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The ability to harvest more solar energy more efficiently (in terms of overall cost-of-implementation as well as quantum efficiency) is a critical challenge facing the scientific community. The development of organic solar cells promises to address several of the disadvantages of traditional solid-state based technologies.  In particular, these offer the possibility of tailoring the absorption spectrum of the component materials, and of solution-processing light-weight, flexible, large area devices at low cost.

The details of the micro- and nanoscale morphology that develop in polymer blend films used in organic photovoltaic (bulk heterojuncction) devices play a critical role in their performance. A schematic diagram of a BHJ solar cell is shown in Figure 1.  The "active layer" is composed of a thin (~100 nm) phase-separated film of an electron donating species (a soluble semiconducting polymer such as a PPV with a soluble electron accepting species such as PCBM. Upon absorption of a photon, an exciton is created which can then migrate within the film.  At interfaces between donor and acceptor domains, the exciton is separated into free holes and electrons.  Under the influence of the inherent bias in the device (created by the different conductive materials used as electrodes), these free carriers can then travel to their respective electrodes. The details of the microscopic phase separation that arises between the interpenetrating donor (polymer) and acceptor (PCBM) networks in these films play a critical role in device performance.  The ideal architecture is one in which light is absorbed efficiently, while at the same time opportunities for exciton and free carrier recombination or annihilation are minimized, thereby simultaneously optimizing charge generation, separation, transport, and collection.  Various length scales are believed to be important in optimizing these different aspects of device performance.  Optimal light absorption requires a polymer layer on the order of at least 100 nm, while efficient charge separation requires a high interfacial area between the donor and acceptor components, ideally with domain sizes comparable to the exciton diffusion length (~10 nm) in order to minimize exciton recombination. The most significant advances in performance achieved for these devices in recent years are attributable to improvements in domain size and ordering within domains. We are interested in exploring the use of fluorocarbon-hydrocarbon interactions as a means of guiding and driving organization and domain size within the active layer of this type of solar cell. 

      Our work thus far has focused on the synthesis of new polyphenylenevinylene (PPV) type materials containing fluorocarbon side chains.  We prepare our polymers via the commonly used base-catalyzed GILCH polymerization (which gives rise to high MW PPVs), and have succeeded in preparing a variety of new fluorocarbon-containing PPVs. Because of solubility constraints encountered with these materials, we have thus far concentrated our efforts on a series of random copolymers (of varying composition) of our fluorinated monomers with hydrocarbon analogs as illustrated here.  (Because the GILCH route is not a well-controlled polymerization, block copolymers are unfortunately not accessible, as is typically the case for conjugated polymers.)  Initial AFM studies of these copolymers indicate that we may already be achieving enough "blockiness" in our random copolymers to give rise to some phase segregation.  AFM images of thin films one example of such a random copolymer are shown below: the films exhibit a fine structure in the phase image (on the right) that is not visible in the height image (on the left) that we attribute to some degree of microphase segregation.  In addition, these materials exhibit an enhanced fluorescence (as compared with hydrocarbon analogs) particularly in the solid state, further suggesting that inclusion of the fluorinated side chains results in some form of enhanced ordering.

       We have also begun investigating a variety of possible strategies that might lead us to "blocky" copolymers.  First, by starting with oligomers of PPV as the monomers for the GILCH polymerization, we may be able to access structures that have enough block-like character to further enhance the kind of phase separation that is our goal.  Another possibility involves copolymerizations of GILCH monomers of different reactivities as a means of preparing gradient polymers. 

      Finally, we have also begun preliminary characterization of diode and photovoltaic behavior of our fluorinated polymers, and are exploring the effect of different surface chemistries on the transport properties of our polymer films.