Understanding the Effected Efficiencies of Polymer Solar Cells Employing Different Fullerene Multiadducts as Acceptors

The application of fullerenes with two or more adducts as acceptors has greatly enhanced the performance of bulk-heterojunction solar cells with poly (3-hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open-circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open-circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multi-adducts such as bis adduct of Phenyl-C61 -butyric acid methyl ester (bis-PCBM) or the indene-C60 bis-adduct (ICBA). Here we investigate the reasons for this change in performance of polymer solar cells(PSCs) based on [6,6]-phenyl C70-butyric acid methyl ester (PC70BM), ICBA and bis-PC70BM as the acceptors and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7) as the donor. The cell configuration is ITO/PEDOT: PSS/active layer /LiF/Al. The PSCs with PC70BM, ICBA and bis-PC70BM show PCEs of 7.29%, 4.92% and 3.33%, respectively. The change of performance of the PSCs could be mainly attributed to the different excition generation and charge collection of PSCs employing different fullerene multi-adducts as acceptors.