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SUMMARY:Loss mechanisms in Polymer-Fullerene Solar Cells - PD Dr. Carsten 
 Deibel\, Experimental Physics VI\, Faculty of Physics and Astronomy Julius
 -Maximilians-University of Würzburg Am Hubland\, Germany 
DTSTART:20130122T143000Z
DTEND:20130122T153000Z
UID:TALK43079@talks.cam.ac.uk
CONTACT:Dr. Girish Lakhwani
DESCRIPTION:In recent years the power conversion efficiency of organic sol
 ar cells based on polymer:fullerene blends improved to beyond 10%. These n
 ew material compositions deserve particularly consideration in order to un
 derstand the crucial steps from photon absorption to photocurrent\, to gai
 n a deeper understanding and aid the ongoing optimisation. I will present 
 some of our work concerning loss mechanisms in P3HT:PC60BM and PTB7:PC70BM
  solar cells. For the latter\, the power conversion efficiency changes dra
 matically from about 3% to about 7% upon adding 1\,8-diiodooctane (DIO) as
  a co-solvent to the chloroform solution are investigated in view of the d
 ominating loss processes. By combining transient photovoltage (TPV)\, volt
 age dependent charge extraction (CE) and time delayed collection field (TD
 CF) measurements\, we show that in the devices processed from chlorobenzen
 e solution severe geminate and nongeminate recombination occur\, while the
  use of DIO facilitates charge photogeneration and strongly reduces gemina
 te losses. From the thus measured charge carrier recombination rate we det
 ermine the nongeminate loss current of the device\, allowing us to reconst
 ruct the current/voltage characteristics across the whole operational rang
 e of the 7% PTB7:PC71BM solar cells. In contrast\, the reconstruction devi
 ates from the measured j/V characteristics for the lower performance devic
 es without DIO. Based on our experiments\, we propose that this discrepanc
 y is due to a combination of field dependent photogeneration and trapped c
 harge carriers in isolated domains of pure fullerene phases. The nongemina
 te recombination dynamics are explained in detail\, and are discussed in v
 iew of Langevin-like and trap-assisted recombination.
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
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