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Theoretical Insights into Excitons, Optical Properties, and Nonradiative Recombination Dynamics in M$_6$CSe$_4$ (M = Ca, Sr) Antiperovskite Carbides

Sanchi Monga, Saswata Bhattacharya

TL;DR

The paper addresses the search for lead-free photovoltaic absorbers in the antiperovskite-carbide family M$_6$CSe$_4$ (M = Ca, Sr) by combining beyond-DFT methods to quantify electronic structure, excitonic effects, and nonradiative carrier dynamics. Using DFT, $G_0W_0$, and BSE, it shows direct-band-gap behavior with gaps of about $1.66$ eV (Ca) and $1.22$ eV (Sr), and reveals bound excitons with $E_b$ of $0.12$ eV and $0.20$ eV, respectively, extending over ~3 unit cells. Finite-temperature TDDFT-NAMD simulations show Ca experiences stronger band-gap fluctuations but weaker nonadiabatic couplings and faster decoherence, resulting in nonradiative lifetimes roughly eleven times longer than Sr. These insights position Ca$_6$CSe$_4$ as a particularly promising lead-free PV candidate and provide a quantitative framework for evaluating excitonic and nonradiative processes in antiperovskite carbides.

Abstract

Theoretically predicted antiperovskite carbides M$_6$CSe$_4$ (M = Ca, Sr) represent an emerging class of optoelectronic materials with potential relevance for photovoltaic applications. In this work, we present a comprehensive first-principles investigation of their electronic, optical, and excitonic properties, together with non-radiative recombination dynamics. Density functional theory (DFT) and many-body perturbation theory (GW) reveal that both compounds are direct band gap semiconductors with gaps spanning the infrared-visible region. Incorporating electron-hole interactions via the Bethe-Salpeter equation leads to pronounced red-shifts in the first peak of optical spectra, indicative of bound excitons with binding energies of 0.12 eV (Ca$_6$CSe$_4$) and 0.20 eV (Sr$_6$CSe$_4$), extending over nearly three unit cells in all directions. Time-dependent DFT combined with nonadiabatic molecular dynamics simulations at 300 K reveals pronounced lattice fluctuations in Ca$_6$CSe$_4$, resulting in 38% larger band gap variations and 28% faster electronic decoherence. Together with 53% weaker nonadiabatic couplings, these effects yield non-radiative recombination lifetimes approximately eleven times longer than in Sr$_6$CSe$_4$. Overall, our results identify M$_6$CSe$_4$ carbides as promising lead-free photovoltaic materials, with Ca$_6$CSe$_4$ exhibiting superior optoelectronic properties and carrier dynamics that motivate further experimental investigation.

Theoretical Insights into Excitons, Optical Properties, and Nonradiative Recombination Dynamics in M$_6$CSe$_4$ (M = Ca, Sr) Antiperovskite Carbides

TL;DR

The paper addresses the search for lead-free photovoltaic absorbers in the antiperovskite-carbide family MCSe (M = Ca, Sr) by combining beyond-DFT methods to quantify electronic structure, excitonic effects, and nonradiative carrier dynamics. Using DFT, , and BSE, it shows direct-band-gap behavior with gaps of about eV (Ca) and eV (Sr), and reveals bound excitons with of eV and eV, respectively, extending over ~3 unit cells. Finite-temperature TDDFT-NAMD simulations show Ca experiences stronger band-gap fluctuations but weaker nonadiabatic couplings and faster decoherence, resulting in nonradiative lifetimes roughly eleven times longer than Sr. These insights position CaCSe as a particularly promising lead-free PV candidate and provide a quantitative framework for evaluating excitonic and nonradiative processes in antiperovskite carbides.

Abstract

Theoretically predicted antiperovskite carbides MCSe (M = Ca, Sr) represent an emerging class of optoelectronic materials with potential relevance for photovoltaic applications. In this work, we present a comprehensive first-principles investigation of their electronic, optical, and excitonic properties, together with non-radiative recombination dynamics. Density functional theory (DFT) and many-body perturbation theory (GW) reveal that both compounds are direct band gap semiconductors with gaps spanning the infrared-visible region. Incorporating electron-hole interactions via the Bethe-Salpeter equation leads to pronounced red-shifts in the first peak of optical spectra, indicative of bound excitons with binding energies of 0.12 eV (CaCSe) and 0.20 eV (SrCSe), extending over nearly three unit cells in all directions. Time-dependent DFT combined with nonadiabatic molecular dynamics simulations at 300 K reveals pronounced lattice fluctuations in CaCSe, resulting in 38% larger band gap variations and 28% faster electronic decoherence. Together with 53% weaker nonadiabatic couplings, these effects yield non-radiative recombination lifetimes approximately eleven times longer than in SrCSe. Overall, our results identify MCSe carbides as promising lead-free photovoltaic materials, with CaCSe exhibiting superior optoelectronic properties and carrier dynamics that motivate further experimental investigation.
Paper Structure (10 sections, 6 equations, 8 figures, 3 tables)

This paper contains 10 sections, 6 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Computational Methods
  3. Results and Discussion
  4. Crystal structure and stability
  5. Static electronic and optical properties
  6. Dynamical structural and electronic properties
  7. Electron-vibrational interactions
  8. Non-radiative electron-hole recombination
  9. Conclusion
  10. Acknowledgment

Figures (8)

  • Figure 1: Structural and stability analysis of M$_6$CSe$_4$ (M = Ca, Sr): (a) primitive unit cell; phonon dispersion obtained from density functional perturbation theory (DFPT) for (b) Ca$_6$CSe$_4$ and (c) Sr$_6$CSe$_4$; total energy as a function of simulation time from $\textit{ab initio}$ molecular dynamics simulations in the canonical (NVT) ensemble at 300 K for (d) Ca$_6$CSe$_4$ and (e) Sr$_6$CSe$4$; and (f) calculated elastic constants $C{ij}$ (in GPa), confirming mechanical stability.
  • Figure 2: Electronic band structures computed using the PBE (orange dashed curves) and $\mathit{G_0W_0}$@PBE (blue solid curves) methods for (a) Ca$_6$CSe$_4$ and (b) Sr$_6$CSe$_4$.
  • Figure 3: Atom- and orbital-resolved density of states for (a) Ca$_6$CSe$_4$ and (b) Sr$_6$CSe$_4$, calculated using the HSE06 functional.
  • Figure 4: Imaginary part of the dielectric function obtained from $\mathit{G_0W_0}$@PBE (blue) and BSE@$\mathit{G_0W_0}$@PBE (magenta) approaches, along with the spatial distribution of the first bright exciton, with the hole fixed 1 Å above the atoms contributing most to the VBM, for Ca$_6$CSe$_4$ (a,b) and Sr$_6$CSe$_4$ (c,d). The green circles in panels (a) and (c) represent the relative oscillator strengths, normalized to a maximum value of 1 and rescaled for visual clarity. The exciton wavefunctions shown in panels (b) and (d) are visualized using an isosurface value of 0.038 e/Å$^3$ over a 6$\times$6$\times$6 supercell. Green, purple, and red spheres denote C, M (Ca/Sr), and Se atoms, respectively.
  • Figure 5: Statistical distributions of (a) intra-octahedral M–C bond lengths, (b) intra-octahedral M–C–M bond angles, (c) C-Se bond distances, and (d) C–Se–C bond angles, obtained from the last 5 ps of AIMD trajectories of the conventional cells for Ca$_6$CSe$_4$ (black) and Sr$_6$CSe$_4$ (red). The standard deviations ($\sigma$) of the bond length and bond angle fluctuations are also reported.
  • ...and 3 more figures