Antimony Tri-Selenide (Sb₂Se₃)

• Antimony tri-selenide (Sb₂Se₃) is a low-symmetry, quasi-one-dimensional material with orthorhombic ribbons that underpin its unique electronic and photonic applications.
• Its electronic structure features an indirect gap near 1.12 eV and highly directional carrier mobilities, crucial for optimizing thin-film photovoltaic performance.
• Complex defect dynamics, including large Fröhlich polarons and multi-stable vacancy states, critically influence charge transport and device efficiency.

Antimony tri-selenide (Sb₂Se₃) is a V–VI binary compound possessing pronounced anisotropy in structure, electronic properties, lattice dynamics, and functional defect chemistry. Its orthorhombic, ribbon-like crystal lattice underpins technologically relevant characteristics spanning photovoltaic conversion, phase-change photonics, and solid-state defect manipulation. Sb₂Se₃ is distinguished by quasi-one-dimensional [Sb₄Se₆]ₙ chains, active lone-pair chemistry on Sb, complex polaron transport phenomena, and multistable vacancy behavior—all of which directly inform its role as a solar absorber and low-loss programmable photonic material.

1. Crystal Structure and Anisotropic Bonding

Sb₂Se₃ crystallizes in the orthorhombic Pbnm (Pnma) space group, forming strongly bonded quasi-1D [Sb₄Se₆]ₙ ribbons oriented along the c-axis (Wang et al., 2021). The unit cell contains three inequivalent selenium sites: Se₁ (ribbon corner, threefold coordinated), Se₂ (internal threefold coordination), and Se₃ (deeply embedded, fivefold coordination) (Herklotz et al., 16 Oct 2025). Lattice parameters typically measured via advanced DFT (HSE06+D3) are $a = 3.95$ Å, $b = 11.55$ Å, and $c = 11.93$ Å.

Inter-ribbon coupling is not restricted to van der Waals interactions; significant electrostatic and lone-pair mediated binding contributes to the bulk cohesion. Calculated inter-ribbon binding energies notably exceed the pure vdW regime (e.g., $E_b(c)/atom ≈ 14.4$ kJ·mol⁻¹), implicating stereochemically active Sb 5s electron pairs in the lattice energetics and electronic polarization (Wang et al., 2021). The lone-pair asymmetry drives structural, dielectric, and carrier transport anisotropy.

2. Electronic Structure and Carrier Dynamics

Electronic band structure calculations reveal an indirect gap of approximately 1.12 eV (VBM near $\Gamma$–Y, CBM near $\Gamma$–X), while the direct gap is within 0.06 eV (Wang et al., 2021). The band dispersion yields highly direction-dependent effective masses: electrons are lightest in the ribbon plane ($m^*_x ≈ 0.14\,m_0$), heavy out-of-plane ($m^*_z ≈ 7.0\,m_0$), whereas holes exhibit quasi-2D–3D mobility.

The Fermi surface topology supports fast, band-like transport in the ab plane, with carrier mobility suppressed perpendicular to ribbons (Wang et al., 2022, Shi et al., 7 Oct 2024). Dielectric constants reflect this anisotropy, with static ($\epsilon_0$) and electronic ($\epsilon_{\infty}$) values exhibiting pronounced variance across axes (e.g., $\epsilon_0^x=85.6$, $\epsilon_0^y=128.2$, $\epsilon_0^z=15.0$) (Wang et al., 2021). These features are critical for device orientation, light absorption, and charge extraction in thin-film photovoltaics.

3. Polaron Formation and Charge Transport

Charge carriers in Sb₂Se₃ are coupled to strong polar optical phonons, leading primarily to the formation of large Fröhlich polarons whose spatial extent encompasses tens of unit cells ($r_p^{(e)} ≈ 40$ Å, $r_p^{(h)} ≈ 32$ Å) (Wang et al., 2022). The Fröhlich coupling constants ($\alpha_e ≈ 1.3$, $\alpha_h ≈ 2.1$) place Sb₂Se₃ in an intermediate regime where small-polaron self-trapping is absent—the carriers remain delocalized, and band-like mobility dominates.

Ultrafast pump–probe experiments corroborate the formation of large, spatially extended, and highly anisotropic polarons following photoexcitation, with lattice distortions propagating predominantly along the ribbon axis at speeds $\sim 3.4$ Å/ps and saturating with radii $\gtrsim 8$ nm (Shi et al., 7 Oct 2024). The time-resolved trapping of carriers by these polarons (lifetime tens to hundreds of ps) can reduce quasi-Fermi-level splitting, affect open-circuit voltages, and impact the efficiency ceiling in photovoltaic applications.

Directional mobility at 300 K reaches $\mu_e ≈ 30$ cm² V⁻¹ s⁻¹, $\mu_h ≈ 8$ cm² V⁻¹ s⁻¹ under minimal defect concentrations; ionized-impurity scattering at higher defect densities ($N_D > 10^{18}$ cm⁻³) collapses $\mu$ to $\ll 1$ cm² V⁻¹ s⁻¹ (Wang et al., 2022).

4. Point Defects, Negative-U Centers, and Efficiency Limits

Sb₂Se₃ hosts intrinsic vacancy defects exhibiting complex charge-state transitions. Both Se and Sb vacancies are amphoteric and capable of forming four-electron negative-U centers, where defect sites trap multiple carriers in strongly reconfigured local environments (Se-Se and Sb-Sb oligomers) (Wang et al., 2023). The four-electron negative-U transitions, quantified by $U_4$ ($U_4 ≈ -0.28 \,\text{eV}$ for Sb, $U_4 ≈ -0.04 \,\text{eV}$ for Se), result in deep thermodynamically stable trap states near mid-gap ($\sim 0.6$–$0.7$ eV above VBM), severely limiting extrinsic doping by self-compensation.

