Variable band gap poly(3,4-alkylenedioxythiophene)-based polymers for photovoltaic and electrochromic applications

The major focus of this work was the synthesis of new narrow band gap (<1.8 eV) conjugated polymers that can be used to effectively absorb sunlight for application in solar cells. The investigated polymers were designed to absorb solar photons, and also to have the proper frontier orbital energies to ensure air stability and effective electron transfer to a fullerene-based acceptor on photoexcitation.;Based on these criteria, a family of soluble poly(arylene-cyanovinylenes) were synthesized by either Knoevenagel polycondensation or Yamamoto coupling polymerization. Here the aromatic rings in the polymer backbone were varied using benzene, thiophene, 3,4-ethylenedioxythiophene, and dialkyl-substituted 3,4-propylenedioxythiophene, and combinations thereof. Optically determined electronic band gaps of the resulting polymers varied from 1.4 to 2.1 eV. It was found that the narrowest band gaps resulted from the strongest donor acceptor interactions along the conjugated backbone.;The polymers were characterized by electrochemical and spectroscopic means. Solar cells were constructed and characterized using a solar simulator for power conversion efficiencies and monochromatic illumination to measure the incident photon to current efficiency (IPCE). Several types of devices were investigated: primarily those based on two-component blends comprising the poly(arylene-cyanovinylenes) and soluble fullerene derivatives, and three-component devices based on the addition of a high-molecular-weight commercial polymer to the aforementioned blends. Commercial conjugated polymers such as MEH-PPV (poly(2-methoxy-5-(2'-ethylhexyloxy)-para-phenylenevinylene)) were found to be effective as blending agents, as the high-molecular-weight polymers improved film quality, and the high band gap of MEH-PPV provided a broadening of the absorption spectrum of the blend. Device efficiencies of ca. 1% were achieved in these cases. The approach taken here of blending several polymers with varying band gaps is a little-used approach and suggests a means of maximizing solar spectrum overlap. The electrochromic properties of this family of polymers were also investigated, indicating that these are multifunctional materials.;The utility of so-called donor-acceptor dyads based on compounds that contain conjugated oligomers covalently attached to fullerenes, was also investigated. Additionally, synthetic routes toward a new class of donor-acceptor polymer based on tetrazine and 3,4-ethylenedioxythiophene were explored.