Copolymerization of 3,3′′′-didodecylquaterthiophene with fluorene and silole units: improving photovoltaic performance by tuning energy levels

We report the design, synthesis and characterization of two copolymers, PQTF and PQTTST, based on 3,3′′′-didodecylquaterthiophene (QT), fluorene and 2,5-dithienylsilole. The spectroscopic, thermal properties and electrochemical properties were investigated. Thermal gravimetric analysis showed that they have good thermal stability with the decomposition temperature higher than 400 °C. The UV-vis absorption and cyclic voltammetry measurements revealed that the introduction of a fluorene unit into the backbone of PQTF resulted in a blue-shift of the longest wavelength absorption and a reduction of the HOMO level to −5.32 eV, while the absorption spectra red-shifted, and the LUMO level reduced to −3.59 eV with the incorporation of a 2,5-dithienylsilole unit into the backbone of PQTTST. In comparison with the homopolymer PQT, the copolymers exhibited disordered alkyl chain stacking. Bulk-heterojunction solar cells fabricated with PQTTST and PC60BM displayed power conversion efficiencies (PCE) of 1.49%, short-circuit currents (Jsc) of 4.77 mA cm−2 and open-circuit voltages (Voc) of 0.72 V. This PCE is the highest recorded for QT-based polymer donor and PC60BM acceptor bulk heterojunction (BHJ) photovoltaic cells.