Band edge evolution of transparent ZnM2IIIO4 (MIII=Co, Rh, Ir) spinels

ZnM2IIIO4 (MIII=Co, Rh, Ir) spinels have been recently identified as promising p-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results, ZnCo2O4 is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased d-d splitting combined with a concomitant lowering of Zn s/p conduction states is found to result in a ZnCo2O4(ZCO)<ZnRh2O4(ZRO)≈ZnIr2O4(ZIO) band gap trend, finally resolving long-standing discrepancies in the literature.