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.
Although research in the field of organic photovoltaics (OPV) still merely focuses on efficiency, efforts to increase the sustainability of the production process and the materials encompassing the device stack are of equally crucial importance to fulfil the promises of a truly renewable source of energy. In this study, a number of steps in this direction are taken. The photoactive polymers all contain an electron-deficient building block inspired on the natural indigo dye, bay-annulated indigo, combined with electron-rich thiophene and 4Hdithieno[3,2-b:2',3'-d]pyrrole units. The synthetic protocol (starting from indigo) is optimized and the final materials are thoroughly analyzed. MALDI-TOF mass spectrometry provides detailed information on the structural composition of the polymers. Best solar cell efficiencies are obtained for polymer:fullerene blends spin-coated from a pristine non-halogenated solvent (o-xylene), which is highly recommended to reduce the ecological footprint of OPV and is imperative for large scale production and commercialization.
Porphyrinoid small molecules can be used as electron donor or acceptor components in bulk heterojunction organic solar cells, which has resulted in steadily improving power conversion efficiencies. However, the effect of material purity is often neglected. In this work, a series of D1-A-π-D2-π-A-D1 conjugated chromophores based on the A 2 B 2 -meso-ethynylporphyrin core is synthesized. Different electron-donating (D1) end groups are chosen to investigate their influence on the material properties and optoelectronic characteristics. The porphyrin small molecules are tested as electron donor materials in bulk heterojunction organic solar cells in combination with PC 71 BM, affording an efficiency up to 4.6% under standard illumination. Furthermore, it is shown that (Glaser type) homocoupled side products are generated during the final Sonogashira cross-coupling reactions, which can be very challenging to remove, and their presence reduces solar cell performance by affecting the open-circuit voltage and fill factor.
Ternary organic photovoltaic devices were prepared through the addition of small amounts of metalloporphyin-terthiophenes to two established low bandgap polymer:fullerene blends. The methodology afforded a PCDTBT:PC 71 BM-based device that displayed an initial power conversion efficiency of 5.1%, an increase of 16% when compared to the binary blend. Among the range of metalloporphyrins considered in this study, the Cu(II)-porphyrin derivatives were found to result in the most significant efficiency improvements.
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