Considerable effort has been put into optimizing photovoltaic devices through tuning the properties of the electron donor, but these systems often rely on a fullerene based electron acceptor. Although fullerene based materials have many desirable structural and electronic properties, their absorption spectra suffer from poor overlap with the solar spectrum, which significantly limits their ability to harness light. The development of non-fullerene based acceptors with good absorption in the visible region of the electromagnetic spectrum can limit our reliance on the donor material for light absorption, broadening the scope of available donor/acceptor combinations for organic photovoltaics.We have designed, synthesized and characterized four BODIPY-based small molecules with good absorption properties. Though these molecules differ in the structure of their core unit, the ?max is consistent (~535 nm). Through tuning of the �donor� strength of the core moiety, the red edge absorption can be selectively modified; stronger donors, such as cyclopentadithiophene, yield a more red shifted charge transfer band and, as a result, a lower band gap (1.54 eV).  These molecules also show low LUMO levels (-3.7 to -3.9 eV), which is desirable for efficient charge dissociation when paired with a wide variety of donor materials. Through preliminary analysis of the charge transport capabilities using field effect transistors, we observed that these molecules consistently show n-type behavior with �ele ~ 10-5 cm2 V-1 s-1. We have also fabricated direct and inverted photovoltaic devices using these molecules as the sole acceptor for the poly(3-hexylthiophene) donor. Preliminary optimization revealed that the device performance responds positively to morphological tuning through the use of solvent additives and annealing processes, criteria that may greatly benefit their use as acceptors in bulk heterojunction devices with other donor materials. A power conversion efficiency of 0.88% was achieved with a remarkable fill factor of 0.54 for ITO/ZnO/P3HT:CPDT  (1:2)/MoO3/Ag device.

Small molecule BODIPY dyes as acceptors in bulk heterojunction organic photovoltaics

247th National Meeting (2014)

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