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SUMMARY:ON THE PHOTON ENERGY AND FIELD-DEPENDENCE OF CHARGE GENERATION AND
  EXTRACTION IN POLYMER-BASED SOLAR CELLS - Prof. Dr. Dieter Neher\, Instit
 ute of Physics and Astronomy\, University of Potsdam\, Germany
DTSTART:20130305T100000Z
DTEND:20130305T110000Z
UID:TALK43083@talks.cam.ac.uk
CONTACT:Dr. Girish Lakhwani
DESCRIPTION:The generation of free charge carriers and their extraction to
  the electrodes are key processes in organic solar cells. There is an ongo
 ing debate on how the electric field across the active layer of such devic
 es affects the efficiency of these processes in competition to geminate an
 d non-geminate recombination. Similar to this\, contradictory results have
  been published regarding the role of hot CT-states in the free carrier ge
 neration process.\n\nWe have used time-delayed extraction experiments to q
 uantify generation and extraction of charge in various polymer-based solar
  cells. We find that for P3HT:PCBM\, the efficiency for free carrier gener
 ation is not affected by the electric field in both poorly-performing as-p
 repared blends as well as in efficient annealed blends\, meaning that the 
 solar cell properties of this material combination are entirely determined
  by the field-driven sweep-out of carriers in competition with non-geminat
 e recombination. Similar conclusions are drawn for polymer-polymer blends 
 based on P3HT with N2200. These blends were shown to exhibit exceptionally
  high fill factors. In contrast\, blends of the low bandgap polymer PCPDTB
 T with PCBM exhibit a pronounced field-dependence of charge generation\, s
 uggesting efficient geminate recombination. We show that the efficiency of
  geminate and non-geminate recombination in these blends is correlated to 
 the blend morphology\, and that both decay channels are strongly reduced i
 n blends with extensive interchain order. Finally\, we address the questio
 n whether free charge generation proceeds predominately via hot CT-states.
  We find that reducing the energy loss upon singlet exciton split-up cause
 s a reduced free carrier formation yield\, with increased field-dependence
 \, however\, both yield and field-dependence are only very slightly affect
 ed when the excess energy is lowered by exclusive excitation of the interf
 acial charge transfer states. This indicates that the free carrier generat
 ion mechanism proceeds via the same CT state manifold\, irrespective of th
 e initial photon energy.
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
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