An Easily Accessible Donor−π-Acceptor-Conjugated Small Molecule from a 4,8-Dialkoxybenzo[1,2‑b:4,5‑b′]dithiophene Unit for Efficient Solution-Processed Organic Solar Cells

A new donor–acceptor-conjugated organic small molecule, BDT(TBT) 2 , comprised of benzo[1,2-b:4,5-b′]dithiophene and 2,1,3-benzothiadiazole units was designed and synthesized. The small molecule BDT(TBT) 2 in its thin film showed an absorption band in the range of 300–700 nm with an absorption edge at 650 nm and an optical band gap of 1.90 eV. As estimated from the cyclic voltammetry measurements, the HOMO and LUMO energy levels of BDT(TBT) 2 were −5.44 and −3.37 eV, respectively. The spin-coated thin film of BDT(TBT) 2 exhibited p-channel output characteristics with a hole mobility of 2.7 × 10–6. BDT(TBT) 2 , when explored as an electron-donor material in solution-processed bulk-heterojunction organic solar cells in conjunction with a PC71BM acceptor with an active layer thickness of 50–55 nm, generated a power conversion efficiency (PCE) of 1.18%. A more impressive PCE of ∼2.9% with a short-circuit current density (J sc) of 7.94 mA cm–2 and an open-circuit voltage (V oc) of 0.89 V was achieved when the active layer of the cell was annealed at higher temperature (∼180 °C).