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

Abstract

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 (V_oc) 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 V_oc = 0.78 V, short circuit current (J_sc) 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.