The Role of Ionicity in Transparent Conducting Materials

In this work, we show that large energy separation between a conduction band minimum (CBM) and higher conduction band (CBM+1) is a universal property in metal oxides, and it decreases when anion ionicity decreases from oxides to sulfides to selenides to tellurides. This originates from the low CBM and high CBM+1 in the oxides. The low CBM in oxides is explained by the high electronegativity of oxygen, which leads to weak coupling between cation_s and O_2s atomic levels and strong coupling between cation_s and unoccupied O_3s atomic levels, thus pushing downward the CBM level. The high CBM+1 mainly originates from strong Coulomb interaction between cation and oxygen due to strong ionicity and short bond length in oxides. Our understanding answered the fundamental and puzzling question, i.e., why most transparent conducting materials are transparent conducting oxides, and highlights the significance of ionicity in the design of high-performance transparent conducting materials.