DENSITY FUNCTIONAL THEORY STUDY OF MATERIAL PROPERTIES OF LANTHANIDE-DOPED ZINC OXIDE FOR DYE SENSITISED SOLAR CELL PHOTOANODE

Abstract

This dissertation presents a comprehensive Density Functional Theory (DFT) investigation into the effects of rare earth (RE) metals doping on zinc oxide (ZnO), targeting its application as a photoanode material in dye-sensitized solar cells (DSSCs). The study conducted an in-depth comparative analysis of structural, mechanical, dynamical, electronic, optical, and transport properties of the undoped cubic (C), Wurtzite (W) and monolayer (M) structural configurations of ZnO. Similar material properties of ZnO modified by individual Ce, Dy, and Eu dopants were further evaluated systematically on the wurtzite phase of ZnO. The effects of Oxygen vacancy and dopant concentration on the material Properties of W-ZnO were evaluated on a supercell (3×3×3) comprising of 108 atoms. The structural optimization was carried out using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The calculated lattice constants of 4.388 Å for C-ZnO, 3.289 Å for W-ZnO and 3.276 Å for M-ZnO were in good agreement with other experimental and computational reports. The rare earth (RE) metal-doped W- ZnO exhibited an average lattice expansion of approximately 1.2% (a = 3.294 Å, 3.291 Å and 3.295 Å) for RE (Ce, Dy, Eu) metal doping respectively. The Mechanical properties were evaluated through calculations of elastic constants and moduli. Doping pristine W-ZnO (Young’s modulus 115.0 GPa, Poisson’s ratio 0.36) with Dy, Eu, and Ce significantly enhances its stiffness to 217.5 GPa, 212.2 GPa, and 215.6 GPa, respectively, indicating synergistic strengthening effects. Additionally, the bulk modulus (B) increased from 136.7 GPa in undoped ZnO to 155.2, 157.3 and 153.6 GPa for RE (Ce, Dy, Eu) doping respectively. Dynamical stability of all doped structures was confirmed via phonon dispersion calculations. No imaginary frequencies were detected across the Brillouin zone, affirming that the RE-doped configurations are dynamically stable. Electronic properties investigations revealed significant modifications in the band structure upon doping. Pristine ZnO has a calculated band gap of 0.76 and 3.19 eV in the DFT and DFT+U formalism respectively. Doping with Ce, Dy, and Eu reduced the effective band gap to approximately 3.17 eV, 3.16 eV, and 3.18 eV, for RE (Ce, Dy, Eu) doping respectively at 1.85 % doping concentration. As the dopant concentration increased from 0-5.56%, the band gap generally decreased for all the RE-doped W-ZnO compounds tested. Optical properties were evaluated by computing the complex dielectric function and absorption spectra. It was observed that, RE doping extended the absorption edge of W-ZnO into the visible region. Specifically, Eu-doped ZnO exhibits a peak absorption coefficient of approximately 2.1 × 10⁵ cm⁻¹ at around 550 nm, while Ce- and Dy-doped variants demonstrate peak coefficients in the range of 2.0–2.1 × 10⁵ cm⁻¹. The refractive index and extinction coefficient analyses corroborate these findings, suggesting that RE doping effectively tunes the optical response to better match the solar spectrum. Transport properties were computed using BoltzTraP code integrated in quantum espresso. This assessment indicated that RE doping significantly improves the charge carrier concentration and conductivity of ZnO. While undoped ZnO exhibits a carrier concentration on the order of 10¹⁸ cm⁻³, the doped systems show enhancements approaching 1020 cm⁻³. This increase is further supported by an elevated density of states near the Fermi level, underscoring the potential for improved electron transport in DSSC photoanodes.