High Photovoltaic Efficiency in Bulk-Stacked One-Dimensional GeSe$_{2}$ van der Waals Crystal

Germanium diselenide (GeSe${2}$) has recently attracted substantial interest as a rare example of one-dimensional (1D) van der Waals material. Here, we investigate the photovoltaic potential of bulk-stacked GeSe${2}$ chains using first-principles calculations within the $GW0$ approximation and the Bethe-Salpeter equation (BSE) to capture quasiparticle and excitonic effects. The bulk GeSe$_{2}$ exhibits indirect GW band gaps of 1.92 eV (type-I) and 1.08 eV (type-II). Optical calculations show markedly stronger visible-light absorption in type-II, yielding a spectroscopically limited maximum efficiency (SLME) of ~25.6% at a 0.5 $μ$m thickness. Phonon and room-temperature ab initio molecular dynamics analyses indicate that type-II is dynamically stable, whereas type-I shows imaginary phonon modes, suggesting a propensity for structural distortion. These results identify type-II GeSe2 as a promising stable absorber for thin-film photovoltaics with enhanced flexibility compared to typical 2D vdW systems.

💡 Research Summary

This study presents a comprehensive first‑principles investigation of bulk‑stacked germanium diselenide (GeSe₂) chains, focusing on two distinct polymorphs: the naturally occurring type‑I and the template‑synthesized type‑II. Using density‑functional theory (DFT) with PBE‑GGA and van‑der‑Waals corrections for structural relaxation, the authors then apply hybrid HSE06 and many‑body perturbation theory within the GW₀ approximation to obtain accurate quasiparticle band structures. GW₀ predicts indirect band gaps of 1.92 eV for type‑I and 1.08 eV for type‑II, substantially larger than the mean‑field DFT values and more realistic for photovoltaic assessment.

To capture excitonic effects, the Bethe–Salpeter equation (BSE) is solved on top of the GW₀ quasiparticle energies. The resulting optical spectra reveal a pronounced excitonic peak near 2 eV for type‑II, whereas type‑I shows only weak excitonic features. Consequently, the absorption coefficient of type‑II is markedly stronger across the visible range (400–700 nm), aligning well with the AM1.5 solar spectrum.

Photovoltaic performance is quantified using the spectroscopically limited maximum efficiency (SLME) metric, which incorporates intrinsic absorption, band‑gap nature, and radiative recombination losses. Thickness‑dependent SLME curves show that at a realistic film thickness of 0.5 µm, type‑I attains ~16 % efficiency, while type‑II reaches ~25.6 %, surpassing many established lead‑free absorbers such as Sb₂S₃ (≈23.5 %) and approaching Sb₂Se₃ (≈29 %).

Stability analyses include phonon dispersion calculations and ab‑initio molecular dynamics (AIMD) at 300 K. Type‑I exhibits imaginary phonon modes, indicating dynamical instability and a propensity for structural distortion. In contrast, type‑II displays only real phonon frequencies, confirming dynamical stability. AIMD simulations over 4 ps reveal stable total energy and temperature fluctuations without any structural collapse, demonstrating thermal robustness at room temperature.

Overall, the work establishes bulk‑stacked type‑II GeSe₂ as a promising thin‑film photovoltaic absorber. Its favorable indirect band gap, strong visible‑light absorption, high SLME, and confirmed dynamical and thermal stability make it a compelling candidate for next‑generation, flexible solar cells. The study also underscores the necessity of GW‑BSE level calculations for accurate prediction of optoelectronic properties in low‑dimensional van‑der‑Waals materials, suggesting that 1D chain‑based crystals can rival or exceed the performance of conventional 2D and 3D semiconductors.