am6b11806_si_001.pdf (2.24 MB)
Systematic Investigation of Benzodithiophene-Benzothiadiazole Isomers for Organic Photovoltaics
journal contribution
posted on 2016-11-09, 00:00 authored by Jia Du, Andria Fortney, Katherine E. Washington, Chandima Bulumulla, Peishen Huang, Dushanthi Dissanayake, Michael C. Biewer, Tomasz Kowalewski, Mihaela C. StefanTwo
new donor–acceptor small molecules based on benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT) were designed and synthesized.
Small molecules 4,4′-[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(2,2′-bithiophene)-5,5′-diyl]bis(benzo[c][1,2,5]thiadiazole) (BDT-TT-BT) and 4,4′-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis[7-(2,2′-bithiophene-5-yl)benzo[c][1,2,5]thiadiazole] (BDT-BT-TT) are structural isomers
with the 2,2-bithiophene unit placed either between the BDT and BT
units or at the end of the BT units. This work is targeted toward
finding the effect of structural variation on optoelectronic properties,
morphology, and photovoltaic performance. On the basis of theoretical
calculations, the molecular geometry and energy levels are different
for these two molecules when the position of the 2,2-bithiophene unit
is changed. Optical and electrochemical properties of these two small
molecules were characterized using UV–vis and cyclic voltammetry.
The results showed that BDT-BT-TT has broader absorption and an elevated
HOMO energy level when compared with those of BDT-TT-BT. The performance
of these two isomers in solar cell devices was tested by blending
with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Power conversion efficiencies as high as 3.22 and 3.71%
were obtained in conventional solar cell structures for BDT-TT-BT
and BDT-BT-TT, respectively. The morphology was studied using grazing
incident wide-angle X-ray scattering and transmission electron microscopy,
which revealed different phase separations of these two molecules
when blended with PC71BM.