Three-Dimensional Nanoscale Organization of Highly Efficient Low Band-Gap Conjugated Polymer Bulk Heterojunction Solar Cells

Scanning probe examination on cross sections of bulk heterojunction blends containing an amorphous conjugated polymer, poly [2,6-(4,4-bis- (2-ethylhexyl)-4H-cyclopenta[2,]-b;3,4-b']-dithiopene)-alt-4,7-(2,1,3bezothiadiazole)], and [6,6]-phenyl C-71-butyric acid methyl ester reveals a structural change in the internal features of hole and electron transporting networks when the film is cast from a solution containing 2% by volume 1,8-octanedithiol. Phase separation of the nanoscalar domains becomes more defined, and the average sizes of hole and electron transporting networks double upon addition of the additive. Devices capable of electron- and hole-only transport show no significant improvement of the charge carrier mobilities. The increase in the size of the domains likely gives rise to less charge recombination.