First-principles defect analyses highlight Se-vacancies (especially V_Se(2)) as dominant nonradiative recombination centers, quantified by electron-capture coefficients $C_n ≈ 5.63 \times 10^{-6}$ cm³/s, and hole-capture $C_p ≈ 1.22 \times 10^{-8}$ cm³/s (Wang et al., 6 Feb 2024). The bulk Shockley–Read–Hall recombination, governed by such deep centers, restricts the maximal achievable device efficiency: under optimal chemical potential and passivation, trap-limited conversion efficiency can reach $\eta_{\text{max}} \approx 25.1\%$ (below the radiative SQ limit of $27.7\%$). Oxygen passivation of Se-vacancies (substitutional $O_\text{Se}$) is thermodynamically favorable, electronic inactive, and effectively suppresses recombination centers.

5. Substitutional Sulfur: Local Vibrational Fingerprints and Site Selectivity

Sulfur incorporation into Sb₂Se₃ as substitutional $S_\text{Se}$ defects occurs with nearly equal formation energies ($\Delta E_{\text{form}} \lesssim 10$ meV) for the three inequivalent Se sites, resulting in statistically random site occupation (Herklotz et al., 16 Oct 2025). Infrared absorption studies reveal four sharp, well-resolved local vibrational modes (LVMs) at 249, 273, 283, and 312 cm⁻¹, each assigned via DFT and polarization analysis to particular $S_\text{Se}$ site/mode symmetries:

• S₁: 249 cm⁻¹ (A″, $S_\text{Se1}$, polarized along c)
• S₂: 273 cm⁻¹ (higher A′, $S_\text{Se1}$, a–b unpolarized)
• S₃: 283 cm⁻¹ (A′, $S_\text{Se2}$)
• S₄: 312 cm⁻¹ (A′, $S_\text{Se3}$, strong b-plane polarization)

Isotope experiments (${}^{34}$S) yield predictable frequency shifts ($\sqrt{32/34} \approx 0.97$) of 5–7 cm⁻¹. All four LVM linewidths are narrow, largely temperature independent, and decoupled from the host phonon continuum. These vibrational fingerprints serve as direct non-destructive probes of substitutional sulfur, enabling compositional mapping and defect profiling in alloyed and surface-modified Sb₂Se₃ (Herklotz et al., 16 Oct 2025).

6. Phase-Change Optical Switching and Photonic Integration

Sb₂Se₃ operates as a robust, low-loss, reversible phase-change material (PCM) for photonic applications in the telecom band. The crystalline phase ($n_{\text{crys}} \approx 3.43$, $k_{\text{crys}} < 10^{-5}$ at 1.55 μm) switches to amorphous ($n_{\text{amo}} \approx 3.35$) with negligible change in absorption (Δk < 10⁻⁵), supporting cycling endurance exceeding $10^7$ cycles at rates up to 20 kHz (Lawson et al., 2023).

Thin films (30–150 nm) can be non-volatilely programmed via nanosecond laser pulses (e.g., 200 ns, 60 mW for amorphization; 20 μs, 12 mW for recrystallization), with characteristic energy densities $E_{\text{amo}} \sim 1 \times 10^{-3}$ J/cm². Structural design considerations (ZnS:SiO₂ encapsulation, waveguide-matched indices) yield high insertion fidelity and exceptional phase stability.

In integrated photonics, patterned Sb₂Se₃ films on silicon MMIs enable matrix-vector programmable transmission for 2×2–5×5 ports, achieving $>90\%$ programming accuracy, $>75\%$ port transmission, and $\sim$15 dB crosstalk isolation over areas three orders of magnitude smaller than conventional interferometer meshes (Radford et al., 22 Nov 2025). The material outperforms established PCMs (GST, GSST) in optical loss and cycling endurance.

7. Device Design, Functional Implications, and Prospects

For photovoltaic applications, the unique crystal anisotropy and high dielectric screening dictate ribbon orientation ([100], [010]) for optimized carrier transport and light absorption (Wang et al., 2021). Film thicknesses in the $0.5$–$2$ μm regime attain near-SQ radiative-limit efficiency ($\sim$28%). Defect passivation (O, Cl, organics), Fermi-level engineering, and controlled quenching protocols are recommended for mitigation of deep trap states and maximization of $\eta$ (Wang et al., 6 Feb 2024).

The site-resolved detection of substitutional S indicates potential for strain and band engineering in complex alloyed and heterostructure devices (Herklotz et al., 16 Oct 2025). The large, directional polarons implicate careful management of lattice softness and interfacial dielectric environment to enhance carrier extraction, while phase-change integration facilitates scalable, zero-power photonic logic and switching devices (Lawson et al., 2023, Radford et al., 22 Nov 2025).

In summary, antimony tri-selenide exemplifies a class of earth-abundant low-symmetry materials whose performance envelope in optoelectronics is governed by the coupling of electronic anisotropy, vibrational mode diversity, polaronic transport, programmable defect behavior, and PCM phase flexibility. Advanced characterization and first-principles modeling have delineated the path for both fundamental and applied improvements.