Synthesis and characterization of π-conjugated copolymers with thieno-imidazole units in the main chain: application for bulk heterojunction polymer solar cellsElectronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00017c

• Main Authors: Keshtov, M. L, Godovsky, D. Yu, Chen, F. C, Khokhlov, A. R, Siddiqui, S. A, Sharma, G. D
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c5cp00017c

In this paper the three new narrow bandgap D-A conjugated copolymers P1 , P2 and P3 based on different weak donor fused thiophene-imidazole containing derivatives and the same benzothiadiazole acceptor unit were synthesized by Stille cross-coupling polymerization and characterized by 1 H NMR, elemental analysis, GPC, TGA, DSC. These copolymers exhibit intensive absorbance in the range 350-900 nm and the optical bandgap lies in the range of 1.50-1.61 eV, which corresponds to the maximum photon flux of the solar spectrum. The electrochemical bandgap derived from cyclic voltammetry varies within the limits 1.47-1.65 eV and is approximately very close to the optical bandgap. The highest occupied molecular orbital (HOMO) energy level of all copolymers is deep lying (−5.24 eV and −5.37 eV and −5.25 eV for P1 , P2 and P2 , respectively) which shows that copolymers have good stability in the air and assured a higher open circuit voltage ( V oc ) for polymer BHJ solar cells. These copolymers were used as donors along with PC 71 BM and the BHJ polymer solar cells based on P1 :PC 71 BM, P2 :PC 71 BM and P3 :PC 71 BM processed from chloroform (CF) solvent with 3 v% DIO as an additive showed an overall PCE of 4.55%, 6.76% and 5.16%, respectively. Three D-A conjugated copolymers P1 , P2 and P3 based on different weak donors and the same acceptor unit, were used as donors for BHJ solar cells and achieved a PCE of 6.76%.