Transparent conducting oxides (TCOs) are critical components in many devices including solar cells and touchscreens. The search and development of new TCOs combining high conductivity and transparency is a major endeavor of modern Materials Science. Novel p-type TCOs are especially greatly sought for as they lie much behind their n-type counterpart and the discovery of a high performance p-type TCO would enable important technological breakthroughs.
There are two types of transport mechanism for carriers in materials: band transport involves delocalized, almost free, carriers while small-polaron transport involves carriers trapped in the crystalline lattice. Materials exhibiting transport through small polarons have been traditionally disregarded for TCO applications as they offer small mobilities. In this work, we use well-established physical models to compare the performances of TCOs based on band- and small-polaron transports. Surprisingly, we demonstrate that small-polaron TCOs can outperform band TCOs in terms of transparency and conductivity, especially p-type. We link this unexpected observation to the absence of collective, Drude-like, optical absorption and reflection in small-polaron materials.
Using our analysis, we outline what materials properties are necessary for high performance small-polaron TCOs, leading to a series of design principles. We also further analyze the recently proposed Sr-doped LaCrO3 p-type TCO presenting small-polaron transport . Using first-principles computations as well as experimental data, we rationalize the good performances of Sr-doped LaCrO3 in view of these design principles and suggest avenues for its improvement. We explain how to obtain the small-polaron properties of a material from first principles and motivate the search for new efficient small-polaron p-type TCOs through our outlined design principles.
 Adv. Mater. 27, 5191–5195 (2015).