4. Active layer morphology

At the start of this article, we saw that there are two types of active layer morphology, the bilayer and the volume heterojunction. The bilayer has an intrinsic limitation that is difficult to overcome, namely its mismatch with the exciton diffusion length. The latter is around 10 to 15 nm, so beyond this distance, the photogenerated excitons are likely to recombine and not contribute effectively to photovoltaic conversion. So the active layer thickness that effectively contributes to absorption in a bilayer is around 30 nm (if we consider a binary mixture with an absorbing NFA), which considerably limits the number of charges generated. On the other hand, the transport of free charges to their respective electrodes is assumed to be very efficient. The volume heterojunction often suffers from the opposite evil, namely, a very broad D/A interface that allows many excitons to dissociate,...