Effects of blend composition on the morphology of Si-PCPDTBT:PC₇₁BM bulk heterojunction organic solar cells

This work investigates the effects of the solution blend composition on the morphology of binary bulk heterojunction organic solar cells composed of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM). The polymer-fullerene ratio was varied from 2:9 to 1:1 (PC₇₁BM weight ratio from 82 to 50 wt%). The dependence of the self-assembly of the polymer phase on the blend composition was investigated using X-ray diffraction measurements. A high polymer loading is required to allow the formation of large crystalline polymer domains in this materials system. X-ray photoelectron spectroscopy measurements revealed that this blend compositional dependence on the crystalline domain size correlated with a gradual alteration of the vertical phase distribution of polymer throughout the active layer. Using photoluminescence and short-circuit current results, we show that the efficacy of exciton dissociation is highly dependent on the solution blend composition. Coupled with mobility measurements, which indicate the hole mobility is also heavily dependent upon the content of polymer in the film, we explain the optimum blend performance of Si-PCPDTBT:PC₇₁BM organic solar cells which occurs at a blend ratio of 2:3.