Low-Energy-Gap Organic Based Acceptor-Donor-Acceptor -Conjugated Small Molecules for Bulk-Heterojunction Organic Solar Cells

We describe the detailed syntheses and characterization of two new electron-donor molecules based on an acceptor-donor-acceptor (A-D-A) structure with carbazole as the electron-rich building block, benzothiadiazole (BT) as the electron-acceptor building block and octylrhodanine as the end group. We also examined the effects of a thiophene group as a spacer between the BT and the carbazole units on the optoelectronic, morphological and photovoltaic properties. The presence of the thiophenes has pronounced effects on both the optical and electrical properties. ECTBT, which contains thiophenes units, showed a red-shifted absorption and a lower HOMO level compared to ECBT, which has no thiophene spacers. Optimized photovoltaic device fabrication based on ECTBT and [6,6]-phenyl-C-70-butyric acid methyl ester (PC71BM) in a 1:1.5 ratio (w/w) exhibited the best power conversion efficiency (PCE), at 3.26%.