Naphtho[1,2-b:5,6-b']dithiophene-Based Conjugated Polymers for Fullerene-Free Inverted Polymer Solar Cells

Three novel copolymers based on zigzag naphthodithiophene (zNDT) with different aromatic rings as pi bridges and different core side substitutions are designed and synthesized (PzNDT-T-1,3-bis(4-(2-ethylhexyl)-thiophen-2- yl)-5,7-bis(2-ethylhexyl) benzo[1,2-c:4,5-c']-dithiophene-4,8-dione (BDD), PzNDT-TT-BDD, and PzNDTP-T-BDD, respectively). The 2D conjugation structure and molecular planarity of the polymers can be effectively altered through the modification of conjugated side chains and pi-bridges. These alterations contribute to the variation in energy levels, light absorption capacity, and morphology compatibility of the polymers. When blended with the nonfullerene acceptor (2,2'-[(4,4,9,9-tetrahexyl-4,9-dihydro-sindaceno[1,2b:5,6-b'] dithiophene-2,7-diyl) bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]] bis-propanedinitrile) (IDIC), PzNDT-T-BDD exhibits the highest power conversion efficiency (PCE) of 9.72% among the three polymers. This result can be attributed to its superior crystallinity and more obvious face-on orientation in blending film. PzNDT-TT-BDD and PzNDTP-T-BDD present PCE values of 8.20% and 4.62%, respectively. The alteration of polymer structure, particularly the modification of conjugated side chains and pi-bridges, is an effective strategy for designing NDT-based polymers with high photovoltaic performance and potential applications in fullerene-free solar cells.