In the beginning 90s, a novel concept was introduced, accounting for the low exciton diffusion length in disordered organic semiconductors, as well as the required thickness for a sufficient light absorption: the so-called bulk heterojunction solar cell [Heeger 1995]. This approach features a distributed junction between donor and acceptor material: both components interpenetrate one another, so that the interface between them is not planar any more, but spatially distributed. It is implemented by spincoating a polymer:fullerene blend, or by coevaporation of conjugated molecules. Bulk heterojunctions have the advantege of being able to dissociate excitons very efficiently over the whole extent of the solar cell, and thus generating polaron pairs anywhere in the film. The disadvantage is that it is somewhat more difficult to separate these polaron pairs due to the increased disorder, or that percolation to the contacts is not always given in the disordered material mixtures. Also, it is more likely that trapped charge carriers recombine with mobile ones. However, the positive effects outweigh the negative.
The most important processes of generation and recombination in disordered organic solar cells are shown in the figure. Excitons are photogenerated, diffuse to a donor-acceptor junction and dissociate to polaron pairs (a) or recombine radiatively (b). If polaron pairs are generated, they can be also separated, now with help of an external electric field; the then free polarons can hop to the corresponding electrodes to generate a photocurrent (a) or recombine with other mobile or trapped charges (c). For an efficient bulk heterojunction solar cell, a good control of the morphology is very important. Rather simple methods of optimisation have been successfully performed only in the new millenium. The choice of solvent [Shaheen 2001] as well as the annealing of the solution processed polymer:fullerene solar cells [Padinger 2003] both lead to a more favourable inner structure in view of polaron pair dissociation and charge transport. Thus, the power conversion efficiency was increased manyfold, in case of the annealing from a bare half percent to above 3 percent. Might not be much, but the steep increase shows the potential. Indeed, optimisation by novel routes is a continuing process. Coevaporated Copper Phthalocyanine / Fullerene solar cells have reached 5.0% efficiency [Xue 2005], and solution processed polythiophene:fullerene cells even 5.8% [Peet 2007].
Next time, we’ll be looking a bit closer into advanced device architectures. Stay tuned;-)