A potential naphtho[2,1-b:3,4-b′]dithiophene-based polymer with large open circuit voltage for efficient use in organic solar cells

A novel copolymer, PNDT-BTN, based on naphtho[2,1-b:3,4-b'] dithiophene (NDT) and alkoxy naphthalene, was designed and synthesized by the Pd-catalyzed Stille-coupling method. The copolymer demonstrated good solubility and film-forming ability, along with good thermal stability. PNDT-BTN exhibited a broad absorption (from 300 to 600 nm), centered at 492 nm. HOMO and LUMO energy levels of the polymer were estimated to be -5.56 eV and -3.48 eV, respectively. The polymer solar cell fabricated from the blend of the polymer (donor) and PC71BM (acceptor) exhibited a high power conversion efficiency of 4.29% with a high V-OC of 0.98 V. To the best of our knowledge, this is among the highest V-OC values of PSCs based on NDT derivatives. This work demonstrates that the replacement of a vinylene group in a conjugated polymer with alkoxy naphthalene moieties is able to significantly lower HOMO energy levels, and therefore, increase the open circuit voltage of solar cells.