title: Reducing regioisomers of fullerene-bisadducts by tether-directed remote functionalization: investigation of electronically and sterically isomeric effects on bulk-heterojunction solar cells.
abstract: C60 bis-adduct containing a mixture of regio-isomers with different LUMO energy levels and steric geometries could greatly affect the morphological and bulk properties. To investigate the regio-isomer effect on solar cell performance, we have successfully designed and synthesized a regio-selective 4-acetatephenyl-4'-methylphenylmethano C60 bis-adduct (S-APM-CBA) by "tether-directed remote functionalization" strategy and a random 4-acetatephenyl-4'-methylphenylmethano C60 bis-adduct denoted as R-APM-CBA by traditional cyclopropanation. The dramatic reduction in the number of regio-isomers in S-APM-CBA is confirmed by the (1)H NMR and HPLC measurements and theoretical calculation. Compared to the R-APM-CBA-based device with a Jsc of 6.63 mA/cm(2), an FF of 44.3% and a PCE of 2.46%, the device using S-APM-CBA yielded a much lower Jsc of 1.48 mA/cm(2), an FF of 32.2%, and a PCE of 0.38%. Consistently, the electron-only device using S-AMP-CBA exhibited lower electron mobility than the R-AMP-CBA-based device. These results imply that the electronic shallow-trap effect ascribed to the LUMO energy variations turned out to be insignificant in the AMP-CBA system. The lower efficiency and mobility of S-AMP-CBA might due to the assumption that the most probable trans-4-III isomer in S-AMP-CBA prevents the intermolecular facial contact of fullerenes, thereby hindering the electron transporting. Furthermore, the nanomorphology of S-AMP-CBA and R-AMP-CBA active layers could be different because of their different three-dimensional structures. This research demonstrated that steric effect of regio-isomers in a given C60 bis-adduct is more crucial than electronic shallow-trap effect.
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan.
authors.forename: Ming-Hung
authors.initials: MH
authors.lastname: Liao
authors.forename: Yin-Yu
authors.initials: YY
authors.lastname: Lai
authors.forename: Yu-Ying
authors.initials: YY
authors.lastname: Lai
authors.forename: Yen-Ting
authors.initials: YT
authors.lastname: Chen
authors.forename: Che-En
authors.initials: CE
authors.lastname: Tsai
authors.forename: Wei-Wei
authors.initials: WW
authors.lastname: Liang
authors.forename: Yen-Ju
authors.initials: YJ
authors.lastname: Cheng