Quasi-Bilayer All-Small-Molecule Solar Cells Based on a Chlorophyll Derivative and Non-Fullerene Materials with Untraditional Energy Alignments

In this study, quasi-bilayers based on a chlorophyll-a derivative named CHL and non-fullerenes (NFs) known as Y6, IT-4F, and IDIC are prepared by sequential spin-coating methods, and they are applied to organic solar cells with a device architecture of indium tin oxide (ITO)/ZnO/CHL/NFs/MoOx/Ag. Such a device has a distinct difference from that of the general one. Specifically, there is a stereotype that NFs are taken as electron acceptors and are consequently arranged next to electron transporting layers in general studies, whereas in this study, spin-coated NFs are placed adjacent to a hole transporting layer of MoOx. Considering that CHL/NF quasi-bilayers prepared by spin-coating share a vertical concentration contribution difference, the fabricated device could work normally with unconventional energy alignments. An optimized power conversion efficiency (PCE) of 2.31% is achieved by a CHL/Y6-based all-small-molecule solar cell followed by a CHL/IT-4F-based device (1.46%) and then a CHL/IDIC-based device (0.99%). This result is considered to be caused by the differences between the light-harvesting ability, phase grain of the active layers, exciton dissociation efficiency, charge transfer resistance, and charge collection efficiency of the devices. Although the PCEs of these CHL/NF-based devices are lower than those of the state-of-the-art ones, the device architecture would break the internal constraints of researchers and provide new inspirations for future device designs. Moreover, the combination of the low cost and abundant storage of natural chlorophyll is also attractive and promising to reach a higher PCE in the future.