Polythiophenes comprising conjugated pendants toward long-term air-stable inverted polymer solar cells with high open circuit voltages

Abstract

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 (−5.26 to −5.39 eV). Furthermore, lower bandgaps were observed for the DBT-derived PTs because of stronger donor–π–acceptor characteristics and more efficient intramolecular charge transfer. Conventional PSCs were fabricated by spin-coating the blend of each PT and the fullerene derivative (PC₇₁BM). 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 : PC₇₁BM (w/w = 1 : 2.5) reached 2.48% with a V_oc of 0.91 V, short circuit current (J_sc) of 6.58 (mA cm⁻²) and fill factor (FF) of 41% under the illumination of AM1.5, 100 mW cm⁻². Furthermore, a PTtCz/PC₇₁BM-based inverted PSC with ZnO_x and MoO₃ 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.