Synthesis and Characterization of a Novel Naphthodithiophene-Based Copolymer for Use in Polymer Solar Cells

A novel naphtho[2,3-b:6,7-d′]dithiophene (NDT)-based copolymer, poly[4,9-bis(2-ethylhexyloxy)naphtho[2,3-b:6,7-d′]dithiophene-2,7-diyl-alt-1,3-(5-heptadecan-9-yl)-4H-thieno[3,4-c]pyrrole-4,6-dione] (P1), was synthesized and characterized for use in polymer solar cells. We systematically investigated the synthesis, thermal stability, optical and electrochemical properties, field-effect carrier mobilities, and photovoltaic characteristics of the resulting polymer. The properties of the NDT-based polymer were compared with those of poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-1,3-(5-heptadecan-9-yl)-4H-thieno[3,4-c]pyrrole-4,6-dione] (P2) prepared via the same synthetic procedure. Polymer solar cells made from the new polymer, P1, yielded a power conversion efficiency of 4.0% with a short-circuit current density of 11.54 mA/cm2, an open-circuit voltage of 0.69 V, and a fill factor of 0.50 under AM 1.5 G irradiation (100 mW/cm2).