Synthesis, characterization and photovoltaic properties of azadipyrromethene-based acceptors: effect of pyrrolic substituents

Azadipyrromethene derivatives are conjugated molecules with high absorptivity in the visible to near-IR region and high electron affinity that have great potential as electron acceptors for solar harvesting applications. To fully take advantage of these molecules, it is necessary to understand their structure-property relationships. We recently showed that phenylethynyl pyrrolic substituents red-shifted the visible absorption band, increased the electron affinity, and significantly improved the device performance in organic photovoltaic devices. Here, we synthesized and characterized a series of azadipyrromethene derivatives and their chelates, where the pyrrolic substituents were chosen to examine the effect of ethynyl group, aryl group and alkyl solubilizing group. Using thienylethynyl or alkoxyphenylethynyl substituents caused a small red-shift of the absorption compared to phenylethynyl substituents. The zinc(II) complexes were blended with regioregular poly(3-hexylthiophene) as the electron donor and tested in organic solar cells. All fullerene-free solar cells studied showed good photovoltaic properties, with power conversion efficiencies ranging from 2% to 4%. The blend films all had similar AFM images with no evidence of large-scale segregation. When comparing solubilizing groups on the pyrrolic substituents, we find that the performance varied as follows: H > t-butyl > 2-ethylhexyl. These results pave the way for developing higher performance non-fullerene acceptors for organic photovoltaic devices.