Ab-Initio investigation of the structural, elastic, electronic, magnetic and optical properties of Mn-doped ZnSe wurtzite: Application in solar cells

This research proposes a comprehensive ab initio study of the structural, electronic, mechanical, magnetic, and optical properties of pure ZnSe and Mn-doped ZnSe at different concentrations using density functional theory (DFT) according to the GGA-PBE+U approximation. Examination of the electronic band structure reveals that pure ZnSe exhibits a direct band gap of 2.69 eV at the $\Gamma$ point, which agrees well with experimental data. Our results demonstrate that the integration of Mn systematically modifies the fundamental characteristics of ZnSe. Structurally, the addition of Mn causes a lattice expansion: for a Mn content of 16.66%, the lattice parameters extend to a = 4.034 Å and c = 6.610 Å, which indicates a lattice expansion while preserving mechanical stability (Young’s modulus $\approx$ 137.7 GPa, Poisson’s ratio $\nu \approx$ 0.29, B/G $\approx$ 2.03). Electronically, the energy gap gradually decreases from 2.51 eV at 6.25% Mn to 1.89 eV at 16.66% Mn, due to Mn 3d-Se 4s hybridization, which is confirmed by density of states and charge density analyses. In terms of magnetism, Mn elicits ferromagnetism with local magnetic moments of about 5 $\mu$B per Mn atom; the ferromagnetic phase is energetically preferential ($\Delta E_1$ = 0.0482 eV/supercell), whereas the non-magnetic phase is significantly less stable ($\Delta E_1$ = 0.8003 eV). The exchange constants ($N_0\alpha \approx$ 0.19 eV; $N_0\beta$ = 1.3–2.0 eV) indicate predominant ferromagnetic interactions. Mn doping significantly enhances optical properties: absorption in the visible and infrared regions increases, reflectivity increases in the infrared and low-visible region while decreasing in the UV, and transmission increases in the IR region. These changes show a quantitative correlation with Mn concentration, illustrating tunable optoelectronic behavior. Photovoltaic simulations also show optimal device performance with 16.66% Mn doping, displaying a Voc of 0.8849 V, a Jsc of 16.90 mA/cm², a fill factor FF of 87.06%, and a PCE energy conversion efficiency of 13.03%, more than double compared to pure ZnSe (6.44%). The external quantum efficiency reaches 100% in the 400–550 nm range and remains high up to 690 nm, with an optimal absorber thickness of approximately $1.8\mu m$. In summary, our research quantitatively establishes the link between structure, magnetism, optical response and photovoltaic efficiency, highlighting Mn-doped ZnSe as a multifunctional material with promising applications in spintronics, optoelectronics, infrared detectors and highly efficient solar cells.