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Molecular designs toward improving organic photovoltaics
Organic photovoltaics (OPVs) that have been studied to date have poor power conversion efficiencies. This dissertation focuses on various molecular designs that could lead to both a fundamental understanding of photoinduced charge separation at a molecular level and also provide a solution to improve bulk properties of organic materials to overcome the poor efficiencies of OPV devices. The effect of molecular architectures on the efficiency of electron transfer, a primary step in OPVs functioning, is evaluated in this work. We have shown that even though dendrimer provides an interesting architecture for efficient electron transfer due to the presence of multiple peripheries around a single core, this architecture leads to trapping of charge at the dendritic core. This results in a decrease in the electron transfer efficiency in solution and also limits the possibility of charge transport to the electron in a photovoltaic device. Non-conjugated polymers containing conductive EDOT units at side chains were also designed and synthesized. The frontier energy levels of these polymers can be easily tuned by changing the conjugation lengths of side chain EDOT oligomers. Moreover, by incorporating crosslinkable units as co-side chains, the absorption bandwidth of these polymers can be manipulated as well
Effects of Dendronization on the Linear and Third-Order Nonlinear Optical Properties of Bis(thiopyrylium) Polymethine Dyes in Solution and the Solid State
Effects of the size and attachment position of benzyl
aryl ether
dendrons covalently attached to bisÂ(thiopyrylium) penta- and heptamethines
on the optical properties of these dyes in solution and in solid films
have been investigated. In dilute solution the low-energy absorption
bands of some of the dendronized species differ from those of the
parent compounds in having much smaller transition dipole moments,
this effect possibly due to differences in ion pairing, while at higher
concentrations, dye–dye interactions lead to a decrease in
the transition dipole moments of the nondendronized species, but not
of the dendronized ones. Consequently, in the high concentration range,
dendronized and nondendronized species exhibit similar values of the
real part of the microscopic third-order polarizability at 1550 nm.
Solid-state film absorption spectra suggest that the dendrons significantly
disrupt the chromophore–chromophore interactions seen for the
nondendronized species, reducing, but not eliminating, linear absorption
losses in the near-IR, and suppressing absorption peaks that are hypsochromically
shifted from the solution spectra maximum: centrally placed dendrons
have a larger effect than terminal dendronization, so that the corresponding
thin-film spectra more closely resemble those seen in solution with
increasing generations of dendronization. Z-scan measurements at 1550
nm indicate that the third-order susceptibility of dendronized heptamethine
guest–host films depend approximately linearly on doping ratio
of dyes and are in reasonable agreement with values extrapolated from
solution-derived third-order polarizabilities; in contrast, the susceptibilities
of films highly doped with an undendronized analogue fall short of
values expected from solution polarizabilities