Colloquium: Hartwin Peelaers

ABSTRACT: Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. To enable these applications, TCO materials have to balance two conflicting properties: transparency and conductivity.

The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption by excitations of electrons to higher conduction-band states. In TCOs these direct transitions to higher conduction band states require an amount of energy larger than that of visible light photons. However, light absorption can still occur due to indirect free-carrier absorption, and a good understanding of these indirect processes is important to improve current materials and applications.

In this talk I will use insights obtained by accurate, parameter-free calculations based on density functional theory to discuss the physics of the processes limiting the transparency, and will compare quantitative results for several widely used TCO materials.