We synthesized a series of polythiophenes (PTs) featuring 2-ethylhexyl-substituted terthiophene (T) or quaterthiophene (BT) as the conjugated unit in the polymer backbone with pendant conjugated tertbutyl-substituted triphenylamine (tTPA)-or carbazole (tCz)-containing moieties as side chains, namely PTtTPA, PBTtTPA, PTtCz and PBTtCz. Incorporating T and BT moieties into the polymer backbone and attaching tTPA or tCz units promoted efficient conjugation within the extended conjugated frameworks of the polymers, resulting in lower band-gap energies and red-shifting of the maximal UVVis absorption wavelength. The higher electron-donating ability of tTPA resulted in broader absorption bands and lower band-gap energies of PTtTPA and PBTtTPA as compared with PTtCz and PBTtCz. Incorporation of the T and BT moieties into the polymer backbone enhanced the compatibility of PT and the fullerene derivative by reducing the side-chain density of PT, thus providing sufficient free volume for efficient incorporation of [6,6]phenyl-C 61 -butyric acid methyl ester (PC 61 BM) into the polymer chains. Polymer solar cells (PSCs) were fabricated by spin-coating a blend of each PT with the fullerene derivative (PC 61 BM) as a composite film-type photoactive layer; PBTtTPA/PC 61 BM-based PSCs showed superior photovoltaic (PV) performance to PTtTPA/ PC 61 BM-based PSCs in terms of conjugation and absorption band broadness. However, PBTtCz/ PC 61 BM-based PSCs showed inferior PV performance to PTtCz/PC 61 BM-based PSCs. The lower HOMO level led to a higher open-circuit voltage (V oc ; 0.74 V) and larger photo-energy conversion efficiency (h; 2.77%) of PTtCz/PC 61 BM-based PSCs.

A series of polythiophenes (PTs) functionalized with bulky conjugated side chains comprising tert-butyl substituted carbazole (tCz) as an electron donor pendant and bisbenzothiazolylvinyl (DBT) as an electron acceptor pendant were synthesized via Stille copolymerization for polymer solar cell (PSC) applications. We use the descriptors PTtCz, PT(tCz) 0.9 (DBT) 0.1 , PT(tCz) 0.64 (DBT) 0.36 , PT(tCz) 0.45 (DBT) 0.55 , and PTDBT to identify each of these conjugated polymers, with the names denoting the compositions of the bulky pendants. The tunable energy levels of the PTs were accomplished by incorporating both tCz as a donor pendant and DBT as an acceptor pendant, while retaining the low-lying HOMO levels (À5.26 to À5.39 eV). Furthermore, lower bandgaps were observed for the DBT-derived PTs because of stronger donor-p-acceptor characteristics and more efficient intramolecular charge transfer.Conventional PSCs were fabricated by spin-coating the blend of each PT and the fullerene derivative (PC 71 BM). The conventional PSC devices exhibited high open circuit voltages (V oc ) of around 0.79-0.91 V. The power conversion efficiency (PCE) of the PSCs based on PTtCz : PC 71 BM (w/w ¼ 1 : 2.5) reached 2.48% with a V oc of 0.91 V, short circuit current (J sc ) of 6.58 (mA cm À2 ) and fill factor (FF) of 41% under the illumination of AM1.5, 100 mW cm À2 . Furthermore, a PTtCz/PC 71 BM-based inverted PSC with ZnO x and MoO 3 as an electron extraction layer and a hole extraction layer respectively was capable of retaining ca. 80% of its original efficiency after storage under ambient conditions (without encapsulation) for 1032 h, according to the ISOS-D-1 shelf protocol. The highly durable inverted PSC accompanied by a large V oc value was achieved for the PT-type polymers.