Functional tuning of A–D–A oligothiophenes: the effect of solvent vapor annealing on blend morphology and solar cell performance

Abstract

A series of solution-processable acceptor–donor–acceptor (A–D–A) oligomers consisting of various central conjugated units, namely, carbazole, benzo[2,1-b:3,4-b′]dithiophene, 2,2′-bithiophene, dithieno[3,2-b:2′,3′-d]silole and dithieno[3,2-b:2′,3′-d]pyrrole were synthesized and developed for application in bulk-heterojunction solar cells (BHJSC). The alteration of the core moiety, while maintaining the shape of the molecular structure, enables fine-tuning of the optical energy gap and highest occupied molecular orbital (HOMO) level of the molecules. Depending on the donor strength of the core, the maximum absorption wavelength of the oligomers ranged from 488 nm to 560 nm in solution and from 530 nm to 694 nm in neat films. HOMO energy levels were shifted in a stepwise fashion from −5.8 to −5.3 eV yielding oligomers with HOMO–LUMO energy gap between 2.04 and 1.60 eV. The structural fine-tuning is further visible in the photovoltaic performance. BHJ solar cells prepared using these oligomers as donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as acceptor demonstrated power conversion efficiencies between 1.4 and 5.9% after solvent vapor annealing. Exposure of the photoactive layer to organic solvent vapor led to re-organization of the donor material within the blend and a large enhancement of J_SC and FF was observed. The role of solvent vapor annealing on the degree of crystallinity and blend morphology was further investigated by grazing incident X-ray diffraction (GIXRD) and atomic force microscopy (AFM) analysis.