Recombination lifetime of free polarons in polymer/fullerene bulk heterojunction solar cells

The recombination lifetime of free polarons was measured using three different methods: electrical field dependent photoresponse, transient photoconductivity, and forward-to-zero bias transient-current response. The average free polaron recombination lifetime is estimated to be a few microseconds for poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells. The competition between sweep-out by the internal field and the loss of photogenerated carriers by recombination is analyzed. The short-circuit free polaron collection efficiency for P3HT:PCBM bulk heterojunction material was determined to be in the range of 80% to 90%. (C) 2012 American Institute of Physics. [doi:10.1063/1.3680879]