Structural design principles for low hole effective mass s-orbital-based p-type oxides
High mobility p-type transparent conducting oxides (TCOs) are critical to current and future optoelectronic devices such as displays, transparent transistors or solar cells. Typical oxides have flat oxygen-based valence bands leading to high hole effective masses and low mobilities. This makes the discovery of high hole mobility oxides very challenging. Sn2+ oxides are known to form Sn-s/O-p mixtures and dispersive valence bands (low hole effective mass). However, not all Sn2+ oxides exhibit low hole effective mass, pointing to the importance of structural factors. Here, we analyze the electronic structure and chemical bonding of three Sn2+ oxides of interest as p-type oxides: SnO and the two K2Sn2O3 polymorphs. We rationalize the differences in their hole effective masses by their Sn–O–Sn angles. As band dispersion is governed by the orbital overlap, Sn–O–Sn angles near 180° maximize the overlap and minimize the hole effective mass. We show that this principle is generalizable to a larger set of Sn2+ oxides. Our work leads to simple structural design principles for the development of low hole effective mass oxides based on Sn2+ (but also other reduced main group cations) offering a new avenue for the ongoing search for high mobility p-type TCOs.