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.