Synthesis and characterization of triphenylamine flanked thiazole-based small molecules for high performance solution processed organic solar cells

Two new small molecules, 5,5-bis(2-triphenylamino-3-decylthiophen-2-yl)-2,2-bithiazole (M1) and 2,5-bis(2-triphenylamino-3-decylthiophen-2-yl) thiazolo[5,4-d]thiazole (M2) based on an electron-donor triphenylamine unit and electron-acceptor thiophene-thiazolothiazole or thiophene-bithiazole units were synthesized by a palladium(0)-catalyzed Suzuki coupling reaction and examined as donor materials for application in organic solar cells. The small molecules had an absorption band in the range of 300-560 nm, with an optical band gap of 2.22 and 2.25 for M1 and M2, respectively. As determined by cyclic voltammetry, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of M1 were -5.27 eV and -3.05 eV, respectively, which were 0.05 eV and 0.02 eV greater than that of M2. Photovoltaic properties of the small molecules were investigated by constructing bulk-heterojunction organic solar cell (OSC) devices using M1 and M2 as donors and fullerene derivatives, 6,6-phenyl-C61-butyric acid methyl ester (PC61BM) and 6,6-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptors with the device architecture ITO/PEDOT:PSS/M1 or M2:PCBM/LiF/Al. The effect of the small molecule/fullerene weight ratio, active layer thickness, and processing solvent were carefully investigated to improve the performance of the OSCs. Under AM 1.5 G 100 mW/cm(2) illumination, the optimized OSC device with M1 and PC71BM at a weight ratio of 1: 3 delivered a power conversion efficiency (PCE) of 1.30%, with a short circuit current of 4.63 mA/cm(2), an open circuit voltage of 0.97 V, and a fill factor of 0.29. In contrast, M2 produced a better performance under identical device conditions. A PCE as high as 2.39% was recorded, with a short circuit current of 6.49 mA/cm(2), an open circuit voltage of 0.94 V, and a fill factor of 0.39. (C) 2011 Elsevier B. V. All rights reserved.