Effect of Spinel Inversion on (Co_xFe_1−x)₃O₄ All-Oxide Solar Cell Performance

Oxide materials have been widely investigated for use as absorber layers in high-performance solar cells, and in this study combinatorial methods were used to specifically investigate cobalt–iron (Co–Fe) oxide composites. Structural inversion of Co–Fe oxides from a normal to an inverse spinel structure occurs at the critical composition of approximately 33 % Fe added to Co, causing changes in the crystallinity, symmetry, sub-lattice vibrational modes, optical bandgap, and electrical resistivity. When used as an absorber layer in all-oxide solar cells with the multi-layered geometry of glass|FTO|TiO₂|Co–Fe–O|Au, enhanced photovoltaic performance was observed, with a maximum V_oc of 534 mV at a composition of approximately 45 % Fe and a 200 % improvement in P_max compared to cobalt-rich devices. Using combinatorial data maps of the various material properties, a significant correlation between the solar cell properties and the chemical composition of the Co–Fe–O layer was revealed. This correlation allows for a better understanding of the Co–Fe–O system, which is a necessary step towards the development of Co–Fe–O-based all-oxide solar cells.