  Metastable defects can decisively influence the diode factor and thus the
efficiency of a solar cell. The diode factor is also influenced by the doping
level and the recombination mechanisms in the solar cell. Here we quantify how
the various parameters change the diode factor by photoluminescence
measurements and simulations. In addition, we show that backside recombination
reduces the open circuit voltage in CuInSe2 solar cells by more than 40 mV.
Passivation by a Ga gradient is shown to be as efficient as a passivation by
dielectric layers. Increased backside recombination reduces the diode factor,
not because of less metastable defect transformation but because of a sublinear
increase in photo generated carriers with excitation. This reduction in diode
factor is unwanted, since the increased recombination reduces the voltage. A
higher doping level, on the other hand, reduces the diode factor, thereby
increasing the fill factor, and at the same time increases the voltage.
