Beyond efficiency: scalability of molecular donor materials for organic photovoltaics

The preparation of various molecules taken as representative examples of some of the main classes of molecular donors for organic solar cells has been analyzed in order to assess the complexity and possible scaling-up of their synthesis. Several parameters are taken into account to evaluate the synthetic complexity index (SC), namely, the number of synthetic steps, the reciprocal yield of each step, the number of operations (in particular column chromatographies) required for the purification and the safety characteristics of the used chemicals. As could be intuitively understood, large complex molecular structures that have led to the highest conversion efficiencies reported so far require more complex lengthy syntheses than smaller systems. These data underline the necessity to define working molecular structures that represent an acceptable trade-off between conversion efficiency, scalability and cost in order to design active photovoltaic molecular materials for possible large-scale production. The preparation of various molecules taken as representative examples of some of the main classes of molecular donors for organic solar cells is discussed in order to assess the complexity and possibilities of scaling-up their synthesis.