Modulation of electronic properties of π-conjugated copolymers derived from naphtho[1,2-b:5,6-b′]dithiophene donor unit: A structure-property relationship study

A set of three donor-acceptor conjugated (D-A) copolymers were designed and synthesized via Stille cross-coupling reactions with the aim of modulating the optical and electronic properties of a newly emerged naphtho[1,2-b:5,6-b′] dithiophene donor unit for polymer solar cell (PSCs) applications. The PTNDTT-BT, PTNDTT-BTz, and PTNDTT-DPP polymers incorporated naphtho[1,2-b:5,6- b′]dithiophene (NDT) as the donor and 2,2′-bithiazole (BTz), benzo[1,2,5]thiadiazole (BT), and pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP), as the acceptor units. A number of experimental techniques such as differential scanning calorimetry, thermogravimetry, UV-vis absorption spectroscopy, cyclic voltammetry, X-ray diffraction, and atomic force microscopy were used to determine the thermal, optical, electrochemical, and morphological properties of the copolymers. By introducing acceptors of varying electron withdrawing strengths, the optical band gaps of these copolymers were effectively tuned between 1.58 and 1.9 eV and their HOMO and LUMO energy levels were varied between -5.14 to -5.26 eV and -3.13 to -3.5 eV, respectively. The spin-coated polymer thin film exhibited p-channel field-effect transistor properties with hole mobilities of 2.73 × 10^-3 to 7.9 × 10^-5 cm² V^-1 s^-1. Initial bulk-heterojunction PSCs fabricated using the copolymers as electron donor materials and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as the acceptor resulted in power conversion efficiencies in the range of 0.67-1.67%.