Cobalt (II) oxide and nickel (II) oxide alloys as potential intermediate-band semiconductors: A theoretical study

Solar cells based on single pn junctions, employing single-gap semiconductors can ideally achieve efficiencies as high as 34%. Developing solar cells based on intermediate-band semiconductors (IBSCs), which can absorb light across multiple band gaps, is a possible way to defy this theoretical limit and achieve efficiencies as high as 60%. Here, we use first principles quantum mechanics methods and introduce CoO and Co_0.25Ni_0.75O as possible IBSCs. We show that the conduction band in both of these materials is divided into two distinct bands separated by a band gap. We further show that the lower conduction band (i.e., the intermediate band) is wider in Co_0.25Ni_0.75O compared with CoO. This should enhance light absorption from the valence band edge to the intermediate band, making Co_0.25Ni_0.75O more appropriate for use as an IBSC. Our findings provide the basis for future attempts to partially populate the intermediate band and to reduce the lower band gap in Co_0.25Ni_0.75O in order to enhance the potential of this material for use in IBSC solar cell technologies. Furthermore, with proper identification of heterojunctions and dopants, CoO and Co_0.25Ni_0.75O could be used in multi-color light emitting diode and laser technologies.