---
title: Formamidinium Tin Triiodide (FASnI3)
url: https://www.emergentmind.com/topics/formamidinium-tin-triiodide-fasni3
type: topic
---

# Formamidinium Tin Triiodide (FASnI3)

Formamidinium tin triiodide (FASnI\(_3\); FA = CH(NH\(_2\))\(_2^+\) or NH\(_2\)CHNH\(_2^+\)) is a lead-free ABX\(_3\) tin halide perovskite in which formamidinium occupies the A site, Sn\(^{2+}\) the B site, and I\(^{-}\) the X site. In the photoactive \(\alpha\)-phase it adopts the ABX\(_3\) perovskite topology, and around room temperature its experimentally relevant structure is a dynamically disordered, nearly cubic perovskite with average symmetry close to \(Pm\bar{3}m\), while local distortions remain strong. FASnI\(_3\) 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 Sn\(^{2+}\)\(\rightarrow\)Sn\(^{4+}\) oxidation, Sn-vacancy-driven p-type self-doping, and a highly nontrivial coupling between local structure, band edges, and interfaces [2511.21254] [2510.05564] [2309.05481].

## 1. Structural motifs and crystallographic descriptions

FASnI\(_3\) supports more than one crystallographic description, and the choice of model depends strongly on the property under study. For the photoactive \(\alpha\)-phase, large-supercell first-principles work describes a nearly cubic average lattice with strong local distortion driven by the stereochemically active Sn(II) \(5s^2\) lone pair. In that description, a \(4\times4\times4\) supercell with randomly oriented FA\(^+\) 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 \(6\times6\times6\) supercell [2511.21254].

A second structural theme is polymorphism between a conventional 3D perovskite motif and a 2D layered motif. First-principles structure prediction found that HC(NH\(_2\))\(_2\)SnI\(_3\) can adopt both a 3D corner-sharing SnI\(_6\) network and a 2D layered structure with alternating SnI\(_6\) octahedral layers and FA-cation layers. The two motifs are nearly degenerate: the reported \(\Delta E = E_{\rm DFT}^{\rm 2D} - E_{\rm DFT}^{\rm 3D}\) ranges from \(+1\) to \(-4\) meV/atom depending on functional, and the 3D\(\leftrightarrow\)2D barrier is \(30\)–\(40\) 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 [1504.06200].

Static structural models used in other contexts are more specific. In one pseudo-cubic relaxation derived from experimental cubic FAPbI\(_3\), FASnI\(_3\) relaxed to \(a = 6.62\) Å, \(b = 6.46\) Å, and \(c = 6.53\) Å, with \(\text{Sn–I}_a\text{–Sn} = 176.7^\circ\), \(\text{Sn–I}_b\text{–Sn} = 180.0^\circ\), and \(\text{Sn–I}_c\text{–Sn} = 171.5^\circ\); the deviation from ideal cubic geometry was attributed largely to FA\(^+\) orientation. In surface-defect calculations, by contrast, the tetragonal phase was used with optimized bulk lattice parameters \(a=b=8.859\) Å and \(c=12.651\) Å. Experimentally, solution-processed films in one additive-engineering study showed a main diffraction peak around \(14^\circ\) assigned to the (100) plane of orthorhombic FASnI\(_3\), and no peak shift upon GeI\(_2\) addition, indicating retention of the FASnI\(_3\) crystal structure rather than bulk Sn–Ge alloying [1907.11347] [2206.06782] [2402.07627].

## 2. Electronic structure and energetic landscape

The electronic structure of FASnI\(_3\) is controlled by the Sn–I framework. In the 3D phase, the valence-band maximum is dominated by antibonding combinations of Sn \(5s\) and I \(5p\) orbitals, while the conduction-band minimum is largely Sn \(5p\) 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 \(1.20\) eV for 3D FASnI\(_3\) and \(2.75\) eV for the 2D layered phase; the 3D electronic structure is described as rather similar to that of MASnI\(_3\) [1504.06200].

For the room-temperature \(\alpha\)-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, \(\langle 111\rangle\)-oriented, and robust against thermal fluctuations [2511.21254].

