  Singlet fission, in which the optical spin-singlet exciton dissociates into
two low energy triplet excitons, has been proposed as a viable approach to
enhance the quantum efficiency of organic solar cells. We show that even when
singlet fission is occurring in the donor molecule, the electronic structure at
the donor$-$acceptor interface must satisfy specific requirements for the solar
cell performance to be enhanced by this process. We focus on the
pentacene$-$C$_{60}$ solar cell, and on the basis of our calculations and
available experimental data, we conclude that there is not enough evidence that
these requirements are met by the donor$-$acceptor interface here. We propose
experiments that can determine whether the minimal requirement for enhanced
performance driven by singlet fission is met in this and other solar cells.
