Porphyrin-Based Bulk Heterojunction Organic Photovoltaics: The Rise of the Colors of Life

Organic photovoltaics (OPV) represent a thin-film PV technology that offers attractive prospects for low-cost and aesthetically appealing (colored, flexible, uniform, semitransparent) solar cells that are printable on large surfaces. In bulk heterojunction (BHJ) OPV devices, organic electron donor and acceptor molecules are intimately mixed within the photoactive layer. Since 2005, the power conversion efficiency of said devices has increased substantially due to insights in the underlying physical processes, device optimization, and chemical engineering of a vast number of novel light-harvesting organic materials, either small molecules or conjugated polymers. As Nature itself has developed porphyrin chromophores for solar light to energy conversion, it seems reasonable to pursue artificial systems based on the same types of molecules. Porphyrins and their analogues have already been successfully implemented in certain device types, notably in dye-sensitized solar cells, but they have remained largely unexplored in BHJ organic solar cells. Very recent successes do show, however, the strong (latent) prospects of porphyrinoid semiconductors as light-harvesting and charge transporting materials in such devices. Here, an overview on the state-of-the-art of porphyrin-based solution-processed BHJ OPV is provided and insights are given into the pathways to follow and hurdles to overcome toward further improvements of porphyrinic materials and devices.