  We investigate nongeminate recombination in organic solar cells based on
copper phthalocyanine (CuPc) and C$_{60}$. Two device architectures, the planar
heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared
in view of differences in charge carrier decay dynamics. We apply a combination
of transient photovoltage (TPV) experiments, yielding the small perturbation
charge carrier lifetime, and charge extraction measurements, providing the
charge carrier density. In organic solar cells, charge photogeneration and
recombination primarily occur at the donor--acceptor heterointerface. Whereas
the BHJ can often be approximated by an effective medium due to rather small
scale phase separation, the PHJ has a well defined two-dimensional
heterointerface. To study recombination dynamics in PHJ devices most relevant
is the charge accumulation at this interface. As from extraction techniques
only the spatially averaged carrier concentration can be determined, we derive
the charge carrier density at the interface $n_{int}$ from the open circuit
voltage. Comparing the experimental results with macroscopic device simulation
we discuss the differences of recombination and charge carrier densities in
CuPc:C$_{60}$ PHJ and BHJ devices with respect to the device performance. The
open circuit voltage of BHJ is larger than for PHJ at low light intensities,
but at 0.3 sun the situation is reversed: here, the PHJ can finally take
advantage of its generally longer charge carrier lifetimes, as the active
recombination region is smaller.
