Downwards tuning the HOMO level of polythiophene by carboxylate substitution for high open-circuit-voltage polymer solar cells

Three polythiophene derivatives with carboxylate substituents, PT–C1, PT–C2 and PT–C3 were synthesized by the Pd-catalyzed Stille-coupling method, for downwards tuning of the highest occupied molecular orbital (HOMO) of polythiophene. The polymers show similar visible absorption spectra to that of regioregular poly(3-hexylthiophene) (P3HT). By attaching the carboxylate substituent, the HOMO energy level downwards shifted to −5.15, −5.11 and −5.10 eV for PT–C1, PT–C2 and PT–C3, respectively, in comparison with that (ca. −4.8 eV) for P3HT. The polymer solar cells (PSCs) based on the three polymers as donor and PC70BM as acceptor all exhibit relatively high open circuit voltage (Voc) of ca. 0.8 V. The power conversion efficiency (PCE) of the PSCs based on PT–C3: PC70BM = 1 : 1.5 (w/w) reached 3.87% with Voc = 0.78 V, short circuit current (Jsc) of 9.68 mA cm−2 and fill factor (FF) of 51.2% under the illumination of AM1.5, 100 mW cm−2. These results indicate that attaching the carboxylate substituent is a simple and effective way to modulate energy levels of polythiophene derivatives and PT–C3 is a promising donor material for PSCs.