Star-shaped heptamers of discotic dyes as new materials for photovoltaic devices
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Star-shaped heptamers of discotic dyes as new materials for photovoltaic devices S. Holger Eichhorn, Nicholas Fox, and Bryan Bornais University of Windsor, Department of Chemistry and Biochemistry, Windsor, Ontario, Canada Abstract Potentially n-type and p-type semi-conducting discotic liquid crystal dyes are linked together to star-shaped heptamers, which might self-organize into super-columns of separated p-type and n-type columnar stacks. Their synthesis, mesomorphism, and electronic properties will be discussed along with their potential use in photovoltaic devices. Introduction Organic photovoltaic devices (OPVDs) have attracted increasing attention from academia and industry because of their potential as a low cost alternative to silicon based devices. OPVDs are not expected to reach the efficiencies of their inorganic counterparts (up to about 20 %) but present efficiencies of only 3 % and low operating stability still are crucial limitations. While high efficiencies are found for the charge carrier generation, the proportion of separated charge carriers remains low because of high recombination rates. It has been generally accepted that effective charge carrier separation takes place only at a donor-acceptor interface. Thus, high efficiencies are expected for materials with high interfacial areas between donor acceptor materials as well as good charge-carrier transport properties (large mean free paths of charge carriers). Optimization of both factors has been proven difficult despite tremendous recent advances in the morphological control of materials down to the molecular level. 1 Presented here is a new attempt of nanostructuring an organic semiconductor based on self-organizing discotic liquid crystals (DLCs). DLCs based on flat polyaromatic cores might co-facially stack into columnar p-type piles with stacking distances as small as 0.32 nm. This short distance allows for an intimate overlap of adjacent π-electron systems within a column that generates anisotropic charge carrier channels with n-type mobilities as high as 0.1 cm2 v-1 s-1.2 Discotic molecules with high electron affinity and with low ionization potential 1 have been investigated as potential n-type and p-type semiconductors. High hole mobilities have been experimentally Figure 1 Simplified cartoon of a photovoltaic device containing a star-shaped hetero-heptamer self-organized confirmed while electron mobilities seem to and self-aligned into nano-separated columns of the same be at least a factor of 103 lower. Recent discotic entity experimental and theoretical studies, however, have improved our understanding of the factors that govern charge carrier mobility in these systems and certainly provide new design criteria for synthetic chemists.3 O
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