  The discovery of Fe-based superconductors (FBS) as the second class of
high-temperature superconducting transition (high-Tc) materials after the
cuprates generated a significant impact on the community of fundamental and
applied superconductivity research. Whenever a new class of high-Tc
superconducting materials is discovered, a lot of effort is devoted to growing
single crystals and epitaxial thin films for exploring basic physical
quantities. Although several properties of FBS are similar to the cuprates
(large upper critical fields, resulting from a short coherence lengths, and
small carrier density), others are distinctly different. For instance, in FBS
the symmetry of the superconducting order parameter is most likely not a d-wave
but an s+-wave, depending on the stoichiometry, crystallographic system, and
doping level. Additionally, the critical current densities of FBS are less
sensitive to the presence of grain boundaries (GBs) than those of the high-Tc
cuprates. These features are highly beneficial for the realization of cheaper
conductors for high-field magnets at low temperatures. Indeed, several groups
have demonstrated FBS thin films on technical metallic substrates and
powder-in-tube processed FBS wires as proof-of-principle studies for conductor
applications. FBS on technical substrates also give many opportunities for
studying how GB networks affect the critical current and how uniaxial strain
impacts the superconducting properties. In this article, we review FBS thin
films, especially on technical metallic substrates, and focus on
application-relevant properties like pinning improvement by natural and
artificial defects as well as the transparency of GBs and GB networks. The
recent development of FBS thin films on technical substrates and their
superconducting properties are presented and the performance gap with respect
to films on single crystals is discussed.
