CPM Seminar

A roadmap to high efficiency intermediate band photovoltaics

Jacob Krich

University of Ottawa

Intermediate band (IB) materials -- semiconductors with an extra set of allowed states deep inside the band gap -- have the potential to be highly efficient and cost effective solar cells. When the IB concept was proposed in 1997, there were no known intermediate band materials. In recent years, great progress has been made in developing materials with intermediate bands, though power conversion efficiencies have remained low. I will discuss two aspects of improving the efficiency of IB solar cells.

First, I will discuss an important proposal for so-called “lifetime recovery.” The proposal claims that when the energy states of the IB are delocalized (i.e., metallic), they do not significantly reduce the nonradiative lifetime. I will show that this proposal is incorrect because the motion of the crystal lattice always re-localizes IB states.

Second, I will discuss the material requirements to increase the efficiencies of IB devices. I will present a figure of merit, consisting of parameters that can be measured without solar cell fabrication. I will present a new model for IB devices, including the behaviour of their junctions with n- and p-type semiconductors. This model can be used to find the optimal thickness of the IB region for given material parameters. I will show the minimum value of the figure of merit required for an IB to improve the efficiency of a device, providing a clear goal for the quality of future IB materials.