The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 137

Fig. 1 a Pyrite (100) surface
showing non-polar
arrangement b Bulk (100)
pyrite surface c Bulk pyrite
(111) surface. Reprinted with
permission from [ 21 ]
terminated and nonpolar. In the {100} facet there are three different formations that
can occur at the surface. The three different terminations are [S-Fe-S], [Fe-S-S], and
[S-S-Fe]. Both [Fe-S-S] and [S-S-Fe] are polar, and show that there is a possibility
for iron-terminated facets. [S-Fe-S] is nonpolar and iron is in a reduced coordi-
nation number of five, whose closeup view is presented in Fig. 1 a. This is of interest
due to iron being in trigonal bipyramidal state, which changes its splitting in ligand
field model. At first it was believed that this could introduce gap states causing the
seen problems of open circuit voltage. With the rise of stronger computational
systems, this has been dismissed as a problem with studies showing that these
differences will not put this surface state in the gap, and is not the cause of the
V oc, although it still deserves more attention [ 21 ]. It has also been shown
computationally that changing sulfur concentration during growth can control the
terminating element of the crystal facet [ 27 ]. With these two facets being quite