints. Mechanical calculations were carried out at 0 K in a static pseudo-cubic structure, surface-passivation work used a tetragonal slab, and several optoelectronic studies emphasize that FA dynamics, phase competition, and supercell size strongly affect the observables. The strictest first-principles benchmark argues that physically sound modeling of 47-FASnI48 requires random FA disorder, explicit pseudo–Jahn–Teller distortions, large supercells, hybrid functionals, spin-orbit coupling, and nonlocal dispersion; this places unusual weight on structural sampling relative to many Pb-perovskite analogues (Venkatanarayanan et al., 26 Nov 2025).
FASnI49 is therefore best understood as a structurally soft, electronically sensitive, and defect-reactive tin halide perovskite whose measurable properties are not fixed by composition alone. Its 3D phase offers a band gap in the 50–51 eV range, mechanically compliant behavior, and suitability for single-junction, tandem, flexible, and near-IR optoelectronic architectures, while its 2D layered polymorphs introduce a distinctly different large-gap, flat-band electronic regime. The persistent scientific problem is not whether FASnI52 can function as a high-quality semiconductor, but under what structural, chemical, and interfacial conditions its near-ideal absorber characteristics can be realized reproducibly.
