Lanthanide-Mediated Spectral Conversion and Electrochemical Charge Dynamics in Gadolinium Oxyselenide for High-Performance Solar Cells

Abstract

To improve visible-light absorption and charge transport in wide-band-gap oxide-based solar absorbers, photovoltaic (PV) devices were developed using terbium (Tb)- and europium (Eu)-doped gadolinium oxyselenide (GOSe) as the photoactive material. GOSe was selected for its thermal stability, optoelectronic tunability, and capacity to accommodate lanthanide dopants that introduce energy levels favorable for photon conversion and defect passivation. The nanophosphors were synthesized using a microwave-assisted solvothermal method to ensure controlled morphology and crystallinity. Structural analysis confirmed hexagonal Gd₂O₂Se phase formation, with dopant-induced modifications in unit cell parameters and bond lengths. Eu doping resulted in denser atomic packing and shorter bond lengths, while Tb doping introduced lattice strain, both influencing optical and charge-transport behavior. Optical characterization showed significant band-gap reduction from 3.8 eV in GOSe to 3.1 eV (GOSeT) and 2.8 eV (GOSeE), expanding absorption into the visible region. Photoluminescence confirmed characteristic 4f emissions at 543 nm (Tb) and 615 nm (Eu), indicating successful energy transfer and validating activation of Tb³⁺ and Eu³⁺ transitions. Electrochemical impedance spectroscopy and voltammetry analyses revealed improved electron mobility, reduced charge-transfer resistance (GOSeE: 0.60 kΩ), and enhanced surface kinetics. The doped nanophosphors were incorporated into thin-film solar cells with the architecture Ag/GOSe:Ln/CdS/ZnO/Al:ZnO/ITO. GOSeE-based devices achieved a power conversion efficiency of 3.22%, with a short-circuit current of 8.20 mA cm^–2 and an open-circuit voltage of 1.20 V, outperforming both GOSeT (1.40%) and undoped GOSe (0.12%). Energy-band analyses of the device layers showed favorable band alignment in doped samples, supporting efficient charge separation. 24-h stability tests under AM 1.5G conditions revealed that GOSeE had better device performance retention, indicating reduced recombination and better structural stability. This study confirm that RE-metal doping is an effective strategy to tune optical and electronic properties of GOSe for use in high-efficiency thin-film PV devices.