Theory and simulation of organic solar cell model compounds: how packing and morphology determine the electronic conductivity.

We approach the electronic conductivity of simple models of organic solar cells containing linear and branched αα'-oligothiophenes and buckminsterfullerene. Close-packed model geometries are generated using a Monte Carlo method, this procedure is verified making use of an analogue model. The electronic structure is described by an extended Su-Schrieffer-Heeger Hamiltonian, the resulting potential energy surfaces relevant to charge transfer can be analyzed using Marcus' theory, leading to local and--via Kirchhoff's rule--global conductivities for uniform oligothiophene and fullerene systems and their mixtures. Dense fullerene systems or subsystems always exhibit a conductivity in excess of 100 S/cm. In contrast, oligothiophenes show a comparable conductivity only for uniform, well-ordered arrangements of layers. Branched oligomers show only a slight improvement over linear oligothiophenes. Our results support the bulk heterojunction approach as a design principle of organic solar cells from a theoretical perspective.