Achieving High-Efficiency Organic Photovoltaics from a New Completely Amorphous Donor Polymer

The development of non-fullerene acceptors (NFAs) such as ITIC and Y6 has greatly improved the efficiency of polymer solar cells. Therefore, focus should now shift towards the design of donor polymers with better compatibility with these NFAs to attempt to push efficiencies to higher levels. The indacenodithienothiophene (IDT) unit has up till now been typically incorporated into donor polymers for fullerene-based solar cells; however, the application of IDT-based polymers in NFA solar cells is rare. In order to increase the number of donor polymer candidates, we have synthesized two new polymers PIDT-T8BT and PIDT-T12BT conjugating IDT and perfluorinated benzothiadiazole moieties through octyl and dodecyl thiophene bridging units, respectively. These polymers were studied and revealed to be completely amorphous without aggregates, and their glass transition temperatures were distinct owing to the different lengths of the alkyl groups attached to the thiophene units. In addition, the length of the side groups greatly affects the solar cell performance. The longer dodecyl side groups in the polymer PIDT-T12BT resulted in a lower glass transition temperature and favorable thermal annealing conditions of the active layer blend with Y6. A promising power conversion efficiency of over 12% was achieved for PIDT-T12BT when paired with the Y6 acceptor and thermally annealed at 170 °C, which is the highest reported value so far for IDT-based donor polymers.