Diindenothieno[2,3-b]thiophene arene for efficient organic photovoltaics with an extra high open-circuit voltage of 1.14 evw

Over the past decade, considerable progress has been made in the development of organic materials for photovoltaic applications. Bulk heterojunction (BHJ) using n-type and a p-type semiconductor materials in the active layer is the most widely adopted device architecture to ensure maximum internal donor–acceptor (D–A) interfacial area for efficient charge separation. The most critical challenge at the molecular level is to combine a suitable pair of p-type and n-type semiconductors in the active layer that can simultaneously produce high open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF). The most straightforward guideline to enlarge Voc is to lower the donor’s highest occupied molecular orbital (HOMO) energy level or raise the acceptor’s lowest unoccupied molecular orbital (LUMO) level. Thiophene and benzene aromatic rings are the most important structural ingredients to comprise p-type conjugated polymers. Benzene-based units such as tricyclic 2,7-fluorene or 2,7-carbazole units have shown to serve as useful building blocks in the construction of donor–acceptor polymers having deep-lying HOMO energy levels that contribute to yield high Voc (40.8 V) for PSCs. However, the intrinsic drawback is that these polymers usually possess relatively large optical band gaps (42 eV) that limit their ability to harvest sunlight and thus result in moderate Jsc. On the other hand, due to the lower aromaticity and easier oxidation, thiophene-based D–A polymers have better light absorption ability to permit greater Jsc. However, their Voc values are generally limited to ca. 0.6 V as a result of the high-lying HOMO levels. It has been demonstrated that forced planarization by covalently fastening adjacent aromatic units in the polymer backbone can reduce the band gap and enhance the intrinsic charge mobility. By extracting benefits from benzene and thiophene moieties to acquire high Voc and Jsc, it is of great interest to integrate benzene units and thiophene units into a molecular entity with forced rigidification to simultaneously extend the conjugation while maintaining the coplanarity. In this regard, we recently developed a pentacyclic diindeno[1,2-b:20,10-d]-thiophene (DIDT) unit that was polymerized to afford an alternating poly(diindenothiophene-altdithienylbenzothiadizole) copolymer PDIDTDTBT (Scheme 1). However, the device using the PDIDTDTBT/PC71BM (1 :2, w/w) blend only gave a Voc of 0.7 V and a Jsc of 5.3 mA cm 2 with a lower PCE of 1.65%. The thieno[3,2-b]thiophene (TT) unit has been an appealing building block for high mobility p-type semiconductors. This fused structure possesses higher aromatic stabilization energy than a thiophene, which can potentially lower the HOMO level for higher Voc. 9 Moreover, the C2h symmetry and coplanar geometry allow more ordered packing and stronger interchain interactions to achieve exceptional hole mobility, which is beneficial for Jsc. 10 However, introducing the