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SUMMARY:Localization of electronic excitations in organic semiconductors: 
 theoretical views from different angles - Dr. Sergei Tretiak\, Theoretical
  Division\, Center for Nonlinear Studies and Center for Integrated Nanotec
 hnologies\, Los Alamos National Laboratory\, USA
DTSTART:20130709T133000Z
DTEND:20130709T143000Z
UID:TALK45751@talks.cam.ac.uk
CONTACT:Dr. Girish Lakhwani
DESCRIPTION:Organic conjugated materials have attracted considerable atten
 tion for unique photophysical and photochemical properties\, low cost\, li
 ght weight\, flexibility\, and convenience of solution-based processing te
 chniques. Synthetic design of new molecular structures targeting specific 
 optoelectronic properties (such as interactions with light and efficient c
 arrier transport)\, requires understanding dynamics and spatial delocaliza
 tion of electronic excitations. In this talk I will overview the interplay
  between the spatial extent/properties of electronic wavefunctions and res
 ulting electronic functionalities on example of several conjugated molecul
 ar systems. The simulation results have been obtained using a broad range 
 of modeling techniques\, ranging from molecular dynamics (MD) to electroni
 c structure theory to solid state effective Hamiltonian approaches. In par
 ticular\, our theoretical characterization of the molecular donor in a sol
 ution processed small molecule bulk-heterojunction photovoltaic device\, e
 lucidates electronic features responsible for record-breaking power conver
 sion efficiency. Combination of classical MD with first-principles density
  functional theory (DFT) details the role of conformational disorder and i
 ntermolecular interactions on the localization properties of electronic st
 ates leading to formation of carrier traps in amorphous aggregates of conj
 ugated polymers. Moreover\, strong electron-phonon coupling further induce
 s dynamical intramolecular localization (self-trapping) of electronic exci
 tations (polarons and singlet/triplet excited states). Embedding platinum 
 atoms in polymer’s backbone tunes the spin-orbit coupling and leads to a
 ppearance of metal-to-ligand charge transfer states\, which results in a r
 ecord intersystem crossing timescale in these polymers with their potentia
 l use for ‘white’ organic light emitting diodes. Finally\, localizatio
 n of excitonic states due to electron-phonon coupling in cycloparaphenylen
 es invalidates Condon approximation and breaks optical selection rules\, m
 aking these materials to be efficient emitters.
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
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