Single and Double Alkoxybenzothiadiazole-Dithienosilole-Based Nonfused Ring Electron Acceptors for Organic Solar Cells

Three alkoxybenzothiadiazole (BTO)-based nonfused ring electron acceptors (BTO-FIC, BTO-T-FIC, and BTO-TT-FIC) were designed and synthesized with a central core unit with a single-BTO (A′) or double-BTO (A′-π-A′) structure. In double-BTO core molecules, thiophene (T) or thienothiophene (TT) π-linkers were introduced to explore the effect of these structural modifications on the optical and electrochemical properties. Tuning of the central core by using a single-BTO or double-BTO structure changed the absorption properties, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, and molecular crystallinity. The BTO-FIC, BTO-T-FIC, and BTO-TT-FIC acceptor films exhibited absorption peaks at 802, 783, and 802 nm, respectively, with strong shoulders, which complement well with those of the PBDB-T donor film. The HOMO/LUMO levels of BTO-FIC, BTO-T-FIC, and BTO-TT-FIC films were −5.79/-4.35, −5.65/-4.18, and −5.61/-4.18 eV, respectively. The HOMO/LUMO levels of the double-BTO core (A′-π-A′)-based acceptors were upshifted by at least ∼0.15 eV compared to those of the single-BTO core (A′)-based acceptor, leading to an increase in V_OC. In addition, the photovoltaic performance of devices based on these acceptors varied considerably, with the BTO-FIC, BTO-T-FIC, and BTO-TT-FIC devices displaying power conversion efficiencies (PCEs) of 10.6, 5.4, and 6.5%, respectively. The superior PCE of the BTO-FIC device is attributed to enhanced hole and electron mobilities, minimized carrier recombination, and high crystallinity, which contribute to higher values of J_SC and PCE.