Synthesis and characterization of low band gap random copolymers based on cyclopentadithiophene and thiophene carboxylates for photovoltaic applications

We have synthesized a series of conjugated random copolymers—P1-COOR′, P2-COOR″, and P3-COOR″—containing electron-donating units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene), alkyl 3-thiophene carboxylate, and thiophene by Stille polycondensation method. The polymers were characterized by 1H NMR spectroscopy, gel permeation chromatography (GPC), UV–Vis absorption spectroscopy, and cyclic voltammetry. The optical band gaps of the polymers varies from 1.88 to 1.93 eV and their absorption characteristics can be tuned by adjusting the ratio of the two electron-donating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene) and thiophene. These polymers exhibit broad and strong absorption between 300 and 700 nm with good solubility and thermal stability. Electrochemical studies indicate sufficiently deep HOMO levels and it gives high open-circuit voltage in the bulk heterojunction device when fullerene derivatives are used as electron acceptors. By changing the ratio of the two donor monomers, the highest occupied molecular orbital energy levels of polymers varies between −5.04 and −5.19 eV and the lowest unoccupied molecular orbital energy levels range from −3.16 to −3.26 eV. Preliminary results on solar cell fabrication using the synthesized polymers show open-circuit voltage in the range of 0.62–0.63 V.