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SUMMARY:The generation of free charges in efficient organic solar cells - 
 Samuel Smith (TCM Group\, University of Cambridge)
DTSTART:20151125T110000Z
DTEND:20151125T114000Z
UID:TALK62662@talks.cam.ac.uk
CONTACT:Joseph Nelson
DESCRIPTION:Organic semiconductors are substantially cheaper than their co
 nventional competitors\, and organic LEDs have already been commercialised
 . It is hoped that the development of efficient organic solar cells might 
 dramatically reduce the cost of renewable solar technologies. However most
  devices manufactured to date perform very poorly. The reason is simple\, 
 organic materials have low dielectric constants (ε ~ 3-4). When light is 
 absorbed it generates a strongly bound electron-hole pair\, which must be 
 separated into free charges before a current can be extracted from the dev
 ice. In most devices\, the charge separation timescale is longer than the 
 electron-hole recombination timescale\, and very little energy can be extr
 acted. In recent years\, a small number of efficient devices have emerged\
 , in which electrons and holes can be separated with near-unity efficiency
 . The origin of the improved device performance in efficient devices is po
 orly understood\, particularly as researchers often assume that the motion
  of charges within organic devices can be described by "hopping" models be
 tween localised electronic states. In a combination of experimental and th
 eoretical work (Gélinas _et al_.\, Science *343*\, 512)\, we conclusively
  demonstrate that the electron and hole can separate by several nm within 
 just 100 fs\; and we show that these results are explained by a simple min
 imal model\, which requires the emergence of a set of delocalised states. 
 The emergence of these states arises from superior crystallinity within ef
 ficient devices.\n\nFree charges diffusing through the device may re-encou
 nter each other\, and run the risk of delayed "non-geminate" recombination
 . I will extend the model developed above to explore this process\, and de
 monstrate that these same delocalised states are also able to suppress thi
 s secondary loss mechanism (Smith and Chin\, PCCP *16*\, 20305)\, consiste
 nt with a range of recent experimental observations\, and providing a comp
 lete picture of the charge generation processes within efficient devices.
LOCATION:TCM Seminar Room\, Cavendish Laboratory
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