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 (λ_max^abs) 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 (V_OC) of 0.73–0.99 V, significantly higher than that of P3HT based cell (V_OC of 0.52 V), which can be ascribed to the lower HOMO energy levels and less condensed molecular packing of these two polymers.