Structural design principles for low hole effective mass s-orbital-based p-type oxides

Abstract

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.