Thin-film experiments sharpen this picture. Additive-free, solvent-free co-evaporation of SnI\(_2\) and FAI yielded highly crystalline FASnI\(_3\) with an optical band gap \(E_g \approx 1.31\) eV, a sharp absorption onset, a steep Urbach tail, and photoluminescence near \(950\) nm with a narrow FWHM of \(\sim 37.2\) nm. The same study reported that typical spin-coated “state-of-the-art” FASnI\(_3\) films often show \(E_g \approx 1.40\) 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 [2510.05564].

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 FASnI\(_3\) containing 10% SnF\(_2\), the Fermi level lies only \(0.01\)–\(0.08\) eV above the valence-band maximum, and Hall, PL/TRPL, and KP–PYS converge to a hole concentration of \(\sim 1.5\times10^{17}\) cm\(^{-3}\). In the same framework, the PEDOT/FASnI\(_3\) valence-band offset is about \(+0.42\) eV, while the FASnI\(_3\)/C\(_{60}\) conduction-band offset is negative, quantified as about \(-0.30\) eV from KP–PYS and as \(\mathrm{CBM}_{\mathrm{C60}}-\mathrm{CBM}_{\mathrm{FASnI_3}} \approx -0.53\) 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 FASnI\(_3\) rather than inherited from Pb-perovskite stacks [2309.05481].

## 3. Elasticity, ductility, and anisotropy

FASnI\(_3\) is mechanically soft even within the already compliant class of hybrid halide perovskites. In the pseudo-cubic cell, the independent elastic constants are \(C_{11}=29.96\), \(C_{22}=25.21\), \(C_{33}=26.02\), \(C_{44}=2.35\), \(C_{55}=4.90\), \(C_{66}=2.41\), \(C_{12}=6.85\), \(C_{13}=8.23\), and \(C_{23}=3.28\) 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: \(C_{11}>C_{22},C_{33}\), \(C_{55}\) is significantly larger than \(C_{44}\) and \(C_{66}\), and \(C_{13}>C_{12},C_{23}\), which correlates with the FA-induced distortions along the \(a\) and \(c\) directions [1907.11347].

Polycrystalline Voigt–Reuss–Hill averages confirm the softness of the material. For FASnI\(_3\), the Hill bulk modulus is \(B=12.96\) GPa, the shear modulus is \(G=5.08\) GPa, and the Young’s modulus is \(E=13.48\) GPa. The Pugh ratio is \(B/G \approx 2.55\), the Poisson ratio is \(\nu = 0.33\), and the universal anisotropy index is \(A^U = 2.60\). Within the criteria used in that study, both \(B/G \gtrsim 1.75\) and \(\nu > 0.26\) indicate ductility, so FASnI\(_3\) is ductile in the Pugh sense. The authors further note that FASnI\(_3\) is softer than FASnBr\(_3\) and slightly softer than FAPbI\(_3\), while also being the least anisotropic of the four FABX\(_3\) systems considered.

The microscopic origin of this mechanical response is twofold. First, the planar FA\(^+\) 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 FA\(^+\) weakening the inorganic framework.” Second, projected crystal orbital Hamilton population analysis gives an averaged \(\mathrm{ICOHP}(\mathrm{Sn{-}I}) = -2.056\) eV, weaker than Pb–I \((-2.115\) eV) and weaker than Sn–Br \((-2.237\) eV). The combination of weaker Sn–I bonding and FA-induced geometric distortion accounts for the low stiffness, easy shear, and directional dependence of FASnI\(_3\) [1907.11347].

## 4. Defects, oxidation, and passivation chemistry

The central defect-chemistry problem in FASnI\(_3\) is the coupling between Sn vacancies and Sn oxidation. In a surface-defect analysis, \(V_{\rm Sn}\) is identified as the dominant defect in both Sn-rich and I-rich conditions, and the degradation pathway is summarized by
\[
3 \,\mathrm{FASnI}_3 + 3 \,\mathrm{O}_2 \rightarrow \mathrm{SnI}_4 + \mathrm{SnO}_2 + 4 \,\mathrm{FAI}.
\]
On the Sn-deficient FASnI\(_3\) (301) surface, removing one surface Sn atom leaves five dangling I atoms, strongly distorts the local octahedral network, shortens nearby Sn–I bonds from \(3.15\) Å to \(\approx 2.90\)–\(2.92\) 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 [2206.06782].

