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SUMMARY:Charge-transfer absorption and emission in organic donor/ acceptor
  solar cells - Koen Vandewal\, Linköping University
DTSTART:20110607T133000Z
DTEND:20110607T143000Z
UID:TALK31402@talks.cam.ac.uk
CONTACT:Jenny Clark
DESCRIPTION:For an efficient conversion of photons to electrons by organic
  materials\, the presence of a material interface between an electron dona
 ting and electron accepting material is crucial. In order to probe interfa
 cial properties directly\, highly sensitive measurements of absorption\, p
 hotocurrent\, photoluminescence and electroluminescence spectra of organic
  D/A\nphotovoltaic devices are performed. For material combinations exhibi
 ting a decent charge carrier generation\, weak absorption and emission inv
 olving\na CT state with energy ECT lower than the lowest optical gap of bo
 th donor and acceptor can be observed. We find that the internal quantum y
 ields of photocurrent generation are similar for both pure phase excitatio
 ns and direct excitation in the low energy part of the CT absorption band\
 , indicating that properties of this relaxed CT state will determine the f
 ree charge carrier\ngeneration rate and short-circuit current (Jsc).\n\nAl
 so the open-circuit voltage (Voc) can be related to CT state properties: T
 he principles of reciprocity and detailed balance provide a relation betwe
 en\nthe CT absorption and emission actions\, and Voc. This relation is sho
 wn to be valid for a range polymer:fullerene photovoltaic devices\, at dif
 ferent\ntemperatures and illumination intensities. When measured under sol
 ar illumination\, we find an energetic difference between ECT and qVoc of 
 ~0.6 eV\nfor this type of photovoltaic devices.\n\nIn order to optimize Vo
 c\, material combinations with ECT close to the optical gap of the main li
 ght absorbing material (Eg) are desirable. A range of D/\nA combinations w
 ith different Eg-ECT values are investigated and possible detrimental effe
 cts on photocurrent generation are discussed.
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
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