ABSTRACT
P3AT being modied with a fullerene (see Section 1.2) or other suitable nanoadducts becomes an active matrix of organic composite capable of converting visible light into electricity [35,51-57]. As a complex exciton is created in such system, there are different pathways its decay into the charge transfer state. The desired pathway would be exciton separation into extractable carriers, which should be displaced from the polymer:fullerene interface. This can happen by diffusion and relaxation of the polarons and electrons into higher-and lower-lying energy levels, respectively, which usually exist, because the energies of the HOMO and LUMO levels of conjugated polymers exhibit a broadened density of states. Relaxation of the carriers into the tails of the density of states can provide sufcient energy to overcome the Coulombic binding energy. With increasing distance from the material interface, the Coulombic attraction becomes less, and nally, the charge carriers become independent of each other. However, there are concurring processes, among them geminate recombination, which means the recombination of spin charge carriers originating from the same photoinitiated exciton. It was found [531] that the mobility and stability of charge carriers become higher considerably in the formation of bulk heterojunctions (BHJ), for example, by the chains of regioregular poly(3-hexylthiophene) with the globes of methanofullerene PC61BM as compared with other polymer:fullerene composites. A much longer charge carrier lifetime achieved in the P3HT:PC61BM BHJ should, therefore, lead to higher concentration of charge carriers and their reduced recombination rate. Specic nanomorphology of such composites could result in a screened Coulombic potential between the radical pairs photoexcited in their BHJ and facilitate their splitting into noninteracting charge carriers with a reduced probability of their further annihilation. It was explained by better structural order in the presence of interface dipoles provoking the creation of a potential barrier for carrier recombination in this composite. This implies that a longer charge carrier lifetime can be achieved at the same concentrations, which nally results in higher photocurrent and larger power conversion efciency of such solar cells. This is why PC61BM has appeared to be the most suitable electron acceptor to be used for a long time in prototypes of plastic solar cells and other molecular device.