Lewis-base passivation was proposed as a chemically guided route to heal such defective surfaces. In that framework, the defective FASnI\(_3\) surface is treated as having a “somehow hard Lewis acid nature,” and passivation performance is correlated with molecular hardness \(\eta = \frac{1}{2}(I-A)\). The hard bases edamine \((\eta = 3.28\ \mathrm{eV})\) and Isatin-Cl \((\eta = 2.42\ \mathrm{eV})\) donate electrons to FASnI\(_3\), with \(e_{\rm transfer}=+1.64\,e\) for edamine and \(+0.40\,e\) for Isatin-Cl; by contrast, p-TA \((\eta = 1.98\ \mathrm{eV})\) and EDT \((\eta = 1.82\ \mathrm{eV})\) show inverse charge transfer, \(e_{\rm transfer}=-4.42\,e\) and \(-3.06\,e\), respectively. The dual effect emphasized in that work is charge redistribution of the \(V_{\rm Sn}\)-related defect state together with saturation of dangling states and reduction of deep gap states [2206.06782].

Processing studies show that the same defect chemistry can be altered at film level. In co-evaporated FASnI\(_3\), XPS gives a dominant Sn \(3d_{5/2}\) peak at \(486.4 \pm 0.2\) eV assigned to Sn\(^{2+}\), only a minor higher-binding-energy component at \(487.1\) eV, and an I \(3d_{5/2}\) single peak at \(619.2\) eV. UPS places the Fermi level near mid-gap, and the resistivity is dramatically larger than in solution-processed films: \(\sim 2.57\) k\(\Omega\cdot\)m for P(FAI0.5), \(\sim 7.4\ \Omega\cdot\)m for P(FAI0.75), \(\sim 13.3\ \Omega\cdot\)m for P(FAI1.0), and \(\sim 42.5\ \Omega\cdot\)m for P(FAI1.5), versus \(10^{-4}\)–\(10^{-2}\ \Omega\cdot\)m for solution films without additives and \(\sim 1.3\times10^{-2}\ \Omega\cdot\)m with 10% SnF\(_2\). The same films show negligible Moss–Burstein shift and are described as nearly intrinsic [2510.05564].

Additive engineering reaches related conclusions by a different route. In GeI\(_2\)-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% GeI\(_2\), the carrier lifetime from single-exponential TRPL rises from \(4.15\) ns to \(12.83\) ns, the Sn\(^{4+}\) XPS signal disappears, the recombination resistance increases from \(\sim 25\) k\(\Omega\) to \(\sim 90\) k\(\Omega\), and the transient photovoltage decay time increases from \(46.7\ \mu\)s to \(96.7\ \mu\)s. In that study, GeI\(_2\) thus acts simultaneously as crystallization regulator, interfacial chemical modifier, and oxidation suppressor [2402.07627].

## 5. Film formation, morphology, and interfacial charge extraction

Vacuum co-evaporation and solution deposition reveal how strongly FASnI\(_3\) properties depend on crystallization pathway. In the co-evaporation route, separate SnI\(_2\) and FAI sources are controlled by quartz crystal microbalances, with a base pressure of \(\sim 5\times10^{-7}\) mTorr and four nominal FAI/SnI\(_2\) 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 \(\sim 400\) nm thick, XRD peaks align closely with ideal \(Pm\bar{3}m\)/Amm2 simulations, and both lower FWHM and small peak shifts toward higher \(2\theta\) indicate reduced microstrain and better crystallinity. By contrast, P(FAI0.5) and P(FAI1.5) give amorphous phases without long-range order [2510.05564].

In solution-processed films, microstructure can be redirected by interfacial seeding. Adding GeI\(_2\) strengthens the FASnI\(_3\) (100) peak at \(\sim 14^\circ\), reduces its FWHM by about \(2\times\) at 5% GeI\(_2\), and increases the average domain size while improving coverage and reducing pinholes. At 10% GeI\(_2\), extra peaks at \(\sim 12.7^\circ\) and \(25.7^\circ\) match SnI\(_2\), 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 FASnI\(_3\) crystals [2402.07627].

