On the role of PTB7-Th:[70]PCBM blend concentration in ortho-xylene on polymer solar cells performance

The use of low-band-gap polymers as donors is a new approach to realize high-efficiency polymer solar cells. Among the low bandgap polymers, poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene]-2-carboxylate-2-6-diyl} (PTB7-Th) is one of the most promising candidates, thanks to the possibility of exploiting deposition protocols (technique, solvent, concentration) from the most-known poly{[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b')dithiophene]-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene]-2-carboxylate-2-6-diyl} (PTB7) and the fact that it shows a power conversion efficiency (PCE) exceeding 10%. In this paper, we focus on the role of the ink concentration for film deposition and the formation of the active layer. The effect of the solid content (PTB7-Th:[70]PCBM; [70]PCBM=[6,6]-phenyl-C-71-butyric acid methyl ester) in an ortho-xylene-based solution on the performance of the polymer solar cell was investigated. Further, a correlation between the solid content and the1,8-diiodooctane additive in the ink formulation with respect to device performance was discussed. A remarkable PCE of 8.2% was demonstrated.