Events

Charge separation by photoexcitation in seimcrystalline polymeric semiconductors: An intrinsic or extrinsic mechanism?

April 5, 2011 at 3pm/36-428

Carlos Silva
University of Montreal

abstract:
Understanding charge generation by light absorption in polymeric semiconductors is of profound scientific importance due to the vigorous drive to develop organic solar cells. Confusion prevails with respect to the intrinsic charge photogeneration mechanism in neat (undoped) semicrystalline films. Numerous publications report charge photogeneration yields (the number of electron-hole pairs produced per absorbed photon) up to 30% on sub-picosecond timescales in neat regioregular poly(3-hexylthiophene) films. This is difficult to reconcile with the accepted picture that Frenkel excitons are the primary photoexcitations. Their binding energy is much higher than the lattice thermal energy at room temperature, such that direct charge generation ought to be improbable. Considering this, two fundamental questions arise: (i) what is the mechanism of direct charge photogeneration in semicrystalline polymer semiconductors? (ii) What is the role of solid-state microstructure in defining it? Here, we combine transient photoluminescence and absorption probes and find that charge photogeneration at 10 K occurs continuously over sub-nanosecond timescales, and not by a diffusion-limited exciton dissociation at defect sites. Rather, we conclude that it is an extrinsic process that occurs efficiently by dissociation of excitons localised at interfaces between crystalline and non-crystalline domains, and is driven by interfacial energetic disorder.

bio:
Carlos Silva, Canada Research Chair in Organic Semiconductor Materials, is Associate Professor of Physics at the Université de Montréal, having joined the department in 2005. He is the 2010 laureate of the Herzberg Medal of the Canadian Association of Physicists. He has extensive expertise in ultrafast optical probes of electronic dynamics in organic semiconductors with applications in optoelectronics. He obtained a PhD in chemical physics from the University of Minnesota in 1998 and was a Postdoctoral Research Associate in the Cavendish Laboratory of the University of Cambridge from 1998 to 2001. In 2001 he obtained an Advanced Research Fellowship from the UK Engineering and Physical Sciences Research Council, which he undertook at the Cavendish Laboratory. The central theme of his research programme concerns the understanding of electronic dynamics in organic semiconductors using transient photoluminescence and absorption spectroscopies.