Copper zinc oxide: investigation into a p-type mixed metal oxide system

In photovoltaics, ZnO is widely used both as an n-type window and buffer layer and as the basis for a range of transparent conducting oxide (TCO) top contact materials. Whilst there are reports of p-type doping, there have been no successful attempts to create a p-type ZnO-based TCO. Synthesis of an effective p-type TCO could be of significant benefit, enabling further development of technologies such as bifacial and multijunction devices. This study investigates the effects of combining ZnO with CuO, an intrinsically p-type narrow band-gap metal oxide, with the aim of synthesising a range of mid to wide band-gap p-type alloys. Alloying ZnO with CuO gave a range of p-type films with varying electrical, optical and structural characteristics. XRD patterns show that as copper content was increased above 24.5% of the total metal content, crystal structure changed from a typical ZnO wurtzite structure to a CuO tenorite structure. A second change occured at 35% copper with the emergence of two further tenorite peaks. These structural changes correlate to significant local increases in band-gap, visible and infrared transmission, and resistivity. These dramatic changes correspond to relatively minor compositional variations. Through careful control of the alloy composition it is therefore theoretically possible to tailor the material properties to suit a wide range of applications, both in photovoltaics and in other fields.