Schedule Oct 08, 2009
Can Theory Help in Understanding Photoexcitations in Organic Semiconductors? An Experimentalist's Perspective.
Suchi Guha, Univ. of Missouri & KITP

Conjugated organic molecules such as short-chain oligomers and long-chain polymers exhibit unique electronic and optical properties that include doping induced conductivity, high quantum efficiency of photoluminescence and nonlinear optical response. The delocalized and highly polarizable character of the pi-orbitals make these materials attractive for photonic and optoelectronic applications.

Polymer photovoltaics based on donor-acceptor (D-A) bulk heterojunctions are becoming viable candidates for the solar cell industry. In the first part of the talk, I will present some highlights of our recent work in utilizing triplet excitons for application in polymer photovoltaics (PV). The PV process in these materials is highly complex involving formation and diffusion of excitons to the D-A sites, formation of intermediate charge transfer complexes (CTC) and their separation into charges. Can ab-initio theoretical methods predict such complex PV processes- what is the diffusion length of singlet and triplet excitons? How accurate are the singlet-triplet splitting energies? How do CTCs form and why is their signature different in the presence of triplet excitons?

In the second part of the talk, I will focus on structure-property relationships in a blue-emitting polymer, polyfluorene, which has a huge potential in display and light-emitting diode applications. Calculations of the vibrational spectrum using density-functional theory (DFT) along with our experimental Raman scattering results reveal the intricate nature of a planar backbone conformation of polyfluorene, which is known to red-shift the photo/electro-luminescence. In the context of Raman intensity calculations, I will discuss a very simple phenomenological model, the so-called bond polarizability model, which has been extensively used in fullerenes.

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