Molecular Engineering of Conjugated Polymers for Solar Cells and Field-Effect Transistors: Side-Chain Versus Main-Chain Electron Acceptors

Abstract

Two model polymers, containing fluorene as an electron-donating moiety and benzothiadiazole (BT) as an electron-accepting moiety, have been synthesized by Suzuki coupling reaction. Both polymers are composed of the same chemical composition, but the BT acceptor can be either at a side-chain (i.e., S-polymer) or along the polymer main chain (i.e., M-polymer). Their optical, electrochemical, and photovoltaic properties, together with the field-effect transistor (FET) characteristics, have been investigated experimentally and theoretically. The FET carrier mobilities were estimated to be 5.20 x 10(-5) and 3.12 x 10(-4) cm(2) V(-1) s(-1) for the S-polymer and M-polymer, respectively. Furthermore, polymeric solar cells (PSCs) with the ITO/PEDOT:PSS/S-polymer or M-polymer: PC(71)BM(1:4)/Al structure were constructed and demonstrated to show a power conversion efficiency of 0.82 and 1.24% for the S-polymer and M-polymer, respectively. The observed superior device performances for the M-polymer in both FET and PSCs are attributable to its relatively low band-gap and close molecular packing for efficient solar light harvesting and charge transport. This study provides important insights into the design of ideal structure-property relationships for conjugate polymers in FETs and PSCs.