Performance of ZnFe2O4 as a photoabsorber in solution-processed all-oxide planar photovoltaics

Low-cost, stable, environment-friendly solar cells are the key aspects of modern-day photovoltaics, and oxide absorbers have become the focus of work towards stable devices. Here we explore the viability of semiconducting transition-metal spinel oxide ZnFe₂O₄ (ZFO) as an absorber layer in solution-processed all-oxide planar solar cells. A 300 nm thick spin-coated ZFO shows a typical cubic spinel structure with a calculated band gap of 1.68 eV, as estimated from density functional theory. Absorption and emission spectra show a direct band gap of 2.19 eV with a maximum absorption coefficient of 10⁵/cm, and the film shows a steady increase in photocurrent on illumination by white light. A conventional solution-grown TiO₂/ZFO/NiO all-oxide thin film heterojunction solar cell prepared on the fluorine-doped tin oxide coated glass substrate shows an open circuit voltage (VOC) of 519 mV under air mass 1.5 solar illumination. Furthermore, upon doping the TiO₂ layer with lithium, enhancement in recombination resistance and reduction of charge transfer resistance results in an increased VOC of 640 mV. Our result introduces spinel oxide as a stable alternative for a photoabsorber in all-oxide photovoltaics, enhancing the multifunctionality of spinel ferrites.