Charge transfer excitons in low band gap polymer based solar cells and the role of processing additives

Organic semiconductor blends yielding efficient charge generation and transport are key components for the development of high performance organic bulk-heterojunction solar cells. In this paper the effect of the processing additive octane-dithiol on the charge transfer emission in poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1-b; 3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is investigated. Despite the fact that blends processed with and without additive show a ground state charge transfer optical absorption only the blend processed without additive shows a corresponding charge transfer emission. The presented experimental data show that the nano-morphology of the bulk-heterojunction blends plays an important role for the formation of emissive charge transfer excitons, and that it is a loss channel in the studied solar cells.