P-type electrical conduction in transparent thin films of CuAlO2

Optically transparent oxides tend to be electrical insulators, by virtue of their large electronic bandgap (greater than or equal to 3.1 eV). The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO-all n-type (electron) conductors-which are widely used as the transparent electrodes in flat-panel displays(1,2). On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a range of novel applications, For example, a combination of the two types of transparent conductor in the form of a pn junction could lead to a 'functional' window that transmits visible light yet generates electricity in response to the absorption of ultraviolet photons. Here we describe a strategy for identifying oxide materials that should combine p-type conductivity with good optical transparency. We illustrate the potential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivity up to 1 S cm(-1). Although the conductivity of our candidate material is significantly lower than that observed for the best n-type conducting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurmountable goal.