Interaction of a Poly(phenylene vinylene) with an Organometallic Lewis Acid Additive. Fundamentals and Application in Polymer Solar Cells

The trimeric perfluoro-o-phenylene mercury compound Hg3 and poly(2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) interact strongly in solution and the solid state. The interaction is attributed to electron donor-acceptor complex formation, where MEH-PPV is the donor, and the Lewis acid Hg3 is the acceptor. The study reported herein explores the effects of the donor acceptor complex formation on the properties of MEH-PPV in solution and in the solid state. Addition of Hg3 to MEH-PPV solution or films leads to a distinct color change, and the change in the visible absorption spectrum and fluorescence of MEH-PPV is consistent with the formation of polymer aggregates. In the solid state, Hg3 induced aggregation is suggested to lead to formation of crystalline domains of the conjugated polymer. Transmission electron microscopy and grazing incidence X-ray scattering results support the hypothesis that the complex formation with Hg3 induces aggregation of the polymer. Transient absorption spectroscopy of the MEH-PPV:Hg3 aggregates in o-dichlorobenzene solution reveals ultrafast exciton dissociation to generate the MEH-PPV positive polaron, formed via photoinduced electron transfer to the Hg3 as an electron acceptor. The Xray crystal structure of a 1:1 complex between oligo(phenylene vinylene) and Hg3 gives insight into the structural interactions that likely account for the Hg3 induced polymer aggregation. Addition of small amounts of Hg3 to MEH-PPV:PC61BM bulk heterojunction solar cells results in a 33% increase in the power conversion efficiency, due to an increase in both the short-circuit photocurrent and open-circuit voltage. These factors are attributed to enhanced exciton dissociation coupled with improved carrier transport, resulting from the MEH-PPV:Hg3 donor-acceptor interaction.