Manipulating\nBackbone Planarity of Ester Functionalized\nConjugated Polymer Constitutional Isomer Derivatives Blended with\nMolecular Acceptors for Controlling Photovoltaic Properties

Exploring both electron donor and acceptor phase components\nin\nbulk heterojunction structures has contributed to the advancement\nof organic photovoltaics (OPV) realizing power conversion efficiencies\nreaching 20%. Being able to control backbone planarity of the donor\npolymer, while understanding its effects on the polymer conformation\nand photophysical properties, fosters the groundwork for further achievements\nin this realm. In this report, three isomeric PM7 derivatives are\ndesigned and synthesized where the benzodithiophene-4,8-dione structure\nis replaced by a quaterthiophene bridge carrying two ester moieties.\nThe placement of these two ester groups varies among three configurational\nisomers, which ultimately influences the chain conformations and aggregation\nbehavior of each polymer. Specifically, PM7-D3 has ester groups attached\nto the inner positions of the outer thiophenes showing moderate solution\naggregation; PM7-D4 has ester groups attached to the inner positions\nof the inner thiophenes featuring a twisted backbone with no solution\naggregation behavior; and PM7-D5 has ester groups attached to the\nouter positions of the inner thiophenes with strong solution aggregation.\nPM7-D5 shows the highest average power conversion efficiency of 11.4%\npaired with the molecular acceptor L8-BO. In addition, the differences\namong the polymer backbones are expressed by their state energies\nand carrier mobility in the corresponding fabricated OPV devices.