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September 26, 2012

Speaker: Dr. David Strubbe, Research Associate, MIT (Materials Science Department with Professor Jeffrey Grossman)

Title: Exciton Self-Trapping In The Optical Response Of Pentacene Crystals From First Principles

Abstract: Pentacene is a prototypical organic semiconductor with optoelectronic and photovoltaic applications. It is known that the lowest-energy singlet excitation has a Stokes shift between absorption and emission of about 0.14 eV, but the structural deformation associated with this self-trapped exciton remains unknown. We begin with a calculation of the optical properties via the first-principles GW/Bethe-Salpeter (BSE) theory [ML Tiago, JE Northrup, and SG Louie, Phys. Rev. B 67, 115212 (2003); S Sharifzadeh, A Biller, L Kronik, and JB Neaton, Phys. Rev. B 85, 125307 (2012)]. We then study the self-trapping phenomenon via our reformulation of the Bethe-Salpeter excited-state forces approximation of Ismail-Beigi and Louie [Phys. Rev. Lett. 90, 076401 (2003)], which can describe the structural relaxation after optical excitation. The formalism is implemented in the BerkeleyGW code, freely available from

Biography: David Strubbe received his B.S. in chemistry and physics from the University of Chicago in 2005, and Ph.D. in physics from the University of California, Berkeley in 2012. His thesis was entitled "Optical and transport properties of organic molecules: Methods and applications," under the direction of Professor Steven Louie. He is currently a postdoctoral associate at MIT in the materials science department with Professor Jeffrey Grossman, where he is working on solar thermal fuels, thermoelectrics and photovoltaics.