Title: Symmetry and Coplanarity of Organic Molecules Affect their Packing and Photovoltaic Properties in Solution-Processed Solar Cells
Authors: Lan, Shang-Che
Raghunath, Putikam
Lu, Yueh-Hsin
Wang, Yi-Chien
Lin, Shu-Wei
Liu, Chih-Ming
Jiang, Jian-Ming
Lin, Ming-Chang
Wei, Kung-Hwa
Department of Materials Science and Engineering
Department of Applied Chemistry
Keywords: bulk heterojunctions;organic solar cell;solution-processed small molecules;crystallization
Issue Date: 25-Jun-2014
Abstract: In this study we synthesized three acceptor donor acceptor (A-D-A) organic molecules, TB3t-BT, TB3t-BTT, and TB3t-BDT, comprising 2,2\'-bithiophene (BT), benzo[1,2-b:3,4-b\':5,6-d \'\']trithiophene (BTT), and benzo[ 1,2-b;4,5-b\']dithiophene (BDT) units, respectively, as central cores (donors), terthiophene (3t) as pi-conjugated spacers, and thiobarbituric acid (TB) units as acceptors. These molecules display different degrees of coplanarity as evidenced by the differences in dihedral angles calculated from density functional theory. By using differential scanning calorimetry and X-ray diffractions for probing their crystallization characteristics and molecular packing in active layers, we found that the symmetry and coplanarity of molecules would significantly affect the melting/crystallization behavior and the formation of crystalline domains in the blend film with fullerene, PC61BM. TB3t-BT and TB3t-BDT, which each possess an inversion center and display high crystallinity in their pristine state, but they have different driving forces in crystallization, presumably because of different degrees of coplanarity. On the other hand, the asymmetrical TB3t-BTT behaved as an amorphous material even though it possesses a coplanar structure. Among our tested systems, the device comprising as-spun TB3t-BDT/PC61BM (6:4, w/w) active layer featured crystalline domains and displayed the highest power conversion efficiency (PCE) of 4.1%. In contrast, the as-spun TB3t-BT/PC61BM (6:4, w/w) active layer showed well-mixed morphology and with a device PCE of 0.2%; it increased to 3.9% after annealing the active layer at 150 degrees C for 15 min. As for TB3t-BTT, it required a higher content of fullerene in the TB3t-BTT/PC61BM (4:6, w/w) active layer to optimize its device PCE to 1.6%.
URI: http://dx.doi.org/10.1021/am501659u
ISSN: 1944-8244
DOI: 10.1021/am501659u
Volume: 6
Issue: 12
Begin Page: 9298
End Page: 9306
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