Introduction. -Materials that display the dual properties of optical transparency and metallic-like conductivity are sparse, but they are almost universally required in modern technologies: as transparent contacts in solar cells; thin film transistors in display and touch-screen technology; 5,6 coatings in smart and low-emission windows. The transparent conducting materials (TCMs) used in the above applications are invariably n-type posttransition metal oxides (such as Sn-doped In 2 O 3 , F-doped SnO 2 and Al-doped ZnO), which routinely display electron conductivity on the order of 1000 S cm -1 to 10 000 S cm -1 and optical transmission above 90%. Meanwhile, the development of p-type TCMs has been less successful -hole conductivity is often 1 to 2 orders of magnitude lower, optical transparency is much harder to achieve, and the chemical and thermal stability of p-type TCMs is often poorer than the n-type metal oxides. This disparity between p-and n-type TCM technology is a major bottleneck in the development of the next generation of opto-electronic devices, which will have at their heart fully transparent p-n junctions.