Browsing by Author "Lu, Yueh-Hsin"

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Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules
(2015-01-01)
Lan, Shang-Che
;
Chang, Chiao-Kai
;
Lu, Yueh-Hsin
;
Lin, Shu-Wei
;
Jen, Alex K. -Y.
In this study we synthesized a series of solution-processable small molecules comprising 2,2'-bithiophene (BTh), terthiophene (TTh), and thiobarbituric acid (TB) units as the central core, pi-conjugated spacer, and acceptor end-capping moieties, respectively, but with alkyl side-chains of different lengths presented from their central BTh units (TBTThBTh-H, TBTThBTh-C4, TBTThBTh-C8, TBTThBTh-C12). We then investigated the structure-property relationships of these compounds in terms of their packing behaviors and bulk heterojunction (BHJ) photovoltaic properties. And we found that the packing of these molecules in neat films is critically dependent of their side-chain lengths, as evidenced by the variations in their lamellar structures determined with grazing-incidence wide-angle X-ray scattering (GIWAXS). The power conversion efficiencies (PCEs) of the photovoltaic BHJ devices comprising these small molecules and PC61BM exhibited zigzag-shaped variations with respect to the alkyl side-chain lengths, with the PCE of devices incorporating TBTThBTh-H and TBTThBTh-C8 being higher than those of devices incorporating TBTThBTh-C4 and TBTThBTh-C12. Using GIWAXS to probe the molecular packing in the BHJ active layers, we found that the alkyl chain lengths of the small molecules had a large impact on the formation of crystallites in the BHJ films; the molecules with more uniform and shorter alkyl side-chain lengths provide stronger intermolecular interactions, being more favorable for the crystallization of these molecules.
Symmetry and Coplanarity of Organic Molecules Affect their Packing and Photovoltaic Properties in Solution-Processed Solar Cells
(2014-06-25)
Lan, Shang-Che
;
Raghunath, Putikam
;
Lu, Yueh-Hsin
;
Wang, Yi-Chien
;
Lin, Shu-Wei
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%.