  Recent research in two-dimensional (2D) materials has boosted a renovated
interest in the p-n junction, one of the oldest electrical components which can
be used in electronics and optoelectronics. 2D materials offer remarkable
flexibility to design novel p-n junction device architectures, not possible
with conventional bulk semiconductors. In this Review we thoroughly describe
the different 2D p-n junction geometries studied so far, focusing on vertical
(out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional
junctions. We discuss the assembly methods developed to fabricate 2D p-n
junctions making a distinction between top-down and bottom-up approaches. We
also revise the literature studying the different applications of these
atomically thin p-n junctions in electronic and optoelectronic devices. We
discuss experiments on 2D p-n junctions used as current rectifiers,
photodetectors, solar cells and light emitting devices. The important
electronics and optoelectronics parameters of the discussed devices are listed
in a table to facilitate their comparison. We conclude the Review with a
critical discussion about the future outlook and challenges of this incipient
research field.
