Manipulating Backbone Planarity of Ester Functionalized Conjugated Polymer Constitutional Isomer Derivatives Blended with Molecular Acceptors for Controlling Photovoltaic Properties

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