Chapter 10: Small Molecule Organic Solar Cells

In this chapter, we review recent progress in small-molecule organic solar cells. First, we introduce the p-i-n-structure which is realized by combining the intrinsic absorber zone with doped transport layers. The doping of the transport layers is realized by mixing the host transport material with dopant molecules which lead to charge transfer in the ground state, thus creating free carriers. Such doped layers offer a number of advantages when used in organic solar cells: they improve the built-in field, allow easy optimization of thin-film optics and thus absorption in the photovoltaically active layers, and lead to good ohmic contacts even when the work function of the electrodes does not fit well to the adjacent organic transport materials. The materials systems used for doped layers and their influence on the built-in field are discussed in detail. We then discuss various absorber materials leading to improved solar cell parameters and overall efficiencies. Among the many potential materials classes for small-molecule organic solar cells, thiophene derivatives have shown excellent properties. By variations of the electronic core and the alkyl side groups, it is possible to independently study the influence of the electronic levels of the molecule and the crystal packing. By photo-induced absorption, we study the exciton separation as a function of orbital energies and temperature. Finally, we discuss optimized cells. The p-i-n concept allows to easily stack cells on top of each other to realize multi-junction organic solar cells. The key challenges here are current matching and optical design optimization. Combining all technologies, efficiencies of 12% have been reached.