Synthesis, molecular structure and photovoltaic performance for polythiophenes with β-carboxylate side chains

To lower the HOMO energy level of polythiophenes, carboxylate groups were introduced to the β-position of the thiophene unit, by which two polythiophenes with tetrathiophene (poly[5,5′′-(bis-3,3′′-((2-butyloctyl)-carboxylate)-2,2′:2′,2′′-terthiophene)-alt-5-thiophene], P-4T-2COOR) or pentathiophene (poly[5,5′′-(bis-3,3′′-((2-butyloctyl)-carboxylate)-2,2′:2′,2′′-terthiophene)-alt-5,5′-(2,2′-bithiophene)], P-5T-2COOR) repeating unit were synthesized. Absorption spectroscopy and cyclic voltammetry measurements revealed that the β-carboxylate substitution red-shifts the maximum absorption wavelength (λmaxabs) in solution owing to the electron accepting nature of the carboxylate group. In addition, the introduction of β-carboxylate reduces the HOMO level from -5.09 eV for P3HT to -5.34 eV and -5.18 eV for P-4T-2COOR and P-5T-2COOR, respectively, which is in good agreement with quantum chemisty calculation results. However, the β-carboxylate side chain showed different orientation to that of P3HT, which leads to weaker intermolecular π-π interaction as confirmed by less red-shited absorption in thin solid film and the quantum calculation results. Polymer solar cells using P-4T-2COOR and P-5T-2COOR as the electron donor, 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno‐[1,2‐b:5,6-b′]di‐thiophene (ITIC) as the electron acceptor were fabricated and tested. The P-4T-2COOR and P-5T-2COOR based cells showed high open circuit (VOC) of 0.73–0.99 V, significantly higher than that of P3HT based cell (VOC of 0.52 V), which can be ascribed to the lower HOMO energy levels and less condensed molecular packing of these two polymers.