EP02.05.04 : Hydrogen Bond Directed Self-Assembly of Molecular Donors for Organic Photovoltaics

9:30 AM–9:45 AM Apr 4, 2018

PCC North, 200 Level, Room 222 BC

Asmerom Weldeab1 Daken Starkenburg1 Danielle Fagnani1 Lei Li1 Zhengtao Xu1 Scott Perry1 Ronald Castellano1 Jiangeng Xue1

1, University of Florida, Gainesville, Florida, United States

The bulk heterojunction (BHJ) photoactive layer design remains attractive for achieving cost effective and efficient organic photovoltaic (OPV) cells. Performance of such OPV devices critically depends on whether the nanoscale morphological structure of the BHJ is conducive to exciton transport to donor-acceptor interfaces and charge transport within each material phase. We are exploring a supramolecular approach based on the hydrogen-bond (HB) directed self-assembly of molecular donors to exercise control over donor-acceptor thin film blend morphology. Our initial work explored simple quaterthiophene donors functionalized with a phthalhydrazide HB unit to promote self-organization. Scanning tunneling microscopy (STM) was used to directly observe the trimer assembly of such HB-functionalized donor molecules. In vacuum-deposited thin film blends (with C60 and C70) the installed H-bonding interactions operate synergistically with π-stacking and have a strong and favorable impact on the absorption properties, molecular donor surface orientation, and phase separation. OPV devices for the H-bond capable donors show improved charge transport characteristics, external quantum efficiencies, and a 2-3 fold increase in power conversion efficiency over devices based on comparator HB-disabled donor molecules with the same donor chromophore design. We further explored the modularity of the supramolecular design with respect to donor chromophore structure and H-bond assembly motif to extend light absorption to longer wavelengths and realize other supramolecular assembly topologies (tetramers and hexamers). OPV cells based on such lower-gap oligomers indeed still possess more than two-fold increase in efficiency than those with HB-disabled comparator molecules, although there remain challenges in controlling the molecular orientation and stacking in these solution-processed films.