The crystal structure of p-type transparent conductive oxide CuBO2

We employed ab initio global structural prediction algorithms to obtain the ground-state structure of CuBO2 This is a very promising p-type transparent conductive oxide that was synthesized recently, and thought to belong to the delafossite family. We proved that the true ground state is certainly not the delafossite structure, and that the most promising candidate is a low symmetry monoclinic phase. This is still a layered structure, but with boron and copper having a different coordination with respect to the delafossite phase. By reporting in 1997, a transparent semiconducting material (the delafossite CuAlO2) with “reasonable” p-type conductivity, Kawazoe [1] and his co-workers opened a new era of “invisible electronics” [2] built on active devices such as diodes. This led to the opening of a variety of fields, including transparent electronics and opto-electronics, organic lightemitting diodes, integrated electro-optical (waveguide) sensors, novel solar cells, and functional smart windows. The major ingredient missing for large-scale development of such technologies is a p-type semiconductor with a large gap, high conductivities andmobilities, together with controlled transparencies that can be manufactured industrially. The most promising materials are still probably the delafossites, such as the original p-type transparent conductive oxide (TCO), CuAlO2 [1]. From all the Cu-based delafossites, CuBO2 is particularly interesting as it possesses large band gaps (direct and indirect) and higher intrinsic conductivity than any other Cu-based TCO [3]. Furthermore, it has a structure symmetry compatible with α-Al2O3 sapphire and ZnO, the latter being already widely used as n-type TCO. This last point is important for the design of a p–n junction, the initial brick of any active electrical