An oligothiophene chromophore with a macrocyclic side chain: synthesis, morphology, charge transport, and photovoltaic performance

Abstract

An oligothiophene chromophore RingBDT(T₃A)₂ has been synthesized, where BDT is benzo[1,2-b:4,5-b′]dithiophene, Ring is a 1,12-dodecylenedioxy cyclic side chain on the benzene of BDT, T₃ is 2,2′:5′,2′′-terthiophene, and A is an electron acceptor. In single crystals, the immediate precursor of RingBDT(T₃A)₂ formed π-dimers and the ring prevented further π-stacking of the dimers. A differential scanning calorimetry study showed that BDT(T₃A)₂, the ringless analog with two 2-ethylhexyloxy side chains on BDT, crystallized quickly from its melt upon cooling, while crystallization of RingBDT(T₃A)₂ melt upon cooling was slow and incomplete. Interestingly, RingBDT(T₃A)₂ solid crystallized fast at ∼110 °C upon heating, but its thin films (200 nm) remained amorphous after annealing at 80 °C. Despite the amorphous nature, the hole mobility of RingBDT(T₃A)₂ films (1.52 × 10⁻³ cm² V⁻¹ s⁻¹) was 144% higher than that of the highly crystalline BDT(T₃A)₂ films (200–80 nm). Solar cells were fabricated from blends of the chromophores and phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM). Thermal annealing at 100 °C for 10 minutes enhanced chromophore π–π interaction, and improved device fill factor and efficiency for the RingBDT(T₃A)₂ blend solar cells, while retaining the amorphous nature of blend. In stark contrast, thermal annealing under the same conditions caused the efficiency of BDT(T₃A)₂ cell efficiency to drop by 82%. This study demonstrates the effectiveness of using a macrocyclic side chain as a strategy for developing amorphous molecular semiconducting materials with improved mobility and morphological stability.