Study of Structural Stability, Mechanical and Optoelectronic Properties of New Earth-Abundant Cu2Ni(Sn,Ge,Si)Se4 Kesterites for Photovoltaic Applications

The urgent demand for cost-effective and environmentally benign materials in photovoltaics has directed significant attention to earth-abundant kesterites. In this study, we perform a comprehensive density functional theory (DFT) analysis of Cu ₂ Ni(Sn,Ge,Si)Se ₄ kesterites, focusing on their structural, mechanical, thermodynamic, electronic, phonon, and optical properties. Structural optimizations were conducted using the SCAN functional, while electronic and optical characteristics were derived using the HSE06 and mBJ + U approaches. The results reveal that substitution of Sn with Ge and Si leads to systematic lattice contraction, enhancement of the mechanical moduli as the bulk modulus increases from 50.4 GPa to 63.6 GPa, and tuning of the bandgap from 0.88 eV to 2.36 eV. All compounds satisfy the Born criteria and exhibit dynamically stable phonon spectra, with no imaginary modes and thermal stability confirmed up to 1000 K. Optical absorption spectra show high coefficients (up to 10 ⁶ cm ^− 1 ), with Cu₂NiSiSe₄ showing strong absorption in the visible and UV regions, while Cu ₂ NiSnSe ₄ is IR-active. The dielectric constant and refractive index correlate inversely with bandgap energy, in agreement with Penn's model. Compared to conventional CZTS compounds, the incorporation of Ni and substitution of Sn with lighter group IV elements allows for modulation of both magnetic and optoelectronic behavior, broadening their applicability. These materials are suitable not only for single-junction solar cells but also for tandem architectures and infrared photodetectors. Notably, Cu ₂ NiSnSe ₄ and Cu ₂ NiGeSe ₄ are well-suited for infrared (IR) applications, while Cu ₂ NiSiSe ₄ is ideal for visible-light absorption.