News & Events
Dr. Michael G. Walter
Posted on October 2, 2015
Date - October 2, 2015
Assistant Professor of Chemistry
“Linking Molecular Structure to the Singlet Exciton Diffusivity in Photoactive Porphyrin Thin Films”
In photosynthesis, photoinduced excitation energy travels through coordinated light-harvesting chlorophyll molecules to reaction centers within femtoseconds (or picoseconds). Rapid and coherent excitonic wave migration in these natural systems has motivated researchers to synthesize and examine new chlorophyll/porphyrin derivatives for use in artificial light-harvesting antenna assemblies. Enhanced light harvesting in solar conversion devices relies on developing materials with long exciton diffusion lengths that allow for dissociation of excited states into free-charge carriers at donor/acceptor interfaces before recombination. Thus, it is important to understand the relationship between the structure/molecular orientation of the porphyrin material and how this affects the distance over which the exciton can migrate before it decays. We have studied the singlet exciton diffusion properties of solution-cast thin films of carboalkoxyphenylporphyrins for use as sensitizers for organic heterojunction solar cells. The photoluminescent singlet decay lifetime PL(t) of pristine porphyrin films and films lightly doped with PCBM were used to obtain a relative quenching efficiency (Q). Q was used in 3D exciton diffusion Monte Carlo simulations to extract the exciton diffusion coefficient (D), LD and the nanocomposition of the blend. Extended PL decay times and exciton diffusion lengths occurred by increasing the alkyl chain length and branching of a meso-substituted carboalkoxyphenyl group, affecting porphyrin-porphyrin excitonic interactions and exciton hopping efficiencies. The effects of the peripheral porphyrin substituents, methods of processing the films, and molecular orientation on the exciton dynamics will be presented.