Multi-Intermediate-Band Character Of Ti-Substituted Cugas2: Implications For Photovoltaic Applications

Intermediate-band (IB) photovoltaic materials are designed to absorb a wider range of the solar spectrum by dividing the gap with a set of bands that plays a mediating role in two-step absorption processes. Thus far, the conventional model of IB absorbers has been focused on a single half-filled IB. We show that a multiple-IB picture with filled and empty bands provides a more convincing explanation of the experimental findings in some cases, and it should be considered as a possible scenario in understanding and engineering IB solar cells. Toward that end, we report the formation of two sets of IBs in a Ti-substituted CuGaS2 compound. Using hybrid functional calculations within the density functional theory framework, we show that the lower IBs are occupied and mainly derive from the Ti 3d(x)(-y)(2)(2) states, while the higher ones originate from the Ti 3d(z)(2) states and are unoccupied. The positions of the IBs within the gap are found to depend weakly on the dopant concentration and the relative distances of the dopant ions. In addition, for a more pertinent comparison to real materials, we present a practical way to combine and analyze the densities of states resulting from the energetically most probable microscopic structures. This multi-intermediate-band picture provides a platform to comprehend unexplained features of the recent experimental study on the material.