Carboxylate-substituted polythiophenes for efficient fullerene-free polymer solar cells: the effect of chlorination on their properties

Two new wide-bandgap polythiophenes, i.e., poly[5,5′-bis(2-hexyldecyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-3-chloro-2,2′-bithiophene] (PDCBT-Cl) and poly[5,5′-bis(2-hexyldecyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-3,3′-dichloro-2,2′-bithiophene] (PDCBT-2Cl) comprising 3-chloro-2,2′-bithiophene and 3,3′-dichloro-2,2′-bithiophene moieties, respectively, were synthesized for fullerene-free polymer solar cells (PSCs). For comparison, three other polymers based on [2,2′-bithiophene]-4,4′-dicarboxylate (DCBT), i.e., PDCBT, PDCBT-F, and PDCBT-2F with 2,2′-bithiophene, 3-fluoro-2,2′-bithiophene, and 3,3′-difluoro-2,2′-bithiophene as comonomers, respectively, were also prepared. PSC devices were fabricated with these polymers as donor materials and ITIC-Th1 as acceptor. The incorporation of chlorine (Cl) or fluorine (F) atoms into polymers both efficiently downshifted the highest occupied molecular orbital (HOMO) energy levels, leading to higher open-circuit voltage (V_oc) in the PSCs. Owing to the proper phase-separated morphology with favorable molecular packing and miscibility, the device based on PDCBT-Cl:ITIC-Th1 exhibited efficient exciton dissociation and charge collection as well as weak charge recombination and thereby displayed the best power conversion efficiency (PCE) up to 12.38%. The devices based on other polymers showed inferior PCEs (8.14% for PDCBT, 10.85% for PDCBT-F, 8.48% for PDCBT-2F, and 6.94% for PDCBT-2Cl). The monomers that are used to make PDCBT-Cl can be synthesized in relatively large scale from commercial available chemicals. All these indicate that PDCBT-Cl is a promising donor material for the large area fabrication of high-performance fullerene-free PSCs.