A new class of low-bandgap copolymers based on benzodithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) units is reported. Chemical modifications of the conjugated backbone promote both high molecular weights and processability while allowing for tuning of the electronic properties. Copolymers with substituted thiophene spacers (alkyl chains facing the BDT unit) or unsubstituted thiophene spacers tend to have low power conversion efficiencies (PCE less than 1%) due to a bad morphology of the active layer, whereas copolymers with substituted thiophene spacers (alkyl chains facing TPD unit) show enhanced morphology and PCEs up to of 3.9%. Finally, BDT-TPD copolymers without any thiophene spacers still show the best performances with power conversion efficiencies up to 5.2%.
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