Interface-resolved spectroscopy shows that FASnI\(_3\) charge extraction is highly asymmetric. Transient surface photovoltage measurements found that PEDOT extracts holes better than NiO\(_x\) and current carbazole-based SAMs, but the PEDOT/FASnI\(_3\) buried interface still suffers from substantial recombination and weak selectivity. On the electron side, C\(_{60}\), PCBM, and ICBA all extract electrons, with C\(_{60}\) giving the largest tr-SPV amplitude, up to \(\approx -47\) 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 \(\mathrm{WF} \approx \mathrm{IE} \approx 4.0\) eV, effectively a degenerated n-type layer that sharpens electron extraction and contributes strongly to the built-in field of ITO/PEDOT/FASnI\(_3\)/C\(_{60}\)/BCP/Ag devices [2309.05481].

## 6. Device implementations, tandem roles, and methodological constraints

FASnI\(_3\) 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 FASnI\(_3\) with 10 mol% SnF\(_2\) and 5 mol% GeI\(_2\), the champion device after light soaking reached a PCE of \(10.8\%\), \(V_{\rm OC}=0.716\) V, \(J_{\rm SC}=20.76\) mA cm\(^{-2}\), and \(\mathrm{FF}=72.9\%\); the 0% GeI\(_2\) reference gave \(8.2\%\), \(0.560\) V, \(20.14\) mA cm\(^{-2}\), and \(72.81\%\). Encapsulated rigid devices with 5% GeI\(_2\) retained \(\sim 80\%\) of initial PCE after \(\sim 150\) h maximum-power-point tracking at \(\sim 42^\circ\)C, while multiple rigid devices stored in N\(_2\) retained \(>91\%\) of their initial PCE after \(\sim 13{,}800\) h [2402.07627].

In multijunction modeling, FASnI\(_3\) is the narrow- to intermediate-gap workhorse of lead-free tandems. One finite-difference time-domain plus finite-element study used \(E_g = 1.4\) eV, \(\varepsilon = 8.2\), \(\mu_n = \mu_p = 22\) cm\(^2\) V\(^{-1}\) s\(^{-1}\), \(\tau_e = \tau_h = 7.93\) ns, \(B_{n,p}=4.37\times10^{-10}\) cm\(^3\) s\(^{-1}\), and \(C_{n,p}=9.3\times10^{-30}\) cm\(^6\) s\(^{-1}\) for the FASnI\(_3\) absorber. In that framework, an optimized single-junction MgF\(_2\)/ITO/ZnO/TiO\(_2\)/FASnI\(_3\)/Spiro-OMeTAD/Au cell reached \(\eta = 26.03\%\), \(J_{\rm sc}=29.80\) mA cm\(^{-2}\), \(V_{\rm oc}=1.029\) V, and \(\mathrm{FF}=84.84\%\). An all-perovskite KSnI\(_3\)/FASnI\(_3\) tandem reached \(\eta = 27.29\%\), \(J_{\rm sc}=14.74\) mA cm\(^{-2}\), \(V_{\rm oc}=2.227\) V, and \(\mathrm{FF}=83.14\%\), while a KSnI\(_3\)/FASnI\(_3\)/ACZTSe triple-junction reached \(\eta = 30.69\%\), \(J_{\rm sc}=13.184\) mA cm\(^{-2}\), \(V_{\rm oc}=2.766\) V, and \(\mathrm{FF}=84.18\%\). In the triple-junction case, FASnI\(_3\) is the current-limiting middle junction [2511.06059].

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 \(\alpha\)-FASnI\(_3\) 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 [2511.21254].

FASnI\(_3\) 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 \(1.2\)–\(1.4\) 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 FASnI\(_3\) can function as a high-quality semiconductor, but under what structural, chemical, and interfacial conditions its near-ideal absorber characteristics can be realized reproducibly.

Source: https://www.emergentmind.com/topics/formamidinium-tin-triiodide-fasni3