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In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A-D-A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C61-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (Jsc) of 9.08 mA cm(-2), an open circuit voltage (Voc) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW cm(-2)). The FFs of these solution-processed small-molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.

A series of low-band-gap (LBG) donor-accepor con- jugated main-chain copolymers (P1-P4) containing planar 2,7- carbazole as electron donors and bithiazole units (4,4 0 -dihexyl- 2,2 0 -bithiazole and 4,4 0 -dihexyl-5,5 0 -di(thiophen-2-yl)-2,2 0 -bithia- zole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with opti- cal band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. More- over, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (Voc) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C61- butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC71BM (1:1.5 w/w) showed the best power conver- sion efficiency value of 1.01% with a short circuit current den- sity (Jsc) of 4.83 mA/cm 2 , a fill factor (FF) of 35%, and Voc ¼ 0.60 V under 100 mW/cm 2 of AM 1.5 white-light illumination.