  As the photovoltaic sector approaches 1 TW in cumulative installed capacity,
we provide an overview of the current challenges to achieve further
technological improvements. On the raw materials side, we see no fundamental
limitation to expansion in capacity of the current market technologies, even
though basic estimates predict that the PV sector will become the largest
consumer of Ag in the world after 2030. On the other hand, recent market data
on PV costs indicates that the largest cost fraction is now infrastructure and
area-related, and nearly independent of the core cell technology. Therefore,
additional value adding is likely to proceed via an increase in energy yield
metrics such as the power density and/or efficiency of the PV module. However,
current market technologies are near their fundamental detailed balance
efficiency limits. The transition to multijunction PV in tandem configurations
is regarded as the most promising path to surpass this limitation and increase
the power per unit area of PV modules. So far, each specific multijunction
concept faces particular obstacles that have prevented their upscaling, but the
field is rapidly improving. In this review work, we provide a global comparison
between the different types of multijunction concepts, including III-Vs,
Si-based tandems and the emergence of perovskite/Si devices. Coupled with
analyses of new notable developments in the field, we discuss the challenges
common to different multijunction cell architectures, and the specific
challenges of each type of device, both on a cell level and on a module
integration level. From the analysis, we conclude that several tandem concepts
are nearing the disruption level where a breakthrough into mainstream PV is
possible.
