Synthesis characterization and bulk-heterojunction photovoltaic applications of new naphtho[1,2-b:5,6-b′]dithiophene-quinoxaline containing narrow band gap D-A conjugated polymers

Alternating donor-acceptor (D-A) π-conjugated copolymers, poly[2,7-bis(3-hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5, 5′-diyl-alt-5,8-bis(4-hexadecylthiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl) quinoxaline-5,5′-diyl] (PTNDTT-QX-I) and poly[2,7-bis(3- hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5, 5′-diyl-alt-5,8-bis(thiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl) quinoxaline-5,5′-diyl] (PTNDTT-QX2-II), were designed and synthesized based on the same thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene donor moiety, differing only at the quinoxaline acceptor counterpart by either additional electron-donating alkyl chain substitution in the thienyl ring attached to the quinoxaline base (in PTNDTT-QX-I) or a change in the location of the outward alkoxy side chain substituent of the phenyl rings (to the meta-position) adjoining the quinoxaline base (in PTNDTT-QX-II). The effect of alkyl chain positioning on the thermal, optical, and electrochemical properties, as well as field effect transistors and solar cell performances of the copolymers, were investigated and the results were compared with a previously published copolymer, PTNDTT-QX, which features a similar quinoxaline unit but is alkoxy substituted at the position para to its peripheral phenyl rings. Both polymers exhibited excellent thermal stability, with thermal decomposition temperatures over 400 °C. They absorbed light in the 300-700 nm range and exhibited optical band gaps of about 1.70 and 1.73 eV for PTNDTT-QX-I and PTNDTT-QX-II, respectively. Precise control of the alkyl/alkoxy chain positioning has made it possible to tune the HOMO energy levels between -5.14 and -5.29 eV and the LUMO energy levels between -3.44 and -3.55 eV. Bulk heterojunction photovoltaic devices of the structure ITO/PEDOT:PSS/polymer: PC₇₁BM/LiF/Al were fabricated by using the polymers as the donors and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) as the acceptor. Power conversion efficiencies (PCEs) of 1.28% and 1.61% respectively were achieved for the photovoltaic devices based on PTNDTT-QX-I/PC₇₁BM and PTNDTT-QX-II/PC₇₁BM under AM 1.5 G simulated 1-sun solar illumination.