Modeling of surface gap state passivation and Fermi level de-pinning in solar cells

The behavior of gap states due to coordination defects (e.g., dangling bonds) and metal induced gap states (MIGS) is compared using density functional supercell calculations. While both types of gap states cause carrier recombination, they are passivated in different ways. Defects can be passivated by shifting their states out of the gap, whereas MIGS lie on normally coordinated atoms and their states cannot be shifted. Their "passivation" requires the insertion of an insulating layer to attenuate them sufficiently before they enter the semiconductor. We show that MIGS also cause Fermi level pinning, inhibiting the control of the work function by the contacts, and so they must also be attenuated to enable certain solar cell designs.