We studied and compared the reported characteristics of 22 different
bulk heterojunction organic solar cells fabricated and characterized by
different research institutes. We only considered bulk heterojunction solar
cells where both the acceptor (the n-type) and
the donor (the p-type) are organic. All cells
were characterized under illumination with the standard A.M. 1.5 spectrum
and an intensity of 100 mW/cm2. The material properties (the highest occupied (HOMO) and lowest
unoccupied molecular orbital (LUMO) of donor and acceptor) and the device
characteristics (the open circuit voltage
Voc, the short circuit current density
Jsc, the fill factor
FF and the efficiency) are compared and related
to each other. One finding is that not the
Voc, but the
Jsc is the limiting factor for obtaining a
high efficiency with the current state of technology. Also an empirical
threshold of 0.2 eV is found between the LUMO's of the donor and acceptor,
necessary for exciton dissociation. There has long been a debate about the
origin of the Voc. In recent literature, it
is proposed that the Voc is not related with
the work function difference of the contacts, but with the energy difference
between the LUMO of the acceptor, and the HOMO of the donor (called `the
interface bandgap'). A relation between the
Voc and the energy levels of donor and
acceptor is searched from our empirical study.