Synthesis and characterization of cyclopentadithiophene-based low bandgap copolymers for all-polymer solar cells

Abstract

Three conjugated copolymers based on cyclopentadithiophene (CPDT) units, namely, poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl} (P1), Poly{4,4-bis(2-ethylhexyl)cyclopeanta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl} (P2) and Poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-N,N′-bis(2-ethylhexyl)-3,4,9,10-perylene diimide-1,7-diyl} (P3) have been synthesized via Stille coupling reaction. The polymers were characterized by 1H NMR spectroscopy, gel permeation chromatography (GPC), UV–vis absorption spectroscopy, and cyclic voltammetry. These new polymers exhibit broad and strong absorption between 500 and 800 nm. The highest occupied molecular orbital energy levels of polymers vary between −4.98 and −5.27 eV and the lowest unoccupied molecular orbital energy levels range from −3.43 to −3.70 eV. By employing P1 and P2 as electron donors (D) and P3 as electron acceptor (A), all-polymer solar cells with bulk heterojunction structure have been fabricated. Preliminary results indicate that these devices show higher open circuit voltage (VOC) in comparison with the traditional polymer/fullerene systems of P1 and P2 blended with the acceptor (6,6)-phenyl C61-butyric acid methyl ester (PCBM).