Synthesis and Annealing Study of Silicon- and Fluorine-containing Low Bandgap Conjugated Polymers for Solar Cell Applications
Two donor-acceptor alternating conjugated copolymers were synthesized as the donor materials of the active layer in polymer solar cells via Stille coupling reaction. Both copolymers consisted of dithienosilole as the donor unit and nonfluorinated 2,1,3-benzooxadiazole or fluorinated 2,1,3-benzooxadiazole as the acceptor unit. The nonfluorinated polymer and fluorinated polymer were designated as P1 and P2, respectively. The structures of copolymers were confirmed by FT-IR, 1H NMR, and 13C NMR. Optoelectronic properties of the polymer were investigated and observed by UV-vis spectrum, photoluminescence spectrum, and cyclic voltammetry. Both polymers exhibited a panchromatic absorption ranging from 300 nm to 1100 nm and displayed low band gaps of 1.46 eV and 1.42 eV, respectively. Both of the synthesized materials were used as the donor material in the bulk heterojunction (BHJ) solar cells and then power conversion efficiency (PCE) measurements were conducted in different weight ratios of the polymer:PC61BM blends.
Compared to the nonfluorinated one, the fluorinated polymer exhibited a higher PCE of 2.67% at room temperature under the illumination of AM 1.5 (100 mW/cm2). Study of the effect of annealing on the performance of the P1/PC61BM devices using a 1:2 blend ratio of P1:PC61BM exhibited the highest short-circuit current density of 5.88 mA/cm2 and a power conversion efficiency of 2.76% at annealing temperature of 125℃. The bulk heterojunction polymer solar cell employing P2 and PC61BM at a blend ratio of 1:2 exhibited the highest short-circuit current density of 6.44 mA/cm2 and a power conversion efficiency of 3.54% at annealing temperature of 150℃.
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