  Singlet fission is a form of multiple exciton generation which occurs in
organic chromophores when a high energy singlet exciton separates into two
lower energy triplet excitons, each with approximately half the singlet energy.
Since this process is spin-allowed it can proceed on an ultrafast timescale of
less than several picoseconds, outcompeting most other loss mechanisms and
reaching quantitative yields approaching 200%.
  Due to this high quantum efficiency, the singlet fission process shows
promise as a means of reducing thermalisation losses in photovoltaic cells.
This would potentially allow for efficiency improvements beyond the
thermodynamic limit in a single junction cell. Efforts to incorporate this
process into solar photovoltaic cells have spanned a wide range of device
structures over the past decade. In this review we compare and categorise these
attempts in order to assess the state of the field and identify the most
promising avenues of future research and development.
