Solution-processed CaMnO3−δ -based all oxide solar cells with high open-circuit voltage

Abstract

Finding a low-cost, stable solar absorber with suitable optical properties is one of the key aspects in promoting solar cell research. Perovskite-structured ${\mathrm{CaMnO}}_{3}$, composed of earth-abundant elements, is an environmentally friendly multifunctional material especially featuring thermoelectricity and G-type antiferromagnetism. In this paper, we explore the potential of $\mathrm{CaMn}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}$(CMO) as an absorber layer in all oxide solar cells. Solution-processed CMO shows strong light absorption capacity in the wavelength range of 400--750 nm with the estimated optical bandgap of 1.77 eV. First-principles calculations show $d\text{\ensuremath{-}}d$ transitions within the upper Mott-Hubbard bands, and lower Mott-Hubbard bands constitute the electronic bandgap, whereas the $p\text{\ensuremath{-}}d$ transitions between the O $2p$ and Mn $3d$ bands contribute to the optical absorption in the visible region. Furthermore, a gradual increase in the current value was observed under illumination in the photoconductivity measurement. We have designed a CMO absorber-layer-based all oxide solar cell using a conventional mesostructured ${\mathrm{TiO}}_{2}$ photo-anode and NiO hole transport layer (HTL). The suitable band alignment of the CMO absorber with both ${\mathrm{TiO}}_{2}$ and NiO enables us to achieve an average open-circuit voltage $({V}_{\mathrm{OC}})$ of 0.92 V with the maximum value of 1.03 V, which is among the highest reported values so far for oxide-based solar cells. Solar cell parameters were also validated using simulations. Our results add a direction in the search for low-cost, stable solar absorber materials.