Amorphous selenium-alloyed TeOx thin-film solar cells
Tellurium dioxide (TeO$_2$) is a wide-bandgap oxide with attractive processing versatility and emerging p-type semiconducting behaviour, making it a promising material system for optoelectronic and photovoltaic applications. However, TeO$_2$-derived thin films prepared by scalable manufacturing processes, such as vacuum sublimation, can contain residual metallic Te$^0$ species that introduce trap states, limit hole transport, and promote carrier recombination. Here, we demonstrate a selenium alloying strategy to control the defect chemistry of vacuum-sublimed TeO$_x$ films. TeO$_2$:Se powder mixtures (90:10~wt\%) were thermally evaporated and subsequently annealed in air at 225~$^\circ$C for up to 9~h, yielding amorphous TeO$_x$:Se films with an optical bandgap of 0.98~eV under optimised conditions. Structural, chemical, optical, and electrical analyses show that prolonged annealing promotes Se redistribution, suppresses residual Te$^0$ to approximately 0.4\%, reduces the film's surface roughness, and shifts the valence-band edge. To evaluate the potential of TeO$_x$:Se for photovoltaic applications, we fabricated thin-film solar cells with an ITO/ZnO/TeO$_x$:Se/MoO$_x$/Ag architecture. Devices based on an 80~nm TeO$_x$:Se active layer annealed for 9~h exhibit optimal performance, with broadband photocurrent generation from 300 to 1000~nm, yielding an open-circuit voltage of 0.50~V, a short-circuit current density of 8.50~mA~cm$^{-2}$, a fill factor of 0.44, and a maximum power conversion efficiency of approximately 1.87\%. These results establish selenium-alloyed TeO$_x$ as a viable oxide-derived absorber for thin-film photovoltaics and highlight chemical-state control as an effective route for improving photovoltaic performance.