Are Small Polarons Always Detrimental to Transparent Conducting Oxides ?

Transparent conducting oxides (TCOs) are critical components in many devices like solar cells or 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 carrier transport mechanism in materials: band transport involves delocalized 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.
Using our analysis, we outline the materials properties necessary for high performance small-polaron TCOs, leading to a series of design principles for the search of new efficient small-polaron p-type TCOs.